Differential expression of the metabotropic glutamate receptor mGluR1α by neurons and axons in the cochlear nucleus: In situ hybridization and immunohistochemistry

mGluR1α is a metabotropic glutamate receptor involved in synaptic modifiability. A differential expression in specific neuronal types could reflect their different connections and response properties in central auditory processing. Using in situ hybridization and immunohistochemistry, we studied mGluR1α receptor expression throughout the cochlear nucleus. Robust labeling occurred in the dorsal cochlear nucleus and small cell shell, with less in the ventral cochlear nucleus. Among the most intensely labeled were the granule cells of the small cell shell. In the dorsal cochlear nucleus, most cell types expressed message and receptor protein, except granule cells. High levels of receptor were expressed by corn cells and cartwheel cells. The terminal dendrites and synaptic spines of cartwheel and fusiform cells contained receptor protein in the molecular layer, where they could synapse with parallel fibers. Fusiform dendrites also expressed mRNA for mGluR1α. The basal dendrites of fusiform cells contained receptor protein in the region where they receive cochlear nerve synapses. Immunostaining of terminal axons was prominent in the molecular layer and the small cell shell, where they were associated with synaptic nests, structures thought to provide long-term changes in excitability. Differential expression levels may reflect different functional requirements of specific cell types, including inhibitory interneurons, like corn cells and cartwheel cells, and excitatory interneurons, like granule cells in the small cell shell, which may participate in local circuits involved in modulatory or gating functions, such as stimulus enhancement or suppression. In presynaptic axons, mGluR1α may relate to the long-term signaling requirements of their modulatory functions. Synapse 28:251–270, 1998. © 1998 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.     

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.     

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.     

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.     

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.     

The cytoarchitecture of the dorsal cochlear nucleus in the 3-month- and 26-month-old C57BL/6 mouse: A golgi impregnation study

The cytoarchitecture of the dorsal cochlear nucleus (DCN) was compared in 3- and 26-month-old C57BL/6 mice. The effects of genetically controlled progressive hearing loss present in the CNS in this mouse strain were analyzed with Nissl-stained and Golgi-impregnated material. The DCN was divided into the superficial molecular, an intermediate fusiform-granule, and the deep polymorphic layers. The molecular layer (ML) consisted of many fibers and a few small ovoid to spherical, fusiform, and granule cells. The fusiform-granule layer (FL) contained large fusiform and many granule cells. Most FL fusiform cells were oriented with their long axes perpendicular to the DCN surface and were present as small aggregations or individually. Cartwheel cells were adjacent to the FL fusiform cells. The deep polymorphic layer (PL) contained spherical, fusiform, granule, and multipolar neurons. The granule cells formed a dorsal cap of the DCN. From this cap, sheets of granule cells separated the DCN from the posterior ventral cochlear nucleus (PVCN) and from the brainstem. The internal organization, neuronal location, orientation, and morphology were similar in both age groups. The granule cells had four to five primary dendrites, varicosities, and few to no dendritic appendages. The FL fusiform cells displayed different dendritic morphology in the two ages. One or two elaborate primary ML apical dendrites in the 3-month-old mice were covered with spikelike dendritic spines. The basal one or two PL dendrites were less elaborate and had few dendrite spines. In contrast, FL fusiform neurons in 26-month-old mice had regular dendritic varicosities and fewer spines which were short and stumpy. Basal dendrites had varicosities and interruptions. Cartwheel neurons in 3-month-old mice had elaborate ML dendritic trees covered with dendritic spines. In 26-month-old mice the dendrites had many varicosities and fewer short blunted dendritic spines. Large multipolar neurons in older mice had thinner dendrities with more varicosities than were in the 3-mcnth group. In both age groups multipolar cells had few dendritic spines limited distally. Small and large spherical cells had two to five primary dendrites with varicosities, little higher-order branching, and spines. Fusiform cells had one or two primary dendrites, little secondary branching, and few to no spines. Minor degenerative changes were noted in spherical and fusiform cells in the two age groups. These included dendritic varicosities, interruptions, and some irregularities of somata surface. Degenerative changes present in the cochlea had significant effects on a limited population of DCN neurons. Finally, the neusronal morphology and architecture of the DCN in C57BL/6 mouse is similar to other mammalian species.     

The neurons in the primate subthalamic nucleus: A Golgi and electron microscopic study

In Golgi preparations of the adult monkey(Macaca mulatta) local interneurons and two varieties of principal neurons, radiating and elongated fusiform, are found in the subthalamic nucleus. The cell bodies of the radiating neurons have a few delicate, somatic spines some of which are occasionally bilobed and trilobed. Five to eight dendritic trunks give rise to branching, tapering dendrites, which may extend for over 400 microns. These dendrites are much thinner than the dendrites in the globus pallidus and the substantia nigra. Some neurons have many and some neurons have few dendritic spines. When numerous the dendritic spines are concentrated on the dendritic trunks and proximal dendrites. The relatively few elongated fusiform neurons are found not only in the capsule but also in the center of the nucleus. Most dendrites emerge from the opposite poles of their smooth surfaced cell bodies. They have a few dendritic spines. Some of these dendrites extend for more than 750 microns. In 1-micron thick plastic sections lipofuscin granules are present in some but not all principal neuron cell bodies of the pig-tail monkey(Macaca mulatta); but these granules are present in all principal neuron cell bodies of the pig-tail monkey(Macaca nemestrina) and of the squirrel monkey (Saimiri sciureus). The local interneurons have small cell bodies and a few relatively long undulating dendrites. The dendrites have bulbous dendritic appendages of varying complexity and beaded axon- like processes. The dendritic appendages and axon-like processes are more numerous distally and on the distal ends of the dendrites they form complex entanglements. Axons coming from the cell body have not been observed. The cell bodies of the local interneurons are identified in cresyl violet stained sections of the monkey (Macaca mulatta), in 1-micron thick plastic sections and electron micrographs of the squirrel monkey(Saimiri sciureus). They have relatively large nuclei surrounded by a thin rim of cytoplasm rich in polyribosomes.     

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.     

Unique somato-dendritic distribution pattern of Kv4.2 channels on hippocampal CA1 pyramidal cells

A-type K(+) current (I(A)) plays a critical role in controlling the excitability of pyramidal cell (PC) dendrites. In vitro dendritic patch-pipette recordings have demonstrated a prominent, sixfold increase in I(A) density along the main apical dendrites of rat hippocampal CA1 PCs. In these cells, I(A) is mediated by Kv4.2 subunits, whose precise subcellular distribution and densities in small-diameter oblique dendrites and dendritic spines are still unknown. Here we examined the densities of the Kv4.2 subunit in 13 axo-somato-dendritic compartments of CA1 PCs using a highly sensitive, high-resolution quantitative immunogold localization method (sodium dodecyl sulphate-digested freeze-fracture replica-labelling). Only an approximately 70% increase in Kv4.2 immunogold density was observed along the proximo-distal axis of main apical dendrites in the stratum radiatum with a slight decrease in density in stratum lacunosum-moleculare. A similar pattern was detected for all dendritic compartments, including main apical dendrites, small-diameter oblique dendrites and dendritic spines. The specificity of the somato-dendritic labelling was confirmed in Kv4.2(-/-) tissue. No specific immunolabelling for the Kv4.2 subunit was found in SNAP-25-containing presynaptic axons. Our results demonstrate a novel distribution pattern of a voltage-gated ion channel along the somato-dendritic surface of CA1 PCs, and suggest that the increase in the I(A) along the proximo-distal axis of PC dendrites cannot be solely explained by a corresponding increase in Kv4.2 channel number.     

Inositol 1,4,5-trisphosphate receptors: Immunocytochemical localization in the dorsal cochlear nucleus

In the cochlear nucleus of mammals, the relatively homogeneous responses of auditory nerve fibers are transformed into a variety of different response patterns by the different classes of resident neurons. The spectrum of these responses is hypothesized to depend on the types and distribution of receptors, ion channels, G proteins, and second messengers that form the signaling capabilities in each cell class. In the present study, we examined the immunocytochemical distribution of the inositol 1,4,5-trisphosphate (IP3) receptor in the dorsal cochlear nucleus to better understand how this second messenger might be involved in shaping the neural signals evoked by sound. Affinity-purified polyclonal antibodies directed against the IP3 receptor labeled a homogeneous population of neurons in the dorsal cochlear nucleus of rats, guinea pigs, mustache bats, cats, New World owl monkeys, rhesus monkeys, and humans. These cells were all darkly immunostained except in the human where the labeling was less intense. Immunoblots of dorsal cochlear nucleus tissue from the rat revealed a single band of protein of molecular weight ～260 kD, which is the same size as the purified receptor, indicating that our antibodies reacted specifically with the IP3 receptor. These immunolabeled neurons were identified as cartwheel cells on the basis of shared characteristics across species, including cell body size and distribution, the presence of a highly invaginated. Nucleus, and a well-developed systain of cisternae. Reaction product was localized along the membranes of rough and smooth endoplasmic reticulum, subsurface cisternae, and the nuclear envelope. This label was distributed throughout the cartwheel cell body and dendritic shafts but not within dendritic spines, axons, or axon terminals. The regular pattern of immunolabeling across mammals suggests that IP3 and cartwheel cells are conserved in evolution and that both play an important but as yet unknown role in hearing. © 1995 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.     

The neurons in the centromedian-parafascicular complex of the monkey (Macaca mulatta): A Golgi Study

The neurons of the nucleus centrum medianum and the neurons of the nucleus parafascicularis were studied in Golgi preparations of the adult monkey(Macaca mulatta) The cell bodies of the principal neurons in the nucleus centrum medianum have a few somatic spines and vary in shape: some are cubical with protruding angles; some are egg-shaped;some are elongated and sausage-shaped. Four to six slightly branched dendrites of unequal thickness radiate from the cell body. Some dendrites extend for nearly 500 microns; all have dendritic spines. In the nucleus parafascicularis there are two varieties of principal neurons:(1) neurons with somatic spines and(2) neurons without somatic spines. The neurons with somatic spines are most numerous. They have polygonal-shaped cell bodies, prominent somatic spines and processes, larger than spines but considerably smaller than dendrites. These processes bear spines and are designated here “microdendrites.” Spines and occasionally a “microdendrite” are found on the axon-hillocks. Five to six dendrites of unequal thickness emerge from the cell bodies. Some extend for more than 500 microns; all have prominent dendritic spines. The neurons without somatic spines are relatively few. Usually three exceptionally long, slightly branched dendrites, one apical and two basal, emerge from their elongated, slim cell bodies. Some dendrites extend for more than 800 microns; all have a few scattered spines. The Golgi type II neurons found in both of these intralaminar nuclei have small cell bodies and a few, relatively long, undulating dendrites, which bear bulbous dendritic appendages and beaded axon-like processes. Distally on these dendrites, where the appendages and processes are more numerous, the dendritic appendages and axon-like processes form complex entanglements. Beaded axons are found on some but not all of the cell bodies. Morphologically these neurons resemble the local interneurons that have been described in various thalamic nuclei.     

Localization of the GABAB receptor 1a/b subunit relative to glutamatergic synapses in the dorsal cochlear nucleus of the rat

Metabotropic gamma-aminobutyric acid receptors (GABA(B)) are involved in pre- and postsynaptic inhibitory effects upon auditory neurons and have been implicated in different aspects of acoustic information processing. To understand better the mechanisms by which GABA(B) receptors mediate their inhibitory effects, we used pre-embedding immunocytochemical techniques combined with quantification of immunogold particles to reveal the precise subcellular distribution of the GABA(B1) subunit in the rat dorsal cochlear nucleus. At the light microscopic level, GABA(B1) was detected in all divisions of the cochlear complex. The most intense immunoreactivity for GABA(B1) was found in the dorsal cochlear nucleus, whereas immunoreactivity in the anteroventral and posteroventral cochlear nuclei was very low. In the dorsal cochlear nucleus, a punctate labeling was observed in the superficial (molecular and fusiform cell) layers. At the electron microscopic level, GABA(B1) was found at both post- and presynaptic locations. Postsynaptically, GABA(B1) was localized mainly in the dendritic spines of presumed fusiform cells. Quantitative immunogold immunocytochemistry revealed that the highest concentration of GABA(B1) in the plasma membrane was in dendritic spines, followed by dendritic shafts and somata. Thus, the most intense immunoreactivity for GABA(B1) was observed in dendritic spines with a high density of immunogold particles at extrasynaptic sites, peaking around 300 nm from glutamatergic synapses. This is in contrast to GABAergic synapses, in which GABA(B1) was only occasionally found. Presynaptically, receptor immunoreactivity was detected primarily in axospinous endings, probably from granule cells, in both the active zone and extrasynaptic sites. The localization of GABA(B1) relative to synaptic sites in the DCN suggests a role for the receptor in the regulation of dendritic excitability and excitatory inputs.     

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.     

Electron microscopic features of physiologically characterized, HRP-labeled fusiform cells in the cat dorsal cochlear nucleus

We report on the anatomy and physiology of three fusiform cells in the dorsal cochlear nucleus (DCN) of the cat. The extra- and intracellular responses of these cells to pure tones showed features typical of the cell type. Peristimulus time histograms (PSTHs) were usually of the pauser or buildup configuration with chopping behavior noted in certain instances. Intracellular records during stimulus presentations revealed sustained depolarizations for the duration of the tone followed by a prolonged after-hyperpolarization (AHP). On rare occasions, a hyperpolarization corresponding to the pause region of the PSTH was noted. Occasionally, a stimulus-induced depolarization would be maintained after stimulus offset. Rebound excitation was also observed after the AHP. Morphologically, all three cells showed the standard fusiform cell features at the light microscopic level. The cell body gave rise to apical and basal dendritic trees. The apical tree branched frequently and displayed numerous spines distally. The basal tree had fewer branches and fewer, more irregular appendages. The axon originated from the cell body and gave rise to one or more collaterals before leaving the nucleus via the dorsal acoustic stria (DAS). At the electron microscopic (EM) level, the axon collaterals may terminate on a variety of cell types in the DCN, including fusiform cells. Their vesicles are round and the terminals closely resemble many unlabeled terminals seen on the cell body and apical and basal dendrites of our labeled fusiform cells. Terminals containing round vesicles, believed to be eighth nerve terminals, were found, with one exception, only on the basal dendrites. The spine-laden, distal apical dendrites received primarily terminals containing round vesicles, presumed to originate from the unmyelinated axons of granule cells. The cell body and unmyelinated initial segment received mostly terminals containing pleomorphic and flat vesicles, which also made up a large percentage of the dendritic input. Some relevant correlations, between the distribution of synaptic terminals and the observed physiology, may be possible.     

Golgi studies of the neurons in layer II of the dorsal horn of the medulla (trigeminal nucleus caudalis)

The translucent band which lies just beneath the spinal V tract at the lower end of the spinal trigeminal nucleus (nucleus caudalis) can be divided into three layers. These three layers are distinguished by textural differences in their neuropil and by the morphology and laminar distribution of the axons and dendrites of their neurons. Layer II contains four different kinds of interneurons. Stalked cells are named after their short, stalk-like branches. Their cell bodies are found in greatest numbers in the outer half of layer II. Their coneshaped dendritic arbors extend medially across layers II and III and sometimes extend into layer IV. Their axons form extensive, canopy-like arborizations in layer I. Stalked cells are considered to be excitatory interneurons receiving input on their dendritic spines from primary axonal endings in the layers II and III glomeruli and transferring it to the dendrites of the layer I projection neurons. Layer II contains three kinds of Golgi type II inteneurons, i.e, neurons whose axons branch repeatedly within the confimes of their dendritic arbors. Islet cells similar to those found in layer III (Gobel), '75a), are found in small clusters in layer II. Their dendrites and axons are largely confined in layer II. The dendrites of the arboreal cell burst, in tree-like fashion, into highly focal dendritic arbors confined largely in layer II while the extensive rostral and caudal dendritic arbors of the II-III border cell lie largely in layers II and III with a few branches extending into layer I. The axons of both of these interneurons arborize in layers II and III with a few collaterals extending into layer I. Islet cells, arboreal cells and II-III border cells are considered to be inhibitory interneurons. They are strategically situated to interrupt transmission between primary axonal endings in layers II and III and the layer I projection neurons by altering the output of the stalked cells.     

Developmental regulation and adult maintenance of potassium channel proteins (Kv 1.1 and Kv 1.2) in the cochlear nucleus of the rat

The development and maintenance of the adult expression and distribution of Kv 1.1 and Kv 1.2, two voltage-dependent potassium channel subunits, were investigated in the anteroventral cochlear nucleus (AVCN) of the rat. Both Kv 1.1 and Kv 1.2 were found in AVCN neuronal cell bodies at birth, as detected by in situ hybridization and immunocytochemistry. However, Kv 1.1 and Kv 1.2 were not seen in axons until the end of the third postnatal week. From postnatal day 21 through adulthood, labeling for both potassium channels was in axonal processes, whereas the number of cell bodies labeled for Kv 1.1 decreased and there were no cell bodies labeled for Kv 1.2. Therefore, these two potassium channel proteins are targeted to their final subcellular destinations in axons well after hearing onset. Once the adult distribution pattern of Kv 1.1 and Kv 1.2 is attained, its maintenance does not depend on signals from auditory nerve synapses. Eliminating auditory nerve input to the cochlear nucleus by means of bilateral cochleotomy did not change Kv 1.1 or Kv 1.2 expression or distribution, as seen by in situ hybridization, immunocytochemistry and Western blot. Thus, normal excitatory synaptic input in adult animals is not a requirement to regulate the expression and cellular and subcellular distribution of these potassium channel proteins.     

Golgi studies in the substantia gelatinosa neurons in the spinal trigeminal nucleus.

This Golgi study identifies three neuronal cell types in the substantia gelatinosa (SG) layer of the spinal trigeminal nucleus. The SG neurons are distinguished from each other based on: (1) dendritic branching pattern, (2) denritic spine distribution, (3) geometric shape of the denritic tree, (4) laminar distribution of the dendrites, (5) axonal branching pattern and (6) laminar distribution of the axonal arbor. The islet cell is found in small clusters and its dendrites and axonal arbor are confined within the SG layer. Its dendrites span the full width of the SG layer and extend up to 500 mum in the long axis of the layer. Dendritic spines are generally sparse with small clusters of spines found on the higher order dendritic branches. The islet cell axon extends for at least 1 mm in the long axis of the layer. Each of its collaterals divide every 50-100 mum with one branch doubling back in the direction of the cell body and the other branch continuing on in the direction of its parent. In this manner each islet cell generates a profuse axonal plexus in the SG layer. The stalked cell is found individually within the SG layer. Its cell body is usually found in the inner half of the SG layer and its sinuous dendrites cross the SG layer and enter the marginal layer. The stalked cell dendrites emit numerous fine stalk-like branches and dentritic spines. Its axon emits branches in the SG and marginal layers. The spiny cell is found singly between groups of islet cells. Its extensive dendritic tree spans up to 500 mum rostrocaudally and mediolaterally crossing into both the marginal and magnocellular layers. Spiny cells have evenly distributed dendritic spines along their dendrites in the SG layer. The spiny cell axon sends branches into all three layers of nucleus caudalis. Numerous branches enter the outer 300 mum of the magnocellular layer where they undergo further branching with some branches returning in recurrent fashion toward the SG layer. The three neuronal cell types of the SG layer satisfy all of the morphological criteria for Golgi type II interneurons. Their highly branched axons generate many collaterals within the confines of their dendritic trees and do not project out of nucleus caudalis. The SG neurons are considered to be inhibitory interneurons interposed between V nerve primary afferent axons which arborize in the SG layer and second order neurons of nucleus caudalis.     

A study of neuronal types in Clarke's column in the adult cat

Three neuronal classes have been identified in Clarke's column in the adult cat. The smallest cells (class A) exhibit variable dendritic branching patterns. Medium-sized neurons (class B) can be subdivided into multipolar and fusiform cells. The majority of the multipolar cells have elongated perikarya and their dendrites project in a radial fashion, while the fusiform neurons often have their long axis perpendicularly oriented. The large Clarke cells (class C) and their dendrites project in the cranio-caudal direction. Their dendrites are generally smooth and often extend for over 1000 μ from the perikaryon, but three types of dendritic specializations have been noted: spines, branchlets and varicosities. These specializations are not strictly restricted to Clarke cells. Dendrites of all three cell types cross the nuclear boundaries. Some enter the dorsal columns via Rexed's lamina V and others enter laminae VI, VII and X. Neurons whose cell bodies lie within laminae V, VI, VII and X occasionally send dendrites into Clarke's column. Class A cells account for at least 60% of the total neuronal population of Clarke's column and outnumber the Clarke cells (class C) by approximately three to one. Class B neurons are the least common and form between 6 and 16% of the population.