Seasonal Increase in Olfactory Receptor Neurons of the Japanese Toad, Bufo japonicus, is Paralleled by an Increase in Olfactory Sensitivity to Isoamyl Acetate

Japanese toads (Bufo japonicus) migrate to and from breeding sites in the early spring, possibly guided by olfactory cues. We previously showed that the electrical activity of olfactory receptor neurons (ORNs) in the toads was enhanced in the breeding period. We undertook morphological and physiological studies of the olfactory epithelium to determine whether any cellular substrate of the epithelium underlies the enhanced electrical activity of ORNs. The ORNs of the toads were labeled by antiserum to olfactory marker protein (OMP), and the morphology of the labeled cells and their distribution in the epithelium were examined throughout the year. The OMP-positive cells, distributed mainly in the basal and intermediate layers of the epithelium, were most numerous in the early breeding period. Cell proliferation in the epithelium detected by 5-bromo-2'-deoxyuridine labeling was most elevated in this period. The electrical activity of ORNs was examined by recording the electroolfactogram (EOG) in the toads throughout the year. Statistical analysis showed a positive correlation between the density of OMP-positive cells in the epithelium and the amplitude of the EOG responses. A greater number of ORNs in the breeding period possibly aids the toads in migrating to their breeding sites.