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PubMed1987

Reciprocal functional connections of the olfactory bulbs and other olfactory related areas with the prefrontal cortex.

Reciprocal putative connections of the prefrontal cortex (PFC) (agranular insular, ventral and lateral orbital region) with the ipsi and contralateral main olfactory bulb (IOB; COB), the mediodorsal thalamic nucleus (MD), the basolateral amygdaloid nucleus (BLA) and the piriform cortex (PC) were investigated with electrophysiological techniques. Evoked field responses and orthodromic unit driving, generated in PFC following electrical stimulation of the above mentioned structures, were abolished following topical application of KCl, except for COB evoked mass potentials. Thus, locally generated activity was elicited in agranular insular cortex following IOB activation, the same region where recently, the taste cortex in the rat was localized. Since gustatory-visceral afferent information reaches insular cortex via 2-3 synaptic relays, autonomic, olfactory and gustatory inputs may interact at this level, and, as suggested previously for the mouse, play a key integrative role in flavor perception. Antidromically invaded neurons, 47% of which were identified by the collision-extinction technique, were also found in PFC areas which overlapped to a considerable extent with those from which orthodromic unit responses were obtained. In particular, closely spaced neurons in ventrolateral orbital (VLO) and lateral orbital (LO) regions were antidromically invaded following IOB and PC shocks; some neurons antidromically discharged by IOB were also transsynaptically activated following PC stimulation. These findings are in agreement with recent neuroanatomical studies which demonstrate axonal projections from PFC neurons to the IOB and COB in the rat and South American armadillo. In addition, stimulation of PFC regions dorsal to the rhinal fissure mostly inhibited spontaneous unit discharges recorded at the mitral cell layer of the IOB, suggesting that this effect may be partially mediated by excitatory inputs of prefrontal axons onto granule cells. The conduction properties, antidromic thresholds and activity-dependent variations in conduction velocity (CV) of bulbopetal neurons in prefrontal cortex were found to be similar to those exhibited by cells projecting to the IOB from olfactory peduncle regions, but not to those present in bulbopetal neurons of the horizontal limb of diagonal band, indicating that the OB may be subjected to centrifugal control by at least two cell groups differing in both histochemical and electrophysiological properties.

Afferent Pathways↗
PubMed1984

Comparison of neurogenesis and cell replacement in the hamster olfactory system with and without a target (olfactory bulb).

The olfactory sensory neurons are unique in the vertebrate nervous system in that they are replaced following experimentally induced degeneration. Unilateral removal of the olfactory bulb in hamster results in degeneration of all mature receptor neurons followed by a neurogenesis and partial replacement of the receptor cell population. To determine if full recovery requires the presence of normal target tissue, a study of sensory neuron replacement was made following a nerve transection procedure, which leaves the olfactory bulb (target) intact. A comparison of quantitative measurements of cell number and thickness in the sensory epithelium showed that the presence of the target tissue alone did not result in improved recovery. One possible explanation is that complete recovery requires that axons of newly formed receptors must first re-establish synaptic contact with the olfactory bulb. To test this possibility, it will be necessary to include longer postoperative recovery times than those used in the present study.

Animals↗
PubMed1988

The effect of microinfusions of drugs into the accessory olfactory bulb on the olfactory block to pregnancy.

Female mice which have mated and are subsequently exposed to the odour (pheromones) of a strange male undergo hormonal changes resulting in a block to their pregnancy. The fact that the stud male's odours can also block pregnancies, that is other than his own, implies the formation of a memory or some form of recognition process by the female for this male's pheromones at the time of mating. The purpose of this study was to evaluate the effect of microinfusions of drugs which interfere with neural transmission, into the accessory olfactory bulbs. This was carried out immediately after mating over a 4-h period during which the "memory" to the stud male's pheromones is formed. Infusions of the alpha-blocker, phentolamine, blocked the formation of the olfactory memory, while the GABA receptor blocker, bicuculline, itself blocked pregnancy, but was without effect on memory formation. Protein synthesis inhibition or calpain inactivation in the accessory bulb was without effect on memory formation at any of the doses used. These studies demonstrate that GABAergic transmitter blockade in the accessory olfactory bulb at the time of mating can prevent subsequent blastocyst implantation some 3 days later, while alpha-noradrenergic blockade can prevent the formation of an olfactory memory to the stud male.

Animals↗
PubMed2001

Chemically and morphologically identifiable glomeruli in the rat olfactory bulb.

Primary olfactory neurons that express the same odorant receptor are distributed mosaically throughout the olfactory neuroepithelium lining the nasal cavity, yet their axons converge and form discrete glomeruli in the olfactory bulb. We previously proposed that cell surface carbohydrates mediate the sorting out and selective fasciculation of primary olfactory axons en route to glomeruli. If this were the case, then axons that terminate in the same glomerulus would express the same complement of cell surface carbohydrates. In this study, we examined the expression of a novel carbohydrate (NOC-3) on neural cell adhesion molecule in the adult rat olfactory system. NOC-3 was expressed by a subset of neurons distributed throughout the olfactory neuroepithelium. The axons of these neurons entered the nerve fiber layer and terminated in a subset of glomeruli. It is interesting to note that we identified three unusually large glomeruli in the lateral, ventrolateral, and ventromedial olfactory bulb that were innervated by axons expressing NOC-3. NOC-3-expressing axons sorted out and fasciculated into discrete fascicles prior to entering these glomeruli. Each of these glomeruli was in a topographically fixed position in the olfactory bulbs of the same animal as well as in different animals, and their lengths were approximately 10% of the total length of the bulb. They could be identified reliably by both their topographical position and their unique morphology. These results reveal that axons expressing the same cell surface carbohydrates consistently target the same topographically fixed glomeruli, which supports a role for these molecules in axon navigation in the primary olfactory nerve pathway.

Animals↗
PubMed1990

Excitatory synaptic transmission in cultures of rat olfactory bulb.

1. Olfactory bulb neurons were dissociated from neonatal rats and plated at low density on a confluent layer of olfactory bulb astrocytes. Intracellular stimulation of presumptive mitral/tufted (M/T) cells evoked monosynaptic excitatory postsynaptic potentials (EPSPs) in adjacent neurons. Whole-cell recording techniques and a flow-pipe drug delivery system were used to compare EPSPs with voltage-clamp recordings of currents evoked by excitatory amino acids (EAA) including N-acetylaspartylglutamate (NAAG), a putative mitral cell transmitter. 2. Cultured olfactory bulb neurons were morphologically and physiologically distinct. Large pyramidal-shaped neurons were present, which were NAAG immunoreactive; stimulation of these neurons invariably evoked EPSPs, suggesting that they were M/T cells. The majority of small bipolar neurons were glutamic acid decarboxylase (GAD) immunoreactive consistent with granule or periglomerular gamma-aminobutyric acid (GABA)ergic interneurons. 3. Monosynaptic EPSPs between M/T cells could be separated into fast and slow components by the use of EAA receptor antagonists. A fast component with a time-to-peak of 7.7 +/- 1.0 (SE) ms and half-width of 31.8 +/- 7.4 ms was blocked by the non-NMDA receptor antagonist 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX, 2.5 microM). The slow component (time-to-peak = 41.4 +/- 7.2 ms; half-width = 218.9 +/- 40.4 ms) was blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP5, 100 microM). 4. Under voltage clamp, flow-pipe applications of NAAG (10-1,000 microM) evoked inward currents at a holding potential of -60 mV in Mg-free solutions.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1997

Gamma activity in the piriform cortex and behavioral thresholds for electrical stimulation in the olfactory bulb.

The olfactory bulb was stimulated by trains of electrical pulses in freely moving rats. Evoked responses resembling damped oscillations at the gamma frequency of 30-60 Hz were recorded in the anterior and posterior piriform cortex. Different types of unconditioned sniffing were induced by stimulation of the olfactory bulb. They were similar to those evoked by an odorant (amylacetate) but differed from the behavioral patterns evoked by non-olfactory (auditory) stimulation. Rats were trained to avoid foot-shocks following electrical pulses into the olfactory bulb as conditioned stimulus in a two way shuttle-box paradigm. Threshold electrical intensities for inducing evoked responses in piriform cortex, unconditioned behavior, and learned avoidance were compared. Thresholds for unconditioned and conditioned behavior were significantly higher in comparison with those for evoking gamma discharges in anterior and posterior piriform cortex. The results suggest that fast time-locked synchronization in the gamma range in the piriform cortex induced by synaptic input from theolfactory bulb is not sufficient for inducing corresponding behavior. Thus behavioral detection and probably also olfactory recognition do not seem to be direct consequences of this fast time-locked neural synchronization. Additional neuronal processes that are connected with further elevation of stimulation intensity seem to be necessary for that.

Animals↗
PubMed2005

Activation of noradrenergic transmission by alpha2-adrenoceptor antagonists counteracts deafferentation-induced neuronal death and cell proliferation in the adult mouse olfactory bulb.

The olfactory bulb is the target of neural progenitor cells that are generated in the subventricular zone of the lateral ventricle in the adult brain. This permanent neurogenesis is likely influenced by olfactory input to the bulb since previous studies have shown that cell proliferation and/or apoptotic death are stimulated by naris closure or surgical transection of the olfactory nerve. Since the olfactory bulb is densely innervated by noradrenergic afferents originating in the locus coeruleus, we have studied the impact of pharmacologically activating this noradrenergic system on cell death and proliferation following unilateral olfactory axotomy in the adult mouse olfactory bulb. We found that noradrenaline release in the olfactory bulb was significantly increased by intraperitoneal injections of the selective alpha(2)-adrenoceptor antagonists, dexefaroxan (0.63 mg/kg) and 5-fluoro-methoxyidazoxan (F 14413; 0.16 mg/kg). A chronic treatment with either compound for 7 days following olfactory axotomy significantly reduced neuronal death, glial activation and cell proliferation in the deafferented olfactory bulb. These data (1) confirm that alpha(2)-adrenoceptor antagonists, presumably by facilitating central noradrenergic transmission, afford neuroprotection in vivo, as previously shown in models of cerebral ischemia, excitotoxicity and devascularization-induced neurodegeneration, and (2) support a role of the locus coeruleus noradrenergic system in promoting survival of neurons in areas of the brain where neurogenesis persists in the adult.

Adrenergic alpha-2 Receptor Antagonists↗
PubMed2003

Neuronal migration from the forebrain to the olfactory bulb requires a new attractant persistent in the olfactory bulb.

Interneurons in the olfactory bulb (OB) are generated not only in the developing embryo but also throughout the postnatal life of mammals from neuronal precursor cells migrating from the anterior subventricular zone (SVZa) of the mammalian forebrain. We discovered that the OB secretes a diffusible activity that attracts these neuronal precursor cells. The attractive activity is present in specific layers in the OB, including the glomerular layer but not the granule cell layer. The attractive activity and the neuronal responsiveness persist from embryonic through neonatal to adult stages. Removal of the rostral OB significantly reduces SVZa migration toward the OB, an effect that can be rescued by a transplant of the OB but not by that of the neocortex. The activity in the OB is not mimicked by the known attractants. These results provide an explanation for the continuous migration of SVZa neurons toward the OB, demonstrate an important role of the OB in neuronal migration, and reveal the existence of a new chemoattractant.

Animals↗
PubMed1994

Neuropeptide tyrosine is expressed in ensheathing cells around the olfactory nerves in the rat olfactory bulb.

The olfactory bulbs of young and adult normal rats and of colchicine-treated rats and of some other species were analysed for the presence of neuropeptide Y and neuropeptide Y messenger RNA, using immunohistochemistry at the light- and electron-microscopic levels and with in situ hybridization. In the rat and mouse, but not in monkey and guinea-pig, neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA were observed in ensheathing cells in the olfactory nerve layer of the olfactory bulb and within nerve bundles in the olfactory mucosa. Double staining experiments revealed that neuropeptide Y-like immunoreactivity was often present in a restricted compartment, mainly the Golgi apparatus, of S-100 protein-positive ensheathing cells. After colchicine treatment a different distribution of neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA was observed. Thus, in the outer olfactory nerve layer both neuropeptide Y-like immunoreactivity and neuropeptide Y messenger RNA disappeared, whereas in the inner part messenger RNA levels remained high and neuropeptide Y-like immunoreactivity was observed in many granule-like structures distributed diffusely in the cytoplasm. The present findings suggest that neuropeptide Y may be involved in the control of regeneration, growth and/or guiding of the axons of the olfactory sensory neurons, the only mammalian neurons known to have a continuous renewal and growth during adult life.

Animals↗
PubMed2003

Sorting and convergence of primary olfactory axons are independent of the olfactory bulb.

Primary olfactory axons expressing the same odorant receptor gene sort out and converge to fixed sites in the olfactory bulb. We examined the guidance of axons expressing the P2 odorant receptor when they were challenged with different cellular environments in vivo. In the mutant extratoes mouse, the olfactory bulb is lacking and is replaced by a fibrocellular mass. In these animals, primary olfactory axons form glomerular-like loci despite the absence of normal postsynaptic targets. P2 axons are able to sort out from other axons in this fibrocellular mass and converge to form loci of like axons. The sites of these loci along mediolateral and ventrodorsal axes were highly variable. Similar convergence was observed for larger subpopulations of axons expressing the same cell surface carbohydrates. The sorting out and convergence of like axons also occurred during regeneration following bulbectomy. Olfactory axon behaviour in these models demonstrates that sorting and convergence of axons are independent of the target, which instead provides distinct topographic cues for guidance.

Animals↗
PubMed2003

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.

Action Potentials↗
PubMed1991

Olfactory deprivation increases dopamine D2 receptor density in the rat olfactory bulb.

Unilateral olfactory deprivation during postnatal development results in significant anatomical and neurochemical changes in the deprived olfactory bulb. Perhaps the most dramatic neurochemical change is the loss of dopaminergic expression by neurons of the glomerular region. We describe here the effects of early olfactory deprivation on other elements of the bulb dopaminergic system, namely the dopamine receptors of the olfactory bulb. Rat pups had a single naris occluded on postnatal day 2 (PN2). On PN20 or PN60, animals were sacrificed and the bulbs were examined for catecholamine levels or D2 and D1 dopamine receptor binding. Receptor densities were quantified by in vitro autoradiography using the tritiated antagonists spiperone (D2) and SCH23390 (D1). Dopamine uptake sites were similarly examined using tritiated mazindol. No significant specific labeling of D1 or mazindol sites was observed in the olfactory bulbs of control or experimental animals at either age. Normal animals displayed prominent labeling of D2 sites in the glomerular and nerve layers. After 60 days of deprivation, deprived bulbs exhibited an average increase in D2 receptor density of 32%. As determined by Scatchard analysis, the mean values for Kd and Bmax were 0.134 nM and 293 fmol/mg protein in normal bulbs, and 0.136 nM and 403 fmol/mg protein in deprived bulbs. The results suggest that, as in the neostriatum, dopamine depletion in the olfactory bulb leads to an upregulation of D2 receptor sites. This change may represent an attempt by the system to adapt neurochemically to reduced dopaminergic activity and thereby maintain bulb function.

Aging↗
PubMed2001

Odor maps in the brain: spatial aspects of odor representation in sensory surface and olfactory bulb.

The olfactory sense detects and distinguishes a multitude of different odors. Recent progress in molecular as well as physiological approaches has elucidated basic principles of neuronal encoding of odorants, common to insects and vertebrates. The construction of neuronal representations for odors begins with the task of mapping the multidimensional odor space onto the two-dimensional sensory surface, and subsequently onto the olfactory bulb or antennal lobe. A distributed expression of odorant receptors, albeit restricted to subregions of the sensory surface (large, intermediate or small for zebrafish, mouse or drosophila, respectively), ensures a robust representation, insensitive to mechanical insult. Olfactory receptor neurons expressing the same odorant receptors converge to form a receptotopic map in the olfactory bulb or antennal lobe. The emerging coding principle is a chemotopic representation of odorants at the first brain level, realized either as combinatorial or as monospecific representation, depending on the odorant.

Animals↗
PubMed1999

Compartmental organization of the olfactory bulb glomerulus.

Olfactory receptor cell (ORC) axons terminate in the olfactory bulb glomerular neuropil, where they synapse with dendrites of mitral, tufted, and periglomerular neurons. We investigated the organization of the glomerular neuropil by using antibodies to both single- and double-label constituents for analyses with confocal microscopy. Electron microscopy (EM) was employed to assess the distribution of synaptic appositions within the glomerulus. Adult Sprague-Dawley rats were processed for immunocytochemistry with olfactory marker protein (OMP), synaptophysin, synapsin 1, glial fibrillary acidic protein (GFAP), and/or microtubule-associated protein 2 (MAP2). Equivalent rats were processed for transmission EM. Double labeling for OMP and MAP2 revealed two distinctive subcompartments within glomeruli: an axonal compartment containing predominately primary afferent axons with individual dendritic inserts and a complementary dendritic compartment that excluded primary afferent axons. Areas not occupied by OMP or MAP2 immunoreactivity were either immunoreactive for GFAP, indicating a glial process, or were blood vessels. Synaptophysin and synapsin 1 also showed differential labeling within the glomerulus. Synaptophysin strongly colocalized with OMP, whereas synapsin 1 was associated most strongly with MAP2. Reconstructions of glomeruli from EM montages revealed interdigitating axonal and dendritic subcompartments. The axonal subcompartments were composed primarily of ORC processes with individual or small groups of dendrites interspersed. Dendritic subcompartments were composed predominately of dendritic processes. Primary afferent axodendritic and local-circuit dendrodendritic synapses segregated within the glomerulus into the axonal and dendritic subcompartments, respectively. The results support the hypothesis of subcompartmental organization within olfactory bulb glomeruli.

Animals↗
PubMed2001

Intranasal inoculation with the olfactory bulb line variant of mouse hepatitis virus causes extensive destruction of the olfactory bulb and accelerated turnover of neurons in the olfactory epithelium of mice.

Viral upper respiratory infections are the most common cause of clinical olfactory dysfunction, but the pathogenesis of dysosmia after viral infection is poorly understood. Biopsies of the olfactory mucosa in patients that complain of dysosmia after viral infection fall into two categories: one in which no olfactory epithelium is seen and another in which the epithelium is disordered and populated mainly by immature neurons. We have used intranasal inoculation with an olfactory bulb line variant of MHV to study the consequences of viral infection on peripheral olfactory structures. MHV OBLV has little direct effect on the olfactory epithelium, but causes extensive spongiotic degeneration and destruction of mitral cells and interneurons in the olfactory bulb such that the axonal projection from the bulb via the lateral olfactory tract is markedly reduced. Moreover, surviving mitral cells apparently remain disconnected from the sensory neuron input to the glomerular layer, judging from retrograde labeling studies using Dil. The damage to the bulb indirectly causes a persistent, long-term increase in the turnover of sensory neurons in the epithelium, i.e. the relative proportion of immature to mature sensory neurons and the rate of basal cell proliferation both increase. The changes that develop after inoculation with MHV OBLV closely resemble the disordering of the olfactory epithelium in some patient biopsies. Thus, damage to the olfactory nerve or bulb may contribute to a form of post-viral olfactory dysfunction and MHV OBLV is a useful model for studying the pathogenesis of this form of dysosmia.

Administration, Intranasal↗
PubMed1994

Dentate gyrus and olfactory bulb responses to olfactory and noxious stimulation in urethane anaesthetized rats.

Recent research has shown that olfactory stimuli such as toluene vapor, but not visual, auditory, tactile or gustatory stimuli, elicit a burst of fast waves (15-30 Hz) in the hilus of the dentate gyrus in waking rats. In urethane-anaesthetized rats, toluene odors elicit similar fast waves. The present study shows that noxious stimulation (tail clamp) produces a blockade of spontaneous slow waves (1-12 Hz) in the hilus of the dentate gyrus but does not increase fast wave activity in urethane-anaesthetized rats. This slow wave blockade, which resembles neocortical activation, is independent of olfaction since it is not affected by tracheotomy. In contrast, tracheotomy abolishes the fast wave response to toluene presentation to the snout unless the toluene vapor is drawn into the nasal passages by suction at the rostral end of the severed trachea. Both the toluene odor-induced fast wave and the tail clamp-induced activation responses are abolished by scopolamine hydrobromide (5.0 mg/kg, i.p.) but not by scopolamine methyl bromide (5.0 mg/kg, i.p.) which does not cross the blood-brain barrier. However, evoked potentials elicited in the dentate hilus by single pulse stimulation of the olfactory bulb are not blocked by scopolamine in urethane-anaesthetized rats. The results suggest that several different types of electrical activity in the hippocampal formation are mediated by cholinergic inputs and that the dentate gyrus plays a role in olfaction.

Anesthesia↗
PubMed2004

Spatial patterns of gene expression in the olfactory bulb.

How olfactory sensory neurons converge on spatially invariant glomeruli in the olfactory bulb is largely unknown. In one model, olfactory sensory neurons interact with spatially restricted guidance cues in the bulb that orient and guide them to their target. Identifying differentially expressed molecules in the olfactory bulb has been extremely difficult, however, hindering a molecular analysis of convergence. Here, we describe several such genes that have been identified in a screen that compiled microarray data to create a three-dimensional model of gene expression within the mouse olfactory bulb. The expression patterns of these identified genes form the basis of a nascent spatial map of differential gene expression in the bulb.

Animals↗
PubMed2002

Optical imaging of odor preference memory in the rat olfactory bulb.

Early olfactory preference learning in rat pups occurs when novel odors are paired with reinforcing tactile stimulation that activate the noradrenergic locus coeruleus. Pairing of odor and a noradrenergic agonist in the olfactory bulb is both necessary and sufficient for odor preference learning. This suggests the memory change occurs in the olfactory bulb. Previous electrophysiological experiments demonstrated that odor preference training induces an increase in the field excitatory postsynaptic potential to olfactory nerve input and an alteration, after training, in glomerular [14C]2- deoxyglucose uptake and in single-unit responses of principal cells. We investigate here whether, 24 h after olfactory preference training, there is an alteration in intrinsic optical signals at the glomerular level. Six-day-old rat pups were trained, as previously, for a peppermint odor preference. Trained pups and control littermates were subjected to imaging of odor-induced intrinsic optical signals 1 day after the training session. Trained pups exhibited significantly larger responses to the peppermint compared with untrained littermates previously exposed to the same odor. The response of trained pups to a control odor (amyl acetate) was, however, not significantly different from that of untrained littermates. These observations demonstrate that odor preference memory can be read-out by optical imaging techniques.

Animals↗