Low and high locomotor responsiveness to cocaine

predicts intravenous cocaine conditioned place preference in

male Sprague-Dawley rats

Richard M. Allen, Carson V. Everett, Anna M. Nelson, Joshua

M. Gulley, Nancy R. Zahniser

Cocaine

• Psychostimulant

• Induces feelings of euphoria and awareness, and locomotion.

• Binds to dopamine transporters (DAT).

• Administration of cocaine causes dopamine (DA) release in the VTA and the Nucleus Accumbens (NAc).

Lange Pharmacology > Chapter 32. Drugs of Abuse > Basic Pharmacology of Drugs of Abuse > Drugs that Bind to Transporters of Biogenic Amines > Cocaine >

Individuals

• Phenotypic differences

• Individual risk

• Example- alcohol

• Animal models- pre-existing rather than drug-induced

Rat #258 injected with 15mg/kg cocaine

Rat #258 injected with equal volume saline

Cocaine-induced Locomotion

High and Low Responders

• Initially identified as high or low responders to novelty

• High responders (HRs)- increased locomotor activity in a novel environment

• HR/LR locomotor responsiveness has been linked to different rates of learning in the self-administration task

• Locomotor response to novelty does not always predict CPP, LRs show a greater low dose cocaine-induced CPP than HRs

HCR vs. LCR

• In male S-D rats acute cocaine induced locomotion is correlated with increased synaptic strength in the VTA

• Used outbred male S-D rats• Classified the rats as either HCR or LCR

based on the median split of their locomotor activity in the open field during the first 30 min after a cocaine injection (10 mg/kg i.p.)

Methods

• 80 male S-D rats

• 12 hour light dark cycle

• Ad libitum access to food and water

Indwelling IV Catheters

• 40 rats

• Externalized access port which protruded from the skin on the rat’s back

• Implanted under ketamine/xylazine

• Recovery was at least 5 days after surgery

• 10 rats had catheter failures during the experiments (blockages)

• 1 rat died during surgery

CPP

• 4 CPP apparati

• Each housed inside a custom made cabinet

• 15 infrared photobeams spaced 5cm apart were used throughout the apparatus (6 in each outer compartment, 3 in the center) to measure horizontal movement

First Experiment

• Characterize rats as HCR or LCR• Rats were habituated to the apparatus for 1 hour

then removed and injected with saline or 10 mg/kg cocaine, i.p. and returned to the chamber for 30 min

• Used the median split of locomotor activity during the first 30 min after cocaine injection in the first session the classify the rats as either HCR or LCR

• Used the CPP to measure locomotion

1st Experiment

• Consecutive beam breaks were used to measure movement

• Rats were tested in the CPP once a day for 7 days

• 24 hours after day 7, a “reversal test” was preformed, with rats receiving cocaine on days 1-7, then receiving saline and vice versa for control rats

Second Experiment

• Used the CPP to assess cocaine-induced locomotion, sensitization, and conditioned place preference in individuals

• As in the first experiment rats were habituated to the apparatus

Day 1 of Conditioning

• Rats were allowed to habituated to the CPP for 1 hour, the first 15 min was used as the rats baseline preference for the CPP compartments

• After 60 min in the CPP rats were removed and injected with either vehicle (saline) or cocaine i.p. or infusion i.v. (10mg/kg or 1mg/kg) and placed back into the CPP for 30 min and their locomotor activity was measured

• This data was used to classify the rats as HCR or LCR

Days 2-5

• Over the next 4 days the rats were exposed to 2 place conditioning sessions each day (at least 4 hours apart)

• Conditioning sessions consisted of 30 min of the rat being confined to either the white or black compartment after receiving vehicle (ip/iv) or cocaine (10 mg/kg ip,1mg/kg iv)

• Rats were randomly assigned to receive cocaine in either the black or the white compartment and they received saline in the other

Day 6

• Day 6, rats were placed in the middle compartment of the CPP with free access to all 3 compartments of the apparatus for 1 hour

• The first 15 min of this hour was used to measure the rats preference in the CPP

• After 1 hour, rats were removed and given an injection of either vehicle or 10 mg/kg ip cocaine or an infusion of vehicle or cocaine 1 mg/kg and placed back in the CPP for 30 min to measure locomotion

Figure 1

After Seven Sessions

• LCRs show a significant increase in locomotor activity

• Locomotor activity elicited by cocaine in LCRs is now greater than cocaine-elicited locomotion in HCRs

• No significant differences observed between groups with the “reversal test”

Figure 2

Fig. 2 A and C

• A and C show rats injected ip with vehicle or 10 mg/kg cocaine

• Show locomotor activity scores from sessions 1 and 6• Significant differences in locomotor score between

LCRs, HCRs, and control in the 1st session• HCRs had significantly greater locomotor activity than

LCRs and controls• Locomotor scores from LCRs and control didn’t differ• Only LCRs demonstrated sensitization to locomotor

effects of cocaine between sessions 1 and 6

Fig. 2 B and D

• Locomotor activity scores from rats infused with vehicle and cocaine (1 mg/kg)

• Locomotor activity scores for HCR, LCR and controls were significantly different at session 1 and 6

• LCRs showed a significant increase in locomotor effects of cocaine between sessions 1 and 6

Figure 3

Fig. 3

• Shows the locomotor activity in the CPP during the 3 min prior to and 10 min after ip injection (left) or iv infusion (right) of cocaine or vehicle

• Locomotor activity scores for ip injection differ significantly from control 4 min after ip injection of cocaine

• Locomotor activity scores for iv infusion differ significantly from control 1 min after iv infusion of cocaine and peaked by 2 min

Figure 4

Fig. 4

• For rats injected ip with either vehicle or cocaine there were no significant differences in CPP preference between baseline (pre-conditioning) or testing (post-conditioning)

• Only LCRs, not HCRs, show CPP preference after iv infusion cocaine conditioning, not ip cocaine conditioning

Discussion

• Differences in rapid trafficking of DATs to and from the cell membrane may contribute to the variability in acute cocaine-induced locomotion

1) Male S-D rats can be classified as HCR/LCR using a CPP following iv cocaine administration

2) Their initial response to cocaine predicts the development of sensitization to cocaines locomotor effects in the CPP

3) The classification also predicts development of CPP with iv infusions of 1 mg/kg cocaine

Development of CPP

• Cocaine CPP developed only under the iv dosing conditions used

• Authors were unsure of why ip failed to develop cocaine CPP

• 10mg/kg cocaine ip is a threshold dose with higher doses (20mg/kg and 40mg/kg) producing greater degrees of conditioning

• LCRs, rats with low locomotor responsiveness to cocaine initially, represent a phenotype that is more susceptible to development of CPP and cocaine locomotor sensitization