The arcuate nucleus mediates facilitating effect of estrogen on glutamate-induced in vitro GnRH release from nerve terminals of female rats

Abstract

It is well accepted that glutamate stimulates in vivo and in vitro gonadotropin-releasing hormone (GnRH) and/or luteinizing hormone (LH) release. The present study aimed to determine whether the arcuate nucleus (ARC) has a role in exerting the modulating effect of estrogen on GnRH response to glutamate. The effects of estrogen on glutamate-induced in vitro GnRH release from either the ARC-median eminence (ME) or ME fragment were examined in female rats. Both high and low doses of estradiol-17 β treatment in ovariectomized OVX animals enhanced glutamate-induced GnRH release from the ARC-ME fragment but not from the ME fragment. This suggests that the ARC mediates the facilitating effect of estrogen on glutamate-induced GnRH release from the ARC-ME. The facilitating effect of estrogen was also observed on GnRH release from the ARC-ME treated with selective excitatory amino acid agonists, i.e., N-methyl-D-aspartate (NMDA), kainate, and α-amino-3-hydroxy-5-methyl-4- isoxazole propionic acid. These results suggest that the ARC plays an important role in facilitating estrogen actions on GnRH response to glutamate via both NMDA and non-NMDA receptors. The estrogen-induced enhancement of GnRH secretory response to glutamate in the ARC-ME region could partly contribute to the induction of preovulatory GnRH/LH surge.