Cisplatin-induced increases in spontaneous neural activity in the dorsal cochlear nucleus and associated outer hair cell loss

Auditory Neural Plasticity in Tinnitus Mechanisms and Management

Tinnitus, which is the perception of sound in the absence of a corresponding external acoustic stimulus, including change of hearing and neural plasticity, has become an increasingly important ailment affecting the daily life of a considerable proportion of the population and causing significant burdens for both the affected individuals and society as a whole. Here, we briefly review the epidemiology and classification of tinnitus, and the currently available treatments are discussed in terms of the available evidence for their mechanisms and efficacy. The conclusion drawn from the available evidence is that there is no specific medication for tinnitus treatment at present, and tinnitus management might provide better solutions. Therapeutic interventions for tinnitus should be based on a comprehensive understanding of the etiology and features of individual cases of tinnitus, and more high quality and large-scale research studies are urgently needed to develop more efficacious medications.

The Role of the Brainstem in Generating and Modulating Tinnitus

Purpose The purpose of this work is to present a perspective article summarizing ideas pertaining to the brainstem's role in generating and modulating tinnitus. It is organized in 4 sections: Part 1, the role of the brainstem as a tinnitus generator; Part 2, the role of the brainstem in modulating tinnitus; Part 3, the role of the brainstem in nonauditory comorbid conditions associated with tinnitus; and Part 4, clinical implications. In Part 1, well-established neurophysiological models are discussed providing the framework of evidence that auditory brainstem nuclei play a role in generating tinnitus. In Part 2, ideas are presented explaining modulatory effects on tinnitus related to underlying pathways originating from or projecting to brainstem auditory and nonauditory nuclei. This section addresses multiple phenomena including somatic-related, attention-mediated, and emotion-mediated changes in the tinnitus percept. In Part 3, the role of the brainstem in common nonauditory comorbidities that occur in patients with tinnitus is discussed. Part 4 presents clinical implications of these new ideas related to the brainstem's involvement in generating and modulating tinnitus. Impact Knowledge of the brainstem's involvement in generating and modulating tinnitus provides a context for health care professionals to understand the temporal relationship between tinnitus and common nonauditory comorbid conditions.

Coenzyme Q10 plus Multivitamin Treatment Prevents Cisplatin Ototoxicity in Rats

Cisplatin (Cpt) is known to induce a high level of oxidative stress, resulting in an increase of reactive oxygen species damaging the inner ear and causing hearing loss at high frequencies. Studies on animal models show that antioxidants may lower Cpt-induced ototoxicity. The aim of this study is to evaluate the ototoxic effects of two different protocols of Cpt administration in a Sprague-Dawley rat model, and to test in the same model the synergic protective effects of a solution of coenzyme Q10 terclatrate and Acuval 400®, a multivitamin supplement containing antioxidant agents and minerals (Acu-Qter). The Cpt was administered intraperitoneally in a single dose (14 mg/kg) or in three daily doses (4.6 mg/kg/day) to rats orally treated or untreated with Acu-Qter for 5 days. The auditory function was assessed by measuring auditory brainstem responses from 2 to 32 kHz at day 0 and 5 days after treatment. Similar hearing threshold and body weight alterations were observed in both Cpt administration protocols, but mortality reduced to zero when Cpt was administered in three daily doses. The Acu-Qter treatment was able to prevent and completely neutralize ototoxicity in rats treated with three daily Cpt doses, supporting the synergic protective effects of coenzyme Q terclatrate and Acuval 400® against Cpt-induced oxidative stress. The administration protocol involving three Cpt doses is more similar to common human chemotherapy protocols, therefore it appears more useful for long-term preclinical studies on ototoxicity prevention.

Neural Hyperactivity of the Central Auditory System in Response to Peripheral Damage

It is increasingly appreciated that cochlear pathology is accompanied by adaptive responses in the central auditory system. The cause of cochlear pathology varies widely, and it seems that few commonalities can be drawn. In fact, despite intricate internal neuroplasticity and diverse external symptoms, several classical injury models provide a feasible path to locate responses to different peripheral cochlear lesions. In these cases, hair cell damage may lead to considerable hyperactivity in the central auditory pathways, mediated by a reduction in inhibition, which may underlie some clinical symptoms associated with hearing loss, such as tinnitus. Homeostatic plasticity, the most discussed and acknowledged mechanism in recent years, is most likely responsible for excited central activity following cochlear damage.

Underlying mechanisms of tinnitus: review and clinical implications

BACKGROUND

The study of tinnitus mechanisms has increased tenfold in the last decade. The common denominator for all of these studies is the goal of elucidating the underlying neural mechanisms of tinnitus with the ultimate purpose of finding a cure. While these basic science findings may not be immediately applicable to the clinician who works directly with patients to assist them in managing their reactions to tinnitus, a clear understanding of these findings is needed to develop the most effective procedures for alleviating tinnitus.

PURPOSE

The goal of this review is to provide audiologists and other health-care professionals with a basic understanding of the neurophysiological changes in the auditory system likely to be responsible for tinnitus.

RESULTS

It is increasingly clear that tinnitus is a pathology involving neuroplastic changes in central auditory structures that take place when the brain is deprived of its normal input by pathology in the cochlea. Cochlear pathology is not always expressed in the audiogram but may be detected by more sensitive measures. Neural changes can occur at the level of synapses between inner hair cells and the auditory nerve and within multiple levels of the central auditory pathway. Long-term maintenance of tinnitus is likely a function of a complex network of structures involving central auditory and nonauditory systems.

CONCLUSIONS

Patients often have expectations that a treatment exists to cure their tinnitus. They should be made aware that research is increasing to discover such a cure and that their reactions to tinnitus can be mitigated through the use of evidence-based behavioral interventions.

Suppression of noise-induced hyperactivity in the dorsal cochlear nucleus following application of the cholinergic agonist, carbachol

Increased spontaneous firing (hyperactivity) is induced in fusiform cells of the dorsal cochlear nucleus (DCN) following intense sound exposure and is implicated as a possible neural correlate of noise-induced tinnitus. Previous studies have shown that in normal hearing animals, fusiform cell activity can be modulated by activation of parallel fibers, which represent the axons of granule cells. The modulation consists of a transient excitation followed by a more prolonged period of inhibition, presumably reflecting direct excitatory inputs to fusiform cells and an indirect inhibitory input to fusiform cells from the granule cell-cartwheel cell system. We hypothesized that since granule cells can be activated by cholinergic inputs, it might be possible to suppress tinnitus-related hyperactivity of fusiform cells using the cholinergic agonist, carbachol. To test this hypothesis, we recorded multiunit spontaneous activity in the fusiform soma layer (FSL) of the DCN in control and tone-exposed hamsters (10 kHz, 115 dB SPL, 4h) before and after application of carbachol to the DCN surface. In both exposed and control animals, 100 μM carbachol had a transient excitatory effect on spontaneous activity followed by a rapid weakening of activity to near or below normal levels. In exposed animals, the weakening of activity was powerful enough to completely abolish the hyperactivity induced by intense sound exposure. This suppressive effect was partially reversed by application of atropine and was usually not associated with significant changes in neural best frequencies (BF) or BF thresholds. These findings demonstrate that noise-induced hyperactivity can be pharmacologically controlled and raise the possibility that attenuation of tinnitus may be achievable by using an agonist of the cholinergic system.

Comparison and contrast of noise-induced hyperactivity in the dorsal cochlear nucleus and inferior colliculus

Induction of hyperactivity in the central auditory system is one of the major physiological hallmarks of animal models of noise-induced tinnitus. Although hyperactivity occurs at various levels of the auditory system, it is not clear to what extent hyperactivity originating in one nucleus contributes to hyperactivity at higher levels of the auditory system. In this study we compared the time courses and tonotopic distribution patterns of hyperactivity in the dorsal cochlear nucleus (DCN) and inferior colliculus (IC). A model of acquisition of hyperactivity in the IC by passive relay from the DCN would predict that the two nuclei show similar time courses and tonotopic profiles of hyperactivity. A model of acquisition of hyperactivity in the IC by compensatory plasticity mechanisms would predict that the IC and DCN would show differences in these features, since each adjusts to changes of spontaneous activity of opposite polarity. To test the role of these two mechanisms, animals were exposed to an intense hyperactivity-inducing tone (10 kHz, 115 dB SPL, 4 h) then studied electrophysiologically at three different post-exposure recovery times (from 1 to 6 weeks after exposure). For each time frame, multiunit spontaneous activity was mapped as a function of location along the tonotopic gradient in the DCN and IC. Comparison of activity profiles from the two nuclei showed a similar progression toward increased activity over time and culminated in the development of a central peak of hyperactivity at a similar tonotopic location. These similarities suggest that the shape of the activity profile is determined primarily by passive relay from the cochlear nucleus. However, the absolute levels of activity were generally much lower in the IC than in the DCN, suggesting that the magnitude of hyperactivity is greatly attenuated by inhibition.

Selective degeneration of synapses in the dorsal cochlear nucleus of chinchilla following acoustic trauma and effects of antioxidant treatment

The purpose of this study was to reveal synaptic plasticity within the dorsal cochlear nucleus (DCN) as a result of noise trauma and to determine whether effective antioxidant protection to the cochlea can also impact plasticity changes in the DCN. Expression of synapse activity markers (synaptophysin and precerebellin) and ultrastructure of synapses were examined in the DCN of chinchilla 10 days after a 105 dB SPL octave-band noise (centered at 4 kHz, 6 h) exposure. One group of chinchilla was treated with a combination of antioxidants (4-hydroxy phenyl N-tert-butylnitrone, N-acetyl-l-cysteine and acetyl-l-carnitine) beginning 4 h after noise exposure. Down-regulated synaptophysin and precerebellin expression, as well as selective degeneration of nerve terminals surrounding cartwheel cells and their primary dendrites were found in the fusiform soma layer in the middle region of the DCN of the noise exposure group. Antioxidant treatment significantly reduced synaptic plasticity changes surrounding cartwheel cells. Results of this study provide further evidence of acoustic trauma-induced neural plasticity in the DCN and suggest that loss of input to cartwheel cells may be an important factor contributing to the emergence of hyperactivity in the DCN after noise exposure. Results further suggest that early antioxidant treatment for acoustic trauma not only rescues cochlear hair cells, but also has impact on central auditory structures.

Tinnitus: Models and mechanisms

Over the past decade, there has been a burgeoning of scientific interest in the neurobiological origins of tinnitus. During this period, numerous behavioral and physiological animal models have been developed which have yielded major clues concerning the likely neural correlates of acute and chronic forms of tinnitus and the processes leading to their induction. The data increasingly converge on the view that tinnitus is a systemic problem stemming from imbalances in the excitatory and inhibitory inputs to auditory neurons. Such changes occur at multiple levels of the auditory system and involve a combination of interacting phenomena that are triggered by loss of normal input from the inner ear. This loss sets in motion a number of plastic readjustments in the central auditory system and sometimes beyond the auditory system that culminate in the induction of aberrant states of activation that include hyperactivity, bursting discharges and increases in neural synchrony. This article will review was has been learned about the biological origins of these alterations, summarize where they occur and examine the cellular and molecular mechanisms that are most likely to underlie them.

Effects of sodium salicylate on spontaneous and evoked spike rate in the dorsal cochlear nucleus

Spontaneous hyperactivity in the dorsal cochlear nucleus (DCN), particularly in fusiform cells, has been proposed as a neural generator of tinnitus. To determine if sodium salicylate, a reliable tinnitus inducer, could evoke hyperactivity in the DCN, we measured the spontaneous and depolarization-evoked spike rate in fusiform and cartwheel cells during salicylate superfusion. Five minute treatment with 1.4 mM salicylate suppressed spontaneous and evoked firing in fusiform cells; this decrease partially recovered after salicylate washout. Less suppression and greater recovery occurred with 3 min treatment using 1.4 mM salicylate. In contrast, salicylate had no effect on the spontaneous or evoked firing of cartwheel cells indicating that salicylate's suppressive effects are specific to fusiform cells. To determine if salicylate's suppressive effects were a consequence of increased synaptic inhibition, spontaneous inhibitory postsynaptic currents (IPSC) were measured during salicylate treatment. Salicylate unexpectedly reduced IPSC thereby ruling out increased inhibition as a mechanism to explain the depressed firing rates in fusiform cells. The salicylate-induced suppression of fusiform spike rate apparently arises from unidentified changes in the cell's intrinsic excitability.

Dorsal cochlear nucleus hyperactivity and tinnitus: are they related?

PURPOSE

Eight lines of evidence implicating the dorsal cochlear nucleus (DCN) as a tinnitus contributing site are reviewed. We now expand the presentation of this model, elaborate on its essential details, and provide answers to commonly asked questions regarding its validity.

CONCLUSIONS

Over the past decade, numerous studies have converged to support the hypothesis that the DCN may be an important brain center in the generation and modulation of tinnitus. Although other auditory centers have been similarly implicated, the DCN deserves special emphasis because, as a primary acoustic nucleus, it occupies a potentially pivotal position in the hierarchy of functional processes leading to the emergence of tinnitus percepts. Moreover, because a great deal is known about the underlying cellular categories and the details of synaptic circuitry within the DCN, this brain center offers a potentially powerful model for probing mechanisms underlying tinnitus.

Tinnitus and inferior colliculus activity in chinchillas related to three distinct patterns of cochlear trauma

A longstanding hypothesis is that tinnitus, the perception of sound without an external acoustic source, is triggered by a distinctive pattern of cochlear hair cell (HC) damage and that this subsequently leads to altered neural activity in the central auditory pathway. This hypothesis was tested by assessing behavioral evidence of tinnitus and spontaneous neural activity in the inferior colliculus (IC) after unilateral cochlear trauma. Chinchillas were assigned to four cochlear treatment groups. Each treatment produced a distinctive pattern of HC damage, as follows: acoustic exposure (AEx): sparse low-frequency inner hair cell (IHC) and outer hair cell (OHC) loss; round window cisplatin (CisEx): pronounced OHC loss mixed with some IHC loss; round window carboplatin (CarbEx): pronounced IHC loss without OHC loss; control: no loss. Compared with controls, all experimental groups displayed significant and similar psychophysical evidence of tinnitus with features resembling a 1-kHz tone. Contralateral IC spontaneous activity was elevated in the AEx and CisEx groups, which showed increased spiking and increased cross-fiber synchrony. A multidimensional analysis identified a subpopulation of neurons more prevalent in animals with tinnitus. These units were characterized by high bursting, low ISI variance, and within-burst peak spiking of approximately 1,000/sec. It was concluded that cochlear trauma in general, rather than its specific features, leads to multiple changes in central activity that underpin tinnitus. Particularly affected was a subpopulation ensemble of IC neurons with the described unique triad of features.

Intense sound-induced plasticity in the dorsal cochlear nucleus of rats: Evidence for cholinergic receptor upregulation

Previous studies in a number of species have demonstrated that spontaneous activity in the dorsal cochlear nucleus (DCN) becomes elevated following exposure to intense sound. This condition of hyperactivity has aroused considerable interest because it may represent an important neural correlate of tinnitus. There is some evidence that neurons in the superficial DCN, such as cartwheel, stellate and fusiform cells, may contribute to the level of hyperactivity induced by intense sound, although the relative importance of these different cell types is unknown. In the present study, we sought to determine the effect of intense sound exposure on multiunit spontaneous activity both at the DCN surface and in the fusiform cell layer and to examine the influence of cholinergic input to DCN circuits on the level of activity in the fusiform cell layer. Rats were studied in two groups, one of which had been exposed to a continuous intense sound (10 kHz 127 dB SPL) for 4h while the other group served as unexposed controls. Between 30 and 52 days post-exposure, recordings of multiunit activity were performed at the DCN surface as well as in the middle of the fusiform cell layer. Changes in fusiform cell layer activity were also studied in response to superficial applications of the cholinergic agonist, carbachol, either alone or following pre-application of the cholinergic antagonist, atropine. The results demonstrated that multiunit spontaneous activity in the rat DCN was generally much higher in both control and exposed animals relative to that which has been observed in other species. This activity was significantly higher at the DCN surface of sound-exposed animals than that of controls. In contrast, hyperactivity could not be demonstrated in the fusiform cell layer of sound-exposed animals. Carbachol administration most commonly caused suppression of fusiform cell layer activity. However, this suppression was considerably stronger in the DCN of sound-exposed animals than in controls. These findings suggest that, hyperactivity at the DCN surface of exposed rats may arise as a consequence of more highly activated neurons in the molecular layer, such as cartwheel and/or stellate cells, and that the lack of hyperactivity in the fusiform cell layer may be the result of inhibition of fusiform cells by these inhibitory interneurons. Although this finding does not rule out fusiform cells as possible sources of hyperactivity in other species, or even in the rat after short post-exposure recovery periods, the enhanced sensitivity of the fusiform cell layer to cholinergic stimulation suggests that in the rat, at least after prolonged post-exposure recovery periods, increased inhibition of activity in this layer by more superficially located neurons may result from an upregulation of receptors for cholinergic input. This upregulation may be greater in rats than in other species due to the relatively heavy cholinergic input that exists in the cochlear nucleus of this species.

Primary afferent dendrite degeneration as a cause of tinnitus

Chronic tinnitus affects millions of people, but the mechanisms responsible for the development of this abnormal sensory state remain poorly understood. This study examined the type and extent of cochlear damage that occurs after acoustic trauma sufficient to induce chronic tinnitus in rats. Tinnitus was evaluated by using a conditioned suppression method of behavioral testing. Cochlear damage was assessed 6 months after acoustic trauma. There was minimal loss of inner and outer hair cells in the exposed cochleas of subjects demonstrating evidence of tinnitus. However, a significant loss of large-diameter fibers in the osseous spiral lamina of exposed cochleas of trauma subjects was observed. The significance of this finding in the context of a model of tinnitus is discussed.

Neuronal nitric oxide synthase expression in the cochlear nucleus in a salicylate model of tinnitus

Although a number of studies suggest that the development of tinnitus is associated with hyperactive neuronal discharges in the brainstem cochlear nucleus (CN), there is relatively little evidence to indicate the neurochemical basis of this phenomenon. While some studies suggest that it may be partly due to a decrease in GABAergic inhibition, it is also possible that increased excitability is a contributing factor. In the current study, we investigated whether the salicylate animal model of tinnitus is associated with changes in the number of CN neurons expressing neuronal nitric oxide synthase (nNOS), one of the NOS isoforms that results in the production of the neurotransmitter, nitric oxide. We used a behavioral conditioning paradigm to confirm that animals receiving salicylate injections experienced tinnitus, and used immunohistochemistry with stereology to quantify the number of nNOS-expressing neurons in the dorsal and ventral CN (DCN and VCN, respectively) in salicylate- and vehicle control-treated animals. We also employed Western blotting to quantify the amount of nNOS protein expression in the total CN (i.e., the DCN and VCN together). We found a significant increase (of approximately 70%) in the number of nNOS-expressing principal neurons in the VCN of salicylate-treated animals compared to controls, with no significant differences in the DCN; nor did we find any significant difference in the overall level of nNOS protein in the total CN using Western blotting. These results suggest that changes in the number of neurons in the VCN expressing nNOS may be implicated in the mechanisms of tinnitus.

The dorsal cochlear nucleus as a participant in the auditory, attentional and emotional components of tinnitus

The dorsal cochlear nucleus (DCN) has been modeled in numerous studies as a possible source of tinnitus-generating signals. This hypothesis was originally developed on the basis of evidence that the DCN becomes hyperactive following exposure to intense noise. Since these early observations, evidence that the DCN is an important contributor to tinnitus has grown considerably. In this paper, the available evidence to date will be summarized. In addition, the DCN hypothesis of tinnitus can now be expanded to include possible involvement in other, non-auditory components of tinnitus. It will be shown by way of literature review that the DCN has direct connections with non-auditory brainstem structures, such as the locus coeruleus, reticular formation and raphe nuclei, that are implicated in the control of attention and emotional responses. The hypothesis will be presented that attentional and emotional disorders, such as anxiety and depression, which are commonly associated with tinnitus, may result from an interplay between these non-auditory brainstem structures and the DCN. Implicit in this hypothesis is that attempts to develop effective anti-tinnitus therapies are likely to benefit from a greater understanding of how the levels of activity in the DCN are influenced by different states of activation of these non-auditory brainstem structures and vice versa.

Reduction of acute cisplatin ototoxicity and nephrotoxicity in rats by oral administration of allopurinol and ebselen

Cisplatin ototoxicity has been associated with the generation of toxic levels of reactive oxygen species (ROS) which can lead to injury or loss of outer hair cells in the organ of Corti, damage to the stria vascularis, and loss of spiral ganglion cells, resulting in permanent hearing loss. In an attempt to reduce the formation of ROS and to bolster the innate oxidative stress defenses of the cochlea, we tested individual and combined formulations of allopurinol, a xanthine oxidase inhibitor, and ebselen, a glutathione peroxidase mimic. We used an acute cisplatin toxicity rat model (16 mg/kg i.p.) to analyze allopurinol and ebselen alone and in combination for their ability to reduce cisplatin associated hearing loss and nephrotoxicity. The results from our studies indicate that a combined formulation of ebselen and allopurinol affords significant protection to the cochlea and kidney from cisplatin toxicity. In the cochlea, protection is dependent on the preservation of outer hair cell number, while in the kidney, protection is associated with the preservation of proximal tubular epithelia. Further evaluation of the chemoprotective effects of ebselen and allopurinol on cisplatin side effects in the presence of tumor appears warranted.

Tinnitus as a plastic phenomenon and its possible neural underpinnings in the dorsal cochlear nucleus

Tinnitus displays many features suggestive of plastic changes in the nervous system. These can be categorized based on the types of manipulations that induce them. We have categorized the various forms of plasticity that characterize tinnitus and searched for their neural underpinnings in the dorsal cochlear nucleus (DCN). This structure has been implicated as a possible site for the generation of tinnitus-producing signals owing to its tendency to become hyperactive following exposure to tinnitus inducing agents such as intense sound and cisplatin. In this paper, we review the many forms of plasticity that have been uncovered in anatomical, physiological and neurochemical studies of the DCN. Some of these plastic changes have been observed as consequences of peripheral injury or as fluctuations in the behavior and chemical activities of DCN neurons, while others can be induced by stimulation of auditory or even non-auditory structures. We show that many parallels can be drawn between the various forms of plasticity displayed by tinnitus and the various forms of neural plasticity which have been defined in the DCN. These parallels lend further support to the hypothesis that the DCN is an important site for the generation and modulation of tinnitus-producing signals.

Effect of unilateral noise exposure on the tonotopic distribution of spontaneous activity in the cochlear nucleus and inferior colliculus in the cortically intact and decorticate rat

Effects of unilateral noise exposure on spontaneous activity (SA) in the anteroventral and dorsal cochlear nuclei (AVCN and DCN) and the central nucleus of the inferior colliculus (ICc) were studied in cortically intact and decorticate rats. SA was measured 1 week following exposure using uptake of 14C-labeled 2-deoxyglucose (2DG) in quiet. Optical density (OD) measurements were obtained in low- and high-frequency (LF and HF) areas of each nucleus. We refer to the ipsilateral AVCN and DCN (side of the noise-exposed ear) and the contralateral ICc as direct nuclei and to their opposite side counterparts as indirect nuclei. Noise exposure altered the tonotopic profile of SA in the direct pathway by causing a decrease in the ratio of HF OD to LF OD (HF/LF ratio). In intact animals, the decreased HF/LF ratio was due to decreased HF OD. In decorticate animals, it was due to decreased HF OD and increased LF OD, the latter occurring mainly in the DCN and ICc. Decorticate-intact differences may reflect corticofugal feedback inhibition. Lesion of the dorsal acoustic stria caused a substantial decrement of SA in the contralateral ICc. Furthermore, strong positive correlations between HF/LF ratios in the DCN, AVCN, and contralateral ICc suggest that the cochlear nucleus is a major contributor to SA in the ICc. Noise exposure had opposite and weaker effects on 2DG uptake in the indirect pathway that were attributed to crossed inhibition. Noise-induced changes in the tonotopic profile of SA may represent a neural correlate of tinnitus.

Cisplatin-induced hyperactivity in the dorsal cochlear nucleus and its relation to outer hair cell loss: relevance to tinnitus

Cisplatin causes both acute and chronic forms of tinnitus as well as increases in spontaneous neural activity (hyperactivity) in the dorsal cochlear nucleus (DCN) of hamsters. It has been hypothesized that the induction of hyperactivity in the DCN may be a consequence of cisplatin's effects on cochlear outer hair cells (OHCs); however, systematic studies testing this hypothesis have yet to appear in the literature. In the present investigation, the relationship between hyperactivity and OHC loss, induced by cisplatin, was examined in detail. Hamsters received five treatments of cisplatin at doses ranging from 1.5 to 3 mg. kg(-1). day(-1), every other day. Beginning 1 mo after initiation of treatment, electrophysiological recordings were carried out on the surface of the DCN to measure spontaneous multiunit activity along a set of coordinates spanning the medial-lateral (tonotopic) axis of the DCN. After recordings, cochleas were removed and studied histologically using a scanning electron microscope. The results revealed that cisplatin-treated animals with little or no loss of OHCs displayed levels of activity similar to those seen in saline-treated controls. In contrast, the majority (75%) of cisplatin-treated animals with severe OHC loss displayed well-developed hyperactivity in the DCN. The induced hyperactivity was seen mainly in the medial (high-frequency) half of the DCN of treated animals. This pattern was consistent with the observation that OHC loss was distributed mainly in the basal half of the cochlea. In several of the animals with severe OHC loss and hyperactivity, there was no significant damage to IHC stereocilia nor any observable irregularities of the reticular lamina that might have interfered with normal IHC function. Hyperactivity was also observed in the DCN of animals showing severe losses of OHCs accompanied by damage to IHCs, although the degree of hyperactivity in these animals was less than in animals with severe OHC loss but intact IHCs. These results support the view that loss of OHC function may be a trigger of tinnitus-related hyperactivity in the DCN and suggest that this hyperactivity may be somewhat offset by damage to IHCs.

Elevated fusiform cell activity in the dorsal cochlear nucleus of chinchillas with psychophysical evidence of tinnitus

Chinchillas with psychophysical evidence of chronic tinnitus were shown to have significantly elevated spontaneous activity and stimulus-evoked responses in putative fusiform cells of the dorsal cochlear nuclei (DCN). Chinchillas were psychophysically trained and tested before and after exposure to a traumatic unilateral 80 dB (sound pressure level) 4 kHz tone. Before exposure, two groups were matched in terms of auditory discrimination performance (noise, and 1, 4, 6, and 10 kHz tones). After exposure, a single psychophysical difference emerged between groups. The exposed group displayed enhanced discrimination of 1 kHz tones (p = 0.00027). Postexposure discrimination of other stimuli was unaffected. It was hypothesized that exposed animals experienced a chronic subjective tone (i.e., tinnitus), resulting from their trauma, and that features of this subjective tone were similar enough to 1 kHz to affect discrimination of 1 kHz objective signals. After psychophysical testing, single-unit recordings were obtained from each animal's DCN fusiform cell layer. Putative fusiform cells of exposed animals showed significantly (p = 0.0136) elevated spontaneous activity, compared with cells of unexposed animals. Putative fusiform cells of exposed animals showed a greater stimulus-evoked response to tones at 1 kHz (p = 0.0000006) and at characteristic-frequency (p = 0.0000009). This increased activity was more pronounced on the exposed side. No increase in stimulus-evoked responses was observed to other frequencies or noise. These parallel psychophysical and electrophysiological results are consistent with the hypothesis that chronic tonal tinnitus is associated with, and may result from, trauma-induced elevation of activity of DCN fusiform cells.

Increases in spontaneous neural activity in the hamster dorsal cochlear nucleus following cisplatin treatment: a possible basis for cisplatin-induced tinnitus

Recent investigations in the hamster have implicated increased spontaneous activity (SA) in the dorsal cochlear nucleus (DCN) as a contributing factor in the etiology of tinnitus induced by intense sound exposure. It might therefore be expected that increased SA would also develop in the DCN of hamsters treated with cisplatin, another cause of tinnitus. We tested this hypothesis by measuring the effects of cisplatin on SA in the DCN. Adult hamsters were divided into three groups, each receiving five injections of cisplatin at one of the following doses: 3 mg/kg, 2.25 mg/kg, or 1.5 mg/kg. Each group had corresponding controls receiving injections of isotonic saline. The effects of cisplatin were studied electrophysiologically 1 month after treatment by recording multiunit SA on the surface of the DCN. Measurements of SA were obtained in three rows of 13-15 locations spaced roughly 100 microm apart and spanning the length of the DCN along the tonotopic axis. Effects of cisplatin were evaluated by comparing plots of mean SA vs. tonotopic locus for cisplatin-treated groups with those of their corresponding untreated control groups. The results demonstrated a consistently higher level of SA in cisplatin-treated groups than in untreated controls. Whereas the highest rates of mean SA in control groups were between 10 and 15 events/s, the highest mean spontaneous rates in cisplatin-treated groups were between 25 and 38 events/s. The cisplatin-induced hyperactivity was greatest in the medial half of the DCN corresponding to the high frequency portion of the tonotopic range. These results suggest that cisplatin treatment is an effective inducer of hyperactivity in the DCN. This hyperactivity may be an important neural correlate of cisplatin-induced tinnitus.