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PubMed1999

A genetic approach to visualization of multisynaptic neural pathways using plant lectin transgene.

The wiring patterns among various types of neurons via specific synaptic connections are the basis of functional logic employed by the brain for information processing. This study introduces a powerful method of analyzing the neuronal connectivity patterns by delivering a tracer selectively to specific types of neurons while simultaneously transsynaptically labeling their target neurons. We developed a novel genetic approach introducing cDNA for a plant lectin, wheat germ agglutinin (WGA), as a transgene under the control of specific promoter elements. Using this method, we demonstrate three examples of visualization of specific transsynaptic neural pathways: the mouse cerebellar efferent pathways, the mouse olfactory pathways, and the Drosophila visual pathways. This strategy should greatly facilitate studies on the anatomical and functional organization of the developing and mature nervous system.

Animals↗

PubMed1990

Development of the anterior olfactory nucleus in normal and unilaterally odor deprived rats.

The development of a second order structure in the olfactory pathway, the anterior olfactory nucleus, was examined in both normal rat pups and in subjects which underwent unilateral naris closure on postnatal day 1 (P1). Naris occlusion in neonatal rats produces a constellation of changes within the first relay in the pathway, the olfactory bulb, including a 25% reduction in total volume. Such large changes suggest that higher order structures might also be affected. Anterior olfactory nucleus development was quantified in several ways. Laminar volumes were computed by using serial section planimetry. In control animals differential development was observed, with regions extending most rostrally (e.g., pars externa and pars lateralis) exhibiting the least growth. The anterior olfactory nucleus on the "deprived" side of subjects with a single naris occluded was identical in size to that observed in controls, development within the pars lateralis was examined in control animals at P10, P20, P30, and adults. Developmental increases in numbers of both branches per cell and spines were noted, but mean branch length remained relatively constant. Finally, the effects of naris occlusion on histological patterns of succinate dehydrogenase (SDH) staining and 2-deoxyglucose uptake within pars lateralis were examined at P20 to test for more subtle effects of naris occlusion. SDH staining was quite similar in deprived and control rats at P20. However, 3H-2-DG uptake was decreased in rostral areas of the anterior olfactory nucleus ipsilateral to the deprived olfactory bulb, suggesting that naris closure does affect the structure.

Animals↗

PubMed1998

Spatiotemporal patterns of expression of extracellular matrix molecules in the developing and adult rat olfactory system.

Using immunocytochemical methods, we have examined extensively the spatial and temporal patterns of expression of three extracellular matrix molecules-laminin, fibronectin, and type IV collagen-in the embryonic, postnatal (days 2 and 11) and adult rat olfactory system. The study started at embryonic day 14 when olfactory fibres and their associated migrating cells course through the nasal mesenchyme. From embryonic day 14 to the adult, a sheet-like pattern of labelling for laminin, fibronectin and type IV collagen was observed along the basal surface of the olfactory epithelium and around the telencephalon. This type of labelling was continuous around the telencephalic vesicle, whereas it appeared disrupted in the basal lamina of the olfactory epithelium to permit exit of the olfactory axons and their associated migrating cells into the mesenchyme. From embryonic day 14 to day 20, punctate labelling for the three molecules studied was observed along the mesenchymal olfactory pathway, the ventral part of the olfactory bulb, the olfactory nerve layer and the presumptive glomerular layer, respectively. By embryonic day 17, the punctate labelling initially detected in the mesenchymal olfactory pathway was replaced by a sheet-like pattern related to the mature basal lamina surrounding the olfactory axon fascicles. Punctate labelling for laminin and type IV collagen persisted in the olfactory nerve layer and around the glomeruli through adult life whereas that of fibronectin declined and disappeared by postnatal day 2. The spatiotemporal distribution of the punctate pattern for laminin, fibronectin and type IV collagen observed in the embryonic olfactory system suggests a role in delineating the pathway for olfactory axon elongation. The continuous expression of laminin and type IV collagen in the adult olfactory bulb may be related to the regenerative activity and high plasticity of the olfactory system.

Animals↗

PubMed1999

beta-amyloid deposition and neurofibrillary tangle formation in the olfactory bulb in ageing and Alzheimer's disease.

Impaired olfaction, hyposmia or anosmia are part of the clinical phenotype in neurodegenerative disorders including Alzheimer's disease (AD). It has been proposed that the most severely affected areas are interconnected with the central olfactory system in contrast to the relative sparing of other sensory areas which lack olfactory connections. The pathology of the first synaptic relay in the olfactory pathway, the olfactory bulb (OB), has been studied in AD, but the results have been inconsistent. In order to define more fully the pathology of the OB, we analysed 15 AD and 15 control cases, using amyloid and tau immunohistochemistry on serial sections. This study demonstrates for the first time that all layers of the OB are severely affected in AD and in normal ageing. The principal effector cells of the OB, the mitral cells, developed neurofibrillary tangles (NFTs) both in AD and in controls. All the cases, with the exception of two of the controls, contained NFTs. Amyloid immunoreactivity was detected in diffuse, primitive, classical and compact deposits in AD, while five control cases contained mainly diffuse deposits. We did not find a correlation between amyloid deposition and NFT formation. Among the control cases, two contained neither amyloid nor NFTs, eight had NFTs but no amyloid and only five had both NFTs and amyloid. All the AD cases had NFT and amyloid deposition. Our data suggest that the earlier pathology in the OB is NFT formation and more than ten NFTs/section is compatible with 93.3% diagnostic accuracy for AD.

Aged↗

PubMed2000

Expression of connexin36 mRNA in adult rodent brain.

A new member of the connexin gene family, named Connexin36 (Cx36) has, recently, been identified in rodents and shown to be preferentially, if not exclusively, expressed in neurones of the adult CNS. In this study we present a detailed in situ hybridization analysis of the expression pattern of mouse Connexin 36 (mCx36) mRNA in the adult mouse brain, with particular regards to the correlation of mCx36 expression to specific neuronal cell classes and systems. We found that mCx36 was strongly and widely expressed in the brain, including areas where the presence of gap junctions was never detected before. Quantitative analysis of the hybridization signal indicated varying levels of expression in different areas. In particular mCx36 was highly expressed in the neurones at different levels of the motor pathway, the olfactory pathway, the hippocampus, and areas related to the generation of respiratory rhythm. On the contrary, mCx36 was more heterogeneously expressed in nuclei of the sensory pathways. These findings show that mCx36 is the first connexin specifically expressed in neuronal cells in the adult rodent brain. The profiles of expression clearly indicate that mCx36 might play specific roles within different neuronal systems.

Age Factors↗

PubMed1995

Identification of cells expressing galectin-1, a galactose-binding receptor, in the rat olfactory system.

Interactions between carbohydrate ligands and their receptors play an important role in cell adhesion and migration in many tissues. Cell-surface carbohydrates that contain terminal galactose have previously been implicated in primary sensory axon growth in the rodent olfactory system. The aim of the present study was to determine whether galectin-1, a galactose-binding receptor, was expressed within the rat primary olfactory pathway. Immunohistochemical and in situ hybridisation analyses revealed expression of galectin-1 by primary sensory olfactory neurons during the major embryonic period of axonogenesis as well as in maturity. In the adult olfactory bulb, galectin-1 was expressed by both second-order projection neurons and interneurons and was selectively localised to the synaptic neuropil layers. Mitral cells, the principal postsynaptic target of primary olfactory axons, began expressing this lectin soon after genesis and maintained high levels into adulthood. The expression of galectin-1 in the primary olfactory pathway and olfactory bulb neuropil suggests a role for this lectin both in the initial formation and in the subsequent maintenance of neuronal connections between the peripheral and the central olfactory neurons as well as between neurons within the bulb.

Animals↗

PubMed1991

Venezuelan equine encephalitis virus propagation in the olfactory tract of normal and immunized mice.

The role of virus spread in the induction of damage to the central nervous system (CNS) of mice infected with Venezuelan equine encephalitis virus (VEEV) via the respiratory route was studied. The virus concentration in various organs and in the blood, the sensitivity to different doses of virus, and ultrastructural lesions in various tissues were examined. It is concluded that VEEV can enter the CNS of nonimmunized mice both by vascular and by olfactory pathways, whereas in immunized mice the olfactory pathway is the main route.

Animals↗

PubMed1991

Ultrastructural evidence for an olfactory-autonomic pathway through the rat central amygdaloid nucleus.

The innervation of medullary projection neurons in the central amygdaloid nucleus (Ce) by afferents from the ventral taenia tecta (VTT) was investigated using combined lesion-induced axonal degeneration and retrograde transport of horseradish peroxidase-conjugated wheat germ agglutinin (HRP-WGA). Injections of HRP-WGA into the nucleus tractus solitarii resulted in retrograde labeling of neurons in the medial Ce. Ultrastructurally HRP-WGA reaction product was identifiable in the perikarya and proximal dendrites of Ce neurons. Degenerating terminals, probably due to damage at the HRP-WGA injection site, were few and confined to the ventral Ce. Electrolytic coagulation of the VTT resulted in approximately 9% of terminals in medial Ce showing signs of degeneration at 5 days post-lesion. Of the terminals sampled, slightly more than 40% were in contact with the dendrites of retrogradely labeled neurons. Where evident, these terminals formed exclusively symmetrical synaptic contacts. These data provide evidence for an oligosynaptic olfactory-autonomic pathway in the rat that may mediate olfactory influences on gastric and cardiovascular aspects of autonomic function.

Amygdala↗

PubMed1997

The central pathway of primary olfactory axons is abnormal in mice lacking the N-CAM-180 isoform.

Although N-CAM has previously been implicated in the growth and fasciculation of axons, the development of axon tracts in transgenic mice with a targeted deletion of the 180-kD isoform of the neural cell adhesion molecule (N-CAM-180) appears grossly normal in comparison to wild-type mice. We examined the organization of the olfactory nerve projection from the olfactory neuroepithelium to glomeruli in the olfactory bulb of postnatal N-CAM-180 null mutant mice. Immunostaining for olfactory marker protein revealed the normal presence of fully mature primary olfactory neurons within the olfactory neuroepithelium of mutant mice. The axons of these neurons form an olfactory nerve, enter the nerve fiber layer of the olfactory bulb, and terminate in olfactory glomeruli as in wild-type control animals. The olfactory bulb is smaller and the nerve fiber layer is relatively thicker in mutants than in wild-type mice. Previous studies have revealed that the plant lectin Dolichos biflorus agglutinin (DBA) clearly stains the perikarya and axons of a subpopulation of primary olfactory neurons. Thus, DBA staining enabled the morphology of the olfactory nerve pathway to be examined at higher resolution in both control and mutant animals. Despite a normal spatial pattern of DBA-stained neurons within the nasal cavity, there was a distorted axonal projection of these neurons onto the surface of the olfactory bulb in N-CAM-180 null mutants. In particular, DBA-stained axons formed fewer and smaller glomeruli in the olfactory bulbs of mutants in comparison to wild-type mice. Many primary olfactory axons failed to exit the nerve fiber layer and contribute to glomerular formation. These results indicate that N-CAM-180 plays an important role in the growth and fasciculation of primary olfactory axons and is essential for normal development of olfactory glomeruli.

Animals↗

PubMed1998

Anatomical distribution of the chemorepellent semaphorin III/collapsin-1 in the adult rat and human brain: predominant expression in structures of the olfactory-hippocampal pathway and the motor system.

Alterations in neuronal connectivity of the mature central nervous system (CNS) appear to depend on a delicate balance between growth-promoting and growth-inhibiting molecules. To begin to address a potential role of the secreted chemorepulsive protein semaphorin(D)III/collapsin-1 (semaIII/coll-1) in structural plasticity during adulthood, we used high-resolution nonradioactive in situ hybridization to identify neural structures that express semaIII/coll-1 mRNA in the mature rat and human brain. SemaIII/coll-1 was expressed in distinct but anatomically and functionally linked structures of the adult nervous system. The olfactory-hippocampal pathway displayed semaIII/coll-1 expression in a continuum of neuronal structures, including mitral and tufted cells of the olfactory bulb, olfactory tubercle, and piriform cortex; and distinct nuclei of the amygdaloid complex, the superficial layers of the entorhinal cortex, and the subiculum of the hippocampal formation. In addition, prominent labeling was found in neuronal components of the motor system, particularly in cerebellar Purkinje cells and in subpopulations of cranial and spinal motoneurons. Retrograde tracing combined with in situ hybridization also revealed that the staining of semaIII/coll-1 within the entorhinal cortex was present in the stellate neurons that project via the perforant path to the molecular layer of the dentate gyrus. Like in the rat, the human brain displayed discrete expression of semaIII/coll-1. Among the structures examined, the most prominent staining was observed in the cellular islands of the superficial layers of the human entorhinal cortex. The constitutive expression of the chemorepellent semaIII/coll-1 in discrete populations of neurons in the mature rat and human CNS raises the possibility that, in addition to its function as repulsive axon guidance cue during development, semaIII/coll-1 might be involved in restricting structural changes that occur in the wiring of the intact CNS.

Adult↗

PubMed1982

Transmission and integration of biologically meaningful olfactory information after bilateral transection of the lateral olfactory tract in the rat.

The contribution of the olfactory inputs conveyed by the lateral olfactory tract (LOT) to the central olfactory areas in the onset of behavioral and electrophysiological responses i.e. multiunit activity of mitral cells was studied in rats which were submitted to a bilateral transection of LOT. The characteristic emotional reactions elicited in sham-operated rats by biologically meaningful odorants--odors of predator or of conspecific--were no longer observed after such lesion. At the olfactory bulb level, the differential habituation of mitral cell electrical responses according to the biological meaning of the stimuli disappeared in the rats with a bilateral section of LOT. However the awaking influence of the odorants when stimulations occurred during slow wave sleep remained unchanged: all the rats were more often awakened by the odor of predator than by the other stimuli. On the other hand, an inhibitory centrifugal influence of LOT on mitral cell electrical responses was noted in rats with a unilateral transection of LOT. The results are discussed in relation to the two olfactory pathways (lateral and medial). During wakefulness, LOT seems essential to the olfactory information processing; in slow wave sleep, the medial olfactory pathways appear to mediate olfactory discrimination and to elicit adapted arousal.

Animals↗

PubMed2001

Cultured olfactory ensheathing cells express nerve growth factor, brain-derived neurotrophic factor, glia cell line-derived neurotrophic factor and their receptors.

In the primary olfactory pathway axons of olfactory neurons (ONs) are accompanied by ensheathing cells (ECs) as the fibres course towards the olfactory bulb. Ensheathing cells are thought to play an important role in promoting and guiding olfactory axons to their appropriate target. In recent years, studies have shown that transplants of ECs into lesions in the central nervous system (CNS) are able to stimulate the growth of axons and in some cases restore functional connections. In an attempt to identify a possible mechanism underlying EC support for olfactory nerve growth and CNS axonal regeneration, this study investigated the production of growth factors and expression of corresponding receptors by these cells. Three techniques immunohistochemistry, enzyme linked immunosorbent assay (ELISA) and reverse transcriptase-polymerase chain reaction (RT-PCR) were used to assess growth factor expression in cultured ECs. Immunohistochemistry showed that ECs expressed nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and glial cell-line derived neurotrophic factor (GDNF). ELISA confirmed the intracellular presence of NGF and BDNF and showed that, compared to BDNF, about seven times as much NGF was secreted by ECs. RT-PCR analysis demonstrated expression of mRNA for NGF, BDNF, GDNF and neurturin (NTN). In addition, ECs also expressed the receptors trkB, GFRalpha-1 and GFRalpha-2. The results of the experiments show that ECs express a number of growth factors and that BDNF in particular could act both in a paracrine and autocrine manner.

Animals↗

PubMed2002

Prolonged seizures increase proliferating neuroblasts in the adult rat subventricular zone-olfactory bulb pathway.

Neuronal precursors in the adult rodent forebrain subventricular zone (SVZ) proliferate, migrate to the olfactory bulb in a restricted pathway known as the rostral migratory stream (RMS), and differentiate into neurons. The effects of injury on this neurogenic region of the mature brain are poorly understood. To determine whether seizure-induced injury modulates SVZ neurogenesis, we induced status epilepticus (SE) in adult rats by systemic chemoconvulsant administration and examined patterns of neuronal precursor proliferation and migration in the SVZ-olfactory bulb pathway. Within 1-2 weeks after pilocarpine-induced SE, bromodeoxyuridine (BrdU) labeling and Nissl staining increased in the rostral forebrain SVZ. These changes were associated with an increase in cells expressing antigenic markers of SVZ neuroblasts 2-3 weeks after prolonged seizures. At these same time points the RMS expanded and contained more proliferating cells and immature neurons. BrdU labeling and stereotactic injections of retroviral reporters into the SVZ showed that prolonged seizures also increased neuroblast migration to the olfactory bulb and induced a portion of the neuronal precursors to exit the RMS prematurely. These findings indicate that SE expands the SVZ neuroblast population and alters neuronal precursor migration in the adult rat forebrain. Identification of the mechanisms underlying the response of neural progenitors to seizure-induced injury may help to advance brain regenerative therapies by using either transplanted or endogenous neural precursor cells.

Animals↗

PubMed1977

Clinical testing of olfaction reassessed.

Odours in current use for testing olfaction (e.g., peppermint, camphor) cause considerable trigeminal nerve stimulation; this would render them relatively inefficacious in the detection of lesions of the main olfactory pathway. Musks and floral odours are considered to be relatively 'pure' olfactory stimulants, acting virtually exclusively via the first cranial nerve. These odours have been compared with standard odours in a group of patients whose olfactory pathways had been damaged by frontal tumours, surgical operation, head injury, multiple sclerosis and miscellaneous causes. Hyposmia or anosmia was detected more frequently and more reliably by musks and floral odours in all groups of patients; a number of patients had gross deficits of odour description without hyposmia or anosmia. Two-thirds of an unselected group of patients with multiple sclerosis had olfactory abnormalities. A substantial improvement in the rate of detection of organic lesions affecting the olfactory pathway can be achieved by substituting odours such as musk ketone, exaltolide, linalyl acetate and coumarin for those in current use.

Humans↗

PubMed2004

Magnetic resonance imaging for diagnosis of congenital anosmia.

Magnetic Resonance Imaging (MRI) was performed on 9 patients who lacked a sense of smell since birth. Seven of them, including two patients with Kallmann syndrome, exhibited abnormality of the olfactory bulb, olfactory tract, olfactory sulcus, or rectus gyrus, with some variation among patients in type and degree of abnormality. The other two patients exhibited normal olfactory pathway morphology, and for them the possibility of acquired sensorineural anosmia could not be ruled out. MRI is useful for determining whether patients with congenital anosmia have olfactory pathway anomalies. Many patients with congenital anosmia and hypoplasty or aplasty of the olfactory pathway nevertheless had no gonadal or endocrinological disorders.

Adolescent↗

PubMed2002

Pheromonal and host-odor processing in the insect antennal lobe: how different?

In the olfactory bulb of vertebrates and the antennal lobe of insects, precise connections between sensory receptor cells and olfactory glomeruli form the basis of a highly organized chemotopic map at the first stage of central processing in the brain. Beyond this basic level of organization, the olfactory system is typically separated into two subsystems: a 'main' olfactory pathway that detects and processes information about most environmental odorants, and an 'accessory' olfactory pathway that is devoted to information about social signals such as sex pheromones. A growing number of studies show, however, that it is not always possible to draw clear functional distinctions between the two subsystems. These findings have led some to speculate that the organizational principles by which olfactory stimuli are represented across glomeruli may be more similar in these two olfactory subsystems than previously thought.

Animals↗

PubMed1998

Spatio-temporal patterns of ensheathing cell differentiation in the rat olfactory system during development.

An immunocytochemical approach with specific glial markers was used to investigate the temporal and spatial patterns of differentiation of ensheathing glia wrapping axon fascicles along the primary olfactory pathway of the rat during development. The two glial markers tested, the proteins S-100 and glial fibrillary acidic protein, are known to be expressed at different stages of maturation in glial cells. The S-100 protein was first weakly expressed in cells accompanying the olfactory axons at embryonic day 14 (E14), while a first faint glial fibrillary acidic protein staining was detected along the olfactory axons at E15 and along the vomeronasal nerves at E16. A strong S-100 immunoreactivity was already present from E16 onwards along the axon fascicles through their course in both the nasal mesenchyme and the subarachnoid space before entering the olfactory nerve layer of the olfactory bulb. A gradual increase in glial fibrillary acidic protein expression was observed along this part of the developing olfactory pathway from E16 up to E20, when an adult-like pattern of staining intensity was seen. By contrast, most of the ensheathing cells residing in the olfactory nerve layer exhibited some delay in their differentiation timing and also a noticeable delayed maturation. It was only from E20 onwards that a weak to moderate S-100 expression was detected in an increasing number of cells throughout this layer, and only few of them appeared weakly glial fibrillary acidic protein positive at postnatal days 1 and 5. The immunocytochemical data indicate that there is a proximodistal gradient of differentiation of ensheathing cells along the developing olfactory pathway. The prolonged immaturity of ensheathing cells in the olfactory nerve layer, which coincides with the formation of the first glomeruli, might facilitate the sorting out of olfactory axons leading to a radical reorganization of afferents before they end in specific glomeruli.

Aging↗

PubMed1986

Functional properties of vertebrate olfactory receptor neurons.

The interaction of an odorant with the chemosensitive membrane of olfactory receptor neurons initiates a sequence of molecular and membrane events leading to sensory transduction, impulse initiation, and the transmission of sensory information to the brain. The main steps in this sequence are summarized in Figure 6. Several lines of evidence support the hypothesis that the initial molecular events and subsequent stages of transduction are mediated by odorant receptor sites and associated ion channels located in the membrane of the cilia and apical dendritic knob of the olfactory receptor neuron. Similarly, the membrane events associated with impulse initiation and propagation are mediated by voltage-gated channels located in the initial axonal segment and the axolemma. The ionic and electrical events associated with the proposed sequence have been characterized in general using a variety of experimental techniques. The identification, localization, and sequence of membrane events are consistent with the neurophysiological properties observed in specific regions of the bipolar receptor neuron. The influence of other cells in the primary olfactory pathway such as the sustentacular cells in the olfactory epithelium, the Schwann cells in the olfactory nerve, and the astrocytes in the olfactory nerve layer in the olfactory bulb on the physiological activity of the olfactory receptor neuron is an emerging area of research interests. The general principles derived from the experimental results described in this review provide only a framework that is both incomplete and of necessity somewhat speculative. As noted in the Introduction, the multidisciplinary study of the primary olfactory pathway is undergoing a renaissance of research interest. The application of modern biophysical, cell, and molecular biological techniques to the basic issues of odorant recognition and membrane excitability will clarify the speculations and lead to the establishment of new hypotheses. Three broad areas of research will benefit from such studies. First, the application of biophysical techniques will lead to a detailed characterization of the membrane properties and associated ion conductance mechanisms. Second, the isolation and biochemical characterization of intrinsic membrane and cytosolic proteins associated with odorant recognition, sensory transduction, and the subsequent electrical events will result from the utilization of cell and molecular biological techniques.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗