An examination of heroin conditioning in preferred and nonpreferred environments and in differentially housed mature and immature rats.

The study addressed two issues. First, we examined the effectiveness of heroin as a conditioning agent in a preferred environment using a place preference paradigm. Four daily injections of 80 micrograms/kg (SC) of heroin HCl were paired with environments that rats initially found to be either preferred or non-preferred. In subsequent tests, only those that had experienced the drug effects in the non-preferred environment increased the percentage of time spent in that environment. Rats conditioned in the test chamber that was initially preferred failed to increase the amount of time spent in that chamber post-conditioning. These results suggest that the conditioned place preference paradigm does not simply assess the rewarding consequence of heroin injections. We also examined the effects of grouped and isolation housing conditions on the heroin-produced conditioned place preference. Rats were housed under these conditions either immediately post weaning or at 120 days of age. There was a difference between the magnitude of the place preference produced by 20 micrograms/kg heroin in the isolated but not in the group housed rats. When isolated at weaning the rats were less sensitive to the drug than were rats isolated at maturity. These data are discussed with particular reference to the development of the endogenous opioid system.