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PubMed2005

Expression and developmental regulation of the K+-Cl- cotransporter KCC2 in the cochlear nucleus.

KCC2 is a neuron-specific Cl- transporter whose role in adult central neurons is to maintain low intracellular Cl- concentrations and, therefore, generate an inward-directed electrochemical gradient for Cl- needed for the hyperpolarizing responses to the inhibitory amino acids GABA and glycine. We report that the KCC2 protein is intensely expressed in CN neurons and preferentially associated with plasma membrane domains, consistent with GABA and glycinergic-mediated inhibition in this auditory nucleus. Postnatal KCC2 expression and distribution patterns are similar in developing and adult CN neurons and do not match the time course of GABergic or glycinergic synaptogenesis. Therefore, in the CN, neither KCC2 protein upregulation nor progressive integration in the plasma membrane seem to be involved in KCC2 developmental regulation. Considering that GABA and glycine are depolarizing during early postnatal development, it is conceivable that KCC2 is in place but inactive during early postnatal development in the CN and becomes active as inhibitory synaptogenesis proceeds. This notion is supported by the finding that the phosphorylation state of KCC2 differs from developing to adult CN, with the phosphorylated form predominating in the latter.

Animals↗
PubMed1997

[Long-term results of multi-channel stimulation of the cochlear nucleus with auditory brain stem prostheses].

Since 1992 11 subjects with bilateral neural deafness due to cerebellopontine angle tumors have been provided with a multichannel auditory brainstem implant. The long term follow-up varies between 12 and 48 months. 10 subjects experienced auditory sensations with various pitch during stimulation of several surface electrodes, 7 of them are still full time users. 1 patient died 18 months after implantation after having been using his device continuously and 1 died due to pneumonic emboly without having been stimulated. 2 subjects revealed uncomfortable side-effects such as motoric or somatosensoric stimulations several months after initial stimulation. 1 of them is not using the device any more. The presented long term results suggest, that deaf neurofibromatosis-patients can regain acoustic contact to the environment, enlarge their communication skills and improve their quality of life by using a multichannel auditory brainstem prosthesis.

Adult↗
PubMed2000

Stimulation of the cochlear nucleus with multichannel auditory brainstem implants and long-term results: Freiburg patients.

Since 1992 18 patients with bilateral retrocochlear deafness have been provided with a multichannel auditory brainstem implant (ABI). The surgical procedure implies tumour removal and ABI implantation in one stage. Most implantations were via the translabyrinthine approach. The long-term follow-up varied between nine and 80 months. In one case auditory perception could not be achieved and in a second case post-operative stimulation was not possible as the subject died due to lung emboli. In all the other cases auditory perception was achieved and only two subjects became non-users during the follow-up period. The presented long-term results suggest that deaf neurofibromatosis type 2 patients regain acoustic contact with the environment, enlarge their communication skills and improve their quality of life by using a multichannel auditory brainstem prosthesis.

Adolescent↗
PubMed1996

Immunocytochemical evidence that glutamate is a neurotransmitter in the cochlear nerve: a quantitative study in the guinea-pig anteroventral cochlear nucleus.

The large so-called type I afferents of the cochlear nerve carry the majority of the auditory input from the cochlea to the cochlear nuclei in the brainstem. These fibres are excitatory and previous studies have suggested they may use glutamate as their neurotransmitter. In the present investigation therefore, antibodies to glutamate and to the glutamate precursor, glutamine, were applied to resin sections of perfusion-fixed brains and of in vitro brain slices subjected to depolarizing levels of potassium before fixation to study glutamate handling and synaptic release. Ultrathin sections were labelled by the immunogold technique, and the immunoreactivity was quantified by recording the density of gold particles over the various tissue profiles. Non-primary, presumably inhibitory, terminals and glial processes were used as reference structures. The cochlear primary terminals proved to be strongly immunoreactive for glutamate. The density of glutamate labelling was higher in primary terminals than in non-primary ones, and lowest in glial processes. The ratio between the mean glutamate and glutamine labelling densities was also higher in primary terminals than in non-primary ones, and lowest in glial processes in each case. In the primary terminals, the glutamate immunoreactivity was higher over vesicle-containing regions than over vesicle-free regions, whilst glutamine was evenly distributed throughout. The in vitro brain slices showed a potassium-induced, partly calcium-dependent depletion of glutamate from the primary terminals but not from the non-primary ones. These observations strongly support the conclusion that glutamate is a neurotransmitter of type I cochlear afferents.

Afferent Pathways↗
PubMed1991

[The surgical approach to the cochlear nucleus area].

The stimulation of the proximal auditory nerve or the Cochlear Nuclear Complex (CNC) situated in the lateral recess of the fourth ventricle is a possibility of hearing rehabilitation for patients suffering from bilateral deafness by bilateral cerebellopontine-angle tumors or by transsections of cochlear nerves on both sides. Using such anatomical landmarks as proximal cochlear nerve, flocculus, choroid plexus and taenia the surgeon can locate exactly the CNC for this stimulation. We investigated location, diameter and surrounding structures of the CNC on 10 human brains fixed in 10% formalin-solution and on 9 histological series of these subjects.

Acoustic Stimulation↗
PubMed2006

The temporal representation of the delay of iterated rippled noise with positive or negative gain by chopper units in the cochlear nucleus.

The role of chopper units in representing the pitch of complex sounds is unresolved. Traditionally chopper units have been regarded as primarily responding to the stimulus envelope of complex stimuli. This has been supported by the response of chopper units to iterated rippled noise (IRN) as they can provide a robust representation of the delay of IRN with positive gain (+) in their first-order interspike intervals and for some chopper units this representation is relatively level independent. The envelope modulation of IRN(+), and pitch, is at the reciprocal of the delay, the pitch of IRN with negative gain (IRN(-)) is often at twice the delay. This distinction between IRN(+) and IRN(-) can be used to help determine whether a unit is simply responding to modulation or to stimulus fine structure. Chopper units with relatively high best frequencies (BF) are unable to represent the distinction between IRN(+) and IRN(-). However, in this study it is shown that at least some chopper units, with low BFs (<1.25 kHz), can represent the pitch of the IRN(-) as perceived perceptually.

Animals↗
PubMed2005

Enhanced temporal response properties of anteroventral cochlear nucleus neurons to broadband noise.

Compared with auditory nerve (AN) fibers, trapezoid body (TB) fibers of the cat show enhanced synchronization to low-frequency tones. This phenomenon probably contributes to the high temporal resolution of binaural processing. We examined whether enhanced synchronization also occurs to sustained broadband noise. We recorded responses to a reference Gaussian noise and its polarity-inverted version in the TB of barbiturate-anesthetized cats. From these we constructed shuffled autocorrelograms (SACs) and quantified spike timing by measuring the amplitude and width of their central peak. Many TB fibers with low characteristic frequency (CF) showed SACs with higher and narrower central peaks than ever observed in the AN, indicating better consistency and precision of temporal coding. Larger peaks were also observed in TB fibers with high CF, but this was mostly caused by higher average firing rates, resulting in a larger number of coincident spikes across stimulus repetitions. The results document monaural preprocessing of the temporal information delivered to binaural nuclei in the olivary complex, which likely contributes to the high sensitivity to interaural time differences.

Acoustic Stimulation↗
PubMed1996

Properties of auditory brainstem responses evoked by intra-operative electrical stimulation of the cochlear nucleus in human subjects.

Electrically evoked auditory brainstem response (EABR) testing can aid placement of the stimulating electrodes during surgical implantation of an auditory brainstem implant (ABI). To facilitate efficient testing, this study of EABR properties examined the effects of various stimulating and recording parameters on the magnitude and clarity of the EABRs obtained from 9 successive ABI patients during intra-operative monitoring. Both stimulus polarities elicited EABRs; the response waveforms were similar and no significant differences between the latencies were found. Stimulus-response relationships displayed thresholds and non-linear growth, characteristic of neural activity, and provided a stimulus amplitude that elicited readily detectable EABRs in all subjects. The stimulus rate could be increased without degrading the EABRs, but usually 50 Hz was used with a 10 ms sampling sweep so that muscle responses, which occurred later than EABRs, could be detected. When 3 stimulating electrodes in a line were tested, the pair with the largest separation consistently provided the largest response. A recording filter passband of 10-3000 Hz was useful for attenuating interference signals because there is negligible energy in the EABR at frequencies above 3 kHz, but there is some energy below 100 Hz.

Brain Stem↗
PubMed2003

Auditory centre projection of the lower brainstem to the dorsal cochlear nucleus in the rabbit.

Afferent projection to dCN from SOC and the periolivary regions was studied in the rabbit by retrograde transport of WGA-HRP. The projection originates primarily from the bilateral TrV and TrL with a very clear contralateral and ipsilateral predominance, respectively. A clear-cut topographical relationship was disclosed between location of neurons in these nuclei and projection sites in dCN. Thus, the medial region of dCN is target of projection arising from the medial regions of TrV and TrL, whereas the lateral region of dCN is supplied by projection from their lateral regions. Although participation in the projection of the ipsilateral TrV is smaller and the contralateral TrL is very weak, the pattern of these preferential connections is also apparent. Minute connections were traced from the other principal olivary nuclei, i.e. MSO, LSO and TrM, mainly from neurons located in their peripheral regions. In the periolivary region the cells of origin of the projection were found in VLPO and VMPO, and in lesser extent in DPO, DMPO, DLPO, RPO and CPO. The present results are discussed in comparison with those of earlier studies and with reference to other inputs to CN.

Animals↗
PubMed2001

Mixed excitatory and inhibitory GABA-mediated transmission in chick cochlear nucleus.

1. Neurons of the chick nucleus magnocellularis (NM) receive depolarizing GABAergic input from the superior olivary nucleus (SON). We examined the response to exogenous GABA or to stimulation of GABAergic fibres in order to identify the ionic basis of GABAergic synaptic transmission and its physiological implications. 2. Reversal potentials of GABA responses (E(GABA)) were determined exclusively by the Cl(-) gradient, measured using whole-cell recording. With gramicidin-perforated patch recording, E(GABA) was -25 +/- 5 mV (mean +/- S.D.), and was stable between embryonic day 17 and post-hatch day 10. With normal intracellular Cl(-), GABA depolarized neurons by 12 mV. 3. In current clamp, repetitive activation of the GABAergic axons reduced the probability of spiking in response to simultaneous stimulation of excitatory axons. However, IPSPs could themselves elicit action potentials, and facilitation of IPSPs by repetitive activation could lead to a characteristic pattern of spiking. 4. These data indicate that IPSPs with reversal potentials positive to spike threshold may have dual functions, depending on the context of their activation.

Animals↗
PubMed1995

Concentration-jump analysis of voltage-dependent conductances activated by glutamate and kainate in neurons of the avian cochlear nucleus.

We have examined the mechanisms underlying the voltage sensitivity of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors in voltage-clamped outside-out patches and whole cells taken from the nucleus magnocellularis of the chick. Responses to either glutamate or kainate had outwardly rectifying current-voltage relations. The rate and extent of desensitization during prolonged exposure to agonist, and the rate of deactivation after brief exposure to agonist, decreased at positive potentials, suggesting that a kinetic transition was sensitive to membrane potential. Voltage dependence of the peak conductance and of the deactivation kinetics persisted when desensitization was reduced with aniracetam or blocked with cyclothiazide. Furthermore, the rate of recovery from desensitization to glutamate was not voltage dependent. Upon reduction of extracellular divalent cation concentration, kainate-evoked currents increased but preserved rectifying current-voltage relations. Rectification was strongest at lower kainate concentrations. Surprisingly, nonstationary variance analysis of desensitizing responses to glutamate or of the current deactivation after kainate removal revealed an increase in the mean single-channel conductance with more positive membrane potentials. These data indicate that the rectification of the peak response to a high agonist concentration reflects an increase in channel conductance, whereas rectification of steady-state current is dominated by voltage-sensitive channel kinetics.

Animals↗
PubMed2001

An improved cell isolation technique for studying intracellular Ca(2+) homeostasis in neurones of the cochlear nucleus.

Neurones isolated from various parts of the brain are used extensively for electrophysiological and immuncytochemical studies, as well as to investigate their Ca(2+) homeostasis. In this work we report on an isolation technique that yielded neurones suitable for functional studies targeting the investigation of their Ca(2+) handling mechanisms. The cell isolation involved enzymatic dissociation with combined collagenase/pronase treatment and gentle mechanical trituration. At the end of the isolation the cells were incubated in a cell culture incubator (CO2 concentration = 5.1%) at 37 degrees C in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% heat-inactivated horse serum. The vitality of the isolated cells was indicated by their low intracellular Ca(2+) concentrations (17.2 +/- 0.5 nM; n = 38) and by their ability to produce large Ca(2+) transients on depolarization. These Ca(2+) transients were rapidly terminated and the resting intracellular Ca(2+) concentration was quickly restored proving that isolation did not compromise the Ca(2+) homeostatic mechanisms of the nerve cells. The technique allowed reliable, long (45-60 min) and reproducible measurements of Ca(2+) currents on these neurones as well as the recording of their intracellular Ca(2+) concentration. Our results indicate that incubation in DMEM with horse serum markedly increases the number of surviving neurones after the enzyme treatment, and their Ca(2+) homeostasis can be studied for significantly longer periods of time.

Animals↗
PubMed2004

Depolarization-activated K+ currents of the bushy neurones of the rat cochlear nucleus in a thin brain slice preparation.

Depolarization-activated outward currents of bushy neurones of 6-14-day-old Wistar rats have been investigated in a brain slice preparation. Under current-clamp, the cells produced a single action potential at the beginning of suprathreshold depolarizing current steps. On voltage-clamp depolarizations, the cells produced a mixed outward K+ current that included a component with rapid activation and rapid inactivation, little TEA+ sensitivity, a half-inactivation voltage of -77 +/- 2 mV (T = 25 degrees C; n = 7; Mean +/- S.E.M.) and single-exponential recovery from inactivation (taurecovery= 12 +/- 1 ms at -100 mV; n=3). This transient component was identified as an A-type K+ current. Bushy cells developed a high-threshold TEA-sensitive K+ current that exhibited less prominent inactivation. These characteristics suggested that this current was associated with the activation of delayed rectifier K+ channels. Bushy neurones also possessed a low-threshold outward K+ current that showed partial inactivation and high 4-aminopyridine sensitivity. Part of this current component was blocked by 200 nmol/l dendrotoxin-I. Application of 100 micromol/l 4-aminopyridine changed the firing behaviour of the bushy neurones from the primary-like pattern to a much less rapidly adapting one, suggesting that the low-threshold current might have important roles in maintaining the physiological function of the cells.

Action Potentials↗
PubMed1997

Firing properties of spherical bushy cells in the anteroventral cochlear nucleus of the gerbil.

In gerbils, spherical bushy cells (SBCs) encode low frequency sound signals into a temporal firing pattern. To investigate the support for the timing in this temporal code, we characterized the membrane electrical properties of visually identified SBCs in brainstem slices. A brief depolarizing subthreshold transient potential (TP) triggered, with relatively invariant latency, a single spike at the onset of a response to depolarizing current pulses. The activation of a subthreshold Na+-conductance, sensitive to blockade with tetrodotoxin, and a high threshold Ca2+-conductance, sensitive to blockade with Co2+ or Cd2+, accelerated the rising phase and amplified the TP. A K+-conductance, sensitive to blockade by 4-aminopyridine (4-AP, 50 microM), shaped the decay of the TP. Following a single spike, voltage-gated activation of transient and sustained K+-conductances suppressed any tendency to repetitively discharge. A reduction in either K+-conductance due to application of 4-AP or tetraethylammonium (TEA, 10 mM), converted the single spike mode to repetitive firing during the depolarizing pulses. A persistent, tetrodotoxin-sensitive Na+-conductance amplified steady-state depolarizing responses. A hyperpolarization-activated conductance, greatly decreased by extracellular Cs+ (3 mM) but resistant to Ba2+ (up to 1 mM), filtered the responses to hyperpolarizing current inputs. A depolarized membrane potential promoted repetitive firing in SBCs. This state, expected in pathophysiological conditions, would corrupt the temporal code.

4-Aminopyridine↗
PubMed2006

Review: cytological characteristics of commissural and tuberculo-ventral neurons in the rat dorsal cochlear nucleus.

The goal of the present review is to summarize the main ultrastructural and immunocytochemical characteristics for glycine and GABA in commissural (COM) and tuberculo-ventral neurons (TV) of the DCN. These neurons are localized in similar areas of the DCN multipolar but are connected to different targets. About 2/3rd of COM-neurons are large to bipolar neurons, mainly glycinergic, often GABA-ergic, with scarce ergastoplasm and axo-somatic boutons. About 1/3rd of COM-neurons are glycine and GABA-negative, and show little ergastoplasm and synaptic coverage. Occasional giant COM-neurons are glycine-positive and GABA-negative, and are covered with synaptic boutons. Other infrequent large neurons, rich in dense core vesicles, glycine- and GABA-negative, are most covered with boutons. TV-neurons are most glycinergic but 9% are glycine-negative. They have little ergastoplasm and a developed Golgi apparatus. Axo-somatic terminals are scarce and mainly contain flat and pleomorphic vesicles, glycine and sometimes GABA (inhibitory). TV-neurons receive a lower number of boutons than COM, which contain mainly flat-pleomorphic terminals. Putative COM-inhibitory boutons contact excitatory pyramidal and giant neurons (monosynaptic inhibition). Some putative inhibitory COM-terminals contact inhibitory cartwheel and tuberculo-ventral neurons. This indicates direct disinhibition and therefore excitation in the DCN (di-three-synaptic). Putative COM-mossy fibers reach the granule areas of the DCN, including unipolar brush cell dendrites, another possible excitatory commissural pathway.

Animals↗
PubMed1999

Breed differences in deafferentation-induced neuronal cell death and shrinkage in chick cochlear nucleus.

Removal of functional presynaptic input can result in a variety of changes in postsynaptic neurons in the central nervous system, including altered metabolism, changes in neuronal cell size, and even death of the postsynaptic cell. Age-dependent neuronal cell death and shrinkage has been documented in second order auditory neurons in the chick brainstem (nucleus magnocellularis, NM) following cochlea removal (Born and Rubel, 1985. J. Comp. Neurol. 231, 435-445). Here we examined whether the extent of neuronal cell death and shrinkage is also breed-dependent. We performed unilateral cochlea removal on both hatchling and adult birds of either a broiler breed (Arbor Acres Cross) or egg layer breed (Hy-Line, H and N) and killed birds one week later. Changes in neuronal cell number and cross sectional area were determined from Nissl-stained sections. We observed 25% neuronal cell loss and a 15-20% decrease in neuronal cross sectional area after cochlea removal in either broiler or egg layer hatchling birds. In adult birds, however, neuronal cell loss is breed-dependent. Adult egg layer birds lose an average of 37% of NM neurons after cochlea removal, while adult broiler birds show no cell loss. In both breeds of adult birds, cochlea removal results in a 20% decrease in neuronal cross sectional area. These results suggest that analysis of differences between breeds as well as ages of birds will prove fruitful in determining how afferent input controls neuronal survival and metabolism.

Animals↗
PubMed2000

Development of primary axosomatic endings in the anteroventral cochlear nucleus of mice.

The endbulb of Held is a large synaptic ending that arises from the myelinated auditory nerve fibers. Endbulbs exhibit an elaborate pattern of terminal branching and produce extensive contact with the postsynaptic cell body. These structural features appear to underlie the tight coupling between presynaptic activity and postsynaptic spike discharges. As a first step toward understanding the relationship between environmental sounds and the development of these neural elements, we examined the age-related changes in the morphology of endbulbs of Held in CBA/J mice, a strain known to retain good hearing throughout life. Neurobiotin was injected into the modiolus of the cochlea in CBA/J mice ranging in age from postnatal day 1 to 7 months. Light microscopic analyses suggest that endbulbs of the CBA/J mice develop from small bouton endings at birth into large, highly branched structures in adults. This increase in structural complexity occurs mostly during the second through eighth postnatal weeks, and general stages of development can be defined. In addition, we compared endbulb structure between adult CBA/J mice and adult shaker-2 mice (Myo15sh2/sh2) and heterozygous littermates (Myo15+/sh2). The shaker-2 mouse carries a mutated myosin 15 gene that results in congenital deafness, presumably due to abnormally short stereocilia in hair cell receptors. Neurobiotin was injected into the modiolus of adult CBA/J, Myo15sh2/sh2, and Myo15+/sh2 mice. Endbulbs of deaf adult Myo15sh2/sh2 mice exhibited a striking reduction in terminal branching compared with those of CBA/J and Myo15+/sh2 mice. Notably, the abnormal endbulbs of Myo15sh2/sh2 mice do not resemble immature endbulbs of normal-hearing mice, suggesting that deafness does not simply arrest development.

Aging↗
PubMed2000

Contributions of ion conductances to the onset responses of octopus cells in the ventral cochlear nucleus: simulation results.

The onset response pattern displayed by octopus cells has been attributed to intrinsic membrane properties, low membrane impedance, and/or synaptic inputs. Although the importance of a low membrane impedance generally is acknowledged as an essential component, views differ on the role that ion channels play in producing the onset response. In this study, we use a computer model to investigate the contributions of ion channels to the responses of octopus cells. Simulations using current ramps indicate that, during the "ramp-up" stage, the membrane depolarizes, activating a low-threshold K(+) channel, K(LT), which increases membrane conductance and dynamically increases the current required to evoke an action potential. As a result, the model is sensitive to the rate that membrane potential changes when initiating an action potential. Results obtained when experimentally recorded spike trains of auditory-nerve fibers served as model inputs (simulating acoustic stimulation) demonstrate that a model with K(LT) conductance as the dominant conductance produces realistic onset response patterns. Systematically replacing the K(LT) conductance by a h-type conductance (which corresponds to a hyperpolarization-activated inward rectifier current, I(h)) or by a leakage conductance reduces the model's sensitivity to rate of change in membrane potential, and the model's response to "acoustic stimulation" becomes more chopper-like. Increasing the h-type conductance while maintaining a large K(LT) conductance causes an increase in threshold to both current steps and acoustic stimulation but does not significantly affect the model's sensitivity to rate of change in membrane potential and the onset response pattern under acoustic stimulation. These findings support the idea that K(LT), which is activated during depolarization, is the primary membrane conductance determining the response properties of octopus cells, and its dynamic role cannot be provided by a static membrane conductance. On the other hand, I(h), which is activated during hyperpolarization, does not play a large role in the basic onset response pattern but may regulate response threshold through its contribution to the membrane conductance.

Acoustic Stimulation↗