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PubMed1981

Topographical and laminar localization of 2-deoxyglucose uptake in rat olfactory bulb induced by electrical stimulation of olfactory nerves.

Experiments were carried out to examine the topographical projection of the olfactory nerves to the olfactory bulb in the rat, using the Sokoloff [14C]2-deoxyglucose (2-DG) technique. Electrical stimulation of a medially located bundle of olfactory nerves produced a discrete zone of 2-DG uptake at the rostral pole of the bulb. Increasing stimulus strength yielded a slightly larger focus at this site. In contrast, electrical stimulation of laterally situated bundles of olfactory nerves resulted in a broad zone of activity extending along the lateral wall of the bulb, and increasing stimulus intensity produced a more extensive area of uptake. Laminar analyses provided information on the relation between activity in the glomerular layer, where the olfactory nerves terminate, and activity in deeper layers. The results support previous studies of the topographical projections of the olfactory nerves to the olfactory bulb. They also support the hypothesis that odor-induced 2-DG uptake in the olfactory bulb represents activation of groups of receptors in the olfactory epithelium whose axons terminate in activated glomerular regions in the olfactory bulb.

Animals↗
PubMed1987

The spatial organization of olfactory nerve projections.

The spatial organization of olfactory nerve projections was examined in the rat. The pathway was traced by orthograde transport of HRP following nasal lavage and by retrograde transport of HRP following injections into the olfactory bulb. The results indicated that there was a broad relationship between the epithelium and the olfactory bulb. Specific regions of the olfactory bulb received input from a large region of the epithelium. However, there was evidence that olfactory nerve terminations were not uniformly dense across their terminal fields. The results suggest that there may be finer sorting of olfactory nerves based on functional specificity.

Animals↗
PubMed1991

Olfactory bulb DA receptors may be located on terminals of the olfactory nerve.

The glomerular layer of the olfactory bulb contains a substantial population of dopaminergic neurons. We determined the quantity and location of D1 and D2 dopamine receptors which are the presumed targets of these neurons. Binding of the D1 selective ligand [3H]SCH23390 was slightly above background and was distributed through all layers of the bulb except the olfactory nerve layer. In contrast there were relatively high levels of [3H]spiperone binding to D2 DA receptors in the glomerular and olfactory nerve layers. The presence of relatively high concentrations of D2 DA receptors in both the nerve layer and glomerular layer suggests the novel hypothesis that these receptors may be localized on terminals of the olfactory nerve.

Animals↗
PubMed1981

Neurogenesis in olfactory epithelium: loss and recovery of transepithelial voltage transients following olfactory nerve section.

1. Unilateral olfactory nerve section was performed on the salamander, Ambystoma tigrinum. Physiological recordings and macroscopic observations were made to investigate the physiological correlates of functional recovery in the olfactory epithelium. 2. Slow transepithelial voltage transients, Veog, evoked by several odorous stimuli systematically decreased in amplitude during the initial 7 days and were not recorded at 10 days following nerve section, suggesting retrograde degeneration of receptor neurons. This was true for negative Veog(-), and positive, Veog(+), response components. Responses obtained from the untreated contralateral side of each animal remained similar to nonaxotomized controls. 3. Progressive recovery of the voltage transients was studied at 24, 45, 80, and 100 days following nerve section. At all stages of recovery, the wave form and time course of the responses were characteristic for each stimulus. This suggested that the response properties of the newly differentiated neuronal population were similar to those of the mature population. 4. At 100 days, response amplitudes evoked by all stimuli were similar to control values at all recording sites on the epithelial surface. The simultaneous loss and recovery of positive and negative components of the Veog indicated that the sources of both are dependent on the presence of functionally mature olfactory receptor neurons. 5. Visual inspection indicated that the olfactory nerve was reconstituted and reconnected to the olfactory bulb between 30-60 days following transection. The fact that physiological activity was recorded in the epithelium prior to this event suggests that molecular recognition and sensory transduction are not dependent on connectivity with the olfactory bulb. 6. It is concluded that physiological recovery of the olfactory receptor cell population occurs following axotomy. The time course of recovery was consistent with morphological evidence (see Ref. 57), indicating that newly differentiated receptor neurons are derived from cells in the basal region of the epithelium and replace the population lost through retrograde degeneration.

Ambystoma↗
PubMed1997

Expression of the dm-20 isoform of the plp gene in olfactory nerve ensheathing cells: evidence from developmental studies.

The olfactory bulb is a specialized area of the CNS with well-defined areas containing both myelinated, and non-myelinated axons. The presence of plp gene transcripts has been demonstrated previously in the olfactory bulb, but no detailed description of plp gene activity in this complex area of the CNS is available. In this study we describe the developmental expression of the two plp gene isoforms, plp and dm-20, and their products in the mouse olfactory bulb. plp gene activity was present in the non-myelinated olfactory nerve layer of the bulb from E14 through to adult ages. Expression in the deeper layers of the bulb, associated with myelination of the second order axons, was apparent from P10 onwards. dm-20 was the predominant isoform expressed at the transcript level in the olfactory nerve layer and was the only isoprotein demonstrable by immunostaining. This expression was associated with the resident glial cell of the non-myelinated olfactory nerve layer, the olfactory nerve ensheathing cell. Selective expression of the DM-20 isoprotein in the non-myelin forming olfactory nerve ensheathing cells implies a role for DM-20 other than as a structural myelin protein. Further study of this specialized glial cell may prove useful in elucidating the specific functions(s) of the DM-20 isoprotein.

Animals↗
PubMed2000

Retinoic acid enhances the rate of olfactory recovery after olfactory nerve transection.

In the olfactory system, retinoic acid (RA) plays an important role in development and may affect growth in the adult animal. To explore the potential effects of RA on recovery after injuries, adult mice were trained in a buried food paradigm and were given a single oral supplement of RA after olfactory nerve transection. Results demonstrate that RA accelerates the recovery of olfactory functions after injury.

Animals↗
PubMed2001

Rhinotopy is disrupted during the re-innervation of the olfactory bulb that follows transection of the olfactory nerve.

Re-innervation of the olfactory bulb was investigated after transection of the olfactory nerve using monoclonal antibody RB-8 to assess whether rhinotopy of the primary olfactory projection is restored. In normal animals RB-8 heavily stains the axons, and their terminals, that project from the ventrolateral olfactory epithelium onto glomeruli of the ventrolateral bulb (termed RB-8(+)). In contrast, axons from dorsomedial epithelium are unlabeled (RB-8(-)) and normally terminate in the dorsomedial bulb. Sprague-Dawley rats underwent unilateral olfactory nerve transection and survived for 6 weeks prior to perfusion, sectioning and immunostaining with RB-8. Nerve lesion does not shift the position of the boundary between RB-8(+) and RB-8(-) regions of the epithelium. However, following transection and bulb re-innervation, the distribution of RB-8(+) and RB-8(-) axons is markedly abnormal. First, in all 10 experimental animals RB-8(-) axons displace RB-8(+) axons from anterior glomeruli. Furthermore, the usual target of the RB-8(-) fibers, i.e. the dorsomedial bulb at more posterior levels of the bulb, remains denervated, judging by the lack of staining with antibodies that label axons derived from all epithelial zones. Finally, RB-8(+) fibers invade foreign territory in the dorsolateral bulb on the lesioned side in some cases. The shifts in terminal territory in the bulb after transection contrast with the restoration of the normal zonal patterning of the projection after recovery from methyl bromide lesion, but is consistent with reports of mistargeting by a receptor-defined subset of neurons after transection.

Animals↗
PubMed1996

Activation of locus coeruleus enhances the responses of olfactory bulb mitral cells to weak olfactory nerve input.

The main olfactory bulb (MOB) receives a dense projection from the pontine nucleus locus coeruleus (LC), the largest collection of norepinephrine (NE)-containing cells in the brain. LC is the sole source of NE innervation of MOB. Previous studies of the actions of exogenously applied NE on mitral cells, the principal output neurons of MOB, are contradictory. The effect of synaptically released NE on mitral cell activity is not known, nor is the influence of NE on responses of mitral cells to olfactory nerve inputs. The goal of the present study was to assess the influence of LC activation on spontaneous and olfactory nerve-evoked activity of mitral cells. In methoxyflurane-anesthetized rats, intracoerulear microinfusions of acetyicholine (ACh) (200 mM; 90-120 nl) evoked a four- to fivefold increase in LC neuronal discharge, and a transient EEG desynchronization and decrease in mitral cell discharge. LC activation increased excitatory responses of mitral cells evoked by weak (i.e., perithreshold) nasal epithelium shocks (1.0 Hz) in 17/18 cells (mean Increase = 67%). The discharge rate of mitral cells at the time that epithelium-evoked responses were increased did not differ significantly from pre-LC activation baseline values. Thus, changes in mitral baseline activity do not account for the increased response to epithelium stimulation. These findings suggest that increased activity in LC-NE projections to MOB may enhance detection of relatively weak odors.

Acetylcholine↗
PubMed1974

Isolation and characterization of plasma membrane fractions from garfish Lepisosteus osseus olfactory nerve.

Garfish Lepisosteus osseus olfactory nerve, because of its large size and the unusually high concentration of axonal membrane, is an excellent source of axonal membrane. A procedure is described for the isolation of two types of plasma membranes from the nerve which are obtained in yields of about 20 mg (fraction I) and 1.5 mg (fraction II) per g of wet nerve. Both membrane fractions consist mostly of rounded membrane vesicles, with a unit membrane thickness of approximately 7.5 nm. The two membrane fractions are different in their lipid to protein ratios, Na-K ATPase activities, polypeptide patterns on sodium dodecyl sulfate (SDS) gel electrophoresis, and fatty acid compositions. They have similar phospholipid composition. On the basis of the relative concentration of axonal and Schwann cell plasma membranes in the nerve, the Na-K ATPase activities of the two membrane fractions and a comparison of the properties of the membrane fractions to those of squid and lobster nerve membrane preparations, fraction I seems to be the axonal membrane and fraction II the Schwann cell plasma membrane. Fraction I has a low protein to lipid ratio. Its polypeptide pattern on SDS gel appears to be much more complex as compared to that of fraction II membrane.

Adenosine Triphosphatases↗
PubMed1990

Ultrastructural organization of receptor cell axons in frog olfactory nerve.

Repeated odorant or electrical stimulation of the frog olfactory nerve leads to long-lasting reduction of excitability of the receptor neurons. In the turtle olfactory nerve electrical stimulation causes an increase in extracellular potassium concentration due to the efflux of potassium from active axons. Elevated potassium concentration depolarizes an axon membrane and inactivates it. If axons travel parallel to each other in the nerve for extended distances, changes in ionic concentration due to activity in one axon will reduce excitability of its neighbors. This phenomenon may have effects like those of lateral inhibition among neighbors first described in Limulus and may act as a filter with a long time constant, for the nervous message. The amphibian olfactory nerve consists of densely packed, small-diameter, unmyelinated fibers. In this study we have examined the ultrastructure of the frog (Rana pipiens) olfactory nerve in longitudinal and cross-sections to determine whether axons follow a sinuous course and change neighbors often along the length of the olfactory nerve, or whether they follow parallel trajectories and thus tend to stay close to the same neighbors. We have found that axons have a very straight course within the nerve and that axons tend to remain adjacent to the same individual axons over long distances. We think that the anatomical substrate of the olfactory nerve favors strong inhibitory axon-axon interaction.

Animals↗
PubMed1999

Norepinephrine increases rat mitral cell excitatory responses to weak olfactory nerve input via alpha-1 receptors in vitro.

A rat olfactory bulb in vitro slice preparation was used to investigate the actions of norepinephrine on spontaneous and afferent (olfactory nerve) evoked activity of mitral cells. Single olfactory nerve shocks elicited a characteristic mitral cell response consisting of distinct, early and late spiking components separated by a brief inhibitory epoch. Bath-applied norepinephrine (1 microM) increased the early spiking component elicited by perithreshold (79% increase, P<0.02), but not by suprathreshold (3% decrease, P>0.05), intensity olfactory nerve shocks. The facilitatory effect of norepinephrine was due to a reduction in the incidence of response failures to perithreshold intensity shocks. Norepinephrine also decreased the inhibitory epoch separating the early and late spiking components by 44% (P<0.05). By contrast, norepinephrine had no consistent effect on the spontaneous discharge rate of the mitral cells. The effects of norepinephrine were mimicked by the al receptor agonist phenylephrine (1 microM, P<0.001). Both norepinephrine and phenylephrine modulation of mitral cell responses were blocked by the al adrenergic antagonist WB-4101 (1 microM). These findings are consistent with observations that the main olfactory bulb exhibits the highest density of alpha1 receptors in the brain. The alpha2 receptor agonist clonidine (100 nM) and the beta receptor agonist isoproterenol (1 microM) had inconsistent effects on mitral cell spontaneous and olfactory nerve-evoked activity. These results indicate that norepinephrine increases mitral cell excitatory responses to weak but not strong olfactory nerve inputs in vitro via activation of al receptors. This is consistent with recent findings in vivo that synaptically released norepinephrine preferentially increases mitral cell excitatory responses to weak olfactory nerve inputs. Taken together, these results suggest that the release of norepinephrine in the olfactory bulb may increase the sensitivity of mitral cells to weak odors. Olfactory cues evoke norepinephrine release in the main olfactory bulb, and norepinephrine plays important roles in early olfactory learning and reproductive/maternal behaviors. By increasing mitral cell responses to olfactory nerve input, norepinephrine may play a critical role in modulating olfactory function, including formation and/or recall of specific olfactory memories.

Afferent Pathways↗
PubMed1995

Development of the olfactory nerve: its relationship to the craniofacies.

Although absence of the olfactory bulbs is a relatively common occurrence seen in holoprosencephaly, in Kallman syndrome, and in a number of malformation syndromes, the extent to which it determines olfactory nerve development, as well as the part it plays in the morphogenesis of the nasal structures, is unknown. Cases of arhinencephaly ascertained at autopsy were studied in an effort to better understand the relationships between the olfactory nerve, bulb, and facies. Based on these studies, it is concluded that both olfactory receptor cells and olfactory nerves are present in arhinencephaly, that the olfactory nerves did not make contact with the brain in these cases, that the presence of olfactory nerves is independent of the severity of the central nervous system malformation, and that the shape of the nasal structures is not dependent on the presence of the olfactory nerve.

Animals↗
PubMed1996

Ultrastructural and cytochemical identification of apoptotic cell death accompanying development of the fetal rat olfactory nerve layer.

It has been previously shown that the embryonic olfactory nerve contains, in addition to glial ensheathing cells, a large population of differentiated neurons that migrate from the developing olfactory epithelium, in close association with the olfactory axon fascicles. The purpose of our study was to verify the hypothesis according to which a process of physiological cell death might be involved in the progressive disappearance of these migrating neurons that has been reported during late embryonic stages in several immunocytochemical studies. To do so, we have investigated the development of the olfactory nerve layer in rat embryos by using light and electron microscopy, with special reference to the presence of cell death processes within this structure. We have also applied the histochemical TUNEL method allowing in situ visualization of cells degenerating by apoptosis. In order to determine if neurons were present among dying cells, a procedure of double-labeling was performed by combining the DNA-specific bisbenzimide with two neuronal markers, the protein B-50/GAP-43 and the lectin Ulex europaeus I. Results brought out the precise temporal and spatial patterns of programmed cell death accompanying the morphogenesis of the olfactory nerve layer. A cell death process was observed within the olfactory nerve layer from its onset at embryonic day 13 (E13). While only few pycnotic cells were observed in E13 and E14 embryos, their number increased from E15 to reach a maximum at E16 and then diminished. Few dying cells were also observed along the olfactory axon fascicles when they penetrated the olfactory nerve layer. Degenerating cells appeared strongly TUNEL-labeled and exhibited morphological features of cell death by apoptosis. Double-labeling experiments revealed that some of the apoptotic cells were neurons. These observations indicate that apoptosis may account for the progressive decrease in the number of migrating neurons present within the embryonic olfactory nerve layer. Otherwise, a zone of massive cell death by apoptosis was observed at E14 within the nasal mesenchyme located ventrally and caudally to the olfactory nerve layer. Double-labeling experiments showed that apoptotic cells present within this zone were not neurons. Our findings strongly suggest that apoptotic cell death of migrating neurons may allow the elimination of non-functional cells whereas that of mesenchymal cells may facilitate outgrowth of the newly formed olfactory axon fascicles by pathway formation.

Animals↗
PubMed1989

Early neurogenesis of the mouse olfactory nerve: Golgi and electron microscopic studies.

The early neurogenesis of the mouse olfactory nerve, from its exist at the nasal epithelium to its entrance into the embryonic telencephalon, has been investigated by using the rapid Golgi method and electron microscopy. Previously unrecognized anatomical and possible functional interrelationships between developing olfactory nerve axons and their sheath cells have been observed: 1) at their exit from sensory epithelium (nasal compartment), 2) at their contact with the CNS surface (intracranial compartment), and 3) at their entrance into the embryonic telencephalon (central nervous tissue compartment). Based on these observations the anatomy of the mouse olfactory nerve is herein redefined. Exiting olfactory nerve axons and sheath cells from the same regions of the nasal epithelium establish an early association which is maintained up to their terminal glomerular neuropile. No disruptions have been found in either the olfactory nerve axons or in the continuity of their sheath cells from exit at the nasal epithelium to entrance into the developing olfactory bulb. Corresponding olfactory nerve axons with their sheath cells enter together and become incorporated into the developing olfactory bulb as units. Consequently, the cellular envelope of the olfactory glomerulus must be composed of olfactory sheath cells rather than of glial (astroglial) cells from the CNS. With this simple anatomical arrangement, a topographic map of the sensory epithelium could be established progressively in the developing olfactory bulb. Eventually, "regenerating" olfactory nerve axons from different nasal regions could be guided by their specific sheath cell conduits toward their target glomeruli; hence, the olfactory message may be maintained undisturbed throughout the life span of the animal. In addition, olfactory nerve axons establish synaptic-like contacts with their corresponding sheath cells prior to or during the perforation of the CNS surface. Reciprocal recognition between corresponding axons and their sheath cells at this crucial stage in their neurogenesis may play a significant role in the establishment of their terminal glomerulus. This new concept of the anatomy of the mammalian olfactory nerve should provide insights helpful in clarifying some of the still-unresolved questions regarding the structural and functional organizations of this primitive system.

Animals↗
PubMed1992

Development of olfactory nerve glia defined by a monoclonal antibody specific for Schwann cells.

Although there is considerable interest in the possible role of olfactory glia in the pathfinding abilities of olfactory nerve axons, the complete development of these glia in vivo has not been described. Using a specific Schwann cell marker, the 1E8 antibody, we have found that olfactory nerve glia can be identified throughout development. These glia appear to originate in the olfactory placode and migrate initially into the periphery of the olfactory nerve, and later into the center of the nerve. Olfactory nerve glia enter the presumptive olfactory bulb with the olfactory receptor neuron axons and distribute themselves along the edge of the olfactory nerve layer. The fact that olfactory nerve glia are specifically immunostained by the 1E8 monoclonal antibody, which recognizes the Schwann cell-specific protein P0, suggests that these cells more closely resemble Schwann cells than astrocytes or enteric glia. These results support and extend previous findings suggesting that olfactory nerve glia have distinctive developmental and anatomical features which may be important to the regenerative capacity of the olfactory system.

Aging↗
PubMed1993

Terminal arborizations of olfactory nerve fibers in the glomeruli of the olfactory bulb.

The glomerulus of the olfactory bulb may serve as a fundamental organizational unit for odor representation. In this context, the axons of olfactory receptor cells with similar response spectra may converge in specific glomeruli. While the topography between the olfactory epithelium and the olfactory bulb glomeruli has been explored, the characteristics of primary afferent terminal fields within glomeruli are poorly understood. To explore this issue, reconstructions of the terminal arbors of single olfactory nerve (ON) fibers within glomeruli were carried out in the rat olfactory bulb at the light microscopic level. Tissue samples prepared with the Golgi-EM technique resulted in distinct impregnation of limited subsets of individual ON fibers. Following camera lucida reconstruction, quantitative analyses were made on selected sets of ON fibers and on the glomeruli they invaded. Most ON fibers began to arborize only after penetrating the glomeruli to a mean depth of approximately 35.9 microns. The fibers gave rise to a complex arbor of branches that was limited in total length (mean 157.7 microns) and the number of en passant varicosities or terminal enlargements (mean = 8.1). The number of varicosities and terminal boutons was proportional to the total length of branches. Also, there was a statistically significant correlation between the total branch length and the area (microns2) encompassed by the total arbor. The intraglomerular region supplied by the terminal arbor of an individual ON fiber was generally restricted. Given the recent molecular specificity attributed to olfactory receptor cells, this may provide a morphological basis for selective processing of signals encoded by specific receptors in the olfactory epithelium.

Animals↗
PubMed1956

Olfactory nerve fibers.

Cross sections of olfactory nerves present a unique appearance. They indicate the presence of large numbers of very small nerve fibers, with a modal diameter of about 0.2 micro and a narrow range for their size variation. From one side of the nasal septum of a pig the yield of fibers was estimated at 6,000,000; the number arising from the turbinates would be considerably larger. The fibers are attached to the membranes of the Schwann sheaths in large bundles through mesaxons longer and more branched than those that have been seen in other nerves. Continuity of the axons between the nerves and the bipolar cells was traced in an examination of the olfactory mucous membrane; and the indication of a one-to-one relationship between cells and axons was reinforced by a comparative count. After the axons leave the bipolar cells they become incased in the central projections of the sustentacular cells. Where the latter come into contact with the basal cells the axons emerge to push back the plasma membranes of the basal cells in the first step in acquiring their nerve sheaths. Later steps are described. When the axons are delivered by the basal cells to the collecting Schwann tubes, they are already aggregated into small bundles with sheaths fundamentally the same as those they will possess until they are delivered to the glia in the olfactory bulb. Some of the aspects of the cytology of the bipolar cells and adjoining sustentacular cells are described. A survey of the physiological properties of olfactory nerve fibers was made in some experiments on the olfactory nerve of the pike. Almost all of the action potential is encompassed within a single elevation, manifesting at its front a conduction velocity of 0.2 m./sec. For a comparison, the last elevation in the C action potential in the sciatic nerve of the frog is cited as an example of conduction at the same velocity. Though expressed through long time constants, the properties of the pike olfactory fibers conform to the generalized schema for properties of vertebrate nerve fibers. This conformity signalizes that they differ from the exceptional properties of the unmedullated fibers of dorsal root origin. An afferent function for unmedullated nerve fibers does not imply that the fibers concerned are alike in their physiological properties.

Action Potentials↗