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Significance of Pituitary Leptin to Gonadotropes

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Pituitary leptin changes in a cyclic manner that indicates it may facilitate or permit the secretion of gonadotropins. There are parallel reductions in pituitary leptin and LH during food deprivation and a parallel recovery mediated by glucose. This circumstantial evidence suggests that pituitary leptin may serve as a metabolic gateway to permit gonadotrope function, if the nutritional status is optimal. This proposed pilot study is designed to develop cellular and animal models that test the hypothesis that pituitary leptin from somatotropes is vital for the normal development and responses of gonadotropes to gonadotropin releasing hormone (GnRH). The studies will test the hypothesis with in vitro approaches in Aim 1 in which siRNA for leptin mRNA will be used to knock out pituitary leptin. In Aim 2, the hypothesis will be tested by in vivo studies, in which leptin expression is deleted in somatotropes by Cre-loxP technology. To ablate the leptin gene in somatotropes, mutant mice bearing the transgene rGHp-Cre will be crossed with mice that bear loxP flanking the leptin gene. In both sets of aims, gonadotrope responses to GnRH will be tested to determine if ablation of pituitary leptin in vitro or in vivo (in somatotropes) affects gonadotrope functions. Gonadotrope development and fertility will also be tested in the mutant mice that have leptin excised in somatotropes. This pilot study provides a focused approach to answer the question of the significance of pituitary leptin to gonadotrope functions, which avoids metabolic problems seen with global leptin knockouts that affect gonadotropes and confound interpretation of leptin functions. The pilot study will lay the foundation for the development of future studies that test the effect of pituitary leptin on gonadotrope responses to other regulators (steroids, neuropeptide Y and activin). Future in vivo studies with Cre-loxP technology can be proposed to test the significance of leptin produced by other pituitary cell types (corticotropes and gonadotropes). The ultimate aim of the study will be to determine how leptin supports reproductive competence and how information about malnutrition may be linked with infertility.

PUBLIC HEALTH RELEVANCE: This proposed pilot study is focused on the significance of pituitary leptin to gonadotropes;testing the hypothesis that pituitary leptin is vital for gonadotrope development and/or functions. Aim 1 will develop an in vitro model;the studies will use siRNA for leptin mRNA to ablate pituitary leptin to determine its effects on gonadotrope function. Aim 2 will develop an in vivo transgenic mouse model in which leptin is ablated specifically in somatotropes by Cre-loxP technology. Transgenic mice, in which leptin is knocked out in somatotropes, will be tested for the time of onset of puberty, fertility, and responses by gonadotropes to stimuli. These studies focus on basic regulatory mechanisms that link nutritional status to fertility.

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