Convergence of segregated pheromonal pathways from the accessory olfactory bulb to the cortex in the mouse.

The accessory olfactory system mediates intraspecies pheromonal communication. Two subsets of spatially segregated vomeronasal sensory neurons, presumably handling functionally and structurally different sets of ligand molecules, can be distinguished. The two subsets of sensory neurons project their axons to segregated zones of the accessory olfactory bulb (AOB) and connect with zonally separated mitral/tufted (M/T) cells, suggesting that the accessory olfactory system is divided into two distinct pathways up to the level of the AOB. To examine whether the segregation is maintained at the accessory olfactory cortical (AOC) regions, we selectively tracer-labelled mitral/tufted cells located in the rostral, caudal or in both zones of the adult mouse AOB. The results demonstrate that the axonal projection patterns of rostral zone and caudal zone M/T cells were indistinguishable in the AOC regions. Mitral/tufted cell axons from either zone of the AOB covered the entire area of all four AOC regions: the bed nucleus of the accessory olfactory tract, the medial amygdaloid nucleus, the posteromedial cortical amygdaloid nucleus and the bed nucleus of the stria terminalis. Therefore, over the entire area of each AOC region, ensembles of cortical neurons receive input from both zonal subsets of M/T cells of the AOB. However, the present results do not rule out the possibility that individual cortical neurons sample information from M/T cells of a single zone. These results are consistent with the idea that the segregation of zonal pathways collapses in the AOC regions. Clusters of cortical neurons in each AOC region may combine information from both families of pheromone receptors and thus handle signals from structurally and functionally different categories of pheromone molecules.

OCAM reveals segregated mitral/tufted cell pathways in developing accessory olfactory bulb.

Two functional subsets of vomeronasal sensory neurons project their axons to two segregated zones in the accessory olfactory bulb (AOB). Using immunohistochemical methods with antibodies against the novel cell adhesion molecule OCAM, we provide evidence that the segregation of functional pathways is maintained at the level of mitral/tufted (M/T) cells of the mouse AOB and that this pattern emerges early in ontogeny. During embryonic and postnatal development OCAM was strongly expressed by M/T cells in the caudal zone of the AOB where OCAM-negative vomeronasal axons terminated. In contrast, rostral zone M/T cells innervated by OCAM-positive vomeronasal axons displayed no or faint OCAM immunoreactivity. Differential expression of OCAM in segregated M/T cell pathways suggests that OCAM may be involved in defining compartments of connectivity and setting up functional subdivisions in the developing AOB.