Behavioral regulation of gonadotropin-releasing hormone

GnRH controls the hypothalamic-pituitary-gonadal axis in all vertebrate species. Yet despite intense efforts, limited progress has been made toward determining how production of this neuropeptide is regulated. An alternative approach to this problem is to study species whose reproductive status is sensitive to behavioral influences and then to use behavioral interactions to probe the GnRH neuronal system. In this review, data on behaviorally induced alterations in the GnRH system will be presented. In several species of fishes, reproductive status and sex can be altered in adults by changes in the social environment. In these fishes, the numbers and size of GnRH-immunoreactive (GnRH-ir) neurons are modified by an individual's social and sexual status. In the ring dove, courtship activates ovarian follicular development and stimulates androgen production in males. During courtship, mast cells containing GnRH-like immunoreactivity enter the brain. This finding presents a new avenue for examining behavioral endocrine and immune interactions. In the musk shrew, most aspects of female reproduction are activated by contact with a male. Interactions with males trigger changes in the numbers of GnRH-ir cells and rapid production of GnRH peptide. The use of comparative animal models provides us with new insights into neuronal plasticity, neuroendoimmunology, and reproductive biology.