Sensitization to the conditioned rewarding effects of cocaine: pharmacological and temporal characteristics

An unbiased place preference conditioning procedure was used to determine whether the repeated administration of cocaine results in sensitization to its conditioned rewarding effects. Rats received noncontingent injections of saline or cocaine (10 mg/kg i.p.) for 5 days. Place preference conditioning commenced 72 hr later. A minimum of three drug conditioning sessions was necessary for the establishment of cocaine-induced conditioned place preferences (CPP) in saline-pretreated rats. The minimum dose producing this effect was 10.0 mg/kg. In contrast, pre-exposure to cocaine resulted in significant place preferences occurring after only two drug conditioning sessions. Furthermore, CPP was observed in response to doses as low as 5.0 mg/kg. This shift in the cocaine dose-response curve was apparent when conditioning commenced either 3 or 7, but not 14, days after the cessation of cocaine pretreatment. An increased sensitivity to cocaine was also observed in rats which received only two cocaine (25.0 mg/kg) injections before conditioning and in those which had received either d-amphetamine (0.5 mg/kg) or morphine (5.0 mg/kg) for 5 days. Repeated administration of the D1 dopamine (DA) receptor antagonist, SCH-23390 (0.01-0.05 mg/kg), or the D2 antagonist, raclopride (0.1-1.0 mg/kg), for 5 days did not modify cocaine-induced place conditioning. Administration of SCH-23390 (0.05 mg/kg) in combination with cocaine, however, prevented the sensitized response to cocaine. In contrast, raclopride did not influence the sensitized response to cocaine. These data demonstrate that sensitization occurs to the conditioned rewarding effects of cocaine and suggest an involvement of D1 DA receptors in the development of this phenomenon.