Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “Cochlear Nucleus”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 595 records · Page 33Linked to original sources
PubMed2005

Effects of cochlear ablation on choline acetyltransferase activity in the rat cochlear nucleus and superior olive.

Using microdissection and quantitative microassay, choline acetyltransferase (ChAT) activity was mapped in the cochlear nucleus (CN) and in the source nuclei of the olivocochlear bundle, the lateral superior olive and ventral nucleus of the trapezoid body. In control rats, gradients of ChAT activity were found within the major subdivisions of the CN and in the lateral superior olive. These gradients correlated with the known tonotopic organizations, with higher activities corresponding to locations representing higher sound frequencies. No gradient was found in the ventral nucleus of the trapezoid body. In rats surviving 7 days or 1 or 2 months after cochlear ablation, ChAT activity was increased 1 month after ablation in the anteroventral CN by 30-50% in most parts of the lesion-side and by 40% in the contralateral ventromedial part. ChAT activity in the lesion-side posteroventral CN was increased by approximately 40-50% at all survival times. Little change was found in the dorsal CN. Decreases of ChAT activity were also found ipsilaterally in the lateral superior olive and bilaterally in the ventral nucleus of the trapezoid body. Our results suggest that cholinergic neurons are involved in plasticity within the CN and superior olive following cochlear lesions.

Animals↗
PubMed1998

Nicotinic acetylcholine receptors in rat cochlear nucleus: [125I]-alpha-bungarotoxin receptor autoradiography and in situ hybridization of alpha 7 nAChR subunit mRNA.

The cochlear nucleus (CN) is the first site in the central nervous system (CNS) for processing auditory information. Acetylcholine in the CN is primarily extrinsic and is an important neurotransmitter in efferent pathways thought to provide CNS modulation of afferent signal processing. Although muscarinic acetylcholine receptors have been studied in the CN, the role of nicotinic receptors has not. We examined the distribution of one nicotinic acetylcholine receptor subtype, the alpha-bungarotoxin receptor (alpha Bgt), in the CN. Quantitative autoradiography was used to localize receptors and in situ hybridization was used to localize alpha 7 mRNA in CN neurons that express the alpha Bgt receptor. Binding sites for alpha Bgt are abundant in the anterior ventral, posterior ventral, and dorsal divisions of the CN, and receptor density is low in the granule cell layer and interstitial nucleus. Heterogeneity in CN subregions is described. Four distinct patterns of alpha Bgt binding were observed: (1) binding over and around neuronal cell bodies, (2) receptors locally surrounding neurons, (3) dense punctate binding in the dorsal CN (DCN) not associated with neuronal cell bodies, and (4) diffuse fields of alpha Bgt receptors prominent in the DCN molecular layer, a field underlying the granule cell layer and in the medial sheet. The perikaryial receptors are abundant in the ventral CN (VCN) and are always associated with neurons expressing mRNA for the receptor. Other neurons in the VCN also express alpha 7 mRNA, but without alpha Bgt receptor expression associated with the cell body. In general, alpha Bgt receptor distribution parallels cholinergic terminal distribution, except in granule cell regions rich in cholinergic markers but low in alpha Bgt receptors. The findings indicate that alpha Bgt receptors are widespread in the CN but are selectively localized on somata, proximal dendrites, or distal dendrites depending on the specific CN subregion. The data are consistent with the hypothesis that descending cholinergic fibers modulate afferent auditory signals by regulating intracellular Ca2+ through alpha Bgt receptors.

Animals↗
PubMed2006

Dorsal cochlear nucleus response properties following acoustic trauma: response maps and spontaneous activity.

Recordings from single neurons in the dorsal cochlear nucleus (DCN) of unanesthetized (decerebrate) cats were done to characterize the effects of acoustic trauma. Trauma was produced by a 250 Hz band of noise centered at 10 kHz, presented at 105-120 dB SPL for 4h. After a one-month recovery period, neurons were recorded in the DCN. The threshold shift, determined from compound action-potential audiograms, showed a sharp threshold elevation of about 60 dB at BFs above an edge frequency of 5-10 kHz. The response maps of neurons with best frequencies (BFs) above the edge did not show the typical organization of excitatory and inhibitory areas seen in the DCN of unexposed animals. Instead, neurons showed no response to sound, weak responses that were hard to tune and characterize, or "tail" responses, consisting of broadly-tuned, predominantly excitatory responses, with a roughly low-pass shape similar to the tuning curves of auditory nerve fibers with similar threshold shifts. In some tail responses whose BFs were near the edge of the threshold elevation, a second weak high-frequency response was seen that suggests convergence of auditory nerve inputs with widely separated BFs on these cells. Spontaneous rates among neurons with elevated thresholds were not increased over those in populations of principal neurons in unexposed animals.

Animals↗
PubMed1997

Localization of a high threshold potassium channel in the rat cochlear nucleus.

Potassium channels play a major role in determining the pattern and frequency of neuronal firing. In the cochlear nucleus (CN), various morphologically defined types of neurons have different responses to a sound. We have previously identified one type of cloned K+ channel, termed Kv3.1, which is highly expressed in many auditory neurons. Expression studies indicate that Kv3.1 channels have an unusually high threshold for activation. In this study, we used both in situ hybridization and immunohistochemistry to examine the expression patterns of the Kv3.1 channel in the CN. In the ventral CN, bushy cells hybridized strongly with Kv3.1 specific probes and a subpopulation of stellate/multipolar cells hybridized with Kv3.1 probes. In the dorsal CN, pyramidal and large multipolar/giant cells expressed Kv3.1 mRNA. Abundant Kv3.1 immunolabeling was also observed in the CN. The pattern of immunolabeling revealed that the Kv3.1 protein is distributed along the soma, proximal dendrites, unmyelinated axons, and axon terminals of stained neurons. In the case of pyramidal and octopus cells, no immunolabeling was detected at the somata, even though these cells expressed Kv3.1 mRNA. Computer simulations were used to explore the functional role of the Kv3.1 channel. The simulations indicate that Kv3.1 conductances may contribute to repolarization of large synaptic potentials. When stimulated at high frequencies, the presence of Kv3.1 enhances the ability of a model cell with some of the features of bushy cells to follow high frequency input with temporal precision.

Amino Acid Sequence↗
PubMed2002

Intracellularly labeled fusiform cells in dorsal cochlear nucleus of the gerbil. II. Comparison of physiology and anatomy.

Fusiform cells represent the major class of dorsal cochlear nucleus (DCN) projection neuron. Although much is understood about their physiology and anatomy, there remain unexplored issues with important functional implications. These include interspecies differences in DCN physiology and the nature of the cell-to-cell variations in fusiform cell physiology. To address these issues, a quantitative examination was made of the physiology and anatomy of 17 fusiform cells from a companion study. The results suggest that the basal dendrites of gerbil fusiform cells may be electrotonically more compact than those of the cat. This relative decrease in the filtering of excitatory inputs might account for the lower incidence of type IV units in that species. These data also suggest that the gerbil DCN lacks the high-frequency specialization described in the cat, because the tonotopic arrangement of the gerbil fusiform cells quantitatively matches the place-frequency map for the gerbil cochlea. Certain physiological properties have anatomical correlates. First, the basal dendrites of low spontaneous rate cells are directed away from the soma only in the caudal direction, while the high spontaneous rate cells have basal dendrites extending rostrally and caudally. Second, input resistance was dominated by the surface area of the apical dendrite. Third, the discharge pattern was correlated with apical dendrite orientation. Finally, there was a spatial gradient of sensitivity to broadband noise organized at least partially within an isofrequency axis. Such trends indicate that neighboring fusiform cells are endowed with different signal processing capabilities.

Animals↗
PubMed2000

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 nondescript 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.

Acoustic Stimulation↗
PubMed1994

Effects of unilateral deafening on the cochlear nucleus of the guinea pig at different ages.

Developmental changes in spherical cell sizes were measured in the ventral cochlear nucleus (VCN) in normal guinea pigs aged 2 days and 2, 7 and 12 weeks to establish the time course of postnatal neuronal growth, as a baseline for our experimental work. A continued growth of spherical cell size was observed in the VCN up to 7 weeks postnatally. Animals were unilaterally deafened by cochlear perfusion with kanamycin sulphate at ages 1 and 6 weeks. After 6 weeks survival the spherical cells were measured in the VCNs of both sides. Unilateral deafferentation at both ages caused an ipsilateral reduction in spherical cell size, neurons of the younger group being smaller than in the older. On the contralateral side these cells in the older group were larger than age-matched controls while in the younger group there was no difference compared with age-matched controls. These findings suggest that the results of deafferentation are age-dependent, and may indicate an ability of the neural circuitry to adapt to the loss of sensory input on the other side.

Aging↗
PubMed2006

Temporal coding by cochlear nucleus bushy cells in DBA/2J mice with early onset hearing loss.

The bushy cells of the anterior ventral cochlear nucleus (AVCN) preserve or improve the temporal coding of sound information arriving from auditory nerve fibers (ANF). The critical cellular mechanisms entailed in this process include the specialized nerve terminals, the endbulbs of Held, and the membrane conductance configuration of the bushy cell. In one strain of mice (DBA/2J), an early-onset hearing loss can cause a reduction in neurotransmitter release probability, and a smaller and slower spontaneous miniature excitatory postsynaptic current (EPSC) at the endbulb synapse. In the present study, by using a brain slice preparation, we tested the hypothesis that these changes in synaptic transmission would degrade the transmission of timing information from the ANF to the AVCN bushy neuron. We show that the electrical excitability of bushy cells in hearing-impaired old DBA mice was different from that in young, normal-hearing DBA mice. We found an increase in the action potential (AP) firing threshold with current injection; a larger AP afterhyperpolarization; and an increase in the number of spikes produced by large depolarizing currents. We also tested the temporal precision of bushy cell responses to high-frequency stimulation of the ANF. The standard deviation of spikes (spike jitter) produced by ANF-evoked excitatory postsynaptic potentials (EPSPs) was largely unaffected in old DBA mice. However, spike entrainment during a 100-Hz volley of EPSPs was significantly reduced. This was not a limitation of the ability of bushy cells to fire APs at this stimulus frequency, because entrainment to trains of current pulses was unaffected. Moreover, the decrease in entrainment is not attributable to increased synaptic depression. Surprisingly, the spike latency was 0.46 ms shorter in old DBA mice, and was apparently attributable to a faster conduction velocity, since the evoked excitatory postsynaptic current (EPSC) latency was shorter in old DBA mice as well. We also tested the contribution of the low-voltage-activated K+ conductance (g (KLV)) on the spike latency by using dynamic clamp. Alteration in g (KLV) had little effect on the spike latency. To test whether these changes in DBA mice were simply a result of continued postnatal maturation, we repeated the experiments in CBA mice, a strain that shows normal hearing thresholds through this age range. CBA mice exhibited no reduction in entrainment or increased spike jitter with age. We conclude that the ability of AVCN bushy neurons to reliably follow ANF EPSPs is compromised in a frequency-dependent fashion in hearing-impaired mice. This effect can be best explained by an increase in spike threshold.

Age Factors↗
PubMed1998

Axonal spheroids in the cochlear nucleus of normal beagle dogs.

In the course of drug evaluation studies, sporadic axonal spheroids were identified in the cochlear nucleus of 8-15-month-old Beagle dogs. These structures were identified by Bielschowski histochemical and anti-neurofilament immunohistochemical stains and by ultrastructural examination. No cellular reaction or significant neuropil alterations were associated with the presence of the spheroids. Their presence was unrelated to treatment and were considered to be an incidental background finding.

Animals↗
PubMed1997

Spongiform degeneration of the gerbil cochlear nucleus: an ultrastructural and immunohistochemical evaluation.

Observations from ultrastructural and immunohistochemical studies suggest that spongiform lesions in the gerbil cochlear nucleus are derived principally from dendrites. Almost one-fifth of the lesion profiles examined ultrastructurally exhibited synaptic contacts with axon terminals. In addition, approximately 80% of lesions are immunopositive for the dendrite-specific microtubule associated protein, MAP2. Ultrastructural studies showed a small percentage (8%) of lesions were derived from myelinated axons, although none were immunohistochemically labelled with antibodies to the tau protein. Staining with the astrocyte-specific markers GFAP, S-100 and vimentin yielded equivocal results, but did not support a major role for astrocytes in lesion formation. The histological profile matches that seen in some other well characterized types of spongiform degeneration.

Animals↗
PubMed1996

Vowel-onset detection by vowel-strength measurement, cochlear-nucleus simulation, and multilayer perceptrons.

An algorithm for detection of vowel onsets in fluent speech was presented by Hermes [j. Acoust. Soc. Am. 87, 866-873 (1990)]. Performance tests showed that detection was good for fluent speech, although the parameter settings had to be modified for application to well-articulated speech. One of the purposes of the algorithm was application to speech by deaf persons, for which it failed completely. In order to improve the algorithm and to make it more generally applicable, two alternative detection strategies have been explored in the present study. These strategies were (a) simulation of transient-chopper responses in the cochlear nucleus and (B) training of multilayer perceptrons. Two large databases of read speech have been used for performance comparison of the original algorithm and the new strategies. The strategy based on simulating cochlear-nucleus responses is found both to result in a higher false-alarm rate than the original algorithm and to be rather level dependent. On the other hand, the performance of a multilayer-perceptron network, trained on mel-scaled spectra, is comparable to the performance of the Hermes algorithm. In more general terms, the results suggest that temporal information on intensity and (rough) spectral envelope are important for human vowel-onset detection behavior. In formation on harmonicity can be used as a secondary source of information to avoid detection of mainly unvoiced, nonvowel onsets.

Algorithms↗
PubMed2006

Somatosensory influence on the cochlear nucleus and beyond.

Interactions between somatosensory and auditory systems occur at peripheral levels in the central nervous system. The cochlear nucleus (CN) receives innervation from trigeminal sensory structures: the ophthalmic division of the trigeminal ganglion and the caudal and interpolar regions of the spinal trigeminal nucleus (Sp5I and Sp5C). These projections terminate primarily in the granule cell domain, but also in magnocellular regions of the ventral and dorsal CN. Additionally, new evidence is presented demonstrating that cells in the lateral paragiganticular regions of the reticular formation (RF) also project to the CN. Not unlike the responses obtained from electrically stimulating the trigeminal system, stimulating RF regions can also result in excitation/inhibition of dorsal CN neurons. The origins and central connections of these projection neurons are associated with systems controlling vocalization and respiration. Electrical stimulation of trigeminal and RF projection neurons can suppress acoustically driven activity of not only CN neurons, but also neurons in the inferior colliculus. Together with the anatomical observations, these physiological observations suggest that one function of somatosensory input to the auditory system is to suppress responses to "expected" body-generated sounds such as vocalization or respiration. This would serve to enhance responses to "unexpected" externally-generated sounds, such as the vocalizations of other animals.

Acoustic Stimulation↗
PubMed2005

Granule cells in the cochlear nucleus sensitive to sound activation detected by Fos protein expression.

Granule cells are the smallest neuronal type in the cochlear nucleus (CN). Due to their small size, it is extremely difficult to record their sound-evoked activity with microelectrodes. Compared with large, non-granule cells, much less is known about their response properties to sound stimulation. Here, we use Fos, the nuclear regulatory protein, as a neuronal activity marker to determine the responsiveness of granule cells to sound in comparison to the larger neurons. The present study determined the threshold sensitivity and activation pattern of neurons in the three subdivisions of the CN with free-field sound stimulation in monaural, awake rats. Immunocytochemical localization of Fos was used as our metric for "sound activation." Neuronal types upregulating Fos expression in response to sound stimulation were further identified with Nissl counterstaining. Our results show that most CN cell types can upregulate Fos expression when sound activated and the number of Fos-expressing neurons is directly related to sound intensity. The threshold for Fos activation in granule cells is lower than that for non-granule cells. The number of Fos activated granule cells saturates at high sound intensity, while the number of Fos activated non-granule cells is a monotonic function. By comparing the patterns of sound-induced Fos expression in different CN cell types, it may be possible to predict features of sound-evoked activity in granule cells.

Acoustic Stimulation↗
PubMed1995

Transformation of temporal discharge patterns in a ventral cochlear nucleus stellate cell model: implications for physiological mechanisms.

1. We have stimulated responses of stellate cells in the anteroventral cochlear nucleus (AVCN) to single-formant stimuli (SFSs) with the use of recorded auditory-nerve fiber (ANF) responses as inputs. In particular, two important features of temporal discharge patterns, the phase locking to best frequency (BF) tones and to stimulus envelopes, were examined in the model output. Our earlier experimental studies with SFSs found an enhancement of the envelope modulation in AVCN chopper units, presumably recorded from stellate cells, as compared with that of ANFs. 2. We simulated in the model three mechanisms for the enhancement in envelope modulation proposed earlier by us, namely, convergence of ANFs, temporal summation and inhibitory input. It was found that the convergence of multiple ANFs alone did not always lead to an enhancement in modulation depth, but was necessary for the model to produce other physiologically plausible envelope features; the temporal summation of subthreshold events can lead to an increase in modulation depth; and the somatic inhibition effectively reduced the envelope minimum and, as a result, increased the modulation depth. In addition, we found that, given the same input configuration, the closer the inputs were located to the soma, the greater modulation depth they produced at the model output. 3. Different types of convergence of ANF inputs were tested in our model. It was found that the convergence of both low and high spontaneous rate (SR) ANFs resulted in an enhancement in modulation depth over a wider range of sound level than that due to the convergence of ANFs from the same SR group.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗
PubMed1999

Voltage-gated Ca2+ conductances in acutely isolated guinea pig dorsal cochlear nucleus neurons.

Although it is known that voltage-gated Ca2+ conductances (VGCCs) contribute to the responses of dorsal cochlear nucleus (DCN) neurons, little is known about the properties of VGCCs in the DCN. In this study, the whole cell voltage-clamp technique was used to examine the pharmacology and voltage dependence of VGCCs in unidentified DCN neurons acutely isolated from guinea pig brain stem. The majority of cells responded to depolarization with sustained inward currents that were enhanced when Ca2+ was replaced by Ba2+, were blocked partially by Ni2+ (100 microM), and were blocked almost completely by Cd2+ (50 microM). Experiments using nifedipine (10 microM), omegaAga IVA (100 nM) and omegaCTX GVIA (500 nM) demonstrated that a variety of VGCC subtypes contributed to the Ba2+ current in most cells, including the L, N, and P/Q types and antagonist-insensitive R type. Although a large depolarization from rest was required to activate VGCCs in DCN neurons, VGCC activation was rapid at depolarized levels, having time constants <1 ms at 22 degrees C. No fast low-threshold inactivation was observed, and a slow high-threshold inactivation was observed at voltages more positive than -20 mV, indicating that Ba2+ currents were carried by high-voltage activated VGCCs. The VGCC subtypes contributing to the overall Ba2+ current had similar voltage-dependent properties, with the exception of the antagonist-insensitive R-type component, which had a slower activation and a more pronounced inactivation than the other components. These data suggest that a variety of VGCCs is present in DCN neurons, and these conductances generate a rapid Ca2+ influx in response to depolarizing stimuli.

Animals↗
PubMed2002

Ultrastructural basis of synaptic transmission between endbulbs of Held and bushy cells in the rat cochlear nucleus.

Auditory nerve fibres make large excitatory synaptic contacts, the endbulbs of Held, with bushy cells in the anteroventral cochlear nucleus (AVCN). We have used serial-section electron microscopy to reconstruct seven endbulbs of Held in contact with three different AVCN bushy cells from a 25-day-old rat, as a basis for interpreting our previous physiological results at this connection. Four endbulbs of Held contacting the same bushy cell were completely reconstructed. The number of separate synaptic specializations within these endbulbs varied from 85 to 217, with a mean of 155. Detailed measurements were obtained from high magnification segments of four endbulbs contacting three different bushy cells. Large variability was found in the size of synaptic specializations within an individual endbulb. The size of postsynaptic densities (PSDs) varied between endbulbs (mean PSD area 0.03, 0.07, 0.07 and 0.18 microm(2); n = 4 endbulbs). The number of morphologically docked vesicles at individual specializations within the same endbulb varied considerably (between 1 and 102). The mean number of morphologically docked vesicles per specialization differed between endbulbs (mean numbers of docked vesicles per specialization = 2.1, 3.7, 5.3, 14.8; n = 4 endbulbs). Despite these large differences, the density of docked vesicles per square micron of PSD was similar between endbulbs (54, 80, 81, 83 docked vesicles per microm(2); n = 4 endbulbs). Within an endbulb, a linear relationship was found between the number of docked vesicles and PSD area, and between PSD area and the number of undocked vesicles within 150 nm of the active zone. The ratio of undocked vesicles (< 150 nm) to docked vesicles ranged from 2 to 5 in different endbulbs (n = 4 endbulbs). These structural observations are discussed in relation to the functional properties of synaptic transmission between endbulbs of Held and bushy cells in the AVCN.

Animals↗
PubMed1999

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.

Acoustic Stimulation↗
PubMed2002

Effects of cochlear ablation on noise induced hyperactivity in the hamster dorsal cochlear nucleus: implications for the origin of noise induced tinnitus.

Chronic increases in multiunit spontaneous activity are induced in the dorsal cochlear nucleus (DCN) following exposures to intense sound. This hyperactivity has been implicated as a neurophysiological correlate of noise induced tinnitus. However, it is not known whether this hyperactivity originates centrally, or instead, reflects an increase in the level of spontaneous input from the auditory nerve. In the present study we addressed this issue by testing whether hyperactivity, induced in the DCN by previous exposure to intense sound, persists after ipsilateral cochlear input to the DCN has been removed. To induce hyperactivity, Syrian golden hamsters were exposed under anesthesia to an intense pure tone (122-127 dB SPL at 10 kHz) for 4 h. Additional hamsters, which were anesthetized for 4 h, but not tone exposed, served as controls. Electrophysiological recordings of spontaneous activity were performed on the surface of the left DCN in animals in which the ipsilateral cochlea was either intact or ablated. The degree of cochlear removal was determined by microdissection and histologic evaluation of the cochlea after completion of each recording session. Comparisons between the levels of activity recorded in animals with and without intact cochleas revealed that the induced hyperactivity in the DCN persisted after both partial and complete cochlear ablations. These results indicate that the maintenance of hyperactivity is not dependent on input from the ipsilateral cochlea, implying that hyperactivity originates centrally.

Animals↗