Stimulation of medial prefrontal cortex serotonin 2C (5-HT(2C)) receptors attenuates cocaine-seeking behavior

Abstract

Serotonin 2C receptor (5-HT(2C)R) agonists administered systemically attenuate both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior. To further elucidate the function of these receptors in addiction-like processes, this study examined the effects of microinfusing the 5-HT(2C)R agonist MK212 (0, 10, 30, 100 ng/side/0.2 microl) into the medial prefrontal cortex (mPFC) on cocaine self-administration and reinstatement of extinguished cocaine-seeking behavior. Male Sprague-Dawley rats were trained to self-administer cocaine (0.75 mg/kg, i.v.) paired with light and tone cues. Once responding stabilized, rats received MK212 microinfusions before tests for maintenance of cocaine self-administration. Next, extinction training to reduce cocaine-seeking behavior, defined as responses performed without cocaine reinforcement available, occurred until low extinction baselines were achieved. Rats then received MK212 microinfusions before tests for reinstatement of extinguished cocaine-seeking behavior elicited by cocaine-priming injections (10 mg/kg, i.p.) or response-contingent presentations of the cocaine-associated cues; operant responses during cocaine-primed reinstatement tests produced no consequences. MK212 microinfusions into the prelimbic and infralimbic, but not anterior cingulate, regions of the mPFC dose-dependently attenuated both cocaine-primed and cue-elicited reinstatement of extinguished cocaine-seeking behavior, but did not reliably affect cocaine self-administration. A subsequent experiment showed that the effects of MK212 (100 ng/side/0.2 microl) on reinstatement of extinguished cocaine-seeking behavior were blocked by co-administration of the 5-HT(2C)R antagonist SB242084 (200 ng/side/0.2 microl). MK212 administered alone into the mPFC as a drug prime produced no discernable effects on cocaine-seeking behavior. These findings suggest that stimulation of 5-HT(2C)Rs in the mPFC attenuates the incentive motivational effects produced by sampling cocaine or exposure to drug-paired cues.

Figure 1

Timeline depicting the order of experimental stages for rats included in the MK212 and MK212+SB242084 experiments: (a) self-administration training; (b) effects of MK212 on maintenance of cocaine self-administration (0.75 mg/kg/0.1 ml, i.v.); (c) extinction training; (d) effects of MK212 or MK212+SB242084 on cue-elicited reinstatement; (e) effects of MK212 or MK212+SB242084 on cocaine-primed reinstatement; and (f) effects of MK212 priming on reinstatement. The sequence of stages (d), (e), and (f) were counterbalanced for order of presentation to control for order effects. The specific number of sessions at each phase varied depending on individual performance. Across all testing phases, rats received a maximum of eight intracranial microinfusions.

Figure 2

Histological reconstructions (left) presenting injector tip placements within the PrL (black), IL (gray), and Cg2 (hatched) subregions of the mPFC of rats included in the analysis; schematic representations (Paxinos and Watson, 2007) were used with permission from Elsevier. Representative photomicrographs showing methylene blue microinfusions for each mPFC subregion taken at magnifications of × 1 (middle) and × 10 (right).

Figure 3

Effects of MK212 microinfused into the PrL, IL, and Cg2 subregions of the mPFC (10, 30, 100 ng/side) on the mean number of reinforcers (+SEM) obtained on a VR5 schedule of cocaine (0.75 mg/kg, i.v.) reinforcement during a 1-h test session. Baselines (white bars) represent mean infusions during the self-administration sessions preceding each test. Rats (n=6–10/group) were pretreated with vehicle (gray bars) before one test and their assigned dose of MK212 (black bars) before the other test, with order of these treatments counterbalanced. IL microinfusions produced a main effect of day regardless of MK212 dosage group. Post hoc comparisons collapsed across dosage groups indicated that the mean (+SEM) number of infusions after vehicle (12.10±0.44) or drug (12.00±0.72) was lower than that obtained during baseline (13.95±0.65; Newman–Keuls, P<0.05).

Figure 4

Effects of MK212 (10, 30, 100 ng/side) microinfused alone into the PrL, IL, and Cg2 subregions of the mPFC on extinguished cocaine-seeking behavior expressed as the mean number of active lever responses (+SEM) during a 1-h test session. Baselines (white bars) represent mean responses during the extinction sessions preceding each test. Rats (n=6–8/group) were pretreated with vehicle (gray bars) before one test and their assigned dose of MK212 (black bars) before the other test, with order of these treatments counterbalanced; responses produced no scheduled consequences during testing nor did rats receive cocaine on the test day.

Figure 5

Effects of MK212 (10, 30, 100 ng/side) microinfused into the PrL, IL, and Cg2 subregions of the mPFC on cue-elicited reinstatement of extinguished cocaine-seeking behavior expressed as the mean number of active lever responses (+SEM) during a 1-h test session. Baselines (white bars) represent mean responses during the extinction sessions preceding each test. Rats (n=6–8/group) were pretreated with vehicle (gray bars) before one test and their assigned dose of MK212 (black bars) before the other test, with order of these treatments counterbalanced. Cues were available response contingently during the test session on an FR1 schedule. Asterisk (^*) represents a difference from baseline (Newman–Keuls, p<0.05). Plus sign (+) represents a difference from vehicle pretreatment test day (Newman–Keuls, p<0.05).

Figure 6

Effects of MK212 (10, 30, 100 ng/side) microinfused into the PrL, IL, and Cg2 subregions of the mPFC on cocaine-primed (10 mg/kg, i.p.) reinstatement of extinguished cocaine-seeking behavior expressed as the mean number of active lever responses (+SEM) during a 1-h test session. Baselines (white bars) represent mean responses during the extinction sessions preceding each test. Rats (n=6–8/group) were pretreated with vehicle (gray bars) before one test and their assigned dose of MK212 (black bars) before the other test, with order of these treatments counterbalanced. The cocaine prime was administered after the intra-mPFC microinfusions and immediately before testing. No cues were presented during the test sessions. Asterisk (^*) represents a difference from baseline (Newman–Keuls, p<0.05). Plus sign (+) represents a difference from vehicle pretreatment test day (Newman–Keuls, p<0.05).

Figure 7

Effects of SB242084 (200 ng/side) on the MK212 (100 ng/side)-induced decrease in cue-elicited reinstatement of extinguished cocaine-seeking behavior expressed as the mean number of active lever responses (+SEM) during a 1-h test session. Baselines (white bars) represent mean responses during the extinction sessions preceding each test. Rats in the vehicle/SB242084 group (n=8–10/group) were pretreated with vehicle (gray bars) before one test and SB242084 (checkered gray bars) before the other test, whereas rats in the MK212/SB242084 group were pretreated with MK212 (black bars) before one test and MK212+SB242084 (checkered black bars) before the other test, with order of these treatments counterbalanced. Cues were available response contingently on an FR1 schedule. Asterisk (^*) represents a difference from baseline (Newman–Keuls, p<0.05). Pound sign (#) represents a difference from MK212+SB242084 pretreatment test day (Newman–Keuls, p<0.05).

Figure 8

Effects of SB242084 (200 ng/side) on the MK212 (100 ng/side)-induced decrease in cocaine-primed reinstatement of extinguished cocaine-seeking behavior expressed as the mean number of active lever responses (+SEM) during a 1-h test session. Baselines (white bars) represent mean responses during the extinction sessions preceding each test. Rats in the vehicle/SB242084 group (n=8–10/group) were pretreated with vehicle (gray bars) before one test and SB242084 (checkered gray bars) before the other test, whereas rats in the MK212/SB242084 group were pretreated with MK212 (black bars) before one test and MK212+SB242084 (checkered black bars) before the other test, with order of these treatments counterbalanced. For cocaine-primed reinstatement, the cocaine prime (10 mg/kg, i.p.) was administered immediately before testing and no cues were presented during the test sessions. Asterisk (^*) represents a difference from baseline (Newman–Keuls, p<0.05). Pound sign (#) represents a difference from MK212+SB242084 pretreatment test day (Newman–Keuls, p<0.05).