Progesterone inhibits behavioral responses and estrogen increases corticosterone levels after acute cocaine administration.

Accumulating evidence suggests that estrogen and progesterone contribute to the sexually dimorphic behavioral response to cocaine. In this study, we tested the hypothesis that varying the level of estrogen or progesterone affects cocaine-induced locomotive behavior in female rats. Ovariectomized (OVX) rats received estrogen (0, 5, 10, 20, or 50 microg) 48 h or progesterone (0, 50, 100, 250, or 500 microg) 24 h before acute saline or cocaine (15 mg/kg) administration. Although estrogen did not affect cocaine-induced ambulatory and rearing behaviors, it affected stereotypic behaviors regardless of cocaine administration (animals receiving 50 microg had higher stereotypic counts than did the OVX group). In contrast, progesterone affected rearing activity dose-dependently: 50 and 500 microg of progesterone inhibited, whereas 100 microg and 250 microg stimulated, rearing in response to cocaine. That estrogen and progesterone did not affect overall baseline behavioral activity suggests their effects are mediated in part through interactions with cocaine. Progesterone administration did not affect corticosterone levels in saline- or cocaine-treated rats. Estrogen administration, however, affected levels of corticosterone both at baseline and after cocaine treatment. After accounting for baseline differences, we found that rats receiving 5 or 10 microg of estrogen and cocaine had higher percentage increases in serum corticosterone levels than did the control group that did not receive estrogen. On the basis of these observations, we suggest that progesterone fluctuations during the estrous cycle impact cocaine-induced behavioral responses, whereas estrogen may affect activity in the hypothalamic-pituitary-adrenal axis. Thus, dose-dependent effects of gonadal hormones may underlie some of the reported sex differences and reproductive cycle effects of cocaine.