The endogenous cannabinoid system and the basal ganglia:

biochemical, pharmacological, and therapeutic aspects

Julian Romeroa, Isabel Lastres-Beckerb, Rosario de Miguelb, Fernando Berrenderob,1,Jose A. Ramosb, Javier Fernandez-Ruizb,*

aLaboratorio de Apoyo a la Investigacion, Fundacion Hospital Alcorcon, 28922-Alcorcon, Madrid, SpainbDepartamento de Bioquımica y Biologıa Molecular III, Facultad de Medicina, Universidad Complutense, Ciudad Universitaria s/n, 28040-Madrid, Spain

Abstract

New data strengthen the idea of a prominent role for endocannabinoids in the modulation of a wide variety of neurobiological functions.

Among these, one of the most important is the control of movement. This finding is supported by 3 lines of evidence: (1) the demonstration

of a powerful action, mostly inhibitory in nature, of synthetic and plant-derived cannabinoids and, more recently, of endocannabinoids on

motor activity; (2) the presence of the cannabinoid CB1 receptor subtype and the recent description of endocannabinoids in the basal ganglia

and the cerebellum, the areas that control movement; and (3) the fact that CB1 receptor binding was altered in the basal ganglia of humans

affected by several neurological diseases and also of rodents with experimentally induced motor disorders. Based on this evidence, it has been

suggested that new synthetic compounds that act at key steps of endocannabinoid activity (i.e., more-stable analogs of endocannabinoids,

inhibitors of endocannabinoid reuptake or metabolism, antagonists of CB1 receptors) might be of interest for their potential use as therapeutic

agents in a variety of pathologies affecting extrapyramidal structures, such as Parkinson’s and Huntington’s diseases. Currently, only a few

data exist in the literature studying such relationships in humans, but an increasing number of journal articles are revealing the importance of

this new neuromodulatory system and arguing in favour of the funding of more extensive research in this field. The present article will review

the current knowledge of this neuromodulatory system, trying to establish the future lines for research on the therapeutic potential of the

endocannabinoid system in motor disorders.

D 2002 Elsevier Science Inc. All rights reserved.

Keywords: Cannabinoids; CB1 receptors; Basal ganglia; Motor disorders

Abbreviations: AEA, arachidonylethanolamide; 2-AG, 2-arachidonoylglycerol; FAAH, fatty acid amide hydrolase; GABA, g-aminobutyric acid; HD,

Huntington’s disease; L-DOPA, levodopa; MPTP, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine; NMDA, N-methyl-D-aspartate; PD, Parkinson’s disease;

THC, tetrahydrocannabinol.

Contents

1. Introduction: a general approach to the endogenous cannabinoid system . . . . . . . . . . . . . 138

2. Role of the endogenous cannabinoid system in the control of motor behavior . . . . . . . . . . 139

2.1. Pharmacological effects of cannabinoids and related compounds on motor activity. . . . 139

2.2. Presence of endocannabinoids and their receptors in basal ganglia structures . . . . . . . 140

2.3. Neurotransmitters affected by cannabinoids in the basal ganglia circuitry. . . . . . . . . 142

2.3.1. g-Aminobutyric acidergic transmission . . . . . . . . . . . . . . . . . . . . . . 142

2.3.2. Dopaminergic transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

2.3.3. Glutamatergic transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143

3. Changes in the endogenous cannabinoid system in motor disorders . . . . . . . . . . . . . . . 143

3.1. Physiological aging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

0163-7258/02/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved.

PII: S0163 -7258 (02 )00253 -X

* Corresponding author. Tel.: +34-91-3941450; fax: +34-91-3941691.

E-mail address: jjfr@med.ucm.es (J. Fernandez-Ruiz).1 Present address: Facultad de Ciencias de la Salud y de la Vida, Universidad Pompeu i Fabra, 08005-Barcelona, Spain.

Pharmacology & Therapeutics 95 (2002) 137–152

3.2. Neurodegenerative diseases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

3.2.1. Huntington’s disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144

3.2.2. Parkinson’s disease . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

3.2.3. Other motor disorders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145

4. Potential therapeutic uses of endogenous cannabinoids and related compounds in motor disorders 146

4.1. Symptomatic treatment with cannabinoid-related compounds . . . . . . . . . . . . . . . 146

4.2. Neuroprotectant effects of cannabinoid-related compounds . . . . . . . . . . . . . . . . 147

5. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148

1. Introduction: a general approach to the endogenous

cannabinoid system

Cannabis sativa preparations (marijuana, hashish) are

among the most widely consumed drugs of abuse around

the world. Their active principles and derivatives, however,

are now being considered as potentially useful therapeutic

molecules, mainly due to the recent description of an

endogenous cannabinoid system (for reviews, see Mechou-

lam et al., 1994; Howlett, 1995; Pertwee, 1997; Di Marzo et

al., 1998), which would contain the molecular targets for the

action of plant-derived cannabinoids. This endogenous

system consists of two types of GTP-binding protein-

coupled receptors, CB1 (present in the CNS and, to a lesser

extent, in peripheral tissues) and CB2 (present outside the

CNS, preferentially in the immune system) (Pertwee, 1997)

and their corresponding endogenous ligands, which are able

to bind and activate these receptors. The endogenous

ligands are mainly derivatives of polyunsaturated fatty

acids, such as the ethanolamide of arachidonic acid termed

‘‘anandamide’’ (AEA, arachidonylethanolamide) (Devane et

al., 1992; Hanus et al., 1993) and 2-arachidonoylglycerol (2-

AG) (Mechoulam et al., 1995; Sugiura et al., 1995).

Although certain selectivity of these endogenous ligands

for CB1 or CB2 receptors recently have been proposed based

on their concentrations and potencies (Sugiura et al., 2000),

it is generally accepted that both ligands bind nonselectively

to both receptor subtypes. These endogenous ligands and

their receptors seem to play a role in a variety of physio-

logical processes, mainly in the brain (for a review, see Di

Marzo et al., 1998), but also in the immune (for reviews, see

Kaminski, 1998; Parolaro, 1999) and cardiovascular (for

reviews, see Wagner et al., 1998; Harris et al., 1999; Hillard,

2000; Randall et al., 2002) systems.

In the brain, the endocannabinoids and their receptors

behave as neurotransmitters or neuromodulators in a variety

of processes, such as the regulation of motor behavior,

cognition, learning and memory, and antinociception (for

reviews, see Di Marzo et al., 1998; Hampson & Deadwyler,

1999; Sanudo-Pena et al., 1999; Walker et al., 1999). They

also play a role in neuronal development (Berrendero et al.,

1998a; Fernandez-Ruiz et al., 1999, 2000). The involvement

of the endocannabinoids in these functions has been pro-

posed based on the distribution of cannabinoid CB1 receptor

binding and mRNA levels in the brain (Herkenham et al.,

1990, 1991a; Mailleux & Vanderhaeghen, 1992a; Tsou et

al., 1998a) and on the well-known pharmacological effects

of plant-derived and synthetic cannabinoids (for reviews,

see Howlett, 1995; Pertwee, 1997). The endocannabinoids

have been shown to be synthesized, released, taken up, and

degraded in neuronal elements by mechanisms similar in

part to those for other neurotransmitters, although their lipid

structure implies some differences with respect to classical

amino acid, amine, and peptide transmitters (Howlett, 1995;

Di Marzo et al., 1998). For instance, AEA is known to be

formed upon demand by receptor-stimulated phospholipase

D-mediated cleavage of a membrane precursor (N-arachi-

donoylphosphatidylethanolamine). However, instead of

accumulating in synaptic vesicles, it is immediately released

into the extracellular milieu and taken up by a specific

carrier-mediated system that is present in both neurons and

glial cells (Hillard et al., 1997; Di Marzo et al., 1998), and

that also works for 2-AG (Piomelli et al., 1999). Once

within the cell, it is degraded by the action of an amidohy-

drolase enzyme [fatty acid amide hydrolase (FAAH)], to

form its two basic components, arachidonic acid and etha-

nolamine (for a review, see Di Marzo et al., 1998). This

enzyme has been located in neuronal elements (Tsou et al.,

1998b). It has also been suggested that this last reaction may

be reversed to increase the synthesis of AEA under special

circumstances (Devane & Axelrod, 1994; Arreaza et al.,

1997). Nevertheless, the precise distribution of AEA-pro-

ducing neurons is still poorly known, thus limiting our

knowledge of the role that this neuromodulator plays in

the normal functioning of the brain.

Since the isolation of these naturally occurring substan-

ces, several laboratories have addressed the synthesis of

compounds with specificity for key proteins of the endo-

cannabinoid system (receptors, transporters, enzymes, etc.)

(for a recent review on novel synthetic compounds with

promising clinical relevance, see Pertwee, 2000). Some of

the lines of research focus on the synthesis of novel agonists

that provide:

(1) higher metabolic stability than anandamide, such as

R-(+)-methanandamide (AM356) (Abadji et al., 1994);

(2) better water solubility, which will improve the mode

of administration of cannabinoids for therapy, such as O-

1057 (Pertwee et al., 2000);

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152138

(3) selective affinity for the different receptor subtypes,

such as some recent AEA analogs, which are potent CB1

receptor agonists, but which bind weakly to CB2 receptors

(Hillard et al., 1999; Pertwee, 2000), and particularly,

compounds such as HU-308 (Hanus et al., 1999), JWH-

133 (Baker et al., 2000), or others (for a review, see Pertwee,

2000), which mainly behave as CB2 receptor agonists. The

advantage of the latter compounds is that they do not

produce any psychotropic effects, as those mediated by

CB1 receptors, but are effective in some CB2 receptor-

mediated effects, such as reduction of blood pressure,

inhibition of intestinal activity, and inflammation, and

promotion of peripheral analgesic activity (Hanus et al.,

1999).

Other recent compounds have been designed to be

selective for other molecular targets of the endocannabinoid

system (for a review, see Giuffrida et al., 2001). Thus,

AM404 or VDM11 behave as inhibitors of endocannabinoid

uptake (Khanolkar et al., 1996; Beltramo et al., 1997;

Jarrahian et al., 2000; De Petrocellis et al., 2000), and have

special relevance since they offer the advantage of poten-

tiating the endogenous tone of endocannabinoids in those

processes where carrier-mediated transport is involved in

terminating responses to these endogenous ligands (Giuf-

frida et al., 2001). These compounds, called indirect ago-

nists, allow a therapeutic strategy that minimizes the

unwanted effects produced by direct CB1 receptor activation

with classic cannabinoids through the control of endocan-

nabinoid levels in a concentration range that avoids psycho-

active side effects (Felder & Glass, 1998). However, some

of these compounds, such as AM404, may also behave as

agonists of the vanilloid receptors (Zygmunt et al., 2000).

Inhibitors of the enzyme involved in the metabolism of

endocannabinoids, FAAH, are also available (Deutsch &

Chin, 1993; Deutsch et al., 1997; Gifford et al., 1999; Boger

et al., 2000) and provide an alternative to the use of uptake

inhibitors with similar results. On the other hand, since

1994, cannabinoid researchers had the first drug capable of

specifically blocking both the in vivo and in vitro effects of

cannabinoids. SR141716A behaves as a specific competi-

tive antagonist for the CB1 receptor subtype (Rinaldi-Car-

mona et al., 1994). Other representatives of cannabinoid

antagonists for both CB1 and CB2 receptors are currently

available (Felder et al., 1998; Rinaldi-Carmona et al., 1998;

Lan et al., 1999; for a review, see Pertwee, 2000).

Hence, cannabinoid pharmacology has experienced a

dramatic impulse in recent years, and has started to be

considered a promising new line for therapeutic treatment of

a variety of diseases, such as brain injury, chronic pain,

glaucoma, asthma, cancer and acquired immunodeficiency

syndrome-associated effects, and other disorders. Neuro-

logical pathologies that involve motor disorders is another

promising field of therapeutic application of endocannabi-

noid-related compounds. Among these, Parkinson’s disease

(PD) and Huntington’s disease (HD) are two of the most

interesting areas of clinical research, due to the direct

relationship of endocannabinoids and their receptors with

neurons that degenerate in the brains of those patients

affected by these disorders. In fact, it is now well accepted

that the control of movement is one of the more relevant

physiological roles of the recently discovered endocannabi-

noids in the brain. This review will summarize our current

knowledge of the role of these endogenous substances in the

control of movement, as well as their potential therapeutic

usefulness in the treatment of motor pathologies.

2. Role of the endogenous cannabinoid system in the

control of motor behavior

The finding that the endocannabinoid system might be

involved in the regulation of motor behavior is based on

three lines of evidence. First, it has been well demonstrated

that synthetic, plant-derived, and endogenous cannabinoids

have powerful actions, mostly inhibitory effects, on motor

activity (Crawley et al., 1993; Fride & Mechoulam, 1993;

Wickens & Pertwee, 1993; Smith et al., 1994; Romero et al.,

1995a, 1995b; for a review, see Sanudo-Pena et al., 1999).

There are differences in both the magnitude and duration of

the motor effects of the different cannabinoids, but these are

attributable to differences in receptor affinity, potency, and/

or metabolic stability. Second, it is also well known that

endocannabinoids and their CB1 receptors are abundantly

distributed in the basal ganglia and the cerebellum, the areas

that control movement (Herkenham et al., 1991a, 1991b;

Mailleux & Vanderhaeghen, 1992a; Tsou et al., 1998a;

Bisogno et al., 1999). Third, an increasing number of

studies have demonstrated that CB1 receptor binding was

altered in the basal ganglia of humans affected by several

neurological diseases (Glass et al., 1993, 2000; Richfield &

Herkenham, 1994; Lastres-Becker et al., 2001a; for a

review, see Consroe, 1998), and also of rodents with

experimentally induced motor disorders (Zeng et al., 1999;

Romero et al., 2000; Page et al., 2000; Lastres-Becker et al.,

2001b, 2002a, 2002b).

2.1. Pharmacological effects of cannabinoids and related

compounds on motor activity

Marijuana consumption affects psychomotor activity in

humans, reflected by a global impairment of performance (es-

pecially in complex and demanding tasks) and resulting in an

increased motor activity, followed by inertia and incoordin-

ation, ataxia, tremulousness, and weakness (for reviews, see

Dewey, 1986; Consroe, 1998). In rodents, the administration

of plant-derived or synthetic cannabinoids, in particular

(� )D9-tetrahydrocannabinol (D9-THC), the prototypical tri-

cyclic cannabinoid derived fromC. sativa (Gaoni &Mechou-

lam, 1968), affects motor behavior, producing motor

impairments in a variety of behavioral tests, paralleled by

changes in the activity of several neurotransmitters in the

basal ganglia (for reviews, see Mechoulam et al., 1994; Di

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152 139

Marzo et al., 1998; Sanudo-Pena et al., 1999). Thus, the acute

administration of D9-THC has been reported to produce,

among others, (1) decreases in spontaneous activity and

induction of catalepsy in mice (Pertwee et al., 1988), (2)

potentiation of reserpine-induced hypokinesia (Moss et al.,

1981), (3) attenuation of spontaneous and induced stereotypic

behaviors (Navarro et al., 1993; Romero et al., 1995b), (4)

enhancement of inactivity in rats (Rodrıguez de Fonseca et

al., 1994; Romero et al., 1995b), (5) reduction of ampheta-

mine-induced hyperlocomotion (Gorriti et al., 1999), (6)

increase in circling behavior (Jarbe et al., 1998), and (7)

disruption of fine motor control (McLaughlin et al., 2000).

However, most of these effects rapidly developed tolerance

when D9-THC administration was prolonged by several days

(Romero et al., 1997). Other plant-derived cannabinoids also

produced motor inhibition (Hiltunen et al., 1988), although

their effects were weak compared with D9-THC in concor-

dance with their lower affinity for the CB1 receptors. In

contrast, synthetic cannabinoids, such as CP-55,940 or

WIN-55,212-2, produced powerful inhibitory effects in a

variety of motor tests and animal models (for reviews, see

Consroe, 1998; Sanudo-Pena et al., 1999).

The first discovered endocannabinoid, AEA, mimicks

D9-THC-induced motor inhibition. Thus, Fride and Mecho-

ulam (1993) reported a decrease in rearing behavior and

immobility in mice. Similar results have been reported by

Crawley et al. (1993) and Smith et al. (1994), as well as by

Wickens and Pertwee (1993), who found that muscimol-

induced catalepsy in rats was potentiated by AEA, as well as

by D9-THC. Furthermore, AEA inhibits motor and stereo-

typic behaviors in a dose-related manner, showing a biphasic

pattern that was related to its low metabolic stability

(Romero et al., 1995a, 1995b). (R)-(+)-methanandamide, a

more stable analog of AEA, produced a dose-dependent

motor inhibition in the open-field test, almost similar in

potency to that produced by D9-THC and of a longer

duration than AEA, almost comparable with D9-THC

(Romero et al., 1996b; Jarbe et al., 1998). On the contrary,

low doses of AEA or other cannabinoids increase motor

behavior in mice, as shown by Souilhac et al. (1995).

Similar results have been reported recently in rats

(Sanudo-Pena et al., 2000). These observations are concord-

ant with the results reported by Fride et al. (1995), who also

demonstrated that low doses of AEA reduced a variety of

pharmacological effects of D9-THC, including motor inhibi-

tion, although these doses were ineffective by themselves.

Recently, the endocannabinoid uptake inhibitor AM404

(Beltramo et al., 1997) has been shown to mimick the

behavioral effects of AEA on motor activity when adminis-

tered alone in rats, as well as to produce similar neuro-

chemical changes (Gonzalez et al., 1999). As will be

described in Section 4, this compound and possible analogs

might be good candidates to be used in hyperkinetic

disorders as HD, although recent evidence has demonstrated

that it is not a selective compound for the endocannabinoid

transporter (Zygmunt et al., 2000).

From a general view, the motor effects of cannabinoid

agonists were usually prevented by SR141716A, a selective

CB1 receptor antagonist (for a review, see Consroe, 1998),

thus suggesting that they were CB1 receptor-mediated.

However, the administration of SR141716A alone caused

hyperlocomotion by itself (Compton et al., 1996). In addi-

tion, cannabinoid-tolerant animals acutely administered

SR141716Aexhibited awithdrawal syndromemainly charac-

terized by an increased motor response (Aceto et al., 1995;

Tsou et al., 1995). All of these data would be compatible

with the idea that the pharmacological blockade of CB1

receptors might be related to hyperlocomotion, which

might support the use of antagonists of the CB1 receptor

for the treatment of hypokinetic signs in PD and related

disorders, an issue that will be discussed in detail in the

Section 4. The recent development of two models of mice

lacking CB1 receptor expression offers a good tool to

examine the involvement of this cannabinoid receptor

subtype in motor activity, as well as to test the hypothesis

of a beneficial effect of the pharmacological blockade of

these receptors in hypokinetic disorders. However, results

have been controversial since a trend to hyperlocomotion

was observed in one of the two models (Ledent et al.,

1999), whereas hypoactivity was evident in the other (Zim-

mer et al., 1999).

2.2. Presence of endocannabinoids and their receptors in

basal ganglia structures

CB1 receptors are widely expressed in the brain, mainly

in neuronal elements (Howlett et al., 1990). Their distri-

bution in cortical, hippocampal, extrapyramidal, and cere-

bellar areas is one of the most abundant (Herkenham et al.,

1991a; Mailleux & Vanderhaeghen, 1992a; Tsou et al.,

1998a), compared with that of other neurotransmitter recep-

tors, and its pattern is well-preserved between different

species (Howlett et al., 1990). Among the brain structures

that contain CB1 receptors, the basal ganglia exhibit the

highest densities (Herkenham et al., 1990, 1991a; Mailleux

& Vanderhaeghen, 1992a). Anatomical studies using lesions

of specific neuronal subpopulations in the basal ganglia

have strongly demonstrated that CB1 receptors are located in

striatal projection neurons (Herkenham et al., 1991b). Fur-

thermore, an autoradiographic study on the distribution of

both receptor binding and mRNA expression for this recep-

tor confirmed this observation (Mailleux & Vanderhaeghen,

1992a). Generally, while an overlapping distribution of

mRNA and protein probably reflects local synthesis of

protein, a lack of correlation tends to suggest transport of

the protein to dendritic or axonal fields. In the case of CB1

receptors, the caudate-putamen and, particularly, the three

nuclei recipient of striatal efferent outputs (the globus

pallidus, entopeduncular nucleus, and substantia nigra pars

reticulata) contain high levels of receptor binding (Herken-

ham et al., 1991a) (see Fig. 1). However, CB1 receptor-

mRNA transcripts are present only in the caudate-putamen,

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152140

(Mailleux & Vanderhaeghen, 1992a) (see Fig. 1), thus

suggesting that CB1 receptors are presynaptically located

in both striatonigral (so-called the ‘‘direct’’ striatal efferent

pathway) and striatopallidal (so-called the ‘‘indirect’’ striatal

efferent pathway) projection neurons, which contain g-

aminobutyric acid (GABA) as neurotransmitter. In both

pathways, CB1 receptors are co-expressed with other

markers, such as glutamic acid decarboxylase, prodynor-

phin, substance P, or proenkephalin, as well as D1 or D2

dopaminergic receptors (Hohmann & Herkenham, 2000). In

contrast, intrinsic striatal neurons, which contain somato-

statin or acetylcholine, do not contain CB1 receptors (Hoh-

mann & Herkenham, 2000). In addition, a small population

of CB1 receptors is likely located on subthalamopallidal

and/or subthalamonigral glutamatergic terminals, revealed

by the presence of measurable levels of mRNA for this

receptor in the subthalamic nucleus, together with the

absence of detectable levels of receptor binding in that

structure (Mailleux & Vanderhaeghen, 1992a). With the

recent development of specific CB1 receptor antibodies,

some immunocytochemical studies have confirmed the cel-

lular distribution of this receptor subtype in the basal ganglia,

as well as their preferential localization on GABAergic neu-

rons (Tsou et al., 1998a). So, these authors reported moder-

ately stained neurons for CB1 receptor immunoreactivity in

the caudate-putamen, which were in the size range and shape

of medium-sized spiny GABAergic neurons, and local-

ization of CB1 receptors in unbeaded fine axons in the

globus pallidus, entopeduncular nucleus, and substantia

nigra (Tsou et al., 1998a). A last aspect that deserves com-

mentary is the existence of latero-medial and dorso-ventral

gradients in the caudate-putamen for both CB1 receptor

binding and mRNA expression (Mailleux & Vanderhaeghen,

1992a) (see Fig. 1). In this sense, it is important to remark

that lateral and dorsal regions of the striatum receive cortical

motor inputs, whereas ventral and medial areas are related

more to the limbic influences. Thus, the existence of these

gradients reinforces the notion that CB1 receptors in the

caudate-putamen are preferentially involved in the mediation

of motor effects of cannabinoids (for a review, see Sanudo-

Pena et al., 1999).

The endogenous cannabinoid ligands AEA and 2-AG are

also present in the basal ganglia (Bisogno et al., 1999;

Berrendero et al., 1999; Di Marzo et al., 2000a) in concen-

trations that are in general higher than those measured in the

whole brain (see Fig. 2 for a comparative analysis). Two key

regions for the control of movement, the globus pallidus and

the substantia nigra, deserve to be mentioned since they

contain the highest levels of endocannabinoids, in particular

AEA, in the brain (Di Marzo et al., 2000b), paralleling the

highest densities of CB1 receptors. This strongly supports a

functional role for the endocannabinoid system in the

control of movement. The precursor of AEA, N-arachido-

noylphosphatidylethanolamine, is also present in the basal

ganglia, which is evidence for the in situ synthesis of this

modulator, production of which is sensitive to different

stimuli (Giuffrida et al., 1999). However, we do not know

about the phenotype of the neurons that produce endocan-

nabinoids in the basal ganglia, mainly due to the lack of

specific markers for these neurons, which will require

further study. FAAH activity is also present in high levels

in all regions of the basal ganglia, particularly in the globus

Fig. 1. Localization of CB1 receptors in the basal ganglia.

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152 141

pallidus and the substantia nigra (Desarnaud et al., 1995),

but this marker lacks the necessary specificity for endocan-

nabinoid transmission.

2.3. Neurotransmitters affected by cannabinoids in the basal

ganglia circuitry

As indicated in Section 2.1, the administration of canna-

binoids induces significant alterations of motor patterns in

rodents and humans, and these effects seem to be produced

presumably by the action of cannabinoids on the activity of

several neurotransmitters that are involved in the control of

movement within the basal ganglia, mainly dopamine,

GABA, and glutamate. Due to the well-known effects that

cannabinoids exert on K + and Ca2+ currents (for reviews,

see Howlett, 1995; Pertwee, 1997) by opening inwardly

rectifying K + channels (Deadwyler et al., 1993; Mackie et

al., 1995; Howlett, 1995) and by inhibiting different types

(N-type and P/Q-type) of Ca2+ channels (Mackie & Hille,

1992; Mackie et al., 1995), it has been suggested that these

compounds could inhibit neuronal activity and neurotrans-

mitter release (Mackie & Hille, 1992), particularly during

periods of intense stimulation. This would be compatible, in

the basal ganglia, with the presynaptic location of CB1

receptors in striatal projection neurons.

2.3.1. g-Aminobutyric acidergic transmission

As expected from the location of CB1 receptors in striatal

GABAergic projection neurons, the activation of these

receptors seems to produce significant effects in GABAergic

activity within the basal ganglia. Thus, electrophysiological

studies indicated that cannabinoids may modulate GABA

release in vivo in the globus pallidus and substantia nigra

(Miller & Walker, 1995, 1996), although these effects were

very modest. More recently, neurochemical studies demon-

strated that the administration of cannabinoids did not affect

GABA synthesis or release in the basal ganglia of naıve

animals (Maneuf et al., 1996; Romero et al., 1998b; Lastres-

Becker et al., 2002b), although cannabinoids were effective

in increasing both parameters in animals with lesions of

striatal GABAergic neurons, as produced in HD (Lastres-

Becker et al., 2002b). In addition, the stimulation of CB1

receptors localized on axonal terminals of striatal GABAer-

gic neurons has been shown to potentiate GABA transmis-

sion by inhibition of the uptake of this neurotransmitter in

globus pallidus slices (Maneuf et al., 1996), a phenomenon

that could underlie the potentiation by cannabinoids of the

catalepsy induced by muscimol administration into the

globus pallidus (Wickens & Pertwee, 1993). The same

inhibition of GABA uptake has been observed in substantia

nigra synaptosomes (Romero et al., 1998b), which is con-

cordant with the observation by Gueudet et al. (1995) that

the blockade of CB1 receptors in striatal projection neurons

with SR141716A reduced the inhibitory GABAergic tone,

thereby allowing the firing of nigrostriatal dopaminergic

neurons. The authors concluded that endocannabinoid trans-

mission might increase the action of striatal GABAergic

neurons in the substantia nigra, producing a decrease of the

stimulation of nigral dopaminergic neurons (Gueudet et al.,

1995). Additional evidence supporting the direct involve-

ment of GABA in cannabinoid-induced motor effects is that

the blockade of GABAB, but not GABAA, receptors can

attenuate most of the signs of motor inhibition caused by the

administration of AEA or D9-THC in rats (Romero et al.,

1996a).

Fig. 2. Concentrations of endocannabinoids in the rat basal ganglia and the

cerebellum, as compared with whole brain. Data from Bisogno et al. (1999)

and Berrendero et al. (1999).

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152142

In contrast to the above studies, mostly supportive of a

role for cannabinoids in increasing GABA transmission in

the basal ganglia, other studies have suggested that canna-

binoids (1) would increase the activity of nigral neurons

without altering GABA influence (Tersigni & Rosenberg,

1996) or (2) would produce an inhibition, rather than a

stimulation, of GABA neurons via a presynaptic action

mediated by the inhibition of Ca2+ channels (Chan et al.,

1998). A similar inhibition of GABA by cannabinoids has

been reported to occur at the level of the striatum (Szabo et

al., 1998). Therefore, further studies will also be required to

elucidate this complex interaction of cannabinoids with

GABA transmission in the basal ganglia.

2.3.2. Dopaminergic transmission

The administration of plant-derived, synthetic or endo-

genous cannabinoids have also been reported to produce

changes in the activity of nigrostriatal dopaminergic neu-

rons (for a review, see Fernandez-Ruiz et al., 1996). We

found a decrease in the activity of tyrosine hydroxylase, the

rate-limiting enzyme for dopamine synthesis, in the striatum

of rats acutely administered AEA (Romero et al., 1995a,

1995b) or AM404 (Gonzalez et al., 1999). However,

although these decreases were consistent and statistically

significant, they occurred more probably as a consequence

of modifications in the activity of striatonigral GABAergic

neurons rather than as a direct effect of cannabinoids on

nigrostriatal dopaminergic projections. In fact, nigrostriatal

dopaminergic neurons do not contain CB1 receptors, at least

in the adult brain, although these receptors co-localize with

D1 or D2 dopaminergic receptors in striatal projection

neurons (Herkenham et al., 1991b), which supports a

potential interaction between both receptor types at the

level of G-protein/adenylyl cyclase signal transduction

mechanisms (Meschler & Howlett, 2001). In addition, it

has been reported that CB1-mRNA gene expression in the

striatum is under the negative control of nigral dopaminer-

gic inputs (Mailleux & Vanderhaeghen, 1993; Romero et

al., 2000; Lastres-Becker et al., 2001a), thus confirming the

reciprocal influence of both neural pathways. Of special

interest are the data obtained in 6-hydroxydopamine-

lesioned rats, a PD animal model, where it has been clearly

demonstrated that the interaction of endocannabinoids with

the dopaminergic system in the basal ganglia is secondary

to their interaction with GABAergic and glutamatergic

projections (Sanudo-Pena et al., 1998b). As these interac-

tions appear to be more relevant in motor disorders than in

the normal state, they will be discussed with more detail in

Sections 3 and 4.

2.3.3. Glutamatergic transmission

Emerging new data indicate that cannabinoids may

also modulate glutamatergic activity in the basal ganglia

by inhibiting the release of this neurotransmitter both in

vivo and in vitro (for a review, see Sanudo-Pena et al.,

1999). Thus, electrophysiological studies have suggested

that cannabinoids may modify the activity of pallidal and

nigral neurons by inhibiting glutamate release from sub-

thalamonigral terminals (Sanudo-Pena & Walker, 1997;

Szabo et al., 2000) and leading to a reduction in motor

activity (Miller et al., 1998). The inhibition of glutamate

release by cannabinoids was reversed by SR141716A

(Szabo et al., 2000), supporting the involvement of CB1

receptors, which, as mentioned in Section 2.2, are also

located in subthalamonigral glutamatergic neurons (Mail-

leux & Vanderhaeghen, 1992a). Mimicking the above

results, a recent electrophysiological study by Gerdeman

& Lovinger (2001) has demonstrated that cannabinoids

are also able to inhibit glutamate release from afferent

terminals in the striatum, this effect also being blocked by

SR141716A. However, it remains to be demonstrated

whether this inhibitory effect of cannabinoids is caused

by the activation of CB1 receptors located presynaptically

on afferent terminals in the striatum or whether it could

be an indirect action mediated by CB1 receptors not

located in these terminals. In this sense, Herkenham et

al. (1991b) proved that excitotoxic lesions of the striatum

led to an almost complete disappearance of CB1 recep-

tors, but doubts about the presence of small receptor

populations remain.

As observed within the basal ganglia, cannabinoids may

also inhibit in vitro glutamate release in the hippocampus

and the cerebellum (Levenes et al., 1998). Furthermore,

synthetic cannabinoids, such as WIN-55212-2 and CP-

55,940, were able to protect hippocampal neurons in vitro

from Ca2+ -evoked excitotoxicity induced by low extracel-

lular Mg2+ concentrations (Shen & Thayer, 1998). Based on

these observations, it has been hypothesized that cannabi-

noid-induced inhibition of glutamate release might have

therapeutic value in neurodegenerative diseases affecting

the basal ganglia and that exhibit excitotoxicity as a part of

the neuropathology (see Section 4.2).

3. Changes in the endogenous cannabinoid system in

motor disorders

As observed for most of the neurotransmitter systems,

endocannabinoid transmission within the basal ganglia is

also influenced by normal senescence (Mailleux & Vander-

haeghen, 1992b; Romero et al., 1998a; Berrendero et al.,

1998b). However, the changes were weak compared with

those observed in the processes of pathological aging

directly or indirectly affecting the basal ganglia (Glass et

al., 1993, 2000; Richfield & Herkenham, 1994; Westlake et

al., 1994). These studies have mainly addressed the altera-

tions in the status of CB1 receptors. There are, however, less

data on endogenous cannabinoid levels either in normal or

pathological postmortem human tissues, probably because

the accuracy of the determinations is uncertain, due to the

marked increase in such levels that take place shortly after

death (Schmid et al., 1995).

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152 143

3.1. Physiological aging

Senescence is a physiological process characterized by a

slow, but progressive, impairment in motor capabilities,

without signs of disease (Schut, 1998). This correlates with

the decrease in the activity of most of the neurotransmitters

acting at the basal ganglia, particularly dopamine and

GABA (for a review, see Francis et al., 1993). Decreases

in CB1 receptor binding, mRNA expression, and activation

of signal transduction mechanisms also have been reported

during normal aging in rats (Mailleux & Vanderhaeghen,

1992b; Romero et al., 1998a; Berrendero et al., 1998b).

These decreases were particularly relevant in the basal

ganglia (Mailleux & Vanderhaeghen, 1992b; Romero et

al., 1998a). In this sense, it is remarkable that the impair-

ment of psychomotor functions associated with marijuana

consumption declines with age (Soueif, 1976), which might

be related to the loss of CB1 receptors in the basal ganglia

during normal senescence.

3.2. Neurodegenerative diseases

Beyond the changes observed in CB1 receptors in the

basal ganglia during normal aging, several studies have also

demonstrated changes in these receptors in the postmortem

basal ganglia of humans affected by several disorders

directly related to motor function, such as HD (Glass et

al., 1993, 2000; Richfield & Herkenham, 1994) or PD

(Lastres-Becker et al., 2001a) or not directly related to the

control of movement, but exhibiting strong motor symp-

toms, such as Alzheimer’s disease (Westlake et al., 1994). In

some cases, these changes appeared before changes in

receptors for other neurotransmitters, even in presympto-

matic phases, which might suggest an involvement of the

endocannabinoid system in the pathogenesis of some motor

disorders (Glass et al., 2000).

3.2.1. Huntington’s disease

HD is a genetic neurodegenerative disorder caused by an

unstable expansion of a CAG repeat in exon 1 of the human

huntingtin gene. Translation through the CAG span results

in a polyglutamine tract near the N-terminus of this protein,

which leads to toxicity predominantly of striatal projection

neurons (for a recent review, see Reddy et al., 1999). The

symptoms of this disease are primarily characterized by

motor disturbances, such as chorea and dystonia, and

secondarily by personality changes and cognition decline

(Reddy et al., 1999). Several studies have clearly demon-

strated that in HD, there exists an almost complete dis-

appearance of CB1 receptor binding in the substantia nigra,

in the lateral part of the globus pallidus, and, to a lesser

extent, in the putamen (Glass et al., 1993, 2000; Richfield &

Herkenham, 1994). This loss of CB1 receptors is concordant

with the characteristic neuronal loss observed in HD that

predominantly affects medium-spiny GABAergic neurons

(Vonsattel et al., 1985), which contain most of the CB1

receptors present in basal ganglia structures (Herkenham et

al., 1991b; Hohmann & Herkenham, 2000). This is also

consistent with the fact that other phenotypic markers for

those neurons, such as substance P, enkephalin, calcineurin,

calbindin, and adenosine and dopamine receptors, are

known to be also depleted in HD (Hersch & Ferrante,

1997). However, recent data in postmortem tissue have

revealed that the loss of CB1 receptors occurred in advance

of other receptor losses, and even before the appearance of

major HD symptomatology. This suggests that losses of

CB1 receptors might be involved in the pathogenesis and/or

progression of the neurodegeneration in HD (Glass et al.,

2000).

Recent studies using rodent models of these motor dis-

orders have validated the data found in postmortem human

tissue. Thus, CB1 receptors are also reduced in the basal

ganglia of HD rat models (Page et al., 2000; Lastres-Becker

et al., 2001b, 2002b). These HD rat models were generated

by lesions of the striato-efferent GABAergic neurons, caused

by the administration of 3-nitropropionic acid, a mitochon-

drial toxin that inhibits succinate dehydrogenase and produ-

ces the same phenomena that have been proposed for the

etiology of the human disease, i.e., failure of energy meta-

bolism, glutamate excitotoxicity, and, to a lesser extent,

oxidative stress, leading to progressive neuronal death (for

a review, see Alexi et al., 1998). Hence, the decrease in CB1

receptors in these rat models occurred in a situation where

there was extensive neuronal death in the striatum, compar-

able with the pattern of cell loss that occurs in advanced

states of the human disease (Alexi et al., 1998). Thus, the

decrease in CB1 receptors might be a mere side-effect of the

3-nitropropionic acid-induced destruction of striatal GABA

projection neurons. However, the decrease in these receptors

in the basal ganglia also occurred in HD animal models,

where cell dysfunction rather than cell death is the major

change that takes place. This occurs in different transgenic

mouse models that express mutated forms of the huntingtin

(Denovan-Wright & Robertson, 2000; Lastres-Becker et al.,

2002a). This observation is concordant with the last results

reported by Glass et al. (2000) in the human disease,

showing that the process of the loss of CB1 receptors in

the basal ganglia starts early in the pathogenesis of HD,

when cell death is minimal. Hence, the data in HD transgenic

mice also suggest that CB1 receptor loss might play a role in

the pathogenesis of this disease.

Although most of the data on endocannabinoid transmis-

sion in HD have almost exclusively addressed the analysis

of CB1 receptors in the basal ganglia, very recently, we have

presented the first evidence on changes in endocannabinoid

ligands in the HD rat model generated by the striatal

application of 3-nitropropionic acid. Thus, we found a de-

crease in the contents of the two endocannabinoids AEA

and 2-AG in the caudate-putamen of lesioned animals

(Lastres-Becker et al., 2001b), which would be compatible

with the decrease found in their receptors. Hence, it can be

concluded that endocannabinoid transmission in the basal

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152144

ganglia in HD seems to be hypoactive, which might

contribute to some extent to the hyperkinesia typical of

HD. In this context, it might be speculated that substances

that elevate the endocannabinoid tonus, such as direct or

indirect receptor agonists, might be useful to improve

movement in this disease, at least in the early hyperactivity

phase, thus reducing choreic movements. As will be

described in Section 4, the above animal models represent

good tools to test the efficiency of cannabinoid-related

compounds to improve movement in this disease.

3.2.2. Parkinson’s disease

PD is a progressive neurodegenerative disorder in which

the capacity of executing voluntary movements is gradually

lost. The major clinical symptomatology in PD includes

tremor, rigidity, and bradykinesia (slowness of movement).

The pathological hallmark of this disease is the degeneration

of melanin-containing dopaminergic neurons of the subs-

tantia nigra pars compacta, which leads to severe dopami-

nergic denervation of the striatum (for a recent review, see

Blandini et al., 2000). Compared with HD, much less data

exist on the status of CB1 receptors in the postmortem basal

ganglia of humans affected by PD. Only recently we have

found that CB1 receptor binding and the activation of G-

proteins by cannabinoid agonists were significantly in-

creased in the basal ganglia as a consequence of the selec-

tive degeneration of nigrostriatal dopaminergic neurons that

occurs in PD patients (Lastres-Becker et al., 2001a). These

increases were not related to the dopaminergic replacement

therapy with levodopa (L-DOPA) that these patients under-

went chronically, since they were also seen in 1-methyl-4-

phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated marmo-

sets, a primate PD model, and disappeared after chronic

L-DOPA administration in these animals (Lastres-Becker et

al., 2001a). This is concordant with previous results in rats

that showed that dopamine exerted a negative effect on CB1

receptor gene expression at the level of the caudate-putamen

(Mailleux & Vanderhaeghen, 1993; Romero et al., 2000).

Interestingly, as demonstrated for HD, the changes in CB1

receptors is also an early event in the molecular pathogen-

esis of PD. Thus, the examination of basal ganglia from

individuals in the early and presymptomatic phase of PD

(incidental Lewy body disease, which is characterized by a

low degree of nigral pathology and the appearance of Lewy

bodies, but not neurological symptoms) demonstrated the

existence of a trend toward an increase in CB1 receptors in

some basal ganglia nuclei (Lastres-Becker et al., 2001a).

These presymptomatic patients were untreated. Thus, we

can conclude again that the increase in CB1 receptors is

related more to PD pathology rather than to L-DOPA

replacement therapy.

These data obtained in humans and non-human primates

are consistent with results found in PD rodent models,

which also exhibited an overactive endocannabinoid trans-

mission in the basal ganglia (Mailleux & Vanderhaeghen,

1993; Romero et al., 2000; Di Marzo et al., 2000b; Lastres-

Becker et al., 2001a), although the subject is controversial.

Thus, Herkenham et al. (1991b) reported no changes in CB1

receptor binding in the striatum following lesions of the

nigrostriatal dopaminergic neurons caused by local applica-

tion of 6-hydroxydopamine in rats. In addition, Zeng et al.

(1999) found an increase in CB1 receptor mRNA levels in

the striatum of 6-hydroxydopamine-treated rats, but only

after the animals had been chronically treated with L-DOPA.

However, these authors proposed that this effect of L-

DOPA, rather than elevating dopamine contents, might be

mediated through an increase in glutamate release, which, in

turn, may induce CB1 receptor mRNA expression in the

caudate-putamen (Mailleux & Vanderhaeghen, 1994). In

contrast, other studies that also used 6-hydroxydopamine-

treated rats have reported a marked increase in CB1 receptor

mRNA gene expression in the caudate-putamen following

degeneration of nigrostriatal dopaminergic neurons (Mail-

leux & Vanderhaeghen, 1993; Romero et al., 2000), in

concordance with data in humans and non-human primates

(Lastres-Becker et al., 2001a). Using the PD rat model

generated by acute treatment with reserpine, Di Marzo et

al. (2000b) reported that an increase in the content of

endocannabinoids in the basal ganglia was paralleled by

hypolocomotion. This effect is strongly related to the

decrease in dopamine transmission caused by reserpine,

because the stimulation of dopaminergic receptors with

selective D1 or D2 agonists was accompanied by a restora-

tion of normal endocannabinoid contents and by a stimu-

lation of locomotion (Di Marzo et al., 2000b). Therefore, we

can assume that endocannabinoid transmission in the basal

ganglia becomes overactive in PD, which is compatible with

the hypokinesia that characterizes this disease. This would

support the suggestion that CB1 receptor antagonists, rather

than agonists, might be useful in alleviating motor deterior-

ation in PD or in reducing the development of dyskinesia

caused by prolonged replacement therapy with L-DOPA

(Brotchie, 2000). However, Brotchie and colleagues, using

the PDmodel of reserpine-treated rats, recently have reported

a decrease in CB1 receptor-mRNA levels in the striatum

following reserpine administration (Silverdale et al., 2001). It

could be argued, however, that reserpine-treated rats are an

acute model of PD, which would not be completely compa-

rable with the chronic state that characterizes the human

disease or other animal models (MPTP-treated marmosets or

6-hydroxydopamine-lesioned rats).

3.2.3. Other motor disorders

To our knowledge, no data exist on the status of can-

nabinoid receptors in other extrapyramidal disorders in the

human, such as tardive dyskinesia, Gilles de la Tourette

syndrome, dystonia, and others, but cannabinoids might be

of interest in these diseases (for a review, see Consroe,

1998). Thus, a relationship between cannabis use and

incidence of tardive dyskinesia has been described in chronic-

ally neuroleptic-treated psychiatric patients (Zaretsky et al.,

1993). Brotchie (1998, 2000) has suggested that cannabi-

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152 145

noid-related substances might be beneficial for the treat-

ment of dyskinesia that develops in PD patients after

chronic treatment with L-DOPA, a fact that will be ad-

dressed in the next section. In addition, plant-derived can-

nabinoids might have potential for the management of (1)

tics and obsessive compulsive behaviors in patients with

Tourette’s syndrome (Hemming & Yellowlees, 1993; Cons-

roe, 1998; Muller-Vahl et al., 1998, 1999c), (2) tremor and

spasticity in multiple sclerosis (for a review, see Pertwee,

2002), and (3) dystonia (for reviews, see Consroe, 1998;

Muller-Vahl et al., 1999a).

4. Potential therapeutic uses of endogenous cannabinoids

and related compounds in motor disorders

From what has been stated in this review, it can be

concluded that compounds acting on the endocannabinoid

system might be of promise in improving motor deteriora-

tion in both hyper- and hypokinetic disorders (for recent

reviews, see Consroe, 1998; Muller-Vahl et al., 1999a). To

date, most of the research has focused on the search for new

symptomatic pharmacotherapies, but evidence has also been

presented that cannabinoid-related compounds might also

be neuroprotective.

4.1. Symptomatic treatment with cannabinoid-related com-

pounds

Compounds able to directly or indirectly activate CB1

receptors in the basal ganglia may be useful in those

diseases in which endocannabinoid transmission is hypo-

functional, as is the case in HD. This is an important

proposal, since HD is a motor disorder where the therapeutic

outcome has been poor and there is a lack of novel

pharmacological therapies with symptomatic and/or neuro-

protective efficacy (Reddy et al., 1999). Therefore, com-

pounds like direct agonists of CB1 receptors, but,

particularly, indirect agonists that through inhibiting endo-

cannabinoid uptake and/or FAAH activity may elevate the

levels of endogenous cannabinoids, might prove effective in

improving the motor deterioration seen in HD (Gonzalez et

al., 1999; Lastres-Becker et al., 2002b). These compounds

might contribute by reducing the excessive activity of the

nigrostriatal dopaminergic neurons that are responsible for

many of the observed neurological symptoms of HD, such

as choreic movements. Other possible interactions within

the basal ganglia should also be considered, as electro-

physiological studies have suggested a possible cannabi-

noid-induced inhibition of GABAergic neurotransmission in

striatonigral and striatopallidal projections in vivo (Miller &

Walker, 1995, 1996). In turn, this could induce a disinhibi-

tion of pallidothalamic and nigrothalamic GABAergic neu-

rons, thus contributing to an amelioration of the hyperkinetic

symptoms (Consroe, 1998). However, it is also important to

consider that the early loss of CB1 receptors found in the basal

ganglia of HD patients (Glass et al., 2000) may preclude

cannabinoids from being effective (Felder & Glass, 1998).

There are some recent studies that support the above

hypotheses. Thus, despite some failed pharmacological

experiences in humans using plant-derived cannabinoids

(for a review, see Consroe, 1998) or some of their synthetic

analogs (Muller-Vahl et al., 1999b), we have demonstrated

that direct agonists such as CP-55,940 or indirect ones such

as AM404 were able to reduce hyperkinesia and to lead to

recovery from GABAergic deficits in rats with striatal

lesions caused by local application of 3-nitropropionic acid

(Lastres-Becker et al., 2002b). AM404 was also able to

normalize motor activity in genetically hyperactive rats

without causing overt cannabimimetic effects (Beltramo et

al., 2000). A priori, these effects of AM404 were expected,

considering the previously reported decrease of endocanna-

binoid levels in the striatum of HD rats (Lastres-Becker et

al., 2001b) and the capability of AM404 to elevate endo-

cannabinoid levels (Beltramo et al., 1997; Pertwee, 2000).

However, the recent demonstration that AM404 also binds

to vanilloid VR1 receptors (Zygmunt et al., 2000) suggests

an involvement of these receptors, alone or in combination

with CB1 receptors, in these antihyperkinetic effects of

AM404. Thus, novel compounds with the capability of

activating both endocannabinoid and endovanilloid mecha-

nisms might be useful for this disease.

In contrast to HD, where the endocannabinoid transmis-

sion is hypoactive, PD, in which nigrostriatal dopaminergic

activity is greatly reduced, is associated with overactivity of

endocannabinoid transmission, so hypokinetic signs in this

disease might be ameliorated by blocking rather than

activating CB1 receptors. In theory, such blockade would

avoid the inhibition of GABA uptake produced by the

activation of CB1 receptors located on striatonigral or

striatopallidal terminals (Maneuf et al., 1996; Romero et

al., 1998b), thus allowing faster removal of this inhibitory

neurotransmitter from the synaptic cleft. However, some

studies have also demonstrated a certain efficacy of canna-

binoid agonists to interact with dopaminergic agonists and

improve motor impairment in PD rat models (Anderson et

al., 1995; Maneuf et al., 1997; Brotchie, 1998; Sanudo-Pena

et al., 1998b). Thus, Anderson et al. (1995) reported that

synthetic cannabinoids markedly attenuated contralateral

rotation induced by D1, but not by D2, agonists in rats with

unilateral 6-hydroxydopamine-induced lesions of the nigro-

striatal pathway. Sanudo-Pena et al. (1998b) reported a more

marked contralateral turning response to CP-55,940, a syn-

thetic cannabinoid agonist, when it was infused into the

substantia nigra of lesioned animals. This did not occur when

the cannabinoid was infused into the striatum or the globus

pallidus. These authors previously had shown that unilateral

microinjections of cannabinoids in intact animals produced

contralateral rotation when the cannabinoid was infused in

the substantia nigra (Sanudo-Pena et al., 1996) and the

striatum (Sanudo-Pena et al., 1998a), whereas ipsilateral

rotation was observed following infusion of cannabinoids

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152146

into the globus pallidus (Sanudo-Pena & Walker, 1998).

However, the effects of cannabinoids varied when they were

administered in combination with D1 or D2 agonists, indic-

ating the extreme complexity of this circuitry with multiple

targets for the action of cannabinoids. Using reserpine-

treated rats, Maneuf et al. (1997) reported a reduction,

instead of an increase, by WIN-55,212-2 of the anti-akinetic

effect of D2, but not D1, receptor agonists.

The capability of cannabinoid agonists to improve motor

deterioration in PD rat models, despite the overactivity of

endocannabinoid transmission found in these animals (Mail-

leux & Vanderhaeghen, 1993; Romero et al., 2000; Di

Marzo et al., 2000b; Lastres-Becker et al., 2001a), might

be related to the hyperactivity of the subthalamic nucleus

found in this disease (for a review, see Blandini et al., 2000).

This hyperactivity seems to be directly involved in the

origin of parkinsonian tremor (Rodrıguez et al., 1998), so

that surgical manipulation of this structure is currently used

in pharmacotherapy-refractant patients (Benabid et al.,

1998). In a recent study, Sanudo-Pena et al. (1998b)

demonstrated that cannabinoid agonists were capable of

compensating for the overactivity of subthalamonigral glu-

tamatergic neurons in the 6-hydroxydopamine model of

hemiparkinsonian rats. As mentioned in Section 2.2, these

neurons contain CB1 receptors (Mailleux & Vanderhaeghen,

1992a) and their activation inhibits glutamate release from

these terminals (Szabo et al., 2000). Therefore, this might be

a plausible explanation for certain beneficial effects of CB1

receptor agonists in PD.

However, despite the data obtained in PD rodent models,

experiences with humans affected by this disease or in non-

human MPTP-lesioned primates have proven that the the-

rapy with plant-derived cannabinoid agonists for attenuating

hypokinetic signs was useless and even enhanced motor

disability (for reviews, see Consroe, 1998; Muller-Vahl et

al., 1999a). It is important to mention that of all the clinical

signs of PD, bradykinesia or akinesia is the best clinical

measure, whereas one of the most robust effects of CB1

receptor agonists in rodents is catalepsy, which is a state of

postural immobility (akinesia). Thus, it is unlikely that CB1

receptor agonists might be useful in PD, as has been largely

demonstrated by Consroe (1998) in a recent review. There-

fore, the newest studies point to CB1 receptor antagonists,

rather than to agonists, as potential beneficial compounds

for the treatment of motor deterioration in PD (Brotchie,

1998, 2000; Lastres-Becker et al., 2001a). These com-

pounds represent a promising alternative treatment, alone

or as a non-dopaminergic adjunct, particularly to reduce the

problem of dyskinesia associated with long-term dopamine

replacement therapy for PD patients (Brotchie, 1998, 2000).

Thus, in MPTP-treated common marmosets, the blockade of

CB1 receptors with SR141716A reduced L-DOPA-induced

dyskinesia without affecting the antiparkinsonism efficacy

of L-DOPA (Brotchie, 1998). Similarly, Di Marzo et al.

(2000b) reported that combined administration of quinpi-

role, a D2 agonist, and SR141716A produced a full restora-

tion of locomotion in reserpine-treated rats. The use of CB1

receptor antagonists in PD also has the advantage of avoid-

ing the unwanted psychotropic effects associated with the

use of classic cannabinoids that behave as agonists for the

CB1 receptor.

4.2. Neuroprotectant effects of cannabinoid-related com-

pounds

Beyond the use of cannabinoid-related compounds to

alleviate the symptomatology in motor disorders, these

compounds also exhibit a potential usefulness as neuro-

protectant substances in a variety of neurodegenerative di-

seases (see Hansen et al., 2002). Therefore, they could be

used not only for ameliorating the motor deterioration, but

also for delaying or arresting the progressive neurodegener-

ation occurring in motor disorders. Cannabinoids may play a

neuroprotective role by means of three different mecha-

nisms. First, cannabinoids have been shown to be potent

antioxidant compounds, although acting through a receptor-

independent mechanism in vitro (Hampson et al., 1998b).

Thus, they might be neuroprotective in disorders associated

with oxidative stress. Of special interest is the effect of

cannabidiol, a non-psychoactive cannabinoid (Howlett,

1995) that exhibits an antioxidant potency even superior to

that of ascorbate and a-tocopherol (Hampson et al., 1998b).

Furthermore, some cannabinoids have been shown to be

effective neuroprotective agents in animal models of cerebral

ischemia (Belayev et al., 1995; Nagayama et al., 1999),

although the mechanism of this effect is not clear. Some

discrepancies in the antioxidant properties of D9-THC have

been found, but the notion of a neuroprotective function of

cannabinoids both in vivo and in vitro is reinforced with

these data (Nagayama et al., 1999). This antioxidant cap-

ability of cannabinoids might prevent neuronal death in

various motor disorders, particularly in HD, where it has

been demonstrated that production of free radicals, a con-

sequence of mitochondrial dysfunction, is one of the major

cytotoxic events that takes place during the pathogenesis of

this motor disorder (Reddy et al., 1999).

Second, and closely related to the former point, cannabi-

noids have the property of inhibiting N-methyl-D-aspartate

(NMDA)-receptor-mediated glutamatergic neurotransmis-

sion (Hampson et al., 1998a). Excitotoxicity is known to be

responsible formuch of the cellular damages that take place in

some neurodegenerative processes, mainly through the acti-

vation of NMDA glutamatergic receptors (Beal, 1995). Both

D9-THC and AEA recently have been shown to inhibit the

activity of those receptors in cortical and cerebellar neuronal

cultures (Hampson et al., 1998a), probably by their ability to

inhibit Ca2+ currents through the activation of CB1 receptors

(Mackie et al., 1995). Interestingly, AEA is capable of

inducing the opposite effect through a receptor-independent

pathway, confirming previous reports of a possible antago-

nism between the effects of AEA and other cannabinoids in

their putative neuroprotectant action (Skaper et al., 1996).

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152 147

Finally, the neuroprotectant effect of cannabinoids may

also be based on their ability to increase the presence of

neurotransmitters in the synaptic cleft in GABA synapses

(Maneuf et al., 1996; Romero et al., 1998b). Therefore, it

has been suggested that one of the mechanisms of neuro-

protection elicited by NMDA receptor blockade implies the

enhancement of GABA transmission (Battaglia et al., 2001).

Cannabinoids increased GABA transmission by themselves

(see Section 2.3.1), so it might be expected that the

inhibition of GABA uptake by cannabinoids could contrib-

ute to the known protectant role of this neurotransmitter and

other GABAmimetics in transneuronal-delayed death (Saji

& Reis, 1987). It is well known that neurons may die when

deprived of their afferent inputs (Cowan, 1984), and this

phenomenon has been demonstrated to occur in striatal

outflow nuclei (Saji & Reis, 1987), revealing the critical

importance of the imbalance between inhibitory and sti-

mulatory innervations. By their action on striatopallidal and/

or striatonigral terminals, cannabinoids could contribute to

the action of other GABAmimetic drugs in the prevention,

at least partially, of the neuronal death in those recipient

nuclei. Furthermore, it recently has been shown that ablation

of the subthalamic nucleus prevents transneuronal death in

substantia nigra pars reticulata neurons (Saji et al., 1996),

thus pointing to an even more promising role for cannabi-

noids in this sense.

5. Concluding remarks

The studies reviewed in this article are all concordant

with the view that control of movement is a key function for

the endocannabinoid transmission in the CNS. We have

reviewed the pharmacological and biochemical bases that

sustain the involvement of the endocannabinoid transmis-

sion in the function of the basal ganglia. We have also

shown that endocannabinoid transmission is altered in

motor disorders, in parallel to the well-known changes in

classic neurotransmitters, such as GABA, dopamine, or

glutamate. This provides the basis for the development of

novel pharmacotherapies with compounds selective for the

different target proteins that constitute the endocannabinoid

system. However, only a few studies have examined,

hitherto, the potential contribution of these compounds in

motor disorders. The importance of this novel system

demands further investigation and the development of novel

promising compounds for the symptomatic and/or neuro-

protectant treatment of basal ganglia pathology.

Acknowledgements

Studies included in this review have been supported by

grants from CAM-PRI (08.5/0029/98) and CICYT (PM99-

0056).

References

Abadji, V., Lin, S., Taha, G., Griffin, G., Stevenson, L. A., Pertwee, R. G.,

& Makriyannis, A. (1994). (R)-Methanandamide: a chiral novel anan-

damide possessing higher potency and metabolic stability. J Med Chem

37, 1889–1893.

Aceto, M. D., Scates, S. M., Lowe, J. A., & Martin, B. R. (1995). Canna-

binoid precipitated withdrawal by the selective cannabinoid receptor

antagonist, SR141716A. Eur J Pharmacol 282, R1–R2.

Alexi, T., Hughes, P. E., Faull, R. L. M., & Williams, L. E. (1998). 3-

Nitropropionic acid’s lethal triplet: cooperative pathways of neurode-

generation. Neuroreport 9, 57–64.

Anderson, L. A., Anderson, J. J., Chase, T. N., & Walters, J. R. (1995). The

cannabinoid agonists WIN 55,212-2 and CP 55,940 attenuate rotational

behaviour induced by a dopamine D1 but not D2 agonist in rats with

unilateral lesions of the nigrostriatal pathway. Brain Res 691, 106–114.

Arreaza, G., Devane, W. A., Omeir, R. L., Sajnani, G., Kunz, J., Cravatt,

B. F., & Deutsch, D. G. (1997). The cloned rat hydrolytic enzyme

responsible for the breakdown of anandamide also catalyzes its forma-

tion via the condensation of arachidonic acid and ethanolamine. Neuro-

sci Lett 234, 59–62.

Baker, D., Pryce, G., Croxford, J. L., Brown, P., Pertwee, R. G., Huffman,

J. W., & Layward, L. (2000). Cannabinoids control spasticity and trem-

or in a multiple sclerosis model. Nature 404, 84–87.

Battaglia, G., Bruno, V., Pisani, A., Centonze, D., Catania, M. V., Calabresi,

P., & Nicoletti, F. (2001). Selective blockade of type-1 metabotropic

glutamate receptors induces neuroprotection by enhancing GABAergic

transmission. Mol Cell Neurosci 17, 1071–1083.

Beal, M. F. (1995). Aging, energy, and oxidative stress in neurodegener-

ative diseases. Ann Neurol 38, 357–366.

Belayev, L., Bar-Joseph, A., Adamchik, J., & Biegon, A. (1995). HU-211, a

nonpsychotropic cannabinoid, improves neurological signs and reduces

brain damage after severe forebrain ischemia in rats. Mol Chem Neuro-

pathol 25, 19–33.

Beltramo, M., Stella, N., Calignano, A., Lin, S. Y., Makriyannis, A., &

Piomelli, D. (1997). Functional role of high-affinity anandamide trans-

port, as revealed by selective inhibition. Science 277, 1094–1097.

Beltramo, M., Rodrıguez de Fonseca, F., Navarro, M., Calignano, A., Gor-

riti, M. A., Grammatikopoulos, G., Sadile, A. G., Giuffrida, A., &

Piomelli, D. (2000). Reversal of dopamine D2 receptor responses by

an anandamide transport inhibitor. J Neurosci 20, 3401–3407.

Benabid, A. L., Benazzouz, A., Hoffmann, D., Limousin, P., Krack, P., &

Pollak, P. (1998). Long-term electrical inhibition of deep brain targets in

movement disorders. Mov Disord 13, 119–125.

Berrendero, F., Garcıa-Gil, L., Hernandez, M. L., Romero, J., Cebeira, M.,

de Miguel, R., Ramos, J. A., & Fernandez-Ruiz, J. J. (1998a). Loca-

lization of mRNA expression and activation of signal transduction

mechanisms for cannabinoid receptor in rat brain during fetal develop-

ment. Development 125, 3179–3188.

Berrendero, F., Romero, J., Garcıa-Gil, L., Suarez, I., de la Cruz, P., Ramos,

J. A., & Fernandez-Ruiz, J. J. (1998b). Changes in cannabinoid receptor

binding and mRNA levels in several brain regions of aged rats. Biochim

Biophys Acta 1407, 205–214.

Berrendero, F., Sepe, N., Ramos, J. A., Di Marzo, V., & Fernandez-Ruiz, J.

J. (1999). Analysis of cannabinoid receptor binding and mRNA expres-

sion and endogenous cannabinoid contents in the developing rat brain

during late gestation and early postnatal period. Synapse 33, 181–191.

Bisogno, T., Berrendero, F., Ambrosino, G., Cebeira, M., Ramos, J. A.,

Fernandez-Ruiz, J. J., & Di Marzo, V. (1999). Brain regional distribu-

tion of endocannabinoids: implications for their biosynthesis and bio-

logical function. Biochem Biophys Res Commun 256, 377–380.

Blandini, F., Nappi, G., Tassorelli, C., & Martignoni, E. (2000). Functional

changes in the basal ganglia circuitry in Parkinson’s disease. Prog

Neurobiol 62, 63–88.

Boger, D. L., Sato, H., Lerner, A. E., Hedrick, M. P., Fecik, R. A., Miyau-

chi, H., Wilkie, G. D., Austin, B. J., Patricelli, M. P., & Cravatt, B. F.

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152148

(2000). Exceptionally potent inhibitors of fatty acid amide hydrolase:

the enzyme responsible for degradation of endogenous oleamide and

anandamide. Proc Natl Acad Sci USA 97, 5044–5049.

Brotchie, J. M. (1998). Adjuncts to dopamine replacement: a pragmatic

approach to reducing the problem of dyskinesia in Parkinson’s disease.

Mov Disord 13, 871–876.

Brotchie, J. M. (2000). The neural mechanisms underlying levodopa-in-

duced dyskinesia in Parkinson’s disease. Ann Neurol 47, S105–S114.

Chan, P. K., Chan, S. C., & Yung, W. H. (1998). Presynaptic inhibition of

GABAergic inputs to rat substantia nigra pars reticulata neurones by a

cannabinoid agonist. Neuroreport 9, 671–775.

Compton, D. R., Aceto, M. D., Lowe, J., & Martin, B. R. (1996). In vivo

characterization of a specific cannabinoid antagonis (SR141716A): in-

hibition of D9 tetrahydrocannabinol-induced responses and apparent

agonist activity. J Pharmacol Exp Ther 277, 586–594.

Consroe, P. (1998). Brain cannabinoid systems as targets for the therapy of

neurological disorders. Neurobiol Dis 5, 534–551.

Cowan, W. M. (1984). Regressive events in neurogenesis. Science 225,

1258–1265.

Crawley, J. N., Corwin, R. L., Robinson, J. K., Felder, Ch. C., Devane,W. A.,

& Axelrod, J. (1993). Anandamide, an endogenous ligand of the canna-

binoid receptor, induces hypomotility and hypothermia in vivo in rodents.

Pharmacol Biochem Behav 46, 967–972.

Deadwyler, S. A., Hampson, R. E., Bennett, B. A., Edwards, T. A., Mu, J.,

Pacheco, M. A., Ward, S. J., & Childers, S. R. (1993). Cannabinoids

modulate potassium current in culture hippocampal neurons. Recept

Channels 1, 121–134.

Denovan-Wright, E. M., & Robertson, H. A. (2000). Cannabinoid receptor

messenger RNA levels decrease in subset neurons of the lateral stria-

tum, cortex and hippocampus of transgenic Huntington’s disease mice.

Neuroscience 98, 705–713.

De Petrocellis, L., Bisogno, T., Davis, J. B., Pertwee, R. G., & Di Marzo, V.

(2000). Overlap between the ligand recognition properties of the anan-

damide transporter and the VR1 vanilloid receptor: inhibitors of anan-

damide uptake with negligible capsaicin-like activity. FEBS Lett 483,

52–56.

Desarnaud, F., Cadas, H., & Piomelli, D. (1995). Anandamide amidohy-

drolase activity in rat brain microsomes. Identification and partial pu-

rification. J Biol Chem 270, 6030–6035.

Deutsch, D. G., & Chin, S. A. (1993). Enzymatic synthesis and degradation

of anandamide, a cannabinoid receptor agonist. Biochem Pharmacol 46,

791–796.

Deutsch, D. G., Lin, S., Hill, W. A. G., Morse, K. L., Solehani, D., Arreaza,

G., Omeir, R. L., & Makriyannis, A. (1997). Fatty acid sulfonyl fluo-

rides inhibit anandamide metabolism and bind to the cannabinoid re-

ceptor. Biochem Biophys Res Commun 231, 217–221.

Devane, W. A., & Axelrod, J. (1994). Enzymatic synthesis of anandamide,

an endogenous ligand for the cannabinoid receptor, by brain mem-

branes. Proc Natl Acad Sci USA 91, 6698–6701.

Devane, W. A., Hanus, L., Breuer, A., Pertwee, R. G., Stevenson, L. A.,

Griffin, G., Gibson, D., Mandelbaum, A., Etinger, A., & Mechoulam,

R. (1992). Isolation and structure of a brain constituent that binds to the

cannabinoid receptor. Science 258, 1946–1949.

Dewey, W. L. (1986). Cannabinoid pharmacology. Pharmacol Rev 38,

151–178.

Di Marzo, V., Melck, D., Bisogno, T., & De Petrocellis, L. (1998). Endo-

cannabinoids: endogenous cannabinoid receptor ligands with neuromo-

dulatory action. Trends Neurosci 21, 521–528.

Di Marzo, V., Berrendero, F., Bisogno, T., Gonzalez, S., Cavaliere, P.,

Romero, J., Cebeira, M., Ramos, J. A., & Fernandez-Ruiz, J. J.

(2000a). Enhancement of anandamide formation in the limbic forebrain

and reduction of endocannabinoid contents in the striatum of D9-tetra-

hydrocannabinol-tolerant rats. J Neurochem 74, 1627–1635.

Di Marzo, V., Hill, M. P., Bisogno, T., Crossman, A. R., & Brotchie, J. M.

(2000b). Enhanced levels of endocannabinoids in the globus pallidus

are associated with a reduction in movement in an animal model of

Parkinson’s disease. FASEB J 14, 1432–1438.

Felder, C. C., & Glass, M. (1998). Cannabinoid receptors and their endog-

enous agonists. Annu Rev Pharmacol Toxicol 38, 179–200.

Felder, C. C., Joyce, K. E., Briley, E. M., Glass, M., Mackie, K. P., Fahey,

K. J., Cullinan, G. J., Hunden, D. C., Johnson, D. W., Chaney, M. O.,

Koppel, G. A., & Brownstein, M. (1998). LY320135, a novel canna-

binoid CB1 receptor antagonist, unmasks coupling of the CB1 receptor

to stimulation of cAMP accumulation. J Pharmacol Exp Ther 284,

291–297.

Fernandez-Ruiz, J. J., Romero, J., Garcıa, L., Garcıa-Palomero, E., & Ra-

mos, J. A. (1996). Dopaminergic neurons as neurochemical substrates

of neurobehavioral effects of marihuana: developmental and adult stud-

ies. In R.J. Beninger, T. Palomo, & T. Archer (Eds.), Dopamine Disease

States (pp. 357–387). Madrid: CYM Press.

Fernandez-Ruiz, J. J., Berrendero, F., Hernandez, M. L., Romero, J., &

Ramos, J. A. (1999). Role of endocannabinoids in brain development.

Life Sci 65, 725–736.

Fernandez-Ruiz, J. J., Berrendero, F., Hernandez, M. L., & Ramos, J. A.

(2000). The endogenous cannabinoid system and brain development.

Trends Neurosci 23, 14–20.

Francis, P. T., Webster, M. T., Chesell, I. P., Holmes, C., Stratmann, G. C.,

Procter, A. W., Cross, A. J., Green, A. R., & Bouen, D. M. (1993).

Neurotransmitters and second messengers in aging and Alzheimer’s

disease. Ann NY Acad Sci 695, 19–26.

Fride, E., & Mechoulam, R. (1993). Pharmacological activity of the can-

nabinoid receptor agonist, anandamide, a brain constituent. Eur J Phar-

macol 231, 313–314.

Fride, E., Barg, J., Levy, R., Saya, D., Heldman, E.,Mechoulam, R., &Vogel,

Z. (1995). Low doses of anandamides inhibit pharmacological effects of

D9-tetrahydrocannabinol. J Pharmacol Exp Ther 272, 699–707.

Gaoni, Y., &Mechoulam, R. (1968). Isolation, structure, and partial synthesis

of an active constituent of hashish. J Am Soc Chem 86, 1646–1647.

Gerdeman, G., & Lovinger, D. M. (2001). CB1 cannabinoid receptor in-

hibits synaptic release of glutamate in rat dorsolateral striatum. J Neuro-

physiol 85, 468–471.

Gifford, A. N., Bruneus, M., Lin, S., Goutopoulos, A., Makriyannis, A.,

Volkow, N. D., & Gatley, S. J. (1999). Potentiation of the action of

anandamide on hippocampal slices by the fatty acid amide hydrolase

inhibitor, palmitylsulphonyl fluoride (AM374). Eur J Pharmacol 383,

9–14.

Giuffrida, A., Parsons, L. H., Kerr, T. M., Rodrıguez de Fonseca, F., Nav-

arro, M., & Piomelli, D. (1999). Dopamine activation of endogenous

cannabinoid signaling in dorsal striatum. Nat Neurosci 2, 358–363.

Giuffrida, A., Beltramo, M., & Piomelli, D. (2001). Mechanisms of endo-

cannabinoid inactivation: biochemistry and pharmacology. J Pharmacol

Exp Ther 298, 7–14.

Glass, M., Faull, R. L. M., & Dragunow, M. (1993). Loss of cannabinoid

receptors in the substantia nigra in Huntington’s disease. Neuroscience

56, 523–527.

Glass, M., Dragunow, M., & Faull, R. L. M. (2000). The pattern of

neurodegeneration in Huntington’s disease: a comparative study of

cannabinoid, dopamine, adenosine and GABA-A receptor alterations

in the human basal ganglia in Huntington’s disease. Neuroscience 97,

505–519.

Gonzalez, S., Romero, J., de Miguel, R., Lastres-Becker, I., Villanua, M. A.,

Makriyannis, A., Ramos, J. A., & Fernandez-Ruiz, J. J. (1999). Extra-

pyramidal and neuroendocrine effects of AM404, an inhibitor of the

carrier-mediated transport of anandamide. Life Sci 65, 327–336.

Gorriti, M. A., Rodrıguez de Fonseca, F., Navarro, M., & Palomo, T.

(1999). Chronic (� )D9-tetrahyrocannabinol treatment induces sensiti-

zation to the psychomotor effects of amphetamine in rats. Eur J Phar-

macol 365, 133–142.

Gueudet, C., Santucci, V., Rinaldi-Carmona, M., Soubrie, P., & Le Fur,

G. (1995). The CB1 cannabinoid receptor antagonist SR141716A

affects A9 dopamine neuronal activity in the rats. Neuroreport 6,

1421–1425.

Hampson, A. J., Bornheim, L. M., Scanziani, M., Yost, C. S., Gray, A. T.,

Hansen, B. M., Leonoudakis, D. J., & Bickler, P. E. (1998a). Dual

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152 149

effects of anandamide on NMDA receptor-mediated responses and neu-

rotransmission. J Neurochem 70, 671–676.

Hampson, A. J., Grimaldi, M., Axelrod, J., & Wink, D. (1998b). Cannabi-

diol and (� )D9-tetrahydrocannabinol are neuroprotective antioxidants.

Proc Natl Acad Sci USA 95, 8268–8273.

Hampson, R. E., & Deadwyler, S. A. (1999). Cannabinoids, hippocampal

function and memory. Life Sci 65, 715–723.

Hansen, H. S., Moesgaard, B., Petersen, G., & Hansen, H. H. (2002).

Putative neuroprotective actions of N-acyl-ethanolamines. Pharmacol

Ther 95, 119–126.

Hanus, L., Gopher, A., Almog, S., & Mechoulam, R. (1993). Two new

unsaturated fatty acid ethanolamides in brain that bind to the cannabi-

noid receptor. J Med Chem 36, 3032–3034.

Hanus, L., Breuer, A., Tchilibon, S., Shiloah, S., Goldenberg, D., Horowitz,

M., Pertwee, R. G., Ross, R. A., Mechoulam, R., & Fride, E. (1999).

HU308: a specific agonist for CB2, a peripheral cannabinoid receptor.

Proc Natl Acad Sci USA 96, 14228–14233.

Harris, D., Kendall, D. A., & Randall, M. D. (1999). Characterization of

cannabinoid receptors coupled to vasorelaxation by endothelium-de-

rived hyperpolarizing factor. Arch Pharmacol 359, 48–52.

Hemming, M., & Yellowlees, P. M. (1993). Effective treatment of Tour-

ette’s syndrome with marijuana. J Psychopharmacol 7, 389–391.

Herkenham, M., Lynn, A. B., Little, M. D., Johnson, M. R., Melvin, L. S.,

de Costa, D. R., & Rice, K. C. (1990). Cannabinoid receptor local-

ization in brain. Proc Natl Acad Sci USA 87, 1932–1936.

Herkenham, M., Lynn, A. B., de Costa, B. R., & Richfield, E. K. (1991a).

Neuronal localization of cannabinoid receptors in the basal ganglia of

the rat. Brain Res 547, 267–274.

Herkenham, M., Lynn, A. B., Little, M. D., Melvin, L. S., Johnson, M. R.,

de Costa, D. R., & Rice, K. C. (1991b). Characterization and local-

ization of cannabinoid receptors in rat brain: a quantitative in vitro

autoradiographic study. J Neurosci 11, 563–583.

Hersch, S. M., & Ferrante, R. J. (1997). Neuropathology and pathophysiol-

ogy of Huntington’s disease. In R.L. Watts, &W.C. Koller (Eds.),Move-

ment Disorders. Neurologic Principles and Practice ( pp. 503–518).

New York: McGraw-Hill.

Hillard, C. J. (2000). Endocannabinoids and vascular function. J Pharmacol

Exp Ther 294, 27–32.

Hillard, C. J., Edgemond, W. S., Jarrahian, A., & Campbell, W. B. (1997).

Accumulation of N-arachidonoylethanolamine (anandamide) into cere-

bellar granule cells occurs via facilitated diffusion. J Neurochem 69,

631–638.

Hillard, C. J., Manna, S., Greenberg, M. J., Dicamelli, R., Ross, R. A.,

Stevenson, L. A., Murphy, V., Pertwee, R. G., & Campbell, W. B.

(1999). Synthesis and characterization of potent and selective agonists

of the neuronal cannabinoid receptor (CB1). J Pharmacol. Exp Ther

289, 1427–1433.

Hiltunen, A. J., Jarbe, T. U., & Wangdahl, K. (1988). Cannabinol and

cannabidiol in combination: temperature, open-field activity, and vocal-

ization. Pharmacol Biochem Behav 30, 675–678.

Hohmann, A. G., & Herkenham, M. (2000). Localization of cannabinoid

CB1 receptor mRNA in neuronal subpopulations of rat striatum: a dou-

ble-label in situ hybridization study. Synapse 37, 71–80.

Howlett, A. C. (1995). Pharmacology of cannabinoid receptors. Annu Rev

Pharmacol Toxicol 35, 607–634.

Howlett, A. C., Bidaut-Rusell, M., Devane, W. A., Melvin, L. S., Johnson,

M. R., & Herkenham, M. (1990). The cannabinoid receptor: biochem-

ical, anatomical and behavioral characterization. Trends Neurosci 13,

420–423.

Jarbe, T. U., Sheppard, R., Lamb, R. J., Makriyannis, A., Lin, S., & Gouto-

poulos, A. (1998). Effects of D9-tetrahydrocannabinol and (R)-metha-

nandamide on open-field behavior in rats.Behav Pharmacol 9, 169–174.

Jarrahian, A., Manna, S., Edgemond, W. S., Campbell, W. B., & Hillard,

C. J. (2000). Structure-activity relationships among N-arachidonyletha-

nolamine (anandamide) head group analogues for the anandamide

transporter. J Neurochem 74, 2597–2606.

Kaminski, N. E. (1998). Regulation of the cAMP cascade, gene expression

and immune function by cannabinoid receptors. J Neuroimmunol 83,

124–132.

Khanolkar, A. D., Abadji, V., Lin, S., Hill, W. A., Taha, G., Abouzid, K.,

Meng, Z., Fan, P., & Makriyannis, A. (1996). Head group analogs of

arachidonylethanolamide, the endogenous cannabinoid ligand. J Med

Chem 39, 4515–4519.

Lan, R., Liu, Q., Lin, S., Fernando, S. R., McCallion, D., Pertwee, R., &

Makriyannis, A. (1999). Structure-activity relationships of pyrazole de-

rivatives as cannabinoid receptor antagonists. J Med Chem 42, 769–776.

Lastres-Becker, I., Cebeira, M., de Ceballos, M., Zeng, B.-Y., Jenner, P.,

Ramos, J. A., & Fernandez-Ruiz, J. J. (2001a). Increased cannabinoid

CB1 receptor binding and activation of GTP-binding proteins in the

basal ganglia of patients with Parkinson’s disease and MPTP-treated

marmosets. Eur J Neurosci 14, 1827–1832.

Lastres-Becker, I., Fezza, F., Cebeira, M., Bisogno, T., Ramos, J. A., Mi-

lone, A., Fernandez-Ruiz, J. J., & Di Marzo, V. (2001b). Changes in

endocannabinoid transmission in the basal ganglia in a rat model of

Huntington’s disease. Neuroreport 12, 2125–2129.

Lastres-Becker, I., Berrendero, F., Lucas, J. J., Martin, E., Yamamoto, A.,

Ramos, J. A., & Fernandez-Ruiz, J. J. (2002a). Loss of mRNA levels,

binding and activation of GTP-binding proteins for cannabinoid CB1

receptors in the basal ganglia of a transgenic model of Huntington’s

disease. Brain Res 929, 236–242.

Lastres-Becker, I., Hansen, H. H., Berrendero, F., de Miguel, R., Perez-

Rosado, A., Manzanares, J., Ramos, J. A., & Fernandez-Ruiz, J. J.

(2002b). Alleviation of motor hyperactivity and neurochemical deficits

by endocannabinoid uptake inhibition in a rat model of Huntington’s

disease. Synapse 44, 23–35.

Ledent, C., Valverde, O., Cossu, G., Petitet, F., Aubert, J. F., Beslot, F.,

Bohme, G. A., Imperato, A., Pedrazzini, T., Roques, B. P., Vassart, G.,

Fratta, W., & Parmentier, M. (1999). Unresponsiveness to cannabinoids

and reduced addictive effects of opiates in CB1 receptor knockout mice.

Science 283, 401–404.

Levenes, C., Daniel, H., Soubrie, P., & Crepel, F. (1998). Cannabinoids

decrease excitatory synaptic transmission and impair long-term depres-

sion in rat cerebellar Purkinje cells. J Physiol 510, 867–879.

Mackie, K., & Hille, B. (1992). Cannabinoids inhibit N-type calcium

channels in neuroblastoma-glioma cells. Proc Natl Acad Sci USA 89,

3825–3829.

Mackie, K., Lai, Y., Westenbroek, R., & Mitchell, R. (1995). Cannabinoids

activate an inwardly rectifying potassium conductance and inhibit Q-

type calcium currents in AtT20 cells transfected with rat brain canna-

binoid receptor. J Neurosci 15, 6552–6561.

Mailleux, P., & Vanderhaeghen, J. J. (1992a). Distribution of neuronal

cannabinoid receptor in the adult rat brain: a comparative receptor bind-

ing radioautography and in situ hybridization histochemistry. Neuro-

science 48, 655–668.

Mailleux, P., & Vanderhaeghen, J. J. (1992b). Age-related loss of cannabi-

noid of cannabinoid receptor binding sites and mRNA in the rat stria-

tum. Neurosci Lett 147, 179–181.

Mailleux, P., & Vanderhaeghen, J. J. (1993). Dopaminergic regulation of

cannabinoid receptor mRNA levels in the rat caudate-putamen: an in

situ hybridization study. J Neurochem 61, 1705–1712.

Mailleux, P., & Vanderhaeghen, J. J. (1994). Glutamatergic regulation of

cannabinoid receptor gene expression in the caudate-putamen. Eur J

Pharmacol 266, 193–196.

Maneuf, Y. P., Nash, J. E., Croosman, A. R., & Brotchie, J. M. (1996).

Activation of the cannabinoid receptor by D9-THC reduces GABA

uptake in the globus pallidus. Eur J Pharmacol 308, 161–164.

Maneuf, Y. P., Croosman, A. R., & Brotchie, J. M. (1997). The cannabinoid

receptor agonist WIN 55,212-2 reduces D2, but not D1, dopamine re-

ceptor-mediated alleviation of akinesia in the reserpine-treated rat mod-

el of Parkinson’s disease. Exp Neurol 148, 265–270.

McLaughlin, P. J., Delevan, C. E., Carnicom, S., Robinson, J. K., & Brener,

J. (2000). Fine motor control in rats is disrupted by D9-tetrahydrocan-

nabinol. Pharmacol Biochem Behav 66, 803–809.

Mechoulam, R., Hanus, L., & Martin, B. R. (1994). Search for endoge-

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152150

nous ligands of the cannabinoid receptors. Biochem Pharmacol 48,

1537–1544.

Mechoulam, R., Ben-Shabat, S., Hanus, L., Ligumsky, M., Kaminski, N.,

Schatz, A. R., Gopher, A., Almog, S., Martin, B. R., Compton, D. R.,

Pertwee, R. G., Griffin, G., Bayewitch, M., Barg, J., & Vogel, Z. (1995).

Identification of an endogenous 2-monoglyceride, present in canine gut,

that binds to cannabinoid receptors. Biochem Pharmacol 50, 83–90.

Meschler, J. P., & Howlett, A. C. (2001). Signal transduction interactions

between CB1 cannabinoid and dopamine rceptors in the rat and monkey

striatum. Neuropharmacology 40, 918–926.

Miller, A., & Walker, J. M. (1995). Effects of a cannabinoid on spontaneous

and evoked neuronal activity in the substantia nigra pars reticulata. Eur

J Pharmacol 279, 179–185.

Miller, A., & Walker, J. M. (1996). Electrophysiological effects of a can-

nabinoid on neural activity in the globus pallidus. Eur J Pharmacol

304, 29–35.

Miller, A., Sanudo-Pena, M. C., & Walker, J. M. (1998). Ipsilateral turning

behavior induced by unilateral microinjections of a cannabinoid into the

rat subthalamic nucleus. Brain Res 793, 7–11.

Moss, D. E., McMaster, S. B., & Rogers, J. (1981). Tetrahydrocannabinol

potentiates reserpine-induced hypokinesia. Pharmacol Biochem Behav

15, 779–783.

Muller-Vahl, K. R., Kolbe, H., Schneider, U., & Emrich, H. M. (1998).

Cannabinoids: possible role in the patho-physiology and therapy of

Gilles de la Tourette syndrome. Acta Psychiatr Scand 98, 502–506.

Muller-Vahl, K. R., Kolbe, H., Schneider, U., & Emrich, H. M. (1999a).

Cannabis in movement disorders. Forsch Komplementarmed 6, 23–27.

Muller-Vahl, K. R., Schneider, U., & Emrich, H. M. (1999b). Nabilone

increases choreatic movements in Huntington’s disease. Mov Disord

14, 1038–1040.

Muller-Vahl, K. R., Schneider, U., Kolbe, H., & Emrich, H. M. (1999c).

Treatment of Tourette-syndrome with D9-tetrahydrocannabinol. Am J

Psychiatry 156, 495.

Nagayama, T., Sinor, A. D., Simon, R. P., Chen, J., Graham, S. H., Jin, K.,

& Greenberg, D. A. (1999). Cannabinoids and neuroprotection in global

and focal cerebral ischemia and in neuronal cultures. J Neurosci 19,

2987–2995.

Navarro, M., Fernandez-Ruiz, J. J., de Miguel, R., Hernandez, M. L.,

Cebeira, M., & Ramos, J. A. (1993). Motor disturbances induced by

an acute dose of D9-tetrahydrocannabinol: possible involvement of ni-

grostriatal dopaminergic alterations. Pharmacol Biochem Behav 45,

291–298.

Page, K. J., Besret, L., Jain, M., Monaghan, E. M., Dunnett, S. B., &

Everitt, B. J. (2000). Effects of systemic 3-nitropropionic acid-induced

lesions of the dorsal striatum on cannabinoid and m-opioid receptor

binding in the basal ganglia. Exp Brain Res 130, 142–150.

Parolaro, D. (1999). Presence and functional regulation of cannabinoid

receptors in immune cells. Life Sci 65, 637–644.

Pertwee, R. G. (1997). Pharmacology of cannabinoid CB1 and CB2 recep-

tors. Pharmacol Ther 74, 129–180.

Pertwee, R. G. (2000). Cannabinoid receptor ligands: clinical and neuro-

pharmacological considerations, relevant to future drug discovery and

development. Expert Opin Invest Drugs 9, 1553–1571.

Pertwee, R. G. (2002). Cannabinoids and multiple sclerosis. Pharmacol

Ther 95, 165–174.

Pertwee, R. G., Greentree, S. G., & Swift, P. A. (1988). Drugs which

stimulate or facilitate central GABAergic transmission interact synerg-

istically with D9-tetrahydrocannabinol to produce marked catalepsy in

mice. Neuropharmacology 27, 1265–1270.

Pertwee, R. G., Gibson, T. M., Stevenson, L. A., Ross, R. A., Banner, W. K.,

Saha, B., Razdan, R. K., &Martin, B. R. (2000). O-1057, a potent water-

soluble cannabinoid receptor agonist with antinociceptive properties. Br

J Pharmacol 129, 1577–1584.

Piomelli, D., Beltramo, M., Glasnapp, S., Lin, S. Y., Goutopoulos, A., Xie,

X. Q., & Makriyannis, A. (1999). Structural determinants for recogni-

tion and translocation by the anandamide transporter. Proc Natl Acad

Sci USA 96, 5802–5807.

Randall, M. D., Harris, D., Kendall, D. A., & Ralevic, V. (2002). Cardio-

vascular effects of cannabinoids. Pharmacol Ther 95, 191–202.

Reddy, P. H., Williams, M., & Tagle, D. A. (1999). Recent advances in

understanding the pathogenesis of Huntington’s disease. Trends Neuro-

sci 22, 248–255.

Richfield, E. K., & Herkenham, M. (1994). Selective vulnerability in Hun-

tington’s disease: preferential loss of cannabinoid receptors in lateral

globus pallidus. Ann Neurol 36, 577–584.

Rinaldi-Carmona, M., Barth, F., Heaulme, M., Shire, D., Calandra, B.,

Congy, C., Martinez, S., Maruani, J., Neliat, G., Caput, D., Ferrara,

P., Soubrie, P., Breliere, J. C., & Le Fur, G. (1994). SR141716A, a

potent and selective antagonist of the brain cannabinoid receptor. FEBS

Lett 350, 240–244.

Rinaldi-Carmona, M., Barth, F., Millan, J., Derocq, J. M., Casellas, P.,

Congy, C., Oustic, D., Sarran, M., Bouaboula, M., Calandra, B., Portier,

M., Shire, D., Breliere, J. C., & Le Fur, G. (1998). SR 144528, the first

potent and selective antagonist of the CB2 cannabinoid receptor. J

Pharmacol Exp Ther 284, 644–650.

Rodrıguez, M. C., Guridi, O. J., Alvarez, L., Mewes, K., Macias, R., Vitek,

J., De Long, M. R., & Obeso, J. A. (1998). The subthalamic nucleus

and tremor in Parkinson’s disease. Mov Disord 13, 111–118.

Rodrıguez de Fonseca, F., Gorriti, M. A., Fernandez-Ruiz, J. J., Palomo, T.,

& Ramos, J. A. (1994). Downregulation of rat brain cannabinoid bind-

ing sites after chronic D9-tetrahydrocannabinol treatment. Pharmacol

Biochem Behav 47, 33–40.

Romero, J., de Miguel, R., Garcıa-Palomero, E., Fernandez-Ruiz, J. J., &

Ramos, J. A. (1995a). Time-course of the effects of anandamide, the

putative endogenous cannabinoid receptor ligand, on extrapyramidal

function. Brain Res 694, 223–232.

Romero, J., Garcıa, L., Cebeira, M., Zadrozny, D., Fernandez-Ruiz, J. J., &

Ramos, J. A. (1995b). The endogenous cannabinoid receptor ligand,

anandamide, inhibits the motor behaviour: role of nigrostriatal dopami-

nergic neurons. Life Sci 56, 2033–2040.

Romero, J., Garcıa-Palomero, E., Fernandez-Ruiz, J. J., & Ramos, J. A.

(1996a). Involvement of GABA-B receptors in the motor inhibition

produced by agonists of brain cannabinoid receptors. Behav Pharmacol

7, 299–302.

Romero, J., Garcıa-Palomero, E., Lin, S. Y., Ramos, J. A., Makriyannis, A.,

& Fernandez-Ruiz, J. J. (1996b). Extrapyramidal effects of methanan-

damide, an analog of anandamide, the endogenous CB1 receptor ligand.

Life Sci 58, 1249–1257.

Romero, J., Garcıa-Palomero, E., Castro, J. G., Garcıa-Gil, L., Ramos,

J. A., & Fernandez-Ruiz, J. J. (1997). Effects of chronic exposure to

D9-tetrahydrocannabinol on cannabinoid receptor binding and mRNA

levels in several rat brain regions. Mol Brain Res 46, 100–108.

Romero, J., Berrendero, F., Garcıa-Gil, L., de la Cruz, P., Ramos, J. A., &

Fernandez-Ruiz, J. J. (1998a). Loss of cannabinoid receptor binding and

messenger RNA levels and cannabinoid agonist-stimulated [35S]-

GTPgS binding in the basal ganglia of aged rats. Neuroscience 84,

1075–1083.

Romero, J., de Miguel, R., Ramos, J. A., & Fernandez-Ruiz, J. J. (1998b).

The activation of cannabinoid receptors in striatonigral neurons in-

hibited GABA uptake. Life Sci 62, 351–363.

Romero, J., Berrendero, F., Perez-Rosado, A., Manzanares, J., Rojo, A.,

Fernandez-Ruiz, J. J., de Yebenes, J. G., & Ramos, J. A. (2000). Uni-

lateral 6-hydroxydopamine lesions of nigrostriatal dopaminergic neu-

rons increased CB1 receptor mRNA levels in the caudate-putamen. Life

Sci 66, 485–494.

Saji, M., & Reis, D. J. (1987). Delayed transneuronal death of substantia

nigra neurons prevented by g-aminobutyric acid agonist. Science 235,

66–69.

Saji, M., Blau, A. D., & Volpe, B. T. (1996). Prevention of transneuronal

degeneration of neurons in the substantia nigra reticulata by ablation of

the subthalamic nucleus. Exp Neurol 141, 120–129.

Sanudo-Pena, M. C., & Walker, J. M. (1997). Role of the subthalamic

nucleus in cannabinoid actions in the substantia nigra of the rat. J

Neurophysiol 77, 1635–1638.

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152 151

Sanudo-Pena, M. C., & Walker, J. M. (1998). Effects of intrapallidal can-

nabinoids on rotational behavior in rats: interactions with the dopami-

nergic system. Synapse 28, 27–32.

Sanudo-Pena, M. C., Patrick, S. L., Patrick, R. L., & Walker, J. M. (1996).

Effects of intranigral cannabinoids on rotational behavior in rats: inter-

actions with the dopaminergic system. Neurosci Lett 206, 21–24.

Sanudo-Pena, M. C., Force, M., Tsou, K., & Walker, J. M. (1998a). Effects

of intrastriatal cannabinoids on rotational behavior in rats: interactions

with the dopaminergic system. Synapse 30, 221–226.

Sanudo-Pena, M. C., Patrick, S. L., Khen, S., Patrick, R. L., Tsou, K., &

Walker, J. M. (1998b). Cannabinoid effects in basal ganglia in a rat

model of Parkinson’s disease. Neurosci Lett 248, 171–174.

Sanudo-Pena, M. C., Tsou, K., & Walker, J. M. (1999). Motor actions of

cannabinoids in the basal ganglia output nuclei. Life Sci 65, 703–713.

Sanudo-Pena, M. C., Romero, J., Seale, G. E., Fernandez-Ruiz, J. J., &

Walker, J. M. (2000). Activational role of cannabinoids on movement.

Eur J Pharmacol 391, 269–274.

Schmid, P. C., Krebsbach, R. J., Perry, S. R., Dettmer, T. M., Maassen, J. L.,

& Schmid, H. H. O. (1995). Occurrence and postmortem generation of

anandamide and other long-chain N-acylethanolamines in mammalian

brain. FEBS Lett 375, 117–120.

Schut, L. J. (1998). Motor system changes in the aging brain: what is

normal and what is not. Geriatrics 53, S16–S19.

Shen, M., & Thayer, S. A. (1998). Cannabinoid receptor agonists protect

cultured rat hippocampal neurons from excitotoxicity. Mol Pharmacol

54, 459–462.

Silverdale, M. A., McGuire, S., McInnes, A., Crossman, A. R., & Brotchie,

J. M. (2001). Striatal cannabinoid CB1 receptor mRNA expression is

decreased in the reserpine-treated rat model of Parkinson’s disease. Exp

Neurol 169, 400–406.

Skaper, S. D., Buriani, A., Dal Toso, R., Petrelli, L., Romanello, L., Facci,

L., & Leon, A. (1996). The aliamide, palmitoylethanolamide and can-

nabinoids, but not anandamide, are protective in a delayed, postgluta-

mate paradigm of excitotoxic death in cerebellar granular neurons. Proc

Natl Acad Sci USA 93, 3984–3989.

Smith, P. B., Compton, D. R., Welch, S. P., Razdan, R. K., Mechoulam, R.,

& Martin, B. R. (1994). The pharmacological activity of anandamide, a

putative endogenous cannabinoid, in mice. J Pharmacol Exp Ther 270,

219–227.

Soueif, M. I. (1976). Differential association between chronic cannabis use

and brain function deficits. Ann NY Acad Sci 282, 323–343.

Souilhac, J., Poncelet, M., Rinaldi-Carmona, M., Le-Fur, G., & Soubrie, P.

(1995). Intrastriatal injection of cannabinoid receptor agonists induced

turning behavior in mice. Pharmacol Biochem Behav 51, 3–7.

Sugiura, T., Kondo, S., Sukagawa, A., Nakane, S., Shinoda, A., Itoh, K.,

Yamashita, A., & Waku, K. (1995). 2-Arachidonoylglycerol: a possible

endogenous cannabinoid receptor ligand in brain. Biochem Biophys Res

Commun 215, 89–97.

Sugiura, T., Kondo, S., Kishimoto, S., Miyashita, T., Nakane, S., Kodaka,

T., Suhara, Y., Takayama, H., & Waku, K. (2000). Evidence that 2-

arachidonoylglycerol but not N-palmitoylethanolamine or anandamide

is the physiological ligand for the cannabinoid CB2 receptor. Compar-

ison of the agonistic activities of various cannabinoid receptor ligands

in HL-60 cells. J Biol Chem 275, 605–612.

Szabo, B., Dorner, L., Pfreundtner, C., Norenberg, W., & Starke, K. (1998).

Inhibition of GABAergic inhibitory postsynaptic currents by cannabi-

noids in rat corpus striatum. Neuroscience 85, 395–403.

Szabo, B., Wallmichrath, I., Mathonia, P., & Pfreundtner, C. (2000). Can-

nabinoids inhibit excitatory neurotransmission in the substantia nigra

pars reticulata. Neuroscience 97, 89–97.

Tersigni, T. J., & Rosenberg, H. C. (1996). Local pressure application of

cannabinoid agonists increases spontaneous activity of rat substantia

nigra pars reticulata neurons without affecting response to iontophoreti-

cally-applied GABA. Brain Res 733, 184–192.

Tsou, K., Patrick, S. L., & Walker, J. M. (1995). Physical withdrawal in rat

tolerants to D9-tetrahydrocannabinol precipitated by a cannabinoid re-

ceptor antagonist. Eur J Pharmacol 280, R13–R15.

Tsou, K., Brown, S., Sanudo-Pena, M. C., Mackie, K., & Walker, J. M.

(1998a). Immunohistochemical distribution of cannabinoid CB1 recep-

tors in the rat central nervous system. Neuroscience 83, 393–411.

Tsou, K., Nogueron, M. I., Muthian, S., Sanudo-Pena, M., Hillard, C. J.,

Deutsch, D. G., & Walker, J. M. (1998b). Fatty acid amide hydrolase is

located preferentially in large neurons in the rat central nervous system

as revealed by immunohistochemistry. Neurosci Lett 254, 137–140.

Vonsattel, J. P., Myers, R. H., Stevens, T. J., Ferrante, R. J., Bird, E. D., &

Richardson, E. P. (1985). Neuropathological classification of Hunting-

ton’s disease. J Neuropathol Exp Neurol 44, 559–577.

Wagner, J. A., Varga, K., & Kunos, G. (1998). Cardiovascular actions

of cannabinoids and their generation during shock. J Mol Med 76,

824–836.

Walker, J. M., Hohmann, A. G., Martin, W. J., Strangman, N. M., Huang,

S. M., & Tsou, K. (1999). The neurobiology of cannabinoid analgesia.

Life Sci 65, 665–673.

Westlake, T. M., Howlett, A. C., Bonner, T. I., Matsuda, L. A., & Herken-

ham, M. (1994). Cannabinoid receptor binding and messenger RNA

expression in human brain: an in vitro receptor autoradiography and

in situ hybridization histochemistry study of normal aged and Alzheim-

er’s brains. Neuroscience 63, 637–652.

Wickens, A. P., & Pertwee, R. G. (1993). D9-Tetrahydrocannabinol and

anandamide enhance the ability of muscimol to induce catalepsy in

the globus pallidus of rats. Eur J Pharmacol 250, 205–208.

Zaretsky, A., Rector, N. A., Seeman, M. V., & Fornazzari, X. (1993).

Current cannabis use and tardive dyskinesia. Schizophr Res 11, 3–8.

Zeng, B.-Y., Dass, B., Owen, A., Rose, S., Cannizzaro, C., Tel, B. C., &

Jenner, P. (1999). Chronic L-DOPA treatment increases striatal canna-

binoid CB1 receptor mRNA expression in 6-hydroxydopamine-lesioned

rats. Neurosci Lett 276, 71–74.

Zimmer, A., Zimmer, A. M., Hohmann, A. G., Herkenham, M., & Bonner,

T. I. (1999). Increased mortality, hypoactivity, and hypoalgesia in can-

nabinoid CB1 receptor knockout mice. Proc Natl Acad Sci USA 96,

5780–5785.

Zygmunt, P. M., Chuang, H., Movahed, P., Julius, D., & Hogestatt, E. D.

(2000). The anandamide transport inhibitor AM404 activates vanilloid

receptors. Eur J Pharmacol 396, 39–42.

J. Romero et al. / Pharmacology & Therapeutics 95 (2002) 137–152152