Cells In Cochlear Nucleus Research Articles

Hyperpolarization-Activated Currents Regulate Excitability in Stellate Cells of the Mammalian Ventral Cochlear Nucleus

The differing biophysical properties of neurons the axons of which form the different pathways from the ventral cochlear nucleus (VCN) determine what acoustic information they can convey. T stellate cells, excitatory neurons the axons of which project locally and to the inferior colliculus, and D stellate cells, inhibitory neurons the axons of which project to the ipsi- and contralateral cochlear nuclei, fire tonically when they are depolarized, and, unlike other cell types in the VCN, their firing rates are sensitive to small changes in resting currents. In both types of neurons, the hyperpolarization-activated current (I(h)) reversed at -40 mV, was activated at voltages negative to -60 mV, and half-activated at approximately -88 mV; maximum hyperpolarization-activated conductances (g(h max)) were 19.1 +/- 2.3 nS in T and 30.3 +/- 2.6 nS in D stellate cells (means +/- SE). Activation and deactivation were slower in T than in D stellate cells. In both types of stellate cells, 50 microM 4(N-ethyl-N-phenylamino)1,2-dimethyl-6-(methylamino) pyridinium chloride (ZD7288) and 2 mM Cs(+) blocked a 6- to 10-fold greater conductance than the voltage-dependent g(h) determined from Boltzmann analyses at -62 mV. The voltage-insensitive, ZD7288-sensitive conductance was proportional to g(h max) and g(input). 8-Br-cAMP shifted the voltage dependence of I(h) in the depolarizing direction, increased the rate of activation, and slowed its deactivation in both T and D stellate cells. Reduction in temperature did not change the voltage dependence but reduced the maximal g(h) with a Q(10) of 1.3 and slowed the kinetics with a Q(10) of 3.3.

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.

Projections from auditory cortex contact cells in the cochlear nucleus that project to the inferior colliculus

Anterograde and retrograde tracing techniques were combined to determine whether auditory cortical axons contact cells in the cochlear nucleus that project to the inferior colliculus. FluoroRuby or fluorescein dextran was injected into auditory cortex to label cortical axons by anterograde transport. Different fluorescent tracers (Fast Blue, FluoroGold, FluoroRuby or fluorescein dextran) were injected into one or both inferior colliculi to label cells in the cochlear nucleus. After 12–15 days, the brain was processed for fluorescence microscopy and the cochlear nuclei were examined for apparent contacts between cortical axons and retrogradely labeled cochlear nucleus cells. The results suggest that axons from the ipsilateral or contralateral cortex contact fusiform and giant cells in the dorsal cochlear nucleus and multipolar cells in the ventral cochlear nucleus that project directly to the inferior colliculus. The contacts occur on cell bodies and dendrites. The target cells in the cochlear nucleus include cells that project ipsilaterally, contralaterally or bilaterally to the inferior colliculus. The results suggest that auditory cortex is in a position to exert direct effects on the monaural pathways that ascend from the cochlear nucleus.

Regulation of the timing of MNTB neurons by short-term and long-term modulation of potassium channels

The firing patterns of neurons in central auditory pathways encode specific features of sound stimuli, such as frequency, intensity and localization in space. The generation of the appropriate pattern depends, to a major extent, on the properties of the voltage-dependent potassium channels in these neurons. The mammalian auditory pathways that compute the direction of a sound source are located in the brainstem and include the connection from bushy cells in the anteroventral cochlear nucleus (AVCN) to the principal neurons of the medial nucleus of the trapezoid body (MNTB). To preserve the fidelity of timing of action potentials that is required for sound localization, these neurons express several types of potassium channels, including the Kv3 and Kv1 families of voltage-dependent channels and the Slick and Slack sodium-dependent channels. These channels determine the pattern of action potentials and the amount of neurotransmitter released during repeated stimulation. The amplitude of currents carried by one of these channels, the Kv3.1b channel, is regulated in the short term by protein phosphorylation, and in the long term, by changes in gene expression, such that the intrinsic excitability of the neurons is constantly being regulated by the ambient auditory environment.

Temperature Affects Voltage-Sensitive Conductances Differentially in Octopus Cells of the Mammalian Cochlear Nucleus

Temperature is an important physiological variable the influence of which on macroscopic electrophysiological measurements in slices is not well documented. We show that each of three voltage-sensitive conductances of octopus cells of the mammalian ventral cochlear nucleus (VCN) is affected differently by changes in temperature. As expected, the kinetics of the currents were faster at higher than at lower temperature. Where they could be measured, time constants of activation, deactivation, and inactivation had Q10 values between 1.8 and 4.6. The magnitude of the peak conductances was differentially affected by temperature. While the peak magnitude of the high-voltage-activated K+ conductance, g(KH), was unaffected by changes in temperature, the peak of the low-voltage-activated K+ conductance, g(KL), was reduced by half when the temperature was lowered from 33 to 23 degrees C (Q10 = 2). Changing the temperature changed the kinetics and the magnitude of the hyperpolarization-activated mixed cation conductance, g(h), but the changes in magnitude were transient. The voltage sensitivity of the three conductances was unaffected by temperature. The action of temperature on these conductances is reflected in the resting potentials and in the shapes of action potentials.

Presence and distribution of three calcium binding proteins in projection neurons of the adult rat cochlear nucleus

The presence and distribution of three cytoplasmic calcium binding proteins, calbindin, calretinin, and parvalbumin, have been investigated in the projection neurons of the cochlear nucleus complex in adult rats by using immunohistochemistry in free-floating slices. Identification of the individual cell types was carried out on the basis of their intranuclear localization, morphological characteristics, and (in the cases of pyramidal and bushy neurons) by retrograde labeling with rhodamine-dextran. The most important findings were confirmed by using confocal microscopy. The data obtained in these experiments are the first to demonstrate the presence of parvalbumin in pyramidal neurons and globular and spherical bushy cells of rat cochlear nucleus, whereas octopus and giant cells did not show positivity for parvalbumin. Calretinin was not present in either Purkinje-like cells or giant neurons. According to the double immunolabeling co-localization experiments, the pyramidal neurons, Purkinje-like cells, globular bushy cells, and octopus cells express two different calcium binding proteins in their cytoplasm (although in different combinations) whereas giant cells and spherical bushy cells contain solely calbindin and parvalbumin, respectively. The presence of calretinin in globular bushy cells provides a tool for distinguishing them from spherical bushy cells. The immunolabeling of the fibers and axonal endings of the acoustic nerve in the ventral part of the cochlear nucleus indicated that these structures are also parvalbumin positive. It is concluded that the heterogenous cell composition of the cochlear nucleus is accompanied by a rather complex expression pattern of the cytoplasmic calcium binding proteins.

Voltage-gated and background K + channel subunits expressed by the bushy cells of the rat cochlear nucleus

Bushy cells of the ventral cochlear nucleus produce a single, short latency action potential at the beginning of long depolarisations. In the present work an immunochemical survey was performed to detect the presence of K + channel subunits which may contribute to the specific membrane properties of the bushy cells. The immunocytochemical experiments conducted on enzymatically isolated bushy cells indicated positive immunolabelling for several subunits known to be responsible for the genesis of rapidly inactivating K + currents. Bushy cells showed strong expression of Kv3.4, 4.2 and 4.3 subunits, with the lack of Kv1.4 specific immunoreaction. The Kv3.4-specific immunoreaction had a specific, patchy appearance. Bushy cells also expressed various members of the Kv1 subunit family, most notably Kv1.1, 1.2, 1.3 and 1.6. Weak positivity could be observed for Kv3.2 subunits. The positive immunolabelling for Kv3.4, Kv4.2 and Kv4.3 was confirmed in free-floating tissue slices. Voltage-clamp experiments performed on positively identified bushy cells in brain slices corroborated the presence and activity of Kv3.4 and Kv4.2/4.3 containing K + channels. Bushy cell showed strong immunopositivity for TASK-1 channels too. The results presented in this work indicate that bushy cells possess several types of voltage-gated K + channel subunits whose activity may contribute to the membrane properties and firing characteristics of these neurones.

Onset neurones in the anteroventral cochlear nucleus project to the dorsal cochlear nucleus.

Considerable circumstantial evidence suggests that cells in the ventral cochlear nucleus, that respond predominantly to the onset of pure tone bursts, have a stellate morphology and project, among other places, to the dorsal cochlear nucleus. The characteristics of such cells make them leading candidates for providing the so-called "wideband inhibitory input" which is an essential part of the processing machinery of the dorsal cochlear nucleus. Here we use juxtacellular labeling with biocytin to demonstrate directly that large stellate cells, with onset responses, terminate profusely in the dorsal cochlear nucleus. They also provide widespread local innervation of the anteroventral cochlear nucleus and a small innervation of the posteroventral cochlear nucleus. In addition, some onset cells project to the contralateral dorsal cochlear nucleus.

An immunogold investigation of the distribution of GABA and glycine in nerve terminals on the somata of spherical bushy cells in the anteroventral cochlear nucleus of guinea pig.

Spherical bushy neurons in the anteroventral cochlear nucleus receive glutamatergic primary terminals from the cochlear nerve and terminals of noncochlear (i.e. nonprimary) origin, many of which colocalize gamma-aminobutyric acid (GABA) and glycine. Here the relationship between GABA and glycine in these terminals has been investigated using postembedding immunogold labelling. A significant negative correlation was found between the density of terminal labelling for GABA and for glycine in four guinea pigs. Terminals could be divided into three categories, GABA-only, glycine-only, or colocalizing depending on whether they had a significantly higher labelling density for either amino acid than the primary terminals. The overall labelling density in all four animals was significantly greater for GABA in GABA-only terminals than colocalizing ones but similar for glycine in both. Within the terminals, the labelling density over synaptic vesicles, nonvesicular regions of cytoplasm and mitochondria was also investigated. No significant difference was detected in the labelling density of vesicles compared with nonvesicular regions for either amino acid. However, a significant difference was found between the overall labelling density over mitochondria and nonvesicular regions for both. There was also significantly more mitochondrial GABA labelling in GABA-only terminals compared to colocalizing terminals but mitochondrial glycine labelling was similar in glycine-only and colocalizing terminals. Thus the level of GABA is higher in single than in colocalizing terminals, particularly in the mitochondria, but similar for glycine in both. It is possible therefore that the presence of glycine affects the level of GABA in the nonprimary terminals but that the presence of GABA does not affect the level of glycine.

HCN channels contribute to the intrinsic activity of cochlear pyramidal cells.

A hyperpolarization-activated current recorded from the pyramidal cells of the dorsal cochlear nucleus was investigated in the present study by using 150- to 200-microm-thick brain slices prepared from 6- to 14-day-old Wistar rats. The pyramidal neurones exhibited a slowly activating inward current on hyperpolarization. The reversal potential of this component was -32 +/- 3 mV (mean +/- SE, n = 6), while its half-activation voltage was -99 +/- 1 mV with a slope factor of 10.9 +/- 0.4 mV (n = 27). This current was highly sensitive to the extracellular application of both 1 mM Cs+ and 10 microM ZD7288. The electrophysiological properties and the pharmacological sensitivity of this current indicated that it corresponded to a hyperpolarization-activated non-specific cationic current (Ih). Our experiments showed that there was a correlation between the availability of the h-current and the spontaneous activity of the pyramidal cells, suggesting that this conductance acts as a pacemaker current in these neurones. Immunocytochemical experiments were also conducted on freshly isolated pyramidal cells to demonstrate the possible subunit composition of the channels responsible for the genesis of the pyramidal h-current. These investigations indicated the presence of HCN1, HCN2 and HCN4 subunits in the pyramidal cells.

Discharge properties of identified cochlear nucleus neurons and auditory nerve fibers in response to repetitive electrical stimulation of the auditory nerve.

Using the in vitro isolated whole brain preparation of the guinea pig maintained at 29 degrees C, we intracellularly recorded and stained cochlear nucleus (CN) neurons and auditory nerve (AN) fibers. Discharge properties of CN cells and AN axons were tested in response to 50-ms trains of electrical pulses delivered to the AN at rates ranging from 100 to 1000 pulses per second (pps). At low stimulation rates (200-300 pps), the discharges of AN fibers and a large proportion of principal cells (bushy, octopus, stellate) in the ventral cochlear nucleus (VCN) followed with high probability each pulse in the train, resulting in synchronization of discharges within large populations of AN fibers and CN cells. In contrast, at high stimulation rates (500 pps and higher), AN fibers and many VCN cells exhibited "primary-like", "onset" and some other discharge patterns resembling those produced by natural sound stimuli. Unlike cells in the VCN, principal cells (pyramidal, giant) of the dorsal CN did not follow the stimulating pulses even at low rates. Instead, they often showed "pauser" and "build-up" patterns of activity, characteristic for these cells in conditions of normal hearing. We hypothesize that, at low stimulation rates, the response behavior of AN fibers and VCN cells is different from the patterns of neuronal activity related to normal auditory processing, whereas high stimulation rates produce more physiologically meaningful discharge patterns. The observed differences in discharge properties of AN fibers and CN cells at different stimulation rates can contribute to significant advantages of high- versus low-rate electrical stimulation of the AN used for coding sounds in modern cochlear implants.

Ultrastructural examination of the somatic innervation of ventrotubercular cells in the rat.

Ventrotubercular cells are multipolar cells in the ventral cochlear nucleus (VCN) that project a collateral axon to the ipsilateral dorsal cochlear nucleus (DCN). These cells are thought to be involved in sensitizing DCN output neurons to spectral shapes that represent the location of a sound source in space. The present report focused on the neuronal composition of this pathway. Intracellular labeling studies in cats and mice have described two types of ventrotubercular cells (Smith and Rhode [1989] J Comp Neurol. 282:595-626; Oertel et al. [1990] J Comp Neurol. 295:136-154). In cats, one difference between the two classes is that type I multipolar neurons have fewer than 35% of their somata apposed by terminals, whereas type II cells have greater than 70% apposition values. Intracellular recordings from single cells, however, are difficult and thus limit the yield of data. We investigated whether a two-component description of the ventrotubercular pathway was representative of a larger population. This issue was addressed by retrogradely labeling ventrotubercular neurons with an extracellular injection of biotinylated dextran amine into the DCN of rats. These injections labeled many VCN neurons, thus providing a more complete view of the pathway than previous studies. Thirty-eight labeled cells were selected for electron microscopic analysis with respect to their location, cell body size, and ultrastructural morphology. We observed labeled type I and type II neurons, but unlike ventrotubercular cells in cats, many of these neurons in rats (17 of 38 cells) had appositions between 35% and 70%. On the basis of this analysis, a third class of ventrotubercular cell, called the adendritic neuron, was revealed. Adendritic neurons have small somata with many filopodial appendages, no observable dendrites, and high percentage of terminal appositions (>80%). The results demonstrated that the ventrotubercular pathway in the rat is diverse.

Morphology of neurons cultured from subdivisions of the mouse cochlear nucleus.

This study was designed to characterize the dendritic organization of cochlear nucleus (CN) cells grown in primary cell culture and to assess differences among cultures grown from different regions of CN. Cultures were prepared from postnatal mice and processed using microtubule-associated protein 2 (MAP2) or gamma-aminobutyric acid (GABA) immunohistochemistry. CN neurons were successfully cultured from preparations grown from either the anteroventral subdivision of the nucleus (AVCN), the posterior region [posteroventral (PVCN) and dorsal (DCN) subnuclei], or the whole CN, although the cultured neurons did not exhibit complex dendritic patterns characteristic of CN neurons in vivo. Neurons cultured from the entire nucleus exhibited an increased rate of survival compared to those cultured from either the anterior or posterior regions, although similar types of cells were observed in all preparations. The majority of cultured CN neurons were GABA-positive and had soma areas that were similar to the areas of immature GABAergic neurons measured in CN sections. Small cells (soma areas <or=60 microm(2)) with one to three symmetrically organized dendrites and large non-GABAergic cells (>or=120 microm(2)) were also present in significant numbers. Overall, CN cultures consisted of a heterogeneous population of neurons that had less elaborate dendritic organizations than cells of corresponding size that have been described in adult animals in vivo.

Interaction of Excitation and Inhibition in Anteroventral Cochlear Nucleus Neurons That Receive Large Endbulb Synaptic Endings

Spherical bushy cells (SBCs) of the anteroventral cochlear nucleus (AVCN) receive their main excitatory input from auditory nerve fibers (ANFs) through large synapses, endbulbs of Held. These cells are also the target of inhibitory inputs whose function is not well understood. The present study examines the role of inhibition in the encoding of low-frequency sounds in the gerbil's AVCN. The presynaptic action potentials of endbulb terminals and postsynaptic action potentials of SBCs were monitored simultaneously in extracellular single-unit recordings in vivo. An input-output analysis of presynaptic and postsynaptic activity was performed for both spontaneous and acoustically driven activity. Two-tone stimulation and neuropharmacological experiments allowed the effects of neuronal inhibition and cochlear suppression on SBC activity to be distinguished. Ninety-one percent of SBCs showed significant neuronal inhibition. Inhibitory sidebands enclosed the high- or low-frequency, or both, sides of the excitatory areas of these units; this was reflected as a presynaptic to postsynaptic increase in frequency selectivity of up to one octave. Inhibition also affected the level-dependent responses at the characteristic frequency. Although in all units the presynaptic recordings showed monotonic rate-level functions, this was the case in only half of the postsynaptic recordings. In the other half of SBCs, postsynaptic inhibitory areas overlapped the excitatory areas, resulting in nonmonotonic rate-level functions. The results demonstrate that the sound-evoked spike activity of SBCs reflects the integration of acoustically driven excitatory and inhibitory input. The inhibition specifically affects the processing of the spectral, temporal, and intensity cues of acoustic signals.

Temporal coding of the pitch of complex sounds by presumed multipolar cells in the ventral cochlear nucleus

Extensive studies of the encoding of fundamental frequency ( f 0) in the auditory nerve indicate that f 0 can be represented by either the timing of the neuronal discharges or the mean discharge rate as a function of characteristic frequency. It is therefore of considerable interest to examine what happens to this information at the next level of the auditory pathway, the cochlear nucleus. Both physiologically and anatomically the cochlear nucleus is considerably more heterogenous than the auditory nerve. There are two main cell types in the ventral division of the cochlear nucleus; bushy and multipolar. Bushy cells give rise to primary-like responses whereas multipolar cells may be characterised by either onset or chopper type responses. Physiological studies have suggested that onset and chopper units may be good at representing the f 0 of complex sounds in their temporal discharge properties. However, in these studies the pitch-producing sounds were usually characterised by highly modulated envelopes and it was not possible to tell if the units were simply responding to the modulation or the temporal fine structure. In this paper we examine the ability of onset and chopper units to encode the f 0 of complex sounds when the modulation cue has been greatly reduced. These stimuli were steady-state vowels in the presence of background noise, and iterated rippled noise (IRN). The response of onset units to the vowel f 0 in the presence of background noise was varied but many still maintained a strong response. In contrast, the majority of chopper units showed a greater reduction in their response to vowel f 0 in the presence of background noise. In keeping with the vowel study, the responses of both types of unit to the delay of the IRN was reduced in comparison with their response to more highly modulated stimuli. Increasing anatomical, pharmacological and physiological evidence would seem to argue against onset units playing a direct role in pitch perception. However, some units, identified as sustained choppers, may be able to represent the pitch of complex sounds in their temporal discharges.

Intracellularly labeled fusiform cells in dorsal cochlear nucleus of the gerbil. I. Physiological response properties.

Fusiform cells in the dorsal cochlear nucleus (DCN) of barbiturate-anesthetized Mongolian gerbils were characterized physiologically and labeled with neurobiotin. This report is based on 17 fusiform cells for which there was reasonable confidence in the association between physiological data and recovered anatomy. The qualitative morphology of these cells was no different from that reported in previous studies. The acoustic response properties were generally consistent with those described in the barbiturate-anesthetized cat. Most responses were of the pauser or buildup type, but a dependence on stimulus frequency and intensity was observed. Stimulus-evoked sustained depolarizations and large, long-lasting afterhyperpolarizations were common membrane potential features. The cells in this study showed a greater tendency to discharge regularly than did those of the cat, likely as a result of the longer interstimulus interval used. Barbiturate anesthesia appears to mask an interspecies difference in DCN physiology that is apparent in unanesthetized, decerebrate preparations. The response of these fusiform cells to a depolarizing current pulse could be altered by the presence of a hyperpolarizing prepulse. Buildup, pauser, and chopper patterns could each be created using appropriate combinations of hyperpolarizing and depolarizing pulse amplitudes. Thus the adult gerbil appears to express the inactivating potassium conductance previously shown to affect fusiform cell firing patterns in vitro. The results further demonstrate that the effects of these potassium currents are readily observed in vivo. Finally, the fusiform cells in this study were quite variable with respect to a number of response properties, including the resting potential, input resistance, spontaneous activity, relative noise index, normalized tone slope, and regularity histogram shape. This diversity likely results from cell-to-cell variations in the balance of activity within the relatively complex network to which the fusiform cells belong, although effects of impalement may contribute to the extent of the diversity.

Octopus cells of the mammalian ventral cochlear nucleus sense the rate of depolarization.

Whole cell patch recordings in slices show that the probability of firing of action potentials in octopus cells of the ventral cochlear nucleus depends on the dynamic properties of depolarization. Octopus cells fired only when the rate of rise of a depolarization exceeded a threshold value that varied between 5 and 15 mV/ms among cells. The threshold rate of rise was independent of whether depolarizations were evoked synaptically or by the intracellular injection of current. Previous work showed that octopus cells are contacted by many auditory nerve fibers, each providing less than 1-mV depolarization. Summation of synaptic input from multiple fibers is required for an octopus cell to reach threshold. In firing only when synaptic depolarization exceeds a threshold rate, octopus cells fire selectively when synaptic input is sufficiently large and synchronized for the small, brief unitary excitatory postsynaptic potentials (EPSPs) to sum to produce a rapidly rising depolarization. The sensitivity to rate of depolarization is governed by a low-threshold, alpha-dendrotoxin-sensitive potassium conductance (g(KL)). This conductance also shapes the peaks of action potentials, contributing to the precision in their timing. Firing in neighboring T stellate cells depends much less strongly on the rate of rise. They lack strong alpha-dendrotoxin-sensitive conductances. Octopus cells appear to be specialized to detect synchronization in the activation of groups of auditory nerve fibers, a common pattern in responses to natural sounds, and convey its occurrence with temporal precision.

The effects of congenital deafness on auditory nerve synapses: Type I and Type II multipolar cells in the anteroventral cochlear nucleus of cats.

Sensory deprivation has been shown to exert detrimental effects on the structure and function of central sensory systems. Congenital deafness represents an extreme form of auditory deprivation, and in the adult white cat, synapses between auditory nerve endings and resident cells of the anteroventral cochlear nucleus exhibited abnormal structure. Endbulbs of Held were reduced in branching and displayed striking hypertrophy of their postsynaptic densities. So-called modified endbulbs showed no change in branching complexity but did exhibit hypertrophy of their postsynaptic densities. These differential pre- and postsynaptic effects prompted the question of how deafness might affect other primary endings and synapses. Thus, we studied type I and type II multipolar cells that receive bouton endings from auditory nerve fibers. Type I multipolar cells project to the contralateral inferior colliculus and have relatively few axosomatic endings; type II multipolar cells project to the contralateral cochlear nucleus and have many axosomatic endings. Compared with normal-hearing cats, bouton endings of congenitally deaf cats were smaller but there was no difference in synaptic vesicle density or size of postsynaptic densities. These data reveal that different classes of primary endings and second-order neurons exhibit different degrees of synaptic anomalies to deafness.

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

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

Commissural glycinergic inhibition of bushy and stellate cells in the anteroventral cochlear nucleus

Synaptic inputs from one cochlear nucleus (CN) to the other can play an important role in modulating the activity of CN neurons. Using the isolated whole brain preparation of the guinea pig, we tested the effects of electrical stimulation of the contralateral auditory nerve (AN) on intracellularly recorded and stained neurons of the anteroventral cochlear nucleus. Stimulation of the contralateral AN evoked only inhibitory postsynaptic potentials (IPSPs) in 63% of recorded neurons, including bushy and stellate cells. The latency of most IPSPs (88%) was in the range 3.3-7.6 ms, consistent with mono- and disynaptic transmission from the contralateral CN. The IPSPs had an average amplitude of 2.6 +/- 1.9 mV and were blocked by strychnine suggesting their glycinergic nature. These data, together with our similar findings in other CN subdivisions, indicate that principal cells of the CN contribute to binaural interactions at earliest stages of acoustic processing.