The effects of sex, estrous cycle, and social contact on cocaine and heroin self-administration in rats.

RATIONALE: Preclinical studies indicate that gonadal hormones are important determinants of drug self-administration. To date, little is known about the influence of sex and estrous cycle on drug self-administration in ecologically relevant social contexts.
OBJECTIVE: The objective of this study was to examine the role of sex and estrous cycle in a rat model during cocaine and heroin self-administration with male-female and female-female social dyads.
METHODS: Male and female virgin rats were trained to self-administer cocaine and heroin in operant conditioning chambers that permitted two rats to self-administer concurrently, but prevented physical contact. Experiment 1 examined cocaine self-administration on a progressive ratio schedule in male-female dyads. Experiments 2 and 3 examined heroin self-administration on a fixed ratio schedule in male-female dyads at constant and varying doses, respectively. Experiment 4 examined heroin self-administration in female-female dyads on a fixed ratio schedule.
RESULTS: Cocaine-maintained breakpoints increased by ∼17 % in females during estrus, but remained consistent in males. Heroin self-administration decreased by ∼70 % during proestrus in females whether they were isolated, housed with males, or housed with females. Heroin self-administration was lower in males than females under some conditions and was not consistently associated with the responding of females.
CONCLUSIONS: Cocaine and heroin self-administration is influenced by the estrous cycle in females when in the presence of a male partner. As a novel finding, these data illustrate that heroin self-administration is reduced in females during proestrus regardless of the social context tested. Finally, these data suggest that drug self-administration in males is only minimally influenced by the hormonal status of a female partner.