Administration of estrogen to ovariectomized rats promotes conditioned place preference and produces moderate levels of estrogen in the nucleus accumbens

Estrogen (E2) can modulate a variety of functional processes, including conditioning. However, the precise relationship between E2 and these processes is not entirely understood. Indeed, the nature of E2's effects on conditioning may depend upon several factors, including, but not limited to, the task examined, route of E2 administration, bioavailability of E2 administered, and/or duration of E2 exposure. The present studies examined the effects of E2 on conditioned place preference (CPP), and E2 levels produced in plasma and the nucleus accumbens. In Experiment 1, ovariectomized, Long–Evans rats were subcutaneously (SC) administered sesame oil vehicle (n = 12), 10 μg (n = 12), or 1 mg (n = 10), E2 immediately prior to placement in the CPP apparatus on conditioning days. Only rats administered 10 μg E2 exhibited a CPP. This regimen of E2 (n = 5/group) also produced moderate levels of E2 in the nucleus accumbens (significantly greater than vehicle and less than 1 mg E2). In Experiment 2, ovariectomized rats were SC administered propylene glycol vehicle (n = 11), 10 μg (n = 13), or 1 mg (n = 14), E2 immediately prior to conditioning. Administration of 1 mg E2 in propylene glycol produced a CPP. Notably, 1 mg E2 in propylene glycol produced moderate levels of E2 in the nucleus accumbens (significantly greater than vehicle or 10 μg E2) that were similar to those produced by 10 μg E2 in sesame oil (n = 5/group). Together, these data suggest that regimen of E2 that can produce a CPP result in moderate levels of E2 in the nucleus accumbens.