Spontaneous spike discharges from single units in the cochlear nucleus after destruction of the cochlea

Effects of conductive hearing loss on gerbil central auditory system activity in silence

Animal models of conductive hearing loss (CHL) show altered structure and function in the central auditory system (CAS), particularly following unilateral deprivation. Assessment of neuronal activity as measured by 2-deoxyglucose (2-DG) uptake following CHL has been reported by two groups of investigators, with different findings. Woolf and colleagues [Brain Res. 274 (1983) 119] found that 2-DG uptake increased in the cochlear nucleus ipsilateral to the CHL, while Tucci et al. [Laryngoscope 109 (1999) 1359] found a decrease in 2-DG uptake in the ipsilateral cochlear nucleus. One significant difference between the protocols in the two studies was that, in the first study, animals were maintained in silence following 2-DG injection, whereas in the Tucci et al. study, animals were exposed to sound. The current study was designed to replicate the protocol used by Woolf et al. Young adult gerbils underwent unilateral malleus removal with bilateral canal ligation (n=6) or a sham procedure (n=7) 48 h prior to 2-DG administration and sacrifice. Optical density measurements were made from CAS nuclei. 2-DG uptake decreased in the ipsilateral cochlear nucleus and contralateral inferior colliculus, and in nuclei of the superior olivary complex bilaterally, supporting the finding that CHL is associated with a decrease in CAS neuronal activity.

Expression of NMDA, AMPA and GABA(A) receptor subunit mRNAs in the rat auditory brainstem. I. Influence of early auditory deprivation

Impact of early post-natal deafening on auditory pathways was investigated in newborn rats deafened by daily amikacin injections from P7 to P16 inducing a complete destruction of the organ of Corti. The expression of mRNAs encoding N-methyl-D-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) and gamma-aminobutyric acid type A (GABA(A)) receptor subunits was then studied by in situ hybridization in the dorsal and ventral cochlear nucleus and in the central nucleus of the inferior colliculus (CNIC). Early post-natal deafening decreased bilaterally the expression of mRNAs encoding NR1, NR2a, NR2b and flop isoforms of AMPA receptors. On the contrary, it increased the expression of mRNAs encoding some GABA(A) subunits (alpha1, beta1, gamma2) and flip isoforms of AMPA receptors. These changes were more pronounced in cochlear nuclei than in CNIC. They suggest that auditory sensation is essential in the normal development of central auditory pathways.

Effects of deafferentation on the electrophysiology of ventral cochlear nucleus neurons

When cochlear pathology impairs the afferent innervation of the ventral cochlear nucleus (VCN), electrical responses of the auditory brainstem are altered and changes in cell and synaptic morphology are observed. However, the impact of deafferentation on the electrical properties of cells in the VCN is unknown. We examined the electrical properties of single neurons in the anterior and posterior VCN following bilateral cochlear removal in young rats. In control animals, two populations of cells were distinguished: those with a linear subthreshold current-voltage relationship and repetitive firing of action potentials with regular interspike intervals (type I), and those with rectifying subthreshold current-voltage relationships and phasic firing of 1-3 action potentials (type II). Measures of action potential shape further distinguished these two groups. Two weeks following cochlear removal, both electrical response patterns were still seen. Type I cells showed a higher input resistance. Deafferented single-spiking type II cells were slightly more depolarized, had smaller action potentials, smaller afterhyperpolarizations and shorter membrane time constants, whereas multiple-spiking type II cells were apparently unaffected. These changes in the electrical properties of VCN neurons following cochlear injury may adversely affect central processing of sounds presented acoustically or electrically by prostheses.

Plasticity in the development of afferent patterns in the inferior colliculus of the rat after unilateral cochlear ablation

The central nucleus of the inferior colliculus (IC) is the site of convergence for nearly all ascending monaural and binaural projections. Several of these inputs, including inhibitory connections from the dorsal nucleus of the lateral lemniscus (DNLL), are highly ordered and organized into series of afferent bands or patches. Although inputs to the IC from the contralateral DNLL are present in the rat by birth [postnatal day 0 (P0)], the earliest indications of band formation are not evident until P4. Subsequently, the initially diffuse projection segregates into a pattern of bands and interband spaces, and by P12 adult-like, afferent-dense patches are established (Gabriele et al., 2000). To determine the role of the auditory periphery in the development of bands and patches before the onset of hearing (P12/P13), unilateral cochlear ablations were performed at P2 (before any evidence of banding). Rat pups were reared to P12, at which time glass pins coated with 1, 1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate were placed in fixed tissue in the commissure of Probst where DNLL fibers cross the midline. The results indicate that a unilateral cochlear ablation disrupts the normal development of afferent patches in the IC. Although the crossed DNLL projections labeled via commissural dye placement always mirrored each other in P12 controls, ablation cases exhibited a consistent, bilateral asymmetry in pattern formation and relative density of the labeled projections. Possible developmental mechanisms likely to be involved in the establishment of afferent bands and patches before the onset of hearing are discussed.

Plasticity of spontaneous neural activity in the dorsal cochlear nucleus after intense sound exposure

Increases in multiunit spontaneous activity (hyperactivity) can be induced in the dorsal cochlear nucleus (DCN) by intense sound exposure. This hyperactivity has been observed in the hamster and rat following exposure to a 10 kHz tone at a level of 125-130 dB SPL for a period of 4 h. The present study demonstrates that the onset of this hyperactivity is not immediate, but develops in the DCN between 2 and 5 days after exposure. Mean rates of multiunit spontaneous activity increased sharply from below normal levels at day 2 to higher than normal levels at day 5. The mean magnitude of activity continued to increase more gradually over the next 6 months. During this period, changes in the distribution of hyperactivity across the tonotopic array were also noted. The hyperactivity was more broadly distributed across the DCN at the early post-exposure times (5 and 14 days) than at later post-exposure recovery times (30 and 180 days), and peak activity was found at increasingly more medial positions over this time frame. These changes over time indicate that the mechanisms leading to hyperactivity following intense sound exposure are more complex than previously realized.

Electrical stimulation of the auditory nerve. III. Response initiation sites and temporal fine structure

Latency, temporal dispersion and input-output characteristics of auditory nerve fiber responses to electrical pulse trains in normal and chronically deafened cat ears were classified and tentatively associated with sites where activity is initiated. Spikes occurred in one or more of four discrete time ranges whose endpoints overlapped partially. A responses had latencies <0.44 ms, exhibited asymptotic temporal dispersion of 8-12 micros and possessed an average dynamic range of 1.2 dB for 200 pulses/s (pps) pulse trains. They likely originated from central processes of spiral ganglion cells. B(1) and B(2) responses (0.45-0.9 ms, 25-40 micros, 1.9 dB) likely stemmed from activity at myelinated and unmyelinated peripheral processes, respectively. C100 micros) likely originated from direct stimulation of inner hair cells, and D8 dB) arose from propagating traveling waves possibly caused by electrically induced motion of outer hair cells. C and D responses were recorded only in acoustically responsive ears. Mean latencies of spikes in all time ranges usually decreased with intensity, and activity at two or even three discrete latencies was often observed in the same spike train. Latency shifts from one discrete time range to another often occurred as intensity increased. Some shifts could be attributed to responses to the opposite-polarity phase of the biphasic pulse. In these cases, temporal dispersion and dynamic range were approximately equal for activity at each latency. A second type of latency shift was also often observed, in which responses at each latency exhibited dissimilar temporal dispersion and dynamic range. This behavior was attributed to a centralward shift in the spike initiation site and it occurred for monophasic as well as biphasic signals. Several fibers exhibited dual latency activity with a 40-90 micros time difference between response peaks. This may have stemmed from spike initiation at nodes on either side of the cell body. Increasing the stimulus pulse rate to 800-1000 pps produced small increases in temporal dispersion and proportionate increases in asymptotic discharge rate and dynamic range, but thresholds did not improve and slopes of rate-intensity functions (in spikes/s/dB) did not change. Responses to high-rate stimuli also exhibited discrete latency increases when discharge rates exceeded 300-400 spikes/s. Spike by spike latencies in these cases depended strongly on the discharge history. Implications for high-rate speech processing strategies are discussed.

Neonatal sensorineural hearing loss affects neurone size in cat auditory midbrain

We examined the effect of a neonatal sensorineural hearing loss on the soma area of neurones in the central nucleus of the inferior colliculus (ICC) in adult cats to evaluate the role of auditory experience on neuronal atrophy within the auditory midbrain. Three groups of animals were used: bilaterally deafened, unilaterally deafened and normal hearing controls. Soma area measurements were made from the laminated central and medial divisions of the ICC of eight deafened and two normal hearing cats. A small but significant reduction in soma area was evident for bilaterally deafened animals compared with normal hearing controls (P<0.05, Dunnett's test). In contrast, there was no significant difference in mean soma area between normal hearing and unilaterally deafened animals (P0.05) irrespective of whether the ICC examined was ipsi- or contralateral to the deafened ear. These results demonstrate that the reduction in soma area of auditory brainstem neurones reported following a sensorineural hearing loss is also evident at the level of the auditory midbrain.

Somatic (craniocervical) tinnitus and the dorsal cochlear nucleus hypothesis

PURPOSE

Of all nonauditory sensory systems, only the somatosensory system seems to be related to tinnitus (eg, temporomandibular joint syndrome and whiplash). The purpose of this study is to describe the distinguishing characteristics of tinnitus associated with somatic events and to use these characteristics to develop a neurological model of somatic tinnitus.

MATERIALS AND METHODS

Case series.

RESULTS

Some patients with tinnitus, but no other hearing complaints, share several clinical features including (1) an associated somatic disorder of the head or upper neck, (2) localization of the tinnitus to the ear ipsilateral to the somatic disorder, (3) no vestibular complaints, and (4) no abnormalities on neurological examination. Pure tone and speech audiometry of the 2 ears is always symmetric and usually within normal limits. Based on these clinical features, it is proposed that somatic (craniocervical) tinnitus, like otic tinnitus, is caused by disinhibition of the ipsilateral dorsal cochlear nucleus. Nerve fibers whose cell bodies lie in the ipsilateral medullary somatosensory nuclei mediate this effect. These neurons receive inputs from nearby spinal trigeminal tract, fasciculus cuneatus, and facial, vagal, and glossopharyngeal nerve fibers innervating the middle and external ear.

CONCLUSIONS

Somatic (craniocervical) modulation of the dorsal cochlear nucleus may account for many previously poorly understood aspects of tinnitus and suggests novel tinnitus treatments.

Conductive hearing loss results in a decrease in central auditory system activity in the young gerbil

OBJECTIVES/HYPOTHESIS

The impact of childhood conductive HL (CHL) on development of auditory function has long been debated. The present study was conducted to define and compare the consequences of CHL and cochlear ablation (CA) in young and adult animals, using 2-deoxyglucose (2-DG) uptake as a measure of metabolic activity. It was hypothesized that, for both ages, CHL would result in a decrease in activity in the major ascending central auditory system pathway of the manipulated ear, but that this decrease would be significantly less than that observed with CA.

STUDY DESIGN

Sham-controlled study of metabolic effects of CHL during sound stimulation.

METHODS

Gerbils (aged 21 days or adult), underwent malleus removal, CA, or a sham procedure. Young animals survived either 48 hours or 3 weeks; adults survived 3 weeks. Each age/survival CHL group contained eight animals; otherwise, each group (CA and sham) contained five animals, for a total number of 54. At the appropriate survival time, animals were given an intracardiac injection of 14C-2-DG, and sacrificed under anesthesia after 45 minutes of exposure to normal laboratory sounds. Tissue sections were prepared for exposure to x-ray film for optical density measurements, and alternate sections stained for identification of nuclei. Measurements from auditory nuclei of experimental animals were corrected against an unaffected control area (abducens nucleus) and compared with measurements taken from animals in the sham group. Auditory evoked potential thresholds to both air- and bone-conducted stimuli were obtained in a second group of neonatal and adult animals.

RESULTS

Both CHL and CA resulted in a marked decrease in 2-DG uptake in the major ascending projection of the manipulated ear, in both the neonatal and adult animals. In young animals, effects of CHL and CA were similar. Effects of CHL in adult animals were less marked and significantly different from either effects of CHL in young animals or effects of CA in adult animals. HL following malleus removal only was purely conductive and ranged from 38 to 55 dB across frequency.

CONCLUSIONS

Results suggest that, particularly in young animals, a unilateral CHL may have profound effects on metabolic activity in the central auditory system.

Effects of unilateral cochlear removal on dendrites in the gerbil medial superior olivary nucleus

For most neurons, dendrites serve as the major pathway for incoming activity from other neurons. It might therefore be expected that dendrites are particularly sensitive to variations in the level of afferent input they receive. In the auditory brainstem, this expectation has been confirmed in neurons of the medial superior olivary nucleus (MSO). The MSO is uniquely suited to studies of afferent influences on dendrites, as lateral and medial dendrites of MSO neurons receive inputs almost exclusively from the ipsilateral and contralateral ears, respectively. Thus, the effects of unilateral afferent manipulations may be compared between defined dendrites on the same neurons. We have used unilateral deafening (by surgical destruction of the cochlea) in immature [postnatal day 18 (P18)] and adult gerbils to study the late maturation and effect of peripheral deafferentation on the dendrites of MSO neurons. In semi-thin, frontal sections from unoperated animals, we found a change between P18 and adulthood from a lateral to a medial bias in the symmetry of MSO dendrites. Cochlear removal in adulthood led to a reduction in the density of dendritic profiles on the side of the ablation in both MSOs. Cochlear removal at P18 led to a rapid (< 3 days) and sustained dendritic atrophy that was most marked in the caudal part of the nucleus. Electron microscope (EM) measurements in the sagittal plane on MSO dendrite profiles of animals unilaterally deafened at P18 showed a reduction in the number, but not in the area, of profiles on the side of the deafened ear. These results demonstrate a developmental change in the symmetry of MSO medial and lateral dendrites, and a rapid and long-lasting reduction in the number of distal dendrites produced by unilateral deafening either in infancy or adulthood.

Glutamergic 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, alpha-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.

Sensorineural hearing loss during development: morphological and physiological response of the cochlea and auditory brainstem

We have investigated the effects of sensorineural hearing loss on the cochlea and central auditory system of profoundly deafened cats. Seventeen adult cats were used: four had normal hearing; 12 were deafened neonatally for periods of < 2.5 years (five bilaterally, seven unilaterally); and one animal had a long-term (approximately 8 years) profound bilateral hearing loss. Bipolar scala tympani stimulating electrodes were bilaterally implanted in each animal, and electrically evoked auditory brainstem responses (EABRs) were recorded in an acute study to evaluate the basic physiologic response properties of the deafened auditory pathway. The cochleae and cochlear nuclei (CN) of each animal were examined with light microscopy. Spiral ganglion cell density in neonatally deafened cochleae was 17% of normal, and only 1.5% of normal in the long-term deaf animal. There was a 46% reduction in total CN volume in neonatally deafened animals compared to normal, and a 60% reduction in the long-term deaf animal. Neural density in the anteroventral CN of bilaterally deafened animals was 37% higher than normal; 44% higher in the long-term deaf animal. Significantly, however, we saw no evidence of a loss of neurones within the anteroventral CN in any deafened animal. There was a significant increase in EABR threshold and wave IV latency in the deafened animals, and a significant decrease in response amplitude and input/output function gradient. Again, these changes were more extensive in the long-term deaf animal. These data show that a sensorineural hearing loss can evoke significant morphological and physiological changes within the cochlea and auditory brainstem, and these changes become greater with duration of deafness. It remains to be seen whether these changes can be reversed following the introduction of afferent activity via chronic electrical stimulation of the auditory nerve.

Responses of anteroventral cochlear nucleus neurons of the unanesthetized decerebrate cat to click pairs as simulated echoes

To elucidate the contribution of the anteroventral cochlear nucleus (AVCN) to 'echo' processing, this study documents the responses of AVCN neurons to simulated echoes and compares them to those of auditory nerve (AN) fibers. Single unit discharges were recorded from 121 units in the AVCN of 21 unanesthetized decerebrate cats in response to click pairs with inter-click intervals ranging from 1 to 32 ms between 45 and 105 dB SPL re 20 microPa. Units were classified according to the post-stimulus time histogram (PSTH) and excitatory-inhibitory response area (EI-area) schemes. Based on their spontaneous rates (SR), units were subdivided into low- ( < 20 spikes/s) and high- ( > 20 spikes/s) SR groups. A majority of the units exhibited second-click responses whose recovery time courses were similar to those of AN fibers. These units included primary-like, chopper and onset units in the PSTH scheme and Types I, I/III and III units in the EI-area scheme. A minority of the units exhibited responses that were distinct from those of AN fibers, in that they had second-click response recovery times that were either markedly reduced or prolonged. This group of units included those with primary-like, chopper and onset PSTHs and Type I/III and III EI-areas. No significant difference was found in the second-click response among various PSTH or EI-area types. High-SR AVCN units exhibited a decrease in the second-click response with increasing level. In contrast, low-SR AVCN units showed little level-dependent change in the second-click responses. This SR-based difference was similar to that previously found among AN fibers. The present results suggest that, although a majority of AVCN units exhibit similar time courses of second-click response recovery to those of AN fibers, there do exist mechanisms in the cochlear nucleus that can substantially alter this representation. Furthermore, the difference between the second-click response recovery functions of low- and high-SR AVCN units and the consistency of this finding between AVCN and AN suggest that SR represents an important dimension for signal representation in the AVCN neurons.

Long-term degeneration in the cochlear nerve and cochlear nucleus of the adult chinchilla following acoustic overstimulation

Adult chinchillas were exposed once to an octave-band noise, centered at 4 kHz, and allowed to survive for 16 days or for 1, 2, 4, and 8 months. Axonal degeneration was mapped in the cochlear nucleus, using the Nauta-Rasmussen silver method, and related to hair cell damage and to loss of myelinated nerve fibers in the osseous spiral lamina of the cochlea. Axonal degeneration in the dorsal cochlear nucleus had already reached a peak by 16 days and disappeared after 1 month. Meanwhile, myelinated nerve fiber degeneration in the cochlea extended basally, followed 2 weeks to 2 months later by spread of axonal degeneration into the corresponding high-frequency region of the ventral cochlear nucleus. Axonal degeneration occurred early in the low-frequency region of the ventral cochlear nucleus, followed 2-4 weeks later by spread of myelinated fiber degeneration into more apical regions of the cochlea. New degeneration of axons in the cochlear nerve and in the ventral cochlear nucleus continued to occur for up to 8 months after stimulation. These findings imply that plastic changes in the central auditory pathways could play a role in the long-term effects of cochlear damage and acoustic overstimulation, possibly leading to a chronic neurodegenerative condition in the ear and in the brain.

Salicylate and quinine selectively increase spontaneous firing rates in secondary auditory cortex

This study presents firing rates for simultaneously recorded spontaneous and stimulus driven multi-unit activity in primary auditory cortex (AI), anterior auditory field (AAF) and secondary auditory cortex (AII) in cats before and after application of salicylate or quinine. From 21 cats, in three cortical areas simultaneously, a total of 1533 multi-unit files were obtained. The data suggest (1) that both salicylate and quinine significantly increase spontaneous firing rates in AII, whereas in AI and AAF both quinine and salicylate reduced the spontaneous rate; (2) the effect of both drugs was to increase spontaneous rates for recording sites with high characteristic frequency (CF) and a tendency to decrease them for low CF sites; (3) the mean stimulus driven firing rates were not affected by either drug except for a decrease produced by quinine in AI; (4) changes in driven firing rate were positively correlated with changes in spontaneous firing rates. This suggests that tinnitus inducing agents selectively increase spontaneous firing rates in the extralemniscal pathway.

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 microM) 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.

Response properties of neurons in the inferior colliculus of the monaurally deafened ferret to acoustic stimulation of the intact ear

Response properties of neurons in the central nucleus of the inferior colliculus (ICC) were investigated after unilateral cochlear removal at various ages during infancy. Nineteen ferrets had the right cochlea surgically ablated, either in adulthood or on postnatal day (P) 5, 25, or 40, 3-18 mo before recording. Adult ablations were made on the same day as ("acute," n = 3), or 2-3 mo before ("chronic," n = 3), recording. Two ferrets were left binaurally intact. Single-unit (n = 702) and multiunit (n = 1,819) recordings were made in the ICC of barbiturate-anesthetized ferrets ipsilateral (all ages) or contralateral (P5 and acute adult only) to the intact ear. In binaurally intact animals, tonal stimulation of the contralateral ear evoked excitatory activity at the majority (94%) of recording loci, whereas stimulation of the ipsilateral ear evoked activity at only 33% of recording loci. In acutely ablated animals, the majority of contralateral (90%) and ipsilateral (70%) loci were excited by tonal stimulation of the intact ear. In chronically ablated animals, 80-90% of loci were excited by ipsilateral stimulation. Single-unit thresholds were generally higher for low-best frequency (BF) than for high-BF units, and higher in the ipsilateral than in the contralateral ICC. Analysis of covariance showed highly significant differences between all of the ipsilateral and contralateral groups, but no effects of age at ablation or survival time following ablation, other than that the group ablated at P25 had higher mean ipsilateral thresholds than the groups ablated at P5 or, acutely, in adulthood. Cochlear ablation at P5, 25, or 40 resulted in a significant increase in dynamic ranges of ipsilateral ICC unit rate-intensity functions relative to acutely ablated animals. Dynamic ranges of units in the contralateral ICC of P5-ablated ferrets were also significantly increased compared with those of acutely ablated animals. Cochlear ablation at P5, 25, or 40 resulted in a significant increase in single-unit spontaneous discharge rates in the ICC ipsilateral but not contralateral (P5 only) to the intact ear. These data show that unilateral cochlear removal in adult ferrets leads to a rapid and dramatic increase in the proportion of neurons in the ICC ipsilateral to the intact ear that is excited by acoustic stimulation of that ear. In addition, the data confirm that, in ferrets, cochlear removal in infancy leads to a further increase in responsiveness of individual neurons in the ipsilateral ICC. Finally, the data show that responses in the ICC contralateral to the intact ear are largely but not completely unchanged by unilateral cochlear removal.

Response map properties of units in the dorsal cochlear nucleus of barbiturate-anesthetized gerbil (Meriones unguiculatus)

The response map scheme introduced by Evans and Nelson (1973) and modified by others, including Davis et al. (1996) for use with gerbils, has been used primarily for classifying units recorded in the cochlear nucleus of unanesthetized decerebrate preparations. Units lacking spontaneous activity (SpAc) have been classified as either type I/III or type II units based on the relative strength of their responses to broad-band noise compared to their responses to best-frequency (BF) tones. The relative noise index (rho), a ratio of these responses after SpAc is subtracted out, provides a convenient measure of this relative strength. In this paper, responses of 320 units recorded in the dorsal cochlear nucleus (DCN) of barbiturate-anesthetized gerbils to short-duration BF tones and broad-band noise were recorded. Since 87.5% of these units lacked SpAc, their response maps resembled those of type II and type I/III units. Units were characterized by rho and the normalized slope (m) of a best line fit to the BF rate versus level plot starting from the sound level corresponding to the first inflection point of the rate curve (typically its maximum value or the start of its sloping saturation). The distributions of rho and m values do not form distinct clusters as they do for units in the decerebrate preparation. Thus, the criteria developed for classifying DCN units in the decerebrate preparation do not appear appropriate for units in the barbiturate-anesthetized preparation. Deposits of horseradish peroxidase were used to locate 52 units. Most of the low SpAc units, 56% with poor noise responses (5/9) and nearly 70% with strong noise responses (25/36), and nearly all of the high SpAc units (6/7), were located either within or below the fusiform cell layer.

Maturational delays in cortical evoked potentials in cochlear implant users

We studied the effects of prolonged auditory deprivation in children in whom auditory stimulation was restored by a cochlear implant. The latency of the P1 component of the late cortical potential was used as the indicator of auditory system maturation. For normal-hearing children there is a gradual evolution of evoked potential features that extends through adolescence with P1 latency becoming adult-like at about age 15. It appears that maturation of P1 latency in normal and implanted children occurs at the same rate, but the time to maturity in implanted subjects is delayed by an amount approximately equal to the duration of deafness.

Susceptibility of developing cochlear nucleus neurons to deafferentation-induced death abruptly ends just before the onset of hearing

To investigate the ability of developing cochlear nucleus (CN) neurons to survive in the absence of afferent input, left cochlear removals were performed on gerbils at 2 day intervals from postnatal (P)3 to P11, and at P18 and P93. After a 3 month postsurgical survival period, Nissl-stained frontal sections through the brainstem were analyzed under the light microscope. CN volume, anteroventral cochlear nucleus (AVCN) neuron cross-sectional area, and total number of neurons in the CN were measured on both sides of the brain. Mean volume reduction of the deafferented CN relative to the intact CN ranged between 76% in the P3 group to 33% in the P11 group and did not differ significantly between P11 and P93. Cochlear removal at all ages reduced AVCN neuron cross-sectional area by approximately 40% in the deafferented CN relative to the intact CN, except for the P93 group where neuron atrophy was significantly less severe (23% mean reduction). Massive loss of CN neurons (>50% of the intact side) was observed following cochlear removal performed during the first postnatal week. However, between P7 and P9, neurons in all areas of the CN lose susceptibility to deafferentation-induced neuron death. No significant neuron loss was observed following cochlear removal after P7. This study shows that an abrupt transition in the ability of CN neurons to survive in the absence of afferent input is coincident with events leading to the onset of hearing.

Neuronal and transneuronal degeneration of auditory axons in the brainstem after cochlear lesions in the chinchilla: cochleotopic and non-cochleotopic patterns

Terminal axonal degeneration in the brain following cochlear lesions was studied with the Nauta-Rasmussen method. Losses of hair cells and myelinated cochlear fibers were assessed. The cochleotopic map projected, from apex to base, on the ventral-to-dorsal axes of the cochlear nuclei. The cochleotopic correspondence was better for loss of cochlear nerve fibers and inner hair cells, than for outer hair cells. Cochlear fibers were traced to all parts of the cochlear nucleus, including the small-cell shell, also to cell-group Y and the flocculus. Terminal axonal degeneration in nuclei of the superior olivary complex, lateral lemniscus, and inferior colliculus was interpreted as transynaptic, since degenerated axons could not be traced to these locations from the cochlear nerve or trapezoid body. Moreover, biotinylated dextran amine injection in the basal turn of scala media of a normal cochlea labeled cochlear nerve fibers projecting to the high-frequency regions of the cochlear nuclei and to the flocculus, but not to more central auditory nuclei. This is the first detailed account of transynaptic degeneration in the ascending auditory pathway resulting from cochlear damage in an adult mammal. These findings are consistent with a dystrophic process depending on hair-cell loss and/or direct damage to cochlear nerve fibers.

Degeneration of axons in the brainstem of the chinchilla after auditory overstimulation

The patterns of axonal degeneration following acoustic overstimulation of the cochlea were traced in the brainstem of adult chinchillas. The Nauta-Rasmussen method for axonal degeneration was used following survivals of 1-32 days after a 105 min exposure to an octave-band noise with a center frequency of 4 kHz and a sound pressure level of 108 dB. Hair-cell and myelinated nerve-fiber loss were assessed in the cochlea. The cochleotopic pattern of terminal degeneration in the ventral cochlear nucleus correlated with the sites of myelinated fiber and inner-hair-cell loss: this correlation was less rigorous with outer-hair-cell loss, especially in the dorsal cochlear nucleus. These results are consistent with a dystrophic process with a slow time course depending on hair-cell loss and/or direct cochlear nerve-fiber damage. However, in a number of cases with no damage in the apical cochlea, fine fiber degeneration occurred with a faster course in low-frequency regions in the dorsal cochlear nucleus and, transynaptically, in a non-cochleotopic pattern in the superior olive and inferior colliculus. These findings suggest that neuronal hyperactivity plays a role in the central degeneration following acoustic overstimulation, possibly by an excitotoxic process.

Morphological changes in the cochlear nucleus of congenitally deaf white cats

Investigations in animal models and humans have indicated that congenital deafness produces degenerative changes in the central auditory pathway. The cochlear nucleus is the first central structure that receives cochlear input, and may be considered the origin of ascending auditory pathways. In this context, we studied congenitally deaf white cats, who express early onset cochlear receptor loss, in order to assess the nature of structural changes in cells of the cochlear nucleus. It is conceivable that pathologic alterations in higher auditory structures are transneuronally distributed through this nucleus. The cochlear nuclei of nonwhite cats with normal hearing were compared to those of deaf white cats exhibiting hearing loss in excess of 70 dB SPL. The cochlear nuclei of the deaf white cats were smaller in volume by roughly 50%, with the ventral and dorsal divisions being equally affected. Cell body silhouette area was determined for spherical bushy cells of the anteroventral cochlear nucleus (AVCN), pyramidal cells of the dorsal cochlear nucleus (DCN), sensory neurons from the principal trigeminal nucleus, and motoneurons of the facial nucleus. We found no statistical difference in neuronal cell body size between nonauditory neurons of these two groups of cats, whereas auditory neurons of deaf white cats were 30.8-39.4% smaller than those of normal cats. These data imply that neuronal changes in congenitally deaf cats are specific to the auditory pathway. Although cochlear nucleus volume loss was uniform for both divisions, there was a differential effect on cell density: AVCN cell density increased by 40%, whereas DCN cell density was relatively unaffected (10% increase). Astrocyte density was also greater in the AVCN (52%) compared to that in the DCN (5%). These observations reveal a differential impact on cells in the cochlear nucleus to congenital deafness, suggesting selective processing impairment at this level. If similar patterns of degeneration occur in humans, such pathologies may underlie reduced processing of input from cochlear implants in congenitally deaf adults.

Evidence for reactive astrocytes in rat vestibular and cochlear nuclei following unilateral inner ear lesion

We investigated whether unilateral removal of the labyrinthine and cochlear receptors induces a macroglial reaction in rat vestibular and cochlear nuclei using vimentin and glial fibrillary acidic protein (GFAP) immunochemical markers. Antibody binding was visualized using the avidin-biotin method and 3,3'-diaminobenzidine as the peroxidase substrate. In addition, double-labelling experiments were performed using specific secondary fluorescent antibodies. Potentially degenerating axon terminals were also studied using a silver impregnation method. In normal adult rats, vimentin was found only in ependymal cells, tanicytes around the fourth ventricle, endothelial cells in the blood vessels and Bergmann glia in the molecular layer of the cerebellum. In lesioned rats, all deafferented vestibular and ventral cochlear nuclei showed strong vimentin immunoreactivity. Furthermore, double-labelling experiments demonstrated that these vimentin-positive cells were also GFAP-positive. The reaction became evident on the second day after the lesion, was intense for 3-8 days and then declined until day 21. No vimentin immunoreactivity could be detected at the level of the ipsilateral dorsal cochlear nucleus. Therefore, unilateral inner ear lesion induced an astroglial reaction within the deafferented vestibular and cochlear nuclei. The decrease in the resting discharge of the primary vestibular afferents and/or in the deafferented central vestibular neurons may induce the glial reaction in the vestibular complex, whereas both degeneration and silence of the cochlear nerve and central cochlear neurons are most probably responsible for the cochlear vimentin-immunoreactive staining. The role of the reactive astrocytes in the vestibular compensation process remains to be determined.

Effects of parallel fiber stimulation on neurons of rat dorsal cochlear nucleus

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.

Lateral superior olive projections to the inferior colliculus in normal and unilaterally deafened ferrets

We have examined the projection from the lateral superior olive (LSO) to the inferior colliculus (IC) in the ferret, with particular interest in the laterality of the projection and in the effects of unilateral cochlear removal in infancy. Large or small deposits of the retrograde tracer wheat germ agglutinin-horseradish peroxidase (WGA-HRP) were made in the IC of anesthetized adult ferrets that either were normally hearing or had been unilaterally deafened in infancy (P5 or P25). After 2 days, the ferrets were perfused, and frontal sections of the brainstem were treated with tetramethyl benzidine. For large deposits of WGA-HRP, equal numbers of labelled neurons were found evenly spread through both LSOs. Smaller deposits of WGA-HRP produced four results that contrasted with previous reports on the cat. First, many more labelled neurons were found in the contralateral than in the ipsilateral LSO. Second, the relative number of labelled neurons in each LSO was independent of whether the deposits were in the ventral or in the dorsal IC. Third, the total number of labelled LSO neurons was independent of whether the deposits were in the ventral or in the dorsal IC. Fourth, the proportion of ipsilateral to contralateral labelled neurons was slightly higher in the medial LSO than in the lateral LSO. Ventral IC deposits resulted in more labelled neurons in the medial LSO, and dorsal IC deposits resulted in more labelled neurons in the lateral LSO, as expected. Neonatal cochlear removal did not change any of these results. We conclude that, in the ferret, the organization of the crossed and uncrossed projections from the LSO to the IC differs from that of the cat, and any similarity with the optic chiasm is not obvious.

Deafness induced cell size changes in rostral AVCN of the guinea pig

The right cochleae of 250-350 g guinea pigs were lesioned by topical administration of neomycin in the middle ear cavity. Eight weeks after the lesion, the cochleae and cochlear nuclei were analyzed. Cochlear hair cell loss was assessed, and cell areas of spherical bushy cells in the rostral anteroventral cochlear nucleus (AVCN) were compared between the lesioned and normal hearing sides for each animal. In five animals with both inner and outer hair cell loss in the lesioned cochlea, the average area of neuronal somata in the rostral AVCN in the lesioned side was 22% smaller than the average area of these cells in the normal hearing side. In two animals with outer hair cell loss but inner hair cells remaining, there was no difference in cell size between the lesioned and non-lesioned AVCN. These results provide evidence that there is significant shrinkage in AVCN cell size in the mature mammal after hearing loss associated with inner hair cell loss.

Effects of unilateral cochlea ablation on the distribution of calretinin mRNA and immunoreactivity in the guinea pig ventral cochlear nucleus

The predominantly neuronal, calcium-binding protein calretinin is highly expressed in the guinea pig auditory system. Within the ventral cochlear nucleus (VCN), calretinin-positive auditory nerve fibers terminate on many calretinin-containing bushy, octopus, and multipolar cells. The abundance of calretinin in the cochlear nucleus provides an ideal system for examining the effects of altered neuronal input on the expression of this calcium-binding protein. The present experiments examined the effects of unilateral cochlea ablation on calretinin immunoreactivity and mRNA levels in the VCN. Calretinin mRNA was labeled by in situ hybridization histochemistry using a radioactive oligonucleotide probe and was quantified by optical density measures on autoradiograms. Survival times of 1, 7, and 56 days postlesion were examined. The results revealed a consistent increase in calretinin mRNA in the rostral portion of the ipsilateral anterior VCN 1 day postlesion but no effect on calretinin mRNA in this region at 7 and 56 days postlesion. The intensity of immunohistochemical label was also increased at 1 and 7 days after surgery. In contrast, calretinin mRNA was not affected 1 day postlesion in the ipsilateral posterior VCN but was decreased at both 7 and 56 days postlesion. The decrease in calretinin mRNA in the posterior VCN at longer survival times was accompanied by decreased immunolabeling of fibers projecting from VCN cells to the superior olivary complex. These results suggest that calretinin gene expression is regulated in part by auditory nerve activity in some cochlear neurons but that additional factors related to the unique cellular milieu also control calretinin expression.

Effect of altered neuronal activity on cell size in the medial nucleus of the trapezoid body and ventral cochlear nucleus of the gerbil

Activity-dependent transneuronal regulation of neuronal soma size has been studied in the medial nucleus of the trapezoid body and ventral cochlear nucleus of adolescent gerbils. Cochlear ablation or tetrodotoxin has been used to eliminate afferent electrical activity in auditory nerve fibers permanently or for 24 or 48 hours. Previous studies have shown that the cross-sectional area of spherical cell somata in the ipsilateral anteroventral cochlear nucleus decreases within 24 hours of electrical activity blockade with tetrodotoxin, which is fully reversible when activity is restored. The present findings extend this work by directly comparing the results of unilateral blockade of auditory nerve action potentials or unilateral cochlear ablation on the size of spherical and globular cell bodies in the ventral cochlear nucleus with changes produced by the same manipulations in third-order cells, principal neurons in the medial nucleus of the trapezoid body. Soma size in both ventral cochlear nucleus cell types decreases reliably by 24 hours after cochlear removal or eighth nerve activity blockade by tetrodotoxin. Soma size of neurons in the contralateral medial nucleus of the trapezoid body decreases 48 hours, but not 24 hours, after either manipulation. When activity in auditory nerve fibers is allowed to resume for 7 days following a 48-hour activity blockade, soma size fully recovers in the medial nucleus of the trapezoid body as well as in ventral cochlear nucleus neurons. We also report that the cross-sectional area of neuronal soma in the medial nucleus of the trapezoid body is larger in lateral regions of medial nucleus of the trapezoid body (low-frequency representation) than in the medial regions of the nucleus (high-frequency representation). We conclude that cell body size changes in brainstem auditory neurons are reversible and that the signals associated with the loss and subsequent recovery of soma size are activity related. However, the delayed effect of activity deprivation in the medial nucleus of the trapezoid body suggests that trophic substances released by afferent axons may contribute to the maintenance of anatomical characteristics.

Cholinergic modulation of spontaneous activity in rat dorsal cochlear nucleus

Extracellular recordings were made from brain stem slices to test the effects of bath application of cholinergic agonists and antagonists on the firing rates of spontaneously active dorsal cochlear nucleus neurons. About 90% of neurons responded to carbachol. A higher proportion responded to muscarine than to nicotine. Muscarine elicited larger responses at lower concentrations than nicotine. Responses to either carbachol or muscarine were always blocked by atropine or scopolamine. The nicotinic antagonists d-tubocurarine, hexamethonium, and mecamylamine blocked the responses to nicotine, but did not decrease the responses to carbachol. Regularly firing neurons showed only increases of firing rate during exposure to cholinergic agonists. About half of responsive bursting neurons showed increased firing; half showed increased followed by decreased firing to 10 microM carbachol or muscarine. All phases of the responses of most bursting neurons were greatly decreased or abolished in low calcium, high magnesium medium, while responses of regular neurons were not detectably affected. Thus, cholinergic agonists appear to act directly on regularly firing neurons, while their actions on bursting neurons may require synaptic activity. The data suggest that cholinergic transmission in the dorsal cochlear nucleus is predominantly muscarinic, and that most regularly firing spontaneously active neurons have muscarinic receptors.

Plastic changes in ipsi-contralateral differences of auditory cortex and inferior colliculus evoked potentials after injury to one ear in the adult guinea pig

In normal adult guinea pigs, evoked potentials recorded at the ipsilateral auditory cortex to monaural high-frequency acoustic stimuli present higher thresholds and lower amplitudes than at the contralateral cortex; in the inferior colliculus, such ipsi-contralateral differences (ICDs) are smaller than in the auditory cortex. Changes in the ICDs were studied after opposite ear injury. Following quasi-complete hair cell destruction induced by sisomicin injection into the contralateral inner ear, threshold ICDs almost disappeared after about two to six days and ipsilateral amplitudes progressively increased in two to three weeks. The occurrence of ICDs at higher auditory centers revealed in this study, indicates peculiar processing of high frequency stimuli in normal guinea pigs. The alteration of ICDs after opposite ear impairment provides a new possibility to study the auditory plasticity in adult animals.

Internally-generated sound stimulates cochlear nucleus units

The body generates many physiological sounds. One of the most prominent is that produced by the blood flowing inside the vessels with each heart beat. On the other hand, the cochlea is a very sensitive receptor with a low threshold. Given the anatomical close proximity of the carotid artery and other vessels to the inner ear, the possibility of its being stimulated is very high. Cochlear nucleus spontaneous as well sound-responding auditory units were studied. A close relationship between the heart beat, that is the blood flow, and the cochlear nucleus firing was demonstrated, in anesthetized and awake guinea-pigs. Temporary mechanical interruption of the blood flow through the ipsilateral carotid artery abolished firing increments at the cochlear nucleus time-locked to the heart beat. We conclude that one component of the so called 'spontaneous' firing in the auditory system is actually evoked activity due to normal body-generated sounds or noises.

The projections of intracellularly labeled auditory nerve fibers to the dorsal cochlear nucleus of cats

The cochlear nucleus receives incoming auditory nerve discharges, preserves or transforms the signals, and distributes outgoing activity to higher centers. The organization of auditory nerve input to the cochlear nucleus will heavily influence the mechanisms by which acoustic information is processed. In order to study structure-function relationships between auditory nerve and cochlear nucleus, the axonal arborizations of type I spiral ganglion cells were labeled with intracellular injections of horseradish peroxidase after first being electrophysiologically characterized by recording with a micropipette inserted into the axon. For each auditory nerve fiber, spontaneous discharge rate (SR) and a frequency tuning curve were determined. The tuning curve yielded the characteristic frequency (CF, that frequency to which the fiber is most sensitive) and CF threshold in dB SPL. Individual axonal arborizations including all terminal swellings were reconstructed through serial sections with the aid of a light microscope and drawing tube. On average, 13.4 +/- 8.1% of the terminal swellings were found in the dorsal cochlear nucleus (DCN) and the remaining terminal swellings were located in the ventral cochlear nucleus. In the DCN, the terminal fields of auditory nerve fibers were restricted to layer III, contributed to cytoarchitectonic striations, and exhibited a systematic relationship between fiber CF and position along the strial (or long) axis of the nucleus. Computer-aided rotations revealed that the terminal fields were anisotropic, being flattened within the trans-strial axis. The maximal width of the terminal fields along the strial axis ranged from 31-321 microns and was inversely related to fiber CF and SR. Variation in the number of terminals or depth of the terminal field within layer III was not related to SR grouping or CF of the fiber.

Electrical stimulation of the auditory nerve in deaf kittens: effects on cochlear nucleus morphology

The present study examines the effects of long-term electrical stimulation of the auditory nerve on the morphology of neurons in the cochlear nucleus in young, sensorineural deaf animals. Kittens, systemically deafened using kanamycin and ethacrynic acid, received bilateral cochlear implants and were stimulated unilaterally for periods of up to four months. After sacrifice, cross-sectional areas of neuron somata were measured with an image-analysis system and compared using nonparametric statistics. The areas of cell somata within the anteroventral cochlear nucleus (AVCN) on the stimulated side were significantly larger than those of corresponding somata on the control, unstimulated side (P less than 0.001). However, there was no statistically significant difference among dorsal cochlear nucleus (DCN) neurons. These results indicate that long-term electrical stimulation of the auditory nerve can at least partially negate some effects of early postnatal auditory deprivation at the level of the cochlear nucleus.

Afferent influences on brainstem auditory nuclei of the chick: nucleus magnocellularis neuronal activity following cochlea removal

Elimination of presynaptic elements often results in marked changes, such as atrophy and death, in postsynaptic neurons in the central nervous system. These transneuronal changes are particularly rapid and profound in young animals. In order to understand the cellular events underlying transneuronal regulation it is necessary to explore changes in the local environment of neurons following manipulations of their afferents. In previous investigations we have documented a variety of rapid and marked cellular changes in neurons of the cochlear nucleus of neonatal chicks (n. magnocellularis) following cochlea removal. In adult chickens, however, these transneuronal changes are either absent or minor. The goals of the studies presented here were to examine changes in the electrical activity of nucleus magnocellularis cells and their afferents following removal of the cochlea and to determine if these changes were similar in adult and neonatal animals. Two measures of electrical activity were used; multiunit recording with microelectrodes and incorporation of radiolabeled 2-deoxyglucose (2-DG). Microelectrode recordings revealed high levels of spontaneous activity in n. magnocellularis and n. laminaris, the binaural target of n. magnocellularis neurons. Neither puncturing of the tympanic membrane nor removal of the columella causes significant changes in spontaneous activity, although the latter results in a profound hearing loss (40-50 dB). Removal of the cochlea, on the other hand, results in immediate cessation of all extracellular electrical activity in the ipsilateral n. magnocellularis. Recordings from the same location for up to 6 h failed to reveal any return of spontaneous activity. When the electrode tip was placed in n. laminaris, unilateral cochlea removal had no discernible effect on extracellularly recorded spontaneous activity, probably due to the high levels of excitatory input from the intact ear. Bilateral cochlea removal, however, completely eliminated activity in n. laminaris. 2-DG studies conducted 1 h to 8 days following unilateral cochlea removal revealed marked decreases in 2-DG incorporation in the ipsilateral n. magnocellularis and bilaterally in the n. laminaris target of the ablated cochlea. No compensatory return of 2-DG incorporation was observed for up to 8 days. Comparisons of adult and neonatal chicks failed to reveal significant differences in the effects of cochlea removal on multiunit activity or 2-DG incorporation, suggesting that age differences in transneuronal regulation are due to intrinsic biochemical differences in young and adult neurons rather than differences in the proportion of synaptic input that has been abolished.

Comparison of the auditory sensitivity of neurons in the cochlear nucleus and inferior colliculus of young and aging C57BL/6J and CBA/J mice

Thresholds of neurons to sounds were compared as a function of central auditory structure [ventral cochlear nucleus (VCN), dorsal cochlear nucleus (DCN), and inferior colliculus (IC)] in young and middle-aged C57BL/6J mice (multiple- and single-unit recordings) and in young and old CBA/J mice (single-unit recordings). Middle-aged C57 mice show progressive loss of sensitivity to high frequencies and noise due to cochlear pathology; CBA mice show little loss of sensitivity through most of their lifespan. Multiple-unit threshold curves (MTCs) for tones indicated that neurons in the C57 VCN suffered a greater degree of age-related loss of sensitivity than neurons in the IC (from an earlier study). Furthermore, whereas the low frequency portions of MTCs in IC neurons in high frequency tonotopic regions typically become 'sensitized' in middle-aged C57 mice (i.e., lower thresholds than young mice), such was not the case for VCN neurons. In contrast to VCN neurons, MTCs of the population of DCN neurons studied were statistically indistinguishable from those of the IC. Measurements of single-unit response areas in C57 mice corroborated the MTCs. In CBA mice, little effect of age was found in comparing single-unit response areas of young and old mice. The findings indicate that sensorineural impairment in middle-aged C57 mice is accompanied by threshold changes that are more severe in the VCN than in the IC or DCN. Because the VCN and DCN are believed to play different roles in hearing, the functions they support should, likewise, be affected to different extents by age-related hearing loss.

Synaptic connections of the auditory nerve in cats: relationship between endbulbs of held and spherical bushy cells

This report focuses on a class of large synaptic endings, the endbulbs of Held. These endings are located in the anteroventral cochlear nucleus and arise from the axons of type I spiral ganglion neurons. Axons were stained with horseradish peroxidase (HRP) using intracellular injections of single fibers or extracellular injections into the auditory nerve. Individual endbulbs or pairs of endbulbs that converged onto the same spherical bushy cell were examined with the aid of a light microscope and subjected to morphometric analyses. Endbulbs of fibers having low spontaneous discharge rates (SR, less than or equal to 18 spikes/sec) have a more complex shape than those of high SR fibers (greater than 18 s/s), a feature represented by systematic differences in endbulb silhouette perimeter without differences in endbulb silhouette area. Consequently, the ratio, silhouette area divided by silhouette perimeter, yields a "form factor" separating endbulbs of high SR from those of low SR. High SR fibers had ratios greater than 0.52 (mean = 0.63 +/- 0.09), whereas low SR fibers had ratios less than 0.52 (mean = 0.45 +/- 0.06). Pairs of endbulbs with unknown physiological properties had similar form factor values, despite the wide range of values observed in the endbulb population. These data imply that endbulbs converging upon the cell body of a spherical bushy cell arise from fibers of the same SR group. Electron microscopic examination was conducted on the endbulb of one physiologically characterized and intracellularly stained auditory nerve fiber (CF = 1.4 kHz; SR = 55 s/s) and its unstained endbulb mate with the aid of serial ultrathin sections. In addition to the well-known axosomatic synapses, these endbulbs formed axodendritic synapses: 11.7% for the HRP-labeled endbulb and 13.3% for the unlabeled endbulb. The axodendritic synapses appear to occur on dendrites of nearby spherical bushy cells and may represent a mechanism whereby single endbulbs can disperse activity to multiple neurons in the cochlear nucleus. We propose that axosomatic synapses preserve fiber SR groupings, whereas axodendritic synapses may not.

Effects of total cochlear haircell loss on integrity of cochlear nucleus. A quantitative study

In cochleas of chincillas treated with amikacin, cochlear sensory cells were totally destroyed in all new-born animals. In animals treated as adults some occasional haircells remained in apical turns. In the neonatally treated animals, the resulting auditory deprivation significantly affected the volume of the ventral cochlear nucleus and large-dark spherical cell area. The density of large-dark spherical cells increased significantly from normal in both neonatally and adult treated groups. Our results suggest that the VCN is more dependent on auditory stimulation for proper development than the DCN. In adult chinchillas treated with amikacin there was a significant change in large-dark spherical cell density without a change in total cell numbers or large-dark spherical cell area volume. Our study indicates that the mature cochlear nucleus is much more resistant to the effects of auditory deprivation than the developing cochlear nucleus and that the maintenance of the mature auditory system is not as dependent on auditory stimulation. Studies such as this examining the morphological effects of profound cochlear deafness on higher levels of the auditory system are essential in cochlear implant research.

Cochlear nucleus cell size is regulated by auditory nerve electrical activity

Accumulating evidence suggests that sensorineural hearing loss in animals is rapidly followed by degenerative changes in central auditory neurons. For example, cochlear removal in birds and mammals results in a reduction in central auditory neuron cell size within 48 hours. A similar decrease in cell size after pharmacologic blockade of auditory nerve electrical activity with tetrodotoxin has been reported. In the present study, we evaluate the reversibility of central auditory changes after a profound sensorineural hearing loss caused by blockade of auditory nerve actions potentials. Tetrodotoxin, which blocks voltage-sensitive sodium channels, was embedded in a slow-release vehicle and placed next to the round window membrane of gerbils. Tetrodotoxin diffused into perilymph and unilaterally blocked electrical activity in auditory nerve axons. Electrical activity blockade was confirmed with recordings of auditory brainstem response. Animals were killed immediately after 24 hours of electrical blockade or 7 days after a transient 24- or 48-hour blockade. Large spherical cells of the anteroventral cochlear nucleus ipsilateral to manipulation were measured and compared to large spherical cells on the opposite, unmanipulated side of the brain. Animals killed immediately after a 24-hour blockade of electrical activity showed a mean decrease of 16% in cell size ipsilateral to the blockade (p less than 0.05). In animals allowed to recover for 7 days after blockade for 24 or 48 hours, cell size returned to previous levels. There was no longer a consistent difference in cell size between the two sides of the brain in these animals (p greater than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Auditory brainstem of the ferret: early cessation of developmental sensitivity of neurons in the cochlear nucleus to removal of the cochlea

The role of primary afferent innervation in the maintenance of neurons in the mammalian auditory system was assessed by performing unilateral removals of the cochlea in neonatal and mature ferrets of known birth dates. Removals were performed under steroid anesthesia and resulted in the complete destruction of the organ of Corti and the loss of at least 80% of type I spiral ganglion neurons. Four main age groups [postnatal days (P)5, P24, P90, and P180] were used. Additional animals received no surgery, partial removals, or complete removals at older ages. Three months after the cochlear removal the animals were reanesthetized and perfused. The brainstem and the temporal bones were wax-embedded, frontally sectioned, and Nissl-stained. Sections of the right and left cochlear nuclei were compared quantitatively. Removal of the cochlea at P5 resulted in the loss of more than 50% of large (nongranular) neurons throughout the ipsilateral cochlear nucleus. Lesions at older ages did not produce any neuron loss. The size of the remaining neurons was reduced by 10-15% in all age groups. Partial lesions at P5 produced a graded response in the cochlear nucleus that was related to the extent of the lesion. The developmental sensitive period for the effects of cochlear removal on the ferret cochlear nucleus is therefore over before the age (P28-P30) at which the animal begins to hear. The present result differs markedly from the chicken, in which the sensitive period for removal of the cochlea persists for at least 2 months after the onset of hearing.

Periaqueductal gray influence on anteroventral cochlear nucleus unitary activity and naloxone effects

The effect of periaqueductal gray (PAG) electrical stimulation on the response properties of auditory and 'spontaneously' firing units (abolished when the cochlea is destroyed) in the anteroventral cochlear nucleus (AVCN) was explored using extracellular recordings in acute guinea-pigs. Significant increases and decreases in firing rate were detected in both neuronal groups: only 4% of the sound-responding units were insensitive to PAG stimulation while the 'spontaneous' units showed significantly smaller changes in firing rate in response to PAG stimulation. The auditory AVCN neurons were categorized both by their sound post stimulus time (PST) histograms at their characteristic frequency (CF) and the changes in the probability of discharge after PAG stimulation while the tone burst was maintained constant. PAG was implicated in pain input modulation through enkephalin actions. Because enkephalins have been also observed at the CN level, a pharmacological approach administering naloxone was carried out. We observed that 1) naloxone abolished the unit discharge shifts observed after PAG stimulation and 2) when the drug was injected without PAG stimulation, it produced changes in the firing, increasing or decreasing, and shifts in the probability of discharge versus time, even in cases in which the firing rate was not altered. An involvement of the auditory efferent pathways to CN is postulated and a possible enkephalinergic factor is suggested as a modulator of the auditory input at this level. The probability of discharge observed in the PSTH at the AVCN is dependent on the auditory input plus the central efferent action to its neurons.

Cytochrome oxidase response to cochlea removal in chicken auditory brainstem neurons

Changes in cytochrome oxidase (CO) activity were studied in the chick brainstem auditory nuclei, n. magnocellularis (NM) and n. laminaris (NL), following unilateral cochlea removal. Chickens aged 10 days or 56 weeks underwent unilateral cochlea removal. Following survival periods of 30 minutes to 14 days for the 10-day-old birds and 6 hours or 14 days for the 56-week-old birds, the animals were perfused with paraformaldehyde/glutaraldehyde fixative. Cryostat sections of the brainstem were then prepared for CO histochemistry. Microdensitometry was used to quantify the difference in CO staining in NM and NL ipsilateral and contralateral to the cochlea removal. Since the cochlea projects to the ipsilateral NM, the contralateral NM was used as a within-animal control. In normal chickens, NM cell bodies and the cell bodies and dendrites of NL neurons stain darkly for CO in both young and adult birds. In 10-day-old birds, there is no significant change in CO staining in NM from 30 minutes to 3 hours after cochlea removal. Then, a rapid biphasic change in CO staining was found in the ipsilateral NM. An increase in staining was observed 6 to 24 hours postoperatively, followed by a decrease in CO staining at 3- to 14-day survival times. In the 56-week-old birds, no increases in CO staining were observed 6 hours after cochlea removal, but a decrease in CO staining was found 14 days postoperatively. In NL, no changes were observed until 3 days (10-day-old birds) or 14 days (56-week-old birds) after cochlea removal. Then a decrease in CO staining was observed in the dendritic and glial/fiber regions of NL containing axons from the deafferented NM. Thus it appears that afferent input has a regulatory effect on the oxidative metabolism of neurons in the chicken auditory brainstem nuclei, an effect that differs with the age of the animal at the time of afferent manipulation.

Cochlear ablation in deafness mutant mice: 2-deoxyglucose analysis suggests no spontaneous activity of cochlear origin

Deafness mutant mice show no stimulus-related cochlear potentials as well as abnormal electrically-evoked responses recorded from the inferior colliculus. Abnormal spontaneous activity in the auditory periphery could result in abnormal development and/or maintenance of the central auditory pathways. We therefore assessed spontaneous activity of cochlear origin in the central nuclei of the mutants by ablating one cochlea and subsequently using the 2-deoxyglucose (2DG) technique to study metabolic activity. Any asymmetries in labeling in a given nucleus should be due to spontaneous activity in the cochlear nerve on the unoperated side. In control animals (+/dn mice undergoing unilateral cochlea ablation), statistically significant decreased 2DG labeling was observed in the ipsilateral PVCN and AVCN, and contralateral MNTB and IC; all receive primary excitatory input from the ablated ear. No significant differences in labeling between right and left sides were observed in any of the nuclei studied in the mutant animals. These findings suggest that there is no spontaneous activity of cochlear origin in these mutants, even though many cochlear nerve fibers and spiral ganglion cells survive.

Rapid changes in cochlear nucleus cell size following blockade of auditory nerve electrical activity in gerbils

Large spherical cells of the mammalian anteroventral cochlear nucleus (AVCN) receive direct excitatory input from auditory nerve axons. Trans-synaptic regulation of neuronal cell size and cell number after cochlear ablation has been previously demonstrated in neonates of several vertebrate species, including the gerbil. Such changes may be related to loss of spontaneous or evoked auditory nerve electrical activity or to loss of activity-independent factors. We have developed a method to chronically, yet reversibly, block auditory nerve electrical activity without violating the integrity of the inner ear. Tetrodotoxin (TTX) was embedded in an ethylene-vinyl acetate copolymer resin (Elvax). A small piece of Elvax containing TTX was placed next to the round window membrane, which allowed TTX to diffuse into the inner ear. As a measure of the effectiveness of manipulation, the onset, duration, and magnitude of the auditory threshold shift were measured by the auditory brainstem response. The sound-evoked response was abolished within 10 minutes of placement of TTX on the round window membrane. The duration of threshold shift was dose-dependent and lasted 24-46 hours. Implants of Elvax without TTX did not produce a significant threshold shift. TTX, which blocks voltage-gated sodium channels, did not abolish the potassium-based cochlear microphonic response. The consequence of blocking afferent electrical activity on gerbil AVCN large spherical cells was examined by measuring their cross-sectional area after each of four manipulations: unilateral auditory nerve action potential blockade with TTX; unilateral surgical cochlear ablation; ipsilateral TTX exposure/contralateral cochlear ablation; and unilateral sham operation (Elvax without TTX).(ABSTRACT TRUNCATED AT 250 WORDS)

Endbulbs of held and spherical bushy cells in cats: morphological correlates with physiological properties

Single auditory nerve fibers of type I spiral ganglion cells in cats were electrophysiologically characterized by recording with micropipettes inserted into the axon and then labeled by intracellular injections of horseradish peroxidase (HRP) through the same pipettes. This method for staining and studying single neurons allowed us to describe structure-function relationships for labeled endbulbs of Held and the somata of their postsynaptic spherical bushy cells. The silhouette areas of terminal endbulbs and the corresponding somata of spherical bushy cells were determined by planimetry from drawings made with a light microscope and drawing tube. On the presynaptic side, endbulb area is related to fiber characteristic frequency (CF, the frequency to which a fiber is most sensitive) such that the largest endbulbs arise from fibers having CFs between 1 and 4 kHz; smaller endbulbs can arise from fibers of any CF. Endbulb area is not correlated with fiber spontaneous discharge rate (SR). Dividing the endbulb's silhouette area by its silhouette perimeter, however, yields a "form factor" that is a reliable indicator of fiber SR: Endbulbs from fibers of low-medium SR (less than or equal to 18 spikes/second) have form factor values less than 0.52, whereas endbulbs of high SR fibers (greater than 18 spikes/second) have values greater than 0.52. This form factor should therefore be predictive of SR groupings in auditory fibers for which physiological data are not available. On the postsynaptic side, the somata of spherical bushy cells receiving endbulbs from low-medium SR fibers are on average smaller than those receiving endbulbs from high SR fibers. In contrast, the nuclei of the spherical bushy cells are the same size regardless of presynaptic fiber SR. Some of the effects of low-medium SR fibers on their postsynaptic targets, when compared to those of high SR fibers, appear to be mimicked by effects of experimentally induced deprivation.

Auditory brainstem of the ferret: effects of unilateral cochlear lesions on cochlear nucleus volume and projections to the inferior colliculus

Unilateral lesions of the right cochlea were made in ferrets aged postnatal day (P)12 to P93. The extent of the lesions was assessed by counting remaining hair cells and ganglion cells in midmodiolar sections through the lesioned cochleas and by comparison with a sample of unlesioned cochleas. The neural effects of the lesions were assessed by measuring the volume of each cochlear nucleus (CN) and by counting the number of neurons in each CN that were retrogradely labeled following injections of WGA-HRP in the left inferior colliculus (IC). Survival times between lesioning and injection of the tracer ranged from 11 to 98 days. CN volume and projections to the IC were also measured in a sample of normal adult ferrets and in normal infants aged P39 to P80. Cochlear lesions resulted in a reduction of the volume of the CN on the lesioned side, relative to the other CN, in animals of all ages and survival times. The extent of the CN volume reduction was negatively correlated with the number of remaining cochlear ganglion cells. However, even where the number of ganglion cells was within the normal range, significant volume reductions occurred. The ventral CN was more severely affected by the lesions than the dorsal CN, but no difference was found between the anteroventral and posteroventral divisions of the nucleus. There was no significant difference in the extent of CN volume reductions between animals of different ages or survival times. Lesions of the right cochlea in younger animals (P14 to P24) resulted, after 90 days survival, in an increase in the number of left CN neurons projecting to the left IC. No significant increase was seen following lesions in older (P90) ferrets or following short (11 or 30 days) survival times in young (P14 to P24) ferrets. The extent of the increase in the ipsilateral CN-IC projection was not related to the number of remaining ganglion cells or to the division of the CN examined. Lesions did not affect the contralateral CN-IC projection. We conclude that cochlear lesions in infant ferrets can alter auditory brainstem morphology and connectivity. The dependence of these alterations on the age of the animal, survival time following lesion, and extent of the lesion varies markedly with the index examined.

Intrinsic properties of neurones in the dorsal cochlear nucleus of mice, in vitro

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices of the cochlear nuclear complex. Probably the larger and most frequent cells were impaled. 2. The steady-state current-voltage (I-V) properties of all cells impaled were nonlinear. The I-V curve was steepest in the voltage range depolarized from the resting potential and most shallow when the cell was hyperpolarized from rest by more than about 10 mV. Thus, the inwardly rectifying I-V characteristics of cells in the DCN distinguish them from those of ventral cochlear nuclear neurones (Oertel, 1983). 3. When depolarized with current, most cells fired trains of large, all-or-none action potentials. The undershoot after single spikes comprised an initial, fast component followed by a second, slower wave. A few cells (15%) generated bursts of smaller, graded spikes in addition to the large ones. 4. Repetitive firing evoked by depolarizing pulses of current was followed by an after-hyperpolarization whose magnitude depended on the strength and duration of the preceding current pulse. 5. Blocking the large action potentials with tetrodotoxin (TTX) revealed Ca2+-dependent spikes in all cells examined. 6. The steady-state I-V relationship became linear in the presence of TTX, suggesting that a persistent Na+ conductance probably mediates the inward rectification seen above the resting potential. 7. Muscarine at micromolar concentrations excited cells and increased their input resistance.

Changes in spontaneous activity and CNS morphology associated with conductive and sensorineural hearing loss in chickens

The effects of a conductive or mixed (conductive and sensorineural) hearing loss on anatomical and physiological properties of the chicken auditory system were examined. Animals used in the anatomical studies underwent either a columella (ossicle) removal, which produced a moderate conductive hearing loss, or an oval window puncture, which produced a severe mixed hearing loss, at 4 days posthatch. In a companion study, multiunit spike counts were obtained from 3-week-old chickens before, during, and after consecutive tympanic membrane puncture, columella removal, and oval window puncture. Tympanic membrane puncture and columella removal (conductive hearing loss) are not associated with either cell area changes in the nucleus magnocellularis or changes in spontaneous neuronal activity. Conversely, an oval window puncture (sensorineural damage) is associated with a cell area reduction of 20%, as well as a marked decline in activity within auditory nuclei.