Development of vomeronasal receptor neuron subclasses and establishment of topographic projections to the accessory olfactory bulb.

Previous studies of the adult vomeronasal system have shown that vomeronasal receptor neurons in the middle layer (expressing Gi alpha 2) and deep layers (expressing Go alpha) of the sensory epithelium project to the anterior and posterior parts of the accessory olfactory bulb (AOB), respectively. In the present study, the development of the two populations of vomeronasal receptor neurons and their segregated projections were investigated in the opossum, Monodelphis domestica. Antibodies to G proteins Gi alpha 2 and Go alpha were used to identify the two subpopulations of receptor neurons. The Gi alpha 2-immunoreactive (ir) cells and Go alpha-ir cells appeared between postnatal day 0 (P0) and postnatal day 3 (P3) and both types of cells increased in number during later development. The differential localization of Gi alpha 2-ir cells in the middle layer and Go alpha-ir cells in the deep layer of the VNO could be seen as early as P3 and became more prominent at later stages. The AOB was clearly identified at P10, and at this stage segregated projections of Gi alpha 2-ir fibers to the anterior part and Go alpha-ir fibers to the posterior part of the AOB were seen. The segregation of the two types of fibers in the AOB resemble that in the adult after P21. These results suggest that Gi alpha 2-ir and Go alpha-ir subpopulations of receptor neurons in the VNO develop in parallel, and that segregation of the two populations of receptor neurons in the VNO and the topographic projection to the AOB are established at very early stages during development.