The cochlear nuclei in monkeys after dihydrostreptomycin or noise exposure

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.

Regulation of NT-3 and BDNF levels in guinea pig auditory brain stem nuclei after unilateral cochlear ablation

Injury to areas of the central nervous system can alter neurotrophin levels, which may influence postlesion neuronal survival and plasticity. To determine if sensorineural hearing loss induces such changes, we used an enzyme-linked immunosorbent assay (ELISA) to measure neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) levels in adult guinea pig brain stem auditory nuclei 3-60 days after a unilateral cochlear ablation (UCA). After UCA, which destroyed the cochlea and cochlear nerve on one side, NT-3 levels were usually depressed at 3 days by 22-44% but became elevated transiently at 7 days by 28-124%. BDNF levels were elevated transiently by 50% on the ablated side in the anteroventral (AVCN) and posteroventral (PVCN) cochlear nucleus at 3 days and may have signaled support for the survival of deafferented neurons. Coincident elevation at 3 and 7 days of BDNF or NT-3 and phosphorylated extracellular signal-regulated protein kinase 2 (ERK2-P) suggested a relationship to stimulated signal transduction activity. Elevated neurotrophin levels may have contributed to synaptogenesis in the AVCN and the superior olive and to changes in the synaptic biochemistry in the auditory nuclei after UCA. In contrast, deficiencies or failure to elevate neurotrophin levels within several days of the UCA correlated with upregulation of phosphorylated stress-activated protein kinase (SAPK-P), suggesting a relationship with stress-activated signal transduction and with the sparse degeneration of fibers observed in some of the auditory nuclei after UCA.

Changes in [14C]-2-deoxyglucose uptake in the auditory pathway of hamsters previously exposed to intense sound

The current study evaluated changes in [14C]-2-deoxyglucose (2-DG) uptake along the auditory pathways of hamsters that were exposed unilaterally to intense sound. The measurement of the acoustically evoked auditory brainstem responses indicated that intense sound exposure caused asymmetrical hearing loss. The 2-DG results revealed some changes in metabolic activity in exposed animals, as compared to unexposed animals. Significant decreases in 2-DG uptake were found in the ipsilateral anteroventral and posteroventral cochlear nucleus, with respect to the exposed left ears. Exposed animals also showed significant increases in the ipsilateral nucleus of the lateral lemniscus, central nucleus of inferior colliculus and medial geniculate body. No significant changes in uptake were observed in the ipsilateral dorsal cochlear nucleus, superior olivary complex, auditory cortex and any contralateral structures. The mechanisms for the observed changes in 2-DG uptake are discussed.

AMPA receptor binding in adult guinea pig brain stem auditory nuclei after unilateral cochlear ablation

This study determined if an asymmetric hearing loss, due to unilateral cochlear ablation, could induce the regulation of intracellular AMPA receptors in brain stem auditory nuclei. In young adult guinea pigs, the high-affinity specific binding of [(3)H]AMPA was measured in the cochlear nucleus (CN), the superior olivary complex (SOC), and the auditory midbrain at 2-147 postlesion days. After correction for tissue shrinkage, changes in specific binding relative to that in age-matched unlesioned controls were interpreted as altered numbers and/or activity of intracellular AMPA receptors. In the CN, transient elevations and/or deficits in binding were evident in most regions, which usually recovered by 147 days. However, persistently deficient binding was evident ipsilaterally in the anterior part of the anteroventral CN (AVCNa). In the SOC, transient elevations in binding were evident at 2 days in the medial limb of the lateral superior olive (LSOmed) and the medial superior olive. Between 7 and 147 days, most SOC nuclei exhibited transient, temporally synchronized postlesion deficits in binding. However, late in the survival period, deficits persisted ipsilaterally in the LSOmed and the lateral (LSOlat) limb of the lateral superior olive. In the midbrain, transient elevations and/or deficits in binding were evident in the dorsal nucleus of the lateral lemniscus as well as in the central and dorsal nucleus of the inferior colliculus. A persistent deficit was evident in the intermediate nucleus of the lateral lemniscus. The findings implied that auditory neurons contain regulatory mechanisms that control the numbers and/or activity of intracellular AMPA receptors. Regulation was induced by cochlear nerve destruction and probably by changes in the excitation of glutamatergic neurons. Many of the regulatory changes were transient, except in the ipsilateral AVCNa and LSO, where postlesion downregulations were persistent. The downregulation in the ipsilateral AVCNa was probably induced directly by the loss of cochlear nerve endings. However, other regulatory changes may have been induced by signals carried on pathways emerging from the ipsilateral CN and on centrifugal auditory pathways.

Glycine receptors in adult guinea pig brain stem auditory nuclei: regulation after unilateral cochlear ablation

In young adult guinea pigs, the effects of unilateral cochlear ablation were determined on the specific binding of [3H]strychnine measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2, 7, 31, 60, and 147 postlesion days. Changes in binding relative to that in age-matched controls were interpreted as altered activity and/or expression of synaptic glycine receptors. Postlesion binding declined ipsilaterally in most of the ventral CN and in the lateral superior olive (LSO). Binding was modestly deficient in the ipsilateral dorsal CN and in the anterior part of the contralateral anteroventral CN. Binding was elevated in the contralateral LSO. Transient changes also occurred. Binding was elevated transiently, between 2 and 31 days, contralaterally in parts of the anteroventral CN, bilaterally in the medial superior olive (MSO), and bilaterally in most of the midbrain nuclei. Binding was deficient transiently, at 60 days, in most of the contralateral CN and bilaterally in the midbrain nuclei. The present findings, together with previously reported postlesion changes in glycine release, were consistent with persistently weakened glycinergic inhibitory transmission ipsilaterally in the ventral CN and the LSO and bilaterally in the dorsal CN. Glycinergic inhibitory transmission was strengthened in the contralateral LSO and transiently strengthened in the MSO bilaterally. A hypothetical model of the findings suggested that glycine receptor regulation may depend on excitatory and glycinergic input to auditory neurons. The present changes in glycine receptor activity may contribute to altered auditory functions, which often accompany hearing loss.

New growth of axons in the cochlear nucleus of adult chinchillas after acoustic trauma

This study determined the effect of acoustic overstimulation of the adult cochlea on axons in the cochlear nucleus. Chinchillas were exposed to an octave-band noise centered at 4 kHz at 108 dB sound pressure level for 1.75 h. One chinchilla was never exposed to the noise, and several others had one ear protected by an ear plug or prior removal of the malleus and incus. Exposure of unprotected ears caused loss of inner and outer hair cells and myelinated nerve fibers, mostly in the basal half of the cochlea. Cochlear nerve fiber degeneration, ipsilateral to the exposed ears, was traced to regions of the cochlear nucleus representing the damaged parts of the cochlea. In silver impregnations of a deafferented zone in the posteroventral cochlear nucleus, the concentration of axons decreased by 43% after 1 month and by 54% after 2 months. However, by 8 months, the concentration of thinner axons, with diameters of less than 0.46 microm, increased by 46-90% over that at 2 months. The concentration of axons with larger diameters did not change. Between 2 and 8 months small axonal endings appeared next to neuronal cell bodies. This later increase of thinner axons and endings is consistent with a reactive growth of new axons of relatively small diameter. The emergence of small perisomatic boutons suggests that the new axons formed synaptic endings, which might contribute to an abnormal reorganization of the central auditory system and to the pathological changes that accompany acoustic overstimulation.

Morphological changes in the anteroventral cochlear nucleus that accompany sensorineural hearing loss in DBA/2J and C57BL/6J mice

Morphological measurements were made on histological sections of the anteroventral cochlear nucleus (AVCN) in mice of the DBA/2J and C57BL/6J strains to determine the effects of sensorineural cochlear pathology on the number, packing density, and size of neurons and on AVCN volume. Both strains possess alleles that cause progressive cochlear pathology initially affecting the organ of Corti: in DBA mice, hearing loss is evident at 4 weeks of age and progresses rapidly; in C57 mice, hearing loss begins after 2 months of age and progresses more slowly. In both strains AVCN volume decreased, some loss of neurons occurred, and these changes paralleled the progression of peripheral hearing loss. Central changes were rapid in DBA mice, but the ultimate magnitude of the changes in 1-year-old mice did not differ between strains. Both strains differed from well-hearing CBA/J mice which exhibited no changes in the AVCN measures. The findings indicate that pathology of the organ of Corti in adult mice results in degenerative changes in the cochlear nucleus. The data also support earlier findings indicating that, if cochlear pathology does not begin prior to young adulthood, the age of onset and duration of sensorineural impairment have little effect on the ultimate magnitude of central effects.

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.

Morphology of the dorsal cochlear nucleus in C57BL/6J and CBA/J mice across the life span

The morphology of the dorsal cochlear nucleus (DCN) was evaluated across the life span in inbred C57BL/6J (C57) and CBA/J (CBA) mice using 5 age groups (young adult to very old). C57 mice exhibit progressive cochlear sensorineural pathology and hearing loss during middle age; CBA mice have only modest sensorineural pathology late in life. DCN layers I, II, and III were evaluated histologically with serial sections stained for Nissl and fibers. DCN volume decreased with age in C57 mice, but the change began earliest and was most pronounced in layer III. In CBA mice, volume increased during the first year of life and decreased only in the oldest mice. All major DCN cell types were found in both strains at all ages. There was an age-related decrease in the mean size of neurons in C57 mice that was first observed in layer III. In CBA mice, only a nonsignificant trend toward smaller neurons was observed in the oldest mice. An age-related decline in the number of neurons in layer III (but not in layers I and II) occurred in C57 mice. Aged CBA mice exhibited no significant loss of DCN neurons. Thus, age-related changes in the DCN were much more pronounced in C57 mice than in CBA mice, and the changes in C57 mice were most pronounced in layer III. Because layer III receives most of the DCN's primary auditory input, it would be directly affected by age-related hearing loss and degeneration of spiral ganglion cells in C57 mice. This suggests that the age-related changes observed in DCN layer III of C57 mice are affected by progressive peripheral degenerative changes; when peripheral loss is minimal (CBA mice), less substantial age-related changes are observed.

Changes in the tonotopic map of the dorsal cochlear nucleus following induction of cochlear lesions by exposure to intense sound

Hamsters were exposed to intense tones (10 kHz) at levels and durations sufficient to cause stereocilia lesions. The purpose was to determine how the tonotopic map of the dorsal cochlear nucleus (DCN) readjusts to loss of receptor sensitivity. Neural population thresholds and tonotopic organization was mapped over the surface of the DCN in normal unexposed animals and those showing tone-induced lesions. The results indicate that cochlear lesions characterized mainly by loss of stereocilia in a restricted portion of the organ of Corti cause changes in a corresponding region of the tonotopic map which reflect primarily changes in the shape and thresholds of neural tuning curves. In many cases the center of the lesion was represented in the DCN as a distinct characteristic frequency (CF) gap in the tonotopic map in which responses were either extremely weak or absent. In almost all cases the map area representing the center of the lesion was bordered by an expanded region of near-constant CF, a feature superficially suggestive of map reorganization. These expanded map areas had abnormal tip thresholds and showed other features suggesting that their CFs had been shifted downward by distortion and deterioration of their original tips. Such changes in neural tuning are similar to those observed by others in the auditory nerve following acoustic trauma, and thus would seem to have a peripheral origin. Thus, it is not necessary to invoke plastic changes in the cochlear nucleus to explain the changes observed in the tonotopic map.

Reduction and recovery of neuronal size in the cochlear nucleus of the chicken following aminoglycoside intoxication

The effect of aminoglycoside intoxication on the cross-sectional area of neurons in nucleus magnocellularis (NM) was studied in neonatal chickens. Birds received daily injections of 100 mg/kg body weight of gentamicin for 10 consecutive days. Cell area was measured at five different tonotopic regions along the posterior-to-anterior dimension of NM (low-to-high frequency) after post-treatment survival times of 8, 23 and 40 days. Gentamicin caused a reversible reduction of cell area that varied as a function of location and survival time. Significant decreases of cell area occurred only in the rostral half of the nucleus. Cell area was reduced at 8 and 23 days survival and recovered to near control values by 40 days post-treatment. Body weight, brain weight and the cross-sectional area of cerebellar Purkinje neurons were also reduced but did not recover. The present results show that aminoglycoside toxicity can affect auditory neurons in the brain. It is suggested that two factors contributed to the changes in NM neuron size: (1) Processes specifically related to the loss and regeneration of cochlear hair cells, most likely changes in afferent activity. (2) A general retardation in growth.

Enhanced evoked response amplitudes in the inferior colliculus of the chinchilla following acoustic trauma

Evoked response amplitude-level functions were measured from electrodes in the inferior colliculus of the chinchilla before and after exposure to a 2 kHz pure tone of 105 dB SPL. The exposure produced approximately 20-30 dB of permanent threshold shift from 2 to 8 kHz, but little or no hearing loss at higher or lower frequencies. Generally less than 60% of the outer hair cells were missing in the region of hearing loss. The amplitude-level functions measured at 4 and 8 kHz generally showed a loss in sensitivity at low sound levels, a reduction in the maximum amplitude and sometimes steeper than normal slopes. The amplitude-level functions measured at 2 kHz also showed a loss in sensitivity; however, the maximum amplitude was often greater than normal. Even though there was no loss in sensitivity at 0.5 kHz, the amplitude-level function was steeper than normal and the maximum amplitude of the evoked response was almost always substantially larger than normal. The enhancement of the evoked response amplitude from the inferior colliculus does not appear to originate in the cochlea, but may reflect a reorganization of neural activity in the central auditory pathway.

Morphology of the octopus cell area of the cochlear nucleus in young and aging C57BL/6J and CBA/J mice

The influence of aging and age-related cochlear impairment on the ventral cochlear nucleus was evaluated by measuring morphological properties of the octopus cell area (OCA) in five age groups of inbred C57BL/6J and CBA/J mice (young adult to very old). The former strain demonstrates progressive cochlear sensorineural pathology and hearing loss during middle age; the latter has only modest sensorineural pathology late in life. Histological sections of the OCA were evaluated with serial sections and several strains for neurons, glia, and fibers, and Golgi impregnations were also used. Aging was associated with a decrease in volume of the OCA, a loss of neurons, slight decrease in neuron size, increased packing density of glial cells, and changes in dendrites ranging from minor to total loss of primary branches. The greatest changes occurred in extreme old age, beyond the median lifespan. Age-related changes were not exacerbated by sensorineural pathology in aging C57BL/6J mice. Individual octopus cells varied greatly in the extent of age-related abnormality.

Speech recognition scores and ABR in cochlear impairment

The relationship between the speech recognition scores (SRS) in noise and the auditory brain-stem evoked response (ABR) was examined in 7 males (age range: 40 to 60 yrs) with fairly similar audiograms but different SRS. The Spearman rank-order correlation coefficient revealed a significant relationship between SRS and latencies of waves I and II of the ABR. The results are discussed in terms of neural degeneration and representation of speech stimuli in the auditory nerve.

A degenerative disorder of the central auditory system of the gerbil

A previously unidentified disorder which affects primarily the cochlear nucleus was observed in two species of gerbils, Meriones unguiculatus and M. libycus. Unusual lesions were observed in the cochlear nucleus bilaterally in all animals examined. In light and electron microscopic specimens these lesions were characterized by the presence of microcysts and vacuolar neuronal degeneration. The microcysts resembled large holes, containing trabeculae, organelles and cellular remnants. Also observed were light and dark degeneration of neuronal perikarya and degenerated axons, dendrites, and synapses, accompanied by phagocytosis. Astrocytosis was not conspicuous. In the one cochlea examined, no microcysts were observed. In young animals the microcysts were prevalent in the cochlear nerve root region and the posteroventral cochlear nucleus. In older animals the microcysts increased in number and area. In the oldest animals, the microcysts had spread to other central auditory structures, including the superior olivary complex, the nuclei of the lateral lemniscus, and the inferior colliculus. Other regions of the brain were largely free of microcysts. The etiology and behavioral manifestations of this disorder are unknown, although it is clearly neurodegenerative and perhaps genetically determined.

Response properties of inferior colliculus neurons in young and very old CBA/J mice

Extracellular recordings were obtained from inferior colliculus (IC) neurons on young (2-month) and very old (greater than 2-year) CBA/J mice in response to contralateral tone and noise stimuli. The old mice had a small loss of spiral ganglion cells throughout the cochlea and moderate hearing loss, manifested as an elevation of neuronal thresholds throughout the IC. There was an age-related increase (3% to 22%) in 'sluggish' neurons (auditory, but poorly driven by sound); however, most neurons responded robustly to sound. Nine response types were derived from post-stimulus time histograms (PSTHs); all types were found in both age groups with no difference in their relative incidence. Sustained responses remained vigorous in old neurons, as indicated by spike counts and temporal discharge patterns. The percentage of neurons that were spontaneously active increased with age in the ventral IC (the area most sensitive to high frequencies, including most of the central nucleus) and decreased in the dorsal IC (the area most sensitive to lower frequencies, including much of the dorsal cortex). Parameters of response areas (range, upper frequency cutoff, best frequency, and rate-best frequency) showed modest age differences, while rate-level functions showed little age-related change. While a significant correlate of old age was attrition of IC neurons from the population capable of responding robustly to sounds, the majority of individual neurons demonstrated a remarkable degree of normalcy in their responses.

Morphometric study of the anteroventral cochlear nucleus of two mouse models of presbycusis

The dimensions and volume of the anterior ventral cochlear nucleus (AVCN), the density and number of AVCN neurons, and the size of neuronal somata nuclei (in Nissl-stained tissue) were determined in two mouse models of age-related hearing loss: the C57BL/6J strain, which undergoes progressive chronic sensorineural hearing loss with onset during young adulthood, and the CBA/J, which demonstrates only moderate hearing loss with onset late in life. Frontal and horizontal AVCN sections, as well as cochleas, were analyzed in 4 C57 age-groups (1, 7, 12, 19+ months) and in 3 CBA groups (1, 10, 22 months). Within each strain no significant changes in AVCN dimensions or volume occur with aging. In C57 mice, packing density and cell number decrease between 1 and 7 months, but remain stable thereafter, despite chronic severe hearing impairment. CBA mice show a reduction in AVCN cell number and packing density only during the second year of life. In aging C57 mice, the size of spherical and perhaps globular cells increases, whereas the size of multipolar cells tends to decrease slightly. In CBA mice, all three AVCN cell types tend to decrease in size with aging. The early cell loss and cell size increases in C57 mice are most consistent in the dorsal (high frequency) region of the AVCN. Likewise, loss of cochlear spiral ganglion cells is most pronounced in the base of the cochlea, which provides input to this region. The data indicate that aging is associated with rather different central effects, depending on AVCN cell type, cochleotopic organization, genotype, and/or the type of peripheral hearing loss involved. The C57 and CBA AVCNs also differ in several aspects irrespective of age. The volume of AVCN and number of AVCN neurons are significantly greater in C57 mice.

Auditory brainstem response in young burn-wound patients treated with ototoxic drugs

Burn-wound patients often require potentially ototoxic doses of aminoglycoside drugs in the treatment of gram-negative sepsis. Cochlear hearing impairment may be an unfortunate consequence of this medical therapy. We evaluated auditory sensitivity with the auditory brainstem response (ABR) in a group of 32 children with acute, severe thermal burns ranging in age from 18 months to 17 years. The mean percent of total body surface area burns was 64%. None of the subjects had a known history of hearing deficits or aminoglycoside therapy, and all yielded a normal baseline ABR upon hospital admission. Eight of the subjects (22%) showed either an abnormal ABR, or no response, at 40 dB prior to hospital discharge. The medical treatment for this group of subjects (gentamicin, amikacin, vancomycin, amphotericin B) was compared to that of a second subgroup of 7 subjects without auditory deficit but with a statistically comparable percentage of burns. The mean dosage of vancomycin was higher for the auditory impairment group than for the unimpaired group. Prediction of ototoxicity in the acute burned patient is extremely difficult as there are numerous factors that may influence the risk of cochlear damage. We conclude, however, that the ABR can be applied in early detection of auditory deficit. Follow-up audiometric assessment is advisable since auditory deficits in this population may be delayed or progressive after discontinuance of drug therapy.

Progressive degeneration in the cochlear nucleus after chemical destruction of the cochlea

In the guinea pig, the organ of Corti and the spiral ganglion cells were destroyed by administering gentamicin into the inner ear. The antero-ventral cochlear nucleus (AVCN) was studied with electron microscopy 15 and 30 days after treatment. There does not seem to be a specific type of degeneration induced by the drug. For nerve terminals, a progressive degeneration occurred: after 15 days, clear and swollen boutons appeared and after 30 days an electron-dense type of degeneration was observed. The cell bodies of the AVCN did not seem altered infirming a rapid, direct or indirect, neurotoxic effect of the drug.