Sex differences in the acquisition of intravenously self-administered cocaine and heroin in rats

Rationale: Despite numerous reports that male and female animals differ in behavioral responses to drugs, few studies have investigated sex differences in drug-reinforced behavior. Objectives: Acquisition of IV cocaine and heroin self-administration was compared in 20 female and 22 male Wistar rats. Methods: An autoshaping procedure was used to train rats to press a lever that resulted in either a 0.2 mg/kg infusion of cocaine or a 0.015 mg/kg infusion of heroin under a fixed-ratio 1 (FR 1) schedule. Daily sessions consisted of six 1-h autoshaping components followed by a 6-h self-administration component. During each autoshaping component, a retractable lever briefly (15 s) extended into the test chamber on a random interval schedule with a mean of either 90 s (cocaine groups) or 480 s (heroin groups) and either ten (cocaine groups) or five (heroin groups) computer-automated infusions were delivered each hour. During each 6-h self-administration component, the lever remained extended and each response on the lever resulted in an infusion of either cocaine (0.2 mg/kg) or heroin (0.015 mg/kg). The criterion for acquisition of cocaine self-administration was a mean of at least 100 infusions and the criterion for heroin self-administration was a mean of at least 20 infusions during the self-administration component over five consecutive sessions. Results: Female rats acquired both cocaine and heroin self-administration more rapidly than males. Acquisition of cocaine self-administration occurred in a greater percentage of female rats compared to males. Female rats self-administered more cocaine than males after acquisition criteria had been met. Conclusions: These findings indicate that female rats were more vulnerable than males to the acquisition of cocaine and heroin self-administration under the conditions of the present experiment.