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PubMed1999

Neuronally modulated transcription of a glycine transporter in rat dorsal cochlear nucleus and nucleus of the medial trapezoid body.

Neurotransmitter transporters limit transmitter concentration at the postsynaptic membrane by removing neurotransmitters from the synaptic cleft. Not only do neurotransmitter transporters contribute to the regulation of synaptic transmission, but they themselves might be dynamically regulated by neuronal activity of the neurons in which they are expressed. In this experiment, we investigated the question of whether the transcription of two different glycine transporters, Glyt1 and Glyt2, is influenced by neuronal activity. These transporters are found in the dorsal cochlear nucleus (DCN) and medial nucleus of the trapezoid body. Glyt1 and Glyt2 mRNA were measured by using hybridization histochemistry and a semiquantitative reverse transcription polymerase chain reaction. Decreases in auditory primary afferent activity, caused by either unilateral labyrinthectomy or disruption of the middle ear ossicles, caused a reduction in Glyt2, but not Glyt1 mRNA in the ipsilateral DCN and in the contralateral medial nucleus of the trapezoid body. Acoustic stimulation at either 10 kHz or 40 kHz was used to provide controlled increases in primary afferent activity, evoking localized increases in Glyt2 mRNA in clusters of neurons in the DCN. The location of these clusters corresponded to the regions of the auditory tonotopic map devoted to these frequencies. The duration of changes in Glyt2 mRNA evoked by unilateral labyrinthectomy, measured with the reverse transcription polymerase chain reaction, was 5-10 days. These data provide the first example of in vivo regulation of transporter transcription by neuronal activity.

Amino Acid Transport Systems, Neutral↗
PubMed2005

Synaptic transmission at the cochlear nucleus endbulb synapse during age-related hearing loss in mice.

Age-related hearing loss (AHL) typically starts from high-frequency regions of the cochlea and over time invades lower-frequency regions. During this progressive hearing loss, sound-evoked activity in spiral ganglion cells is reduced. DBA mice have an early onset of AHL. In this study, we examined synaptic transmission at the endbulb of Held synapse between auditory nerve fibers and bushy cells in the anterior ventral cochlear nucleus (AVCN). Synaptic transmission in hearing-impaired high-frequency areas of the AVCN was altered in old DBA mice. The spontaneous miniature excitatory postsynaptic current (mEPSC) frequency was substantially reduced (about 60%), and mEPSCs were significantly slower (about 115%) and smaller (about 70%) in high-frequency regions of old (average age 45 days) DBA mice compared with tonotopically matched regions of young (average age 22 days) DBA mice. Moreover, synaptic release probability was about 30% higher in high-frequency regions of young DBA than that in old DBA mice. Auditory nerve-evoked EPSCs showed less rectification in old DBA mice, suggesting recruitment of GluR2 subunits into the AMPA receptor complex. No similar age-related changes in synaptic release or EPSCs were found in age-matched, normal hearing young and old CBA mice. Taken together, our results suggest that auditory nerve activity plays a critical role in maintaining normal synaptic function at the endbulb of Held synapse after the onset of hearing. Auditory nerve activity regulates both presynaptic (release probability) and postsynaptic (receptor composition and kinetics) function at the endbulb synapse after the onset of hearing.

Aging↗
PubMed1985

Functional organization of the cochlear nucleus of rufous horseshoe bats (Rhinolophus rouxi): frequencies and internal connections are arranged in slabs.

The functional organization of the cochlear nucleus (CN) was studied with physiological recording and anatomical tracing techniques. Recordings were made from single CN neurons to examine their temporal firing patterns to tone burst stimuli and their frequency tuning characteristics. Recording loci of individual neurons were carefully monitored in order to understand how the functional properties of a cell relate to its location within the CN. We found that tonal frequencies were systematically represented in each of the three CN divisions (anteroventral, AVCN; posteroventral, PVCN; dorsal, DCN). Eight temporal response patterns were observed in CN neurons when stimulated at units' best excitatory frequencies (BF). With a few exceptions, neurons in each CN division could generate all eight firing patterns with different distributions for the three division. A focal injection of horseradish peroxidase (HRP), at the end of the physiological study, to a group of neurons possessing a similar BF in one CN division resulted in anterograde labeling of nerve terminals in the other two divisions at precisely the areas where the same frequency band was processed in these divisions. Labeled terminals in each division were closely congregated in the form of a thin slab. The slab orientation was division specific whereas its location was frequency specific, which could be predicted on the basis of physiological data. HRP injections into the DCN also resulted in retrograde labeling of somata in the AVCN and PVCN. On the other hand, only DCN neurons were retrogradely labeled when HRP was injected into the AVCN or the PVCN. These data showed how the three CN divisions are internally connected. Furthermore, retrogradely labeled cells occupied the same slabs where we found anterogradely labeled nerve terminals. Additionally, in a group of bats, HRP was injected into various functionally (i.e., BF) identified regions of the central nucleus of the inferior coliculus (IC) to clarify the type and location of CN projecting neurons. Retrogradely labeled cells in individual CN divisions likewise were arranged in slabs whose locations in the CN nuclei depended on the BFs of neurons at the injection site in the IC. These results show that slabs represent units of functional organization (i.e., tonal frequency, local connection and central projection) in the CN.

Animals↗
PubMedDate not supplied

Summation of spatiotemporal input patterns in leaky integrate-and-fire neurons: application to neurons in the cochlear nucleus receiving converging auditory nerve fiber input.

The response of leaky integrate-and-fire neurons is analyzed for periodic inputs whose phases vary with their spatial location. The model gives the relationship between the spatial summation distance and the degree of phase locking of the output spikes (i.e., locking to the periodic stochastic inputs, measured by the synchronization index). The synaptic inputs are modeled as an inhomogeneous Poisson process, and the analysis is carried out in the Gaussian approximation. The model has been applied to globular bushy cells of the cochlear nucleus, which receive converging inputs from auditory nerve fibers that originate at neighboring sites in the cochlea. The model elucidates the roles played by spatial summation and coincidence detection, showing how synchronization decreases with an increase in both frequency and spatial spread of inputs. It also shows under what conditions an enhancement of synchronization of the output relative to the input takes place.

Action Potentials↗
PubMed1985

The dorsal cochlear nucleus of the mouse: a light microscopic analysis of neurons that project to the inferior colliculus.

In the mouse dorsal cochlear nucleus (DCN), all members of a distinct class of large multipolar neurons were shown to project to the contralateral inferior colliculus by using retrograde horseradish peroxidase techniques. Typically, these multipolar neurons have the largest cell bodies in the nucleus and are distributed in layers II, III, and IV. Each contains a round, pale nucleus with a prominent nucleolus and conspicuous Nissl bodies. In Golgi preparations, however, two types of large cells could be distinguished on the basis of dendritic characteristics. Pyramidal cells form relatively flattened, slablike dendritic fields whose alignment contributes to the laminar organization of the DCN. They represent 75-80% of the large cell population and are found in layer II and the superficial region of layer III. Giant cells represent the other type of large multipolar neuron and are distributed in the deeper regions of layer III and in layer IV. Their ellipsoidal dendritic fields are formed by long and relatively unbranched dendrites that project across the laminae. The differences in dendritic morphology imply that each cell class segregates its afferent input in distinct ways and subserves different auditory functions.

Animals↗
PubMed1985

Projections from the cochlear nucleus to the inferior colliculus in normal and neonatally cochlea-ablated gerbils.

The distribution of the projection from one cochlear nucleus (CN) within each inferior colliculus (IC) was studied in adult, normal gerbils and adult gerbils subjected to unilateral ablation of the contralateral cochlea at 2 days of age. The projection was studied by using the Fink-Heimer technique for impregnating degenerating axons and their terminal processes with silver. Following an extensive, unilateral lesion of the CN, degeneration was seen in both ICs of all animals. In normal animals, degeneration was both more widespread and heavier in the contralateral than in the ipsilateral central nucleus of IC (ICC). Degeneration was most widespread in the rostral and lateral parts of both ICCs and in the ventral part of the contralateral ICC. Degeneration was observed in 26% of the area examined in ipsilateral ICC and in 73% of the area examined in contralateral ICC. In cochlea-ablated animals there was a much greater similarity in the area of degeneration in the ICC ipsilateral (57%) and contralateral (67%) to the CN lesion. The same regional distributions of degeneration were observed as in the normal animals except that the distribution of degeneration in the ipsilateral ICC more closely resembled the normal contralateral than the normal ipsilateral profile. We conclude that the normal distribution of projections from the CN within the ipsilateral ICC is substantially modified by neonatal ablation of the contralateral cochlea.

Animals↗
PubMed2001

Distribution of immunophilin FKBP-12 protein and mRNA within the mammalian cochlea and cochlear nucleus.

Immunophilin FK binding protein-12 (FKBP-12), the soluble receptor for the immunosuppressant drug FK506, is involved in a number of neuronal activities including increased nerve regeneration in the peripheral nervous system and enhanced recovery in animal models of neurodegenerative diseases. In addition, FKBP-12 is tightly bound to the calcium release channel ryanodine receptor and physiologically interacts with the inositol 1,4,5-trisphosphate receptor. In nearly all cell types, release of intracellular Ca(2+) and subsequent second messenger signaling involves activation of these ion channels. We determined the distribution of FKBP-12 within the mammalian cochlea and dorsal cochlear nucleus (DCN) in order to gain insight into Ca(2+) regulation within the cochlea and to possibly identify potential cellular targets for neuroimmunophilin ligands that may prove useful in protection and recovery following ototoxic insult. FKBP-12 protein and mRNA were found to be abundant throughout rat and guinea pig cochlea and DCN.

Animals↗
PubMed2005

Responses of ventral cochlear nucleus neurons to contralateral sound after conductive hearing loss.

Conductive hearing loss (CHL) is an attenuation of signals stimulating the cochlea, without damage to the auditory end organ. It can cause central auditory processing deficits that outlast the CHL itself. Measures of oxidative metabolism show a decrease in activity of nuclei receiving input originating at the affected ear but, surprisingly, an increase in the activity of second-order neurons of the opposite ear. In normal hearing animals, contralateral sound produces an inhibitory response to broadband noise in approximately one third of ventral cochlear nucleus (VCN) neurons. Excitatory responses also occur but are very rare. We looked for changes in the binaural properties of neurons in the VCN of guinea pigs at intervals immediately, 1 day, 1 wk, and 2 wk after the induction of a unilateral CHL by ossicular disruption. CHL was always induced in the ear ipsilateral to the VCN from which recordings were made. The main observations were as follows: 1) ipsilateral excitatory thresholds were raised by at least 40 dB; 2) contralateral inhibitory responses showed a small but statistically significant immediate decrease followed by an increase, returning to normal by 14 days; and 3) there was a large increase in the proportion of units with excitatory responses to contralateral BBN. The increase was immediate and lasting. The latencies of the excitatory responses were at least 6 ms, consistent with activation by a path involving several synapses and inconsistent with cross talk. The latencies and rate-level functions of contralateral excitation were similar to those seen occasionally in normal hearing animals, suggesting an upregulation of an existing pathway. In conclusion, contralateral excitatory inputs to the VCN exist, which are not normally effective, and can compensate rapidly for large changes in afferent input.

Acoustic Stimulation↗
PubMed1989

Structural and functional properties distinguish two types of multipolar cells in the ventral cochlear nucleus.

We distinguish two types of large multipolar cells designated sustained (CS) and onset (OC) choppers in the anterior posteroventral cochlear nucleus (A-PVCN)/nerve root region on the basis of certain anatomical and physiological features. CS axons head into the trapezoid body, while OC axons use the intermediate acoustic stria of Held. At the electron microscopic (EM) level, collateral terminals of OC axons contain pleomorphic vesicles; CS terminals contain small round vesicles. CS dendritic trees tend to be distributed in a stellate fashion while OC dendritic trees tend to be elongated. At the EM level the sustained chopper somata are sparsely innervated while the proximal dendritic tree receives considerably more input. The OC somata are highly innervated and this heavy innervation continues out onto the proximal dendrites. Distally the dendritic innervation falls off considerably for both categories. Physiologically, members of the OC population have wider dynamic ranges at the characteristic frequency (CF), wider response areas that are typically not flanked by inhibitory sidebands, and responses to short tones that do not show the same form of regularity expressed by sustained choppers. Intracellularly the sustained choppers exhibit sustained depolarization to short tones for the duration of the stimulus with resultant regular spiking at a rate that is stimulus level dependent. The response to swept tone shows this same level-dependent regularity. In response to tones, the OC cells also show a sustained depolarization whose amplitude is stimulus-level dependent but whose range is much greater and whose onset is initiated more abruptly. Although the onset component of the OC spike output is reliably initiated by these levels of depolarization, regular firing to the sustained depolarization is not initiated at levels of depolarization that would surely generate regular firing in sustained choppers. This regularity is also absent in the swept tone response despite marked levels of excitation.

Acoustic Stimulation↗
PubMed1985

Evidence for the involvement of kainate receptors in synaptic transmission in the avian cochlear nucleus.

Previous studies using various excitatory amino acid antagonists have shown that synaptic transmission between the auditory nerve and the cochlear nucleus of chickens (nuc. magnocellularis; NM) is mediated by non-N-methyl-D-aspartate (non-NMDA) receptors. In the present study we have attempted to define the subclass of non-NMDA receptor in the NM by examining the effects of various excitatory amino acid agonists on synaptically evoked field potentials in an in vitro preparation of the chicken brain stem. Both quisqualate and DL-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA), whose actions operationally define the quisqualate receptor class, caused variable and weak depression of evoked responses in the NM, as did L-glutamate. Kainic acid, on the other hand, completely blocked postsynaptic responses at micromolar concentrations. We conclude that kainate-preferring non-NMDA receptors play a predominant role in mediating transmission in the NM.

Animals↗
PubMed1983

Lateral inhibition in the anteroventral cochlear nucleus of the cat: a microiontophoretic study.

The spontaneous firing rates of non-prepotential (NPP) units of the anteroventral cochlear nucleus are quite low so it has not been possible to determine whether side band tones are inhibitory when presented alone. Microiontophoretically-applied excitatory amino acids can be used to excite non-spontaneous cells directly. Using this technique it can be shown that side band tone bursts 1/2 to 3/4 octave above the characteristic frequency (CF) of a NPP unit inhibit the amino acid-induced firing. Side band tones which inhibited the amino acid-induced firing were beyond the tuning curve. Side band tones within the tuning curve produced excitation. Both, however, usually reduced the activity evoked by a CF tone burst (i.e., two-tone interaction). The data suggests that lateral inhibition and two-tone interactions are separate phenomena in the auditory system and that lateral inhibition may play a critical role in determining the shape of the tuning curve of NPP units.

Amino Acids↗
PubMed1984

Wiener kernel analysis of responses from anteroventral cochlear nucleus neurons.

Responses to pseudo-random Gaussian white noise, tones and clics were recorded from neurons in the anteroventral cochlear nucleus (AVCN) of barbiturate anesthetized cats. The responses to white noise were used to calculate estimates of the zero-, first- and second-order Wiener kernels for these neurons. The Wiener kernels did contain useful information on the fundamental, DC and second harmonic components of the responses of AVCN neurons to tones, clicks and noise. However, they generally did not provide predictions of the difference tone distortion products found in the peripheral auditory system. Overall, the addition of the second kernel improved a prediction based on the zero- and first-order kernels, but not by very much. If the estimates of the Wiener kernels were not very good, then a second-order prediction could be worse than a first-order one. To produce good estimates of the Wiener kernels, many repetitions of very long Gaussian white noise stimuli are necessary. Therefore the technique does not permit rapid data collection. Further, exposure to long duration high intensity noise can result in acoustic trauma. This damage effects the mechanism that generates the difference tone distortion products, and it can also affect the tuning of the auditory neurons. Thus Wiener's nonlinear system identification theory has only limited usefulness in the analysis of the peripheral auditory system.

Acoustic Stimulation↗
PubMed1992

Stimulation with chronically implanted microelectrodes in the cochlear nucleus of the cat: histologic and physiologic effects.

The effects of several hours of continuous electrical stimulation in the cats' cochlear nucleus with chronically implanted activated iridium microelectrodes was investigated from the changes in the evoked response near the inferior colliculus and also by histologic evaluation of the stimulated tissue. The stimulating microelectrodes had geometric surface areas of 75-500 microns2. They were pulsed continuously for 4 h, at a pulse repetition rate of 200 Hz, using charge-balanced pulse pairs. The charge per phase was 1.8 or 3.6 nC/ph. The animals were sacrificed for histologic evaluation 2 h, or several days later. The only remarkable histologic change resulting from the 4 h of stimulation was some aggregation of lymphocytes at the site of stimulation. However, depression of the electrical excitability of neurons near the sites often persisted for several days after 4 h of stimulation at 3.6 nC/phase. The charge per phase of the stimulus pulse pair was correlated strongly with the depression of excitability, and there was a weaker correlation between the depression and the amplitude of the first phase of voltage transient induced across the electrode-tissue interface. The charge density, calculated from the geometric surface area of the stimulating electrodes, was poorly correlated with the severity of the depression. The findings suggest a means of detecting impending stimulation-induced neural damage while it is still reversible.

Animals↗
PubMed1993

Functional changes in the ventral cochlear nucleus following acute acoustic overstimulation.

The effects of acute acoustic overstimulation on the discharge patterns of neurons in the ventral cochlear nucleus (VCN) were evaluated in anesthetized chinchillas. Response measures were obtained from the same neuron before and after presenting a 3- to 5-min intense tone (90-105 dB SPL) located one-half oct above the unit's characteristic frequency (CF). If a unit had an inhibitory response area at frequencies above CF and if the traumatizing tone reduced the magnitude of the inhibitory response, then the neuron's discharge rate to suprathreshold tones at CF increased ("enhancement") by as much as 25%. However, if a unit lacked an inhibitory response area at frequencies above CF, then the traumatizing tone typically caused either no change or a decrease in the unit's discharge rate at CF. The traumatizing tone did not alter the shape of the post-stimulus time histograms. Moreover, the width of the excitatory response area was not altered by the exposure even when traumatizing stimulus reduced the magnitude of the inhibitory response above CF. The enhanced firing rate at CF following the exposure could conceivably contribute to the enhanced evoked-potential amplitudes observed in the auditory brain stem following acoustic trauma.

Acoustic Stimulation↗
PubMed2004

Responses of dorsal cochlear nucleus neurons to signals in the presence of modulated maskers.

The detection of a signal in noise is enhanced when the masking noise is coherently modulated over a wide range of frequencies. This phenomenon, known as comodulation masking release (CMR), has been attributed to across-channel processing; however, the relative contribution of different stages in the auditory system to such across-channel processing is unknown. It has been hypothesized that wideband or lateral inhibition may underlie a physiological correlate of CMR. To further test this hypothesis, we have measured the responses of single units from the dorsal cochlear nucleus in which wideband inhibition is particularly pronounced. Using a sinusoidally amplitude-modulated tone at the best frequency of each unit as a masker, a pure-tone signal was added in the dips of the masker modulation. Flanking bands (FBs, also amplitude-modulated pure tones) were positioned to fall within the inhibitory sidebands of the receptive field of the unit. The FBs were either in phase (comodulated) or out of phase (codeviant) with the on-frequency masker. For the majority of units, the addition of the comodulated FBs produced a strong reduction in the response to the masker modulation, making the signal more salient in the post stimulus time histograms. The change in spike rate in response to the signal between the masker and signal-plus-masker conditions was greatest for the comodulated condition for 29 of 45 units. These results are consistent with the hypothesis that wideband inhibition may play a role in across-channel processing at an early stage in the auditory pathway.

Acoustic Stimulation↗
PubMed1995

Vowel and vowel sequence processing by cochlear nucleus neurons.

This study examined neuronal discharge rates and temporal patterns to vowels and vowel sequences in chinchilla. The properties of primary-like, chopper, and onset neurons were studied using vowels /i/, /a/, and /u/ individually and paired with separations (0-100 ms), at sound levels above and below thresholds. The interspike interval, period, and post-stimulus-time histograms of all neuronal types to a vowel were modified when in a sequence. Primary-like and chopper discharges were reduced and enhanced depending on vowel sequence parameters; onset neurons exhibited discharge rate reductions only and not enhancements. In addition to rate changes, novel discharge intervals appeared with vowel pairs. An unexpected finding on choppers was that subthreshold levels of the preceding vowel in a paired sequence enhanced discharges to the succeeding one. Reducing levels of preceding or increasing levels of following vowels evoked changes not predictable from single vowel data. Thus the responses to paired vowels in a sequence are interactive. Patterns of discharges and rate functions to vowel sounds from neurons of the same type varied greatly. The cochlear nuclei harbor anatomically and functionally diverse neurons. Because of this heterogeneity, the neural transformations of vowel segments by all cochlear nucleus neuronal types can not be predicted from sinusoidal data.

Acoustic Stimulation↗
PubMed2006

Effects of exposing DBA/2J mice to a high-frequency augmented acoustic environment on the cochlea and anteroventral cochlear nucleus.

DBA/2J (D2) mice, which exhibit very early progressive sensorineural hearing loss, were treated for 12h nightly with an augmented acoustic environment (AAE) initiated before the onset of hearing. The AAE consisted of repetitive bursts of a 70 dB sound pressure level, half-octave noise band centered at 20 kHz (i.e. low frequencies were excluded). At 55 days of age, AAE-treated mice, compared to control mice, exhibited less elevation of auditory brainstem response thresholds for tone frequencies from 16 to 32 kHz and fewer missing outer hair cells in the high-frequency tonotopic region of the cochlea. The dorsal region of their anteroventral cochlear nucleus (most strongly stimulated by the AAE) was larger, had more surviving neurons, and larger neurons than those of untreated control mice. These and previous findings using an AAE band containing lower frequencies indicate that AAE treatment effects are frequency-related. The findings provide support for the hypothesis that the beneficial effects of AAE treatment on the cochlea are associated with increased physiological activity evoked by the AAE, and the central AAE effects result from increased AAE-evoked neural activity and a healthier cochlea providing the auditory input.

Analysis of Variance↗