Studies of the GnRH pulse generator. Developmental and location-specific regulation of GnRH secretion

The hypothalamo-pituitary-gonadal (HPG) axis governs reproductive function. Gonadotropin-releasing hormone (GnRH) neurons play a critical role in the central regulation of the HPG axis through the episodic release of the GnRH peptide to the hypophyseal blood. In addition to its neuroendocrine function, GnRH has been proposed to exert neuromodulatory role within the brain. GnRH neuron cell bodies are spread throughout the medial and basal hypothalamus and are found primarily in the preoptic area. They project their axons to the median eminence, where GnRH is secreted in a highly coordinated manner to regulate pituitary response. Interestingly, GnRH neuron physiological function appears to depend on their anatomical location. Therefore, GnRH secretion at different sites may be regulated by diverse mechanisms.
In this dissertation I provide new insights into the physiology of the GnRH neuronal system. These discoveries were enabled by a novel methodological approach for the direct detection of the GnRH release in the brain tissue. Previously the lack of such methods hindered the field from addressing important questions about mechanisms regulating GnRH secretion. Specifically, I adopt fast-scan cyclic voltammetry (FSCV) to measure both evoked and spontaneous GnRH release. I demonstrate via a series of control experiments that FSCV is a specific, sensitive tool for the monitoring of GnRH release with an excellent spatial and temporal resolution.
I applied the newly developed method to investigate mechanisms governing GnRH secretion in the preoptic area and the median eminence. These data provide evidence for significant regional differences in the role of action potential firing as well as modulation of GnRH release by kisspeptin and gonadotropin inhibitory hormone (GnIH). GnRH secretion in the median eminence requires firing activity in the neighboring network that promotes kisspeptin release. Kisspeptin can act directly on the GnRH neuron terminals to increase intracellular Ca2+ concentration and trigger GnRH release. This is achieved via mechanisms involving mobilization of intracellular Ca2+ stores and Ca2+ influx through the cadmium-sensitive calcium channels. On the other hand, GnIH can prevent kisspeptin-induced GnRH release form the median eminence terminals. It acts most likely through the upstream network that is distal to the sites of kisspeptin action. In contrast to the median eminence GnRH release, in the preoptic area action potentials are not required for GnRH secretion and kisspeptin and GnIH targets tend to occur in the proximity to the GnRH release sites.
Next, the changes in the pattern of GnRH secretion from the median eminence and the development of pituitary response to GnRH during sexual maturation were investigated. Findings of these studies demonstrate that GnRH is released with an unexpectedly high frequency during perinatal life in male mice. Elevated frequency of GnRH release persists in one-week-old animals and appears to be due to the insufficient endogenous inhibitory signaling, but not an active excitatory drive. By two weeks of age GnRH release is almost completely abolished. This switch coincides with the pituitary gaining responsiveness to the GnRH signal. During the early life pituitary appears to be insensitive to the high activity of the GnRH system; a possible mechanism to prevent premature activation of the HPG axis.