Primary olfactory fibres 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 teleost, the brown trout, with the aims of clarifying whether the caudal projection pertains to the olfactory or to the terminal nerve system, of identifying the brain regions receiving this projection, and of investigating its possible functional significance. As olfactory markers (OMs) we used two polyclonal antibodies (to substance P and to alpha-melanocyte-stimulating hormone) that were found to label the olfactory projection 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 axons of olfactory neurons. In adults, olfactory neurons projected not only to olfactory glomeruli in the olfactory bulb but also, as has been reported previously, to more caudal targets in the forebrain through the medial olfactory tract. Our results show that these targets include the ventral and commissural nuclei of the area ventralis telencephali, the periventricular preoptic region, and the organum vasculosum laminae terminalis. Glomeruli were not observed before hatching, and the extrabulbar olfactory projections appear late in development. Extensive periventricular preoptic olfactory plexuses and olfactory innervation of the organum vasculosum laminae terminalis did not appear until adulthood. The cells of the ganglion nervus terminalis, which form ganglionic groups along the olfactory nerves, were not stained with these olfactory markers at any developmental stage studied, nor was the medial olfactory tract FMRP-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 implicated in sexual behaviour: We discuss the related possibility that, in teleosts, these extrabulbar olfactory projections (rather than projections of the terminal nerve, as is widely held) are the primary mediators of neuroendocrine response to pheromones.