Effects of antioxidants on auditory nerve function and survival in deafened guinea pigs

Protective Effects of Vitamin C against Neomycin-Induced Apoptosis in HEI-OC1 Auditory Cell

Ototoxic hearing loss results from hair cell death via reactive oxygen species (ROS) overproduction and consequent apoptosis. We investigated the effects of vitamin C (VC) on neomycin-induced HEI-OC1 cell damage, as well as the mechanism of inhibition. HEI-OC1 cells were treated with neomycin or with vitamin C (VC). The results indicated that VC had a protective effect on neomycin-induced HEI-OC1 cell death. Mechanistically, VC decreased neomycin-induced ROS generation, suppressed cell death, and increased cell viability. VC inhibited neomycin-induced apoptosis, ameliorated neomycin reduced antiapoptotic Bcl-2 expression, and suppressed neomycin increased expression of proapoptotic Bax, caspase-3 cleavage and caspase-8. TUNEL labeling demonstrated that VC blocked neomycin-induced apoptosis. Further study revealed that the effect of VC on neomycin-induced hair cell death was through interference with JNK activation and p38 phosphorylation. These results indicate that VC via suppressed ROS generation, which inhibited cell death by counteracting apoptotic signaling induced by neomycin in cells. Hence, VC is a potential candidate for protection agent against neomycin-induced HEI-OC1 cell ototoxicity.

Protective Effects of N-Acetyl-L-Cysteine on the Density of Spiral Ganglion Cells and Histological Changes Induced by Continuous Noise Exposure in Rats

Background

Noise exposure causes loss of cochlea hair cells, leading to permanent sensorineural hearing loss, and initiates pathological changes to the bipolar primary auditory neurons (ANs). This study focuses on the effects of N-acetyl-l-cysteine (NAC) in protecting the density of spiral ganglion cells and in histological changes induced by continuous noise exposure in rats.

Methods

Twenty-four male Wistar rats were randomly allocated into four experimental groups to receive NAC, saline, noise, or both noise and NAC. Noise exposure continued for ten days. Saline and NAC were injected daily during the noise exposure, and 2 days before and after the noise exposure. Evaluation of cochlear histopathology and the density of spiral ganglion cells was performed 21 days after exposure.

Results

In the animals exposed to noise, a reduction in the density of spiral ganglion cells was evident in both the basal and middle turns of the cochlea. This improved on receiving NAC treatment (P = 0.046). In the histopathology evaluation, some histological changes, such as disorganised architecture of the outer hair and supporting cells and a slightly thickened basilar membrane, were found in the basal turns in the noise group.

Conclusion

NAC offered partial protection against noise exposure by improving the density of spiral ganglion cells and reducing morphological changes.

Effects of subcutaneous alfaxalone alone and in combination with dexmedetomidine and buprenorphine in guinea pigs (Cavia porcellus)

OBJECTIVE

To characterize alfaxalone administered subcutaneously (SC) in guinea pigs, both alone and in combination with dexmedetomidine and buprenorphine.

STUDY DESIGN

Prospective, blinded, crossover study.

ANIMALS

A total of 15 healthy female guinea pigs weighing 400-600 g.

METHODS

Alfaxalone (10, 20 and 40 mg kg^-1) was administered SC to three guinea pigs as a pilot dose-finding study. Alfaxalone (20 mg kg^-1; A₂₀) was selected for comparison against combination protocols of alfaxalone (15 and 20 mg kg^-1) with dexmedetomidine (0.25 mg kg^-1) and buprenorphine (0.05 mg kg^-1; A₁₅DB, A₂₀DB). Each protocol was randomly administered to 12 guinea pigs separated by ≥7 days. Time and quality of induction and recovery, heart rate, respiratory rate, peripheral hemoglobin oxygen saturation, rectal temperature, pedal withdrawal reflex and adverse effects were recorded.

RESULTS

The median time to induction for A₂₀, A₁₅DB and A₂₀DB was 6.8-8.0 minutes with no significant difference between treatments. Mean duration of recumbency for A₂₀ was 73.6 ± 19.6 minutes. Recumbency duration for A₁₅DB and A₂₀DB extended to 90 minutes, at which time dexmedetomidine was antagonized using atipamezole (0.025 mg kg^-1 SC). Physiological variables were within normal limits with the exception of one animal that died 45 minutes following treatment with A₂₀DB. Pedal withdrawal reflex remained intact with all treatments. Minor side effects such as twitching or bruxism occurred sporadically with treatment A₂₀ but not with A₁₅DB and A₂₀DB.

CONCLUSIONS AND CLINICAL RELEVANCE

SC alfaxalone produced uncomplicated sedation that may be recommended for nonpainful procedures that do not require complete immobility. The addition of dexmedetomidine and buprenorphine increased the duration of sedation and immobility, but did not result in general anesthesia. This combination sedation protocol may be useful for nonpainful procedures requiring extended immobility.

Local inner ear application of dexamethasone in cochlear implant models is safe for auditory neurons and increases the neuroprotective effect of chronic electrical stimulation

Dexamethasone (DEX) can reduce fibrous tissue growth as well as loss of residual hearing which may occur after cochlear implantation. Little is known about the effect of local inner ear DEX treatment on the spiral ganglion neurons (SGN), which are the target of the electrical stimulation with a cochlear implant (CI). Three different clinically relevant strategies of DEX-delivery into the inner ear were used. DEX was either eluted from the electrode carriers' silicone, released from a reservoir by passive diffusion, or actively applied using a pump based system. The effect of the locally applied DEX on SGN density, size and function was evaluated. DEX did not affect the SGN density compared to the relevant control groups. Simultaneously applied with chronic electrical stimulation (ES), DEX increased the neuroprotective effect of ES in the basal region and the hearing threshold tended to decrease. The EABR thresholds did not correlate with the relevant SGN density. When correlating the SGN number with fibrosis, no dependency was observed. DEX concentrations as applied in these animal models are safe for inner ear delivery in terms of their effect on SGN density. Additionally, DEX tends to improve the neuroprotective effect of chronic electrical stimulation by increasing the number of surviving neurons. This is an important finding in regard to clinical applications of DEX for local treatment of the inner ear in view of cochlear implantation and other applications.

Spiral ganglion stem cells can be propagated and differentiated into neurons and glia

The spiral ganglion is an essential functional component of the peripheral auditory system. Most types of hearing loss are associated with spiral ganglion cell degeneration which is irreversible due to the inner ear's lack of regenerative capacity. Recent studies revealed the existence of stem cells in the postnatal spiral ganglion, which gives rise to the hope that these cells might be useful for regenerative inner ear therapies. Here, we provide an in-depth analysis of sphere-forming stem cells isolated from the spiral ganglion of postnatal mice. We show that spiral ganglion spheres have characteristics similar to neurospheres isolated from the brain. Importantly, spiral ganglion sphere cells maintain their major stem cell characteristics after repeated propagation, which enables the culture of spheres for an extended period of time. In this work, we also demonstrate that differentiated sphere-derived cell populations not only adopt the immunophenotype of mature spiral ganglion cells but also develop distinct ultrastructural features of neurons and glial cells. Thus, our work provides further evidence that self-renewing spiral ganglion stem cells might serve as a promising source for the regeneration of lost auditory neurons.

Protective effect of trimetazidine on amikacin-induced ototoxicity in rats

OBJECTIVE

Aminoglycoside antibiotics are known to have ototoxic effects and may induce sensorineural hearing loss. This study investigated the protective effect of trimetazidine, which has antioxidant and cytoprotective properties, against amikacin ototoxicity.

METHODS

Thirty-two male rats were divided into four groups - amikacin, amikacin + trimetazidine, trimetazidine, and control groups. Trimetazidine, 10 mg/kg per day, was given for 14 days by oral gavage. Amikacin, 600 mg/kg per day, was also given for 14 days, by the intramuscular route. Distortion product otoacoustic emission (DPOAE) and auditory brainstem response (ABR) tests were applied to the rats for hearing assessment. At the termination of the study, the biochemical parameters were calculated to evaluate the oxidative status.

RESULTS

The DPOAE values of the amikacin group were significantly lower on the 7th and 14th days than those of the trimetazidine + amikacin group and there was an increase in the ABR thresholds. The ABR thresholds for the amikacin group on the 7th and 14th days were significantly higher than the levels on the first day of the study, while there was no significant increase in those values in the trimetazidine + amikacin group. The total oxidant status (TOS) and oxidant status index (OSI) values of the amikacin group were significantly higher than those of the trimetazidine + amikacin group. The total antioxidant status (TAS) values of the amikacin group were lower than those of the trimetazidine + amikacin group.

CONCLUSIONS

The audiologic tests and biochemical parameters investigated in this study both point to the protective effect of trimetazidine against amikacin-induced ototoxicity.

L-cysteine attenuates peroxynitrite-elicited cytotoxicity to spiral ganglion neurons: possible relation to hearing loss

OBJECTIVES

The purpose of this work was to investigate whether L-cysteine was able to protect spiral ganglion neurons (SGNs) against peroxynitrite (ONOO(-))-elicited toxicity.

METHODS

The rat SGNs were isolated and cultured in this work. Cell viability was assessed by 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay. The morphological changes were examined under inverted phase contrast microscope. Cells underwent apoptosis were determined by TdT-mediated dUTP nick end labeling (TUNEL) assay. Intracellular glutathione (GSH) content, superoxide dismutase (SOD) activity and malonaldehyde (MDA) level were detected by biochemical methods. Laser scanning confocal microscope was employed to analyze cytosolic Ca(2+) concentration.

RESULTS

Results showed that ONOO(-) reduced the cell viability of SGNs in a time- and dose-dependent manner. ONOO(-)-triggered cell damage was further confirmed via apoptotic pathway rather than necrosis. Pretreatment with L-cysteine (5 mM) for 12 hours could almost completely rescue SGNs from ONOO(-)-induced damage. The decrease in intracellular GSH content and SOD activity, as well as the increase in MDA level induced by ONOO(-) were correspondingly antagonized by the administration of L-cysteine. Furthermore, L-cysteine can significantly inhibit elevation of Ca(2+) concentration induced by ONOO(-).

DISCUSSION

Our findings indicate that L-cysteine protects SGNs from ONOO(-)-induced damage via enhancing the antioxidative activity and, suppressing the lipid peroxidation as well as the release of cytosolic Ca(2+), thereby indicating that oxidation resistance was useful to prevent audiological diseases initiated by oxidative stress.

Post-treatment effects of local GDNF administration to the inner ears of deafened guinea pigs

For patients with profound hearing loss, a cochlear implant is the only treatment available today. The function of a cochlear implant depends in part on the function and survival of spiral ganglion neurons. Following deafferentation, glial cell-derived neurotrophic factor (GDNF) is known to affect spiral ganglion neuron survival. The purpose of this study was to assess delayed GDNF treatment after deafening, the effects of cessation of GDNF treatment, and the effects of subsequent antioxidants on responsiveness and survival of the spiral ganglion neurons. Three-week deafened (by local neomycin administration) guinea pigs were implanted in the scala tympani with a combined cochlear implant electrode and cannula. GDNF (1 μg/mL) or artificial perilymph was then delivered for 4 weeks, following which the animals received systemic ascorbic acid  +  Trolox or saline for an additional 4 weeks. Thresholds for electrically-evoked auditory brain stem responses (eABRs) were significantly elevated at 3 weeks with deafness, stabilized with GDNF, and showed no change with GDNF cessation and treatment with antioxidants or saline. The populations of spiral ganglion neurons were reduced with deafness (by 40% at 3 weeks and 70% at 11 weeks), and rescued from cell death by GDNF with no further reduction at 8 weeks following 4 weeks of cessation of GDNF treatment equally in both the antioxidant- and saline-treated groups. Local growth factor treatment of the deaf ear may prevent deterioration in electrical responsiveness and rescue auditory nerve cells from death; these effects outlast the period of treatment, and may enhance the benefits of cochlear implant therapy for the deaf.

Nutrient plasma levels achieved during treatment that reduces noise-induced hearing loss

Hearing loss encompasses both temporary and permanent deficits. If temporary threshold shift (TTS) and permanent threshold shift (PTS) share common pathological mechanisms, then agents that reduce PTS also should reduce TTS. Several antioxidant agents have reduced PTS in rodent models; however, reductions in TTS have been inconsistent. This study first determined whether dietary antioxidants (beta-carotene and vitamins C and E) delivered in combination with magnesium (Mg) reliably increase plasma concentrations of the active agents. Then, additional manipulations tested the hypothesis that these nutrients reduce acute TTS insult in the first 24 h after loud sound as well as longer lasting changes in hearing measured up to 7 days postnoise. Saline or nutrients were administered to guinea pigs prior to and after noise exposure. Sound-evoked electrophysiological responses were measured before noise, with tests repeated 1-h postnoise, as well as 1-day, 3-days, 5-days, and 7-days postnoise. All subjects showed significant functional recovery; subjects treated with nutrients recovered more rapidly and had better hearing outcomes at early postnoise times as well as the final test time. Thus, this combination of nutrients, which produced significant increases in plasma concentrations of vitamins C and E and Mg, effectively reduced hearing loss at multiple postnoise times. These data suggest that free radical formation contributes to TTS as well as PTS insults and suggest a potential opportunity to prevent TTS in human populations.

Biomaterials in cochlear implants

The cochlear implant (CI) represents, for almost 25 years now, the gold standard in the treatment of children born deaf and for postlingually deafened adults. These devices thus constitute the greatest success story in the field of ‘neurobionic’ prostheses. Their (now routine) fitting in adults, and especially in young children and even babies, places exacting demands on these implants, particularly with regard to the biocompatibility of a CI’s surface components. Furthermore, certain parts of the implant face considerable mechanical challenges, such as the need for the electrode array to be flexible and resistant to breakage, and for the implant casing to be able to withstand external forces.

As these implants are in the immediate vicinity of the middle-ear mucosa and of the junction to the perilymph of the cochlea, the risk exists – at least in principle – that bacteria may spread along the electrode array into the cochlea. The wide-ranging requirements made of the CI in terms of biocompatibility and the electrode mechanism mean that there is still further scope – despite the fact that CIs are already technically highly sophisticated – for ongoing improvements to the properties of these implants and their constituent materials, thus enhancing the effectiveness of these devices.

This paper will therefore discuss fundamental material aspects of CIs as well as the potential for their future development.

Spiral ganglion cell survival after round window membrane application of brain-derived neurotrophic factor using gelfoam as carrier

Several studies have shown that treatment with various neurotrophins protects spiral ganglion cells (SGCs) from degeneration in hair-cell deprived cochleas. In most of these studies the neurotrophins are delivered by means of intracochlear delivery methods. Recently, other application methods that might be more suited in cochlear implant patients have been developed. We have examined if round window membrane application of gelfoam infiltrated with a neurotrophin resulted in SGC survival in deafened guinea pigs. Two weeks after deafening, gelfoam cubes infiltrated with 6 μg of brain-derived neurotrophic factor (BDNF) were deposited onto the round window membrane of the right cochleas. Electric pulses were delivered through an electrode positioned within the round window niche to electrically evoke auditory brainstem responses (eABRs). Two or four weeks after deposition of the gelfoam all cochleas were histologically examined. We found that local BDNF treatment enhances the survival of SGCs in the basal cochlear turn after two and four weeks. The treatment had no effect on SGC size or shape. In animals treated with BDNF, eABR amplitudes were smaller than in normal-hearing control animals and similar to those in deafened controls. We conclude that BDNF delivered by means of local gelfoam application provides a protective effect, which is limited compared to intracochlear delivery methods.

In vivo infusion of UTP and uridine to the deafened guinea pig inner ear: effects on response thresholds and neural survival

Nucleotides and nucleosides are known to function as neurotransmitters and neuromodulators but have recently been shown to have a trophic effect on neurons. It has previously been shown, in an animal model for cochlear implants, that local infusion of neurotrophic factors intervenes with the degenerative processes occurring after deafening and protects the auditory spiral ganglion neurons so that electrical responsiveness is maintained. Here we test the hypothesis that nucleosides and nucleotides have a similar effect on the acutely damaged inner ear. Pigmented guinea pigs received a cochlear implant electrode for measuring electrically evoked auditory brainstem responses and a miniosmotic pump for delivering drugs directly to the cochlea. The animals were deafened by a 48-hr infusion with 10% neomycin, followed by 23 days of treatment with primarily UTP, uridine nucleotides, or as control artificial perilymph. Electrically evoked responses were measured weekly, and at the end of the experiment the cochleae were collected and processed for morphological analysis and spiral ganglion neuron counting. Both UTP- and uridine-treated groups showed significantly better response after 23 days of treatment compared with the control group. The densities of spiral ganglion neuron were significantly higher for both treated groups compared with the control group treated with artificial perilymph. The results demonstrate that UTP and uridine rescue auditory neurons and suggest that drugs acting on purinoceptors could be of clinical importance.

Effects of delayed treatment with combined GDNF and continuous electrical stimulation on spiral ganglion cell survival in deafened guinea pigs

Electrical stimulation (ES) of spiral ganglion cells (SGC) via a cochlear implant is the standard treatment for profound sensor neural hearing loss. However, loss of hair cells as the morphological correlate of sensor neural hearing loss leads to deafferentation and death of SGC. Although immediate treatment with ES or glial cell line-derived neurotrophic factor (GDNF) can prevent degeneration of SGC, only few studies address the effectiveness of delayed treatment. We hypothesize that both interventions have a synergistic effect and that even delayed treatment would protect SGC. Therefore, an electrode connected to a pump was implanted into the left cochlea of guinea pigs 3 weeks after deafening. The contralateral untreated cochleae served as deafened intraindividual controls. Four groups were set up. Control animals received intracochlear infusion of artificial perilymph (AP/-). The experimental groups consisted of animals treated with AP in addition to continuous ES (AP/ES) or treated with GDNF alone (GDNF/-) or GDNF combined with continuous ES (GDNF/ES). Acoustically and electrically evoked auditory brain stem responses were recorded. All animals were killed 48 days after deafening; their cochleae were histologically evaluated. Survival of SGC increased significantly in the GDNF/- and AP/ES group compared with the AP/- group. A highly significant increase in SGC density was observed in the GDNF/ES group compared with the control group. Additionally, animals in the GDNF/ES group showed reduced EABR thresholds. Thus, delayed treatment with GDNF and ES can protect SGC from degeneration and may improve the benefits of cochlear implants.

Regional distribution of manganese superoxide dismutase 2 (Mn SOD2) expression in rodent and primate spiral ganglion cells

Manganese superoxide dismutase 2 (SOD2) is a key metabolic anti-oxidant enzyme for detoxifying free radicals inside mitochondria. This study documents a gradient in expression of SOD2 by spiral ganglion cells in basal versus apical turn of cochlea that is consistent with differential vulnerability of high frequency hearing to free radical damage. Immunohistochemical methods were used to identify distribution of SOD2 in temporal bone sections from mice, rats, macaques, and humans. In mice and rats, both the proportion of SOD2 immunopositive type 1 spiral ganglion cells and the intensity of immunoreactivity were elevated near cochlear apex. In macaques and humans, the proportion of SO2 immunopositive spiral ganglion cells was equal across cochlear turn, but the intensity of immunoreactivity remained highest for ganglion cells near cochlear apex. Strong SOD2 immunoreactivity was also observed in human type 1 spiral ganglion cells. The average area density of SOD2 immunoreactivity in ganglion cells for each species and cochlear turn showed an allometric relationship with body weight, which is consistent with a conserved basal metabolic characteristic. These findings suggest that spiral ganglion cell responses to ROS exposure may vary along cochlear spiral with lower response capacity at cochlear base contributing to cumulative susceptibility to high frequency hearing loss.

Neurotrophic support and oxidative stress: converging effects in the normal and diseased nervous system

Oxidative stress and loss of neurotrophic support play major roles in the development of various diseases of the central and peripheral nervous systems. In disorders of the central nervous system such as Alzheimer's, Parkinson's, and Huntington's diseases, oxidative stress appears inextricably linked to the loss of neurotrophic support. A similar situation is seen in the peripheral nervous system in diseases of olfaction, hearing, and vision. Neurotrophic factors act to up-regulate antioxidant enzymes and promote the expression of antioxidant proteins. On the other hand, oxidative stress can cause down-regulation of neurotrophic factors. We propose that normal functioning of the nervous systems involves a positive feedback loop between antioxidant processes and neurotrophic support. Breakdown of this feedback loop in disease states leads to increased oxidative stress and reduced neurotrophic support.

Water-soluble Coenzyme Q10 formulation (Q-ter) promotes outer hair cell survival in a guinea pig model of noise induced hearing loss (NIHL)

The mitochondrial respiratory chain is a powerful source of reactive oxygen species (ROS) also in noise induced hearing loss (NIHL) and anti-oxidants and free-radicals scavengers have been shown to attenuate the damage. Coenzyme Q(10) (CoQ(10)) or ubiquinone has a bioenergetic role as a component of the mithocondrial respiratory chain, it inhibits mitochondrial lipid peroxidation, inducing ATP production and it is involved in ROS removal and prevention of oxidative stress-induced apoptosis. However the therapeutic application of CoQ(10) is limited by the lack of solubility and poor bio- availability, therefore it is a challenge to improve its water solubility in order to ameliorate the efficacy in tissues and fluids. This study was conducted in a model of acoustic trauma in the guinea pig where the effectiveness of CoQ(10) was compared with a soluble formulation of CoQ(10) (multicomposite CoQ(10) Terclatrate, Q-ter) given intraperitoneally 1 h before and once daily for 3 days after pure tone noise exposure (6 kHz for 1 h at 120 dB SPL). Functional and morphological studies were carried out by measuring auditory brainstem responses, scanning electron microscopy for hair cell loss count, active caspase 3 staining and terminal deoxynucleotidyl transferase-mediated dUTP labelling assay in order to identify initial signs of apoptosis. Treatments decreased active caspase 3 expression and the number of apoptotic cells, but animals injected with Q-ter showed a greater degree of activity in preventing apoptosis and thus in improving hearing. These data confirm that solubility of Coenzyme Q(10) improves the ability of CoQ(10) in preventing oxidative injuries that result from mitochondrial dysfunction.

Antioxidant protection against acoustic trauma by coadministration of idebenone and vitamin E

Idebenone, a synthetic analogue of coenzyme Q, attenuates noise-induced hearing loss by virtue of its antioxidant properties. This study involves a guinea pig model of acoustic trauma where the effectiveness of idebenone is analyzed in comparison with Vitamin E (alpha-tocopherol) that exhibits a potent antioxidant activity in the inner ear. Idebenone and vitamin E were injected intraperitoneally 1 h before noise exposure and once daily for three days; functional and morphological studies were then carried out, respectively, by auditory brainstem responses evaluation, scanning electron microscopy and terminal deoxynucleotidyl transferase-mediated dUTP nick end labelling assay identification of missing and apoptotic cells was also performed. The results showed that the protective effects of idebenone and vitamin E were not additive implying that the two antioxidants may share competitive mechanisms.

Delayed neurotrophin treatment following deafness rescues spiral ganglion cells from death and promotes regrowth of auditory nerve peripheral processes: effects of brain-derived neurotrophic factor and fibroblast growth factor

The extent to which neurotrophic factors are able to not only rescue the auditory nerve from deafferentation-induced degeneration but also promote process regrowth is of basic and clinical interest, as regrowth may enhance the therapeutic efficacy of cochlear prostheses. The use of neurotrophic factors is also relevant to interventions to promote regrowth and repair at other sites of nerve trauma. Therefore, auditory nerve survival and peripheral process regrowth were assessed in the guinea pig cochlea following chronic infusion of BDNF + FGF(1) into scala tympani, with treatment initiated 4 days, 3 weeks, or 6 weeks after deafferentation from deafening. Survival of auditory nerve somata (spiral ganglion neurons) was assessed from midmodiolar sections. Peripheral process regrowth was assessed using pan-Trk immunostaining to selectively label afferent fibers. Significantly enhanced survival was seen in each of the treatment groups compared to controls receiving artificial perilymph. A large increase in peripheral processes was found with BDNF + FGF(1) treatment after a 3-week delay compared to the artificial perilymph controls and a smaller enhancement after a 6-week delay. Neurotrophic factor treatment therefore has the potential to improve the benefits of cochlear implants by maintaining a larger excitable population of neurons and inducing neural regrowth.

Glial cell line-derived neurotrophic factor and antioxidants preserve the electrical responsiveness of the spiral ganglion neurons after experimentally induced deafness

Cochlear implant surgery is currently the therapy of choice for profoundly deaf patients. However, the functionality of cochlear implants depends on the integrity of the auditory spiral ganglion neurons. This study assesses the combined efficacy of two classes of agents found effective in preventing degeneration of the auditory nerve following deafness, neurotrophic factors, and antioxidants. Guinea pigs were deafened and treated for 4 weeks with either local administration of GDNF or a combination of GDNF and systemic injections of the antioxidants ascorbic acid and Trolox. The density of surviving spiral ganglion cells was significantly enhanced and the thresholds for eliciting an electrically evoked brain stem response were significantly reduced in GDNF treated animals compared to deafened-untreated. The addition of antioxidants significantly enhanced the evoked responsiveness over that observed with GDNF alone. The results suggest multiple sites of intervention in the rescue of these cells from deafferentation-induced cell death.