Localization of drug reward mechanisms by intracranial injections

Extinction of cocaine-induced place approach in rats: a validation of the "biased" conditioning procedure

It has often been demonstrated that when a rat is conditioned in a cue-specific environment that has been repeatedly paired with cocaine injections, it will spend more time in that environment than it does in a saline-paired environment. This behavioral procedure is commonly known as the conditioned place preference (CPP)-test. At present, a firm theoretical understanding of the mechanisms underlying the production of a CPP are unknown. It is insufficient merely to know that a CPP can result after repeated drug pairings. Rather, it is necessary that the procedure is validated within a learning theory framework. The objective of the present study was, therefore, to establish that what is observed in place preference studies was, indeed, conditioning. This was accomplished by determining whether a cocaine-induced increase in time spent in a drug-paired environment was subject to attenuation following extinction trials. Rats were tested for their initial bias in spending more time in one of two stimulus-specific chambers of a place-conditioning apparatus. On four occasions, rats were injected with 2.5 mg/kg cocaine and confined to their less-preferred chamber whereas, on four alternating sessions, they were conditioned with saline (vehicle) in their preferred chamber. Subsequent testing in the nondrugged state revealed that these rats displayed a significant increase in the time spent in their initially least-preferred environment compared to baseline measurements. Following establishment of this cocaine-induced CPP, the rats were injected only with saline and conditioned for an equal number of sessions (i.e., four).(ABSTRACT TRUNCATED AT 250 WORDS)