On the topographic targeting of basal vomeronasal axons through Slit-mediated chemorepulsion

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Abstract
The vomeronasal projection conveys information provided by pheromones and detected by neurones in the vomeronasal organ (VNO) to the accessory olfactory bulb (AOB) and thence to other regions of the brain such as the amygdala. The VNO-AOB projection is topographically organised such that axons from apical and basal parts of the VNO terminate in the anterior and posterior AOB respectively. We provide evidence that the Slit family of axon guidance molecules and their Robo receptors contribute to the topographic targeting of basal vomeronasal axons. Robo receptor expression is confined largely to basal VNO axons, while Slits are differentially expressed in the AOB with a higher concentration in the anterior part, which basal axons do not invade. Immunohistochemistry using a Robo-specific antibody reveals a zone-specific targeting of VNO axons in the AOB well before cell bodies of these neurones in the VNO acquire their final zonal position. In vitro assays show that Slit1-Slit3 chemorepel VNO axons, suggesting that basal axons are guided to the posterior AOB due to chemorepulsive activity of Slits in the anterior AOB. These data in combination with recently obtained other data suggest a model for the topographic targeting in the vomeronasal projection where ephrin-As and neuropilins guide apical VNO axons, while Robo/Slit interactions are important components in the targeting of basal VNO axons.