Masato Matsuoka

Specific expression pattern of Fos in the accessory olfactory bulb of male mice after exposure to soiled bedding of females.

The heterogeneous structure of the accessory olfactory bulb (AOB) has been demonstrated immunocytochemically. In this study, we analyzed the expression of an immediate-early gene protein, c-Fos, as a marker of neuronal activity in response to chemosensory cues was analyzed. The number of c-Fos-immunoreactive (Fos-ir) cells was measured in the rostral and caudal zones of the AOB in male ICR mice after exposure to the soiled bedding of female mice. The results revealed no significant difference in the number of Fos-ir cells in the caudal zone of the AOB between exposure to the soiled bedding of female ICR mice (ICR group) and exposure to that of female Balb mice (Balb group). In the rostral zone, however, the number of Fos-ir cells in the glomerular layer and granule cell layer was larger in the ICR group than in the Balb group. The difference in the expression of c-Fos in response to different pheromonal stimuli between the rostral and caudal zones in the mouse AOB has been shown for the first time in this study. These results strongly suggest that the heterogeneous structure of the AOB has an important role in the perception and processing of pheromones.

Expression and regulation of the immediate-early gene product arc in the accessory olfactory bulb after mating in male rat

Recent studies of the accessory olfactory bulb have shown that the expression of immediate-early genes, e.g., c-fos, c-jun and egr-1, can be used as a marker of neuronal activity in response to pheromonal cues. In this study, we analyzed the expression pattern, in response to mating, of the novel immediate-early gene product Arc (an activity-regulated cytoskeleton-associated protein). Arc is hypothesized to play a role in activity-dependent neuronal plasticity in the hippocampus. In a control group of male rats, only a small number of Arc-immunoreactive cells were observed in the accessory olfactory bulb. In a mating group, however, a marked increase in the number of Arc-immunoreactive cells was observed only in the granule cell layer of the accessory olfactory bulb. The increase in the number of Arc-immunoreactive cells after mating was similar to that observed for other immediate-early genes. However, for the mating group, the increase in Arc-positive cells was limited to the granule cell layer. Granule cells have been shown to exhibit a strong synaptic plasticity in response to pheromonal stimulation. From these findings we suggest that Arc plays an important role in neuronal plasticity in the accessory olfactory bulb.

Morphological changes of synapses induced by urinary stimulation in the hamster accessory olfactory bulb.

Urinary pheromones are considered to regulate reproductive functions in various rodent species. The effects of urinary stimuli on synaptic plasticity in the accessory olfactory bulb (AOB), which is the primary nucleus of the vomeronasal system, were studied. Adult male hamsters were divided into four groups and each group was exposed to one of the following four materials: distilled water, female hamster urine, female rat urine, and male hamster urine. After 15 days, the sizes of synapses in the glomerular and the mitral/tufted (MT) cell layers of the AOB were measured. The glomerular synapses, located between the axons of sensory cells and the dendrites of MT cells, were larger in the groups exposed to either the female hamster or the female rat urine compared with those for the distilled water and male hamster urine groups. In the MT cell layer, the synapses are of two types: asymmetrical excitatory synapses and symmetrical inhibitory ones. Exposure of adult male hamsters to female hamster urine induced a reduction in the size of asymmetrical synapses, while on exposure to other kinds of urine there was no synaptic change. The sizes of the symmetrical synapses were not changed by any urinary stimulus. The present study revealed that morphological changes of synapses in the AOB were induced by urinary stimuli. Different urines induced different morphological responses. It is suggested that this synaptic plasticity is responsible for regulation of the output of pheromonal information from the AOB to the higher centers of the vomeronasal system. Synapse 28:160–166, 1998.

Continual neurogenesis of vomeronasal neurons in vitro

We developed a culture system of vomeronasal neurons in which continuous degeneration and regeneration of axon bundles were observed. Partially dissociated vomeronasal cells from rat embryonic day 15 were grown in culture and formed a miniature vomeronasal-like epithelium. We called these structures vomeronasal pockets. They contained both vomeronasal neurons and supporting cells. They formed a spherical structure with a central cavity where microvilli protruded from supporting cells. Mature vomeronasal neurons with well-developed microvilli were not observed in the vomeronasal pocket. The time period between degeneration of axon bundles and the next was about 2 weeks. When vomeronasal pockets were incubated with 5 microgram/mL aphidicolin, an inhibitor of cell division, regeneration of axon bundles was not observed after degeneration. These results suggest that vomeronasal neurons in culture undergo continuous regeneration but do not fully mature. In this culture system, vomeronasal pockets survived for over 1 year.

Social environment affects synaptic structure in the glomerulus of the accessory olfactory bulb of the hamster

The accessory olfactory bulb (AOB) is the primary center of the vomeronasal chemosensory system. The effects of differential rearing on the numerical density and size of synapse as well as neuronal size were examined in the AOB of adult hamster. At 30 days from birth, male littermates were assigned to one of three experimental conditions. (1) the IC (isolated condition), where a male animal was housed alone, (2) the NC (neighbor condition), where one male was separated from two females by wire shields, and (3) the SC (social condition), where two males and two females were housed together. After 2 months of differential rearing, the AOBs of male littermates from each experimental set were prepared for morphological examination. The lengths of synaptic contact zones and the density of synapses in the glomeruli of the AOB as well as the area of somata of mitral/tufted cells were measured with an image analyzer. The synaptic contact zone was longer in the SC compared with both the IC and the NC while there was no difference in the synaptic density among the three groups. The somal area of mitral/tufted cells was larger in both the SC and the NC compared with the IC. These results indicate that the exposure to different rearing conditions induces differential morphological changes in both synapses and somata in the AOB of adult hamster.

Mating behavior induces differential Arc expression in the main and accessory olfactory bulbs of adult rats

The expression of activity-related cytoskeleton-associated protein, Arc, could be useful as a marker for neuronal activity. We investigated Arc-immunoreactivity in both the accessory olfactory bulb (AOB) and the main olfactory bulb (MOB) of adult male rats in response to mating or exposure to female pheromones. Mating behavior strongly enhanced the Arc-immunoreactivity in the granule cell layer of the AOB. However, the enhancement of Arc-immunoreactivity by mating behavior was not observed in the MOB. These results showed that Arc-immunoreactivity was enhanced when the AOB received both afferent and efferent information during mating behavior. Hence, the expression of Arc in the AOB directly associates the pheromonal information with mating behavior. The AOB will provide a useful model to investigate the function of Arc protein.

Effects of differential rearing on the structure of perforated synapses in the granule cell layer of the rat's accessory olfactory bulb

The effects of differential rearing on synaptic morphology in the granule cell layer of the accessory olfactory bulb (AOB) were examined in adult rats. Forty-day-old male rats were housed in one of three ways: individually (isolated condition, IC); with four males per cage (unisexual condition, UC); or with two males and two females per cage (social condition, SC). After 2 months, the animals were killed for electron microscopy. Two types of synapses were classified: (1) perforated synapses, which are characterized by discontinuities in their postsynaptic thickenings, and (2) non-perforated synapses. The length of the synaptic contact zone as well as the area and the length of the perimeter of both the pre- and postsynaptic terminals were measured in each observed synapse. For perforated synapses, the length of the synaptic contact zone was significantly greater in the UC and the SC than in the IC. For the presynaptic terminals of perforated synapses, the area was greater in the UC and the SC than in the IC, and the length of the perimeter was greater in the UC than in the IC. In contrast, there was no difference in the area and the length of the perimeter among the three groups for postsynaptic terminals. For non-perforated synapses, there was no statistically significant difference in either the area or the length of the perimeter of pre- and postsynaptic terminals among the three experimental groups, although the length of the synaptic contact zone was greater in the SC than in the IC.(ABSTRACT TRUNCATED AT 250 WORDS)

Social Environment Affects Synapses in the Accessory Olfactory Bulb of Adult Hamster: Quantitative Electron Microscopic Study

There is increasing evidence that the complexity of the environment to which animals are exposed has an effect on fine synaptic structure, and that the effect varies among different regions of the central nervous system [1,2]. The vomeronasal system, consisting of the vomeronasal organ, the accessory olfactory bulb (AOB), and the higher vomeronasal centers receiving efferents from the AOB, is now thought to play a critical role in the perception and processing of conspecific chemical signals (pheromones) in mammals [3]. We have reported that differential rearing affects the synapses in the granule cell layer of the AOB and the molecular layer of the medial amygdaloid nucleus in adult rats [4,5]. The AOB is the primary nucleus in the vomeronasal system. Since synapses in the glomeruli of the AOB are formed between axons of vomeronasal sensory cells and dendrites of mitral/tufted (M/T) cells [6], they are the first synaptic contact in the vomeronasal pathway. The present study was therefore carried out to examine whether or not different pheromonal environments could affect synaptic structure in the vomeronasal system.