Involvement of Gα _olf-expressing neurons in the vomeronasal system of Bufo japonicus

Most terrestrial vertebrates possess anatomically distinct olfactory organs: the olfactory epithelium (OE) and the vomeronasal organ (VNO). In rodents, olfactory receptors coupled to Gα _olf are expressed in the OE, whereas vomeronasal receptors type 1 (V1R) and vomeronasal receptors type 2 (V2R), coupled to Gα _i2 and Gα _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 Gα _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 DiI labeling, we found that B. japonicus Gα _olf and Gα _o were expressed in the apical and middle-to-basal layer of the vomeronasal neuroepithelium, and that the axons of these Gα _olf- and Gα _o-expressing vomeronasal neurons projected to the rostral and caudal accessory olfactory bulb, respectively. These results strongly suggest that both the Gα _olf- and Gα _o-mediated signal transduction pathways function in the B. japonicus VNO. The expression of Gα _olf in the B. japonicus VNO may correlate with the detection of airborne chemical cues and with a terrestrial life.