Role of membrane estrogen receptor alpha on the positive feedback of estrogens on Kisspeptin and GnRH neurons.

Abstract

Estrogens act through nuclear and membrane-initiated signaling. Estrogen receptor alpha (ERα) is critical for reproduction, but the relative contribution of its nuclear and membrane signaling to the central regulation of reproduction is unclear. To address this question, two complementary approaches were used: estetrol (E₄) a natural estrogen acting as an agonist of nuclear ERs but as an antagonist of their membrane fraction and the C451A-ERα mouse lacking mERα. E₄ dose-dependently blocks ovulation in female rats, but the central mechanism underlying this effect is unknown. To determine whether E₄ acts centrally to control ovulation, its effect was tested on the positive feedback of estradiol (E₂) on neural circuits underlying LH secretion. In ovariectomized females chronically exposed to a low dose of E₂, estradiol benzoate (EB) alone or combined with progesterone (P) induced an increase in the number of kisspeptin (Kp) and gonadotropin-releasing hormone (GnRH) neurons co-expressing Fos, as a marker of neuronal activation. However, E₄ blocked these effects of EB when provided alone, but not when combined to P. These results indicate that E₄ blocked the central induction of the positive feedback in the absence of P, suggesting an antagonistic effect of E₄ on mERα in the brain as shown in peripheral tissues. In parallel, as opposed to wild-type females, C451A-ERα females did not show the activation of Kp and GnRH neurons in response to EB unless they are treated with P. Together these effects supports a role for membrane-initiated estrogen signaling in the activation of the circuit mediating the LH surge.Significance statement Estrogen receptor alpha (ERα) is critical for the activation of the neural circuits underlying ovulation. However, the relative contribution of its nuclear and membrane signaling to this neuroendocrine phenomenon is unclear. Using two complementary approaches to block membrane ERα signaling the present study reveals that membrane ERα signaling is required for the activation by estrogens of gonadotropin-releasing hormone (GnRH) and kisspeptin (Kp) neurons, two key neuronal populations underlying the surge of luteinizing hormone (LH) which triggers ovulation. Interestingly, the absence of activation of Kp and GnRH neurons is alleviated in both models by progesterone (P). Collectively the results of these two approaches converge to provide evidence that membrane estrogen signaling contributes to this key event for the central regulation of reproduction.