Changes in the tonotopic map of the dorsal cochlear nucleus in hamsters with hair cell loss and radial nerve bundle degeneration

Amino acid and acetylcholine chemistry in mountain beaver cochlear nucleus and comparisons to pocket gopher, other rodents, and cat

The mountain beaver and pocket gopher are two rodents that live mostly underground in tunnel systems. Previous studies have suggested that their cochlear nucleus structure, particularly that of the dorsal cochlear nucleus (DCN), differs significantly from that of other mammals, that the hearing ability of the pocket gopher is deficient compared to that of other rodents, and that the DCN of the mountain beaver is more responsive to slow oscillations of air pressure than to sounds. We conducted some electrophysiological recordings from mountain beaver DCN and then used microchemical methods to map in mountain beaver cochlear nuclei the distributions of amino acids, including the major neurotransmitters of the brain, and enzyme activities related to the metabolism of the neurotransmitter acetylcholine, which functions in centrifugal pathways to the cochlear nucleus. Similar measurements were made for a pocket gopher cochlear nucleus. Responses to tonal stimuli were found in mountain beaver DCN. Distributions and magnitudes of neurotransmitter and related amino acids within mountain beaver and pocket gopher cochlear nuclei were not very different from those of other rodents and cat. However, the enzyme of synthesis for acetylcholine, choline acetyltransferase, had only low activities in the DCN of both mountain beaver and pocket gopher. The chemical distributions in the mountain beaver DCN support a conclusion that it corresponds to just the superficial DCN portion of other mammals. High correlations between the concentrations of γ-aminobutyrate (GABA) and glycine were found for both mountain beaver and pocket gopher cochlear nuclei, suggesting that their co-localization in cochlear nucleus synapses may be especially prominent in these animals. Previous evidence suggests convergence of somatosensory and auditory information in the DCN, and this may be especially true in animals spending most of their time underground. Our results suggest that the enlarged DCN of the mountain beaver and that of the pocket gopher are not very different from those of other rodents with respect to involvement of amino acid neurotransmitters, but they appear to have reduced cholinergic innervation.

Cochlear damage affects neurotransmitter chemistry in the central auditory system

Tinnitus, the perception of a monotonous sound not actually present in the environment, affects nearly 20% of the population of the United States. Although there has been great progress in tinnitus research over the past 25 years, the neurochemical basis of tinnitus is still poorly understood. We review current research about the effects of various types of cochlear damage on the neurotransmitter chemistry in the central auditory system and document evidence that different changes in this chemistry can underlie similar behaviorally measured tinnitus symptoms. Most available data have been obtained from rodents following cochlear damage produced by cochlear ablation, intense sound, or ototoxic drugs. Effects on neurotransmitter systems have been measured as changes in neurotransmitter level, synthesis, release, uptake, and receptors. In this review, magnitudes of changes are presented for neurotransmitter-related amino acids, acetylcholine, and serotonin. A variety of effects have been found in these studies that may be related to animal model, survival time, type and/or magnitude of cochlear damage, or methodology. The overall impression from the evidence presented is that any imbalance of neurotransmitter-related chemistry could disrupt auditory processing in such a way as to produce tinnitus.

Amino acid concentrations in the hamster central auditory system and long-term effects of intense tone exposure

Exposure to intense sounds often leads to loss of hearing of environmental sounds and hearing of a monotonous tonal sound not actually present, a condition known as tinnitus. Chronic physiological effects of exposure to intense tones have been reported for animals and should be accompanied by chemical changes present at long times after the intense sound exposure. By using a microdissection mapping procedure combined with high-performance liquid chromatography (HPLC), we have measured concentrations of nine amino acids, including those used as neurotransmitters, in the cochlear nucleus, inferior colliculus, medial geniculate, and auditory cortex of hamsters 5 months after exposure to an intense tone, compared with control hamsters of the same age. No very large differences in amino acid concentrations were found between exposed and control hamsters. However, increases of glutamate and γ-aminobutyrate (GABA) in some parts of the inferior colliculus of exposed hamsters were statistically significant. The most consistent differences between exposed and control hamsters were higher aspartate and lower taurine concentrations in virtually all regions of exposed hamsters, which reached statistical significance in many cases. Although these amino acids are not considered likely neurotransmitters, they indirectly have roles in excitatory and inhibitory neurotransmission, respectively. Thus, there is evidence for small, widespread, long-term increases in excitatory transmission and decreases in inhibitory transmission after a level of acoustic trauma previously shown to produce hearing loss and tinnitus.

Stimulus-dependent changes in optical responses of the dorsal cochlear nucleus using voltage-sensitive dye

Optical imaging with voltage-sensitive dye was used to examine the spatiotemporal dynamics of stimulus-driven activity on the surface of the dorsal cochlear nucleus (DCN). Stimulation with tones at low to moderate levels produced localized regions of activation that were most commonly elongated rostrocaudally. The size of these activation areas expanded with increases in sound level, while their centers shifted from the lateral direction to the medial direction with increases in stimulus frequency. In contrast to the tonotopic patterns of activation evoked by tones, electrical stimulation of the DCN surface resulted in bands of activation that were elongated along the medial-lateral axis; response latencies increased with distance along these bands from the point of stimulation. Shifting the site of electrical stimulation from the rostral direction to the caudal direction induced corresponding shifts in the rostrocaudal location of the activation band; moving the electrode tip to subsurface depths resulted in loss of the elongated band. Transecting the DCN along the rostrocaudal axis midway between its medial and lateral extremities blocked propagation of the response to the half of the DCN distal to but not proximal to the stimulating electrode. The results suggest that the two modes of stimulation activated two distinct populations of neurons, one involving primarily tonotopically organized cells and the other crossing these tonotopic zones and likely representing the activation of parallel fibers. These results reveal a number of new features in the spatial patterns of tone-elicited activation that are not readily predicted by responses recorded electrophysiologically.

Three-dimensional tonotopic organization of the C57 mouse cochlear nucleus

The cochlear nucleus (CN) is the first sound processing center in the central auditory system that receives the almost unprocessed auditory information from the auditory periphery. The functional organization of the CN has been studied to a great extent in many mammals, including the cat, rat and bat. Yet, despite the general usefulness of the mouse, including the availability of various inbred strains and gene-manipulated lines, our current understanding of the mouse CN remains limited. The purpose of this study was to illustrate the functional organization of the CN in C57 mice, using an electrophysiological approach. Our results showed that the auditory response properties of CN neurons were similar in all three of the CN subdivisions. Sound frequency was systematically represented in each of the three CN subdivisions, i.e., the anteroventral, posteroventral and the dorsal divisions. The best frequency of CN neurons decreased along the dorsomedial-to-ventrolateral axis in an orderly progression whereas the tonotopic axes were relatively indistinct in the rostrocaudal plane. There was no disruption of the tonotopic map within each subdivision of the CN. The findings indicate that the CN tonotopic organization in the C57 mouse is similar to that in the cat and other mammals.

Age-related hearing loss in C57BL/6J mice has both frequency-specific and non-frequency-specific components that produce a hyperacusis-like exaggeration of the acoustic startle reflex

Auditory brainstem-evoked response (ABR) thresholds were obtained in a longitudinal study of C57BL/6J mice between 10 and 53 weeks old, with repeated testing every 2 weeks. On alternate weeks, acoustic startle reflex (ASR) amplitudes were measured, elicited by tone pips with stimulus frequencies of 3, 6, 12, and 24 kHz, and intensities from subthreshold up to 110 dB sound pressure level. The increase in ABR thresholds for 3 and 6 kHz test stimuli followed a linear time course with increasing age from 10 to 53 weeks, with a slope of about 0.7 dB/week, and for 48 kHz a second linear time course, but beginning at 10 weeks with a slope of about 2.3 dB/week. ABR thresholds for 12, 24, and 32 kHz increased after one linear segment with a 0.7 dB slope, then after a variable delay related to the test frequency, shifted to a second segment having slopes of 3-5 dB/week. Hearing loss initially reduced the ASR for all eliciting stimuli, but at about 6 months of age, the response elicited by intense 3 and 6 kHz stimuli began to increase to reach values about three times above normal, and previously subthreshold stimuli came to elicit vigorous responses seen at first only for the intense stimuli. This hyperacusis-like effect appeared in all mice but was especially pronounced in mice with more serious hearing loss. These ABR data, together with a review of histopathological data in the C57BL/6 literature, suggest that the non-frequency-specific slow time course of hearing loss results from pathology in the lateral wall of the cochlea, whereas the stimulus-specific hearing loss with a rapid time course results from hair cell loss. Delayed exaggeration of the ASR with hearing loss reveals a deficit in centrifugal inhibitory control over the afferent reflex pathways after central neural reorganization, suggesting that this mouse may provide a useful model of age-related tinnitus and associated hyperacusis.

The cochleogram of the guinea pig

The cochleogram is an important tool to relate properties of the cochlea (e.g. hair cell loss, damaged hair cells) to their position in the cochlear turns, to calculate the average hair cell density, and to measure the length of the whole cochlea. In this work different methods of plotting cochleograms are compared. We suggest that a sector-wise division of the cochlea for counting a cochleogram has advantages over line diagrams that provide a higher spatial resolution but might lead to misinterpretations of the degree of missing hair cells. The scanning electron microscopic analysis of 171 guinea pig cochleas revealed a mean basilar membrane length of 16.4 +/- 1.4 mm (mean +/- standard deviation) with sector lengths of 6.9, 4.2, 3.2, and 1.9 mm, thus adding relevant information to the morphology of the guinea pig cochlea.

Development of tinnitus-related neuronal hyperactivity through homeostatic plasticity after hearing loss: a computational model

Tinnitus, the perception of a sound in the absence of acoustic stimulation, is often associated with hearing loss. Animal studies indicate that hearing loss through cochlear damage can lead to behavioral signs of tinnitus that are correlated with pathologically increased spontaneous firing rates, or hyperactivity, of neurons in the auditory pathway. Mechanisms that lead to the development of this hyperactivity, however, have remained unclear. We address this question by using a computational model of auditory nerve fibers and downstream auditory neurons. The key idea is that mean firing rates of these neurons are stabilized through a homeostatic plasticity mechanism. This homeostatic compensation can give rise to hyperactivity in the model neurons if the healthy ratio between mean and spontaneous firing rate of the auditory nerve is decreased, for example through a loss of outer hair cells or damage to hair cell stereocilia. Homeostasis can also amplify non-auditory inputs, which then contribute to hyperactivity. Our computational model predicts how appropriate additional acoustic stimulation can reverse the development of such hyperactivity, which could provide a new basis for treatment strategies.

Effects of cochlear ablation on noise induced hyperactivity in the hamster dorsal cochlear nucleus: implications for the origin of noise induced tinnitus

Chronic increases in multiunit spontaneous activity are induced in the dorsal cochlear nucleus (DCN) following exposures to intense sound. This hyperactivity has been implicated as a neurophysiological correlate of noise induced tinnitus. However, it is not known whether this hyperactivity originates centrally, or instead, reflects an increase in the level of spontaneous input from the auditory nerve. In the present study we addressed this issue by testing whether hyperactivity, induced in the DCN by previous exposure to intense sound, persists after ipsilateral cochlear input to the DCN has been removed. To induce hyperactivity, Syrian golden hamsters were exposed under anesthesia to an intense pure tone (122-127 dB SPL at 10 kHz) for 4 h. Additional hamsters, which were anesthetized for 4 h, but not tone exposed, served as controls. Electrophysiological recordings of spontaneous activity were performed on the surface of the left DCN in animals in which the ipsilateral cochlea was either intact or ablated. The degree of cochlear removal was determined by microdissection and histologic evaluation of the cochlea after completion of each recording session. Comparisons between the levels of activity recorded in animals with and without intact cochleas revealed that the induced hyperactivity in the DCN persisted after both partial and complete cochlear ablations. These results indicate that the maintenance of hyperactivity is not dependent on input from the ipsilateral cochlea, implying that hyperactivity originates centrally.

Dynamic representational plasticity in sensory cortex

Studies of the effects of peripheral and central lesions, perceptual learning and neurochemical modification on the sensory representations in cortex have had a dramatic effect in alerting neuroscientists and therapists to the reorganizational capacity of the adult brain. An intriguing aspect of some of these investigations, such as partial peripheral denervation, is the short-term expression of these changes. Indeed, in visual cortex, auditory cortex and somatosensory cortex loss of input from a region of the peripheral receptor epithelium (retinal, basilar and cutaneous, respectively) induces rapid expression of ectopic, or expanded, receptive fields of affected neurons and reorganization of topographic maps to fill in the representation of the denervated area. The extent of these changes can, in some cases, match the maximal extents demonstrated with chronic manipulations. The rapidity, and reversibility, of the effects rules out many possible explanations which involve synaptic plasticity and points to a capacity for representational plasticity being inherent in the circuitry of a topographic pathway. Consequently, topographic representations must be considered as manifestations of physiological interaction rather than as anatomical constructs. Interference with this interaction can produce an unmasking of previously inhibited responsiveness. Consideration of the nature of masking inhibition which is consistent with the precision and order of a topographic representation and which has a capacity for rapid plasticity requires, in addition to stimulus-driven inhibition, a source of tonic input from the periphery. Such input, acting locally to provide tonic inhibition, has been directly demonstrated in the somatosensory system and is consistent with results obtained in auditory and visual systems.

Deafferentation-induced changes in protein kinase C expression in the rat cochlear nucleus

Isoforms of the signal transducing molecule, protein kinase C (PKC), may play a role in neural plasticity following sensory deafferentation. To explore the role of PKC in central auditory plasticity, we studied the effect of auditory deafferentation on the expression of PKC betaI, betaII, gamma, and delta in the rat dorsal (DCN) and ventral cochlear nucleus (VCN), using immunocytochemistry. Male rats were treated with kanamycin and furosemide to induce hair cell loss. At various intervals post-treatment, brains were perfusion-fixed and processed for immunocytochemistry. Following deafferentation, we observed a gradual increase in PKC betaI immunoreactivity (ir) in the deepest layers of the DCN, possibly representing synapses of primary afferents or parallel fibers on unlabeled neurons. Correlated with this, we observed an increase in the number of neurons in the deep DCN that showed PKC delta ir. In controls, we observed PKC gamma ir in small ovoid cells concentrated in the middle layer of the DCN. From days 4 through 14 after deafferentation, we found an increase in the intensity of staining of these cells, with a return toward control levels by day 28. Finally, Purkinje-like cells (PLC) in the VCN, which express only PKC delta in control rats, began to express PKC gamma after deafferentation, correlated with increased expression of calbindin D28k in PLC. Thus PKC isoforms are differentially regulated in the CN following deafferentation, supporting a role for PKC in auditory plasticity.

Plasticity of spontaneous neural activity in the dorsal cochlear nucleus after intense sound exposure

Increases in multiunit spontaneous activity (hyperactivity) can be induced in the dorsal cochlear nucleus (DCN) by intense sound exposure. This hyperactivity has been observed in the hamster and rat following exposure to a 10 kHz tone at a level of 125-130 dB SPL for a period of 4 h. The present study demonstrates that the onset of this hyperactivity is not immediate, but develops in the DCN between 2 and 5 days after exposure. Mean rates of multiunit spontaneous activity increased sharply from below normal levels at day 2 to higher than normal levels at day 5. The mean magnitude of activity continued to increase more gradually over the next 6 months. During this period, changes in the distribution of hyperactivity across the tonotopic array were also noted. The hyperactivity was more broadly distributed across the DCN at the early post-exposure times (5 and 14 days) than at later post-exposure recovery times (30 and 180 days), and peak activity was found at increasingly more medial positions over this time frame. These changes over time indicate that the mechanisms leading to hyperactivity following intense sound exposure are more complex than previously realized.

Hyperactivity in the dorsal cochlear nucleus after intense sound exposure and its resemblance to tone-evoked activity: a physiological model for tinnitus

Intense tone exposure induces increased spontaneous activity (hyperactivity) in the dorsal cochlear nucleus (DCN) of hamsters. This increase may represent an important neural correlate of noise-induced tinnitus, a condition in which sound, typically of very high pitch, is perceived in the absence of a corresponding acoustic stimulus. Since high pitch sounds are thought to be represented in central auditory structures by the place of activation across the tonotopic array; it is therefore possible that the high pitch of noise-induced tinnitus occurs because intense sound exposure induces a tonotopic distribution of chronic hyperactivity in the DCN similar to that normally evoked only under conditions of high frequency stimulation. To investigate this possibility we compared this tone-induced hyperactivity with the activity evoked in normal animals by presentation of a tone. This comparison revealed that the activity in the DCN of animals which had been exposed to an intense 10 kHz tone 1 month previously showed a striking similarity to the activity in the DCN of normal animals during presentation of low to moderate level tonal stimuli of the same frequency. In both test conditions similar patterns were seen in the topographic distribution of the increased activity along the tonotopic axis. The magnitude of hyperactivity in exposed animals was similar to the evoked activity in the normal DCN responding to a stimulus at a level of 20 dB SL. These results suggest that the altered DCN following intense tone exposure behaves physiologically as though it is responding to a tone in the absence of a corresponding acoustic stimulus. The relevance of these findings to noise-induced tinnitus and their implications for understanding its underlying mechanisms are discussed.

Changes in spontaneous neural activity in the dorsal cochlear nucleus following exposure to intense sound: relation to threshold shift

Previous studies have shown that the dorsal cochlear nucleus exhibits increased spontaneous activity after exposure to intense sound. Such increases were apparent 1-2 months after the exposure and were generally proportional to the shift in response thresholds induced by the same exposure. The purpose of the present study was to determine whether this sound-induced increase in spontaneous activity is an early event which can be observed shortly after exposure. As in previous studies, anesthetized hamsters ranging in postnatal age from 60-70 days were exposed to a 10-kHz tone at levels between 125 and 130 dB SPL for a period of 4 h. Control animals were similarly anesthetized but were not exposed to the intense tone. Exposed animals were examined in two groups, one at 30 days after exposure, the other at 2 days after exposure. Time of exposure was adjusted so that all animals were between 90 and 100 days of age when spontaneous activity was studied electrophysiologically. The results showed that the increases in spontaneous activity, which were evident at 30 days after exposure, were not observed in animals studied 2 days after exposure. This result contrasted with the effect of the intense tone exposure on neural response thresholds. That is, the shifts in response thresholds seen 2 days after exposure were similar to those observed in animals studied 30 days after exposure. These results indicate that changes in spontaneous activity reflect a more slowly developing phenomenon and occur secondarily after induction of threshold shift.