PRIMARY OLFACTORY FIBERS PROJECT TO THE VENTRAL TELENCEPHALON AND PREOPTIC REGION IN TROUT (SALMO-TRUTTA) - A DEVELOPMENTAL IMMUNOCYTOCHEMICAL STUDY

We studied the development of the primary olfactory system of a teleos t, the brown trout, with the aims of clarifying whether the caudal pro jection pertains to the olfactory or to the terminal nerve system, of identifying the brain regions receiving this projection, and of invest igating its possible functional significance. As olfactory markers (OM s) we used two polyclonal antibodies (to substance P and to a-melanocy te-stimulating hormone) that were found to label the olfactory project ion strongly after preadsortion of the antibody with the corresponding antigen (OMs), and as a terminal nerve marker we used an antiserum to FMRF-amide peptide. OM labelling was observed in both perikarya and a xons of olfactory neurons. In adults, olfactory neurons projected not only to olfactory glomeruli in the olfactory bulb but also, as has bee n reported previously, to more caudal targets in the forebrain through the medial olfactory tract. Our results show that these targets inclu de the ventral and commissural nuclei of the area ventralis telencepha li, the periventricular preoptic region, and the organum vasculosum la minae terminalis. Glomeruli were not observed before hatching, and the extrabulbar olfactory projections appear late in development. Extensi ve periventricular preoptic olfactory plexuses and olfactory innervati on of the organum vasculosum laminae terminalis did not appear until a dulthood. The cells of the ganglion nervus terminalis, which form gang lionic groups along the olfactory nerves, were not stained with these olfactory markers at any developmental stage studied, nor was the medi al olfactory tract FMRF-amide peptide immunoreactive. Our results thus confirm the existence of primary olfactory projections to extrabulbar targets in trout. The target regions identified in this study are imp licated in sexual behaviour: We discuss the related possibility that, in teleosts, these extrabulbar olfactory projections (rather than proj ections of the terminal nerve, as is widely held) are the primary medi ators of neuroendocrine response to pheromones. (C) 1994 Wiley-Liss, I nc.