Olfactory bulb ventricles as a frequent finding in magnetic resonance imaging studies of the olfactory system

Background. In some species an embryologic cavity inside the olfactory bulb (OB) persists and is called olfactory bulb ventricle (OBV). It is generally assumed that OBVs in humans are solitary findings representing remnants of embryologic structures that were not fully regressed, although the incidence of OBVs was never examined. Using magnetic resonance imaging (MRI), the present study aimed to study the incidence of OBVs in healthy human subjects. Material and methods. A total of 122 individuals participated. Volumes of the right and left OB were determined using MRI scans and a standardized protocol for OB analysis. For comparison, OBs of 42 cadavers were collected and sectioned. Results. The main finding of this study was the high incidence of OBV-like structures in our study group. Seventy-two out of 122 (59%) participants yielded signs for an OBV whereas three out of 42 postmortem OBs contained histologically detectable OBV. Discussion: This stands in disagreement with the previous assumption of complete obliteration at the time of birth. This discrepancy may be explained by the fact that our present findings are based on modern MRI techniques with much higher resolution than 10 or 20 years ago. Another possible explanation for the discrepancy between studies based on MRI and histopathology might relate to postmortem resorption of cerebrospinal fluid from OBVs. Especially with a long postmortem interval OBVs may collapse and may no longer appear as an open cavity.     

Hyperreactivity following temporary chemical lesions in the region ventral to the anterior septum but not in the anterior olfactory nucleus,the lateral olfactory tract,or the olfactory bulb

Bilateral temporary chemical lesions were produced by slow infusion of the general blocking agent lidocaine into several anterior forebrain sites which have been implicated in the increase in reactivity following lesions of the olfactory bulbs. There was an increase in reactivity with infusions of the blocking agent lidocaine into the region ventral to the anterior septum but not with infusions into the region of the anterior olfactory nucleus, or the olfactory bulbs. A control group in which lidocaine was infused into the vicinity of the lateral olfactory tract also showed no increase in reactivity to the experimenter. Infusions of 0.9% NaCl produced no change in reactivity at any of these sites. The results are consistent with previous evidence that lesions of the olfactory bulbs increase reactivity to the experimenter as a result of incidental damage to more caudal tissue. The present findings suggest that the important caudal region lies ventral to the anterior septum between the vertical arm of the diagonal band of Broca and the rostral limb of the anterior commissure.     

Somatostatin-14-like immunoreactive neurons and fibres in the human olfactory bulb

Summary This study describes the morphological features and the distribution pattern of neurons in the human olfactory bulb which are immunoreactive for an antiserum against the neuropeptide somatostatin-14.Immunoreactive nerve cell bodies were mainly found in the white matter surrounding the cell clusters of the anterior olfactory nucleus. Some immunoreactive neurons were also found scattered throughout the anterior olfactory nucleus and the deeper parts of the inner granule cell layer. Only a few immunoreactive neurons were localized in the glomerular layer and the outer granule cell layer.Immunoreactive fibres were found in all layers of the olfactory bulb. In addition, an impressive number of coiled and kinked immunoreactive fibres were localized within the anterior olfactory nucleus forming a dense plexus. Accumulations of twisted and coiled branches of immunoreactive fibres were rarely found either surrounding or within the olfactory glomerula.The characteristics of somatostatin-14 immunoreactive neurons as seen in the combined pigment-Nissl preparation were studied after decolourizing the chromogen and restaining the preparations with aldehydefuchsin in order to demonstrate the lipofuscin pigment and gallocyanin chrome alum for Nissl material. About 90% of the immunoreactive neurons studied in this manner turned out to be devoid of lipofuscin granules. The remaining 10% displayed different patterns of pigmentation.These findings suggest the presence of different types of somatostatin-14-like immunoreactive neurons in the olfactory bulb of the human adult.     

Olfactory cues and accessory olfactory bulb lesion: effects on sexual behavior in the cyclic female rat

The influence of olfactory cues and accessory olfactory bulb (AOB) lesion on female sexual behavior was studied in virgin female Wistar rats. In Experiment 1, it appeared that distance or contact exposure to male urine soiled bedding for 8 hours before testing increased sexual receptivity, i.e., the number of receptive females at 18:00-19:00 on proestrus. In Experiment 2, we observed that sexual receptivity at 18:00-19:00 on proestrus was not affected by AOB lesion as compared to sham-operated females. In Experiment 3 the effects of both AOB lesion and olfactory cues were analyzed. Sexual receptivity at 18:00-19:00 on proestrus did not significantly differ in sham-operated and accessory olfactory bulbectomized females both exposed to the odor of male urine. Regarding lordosis quotient in the three experiments, no significant difference was observed. Mechanisms whereby olfactory cues and/or AOB lesion modified female sexual behavior on proestrus in virgin female rats were discussed in the light of previous and present observations.     

Spontaneous activity of single neurons in the olfactory bulb of the rainbow trout (Salmo gairdneri) and its modulation by olfactory stimulation with amino acids

Summary Spontaneous action potentials recorded from 126 neurons in the olfactory bulb of Salmo gairdneri show a higher tendency towards bursting patterns of activity than those shown in previous reports about other fish. Granule cells and periglomerular cells are more likely to fire in bursts than mitral cells. Natural chemical stimulation of the olfactory mucosa with amino acid solutions produced a unique pattern of excitatory and inhibitory responses across all units. Chi-square values were calculated for stimulatory effectiveness between forty-five pairs of odours. L-serine and L-alanine consistently showed a high degree of similarity with several other odours. The converse was true for GABA and L-histidine, although this pair had a high chi-square value when mutually compared. Enantiomeric pairs of amino acids were often found to have opposite stimulatory effects on bulbar units. These results are discussed in relation to the possible properties and configurations of odorant receptor sites for amino acids in the fish olfactory mucosa.     

Patterns of expression of the immediate-early gene egr-1 in the accessory olfactory bulb of female mice exposed to pheromonal constituents of male urine

Male mice excrete large quantities of major urinary proteins that have been proposed to have an important pheromonal role either alone or by way of their bound ligands. We have found that these major urinary proteins are not only likely to mediate the pregnancy blocking effects of male urine, but that they also convey the strain recognition signal of the male pheromone. Recent molecular biological investigations have characterized two classes of pheromonal receptor in the vomeronasal organ that appear to project separately to anterior and posterior regions of the accessory olfactory bulb. However, it is not known whether these separate pathways handle fundamentally different types of pheromonal information. We have attempted to investigate this question using the expression of the immediate-early gene egr-1 as a marker for activity of neurons in the accessory olfactory bulb of female mice in response to putative pheromonal constituents. Exposure to 2,3 dihydro-exo-brevicomin and 2-sec-butyl-4,5-dihydro-thiazole, the main ligands bound to the major urinary proteins, elicited expression of egr-1 in clusters of presumed mitral neurons at the medial and lateral margins of the posterior accessory olfactory bulb. Whole male urine and a preparation of major urinary proteins that had been stripped of their ligands induced egr-1 expression in mitral cells of the anterior half of the accessory olfactory bulb in addition to the posterior clusters.

This would suggest that the anterior and posterior halves of the accessory olfactory bulb are processing different aspects of the male pheromone signal with the anterior region, which responds preferentially to major urinary proteins, being principally concerned with the strain recognition component.     

Complex neural representation of odour information in the olfactory bulb

The most important task of the olfactory system is to generate a precise representation of odour information under different brain and behavioural states. As the first processing stage in the olfactory system and a crucial hub, the olfactory bulb plays a key role in the neural representation of odours, encoding odour identity, intensity and timing. Although the neural circuits and coding strategies used by the olfactory bulb for odour representation were initially identified in anaesthetized animals, a large number of recent studies focused on neural representation of odorants in the olfactory bulb in awake behaving animals. In this review, we discuss these recent findings, covering (a) the neural circuits for odour representation both within the olfactory bulb and the functional connections between the olfactory bulb and the higher order processing centres; (b) how related factors such as sniffing affect and shape the representation; (c) how the representation changes under different states; and (d) recent progress on the processing of temporal aspects of odour presentation in awake, behaving rodents. We highlight discussion of the current views and emerging proposals on the neural representation of odorants in the olfactory bulb.     

Estrogen-induced region specific decrease in the density of 5-bromo-2-deoxyuridine-labeled cells in the olfactory bulb of adult female rats

Effects of chronic estrogen treatment on the survival rate of newly integrated interneurons were studied in the olfactory bulb of adult (250-300 g) female rats. Ovariectomized rats received 17-beta estradiol dissolved in sesame oil (i.p., 100 microg/100 g body weight [b.w.]) during six consecutive days, and on day 6 they were also injected with the mitotic marker 5-bromo-2-deoxyuridine (BrdU, i.p., 50 mg/kg b.w.) in every 2 hours during 8 hours. After 21 days of survival animals were killed and the density of BrdU-immunoreactive cells was analyzed in the granule cell and glomerular layer both in the main and accessory olfactory bulb. A significant decrease was found in the density of BrdU-labeled cells in both layers examined in the accessory olfactory bulb of ovariectomized and estradiol-treated rats when compared with those of ovariectomized and vehicle-treated animals. In the main olfactory bulb, in contrast, no difference was observed in the density of BrdU-immunoreactive cells in either of the two layers. Our results suggest that cells destined to the glomerular and granule cell layers react in the same way to chronic estrogen treatment, and the effect of estradiol is region specific, at least, within the olfactory bulb. 17-Beta estradiol reduces the density of newly generated cells in the accessory olfactory bulb, an area involved in the perception of pheromones, thus having a role in regulating sexual behavior, while the rate of integration and survival of newly born cells in the first relay station of the main olfactory pathway, i.e. the main olfactory bulb, remains unchanged.     

Oxytocin facilitates the induction of long-term potentiation in the accessory olfactory bulb

When female mice are mated, they form a memory to the pheromonal signal of their male partner. Several lines of evidence indicate that the neural changes underlying this memory occur in the accessory olfactory bulb (AOB) at the first stage of the vomeronasal system. The formation of this memory depends on the mating-induced release of noradrenaline in the AOB. In addition to noradrenaline, the neuropeptide oxytocin (OT) is also released within the central nervous system during mating. Because OT has been implicated in social memory and its receptors are expressed in the AOB, we hypothesized that OT might promote the strength of synaptic transmission from mitral to granule cells in the AOB. To test this hypothesis, we analyzed the lateral olfactory tract-evoked field potential that represents the granule cell response to mitral cell activation and its plasticity in parasagittal slices of the AOB. Of the 10-, 20-, 50-, and 100-Hz stimulations tested, the 100-Hz stimulation was optimal for inducing long-term potentiation (LTP). OT paired with 100-Hz stimulation that only produced short-term potentiation enhanced LTP induction in a dose-dependent manner. OT-paired LTP was blocked by both the selective OT antagonist desGly-NH₂,d(CH₂)₅[Tyr(Me)²,Thr⁴]-ornithine vasotocin and the N-methyl-d-aspartate (NMDA) receptor antagonist dl-2-amino-5-phosphonovaleric acid. These results indicate that OT can function as a gate to modulate the establishment of NMDA receptor-dependent LTP at the mitral-to-granule cell synapse in the AOB.     

Proteinase-activated receptors 1 and 2 in rat olfactory system: layer-specific regulation of multiple signaling pathways in the main olfactory bulb and induction of neurite retraction in olfactory sensory neurons

Proteinase-activated receptors (PARs) are a family of four G protein-coupled receptors that are widely distributed in the CNS and involved in neural cell proliferation, differentiation and survival. The olfactory system undergoes continuous neurogenesis throughout life and may represent a critical target of PAR cellular actions. In the present study we investigated the functional activity of PAR1 and PAR2 in microdissected tissue preparations of olfactory nerve-glomerular layer (ON-GL), external plexiform layer (EPL) and granule cell layer (GRL) of the rat main olfactory bulb and in primary cultures of olfactory neuroepithelial cells. Activation of either PAR1 or PAR2 regulated multiple signaling pathways, including activation of pertussis-toxin sensitive Gi/o proteins, inhibition of cyclic AMP formation, stimulation of Gq/11-mediated phosphoinositide (PI) hydrolysis, phosphorylation of Ca2+/calmodulin-dependent protein kinase II and activation of the monomeric G protein Rho, predominantly in ON-GL, whereas only activation of Rho was detected in the deeper layers. Olfactory nerve lesion by nasal irrigation with ZnSO4 induced a marked decrease of PAR signaling in ON-GL. In primary cultures of olfactory neurons, double immunofluorescence analysis showed the localization of PAR1 and PAR2 in cells positive for olfactory-marker protein and neuron-specific enolase. Cell exposure to either nanomolar concentrations of thrombin and trypsin or PAR-activating peptides caused rapid neurite retraction. This study provides the first characterization of the laminar distribution of PAR1 and PAR2 signaling in rat olfactory bulb, demonstrates the presence of the receptors in olfactory sensory neurons and suggests a role of PARs in olfactory sensory neuron neuritogenesis.     

Field potentials in the olfactory bulb of the rainbow trout (Salmo gairdneri): Evidence for a dendrodendritic inhibitory pathway

Summary Field potentials have been recorded from the olfactory bulb of the teleost fish, Salmo gairdneri. Stimulation of the olfactory nerve generates a complex wave pattern, which when recorded near the surface in the centre of the bulb consists of one positive and three negative components, P, N₁, N₂ and N₃. The N₁ has been attributed to depolarization of mitral cell dendrites, N₂ to depolarization of the superficial dendrites of granule cells, and P to subsequent hyperpolarization of mitral cells. Possible origins of the N₃ wave are discussed. Spikes from single units recorded in the mitral cell layer usually occur superimposed on the N₁ wave, and single units recorded in the region of granule cells are usually found coincident with the training edge of the deep positive wave. With the exception of N₃ the patterns of activity observed are very similar to previously published results obtained from other vertebrates. It is suggested that the dendrodendritic inhibitory pathway proposed for the mammalian olfactory bulb (Rall et al., 1966; Rall and Shepherd, 1968) is common to all vertebrates.     

Distribution of DNA, proteins, and lipids in cells of the olfactory bulb in rats of different ages

Luminescence and absorption stains specific for DNA (acridine orange, ethidium bromide), proteins (silver nitrate), and lipids (Sudan III) were used to study the distribution of DNA, proteins, and lipids in sections of the olfactory bulb in rats, studies being performed after fixation of brains with paraformaldehyde. DNA was found to be more abundant in the glomerular cell layer than the mitral cell layer. Higher quantities of DNA were present in the granular layer, located beneath the mitral layer. The characteristics of cell layers in the olfactory bulb were studied in rats aged two days and one month. There were differences between the layers of rats of different ages in terms of the content and distribution of DNA, though there were no differences in the total protein or lipid contents. Glomeruli were immature in two-day-old rats. __________ Translated from Morfologiya, Vol. 127, No. 3, pp. 30–33, May–June, 2005.     

Olfactory deprivation enhances normal spine loss in the olfactory bulb of developing ferrets

Ferrets show a sensitive phase in their postnatal development during which they can become imprinted to food odors. At the same time the number of granule cell spines in the olfactory bulb reaches a maximum, declining significantly thereafter. In ferrets, exposed continuously to saturated levels of geraniol odor in the cage environment, the normal decline in spine number (occurring between day 60 and 90) is significantly enhanced. No such effects were observed during earlier ontogenetic phases. This late postnatal phase is further associated with a marked and significant decrease in total brain weight. The significance of these events to olfactory imprinting and plasticity in the developing brain is discussed.