Pharmacokinetics of repeated doses of intravenous cocaine across the menstrual cycle in rhesus monkeys

Numerous studies in rodents suggest that there are sex differences in response to cocaine that are related to fluctuations in the ovarian hormones of females. Given that female rhesus monkeys have menstrual cycles that are remarkably similar to those of humans, they provide an ideal laboratory animal model for assessing the effects of cocaine across the menstrual cycle. The present study assessed the effects of 4 injections of intravenous (i.v.) cocaine (0.00, 0.25 or 0.50 mg/kg), spaced 15 min apart, in 4 female rhesus monkeys. Each monkey was tested with each dose during 4 phases of the menstrual cycle: menses, midfollicular, periovulatory and midluteal. Estradiol and progesterone levels were measured each session before cocaine administration to verify phase of the menstrual cycle. Cocaine and cocaine metabolite levels were measured 5 min after each cocaine dose and 5, 15, 30, 45, 60 and 120 min after the last cocaine dose. Similarly, levels of luteinizing hormone (LH) and prolactin levels were measured before, 5, 15, 30, 45, 60 and 120 min after the last cocaine dose. Cocaine and metabolite levels increased as a function of dose, but there were minimal differences across the menstrual cycle following repeated injections of cocaine. With a few exceptions, LH levels decreased as a function of time within the session, with no differences as a function of cocaine dose. Cocaine produced transient increases in LH levels during the luteal phase, with maximal levels occurring after the second cocaine injection. Lastly, cocaine substantially decreased prolactin levels across all menstrual cycle phases. Taken together, these data indicate that any behavioral differences observed either across the menstrual cycle or between males and females, are probably not related to alterations in the pharmacokinetics of cocaine across the menstrual cycle.