Modification of synapse formation of accessory olfactory bulb neurons by coculture with vomeronasal neurons

Vomeronasal neurons promote synaptic formation on dendritic spines but not dendritic shafts in primary culture of accessory olfactory bulb neurons

To investigate the morphological changes of accessory olfactory bulb (AOB) neurons arising from pheromonal signals, a coculture system of AOB neurons and vomeronasal (VN) neurons had been established. Our previous study indicates that under coculture condition, the density of dendritic spines of an AOB neuron is less and the individual spine-head volume is larger than those under monoculture condition. In this study, to determine whether these differences in the dendrites of AOB neurons reflect the differences in synapse formation and synaptic properties, we observed these cultured cells by electron microscopy. Various synapses were observed under each culture condition. Synapses were classified on the basis of their postsynaptic structure and the size of postsynaptic density (PSD) was measured. Under the coculture condition with VN neurons, synapses on dendritic spines, which formed between AOB neurons, were observed frequently. In contrast, many synapses were formed on dendritic shafts under monoculture condition. The PSD of asymmetrical synapses on the spines under coculture condition was larger than that under monoculture condition. Moreover, some dendrodendritic reciprocal synapses were found only in coculture. We confirmed synapse formation between VN axons and AOB dendrites by immunohistochemical electron microscopy; thus, the characteristics of synapses between AOB neurons are considered to be modified by the synaptic contacts with VN axons.

Target regulation of V2R expression and functional maturation in vomeronasal sensory neurons in vitro

Vomeronasal receptors from the V1R and V2R gene families mediate the detection of chemical stimuli such as pheromones via the vomeronasal organ (VNO). The differential expression of vomeronasal receptors might contribute in part to a variety of pheromonal effects, which are different sexually, developmentally and even individually. However, little is known about the mechanisms controlling vomeronasal receptor expression. Cultured vomeronasal sensory neurons (VSNs) bear phenotypic resemblance to the intact VNO but they remain immature. Because indices of VSN maturation are increased by coculture with the target cells for VSNs, accessory olfactory bulb (AOB) neurons, AOB neurons may regulate vomeronasal receptor expression and functional maturation in VSNs. To test this hypothesis, we examined the expression of V2R-type vomeronasal receptors (VR1 and VR4) and chemosensory responsiveness in VNOs cocultured with AOB neurons. Immunoblot and immunocytochemical analysis revealed that the coculture of VNOs with AOB neurons resulted in a greater expression of VR1 and VR4 after 10 days than VNOs cultured alone. Moreover, calcium imaging analysis showed that cocultured VNOs responded to urine components applied iontophoretically into their cavities with a time course similar to the V2R expression, in contrast to singly cultured VNOs that displayed no response. These results demonstrate that AOB neurons induce the expression of vomeronasal receptors in VSNs, allowing them to function.