Abstract
Olfactory cues appear to guide the Japanese toads (Bufo japonicus) to migrate to the spawning site in the breeding season, in which large responses in electoro-olfactogram (EOG) are usually observed. Olfactory receptor neurons (ORNs) in the toads collected throughout a year were labeled by antiserum to olfactory marker protein (OMP), a marker for mature ORNs. The number of ORNs started to increase at a beginning of winter torpid period (December), reached a maximum in the breeding period (mostly February) and gradually decreased during the foraging period (April–October). In the winter torpid period the ORNs with small round-shaped cell body were densely distributed in the basal layer of the olfactory epithelium. In the breeding period the ORNs with large cell body appeared in the intermediate layer, while the ORNs became sparse in the basal layer. These results suggest that the ORNs migrate to the intermediate layer to mature in the breeding period. EOG to odor stimulus (isoamyl acetate) was recorded in the toads of the breeding and foraging periods. The threshold of odor response in the breeding period was about 10 times lower than that in the foraging period. Moreover, in the breeding period maximum odor responses were elevated to the level of about 7 times as large as those in the foraging period. Higher odor sensitivities of the toads in the breeding period may be induced by the maturation of ORNs and concomitant increase in the number of those neurons. [Jpn J Physiol 55 Suppl:S167 (2005)]
