Regulation of gonadotropin-releasing hormone and luteinizing hormone secretion by AMPA receptors. Evidence for a physiological role of AMPA receptors in the steroid-induced luteinizing hormone surge.

Recent work has demonstrated that glutamate functions as a major transmitter involved in the regulation of gonadotropin-releasing hormone (GnRH) and luteinizing hormone (LH) secretion in female animals, although the specific receptors and mechanisms mediating its effects have not been completely worked out. The purpose of the present study, therefore, was to examine the role of the AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid)-type glutamate receptor in the control of GnRH and LH secretion in female animals. Toward this end, the dose- and steroid-dependent effects of AMPA on GnRH and LH secretion in female rats were investigated using both in vitro and in vivo approaches, and the role of AMPA receptors in the production of the steroid-induced LH surge was also assessed. The results of the study revealed that central administration of AMPA resulted in a stimulation of LH release in the estrogen-primed ovariectomized adult rat. AMPA was also found to potently stimulate GnRH release in vitro from mediobasal hypothalamic (MBH) fragments obtained from estrogen-primed ovariectomized adult rats, and this effect was blocked by the selective AMPA receptor antagonist, NBQX. The mechanism of action of AMPA appeared to differ from that of N-methyl-D-aspartate (NMDA) as AMPA, in contrast to NMDA, failed to elevate nitric oxide synthase activity in the hypothalamus. The effect of AMPA on LH secretion was demonstrated to be steroid dependent, as central administration of AMPA stimulated LH release in estrogen-primed ovariectomized rats, but was inhibitory to LH release in non-estrogen-primed ovariectomized rats. In contrast, AMPA stimulated GnRH release equally well from MBH fragments obtained from estrogen-primed or non-estrogen-primed ovariectomized rats. The different effects of AMPA on LH release may be due to different pituitary sensitivities between the two models, or alternatively, AMPA may stimulate the release of LH inhibitory factors in the ovariectomized rat in the absence of estrogen. Finally, a physiological role for AMPA receptors in the production of the steroid-induced LH surge was suggested, based on the finding that central administration of the selective AMPA receptor antagonist, NBQX, into the third cerebroventricle significantly attenuated the steroid-induced LH surge in the ovariectomized adult female rat.