Primary olfactory terminations in the forebrain of amphibia: a comparative study with soybean agglutinin.

Ambystoma and Xenopus were used to investigate olfactory terminations in the forebrain by exploiting the selective binding of soybean agglutinin to primary olfactory projections. Lectin binding and tridimensional reconstruction provided new information on the distribution of olfactory glomeruli and on regional and interspecific differences. Ambystoma showed an uniform labelling of all olfactory terminations, which were localized in the rostro-lateral forebrain without obvious segregation of different terminal fields, except at dorsal levels. On the contrary, the binding pattern of Xenopus confirmed the existence of three sets of primary olfactory neurons characterized by clear differences in lectin binding. Selective soybean agglutinin binding and a detailed tridimensional reconstruction provided evidence for the existence of a set of neuronal projections, described for the first time, which terminate in a discrete bilateral region of the forebrain localized below the main olfactory bulb. The presence of three different sets of primary olfactory neurons, the experimental potentials and the possible functional significance of SBA binding sites are discussed.