Rationale and objective: It is well established that stress induces reinstatement of drug seeking in an animal model of relapse. Here we studied the role of the medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) in foot-shock stress-induced reinstatement of cocaine seeking.
Methods: Groups of rats were trained to self-administer cocaine (0.5 mg/kg per infusion, i.v., 3 h/day for 9 days) and after ten drug-free days were exposed to extinction and reinstatement test sessions. Each 60 min of extinction was separated by a 30-min time-out period after which the lever and stimulus lights were reintroduced. Rats were given four 1-h extinction sessions on day 1 and then on subsequent days were given two to three 1-h extinction sessions that were followed by a 3-h test for reinstatement. Tests were run every 48 h. In one set of experiments, the effects of inactivation of the prelimbic (PL), infralimbic (IL) or OFC by tetrodotoxin (TTX, 5 ng/0.5 micro l per side) on reinstatement induced by foot shock (5 min, intermittent, 1 mA) or priming injections of cocaine (20 mg/kg, i.p.) were determined. In a second set, the effects of infusions of the D1-like and D2-like dopamine receptor antagonists (SCH 23390 and raclopride) were studied using the same methods.
Results: TTX infusions into the PL cortex blocked both foot shock and cocaine-induced reinstatement. TTX into OFC attenuated foot-shock-induced, but not cocaine-induced reinstatement. Infusions into IL were ineffective. Infusions of SCH 22390 (0.25 micro g/0.5 micro l per side) into either PL or OFC blocked foot-shock-induced reinstatement, but infusions into PL had no effect on cocaine-induced reinstatement. Raclopride (5 micro g/0.5 micro l per side) had no effect on foot-shock-induced reinstatement in either PL or OFC or on cocaine-induced reinstatement when infused into PL. Neither TTX nor SCH23390 infusions into PL or OFC had any effect on lever pressing for sucrose.
Conclusions: These results suggest that the PL and OFC regions form part of the circuitry mediating the effects of foot shock stress on reinstatement of drug seeking and that the PL region may be a common pathway for cue, drug and foot-shock stress-induced reinstatement of drug seeking.