We investigated the properties of small cells in the vomeronasal sensory epithelium of adult rats. The sensory neurons in the sensory epithelium were stained by antibodies to G(i2alpha) and G(oalpha) in their cell bodies and dendrites, while the small cells, which formed a cluster in the epithelium, were not stained at all. Voltage-activated inward currents were not detected by patch-clamp recordings, but outward currents were induced by the application of voltage step pulses. These results suggest that the small cells are different from the vomeronasal sensory neurons. Bromodeoxyuridine (BrdU) labeling indicated that dividing cells existed in the cluster of small cells.

We studied Fos-immunoreactive (Fos-ir) structures in the accessory olfactory bulb (AOB) of rats after the vomeronasal organ was exposed to urine. Exposure of the vomeronasal organ of male Wistar rats to oestrous and dioestrous female Wistar urine led to the appearance of many Fos-ir cells in the rostral region of the periglomerular cell (PGC) layer, but induced few Fos-ir cells in the caudal region. These results suggest that the regionalization of Fos-ir cells after exposure to female urine is remarkable in the PGC layer of the AOB. Sexually experienced male rats have been shown to prefer oestrous to dioestrous female urine, while sexually inexperienced males do not exhibit these preferences. In the present study, we compared the expression of Fos-ir cells in the AOB of sexually experienced and sexually inexperienced male rats following exposure to oestrous and dioestrous urine. In the localized region (lateral and rostral sectors) of the PGC layer, many more Fos-ir cells were expressed in the sexually experienced rats than in the inexperienced rats. These results suggest that sexual experience in males enhances the transmission of reproductively salient information concerning potential oestrous status to a specific PGC region of the AOB.

Neurogenesis occurs in the peripheral and central olfactory systems throughout life. The neurogenetic process leading to the formation of primary sensory neurons persists into adult life in the olfactory epithelium of mammals.1) The forebrain subventricular zone (SVZ) generates an immense number of neurons, 2) which migrate via the rostral migratory stream to the main olfactory bulb (MOB). Morphological and electrophysiological studies have shown that these cells differentiate into granule cells and periglomerular cells in the MOB and accessory olfactory bulb (AOB), even during adulthood.1-6) Newly generated neurons play important roles in odor discrimination 7) and odor memory. 8)Sexually experienced male rats show increases in levels of testosterone and prolactin after mating.9) The treatment of testosterone propionate in castrated male meadow voles displays a significant increase in the density of proliferated cells in the amygdala.10) The production of neuronal progenitors is stimulated in the SVZ of female mice during pregnancy via the hormone prolactin, 11) suggesting that forebrain olfactory neurogenesis may contribute to reproductive behaviors such as mating and pregnancy in males and females. Pheromonal signals, which are mainly received by the vomeronasal organ (VNO), provide specific information concerning the reproductive state, identity, and social status of different members of the population in a variety of mammals. [12][13][14] Various gonadal functions of females are controlled by pheromones in many vertebrates. Pheromones have been found in saliva, skin gland secretions, and urine. For example, pheromones in the urine excreted from male and female rats induce various changes in gonadal function and endocrine state, including reflex ovulation in the absence of coitus and mounting, 15) and a reduction in the oestrous cycle of female rats from 5 to 4 d, 16) and oestrous synchrony among female rats housed together.17) The VNO exists in many vertebrates for the purpose of receiving pheromones, which in turn affect sexual and social behavior. 18,19) Vomeronasal sensory neurons project to the AOB located on the dorso-caudal surface of the MOB.20) The AOB is an important site, not only for the discrimination of pheromones but also for memories concerning the reproduction. 21)Sexually experienced male rats have been shown to prefer oestrous to dioestrous female urine, while sexually inexperienced males do not exhibit these preferences. 22,23) In a previous study, we have showed that sexual experience in males enhances the transmission of reproductively salient information concerning potential oestrous status to the localized region (lateral and rostral section) of the glomerular layer (GL) of the AOB. 24) In the vomeronasal epithelium, the sensory neurons are continuously replaced with a few proliferating cells throughout life. 20,25,26) The newly-added cells in the AOB of the adult rat and opossum correspond to the neuronal precursors originating from the rostral migratory stream. 26,27) To explore rol...