Subdivisions Of Cochlear Nucleus Research Articles

Do cochlear nuclei contribute to auditory lateralization? A stereological evaluation of neuron numbers.

The aim of this study was to estimate the total number of cells in the subdivisions of the cochlear nucleus (CN) of the rat with unbiased stereological methods. The total number of neurons was determined in both the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN) to compare the right and left sides. The total cell numbers were 15,280 in the left VCN, 15,400 in the right VCN, 10,260 in the left DCN, and 10,860 in the right DCN. Comparison of the right and left major subdivisions of the CN of the rat showed that there was no significant difference between the right and left sides of the rat CN. This result indicates that the CNs do not contribute to auditory lateralization in the rat in regard to cell numbers.

Distribution of BDNF, NT-3 and NT-4 in the developing auditory brainstem

This study describes the developmental expression of three neurotrophins, brain derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and neurotrophin (NT-4) in the rat auditory brainstem using immunohistochemistry. At postnatal day 0 (PND 0), neurotrophins expression was virtually absent from all auditory nuclei in the brainstem, even though some positive neurons were observed in the mesencephalic trigeminal nucleus at this age. However, BDNF, NT-3 and NT-4 positive neurons were observed in most brainstem auditory nuclei by PND 6. At the following stages, there was a general increase in the intensity of the neurotrophins immunoreactivity and BDNF labeling was particularly prominent in most cochlear nucleus neurons. A differential pattern of staining emerged in cochlear nucleus subdivisions, with more intense staining present in the ventral part. The superior olivary complex nuclei followed a similar pattern of BDNF staining compared to the cochlear nucleus. In the adult, BDNF heavily labeled most neurons of the superior olivary nuclei and moderately labeled neurons of the inferior colliculus (IC). NT-3 and NT-4 showed a similar pattern of staining in most auditory brainstem nuclei. The first staining was observed by PND 6 in some neuronal cell bodies. NT-3 and NT-4 immunoreactivity increased in the following stages and in the adult moderate labelings were observed in most neurons of the cochlear nucleus, the superior olivary nuclei and the IC.These results show that neurotrophins are expressed 1week before the onset of hearing and the increase of their expressions correlate with the appearance of sound-evoked activity in the system. The temporal distribution of neurotrophins does not correlate with neuronal birth, axonal outgrowth or the formation of connection in the auditory structures, suggesting a role primarily in the maintenance and/or modulation of postnatal and adult functions.

The Postnatal Growth of Cochlear Nucleus Subdivisions and Neuronal Somata of the Anteroventral Cochlear Nucleus in the Mongolian Gerbil (Meriones unguiculatus)

We have analysed the postnatal volume increase of the cochlear nucleus subdivisions from birth to senescence in the Mongolian gerbil. All subdivisions showed the most prominent growth in the 2nd week of life before the onset of hearing on day 12. After the onset of hearing, the dorsal and the postero-ventral portion of the cochlear nucleus showed only a moderate degree of further growth. The antero-ventral cochlear nucleus, however, almost doubled in volume after the onset of hearing, reaching a maximum in 4-month-old animals. In ageing gerbils, we observed a small but systematic reduction of the cochlear nucleus volume. To further assess which elements contribute to the growth of the antero-ventral cochlear nucleus after the onset of hearing, we analysed the somal size of the neurones. Before the onset of hearing, somal and volume growth were highly correlated. However, somata had reached an adult size by the onset of hearing. We conclude that the growth of the antero-ventral cochlear nucleus after the onset of hearing is due to changes in the neuropil.

Quantitative analysis of spiral ganglion projections to the cat cochlear nucleus

A quantitative examination of the tonotopic organization of primary afferent projections to the cochlear nucleus (CN) in adult cats was conducted by using focal extracellular injections of Neurobiotin (NB) into the spiral ganglion of the basal cochlea. One to three injections separated by intervals of at least 2 mm were positioned along the basal one-third of the cochlea. Each injection produced discrete projection laminae that appeared as parallel horizontal sheets of labeled axons terminals distributed sequentially dorsally to ventrally across each major CN subdivision: the anteroventral, posteroventral, and dorsal cochlear nucleus, (AVCN, PVCN, and DCN, respectively). The length (rostrocaudal dimension), width (mediolateral dimension), thickness (dorsoventral dimension), and relative placement of 18 "frequency-band" laminae were measured in 10 adult cochlear nuclei. The average AVCN projection thickness was approximately twice that of the PVCN and DCN projections. In double injection cases, the center-to-center separation between AVCN laminae was also approximately twice that in the PVCN and equal to that in the DCN. Lamina thickness did not differ significantly as a function of frequency representation. However, in both width and length, mid-frequency laminae were up to two times larger than high-frequency laminae. Thus, the results indicate that DCN projections are the most discrete (i.e., are the thinnest and have the least overlap between adjacent frequency projections), whereas the AVCN projections are the largest but are as discrete as PVCN projections. In addition, high-frequency projections are smaller and more discrete than mild-frequency projections, which are larger and have greater overlap with adjacent frequency projections.

Age-Dependent Effects of the Onset of a Conductive Hearing Loss on the Volume of the Cochlear Nucleus Subdivisions and the Expression of c- fos in the Mongolian Gerbil (Meriones unguiculatus)

A monaural conductive hearing loss was induced by interrupting the chain of the middle ear ossicles on the right side in gerbils of four different age groups (P12-14, P20-21, P42 and P84). The volumes of the cochlear nucleus subdivisions and the number of cells that expressed immunoreactivity for c-fos after noise stimulation were determined on the left and right side in the deprived animals, and in undeprived control animals when they reached the age of 6 months. The anteroventral cochlear nucleus on the deprived side was reduced in volume when the deprivation started before the age of 3 months. The other cochlear nucleus subdivisions showed no systematic age-dependent reductions. The expression of c-fos in the dorsal cochlear nucleus appeared more resistant to a hearing loss, with deprivation being more effective in younger animals. c-fos expression was also dramatically reduced in the ventral cochlear nucleus, regardless of age at the onset of hearing loss.

Morphology of the cochlear nucleus in CBA/J mice with chronic, severe sensorineural cochlear pathology induced during adulthood

The effects of chronic cochlear impairment on morphological features of the adult cochlear nucleus (CN) were assessed in CBA/J mice in which severe sensorineural damage had been induced by exposure to intense noise. Sections from various CN subdivisions, stained for Nissl substance and fibers, were quantitatively evaluated in four groups of noise-exposed mice that differed with regard to the age at noise exposure (2, 6, or 11 months), age at the time the CN was evaluated (6, 11, or 24 months), and the duration (chronicity) of sensorineural impairment (4, 5, 13, or 18 months). Like-aged, non-exposed CBA mice were used as controls, so the effects of peripheral damage and aging could be compared. Cochlear damage produced significant changes in CN subdivisions thought to receive the heaviest input from cochlear afferents (anteroventral CN, octopus cell area, dorsal CN layer III). These changes included a reduction of neuropil volume, reductions in neuron size, and increases in neuronal packing density that were complementary to reduced volume in these subdivisions. Effects on neuron number were minimal in all subdivisions. Central changes in noise-exposed mice were absent or diminished in DCN layers I and II, which receive relatively less input from primary fibers. The age at onset and chronicity of damage had little to do with the severity of central effects of cochlear damage. The effects of cochlear damage were not additive with age-related changes seen in the old controls.

Central projections of auditory nerve fibers of differing spontaneous rate, II: Posteroventral and dorsal cochlear nuclei

Response properties of auditory nerve fibers (ANFs), including threshold sensitivity, vary systematically with spontaneous discharge rate (SR) (Liberman, M.C.: J. Acoust. Soc Amer. 63:442-455, 1978). Thus, an understanding of the mechanisms underlying signal transformation in the cochlear nucleus (CN) must include a description of any SR-based difference in ANF projections. This study is the second of a pair describing the CN projections of intracellularly labeled ANFs of known SR, the first of which summarized projection to the anteroventral CN (Liberman, M.C.: J. Comp. Neurol. 313:240-258, 1991). For each swelling from each labeled fiber, the position (within CN subdivisions), the size, and the type of cell contacted (if determinable) was noted: roughly one in four labeled swellings appeared in intimate contact with the soma or proximal dendrites of a CN cell. In all such cases, cell size and swelling size were measured. As reported for auteroventral cochlear nucleus, the ANF innervation of the small-cell regions of posteroventral CN (PVCN) was almost exclusively by low- and medium-SR fibers. Other significant SR-based trends in ANF projections included 1) a tendency for high-SR fibers to contact larger cells in PVCN, 2) a meager projection of low- and medium-SR fibers to octopus cells, and 3) a tendency in the dorsal CN (DCN) for low-SR terminals to end closer to the fusiform cell layer than high-SR terminals. There were no significant SR-based difference in ANF swelling sizes in any subdivision. A consideration of the average cell sizes, ANF swelling sizes and estimated numbers of ANFs of different CF and SR converging on each CN cell help explain some of the differences in response transformation associated with different cell types in the CN.

Effects of neonatal deafening and chronic intracochlear electrical stimulation on the cochlear nucleus in cats

Four newborn kittens were deafened by daily intramuscular injections of neomycin sulfate, beginning the day after birth and continuing for 14–16 days. At 10–16 weeks of age the deaf kittens were implanted unilaterally with a four wire intracochlear electrode array. The animals were stimulated daily (starting at 13–18 weeks of age), for a period of one hour, at 6 dB above the electrically evoked auditory brainstem response threshold. After 3 months of chronic intracochlear electrical stimulation, animals were studied in acute electrophysiological experiments and euthanized for histological studies. This study compares the stimulated and control cochlear nuclei (CN) of these deafened animals to the CN of four normal adult cats. Statistical comparisons of spherical cell densities in the anteroventral cochlear nucleus (AVCN), cross-sectional spherical cell areas, and volumes of the cochlear nucleus subdivisions were included in the analysis. The results indicate that, by all of these measures, the cochlear nuclei in neonatally deafened animals were significantly different from the cochlear nuclei of control animals. As a result of deafening, the density of spherical cells was decreased by 30%, the cross-sectional areas of spherical cells were reduced by 20%. and the volume of the cochlear nucleus was reduced by 25%. These changes were observed in both cochlear nuclei (ipsilateral to both stimulated and unstimulated ears) of the deafened animals. With the measures employed, no significant difference was demonstrated in comparisons between the deafened/unstimulated and the deafened/stimulated cochlear nuclei. That is, no reversal of the profound effects of deafening was observed in the cochlear nuclei as a consequence of chronic intracochlear electrical stimulation which was begun 11 to 16 weeks after deafening.

Quantitative comparison of frequency representation in the auditory brainstem nuclei of the gerbil, Pachyuromys duprasi

Frequency representation in the auditory brainstem nuclei of the fattailed gerbil (Pachyuromys duprasi) was studied by focal iontophoretic HRP injections into the cochlear nucleus at physiologically characterized positions. Tuning curves of cochlear nucleus neurons were determined by extracellular recording of multi-unit spike activity prior to HRP application. Neurons with characteristic frequencies of 0.55 to 17.5 kHz were recorded. Anterograde labeling of synaptic terminal fields demonstrated a tonotopic arrangement of frequencies within the cochlear nucleus subdivisions and the lateral and medial superior olivary nuclei. In all nuclei investigated terminal fields were arranged in isofrequency planes. In the subdivisions of the cochlear nucleus and in the lateral superior olive the frequency range from 0.55 to 17.5 kHz was represented. In the medial superior olive on the other hand, frequency representation was restricted to frequencies below 3.5 kHz. Quantitative evaluation of tonotopy showed that the frequency representation in the anteroventral and posteroventral cochlear nucleus and lateral superior olive were very similar. The amount of tissue-volume processing one octave varied within the hearing range. The frequency range of highest auditory sensitivity (between 1 and 5 kHz) occupied up to 25% nuclear tissue/octave, while at the upper and lower limits of the hearing range, frequencies were represented by a ratio of 10-15% nuclear tissue/octave. The frequency representation pattern in these brainstem nuclei reflected exactly the frequency place map of the cochlea.

Topographic organization of the central projections of the spiral ganglion in cats

The morphological organization of inputs from restricted sectors of the cat cochlear spiral ganglion into the cochlear nucleus was studied by making focal extracellular injections of horseradish peroxidase (HRP) into the spiral ganglion. Injections resulted in Golgi-like labeling of a small cluster of spiral ganglion cells and their peripheral and central axons. Large injections involved most of the cells within Rosenthal's canal in sectors of the spiral ganglion innervating greater than or equal to 1 mm of the basilar membrane and resulted in narrow, complete laminae of labeled axons and preterminal fields within each cochlear nucleus subdivision. The positions of these bands were consistent with the "isofrequency laminae" appropriate for the frequencies represented at the injection sites, with high frequency laminae situated more dorsally, and lower frequencies progressively more ventral. A discrete projection to the small cell cap area was observed that was discontinuous with the main projection laminae in the ventral cochlear nuclei (VCN). In the dorsal cochlear nucleus, projecting fibers and terminals were excluded from the molecular cell layer. No labeled fibers entered the granule cell areas. In contrast to larger injections, very small HRP deposits labeled only part of an isofrequency lamina. Specifically, injections restricted to the scala tympani aspect of the spiral ganglion labeled only the lateral part of VCN isofrequency laminae, whereas injections limited to the scala vestibuli aspect of the ganglion labeled the medial aspect of the isofrequency planes. Thus these data indicate a previously unrecognized topographic representation of the vertical dimension of the spiral ganglion across VCN isofrequency laminae. Some possible functional implications of this projection organization are discussed.

Intrinsic connections within and between cochlear nucleus subdivisions in cat.

The cat cochlear nuclear complex (CNC) is divided into three major subdivisions: the anteroventral, the posteroventral, and the dorsal cochlear nuclei (AVCN, PVCN, and DCN, respectively). Each of these subdivisions receives a topographic projection from the cochlea and each consists of a number of different cell types. The interconnections between these subdivisions and the cell types which give rise to them were studied by means of small injections of horseradish peroxidase (HRP) made at physiologically identified locations. DCN injections resulted in few labeled cells in the DCN, suggesting that its internal connections are very limited. In contrast, these same DCN injections resulted in numerous labeled cells in the PVCN and AVCN. Labeled PVCN cells, consisting of multipolar, octopus, and small spindle-shaped cells, were located in spatially restricted laminae stretching the entire rostrocaudal length of the nucleus, while labeled AVCN cells consisting of multipolar, globular, small spindle-shaped and small spherical cells were broadly distributed over the posterior half of the nucleus. Similar injections placed in the PVCN resulted in numerous labeled cells in all three subdivisions. The PVCN and AVCN cells labeled after PVCN injections were widely distributed across the isofrequency representations in both nuclei, while the labeled DCN cells were restricted to locations over the injection sites. Injections placed in the posterior half of the AVCN resulted in only very few labeled cells in the DCN. No cells were labeled following injections in the rostral AVCN.

Uptake and release of glycine in the guinea pig cochlear nucleus after axotomy of afferent or centrifugal fibers.

Glycine may be an inhibitory transmitter in the mammalian cochlear nucleus (CN). This study attempts to determine if cochlear and/or centrifugal projections to the CN use glycine as a transmitter. The high-affinity uptake and electrically evoked release of exogenous [14C]glycine were measured in vitro in the three major subdivisions of the guinea pig CN: the anteroventral, posteroventral, and dorsal cochlear nuclei (AVCN, PVCN, and DCN, respectively). [14C]Glycine (3.4 microM) was taken up by each subdivision, reaching tissue concentrations six to seven times that in the medium. Subsequent electrical stimulation evoked a Ca2+-dependent release of [14C]glycine from each subdivision. These activities were compared in subdivisions fr0m unlesioned animals, and from animals with lesions of centrifugal or cochlear projections to the CN. Two knife-cut lesions were made to interrupt centrifugal projections to the CN lying in the right acoustic striae and trapezoid body. In one group of animals, centrifugal fibers projecting mainly to the right AVCN and PVCN were severed, which reduced [14C]glycine uptake and release by 44-53% in these subdivisions, but not in the right DCN. In another group of animals, fibers projecting mainly to the right PVCN and DCN were severed, which reduced [14C]glycine uptake and release by 33-47% in these subdivisions, but not in the right AVCN. In CN subdivisions contralateral to either lesion there was no significant change in [14C]glycine uptake or release. Neither of these lesions altered the uptake or release of D-[3H]aspartate in the right or the left CN. Ablation of the left cochlea, which presumably destroyed cochlear nerve fibers unilaterally, had no effect on [14C]glycine uptake and release. These observations suggest that centrifugal projections contribute a proportion of the glycinergic synaptic endings in the CN. In addition, some glycinergic endings probably arise from neurons intrinsic to the CN. The cochlear nerve contains very few, if any, glycinergic fibers.

Uptake and release of glycine in the guinea pig cochlear nucleus.

This study attempts to determine if the cochlear nucleus (CN) contains glycinergic synaptic endings. The uptake and release of exogenous radiolabeled glycine were measured in vitro in the three major subdivisions of the guinea pig CN: anteroventral, posteroventral, and dorsal. A kinetic analysis of [3H]glycine uptake revealed the presence in each CN subdivision of a high- and a low-affinity uptake mechanism. The high-affinity mechanism had a Km of 25.2-30.5 microM and a Vmax of 3.8-4.8 nmol/10 mg of cell water/5 min, whereas the low-affinity mechanism had a Km of 633-718 microM and a Vmax of 26.6-37.1 nmol/10 mg of cell water/5 min. At steady state, the high-affinity mechanism accumulated 10 microM [3H]glycine from the medium, achieving tissue concentrations that were 13-24 times that in the medium. The high-affinity uptake was dependent on the temperature and on the concentrations of NaCl and glucose in the incubation medium. It exhibited a high degree of substrate specificity, as determined by the effects of structural analogues of glycine on the uptake of [3H]glycine. Each CN subdivision also contained two mechanisms mediating [14C]glycine release. One was activated by depolarizing electrical stimuli, produced a rapid transient release of [14C]glycine, and was dependent on the presence of extracellular Ca2+. The other was continuous, producing a slow spontaneous efflux of [14C]glycine. Released glycine could be removed primarily by uptake, because during release measurements, the amount of [14C]glycine detected in the medium decreased when glycine uptake activity was optimized. The electrically evoked, Ca2+-dependent release and the high-affinity uptake of glycine may mediate the synaptic release and inactivation of glycine, respectively. These findings, therefore, support the presence of glycinergic synaptic endings in each CN subdivision.

Organization and response properties of neurons in the cochlear nuclei of DBA/2 mice

Neurons in the inferior colliculus (IC) of DBA/2 mice (an inbred strain with genetically determined hearing loss and susceptibility to audiogenic seizures—AGS) have a high incidence of afterdischarges (ADs: a continuation of sustained discharges after termination of relatively intense sounds, indicating increased excitability) [J. F. Willott, J. Neurophysiol. 45, 35–47 (1981)]. The present study obtained recordings from the cochlear nucleus (CN) using the same preparation and found no ADs in any CN subdivisions of this strain, indicating that ADs arise at higher auditory nuclei, perhaps involving synaptic mechanisms or a particular neurotransmitter system. Thus, peripheral hearing impairment and/or other genetic factors affected neural responses differently at different levels of the auditory CNS. A second major finding of this study is that tonotopic organization and the shape of tuning curves is abnormal in DBA/2 mice when compared to C57BL/6 mice used as “controls.” In the latter strain, best frequencies (BFs) of tuning curves showed the typical mammalian tonotopic organization, with dorsoventral progression from high to low BFs. In DBA/2 mice, however, virtually all BFs were near 14–16 kHz, regardless of dorsoventral level. In addition, there was sensitization of thresholds for some portions of tuning curves relative to what is seen at a given level in the C57BL/6 strain.

Projections of auditory nerve in the cat as seen by anterograde transport methods

Ascending projections of the auditory nerve of the cat were studied by autoradiographic methods after injection of labeled precursors into the cochlea. In animals in which [ 3H]leucine was injected, transport to the cochlear nucleus was observed after survival periods of 2–7 days, but there was no evidence of transport to more central structures in the auditory pathways. In these cases the pattern of labeling around cell bodies depended on the type of cell and on the location of the cell within subdivisions of the cochlear nucleus. For example, there was dense perisomatic labeling around spherical cells in the anteroventral cochlear nucleus and around octopus cells in the posteroventral cochlear nucleus. Some other types of cells in subdivisions of both anteroventral and posteroventral cochlear nucleus had little perisomatic labeling. In the dorsal cochlear nucleus, the most densely labeled area consisted of a band just central to the layer of fusiform cells. Labeled fibers were not found peripheral to this band in the granular layer, molecular layer and peripheral part of the layer of fusiform cells. In the dorsal nucleus there was no evidence of dense perisomatic labeling such as was found around some cells in the ventral divisions of the cochlear nucleus. Following injections of a mixture of [ 3H]proline and [ 3H]fucose into the cochlea, labeling was seen in the superior olivary complex. Transneuronal transport was suggested as the explanation for this finding because of (1) the choice of the labeled precursors, (2) the length of the survival periods (14 or 20 days), and (3) the evidence that isotope was within neurons in the cochlear nucleus. We concluded that under appropriate conditions, unique patterns of silver grains are found around different types of cells in the various subdivisions of the cochlear nucleus, apparently because fibers and terminals of the auditory nerve become saturated with the radioisotope. As a result, a highly detailed survey of projections of the auditory nerve to each subdivision of the cochlear nucleus is revealed at the light-microscopic level. Under different experimental conditions labeled fibers are found in the superior olivary complex, but these conditions are those most conducive to transneuronal transport and so such experiments cannot be used as evidence of projection of auditory nerve fibers beyond the cochlear nucleus.