Regions Of Cochlear Nucleus Research Articles

Effects of brainstem lesions on amino acid levels in the rat cochlear nucleus

There is evidence for glutamate, γ-amino butyric acid (GABA), and glycine as neurotransmitters of centrifugal pathways to the cochlear nucleus, but the quantitative extent of their contributions to amino acid neurotransmission in cochlear nucleus regions has not been known. We used microdissection of freeze-dried tissue sections of rat cochlear nucleus, with mapping of sample locations, combined with a high performance liquid chromatography (HPLC) assay, to measure amino acid levels in cochlear nucleus subregions of rats with unilateral lesions of centrifugal pathways to the cochlear nucleus. In rats with lesions transecting all or almost all pathways to the cochlear nucleus from brain stem regions, GABA, aspartate, and glutamate levels were reduced, compared to contralateral values, in almost all ipsilateral cochlear nucleus regions. The largest reductions, in dorsal (DCN), anteroventral (AVCN), and posteroventral (PVCN) cochlear nucleus regions, approached 50% for GABA, 40% for aspartate, and 30% for glutamate. In contrast, glutamine and taurine levels were typically higher in lesioned-side cochlear nucleus regions than contralaterally. Effects on glycine levels were mixed but usually included increased lesioned-side values compared to contralateral, probably reflecting a balance between increases during protein breakdown and decreases of free glycine in transected pathways. More limited lesions transecting just dorsal pathways showed much less effect on amino acid levels. Lesion of the ipsilateral trapezoid body connection plus ipsilateral superior olivary nuclei resulted in decreases of GABA, aspartate, and glutamate levels especially in ventral cochlear nucleus regions. No clear contralateral effects of this lesion could be shown. The results most strongly support centrifugal GABAergic pathways to the cochlear nucleus, providing almost half of GABAergic neurotransmission in most regions. Our results support and extend previously published measurements of lesion effects on GABA uptake and release in cochlear nucleus subdivisions.

Volumes of cochlear nucleus regions in rodents

The cochlear nucleus receives all the coded information about sound from the cochlea and is the source of auditory information for the rest of the central auditory system. As such, it is a critical auditory nucleus. The sizes of the cochlear nucleus as a whole and its three major subdivisions – anteroventral cochlear nucleus (AVCN), posteroventral cochlear nucleus (PVCN), and dorsal cochlear nucleus (DCN) - have been measured in a large number of mammals, but measurements of its subregions at a more detailed level for a variety of species have not previously been made. Size measurements are reported here for the summed granular regions, DCN layers, AVCN, PVCN, and interstitial nucleus in 15 different rodent species, as well as a lagomorph, carnivore, and small primate. This further refinement of measurements is important because the granular regions and superficial layers of the DCN appear to have some different functions than the other cochlear nucleus regions. Except for DCN layers in the mountain beaver, all regions were clearly identifiable in all the animals studied. Relative regional size differences among most of the rodents, and even the 3 non-rodents, were not large and did not show a consistent relation to their wide range of lifestyles and hearing parameters. However, the mountain beaver, and to a lesser extent the pocket gopher, two rodents that live in tunnel systems, had relative sizes of summed granular regions and DCN molecular layer distinctly larger than those of the other mammals. Among all the mammals studied, there was a high correlation between the size per body weight of summed granular regions and that of the DCN molecular layer, consistent with other evidence for a close relationship between granule cells and superficial DCN neurons.

Pathogenic plasticity of Kv7.2/3 channel activity is essential for the induction of tinnitus

Tinnitus, the perception of phantom sound, is often a debilitating condition that affects many millions of people. Little is known, however, about the molecules that participate in the induction of tinnitus. In brain slices containing the dorsal cochlear nucleus, we reveal a tinnitus-specific increase in the spontaneous firing rate of principal neurons (hyperactivity). This hyperactivity is observed only in noise-exposed mice that develop tinnitus and only in the dorsal cochlear nucleus regions that are sensitive to high frequency sounds. We show that a reduction in Kv7.2/3 channel activity is essential for tinnitus induction and for the tinnitus-specific hyperactivity. This reduction is due to a shift in the voltage dependence of Kv7 channel activation to more positive voltages. Our in vivo studies demonstrate that a pharmacological manipulation that shifts the voltage dependence of Kv7 to more negative voltages prevents the development of tinnitus. Together, our studies provide an important link between the biophysical properties of the Kv7 channel and the generation of tinnitus. Moreover, our findings point to previously unknown biological targets for designing therapeutic drugs that may prevent the development of tinnitus in humans.

Plasticity of somatosensory inputs to the cochlear nucleus – Implications for tinnitus

This chapter reviews evidence for functional connections of the somatosensory and auditory systems at the very lowest levels of the nervous system. Neural inputs from the dosal root and trigeminal ganglia, as well as their brain stem nuclei, cuneate, gracillis and trigeminal, terminate in the cochlear nuclei. Terminations are primarily in the shell regions surrounding the cochlear nuclei but some terminals are found in the magnocellular regions of cochlear nucleus. The effects of stimulating these inputs on multisensory integration are shown as short and long-term, both suppressive and enhancing. Evidence that these projections are glutamatergic and are altered after cochlear damage is provided in the light of probable influences on the modulation and generation of tinnitus.

Effects of High-Intensity Sound Exposure on Neurotransmitter Chemistry in the Central Auditory System

Exposure to intense sound often leads to tinnitus, the perception of a monotonous sound not actually present. Increased neural spontaneous activity in the central auditory system found in animal models of tinnitus should have a basis in their chemistry. Most chemical studies so far have focused on neurotransmitters, by which neurons communicate with each other, because alteration of this chemistry could easily lead to abnormal neural activity that might be perceived as tinnitus. Although increased spontaneous activity has been observed in the hamster dorsal cochlear nucleus (DCN) a month after intense tone exposure, we did not find increased glutamate concentrations in the 3 layers of the hamster dorsal DCN at that time. We did, however, find decreased glutamate concentrations 2 days after exposure that might correlate with slightly decreased spontaneous activity observed then. Others have provided evidence for decreased glutamate release in the chinchilla DCN 2 days after intense sound exposure. Other intense-sound-induced changes are increased choline acetyltransferase activity in some cochlear nucleus regions, increased acetylcholine receptor sensitivity in some DCN neurons, and some changes in the γ-aminobutyric acid (GABA) neurotransmitter system in the inferior colliculus. There is a need for more study of these and other neurotransmitter systems to determine their possible roles in tinnitus.

Age-related Decline in Kv3.1b Expression in the Mouse Auditory Brainstem Correlates with Functional Deficits in the Medial Olivocochlear Efferent System

Kv3.1b channel protein is widely distributed in the mammalian auditory brainstem, but studies have focused mainly on regions critical for temporal processing, including the medial nucleus of the trapezoid body (MNTB) and anteroventral cochlear nucleus (AVCN). Because temporal processing declines with age, this study was undertaken to determine if the expression of Kv3.1b likewise declines, and if changes are specific to these nuclei. Immunocytochemistry using an anti-Kv3.1b antibody was performed, and the relative optical density of cells and neuropil was determined from CBA/CaJ mice of four age groups. Declines in expression in AVCN, MNTB, and lateral superior olive (35, 26, and 23%) were found, but changes were limited to neuropil. Interestingly, cellular optical density declines were found in superior paraolivary nucleus, ventral nucleus of the trapezoid body, and lateral nucleus of the trapezoid body (24, 29, and 26%), which comprise the medial olivocochlear (MOC) feedback system. All declines occurred by middle age (15 months old). No age-related changes were found in the remaining regions of cochlear nucleus or in the inferior colliculus. Contralateral suppression of distortion-product otoacoustic emission amplitudes of age-matched littermates also declined by middle age, suggesting a correlation between Kv3.1 expression and MOC function. In search of more direct evidence for such a correlation, Kv3.1b knockout mice were examined. Knockouts show poor MOC function as compared to +/+ and +/- genotypes. Thus, Kv3.1b expression declines in MOC neurons by middle age, and these changes appear to correlate with functional declines in efferent activity in both middle-aged CBA mice and Kv3.1b knockout mice.

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, γ-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.

Serotonin projection patterns to the cochlear nucleus

The cochlear nucleus is well known as an obligatory relay center for primary auditory nerve fibers. Perhaps not so well known is the neural input to the cochlear nucleus from cells containing serotonin that reside near the midline in the midbrain raphe region. Although the specific locations of the main, if not sole, sources of serotonin within the dorsal cochlear nucleus subdivision are known to be the dorsal and median raphe nuclei, sources of serotonin located within other cochlear nucleus subdivisions are not currently known. Anterograde tract tracing was used to label fibers originating from the dorsal and median raphe nuclei while fluorescence immunohistochemistry was used to simultaneously label specific serotonin fibers in cat. Biotinylated dextran amine was injected into the dorsal and median raphe nuclei and was visualized with Texas Red, while serotonin was visualized with fluorescein. Thus, double-labeled fibers were unequivocally identified as serotoninergic and originating from one of the labeled neurons within the dorsal and median raphe nuclei. Double-labeled fiber segments, typically of fine caliber with oval varicosities, were observed in many areas of the cochlear nucleus. They were found in the molecular layer of the dorsal cochlear nucleus, in the small cell cap region, and in the granule cell and external regions of the cochlear nuclei, bilaterally, of all cats. However, the density of these double-labeled fiber segments varied considerably depending upon the exact region in which they were found. Fiber segments were most dense in the dorsal cochlear nucleus (especially in the molecular layer) and the large spherical cell area of the anteroventral cochlear nucleus; they were moderately dense in the small cell cap region; and fiber segments were least dense in the octopus and multipolar cell regions of the posteroventral cochlear nucleus. Because of the presence of labeled fiber segments in subdivisions of the cochlear nucleus other than the dorsal cochlear nucleus, we concluded that the serotoninergic projection pattern to the cochlear nucleus is divergent and non-specific. Double-labeled fiber segments were also present, but sparse, in the superior olive, localized mainly in periolivary regions; this indicated that the divergence of dorsal and median raphe neurons that extends throughout regions of the cochlear nucleus also extended well beyond the cochlear nucleus to include at least the superior olivary complex as well.

Synaptophysin immunoreactivity in the cat cochlear nuclei.

The presence of synaptophysin, a presynaptic Ca2+ binding glycoprotein, has been analyzed in the cochlear nucleus complex of the adult cat using an antisynaptophysin monoclonal antibody. Synaptophysin immunoreactivity was differently distributed between regions of cochlear nuclei. Terminal boutons contacting directly with neurons (cell bodies and dendrites) or in the neuropil of ventral and dorsal cochlear nuclei appeared immunostained. In the ventral cochlear nucleus, synaptophysin-labelled boutons were clearly defined in contact with spherical, globular, multipolar, octopus and cochlear-nerve root neurons. However, the dorsal cochlear nucleus showed a dense immunostained network of synaptophysin-labelled puncta, inside which some neuron cell bodies were observed. The present findings show a differential distribution of synaptophysin-immunostained boutons between the ventral and the dorsal cochlear nuclei of the cat. The present description of the different synaptophysin-labelled terminal boutons in the cochlear nuclei complex and their pattern of distribution, will be useful for further studies on development, degeneration or regeneration of the peripheral auditory pathway.

Synapses from labeled type II axons in the mouse cochlear nucleus

This study investigates the ultrastructure and central targets in the cochlear nucleus of axonal swellings of type II primary afferent neurons. Type II axons comprise only 5–10% of the axons of the auditory nerve of mammals, but they alone provide the afferent innervation of the outer hair cells. In this study, type 11 axons were labeled with horseradish peroxidase, and serial-section electron microscopy was used to examine their swellings in: (1) the granule-cell lamina at its boundary with posteroventral cochlear nucleus, (2) the rostral anteroventral cochlear nucleus, and (3) the auditory nerve root. Only some (18%) of the type II terminal and en-passant swellings formed synapses. The synapses were asymmetric and contained clear round synaptic vesicles, suggesting that they are excitatory. Type 11 synapses were compared to those from type I fibers, the fibers providing the afferent innervation of the inner hair cells. Type II synapses tended to have slightly smaller and fewer synaptic vesicles, had a greater proportion of the membrane apposition accompanied by a postsynaptic density, and often had densities that were discontinuous or `perforated'. In all cochlear nucleus regions examined, the postsynaptic targets of type II synapses had characteristics of dendrites; in most cases these dendrites could not be traced to their cell bodies of origin. Some evidence suggests, however, that targets may include granule cells, spherical cells, and other cells in the nerve root. These results suggest afferent information from outer hair cells reaches diverse regions and targets within the cochlear nucleus.

Poster 76: Volumes of Cochlear Nucleus Regions in Mountain Beaver Compared with Other Rodents

Poster 76: Volumes of Cochlear Nucleus Regions in Mountain Beaver Compared with Other Rodents

Effect of transient neonatal hypothyroidism on the free aspartic acid, glutamic acid, and GABA content of different central auditory regions in the rat.

The aspartic acid, glutamic acid, and gamma-aminobutyric acid (GABA) contents were determined in four central auditory system regions in rats with transient neonatal hypothyroidism compared with control ones: the ventral and dorsal parts of the cochlear nucleus, the central nucleus of the inferior colliculus, the auditory cortex, and in an extra-auditory structure, the substantia nigra pars reticulata. The animals were sacrificed at 50 days of age, brain tissue samples were taken out by microdissection, and the free amino acids were extracted. The amino acid content was assessed by double-isotope labelling following two-dimensional thin-layer chromatography separation. GABA content was significantly decreased in both cochlear nucleus regions and glutamic acid was elevated in the inferior colliculus. Neonatal hypothyroidism had no significant effect on the aspartic acid levels in the regions studied. The results suggest an effect of neonatal hypothyroidism on regional contents of free amino acids known as candidate neurotransmitters in the auditory system.

An HRP-study of the frequency-place map of the horseshoe bat cochlea: morphological correlates of the sharp tuning to a narrow frequency band.

The frequency-place map of the horseshoe bat cochlea was studied with the horseradish peroxidase (HRP) technique involving focal injections into various, physiologically defined regions of cochlear nucleus (CN). The locations of labeled spiral ganglion cells and their termination sites on inner hair cells of the organ of Corti from injections into CN-regions responsive to different frequencies were analyzed in three dimensional reconstructions of the cochlea. Horseshoe bats from different geographical populations were investigated. They emit orientation cells with constant frequency (CF) components around 77 kHz (Rhinolophus rouxi from Ceylon) and 84 kHz (Rhinolophus rouxi from India) and their auditory systems are sharply tuned to the respective CF-components. The HRP-map shows that in both populations: the frequency range around the CF-component of the echolocation signal is processed in the second half-turn of the cochlea, where basilar membrane (BM) is not thickened, secondary spiral lamina (LSS) is still present and innervation density is maximal; frequencies more than 5 kHz above the CF-component are processed in the first half-turn, where the thickened BM is accompanied by LSS and innervation density is low; frequencies below the spectral content of the orientation call are represented in apical turns showing no morphological specializations. The data demonstrate that the cochlea of horseshoe bats is normalized to the frequency of the individual specific CF-component of the echolocation call. The HRP-map can account for the overrepresentation of neurons sharply tuned to the CF-signal found in the central auditory system. A comparison of the HRP-map with a map derived with the 'swollen nuclei technique' following loud sound exposure (Bruns 1976b) reveals that the latter is shifted towards cochlear base by about 4 mm. This discrepancy warrants a new interpretation of the functional role of specialized morphological structures of the cochlea within the mechanisms giving rise to the exceptionally high frequency selectivity of the auditory system.

The cochlear frequency map of the mustache bat, Pteronotus parnellii.

The frequency-place map of the cochlea of mustache bats was constructed by the analysis of HRP-transport patterns in spiral ganglion cells following iontophoretic tracer injections into cochlear nucleus regions responsive to different frequencies. The cochlea consists of 5 half turns (total length 14.3 mm) and the representation of certain frequency bands can be assigned to specific cochlear regions: The broad high frequency range between 70 and 111 kHz is represented in the most basal half turn within only 3.2 mm. This region is terminated apically by a distinct narrowing of the scala vestibuli that coincides with a pronounced increase in basilar membrane (BM) thickness. The narrow intermediate frequency range between 54 and 70 kHz is expanded onto 50% of cochlear length between 4.0 and 11.1 mm distance from apex. The frequency range around 60 kHz, where the tuning characteristics of the auditory system are exceptionally sharp, is located in the center of this expanded BM-region in the second half turn within a maximum of innervation density. These data can account for the vast overrepresentation of neurons sharply tuned to about 60 kHz at central stations of the auditory pathway. In the cochlear region just basal to the innervation maximum, where label from injections at 66 and 70 kHz was found, a number of morphological specializations coincide: the BM is maximally thickened, innervation density is low, the spiral ligament is locally enlarged, and the 'thick lining', a dense covering of the scala tympani throughout the basal halfturn, suddenly disappears. Low frequencies up to 54 kHz are represented within the apical half turns over a 4 mm span of the basilar membrane. The data are compared to the cochlea of horseshoe bats and the possible functional role of the morphological discontinuities for sharp tuning and the generation of otoacoustic emissions is discussed.