GONADOTROPIN-RELEASING-HORMONE CONTAINING NEURONS AND OLFACTORY FIBERS DURING DEVELOPMENT - FROM LAMPREY TO MAMMALS

Gonadotropin releasing-hormone (GnRH) regulates the hypothalamo-pituit ary-gonadal axis in all vertebrates, The vast majority of GnRH neurons are thought to be derived from progenitor cells in medial olfactory p lacodes, Several antibodies and lectins that recognize cell surface ca rbohydrates have been useful for delineating the migratory pathway fro m the olfactory placodes and vomeronasal organ, through the nasal comp artment, and across the cribriform plate into the brain. In rats, alph a-galactosyl-linked glycoconjugates (immunoreactive with the CC2 monoc lonal antibody) are expressed on fibers along the GnRH migration pathw ay and approximately 10% of the GnRH neuronal population, In lamprey, the alpha-galactosyl binding lectin, Grifonia simplicifolia-1 (GS-1), identifies cells and fibers of the developing olfactory system, In con trast to the CC2 immunoreactive GnRH neurons in rats, the GS-1 does no t label a subpopulation of presumptive GnRH neurons in lamprey, Result s from these and other experiments suggest that GnRH neurons in develo ping lamprey do not originate within the olfactory placode, but rather within proliferative zones of the diencephalon, However, the overlap of olfactory- and GnRH-containing fibers from prolarval stages to meta morphosis, suggest that olfactory stimuli may play a major role in the regulation of GnRH secretion in lamprey throughout life. By contrast, olfactory fibers are directly relevant to the migration of GnRH neuro ns from the olfactory placodes in mammalian species, Primary interacti ons between olfactory fibers and GnRH neurons are likely transient in mammals, and so in later life olfactory modulation of GnRH secretion i s likely to be indirect. (C) 1997 Elsevier Science Inc.