Estimation of numbers of vomeronasal synapses in the glomerular layer of the accessory olfactory bulb of the mouse at different ages

doi:10.1016/0006-8993(81)90336-X

Brain Research

Volume 205, Issue 2, 2 February 1981, Pages 245-253

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Abstract

The total number of vomeronasal nerve synapses was estimated in a series of mice from 1 to 12 months of age. The volume of the glomerular layer of the accessory olfactory bulb was derived from area measurements through a series of semithin sections. The specialized appositions constituting the vomeronasal nerve synapses were identified in electron micrographs of the glomerular layer by the characteristic electron dense matrix, vesicles, shape and arrangement of the presynaptic elements, and the pale, vesicle-containing dendritic postsynaptic elements.

The volume of the glomerular layer showed an overall increase from 66 x 10³ cu. μm at 1 month of age to 82 x 10⁶ cu. μm at 12 months (although there was almost equally large intergroup variation probably not associated with age). The number of synapses per unit area was between 0.114 and 0.131 per sq. μm and unrelated to age. The average length of the synaptic appositions increased from0.32 ± 0.01 μm at 1 month to0.36 ± 0.02 μm at 12 months (the major increase occurring between 1 and 4 months). The calculated total numbers of synapses in the glomerular layer were between 20 and 21 millions at 1, 4 and 8 months and showed a statistically insignificant increase to slightly over 23 millions at 12 months. Since there are about 140 thousand vomeronasal axons, each axon makes on the average about 150 synapses.

New vomeronasal neurosensory cells are continually being formed during life and the new cells develop axons which grow to the bulb. However, both the overall number of cells in the epithelium and the total number of synapses in the bulb are constant after about 8 months of age. This could be achieved without synaptic turnover if the newly formed cells die before making synapses. Alternatively, it could be achieved if the entire population of neurosensory cells and their synapses are undergoing continual replacement.

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To investigate the interaction between vomeronasal receptor neurons and accessory olfactory bulb neurons during pheromonal signal processing and specific synapse formation, partially dissociated rat vomeronasal receptor neurons were co-cultured with accessory olfactory bulb neurons. Between 7 and 14 days in co-culture, a few bundles of fibers from a spherical structure, termed the vomeronasal pocket, of cultured vomeronasal receptor neurons extended to the accessory olfactory bulb neurons. An optical recording of the intracellular Ca²⁺ concentration was used to monitor the synaptic activation of cultured accessory olfactory bulb neurons. Electrical stimulation of the vomeronasal pocket between 7 and 14 days in co-culture had no effects on most of the cultured neurons tested, although it occasionally evoked weak responses in a small number of neurons. In contrast, vomeronasal pocket stimulation after 21 days in co-culture evoked clear calcium transients in a substantial number of cultured accessory olfactory bulb neurons. These responses of accessory olfactory bulb neurons were reversibly suppressed by the application of 6-cyano-7-nitroquinoxaline-2,3-dione; the calcium transients disappeared in most of the neurons and were diminished in the others. The application of d-2-amino-5-phosphonopentanoic acid partially affected the calcium transients, but blocked spontaneous calcium increases, which were observed repeatedly in accessory olfactory bulb-alone cultures. The application of both 6-cyano-7-nitroquinoxaline-2,3-dione and d-2-amino-5-phosphonopentanoic acid completely blocked the evoked calcium transients. These results suggest that functional glutamatergic synapses between vomeronasal receptor neurons and accessory olfactory bulb neurons were formed at around 21 days in co-culture.
Specific expression pattern of Fos in the accessory olfactory bulb of male mice after exposure to soiled bedding of females
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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 chemonsensory 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.
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Social environment affects synaptic structure in the glomerulus of the accessory olfactory bulb of the hamster
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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.
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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. These results demonstrated that exposure to different rearing conditions, in which the pheromonal environment may be substantially different, can induce striking morphological changes in the perforated synapses of the AOB in adult rats.
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Using the rapid Golgi method, the accessory olfactory bulb (AOB) of the adult rat was examined. The principal neurons of the AOB were very variable in terms of their dendritic branching pattern and morphology of the glomerular arbors. Some glomerular arbors occupied small portions, other large portions of a single glomerulus. These morphological patterns suggest that complex integrative events occur in the AOB.

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Estimation of numbers of vomeronasal synapses in the glomerular layer of the accessory olfactory bulb of the mouse at different ages

Abstract

Brain Research

Brain Research

Brain Research

Brain Research

Brain Research

Estimation of nuclear population from microtome sections

Anat. Rec.

Estimation of the number of synapses in a volume of nervous tissue from counts in thin sections by electron microscopy

J. Neurocytol.

The structure of the accessory olfactory lobule