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PubMed1989

Long-distance fiber outgrowth from the heterotopically transplanted olfactory bulb in the rat.

An embryonic olfactory bulb was heterotopically inserted and allowed to mature in young adult rat brains. The projection of the transplanted olfactory bulb to the host brain was examined by injections of peroxidase-labeled wheatgerm agglutinin into the host olfactory bulb (and anterior olfactory nucleus). Neurite elongation to the host olfactory area occurred most frequently from the transplant which had been inserted into the anterior horn of the lateral ventricle and fused medially with the lateral septum in host brains with no detectable damage of host olfactory connections. Transplants in the septum, olfactory tubercle, nucleus of the horizontal limb of the diagonal band, or anterior piriform cortex also showed the projection to the host olfactory area. These results indicate that the transplanted olfactory bulb projection neurons have potent abilities to detect the target and project to it even if there is a considerable distance (2-5 mm).

Animals↗
PubMed1998

Deafferentation up-regulates the expression of the mGlu1a metabotropic glutamate receptor protein in the olfactory bulb.

Chemical lesion of olfactory neuroepitheium induced an up-regulation of the mGlu1a metabotropic glutamate receptor protein in the olfactory bulb, as shown by Western blot analysis. At 2 days after the lesion, the increase in the receptor protein was associated with an increase in mGlu1a mRNA levels; in contrast, at longer times after the lesion (16 days), mRNA levels were reduced in spite of the high expression of the receptor protein, perhaps as a result of product-inhibition of mGlu1 gene expression. Immunohistochemical analysis showed that the increase in mGlu1a induced by olfactory denervation was confined to the glomeruli, which occupy the external portion of the olfactory bulb. Within these structures, mGlu1a receptors are mainly localized on the distal dendrites of mitral cells, which are innervated by the glutamatergic axons of the olfactory nerve. These results demonstrate that the expression of mGlu1a receptors is up-regulated in response to glutamatergic deafferentation, supporting a role for this particular receptor subtype in the physiology of synaptic transmission.

Amino Acid Sequence↗
PubMed1976

Fiber trajectories of olfactory bulb efferents in the hamster.

Olfactory bulb efferents sweep caudally over the surface of the piriform lobe in a broad fiber sheet. The internal organization of this axon population was analysed by topologically transforming the cortical surface from its in situ cylindrical form into an unrolled (flattened) map. The distribution of degeneration elicited by restricted bulb lesions and fiber transections was then reconstructed onto this map. Most of the projection cortex of the main olfactory bulb is innervated in a widespread, non-topographic manner by axons that collect in the compact bundle of the lateral olfactory tract (LOT). LOT collateral branches bound for the prepiriform cortex veer laterally off the main trajectory of the tract at an angle of 50 degrees or less. Thus, transection of discrete fiber populations leaves only a small wedge-shaped pocket of totally denervated cortex distal to the cut. The medial half of the olfactory tubercle and the hippocampal rudiment receive their bulbar input along medially disposed fibers that do not join the LOT proper. The lateral half of the olfactory tubercle, however, receives an input from LOT fiber collaterals as well as these medial bulb efferents. Finally, much of the corticomedial amygdaloid complex receives fibers from the accessory olfactory bulb along a specialized subdivision of the LOT, the accessory olfactory tract. These observations are expressed in a schematic summary of the trajectories of olfactory bulb efferents as they appear in the unrolled map and in the more standard ventral view of the hamster brain.

Animals↗
PubMed2003

Dendrodendritic inhibition and simulated odor responses in a detailed olfactory bulb network model.

In the olfactory bulb, both the spatial distribution and the temporal structure of neuronal activity appear to be important for processing odor information, but it is currently impossible to measure both of these simultaneously with high resolution and in all layers of the bulb. We have developed a biologically realistic model of the mammalian olfactory bulb, incorporating the mitral and granule cells and the dendrodendritic synapses between them, which allows us to observe the network behavior in detail. The cell models were based on previously published work. The attributes of the synapses were obtained from the literature. The pattern of synaptic connections was based on the limited experimental data in the literature on the statistics of connections between neurons in the bulb. The results of simulation experiments with electrical stimulation agree closely in most details with published experimental data. This gives confidence that the model is capturing features of network interactions in the real olfactory bulb. The model predicts that the time course of dendrodendritic inhibition is dependent on the network connectivity as well as on the intrinsic parameters of the synapses. In response to simulated odor stimulation, strongly activated mitral cells tend to suppress neighboring cells, the mitral cells readily synchronize their firing, and increasing the stimulus intensity increases the degree of synchronization. Preliminary experiments suggest that slow temporal changes in the degree of synchronization are more useful in distinguishing between very similar odorants than is the spatial distribution of mean firing rate.

Dendrites↗
PubMed2003

GABA(B) receptors inhibit dendrodendritic transmission in the rat olfactory bulb.

In the mammalian olfactory bulb, mitral cell dendrites release glutamate onto the dendritic spines of granule cells, which in turn release GABA back onto mitral dendrites. This local synaptic circuit forms the basis for reciprocal dendrodendritic inhibition mediated by ionotropic GABA(A) receptors in mitral cells. Surprisingly little is known about neurotransmitter modulation of dendrodendritic signaling in the olfactory bulb. In this study, we examine whether metabotropic GABA(B) receptors modulate dendrodendritic signaling between mitral and granule cells. We find that the selective GABA(B) agonist baclofen reduces mitral cell recurrent inhibition mediated by dendrodendritic synapses. GABA(B) receptor activation causes only a weak inhibition of field EPSCs in the external plexiform layer and only slightly reduces glutamate-mediated mitral cell self-excitation. Although GABA(B) receptors depress mitral cell glutamate release only weakly, baclofen causes a marked reduction in the amplitude of granule-cell-evoked, GABA(A)-mediated IPSCs in mitral cells. In addition to reducing the amplitude of granule-cell-evoked IPSCs, baclofen causes a change from paired-pulse depression to paired-pulse facilitation, suggesting that GABA(B) receptors modulate GABA release from granule cells. To explore the mechanism of action of GABA(B) receptors further, we show that baclofen inhibits high-voltage-activated calcium currents in granule cells. Together, these findings suggest that GABA(B) receptors modulate dendrodendritic inhibition primarily by inhibiting granule cell calcium channels and reducing the release of GABA. Furthermore, we show that endogenous GABA regulates the strength of dendrodendritic inhibition via the activation of GABA(B) autoreceptors.

Animals↗
PubMed1991

Serial reconstructions of granule cell spines in the mammalian olfactory bulb.

The morphology of olfactory bulb granule cell spines and their dendrodendritic synaptic relations with mitral and tufted cell dendrites were examined using serial electron micrographs and 3D computer reconstructions. Most granule cell spines were pedunculated with large elliptical heads and necks (stems) longer than those described for exclusively postsynaptic spines elsewhere in the nervous system. The spines typically contained a mitochondrion, which most likely reflects the metabolic requirements of the presynaptic functions of these spines. In several cases multiple spine heads were observed connected to the parent dendritic trunk via a common neck. In addition, dendritic varicosities making synaptic connections were noted. In the data set sampled, all of the reconstructions supported the hypothesis of divergence of granule cell connectivity: in no instance was a granule cell found to contact repeatedly the same mitral or tufted cell dendrite. Examination of the topological organization of reciprocal dendrodendritic synaptic connections with mitral/tufted cell dendrites revealed parallel rows of spine heads on mitral/tufted secondary dendrites separated by intervening zones of several microns in which no synaptic appositions were found. The results provide evidence regarding rules of connectivity underlying the function of local circuits in mediating lateral inhibition in the external plexiform layer of the olfactory bulb.

Animals↗
PubMed2002

Novel microglomerular structures in the olfactory bulb of mice.

The murine olfactory system consists of two primary divisions: (1) a main olfactory system, in which olfactory sensory neurons (OSNs) located in the main olfactory epithelium (MOE) send their axons to glomeruli in the main olfactory bulb (MOB); and (2) an accessory olfactory system, in which OSNs located in the vomeronasal organ send their axons to glomeruli in the accessory olfactory bulb (AOB). In labeling studies using the lectin Ulex europaeus agglutinin (UEA), we discovered a novel subset of small neuropilar structures in the MOB that are distinct from other glomeruli both in the MOB and AOB. These "microglomeruli" are morphologically similar to MOB glomeruli in many respects: they receive innervation from processes present in the olfactory nerve layer and are isolated from other glomeruli by juxtaglomerular cells; in addition, the compartmental pattern of UEA labeling suggests the presence of UEA (-) processes within their neuropil. Microglomeruli contained processes that express the olfactory marker protein, a marker common to mature OSN axons. However, unlike other glomerular structures, the microglomeruli did not contain neural cell adhesion molecule-labeled processes. Within microglomeruli, UEA(+) processes interdigitated with MAP2(+) dendrites, some of which likely originate from interneurons, as indicated by glutamic acid decarboxylase labeling. Synaptophysin labeling in microglomeruli strongly suggested that synapses occur between UEA(+) processes and dendrites. Anterograde labeling of OSNs, by injection of rhodamine-dextran into one naris, demonstrated that UEA(+) processes in microglomeruli originated in the MOE. The unique morphology, protein expression, and location of microglomeruli have led us to hypothesize that they represent a novel class of glomerular structures in the murine olfactory system.

Animals↗
PubMed1999

Mediation by G protein betagamma subunits of the opioid stimulation of adenylyl cyclase activity in rat olfactory bulb.

In the rat olfactory bulb, activation of opioid receptors enhances basal adenylyl cyclase (EC 4.6.1.1) activity and potentiates enzyme stimulation by Gs-coupled neurotransmitter receptors in a pertussis toxin-sensitive manner. In the present study, we investigated the involvement of G protein betagamma subunits by examining the effects of betagamma scavengers and exogenously added betagamma subunits of transducin (betagamma(t)). The QEHA fragment of type II adenylyl cyclase (50 microM), a peptide that binds to and inactivates betagamma, inhibited the maximal stimulation of adenylyl cyclase activity elicited by Leu-enkephalin (Leu-enk) by about 50%. Similarly, the GDP-bound form of the alpha subunit of transducin (5 nM-1.5 microM), another betagamma scavenger, reduced both the opioid stimulation of basal adenylyl cyclase activity and the potentiation of vasoactive intestinal peptide-stimulated enzyme activity. Under the same experimental conditions, these agents failed to affect the stimulation of the enzyme activity elicited by activation of beta-adrenergic receptors with 1-isoproterenol. Moreover, the addition of betagamma(t)(400 nM) stimulated basal adenylyl cyclase by 80%, and this effect was not additive with that produced by Leu-enk. The data indicate that opioids enhance adenylyl cyclase activity in rat olfactory bulb by promoting the release of betagamma subunits from pertussis toxin-sensitive G proteins Gi/Go.

Adenylyl Cyclase Inhibitors↗
PubMed1995

Presynaptic metabotropic glutamate receptors in adult and developing neurons: autoexcitation in the olfactory bulb.

The mitral cell of the olfactory bulb is the primary relay neuron that transmits information from the olfactory receptors to the rest of the brain. This excitatory neuron releases glutamate from presynaptic dendrites and axon terminals. All rat mitral cells studied showed strong, selective, and widespread metabotropic glutamate receptor mGluR1 alpha immunoreactivity on the presynaptic membrane of dendrites, often at the synaptic vesicle release site, when examined with light and electron microscopy. The finding of glutamate receptors on mitral cell secondary dendrites supports the conclusion that not all dendritic membrane with glutamate receptors necessarily have gray type I asymmetrical synaptic specializations. In contrast, the metabotropic glutamate receptor mGluR5 was not found in mitral cells but was expressed by granule cells and astrocytes around mitral dendrites. Both mGluR1 alpha and mGluR5 were expressed early in development, with strong immunostaining present by postnatal day 1. MGluR1 alpha staining at birth mirrored the adult staining pattern. MGluR5 staining at birth showed different patterns of immunostaining than that found in the adult, particularly in the external plexiform layer. In vitro olfactory bulb neurons and their dendrites from embryonic day (E) 18 olfactory bulbs responded to t-ACPD and quisqualate, selective and nonselective metabotropic glutamate receptor agonists, and to several ionotropic glutamate agonists with increases in intracellular Ca2+ as studied with fura-2 digital imaging. These data indicate that the receptors were functionally active at an early stage of development. Application of the glutamate receptor blockers d-2-amino-5-phosphonovalerate (AP5) and 6-cyano-7-nitroquinoxaline (CNQX) to E17 olfactory bulb neurons after only 4 days in vitro resulted in a dramatic decrease in Ca2+ levels in 70% of 128 cells tested, suggesting that embryonic neurons after a short time in vitro can actively secrete glutamate. The presence of glutamate receptors on the long mitral cell dendrite suggests that it would be able to respond to release of its own excitatory transmitter, probably at an early stage of development. In the probable absence of other excitatory input to the secondary mitral dendrites, it would be the only excitatory "input." This autoexcitatory response would be modulated by release of GABA from olfactory interneurons occurring milliseconds after glutamate release induced by olfactory nerve activation. This novel type of neuronal microcircuitry would potentially amplify signal transmission and current spread along the long mitral dendrites and could play an important role in lateral inhibition of olfactory neurons.

Amino Acid Sequence↗
PubMed1984

The glial cells in the nerve fiber layer of the rat olfactory bulb.

In mammals the olfactory receptor neurons are the only ones that are known to undergo continuous cell renewal in the adult animal. This means that the axon of each newly formed neuron must grow into the olfactory bulb to find its appropriate target cell. It is presumed that astrocytes ensheath the olfactory axons as they course through the nerve fiber layer of the bulb even though the cells in question differ ultrastructurally from typical astrocytes. The purpose of the present study was to examine the glial cells in the nerve fiber layer of the rat olfactory bulb in an effort to resolve this apparent discrepancy. Two morphologically distinct types of glial cell were found in the nerve fiber layer. One type, which resembled the typical astrocytes that are present in other areas of the central nervous system, contained bundles of filaments in an electron-lucent cytoplasm. These cells also formed endfeet on blood vessels and formed part of the external glial limiting membrane. They did not, however, ensheath the olfactory axons. The cytoplasm of the other type of glial cell was denser than that of typical astrocytes and contained fewer filaments, which were seldom grouped into bundles. These cells also formed part of the glial limiting membrane at the surface of the bulb and were the only ones that ensheathed the olfactory axons. It is concluded that the cell ensheathing the olfactory axons in the nerve fiber layer of the rat olfactory bulb is a morphological variant of the typical astrocyte.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2006

Olfactory function and olfactory bulb volume in patients with postinfectious olfactory loss.

OBJECTIVE: The study aimed to investigate whether the degree of postinfectious olfactory loss is reflected in volume of the olfactory bulb (OB). STUDY DESIGN: Retrospective study of 26 patients with postinfectious olfactory loss. MATERIALS AND METHODS: Olfactory function was assessed with the "Sniffin' Sticks" test kit, and the magnetic resonance imaging study focused on OB volume and the olfactory sulcus. RESULTS: The study revealed that 1) OB volume varies with regard to olfactory function, 2) OB volume decreases with duration of olfactory loss, and 3) patients with parosmia had smaller OB volumes than patients who did not report such smell distortions, although their overall olfactory function was not significantly different from each other. CONCLUSION: The study emphasizes that OB volume is a gauge of olfactory function.

Adult↗
PubMed2005

Heterogeneous targeting of centrifugal inputs to the glomerular layer of the main olfactory bulb.

The centrifugal systems innervating the olfactory bulb are important elements in the functional regulation of the olfactory pathway. In this study, the selective innervation of specific glomeruli by serotonergic, noradrenergic and cholinergic centrifugal axons was analyzed. Thus, the morphology, distribution and density of positive axons were studied in the glomerular layer of the main olfactory bulb of the rat, using serotonin-, serotonin transporter- and dopamine-beta-hydroxylase-immunohistochemistry and acetylcholinesterase histochemistry in serial sections. Serotonin-, serotonin transporter-immunostaining and acetylcholinesterase-staining revealed a higher heterogeneity in the glomerular layer of the main olfactory bulb than previously reported. In this sense, four types of glomeruli could be identified according to their serotonergic innervation. The main distinctive feature of these four types of glomeruli was their serotonergic fibre density, although they also differed in their size, morphology and relative position throughout the rostro-caudal main olfactory bulb. In this sense, some specific regions of the glomerular layer were occupied by glomeruli with a particular morphology and a characteristic serotonergic innervation pattern that was consistent from animal to animal. Regarding the cholinergic system, we offer a new subclassification of glomeruli based on the distribution of cholinergic fibres in the glomerular structure. Finally, the serotonergic and cholinergic innervation patterns were compared in the glomerular layer. Sexual differences concerning the density of serotonergic fibres were observed in the atypical glomeruli (characterized by their strong cholinergic innervation). The present report provides new data on the heterogeneity of the centrifugal innervation of the glomerular layer that constitutes the morphological substrate supporting the existence of differential modulatory levels among the entire glomerular population.

Acetylcholine↗
PubMed1982

Light microscope differentiation of two populations of rat olfactory bulb granule cells.

Examination of glycolmethacrylate embedded olfactory bulbs of normal rats revealed that the granule cells of the accessory olfactory bulb were dissimilar from the majority of main olfactory bulb granule cells. Cells from these structures can be discriminated on the basis of spherical form. nuclear diameter, nucleoplasm staining, basal dendritic arborizations, and susceptibility to loss after neonatal X-irradiation. Based on their staining qualities, we have called these cells light and dark granule cells. In the normal rat the dark granules make up about 85% of granule cells in the main olfactory bulb and is the cell type preferentially killed by neonatal X-irradiation. Timing of postnatal X-irradiation and consequent differential loss of the dark type of main olfactory bulb granule cell suggests that the light type is largely prenatally formed.

Animals↗
PubMed1989

Morphometric modifications associated with early sensory experience in the rat olfactory bulb: II. Stereological study of the population of olfactory glomeruli.

The present study explores the local variations of size and number of olfactory glomeruli induced by the exposure of young rats to long-term stimulation with a single odor. Three groups of 5 rats were used that were either: (1) stimulated with ethyl acetoacetate from birth to 1 month of age, (2) unilaterally deprived following early occlusion of one nare, or (3) normal animals of the same age. Areas and coordinates of all glomerular profiles were measured in 14 coronal sections uniformly distributed along the rostrocaudal axis of the olfactory bulb. A distribution-free stereological method was applied to compute the size and number of glomeruli either along the bulbar rostrocaudal extent or in the bulbar coronal plane. Following complete sensory deprivation or long-term stimulation with ethyl acetoacetate, the mean diameter of glomeruli was significantly reduced everywhere, except in the ventrolateral and ventromedial regions of the posterior olfactory bulb in rats reared with a single odor. In both of these areas, the number of glomeruli was either significantly increased following long duration exposure or significantly reduced following unilateral deprivation. Thus these results show that selective modifications of the olfactory environment during postnatal maturation induce morphometric variations in specific areas of the glomerular layer. These data are discussed with respect to the concept of the topographical coding of odor quality at the level of the glomeruli and plasticity of the olfactory system during postnatal development.

Acetoacetates↗
PubMed1995

Localization of 5-HT2A receptor mRNA by in situ hybridization in the olfactory bulb of the postnatal rat.

The olfactory bulb receives a dense serotonergic input and appears to require serotonergic input in early olfactory associational learning. However, it is not known which cell types receive the serotonergic input or whether the cells express markers for the input throughout life. These issues need resolution in order for the mechanisms of serotonergic interactions to be better understood. The mRNA for the 5-HT2A receptor was localized in the olfactory bulb of postnatal day 1, 2, 14 and 9-month-old Sprague-Dawley rats as well as in the bulb of adult (6 months) and aged (22-30 month) Fisher 344 rats by in situ hybridization using an 35S-labelled 5-HT2A- specific oligonucleotide probe mixture. In all animals, hybridization was observed in mitral cells which are the major output cells of the main olfactory bulb. Tufted cells, located in the external plexiform layer and juxtaglomerular region, were readily observed in adult and aged animals and were also observed, albeit not as readily, in neonate pups. Quantitative analysis of the silver grain density over cells confirmed qualitative observations and showed that mitral and tufted cells were labelled in the neonate as well as in adult and aged animals. Labelled cells were also numerous in the external division of the anterior olfactory nucleus in all animals. 5-HT2A receptor mRNA could not be detected either qualitatively or quantitatively by in situ hybridization in the accessory olfactory bulb, nor could it be observed in the olfactory epithelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Age Factors↗