Neuro biochemistry
Dr. SarYOno, SKp.,MKes. Medical biochemistry unit
Neuron Communication by neurons is based on
changes in the membrane’s permeability to ions.
A typical neuron has a dendritic region and an axonal region.
The dendritic region is specialized to receive information, typically neurotransmitters; it then undergoes graded potentials.
The axonal region is specialized to deliver information: after undergoing action potentials, neurotransmitters are released from the axon terminal.
Synapses An interneuronal junctions Two kinds of synapses
Chemical synapses Electric synapses
Chemical Synapses Vesicles contain neurotransmitters that can
alter the ionic conductivity of the postsynaptic membrane
The postsynaptic membrane is separated from the presynaptic membrane by a synaptic cleft having a width of 20 nm
An increase in sodium ion/Na+, permeability tends to depolarize the postsynaptic membrane (excitatory)
An increase in potassium ion, permeability hyperpolarizes the membrane (inhibitory)
Chemical vs. Electric SynapseElectrical synapse• Impulses can be regenerated without interruption in adjacent cells• Gap junctions
– Adjacent cells electrically coupled through a channel– Each gap junction is composed of 12 connexin proteins
• Examples– Smooth and cardiac muscles, brain, and glial cells
Chemical synapse• Terminal bouton is separated from postsynaptic cell by synaptic cleft• NTs are released from synaptic vesicles• Vesicles fuse with axon membrane and NT released by exocytosis• Amount of NTs released depends upon frequency of APExamples: otot lurik/rangka
Chemical vs. Electric Synapse
Chemical Synapse• Unidirectional• Current Flow Limited By Neurotransmitter Diffusion• Nonlinear
Electric Synapse• Bidirectional• Very Fast Current Flow• Linear
Ion gating in axon Changes in membrane potential caused by ion flow
through ion channels Voltage gated (VG) channels open in response to change
in membrane potential Gated channels are part of proteins that comprise the
channel Can be open or closed in response to change
2 types of channels for K+ 1 always open 1 closed in resting cell
Channel for Na+ Always closed in resting cells
• Some Na+ does leak into the cells
How A Nerve Cell Fires Nerve cell membrane is a lipid bilayer with
embedded proteins. ATP-powered ion pumps keep outside of
membrane + charged, inside – charged. Channels in membrane can let + ions pass
through. Channels normally closed. Neurotransmitter gated channels collapse
(‘depolarize’) voltage gradient. Voltage gated channels propagate
depolarization in a wave down axon.
Action potentials Stimulus causes depolarization to threshold Voltage gated (VG) Na+ channels open
Electrochemical gradient inward + feedback loop
Rapid reversal in membrane potential from –70 to + 30 mV VG Na+ channels become inactivated
VG K+ channels open Electrochemical gradient outward -feedback loop
Action potentials Depolarization and repolarization occur via diffusion, do not
require active transport Once AP completed, Na+/K+ ATPase pump extrudes Na+,
and recovers K+ All or none
When threshold reached, maximum potential change occurs Amplitude does not normally become more positive than + 30
mV because VG Na+ channels close quickly and VG K+ channels open
Duration is the same, only open for a fixed period of time Coding for Stimulus Intensity
Increased frequency of AP indicates greater stimulus strength Recruitment
Stronger stimuli can activate more axons with a higher threshold
(Potensial berjenjang)
Synaptic transmission NT release is rapid because many vesicles form fusion
complexes at docking site AP travels down axon to bouton VG Ca2+ channels open
– Ca2+ enters bouton down concentration gradient– Inward diffusion triggers rapid fusion of synaptic vesicles and release of NTs
Ca2+ activates calmodulin, which activates protein kinase Protein kinase phosphorylates synapsins
– Synapsins aid in the fusion of synaptic vesicles
Inhibit transmiter release
Inhibit transmiter uptake
organofosfat
The Dopamine The Dopamine HypothesisHypothesis
F. I. Carroll et al, Journal of Medical Chemistry 42, 2721-36 (1999)
Synapses are major targets of neuroactive drugs
CocaineX
Adenosine
CaffeineX
Nicotine
•Caffeine: inhibits adenosine receptors•Nicotine: activates acetylcholine receptors•Cocaine: inhibits uptake of DA, NE, 5HT•Ethanol?
AChReceptor
Synapses are the targets of therapeutic drugs Synapses are also the sites of actions of many classes of
therapeutic drugs which act upon the brain. 1) Antidepressant drugs generally act by inhibiting the
uptake of serotonin and norepinephrine 2) An important class of analgesic drugs, the opiate
analgesics, activate receptors for neurotransmitters known as the endorphins and enkephalins
3) Antipsychotic drugs block or inhibit receptors for dopamine
4) Some types of anticonvulsant drugs potentiate the effects of the neurotransmitter GABA
•Antidepressant drugs: serotonin uptake inhibitors• Analgesics (morphine): opiate receptor agonists• Antipsychotic drugs: DA receptor antagonists•Anticonvulsant drugs; GABA, modulators•Antianxiety agents: GABA, modulators
Zigmond et al
(1999) Fig 8.3
Stages 1 & 2 Accumulation of a precursor amino acid into the neuron which is metabolized to yield the mature transmitter (ZZ)
Stage 3 Transmitter is then accumulated into vesicles by the vesicular transporter for storage and release. Stages 4 & 5 Transmitter is released into synaptic cleft to interact with post-synaptic receptors or autoreceptors that
regulate transmitter release, synthesis or firing rate.Stage 6 – 9 Inactivation and termination of the action of the released transmitter by reuptake through neuronal
transporter proteins, enzymatic degradation, uptake by glial cells or passive diffusion
LIFE CYCLE OF A TRANSMITTER
Neurotransmiter (bekerja cepat)
Klas I ; asetilkolin Klas II (amina); norepinefrin, epinefrin,
dopamin, serotonin, histamin Klas III (asam amino); GABA, glisin,
glutamat, aspartat Klas IV; NO
Neuropeptida (bekerja lambat)
Releasing hormon Peptida hipofise; LH, GH, endorfin, prolaktin,
oksitosin dll Peptida usus dan otak; enkefalin, substansi P,
gastrin, insulin, glukagon dll Dari jaringan lain; angiotensin, bradikinin,
kalsitonin dll
Neurotransmiter vs neuropeptida
Bekerja cepat Molekul kecil Disintesis di
sitosol Tempat kerja
di membran pasca sinap
Bekerja lambat
Molekul besar
Disintesis di ribosom, retikulum endoplasma, aparatus golgi
Tempat non sinaps, di sel prasinaps maupun pasca sinaps
Letak neurotransmiter penting
Asetilkolin-disekresi di sebagian besar otak, otot rangka dll
Norepinefrin- disekresi di neuron yang badan selnya di batang otak dan hipotalamus
Dopamin – di substansia nigra GABA- di medula spinalis, serebelum, ganglia
basalis, sebagian korteks Serotonin- di rafe medial batang otak Glutamat – di presinap sensorik korteks
Neurotransmitter in the CNS -There is a broad set of different neurotransmitters in the CNS-Neurotransmitters can be activating or inhibitory-Neurotransmitter bind to specific receptors and cause excitatory post synaptic potential (EPSP) or inhibitory post synaptic potentials (IPSP) -Activating transmitter act by opening kation-channels (Na+,K+, Ca2+)-Inhibitory transmitter act by opening anion (mainly Cl- -Channels)-Important neurotransmitter:
-Acetylcholine (Transmitter of Neuro-muscular junctions)-Dopamine-Serotonin-Glycin (inhibitory)-Aspartate-Glutamate-GABA (-amino-butyric acid, inhibitory)-Adrenaline/Noradrenaline (Autonomous nervous system)
Neurotransmitters
DopamineNorepinephrineEpinephrine
Glutamate -Aminobutyrate
Serotonin
Tyrosine
Tryptophan
PLP (vit B6)
deCO2ase
PLP (vit B6)
deCO2ase
PLP (vit B6)
deCO2ase
Amino AcidPrecursors
PLP:piridoksal fosfat
Pathway
Dopamine
Norepinephrine
Serotonin(5-HT)
Tyrosine
Tryptophan
PLP (vit B6)
AAA deCO2ase
PLP (vit B6)
AAA deCO2ase
Catecholamines
DOPA
DOHase Vit C, O2
Tyr OHase
THBP, O2
5HTPTrp OHase
O2THBP,
EpinephrinePNMT
SAMSAHC
THBP: tetrahydrobiopterin
Acetylcholine+ cholineAcetyl-CoAPyruvate
PDH complex(FAD, lipoamide, TPP)
Choline acetyltransferase
Acetylcholine
Acetylcholinesterase
Acetate + choline
Reuptake or diet
Catecholamine Biosynthesis
CH2CHCO2-
NH3+
HO
CH2CHCO2-
NH3+
HO
HO
CH2CH2NH2
HO
HO
CHCH2NH2
HO
HO
OH
CHCH2NHCH3
HO
HO
OH
Tyr hydroxylase
O2
Tyrosine Dihydroxyphenylalanine (DOPA)
Dopamine
DOPAdecarboxylase CO2
Dopaminehydroxylase
Norepinephrine
Catechol
Epinephrine(Adrenaline)
SAM
S-Adenosyl-homocysteine
Methyl transferase
DOPA, dopamine, norepinephrine,and epinephrine are all neurotransmitters
L-DOPA in Parkinsonism
Blood Brain
Blood Brain Barrier
L-DOPA L-DOPA Dopamine
Dopamine
HO
HO CH2-C-CO2H
CH3
NHNH2Carbidopa
Blocks
Parkinsonism associated with dopamine in brain through loss ofneurons in basal ganglia.Carbidopa + L-DOPA
Monoamine Oxidase (MAO)MAO
(in mitochondria)
R R’OH H NorepiOH CH3 EpiH H Dopamine
CHCH2NHR'
HO
HO
RCHCHO
HO
HO
R
CHCO2H
HO
HO
RUrinary metaboliteMAO inhibitors (e.g., tranylcypromine) are useful
in the treatment of depression Brain levels of dopamine and norepi.; also serotonin
Aldehydedehydrogenase
R=OH Vanillylmandelic acid (VMA)R=H Homovanillic acid (HVA)
Catechol-O-Methyl Transferase (COMT)
CHCH2NHR'
HO
HO
RCHCH2NHR'
HO
CH3O
R
COMT
Inactive metabolite
SAM S-Adenosyl-homocysteine
• COMT found in cytoplasm• Terminates activity of catecholamines• Catecholamine excretion products result from combined actions of MAO and COMT• Inhibitors of COMT (e.g., tolcapone) useful in Parkinson’s disease
Active catecholamine
Tryptophan Metabolism: Serotonin Formation
NH
CH2CHCO2-
NH3+
NH
CH2CHCO2-
NH3
HO
+
NH
CH2CH2NH2
HO
Tryptophan(Trp)
Indole ring
Trphydroxylase
O2
5-Hydroxy-tryptophan
Decarboxylase
CO2 5-Hydroxy-tryptamine (5-HT);Serotonin
Serotonin• Serotonin formed in:
• Brain (neurotransmitter; regulation of sleep, mood, appetite) • Platelets (platelet aggregation, vasoconstriction)• Smooth muscle (contraction) • Gastrointestinal tract (enterochromaffin cells - major storage site)
• Drugs affecting serotonin actions used to treat: • Depression
•Serotonin-selective reuptake inhibitors (SSRI) • Migraine• Schizophrenia• Obsessive-compulsive disorders • Chemotherapy-induced emesis
• Some hallucinogens (e.g., LSD) act as serotonin agonists
Serotonin Metabolism: 5-HIAA
NH
CH2CH2NH2
HO
NH
CH2CHOHO
NH
CH2CO2HHO
Serotonin
MAO
Dehydrogenase
5-Hydroxyindole acetic acid (5-HIAA) (Urine)
Carcinoid tumors: • Malignant GI tumor type• Excretion of large amounts of 5-HIAA
Serotonin Metabolism: Melatonin
NH
CH2CH2NHCOCH3
H3CO
NH
CH2CH2NH2
HO2 Steps
Serotonin Melatonin
Melatonin:• Formed principally in pineal gland• Synthesis controlled by light, among other factors• Induces skin lightening• Suppresses ovarian function• Possible use in sleep disorders
Nitric Oxide• Cell messenger• Implicated in a wide range of physiological and pathophysiological events:• Vasodilation:
• Activates guanylyl cyclase cGMP• Nitroglycerin Glycerin + NO• Sildenafil (Viagra): in vascular smooth muscle:
NO cGMP GMPPhospho-diesterase-5
Blocks
Synthesis of Nitric Oxide
NH3+NH2
H2N=C-HNCH2CH2CH2CHCO 2-
+
NH3+
NH2CONH CH2CH2CH2CHCO 2- + NO
Nitric oxide synthase (NOS)
Arginine
Citrulline
Tryptophan Metabolism:Biosynthesis of Nicotinic Acid
NH
CH2CHCO2-
NH3+
TryptophanN
CO2H
Nicotinic acid (Niacin)
Several steps
Nicotinamide adenine dinucleotide (NAD)
GABA
GLUTAMATE GABA + CO2
GLU DECARBOXYLASE GABA IS THE MAJOR INHIBITORY NEURO-TRANSMITTER IN
BRAIN GLU IS THE MAJOR EXCITATORY NEURO-TRANSMITTER
STIMULATION OF NEURONS BY GABA PERMEABILITY TO CHLORIDE IONS
BENZODIAZEPINES (VALIUM) ENHANCE MEMBRANE PERMEABILITY OF Cl IONS BY GABA
GABAPENTIN PROTECTS AGAINST GLU EXCITOTOXICITY
HISTAMINE
HISTIDINE HISTAMINE + CO2
HIS DECARBOXYLASE HISTAMINES INVOLVED IN
ALLERGIC RESPONSE H1 RECEPTORS IN GUT, BRONCHI
• STIMULATION SMOOTH MUSCLE CONTRN’• H1 RECEPTOR ANTAGONISTS
– CLARITIN, ZYRTEC, ETC
HISTAMINE
HISTAMINES INVOLVED IN CONTROL OF ACID SECRETION IN STOMACH
H2 RECEPTORS • STIMULATION HCl SECRETION• H2 ANTAGONISTS
– CIMETIDINE– RANITIDINE
H2 RECEPTORS IN HEART STIMULATION HEART RATE
SEROTONIN TRP 5-HYDROXYTRYPTOPHAN
TRP HYDROXYLASE REQUIRES 5,6,7,8
TETRAHYDROBIOPTERIN 5-HT SEROTONIN + CO2
AROMATIC ACID DECARBOXYLASE SEROTONIN CAUSES
SMOOTH MUSCLE CONTRACTION BRAIN NEUROTRANSMITTER MELATONIN SYNTHESIZED IN PINEAL
GLAND
CATECHOLAMINES EPI, NOREPINEPHRINE, DOPAMINE
AMINE DERIVATIVES OF CATECHOL
REACTIONS:
TYR L-DOPA TYR HYDROXYLASE
L-DOPA DOPAMINE + CO2 AROMATIC ACID DECARBOXYLASE
DOPAMINE NOREPINEPHRINE DOPAMINE β-HYDROXYLASE
NOREPINEPHRINE EPINEPHRINE REQUIRES SAM
L-DOPA AND DOPAMINE IN SUBSTANTIA NIGRA, CATECHOLAMINE
PRODUCTION STOPS AT DOPAMINE PARKINSON’S DISEASE: DEGENERATION OF
SUBSTANTIA NIGRA DOPAMINE TREAT BY GIVING PRECURSOR, L-DOPA DOPAMINE CANNOT CROSS BLOOD/BRAIN
BARRIER TRANSPLANTATION OF ADR. MEDULLA CELLS TO
BRAIN L-DOPA A PRECURSOR OF MELANIN
PRODUCTION
NOREPINEPHRINE EPINEPHRINE
S-ADENOSYLMETHIONINE
ACTIONS OF NOREPINEPHRINE NOT NEARLY AS ACTIVE AS EPINEPHRINE
DURING EXTREME STRESS CIRCULATORY SYSTEM
CONSTRICTS GREAT VEINS (2) VASOCONSTRICTIVE TO SKIN (1) VASOCONSTRICTION (1) EFFECTS ON
GI TRACT SPLEEN PANCREAS KIDNEYS
NEUROTRANSMITTER IN THE BRAIN
ACTIONS OF EPINEPHRINE
AS AN INSULIN ANTAGONIST ACTIVATES MUSCLE GLYCOGEN PHOSPHORYLASE
GLUCOSE-6-P USED IN GLYCOLYSIS TRIGGERS PHOSPHORYLATION (ACTIVATION) OF
HORMONE-SENSITIVE LIPASE IN FAT CELLS MOBILIZES FAT BY HYDROLYZING TGs
GLYCOGEN BREAKDOWN IN LIVER ACTIVATES GLUCONEOGENESIS IN LIVER INHIBITS FATTY ACID SYNTHESIS
ACTIONS OF EPINEPHRINE
ON CARDIAC MUSCLE β1 -ADRENERGIC RECEPTOR STIMULATION
HEART RATE AND CARDIAC OUTPUT• β-BLOCKERS BLOOD PRESSURE
DILATES CORONARY ARTERIES (β2) ON SMOOTH MUSCLE (β2-ADRENERGIC)
IN BRONCHIOLES, FOR EXAMPLE MUSCLE RELAXATION
ACTIVATION OF G-PROTEINS• cAMP , ETC
ASTHMA MEDICATIONS
Phenylalanine and tyrosine are precursors of the
catecholamines, dopamine, norepinephrine, and epinephrine.
• Dopamine has a role in the pathogenic mechanisms
that cause Parkinson’s disease.
• Parkinson’s disease is characterized by a loss of
dopaminergic neurons that results in diminished
levels of dopamine in the striatum.
Dopamine is synthesized in the cytoplasm and (normally) immediately sequestered in intracellular storage vesicles through the agency of α-synuclein.
2. Biosynthesis of the catecholamines
phenylalanine
tyrosine
Tyrosine 3-monooxygenase*
dihydroxyphenylalanine (DOPA)
dopamine
norepinephrine
epinephrine
*Rate-determining step
•
Tryptophanprecursor of 5-hydroxy-tryptamine (serotonin).
The broad actions of the neurotransmitter 5-hydroxytryptamine involve effects upon emotion, mood, and reward.
suicide and depression have been associated with reduced serotonergic transmission.
One recent postmortem study examined the capacity to synthesize 5-hydroxytryptamine and the capacity to bind 5-hydroxytryptamine.
Their findings indicated that the brains of suicide victims had had reduced serotonergic function.
2. Biosynthesis of 5-hydroxytryptamine
In the conversion of tryptophan into 5-hydroxy-
tryptamine, tryptophan undergoes hydroxylation to
yield 5-hydroxytryptophan that is then decarboxylated
to yield 5-hydroxytryptamine.
Reactions that convert tyrosine or tryptophan into neurotransmitters.
It has been shown that that the intensity and
duration of catecholamine signaling at the synapse is
governed by the efficiency of the re-uptake of released
neurotransmitter. Re-uptake occurs by means of high-
affinity membrane transporters. The finding that a
dopamine “re-uptake” transporter can serve as a
cocaine “receptor” has provided new insight into
mechanisms of addiction.
Cartoon depicting the binding of cocaine (and other agents) by the membrane transporters that facilitate reuptake.
D. Arginine is the precursor of the second
messenger nitric oxide. This a second messenger
that appears to have distinctly different functions in
different cell types. For example, the nitric oxide
synthesized by platelets is seen to inhibit platelet
aggregation and adherence. In this way it
contributes to the anti-thrombogenic properties of
the endothelium.
In the kidney (in experimental animals), a diminution in
nitric oxide was associated with glucocorticoid-induced
hypertension. Such studies were carried out because it
had been suggested that decreased nitric oxide
contributes to the impaired endothelium-dependent
vasodilatation seen in essential hypertension.
On the other hand, cortical cells from patients with
Alzheimer’s disease showed increased levels of nitric
oxide mRNA and protein. Hence, nitric oxide has been
associated with the progression of Alzheimer’s disease.
From an overview however, nitric oxide is generally
considered to be a vasodilator.
As a second messenger, nitric oxide up regulates
the activity of the enzyme that synthesizes cyclic GMP,
i.e., guanylyl cyclase. The cyclic GMP then up
regulates a cyclic GMP-dependent protein kinase. The
protein kinase, in turn, phosphorylates specific
substrates in order to bring about its effects.
2. Biosynthesis of nitric oxide
Nitric oxide is synthesized by the enzyme nitric
oxide synthase (NOS). There are three types of
enzymes, the neuronal nitric oxide synthase (nNOS),
the endothelial nitric oxide synthase (eNOS), and the
inducible nitric oxide synthase (iNOS). They each
catalyze the complex oxidation/reduction reaction that
utilizes one of arginine’s nitrogen atoms in order to
make the “free radical” nitric oxide.
The NOS reaction requires NADPH, FAD, and FMN along with tetrahydrobiopterin (BH4) and heme (Fe).
An electron transport chain has been formulated for the reaction.
V. Glutamate and its Receptors and Signaling The amino acid glutamate is an endogenous excitatory
neurotransmitter, utilized by neurons in the human
central nervous system and the peripheral nervous system. Glutamate acts by binding glutamate receptors that allow it to play a pivotal role in synaptic mechanisms
involved in learning and memory.
Learning and memory involve synaptic plasticity, the
phenomenon in which the efficacy, or efficiency, of
synaptic transmission varies in an activity-dependent
manner. The effect can be transient (short-term) or
persistent (long-term). Signal transduction involving
glutamate receptors governs the reactions involved in
these processes..
Cartoon depicting signal transduction involving the glutamate receptor in the phenomenon of synaptic plasticity.
Glutamate binds two types of receptors, the
ionotropic glutamate receptors and the metabotropic
receptors. There are three of the former, the NMDA
(N-methyl-D-aspartate), the AMPA (α-amino-3-
hydroxy-5-methyl-4-isoxazolepropionic acid), and the
kainate receptors. They are referred to as ionotropic
because they associate to form cation channels that
facilitate the entry of Ca2+ (or Na+) into the cell.
Glutamate receptor molecules form dimers and then tetramers that function as ion channels.
These ion channels are ligand-gated channels that
assume an “open” configuration upon glutamate (or
agonist) binding. They are locked in a “closed”
configuration when an antagonist is bound. It should
be remembered that when a protein binds a ligand, the
protein generally undergoes a conformational change
that can alter its properties and its activity.
Effect of an antagonist, a partial agonist, and the full agonist, glutamate upon the conformation of the glutamate receptor.
Excessive glutamate binding to glutamate
receptors in the brain can bring about excitotoxic
neuronal cell death, however. Excessively activated
receptors lead to brain damage seen with cerebral
ischemia, traumatic brain injury, and the neuro-
degeneration associated with Huntington’s chorea,
epileptic disorders, and Alzheimer’s disease.
These excitotoxic effects are due, in part, to
signaling downstream of certain glutamate receptors that
leads to in increased intracellular Ca2+. The ionotropic
N-methyl-D-aspartate (NMDA) glutamate receptor is
one of the principal ones that facilitates the entry of
extracellular calcium (Ca2+) into the neuronal cell.
Calcium (Ca2+) that enters the neuronal cell by way of
NMDA channels can, among other actions, up regulate
the synthesis of nitric oxide by the nNOS.
As a result of increasing intracellular Ca2+ NMDA glutamate receptors have broad influence, modulating the activities of a variety of protein kinases and phosphoprotein phosphatases.
Recently, it has been suggested that the NMDA glutamate receptor and a (D1) dopamine receptor cooperate in the molecular mechanisms of compulsion and persistence as related to drug adiction.