Blockade of 5-HT2A receptors in the medial prefrontal cortex attenuates reinstatement of cue-elicited cocaine-seeking behavior in rats

Abstract

Rationale: The action of serotonin (5-HT) at the 5-HT(2A) receptor subtype is thought to be involved in cocaine-seeking behavior that is motivated by exposure to drug-associated cues and drug priming. 5-HT(2A) receptors are densely clustered in the ventromedial prefrontal cortex (vmPFC), an area that plays a role in mediating cocaine-seeking behavior.

Objectives: This study examined the hypothesis that M100907, a 5-HT(2A) receptor antagonist, infused directly in the vmPFC attenuates cue- and cocaine-primed reinstatement of cocaine-seeking behavior.

Methods: Rats trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues underwent extinction training during which operant responses produced no consequences. Once behavior extinguished, rats were tested for reinstatement of responding elicited by either response-contingent presentations of the cocaine-paired light/tone cues or by cocaine-priming injections (10 mg/kg, i.p.) within 1 min after pretreatment with microinfusions of M100907 (0.1, 0.3, 1.0, or 1.5 μg/0.2 μl/side) into the vmPFC.

Results: Intra-vmPFC M100907 decreased cue-elicited reinstatement at the two highest doses (1.0 and 1.5 μg) but produced only a slight decrease in cocaine-primed reinstatement that was not dose dependent. The decrease in cue reinstatement was not likely due to impaired ability to respond since intra-vmPFC M100907 infusions had minimal effect on cocaine self-administration and no effect on cue-elicited sucrose-seeking behavior, or spontaneous or cocaine-induced locomotion. M100907 infusions into the adjacent anterior cingulate cortex had no effect on cue reinstatement.

Conclusions: The results suggest that the blockade of 5-HT(2A) receptors in the vmPFC selectively attenuates the incentive motivational effects of cocaine-paired cues.

Fig. 1

Thionin-stained sections taken in the coronal plane demonstrating representative cannula placements in the vmPFC (a) and Cg2 (b)

Fig. 2

Effects of M100907 on cocaine self-administration, expressed as the mean ± SEM number of reinforcers (infusions of cocaine with cues) received over a 1-h test session in each dosage group (a) and collapsed across dosage groups (b). Animals assigned to receive 0.1 (n=6), 0.3 (n= 6), 1.0 (n=6), or 1.5 (n=5) μg/0.2 μl/side M100907 into the vmPFC were tested on 1 day with their assigned dose (striped bar) and on another day with the vehicle (black bar), with order counterbalanced. Baselines (white bar) were calculated as the average number of reinforcers obtained during the first hour of the self-administration sessions immediately preceding each test. There was a small, but significant decrease in responding on the M100907 test day relative to baseline when collapsed across dose (i.e., main effect of test day). The asterisk (*) represents a significant difference from extinction baseline, test of simple main effects, P<0.05

Fig. 3

Effects of M100907 pretreatment on cue-elicited reinstatement of extinguished cocaine-seeking behavior when injected directly into the vmPFC, expressed as mean responses/hour ± SEM on the active lever. Animals assigned to receive 0.1 (n=13), 0.3 (n=12), 1.0 (n= 14), or 1.5 (n=17) μg/0.2 μl/side M100907 into the vmPFC were tested on 1 day with their assigned dose (striped bar) and on another day with the vehicle (black bar), with order counterbalanced. These pretreatments were infused within 1 min before placing the animals into the self-administration chambers, where light and tone cues were available response-contingently on an FR1 schedule. Baselines (white bar) were calculated as the average number of active lever presses during the extinction sessions immediately preceding each test. The asterisk (*) represents a significant difference from extinction baseline, test of simple main effects, P<0.05. The plus sign (+) represents a significant difference from vehicle pretreatment session, planned t tests, P<0.05

Fig. 4

The effects of 1.5μg/0.2μl/side M100907 on cue-elicited reinstatement of extinguished cocaine-seeking behavior when injected directly into the Cg2 region of the anterior cingulate cortex (n=8), which served as an anatomical control site. Animals received 1.5 μg/0.2 μl/side M100907 and were tested on 1 day with their assigned dose (striped bar) and on another day with the vehicle (black bar), with order counterbalanced. These pretreatments were infused within 1 min before placing the animals into the self-administration chambers, where light and tone cues were available response-contingently on an FR1 schedule. Baselines (white bar) were calculated as the average number of active lever presses during the extinction sessions immediately preceding each test. The asterisk (*) represents a significant difference from extinction baseline, test of simple main effects, P<0.05

Fig. 5

Effects of M100907 pretreatment on cocaine-primed reinstatement of extinguished cocaine-seeking behavior, expressed as mean responses/hour ± SEM on the active lever in each dosage group (a) and collapsed across dosage groups (b). Animals assigned to receive 0.1 (n=13), 0.3 (n=13), 1.0 (n=13), or 1.5 (n=14) μg/0.2 μl/side M100907 into the vmPFC were tested on 1 day with their assigned dose (striped bar) and on another day with the vehicle (black bar), with order counterbalanced. These pretreatments were infused immediately before the animals received the cocaine prime (10 mg/kg, i.p.) and were then immediately placed into the self-administration chambers. No cues were presented during the test sessions. Baselines (white bar) were calculated as the average number of active lever presses during the extinction sessions immediately preceding each test. When collapsed across doses (b), there was a significant increase in responding on both the vehicle and M100907 test days relative to extinction baseline and a significant decrease in responding on the M100907 test day relative to the vehicle test day. The star (★) represents a significant difference from extinction baseline, test of simple main effects, P<0.05. The plus sign (+) represents a significant difference from vehicle test day, test of simple main effects, P<0.05

Fig. 6

Effects of M100907 priming injections on reinstatement of extinguished cocaine-seeking behavior, expressed as mean responses/hour ± SEM on the active lever. Animals received 0.1 (n=13), 0.3 (n= 13), 1.0 (n=14), or 1.5 (n=13) μg/0.2 μl/side M100907 infused into the vmPFC on one test day (striped bar) and vehicle on another day (black bar), with order counterbalanced. They were placed into the self-administration chambers immediately after these pretreatments. Baselines (white bar) were calculated as the average number of active lever presses obtained during the extinction sessions immediately preceding each test. No cues were presented during the test sessions

Fig. 7

Effects of M100907 pretreatment on cue reinstatement of sucrose-seeking behavior (n=10), expressed as mean responses/hour ± SEM on the active lever. Animals were tested on 1 day with 1.5 μg/side M100907 (striped bar) and on another day with the vehicle (black bar), with order counterbalanced. These pretreatments were infused within 1 min before placing the animals into the self-administration chambers, where light and tone cues were available response-contingently on an FR1 schedule. Baselines (white bar) were calculated as the average number of active lever presses obtained during the extinction sessions immediately preceding each test. There was a significant increase in responding on both the vehicle and M100907 test days relative to extinction baseline. The star (★) represents a significant difference from extinction baseline, test of simple main effects, P<0.05

Fig. 8

Effects of M100907 pretreatment on locomotor activity, expressed as total movement (m) during a 90-min test session. Animals received either vehicle or 1.0 μg/side M100907 into the vmPFC (n=12/group) and were given an injection 1 min later of cocaine (10 mg/kg, i.p.) for one test and saline (10 mg/kg, i.p.) for the other test, with order counterbalanced. They were then immediately placed into the test chambers. A pound sign (#) indicates a difference from saline test day, ANOVA main effect, P<0.05