The aim of this proposal is to elucidate the roles of the pattern of the gonadotropin-releasing hormone (GnRH) pulse signal and the ambient ovarian hormonal milieu, in regulating expression of the 3 gonadotropin subunit genes in female rats. We also plan to explore a possible mechanism of steroidal regulation of subunit gene transcription, namely regulation of the duration of transcription. Reproduction in mammals is controlled by the pituitary hormones, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). LH and FSH consist of a common alpha subunit and specific beta subunits, which in turn are coded by 3 genes in the gonadotrope cell. Only a single hypothalamic GnRH is known to regulate subunit gene expression and hormone secretion, though differential regulation of LH and FSH synthesis and secretion is known to occur in normal physiology. Thus differential synthesis and secretion appear to be regulated by different patterns (amplitude and frequency) of GnRH pulsatile secretion, together with the direct feedback effects of gonadal steroids and peptides on the pituitary gonadotrope cell. We propose to validate a GnRH deficient female rat model; assess effects of different patterns of exogenous GnRH pulses on subunit mRNA expression; establish if other neuropeptides are required to rapidly increase LH beta mRNA; and determine if responses are modulated by estradiol (E2), progesterone (P), inhibin, and follistatin; investigate the physiologic mechanisms regulating gene expression during the rat estrous cycle. The second goal is to test our hypothesis that the duration of increased subunit gene transcription in response to pulsatile Gn RH signals is regulated by gonadal steroid hormones. We have preliminary evidence that in response to Gn RH pulses, transcription occurs for a shorter duration in the presence of steroids and we will evaluate the roles of gonadal steroids in this respect. If proven to be true, this would be a novel mechanism of steroid regulation of gonadotropin synthesis and secretion. The critical importance of pulsatile GnRH stimulus to initiate subunit gene transcription and to maintain hormonal secretion is well documented. The pattern of GnRH pulses is known to change during ovulatory cycles and abnormalities of the mechanisms regulating GnRH frequency and amplitude appear to be an important part of syndromes causing anovulation in women. GnRH pulses are slow in hypothalamic amenorrhea (HA) and hyperprolactinemia and occur at fast frequency in polycystic ovarian disease (PCO). Our prior data in rats showed that slow frequency GnRH stimuli favor FSH beta mRNA expression and FSH secretion (as seen in HA and hyperprolactinemia), and fast frequency GnRH pulses favor alpha and LH beta mRNA expression and LH secretion (as occurs in PCO). This provides strong support to our contention that the proposed studies of the mechanisms of GnRH action in regulating gene expression in rats will allow insight into normal physiologic mechanisms, and ultimately improved methods of treating human infertility due to anovulation.
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