Multiple GnRH Molecules, Phylogeny and Evolution
The hypophysiotropic GnRH molecule is found in hypothalamic and preoptic regions of vertebrate brains. This localization enables the GnRH neuroterminals to have access to the portal capillary system leading to the anterior pituitary, or directly to the pituitary gland itself in fish. While it was thought for many years that this was the only location of GnRH cells in the brain, at least two other populations of GnRH neurons have been discovered in the past decade. The hypophysiotropic GnRH neurons found in the preoptic area (POA)-anterior hypothalamus (AH) of most species are referred to here as the GnRH-1 system (see below for nomenclature). A second population, found predominantly in the midbrain is referred to as the GnRH-2 neurons. A terminal nerve/olfactory population of GnRH cells that is distinct from the other populations, henceforth called GnRH-3 cells, was recently identified in several species. In fact, two or even three distinct populations of GnRH cells have been identified in the brain of a single species. While the functions of the GnRH-2 and GnRH-3 cells are not understood, their localization and lack of a hypophysiotropic projection suggest a function distinct from gonadotropin release. It has been speculated that these latter cells play a role in the mediation of sexual behavior and pheromonal cues, although this is yet to be proven.
KeywordsGnRH Neuron GnRH Gene Midbrain Tegmentum GnRH Cell Jawless Fish
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