Abstract
Excitatory amino acids (EAAs), particularly glutamate, have been implicated in the control of luteinizing hormone (LH) secretion through facilitation of gonadotropin-releasing hormone release. The effects of EAAs are mediated by means of ionotropic glutamate receptors, which are divided into N-methyl-D-aspartate (NMDA) and non-NMDA (kainate and AMPA) subtypes. Moreover, ovarian steroids are responsible for inducing the preovulatory surge of LH and are involved in the actions of EAAs on LH release. Progesterone is directly involved in the potentiating effect of ovarian steroids on the stimulating effect of AMPA neurotransmission on gonadotropin secretion. To broaden our understanding of the role of hypothalamic AMPA receptors in the steroid-induced LH surge, we determined the cellular localization of AMPA receptors in the hypothalamus of guinea pigs by using antibodies that recognize the GluR1, GluR2, GluR2/3, or GluR4 subunits, and then we examined the neuroanatomic relationships between these receptors and the progesterone receptor (PR). Different patterns of immunostaining within the preoptic area and hypothalamus were evident with the antibodies to the four subunits with marked contrasts between moderate staining for GluR1, intensely stained structures for GluR2 and GluR2/3, and little specific staining for GluR4. Immunoreactive (IR) neurons were visualized in many regions, including the two regions known to contain a dense population of estradiol-induced PR-IR cells: the preoptic periventricular and ventrolateral hypothalamic nuclei. Approximately 60% of GluR1-IR and 39% of GluR2-IR cells in the preoptic region possessed PR, whereas 46% of GluR1-IR and 54% of GluR2-IR cells in the ventrolateral nucleus expressed PR. These neuroanatomic results suggest that the coordinated actions of progesterone and glutamatergic inputs on mammalian reproductive functions are integrated at the cellular level.
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