The gene encoding GnRH and its associated peptide GAP: some insights into hypogonadism

Hormonal and neurotransmitter regulation of GnRH gene expression and related reproductive behaviors

Gonadotropin-releasing hormone (GnRH), having a highly conserved structure across mammalian species, plays a pivotal role in the control of the neuroendocrine events and the inherent sexual behaviors essential for reproductive function. Recent advances in molecular genetic technology have contributed greatly to the investigation of several aspects of GnRH physiology, particularly steroid hormone and neurotransmitter regulation of GnRH gene expression. Behavioral studies have focused on the actions of GnRH in steroid-sensitive brain regions to understand better its role in the facilitation of mating behavior. To date, however, there are no published reports which directly correlate GnRH gene expression and reproductive behavior. The intent of this article is to review the current understanding of the way in which changes in GnRH gene expression, and modifications of GnRH neuronal activity, may ultimately influence reproductive behavior.

Social regulation of the brain-pituitary-gonadal axis

Reproduction in vertebrates is regulated by the hypothalamic-pituitary-gonadal axis via neural and hormonal feedback. This axis is also subject to exogenous influences, particularly social signals. In the African cichlid fish Haplochromis burtoni, gonadal development in males is socially regulated. A small fraction of the males, which are brightly colored, maintain territories and aggressively dominate inconspicuously colored nonterritorial males. Here we show through manipulation of the social and endocrine environment that changes in social status and gonadal state are accompanied by soma size changes in a population of gonadotropin-releasing hormone-containing neurons in the ventral forebrain. In territorial males, these cells are significantly larger than in nonterritorial males. When an animal switches from being territorial to nonterritorial through a change in social situation, these cells shrink; in animals that change from nonterritorial to territorial status, the cells enlarge. These gonadotropin-releasing hormone-containing cells project to the pituitary and are ultimately responsible for regulating gonadal growth. This mechanism of socially induced cell size change provides the potential for relatively quick adaptive changes in the neuron-endocrine system without nerve cell addition or death. Since the structure of this regulatory axis is conserved among all vertebrates, other species with socially modulated reproductive physiology may exhibit a similar form of physiological regulation.

The Atlantic salmon prepro-gonadotropin releasing hormone gene and mRNA

Screening for the gene encoding salmon gonadotropin releasing hormone (sGnRH) in an Atlantic salmon (Salmo salar) genomic library resulted in isolation of a positive clone designated lambda sGnRH-1. An anchor polymerase chain reaction (PCR) technique was used to amplify GnRH cDNA derived from salmon hypothalamic mRNA. The cDNA sequence was aligned to the 7607 base pair genomic sequence which was shown to encode the entire prepro-GnRH gene. The cDNA proved that the cloned gene is expressed in the hypothalamus of mature salmon. The coding domain of sGnRH differs from the mammalian GnRH by six nucleotide changes which allow the two amino acid differences between the two GnRH variants. Salmon GnRH associated peptide (GAP) differs extensively in sequence and size from the mammalian counterpart. Compared to the GnRH cDNA of a cichlid species the similarity is 69.3% in the protein coding sequence.