Neural Pathway Underlying Pheromone Detection in Fish

Neural substrate for the detection of fish sex pheromones has been controversial during the last five years since Demski and Northcutt(1983) first suggested that the terminal nerve(TN), rather than the olfactory system, mediates responses to sex pheromones.  The evidence supporting this hypothesis is multiple, but indirect.  Thus, the first question I address in this talk is:Is it the terminal nerve or the olfactory system that carries sex pheromone information?  Recently-identified sex pheromones in goldfish, when applied to the olfactory epithelium, evoke clear responses in some olfactory bulb neurons, especially in those located in the medial part of the olfactory bulb.  In contrast, no ganglion cells of the TN respond to any of these sex pheromones or any other odorants we have tested.  Thus, our evidence indicates that sex pheromone information is conveyed by the classical olfactory system, not by the TN.  The above results and those by others show that sexually relevant olfactory information is processed mainly in the medial part of the olfactory bulb, but not in the lateral part.  This raises the second question of the talk: How can the medial and lateral part of the olfactory bulb work independently of each other?  Intracellular and field potential recording experiments demonstrate that the medial and lateral parts of the olfactory bulb possess their own input fibers, inhibitory interneurons, projection routes and projection areas.  Mitral cells in teleosts have limited dendritic extensions (400 um), which confine the dendrodendritic interaction with granule cells to limited areas.  All these structural features may underline the functional differntiation of the olfactory bulb into the two parts.