On the functional relationship between the dorsal and ventral divisions of the cochlear nucleus of the cat

Synaptic reorganization in the adult rat's ventral cochlear nucleus following its total sensory deafferentation

Ablation of a cochlea causes total sensory deafferentation of the cochlear nucleus in the brainstem, providing a model to investigate nervous degeneration and formation of new synaptic contacts in the adult brain. In a quantitative electron microscopical study on the plasticity of the central auditory system of the Wistar rat, we first determined what fraction of the total number of synaptic contact zones (SCZs) in the anteroventral cochlear nucleus (AVCN) is attributable to primary sensory innervation and how many synapses remain after total unilateral cochlear ablation. Second, we attempted to identify the potential for a deafferentation-dependent synaptogenesis. SCZs were ultrastructurally identified before and after deafferentation in tissue treated for ethanolic phosphotungstic acid (EPTA) staining. This was combined with pre-embedding immunocytochemistry for gephyrin identifying inhibitory SCZs, the growth-associated protein GAP-43, glutamate, and choline acetyltransferase. A stereological analysis of EPTA stained sections revealed 1.11±0.09 (S.E.M.)×10(9) SCZs per mm(3) of AVCN tissue. Within 7 days of deafferentation, this number was down by 46%. Excitatory and inhibitory synapses were differentially affected on the side of deafferentation. Excitatory synapses were quickly reduced and then began to increase in number again, necessarily being complemented from sources other than cochlear neurons, while inhibitory synapses were reduced more slowly and continuously. The result was a transient rise of the relative fraction of inhibitory synapses with a decline below original levels thereafter. Synaptogenesis was inferred by the emergence of morphologically immature SCZs that were consistently associated with GAP-43 immunoreactivity. SCZs of this type were estimated to make up a fraction of close to 30% of the total synaptic population present by ten weeks after sensory deafferentation. In conclusion, there appears to be a substantial potential for network reorganization and synaptogenesis in the auditory brainstem after loss of hearing, even in the adult brain.

Evidence of key tinnitus-related brain regions documented by a unique combination of manganese-enhanced MRI and acoustic startle reflex testing

Animal models continue to improve our understanding of tinnitus pathogenesis and aid in development of new treatments. However, there are no diagnostic biomarkers for tinnitus-related pathophysiology for use in awake, freely moving animals. To address this disparity, two complementary methods were combined to examine reliable tinnitus models (rats repeatedly administered salicylate or exposed to a single noise event): inhibition of acoustic startle and manganese-enhanced MRI. Salicylate-induced tinnitus resulted in wide spread supernormal manganese uptake compared to noise-induced tinnitus. Neither model demonstrated significant differences in the auditory cortex. Only in the dorsal cortex of the inferior colliculus (DCIC) did both models exhibit supernormal uptake. Therefore, abnormal membrane depolarization in the DCIC appears to be important in tinnitus-mediated activity. Our results provide the foundation for future studies correlating the severity and longevity of tinnitus with hearing loss and neuronal activity in specific brain regions and tools for evaluating treatment efficacy across paradigms.

A 3-d 160-site microelectrode array for cochlear nucleus mapping

A 3-D application-specific microelectrode array has been developed for physiological studies in guinea pig cochlear nucleus (CN). The batch-fabricated silicon probes contain integrated parylene cables and use a boron etch-stop to define 15μm-thick shanks and limit tissue displacement. Targeting the ventral (three probes) and dorsal (two probes) subnuclei, the custom four-shank 32-site probes are combined in a slotted block platform having a 1.18-mm (2) footprint. The device has permitted, for the first time, high-density 3-D in vivo studies of ventral CN to dorsal CN connections, stimulating with 1000 μm (2) sites in one subnucleus while recording with 177 μm (2) sites in the other. Through these experiments, it has demonstrated the efficacy of bimodal silicon arrays to better understand the central nervous system at the circuit level. The 160 electrode sites also provide a high-density neural interface, which is an essential aspect of auditory prosthesis prototypes.

Auditory brainstem circuits that mediate the middle ear muscle reflex

The middle ear muscle (MEM) reflex is one of two major descending systems to the auditory periphery. There are two middle ear muscles (MEMs): the stapedius and the tensor tympani. In man, the stapedius contracts in response to intense low frequency acoustic stimuli, exerting forces perpendicular to the stapes superstructure, increasing middle ear impedance and attenuating the intensity of sound energy reaching the inner ear (cochlea). The tensor tympani is believed to contract in response to self-generated noise (chewing, swallowing) and non-auditory stimuli. The MEM reflex pathways begin with sound presented to the ear. Transduction of sound occurs in the cochlea, resulting in an action potential that is transmitted along the auditory nerve to the cochlear nucleus in the brainstem (the first relay station for all ascending sound information originating in the ear). Unknown interneurons in the ventral cochlear nucleus project either directly or indirectly to MEM motoneurons located elsewhere in the brainstem. Motoneurons provide efferent innervation to the MEMs. Although the ascending and descending limbs of these reflex pathways have been well characterized, the identity of the reflex interneurons is not known, as are the source of modulatory inputs to these pathways. The aim of this article is to (a) provide an overview of MEM reflex anatomy and physiology, (b) present new data on MEM reflex anatomy and physiology from our laboratory and others, and (c) describe the clinical implications of our research.

Inhibitory neurotransmission, plasticity and aging in the mammalian central auditory system

Aging and acoustic trauma may result in partial peripheral deafferentation in the central auditory pathway of the mammalian brain. In accord with homeostatic plasticity, loss of sensory input results in a change in pre- and postsynaptic GABAergic and glycinergic inhibitory neurotransmission. As seen in development, age-related changes may be activity dependent. Age-related presynaptic changes in the cochlear nucleus include reduced glycine levels, while in the auditory midbrain and cortex, GABA synthesis and release are altered. Presumably, in response to age-related decreases in presynaptic release of inhibitory neurotransmitters, there are age-related postsynaptic subunit changes in the composition of the glycine (GlyR) and GABA(A) (GABA(A)R) receptors. Age-related changes in the subunit makeup of inhibitory pentameric receptor constructs result in altered pharmacological and physiological responses consistent with a net down-regulation of functional inhibition. Age-related functional changes associated with glycine neurotransmission in dorsal cochlear nucleus (DCN) include altered intensity and temporal coding by DCN projection neurons. Loss of synaptic inhibition in the superior olivary complex (SOC) and the inferior colliculus (IC) likely affect the ability of aged animals to localize sounds in their natural environment. Age-related postsynaptic GABA(A)R changes in IC and primary auditory cortex (A1) involve changes in the subunit makeup of GABA(A)Rs. In turn, these changes cause age-related changes in the pharmacology and response properties of neurons in IC and A1 circuits, which collectively may affect temporal processing and response reliability. Findings of age-related inhibitory changes within mammalian auditory circuits are similar to age and deafferentation plasticity changes observed in other sensory systems. Although few studies have examined sensory aging in the wild, these age-related changes would likely compromise an animal's ability to avoid predation or to be a successful predator in their natural environment.

Auditory nerve inputs to cochlear nucleus neurons studied with cross-correlation

The strength of synapses between auditory nerve (AN) fibers and ventral cochlear nucleus (VCN) neurons is an important factor in determining the nature of neural integration in VCN neurons of different response types. Synaptic strength was analyzed using cross-correlation of spike trains recorded simultaneously from an AN fiber and a VCN neuron in anesthetized cats. VCN neurons were classified as chopper, primarylike, and onset using previously defined criteria, although onset neurons usually were not analyzed because of their low discharge rates. The correlograms showed an excitatory peak (EP), consistent with monosynaptic excitation, in AN-VCN pairs with similar best frequencies (49% 24/49 of pairs with best frequencies within +/-5%). Chopper and primarylike neurons showed similar EPs, except that the primarylike neurons had shorter latencies and shorter-duration EPs. Large EPs consistent with end bulb terminals on spherical bushy cells were not observed, probably because of the low probability of recording from one. The small EPs observed in primarylike neurons, presumably spherical bushy cells, could be derived from small terminals that accompany end bulbs on these cells. EPs on chopper or primarylike-with-notch neurons were consistent with the smaller synaptic terminals on multipolar and globular bushy cells. Unexpectedly, EPs were observed only at sound levels within about 20 dB of threshold, showing that VCN responses to steady tones shift from a 1:1 relationship between AN and VCN spikes at low sound levels to a more autonomous mode of firing at high levels. In the high level mode, the pattern of output spikes seems to be determined by the properties of the postsynaptic spike generator rather than the input spike patterns. The EP amplitudes did not change significantly when the presynaptic spike was preceded by either a short or long interspike interval, suggesting that synaptic depression and facilitation have little effect under the conditions studied here.

Inferior colliculus responses to multichannel microstimulation of the ventral cochlear nucleus: implications for auditory brain stem implants

Multichannel techniques were used to assess the frequency specificity of activation in the central nucleus of the inferior colliculus (CIC) produced by electrical stimulation of localized regions within the ventral cochlear nucleus (VCN). Data were recorded in response to pure tones from 141 and 193 multiunit clusters in the rat VCN and the CIC, respectively. Of 141 VCN sites, 126 were individually stimulated while recording responses in the CIC. A variety of CIC response types were seen with an increase in both electrical and acoustic stimulation levels. The majority of sites exhibited monotonic rate-level types acoustically, whereas spike rate saturation was achieved predominantly with electrical stimulation. In 20.6% of the 364 characteristic frequency aligned VCN-CIC pairs, the CIC sites did not respond to stimulation. In 26% of the 193 CIC sites, a high correlation was observed between acoustic tuning and electrical tuning obtained through VCN stimulation. A high degree of frequency specificity was found in 58% of the 118 lowest threshold VCN-CIC pairs. This was dependent on electrode placement within the VCN because a higher degree of frequency specificity was achieved with stimulation of medial, central, and posterolateral VCN regions than more anterolateral regions. Broadness of acoustic tuning in the CIC played a role in frequency-specific activation. Narrowly tuned CIC sites showed the lowest degree of frequency specificity on stimulation of the anterolateral VCN regions. These data provide significant implications for auditory brain stem implant electrode placement, current localization, power requirements, and facilitation of information transfer to higher brain centers.

Auditory responses in the cochlear nucleus of awake mustached bats: precursors to spectral integration in the auditory midbrain

In the cochlear nucleus (CN) of awake mustached bats, single- and two-tone stimuli were used to examine how responses in major CN subdivisions contribute to spectrotemporal integrative features in the inferior colliculus (IC). Across CN subdivisions, the proportional representation of frequencies differed. A striking result was the substantial number of units tuned to frequencies <23 kHz. Across frequency bands, temporal response patterns, latency, and spontaneous discharge differed. For example, the 23- to 30-kHz representation, which comprises the fundamental of the sonar call, had an unusually high proportion of units with onset responses (39%) and low spontaneous rates (53%). Units tuned to 58-59 kHz, corresponding to the sharply tuned cochlear resonance, had slightly but significantly longer latencies than other bands. In units tuned to frequencies >30 kHz, 31% displayed a secondary excitatory peak, usually between 10 and 22 kHz. The secondary peak may originate in cochlear mechanisms for some units, but in others it may result from convergent input onto CN neurons. In 20% of units tested with two-tone stimuli, suppression of best frequency (BF) responses was tuned at least an octave below BF. These properties may underlie similar IC responses. However, other forms of spectral interaction present in IC were absent in CN: we found no facilitatory combination-sensitive interactions and very few combination-sensitive inhibitory interactions of the dominant IC type in which inhibition was tuned to 23-30 kHz. Such interactions arise above CN. Distinct forms of spectral integration thus originate at different levels of the ascending auditory pathway.

Effects of carboplatin on amino acid chemistry in chinchilla cochlear nucleus

Carboplatin, a drug widely used against solid head and neck tumors, selectively destroys cochlear inner hair cells and type I auditory nerve fibers in chinchilla. This should affect neurotransmitter chemistry, involving amino acids, where the type I auditory nerve fibers terminate in the cochlear nucleus. Using microdissection combined with high-performance liquid chromatography, amino acid concentrations were mapped in the cochlear nuclei of chinchillas injected intraperitoneally 6-8 weeks earlier with 100 mg/kg carboplatin and in those of control animals. Glutamate concentrations were 23% lower in the anteroventral cochlear nucleus (AVCN) and 40% lower in the posteroventral cochlear nucleus (PVCN) of carboplatin-injected chinchillas as compared to controls, while aspartate concentrations were 18% lower in AVCN and 27% lower in PVCN. Using a fluorometric assay, activities of glutaminase, an enzyme which catalyzes glutamate synthesis, were 30% lower in AVCN and 38% lower in PVCN of carboplatin-injected chinchillas. Concentrations of glutamine, gamma-aminobutyrate, and glycine were also lower in some ventral and dorsal cochlear nucleus regions of treated animals. These changes probably result mainly from both primary and later effects of reduced type I auditory nerve fiber input to the cochlear nucleus.

Intracellular responses of onset chopper neurons in the ventral cochlear nucleus to tones: evidence for dual-component processing

Intracellular responses of onset chopper neurons in the ventral cochlear nucleus to tones: evidence for dual-component processing. The ventral cochlear nucleus (VCN) contains a heterogeneous collection of cell types reflecting the multiple processing tasks undertaken by this nucleus. This in vivo study in the rat used intracellular recordings and dye filling to examine membrane potential changes and firing characteristics of onset chopper (OC) neurons to acoustic stimulation (50 ms pure tones, 5 ms r/f time). Stable impalements were made from 15 OC neurons, 7 identified as multipolar cells. Neurons responded to characteristic frequency (CF) tones with sustained depolarization below spike threshold. With increasing stimulus intensity, the depolarization during the initial 10 ms of the response became peaked, and with further increases in intensity the peak became narrower. Onset spikes were generated during this initial depolarization. Tones presented below CF resulted in a broadening of this initial depolarizing component with high stimulus intensities required to initiate onset spikes. This initial component was followed by a sustained depolarizing component lasting until stimulus cessation. The amplitude of the sustained depolarizing component was greatest when frequencies were presented at high intensities below CF resulting in increased action potential firing during this period when compared with comparable high intensities at CF. During the presentation of tones at or above the high-frequency edge of a cell's response area, hyperpolarization was evident during the sustained component. The presence of hyperpolarization and the differences seen in the level of sustained depolarization during CF and off CF tones suggests that changes in membrane responsiveness between the initial and sustained components may be attributed to polysynaptic inhibitory mechanisms. The dual-component processing resulting from convergent auditory nerve excitation and polysynaptic inhibition enables OC neurons to respond in a unique fashion to intensity and frequency features contained within an acoustic stimulus.

Intracellular responses of the rat anteroventral cochlear nucleus to intracochlear electrical stimulation

The anteroventral cochlear nucleus (AVCN) is the first central processing site for acoustic information. The influence and extent of convergent auditory nerve input to AVCN neurons was investigated using brief (<0.2 ms) intracochlear electrical activation of spiral ganglion cells. In 40 neurons recorded in vivo, the major intracellular response to stimulation was an excitatory postsynaptic potential (EPSP) with short latency (approximately 1 ms) and fast rise time (<1 ms). Graduated EPSP amplitude increases were also seen with increasing stimulation strength resulting in spike generation. Hyperpolarization followed excitation in most neurons, its extent distinguished three response types: Type I showed no hyperpolarization; Type II and Type III displayed short (<10 ms) and long (>19 ms) duration hyperpolarization, respectively. Hyperpolarization was attributed to an inhibitory postsynaptic potential (IPSP) in addition to spike after hyperpolarization. Neurobiotin filling identified Type I and II neurons as stellate and Type III as bushy cells. These results suggests that AVCN neurons receive direct, possibly convergent, excitatory input from auditory nerves emanating from spiral ganglion cells with hyperpolarization resulting from polysynaptic inhibitory input.

Development of single- and two-tone responses of anteroventral cochlear nucleus neurons in gerbil

Responses of anteroventral cochlear nucleus (AVCN) neurons in developing gerbils were obtained to single-tone stimuli, and two-tone stimuli elicited by best frequency probes presented over a range of intensities. Neurons displayed Type I, Type I/III, and Type III receptive field patterns. Best frequencies ranged from 1.5 to 10.0 kHz. Two-tone suppression (2TS) was first observed in 5 of 16 neurons examined at 14 dab. and in all neurons examined in gerbils aged 15 to 60 dab. Suppression areas grew larger, and discharge rate reductions became greater with age. Features of the two-tone responses that were highly correlated with single-tone responses across age groups include maximum rate reductions and suppression area thresholds. The intensity level of the CF probe-tone also influenced these features of 2TS. Maximum rate reductions to below spontaneous rate levels of activity were common across age groups. Results suggest that the cochlear amplifier is present and fundamentally adult-like by 15 dab for the regions of the cochlea coding the mid frequencies in gerbil. Over the subsequent week, contributions to the developing two-tone responses by the cochlear amplifier increase slightly. Two-tone responses are influenced by central inhibitory mechanisms as early as 14 dab.

Membrane currents influencing action potential latency in granule neurons of the rat cochlear nucleus

Granule cells are the most numerous neurons in the cochlear nucleus, but, because of their small size, little information on their membrane properties and ionic currents is available. We used an in vitro slice preparation of the rat ventral cochlear nucleus to make whole-cell recordings from these cells. Under current clamp, some granule neurons fired spontaneous action potentials and all generated a train of action potentials on depolarization (threshold current, 10-35 pA). Hyperpolarization increased the latency to the first action potential evoked during a subsequent depolarization. We examined which voltage-gated currents might underlie this latency shift. In addition to a fast inward Na+ current, depolarization activated two outward potassium currents. A transient current was rapidly inactivated by membrane potentials positive to -60 mV, while a second, more slowly inactivating current was observed following the decay of the transient current. No hyperpolarization-activated conductances were observed in these cells. Modelling of the currents suggests that removal of inactivation on hyperpolarization accounts for the increased action potential latency in granule cells. Such a mechanism could account for the 'pauser'-type firing patterns of the fusiform cells which receive a prominent projection from the granule cells in the dorsal cochlear nucleus.

Paired tone facilitation in dorsal cochlear nucleus neurons: a short-term potentiation model testable in vivo

It has been suggested that the dorsal cochlear nucleus (DCN) is involved in coding stimulus history or prior auditory activity [Manis (1989) J. Neurophys., 61, 149-161; Manis (1990) J. Neurosci., 10, 2338-2351]. The major output neurons of the DCN are the fusiform (pyramidal) cells which are thought to receive excitatory inputs from the descending branch of the acoustic nerve onto their basal dendrites and significant inhibitory glycinergic and GABAergic inputs to the soma and dendrites. The apical dendrites of these neurons lie within the molecular layer of the DCN and encounter parallel fibers which are thought to utilize the excitatory amino acid neurotransmitter glutamate. In this study of anesthetized chinchillas, we found that, in contrast to the responses of acoustic nerve fibers and most cochlear nucleus neurons which are masked by an appropriate preceding signal, many DCN principal cells are facilitated during the second of two identical stimuli. Facilitated DCN responses often have a reduced interspike interval and a more chopper-like temporal response pattern to the second characteristic frequency tone. This paired tone facilitation in the chinchilla DCN provides as in vivo model of short-term potentiation elicited by sensory stimulation similar to the paired-pulse facilitation observed with electrical stimulation in other models.

Development of GAD-immunoreactivity in the dorsal cochlear nucleus of the hamster and cat: light and electron microscopic observations

Physiologic and pharmacologic evidence suggests that inhibitory influences are active in the mammalian dorsal cochlear nucleus (DCN) by the onset of hearing, while anatomical evidence suggests that inhibitory synapses are not present until days or weeks later. One inhibitory neurotransmitter in the DCN is gamma aminobutyric acid (GABA) and its presence can be indexed by immunohistochemical localization of its synthetic enzyme glutamic acid decarboxylase (GAD). The present study investigated the ingrowth and synapse formation of GAD-immunoreactive inputs in the DCN of cat and hamster. GAD-immunoreactive puncta are present in the DCN of the cat at birth and of the hamster on postnatal day (PND) 3. Thus, the present data correlate well with the physiologic and pharmacologic evidence. In both species the first labelled puncta are near the dorsal acoustic stria and may originate from efferent axons in the stria. Several days later a band of labelled puncta is found in the fusiform cell layer. This location is equivalent to the termination zone of cartwheel cells, GAD-immunoreactive interneurons in the DCN. Based on this spatiotemporal sequence in the appearance of GAD-immunoreactive puncta, we suggest that sources of GABA extrinsic to the DCN mature first, followed by intrinsic sources.

Varieties of inhibition in the processing and control of processing in the mammalian cochlear nucleus

Seven-barrel micropipettes were used to apply drugs microiontophoretically to single units in the dorsal cochlear nucleus (DCN) in chloralose-anaesthetised guinea-pigs. While both agonists and antagonists of putative neurotransmitters in the cochlear nucleus have been investigated in these experiments, the main thrust has been to explore the influence of specific antagonists on cells' spectral and temporal properties, thus elucidating the effects of naturally occurring inhibitory transmitters. At least five types of inhibition appear to be pharmacologically/physiologically separable: (1) Stimulus-evoked tonic "lateral/sideband" inhibition: glycinergic; (blocked by strychnine); responsible for the lateral inhibition of dorsal cochlear nucleus (DCN) type III and IV cells. Strychnine has its predominant effect on sustained (lateral) inhibition compared with the more transient forms of inhibition. Subtraction of receptive field maps enables us to visualise the extent of the inhibitory receptive field. It extends virtually throughout the unit's response field for both these classes but is generally, especially in type IV cells, maximal at the characteristic frequency (CF). This type of inhibition will primarily be responsible for enhancing spectral contrasts in the way that, in the visual system, surround inhibition enhances visual contrast. Furthermore, lateral inhibitory sidebands can "bias" the "working point" of a cell's response so that the dynamic range of effective stimuli and response can be extended. (2) "Background" tonic inhibition: GABAAergic; (blocked by bicuculline). Blocking this inhibition generally results in an increase in the background (i.e., spontaneous) activity. This inhibition is probably responsible for adjusting excitatory-inhibitory contrasts in both spectral and temporal domains. (3) Stimulus-related off-inhibition appears to be neither glycinergic nor GABAAergic. Blocking these receptors actually enhances off-inhibition. Nicotinic cholinergic blockers may have a small effect on off-inhibition, but so far we have not been able to block it entirely. This off-inhibition is important for enhancing temporal contrast. This inhibition must, therefore, be mediated by other transmitters, yet undetermined, or by a local feedback circuit or, less likely, be a membrane-based after-effect of stimulation. (4) Pre-synaptic inhibition, mediated by GABAB receptors presumed to act on primary afferent terminals, thus controlling afferent input to DCN principal cells. (5) Short-latency contralateral inhibition, mediated by glycine.

An atlas of glycine- and GABA-like immunoreactivity and colocalization in the cochlear nuclear complex of the guinea pig

The distribution and colocalization of gamma-aminobutyric acid (GABA)- and glycine-like immunoreactivity in the cochlear nuclear complex of the guinea pig have been studied to produce a light microscopic atlas. The method used was based on post-embedding immunocytochemistry in pairs of 0.5-micron-thick plastic sections treated with polyclonal antibodies against conjugated GABA and glycine respectively. Immunoreactive cells, presumably short axon neurones, predominated in the dorsal cochlear nucleus, with mostly single-GABA-labelled cells in the superficial layer, double-labelled in the middle, and single-glycine-labelled in the deep layers. A few large single-glycine-labelled cells, interpreted as commissural neurons, occurred in the ventral nucleus. Scattered double-labelled cells, probably Golgi cells, were seen in the granule cell domain. Immunolabeled puncta of all three staining categories occurred in large numbers throughout the complex, apposed to somata and in the neuropil, showing a differential distribution onto different types of neuron. Three immunolabeled tracts were noted: the tuberculoventral tract, the commissural acoustic stria, and the trapezoidal descending fibres. Most of the fibres in these tracts were single-labelled for glycine, although in the last mentioned tract single-GABA- and double-labelled fibres were also found. Some of the immunolabeled cell types described here are proposed as the origins of the similarly labelled puncta and fibres on the basis of known intrinsic connections.

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

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

Descending projections to the dorsal and ventral divisions of the cochlear nucleus in guinea pig

The origins of extrinsic projections to the guinea pig dorsal and ventral cochlear nuclei were identified by examining the retrograde transport of horseradish peroxidase conjugated to wheatgerm agglutinin following its injection into each of these divisions. Major projections originated in periolivary regions of the superior olivary complex, the contralateral cochlear nucleus and the inferior colliculus. There was no contribution from the nuclei of the lateral lemniscus to these pathways. The heaviest projection from the periolivary regions to both divisions of the cochlear nucleus arose bilaterally in the ventral nucleus of the trapezoid body. The ipsilateral lateral nucleus of the trapezoid body also projected heavily to dorsal and ventral cochlear nucleus. In addition, the ventral cochlear nucleus received a substantial projection from the dorsal aspect of the ipsilateral dorsomedial periolivary nucleus. Projections originating bilaterally in the central nucleus of the inferior colliculus terminated in the deep layers of dorsal cochlear nucleus. These projections appear to be more strongly ipsilateral and specific than those reported in the cat.

Anatomy of the cochlear nuclear complex of guinea pig

The cyto- and fibre-architecture of the cochlear nuclear complex of the guinea-pig has been studied in serial sections using Nissl, Golgi and combined cell-myelin staining of normal material, and a silver degeneration method after cochlear ablation. The nuclear subdivisions and major cell types can be recognised on the basis of those found in the cat, but there are some differences between the two species in the precise distribution and morphology of the neurons. The rostrodorsal part of the anteroventral cochlear nucleus (AVCN) contains predominantly spherical bushy cells, but these cannot be readily divided into large and small types as in the cat. Globular bushy cells are seen in the caudal region of the AVCN, but the majority occur in the posteroventral cochlear nucleus (PVCN), in an area extending from the nerve root right up to the boundary of the dorsal cochlear nucleus (DCN). The octopus cells constitute a distinct region in the most dorsomedial part of the PVCN underneath the DCN. Giant cells are seen scattered around the nerve root region. Multipolar and small cells are seen throughout the non-granular regions of the ventral cochlear nucleus (VCN) except for the octopus cell area, but occur mainly in the more rostral regions of the PVCN. Small cells occur in greatest abundance in the thin cap area at the dorsal edge of the VCN below a superficial granule cell layer. The latter covers the dorsolateral surface of the VCN, and a lamina of granule cells partially separates the PVCN from the DCN. The DCN can be divided into four layers. The outermost molecular layer (layer 1) is separated from the deeper regions by a prominent layer of granule cells (layer 2) which also contains the pyramidal cells. Molecular layer stellate cells are seen in layer 1 and a staggered row of cartwheel neurons is found at the boundary between layers 1 and 2. Layer 3 contains the basal dendrites of the pyramidal cells and some small (vertical) cells, and is innervated by the descending branches of the cochlear nerve. The deepest layer 4, which contains multipolar cells and giant cells, does not appear to receive this direct cochlear input.

Coordinated activity of neuron pairs in anesthetized rat dorsal cochlear nucleus

We have recorded from small groups of neurons in the dorsal cochlear nucleus of anesthetized rats in an effort to study neuronal interactions. Multi-unit recordings on each single electrode were sorted by waveform into spike trains from individual neurons using a principal components spike sorter. Pairs of such sorted spike trains were studied with cross-correlation analysis to detect excitatory and/or inhibitory interactions. In a few cases recordings were obtained from two electrodes simultaneously, thus allowing cross-correlation studies without the consequences of spike train waveform sorting. All neurons were characterized by their strongest response frequency (at a fixed sound pressure level) and peristimulus histogram responses to 55 ms tone bursts. Fifty-eight percent of the neuron pairs studied showed peaks in their cross-correlograms indicative of coordinated neural activity. Of these pairs, 86% showed peak configurations (i.e. correlograms with asymmetrically located peaks) consistent with the interpretation that one cell induced the other to discharge. The remaining correlograms contained symmetric peaks which were centrally located, possibly due to shared input to these neuron pairs. Latencies of asymmetric peaks in cross-correlograms were typically 2 ms; consequently, an intervening excitatory synapse may be involved. Similar results were obtained from at least one pair of neurons where each neuron was recorded by a separate electrode. Strongest response frequencies of each neuron pair, for which they could be determined, were within 0.17 log units. Peristimulus histograms from each neuron in these pairs revealed that it was common for adjacent cells to respond with differing time patterns under the same stimulus conditions. The variations in histogram patterns of interconnected neurons suggests some relatively complex integrative function for these circuits.

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

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

Synaptic connections in the dorsal cochlear nucleus of mice, in vitro

1. Intracellular recordings were made from the dorsal cochlear nucleus (DCN) in slices that contained the root of the auditory nerve and parts of the dorsal and ventral cochlear nuclei. Probably the largest and most common cells were impaled. 2. Weak shocks to the nerve usually evoked an excitatory postsynaptic potential (EPSP) that lasted about 90 ms and whose latency was often less than 1.2 ms, indicating monosynaptic input. 3. Stronger shocks elicited a larger EPSP and a later train of inhibitory postsynaptic potentials (IPSPs). Increasing the stimulus voltage shortened the latency of the train of IPSPs and increased its efficacy so that at large stimulus strengths inhibition dominated the synaptic response. 4. To determine whether any of the neuronal circuitry which generated the synaptic responses involved the ventral cochlear nucleus, recordings were made from slices containing only the dorsal nucleus. Synaptic responses to stimulation of the pial surface of the isolated DCN resembled those driven from the nerve root. That is, weak shocks evoked long-lasting, monosynaptic EPSPs and stronger stimuli elicited a larger EPSP followed by trains of IPSPs. The DCN, therefore, contains intrinsic inhibitory interneurones. 5. The parallel fibres of the DCN course superficially, near the stimulating electrodes, whereas the axons of the auditory nerve terminate in deeper areas. Thus, the monosynaptic EPSPs evoked from the pial surface are probably generated by parallel fibres. Apparently the inhibitory interneurones are also excited by a circuit including parallel fibres. 6. The putative neurotransmitter of parallel fibres, glutamate, excited all neurones tested. 7. Cells were sensitive both to glycine and to gamma-aminobutyric acid (GABA). Only strychnine, however, not picrotoxin or bicuculline, blocked IPSPs.

Inhibitory connections between AVCN and DCN: evidence from lidocaine injection in AVCN

Principal cells of the dorsal cochlear nucleus (DCN) receive a variety of inhibitory influences, including some that are associated with the anteroventral cochlear nucleus (AVCN). Electrical stimulation in the caudal part of the AVCN produces long-lasting inhibition of DCN units [(1973) Exp. Brain Res. 17, 428-442], including unit type IV, which is associated with principal cells. This inhibition may be mediated either by orthodromic activation of AVCN axons that terminate in DCN or by antidromic stimulation of DCN inhibitory interneurons whose axons send collaterals to the AVCN. These two possibilities cannot be distinguished with electrical stimulation alone. In this study, microinjections of lidocaine were made in the AVCN. These injections should block AVCN to DCN axons without directly affecting the activity, within the DCN, of DCN interneurons. Two effects of lidocaine injections were observed. In some cases, all activity of DCN cells was abolished by lidocaine. These cases involved either spread of lidocaine to the recording site or into the auditory nerve. In other cases, the inhibitory responses of DCN type IV units were weakened by lidocaine without changing the units' thresholds or spontaneous activity. The latter result demonstrates that there are inhibitory pathways originating in AVCN or passing through AVCN and terminating in the DCN.

Short-latency auditory evoked potentials in the monkey. II. Intracranial generators

The generators of the short-latency auditory evoked potentials (SLAEPs) in the monkey have been defined by intracranial mapping from cochlea to auditory cortex. SLAEP components other than 1a and the slow negativity (SN) following wave 7 derive from compound action potentials propagated in subcortical auditory pathways. The component generators are complex due to the presence of two bursts of activity in the eighth nerve, to the fact that the ascending auditory fibers both synapse on and bypass specific relay nuclei, and to the differences in orientation of segments of the auditory pathways. Most SLAEP components recorded at the surface reflect the summation of activity from multiple generators. However, much of the activity seen within subcortical structures cannot be traced to the surface of the brain. Component 1a is identified with the cochlear summating potential, while 1b reflects the initial afferent volley in the distal portion of the eighth nerve. Component 2 represents the initial depolarization of the eighth nerve terminals within the ipsilateral cochlear nucleus. Component 3h reflects the second volley of activity in the distal portion of the eighth nerve and the outflow of the cochlear nucleus which decussates in the trapezoid body. Component 3v represents the initial cochlear nucleus outflow volley ascending the lateral lemniscus. Component 4 principally reflects the second volley of activity within the eighth nerve terminals, and outflow from the ipsilateral superior olivary complex ascending in that lateral lemniscus, with a possible contribution from activity in the contralateral CNC. Component 5 represents the outflow of the contralateral superior olivary complex ascending in that lateral lemniscus. Component 6 reflects another volley from the ipsilateral superior olivary complex ascending in that lateral lemniscus, as well as outflow from both inferior colliculi propagating in their brachii. The generators of component 7 are the most complex encountered, representing volleys in both lateral lemnisci, activity of the contralateral inferior colliculus, and activity in both auditory radiations. A component that follows wave 7, seen best in mastoid-to-mastoid recording linkage, represents outflow from both inferior colliculi propagating in their brachia. Components 8 and 9 principally reflect propagated action potentials in the auditory radiations bilaterally, with an additional contribution from activity of both inferior colliculi. The SN mainly represents volume-conducted postsynaptic potentials from both inferior colliculi and cochlear nuclei.

Distribution and fine structure of neuronal elements containing glutamate decarboxylase in the rat cochlear nucleus

Distribution and fine structure of gamma-aminobutyric acid (GABA)-containing structures were examined in the rat cochlear nuclear complex by means of immunohistochemistry using glutamate decarboxylase (GAD) as a marker. GAD-like immunoreactive (GADI) terminals were diffusely distributed in the dorsal cochlear nucleus, while in the ventral cochlear nucleus numerous immunoreactive fibers were situated around the cell bodies. These light-microscopic observations were confirmed by electron microscopy. Evidence suggesting that many of GADI boutons in the cochlear nucleus are of intrinsic origin was also shown.

Noise masking of tone responses and critical ratios in single units of the mouse cochlear nerve and cochlear nucleus

Responses of single units in the cochlear nerve and cochlear nucleus to tone bursts in a background of continuous white broadband noise were recorded. Tone and noise intensities ranged from threshold to saturation levels. Masking of the tone response by the noise was demonstrated by comparing peristimulus-time histograms and spike rates recorded during the tone and between tone presentations. The response of a unit to masking was found to be predictable based upon the difference in its rate of response to the tone and to the noise when the tone was masked. Several nonlinearities of the masking process are described. The most prominent one is an increase in the difference between tone and noise levels at the threshold of masking with increasing tone levels, i.e. neural critical ratios increase with increasing tone level. On the average, the frequency dependence of single unit effective bandwidths and of critical ratio bandwidths is similar to that of mean behavioral critical ratio bands.

Response properties of type II and type III units in dorsal cochlear nucleus

Type II and type III units in the dorsal cochlear nucleus (DCN) of unanesthetized (decerebrate) cats are those with excitatory responses to best frequency (BF) tones at all levels above threshold. They are distinguished from type IV units which give predominantly inhibitory responses to tones. Type II and type III units are distinguished from one another by their rates of spontaneous activity. Type II units have low rates of spontaneous activity (below 2.5 spikes/s), whereas type III units are spontaneously active at rates up to 95 spikes/s. In this paper we show that segregation of units according to spontaneous activity produces a segregation of several other properties as well. A typical type II unit responds vigorously to BF tones (median maximum rate of 242 spikes/s), has slightly nonmonotonic rate versus level function, and responds weakly or not at all to broadband noise. These units tend to be located in the deep layer of the DCN. Type III units tend to respond to BF tones at moderate rates (median maximum driven rate of 124 spikes/s), have monotonic or nonmonotonic rate versus level functions, and respond to noise at rates comparable to their tone rates. They are distributed somewhat more uniformly in all DCN layers, but a majority were found in the fusiform cell and molecular layers. Most evidence indicates that type II units are recorded from inhibitory interneurons in the DCN.

Identification of response properties of ascending axons from dorsal cochlear nucleus

Electrical stimulation in the dorsal acoustic stria (DAS) and the posterior and dorsal part of the ventral cochlear nucleus (VCN) was used to identify the response properties of efferent axons from the dorsal cochlear nucleus (DCN) in unanesthetized, decerebrate cats. Responses recorded in the DCN of this preparation can be divided into two broad classes, type II/III and type IV. Results of previous studies suggest that one subclass of type II/III units, those without spontaneous activity, are inhibitory interneurons terminating on type IV cells. Most type IV units (34/43) could be antidromically activated from the DAS and most units which could be activated from the DAS were of this type (34/39), suggesting that type IV responses are recorded from the projection cells of the DCN. Few type II/III cells without spontaneous activity could be activated from the DAS (1/30). A few cells of this type (6/32) could be antidromically stimulated from regions of the posterior AVCN or dorsal PVCN through which axon collaterals of some small cell interneurons of the DCN have been reported to pass. No type IV cells could be activated from the VCN (0/22). Thus the results are consistent with some type II/III responses originating in DCN interneurons. Spontaneously active type II/III units had intermediate properties.

Effects of neonatal conductive hearing loss on brain stem auditory nuclei

Both postnatal auditory deprivation and experimentally produced conductive hearing losses in mice result in incomplete maturation of most brain stem auditory neurons. The affected groups are: octopus cell, globular cell, small spherical cell, and large spherical cell groups in ventral cochlear nuclei; and the lateral superior olive and medial nucleus of the trapezoid body of the superior olivary complex. When 45 days of auditory deprivation are followed by 45 days of normal acoustic stimulation, there is incomplete maturation of neurons in: multipolar cell, globular cell, small spherical cell, and large spherical cell groups in ventral cochlear nuclei; lateral superior olive and medial nucleus of trapezoid body in superior olivary complex; and central nucleus of inferior colliculus. A critical period exists when adequate sound stimulation is needed for full development of these neurons.

Descending connections between superior olivary and cochlear nuclear complexes in the cat studied by autoradiographic and horseradish peroxidase methods

The descending pathways from the superior olivary complex (SO) to the cochlear nuclear complex (CN) were investigated in 58 cats using labelled aminoacid and horseradish peroxidase transport techniques. Descending connections were found coursing bilaterally through the trapezoid body (TB) and ipsilaterally in the intermediate and dorsal acoustic striae. The dorsolateral periolivary nucleus (DLPO) sends fibres through both the intermediate and dorsal striae, which are joined by others from the lateral preolivary nucleus (LPO). Both the latter nucleus and the medial preolivary nucleus (MPO) give rise to a bilateral descending projection which traverses TB. The distribution of these descending pathways within CN is described (although the technique did not permit precise synaptic identification). The possible implications of these pathways for response patterns at the level of CN are discussed.

Dynamic properties of excitation and two-tone inhibition in the cochlear nucleus studied using amplitude-modulated tones

The dynamic properties of excitation and two-tone inhibition in the cochlear nucleus were studied from extracellularly recorded unit responses to two simultaneously presented tones. One tone was presented at the unit's characteristic frequency, CF, the other at the unit's best inhibitory frequency, BIF. One or both of the tones were amplitude-modulated with pseudorandom noise. The system under study is in general nonlinear, but can be considered to function as a linear system for small changes in sound intensity around a certain operating point. The dynamic properties are likely to be different at different operating points. A suitable method for the study of dynamic properties of such a system employs tones that are amplitude-modulated with pseudorandom noise. In the present study, the dynamic properties were assessed by cross-correlating the unit discharge rate with the modulation. This was accomplished by computing the cross-covariance function between a period of noise and a period histogram of the discharges, the histogram being locked to the periodicity of the pseudorandom noise. In this way, it has been shown in previous works (Moller, 1973, 1974b), that the cross-covariance function is a valid approximation of the system's impulse response function at a certain sound intensity, provided the modulation is kept at a low value. In the present study the computed cross-covariance function is thus an approximation of the change in discharge rate of the cochlear nucleus units in response to a brief increase in stimulus intensity. As the response of the system under the given circumstances is approximately that of a linear system, the integrated cross-covariance is an approximation of the system's step response function, i.e the change in discharge rate that resulte from a hypothetical step increase in stimulus intensity. The results of the present study can be summarized as follows: 1. The impulse and step response functions computed from the responses to the modulated inhibitory tone of the great majority of units from which recording was made were found to be virtual mirror images of those obtained when the excitatory tone was modulated, the inhibitory response being somewhat smaller in amplitude than the excitatory. 2. When both tones were modulated simultaneously, the step response function was approximately the algebraic sum of the two responses obtained when the tones were modulated singly, further indicating that the system functions as a linear system when the stimulus amplitude is varied slightly around a certain operating point. 3. The shape of the cross-covariance functions is similar for all three stimulus situations, but varies with stimulus intensity and is different in different units. 4. The implication of the results is that the inhibition studied may either originate from the inhibition (suppression) seen in primary fibers or it may be the result of a true neural inhibition in the cochlear nucleus that occurs without any interneurons.

Spectral and temporal characteristics of activation and suppression of units in the cochlear nuclei of the anaesthetized cat

1. The responses are described of cochlear nucleus neurons of anaesthetized cats as a function of time in dependence on intensity and frequency of tonal stimuli. Depending on spectral properties three types are distinguished in the group of spontaneously active units: A type (activation only) AS type (activation and suppression) and S type (suppression only). The A(S) neurons have insufficient spontaneous activity to judge presence or absence of suppression. 2. Four temporal patterns of response are distinguished: transient, sustained, build up and complex. Units of the A type display a sustained time course of activation and have properties similar to those of auditory nerve fibres. S type units show sustained suppression. Temporal patterns of activation other than sustained were found only in the AS and A(S) units. 3. The recordings indicate that on suppression and off suppression are present more frequently in VCN neurons than previously found. The suppression phenomena in the DCN are, however, still more wide spread and more dramatic in appearance. 4. In contrast to earlier findings, off suppression was always observed and never seen to extent beyond the on activation band in A neurons. Data from AS neurons indicate that off suppression is neither simply an affect of high firing rates nor simply a continuation of on suppresion. 5. The relations between off suppression and spectral and temporal characteristics of on activation and suppression can be matched with a model featuring overlapping antagonistic inputs and postexcitatory inhibition.

Dynamic properties of excitation and inhibition in the cochlear nucleus

The dynamic properties of inhibition and excitation of single units in the cochlear nucleus were studied using tones that were amplitude modulated either sinusoidally or with pseudorandom noise. The cross-correlation analysis of the unit discharge rate and the pseudorandom noise modulation of the stimuli showed that the dynamic properties determined from the responses to a single modulated excitatory tone (at CF), to a modulated excitatory tone together with an unmodulated inhibitory tone (above CF), and to a modulated inhibitory tone together with an an unmodulated excitatory tone were almost identical with regard to latency as well as to delay of the peak of the cross-covariance function. On the basis hereof it is inferred that the inhibition is either a cochlear phenomenon or that it is transmitted over pathways with identical temporal properties as those of the excitation. When 2 tones were presented simultaneously the modulation of the excitatory tone usually gave a higher degree of modulation of the discharge rate than did modulation of an inhibitory tone. Addition of an unmodulated inhibitory tone to a modulated tone at CF resulted in an extension of the range of intensities over which the maximal gain was relatively constant. In many units this range extended from about 10dB above the unit's threshold to 60 dB or more above.

Postnatal maturation of the cochlear neclei in the cat: a neurophysiological study

The study of the postnatal maturation of the ventral cochlear nuclei (VCN) and the dorsal cochlear nuclei (DCN) was carried out on the cat by means of recordings of the extra-cellular neuronal activity. At birth it is already possible to obtain toneburst responses in the VCN and DCN. At this age the responses are characterised by a small number of spikes grouped in three bursts when the tone-bursts lasted 500 msec. Subsequently, the number of bursts increases until, from 9 or 10 days onwards the responses become sustained. These responses do not acquire their adult characteristics until more than a month after birth. During postnatal maturation of the cochlear nuclei, the VCN is distinguishable from the DCN by the greater number of units revealing spontaneous activity. Another criterion, such as latency, differentiates the VCN from the DCN from the point of view of the kinetics of maturation.