MIGRATING LUTEINIZING-HORMONE-RELEASING HORMONE (LHRH) NEURONS AND PROCESSES ARE ASSOCIATED WITH A SUBSTRATE THAT EXPRESSES S100

Luteinizing hormone-releasing hormone (LHRH) containing neurons arise in the region of the medial olfactory placode and migrate into the dev eloping olfactory bulbs and basal forebrain along branches of the term inal and vomeronasal nerves. The neurons ultimately come to reside in olfactory and septo-preoptic areas and project extensively to several brain regions, including the preoptic area and median eminence. The pr esent study examined the expression of a glial-associated guidance mol ecule, S100, as a possible substrate for this migration. Monodelphis d omestica (the Brazilian grey, short-tailed opossum) was studied since this species gives birth to very immature young, allowing access to ea rly periods of mammalian forebrain development. Immunoreactivity for b oth S100 and LHRH-containing neurons and fibers was observed to be clo sely associated along the entire LHRH migratory route from the vomeron asal organ to the septo-preoptic area as early as the day of birth (PO ). By P10, S100-immunoreactivity was also seen in areas containing LHR H-immunoreactive fibers such as the preoptic area and median eminence. We suggest that S100, a protein with neurotrophic properties in vitro , acts as a guidance molecule for migrating LHRH-immunoreactive neuron s and elongating processes.