The bilateral bulbar projections of the primary olfactory neurons in the frog

Whether or not the frog olfactory neuroreceptor cells project bilaterally to the olfactory bulb is still a debated question. We therefore decided to ascertain whether bilateral projections of the primary olfactory input exist and if so to investigate their extent. Reproducible extracellular bilateral bulbar potentials were recorded in the frog following electrical stimulation of dorsal or ventral olfactory nerve bundles. The general features of the contralateral evoked responses were very similar to those of the ipsilateral response. The contralateral response disappeared after transection of the rostral part of the olfactory interbulbar adhesion but not following transection of the habenular or anterior commissures. Horseradish peroxidase labelling showed that the fiber terminations of the olfactory nerve bundle was not restricted to the ipsilateral olfactory bulb but included the medial aspects of the contralateral bulb. The intertelencephalic sections increased the magnitude of the ipsilateral evoked responses. Olfactory bulb isopotential maps revealed a rough topographical correspondence between the olfactory neuroepithelium and bulb along the medio-lateral axis as well as along the dorso-ventral axis. In addition, a projection of the medial and central part of the olfactory sac to the medial part of the contralateral olfactory bulb through the interbulbar adhesion was confirmed. These findings suggest first, that the fibers from the neuro-receptors located in either the ventral or the dorsal olfactory mucosae project to both olfactory bulbs, and second, that the left and right bulbs exert a constant inhibition on each other via the habenular commissure.