Central regulation of reproduction in amphibians

This review article includes a literature review of synteny analysis of the amphibian gonadotropin-releasing hormone (GnRH) genes, the distribution of GnRH 1 and GnRH2 neurons in the central nervous system of amphibians, the function and regulation of hypophysiotropic GnRH1, and the function of GnRH1 in amphibian reproductive behaviors. It is generally accepted that GnRH is the key regulator of the hypothalamic-pituitary-gonadal axis. Three independent GnRH genes, GnRH1, GnRH2, and GnRH3, have been identified in vertebrates. Previous genome synteny analyses suggest that there are likely just two genes, gnrh1 and gnrh2, in amphibians. In three groups of amphibians: Anura, Urodela, and Gymnophiona, the distributions of GnRH1 and GnRH2 neurons in the central nervous system have also been previously reported. Moreover, these neuronal networks were determined to be structurally independent in all species examined. The somata of GnRH1 neurons are located in the terminal nerve, medial septum (MS), and preoptic area (POA), and some GnRH1 neurons in the MS and POA project into the median eminence. In contrast, the somata of GnRH2 neurons are located in the midbrain tegmentum. In amphibians, GnRH1 neurons originate from the embryonic olfactory placode, while GnRH2 neurons originate from the midbrain. The characterization and feedback regulation mechanisms of hypophysiotropic GnRH1 neurons in amphibians, the involvement of GnRH1 in amphibian reproductive behavior, and its possible mechanism of action should be elucidated in future.