Regionalization of Fos immunostaining in rat accessory olfactory bulb when the vomeronasal organ was exposed to urine

The distribution of Fos‐immunoreactive (Fos‐ir) cells in the accessory olfactory bulb (AOB) of rats following vomeronasal organ exposure to urine was studied. Following exposure to male and female Wistar rat urine, Fos‐ir cells were found in the mitral/tufted cell layer, granule cell layer and periglomerular cell layer of the AOB of female Wistar rat, with the highest number in the granule cell layer. Exposure to water or removal of the vomeronasal organ suppressed the expression of Fos‐ir cells. These results suggest that female Wistar rats specifically detect urinary substances derived from male or female Wistar rats via the vomeronasal organ. Exposure of the vomeronasal organ of female Wistar rats to male Wistar urine induced the appearance of many more Fos‐ir cells in all layers of the AOB than exposure to female Wistar urine. As for the mitral/tufted cell layer, the density of Fos‐ir cells in the rostral portion (Gi2α‐positive) of all regions of the AOB was about twice as high as that in the caudal portion when male urine was given. The distribution pattern of Fos‐ir cells in response to female urine was not identical to that in response to male urine. That is, the density of Fos‐ir cells in the caudal portion was slightly larger than that in the rostral portion in the lateral region, while in other regions the density in the rostral portion was higher than that in the caudal portion. It is likely that information from different pheromones is transmitted to the higher brain regions through the different regions of the AOB.