RELATIONSHIP OF GONADOTROPIN-RELEASING-HORMONE (GNRH) NEURONS TO THE OLFACTORY SYSTEM IN DEVELOPING LAMPREY (PETROMYZON-MARINUS)

Gonadotropin releasing-hormone (GnRH) regulates the hypothalamo-pituit ary-gonadal axis in vertebrates. The regulation of GnRH is intimately related to information from the olfactory system. Additionally, GnRH n eurons are thought to be derived from progenitor cells in medial olfac tory placodes. The present experiments were conducted to characterize the earliest development of GnRH neurons in lamprey and to determine t heir relationship to cells and fibers derived from the olfactory syste m. Eggs from fertile adult sea lamprey were fertilized in the laborato ry, and larvae were maintained for up to 100 days. GnRH neurons were v isualized within the lamprey preoptic area and hypothalamus as soon as GnRH was detectable (22 days after fertilization). The number of neur ons increased with age through day 100. GnRH neurons were never seen w ithin the olfactory system. The cells and fibers of the olfactory syst em were identified using the lectin, Grifonia Simplicifolia-1 (GS-1). Overlap between the olfactory and GnRH systems were at the level of fi ber projections. GS-1 reactive cells of apparent placodal origin did n ot enter the region of the preoptic area or hypothalamus that containe d GnRH neurons. Recently divided cells were labeled with the thymidine analog, bromodeoxyuridine (BrdU). The positions of BrdU-labeled cells after different survival times suggest a predominant medial-lateral r adial neuron migration with a small number in positions suggestive of migration between the olfactory epithelium and the telencephalic lobes . Regardless of survival time, these cells were always found close to their entry point into the brain, suggesting minimal rostral-caudal mi gration. Based on these results, we hypothesize that GnRH neurons in d eveloping lamprey originate within proliferative zones of the dienceph alon and not in the olfactory system. Based on the overlap of olfactor y- and GnRH-containing fibers from prolarval stages to metamorphosis, olfactory stimuli may play a major role in the regulation of GnRH secr etion in lamprey. (C) 1996 Wiley-Liss, Inc.