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The vomeronasal (VN) systems of rodents and opossums are of the segregated type, i.e alpha-subtype G protein Gi2- or Go-expressing VN neurons, which are sensory cells, project discretely to the rostral or caudal region of the accessory olfactory bulb (AOB). Although this zone-specific projection is believed to be a common feature for processing pheromones in mammals, we previously found a uniform-type VN system in goat in which only Gi2-expressing VN axons terminate at the AOB. In most mammals, it remains unclear whether their VN systems are of the segregated or uniform type. Therefore, we investigated morphologically the VN systems of different mammalian species (dog, horse, musk shrew and common marmoset). Consequently, all VN axons of the examined animals were positively stained with immunohistochemistry for Gi2 in the same way as that in the goat. On the other hand, we observed immunoreactivities against Go in the olfactory axons, but not in the VN axons. These results suggest that many mammals have uniform-type VN systems, and at least two types of VN systems exist in terrestrial mammals. This morphological evidence will help us determine the processing function of VN systems.

Goats have a well-developed vomeronasal (VN) system and exhibit pheromone-induced reproductive facilitation, but there are no reports on the projection pattern of VN neurons in this species. Rodent, guinea pig and opossum accessory olfactory bulbs (AOBs) have been shown to have a segregated pattern of projection of the VN neurons, which express the two alpha-subtypes of the G-protein, namely Gi2 and Go, to the rostral and caudal regions of the AOB, respectively. In this study we investigated the projection pattern of VN nerve terminals by immunocytochemical staining of the goat vomeronasal organ (VNO) and the AOB with antibodies to Gi2 and Go. Gi2-immunoreactivity was found on the luminal surface of the sensory epithelium of the VNO, and in the VN nerve and glomerular layer throughout the AOB. On the other hand, Go-immunoreactivity was not identified in either the VNO or the VN nerve layer of the AOB. These results indicate that the projection pattern of VN neurons from the VNO to the AOB in the goat is considerably different from that in rodents which show a distinct segregated pattern.