論文

Most terrestrial vertebrates possess anatomically distinct olfactory organs: the olfactory epithelium (OE) and the vomeronasal organ (VNO). In rodents, olfactory receptors coupled to G alpha(olf) are expressed in the OE, whereas vomeronasal receptors type 1 (V1R) and vomeronasal receptors type 2 (V2R), coupled to G alpha(i2) and G alpha(o), respectively, are expressed in the VNO. These receptors and G proteins are thought to play important roles in olfactory perception. However, we previously reported that only V2R and G alpha(o) expression is detected in the Xenopus laevis VNO. As X. laevis spends its entire life in water, we considered that expression of limited types of chemosensory machinery in the VNO might be due to adaptation of the VNO to aquatic life. Thus, we analyzed the expression of G proteins in the VNO and the accessory olfactory bulb (AOB) of the adult Japanese toad, Bufo japonicus, because this species is well adapted to a terrestrial life. By using immunohistochemical analysis in combination with in situ hybridization and Dil labeling, we found that B. japonicus G alpha(olf) and G alpha(o) were expressed in the apical and middle-to-basal layer of the vomeronasal neuroepithelium, and that the axons of these G alpha(olf)- and G alpha(o)-expressing vomeronasal neurons projected to the rostral and caudal accessory olfactory bulb, respectively. These results strongly suggest that both the G alpha(olf)- and G alpha(o)-mediated signal transduction pathways function in the B. japonicus VNO. The expression of G alpha(olf) in the B. japonicus VNO may correlate with the detection of airborne chemical cues and with a terrestrial life. J. Comp. Neurol. 519:3189-3201, 2011. (C) 2011 Wiley- Liss, Inc.

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.