Date of Graduation

2018

Document Type

Dissertation

Degree Type

PhD

College

School of Medicine

Department

Neurology

Committee Chair

Stanley Hileman

Committee Co-Chair

Robert Dailey

Committee Member

Robert Goodman

Committee Member

Steven Hardy

Committee Member

James Simpkins

Abstract

Gonadotropin-releasing hormone (GnRH) secretion from the hypothalamus is an important regulator of ovarian function and reproductive cyclicity. During the reproductive cycle, the pattern of GnRH secretion from the hypothalamus, and subsequently luteinizing hormone (LH) secretion from the anterior pituitary, is modulated by ovarian hormones. However, the effects of estradiol (E2) and progesterone cannot be directly communicated to GnRH neurons because these neurons do not express progesterone receptors or estrogen receptor alpha (ERalpha), the estrogen receptor that influences estrogenic regulation of GnRH secretion. Thus, neuronal intermediates that do express these receptors must exist to communicate steroidal feedback to GnRH neurons. Kisspeptin, neurokinin B (NKB), dynorphin, and neuronal nitric oxide synthase (nNOS) are all neuronal intermediates that may participate in the communication of steroidal feedback.;Increased GnRH secretion is critical for puberty onset. Neurokinin B and dynorphin may play a role in regulating GnRH secretion during pubertal development, as senktide, an NKB receptor (NK3R) agonist, and nor-BNI, a kappa-opioid receptor (KOR) antagonist, stimulate LH secretion in prepubertal ewes. However, where these effects occurred within the hypothalamus was unknown. Thus, we examined whether senktide or nor-BNI placement in the arcuate nucleus (ARC) or preoptic area (POA) would increase LH secretion in prepubertal ewes. Placement of senktide-containing microimplants into either the ARC or POA significantly increased mean LH concentrations. In contrast, nor-BNI-containing microimplants had no effect on LH secretion when placed in either the ARC or POA. Additionally, the influence of E2 or age on NK3R expression in several hypothalamic areas was also investigated. Estradiol did not significantly alter NK3R cell numbers in any of the areas examined. However, there was a significant increase in the number of NK3R neurons in the POA of adult ewes compared to prepubertal ewes. Further work will be necessary to determine how and where dynorphin is acting in prepubertal ewes to inhibit LH secretion. However, changes in the NKB/NK3R system, particularly within the POA, may play a role in the pubertal increase in LH secretion in ewes.;Nitric oxide (NO), a gaseous neurotransmitter synthesized by nitric oxide synthase (NOS), is another neuronal intermediate that may play an integral role in GnRH secretion and puberty onset. Therefore, we sought to neuroanatomically characterize nNOS expression in prepubertal ewes and determine if E2 would exert effects on this system. Neurons immunoreactive for nNOS were identified in several areas in prepubertal ewes, including the POA, ARC, and ventrolateral portion of the ventromedial hypothalamus. Moreover, nNOS neurons colocalized with ERalpha, kisspeptin, and GnRH. However, neither the number of nNOS neurons in the POA or hypothalamus, nor the percentage of nNOS coexpression with ERalpha, kisspeptin, or GnRH were influenced by E2. These experiments reveal that nNOS is abundantly expressed in the prepubertal sheep hypothalamus, a portion of nNOS neurons contain ERalpha, and a neuroanatomical relationship exists between both nNOS and kisspeptin and nNOS and GnRH. Therefore, nNOS may act both directly and indirectly to influence GnRH secretion in prepubertal sheep.;Because our results in prepubertal sheep were in contrast to what has previously been reported in adult rodents, we also determined if nNOS colocalized with kisspeptin and GnRH in adult female sheep and primates. Similar to prepubertal ewes, kisspeptin neurons in adult ewes were highly colocalized with nNOS. In contrast, no colocalization was observed between kisspeptin and nNOS in adult female primates. Gonadotropin-releasing hormone neurons also colocalized with nNOS in adult ewes and primates, albeit to a much lesser extent than what was observed in prepubertal ewes. Thus, although there are species differences, a neuroanatomical relationship exists between nNOS and kisspeptin and nNOS and GnRH in adult female primates and sheep.

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