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

Fatty acid-binding protein 5 differentially impacts dopamine signaling independent of sex and environment

Epidermal/brain fatty acid-binding protein 5 (FABP5) plays an integral role in the intracellular trafficking of bioactive lipids/endocannabinoids and the subsequent initiation of cellular cascades affecting cannabinoid and dopamine (DA) systems. Social isolation (SI) and environmental enrichment (EE) during adolescence have been shown to impact DA signaling, and, specifically, DA transporter (DAT) and receptor levels of DA type 1 (D1) and 2 (D2); however, the relationship between FABP5, environment and DA signaling remains unclear. The present study quantified DAT and DA receptor levels in male/female FABP5-/- and FABP5+/+ mice raised in either SI or EE. Results showed that FABP5-/- mice had 6.09-8.81% greater D1 levels in striatal sub-regions of the caudal brain, independent of sex or environment. D1 levels were 8.03% greater only in the olfactory tubercle of enrichment-reared animals. In summary, these results supported that FABP5 plays an important function in regulating striatal DA signaling, and this may have important implications as a target with therapeutic potential for various psychiatric disorders.

Can social behaviour drive accessory olfactory bulb asymmetries? Sister species of caviomorph rodents as a case in point

In mammals, the accessory olfactory or vomeronasal system exhibits a wide variety of anatomical arrangements. In caviomorph rodents, the accessory olfactory bulb (AOB) exhibits a dichotomic conformation, in which two subdomains, the anterior (aAOB) and the posterior (pAOB), can be readily distinguished. Interestingly, different species of this group exhibit bias of different sign between the AOB subdomains (aAOB larger than pAOB or vice versa). Such species-specific biases have been related with contrasting differences in the habitat of the different species (e.g. arid vs. humid environments). Aiming to deepen these observations, we performed a morphometric comparison of the AOB subdomains between two sister species of octodontid rodents, Octodon lunatus and Octodon degus. These species are interesting for comparative purposes, as they inhabit similar landscapes but exhibit contrasting social habits. Previous reports have shown that O. degus, a highly social species, exhibits a greatly asymmetric AOB, in which the aAOB has twice the size of the pAOB and features more and larger glomeruli in its glomerular layer (GL). We found that the same as in O. degus, the far less social O. lunatus also exhibits a bias, albeit less pronounced, to a larger aAOB. In both species, this bias was also evident for the mitral/tufted cells number. But unlike in O. degus, in O. lunatus this bias was not present at the GL. In comparison with O. degus, in O. lunatus the aAOB GL was significantly reduced in volume, while the pAOB GL displayed a similar volume. We conclude that these sister species exhibit a very sharp difference in the anatomical conformation of the AOB, namely, the relative size of the GL of the aAOB subdomain, which is larger in O. degus than in O. lunatus. We discuss these results in the context of the differences in the lifestyle of these species, highlighting the differences in social behaviour as a possible factor driving to distinct AOB morphometries.

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.

Bicuculline induces synapse formation on primary cultured accessory olfactory bulb neurons

To investigate the roles of the GABAergic inhibitory system of accessory olfactory bulb (AOB) in pheromonal memory formation, we have developed a primary culture system of AOB neurons, which had numerous excitatory and inhibitory synapses. Using this culture system of AOB neurons, we examined the correlation in rats between neuronal excitation and synaptic morphology by bicuculline-induced disinhibition of cultured AOB neurons. The exposure to bicuculline induced long-lasting oscillatory changes in the intracellular calcium level ([Ca2+]in) of cultured non-GABAergic multipolar neurons, which were identified as mitral/tufted cells (MT cells). These MT cells exhibited the appearance of dendritic filopodia structures after a 10-min treatment with bicuculline. By labelling presynaptic terminals with FM4-64, the appearance of new presynaptic terminals was clearly observed on newly formed filopodia after 120 min treatment with bicuculline. These results suggest that bicuculline-induced [Ca2+]in oscillation of MT cells induces the growth of filopodia and subsequently the formation of new presynaptic terminals. Furthermore, tetrodotoxin or the deprivation of extracellular calcium blocked bicuculline-induced synapse formation. The present results indicate that the long-lasting [Ca2+]in oscillation caused by bicuculline-induced disinhibition of cultured MT cells is significantly implicated in the mechanism underlying synapse formation on cultured AOB neurons. Our established culture system of AOB neurons will aid in clarifying the mechanism of synapse formation between AOB neurons and the molecular mechanism of pheromonal memory formation.

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.

A comparative immunocytochemical study of development and regeneration of chemosensory neurons in the rat vomeronasal system

Vomeronasal neurons undergo continuous neurogenesis during development and after neuronal injury. We used immunocytochemical methods to compare different stages of the vomeronasal organ development to those of regeneration following vomeronasal nerve transection. At E15 and at 6 to 10 days after injury, nestin-positive cells were observed throughout the sensory epithelium. We did not find nestin immunoreactivity to be localized to the boundary region of the epithelium. The early appearance and wide distribution of nestin-positive cells suggests that they represent chemosensory precursor cells that develop and migrate vertically in the epithelium. Vomeronasal receptor cells degenerated 6 to 8 days after nerve transection, but axon terminals in the accessory olfactory bulb (AOB) continued to show the presence of the chemosensory specific marker (OMP) for up to ten days, a significant finding observed in this study. It is likely that the distance from the site of nerve transection may contribute to differences in the time course of anterograde and retrograde axon degradation. OMP-positive neurons were observed in the normal adult epithelium and to a much lesser extent 10-60 days after recovery from nerve transection. Axons from regenerated receptor cells did not reach the AOB during this time period. This failure to reestablish connections with target cells in the AOB could explain why OMP-positive cells were rarely observed among the regenerated cells in the vomeronasal epithelium.

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.

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.

Synaptic plasticity in olfactory memory formation in female mice

Female mice develop a long-lasting olfactory recognition memory of a partner male at the first relay in the vomeronasal system. In this study the synaptic plasticity relevant to this phenomenon was examined at reciprocal dendrodendritic synapses in the accessory olfactory bulb of female mice by electron microscopy. The size of asymmetrical excitatory synapses (mitral/tufted to granule cells) of the reciprocal synapses was significantly larger in the group of female mice which were subjected to a treatment intended to induce olfactory memory formation than in the control group. It is suggested that olfactory memory formation is associated with a conformational change at the level of synaptic structure of the accessory olfactory bulb.

Coculture of the vomeronasal organ and olfactory bulb of the fetal rat

The vomeronasal organ and the olfactory bulb of the rat were cocultured from 15-day embryo siblings on collagen-coated membrane in Dulbecco's modified Eagle's medium containing fetal calf serum, horse serum, and antibiotics. At 4 days in vitro (DIV), vomeronasal axons forming two to three large fascicles were seen originating from the explants of the vomeronasal organ. Differential axonal growth was observed. Some fascicles made connections with the explants of the olfactory bulb. Twenty percent of the cocultures studied here showed the formation of connections. At 6-10 DIV many fascicles that did not connect with the olfactory bulb had degenerated, and large fascicles that were connected with the olfactory bulb survived for more than 10 DIV. The formation of connections between the vomeronasal organ and the olfactory bulb in coculture favors the survival of large nerve fascicles, but it could not be determined whether or not the presence of the olfactory bulb affects the initial orientation of the fibers and fascicles from the explants of the vomeronasal organ.

Plastic effect of soiled bedding on the structure of synapses in rat accessory olfactory bulb

The effects of exposure to soiled bedding on synaptic morphology in the accessory olfactory bulb (AOB) were examined in adult male rats. Forty-day-old male rats were isolated. One group was exposed to bedding soiled by male and female rats (EC). Another group was exposed only to male-soiled bedding (SC). A third group was exposed to clean bedding (IC). After 2 months, the animals were sacrificed for electron microscopy. The size and the numerical density of synapses were measured in the glomerulus and the granule cell layer. In the glomerulus, the mean size of the synapses was significantly greater in the EC than in the IC group, whereas that in the SC group appeared to be intermediate between those in the EC and the IC groups but was not significantly different from those in the EC and the IC groups. There was no statistically significant difference in the density of synapses among the three groups. Synapses in the granule cell layer are classified into two types: 1) perforated synapses, which are characterized by discontinuities in their postsynaptic thickenings, and 2) nonperforated synapses. The mean size of perforated synapses was significantly greater in the EC than in the IC and SC groups; however, no statistically significant difference was observed in the size of nonperforated synapses among the three groups. Moreover, no statistically significant difference was observed in the numerical densities of either perforated or nonperforated synapses among the three groups. These results suggested that exposure to a more complex soiled bedding environment (i.e., to bedding soiled by both male and female rats) can induce greater structural changes of the synapses in the AOB of male adult rats.

An analysis of contemporary morphological concepts of synaptic remodelling in the CNS: perforated synapses revisited

Perforated synapses refer to a synaptic type found in the central nervous system. They are characterized by their large size and by a discontinuity of the postsynaptic density when viewed in transverse sections, and by a doughnut or horseshoe shape when viewed in en face views. Of recent morphological studies, one approach has followed their characteristics throughout development and maturity, while others have concentrated on their probable roles in activities including kindling, long-term potentiation, spatial working memory, differential rearing, and the functioning of neuroleptics. An assessment is made of the hypotheses and models that have proved determinative in the emergence of perforated synapses as being significant in synaptic plasticity. Their distribution and frequency are summarized, with emphasis on the importance of unbiased stereological procedures in their analysis. Using three-dimensional approaches various subtypes are recognized. Of these, a complex or fragmented subtype appears of especial significance in synaptic plasticity. Ideas regarding the life-cycle of perforated synapses are examined. The view that they originate from conventional, non-perforated synapses, enlarge, and subsequently split to give rise to a new generation of non-perforated synapses, is critically assessed. According to an alternative model, perforated and non-perforated synapses constitute separate populations from early in their development, each representing complementary forms of synaptic plasticity. An attempt is also made to discover whether synaptic studies on the human brain in normal aging and in Alzheimer's disease throw light on the role of perforated synapses in synaptic plasticity. The loss of synapses in Alzheimer's disease may include a loss of perforated synapses - of particular relevance for an understanding of certain neuropathological conditions.

Morphology of vomeronasal organ cultures from fetal rat

The vomeronasal organs (VNOs) of rats were cultured from embryonic 15-day littermates on collagen-coated membrane in Dulbecco's modified Eagle's medium containing serum. The explants were observed sequentially and fixed at 4, 6, 8, 10 and 14 days in vitro (DIV). Organogenesis of VNOs and cell differentiation took place in vitro. Patterns of organogenesis of the VNO in vitro were different from those in vivo. Both sensory and supporting cells in the sensory epithelium had microvilli on their surface. Epithelial cells in aggregates of non-sensory epithelial cells had cilia and microvilli on their surface. Vomeronasal axons forming two to three large fascicles were seen originating from the VNO at 4, 6, and 8 DIV, and degenerated at 10 or 14 DIV. Glial cells (ensheathing cells) were observed in the fascicles. These morphological characteristics of VNO cells in vitro were similar to those observed in vivo.