Estrogen reduces serotonin-1A receptor-mediated oxytocin release and Gα_i/o/z proteins in the hypothalamus of ovariectomized rats

The present study examined the effect of estradiol on hypothalamic serotonin-1A (5-HT_1A) receptor signaling in female rats. We first examined the time-course effects of a single injection of the 5-HT_1A receptor agonist (±)8-OH-DPAT (5, 15 or 30 min prior to decapitation), and dose response of (+)8-OH-DPAT (50, 100, 200 or 500 μg/kg, s.c.) on plasma hormones in ovariectomized rats that received a daily injection of β-estradiol 3-benzoate (10 μg/day, s.c.) or vehicle (sesame oil) for 2 days. In vehicle-and estrogen-treated rats, the peak response of hormones occurred at 15 min after injection and the time-course of oxytocin and adrenocorticotropic hormone (ACTH) responses to an injection of 8-OH-DPAT were comparable. However, only the oxytocin response was reduced by estrogen treatment. A second experiment compared the ACTH and oxytocin responses with doses of 50 or 200 μg/kg, s.c. of (+)8-OH-DPAT vs. (±)8-OH-DPAT in ovariectomized rats that were treated with oil or β-estradiol 3-benzoate (10 μg/day, s.c.) for 2 days. (+)8-OH-DPAT and (±)8-OH-DPAT produced a similar magnitude of increase in plasma levels of ACTH and oxytocin. Treatment with β-estradiol 3-benzoate produced a significant and comparable reduction in the oxytocin response to the highest dose (200 μg/kg, s.c.) of both (+)8-OH-DPAT and (±)8-OH-DPAT but did not alter the ACTH response to either (+)8-OH-DPAT or (±)8-OH-DPAT. In the dose-response experiment, a dose of 50 μg/kg of (+)8-OH-DPAT produced a maximal increase in plasma levels of ACTH, while the maximal oxytocin response was achieved with a dose of 200 βg/kg, s.c. Treatment with β-estradiol 3-benzoate reduced the maximal oxytocin response to (+)8-OH-DPAT (by 29%) but did not alter the ACTH response to any doses of (+)8-OH-DPAT. To examine potential mechanisms mediating the effects of estrogen on 5-HT_1A receptor signaling, we measured the levels of Gα_i, Gα_o and Gα_z proteins, which couple 5-HT_1A receptors to their effector enzymes, in two subregions of the hypothalamus. The levels of Gα_z protein were reduced in the mediobasal hypothalamus (containing the ventromedial and arcuate nuclei), which mainly expresses estrogen receptor-α, but not in the paraventricular hypothalamus, which mainly expresses estrogen receptor-β. Estradiol reduced the levels of Gα_i2 and Gα_i3 proteins in both hypothalamic regions but did not affect Gα_i1 levels in either area. Combined, the data suggest that racemic and stereoselective 8-OH-DPAT have similar neuroendocrine effects and that both estrogen receptor-α and estrogen receptor-β mediate the reduction in levels of Gα_i2,3 proteins.