Ovarian hormones do not affect the initiation and maintenance of intravenous self-administration of heroin in the female rat

Abstract

The effect of ovarian hormones on the initiation and maintenance of heroin intravenous self-administration was studied in ovariectomized female rats. In Experiment 1 (initiation), the behavior of groups of females (n = 8) ovariectomized (OVX), injected with 10 µg estradiol benzoate (EB) every 3 days (OVX+EB), and intact, was compared with that of intact males (n = 7) exposed to an ascending series of doses of heroin (6.125–50 µg/kg per infusion) on a fixed ratio—FR-1—reinforcement schedule. Twelve 3-h sessions per dose were given, 2 per day in the dark and 2 in the light period of a 12:12-h light:dark cycle. No differences in rate of responding between groups were observed at any of the heroin doses during acquisition or during tests given after acquisition on a descending series of doses. In Experiment 2 (maintenance), 7 OVX female rats were trained to self-administer heroin on an FR-1 reinforcement schedule. They were then switched to a progressive ratio schedule, 4 sessions per day, 4 h each, and a regimen of EB and progesterone (P) injections was initiated during which all animals were tested over a 5-day period (cycle) at each of four doses of heroin (50, 25, 12.5, and 0.0 µg/kg per infusion). Neither EB, given on Day 3, nor P, given on Day 5 of each cycle, affected the highest ratio achieved (breakpoint) or number of heroin infusions taken. These results suggest that the sensitivity of female and male rats to the reinforcing effects of heroin is not different, and that, in females, the reinforcing effects are not affected by circulating ovarian hormones.