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PubMed1990

Evidence for the Involvement of Rat Olfactory Bulb in Processes Supporting Long-Term Olfactory Memory.

Current advances in the neurobiology of learning and memory suggest the existence of experience-induced plasticity in sensorial pathways conveying relevant information to higher integrative brain structures. For instance, olfactory learning is known to induce long-lasting modifications of neural activity at the level of the first relay structure of the olfactory system, the olfactory bulb. The observed forms of plasticity depend on the action exerted during learning by ascending neuromodulatory systems, such as the noradrenergic (NA) system originating from the locus ceruleus. This study was aimed at investigating the importance of olfactory bulb plasticity in learning and retention of an olfactory task. In a daily training schedule animals had to learn to use multi-site electrical stimulation patterns of the olfactory bulb as discriminative cues for choosing between a palatable and a nonpalatable solution. We first examined the effects of a continuous intrabulbar infusion of propranolol (a beta-NA receptor antagonist) carried out during the learning period. We found that this treatment neither impaired the retention of a previously learned task nor the learning of a new task. However, the animals presented a severe deficit in long-term retention (>5 days) of the task learned under perfusion. Unexpectedly, this effect cannot be ascribed to a selective blockade of beta-NA receptors since infusion of the drug vehicle (saline-ascorbate) produced exactly the same deficit while a saline solution remained without effect. A final experiment showed that the selective deficit in long-term retention was not observed when the infusion of the saline-ascorbate solution started on the day following completion of learning. Taken together, these results suggest that ascorbate-sensitive neural processes occurring within the olfactory bulb during learning are of functional importance for long-term storage of olfactory information.

Journal Article↗
PubMed2002

Sublaminar organization of the mouse olfactory bulb nerve layer.

Olfactory sensory neuron (OSN) axons coalesce to form the olfactory nerve (ON) and then grow from the olfactory epithelium to the olfactory bulb (OB), enter the olfactory nerve layer (ONL), reorganize extensively, and innervate specific glomeruli. Within the ON and ONL a population of glial cells, the olfactory ensheathing cells (OECs), surround OSN axon fascicles. To better understand the relationship between OECs and axon fascicles in the ONL of the adult mouse, we used confocal microscopy and antibodies to the low affinity nerve growth factor receptor p75 (p75), glial fibrillary acidic protein (GFAP), neuropeptide Y (NPY), and S-100 to identify glia. Antibodies to olfactory marker protein (OMP) and neuronal cell adhesion molecule (NCAM) were used to identify OSN axons. Electron microscopy characterized the ONL ultrastructure. We found that glial processes were not uniformly distributed in the ONL of the mouse. The p75(+) OEC processes were restricted to the ON and the outer ONL sublamina, and oriented parallel to the plane of the OB layers. In the inner ONL NPY(+) OEC-like processes were seen. GFAP(+) processes were restricted to the inner ONL sublamina, the ONL/GL boundary, and the GL, where they delineated loosely aggregated axon fascicles that entered the glomeruli obliquely. S-100(+) processes and somata were distributed throughout the ONL; the outer and inner ONL were equivalent in their S-100 staining. Ultrastructural studies showed that, although OECs could be identified in both the outer and inner ONL, in the latter, their relationship to bundles of OEC axons appeared less orderly than seen in the outer ONL. Our data demonstrate a differential organization of the ONL that could subserve distinct functions; axon extension may occur predominantly in the outermost ONL, whereas glomerular targeting occurs in the inner sublamina of the ONL.

Animals↗
PubMed2004

Characterization of the synaptic properties of olfactory bulb projections.

The olfactory bulb directly projects to several diverse telencephalic structures, but, to date, few studies have investigated the physiological characteristics of most of these areas. As an initial step towards understanding the odor processing functions of these secondary olfactory structures, we recorded evoked field potentials in response to lateral olfactory tract stimulation in vivo in urethane-anesthetized Sprague-Dawley rats in the following brain structures: anterior olfactory nucleus, ventral and dorsal tenia tecta, olfactory tubercle, anterior and posterior piriform cortex, the anterior cortical nucleus of the amygdala, and lateral entorhinal cortex. Using paired-pulse stimulation with interpulse intervals of 25-1000 ms, we observed facilitation of the response to the second pulse in every structure examined, although the degree of facilitation varied among the target structures. Additionally, pulse train stimulation at three different frequencies (40, 10 and 2 Hz) produced facilitation of evoked field potentials that also varied among target structures. We discuss the potential utility of such short-term facilitation in olfactory processing.

Animals↗
PubMed2003

Nitric oxide synthase containing periglomerular cells are GABAergic in the rat olfactory bulb.

In the olfactory glomeruli of the rat olfactory bulb, there is a population of periglomerular cells (PG) that contains the neuronal isoform of the nitric oxide synthase (nNOS). To date, these PG have not been characterized neurochemically and it has not been determined whether they are type 1 (GABAergic PG that receive synaptic contacts from the olfactory axons) or type 2 PG (non-GABAergic PG that do not receive synapses from the olfactory axons). Combining pre-embedding NADPH-diaphorase histochemistry and post-embedding immunoperoxidase detection of GABA, we demonstrate that nNOS-containing PG are GABAergic and therefore, belong to the type 1 PG. The possible actions of nitric oxide in the olfactory glomeruli are discussed.

Animals↗
PubMed1996

Transplanted glial scar impedes olfactory bulb reinnervation.

The olfactory system is the only region of the mammalian central nervous system in which degeneration of the primary sensory neurons results in the development of new neurons and reinnervation of the secondary sensory neurons. Axotomy of the olfactory nerve at the cribriform plate does not cause the formation of a glial scar which blocks nerve regeneration. The purpose of this study was to determine whether a glial scar that formed in the optic nerve would suppress axonal regeneration when transplanted to the site of olfactory nerve axotomy. Primary olfactory neurons were axotomized along the cribriform plate in adult rats. A compact glial scar formed by transection of an adult rat optic nerve 50 to 60 days prior to removal was transplanted into the olfactory nerve axotomy site. The rats were allowed to survive for 1, 2, 3, or 4 weeks. Transport of horseradish peroxidase (HRP) from the nasal cavity by the olfactory neurons was used to examine the temporal and spatial pattern of regeneration of the olfactory nerve after axotomy and axotomy followed by glial scar transplantation. Twenty-one days after olfactory nerve axotomy, HRP was found in the glomerular layer where the primary olfactory axons synapse on the apical dendrites of the secondary olfactory neurons. In the presence of the transplanted glial scar, HRP labeling was not found in certain glomeruli even at 4 weeks postaxotomy. Glial scars formed within the optic nerve impede reinnervation of the olfactory bulb by neurons which have an exceptional regenerative capacity due in part to the ensheathing glia.

Age Factors↗
PubMed1993

Purification of olfactory nerve ensheathing cells from the olfactory bulb.

Cells of the olfactory nerve layer of the olfactory bulb are labeled with the O4 antibody, which also labels Schwann cells, oligodendrocytes, and many oligodendrocyte-type-2 astrocyte (O-2A) progenitors. Purification of cells which were O4+, but did not express galactocerebroside (GC), from spinal cord, hippocampus, corpus callosum, and cerebellum of perinatal rats always yielded cells with the expected characteristics of O-2A progenitors. In contrast, the great majority of O4+GC- cells isolated from the olfactory bulb expressed an antigenic phenotype and pattern of differentiation in vitro which did not correspond with that of any previously described glial populations. As the localization of O4 staining in the olfactory bulb indicates that these cells are olfactory nerve ensheathing cells (ONECs), our results therefore suggest that ONECs represent a novel glial cell-type which can now be analyzed as a purified population.

Animals↗
PubMed2002

Trigeminal collaterals in the nasal epithelium and olfactory bulb: a potential route for direct modulation of olfactory information by trigeminal stimuli.

The nasal epithelium is richly invested with peptidergic (substance P and calcitonin gene-related peptide [CGRP]) trigeminal polymodal nociceptors, which respond to numerous odorants as well as irritants. Peptidergic trigeminal sensory fibers also enter the glomerular layer of the olfactory bulb. To test whether the trigeminal fibers in the olfactory bulb are collaterals of the epithelial trigeminal fibers, we utilized dual retrograde labeling techniques in rats to identify the trigeminal ganglion cells innervating each of these territories. Nuclear Yellow was injected into the dorsal nasal epithelium, and True Blue was injected into the olfactory bulb of the same side. Following a survival period of 3-7 days, the trigeminal ganglion contained double-labeled, small (11.8 x 8.0 microm), ellipsoid ganglion cells within the ethmoid nerve region of the ganglion. Tracer injections into the spinal trigeminal complex established that these branched trigeminal ganglion cells also extended an axon into the brainstem. These results indicate that some trigeminal ganglion cells with sensory endings in the nasal epithelium also have branches reaching directly into both the olfactory bulb and the spinal trigeminal complex. These trigeminal ganglion cells are unique among primary sensory neurons in having two branches entering the central nervous system at widely distant points. Furthermore, the collateral innervation of the epithelium and bulb may provide an avenue whereby nasal irritants could affect processing of coincident olfactory stimuli.

Animals↗
PubMed2005

Effects of functional group position on spatial representations of aliphatic odorants in the rat olfactory bulb.

Principles of olfactory coding can be clarified by studying the olfactory bulb activity patterns that are evoked by odorants differing systematically in chemical structure. In the present study, we used series of aliphatic esters, ketones, and alcohols (27 odorants total) to determine the effects of functional group position on glomerular-layer activity patterns. These patterns were measured as uptake of [(14)C]2-deoxyglucose and were mapped into standardized data matrices for statistical comparison across different rats. The magnitude of the effect of position differed greatly for the different functional groups. For ketones, there was little or no effect of position on evoked patterns. For esters, uptake in individual glomerular modules increased, whereas uptake in others decreased with changing group position, yet the overall patterns remained similar. For alcohols, group position had a profound effect on evoked activity patterns. For example, moving the hydroxyl group in either heptanol or nonanol from the first to the fourth carbon changed the activity patterns so greatly that the major areas of response did not overlap. Within every functional group series, however, responses were globally chemotopic, such that pairs of odorants with the smallest difference in functional group position evoked the most similar patterns. These results help to define further the specificities of glomeruli within previously described glomerular modules, and they show that functional group position can be an important feature in encoding an odorant molecule.

Alcohols↗
PubMed1987

Number of mitral cells and the bulb volume in the aging human olfactory bulb: a quantitative morphological study.

The present study documents the morphological changes in the aging human olfactory bulb. Eight bulb pairs from white females between the ages of 25 and 102 years were used. The number of mitral cells in layers IV and III was determined for each bulb and corrected for split cell error. Counts were made on 10-micron thick Nissl-stained sections at 250-micron intervals. The mean number of mitral cells per olfactory bulb at age 25 was estimated from linear regression to be 50,935; at age 60, 32,718; and at age 95, 14,501. The average loss over the time interval studied was 520 mitral cells per year. The volume of each bulb layer, except layer IV, was determined. The difference in the volume of each layer within individuals with age and the total volume with age was not significant (P less than 0.11). The estimated mean bulb volume was found to be 50.02, 43.35, and 36.68 mm3 at ages 25, 60, and 95 years, respectively. The estimated reduction in bulb volume per year increase of age was 0.19 mm3. The ratio of mitral cells to bulb volume for layer III decreased by 19.4 units for every year increase in age. No significant difference was found between the left and the right bulbs in regard to the number of mitral cells and the bulb volume. Histologically, the glomerular layer thickness as well as the mitral cell size and concentration per unit area decreased with age. The intrabulbar anterior olfactory nucleus was discontinuous, highly variable in size, and presumably variable even in neuronal numbers.

Adult↗
PubMed1993

Local inhibition of nitric oxide synthase activity in the accessory olfactory bulb does not prevent the formation of an olfactory memory in mice.

The putative intercellular transmitter nitric oxide has been suggested to play a role in synaptic plasticity in several models of learning and memory. We have investigated the cellular localisation of nitric oxide synthase in the accessory olfactory bulb of the mouse, using immunohistochemistry and NADPH diaphorase histochemistry. The strikingly high levels of nitric oxide synthase observed in the accessory olfactory bulb were found to be due almost exclusively to its localisation in granule cell interneurons. In mice the accessory olfactory bulb has been proposed as the site of synaptic changes occurring during the formation of an olfactory memory to male pheromones. In an attempt to disrupt the formation of this olfactory memory, we used local infusions of the nitric oxide synthase inhibitor L-NG-nitroarginine, into the olfactory bulb over the critical period for memory formation. Infusions of L-NG-nitroarginine at doses that effectively inhibited nitric oxide synthase activity did not prevent memory formation. The apparent resistance of this memory to inhibition of nitric oxide synthase activity may reflect the special nature of the mitral cell to granule cell reciprocal synapse in the accessory olfactory bulb.

Amino Acid Oxidoreductases↗
PubMed1999

Errors in lamina growth of primary olfactory axons in the rat and mouse olfactory bulb.

In the adult olfactory nerve pathway of rodents, each primary olfactory axon forms a terminal arbor in a single glomerulus in the olfactory bulb. During development, axons are believed to project directly to and terminate precisely within a glomerulus without any exuberant growth or mistargeting. To gain insight into mechanisms underlying this process, the trajectories of primary olfactory axons during glomerular formation were studied in the neonatal period. Histochemical staining of mouse olfactory bulb sections with the lectin Dolichos biflorus-agglutinin revealed that many olfactory axons overshoot the glomerular layer and course into the deeper laminae of the bulb in the early postnatal period. Single primary olfactory axons were anterogradely labelled either with the lipophilic carbocyanine dye, 1,1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), or with horse-radish peroxidase (HRP) by localized microinjections into the nerve fiber layer of the rat olfactory bulb. Five distinct trajectories of primary olfactory axons were observed in DiI-labelled preparations at postnatal day 1.5 (P1.5). Axons either coursed directly to and terminated specifically within a glomerulus, branched before terminating in a glomerulus, bypassed glomeruli and entered the underlying external plexiform layer, passed through the glomerular layer with side branches into glomeruli, or branched into more than one glomerulus. HRP-labelled axon arbors from eight postnatal ages were reconstructed by camera lucida and were used to determine arbor length, arbor area, and arbor branch number. Whereas primary olfactory axons display errors in laminar targeting in the mammalian olfactory bulb, axon arbors typically achieve their adult morphology without exuberant growth. Many olfactory axons appear not to recognize appropriate cues to terminate within the glomerular layer during the early postnatal period. However, primary olfactory axons exhibit precise targeting in the glomerular layer after P5.5, indicating temporal differences in either the presence of guidance cues or the ability of axons to respond to these cues.

Aging↗
PubMed1985

Influence of olfactory bulb on dendritic knob density of rat olfactory receptor neurons in vitro.

The maturation of olfactory receptor cells is facilitated by the presence of their target tissue, the olfactory bulb. Organ cultures of embryonic rat olfactory mucosa (OM) maintained in the presence of the presumptive olfactory bulb (POB) had a significantly higher (1.6 X) density of ciliated dendritic knobs than those without the POB. No significant difference was found in the density of non-ciliated dendritic knobs and total knob density in these two groups of cultures. A control group of explants in which the OM and POB had been separated and recombined also showed an increased ciliated dendritic knob density. The area of the olfactory epithelium was the same whether or not the POB was present. In addition, scanning electron microscopy observations revealed a high degree of heterogeneity in the surface morphology of the olfactory epithelium.

Animals↗
PubMed2001

Electrophysiology of interneurons in the glomerular layer of the rat olfactory bulb.

In the mammalian olfactory bulb, glomeruli are surrounded by a heterogeneous population of interneurons called juxtaglomerular neurons. As they receive direct input from olfactory receptor neurons and connect with mitral cells, they are involved in the initial stages of olfactory information processing, but little is known about their detailed physiological properties. Using whole cell patch-clamp techniques, we recorded from juxtaglomerular neurons in rat olfactory bulb slices. Based on their response to depolarizing pulses, juxtaglomerular neurons could be divided into two physiological classes: bursting and standard firing. When depolarized, the standard firing neurons exhibited a range of responses: accommodating, nonaccommodating, irregular firing, and delayed to firing patterns of action potentials. Although the firing pattern was not rigorously predictive of a particular neuronal morphology, most short axon cells fired accommodating trains of action potentials, while most delayed to firing cells were external tufted cells. In contrast to the standard firing neurons, bursting neurons produced a calcium-channel-dependent low-threshold spike when depolarized either by current injection or by spontaneous or evoked postsynaptic potentials. Bursting neurons also could oscillate spontaneously. Most bursting cells were either periglomerular cells or external tufted cells. Based on their mode of firing and placement in the bulb circuit, these bursting cells are well situated to drive synchronous oscillations in the olfactory bulb.

2-Amino-5-phosphonovalerate↗
PubMed2006

Differential effects of unilateral olfactory deprivation on noradrenergic and cholinergic systems in the main olfactory bulb of the rat.

The lack of environmental olfactory stimulation produced by sensory deprivation causes significant changes in the deprived olfactory bulb. Olfactory transmission in the main olfactory bulb (MOB) is strongly modulated by centrifugal systems. The present report examines the effects of unilateral deprivation on the noradrenergic and cholinergic centrifugal systems innervating the MOB. The morphology, distribution, and density of positive axons were studied in the MOBs of control and deprived rats, using dopamine-beta-hydroxylase (DBH)-immunohistochemistry and acetylcholinesterase (AChE) histochemistry in serial sections. Catecholamine content was compared among the different groups of MOBs (control, contralateral, and ipsilateral to the deprivation) using high-performance liquid chromatography analysis. Sensory deprivation revealed that the noradrenergic system developed adaptive plastic changes after olfactory deprivation, including important modifications in its fiber density and distribution, while no differences in cholinergic innervation were observed under the same conditions. The noradrenergic system underwent an important alteration in the glomerular layer, in which some glomeruli showed a dense noradrenergic innervation that was not detected in control animals. The DBH-positive glomeruli with the highest noradrenergic fiber density were compared with AChE-stained sections and it was observed that the strongly noradrenergic-innervated glomeruli were always atypical glomeruli (characterized by their strong degree of cholinergic innervation). In addition to the morphological findings, our biochemical data revealed that olfactory deprivation caused a decrease in the content of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid in the ipsilateral MOB in comparison to the contralateral and control MOBs, together with an increase in noradrenaline levels in both the ipsilateral and contralateral MOBs. Our results show that regulation of the noradrenergic centrifugal system in the MOB depends on environmental olfactory stimulation and that it is highly reactive to sensory deprivation. By contrast, the cholinergic system is fairly stable and does not exhibit clear changes after the loss of sensory inputs.

3,4-Dihydroxyphenylacetic Acid↗
PubMedDate not supplied

A surgical approach to the canine olfactory bulb for meningioma removal.

Olfactory bulb lesions were diagnosed in four dogs presented for generalized seizure disorders. Surgery was performed on each dog using a transfrontal craniotomy. A free fascial-fat graft was used to cover the dural defect resulting from surgery. No major complications were observed during the immediate postoperative period. The histopathologic diagnosis in each case was meningioma. Generalized seizures recurred in all dogs, and three dogs were euthanized for this reason from 9 to 29 weeks postoperatively. One dog was euthanized 12 weeks after surgery due to pancreatitis and pneumonia. Necropsy showed that two dogs had recurrent olfactory bulb meningiomas, one dog had a meningioma of the opposite olfactory bulb, and one dog was tumor free.

Adipose Tissue↗
PubMed1984

Topographic organization of connections between the main olfactory bulb and pars externa of the anterior olfactory nucleus in the hamster.

The organization of connections between the main olfactory bulb (MOB) and pars externa (pE) of the anterior olfactory nucleus was studied in hamsters by using wheat germ agglutinin-horseradish peroxidase as both an anterograde and a retrograde neuronal tracer. Bulbar efferents of pE project exclusively to the contralateral MOB. A topographic organization is evident in these efferents, such that distinct sectors of pE project predominantly to certain sectors of the contralateral MOB and lightly to other sectors. The predominant projection to any bulbar sector in the coronal plane is repeated at nearly all rostral-caudal levels, i.e., the pE efferents to the contralateral MOB terminate within long strips or wedges that show a sector-to-sector topographic organization with respect to the medial-lateral and dorsal-ventral axes but not the rostral-caudal axis of the MOB. Afferents to pE arising in the ipsilateral MOB also show a sector-to-sector topographic organization. Injections into restricted sectors along the circumference of pE label all classes of output neurons (mitral cells and internal, middle, and external tufted cells) in restricted sectors of the ipsilateral MOB, and the sectors that have retrograde neuronal labeling are homotopic to those in the contralateral MOB that have dense anterograde terminal labeling. External tufted cells are not labeled and the other classes of MOB output neurons do not have prominent topographic patterns of labeling in cases with injections caudal to pE. The somata of external tufted cell that project to pE are predominantly in the deep part of the glomerular layer; most of the external tufted cells that lie more superficially in the glomerular layer do not appear to have projections extrinsic to the MOB. These results indicate that both the afferent and efferent connections of pE with the MOB are topographically organized and provide a short synaptic pathway between homotopic sectors of the two main olfactory bulbs.

Animals↗
PubMed2005

Reduced olfactory bulb volume in post-traumatic and post-infectious olfactory dysfunction.

The olfactory bulb is a highly plastic structure the volume of which partly reflects the degree of afferent neural activity. In this study, 22 patients with post-infectious olfactory deficit, nine participants with post-traumatic olfactory deficit, and 17 healthy controls underwent magnetic resonance volumetry of the olfactory bulb. Patients presented with significantly smaller olfactory bulb volumes than controls; significant correlations between olfactory function and bulb volume were observed. Patients with parosmia exhibited smaller olfactory bulb volumes than those without parosmia. Findings indicate that smell deficits leading to a reduced sensory input to the olfactory bulb result in structural changes at the level of the bulb. Reduced olfactory bulb volumes may also be considered to be characteristic of parosmia.

Adult↗