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dc.contributor.advisorWilliams, Gary L.
dc.contributor.advisorSpencer, Thomas E.
dc.creatorAmstalden, Marcel
dc.date.accessioned2004-09-30T02:06:23Z
dc.date.available2004-09-30T02:06:23Z
dc.date.created2003-12
dc.date.issued2004-09-30
dc.identifier.urihttp://hdl.handle.net/1969.1/507
dc.description.abstractThe physiological mechanisms through which nutrition mediates its effects in controlling reproduction are not well characterized. Both neural and endocrine components have been implicated in the communication of nutritional status to the central nervous system. Leptin, a hormone synthesized and secreted mainly by adipocytes, is heavily involved in this communication network. The objectives of studies reported herein were 1) to determine the effects of short-term restriction of nutrients on circulating leptin, leptin gene expression in adipose tissue, and leptin receptor (LR) gene expression in the adenohypophysis of ovariectomized cows; and 2) to investigate the responsiveness of the hypothalamic-adenohypophyseal (AP) axis of fasted and non-fasted cattle to leptin. Studies demonstrated that circulating concentrations of leptin and leptin gene expression in subcutaneous adipose tissue are decreased by fasting. Although 2 to 3 days of fasting did not affect patterns of release of luteinizing hormone (LH), cerebroventricular infusions of leptin increased mean circulating concentrations of LH in fasted, but not normal-fed cows, without affecting frequency or amplitude of pulses of LH. In vitro studies were conducted to determine whether the in vivo effects of leptin could be accounted for at the hypothalamic and/or AP levels. Leptin did not affect the release of gonadotropin-releasing hormone (GnRH) from hypothalamic-infundibular explants from either normal-fed or fasted cattle. Moreover, leptin did not affect the basal release of LH from bovine AP cells or AP explants from normal-fed cows. However, leptin induced a higher basal release of LH from AP explants of fasted cows and increased GnRH-stimulated release of LH from AP explants of normal-fed cows. Results demonstrate that leptin acts directly at the AP level to modulate the secretion of LH, and its effects are dependent upon nutritional status. Cellular mechanisms associated with the increased responsiveness of gonadotropes to leptin in fasted cows were investigated. Expression of LR and suppressor of cytokine signaling-3 (SOCS-3) in the adenohypophysis did not account for the increased responsiveness of fasted cows to leptin. Therefore, although leptin clearly stimulates the hypothalamic-gonadotropic axis in nutrient-restricted cattle, it is unclear why cattle maintained under neutral or positive energy balance are resistant to leptin.en
dc.format.extent2492813 bytesen
dc.format.extent284001 bytesen
dc.format.mediumelectronicen
dc.format.mimetypeapplication/pdf
dc.format.mimetypetext/plain
dc.language.isoen_US
dc.publisherTexas A&M University
dc.subjectBovineen
dc.subjectLeptinen
dc.subjectLHen
dc.subjectGnRHen
dc.subjectNeuroendocrinologyen
dc.subjectAdenohypophysisen
dc.subjectHypothalamusen
dc.titleRole of leptin in regulating the bovine hypothalamic-gonadotropic axisen
dc.typeBooken
dc.typeThesisen
thesis.degree.departmentAnimal Scienceen
thesis.degree.disciplinePhysiology of Reproductionen
thesis.degree.grantorTexas A&M Universityen
thesis.degree.nameDoctor of Philosophyen
thesis.degree.levelDoctoralen
dc.contributor.committeeMemberHarms, Paul G.
dc.contributor.committeeMemberWelsh, Thomas H., Jr.
dc.contributor.committeeMemberMiranda, Rajesh C. G.
dc.type.genreElectronic Dissertationen
dc.type.materialtexten
dc.format.digitalOriginborn digitalen


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