Isolation versus grouped housing in rats: differential effects of low doses of heroin in the place preference paradigm.

Male Long Evans rats were reared from weaning (21-23 days) either in isolation or in groups of four for 40 days. Animals were then individually introduced to a testing apparatus consisting of two distinct chambers. A modified place preference paradigm was used consisting of 3 phases: (1) An habituation phase (4 days) during which rats were allowed free access to the entire test apparatus for 15 min. periods daily; (2) A conditioning phase (4 days) during which rats were confined to their non-preferred side for 15 minutes each day immediately following subcutaneous injection of 0, 20, 40 and 80 micrograms/kg of heroin HCl; (3) A test phase (1 day) during which rats were again allowed free access to the testing chamber following injection of vehicle. The difference in time spent on the conditioned side during habituation and test periods was determined. The group-reared rats showed similar effects for all doses of heroin whereas the same magnitude of drug effect was attained only at the highest dose used in the isolated rats. This differential sensitivity to heroin in the place preference paradigm is discussed in terms of the modification of behavioral effects of opiates by environmental influences.