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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.

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anterior olfactory nucleus


olfactory neuroreceptor axon


bulbar adhesion

DI :

latero-dorsal olfactory nerve bundle


centro-dorsal olfactory nerve bundle


mediodorsal olfactory nerve bundle


external plexiform layer


glomerular layer




granular cell layer


main olfactory bulb


mitral cell


mitral cell body layer


olfactory nerve


lateral ventricule

VI :

latero-ventral ol-factory nerve bundle


centro-ventral olfactory nerve bundle


medio-ventral olfactory nerve bundle


vomero-nasal nerve


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Correspondence to J. Leveteau.

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Leveteau, J., Andriason, I. & Mac Leod, P. The bilateral bulbar projections of the primary olfactory neurons in the frog. Exp Brain Res 89, 93–104 (1992).

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Key words

  • Bilateral bulbar projections
  • Olfactory bulbs
  • Interbulbar connections
  • Horse-radish peroxidase
  • Field potential mapping
  • Frog