The somatotopic organization of the olfactory bulb in elasmobranchs

The olfactory bulbs (OBs) are bilaterally paired structures in the vertebrate forebrain that receive and process odor information from the olfactory receptor neurons (ORNs) in the periphery. Virtually all vertebrate OBs are arranged chemotopically, with different regions of the OB processing different types of odorants. However, there is some evidence that elasmobranch fishes (sharks, rays, and skates) may possess a gross somatotopic organization instead. To test this hypothesis, we used histological staining and retrograde tracing techniques to examine the morphology and organization of ORN projections from the olfactory epithelium (OE) to the OB in three elasmobranch species with varying OB morphologies. In all three species, glomeruli in the OB received projections from ORNs located on only the three to five lamellae situated immediately anterior within the OE. These results support that the gross arrangement of the elasmobranch OB is somatotopic, an organization unique among fishes and most other vertebrates. In addition, certain elasmobranch species possess a unique OB morphology in which each OB is physically subdivided into two or more “hemi‐olfactory bulbs.” Somatotopy could provide a preadaptation which facilitated the evolution of olfactory hemibulbs in these species. J. Morphol., 2013. © 2012 Wiley Periodicals, Inc.     

Structural organization and connections of cell ensembles of the olfactory bulb of the cat brain

The cat cerebral olfactory tubercle (OT) includes into its composition certain cellular ensembles distinguished by means of morphological criteria. The ensembles consist of three cellular components: clusters of granular neurons (islands of Calleja), pyramid-like neurons of the layer II, situating along the periphery of the islands, and groupings made by polygonal and spindle-like neurons of the layer III. Morphometrical analysis of every of the three cellular complex components has been carried out. About 7-10 islands of Calleja are situated on the OT territory, which makes nearly 75% of the whole surface of the OT. Neuronal composition of the cellular ensembles has been studied by Golgi method. Varieties of long and short axonal neurons, included into the ensemble composition, have been characterized. Presence of projections of the macrocellular neurons of the layer III (a part of the third component of the ensemble) has been revealed in the posterior part of the lateral hypothalamus and in the field of Forel H1. Possible role of the cellular ensembles is discussed for ensuring various functions of the OT.     

Representation of natural stimuli in the rodent main olfactory bulb

Natural odorants are complex mixtures of diverse chemical compounds. Monomolecular odorants are represented in the main olfactory bulb by distinct spatial patterns of activated glomeruli. However, it remains unclear how individual compounds contribute to population representations of natural stimuli, which appear to be unexpectedly sparse. We combined gas chromatography and intrinsic signal imaging to visualize glomerular responses to natural stimuli and their fractionated components. While whole stimuli activated up to 20 visible glomeruli, each fractionated component activated only one or few glomeruli, and most glomeruli were activated by only one component. Thus, responses to complex mixtures reflected activation by multiple components, with each contributing only a small part of the overall representation. We conclude that the population response to a complex stimulus is largely the sum of the responses to its individual components, and activation of an individual glomerulus independently signals the presence of a specific component.     

The postnatal development of the main olfactory bulb of the rat

The postnatal development from birth to 1 year of the main olfactory bulb was examined quantitatively. The volume of the main olfactory bulb increased over seven-fold by day 30 and remained unchanged thereafter. During the same period the volume of the granular layer increased 18-fold and the mean areas of the olfactory glomeruli increased seven-fold. The mean areas of mitral cell perikarya doubled between the neonatal and juvenile periods. The total number of the mitral cells, however, declined during the first three postnatal weeks. In the internal granular layer of the main olfactory bulb, 89% of the granule cells were acquired postnatally. Much of the cellular gain occurred during the first 3 weeks, with the period of maximum acquisition between days 8 and 14. The number of subependymal cells, the precursors of granule cells, reached a peak at 12 days and gradually declined. But some primitive cells could still be found at one year of age and there was an increase in the total number of granule cells beyond day 30. The mean nuber of internal granular layer cells in a single main olfactory bulb of adult rats was about 5 X 10(6); the number of mitral cells about 4 X 10(4). In the animals injected with 3H-thymidine on day 20 and killed 2 h after injection a small but significant proportion of cells was labelled in the subependymal layer but few in the internal granular layer. In the animals killed 20 and 40 days after injection there was a 10--11-fold rise in the proportion of labelled internal granular layer cells. The proportion of labelled internal granular layer cells decreased in longer survival groups but the total number of labelled cells remained the same, even in year-old animals. However, the total number of internal granular layer cells in the sections examined increased with age.     

GABA immunoreactivity in the main olfactory bulb of the frog Rana temporaria

Immunoreactivity for gamma-aminobutyric acid (GABA) was localized at the light microscopic level in the main olfactory bulb (MOB) of the frog, Rana temporaria. By means of free-floating peroxidase-antiperoxidase immunocytochemical technique, GABA was found in a large number of neurons in the granular cell layer, in a few small somata in the mitral cell layer and in two different types of cell somata in the glomerular layer. Individual GABA-immunopositive cells were found in the olfactory nerve layer. GABA immunostaining was also localized in cell processes and fiber fragments. There were many immunoreactive puncta in all layers of the MOB. GABA-positive punctate structures often outlined immunonegative cells in the mitral cell and glomerular layers. Rounded tightly packed groups of immunoreactive puncta were found only along ventral border of the glomerular layer. The results are discussed in comparison with data obtained on mammalian MOB in terms of MOB functional organization.     

Age-related changes of parvalbumin immunoreactive neurons in the rat main olfactory bulb

Parvalbumin (PV) is found in the olfactory system, including the main olfactory bulb, and is thought to be one of the neuroactive substances in olfaction. Changes in PV immunoreactivity in the olfactory system during aging have not been examined. We investigated such changes in the main olfactory bulb (MOB) of the rat at postnatal month 1 (PM 1), PM 3, PM 6, PM 12 and PM 24. PV-IR neurons were almost completely restricted to the external plexiform layer. At PM 1 there were only a few PV-IR neurons; at PM 3, the number of PV-IR neurons was at its greatest but they were not well developed morphologically. At PM 6, the number of PV-IR neurons was similar to that at PM 3 and they had satellite somata with well-developed processes with many varicosities. By PM 12 the number of neurons and processes had declined, and by PM 24, they had fallen even further and the remaining processes had lost most of their varicosities. We conclude that age-related degeneration of PV-IR neurons in the MOB may reduce calcium buffering and affect olfactory function in senile species.     

The source of spontaneous activity in the main olfactory bulb of the rat

Introduction

In vivo, most neurons in the main olfactory bulb exhibit robust spontaneous activity. This paper tests the hypothesis that spontaneous activity in olfactory receptor neurons drives much of the spontaneous activity in mitral and tufted cells via excitatory synapses.

Methods

Single units were recorded in vivo from the main olfactory bulb of a rat before, during, and after application of lidocaine to the olfactory nerve. The effect of lidocaine on the conduction of action potentials from the olfactory epithelium to the olfactory bulb was assessed by electrically stimulating the olfactory nerve rostral to the application site and monitoring the field potential evoked in the bulb.

Results

Lidocaine caused a significant decrease in the amplitude of the olfactory nerve evoked field potential that was recorded in the olfactory bulb. By contrast, the lidocaine block did not significantly alter the spontaneous activity of single units in the bulb, nor did it alter the field potential evoked by electrical stimulation of the lateral olfactory tract. Lidocaine block also did not change the temporal patters of action potential or their synchronization with respiration.

Conclusions

Spontaneous activity in neurons of the main olfactory bulb is not driven mainly by activity in olfactory receptor neurons despite the extensive convergence onto mitral and tufted cells. These results suggest that spontaneous activity of mitral and tufted is either an inherent property of these cells or is driven by centrifugal inputs to the bulb.     

Immunocytochemical identification of GABAergic neurons in the main olfactory bulb of the rat

With the aid of a sheep antiserum against rat brain glutamate decarboxylase (GAD), the endogenous marker for GABAergic neurons, we have labeled immunocytochemically various types of nerve cells in the main olfactory bulb of rats, with and without topic injections of colchicine. The peroxidase-antiperoxidase procedure was applied to floating Vibratome and frozen sections. A large part of the periglomerular cell population and practically all granule cells in the deep layers contain GAD-like immunoreactivity in untreated rats, while tufted and mitral cells (the projection neurons) are unstained. This observation confirms a previous study with a rabbit antiserum against mouse brain GAD, which suggested that GABAergic neurons with presynaptic dendrites contain high somatal concentrations of GAD. We show, however, that immunostaining of granule cell bodies decreases progressively from the internal plexiform layer to the deep portion of the granule cell layer. Many cell processes in the glomeruli are densely stained. They presumably represent synaptic gemmules of the numerous GAD-positive periglomerular cells, which thus could provide initial, inhibitory modulation of the afferent input. In the external plexiform layer immunostaining of the neuropil is substantially denser in the superficial half than in the deep half. This may reflect a corresponding gradient of inhibition related to unequal frequency of occurrence of synaptic gemmules of granule cell dendrites. Alternatively such a graded immunostaining of cell processes could be related to the corresponding gradient in the density of immunostaining of granule cell bodies in the deep layers, in accordance with recent data indicating that superficial and deep granule cells project their ascending dendrites respectively to superficial and deep portions of the external plexiform layer. Furthermore, we have demonstrated the presence of additional classes of GAD-positive neurons, microneurons in the external plexiform layer, small neurons in the periglomerular region, the external plexiform layer, the mitral cell layer, the internal plexiform layer, and medium-size neurons in the granule layer and the white matter. The small- and medium-size GAD-positive neurons appear weakly immunoreactive in untreated rats, but become densely stained after topic colchicine injection. Such cells presumably lack presynaptic dendrites and may correspond to different types of short axon cells demonstrated by the Golgi method.(ABSTRACT TRUNCATED AT 400 WORDS)     

Comparison of social interaction and neural activation in the main olfactory bulb and the accessory olfactory bulb between Microtus mandarinus and Microtus fortis

To gain insight into the function of AOB and MOB during different social interaction and in different vole species,the behaviors and neural activation of the olfactory bulbs in social interactions of mandarin voles Microtus mandarinus and reed voles Microtus fortis were compared in the present research.Mandarin voles spent significantly more time attacking and sniffing their opponents and sniffing sawdust than reed voles.During same sex encounters,mandarin voles attacked their opponents for a significantly ...     

Zonal organization of the mammalian main and accessory olfactory systems

Zonal organization is one of the characteristic features observed in both main and accessory olfactory systems. In the main olfactory system, most of the odorant receptors are classified into four groups according to their zonal expression patterns in the olfactory epithelium. Each group of odorant receptors is expressed by sensory neurons distributed within one of four circumscribed zones. Olfactory sensory neurons in a given zone of the epithelium project their axons to the glomeruli in a corresponding zone of the main olfactory bulb. Glomeruli in the same zone tend to represent similar odorant receptors having similar tuning specificity to odorants. Vomeronasal receptors (or pheromone receptors) are classified into two groups in the accessory olfactory system. Each group of receptors is expressed by vomeronasal sensory neurons in either the apical or basal zone of the vomeronasal epithelium. Sensory neurons in the apical zone project their axons to the rostral zone of the accessory olfactory bulb and form synaptic connections with mitral tufted cells belonging to the rostral zone. Signals originated from basal zone sensory neurons are sent to mitral tufted cells in the caudal zone of the accessory olfactory bulb. We discuss functional implications of the zonal organization in both main and accessory olfactory systems.     

Telencephalic sources of the afferents of the main olfactory bulb in the frog Rana temporaria

Following horseradish peroxidase iontophoretic application into the main olfactory bulb (MOB) retrograde neuronal labeling was examined in the telencephalon in the frog. Labeled neurons, the sources of the MOB afferents are found in the mitral cell layer of the contralateral MOB, pallial and some subpallial areas. Very heavy labeling is observed in the pars ventralis of the lateral pallium, and to a lesser extent in the medial pallium, pars dorsalis of the lateral pallium and in the dorsal pallium. In subpallium labeled neurons are found in the eminentia postolfactoria, the rostral part of the medial septal nucleus, and in the nucleus of the ventro-medial telencephalic wall, which is probably homologous to the nucleus of the diagonal band (Broca) of mammals. No labelled neurons were found in the caudal portion of the MOB granular layer, usually referred to as the anterior olfactory nucleus. The arrangement of the MOB centrifugal innervation in amphibians is discussed in comparison with that in mammals.     

Heterogeneous expression of glycoconjugates among individual glomeruli of the hamster main olfactory bulb

Glomeruli within the main olfactory bulb (MOB) are known as areas of synapse formation between axon terminals of olfactory neurons in the olfactory epithelium and dendrites of the first relay neurons (mitral and tufted cells) in the MOB, so that they serve as functional units in olfaction. We examined expression patterns of glycoconjugates in the glomeruli of the hamster MOB by lectin histochemistry using 21 biotinylated lectins. Thirteen lectins, WGA, s-WGA, DSL, DBA, SBA, WA, SJA, RCA-I, PNA, ECL, UEA-I, PSA and LCA, showed differential binding patterns among the glomeruli. To evaluate these differential binding patterns of lectins, we analysed staining intensity of each of the 13 lectins on the level of individual glomeruli by image analysis, and classified staining intensity into five grades (negative, 1+, 2+, 3+, 4+) on the basis of results obtained. This classification enables us to make detailed comparison among individual glomeruli. We further analysed the grade of staining intensity of each of the 13 lectins in the same glomerulus in adjacent serial sections by image analysis, and found that individual glomeruli varied in combination of grades of staining intensity and kinds of lectins. These results indicate that glycoconjugates are expressed heterogeneously in individual glomeruli, and that heterogeneous expression may contribute to the topographic organization of the primary olfactory pathway.     

Structural and immunohistological modifications in olfactory bulb of the staggerer mutant mouse.

In the present study, we describe the structural and cytological changes observed in staggerer mutant olfactory bulbs, as compared to normal mice. On the basis of photonic and ultrastructural observations we tried to define the alterations induced by the mutation: i.e. a reduction of bulb size, a reduction in the volume of three out of the six architectonic layers (glomerular, external and internal plexiform), a reduction of glomeruli size, a loss of half the mitral cells and a slight decrease in juxtaglomerular interneuron number. In staggerer, an hypertrophy of glial ensheathing cell processes was especially evident at the level of each glomerulus, whereas the density of the astrocyte network was weaker in the granular layer and the nerve layer not apparently impaired. An immunofluorescent labelling study combined with confocal scanning microscopy was performed in order to identify the cellular type and the differentiation degree of the various elements. Antibodies anti-GFAP, a protein present in both ensheathing cells and astrocytes, and anti-OMP, the specific maturation protein of the nerve layer, were used for that purpose. Data confirmed the reality of the gliosis and the persistence of the sensory component in the mutant. All the structural alterations described in staggerer olfactory bulb were in close agreement with the functional troubles previously recorded. Our results are discussed in connection with the present knowledge on embryonal origin, fetal development and adult cellular renewal of the olfactory bulb.     

The effects of analgesic supplements on neural activity in the main olfactory bulb of the mouse

We evaluated ketoprofen, a nonsteroidal anti-inflammatory drug (NSAID), as an antinociceptive supplement to chloral hydrate anesthesia in mouse. Effects of ketoprofen on main olfactory bulb (MOB) neuronal spontaneous activity were investigated using extracellular recordings in mouse in vivo. These effects were compared with those of another nociceptive supplement, the mu-opioid agonist buprenorphine. Ketoprofen (100 or 200 mg/kg) did not significantly alter MOB single-unit spontaneous rates in either ICR or C57BL/6J mice. In contrast, buprenorphine, at doses of 0.02, 0.05, and 0.20 mg/kg, inhibited MOB neuronal spontaneous rates by 19%, 49%, and 57%, respectively. Neither drug altered the temporal patterning of single-unit spike trains, as measured by the interspike interval (ISI) coefficient of variation (CV). We also investigated the ability of ketoprofen and buprenorphine to induce antinociception in the anesthetized mouse. The electroencephalogram (EEG) was used to measure the anesthetic plane. Both ketoprofen and buprenorphine altered the EEG trace and ketoprofen altered the power spectrum in a manner consistent with deepening anesthesia. Lastly, when applied at the time of anesthesia induction, ketoprofen decreased the amount of chloral hydrate necessary to maintain a defined anesthetic plane during the rest of the experiment. These results suggest that ketoprofen induces antinociception under chloral hydrate anesthesia without significantly inhibiting spontaneous activity of MOB neurons. Ketoprofen is therefore suitable as an antinociceptive supplement to chloral hydrate anesthesia during in vivo electrophysiologic recordings of the mouse MOB.     

Electrical signaling in the olfactory bulb

The olfactory bulb employs lateral and feedback inhibitory pathways to distribute odor information across parallel assemblies of mitral and granule cells. The pathways involve dendritic action potentials that can interact with a variety of voltage-dependent conductances and synaptic transmission to produce complex and dynamic patterns of activity. Electrical coupling also helps to ensure proper coordination and synchronization of these patterns. These mechanisms provide numerous options for dynamic modulation and control of signaling in the olfactory bulb.     

Inhibition in the fish olfactory bulb

Inhibition in the olfactory bulb of the carp was studied by recording potentials from secondary neurons intracellularly. Three types of inhibition — trace, early, and late — can arise in neurons of the olfactory bulb. Trace inhibition corresponds to hyperpolarization about 20 msec in duration, which is closely connected with the spike, but it is not after-hyperpolarization but an IPSP. Early and late inhibition correspond to IPSPs of different parameters. The first has a latency of 0–50 msec (relative to the spike) and a duration of 60–400 msec; the corresponding values for the second are 100–400 msec and 0.5–3 sec. The possible mechanisms of these types of inhibition are discussed.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 650–656, November–December, 1971.     

Neuronal organization of the accessory olfactory bulb of the lizard Podarcis hispanica: Golgi study

The neuronal organization of the accessory olfactory bulb (AOB), which receives sensory information from the vomeronasal organ, was described in a squamate reptile (Podarcis hispanica) by means of light microscopy. Using the Golgi-impregnation method, seven neuronal types could be distinguished: Periglomerular cells constitute a morphologically heterogeneous population of small neurons located between and around the glomeruli. The mitral cells are diffusely distributed in the AOB. Their cell bodies are usually located within the mitral cell layer, but some of them could be also observed in the plexiform layers. Mitral cells were classified into three subgroups on the basis of their sizes and dendritic tree morphologies. Thus, the “outer mitral cells” have the biggest cell bodies, and their distal secondary dendrites are mainly distributed rostrocaudally in the external plexiform layer. The “inner mitral cells” have large cell bodies, and their secondary dendrites are distributed dorsoventrally and are located deeper than those of the other two subgroups. The third type, the “small mitral cells,” is the smallest one among mitral cells in the AOB, and from their cell bodies, only two main dendritic trunks arise. The granule cells are composed of several categories based on their different cell body locations and dendritic tree morphologies. Thus, the “superficial granule cells” are located exclusively in the external plexiform layer and have small dendritic fields. The “middle granule cells” have fusiform cell bodies—situated in the internal plexiform layer—and present a wide dendritic projection area. Finally, the “deep granule cells” are distributed throughout the granule cell layer and include a great variety of dendritic tree morphologies. The distribution and morphological features of all neuronal types constituting the AOB of Podarcis were compared with those reported on other vertebrates. The results suggest that the lamination pattern and neuronal organization of the AOB in lizards are more similar to that of mammals than to that of the remaining vertebrates.