| Neurons that synthesize and secrete the decapeptide gonadotropin releasing hormone (GnRH) are the highest order of the hypothalamic pituitary gonadal (HPG) axis, and thus, govern the entire reproductive system. Many internal (e.g., steroid hormones) and external (e.g., chemosensory cues) factors influence GnRH neuronal activity, but mechanisms underlying regulation of these cells remain elusive. GnRH neurons exist as anatomically distinct subpopulations of cells in (rostral to caudal forebrain) tenia tecta, medial septum (MS), diagonal band of Broca/organum vasculosum of the lamina terminalis (DBB/OVLT), and caudal preoptic area (cPOA). The experiments of this dissertation confirmed that a robust developmental event (puberty), an internal hormonal stimulus (testosterone), and an external sensory stimulus (female pheromones) in male Syrian hamsters all affect GnRH neurons in a brain region-dependent manner.; The most rostral cell groups were robustly affected by puberty and testosterone. These same cells groups also had a higher proportion of close appositions from GnRH fibers, suggesting more abundant GnRH-GnRH communication within these cell groups. The most caudal cells (cPOA) may be important for generating the neuroendocrine response to female chemosensory cues, as this is the brain area that has the highest proportion of close appositions from fibers of the medial amygdala, a chemoensory processing nucleus.; In summary, there is heterogeneity within the GnRH system. GnRH cell populations respond to puberty and to internal and external stimuli differently depending on where they reside in the brain. Communication within the GnRH system and between non-GnRH and GnRH neurons also varies with brain region. Future work determining the types of connections within the GnRH system (synaptic or non-synaptic), the phenotype of neuronal or non-neuronal cells projecting to the GnRH system, and changes in receptor expression within these different GnRH subpopulations would advance understanding of how these variables are integrated by GnRH system to impact reproduction. |
