Sex differences in the acquisition of intravenously self-administered cocaine and heroin in rats.
BACKGROUND
Despite numerous reports that male and female animals differ in behavioral responses to drugs, few studies have investigated sex differences in drug-reinforced behavior.


OBJECTIVE
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.
Lynch
W J
WJ
Department of Psychiatry, University of Minnesota, Minneapolis 55455, USA. lynch020@maroon.tc.umn.edu
Carroll
M E
ME
D000818:Animals; D003042:Cocaine / Q000008:administration & dosage*; D005260:Female; D003932:Heroin / Q000008:administration & dosage*; D007275:Injections, Intravenous; D008297:Male; D051381:Rats; D017208:Rats, Wistar; D012646:Self Administration; D012727:Sex Characteristics; D013997:Time Factors
D003932:Heroin; D003042:Cocaine