A note on the organization of the amphibian olfactory bulb.

After horseradish peroxidase (HRP) injections were made in limited sectors of the main olfactory bulb in the adult frog Rana pipiens, the cellular morphology of mitral cells and granule cells impregnated with HRP were examined in uninjected regions of the bulb. Mitral cells were observed to possess glomerular dendrites and prominent secondary dendrites, both of which have smooth shafts. The glomerular dendrites may be multiple, are often branched, and may arise from secondary dendrites, as well as from the cell body. The axon may also arise from a secondary dendrite. Granule cells have simple or branched peripheral dendrites, and these are spiny, where they intermingle with the mitral cell secondary dendrites. The prominence of the secondary dendrites of frog mitral cells contrasts sharply with their reported insignificance in urodeles, as studied in earlier literature. The layers of the main olfactory bulb are not as fully concentric in the frog, as they are in mammals. The implantation cone and glomerular layer occupy a small part of the surface area of the olfactory bulb on its anteroventral aspect, while the perimeters of the subjacent layers extend farther posteriorly and dorsally in successive steps. The granule cell core extends well beyond the perimeter of the mitral cell layer in a posterior direction. Long secondary dendrites of mitral cells also extend posteriorly beyond the perimeter of the mitral cell-external plexiform layer and interlace with granule cell peripheral dendrites in a plexiform layer external to the posterior region of the granule cell core. This layer, the superficial plexiform layer, forms an apron around the posterior segment of the olfactory bulb and contributes to the interbulbar adhesion.(ABSTRACT TRUNCATED AT 250 WORDS)     

Nectin-1 spots regulate the branching of olfactory mitral cell dendrites

Olfactory mitral cells extend lateral secondary dendrites that contact the lateral secondary and apical primary dendrites of other mitral cells in the external plexiform layer (EPL) of the olfactory bulb. The lateral dendrites further contact granule cell dendrites, forming dendrodendritic reciprocal synapses in the EPL. These dendritic structures are critical for odor information processing, but it remains unknown how they are formed. We recently showed that the immunoglobulin-like cell adhesion molecule nectin-1 constitutes a novel adhesion apparatus at the contacts between mitral cell lateral dendrites, between mitral cell lateral and apical dendrites, and between mitral cell lateral dendrites and granule cell dendritic spine necks in the deep sub-lamina of the EPL of the developing mouse olfactory bulb and named them nectin-1 spots. We investigated here the role of the nectin-1 spots in the formation of dendritic structures in the EPL of the mouse olfactory bulb. We showed that in cultured nectin-1-knockout mitral cells, the number of branching points of mitral cell dendrites was reduced compared to that in the control cells. In the deep sub-lamina of the EPL in the nectin-1-knockout olfactory bulb, the number of branching points of mitral cell lateral dendrites and the number of dendrodendritic reciprocal synapses were reduced compared to those in the control olfactory bulb. These results indicate that the nectin-1 spots regulate the branching of mitral cell dendrites in the deep sub-lamina of the EPL and suggest that the nectin-1 spots are required for odor information processing in the olfactory bulb.     

Differential distribution of ionotropic glutamate receptor subunits in the rat olfactory bulb

The subcellular localization of ionotropic glutamate receptor (GluR) subunits was examined with light and electron microscopy in the rat olfactory bulb by using antibodies to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor subunits: GluR1, GluR2/3, and GluR4; and kainate (KA) receptor subunits: GluR5/6/7. Immunoreactivity to GluR1 was heavy in the glomerular layer, moderate in the external plexiform layer, and localized to periglomerular somata and dendrites, short axon somata and dendrites, mitral cell somata, and mitral/tufted dendrites. GluR2/3 immunoreactivity was heavy in the external plexiform and glomerular layers and localized to periglomerular somata and dendrites, mitral cell somata, mitral/tufted dendrites, granule cell somata, and olfactory nerve-associated glia. GluR4 immunoreactivity showed heavy staining in the external plexiform and olfactory nerve layers with localization to mitral cells, mitral/tufted dendritic processes, and olfactory nerve glial processes. GluR5/6/7 immunoreactivity was heavy in the external plexiform layer, moderate in the olfactory nerve and glomerular layers, and localized to granule cells, mitral cells, and mitral/tufted dendritic processes. Ultrastructural immunolabeling for all antibodies examined showed immunoreactivity in the postsynaptic membrane and densities, adjacent dendritic cytoplasm, and somatic cytoplasm. These data demonstrate a highly specific laminar, cellular, and subcellular distribution of ionotropic GluR subunits within the primary afferent and local synaptic circuits of the olfactory bulb. The results are consistent with the notion that the different roles subserved by glutamate in the olfactory bulb are actuated, in part, by a differential distribution of GluR subunits.     

Local information processing in dendritic trees: subsets of spines in granule cells of the mammalian olfactory bulb

The anaxonic granule cell of the olfactory bulb is believed to inhibit mitral and tufted cells through reciprocal dendrodendritic synapses. However, little is known about the detailed input-output properties of the granule cell. This study explores the functional properties of granule cells by using detailed reconstructions of Golgi-impregnated granule cells as the basis for computational models. Three Golgi-impregnated granule cells from the olfactory bulbs of C57BL/6j mice were selected for detailed analysis. Measurements were made of the diameter and length of all spine heads, spine necks, and dendritic branches. These measurements formed the basis of a compartmental model of each cell in which simulations of the spread of synaptic potentials within the dendritic tree were performed with SABER (Analogy, Inc.), a circuit analysis program. The results show that the degree of spread of synaptic potentials can define functionally related subsets of spines within the dendritic tree. The size of these subsets varies with the anatomical location of the input spine, the magnitude of the input, the time course of the input, the size of the spine neck resistance, and the activity of other spines. The data indicate that the functional organization of granule cell dendritic arbors is more complex than previously thought: between the level of the individual spine and the entire dendritic tree are several levels of subsets of spines that can mediate discrete localized inhibition onto subsets of mitral or tufted cell secondary dendrites within the external plexiform layer of the olfactory bulb.     

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.     

The olfactory granule cell: from classical enigma to central role in olfactory processing

The granule cell of the olfactory bulb was first described by Golgi in 1875 and Cajal and his contemporaries in the 1890s as an enigmatic cell without an axon, whose status as a nerve cell was questionable. Insight into its functions began in the 1960s with evidence that it acted as an interneuron to mediate powerful inhibition of mitral cells. The circuit was found to involve dendrodendritic synapses for activation by mitral cell lateral dendrites of the granule cell dendritic spines and inhibition of the same and neighboring mitral cell lateral dendrites. Subsequent studies established the roles of glutamatergic receptors and GABAergic receptors in this circuit. The lateral inhibition is believed to be involved in contrast enhancement between mitral cells responding to different odor molecules. Current studies are analysing how the lateral inhibition can be mediated over arbitrary distances between columns of granule cells through action potential propagation in the mitral cell secondary dendrites. Among other important properties, granule cells undergo neurogenesis from precursor cells throughout adult life. This originally enigmatic cell thus appears to play a critical role in olfactory processing.     

Synaptic input to dentate granule cell basal dendrites in a rat model of temporal lobe epilepsy

In patients with temporal lobe epilepsy some dentate granule cells develop basal dendrites. The extent of excitatory synaptic input to basal dendrites is unclear, nor is it known whether basal dendrites receive inhibitory synapses. We used biocytin to intracellularly label individual granule cells with basal dendrites in epileptic pilocarpine-treated rats. An average basal dendrite had 3.9 branches, was 612 microm long, and accounted for 16% of a cell's total dendritic length. In vivo intracellular labeling and postembedding GABA-immunocytochemistry were used to evaluate synapses with basal dendrites reconstructed from serial electron micrographs. An average of 7% of 1,802 putative synapses were formed by GABA-positive axon terminals, indicating synaptogenesis by interneurons. Ninety-three percent of the identified synapses were GABA-negative. Most GABA-negative synapses were with spines, but at least 10% were with dendritic shafts. Multiplying basal dendrite length/cell and synapse density yielded an estimate of 180 inhibitory and 2,140 excitatory synapses per granule cell basal dendrite. Based on previous estimates of synaptic input to granule cells in control rats, these findings suggest an average basal dendrite receives approximately 14% of the total inhibitory and 19% of excitatory synapses of a cell. These findings reveal that basal dendrites are a novel source of inhibitory input, but they primarily receive excitatory synapses.     

Zinc-enriched (ZEN) terminals in mouse olfactory bulb

The present study was designed to localize zinc-enriched (ZEN) terminals in mouse olfactory bulb by means of ZnT3 immunocytochemistry (ICC) and zinc autometallography (AMG). The immunocytochemical staining of ZnT3 was closely correlated with the AMG pattern. ZEN terminals were defined as terminals showing both ZnT3 immunoreactivities and AMG granules. At the light microscopic level, dense staining patterns for ZnT3 immunoreactivity were seen in the granule cell layer and the olfactory glomerular layer. At the ultrastructural level, ZEN terminals were restricted to presynaptic terminals with single or multiple postsynaptic thickenings. The postsynaptic profiles contacting ZEN terminals appeared to be dendrites or somata of granule cells in the granule cell layer and periglomerular cells and mitral/tufted (M/T) cells in the olfactory glomerular layer. This suggests that two main sources of ZEN terminals are present in mouse olfactory bulb: (1) centrifugal fibres making asymmetrical synapses with granule cells and periglomerular cells, and (2) olfactory receptor terminals contacting dendritic profiles of M/T cells or periglomerular cells. The close correlation between ZEN terminals and the glutamatergic system is discussed.     

Plasticity of dendrodendritic microcircuits following mitral cell loss in the olfactory bulb of the murine mutant Purkinje cell degeneration

Mitral cells of the olfactory bulb typically form reciprocal dendrodendritic synapses with anaxonic interneurons, granule cells, within a sublamina of the external plexiform layer. As a result of mitral cell loss in the murine mutant Purkinjie cell degeneration (PCD), subpopulations of these granule cells are denervated. The present report examines the capacity of these denervated interneurons to form new dendrodendritic microcircuits with a second population of olfactory bulb neurons, tufted cells. Quantitative ultrastructural assessments were made of the morphology and distribution of dendrodendritic circuits in the olfactory bulbs of normal heterozygous littermates and affected homozygous recessive PCD mice following mitral cell loss. There were no apparent morphological characteristics that distinguished the reciprocal synaptic connections formed by mitral cells from those formed by tufted cells. However, the segregation of mitral cell dendrodendritic circuits in the deep sublamina of the external plexiform layer (EPL) and tufted cell circuits in the superficial sublamina provided the basis for a comparative analysis of synaptic organization following mitral cell loss. Following mitral cell loss there was a significant reduction in the area occupied by characteristic mitral cell dendrites within the deep sublamina of the EPL. A slight but nonsignificant increase in the area occupied by granule cell spines was also observed. The number of synaptic appositions involving granule cells decreased slightly, the number involving tufted cells increased significantly in the mutant mice. This indicates that many granule cell spines survive denervation and establish new reciprocal dendrodendritic synapses at available sites on tufted cells. In both the control and mutant mice the ratios of symmetrical:asymmetrical dendrodendritic synapses closely approached 1. This demonstrates that not only do the denervated spines receive new afferent input from tufted cell dendrites, but they also establish the reciprocal efferent projection. These data are discussed in terms of the sublaminar organization of dendrodendritic microcircuits in the olfactory bulb and their capacity of plasticity and reorganization following pertubation.     

Light microscopic Golgi study of mitral/tufted cells in the accessory olfactory bulb of the adult rat.

Mitral/tufted cells (MTCs) of the accessory olfactory bulb (AOB) of adult rats were investigated light microscopically with the rapid Golgi method. The somata of the MTCs, appearing ovoid or triangular in shape, are distributed throughout the external plexiform layer. The soma size varies from small to large (12-26 microns). Apical dendrites originating from the soma enter the glomerular layer to provide branches that form the glomerular arbors. After making a glomerular arbor, some dendrites develop a second arbor (en passant and terminal arbors, respectively). The MTCs have a very diverse dendritic branching pattern and most have a variable number of glomerular arbors per cell (up to 6); we have tentatively classified the MTCs into simple, intermediate, and complex. Of the glomerular arbors, 80% have a diameter of less than 50 microns. The glomerular arbors have been classified as baskets (small spherical or ovoid) with short loopy processes; balls of yarn (large and nearly spherical) with loosely intermingled thick loops; and bushes (small to large and rather polymorphic) with irregular processes. The MTCs send dendritic arbors to terminate in one or more glomeruli where they are arranged in several different types of endings. Since it is generally believed that the dendrites of mitral and tufted cells of the main olfactory bulb terminate in only one glomerulus, the difference in the termination of the dendrites of the MTCs may represent a morphological characteristic that is relevant to the coding and/or integration of sensory information.     

Morphology of physiologically identified mitral cells in the carp olfactory bulb: a light microscopic study after intracellular staining with horseradish peroxidase

Physiologically identified mitral cells in the carp olfactory bulb were stained by intracellular injection of horseradish peroxidase in order to study the morphology in detail. The somata were fusiform, elongated, oval, triangular, or irregular. The mean diameters of the somata were 30 microns X 14 microns. Two to five thick dendrites arose from the somata and frequently gave off branches to form glomerular tufts. The dendrites extended less than 400 microns; the dendritic field of single mitral cells in the medial or lateral part of the olfactory bulb was confined within the respective part of the bulb. The axons arose from either the somata or the dendrites and had a conical initial portion, usually with a smooth contour. Some cells had poorly developed intrabulbar axon collaterals. No difference between the mitral cells in the medial part of the olfactory bulb and those in the lateral part was found in the soma diameter, the dendritic diameter at the base, or the number of first-order dendrites. However, there was a difference in the site of the origin of the axon between them: most of the axons of the mitral cells in the medial part arose from the dendrites, while most of the axons of the mitral cells in the lateral part arose from the somata. The morphology of physiologically identified mitral cells is basically consistent with that reported in the Golgi studies of teleosts. The limited dendritic fields of mitral cells may underlie the previously reported functional separation of the olfactory bulb into medial and lateral parts. The results also indicate that the two parts of the teleost olfactory bulb are differentiated not only functionally but also morphologically.     

Golgi analyses of dendritic organization among denervated olfactory bulb granule cells

In the vertebrate olfactory bulb, the primary projection neurons, mitral and tufted cells, have reciprocal dendrodendritic synapses with respective subpopulations of anaxonic interneurons called granule cells. In the neurological murine mutant Purkinje Cell Degeneration (PCD), all mitral cells are lost during early adulthood. As a consequence, a subpopulation of granule cells is deprived of both afferent input and efferent targets. The effect of this event on the morphology and sublaminar distribution of granule cells was studied with light microscopic Golgi procedures in affected homozygous recessive PCD mutants and normal heterozygous littermate controls. In the control mice, a minimum of three subpopulations were identified predominantly on the basis of the topology of apical dendrites and their spinous processes within the external plexiform layer (EPL) of the olfactory bulb: type I had dendrites extending across the full width of the EPL and a homogeneous distribution of spines; type II had dendritic arbors confined to the deeper EPL; type III had apical dendrites that arborized extensively within the superficial EPL with no arbors or spines present in the deeper EPL. Prior studies suggest that type II cells form connections with mitral cells; type III cells form connections with tufted cells; and type I cells may integrate information from both populations of projection neurons. In the mutant PCD mice, the classification of subpopulations of granule cells proved difficult due to a compression of dendritic arbors within the EPL. Dendritic processes followed a more horizontal tangent relative to the radial orientation seen in control mice. The length of dendritic branches was reduced by approximately 20% with a corresponding decrease in the number of spines. The density of spines (#/1 micron of dendrite) was constant in both controls and mutants at approximately 0.21. Truncation of the dendrites in the PCD mutants appeared to occur at terminal portions because the number of dendritic bifurcations was equal in both groups of mice. The data are discussed in terms of subpopulations of granule cells in the mouse olfactory bulb, the sublaminar organization of olfactory bulb circuits, and the capacity for survival and plasticity in the reciprocal dendrodendritic circuits mediated by the granule cell spines.     

Distribution of immunoreactivity for gamma-aminobutyric acid in the salamander olfactory bulb.

Intrinsic neurons provide inhibitory synaptic input to mitral (and tufted) output cells within several laminae of the olfactory bulb. In rodents, the two main types of intrinsic neurons are granule and periglomerular cells, both of which contain gamma-aminobutyric acid (GABA). In the present study, immunocytochemical techniques were used to determine whether intrinsic neurons in the salamander olfactory bulb might also contain GABA. With the aid of two antisera to different GABA-conjugates, immunoreactivity for GABA was localized within the olfactory bulb laminae. In the glomerular layer, periglomerular cells, which were strongly immunoreactive, were concentrated in clusters along the border with the olfactory nerve layer. Dendrites of the cells encircled nearby glomeruli and were presumably a primary source of intraglomerular processes that were also stained. In the subglomerular region and external plexiform layer, relatively few immunoreactive cells were observed, most of which appeared to be periglomerular and tufted cell types with glomerular dendrites. Throughout the external plexiform and mitral cell layers, however, a dense matrix of spiny processes and puncta was stained, outlining large, unstained dendrites derived from the large, unstained cell bodies of mitral cells. The spiny processes and puncta appeared to be derived from granule cells, which were the most abundant immunoreactive cells in the bulb. Granule cell bodies filled the granule cell layer. In tissue fixed with 0.1-0.2% glutaraldehyde, staining in the olfactory bulb laminae was blocked by preadsorption of the two antisera with glutaraldehyde-conjugated GABA-bovine serum albumin. The staining therefore appeared to be specific for fixed GABA.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Synaptic excitatory and inhibitory interactions at distal dendritic sites on mitral cells in the isolated turtle olfactory bulb

The olfactory nerves terminate exclusively on the distal dendritic tufts of mitral cells in the olfactory bulb, which makes this a favorable model for analysis of synaptic responses in distal dendrites. Intracellular recordings of responses to olfactory nerve volleys have been obtained in the isolated turtle olfactory bulb. Single mitral cells usually responded with excitatory postsynaptic potentials (EPSPs) to volleys in two different bundles, indicating convergence from separate receptor neuron populations. Paired volleys revealed long-lasting inhibition of a test EPSP by a conditioning volley. This could be shown to be independent of the inhibition of mitral cells by granule cell interneurons in the deeper layers. The results suggest that excitatory and inhibitory synaptic interactions in the glomerular layer are important in the processing of olfactory inputs. The results also provide an exception to the classical doctrine that synaptic inhibition is preferentially sited near the cell body and axon hillock in order to control impulse generation there. Our findings of inhibitory actions on distal dendrites of mitral cells may provide a model for similar actions on distal dendrites of other central neurons.     

Ultrastructural identification of physiologically recorded neurons in the cat cerebellum

Neurons from the cerebellar cortex of cats were examined in electron microscopic preparations after intracellular recording and pressure injections of horseradish peroxidase (HRP). Neurons that contained HRP reaction product within dendrites were identified as either Purkinje or Golgi II cells. This identification was based on specific ultrastructural criteria that included the presence of synapses on the surfaces of dendritic shafts or spines, the identification of the presynaptic component of these synapses, and the presence of certain visible intracellular organelles. In addition, we examined specimens that contained two types of labeled dendrites after a single HRP injection. These dendrites were identified as arising from Purkinje and Golgi II cells and were shown to interdigitate with each other in a dendritic glomerulus. Dendritic appendages or sheet-like spines emanated from the Purkinje cell dendrite and sent small finger-like protrusions that surrounded and invaginated the Golgi II cell dendrite. In this glomerulus, the dendrites were shown to approach each other, and preliminary results suggest the presence of a gap junction at this site of direct apposition. This finding supports physiologic data which suggest electrical coupling between Purkinje and Golgi II cells. In addition, the results of this study demonstrate the usefulness of combining intracellular electrophysiology with HRP staining for the ultrastructural identification of recorded neurons.     

Subdivisions in the Multiple GABAergic Innervation of Granule Cells in the Dentate Gyrus of the Rat Hippocampus

The sources of GABAergic innervation to granule cells were studied to establish how the basic cortical circuit is implemented in the dentate gyrus. Five types of neuron having extensive local axons were recorded electrophysiologically in vitro and filled intracellularly with biocytin (Han et al., 1993). They were processed for electron microscopy in order to reveal their synaptic organization and postsynaptic targets, and to test whether their terminals contained GABA. (1) The hilar cell, with axon terminals in the commissural and association pathway termination field (HICAP cell), formed Gray's type 2 (symmetrical) synapses with large proximal dendritic shafts (n= 18), two-thirds of which could be shown to emit spines, and with small dendritic branches (n= 6). Other boutons of the HICAP neuron were found to make either Gray's type 1 (asymmetrical) synapses (n= 4) or type 2 synapses (n= 6) with dendritic spines. Using a highly sensitive silver-intensified immunogold method for the postembedding visualization of GABA immunoreactivity, both the terminals and the dendrites of the HICAP cell were found to be immunopositive, whereas its postsynaptic targets were GABA-immunonegative. The dendritic shafts of the HICAP cell received synapses from both GABA-negative and GABA-positive boutons; the dendritic spines which densely covered the main apical dendrite in the medial one-third of the molecular layer received synapses from GABA-negative boutons. (2) The hilar cell, with axon terminals distributed in conjunction with the perforant path termination field (HIPP cell), established type 2 synapses with distal dendritic shafts (n= 17), most of which could be shown to emit spines, small-calibre dendritic profiles (n= 2) and dendritic spines (n= 6), all showing characteristics of granule cell dendrites. The sparsely spiny dendrites of the HIPP cell were covered with many synaptic boutons on both their shafts and their spines. (3) The cell with soma in the molecular layer had an axon associated with the perforant path termination field (MOPP cell). This GABA-immunoreactive cell made type 2 synapses exclusively on dendritic shafts (n= 20), 60% of which could be shown to emit spines. The smooth dendrites of the MOPP cell were also restricted to the outer two-thirds of the molecular layer, where they received both GABA-negative and GABA-positive synaptic inputs. (4) The extensive axonal arborization of the dentate basket cell terminated mainly on somata (n= 26) and proximal dendrites (n= 9) in the granule cell layer, and some boutons made synapses on somatic spines (n= 6); all boutons established type 2 synapses. (5) The dentate axo-axonic cell established type 2 synapses (n= 14) exclusively on axon initial segments of granule cells in the granule cell layer, and on initial segments of presumed mossy cells in the hilus. The results demonstrate that granule cells receive inputs from the local circuit axons of at least five distinct types of dentate neuron terminating in mutually exclusive domains of the cell's surface in four out of five cases. Four of the cell types (HICAP cell, MOPP cell, basket cell, axo-axonic cell) contain GABA, and the HIPP cell may also be inhibitory. The specific local inhibitory neurons terminating in conjunction with particular excitatory amino acid inputs to the granule cells (types 1 – 3) are in a position to interact selectively with the specific inputs on the same dendritic segment. This arrangement provides a possibility for the independent regulation of the gain and long-term potentiation of separate excitatory inputs, through different sets of GABAergic local circuit neurons. The pairing of excitatory and inhibitory inputs may also provide a mechanism for the downward reseating of excitatory postsynaptic potentials, thereby extending their dynamic range.     

Distribution of dendrites of mitral, displaced mitral, tufted, and granule cells in the rabbit olfactory bulb

To determine the dendritic fields, mitral, displaced mitral, middle tufted, and granule cells in the rabbit olfactory bulb were stained by intracellular injection of HRP. The secondary dendrites of mitral cells were distributed mostly in the inner half of the external plexiform layer (EPL). Those of displaced mitral cells extended mainly into the middle and superficial sublayers in the EPL. The secondary dendrites of middle tufted cells were distributed mostly in the superficial portion of the EPL. Mitral cells extended their secondary dendrites in virtually all directions within a plane tangential to the mitral cell layer (MCL) and thus had a disklike projection field with a radius of about 850 microns. Displaced mitral cells had similar dendritic projection fields in the tangential plane but with somewhat distorted shapes. The secondary dendrites of middle tufted cells had a tendency to extend in particular directions. From the projection pattern of the gemmules on the peripheral processes, granule cells were classified into three types. Type I granule cells had gemmules both in the superficial and in the deep sublayers of the EPL. The peripheral processes of Type II granule cells were confined to the deep half of the EPL. The gemmules of Type III granule cells ere distributed in the superficial half of the EPL. The differing dendritic ramification among mitral, displaced mitral, and middle tufted cells suggests the separation of the dendrodendritic synaptic interactions with granule cells in different sublayers in the EPL. It also suggests a functional separation of the sublayers of the EPL.     

Glomerular formation in the developing rat olfactory bulb.

Using the confocal microscope together with markers for the cellular components of glomeruli, we examined the spatiotemporal cellular interactions that occur between the axons of olfactory receptor cells, their dendritic targets, and glial cells during the critical period of glomerular formation. We have employed markers of immature and mature olfactory receptor cell axons, mitral/tufted cell dendrites, and glial cells as well as a synapse-associated protein for double- and triple-label immunocytochemistry. Axons of olfactory receptor cells grew into a dense dendritic zone of the olfactory bulb (comprising the dendrites of both mitral and tufted cells) between E17 and E18. At E19, these axons coalesced into protoglomeruli, which continued to develop until birth, when the basic anatomical structure of adult glomeruli emerged. Neither mitral/tufted cell dendrites nor olfactory bulb astrocytes became specifically associated with these protoglomeruli until E21. Ensheathing cells remained restricted to the outer nerve fiber layer and did not appear to contribute to glomerular formation. Finally, the synaptophysin staining has shown that synaptic constituents are expressed as early as E17, prior to the appearance of mature olfactory receptor cell axons. Based on these data, we have established a time line detailing the temporal and spatial interactions that occur between cell types during late embryonic rat olfactory bulb development. We conclude that the initial event in the formation of glomeruli is the penetration of the mitral/tufted cell dendritic zone by olfactory receptor cell axons. The coalescence of dendritic and glial processes into glomerular structures appears secondary to the arrival of the olfactory receptor cell axons.     

The ultrastructure of the cat olfactory bulb

The fine structure of the olfactory bulb was investigated in the cat. Five types of bulbar neurons were identified on the basis of size, location, cytoplasmic organelles and synaptic complexes with the neuron perikaryon. The smallest diameter neurons were predominatly located in the glomerular and granule cell layers. Ultrastructural characteristics for these two neuron populations, viz., periglomerular and internal granule cells, were identical. Tufted cells, smaller in diameter than mitral cells, were located in the external plexiform and glomerular layers and possessed all the morphological characteristics of the mitral cells. Mitral cells were the largest bulbar neurons and were located in a single lamina. Stellate (short-axon) neurons were found in the glomerular, external plexiform and granule cell layers. Reciprocal synapses were observed in all laminae of the bulb except the primary olfactory nerve and deeper granule cell layers. Such synapses were found in the glomeruli, on tufted and mitral cells, in the neuropil of the external plexiform layer, and on the axon hillock and initial segments of the mitral cells. The morphology in the cat olfactory bulb corroborated bulbar ultrastructure described for other mammals.