The influence of environment on the induction of sensitization to the psychomotor activating effects of intravenous cocaine in rats is dose-dependent

The acute psychomotor response and development of sensitization to amphetamine is attenuated if IP injections are given in the cage where a rat lives relative to when injections are given in a novel but physically identical test environment. Furthermore, when the environmental cues predicting IP injections are completely eliminated by using remotely activated IV injections in the home caage, 1.0 mg/kg amphetamine produces a very small acute response and no sensitization. The same treatments do produce sensitization if IV injections are signaled by placement of the rat in a novel test cage. The present experiment was designed to determine if there is a similar effect of environmental condition on the response to IV cocaine, and to what extent the effect may be dose-dependent. This was accomplished by comparing the psychomotor activating effects (rotational behavior) of repeated IV administrations of one of eight doses of cocaine (0.0, 0.3, 0.6, 1.2, 2.4, 3.6, 4.8, or 7.2 mg/kg) given in the home cage, with infusions of the same doses given in a novel test cage. There was no effect of environment on the acute psychomotor response to cocaine. There was, however, a significant effect of environment on the induction of sensitization. A higher dose of cocaine was required to induce sensitization when IV administrations were given in the home cage than when they were given in a physically identical but novel test environment. At high doses, however, cocaine induced sensitization regardless of environmental condition. The results suggest that the effect of this environmental manipulation is to shift the dose-effect curve for the induction of sensitization, and support the notion that the ability of psychostimulant drugs to induce sensitization can be modulated by the circumstances surrounding drug administration.