Cartwheel Cells Research Articles

Bidirectional synaptic plasticity in the cerebellum-like mammalian dorsal cochlear nucleus.

The dorsal cochlear nucleus integrates acoustic with multimodal sensory inputs from widespread areas of the brain. Multimodal inputs are brought to spiny dendrites of fusiform and cartwheel cells in the molecular layer by parallel fibers through synapses that are subject to long-term potentiation and long-term depression. Acoustic cues are brought to smooth dendrites of fusiform cells in the deep layer by auditory nerve fibers through synapses that do not show plasticity. Plasticity requires Ca(2+)-induced Ca(2+) release; its sensitivity to antagonists of N-methyl-d-aspartate and metabotropic glutamate receptors differs in fusiform and cartwheel cells.

Effects of acoustic trauma on dorsal cochlear nucleus neuron activity in slices

Previous studies found increased multi-unit spontaneous activity in the dorsal cochlear nucleus (DCN) of animals that had been exposed to intense sound. Such activity may be related to tinnitus. Our study examined effects of previous exposure to intense sound on single neurons in the DCN, by measuring spontaneous activities and sensitivities to acetylcholine, an important neurotransmitter of centrifugal pathways to the cochlear nucleus, in brain slices. Spontaneous discharges were recorded extracellularly in the DCN portion of brain slices from control and intense-tone-exposed rats. Slices from exposed rats showed increased prevalence of bursting and decreased regular spontaneous activity. Since regular neurons include fusiform cells, and bursting neurons include cartwheel cells, intense tone exposure may lead to increased activity of DCN cartwheel cells and decreased activity of fusiform cells. Alternatively, the activity of some fusiform cells might change to bursting. Intense tone exposure also appeared to increase bursting neuron sensitivity to carbachol. This suggests that changes in DCN cartwheel cell spontaneous activity may reflect changes in effects of cholinergic centrifugal pathways following intense tone exposure. We conclude that acoustic trauma may lead to changes in the physiology and pharmacology of DCN neurons. These changes may be related to underlying mechanisms of central tinnitus.

Cellular and subcellular distribution of AMPA-type glutamate receptor subunits and metabotropic glutamate receptor 1α in the cochlear nucleus of the horseshoe bat ( Rhinolophus rouxi)

Ionotropic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) selective glutamate receptors (GluRs) are the main mediators of fast excitatory neurotransmission and composed of a variable combination of four different subunits (GluR1–4). The metabotropic glutamate receptor 1α (mGluR1α) is involved in plastic synaptic events. Since horseshoe bats strongly depend on temporal cues for acoustic imaging by echolocation and exhibit prominent species specific specializations of the cochlear nucleus (CN), the subunit distribution of AMPA selective GluRs and the distribution of mGluR1α was studied at the light and electron microscopic level with preembedding immunocytochemistry. Immunoreactivity to GluR1 was low throughout the CN. All types of projection neurons of the ventral CN expressed distinct GluR2/3 and GluR4 immunoreactivity with GluR4-labeling especially prominent in multipolar and octopus cell-like neurons of the posteroventral CN. The AMPA and metabotropic receptor inventory of the laminated ventral subdivision of the dorsal CN (DCNv) agreed with that reported in other mammals, whereas the specialized dorsal non-laminated subdivision of DCN (DCNd) lacked the prominent labeling for GluR2/3 and mGluR1α that characterizes cartwheel cells of DCNv. Distinct GluR2/3 and GluR4 immunoreactivity combined with low expression of mGluR1α immunoreactivity was characteristic for fusiform cells of DCNv and DCNd. Tuberculoventral cells of both the deep DCNv and the DCNd exhibited light to moderate GluR2/3 and GluR4 immunoreactivity. The staining patterns in DCNd thus indicate a loss of cerebellar-like microcircuits and a conservation of frequency specific circuitry of the deep and fusiform cell layers of the mammalian DCN.

Expression of GABA A receptor subunits in rat brainstem auditory pathways: cochlear nuclei, superior olivary complex and nucleus of the lateral lemniscus

Inhibition by GABA is important for auditory processing, but any adaptations of the ionotropic type A receptors are unknown. Here we describe, using in situ hybridization, the subunit expression patterns of GABA A receptors in the rat cochlear nucleus, superior olivary complex, and dorsal and ventral nuclei of the lateral lemniscus. All neurons express the β3 and γ2L subunit messenger RNAs, but use different α subunits. In the dorsal cochlear nucleus, fusiform (pyramidal) and giant cells express α1, α3, β3 and γ2L. Dorsal cochlear nucleus interneurons, particularly vertical or tuberculoventral cells and cartwheel cells, express α3, β3 and γ2L. In the ventral cochlear nucleus, octopus cells express α1, β3, γ2L and δ. Spherical cells express α1, α3, α5, β3 and γ2L. In the superior olivary complex, the expression profile is α3, α5, β3 and γ2L. Both dorsal and ventral cochlear nucleus granule cells express α1, α6, β3 and γ2L; unlike their cerebellar granule cell counterparts, they do not express β2, γ2S or the δ subunit genes. The δ subunit's absence from cochlear nucleus granule cells may mean that tonic inhibition mediated by extrasynaptic GABA A receptors is less important for this cell type. In both the dorsal and ventral nuclei of the lateral lemniscus, α1, β3 and γ2L are the main subunit messenger RNAs; the ventral nucleus also expresses the δ subunit. We have mapped, using in situ hybridization, the subunit expression patterns of the GABA A receptor in the auditory brainstem nuclei. In contrast to many brain regions, the β2 subunit gene and γ2S splice forms are not highly expressed in auditory brainstem nuclei. GABA A receptors containing β3 and γ2L may be particularly well suited to auditory processing, possibly because of the unique phosphorylation profile of this subunit combination.

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.

Purkinje cell degeneration and control mice: responses of single units in the dorsal cochlear nucleus and the acoustic startle response

The cartwheel cell is the most numerous inhibitory interneuron of the dorsal cochlear nucleus (DCN). It is expected to be an important determinant of DCN function. To assess the contribution of the cartwheel cell, we examined the discharge characteristics of DCN neurons and behavioral measures in the Purkinje cell degeneration (pcd) mice, which lack cartwheel cells, and compared them to those of the control mice. Distortion product otoacoustic emissions and auditory brainstem-evoked response thresholds were similar between the two groups. Extracellularly recorded DCN single units in ketamine/xylazine-anesthetized mice were classified according to post-stimulus time histogram (PSTH) and excitatory–inhibitory response area (EI-area) schemes. PSTHs recorded in mouse DCN included chopper, pauser/buildup, onset, inhibited and non-descript types. EI-areas recorded included Types I, II, III, I/III, IV and V. There were no significant differences in the proportions of various unit types between the pcd and control mice. The pcd units had slightly lower thresholds to characteristic frequency tones; however, they had spontaneous rates, thresholds to noise, and maximum driven rates to noise that were similar to those of the control units. Pcd mice had smaller startle amplitudes, but startle latency, prepulse inhibition/augmentation and facilitation by a background tone were comparable between the two groups. From these results, we conclude that DCN function in response to relatively simple acoustic stimuli is minimally affected by the absence of the cartwheel cells. Future studies employing more complex and/or multimodal stimuli should help assess the role of the cartwheel cells.

Differential postsynaptic distribution of GluRs 1–4 on cartwheel and octopus cell somata in the gerbil cochlear nucleus

Differences were demonstrated in the distribution of glutamate receptors (GluR) 1, 2, 2/3 and 4 postsynaptic immunoreactivity (PSIR) on the somata of cartwheel and octopus cells in the adult gerbil cochlear nucleus (CN). Montages of electron micrographs of cartwheel and octopus cells immunoreacted with antibodies to GluR 1, 2, 2/3 and 4 were prepared. The number of synaptic terminals with PSIR were counted on all cells for each antibody, normalized to the total length of somatic surface analyzed. The density of terminals apposed to PSIR on octopus cells was similar for the antibodies GluR1, 2/3 and 4, but significantly less for GluR2. On cartwheel somata the numbers of terminals apposed to immunoreactive postsynaptic specializations with GluR1, 2, 2/3 or 4 were not significantly different from each other. The density of terminals apposed to GluR2/3 and 4 positive postsynaptic specializations was significantly less on cartwheel cells than on octopus somata. The data suggest that the decreased presence of the GluR2 subunit, which confers calcium impermeability to the assembled receptor and slower gating kinetics to receptors with a high GluR4 content, is the major difference in the AMPA receptors on the somata of these cell types. The presence on cartwheel cells of a majority of AMPA receptors which contain GluR2 may account for the fact that cartwheel cells respond to shocks to the auditory nerve with 100 ms excitatory postsynaptic potentials (EPSPs), while octopus cells, most of whose AMPA receptors lack GluR2, respond with 1 ms EPSPs.

Differential distribution of glutamate receptors in the cochlear nuclei

Glutamate receptors are the major excitatory neurotransmitter receptors of the mammalian central nervous system, and include AMPA, kainate, delta, NMDA, and metabotropic types. In the cochlear nucleus (CN), the AMPA receptor subunits GluR2-4 are found in major kinds of neurons, while GluR1 subunit distribution is more restricted. GluR2 is low in the anteroventral CN, suggesting that many AMPA receptors here are calcium-permeable. Delta receptors are most prevalent in cartwheel cells in the dorsal CN. Of the NMDA receptors, NR1 is widespread while the NR2 subunits show more restricted distributions. Of the metabotropic glutamate receptors, mGluR1α is most prevalent in the dorsal CN, and mGluR2 is concentrated in Golgi cells and unipolar brush cells. AMPA receptors in endbulb synapses in the anteroventral CN are mainly GluR3+4 complexes: probably an adaptation for rapid auditory neurotransmission. Glutamate receptors are differentially distributed in synapses of fusiform cells of the dorsal CN; GluR4 and mGluR1α are present only at basal dendrite synapses (auditory nerve), while other glutamate receptors occupy both apical and basal synapses. Analysis of cytoplasmic distribution suggests that a selective targeting mechanism may restrict movement of GluR4 and mGluR1α to basal dendrites, although other targeting mechanisms may be present.

Pharmacological evidence of inhibitory and disinhibitory neuronal circuits in dorsal cochlear nucleus.

The dorsal cochlear nucleus (DCN) is rich in both glycine and GABA inhibitory neurotransmitter systems, and the response properties of its principal cells (pyramidal and giant cells) are strongly shaped by inhibitory inputs. For example, DCN principal cells often display highly nonmonotonic (so-called type IV) input-output functions in response to best-frequency (BF) tones. In this study, the inhibitory inputs onto the principal cell types and onto response types of known inhibitory interneurons were compared before and during iontophoretic application of the glycine- and GABA(A)-receptor antagonists, strychnine and bicuculline. Strychnine eliminates the central (on-BF) inhibitory area in type IV units, resulting in monotonic BF rate-level curves. Unexpectedly, bicuculline primarily enhances inhibition in principal-cell types; for example, type IV units are inhibited at lower sound levels in the presence of bicuculline. Principal cell types with weaker inhibitory inputs (type IV-T and type III units) are more strongly inhibited in the presence of bicuculline and usually are converted into type IV units. This enhancement of on-BF inhibition by bicuculline suggests a disinhibitory process involving GABA(A) action on a non-GABA(A)ergic inhibitory pathway. This latter pathway is probably glycinergic and involves type II units (deep-layer vertical cells) and/or complex-spiking units (superficial cartwheel cells) because both of these unit types are disinhibited by bicuculline. One intrinsic GABA(A) source could be the superficial stellate cells in DCN because bicuculline partly blocks the inhibition evoked by somatosensory-stimulated activation of the superficial granule-cell circuitry in DCN. Taken together, the results suggest that glycinergic circuits mediate directly the inhibition of DCN principal cells, but that GABA(A)ergic circuits modulate the strength of the inhibition.

Acoustic and current-pulse responses of identified neurons in the dorsal cochlear nucleus of unanesthetized, decerebrate gerbils.

In an effort to establish relationships between cell physiology and morphology in the dorsal cochlear nucleus (DCN), intracellular single-unit recording and marking experiments were conducted on decerebrate gerbils using horseradish peroxidase (HRP)- or neurobiotin-filled micropipettes. Intracellular responses to acoustic (tone and broadband noise bursts) and electric current-pulse stimuli were recorded and associated with cell morphology. Units were classified according to the response map scheme (type I to type V). Results from 19 identified neurons, including 13 fusiform cells, 2 giant cells, and 4 cartwheel cells, reveal correlations between cell morphology of these neurons and their acoustic responses. Most fusiform cells (8/13) are associated with type III unit response properties. A subset of fusiform cells was type I/III units (2), type III-i units (2), and a type IV-T unit. The giant cells were associated with type IV-i unit response properties. Cartwheel cells all had weak acoustic responses that were difficult to classify. Some measures of membrane properties also were correlated with cell morphology but to a lesser degree. Giant cells and all but one fusiform cell fired only simple action potentials (APs), whereas all cartwheel cells discharged complex APs. Giant and fusiform cells all had monotonic rate versus current level curves, whereas cartwheel cells had nonmonotonic curves. This implies that inhibitory acoustic responses, resulting in nonmonotonic rate versus sound level curves, are due to local inhibitory interactions rather than strictly to membrane properties. A complex-spiking fusiform cell with type III unit properties suggests that cartwheel cells are not the only complex-spiking cells in DCN. The diverse response properties of the DCN's fusiform cells suggests that they are very sensitive to the specific complement of excitatory and inhibitory inputs they receive.

Time Course and Permeation of Synaptic AMPA Receptors in Cochlear Nuclear Neurons Correlate with Input

AMPA receptors mediate rapid glutamatergic synaptic transmission. In the mammalian cochlear nuclei, neurons receive excitatory input from either auditory nerve fibers, parallel fibers, or both fiber systems. The functional correlates of differences in the source of input were examined by recording AMPA receptor-mediated, miniature EPSCs (mEPSCs) in whole-cell voltage-clamp mode from identified neurons. Bushy, octopus, and T-stellate cells of the ventral cochlear nucleus (VCN) and tuberculoventral cells of the dorsal cochlear nucleus (DCN) receive most of their excitatory input from the auditory nerve; fusiform cells receive excitatory inputs from both the auditory nerve and parallel fibers; cartwheel cells receive excitatory input from parallel fibers alone. mEPSCs from bushy, octopus, T-stellate, and tuberculoventral cells had significantly faster decay time constants (0.35-0.40 msec) than did those from fusiform and cartwheel cells (1.32-1.79 msec). Some fusiform cells had two populations of mEPSCs with distinct time courses. mEPSCs in cells with auditory nerve input alone were inhibited by philanthotoxin, a blocker of calcium-permeable AMPA receptors, whereas mEPSCs in cells with parallel fiber input were not. Thus AMPA receptors postsynaptic to the auditory nerve differ from those postsynaptic to parallel fibers both in channel-gating kinetics and in their permeability to calcium. These results confirm the conclusion that synaptic AMPA receptors are specialized according to the source of input (Hunter et al., 1993; Rubio and Wenthold, 1997; Wang et al., 1998).

Glycine immunoreactivity of multipolar neurons in the ventral cochlear nucleus which project to the dorsal cochlear nucleus

Certain distinct populations of neurons in the dorsal cochlear nucleus are inhibited by a neural source that is responsive to a wide range of acoustic frequencies. In this study, we examined the glycine immunoreactivity of two types of ventral cochlear nucleus neurons (planar and radiate) in the rat which project to the dorsal cochlear nucleus (DCN) and thus, might be responsible for this inhibition. Previously, we proposed that planar neurons provided a tonotopic and narrowly tuned input to the DCN, whereas radiate neurons provided a broadly tuned input and thus, were strong candidates as the source of broadband inhibition (Doucet and Ryugo [1997] J. Comp. Neurol. 385:245–264). We tested this idea by combining retrograde labeling and glycine immunohistochemical protocols. Planar and radiate neurons were first retrogradely labeled by injecting biotinylated dextran amine into a restricted region of the dorsal cochlear nucleus. The labeled cells were visualized using streptavidin conjugated to indocarbocyanine (Cy3), a fluorescent marker. Sections that contained planar or radiate neurons were then processed for glycine immunocytochemistry using diaminobenzidine as the chromogen. Immunostaining of planar neurons was light, comparable to that of excitatory neurons (pyramidal neurons in the DCN), whereas immunostaining of radiate neurons was dark, comparable to that of glycinergic neurons (cartwheel cells in the dorsal cochlear nucleus and principal cells in the medial nucleus of the trapezoid body). These results are consistent with the hypothesis that radiate neurons in the ventral cochlear nucleus subserve the wideband inhibition observed in the dorsal cochlear nucleus. J. Comp. Neurol. 408:515–531, 1999. © 1999 Wiley-Liss, Inc.

Glycine immunoreactivity of multipolar neurons in the ventral cochlear nucleus which project to the dorsal cochlear nucleus

Certain distinct populations of neurons in the dorsal cochlear nucleus are inhibited by a neural source that is responsive to a wide range of acoustic frequencies. In this study, we examined the glycine immunoreactivity of two types of ventral cochlear nucleus neurons (planar and radiate) in the rat which project to the dorsal cochlear nucleus (DCN) and thus, might be responsible for this inhibition. Previously, we proposed that planar neurons provided a tonotopic and narrowly tuned input to the DCN, whereas radiate neurons provided a broadly tuned input and thus, were strong candidates as the source of broadband inhibition (Doucet and Ryugo [1997] J. Comp. Neurol. 385:245-264). We tested this idea by combining retrograde labeling and glycine immunohistochemical protocols. Planar and radiate neurons were first retrogradely labeled by injecting biotinylated dextran amine into a restricted region of the dorsal cochlear nucleus. The labeled cells were visualized using streptavidin conjugated to indocarbocyanine (Cy3), a fluorescent marker. Sections that contained planar or radiate neurons were then processed for glycine immunocytochemistry using diaminobenzidine as the chromogen. Immunostaining of planar neurons was light, comparable to that of excitatory neurons (pyramidal neurons in the DCN), whereas immunostaining of radiate neurons was dark, comparable to that of glycinergic neurons (cartwheel cells in the dorsal cochlear nucleus and principal cells in the medial nucleus of the trapezoid body). These results are consistent with the hypothesis that radiate neurons in the ventral cochlear nucleus subserve the wideband inhibition observed in the dorsal cochlear nucleus.

Glutamatergic transmission of neuronal responses to carbachol in rat dorsal cochlear nucleus slices

This study found that glutamate receptor antagonists block the excitatory effects of carbachol, a cholinergic agonist, on bursting neurons in the dorsal cochlear nucleus of rat brain slices. Among antagonists for glutamate receptor subtypes, those for non- N-methyl- d-aspartate ionotropic glutamate receptors were more potent than those for N-methyl- d-aspartate receptors. The glutamate receptor antagonists did not block the effects of carbachol on regularly firing neurons in the dorsal cochlear nucleus of the same slices. Antagonists for GABA or glycine receptors did not alter the effects of carbachol on bursting neurons. Effects of carbachol on bursting activity could be mimicked by application of glutamate or its agonist, α-amino-3-hydroxy-5-methylisoxazole-4-propionate, whose effects were not blocked by synaptic blockade. During carbachol application, increased release of glutamate and glycine from the dorsal cochlear nucleus part of brain slices was measured using high-performance liquid chromatography. Release of other amino acids showed no significant change. The results suggest that, in rat dorsal cochlear nucleus, cholinergic effects on regular and bursting spontaneous firing occur through different mechanisms. Cholinergic effects on regular neurons (which include fusiform cells) are direct, through muscarinic receptors. Cholinergic effects on bursting neurons (which include cartwheel cells) are indirect and involve glutamatergic neurotransmission, mostly via non- N-methyl- d-aspartate ionotropic receptors. The granule cell–parallel fiber pathway may be involved in this glutamatergic transmission.

Facilitation of acoustic responses of cartwheel neurons of the cat dorsal cochlear nucleus.

Responses to clicks were increased in cartwheel cells of the dorsal cochlear nucleus of cats after pairing presentations of the clicks with local iontophoretic delivery of glutamate. The cells were identified by bursting discharges, and were recorded intracellularly in vivo. The findings indicate that inhibitory interneurons such as cartwheel cells can participate in complex adaptive acoustic signal processing. Each cell displayed doublet discharges of > 800 Hz. In 70% of the cells, some of the doublet discharges reached rates > 1000 Hz.

Differential expression of the metabotropic glutamate receptor mGluR1alpha by neurons and axons in the cochlear nucleus: in situ hybridization and immunohistochemistry.

mGluR1alpha 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 mGluR1alpha 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 mGluR1alpha. 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, mGluR1alpha may relate to the long-term signaling requirements of their modulatory functions.

Effects of endogenous acetylcholine on spontaneous activity in rat dorsal cochlear nucleus slices

We have examined the contribution of endogenous acetylcholine (ACh) release to the spontaneous firing of both regular (probably fusiform cells) and bursting neurons (probably cartwheel cells) in the dorsal cochlear nucleus (DCN) in rat brainstem slices. The muscarinic antagonists atropine, scopolamine, and tropicamide (1–2 μM) caused substantial decreases of firing rates in a majority of the neurons. Reversible acetylcholinesterase (AChE) inhibitors typically caused large transient increases in firing that decayed more slowly than responses to carbachol. The irreversible AChE inhibitor diisopropyl fluorophosphate (DFP) usually caused a sustained increase, with an initial peak followed by a gradual change to a final level higher than before DFP. Tropicamide caused large decreases in firing after DFP, confirming sustained ACh release. Both neostigmine and DFP applied after AChE inhibition by DFP sometimes elicited a transient response. We conclude that the level of sustained response to DFP is determined by the rate of endogenous ACh release, and that DFP and reversible AChE inhibitors exert an initial transient agonist effect that overlaps the initial effect of acetylcholinesterase inhibition. The slice experiments provide a model for cholinergic mechanisms in vivo, confirm that the release of endogenous ACh increases the firing rates of regular and bursting neurons in superficial DCN, and support the hypothesis that spontaneous firing of DCN neurons is sustained in part by cholinergic inputs.

Arginine 8-vasopressin metabolite, AVP4–9, stimulates calcium signals in rat hippocampus

We have compared the effects of parallel fiber stimuli on extracellularly recorded neurons showing regular or bursting spontaneous activity patterns in the dorsal cochlear nucleus of rat brainstem slices. Ninety percent of regular neurons failed to respond to stimulus currents (1.4 ± 0.28 mA, mean ± SEM) significantly greater than those (0.4 ± 0.07 mA) that elicited responses from 96% of bursting neurons. Responses of bursting neurons were elicited from widely separated loci along the molecular layer. Kynurenic acid and CNQX or DNQX blocked both spontaneous firing and responses to parallel fiber stimuli of bursting neurons. The same agents also blocked responses of regular neurons but had little or no effect on their spontaneous firing rates. AP-5 caused small decreases in spontaneous rates of both bursting and regular neurons but did not appear to affect responses to stimuli. The data support the hypothesis that the responses of both regular and bursting neurons to parallel fiber stimulation are mediated by glutamate, acting mainly through non-NMDA receptors. Spontaneous activity of bursting, but not regular, neurons also requires non-NMDA glutamatergic transmission, suggesting that the spontaneous firing of bursting neurons, consisting largely of cartwheel cells, may depend upon granule cell activity.

Granule Cell Activation of Complex-Spiking Neurons in Dorsal Cochlear Nucleus

Dorsal cochlear nucleus (DCN) principal cells receive, in addition to their well known auditory inputs, various nonauditory inputs via a cerebellar-like granule cell circuit located in the superficial layers of the DCN. Activation of this circuit (granule cell axons make excitatory synapses on the principal cells but also contact inhibitory interneurons that project to the principal cells) produces strong inhibition of the principal cells. Here we investigate the role of cartwheel cells, homologs of cerebellar Purkinje cells, in producing this inhibition. The responses of type IV units (one type of principal cells) and of cartwheel cells were recorded to ortho- and antidromic activation of the granule cells (i. e., by stimulation of their inputs from the somatosensory cuneate and spinal trigeminal nuclei and by direct stimulation of their parallel fiber axons). Cartwheel cells were identified on the basis of recording depth and complex action potential shape. A four-pulse facilitation paradigm (four pulses at 50 msec intervals) was used; this stimulus allows separation of the apparently simple inhibitory somatosensory response of type IV units into a three-component (inhibition-excitation-inhibition) response. As expected, cartwheel cells are excited by granule cell activation; the latencies and four-pulse amplitudes of these responses correspond to the properties of the second, long-latency inhibitory component of type IV responses. The source of the first, short-latency inhibitory response is still unknown. Nevertheless, these results show that cartwheel cells convey inhibitory polysensory information to DCN principal cells.

Physiological identification of the targets of cartwheel cells in the dorsal cochlear nucleus.

The integrative contribution of cartwheel cells of the dorsal cochlear nucleus (DCN) was assessed with intracellular recordings from anatomically identified cells. Recordings were made, in slices of the cochlear nuclei of mice, from 58 cartwheel cells, 22 fusiform cells, 3 giant cells, 5 tuberculoventral cells, and 1 cell that is either a superficial stellate or Golgi cell. Cartwheel cells can be distinguished electrophysiologically from other cells of the cochlear nuclei by their complex spikes, which comprised two to four rapid action potentials superimposed on a slower depolarization. The rapid action potentials were blocked by tetrodotoxin (n = 17) and were therefore mediated by voltage-sensitive sodium currents. The slow spikes were eliminated by the removal of calcium from the extracellular saline (n = 3) and thus were mediated by voltage-sensitive calcium currents. The spontaneous and evoked firing patterns of cartwheel cells were distinctive. Cartwheel cells usually fired single and complex spikes spontaneously at irregular intervals of between 100 ms and several seconds. Shocks to the DCN elicited firing that lasted tens to hundreds of milliseconds. With the use of these distinctive firing patterns, together with a pharmacological dissection of postsynaptic potentials (PSPs), possible targets of cartwheel cells were identified and the function of the connections was examined. Not only cartwheel and fusiform cells, but also giant cells, received patterns of synaptic input consistent with their having originated from cartwheel cells. These cell types responded to shocks of the DCN with variable trains of PSPs that lasted hundreds of milliseconds. PSPs within these trains appeared both singly and in bursts of two to four, and were blocked by 0.5 or 1 microM strychnine (n = 4 cartwheel, 4 fusiform, and 2 giant cells), indicating that cartwheel cells are likely to be glycinergic. In contrast with cartwheel cells, which are weakly excited by glycinergic input, glycinergic PSPs consistently inhibited fusiform and giant cells. Tuberculoventral cells and the putative superficial stellate cell received little or no spontaneous synaptic activity. Shocks to the DCN evoked synaptic activity that lasted approximately 5 ms. These cells therefore probably do not receive input from cartwheel cells. In addition, the brief firing of tuberculoventral cells and of the putative superficial stellate cell in response to shocks indicates that these cells are unlikely to contribute to the late, glycinergic synaptic potentials observed in cartwheel, fusiform, and giant cells.