Morphological Evidence for Two Types of Mammalian Vomeronasal System

Abstract

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.

Diverse Systems for Pheromone Perception: Multiple Receptor Families in Two Olfactory Systems

“…A lack of Go-mediated pathways in the VNO has also been observed in various other mammals, such as suncuses (Insectivora), dogs (Carnivora), horses (Perissodactyla), and marmosets (Primata) (Takigami et al, 2004). In these animals, only Gi2-expression was observed in the AOBs.…”

Comparative genomic analysis identifies an evolutionary shift of vomeronasal receptor gene repertoires in the vertebrate transition from water to land

“…The uniform system is found in all other examined mammals that have VNOs (goat, dog, horse, musk shrew, and marmoset). These organisms contain only G␣ i2 -expressing vomeronasal sensory neurons (Takigami et al 2004). Because V1Rs are expressed in G␣ i2 -positive neurons and V2Rs are expressed in G␣ O -positive neurons (Mombaerts 2004), we hypothesize that only a small number of mammalian lineages such as rodents and opossums possess functional V2Rs.…”

“…1) (Wakabayashi et al 2002). A recent study identified two types of vomeronasal systems in mammals (Takigami et al 2004). The segregated system is found in rodents and opossums, which have both G␣ i2 -and G␣ O -expressing vomeronasal sensory neurons that separately project their axons to segregated (anterior and posterior) regions of the accessory olfactory bulb (Takigami et al 2004).…”

Dramatic variation of the vomeronasal pheromone receptor gene repertoire among five orders of placental and marsupial mammals

“…Although rodents have a complex VNO with a thick layer of vomeronasal sensory epithelium (VNSE), where vomeronasal receptors are found, carnivores have a different type of VNO with a much thinner VNSE (14,52). Furthermore, the VNO of the ferret Mustela putorius, another carnivore, is rudimentary in size and development and does not change with respect to season, which is in contrast to the seasonal variation observed in other ferret organs involved in sexual reproduction and behavior and in rodent VNOs (57).…”

“…Among artiodactyls, cow is the only species with a fully sequenced genome. However, a few studies have focused on the VNO of the goat, which belongs to the same family as the cow (17,52), and a genomic Southern analysis suggested that the goat V1R repertoire is also significantly smaller than that in rodents (17). However, even between mouse and rat, there is a nearly 2-fold difference in the number of functional V1R genes, indicating that ordinal generalizations of V1R repertoire sizes from one species should be drawn cautiously.…”