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PubMed1998

Vowel representations in the ventral cochlear nucleus of the cat: effects of level, background noise, and behavioral state.

Single-unit responses were studied in the ventral cochlear nucleus (VCN) of cats as formant and trough features of the vowel /epsilon/ were shifted in the frequency domain to each unit's best frequency (BF; the frequency of greatest sensitivity). Discharge rates sampled with this spectrum manipulation procedure (SMP) were used to estimate vowel representations provided by populations of VCN neurons. In traditional population measures, a good representation of a vowel's formant structure is based on relatively high discharge rates among units with BFs near high-energy formant features and low rates for units with BFs near low-energy spectral troughs. At most vowel levels and in the presence of background noise, chopper units exhibited formant-to-trough rate differences that were larger than VCN primary-like units and auditory-nerve fibers. By contrast, vowel encoding by primary-like units resembled auditory nerve representations for most stimulus conditions. As is seen in the auditory nerve, primary-like units with low spontaneous rates (SR <18 spikes/s) produced better representations than high SR primary-like units at all but the lowest vowel levels. Awake cats exhibited the same general response properties as anesthetized cats but larger between-subject differences in vowel driven rates. The vowel encoding properties of VCN chopper units support previous interpretations that patterns of auditory nerve convergence on cochlear nucleus neurons compensate for limitations in the dynamic range of peripheral neurons.

Acoustic Stimulation↗
PubMed1992

Postnatal development of GABAergic neurons in the gerbil cochlear nucleus: pre-embedding and post-embedding immunocytochemical staining.

The purpose of this investigation was to compare the results obtained when plastic-embedded sections and vibratome-sliced sections were used to localize gamma-aminobutyric acid immunoreactivity (GABA-IR) in the gerbil cochlear nucleus (CN) during postnatal development. GABA-IR was mainly located in the perikarya of the neurons of the gerbil CN. At two days old, GABA-IR was found in plastic sections. No discernible GABA-IR was found in the vibratome slices at three to four days. At eight days, anti-GABA labeled cells were randomly located in the superficial and deep layer of the dorsal cochlear nucleus (DCN) in the vibratome sections, whereas they were mainly accumulated in the vicinity of the granule layer between the junction of the DCN and posteroventral cochlear nucleus (PVCN) in the plastic sections. At 14 days, anti-GABA labeled cells were reduced in number in the superficial third of the DCN in the vibratome slices and reduced in number in the junction between the DCN and PVCN in the plastic sections. At that time, a striking change was the formation of lamellation of the anti-GABA labeled cells mainly located in the middle third (fusiform layer) of the DCN in the plastic sections. A similar pattern of lamellation was found in the vibratome slices at 17-18 days. At three, four, six and eight weeks, the anti-GABA labeled cells were mainly located in the ventral part of the DCN close to the choroidal plexuses and the ventral part of the PVCN in the plastic sections. At six to 15 months old, distinct anti-GABA labeled cells were located in the fusiform layer of the DCN and scattered in the PVCN in the plastic sections. Another striking change at this age was that numerous vacuoles with a mesh-like network were present in the PVCN.

Animals↗
PubMed1997

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.

Action Potentials↗
PubMedDate not supplied

Coding of time-varying sounds in the cochlear nucleus.

The results presented in the present paper show that single nerve cells in the cochlear nucleus have different discharge patterns in response to tones whose frequency varies rapidly compared to tones with slowly varying frequency. Moreover, small, rapid changes in the intensity of a tone are reproduced in such a way that a modulation of a few decibels can give rise to a nearly 100% modulation in the discharge pattern. Amplitude modulation is reproduced in the discharge pattern of these neurons throughout a large intensity range, in many units more than 70 dB. This intensity range is much larger than the intensity range where the mean discharge rate is a function of the sound intensity, meaning that changes in frequency as well as in intensity are enhanced in these units. Though no doubt but a small part of the processing that occurs in the cochlear nucleus, such information seems likely to be extracted somewhat differently by different neurons.

Acoustic Stimulation↗
PubMed1985

Ventral cochlear nucleus neural discharge characteristics in the absence of outer hair cells.

The role of the cochlear outer hair cell (OHC) in auditory processing remains poorly understood. The OHCs possess an independent afferent innervation which constitutes 5-10% of cochlear afferent neurons and which appears to project to the cochlear nucleus (CN). Whether the OHCs contribute to the processing of auditory signals in the CN has not been determined. To address this question, kanamycin ototoxicity was used to produce selective OHC loss while leaving the inner hair cell (IHC) population largely intact, in the basal portion of the cochlea of chinchillas. Single unit responses were then recorded in the ventral cochlear nucleus (VCN), and compared to responses in untreated subjects. Many of the changes observed in VCN neural responses reflected changes which have previously been reported in the VIIIth nerve. However, frequency tuning curve tip segments which were normal in both bandwidth and length were observed in approximately 22% of the units associated with regions of complete OHC loss and preservation of IHCs. This has not been reported in previous OHC lesion studies. Also, first spike latency was found to be significantly lengthened for units associated with the OHC free regions. Those features of VCN neural responses which first arise within the CN, such as non-primary-like post-stimulus-time histogram response patterns, were unaffected by OHC loss. These results suggest that afferent fibers associated with OHCs do not play a major role in signal processing in the VCN.

Animals↗
PubMed1983

Influence of temperature on tuning of primary-like units in the guinea pig cochlear nucleus.

The effect of temperature changes on the response area of primary-like single units recorded extracellularly in the anteroventral cochlear nucleus of the guinea pig was studied for cochlear temperatures in the range 27.4-40.3 degrees C, while maintaining a normal rectal temperature of 38.5 degrees C. A new approach to the cochlear nucleus was developed which involved opening of the temporal bone overlying the flocculus cavity and aspirating a small portion of the paraflocculus. Irrespective of characteristic frequency CF (0.8-16 kHz), hypothermia caused a reversible elevation of CF thresholds (1.6 dB/degrees C), a small loss of the sharpness of threshold tuning and reductions of the saturation and mean spontaneous rates. The CFs were unaffected, in contrast to single units in birds and cold-blooded animals. From a comparison of the effects of temperature on different species it is proposed that the processes that define CF and sharpness of tuning are different within a cochlea, and that these processes can be differentially influenced.

Animals↗
PubMed1992

Developmental changes in the effects of drugs acting at NMDA or non-NMDA receptors on synaptic transmission in the chick cochlear nucleus (nuc. magnocellularis).

The developmental pharmacology of excitatory amino acid (EAA) receptors in the chick cochlear nucleus (nucleus magnocellularis, NM) was studied by means of bath application of drugs and recording of synaptically-evoked field potentials in brain slices taken from chicks aged embryonic day (E) 14 through hatching (E21). The abilities of various EAA agonists (N-methyl-D-aspartate [NMDA], kainic acid, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid [AMPA]) to suppress postsynaptic responses by depolarization block and of EAA antagonists ((3-[RS]-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid [CCP], dizocilpine [MK-801], 6-nitro-7-sulfamoyl-benzo(F)quinoxaline-2,3 dione [NBQX], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX] and 6,7-dinitroquinoxaline-2,3-dione [DNQX]) to suppress these responses directly were assessed quantitatively. The results support the existence of NMDA receptors in NM and suggest that the ability of these receptors to influence synaptically-evoked responses declines dramatically during the last week of embryonic life. The results similarly suggest that the non-NMDA receptors in NM undergo changes in density and/or function during a period of development when the cochlear nucleus is undergoing a variety of morphological and functional transformations.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed2006

Intense sound-induced plasticity in the dorsal cochlear nucleus of rats: evidence for cholinergic receptor upregulation.

Previous studies in a number of species have demonstrated that spontaneous activity in the dorsal cochlear nucleus (DCN) becomes elevated following exposure to intense sound. This condition of hyperactivity has aroused considerable interest because it may represent an important neural correlate of tinnitus. There is some evidence that neurons in the superficial DCN, such as cartwheel, stellate and fusiform cells, may contribute to the level of hyperactivity induced by intense sound, although the relative importance of these different cell types is unknown. In the present study, we sought to determine the effect of intense sound exposure on multiunit spontaneous activity both at the DCN surface and in the fusiform cell layer and to examine the influence of cholinergic input to DCN circuits on the level of activity in the fusiform cell layer. Rats were studied in two groups, one of which had been exposed to a continuous intense sound (10 kHz 127 dB SPL) for 4h while the other group served as unexposed controls. Between 30 and 52 days post-exposure, recordings of multiunit activity were performed at the DCN surface as well as in the middle of the fusiform cell layer. Changes in fusiform cell layer activity were also studied in response to superficial applications of the cholinergic agonist, carbachol, either alone or following pre-application of the cholinergic antagonist, atropine. The results demonstrated that multiunit spontaneous activity in the rat DCN was generally much higher in both control and exposed animals relative to that which has been observed in other species. This activity was significantly higher at the DCN surface of sound-exposed animals than that of controls. In contrast, hyperactivity could not be demonstrated in the fusiform cell layer of sound-exposed animals. Carbachol administration most commonly caused suppression of fusiform cell layer activity. However, this suppression was considerably stronger in the DCN of sound-exposed animals than in controls. These findings suggest that, hyperactivity at the DCN surface of exposed rats may arise as a consequence of more highly activated neurons in the molecular layer, such as cartwheel and/or stellate cells, and that the lack of hyperactivity in the fusiform cell layer may be the result of inhibition of fusiform cells by these inhibitory interneurons. Although this finding does not rule out fusiform cells as possible sources of hyperactivity in other species, or even in the rat after short post-exposure recovery periods, the enhanced sensitivity of the fusiform cell layer to cholinergic stimulation suggests that in the rat, at least after prolonged post-exposure recovery periods, increased inhibition of activity in this layer by more superficially located neurons may result from an upregulation of receptors for cholinergic input. This upregulation may be greater in rats than in other species due to the relatively heavy cholinergic input that exists in the cochlear nucleus of this species.

Acoustic Stimulation↗
PubMedDate not supplied

Neurotransmitters in the cochlea and the cochlear nucleus.

The inhibitory efferent transmitter in the cochlear is most likely acetylcholine. The afferent transmitter (between hair cells and primary afferent fibres) is not known. There is some evidence for glutamate (or aspartate) but the high concentrations necessary to activate the afferents when these amino-acids are applied intracochlearly may indicate that their effects is unspecific. A number of other transmitter candidates can be safely ruled out at these synapses. In the cochlear nucleus of transmitter between primary afferents and secondary cells is probably glutamate (or aspartate).

Acetylcholine↗
PubMed1994

Auditory nerve terminals in the cochlear nucleus magnocellularis: differences between low and high frequencies.

Primary auditory nerve fibers were labelled in the barn owl by localized horseradish peroxidase (HRP) injections into the cochlear nucleus angularis. They were followed to their terminal sites in the hearing organ (basilar papilla), confirming that they were auditory, and to the cochlear nucleus magnocellularis. The terminal sites of low-frequency fibers within nucleus magnocellularis always included an area previously described as the lagenar part, i.e., an area receiving primary input which is probably only vestibular. Furthermore, a number of differences were recognized between these low-frequency (up to 0.64 kHz) and the high-frequency (1.8 kHz and above) auditory nerve projections to nucleus magnocellularis. Most importantly, the collaterals given off by low-frequency fibers into the nucleus typically showed multiple terminal branching, with both en passant and terminal bouton-like swellings. High-frequency fiber collaterals, in contrast, terminated unbranched in a single endbulb of Held. Nucleus magnocellularis is the first station in a brainstem auditory pathway processing stimulus timing information, coded through neuronal phase locking. The prominent difference in terminal shape found between its low- and high-frequency input fibers is interpreted as reflecting different requirements of the absolute temporal precision for significant phase locking. Terminals in the shape of endbulbs of Held are probably a specialization to improve the temporal precision of synaptic transmission, allowing phase locking to higher frequencies.

Acoustic Stimulation↗
PubMed1997

Evidence of stimulus-dependent correlated activity in the dorsal cochlear nucleus of decerebrate gerbils.

Cross-correlation analysis of simultaneously recorded spike trains was used to study the internal organization of the dorsal cochlear nucleus (DCN) of unanesthetized decerebrate Mongolian gerbils. The goal was to test the model (adapted from cat) that its principal cells (type III and type IV units) receive three sources of shared auditory input: excitatory input from the auditory nerve; inhibitory input from DCN interneurons (vertical cells; type II and type II-i units) that respond vigorously to tones; and inhibitory input from ventral cochlear nucleus principal cells (D-stellate cells; wideband inhibitors) that conversely respond vigorously to noise. Records of spontaneous and/or driven activities (to long-duration tones and frozen broadband noise) were obtained for 51 pairs consisting of type II, type III, and type IV units; type III units inhibited by low-level noise were subclassified as type III-i units. Pairs were isolated with two electrodes to study the effect of differences in unit best frequencies (BFs) on correlation. All correlated pairs composed of type III and type IV units (17 of 31 pairs) showed central mounds (CMs), indicative of shared input, in their cross-correlograms. These data exhibited two important properties: pairs with similar BFs were more likely to show CMs, and the shape of the CMs was stimulus dependent. That is, CM width typically changed sharply from wide to narrow with increasing level; significantly, transition-level CMs were either a composite of these shapes or not present. The transition to only narrow CMs occurred above the thresholds of type II and type III-i units to tones, but below their thresholds to noise. Cross-correlograms derived from the tone-evoked activities of pairs involving type II units (3 of 6 pairs) showed inhibitory troughs (ITs); unexpectedly, type III-i units were involved in both IT and CM pairs, suggesting that this unit type may reflect recordings from both vertical and principal cells. Overall, the results are interpretable in terms of the model of gerbil DCN that was adapted from cat, suggesting that the model generalizes across species. Compared with cat, however, gerbil principal cell responses (predominantly type III unit properties) are less dominated by inhibition.

Acoustic Stimulation↗
PubMed1998

Differential distribution of NMDA receptor subunit mRNA in the rat cochlear nucleus.

The distribution and expression of mRNAs for different subunits of the N-methyl-D-aspartate receptor (NMDAR) were examined in the cochlear nucleus (CN) of the rat using radioactive in situ hybridization methods. Heavy labeling for NMDAR1 subunit mRNA was observed in all major CN neuronal types with lower labeling for NMDAR2A, 2B, 2C, and 2D mRNA. Silver grain counting was used to compare expression of different NMDAR2 subunits between six of the major CN cell types. Small cells of the small cell cap/shell area had the highest expression of NMDAR2A-C subunit mRNAs of the cell types assessed. These small cells as well as fusiform and corn cells of the dorsal cochlear nucleus had higher NMDAR2C than other NMDAR2 subunits, providing these neuron types with a distinct expression pattern or profile. The other three cell types assessed, spherical bushy cells, granule cells, and octopus cells had relatively equivalent levels of NMDAR2A-C subunit expressions, providing a second distinct profile. NMDAR2D mRNA had low expression in all six cell types assessed.

Animals↗
PubMed1975

Single unit activity in the dorsal cochlear nucleus of the cat.

Single unit activity was examined in three component layers of the dorsal cochlear nucleus (DCN): the molecular layer, the fusiform cell layer, and the polymorphic layer (deep DCN). Electrophysiological units were classified into types on the basis of their activity under a variety of stimulus conditions. In the molecular layer spike activity was small and difficult to isolate. Almost all units in the fusiform cell layer could be classified as either "pauser" or "buildup" units. Classification of units in the deep DCN was sometimes difficult, but "pauser," "chopper," and some "on" units were found. The "on" types of units tended to be located in the more superficial part of the deep DCN. Unit locations were referred to a three-dimensional block model of the cochlear nucleus.

Acoustic Stimulation↗
PubMed1989

The form and distribution of GABAergic synapses on the principal cell types of the ventral cochlear nucleus of the cat.

The distribution of GABAergic endings was examined histochemically in the ventral cochlear nucleus (VCN) of the cat using an antibody to glutamate decarboxylase (GAD), the synthetic enzyme for GABA. Immunoreactive (GAD+) endings appeared in all subdivisions of the cat VCN. Each of the principal cell types had a characteristic labeling pattern, based on the size, concentration, and distribution of GAD+ endings on its soma. Spherical bushy cell somata were typically contacted by many small (less than 1.5 microns in diameter) and medium-sized (1.5-2 microns in diameter) endings, many of which aggregated into tight clusters. Globular bushy cells had a similar pattern, but the clusters of GAD+ endings were less tightly packed. Reactive endings on stellate cells were more evenly distributed. GAD+ endings on octopus cells were larger (up to 2.5 microns in diameter) than those on the bushy cells and tended to aggregate into small clusters or rows on the somata and dendrites. Reactive endings contained small pleomorphic vesicles and formed symmetrical synaptic contacts on each of the cell types examined. The patterns formed by GAD+ endings on each type of neuron resemble those of certain types of non-cochlear axons previously described with the Golgi methods as projecting from the dorsal cochlear nucleus and the trapezoid body.

Animals↗
PubMed1980

Identification of response properties of ascending axons from dorsal cochlear nucleus.

Electrical stimulation in the dorsal acoustic stria (DAS) and the posterior and dorsal part of the ventral cochlear nucleus (VCN) was used to identify the response properties of efferent axons from the dorsal cochlear nucleus (DCN) in unanesthetized, decerebrate cats. Responses recorded in the DCN of this preparation can be divided into two broad classes, type II/III and type IV. Results of previous studies suggest that one subclass of type II/III units, those without spontaneous activity, are inhibitory interneurons terminating on type IV cells. Most type IV units (34/43) could be antidromically activated from the DAS and most units which could be activated from the DAS were of this type (34/39), suggesting that type IV responses are recorded from the projection cells of the DCN. Few type II/III cells without spontaneous activity could be activated from the DAS (1/30). A few cells of this type (6/32) could be antidromically stimulated from regions of the posterior AVCN or dorsal PVCN through which axon collaterals of some small cell interneurons of the DCN have been reported to pass. No type IV cells could be activated from the VCN (0/22). Thus the results are consistent with some type II/III responses originating in DCN interneurons. Spontaneously active type II/III units had intermediate properties.

Acoustic Stimulation↗
PubMed1988

Influence of developmental auditory deprivation on neuronal ultrastructure in the mouse anteroventral cochlear nucleus.

Developmental auditory deprivation caused mouse anteroventral cochlear nucleus neurons to have significantly fewer auditory nerve terminals and more non-auditory nerve terminals. This suggested that stimulation regulated the developmental arborization of auditory nerve terminals and competition for synaptic space. Intracellularly, mitochondria were smaller and darker in the deprived neurons and appeared less active metabolically. Interference with these neuronal processes may underlie the impaired development seen in auditory deprivation.

Afferent Pathways↗
PubMed2000

Potassium-depolarization-induced cytoplasmic [Ca2+] transient in freshly dissociated pyramidal neurones of the rat dorsal cochlear nucleus.

The significance of voltage-activated Ca2+ currents in eliciting cytoplasmic Ca2+ transients was studied in pyramidal neurones isolated from the rat dorsal cochlear nucleus using combined enzyme treatment/mechanical trituration. Increases in cytoplasmic Ca2+ concentration ([Ca2+]i) were evoked by K+-induced depolarizations (10-50 mM) and monitored by the Fura-2 fluorimetric technique. The acutely dissociated neurones had a resting [Ca2+]i of 17.2+/-0.5 nM. They possessed caffeine-sensitive Ca2+ stores which were empty at rest; these stores could be filled with Ca2+ entering from the extracellular space and were re-emptied quickly. The effects of various specific high-voltage-activated (HVA) Ca2+ channel antagonists (nifedipine, omega-agatoxin IVA and omega-conotoxin GVIA) on [Ca2+]i transients were tested. Analysis of the blocking effects of these agents on the [Ca2+]i, transients indicates that, in the pyramidal neurones of the dorsal cochlear nucleus, N-type Ca2+ channels are primarily responsible for producing the depolarization-induced increases in [Ca2+]i.

Animals↗
PubMed1984

Noise masking of tone responses and critical ratios in single units of the mouse cochlear nerve and cochlear nucleus.

Responses of single units in the cochlear nerve and cochlear nucleus to tone bursts in a background of continuous white broadband noise were recorded. Tone and noise intensities ranged from threshold to saturation levels. Masking of the tone response by the noise was demonstrated by comparing peristimulus-time histograms and spike rates recorded during the tone and between tone presentations. The response of a unit to masking was found to be predictable based upon the difference in its rate of response to the tone and to the noise when the tone was masked. Several nonlinearities of the masking process are described. The most prominent one is an increase in the difference between tone and noise levels at the threshold of masking with increasing tone levels, i.e. neural critical ratios increase with increasing tone level. On the average, the frequency dependence of single unit effective bandwidths and of critical ratio bandwidths is similar to that of mean behavioral critical ratio bands.

Action Potentials↗