The present study investigated the effects of long-term estradiol withdrawal (ovariectomy) on hypothalamic serotonin-1A (5-HT(1A)) receptor signaling. Changes in neuroendocrine responses to the 5-HT(1A) agonist 8-OH-DPAT and levels of G(z) protein in the hypothalamus were used to examine 5-HT(1A) receptor signaling. Five days following ovariectomy, rats received daily injections of either 2 microg of beta-estradiol 3-benzoate or vehicle (subcutaneously) for 2, 4 or 14 days. Twenty-four hours after the last injection, and 15 min prior to sacrifice, rats were injected with (+/-)8-OH-DPAT (50 micro;g/kg, s.c.) or saline. Estradiol treatment did not alter basal corticotropin (ACTH) or oxytocin levels. Injection of (+/-)8-OH-DPAT produced significant increases in plasma ACTH and oxytocin levels. In the vehicle-treated rats, hormone responses to 8-OH-DPAT were enhanced in rats that received injections for 14 days compared with rats that received injections for either 2 or 4 days. Estradiol treatment for 4 or 14 days blunted this enhanced ACTH response to 8-OH-DPAT, whereas the oxytocin response to 8-OH-DPAT was only blunted after 14 daily injections of beta-estradiol 3-benzoate. The treatment with beta-estradiol 3-benzoate (2 microg/rat) did not reduce membrane-associated G(z) protein levels in the paraventricular nucleus of the hypothalamus. Hence, the inhibitory influence of a low dose of beta-estradiol 3-benzoate on 5-HT(1A) receptor signaling in the hypothalamus is not accompanied by a change in the levels of G(z) protein in the paraventricular hypothalamic nucleus. Results from the present study indicate a supersensitivity of 5-HT(1A) receptors after withdrawal of estradiol and suggest that estradiol suppresses 5-HT(1A) receptor signaling.
Copyright 2002 S. Karger AG, Basel