CONTROLLING INTERINDIVIDUAL DIFFERENCES IN THE UNCONDITIONED RESPONSETO AMPHETAMINE IN THE STUDY OF ENVIRONMENT-DEPENDENT SENSITIZATION

Two subgroups of rats selected on the basis of their emergence latency in a light-dark box test were shown to exhibit significantly differen t unconditioned responses to d-amphetamine (AMPH, 1 mg/kg). The rats p resenting a low latency to emerge from the dark side (LL subgroup) res ponded more to AMPH than the rats presenting a high latency (HL subgro up). These two subgroups were compared for environment-dependent and e nvironment-independent sensitization. The major findings were as follo ws: (a) when these two subgroups underwent a conditioning procedure to study environment-dependent sensitization, in which the paired groups received AMPH in Environment A (activity cages) and saline in Environ ment B (plastic housing cages), the unpaired groups received saline in A and AMPH in B, and the control groups received saline in both envir onments, only the LL subgroup showed conditioned activity and environm ent-dependent sensitization; (b) when LL and HL subgroups were submitt ed to a sensitization procedure designed to rule out any conditioning processes (environment-independent sensitization), there was no signif icant difference in the development and magnitude of sensitization alt hough the amplitude of the response following each injection remained lower in the HL compared with the LL subgroup; (c) when unconditioned responses to AMPH for the two subgroups were equated by increasing the dose of AMPH for the HL rats (1.25 mg/kg), there was no longer a sign ificant difference between the two subgroups with respect to condition ed activity and environment-dependent sensitization; (d) in the LL sub group, an extinction procedure (in which all animals received vehicle in both environments) that completely abolished the conditioned activi ty in the paired group, suppressed the difference between paired and u npaired groups during the test for environment-dependent sensitization , by reducing the response of the former. Overall, these results provi de two major contributions: first, they show that interindividual diff erences in the unconditioned response to AMPH influence the outcome of the study of environment-dependent sensitization; second, when these differences are controlled, they suggest that environment-dependent se nsitization appears to be the result of the addition between condition ed activity and environment-independent effects of AMPH.