| In five separate experiments, rats were raised from weaning to 50-60 days of age in either an enriched condition (EC), social condition (SC: Experiment 3 only), or isolated condition (IC) environment. EC rats were group caged (14/box) and were exposed to novel environmental stimuli daily. IC rats were singly caged in individual standard laboratory cages. SC rats were housed 3/cage in large standard laboratory cages.;Results showed that EC rats have a lower level of basal dopaminergic functioning in the nucleus accumbens and striatum than IC rats (Experiments 1 and 2). With amphetamine challenge, EC rats showed a greater neurochemical response than IC, when compared to their own control groups. This effect was only found in vivo, not in vitro, and suggests that neural differences alone cannot account for the differential responses to amphetamine.;Behaviorally, IC control rats showed greater levels of locomotor activity than EC rats (Experiment 1, 3, 4, and 5). With acute amphetamine administration, EC rats showed greater increases in locomotion than IC, which was probably related to a greater neurochemical response to amphetamine in EC rats. With repeated amphetamine administration, however, IC rats showed a greater locomotor response from the first to last amphetamine administration than EC.;To assess differences in the reinforcing effect of amphetamine, the conditioned place preference (CPP) paradigm was used. Conditioned animals preferred the amphetamine-associated environment when given a free choice access to both a drug-paired and nonpaired environment. EC rats showed a significant CPP at lower doses than IC (0.5 mg/kg in Experiment 3; 0.1, 0.3, and 1.0 in Experiment 4).;In the final experiment (Experiment 5), an attempt to sensitize rats to the reinforcing and locomotor-stimulating effects of amphetamine through amphetamine pre-exposure failed. It was found though, that EC and IC rats pre-exposed and conditioned with amphetamine extinguished CPP on the second day of testing.;In summary, EC rats are less active, and have lower levels of dopamine activity than IC. With amphetamine challenge, EC rats showed a greater behavioral response than IC when compared to their own control groups, which was probably due to an increased neurochemical response in EC rats. With repeated administration, IC rats showed a greater increase in locomotion than EC, perhaps due to stimulation of the mesolimbic pathway. (Abstract shortened with permission of author.). |
