Erratum: Schaefer ML,et al. 2002. Trigeminal collaterals in the nasal epithelium and olfactory bulb: A potential route for direct modulation of olfactory information by trigeminal stimuli. J Comp Neurol 444:221–226.

The original article to which this Erratum refers was published in the Journal of Comparative Neurology (2002) 444(3):221–226.     

Trigeminal ganglion neurons which project by way of axon collaterals to both the caudal spinal trigeminal and the principal sensory trigeminal nuclei

Employing a combination of fluorescent retrograde double labeling and immunofluorescence histochemistry, we found that some single neurons in the trigeminal ganglion of the rat projected by way of axon collaterals both to the caudal spinal trigeminal nucleus and to the principal sensory trigeminal nucleus, and that about 40% or 57% of these neurons showed respectively substance P- or calcitonin gene-related peptide-like immunoreactivity.     

Nitrergic innervation of trigeminal and hypoglossal motoneurons in the cat

The present study was undertaken to determine the location of trigeminal and hypoglossal premotor neurons that express neuronal nitric oxide synthase (nNOS) in the cat. Cholera toxin subunit b (CTb) was injected into the trigeminal (mV) or the hypoglossal (mXII) motor nuclei in order to label the corresponding premotor neurons. CTb immunocytochemistry was combined with NADPH-d histochemistry or nNOS immunocytochemistry to identify premotor nitrergic (NADPH-d(+)/CTb(+) or nNOS(+)/ CTb(+) double-labeled) neurons. Premotor trigeminal as well as premotor hypoglossal neurons were located in the ventro-medial medullary reticular formation in a region corresponding to the nucleus magnocellularis (Mc) and the ventral aspect of the nucleus reticularis gigantocellularis (NRGc). Following the injection of CTb into the mV, this region was found to contain a total of 60 +/- 15 double-labeled neurons on the ipsilateral side and 33 +/- 14 on the contralateral side. CTb injections into the mXII resulted in 40 +/- 17 double-labeled neurons in this region on the ipsilateral side and 16 +/- 5 on the contralateral side. Thus, we conclude that premotor trigeminal and premotor hypoglossal nitrergic cells coexist in the same medullary region. They are colocalized with a larger population of nitrergic cells (7200 +/- 23). Premotor neurons in other locations did not express nNOS. The present data demonstrate that a population of neurons within the Mc and the NRGc are the source of the nitrergic innervation of trigeminal and hypoglossal motoneurons. Based on the characteristics of nitric oxide actions and its diffusibility, we postulate that these neurons may serve to synchronize the activity of mV and mXII motoneurons.     

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.     

Heterogeneous expression of connexin 36 in the olfactory epithelium and glomerular layer of the olfactory bulb

Gap junctions regulate a variety of cell functions by directly connecting two cells through intercellular channels. Connexins are gap junction channel-forming protein subunits. In this study, we studied the expression of connexin 36 (Cx36) in the olfactory epithelium and olfactory bulb of adult mice. In situ hybridization revealed that mRNA for Cx36 was expressed in the olfactory sensory epithelium, main olfactory bulb and accessory olfactory bulb. Expression of mRNA encoding Cx36 was observed in the olfactory epithelium mainly in ventral and lateral regions of the turbinates. Immunohistochemical determination of Cx36 protein expression showed sparse punctuate staining in the olfactory epithelial layer. Intense Cx36-like immunostaining was found in the olfactory nerve bundles underlying the olfactory epithelium and in the olfactory nerve layer and glomerular layer of the olfactory bulb. Mapping of the intensity of Cx36-like immunofluorescence in glomeruli throughout the main olfactory bulb indicated a heterogeneous distribution. A set of approximately 50 glomeruli located in the anterior and posterior limits of the olfactory bulb was more intensely labeled than other glomeruli. There was intense immunofluorescence signal in the glomerular layer of the accessory olfactory bulb and in the vomeronasal nerve. beta-Galactosidase distribution in the olfactory epithelium and olfactory bulb in Cx36 knockout mice (Deans et al. [2001] Neuron 31:477-485) supported the findings with immunofluorescence. Cx36-like immunofluorescence was absent in the olfactory nerve bundles in Cx36 knockout mice. The immunolocalization of Cx36 to the olfactory and vomeronasal nerves, and a subset of olfactory glomeruli suggest a functional role for Cx36 in odor coding.     

Aging in the rat olfactory system: relative stability of piriform cortex contrasts with changes in olfactory bulb and olfactory epithelium

Previous studies have quantified growth and atrophy of the olfactory bulb and olfactory epithelium of the Sprague-Dawley rat from maturity to senescence. Major events occurring in these structures include changes in the volume of mitral cells and changes in the number of septal olfactory receptors. These effects are large, consist of a growth phase followed by atrophy, and are temporally related in that events in the olfactory epithelium precede those in the mitral cells. A hypothesis of aging based on transneuronal effects would predict that these changes would be similarly transmitted to the next synaptic station in the olfactory pathway. Therefore, cells and synapses of the piriform cortex were studied in rats 3, 12, 18, 24, 27, 30, and 33 months of age. Alternate Vibratome sections through brains perfused with mixed aldehydes were processed for light and electron microscopy. No significant age effects were found for the volumes of cortical laminae Ia and Ib. Both numerical and surface density of synaptic apposition zones in layer Ia, formed primarily by mitral cell axons, were stable with age. A modest (18%) but significant decline in the proportion of layer Ia occupied by dendrites and spines was mirrored by an increase in the proportion of glial processes; no change in the proportion of axons and terminals was observed. Neither nuclear volume, nor soma volume, nor numerical density of layer II neurons changed with age. Thus, contacts made in the piriform cortex by mitral cell axons remain relatively stable in senescence, despite the marked volumetric changes in the mitral cell somata, changes which were confirmed again in this study. Age-related dendritic regression in layer II neurons may be attributable to functional deafferentation subsequent to reduced receptor input to mitral cells.     

Oxidative stress in the aging murine olfactory bulb: redox proteomics and cellular localization

A recent proteomics analysis from our laboratory demonstrated that several oxidative stress response proteins showed significant changes in steady-state levels in olfactory bulbs (OBs) of 20- vs. 1.5-month-old mice. Oxidative stress may result in protein oxidation. In this study, we investigated two forms of protein oxidative modification in murine OBs: carbonylation and nitration. Redox proteomics with two-dimensional gel electrophoresis, Western blotting, protein digestion, and mass spectrometry was used to quantify total and specific protein carbonylation and to identify differentially carbonylated proteins and determine the carbonylation status of previously identified proteins in OBs of 1.5- and 20-month-old mice. Immunohistochemistry was used to demonstrate the relative intensity and localization of protein nitration in OBs of 1.5-, 6-, and 20-month-old mice. Total protein carbonylation was significantly greater in OBs of 20- vs. 1.5-month-old mice. Aldolase 1 (ALDO1) showed significantly more carbonylation in OBs from 20- vs. 1.5-month-old mice; heat shock protein 9A and dihydropyrimidinase-like 2 showed significantly less. Several previously investigated proteins were also carbonylated, including ferritin heavy chain (FTH). Nitration, identified by 3-nitrotyrosine immunoreactivity, was least abundant at 1.5 months, intermediate at 6 months, and greatest at 20 months and was localized primarily in blood vessels. Proteins that were specific targets of oxidation were also localized: ALDO1 in astrocytes of the granule cell layer and FTH in mitral/tufted cells. These results indicate that specific carbonylated proteins, including those in astrocytes and mitral/tufted neurons, and nitrated proteins in the vasculature are molecular substrates of age-related olfactory dysfunction.     

Similarities and differences in the innervation of mystacial vibrissal follicle-sinus complexes in the rat and cat: a confocal microscopic study

Our confocal three-dimensional analyses revealed substantial differences in the innervation to vibrissal follicle-sinus complexes (FSCs) in the rat and cat. This is the first study using anti-protein gene product 9.5 (PGP9.5) immunolabeling and confocal microscopy on thick sections to examine systematically the terminal arborizations of the various FSC endings and to compare them between two species, the rat and the cat, that have similar-appearing FSCs but different exploratory behaviors, such as existence or absence of whisking. At least eight distinct endings were clearly discriminated three dimensionally in this study: 1) Merkel endings at the rete ridge collar, 2) circumferentially oriented lanceolate endings, 3) Merkel endings at the level of the ring sinus, 4) longitudinally oriented lanceolate endings, 5) club-like ringwulst endings, 6) reticular endings, 7) spiny endings, and 8) encapsulated endings. Of particular contrast, each nerve fiber that innervates Merkel cells at the level of the ring sinus in the rat usually terminates as a single, relatively small cluster of endings, whereas in the cat they terminate en passant as several large clusters of endings. Also, individual arbors of reticular endings in the rat ramify parallel to the vibrissae and distribute over wide, overlapping territories, whereas those in the cat ramify perpendicular and terminate in tightly circumscribed territories. Otherwise, the inner conical body of rat FSCs contains en passant, circumferentially oriented lanceolate endings that are lacking in the cat, whereas the cavernous sinus of the cat has en passant corpuscular endings that are lacking in the rat. Surprisingly, the one type of innervation that is the most similar in both species is a major set of simple, club-like endings, located at the attachment of the ringwulst, that had not previously been recognized as a morphologically unique type of innervation. Although the basic structure of the FSCs is similar in the rat and cat, the numerous differences in innervation suggest that these species would have different tactile capabilities and perceptions possibly related to their different vibrissa-related exploratory behaviors.     

Possible role of calcitonin gene‐related peptide in trigeminal modulation of glomerular microcircuits of the rodent olfactory bulb

Chemosensation in the mammalian nose comprises detection of odorants, irritants and pheromones. While the traditional view assigned one distinct sub‐system to each stimulus type, recent research has produced a more complex picture. Odorants are not only detected by olfactory sensory neurons but also by the trigeminal system. Irritants, in turn, may have a distinct odor, and some pheromones are detected by the olfactory epithelium. Moreover, it is well‐established that irritants change odor perception and vice versa. A wealth of psychophysical evidence on olfactory‐trigeminal interactions in humans contrasts with a paucity of structural insight. In particular, it is unclear whether the two systems communicate just by sharing stimuli, or whether neuronal connections mediate cross‐modal signaling. One connection could exist in the olfactory bulb which performs the primary processing of olfactory signals and receives trigeminal innervation. In the present study, neuroanatomical tracing of the mouse ethmoid nerve illustrates how peptidergic fibers enter the glomerular layer of the olfactory bulb, where local microcircuits process and filter the afferent signal. Biochemical assays reveal release of calcitonin gene‐related peptide from olfactory bulb slices and attenuation of cAMP signaling by the neuropeptide. In the non‐stimulated tissue, the neuropeptide specifically inhibited the basal activity of calbindin‐expressing periglomerular interneurons, but did not affect the basal activity of neurons expressing calretinin, parvalbumin, or tyrosine hydroxylase, nor the activity of astrocytes. This study represents a first step toward understanding trigeminal neuromodulation of olfactory‐bulb microcircuits and provides a working hypothesis for trigeminal inhibition of olfactory signal processing.     

Linear correlation between the number of olfactory sensory neurons expressing a given mouse odorant receptor gene and the total volume of the corresponding glomeruli in the olfactory bulb

Chemosensory specificity in the main olfactory system of the mouse relies on the expression of ～1,100 odorant receptor (OR) genes across millions of olfactory sensory neurons (OSNs) in the main olfactory epithelium (MOE), and on the coalescence of OSN axons into ～3,600 glomeruli in the olfactory bulb. A traditional approach for visualizing OSNs and their axons consists of tagging an OR gene genetically with an axonal marker that is cotranslated with the OR by virtue of an internal ribosome entry site (IRES). Here we report full cell counts for 15 gene‐targeted strains of the OR‐IRES‐marker design coexpressing a fluorescent protein. These strains represent 11 targeted OR genes, a 1% sample of the OR gene repertoire. We took an empirical, “count every cell” strategy: we counted all fluorescent cell profiles with a nuclear profile within the cytoplasm, on all serial coronal sections under a confocal microscope, a total of 685,673 cells in 56 mice at postnatal day 21. We then applied a strain‐specific Abercrombie correction to these OSN counts in order to obtain a closer approximation of the true OSN numbers. We found a 17‐fold range in the average (corrected) OSN number across these 11 OR genes. In the same series of coronal sections, we then determined the total volume of the glomeruli (TGV) formed by coalescence of the fluorescent axons. We found a strong linear correlation between OSN number and TGV, suggesting that TGV can be used as a surrogate measurement for estimating OSN numbers in these gene‐targeted strains. J. Comp. Neurol. 524:199–209, 2016. © 2015 Wiley Periodicals, Inc.     

Noradrenergic modulation of behavioral odor detection and discrimination thresholds in the olfactory bulb

The mammalian main olfactory bulb (MOB) receives a significant noradrenergic input from the locus coeruleus. Norepinephrine (NE) is involved in the acquisition of conditioned odor preferences in neonatal animals and in some species‐specific odor‐dependent behaviors. Thus far, the role of NE in odor processing in adult rats remains less studied. We investigated the role of noradrenergic modulation in the MOB on odor detection and discrimination thresholds using behavioral and computational modeling approaches. Adult rats received bilateral MOB injections of vehicle, NE (0.1–1000 μm ), noradrenergic receptor antagonists and NE + receptor antagonists combined. NE infusion improved odor detection and discrimination as a function of NE and odor concentration. The effect of NE on detection and discrimination magnitude at any given odor concentration varied in a non‐linear function with respect to NE concentration. Receptor antagonist infusion demonstrated that α1 receptor activation is necessary for the modulatory effect of NE. Computational modeling showed that increases in the strength of α1 receptor activation leads to improved odor signal‐to‐noise ratio and spike synchronization in mitral cells that may underlie the behaviorally observed decrease of detection and discrimination thresholds. Our results are the first to show that direct infusion of NE or noradrenergic receptor antagonists into a primary sensory network modulates sensory detection and discrimination thresholds at very low stimulus concentrations.     

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.     

Mitral cells in the olfactory bulb of adult zebrafish (Danio rerio): morphology and distribution

The mitral cell is the primary output neuron and central relay in the olfactory bulb of vertebrates. The morphology of these cells has been studied extensively in mammalian systems and to a lesser degree in teleosts. This study uses retrograde tract tracing and other techniques to characterize the morphology and distribution of mitral cells in the olfactory bulb of adult zebrafish, Danio rerio. These output neurons, located primarily in the glomerular layer and superficial internal cell layer, had variable-shaped somata that ranged in size from 4-18 microm in diameter and 31-96 microm2 in cross-sectional area. The mitral cells exhibited two main types of morphologies with regard to their dendrites: the unidendritic morphology was a single primary dendrite with one or more tufts, but multidendritic cells with several dendritic projections also were seen. The axons of these cells projected to either the medial or the lateral olfactory tract and, in general, the location of the cell on the medial or lateral side of the bulb was indicative of the tract to which it would project. Further, this study shows that the majority of zebrafish mitral cells likely innervate a single glomerulus rather than multiple glomeruli. This information is contrary to the multiple innervation pattern suggested for all teleost mitral cells. Our findings suggest that mitral cells in zebrafish may be more similar to mammalian mitral cells than previously believed, despite variation in size and structure. This information provides a revised anatomical framework for olfactory processing studies in this key model system.     

Central projections and trigeminal ganglion location of corneal afferent neurons in the monkey, Macaca fascicularis

The method of transganglionic transport of horseradish peroxidase-wheat germ agglutinin conjugate (HRP-WGA) was used to determine the location within the monkey trigeminal ganglion of the primary afferent neurons that innervate the cornea, and the brainstem and spinal cord termination sites of these cells. In each of four animals. Gelfoam pledgets were saturated with 2% HRP-WGA in saline and applied to the scratched surface of the central cornea for 30 minutes. Postmortem examination of the corneal whole mounts revealed that the tracer solution remained confined to approximately the central one-fourth of the cornea with no spread into the peripheral cornea or limbus. Seventy-two to 96 hours after tracer application, 126-242 labeled cell bodies were observed in the medial region of the ipsilateral trigeminal ganglion. The majority of neurons were concentrated in an area of the ganglion that lay directly caudal to the entering fibers of the ophthalmic nerve, but smaller numbers of cells lay somewhat more laterally, near the region where the ophthalmic and maxillary nerves come together. A very small number of neurons in one animal innervated the cornea by sending their fibers into the maxillary nerve. HRP-WGA-labeled terminal fields were present to some extent in all four major rostrocaudal subdivisions of the ipsilateral trigeminal brainstem nuclear complex (TBNC), but the size of the terminal fields and the intensity of labeling differed markedly from one level of the TBNC to the next. Labeled fibers projected heavily to the transitional zone between caudal pars interpolaris and rostral pars caudalis (i.e., the "periobex" region of the TBNC) and moderately to the trigeminal main sensory nucleus, pars oralis, and caudal pars caudalis at the level of the pyramidal decussation. Remaining areas of the TBNC, including rostral pars interpolaris and the midlevel of pars caudalis, received few, if any, corneal afferent projections. Occasional labeled fibers were observed in the dorsal horn of C1 and in the rostral half of C2. It is hoped that data generated in the current investigation of nonhuman primates will contribute to a better understanding of the neural substrates that subserve corneal sensation and the blink reflex in humans.     

Anatomical and functional evidence for a role of arginine-vasopressin (AVP) in rat olfactory epithelium cells

The olfactory epithelium (OE) is composed of olfactory sensory neurons (OSNs) and sustentacular cells; it lies in the nasal cavity where it is protected by a thin mucus layer. The finely regulated composition of this mucus provides OSN with a suitable ionic environment. To maintain the functional integrity of the epithelium despite permanent physical, chemical and microbial aggressions, both OSNs and surrounding sustentacular cells are continuously renewed from globose basal cells. Moreover, the sense of smell is involved in so numerous behaviours (feeding, reproduction, etc.) that it has to cross-talk with the endocrine and neuroendocrine systems. Thus, besides its sensory function, the olfactory epithelium is thought to undergo a lot of complex regulatory processes. We therefore studied the effects of various neuropeptides on primary cultures of Sprague-Dawley rat olfactory epithelium cells. We found that arginine-vasopressin (AVP) triggered a robust, dose-dependent calcium increase in these cells. The cell response was essentially ascribed to the V1a AVP receptor, whose presence was confirmed by RT-PCR and immunolabelling. In the culture, V1a but not V1b receptors were present, mainly localized in neurons. In the epithelium, both subtypes were found differentially distributed. V1a-R were localized mainly in globose basal cells and at the apical side of the epithelium, in the area of the dendritic knobs of OSNs. V1b-R were strongly associated with Bowman's gland cells and globose basal cells. These localizations suggested potential multifaceted roles of a hormone, AVP, in the olfactory epithelium.     

Distribution of astroglia in glomeruli of the rat main olfactory bulb: Exclusion from the sensory subcompartment of neuropil

During an entire lifetime, sensory axons of regenerating olfactory receptor neurons can enter glomeruli in the olfactory bulb and establish synaptic junctions with central neurons. The role played by astrocytes in this unique permissiveness is still unclear. Glomerular astrocytes have been identified by immunocytochemistry for glial fibrillary acidic protein and S100 proteins at the light and electron microscopic levels. The latter labeling included submicroscopic lamellar and filopodial extensions of astroglial processes. Cell bodies and processes accumulate along the border between juxtaglomerular walls and glomerular neuropil. Within glomeruli, a network of astroglial processes encloses mesh-like neuropil zones devoid of astroglia. Electron microscopy confirmed the division into subcompartments of glomerular neuropil: 1) The “sensory-synaptic subcompartment” includes all sensory axon terminals and terminal dendritic branches receiving sensory input, whereas astroglia are excluded; 2) in the “central-synaptic subcompartment,” astroglial processes are intermingled with other neuropil components: dendrites of relay cells and interneurons, dendrodendritic synapses, centrifugal (cholinergic and serotonergic) axons, their axodendritic synapses, and blood vessels. Unevenly distributed astroglial processes in this subcompartment are attached to vascular basal laminae, stem dendrites, and subpopulations of dendrodendritic synapses, especially those colocalized with centrifugal projections (“triadic synapses”). Astroglia-free parts of the “central” subcompartment contain segments of dendrites and subpopulations of dendrodendritic synapses. Because of the subdivision of the glomerular neuropil into portions with and without glial components, glia do not completely demarcate the border between the “sensory” and the “central” subcompartments. Interdigitation between the subcompartments varies among glomeruli and even within a single glomerulus. The mesh width of astroglial networks covaries with numerical relations between sensory and dendrodendritic synapses. This distribution pattern of astrocytes suggests that these glial cells monitor brain-derived effects on olfactory glomerular neuropil rather than olfactory input and that astroglial processes are (re-)arranged accordingly. J. Comp. Neurol. 388:191–210, 1997. © 1997 Wiley-Liss, Inc.     

Neurogenesis in the olfactory bulb of the frogXenopus Laevis shows unique patterns during embryonic development and metamorphosis

We determined the time of origin of neurons in the olfactory bulb of the South African clawed frog,Xenopus laevis. Tritiated thymidine injections were administered to frog embryos and tadpoles from gastrulation (stage 11/12) through metamorphosis (stage 65), paraffin sections were processed for autoradiography, and the distribution of heavily and lightly labeled cells was examined. In the ventral olfactory bulb, we observed that the mitral cells were born as early as stage 11/12 and continued to be generated through the end of metamorphosis. Interneurons (periglomerular and granule cells) were not born in the ventral bulb until stage 41, and birth of these cells also continued through metamorphosis. Labeled cells were observed in the accessory olfactory bulb, beginning at stage 41. In contrast, the cells of the dorsal olfactory bulb were not born until the onset of metamorphosis (stage 54); at this stage in the dorsal bulb, the genesis of mitral cells, interneurons, and glial cells completely overlapped. The results indicate that olfactory axon innervation is not necessary to induce early stages of neurogenesis in the ventral olfactory bulb. On the other hand, the results on the dorsal olfactory bulb are consistent with the hypothesis that innervation from new or transformed sensory neurons in the principal cavity induces neurogenesis in the dorsal bulb.     

Distribution of local axon collaterals of mitral, displaced mitral, and tufted cells in the rabbit olfactory bulb

To determine the projection fields of intrabulbar axon collaterals, mitral, displaced mitral, and middle tufted cells in the rabbit olfactory bulb were stained by intracellular injection of HRP. The axon collaterals of mitral cells and displaced mitral cells were distributed exclusively within the granule cell layer (GrL). Those of middle tufted cells were distributed mostly in the GrL and on rare occasions in the mitral cell layer. None of these collaterals entered the external plexiform layer. Axon collaterals of mitral cells, emitted at the depths of the GrL, were distributed widely from the deep portion to the most superficial portion of the GrL. Collaterals of displaced mitral cells were also emitted in the deep part, but they tended to be distributed more superficially in the GrL than mitral cells. Collaterals of middle tufted cells were released and distributed superficially in the GrL. The axon collaterals of these principal cells typically extended for a longer distance than the secondary dendrites, and they sometimes formed bushlike terminal arborizations. The results indicate that the projection fields of the axon collaterals of principal cells are spatially separated from the dendritic projection fields. This suggests that the output of these principal cells through the collaterals has a functionally different role from the output through the dendrites. The observation that the three types of principal cells differ in the distribution pattern of their axon collaterals in the GrL suggests that there is a functional separation of the sublayers in the GrL.     

Voltage- and current-clamp recordings of the receptor potential in olfactory receptor cells in situ

Intracellular recordings obtained using the whole cell configuration of the patch recording technique show that isolated somas of olfactory receptor cells are electrogenic, producing fast overshooting action potentials when depolarized. In situ, these cells produce graded receptor potentials and action potentials when stimulated chemically. The receptor potential can be voltage clamped for analysis of the ionic basis of sensory transduction in olfaction.     

Morphometric modifications associated with early sensory experience in the rat olfactory bulb: I. Volumetric study of the bulbar layers

Current models of sensory coding in the olfactory bulb are based on the notion of topographical specificity in the processing of stimuli. Part of this hypothesis comes from studies of patterns of radiolabelled 2-deoxyglucose uptake, and local morphometric variations of the mitral cell size observed following prolonged exposure to an odor. The present study explored the possibility that exposing young rats to a long-term stimulation with an odor would induce spatially distributed volumetric variations of the bulbar layers. Three groups of 5 rats have been studied: (1) stimulated with ethyl acetoacetate from birth to 1 month of age, (2) unilaterally deprived following early cauterization of one nostril, and (3) normal animals of same age. Fourteen frontal histological sections uniformly distributed along the rostrocaudal axis of the olfactory bulb were used for this study. Areas of the bulbar layers were measured with the aid of an image analyser and the volume of the corresponding layers deduced by computation. Following complete sensory deprivation, the volume of all bulbar layers, except the periventricular core, was homogeneously reduced along the rostrocaudal axis of the olfactory bulb. Following long-term stimulation with ethyl acetoacetate, volume reduction was significantly higher in anterior and middle regions than in the posterior part of the olfactory bulb. It is assumed that neuronal pathways activated by ethyl acetoacetate stimulation are mainly located in the posterior part of the olfactory bulb. Functional interpretations of these results are discussed with respect to the spatial dimension in olfactory coding.