Immunocytochemical localization of the mGluR1α metabotropic glutamate receptor in the dorsal cochlear nucleus

We demonstrate that the metabotropic glutamate receptor mGluR1α is enriched in two interneuron cell populations in the dorsal division of the cochlear nucleus. Electron microscopic analysis confirms that mGluR1α immunoreactivity is concentrated in the dendritic spines of cartwheel cells and in dendrites of the recently described unipolar brush cells. The cartwheel cells, which have many similarities to the Purkinje cells of the cerebellum, participate in a local neuronal circuit that modulates the output of the dorsal cochlear nucleus. Immunostained unipolar brush cells were observed in granule cell regions of the cochlear nucleus and the vestibulocerebellum. The presence of analogous cell types with similar patterns of immunolabeling in the cerebellum and in the dorsal cochlear nucleus suggests that a shared but as yet unknown mode of processing may occur in both structures. © 1996 Wiley-Liss, Inc.     

Subcellular compartmentalization of a potassium channel (Kv1.4): preferential distribution in dendrites and dendritic spines of neurons in the dorsal cochlear nucleus

Voltage-dependent ion channels have specific patterns of distribution along the neuronal plasma membrane of dendrites, cell bodies and axons, which need to be unravelled in order to understand their contribution to neuronal excitability and firing patterns. We have investigated the subcellular compartmentalization of Kv1.4, a transient, fast-inactivating potassium channel, in fusiform cells and related interneurons of the rat dorsal cochlear nucleus. A polyclonal antibody which binds to a region near the N-terminus domain of a Kv1.4 channel was raised in rabbits. Using a high-resolution combination of immunocytochemical methods, Kv1.4 was localized mainly in the apical dendritic trunks and cell bodies of fusiform cells, as well as in dendrites and cell bodies of interneurons of the dorsal cochlear nucleus, likely cartwheel cells. Quantitative immunogold immunocytochemistry revealed a pronounced distal to proximal gradient in the dendrosomatic distribution of Kv1. 4. In plasma membrane localizations, Kv1.4 was preferentially present in dendritic spines, either in the spine neck or in perisynaptic locations, always away from the postsynaptic density. These findings indicate that Kv1.4 is largely distributed in dendritic compartments of fusiform and cartwheel cells of the dorsal cochlear nucleus. Its preferential localization in dendritic spines, where granule cell axons make powerful excitatory synapses, suggests a role for this voltage-dependent ion channel in the regulation of dendritic excitability and excitatory inputs.     

Synaptic organization in the dorsal cochlear nucleus of the cat: a light and electron microscopic study

This report describes some observations of the synaptic organization of one region of the cat dorsal cochlear nucleus (DCN). The large “fusiform cell” and its innervation from the cochlea are emphasized. The morphology of the mature fusiform cell and its postnatal development are described in rapid Golgi impregnations of perfusion-fixed littermate cats. The mature features are correlated with profiles of fusiform cell bodies, apical dendrites, and basal dendritic trunks in electron micrographs from adult cat brains. Small neurons and granule cells are also identified in electron micrographs. In Golgi impregnations, axons of small cells and granule cells may terminate upon fusiform cells. Six classes of axons can be distinguished in rapid Golgi impregnations of the DCN. Two classes are of cochlear origin. One axonal class arises from small cells. The sources of the remaining axonal classes have not been identified in this study. Primary afferents can terminate as large, mossy endings in the DCN neuropil. They can also participate in axonal nests along with axons and dendrites of small cells. In electron micrographs, four synaptic endings can be distinguished. Primary cochlear fibers end in large terminals with asymmetrical synaptic complexes and round, clear vesicles. Primary axons can end in glomeruli, resembling those of the cerebellum, or in synaptic nests which are conglomerates of neuronal processes including other types of endings. The origins of the other synaptic types are not yet known. According to this study, primary afferent input could influence fusiform cells directly or indirectly, via small cells and granule cells.     

Ionotropic and metabotropic glutamate receptors show unique postsynaptic,presynaptic, and glial localizations in the dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) is a major brain center for integration of auditory information, and excitatory amino acid neurotransmission plays a central role in the processing of this information. In this study, the distribution of glutamate receptors was examined with preembedding immunocytochemistry, using 14 antibodies to ionotropic (GluR1, GluR2/3, GluR4, GluR5-7, GluR6/7, KA2, NR1, NR2A/B, delta 1/2) and metabotropic (mGluR1α, mGluR2/3, mGluR5) glutamate receptor subtypes. Each of these antibodies produced a specific immunolabeling pattern, including a variety of postsynaptic, presynaptic, and glial localizations. Some antibodies showed widespread distribution patterns, notably the antibodies to the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor subunits, GluR2 and GluR3, and the N-methyl-D-aspartate (NMDA) receptor subunit, NR1. In contrast, antibodies to other glutamate receptor subunits produced more restricted distribution patterns, especially that to GluR1, which stained the outer neuropil of the DCN, cartwheel cells, and a small population of presumptive interneurons associated with the dorsal acoustic stria, but produced little or no staining in fusiform cells or deep DCN neurons. Staining of the postsynaptic density and membrane of the granule cell-parallel fiber/cartwheel cell spine synapse was most prevalent with delta 1/2 and mGluR1α antibodies. A unique pattern of staining was found with mGluR2/3 antibody—with staining concentrated in Golgi cells and unipolar brush cells of the middle to deep DCN. Distribution of some glutamate receptors in the DCN shows similarities to that of the cerebellum, where delta 2 and mGluR1α may modulate neurotransmission at parallel fiber synapses, while mGluR2 and/or mGluR3 may modulate mossy terminal function. © 1996 Wiley-Liss, Inc.     

Patterns of glutamate decarboxylase immunostaining in the feline cochlear nuclear complex studied with silver enhancement and electron microscopy

The cochlear nuclear complex of the cat was immunostained with an antiserum to glutamate decarboxylase (GAD), the biosynthetic enzyme for the inhibitory neurotransmitter GABA, and studied with different procedures, including silver intensification, topical colchicine injections, semithin sections, and immunoelectron microscopy. Immunostaining was found in all portions of the nucleus. Relatively few immunostained cell bodies were observed: most of these were in the dorsal cochlear nucleus and included stellate cells, cartwheel cells, Golgi cells, and unidentified cells in the deep layers. An accumulation of immunoreactive cells was also found within the small cell cap and along the medial border of the ventral cochlear nucleus. Immunostained cells were sparse in magnocellular portions of the ventral nucleus. Most staining within the nucleus was of nerve terminals. These included small boutons that were prominent in the neuropil of the dorsal cochlear nucleus, the granule cell domain, in a region beneath the superficial granule cell layer within the small cell cap region, and along the medial border of the ventral nucleus. Octopus cells showed small, GAD-positive terminals distributed at moderate density on both cell bodies and dendrites. Larger, more distinctive terminals were identified on the large cells in the ventral nucleus, in particular on spherical cells and globular cells. There was a striking positive correlation of the size, location, and complexity of GAD-positive terminals with the size, location, and complexity of primary fiber endings on the same cells. This correlation did not hold in the dorsal nucleus, where pyramidal cells receive many large GAD-positive somatic terminals despite the paucity of primary endings on their cell bodies. The GAD-positive terminals contained pleomorphic synaptic vesicles and formed symmetric synaptic junctions that occupied a substantial portion of the appositional surface to cell bodies, dendrites, axon hillocks, and the beginning portion of the initial axon segments. Thus, the cells provided with large terminals can be subjected to considerable inhibition that may be activated indirectly through primary fibers and interneurons or by descending inputs from the auditory brainstem.     

Morphology and physiology of cells in slice preparations of the dorsal cochlear nucleus of mice

Horseradish peroxidase (HRP) was injected into cells from which intracellular recordings were made in slices of the dorsal cochlear nucleus (DCN) in order to correlate physiology with morphology. In general, the morphology of cells labeled intracellularly with HRP corresponded to those made with Golgi impregnations in mice and other mammals. The following cells were labeled: one granule cell, four cartwheel cells, eight fusiform cells, two other cells in the fusiform cell layer, and two tuberculoventral association cells in the deep layers of the DCN. The axon of the granule cell runs parallel to isofrequency laminae with collaterals branching perpendicularly and running along the tonotopic axis. The cartwheel cells have dendrites in the molecular layer that are densely covered with spines. The axon of one cell terminates just dorsally to the cell body. Fusiform cells have the characteristic spiny, apical and smooth, basal dendrites. The basal dendrites are conspicuously oriented parallel to isofrequency laminae. Axons of the fusiform cells exit through the dorsal acoustic stria without branching. The two tuberculoventral association cells in the deep DCN have axons that terminate both in the deep DCN, within the same isofrequency lamina that contains the cell body, and in the ventral cochlear nucleus (VCN). Intracellular recordings from 11 of these cells show that they cannot be distinguished on the basis of their responses to intracellularly injected current. All cell types fired large action potentials that were followed by a fast and a slower undershoot, distinguishing them from cells of the VCN but not from one another. Most cells responded to shocks of the auditory nerve root with early EPSPs and later IPSPs. The latencies of EPSPs show that some were monosynaptic and others polysynaptic. That there was no systematic relationship between the latencies of EPSPs and the cell types from which they were recorded shows that shocks to the nerve root may have activated more than just the large, myelinated, auditory nerve fibers.     

Comparative analysis of the subcellular and subsynaptic localization of mGluR1a and mGluR5 metabotropic glutamate receptors in the shell and core of the nucleus accumbens in rat and monkey

Group I metabotropic glutamate receptors (mGluRs) play critical roles in synaptic plasticity and drug addiction. To characterize potential sites whereby these receptors mediate their effects in the ventral striatum, we studied the subcellular and subsynaptic localization of mGluR1a and mGluR5 in the shell and core of the nucleus accumbens in rat and monkey. In both species, group I mGluRs are mainly postsynaptic in dendrites and spines, with rare presynaptic labeling in unmyelinated axons. Minor, yet significant, differences in proportions of specific immunoreactive elements were found between the accumbens shell and the accumbens core in monkey. At the subsynaptic level, significant differences were found in the proportion of plasma membrane-bound mGluR5 labeling between species. In dendrites, spines, and unmyelinated axons, a significantly larger proportion of mGluR5 labeling was bound to the plasma membrane in rats (50-70%) than in monkeys (30-50%). Conversely, mGluR1a displayed the same pattern of immunogold labeling in the two species. Electron microscopic colocalization studies revealed 30% colocalization of mGluR1a and mGluR5 in dendrites and as much as 50-65% in spines in both compartments of the rat accumbens. Both group I mGluRs were significantly expressed in D1-immunoreactive dendritic processes (60-75% colocalization) and spines (30-50%) of striatal projection neurons as well as dendrites of cholinergic (30-70%) and parvalbumin-containing (70-85%) interneurons. These findings highlight the widespread expression of group I mGluRs in projection neurons and interneurons of the shell and core of the nucleus accumbens, providing a solid foundation for regulatory and therapeutic functions of group I mGluRs in reward-related behaviors and drug addiction.     

Physiology and morphology of complex spiking neurons in the guinea pig dorsal cochlear nucleus

Intracellular recordings from the dorsal cochlear nucleus have identified cells with both simple and complex action potential waveforms. We investigated the hypothesis that cartwheel cells are a specific cell type that generates complex action potentials, based on their analogous anatomical, developmental, and biochemical similarities to cerebellar Purkinje cells, which are known to discharge complex action potentials. Intracellular recordings were made from a brain slice preparation of the guinea pig dorsal cochlear nucleus. A subpopulation of cells discharged a series of two or three action potentials riding on a slow depolarization as an all-or-none event; this discharge pattern is called a complex spike or burst. These cells also exhibited anodal break bursts, anomalous rectification, subthreshold inward rectification, and frequent inhibitory postsynaptic potentials (IPSPs). Seven complex-spiking cells were stained with intracellular dyes and subsequently identified as cartwheel neurons. In contrast, six identified simplespiking cells recorded in concurrent experiments were pyramidal cells. The cartwheel cell bodies reside in the lower part of layer 1 and the upper part of layer 2 of the nucleus. The cells are characterized by spiny dendrites penetrating the molecular layer, a lack of basal dendritic processes, and an axonal plexus invading layers 2 and 3, and the inner regions of layer 1. The cartwheel cell axons made putative synaptic contacts at the light microscopic level with pyramidal cells and small cells, including stellate cells, granule cells, and other cartwheel cells in layers 1 and 2. The axonal plexus of individual cartwheel cells suggests that they can inhibit cells receiving inpt;t from either the same or adjacent parallel fibers and that this inhibition is distributed along the isofrequency contours of the nucleus. © 1994 Wiley-Liss, Inc.     

Cochlear nerve projections to the small cell shell of the cochlear nucleus: The neuroanatomy of extremely thin sensory axons

Labeling cochlear nerve fibers in the inner ear of chinchillas with biotinylated dextran polyamine was used to trace the thin fibers (Type II), which likely innervate outer hair cells. These axons, 0.1–0.5 μm in diameter, were distinguished from the thicker Type I, fibers innervating inner hair cells, and traced to small‐cell clusters in the cochlear nucleus. This study provided two major new insights into the outer hair cell connections in the cochlear nucleus and the potential significance of very thin axons and synaptic nests, which are widespread in the CNS. 1) EM serial reconstructions of labeled and unlabeled material revealed that Type II axons rarely formed synapses with conventional features (vesicles gathered at junctions). Rather, their endings contained arrays of endoplasmic reticulum and small spherical vesicles without junctions. 2) Type II axons projected predominantly to synaptic nests, where they contacted other endings and dendrites of local interneurons (small stellate and mitt cells, but not granule cells). Synaptic nests lacked intrinsic glia and, presumably, their high‐affinity amino acid transporters. As functional units, nests and their Type II inputs from outer hair cells may contribute to an analog processing mode, which is slower, more diffuse, longer‐lasting, and potentially more plastic than the digital processors addressed by inner hair cells. Synapse 33:83–117, 1999. © 1999 Wiley‐Liss, Inc.     

Differentiation of the giant and fusiform cells in the dorsal cochlear nucleus of the hamster

Two categories of large neurons--fusiform cells and giant cells--can be distinguished in the dorsal cochlear nucleus of the hamster. In the adult, these neurons are located in separate laminae in the nucleus and have distinct dendritic morphology. However, the two cell types are not distinguishable in the newborn hamster. At birth the large cells in the dorsal cochlear nucleus are clustered into one group and are alike morphologically. On postnatal day 5, laminae are still not apparent, but the neurons have begun to acquire their adult shapes. By day 15 laminae have formed, and the cells appear mature with the one exception that the apical dendrites of the fusiform cells have not acquired the spines which will cover their surface in the adult. The appearance of laminae coincides with the growth of axons and dendrites into a interstitial zone between the layers of cell bodies. Dendritic growth occurs during the time of axonal ingrowth and establishment of contacts between the axons and dendrites. The growth of the apical dendrites of fusiform cells, which are not contacted by these fibers, lags behind. These results demonstrate that afferent ingrowth and the differentiation of dendrites in the dorsal cochlear nucleus are temporally related. The synchronous development may serve to ensure a specific synaptic arrangement between the axons and their target dendrites.     

Differential distribution of synaptic endings containing glutamate, glycine, and GABA in the rat dorsal cochlear nucleus

The dorsal cochlear nucleus (DCN) integrates the synaptic information depending on the organization of the excitatory and inhibitory connections. This study provides, qualitatively and quantitatively, analyses of the organization and distribution of excitatory and inhibitory input on projection neurons (fusiform cells), and inhibitory interneurons (vertical and cartwheel cells) in the DCN, using a combination of high-resolution ultrastructural techniques together with postembedding immunogold labeling. The combination of ultrastructural morphometry together with immunogold labeling enables the identification and quantification of four major synaptic inputs according to their neurotransmitter content. Only one category of synaptic ending was immunoreactive for glutamate and three for glycine and/or gamma-aminobutyric-acid (GABA). Among those, nine subtypes of synaptic endings were identified. These differed in their ultrastructural characteristics and distribution in the nucleus and on three cell types analyzed. Four of the subtypes were immunoreactive for glutamate and contained round synaptic vesicles, whereas five were immunoreactive for glycine and/or GABA and contained flattened or pleomorphic synaptic vesicles. The analysis of the distribution of the nine synaptic endings on the cell types revealed that eight distributed on fusiform cells, six on vertical cells and five on cartwheel cells. In addition, postembedding immunogold labeling of the glycine receptor alpha1 subunit showed that it was present at postsynaptic membranes in apposition to synaptic endings containing flattened or pleomorphic synaptic vesicles and immunoreactive for glycine and/or GABA on the three cells analyzed. This information is valuable to our understanding of the response properties of DCN neurons.     

Cartwheel neurons of the dorsal cochlear nucleus: a Golgi-electron microscopic study in rat

Cartwheel neurons in rat dorsal cochlear nucleus (DCN) were studied by Golgi impregnation-electron microscopy. Usually situated in layers 1-2, cartwheel neurons (10-14 micrometers in mean cell body diameter) have dendritic trees predominantly in layer 1. The dendrites branch at wide angles. Most primary dendrites are short, nontapering, and bear only a few sessile spines. Secondary and tertiary dendrites are short, curved, and spine-laden. The perikaryon forms symmetric synapses with at least two kinds of boutons containing pleomorphic vesicles. The euchromatic nucleus is indented and has an eccentric nucleolus. The cytoplasm shows several small Nissl bodies, a conspicuous Golgi apparatus, and numerous subsurface and cytoplasmic cisterns of endoplasmic reticulum with a narrow lumen, joined by mitochondria in single or multiple assemblies. In primary dendrites mitochondria are situated peripherally, while in distal branches they become ubiquitous and relatively more numerous. Dendritic shafts usually form symmetric synapses with boutons that contain pleomorphic vesicles. The majority of the dendritic spines are provided with a vesiculo-saccular spine apparatus. All dendritic spines have asymmetric synapses. Most of these are formed with varicosities of thin, unmyelinated fibers (presumably axons of granule cells) running parallel to the long axis of the DCN or radially. These varicosities contain round, clear synaptic vesicles. On the initial axon segment few symmetric synapses are present. The axon acquires a thin myelin sheath after a short trajectory. Cartwheel neurons outnumber all other neurons in layers 1-2 (with the exception of granule cells), and presumably correspond to type C cells with thinly myelinated axons described by Lorente de Nó. The axons of these neurons provide a dense plexus in the superficial layers without leaving the DCN. The possible functional role of cartwheel neurons is discussed.     

Immunocytochemical localization of GABA in the cochlear nucleus of the guinea pig

The immunocytochemical distribution of gamma-aminobutyric acid (GABA) was determined in the cochlear nucleus of the guinea pig using affinity-purified antibodies made against GABA conjugated to bovine serum albumin. Light microscopic immunocytochemistry shows immunoreactive puncta, which appear to be GABA-positive presynaptic terminals, distributed throughout the cochlear nucleus. In the ventral cochlear nucleus, these puncta are often found around unlabeled neuronal cell bodies. While occasional labeled small cells are found in the ventral cochlear nucleus, most GABA-immunoreactive cell bodies are present in the superficial layers of the dorsal cochlear nucleus. Based on size and shape, immunoreactive cells in the dorsal cochlear nucleus are divided into 3 classes: medium round cells with diameters averaging 16 microns, small round cells with average diameters of 9 microns and small flattened cells with major and minor diameters averaging 11 and 6 microns, respectively. Labeled fusiform and granule cells are not seen. A similar distribution of label was seen using antibodies against glutamic acid decarboxylase. Electron microscopic immunocytochemistry of the anteroventral cochlear nucleus shows GABA immunoreactive boutons containing oval/pleomorphic synaptic vesicles on cell bodies and dendrites. Other major classes of terminals, including those with small round, large round and flattened synaptic vesicles are unlabeled.     

Distribution of mGluR1alpha and mGluR5 immunolabeling in primate prefrontal cortex

Metabotropic glutamate receptors (mGluRs) mediate important modulatory glutamatergic influences throughout the brain. However, the specific localization and functions of group I mGluR subtypes (mGluR1alpha and mGluR5) in cortical neurotransmission are not well known, particularly in primates. To address this issue, we used immunoelectron microscopy to compare the subcellular localizations of mGluR1alpha and mGluR5 in the prefrontal cortex of macaque monkeys. Both receptor subtypes were found in a variety of subcellular compartments, including spines, dendrites, preterminal axons, axon terminals, and glia; however, quantitative differences were found in the relative abundance of labeled elements for each receptor. The mGluR1alpha-immunoreactive (-IR) elements were overwhelmingly the spines and dendrites, with labeled terminals, axons, and glia seen more rarely. The mGluR5-IR elements were also mostly spines and dendrites, but the proportion of labeled unmyelinated axons, terminals, and glia was higher than for mGluR1alpha-IR elements. Double labeling with SMI-32 and parvalbumin confirmed that both receptors were found in pyramidal cell and interneuron dendrites. The localization of mGluR1alpha to pyramidal cells in primate cortex contrasts with reports that mGluR1alpha is found almost exclusively in interneurons in rodent cortex. By using double labeling, we found no evidence for mGluR1alpha or mGluR5 in dopaminergic afferents to prefrontal cortex. The data presented here provide an anatomical substrate for a differential role of mGluR1alpha and mGluR5 in post-and presynaptic actions of glutamate in primate prefrontal cortex. They further suggest differences in the cortical distribution of group I mGluRs between primates and rodents.     

The fine structure of the lateral superior olivary nucleus of the cat

The synaptic organization of the lateral superior olivary nucleus of the cat was analyzed under the electron microscope. The predominant cell type, the fusiform cell, has dendrites that extend from opposite poles of the cell body toward the margins of the nucleus, where they terminate in spinous branches. The fusiform cells are contacted by three types of synaptic terminals that can be distinguished by the size and shape of their synaptic vesicles. The somatic and proximal dendritic surfaces are apposed by synaptic terminals containing small, flat synaptic vesicles. Further from the cell body, the dendrites form numerous synaptic contacts with terminals containing large round vesicles as well as with the terminals containing small, flat vesicles. The most distal dendritic branches and their spiny appendages appear to form synapses almost exclusively with the terminals with large, round vesicles. A relatively rare type of terminal that contains small, round vesicles may form synapses with either the somatic or dendritic surfaces. A few small cells are interspersed among the fusiform cells, but they are more commonly located around the margins of the nucleus. The small cells form few axosomatic contacts. The simplest interpretation of the findings is that the terminals with small, flat vesicles arise in the medial nucleus of the trapezoid body and are inhibitory in function, whereas the terminals with large, round vesicles arise in the anteroventral cochlear nucleus and are excitatory; however, this remains to be demonstrated experimentally. In any case, the differential distribution of these two types of inputs on the somatic and dendritic surfaces must be an important determinant of the physiological response properties of the fusiform cells to binaural acoustic stimuli.     

Fine structure of the cell clusters in the cochlear nerve root: Stellate,granule, and mitt cells offer insights into the synaptic organization of local circuit neurons

The small cell shell of the cochlear nucleus contains a complex integrative machinery which can be used to study the roles of interneurons in sensory processing. The cell clusters in the cochlear nerve root of the chinchilla provide the simplest example of this structure. Reported here are the neuronal architecture and synaptic organization of the three principal cell types and the three distinctive neuropil structures that could be characterized with the Nissl and Golgi methods and electron microscopy. Granule cells were characterized by several dendrites with claw-like terminals that received synaptic contacts from multiple excitatory mossy fiber rosettes. Given their relatively large number and their prolific parallel fiber synapses, the granule cells provide a suitable substrate for a tangential spread of excitatory activity, which could build to considerable proportions. The mitt cells had a thickened, single dendrite, its terminal branches arranged in a shape reminiscent of a baseball catcher's mitt. The dendritic mitt enclosed an enormous, convoluted mossy fiber rosette forming many excitatory synapses on just one cell. This could provide for a discrete, comparatively fast input-output relay of signals. Small stellate cells had longer, radiating dendrites that engaged the synaptic nests. These nests were strung in long strands, containing heterogeneous synapses from putative excitatory and inhibitory inputs. Given the prevalence of the synaptic nests, the small stellate cells appear to have the greatest integrative capacity. They provide the main output of the synaptic nests. © 1996 Wiley-Liss, Inc.     

Heterogeneity of glycinergic and gabaergic interneurons in the granule cell layer of mouse cerebellum

Interneurons of the cerebellum granule cell layer (GCL) form distinct populations. Golgi cells extend dendrites in the molecular layer (ML) and innervate granule cells. In contrast, Lugaro cells have dendrites confined to the GCL but innervate interneurons in the ML, and globular cells have both their dendrites and axons in the ML. The latter cells were described recently and remain poorly characterized. Although several neurochemical markers have been associated selectively with GCL interneurons, it is unclear how they relate to their morphological classification and neurochemical phenotype (glycinergic and/or gamma-aminobutyric acid [GABA]ergic). Here, we performed a detailed characterization of GCL interneurons in mice expressing enhanced green fluorescent protein (GFP) in glycinergic and GABAergic neurons, respectively. By using immunofluorescence for metabotropic glutamate receptor 2 (mGluR2) and neurogranin as markers, we demonstrate the existence of five non-overlapping subsets of Golgi cells: about 65% are glycinergic/GABAergic and co-express both markers. Two small subsets (5-10%) also contain both neurotransmitters but express only mGluR2; they are distinguished by cell body size and location in the GCL. The fourth subset (15%) is GABAergic only and expresses neurogranin. The fifth subset (5%) is glycinergic only and lacks both markers. Thus, the heterogeneity of Golgi cells suggests that they belong to specific functional circuits and are differentially regulated by mGluRs and Ca(2+)-calmodulin-dependent signaling pathways. In contrast to Golgi cells, Lugaro and globular cells are glycinergic/GABAergic and lack mGluR2 and neurogranin. They each represent at least 15% of GCL interneurons and extensively innervate stellate and basket cells, but not Purkinje cells, emphasizing their contribution to inhibitory control of ML interneurons.     

Projections from auditory cortex to the cochlear nucleus in rats: Synapses on granule cell dendrites

Previous work has demonstrated that layer V pyramidal cells of primary auditory cortex project directly to the cochlear nucleus. The postsynaptic targets of these centrifugal projections, however, are not known. For the present study, biotinylated dextran amine, an anterograde tracer, was injected into the auditory cortex of rats, and labeled terminals were examined with light and electron microscopy. Labeled corticobulbar axons and terminals in the cochlear nucleus are found almost exclusively in the granule cell domain, and the terminals appear as boutons (1–2 μm in diameter) or as small mossy fiber endings (2–5 μm in diameter). These cortical endings contain round synaptic vesicles and form asymmetric synapses on hairy dendritic profiles, from which thin (0.1 μm in diameter), nonsynaptic “hairs” protrude deep into the labeled endings. These postsynaptic dendrites, which are typical of granule cells, surround and receive synapses from large, unlabeled mossy fiber endings containing round synaptic vesicles and are also postsynaptic to unlabeled axon terminals containing pleomorphic synaptic vesicles. No labeled fibers were observed synapsing on profiles that did not fit the characteristics of granule cell dendrites. We describe a circuit in the auditory system by which ascending information in the cochlear nucleus can be modified directly by descending cortical influences. © 1996 Wiley-Liss, Inc.     

Origins and targets of commissural connections between the cochlear nuclei in guinea pigs

Our objective was to identify the origins and targets of axons that project from one cochlear nucleus to the other. First, retrograde tracers were injected into one cochlear nucleus to label commissural cells in the opposite nucleus. In the dorsal cochlear nucleus, a few cells in the deep layers were labeled; they were not further classified according to type. In the ventral cochlear nucleus, all commissural cells that could be classified were multipolar cells. Second, an anterograde tracer was injected into one cochlear nucleus, and the distribution of boutons in the opposite cochlear nucleus was examined. Labeled boutons were present throughout the ventral cochlear nucleus, where they appeared to contact multipolar cells, spherical and globular bushy cells, and octopus cells. In the dorsal cochlear nucleus, labeled boutons were present in the fusiform cell and deep layers and appeared to contact fusiform cells and cells of unknown type. Many labeled terminals were also present in the granule cell regions. Injections into regions associated with high or low frequencies labeled boutons in corresponding regions in the contralateral ventral cochlear nucleus. Third, multiple tracers were used to determine whether cells that project to the inferior colliculus are contacted by commissural axons. Boutons labeled by anterograde transport of one tracer placed in the cochlear nucleus were frequently observed to be apposed to cells that were labeled by retrograde transport of a different tracer placed in the contralateral inferior colliculus. We conclude that commissural projections originate from multipolar cells throughout the ventral cochlear nucleus (and from a small number of cells in the dorsal cochlear nucleus) and make contact with all major cell types of the cochlear nuclei, including at least some of those that project to the inferior colliculus. © 1996 Wiley-Liss, Inc.     

Somatostatin-containing neurons in rat organotypic hippocampal slice cultures: Light and electron microscopic immunocytochemistry

Light and electron microscopic immunocytochemical techniques were used to study the interneuron population staining for somatostain (SRIF) in cultured slices of rat hippocampus. The SRIF immunoreactive somata were most dense in stratum oriens of areas CA1 and CA3, and in the dentate hilus. Somatostain immunoreactive cells in areas CA1 and CA3 were characteristically fusiform in shape, with dendrites that extended both parallel to and into the alveus. The axonal plexus in areas CA1 and CA3 was most dense in stratum lacunosum-moleculare and in stratum pyramidale. Electron microscopic analysis of this area revealed that the largest number of symmetric synaptic contacts from SRIF immunoreactive axons were onto pyramidal cell somata and onto dendrites in stratum lacunosum-moleculare. In the dentate gyrus, SRIF somata and dendrites were localized in the hilus. Hilar SRIF immunoreactive neurons were fusiform in shape and similar in size to those seen in CA1 and CA3. Axon collaterals coursed throughout the hilus, projected between the granule cells and into the outer molecular layer. The highest number of SRIF synaptic contacts in the dentate gyrus were seen on granule cell dendrites in the outer molecular layer. Synaptic contacts were also observed on hilar neurons and granule cell somata. SRIF synaptic profiles were seen on somata and dendrites of interneurons in all regions. The morphology and synaptic connectivity of SRIF neurons in hippocampal slice cultures appeared generally similar to intact hippocampus. © 1994 Wiley-Liss, Inc.