DIURNAL PATTERNS OF COCAINE AND HEROIN SELF-ADMINISTRATION IN RHESUS-MONKEYS RESPONDING UNDER A SCHEDULE OF MULTIPLE DAILY SESSIONS

A number of non-pharmacological factors have been shown to influence d rug self-administration in experimental animals. This report examines diurnal changes in drug self-administration by rhesus monkeys trained to self-administer food (1 gm fruit-flavored pellets) and cocaine (0.0 1 or 0.032 mg/kg/injection) under a second order FR4 (VR16:S) schedule during four daily food and drug self-administration sessions. Saline, different unit doses of cocaine (0.001-0.1 mg/kg/injection) or differ ent unit doses of heroin (0.0001-0.01 mg/kg/injection) were substitute d for the maintenance dose of cocaine during drug sessions. Dose-effec t curves relating unit dose of cocaine or heroin to the number of inje ctions per session displayed an inverted U-shape during each of the fo ur daily drug sessions. When 0.032 mg/kg/injection cocaine or 0.0032 m g/kg/injection heroin were available, monkeys usually self-administere d the maximum number of injections during all four drug sessions. Subs titution of saline or lower unit doses of cocaine (0.001-0.01 mg/kg/in jection) or heroin (0.0001-0.001 mg/kg/injection) decreased the number of injections/session; however, these decreases were consistently gre ater during the evening (20.0021.00 h) and morning (07.00-08.00 h) ses sions than during the afternoon sessions (12.00-13.00 h and 16.00-17.0 0 h). As a result, the ascending limbs of the cocaine and heroin dose- effect curves for the evening and morning sessions were shifted to the right of the ascending limbs of the dose-effect curves for the aftern oon sessions. Moreover, when saline was substituted for cocaine for on ly two sessions per day, drug self-administration decreased more durin g the evening and morning sessions even when the cocaine was available during those sessions. These findings suggest a diurnal variation in cocaine and heroin self-administration. Specifically, drug self-admini stration during the evening and morning sessions appears to be more se nsitive to a decrease in reinforcer magnitude than responding during t he afternoon sessions. These findings confirm and extend previous repo rts of the influence of non-pharmacological factors on drug self-admin istration.