The gonadotropin-releasing hormone (GnRH) neurons are the final output neurons of a complex neuronal network that controls fertility in all mammals. The GnRH neurons reside in a scattered continuum throughout the anterior hypothalamus. The majority of GnRH neurons project an axon to the median eminence where GnRH is secreted into the hypophyseal-pituitary portal vessels from whence it travels to the anterior pituitary gland. GnRH acts on the gonadotrophs of the anterior pituitary gland to cause the secretion of luteinising hormone (LH) and follicle stimulating hormone (FSH) into the peripheral circulation. LH and FSH act on the gonads to control gametogenesis and steroidogenesis. This thesis focuses on two unanswered questions in reproductive neurobiology that are fundamental to fertility 1) how the GnRH neurons become activated at puberty to produce patterned GnRH secretion and 2) the nature of the positive feedback mechanism that drives the preovulatory GnRH and LH surges.
Recently, a novel neuropeptide called kisspeptin and its G-protein coupled receptor GPR-54 were found to be essential for pubertal activation of GnRH neurons, with GPR-54 mutation or deletion resulting in failed puberty and infertility in humans and mice. In addition, kisspeptin administration potently stimulates GnRH neuron-mediated gonadotropin secretion and advances the onset of pubertal maturation suggesting an important role for kisspeptin in the activation and perhaps post-pubertal modulation of GnRH neurons.
In this thesis I have used immunocytochemical, whole animal manipulations and knockout mouse approaches to investigate the role of kisspeptin in both the activation of GnRH neurons at puberty and in the estrogen positive feedback mechanism in the mouse.
I have demonstrated that kisspeptin neurons are located principally in the rostral periventricular area of the third ventricle (RP3V) and the arcuate nucleus (ARN), which are both known to be important areas for the modulation of GnRH neuronal activity. Kisspeptin fibres are found in abundance throughout the hypothalamus, but of particular interest are the kisspeptin fibres found in close apposition with a subset of GnRH neurons in the rostral preoptic area (rPOA).
The kisspeptin neurons in the RP3V are sexually dimorphic with up to ten times more neurons in the female than the male. The number of kisspeptin neurons in the RP3V increases throughout pubertal development reaching adult levels at the time of puberty in both males and females. In concert with the increase in the number of kisspeptin neurons in the RP3V there is an increase in the percentage of GnRH neurons in the rPOA which exhibited a close apposition with a kisspeptin fibre indicating that kisspeptin neurons may target GnRH neurons to activate them at puberty. Additionally, I demonstrate that the increase in the number of neurons in the RP3V of the female mouse approaching puberty is driven by estrogen secreted from the ovary.
A significant number of kisspeptin neurons in the RP3V were shown to express tyrosine hydroxylase (TH). The number and percentage of kisspeptin cells colocalised with TH cells in the RP3V did not change throughout the estrous cycle. Some colocalisation of kisspeptin and TH was observed at terminal appositions with GnRH neurons in the rPOA, though the magnitude of colocalisation also did not change throughout the estrous cycle.
I demonstrate that RP3V kisspeptin neurons are a critical part of the estrogen positive feedback mechanism which drives the preovulatory GnRH and LH surges. Kisspeptin neurons in the RP3V express steroid receptors and are activated by estrogen positive feedback. Loss of kisspeptin-GPR-54 signalling prevents the GnRH neurons from being activated by estrogen positive feedback indicating that the RP3V kisspeptin neurons not only contribute to the estrogen positive feedback mechanism, but are a critical component of the mechanism.
The results of these studies demonstrate that kisspeptin is an integral component in both the activation of GnRH neurons at puberty and in the estrogen positive feedback mechanism which drives the preovulatory GnRH and LH surges. Therefore, kisspeptin plays an important role in the neuroendocrine control of reproduction in the mouse.
| Identifer | oai:union.ndltd.org:ADTP/208070 |
| Date | January 2008 |
| Creators | Clarkson, Jenny, n/a |
| Publisher | University of Otago. Department of Physiology |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://policy01.otago.ac.nz/policies/FMPro?-db=policies.fm&-format=viewpolicy.html&-lay=viewpolicy&-sortfield=Title&Type=Academic&-recid=33025&-find), Copyright Jenny Clarkson |
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