Within-session determination of dose-response curves for heroin self-administration in rats: Comparison with between-session determination and effects of naltrexone

A procedure was employed in the present study to obtain dose-response curves for heroin self-administration within each experimental session. The data generated rising this procedure were compared to dose-response data obtained using between-session dose manipulations. The dose of heroin (18, 30, 60 or 100 mu g/kg/int) was varied across 4-hourly segments separated by a 20-min time-out period during which heroin was not available. The within-session dose-response procedure yielded data similar to those obtained using between-session dose manipulations when the order of dose presentation was increasing or random. However, the dose-response curve for total drug intake was flat when the doses were presented in decreasing order. Further analysis of the dose-response curves in the within-session procedure demonstrated that the rate of heroin intake increased in the third and fourth hourly components compared to the first component, suggesting acute tolerance to the reinforcing and/or rate-suppressive effects of heroin. Furthermore, using a random order of dose presentation, administration of 3.0 mg/kg of naltrexone prior to the session shifted the dose-response curve for heroin self-administration 5-fold to the right in the within-session procedure. The data indicate that the within-session dose-response procedure can be used to investigate the pharmacology of heroin self-administration in rodents.