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PubMed1993

Regularity of cochlear nucleus stellate cells: a computational modeling study.

This article reports on a computational modeling study designed to investigate the generation of the transient chopper response of cochlear nucleus stellate cells. The model is based on a simulation of the auditory periphery which feeds a generic stellate-cell model. Physiological recordings of transient chopper units in response to short, best frequency, tone bursts show a brief initial period (typically < 10 ms) of rapid rate adaptation as evidenced by a rapid rise in mean interspike interval. Associated with this rate adaptation is a significant increase in firing irregularity. The changes in rate and irregularity have recently been attributed to the activation of noisy inhibitory inputs on the cell [e.g., Banks and Sachs, J. Neurophysiol. 65, 606-629 (1991)]. However, the results show that the transient chopper response pattern can be generated without the need for inhibitory inputs. The transience of the initial chopping pattern is sensitive to the following model parameters: (a) the firing threshold of the cell, (b) the number of excitatory inputs that converge on the cell, and (c) the magnitude of the current delivered to the cell for each active input. The response was also found to be relatively insensitive to changes in the degree of dendritic filtering imposed on the auditory-nerve input. The results of each simulation can be explained by considering the pattern of depolarization the cell receives during the course of a tone burst.

Acoustic Stimulation↗
PubMed1993

Altered distribution of synaptic densities at aberrant synapses in the chick cochlear nucleus.

The role of axon-target cell interactions in shaping the prevalence and distribution of synaptic densities was studied in an experimentally-induced aberrant functional projection from the chick cochlear nucleus (nuc. magnocellularis, NM) to the contralateral NM. Contact with an abnormal target appears to induce in the aberrant axons a pattern of presynaptic densities resembling that in normal cochlear nerve endings in NM. NM axon terminals induced similar numbers of postsynaptic densities (PSDs) per unit length of membrane apposition in both their normal and abnormal targets but the longer membrane apposition in the highly invaginated aberrant terminal in NM results in a significantly greater amount of postsynaptic density per ending. These auditory neurons thus appear able to adjust a variety of features to permit assembly and maintenance of a novel functional synapse.

Animals↗
PubMed1990

Temporal responses of primarylike anteroventral cochlear nucleus units to the steady-state vowel /i/.

It has previously been shown that a population of units classified as "primarylike" and located in the ventral cochlear nucleus of the anesthetized guinea pig was unable to signal the position of the higher formant-related peaks (greater than 1.5 kHz) of steady-state vowels in terms of a temporal-place representation [Palmer et al., J. Acoust. Soc. Am. 79, 100-113 (1986)]. In this paper, it is demonstrated that units characterized by a prepotential in their spike waveform and a primarylike post-stimulus time histogram shape can encode the relative position of the formant peaks present in the spectra of steady-state vowels in terms of a temporal-place code. The possible reasons for the differences in the present results and the previous report are discussed.

Animals↗
PubMed1984

Stellate neurons in rat dorsal cochlear nucleus studies with combined Golgi impregnation and electron microscopy: synaptic connections and mutual coupling by gap junctions.

Stellate neurons in the outer two layers of the rat dorsal cochlear nucleus (DCN) were studied by the Golgi-EM method. Stellate cell bodies are usually spherical or ovoidal and range from 9 microns to 14 microns in mean diameter. The smallest cells are situated underneath the ependymal layer and the largest cells in layer 2. Primary dendrites are short, thin and smooth and arise abruptly from the perikaryon, without a tapering main stem. Meandering secondary and tertiary dendrites extend in all directions, carry few pleomorphic spines lacking a spine apparatus and often show artifactual beading. The axons are impregnated only for a short distance (10-45 microns). The nucleus is indented, the nucleolus varies in position, and the chromatin, evenly dispersed in the centre, forms small clumps along the nuclear envelope. The cytoplasm is rich in free polyribosomes and contains scattered cisterns of granular endoplasmic reticulum. Varicosities of thin fibres, containing round synaptic vesicles, form asymmetric synapses on perikarya, dendritic shafts and spines of stellate cells. Such fibres run parallel to the long axis of the DCN or are oriented radially and are interpreted as axons of cochlear granule cells. Two kinds of bouton containing pleomorphic vesicles, one kind electron lucent and the other electron dense, form symmetric synapses on perikarya and dendritic shafts of stellate cells. The lucent boutons occur more frequently than the dense boutons, especially on the distal dendritic branches. The boutons with pleomorphic vesicles presumably represent terminals of local circuit neurons, probably the stellate and cartwheel cells. In addition, stellate cells show numerous dendro-somatic and dendro-dendritic appositions characterized by gap junctions and puncta adhaerentia. Most of the dendrites involved in these appositions resemble stellate cell dendrites and it is concluded that DCN stellate cells are coupled electrotonically with one another. The axons of stellate cells acquire a thin myelin sheath. Since the Golgi impregnation did not stain axons of stellate cells past this point, we were unable to demonstrate the synaptic targets of stellate cells.

Animals↗
PubMed1986

Autoradiographic localization of receptors in the cochlear nucleus of the mouse.

Light microscopic autoradiography of bound radiolabeled ligands was used to describe the distribution of six receptor types in the dorsal and ventral mouse cochlear nuclei: Glycine receptor ([3H]strychnine); GABA receptor ([3H]muscimol); benzodiazepine receptor ([3H]flunitrazepam); adenosine receptor ([3H]cyclohexyladenosine); muscarinic ACh receptor ([3H]quinuclidinyl benzilate); histamine receptor ([3H]mepyramine). The most intense [3H]strychnine labeling was observed in the deep region of the dorsal cochlear nucleus (DCN), with slightly lower levels in the molecular and pyramidal layers. Highest density of [3H]muscimol binding sites was observed in the granule cell layer of the posterior ventral nucleus (PVCN) and in the pyramidal layer of the DCN. Diffuse [3H]flunitrazepam labeling was distributed over all laminar regions of the DCN; the highest grain density was observed over the granule cell layer of the PVCN. Intense [3H]cyclohexyladenosine labeling was seen over the molecular layer, possibly extending into the pyramidal layer, of the DCN. The granule cell layer of the PVCN was also densely labeled. High concentrations of [3H]quinuclidinyl benzilate sites were seen in the molecular layer, possibly extending into the pyramidal layer, of the DCN. A thin band of high grain density was also visible over the granule cell layer of the PVCN. Moderate, diffuse [3H]mepyramine labeling was visible throughout the DCN, with slightly higher grain density over the molecular, and possibly the pyramidal layers, than over the deep region of the DCN.

Animals↗
PubMed1982

Intensity coding in the auditory periphery of the cat: responses of cochlear nerve and cochlear nucleus neurons to signals in the presence of bandstop masking noise.

The dynamic range over which fine intensity discrimination is possible has been reported to be largely unaffected by limitation of the spread of neuronal activity to neighbouring frequency regions by bandstop noise masking. We have therefore examined the responses of cochlear nerve and nucleus neurons to tone and noise signals in the presence of a bandstop masking noise designed to be comparable to that employed in the psychophysical experiments. Under these conditions, the vast majority of cochlear nerve fibres were saturated by sound levels at which some 50% of our sample of cochlear nucleus neurons still responded to signal level differences. The extended dynamic ranges of these cochlear nucleus neurons was shown to be a result of activation, by the masking noise, of the lateral inhibitory side-bands 'biassing' the neuron's discharge. A small proportion of cochlear fibres, having low spontaneous discharge rates and showing strong two-tone suppression effects, demonstrated analogous but not so pronounced effects. It is unclear in what form information on the level of stimuli under these conditions is transmitted by the majority of apparently saturated cochlear nerve fibres, but several possible mechanisms are discussed.

Animals↗
PubMed1990

GABA actions within the caudal cochlear nucleus of developing kittens.

1. The effects of gamma-aminobutyric acid (GABA), microionophoretically applied onto neurons within the dorsal and posteroventral divisions (i.e., caudal regions) of the cochlear nucleus (CN), were studied during postnatal development in kittens with the use of extracellular recording techniques. Approximately 80% of all neurons encountered within the caudal CN responded to exogenously applied GABA regardless of neuronal response type or postnatal age. 2. GABA reduced acoustically evoked as well as spontaneous discharge rates in a dose-dependent manner at all ages studied, and generally abolished discharge activity at sufficiently high doses (i.e., ejection currents). Dose-response curves generated during acoustic stimulation by varying GABA ejection current were sigmoidal at all ages studied, and the range of slopes relating discharge rate to applied currents increased during the first 10 postnatal days. 3. Neural thresholds to acoustic stimuli were elevated, and slopes of discharge-rate-versus-sound-pressure-level curves were depressed regardless of age when GABA was microionophoresed onto CN neurons. 4. GABA's capacity to reduce spontaneous or acoustically evoked discharge rates was a voltage-dependent phenomenon directly related to control discharge rates (i.e., efficacy was high when discharge rates were high) for neurons recorded from both immature and mature animals. 5. A small set of neurons recorded from animals younger than 2 wk exhibited prolonged GABA "activation" and "deactivation" times (i.e., times required to achieve 90% of the maximal effect evoked by GABA and to recover from that effect, respectively) and may represent a group of actively differentiating units. 6. Bicuculline microionophoresis effectively blocked the actions of exogenously applied GABA and endogenous GABA, which, presumably, was synaptically released as a result of acoustic stimulation. The actions of bicuculline were dose dependent in animals ranging in age from 2 postnatal days to adulthood. These results suggest that recognition sites for GABA and bicuculline and the ionophore associated with the GABA receptor are present and functionally coupled in the caudal CN before the developmental period during which the full complement of inhibitory projections form synaptic contacts with these cells.

Acoustic Stimulation↗
PubMed1990

Collaterals from lateral and medial olivocochlear efferent neurons innervate different regions of the cochlear nucleus and adjacent brainstem.

Two populations of superior olivary neurons which project to different sensory cell regions in the cochlea also give off collateral projections to the ventral cochlear nucleus (VCN) and adjacent brainstem. To determine whether these VCN projections also have different targets they were characterized by selective retrograde amino acid transport. Retrograde transport of 3H-d-aspartate (D-ASP) selectively labeled the unmyelinated fibers and neurons of the lateral olivocochlear (OC) system including a dense collateral projection to the central VCN. Retrograde transport of 3H-nipecotic acid (NIP) labeled the myelinated fibers and neurons of the medial OC system, including collateral projections to the peripheral VCN, subpeduncular granule cells, and nucleus Y. Medial and lateral OC efferent collaterals thus innervate different regions of the CN. Lateral system collaterals overlap extensively with Type I spiral ganglion cell afferent input. They are well positioned to play a role in modulating afferent input to the central auditory system, as is the primary projection of these efferents to the cochlea. The medial system collaterals project near the recently described afferent projections of Type II spiral ganglion cells. The medial system collaterals may therefore be related to the function of outer hair cells, as the medial system primary axons appear to be in the cochlea.

Animals↗
PubMed1992

Growth cones and structural variation of synaptic end-bulbs in the cochlear nucleus of the adult cat brain.

To explore the potential for structural variation and new growth at the synapse, we studied the morphological patterns of the end-bulbs of cochlear nerve axons in adult cats by using rapid Golgi, reduced silver, and electron microscopic methods. Horseradish peroxidase labeling of these endings in the anterior division of the antroventral cochlear nucleus was produced by anterograde transport following injection into the cochlea. Three types of end-bulbs were distinguished, regardless of method: reticular, coalescent, and ringed forms, all synapsing on spherical bushy cells. The reticular variety corresponds to the classically described end-bulb and constitutes the majority in all regions of the tonotopic map. The ringed end-bulb, described here for the first time, forms an excitatory synaptic cuff around the base of a bushy cell's main dendrite; these endings were localized to the region receiving cochlear input in the 1-6 kHz range, which is used in vocalization. The coalescent ending forms a small fraction of the end-bulb population throughout the region studied. The findings raise the possibility of functional differences between these synaptic types. Growth cones and retraction clubs were present on most, if not all, of the end-bulbs in every adult cat studied. A systematic survey of the end-bulb patterns revealed a continuous gradient of variation, in which each synaptic type forms a distinct mode. These findings lead us to hypothesize that the end-bulbs are in a continual state of structural and functional flux. These endings should prove useful for studies on the modifiable properties of central synapses.

Animals↗
PubMed1975

Relations between auditory nerve endings and cell types in the cat's anteroventral cochlear nucleus seen with the Golgi method and Nomarski optics.

Rapid Golgi impregnations of the ascending branches of the auditory nerve fibers and of the types of neurons in the anteroventral cochlear nucleus (AVCN) were studied. Entire ascending branches could be observed, some of these branches project to each subdivision, others do not. There are two main typesof large neurons: the bushy and stellate cells. Criteria were established for identifying unimpregnated bushy and stellate perikarya by means of Nomarski optics, and these criteria were checked by Momarski observations on neurons which had either impregnated dendrites and unimpregnated cell bodies or impregnated portions of perikarya. In this way, the relations of unimpregnated cell bodies to auditory nerve endings were observed. Furthermore, with Nomarski optics, the cytoarchitectonic subdivisions of AVCN could be determined. Differences in the end-bulbs and collateral endings formed by the auditory nerve fibers were distinguished in three of the cytoarchitectonic subdivisions of the AVCN. End-bulbs in the anterior division were much larger than those in the dorsal and ventral parts of the posterior division. The large end-bulbs of Held in the anterior division of the AVCN were consistently associated with the perikarya of bushy cells and not with those of stellate cells. The large end-bulbs are not observed in the posterior division. Thus, bushy cells in the posterior division, although morphologically similar to those in the anterior division, must have a different synaptic organization. This difference may correspond to electrophysiological distinctions in the time-patterns of response recorded in these regions following acoustic stimulation.

Animals↗
PubMed1990

Cross-correlation analysis of inhibitory interactions in dorsal cochlear nucleus.

1. Cross-correlation analysis was used to study the organization of inhibitory connections between type II or type III units and type IV principal cells in cat dorsal cochlear nucleus (DCN). Pairs of units were isolated using two microelectrodes so that information about the distance over which connections are made could be analyzed. Data were obtained from 51 pairs consisting of a type II and a type IV unit and from 22 pairs consisting of a type III and a type IV unit. The analyses in this paper concentrate on type II-type IV pairs. 2. Inhibitory troughs (ITs) are observed in the cross-correlograms of type II-type IV pairs (21/51 cases). An IT is a transient decrease in discharge probability in the postsynaptic (type IV) unit immediately after spikes in the presynaptic unit (type II). The average latency to the start of ITs is 0.73 ms, and the troughs are asymmetric with a faster leading phase. Small excitatory peaks accompany the ITs in type II units, but these are probably secondary effects associated with the IT. ITs are consistent with a monosynaptic, inhibitory connection between type II and type IV units. A variety of evidence suggests that type II responses are recorded from vertical cells, an interneuron in the deep layer of the DCN that may be glycinergic. 3. The cross-correlograms of type III-type IV pairs are more complex and variable than those of type II-type IV pairs--ITs are seen in 4/22 cases, and peaks of correlation that are symmetrically located around the origin (central mound or CM) are seen in 4/22 cases; two cases have both an IT and a CM. CMs result from shared sources of input. Whereas type II-type IV correlogram features change primarily in amplitude as stimulus conditions change, correlogram features in some type III-type IV pairs change qualitatively with stimulus conditions; correlograms are flat for some stimuli and show ITs or CMs or mixtures of the two for others. This variability suggests that the circuitry associated with type III-type IV pairs is more complex than a monosynaptic connection, and further analysis of type III-type IV pairs was not done. 4. The strength of inhibition for an IT is measured as the area under the IT (effectiveness) and as effectiveness divided by the postsynaptic discharge rate (association index).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1984

Cells in the anteroventral cochlear nucleus are insensitive to L-glutamate and L-aspartate; excitatory synaptic responses are not blocked by D-alpha-aminoadipate.

Auditory nerve fibers transmit signals from the cochlea to the 3 regions of the cochlear nuclear complex, the anteroventral (AVCN), posteroventral, and dorsal cochlear nucleus in the brainstem. It has been suggested that the amino acids L-aspartate and L-glutamate might serve as a neurotransmitter in auditory nerve fibers. The sensitivity of postsynaptic cells in the cochlear nuclei to these amino acids has been tested by iontophoretic techniques. One difficulty with these experiments is that responses were recorded only extracellularly. A second difficulty is that the concentrations needed to affect cells could not be determined. To avoid these difficulties a brain slice preparation was used to test the sensitivity of cells in the AVCN to bath applied L-glutamate and L-aspartate at concentrations ranging from 10(-5) to 10(-2) M. All cells that were tested in the cochlear nuclear complex were insensitive at all concentrations used; the resting potentials and the input resistances remained unchanged and the synaptic responses to electrical stimulation of the auditory nerve were not desensitized. All cells that were tested in the hippocampus, however, depolarized in the presence of 10(-4) M L-glutamate and L-aspartate. The synaptic responses to electrical stimulation of the auditory nerve were not blocked by D-alpha-aminoadipate, an amino acid which has been shown to block excitation of cells in the cochlear nuclei by auditory nerve fibers. The results are not consistent with L-glutamate and L-aspartate serving as neurotransmitters in the AVCN.

2-Aminoadipic Acid↗
PubMed1988

Decreased protein synthesis in cochlear nucleus following developmental auditory deprivation. Use of vascular saline perfusion to improve small tissue sample analysis.

The incorporation of tritiated leucine was used as an index of protein synthesis in the cochlear nucleus (CN) of mice unilaterally (right side) hearing deprived throughout the period of hearing development. Right-left differences in radiolabel concentration were measured by scintillation counting of whole tissue homogenates. To improve upon the detection of small differences in radiolabel incorporation, the brain was perfused with saline prior to removal of CN tissue and the results compared with the standard nonperfusion method of tissue collection. Statistical analyses demonstrated the perfusion significantly reduced the acid soluble (unbound) label in CN without affecting the amount of protein bound label. Furthermore, a significant right side decrease in leucine incorporation was seen with the perfusion treatment, but not in the nonperfused treatment. This demonstrated that developmental auditory deprivation led to a decrease in protein synthesis at maturity. The results also demonstrated that mechanisms for leucine uptake were not impaired and the decrease in protein synthesis was not due to reduced availability of precursor amino acid. Thus, the use of saline perfusion prior to tissue collection facilitated the identification of protein synthesis differences that were unidentified by the traditional method.

Acoustic Stimulation↗
PubMed1991

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)

Animals↗
PubMedDate not supplied

Progressive degeneration in the cochlear nucleus after chemical destruction of the cochlea.

In the guinea pig, the organ of Corti and the spiral ganglion cells were destroyed by administering gentamicin into the inner ear. The antero-ventral cochlear nucleus (AVCN) was studied with electron microscopy 15 and 30 days after treatment. There does not seem to be a specific type of degeneration induced by the drug. For nerve terminals, a progressive degeneration occurred: after 15 days, clear and swollen boutons appeared and after 30 days an electron-dense type of degeneration was observed. The cell bodies of the AVCN did not seem altered infirming a rapid, direct or indirect, neurotoxic effect of the drug.

Animals↗
PubMed1990

Encoding of amplitude modulation in the gerbil cochlear nucleus: I. A hierarchy of enhancement.

The main goal of the present study was to investigate the encoding of a biologically-relevant acoustic feature--amplitude modulation (AM)--in single neurons of the auditory nerve and ventral cochlear nucleus (VCN). In the anesthetized gerbil auditory-nerve fibers and VCN units show strong synchronous responses to low-intensity, low-frequency AM. As frequency increases, the strength of the synchronous response decreases. In the auditory nerve the strength of the synchronous response is substantially less at high intensities than at low intensities and does not change significantly with AM frequency at high intensities. In contrast to the auditory nerve, VCN units show strong responses at high intensities. They have a particular AM frequency to which they are maximally responsive, and this frequency varies from unit to unit. Therefore, VCN units transform their ascending inputs by enhancing the synchronous response to AM. A correlation exists between a unit's ability to encode AM and its responses to simple sounds. Specifically, onset units show the strongest synchronous responses, followed in order by chopper, primarylike-with-notch and primarylike units. This enhancement is greatest at high intensities and can occur up to 90 dB above a unit's threshold. Thus, a hierarchy of enhancement for AM processing exists in the most peripheral nucleus of the central auditory system.

Acoustic Stimulation↗
PubMed1978

Kainic acid injections result in degeneration of cochlear nucleus cells innervated by the auditory nerve.

When kainic acid, a putative neurotoxin for neurons with glutamatergic input, is injected into the brainstem, it produces a selective pattern of degeneration in the cochlear nucleus. The rate and extent of degeneration is correlated with the distribution of the primary auditory fibers. This evidence supports the hypothesis that glutamate is the neurotransmitter for primary auditory fibers.

Animals↗
PubMed1986

Cochlear nucleus lesions in the adult gerbil: effects on neurone responses in the contralateral inferior colliculus.

Unilateral cochlea ablation in neonatal gerbils has previously been shown to result in transneuronal degeneration of the ventral cochlear nucleus (CN) and enhanced responses of neurones in the contralateral inferior colliculus (IC) to ipsilateral stimulation of the non-ablated ear. Neither effect occurs in adult-ablated animals. To determine whether the lack of physiological change in the adult is due to the persistence of the ventral CN we lesioned the left CN of adult gerbils and recorded neurone responses in the right IC to stimulation of the right ear. Neurone thresholds, latency and intensity/response functions were unaffected by the lesion. The proportion of recording loci in the IC at which excitatory responses were obtained was also unaffected. A transient increase was observed in the maximum unit discharge level. The findings suggest that CN lesions in adults do not produce either the range or degree of neuronal changes resulting from neonatal cochlea ablation and the subsequent transneuronal degeneration of the CN. Thus, the effects of cochlea ablation are age-dependent.

Animals↗