Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Neuroscience

Supervisor

Michael N. Lehman

Abstract

The gonadotropin-releasing hormone (GnRH) neurons in the hypothalamus represent the final common output in the central control of reproduction. GnRH secretion is modulated indirectly by steroid feedback action of gonadal steroids on afferent interneurons. In recent years, the neuropeptide, kisspeptin, has emerged as a key mediator of steroid feedback onto GnRH neurons and the reproductive system. Kisspeptin neurons located in the preoptic area (POA) mediate estradiol (E2) positive feedback leading to the preovulatory surge, and kisspeptin cells in the arcuate nucleus (ARC) have been shown to mediate the negative feedback actions of E2 on pulsatile GnRH secretion. ARC kisspeptin neurons are distinguished from those in POA by their extensive reciprocal connections with each other, and their co-expression of two other neuropeptides, neurokinin B (NKB) and dynorphin. Because of this co-expression, the ARC kisspeptin neurons have been termed “KNDy” (Kisspeptin/NKB/Dynorphin) cells. Using sheep and mice as neuroendocrine models, the goal of this dissertation was to investigate the functional organization of KNDy cells and their projections to GnRH neurons, in order to gain insight into their role as mediators of steroid feedback. First, we showed that, contrary to expectations based on rodent work, KNDy neurons in the sheep are activated during both positive and negative feedback of E2 suggesting that they are common mediators of both GnRH pulses and the GnRH surge. Next, using KNDy cell peptides as markers, we showed that KNDy neurons in the sheep send direct neuronal projections to a majority of GnRH cell bodies spread over multiple regions, as well as input to GnRH terminals in the median eminence. We then explored the potential plasticity of synaptic inputs onto KNDy and GnRH neurons across the ovine estrous cycle, and found a significant increase in the total number of inputs to KNDy neurons, as well as KNDy inputs to GnRH cells, at the time of the preovulatory GnRH surge. Performing similar experiments in transgenic mice, we tested the hypothesis that plasticity in these inputs is regulated, in part, by E2. Finally, we began to investigate the possibility that NKB released by KNDy cells might play a functional role in the GnRH surge, by examining internalization of the NKB receptor, NK3R, across the estrous cycle. Taken together, these findings provided novel information on the roles of kisspeptin and KNDy neurons in steroid feedback control of GnRH secretion, and set the stage for future experiments to explore the mechanisms for steroid feedback in individual neurons, the functional role of synaptic plasticity in this system, and the role of KNDy peptides in regulating the reproductive neuroendocrine system across the estrous cycle.

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