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

Estrogen (E-2) can modulate a variety of functional processes, including conditioning. However, the precise relationship between E-2 and these processes is not entirely understood. Indeed, the nature of E-2's effects on conditioning may depend upon several factors, including, but not limited to, the task examined, route of E-2 administration, bioavailability of E-2 administered, and/or duration of E-2 exposure. The present studies examined the effects of E-2 on conditioned place preference (CPP), and E-2 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 mu g (n = 12), or 1 mg (n = 10), E-2 immediately prior to placement in the CPP apparatus on conditioning days. Only rats administered 10 mu g E-2 exhibited a CPP. This regimen of E-2 (n = 5/group) also produced moderate levels of E-2 in the nucleus accumbens (significantly greater than vehicle and less than 1 mg E-2). In Experiment 2, ovariectomized rats were SC administered propylene glycol vehicle (n = 11), 10 mu g (n = 13), or 1 mg (n = 14), E-2 immediately prior to conditioning. Administration of 1 mg E-2 in propylene glycol produced a CPP. Notably, 1 mg E-2 in propylene glycol produced moderate levels of E-2 in the nucleus accumbens (significantly greater than vehicle or 10 mu g E-2) that were similar to those produced by 10 mu g E-2 in sesame oil (n = 5/group). Together, these data suggest that regimen of E-2 that can produce a CPP result in moderate levels of E-2 in the nucleus accumbens. (c) 2005 Elsevier B.V. All rights reserved.