Training dose and session time as contextual determinants of heroin self-administration in rats

In this study a rat self-administration model was used to examine the effects of training dose and time in the session on the dose-effect curve for heroin. Doses of heroin lower than 5.4 mu g/inf maintained higher rates of drug intake in animals trained with 5.4 mu g/inf compared to 18 mu g/inf. Doses greater than 5.4 mu g/inf maintained similar rates of intake in both groups of animals. The dose-response curve was shifted downward and to the right as the session progressed for animals trained with 5.4 mu g/inf of heroin; however, the shift in the dose-intake curve over the session was less pronounced when the training dose was 18 mu g/inf. Naltrexone and naltrindole were administered to animals in which responding was engendered with infusions of 5.4 mu g of heroin to determine the effects of these antagonists in the context of time is the session. The potency of naltrexone decreased across the 4 h of the session with a time course that was consistent with literature reports on the elimination kinetics of naltrexone in rat brain. In contrast, there was not a significant interaction between naltrindole dose and session time. Therefore, the rates of heroin intake in rats are dependent not only upon the dose available for self-administration, but upon the session time and training dose as well. (C) 1998 Elsevier Science Inc.