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PubMed1993

A pilot study on morphological compartmentalization and heterogeneity in the elasmobranch olfactory bulb.

The olfactory bulb of many elasmobranch fishes is morphologically subdivided into distinct units or sub-bulbs immediately adjacent to the olfactory epithelium. We investigated this morphological feature in two species of shark and one species of ray in order to understand its impact on the arrangement of the primary olfactory projections onto the bulb. Using anterograde tracing methods in vitro (biocytin) as well as in fixed tissue (DiI), we observed a direct segregated projection of the olfactory afferents onto the bulb. Application of tracers to the lateral part of the olfactory epithelium resulted in staining restricted to this region of the bulb, whereas the same tracers applied to the medial part of the epithelium resulted in staining of the medial olfactory bulb. The sub-bulbs appear to be individual anatomical units that each receive input from the olfactory lamellae. Nissl and myelin staining as well as the Golgi method show that the cytoarchitecture of the sub-bulbs is not substantially different from that of other anamniotes. However, we did note the existence of two types of mitral cell, based on the morphology of their dendritic arborization. Type L cells exhibit a loose dendritic arborization, whereas type T cells are characterized by a dense, bush-like dendritic arborization. Both types of mitral cells lack basal dendrites.

Animals↗
PubMed1984

Synaptic excitatory and inhibitory interactions at distal dendritic sites on mitral cells in the isolated turtle olfactory bulb.

The olfactory nerves terminate exclusively on the distal dendritic tufts of mitral cells in the olfactory bulb, which makes this a favorable model for analysis of synaptic responses in distal dendrites. Intracellular recordings of responses to olfactory nerve volleys have been obtained in the isolated turtle olfactory bulb. Single mitral cells usually responded with excitatory postsynaptic potentials (EPSPs) to volleys in two different bundles, indicating convergence from separate receptor neuron populations. Paired volleys revealed long-lasting inhibition of a test EPSP by a conditioning volley. This could be shown to be independent of the inhibition of mitral cells by granule cell interneurons in the deeper layers. The results suggest that excitatory and inhibitory synaptic interactions in the glomerular layer are important in the processing of olfactory inputs. The results also provide an exception to the classical doctrine that synaptic inhibition is preferentially sited near the cell body and axon hillock in order to control impulse generation there. Our findings of inhibitory actions on distal dendrites of mitral cells may provide a model for similar actions on distal dendrites of other central neurons.

Animals↗
PubMed2005

Functional properties of dopaminergic neurones in the mouse olfactory bulb.

The olfactory bulb of mammals contains a large population of dopaminergic interneurones within the glomerular layer. Dopamine has been shown both in vivo and in vitro to modulate several aspects of olfactory information processing, but the functional properties of dopaminergic neurones have never been described due to the inability to recognize these cells in living preparations. To overcome this difficulty, we used a transgenic mouse strain harbouring an eGFP (enhanced green fluorescent protein) reporter construct under the promoter of tyrosine hydroxylase, the rate-limiting enzyme for cathecolamine synthesis. As a result, we were able to identify dopaminergic neurones (TH-GFP cells) in living preparations and, for the first time, we could study the functional properties of such neurones in the olfactory bulb, in both slices and dissociated cells. The most prominent feature of these cells was the autorhythmicity. In these cells we identified five main voltage-dependent conductances: the two having largest amplitude were a fast transient Na(+) current and a delayed rectifier K(+) current. In addition, we observed three smaller inward currents, sustained by Na(+) ions (persistent type) and by Ca(2)(+) ions (LVA and HVA). Using pharmacological tools and ion substitution methods we showed that the pacemaking process is supported by the interplay of the persistent Na(+) current and of a T-type Ca(2)(+) current. We carried out a complete kinetical analysis of the five conductances present in these cells, and developed a Hodgkin-Huxley model of TH-GFP cells, capable of reproducing accurately the properties of living cells, including autorhytmicity, and allowing a precise understanding of the process.

Animals↗
PubMedDate not supplied

[Computer modeling of mechanisms of the information processing in the olfactory bulb. I. Model of the structure-functional organizations of neuronal elements in the olfactory bulb and receptor epithelium].

A computer model of the olfactory bulb was constructed. The paper describes: 1) the general architecture of a model neuron network that reflects the neurophysiological experimental and theoretical data on the structural and functional organization of the peripheral part of the olfactory system, the olfactory bulb with inputs from olfactory receptor neurons; 2) the organization of each of three levels of the model: receptors, olfactory glomeruli, and basic neurons; and 3) a scenario of the computer model work. In some aspects, in particular, in the principle of information presentation, the treatment of the role of basic neurons (mitral and tufted cells), and their interrelations in modules, the model favorably differs from the available olfactory bulb models. The model is basic and provides further refinement of the architecture, an increase in the number of modules, and the modeling of the learning process.

Animals↗
PubMed1978

Forebrain projections of the pigeon olfactory bulb.

The olfactory system of the pigeon (Columba livia) was examined. Our electrophysiological and experimental neuroanatomical (Fink-Heimer technique) data showed that axons from the olfactory bulb terminated in both sides of the forebrain. The cortex prepiriformis (olfactory cortex), the hyperstriatum ventrale and the lobus parolfactorius comprised the uncrossed terminal field. The crossed field included the paleostriatum primitivum and the caudal portion of the lobus parolfactorius, areas which were reached through the anterior commissure. In this report the relationships between areas that receive olfactory information and the possible roles that olfaction plays in the birds' behavior are discussed.

Animals↗
PubMed1996

Peripherin fibers in the main olfactory bulb are different from olfactory fibers and from LHRH fibers: an immunocytochemical and DiI study.

Fibers coming from the olfactory epithelium and entering the olfactory bulb expressed peripherin. These were not sensory olfactory fibers because: (1) they did not form glomeruli, (2) they did not express olfactory marker protein (OMP), (3) they entered deeply into the olfactory bulb, reaching its white matter. Cells and fibers in the olfactory bulb expressing luteinizing hormone-releasing hormone (LHRH) differed from those expressing peripherin because LHRH and peripherin did not coexist and the distributions of LHRH-positive and peripherin-positive fibers were found to be different.

Animals↗
PubMedDate not supplied

[Computer modeling of mechanisms of the information processing in the olfactory bulb. II. Mechanisms of identification and short-term storage in the olfactory bulb: results of the computer experimentation].

The results of experimentation with the computer model of the olfactory bulb are presented. The architecture and scenario of the work of the model were described previously. The dynamic character of the identification process and the mechanism of memorizing short-term of smell stimuli are described. During the identification, a self-adjustment of the olfactory bulb to incoming signals occurs. The self-modification of mitral and tufted cell synapses enhances responses of the cells; upon subsequent presentation of the stimulus, the olfactory bulb responds with a higher activity. The modeling confirmed the validity of the assumption that the functions of mitral and tufted cells are to identify the components of a complex smell and the image of the smell as the whole.

Animals↗
PubMed2006

Coding and synaptic processing of sensory information in the glomerular layer of the olfactory bulb.

Input from olfactory receptor neurons is first organized and processed in the glomerular layer of the olfactory bulb. Olfactory glomeruli serve as functional units in coding olfactory information and contain a complex network of synaptic connections. Odor information has long been thought to be represented by spatial patterns of glomerular activation; recent work has, additionally, shown that these patterns are temporally dynamic. At the same time, recent advances in our understanding of the glomerular network suggest that glomerular processing serves to temporally sharpen these dynamics and to modulate spatial patterns of glomerular activity. We speculate that odor representations and their postsynaptic processing are tuned to and shaped by the sniffing behavior of the animal.

Animals↗
PubMed2006

Immunocytochemical localization of the GABAB2 receptor subunit in the glomeruli of the mouse main olfactory bulb.

The olfactory input to the brain is carried out by olfactory nerve axons that terminate in the olfactory bulb glomeruli and make synapses onto dendrites of glutamatergic projection neurons, mitral and tufted cells, and GABAergic interneurons, periglomerular cells. The dendrites are reciprocally connected through asymmetric synapses of mitral/tufted cells with periglomerular cells and symmetric synapses of the opposite direction. Transmission at the first synapse in the olfactory pathway is regulated presynaptically, and this regulation is mediated, in part, by metabotropic GABAB receptors that, when activated, inhibit transmitter release from the olfactory nerve. Functional GABAB receptors are heterodimers composed of the GABAB1 and GABAB2 subunits. Studies using double immunofluorescence have shown colocalization of both subunits in the glomerular neuropil, and ultrastructural studies have localized GABAB1 to extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of olfactory nerve terminals. We studied the subcellular localization of GABAB2 in the mouse olfactory glomeruli using a subunit-specific antibody and preembedding immunogold labeling. Immunoreactivity for GABAB2 was associated with symmetric dendrodendritic synapses of periglomerular cells with mitral/tufted cells and was localized to the extrasynaptic plasma membrane of presynaptic dendrites, and extrasynaptic, synaptic, and perisynaptic sites on the plasma membrane of postsynaptic dendrites. The results suggest that postsynaptic, and perhaps presynaptic, GABAB receptors may be expressed at GABAergic synapses between dendrites of periglomerular interneurons and projection neurons. Immunolabeling was observed at junctions of the olfactory nerve with mitral/tufted cell dendrites, providing ultrastructural evidence for the expression of the GABAB2 subunit at the primary olfactory synapse.

Animals↗
PubMed2006

Olfactory experience accelerates glomerular refinement in the mammalian olfactory bulb.

Primary olfactory sensory neurons (OSNs) impart both molecular and functional organization to the olfactory bulb. Because OSNs can be selectively activated by odorants in vivo, we sought to determine whether odorant experience alters the cellular dynamics of specific OSNs or their axonal projections. Using mice, we found that odorant stimulation associated with behavioral conditioning influenced OSN wiring by accelerating glomerular refinement independent of OSN number; furthermore, this wiring was strongly associated with olfactory learning.

Aldehydes↗
PubMed2002

Minigenes impart odorant receptor-specific axon guidance in the olfactory bulb.

An olfactory sensory neuron (OSN) expresses selectively one member from a repertoire of approximately 1000 odorant receptor (OR) genes and projects its axon to a specific glomerulus in the olfactory bulb. Both processes are here recapitulated by MOR23 and M71 OR minigenes, introduced into mice. Minigenes of 9 kb and as short as 2.2 kb are selectively expressed by neurons that do not coexpress the endogenous gene but coproject their axons to the same glomeruli. Deletion of a 395 bp upstream region in the MOR23 minigene abolishes expression. In this region we recognize sequence motifs conserved in many OR genes. Transgenic lines expressing the OR in ectopic epithelial zones form ectopic glomeruli, which also receive input from OSNs expressing the cognate endogenous receptor. This suggests a recruitment through homotypic interactions between OSNs expressing the same OR.

Animals↗
PubMed1984

Selective retrograde transport of tritiated D-aspartate from the olfactory bulb to the anterior olfactory nucleus, pyriform cortex and nucleus of the lateral olfactory tract in the rat.

After an injection of [3H]D-aspartate into the olfactory bulb of the rat, retrogradely labeled cells were detected bilaterally in the anterior olfactory nucleus (AON), and ipsilaterally in the pyriform cortex (PC) and nucleus of the lateral olfactory tract (NLOT). These results suggest a certain selective retrograde transport of this amino acid, and are discussed in relation to transmitter candidates in the olfactory bulb.

Animals↗
PubMed2003

Axon mis-targeting in the olfactory bulb during regeneration of olfactory neuroepithelium.

During development, primary olfactory axons typically grow to their topographically correct target zone without extensive remodelling. Similarly, in adults, new axons arising from the normal turnover of sensory neurons essentially project to their target without error. In the present study we have examined axon targeting in the olfactory pathway following extensive chemical ablation of the olfactory neuroepithelium in the P2-tau:LacZ line of mice. These mice express LacZ in the P2 subpopulation of primary olfactory neurons whose axons target topographically fixed glomeruli on the medial and lateral surfaces of the olfactory bulb. Intraperitoneal injections of dichlobenil selectively destroyed the sensory neuroepithelium of the nasal cavity without direct physical insult to the olfactory neuron pathway. Primary olfactory neurons regenerated and LacZ staining revealed the trajectory of the P2 axons. Rather than project solely to their topographically appropriate glomeruli, the regenerating P2 axons now terminated in numerous inappropriate glomeruli which were widely dispersed over the olfactory bulb. While these errors in targeting were refined over time, there was still considerable mis-targeting after four months of regeneration.

Aging↗
PubMed1992

Neural circuit computation: complex patterns in the olfactory bulb.

The olfactory bulb principal neurons show complex responses during olfactory stimulation that are characterized by periods of profound suppression, temporal patterns of activity, and nonmonotonic intensity response functions. The model presented here suggests that these features may arise from changing spatial patterns of activity across the bulbar surface due to restricted (chemotopic) activation in bulbar glomeruli. The model is based on a simplified version of the bulbar neural circuit as it is presently known from anatomical and physiological investigations and provides explanations for several anomalous bulbar response patterns.

Axons↗
PubMed1998

Human olfactory bulb: aging of glomeruli and mitral cells and a search for the accessory olfactory bulb.

The aims of this study on the human olfactory bulb were two. First morphometry of the bulbs revealed marked declines during aging in the numbers of mitral cells and glomeruli, the bulb's principal integrative and relay elements. Numbers of glomeruli and mitral cells in each bulb of the young adult human were found to be approximately 8,000 and 40,000, respectively; these numbers declined steadily with age at an approximate rate of 10% per decade, so that in the ninth and tenth decades less than 30% of these elements remain in place. Such a marked decline with aging is suggested to underlie in part the decline in olfactory abilities (odor detection and identification) of humans with aging. In a separate study a systematic search for presence of an accessory olfactory bulb in the adult and aging bulbs was undertaken. No positive evidence for such an organized formation was found in the various regions of the adult bulbs of different age groups. The implications of these negative findings for the recent theories on human vomeronasal function and pheromonal perception are discussed.

Adult↗
PubMed1993

Morphological and quantitative evaluation of olfactory bulb development in Xenopus after olfactory placode transplantation.

We found previously that the number of olfactory axons is correlated with the number of mitral/tufted cells (output neurons of the olfactory bulb) during normal larval development. To examine the significance of this quantitative relationship, we evaluated the effects of transplanting an extra olfactory placode on the development of the larval olfactory bulb. We found that the transplanted tissue retained the normal, pseudostratified, columnar appearance and had the same cell types as normal olfactory epithelium, and the olfactory bulbs had the same laminar organization as control bulbs. With gross examination of the olfactory bulb, the side innervated by the transplant appeared slightly larger than the contralateral side in animals analyzed at a young larval stage (stage 50) and in 2 of the 9 animals examined at late larval stages (57/58). Tissue sections and area measurements, however, revealed that the volume of the olfactory bulbs in animals with a transplant was not significantly different from control values. Our quantitative analysis also showed that in stage-50 animals with a transplant, the total number of olfactory axons (in nerves from the transplanted and host olfactory organs) appeared to be greater than in control animals, but not to a statistically significant level. The number of mitral/tufted cells was not different from controls. In animals examined at stage 57/58, there was no difference from controls in either the total number of olfactory axons, total number of mitral/tufted cells, or convergence ratio of olfactory axons onto mitral/tufted cells. Thus, in the late-stage larvae, the quantitative relationship between olfactory axons and mitral/tufted cells was not altered by the experimental manipulation. These results suggest that the olfactory bulb can regulate the number of afferent fibers.

Animals↗
PubMed2006

Maps of odorant molecular features in the Mammalian olfactory bulb.

The olfactory bulb (OB) is the first relay station of the central olfactory system in the mammalian brain and contains a few thousand glomeruli on its surface. Because individual glomeruli represent a single odorant receptor, the glomerular sheet of the OB forms odorant receptor maps. This review summarizes the emerging view of the spatial organization of the odorant receptor maps. Recent studies suggest that individual odorant receptors are molecular-feature detecting units, and so are individual glomeruli in the OB. How are the molecular-feature detecting units spatially arranged in the glomerular sheet? To characterize the molecular-feature specificity of an individual glomerulus, it is necessary to determine the molecular receptive range (MRR) of the glomerulus and to compare the molecular structure of odorants within the MRR. Studies of the MRR mapping show that 1) individual glomeruli typically respond to a range of odorants that share a specific combination of molecular features, 2) each glomerulus appears to be unique in its MRR property, and 3) glomeruli with similar MRR properties gather together in proximity and form molecular-feature clusters. The molecular-feature clusters are located at stereotypical positions in the OB and might be part of the neural representation of basic odor quality. Detailed studies suggest that the glomerular sheet represents the characteristic molecular features in a systematic, gradual, and multidimensional fashion. The molecular-feature maps provide a basis for understanding how the olfactory cortex reads the odor maps of the OB.

Animals↗