The role of acidic fibroblast growth factor in recovery of acoustic trauma

Effects of substance P during the recovery of hearing function after noise-induced hearing loss

Substance P (SP) is a widely distributed neurotransmitter in living tissues and is involved in various repair processes. We investigated the possibility that SP may ameliorate cochlear hair cell damage produced by noise exposure. The present study examined the effect of SP in protecting the cochlea from noise damage in guinea pigs exposed to noise after an infusion of SP into the inner ear. Changes in the hearing threshold (auditory brain response, ABR), number of synaptic ribbons, and the appearance of the outer hair cells after noise exposure were analyzed at 2 severity levels of noise-induced hearing loss. The moderate noise-induced hearing loss (110dB, 3h) group showed recovery in the ABR threshold over time, finally reaching a level slightly above pre-exposure levels, with only slight injury to the synaptic ribbons and minimal changes in the appearance of the outer hair cells. Our results indicated that in moderate hearing loss, SP exhibited a protective effect on the inner ear, both functionally and structurally. While the final magnitude of ABR threshold elevation was greater in severe noise-induced hearing loss, the synaptic ribbons and outer hair cells showed signs of severe damage.

Free radical scavengers vitamins A, C, and E plus magnesium reduce noise trauma

Free radical formation in the cochlea plays a key role in the development of noise-induced hearing loss (NIHL). The amount, distribution, and time course of free radical formation have been defined, including a clinically significant formation of both reactive oxygen species and reactive nitrogen species 7-10 days after noise exposure. Reduction in cochlear blood flow as a result of free radical formation has also been described. Here we report that the antioxidant agents vitamins A, C, and E act in synergy with magnesium to effectively prevent noise-induced trauma. Neither the antioxidant agents nor the magnesium reliably reduced NIHL or sensory cell death with the doses we used when these agents were delivered alone. In combination, however, they were highly effective in reducing both hearing loss and cell death even with treatment initiated just 1 h before noise exposure. This study supports roles for both free radical formation and noise-induced vasoconstriction in the onset and progression of NIHL. Identification of this safe and effective antioxidant intervention that attenuates NIHL provides a compelling rationale for human trials in which free radical scavengers are used to eliminate this single major cause of acquired hearing loss.

Mechanisms of noise-induced hearing loss indicate multiple methods of prevention

Recent research has shown the essential role of reduced blood flow and free radical formation in the cochlea in noise-induced hearing loss (NIHL). The amount, distribution, and time course of free radical formation have been defined, including a clinically significant late formation 7-10 days following noise exposure, and one mechanism underlying noise-induced reduction in cochlear blood flow has finally been identified. These new insights have led to the formulation of new hypotheses regarding the molecular mechanisms of NIHL; and, from these, we have identified interventions that prevent NIHL, even with treatment onset delayed up to 3 days post-noise. It is essential to now assess the additive effects of agents intervening at different points in the cell death pathway to optimize treatment efficacy. Finding safe and effective interventions that attenuate NIHL will provide a compelling scientific rationale to justify human trials to eliminate this single major cause of acquired hearing loss.

Edaravone protects the vestibular periphery from free radical-induced toxicity in response to perilymphatic application of (±)-α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid

Intracochlear infusion of (+/-)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) was performed with a syringe pump in guinea pigs, and peripheral vestibular dysfunction was induced. Animals were administered edaravone systemically or topically. In the systemic application group, animals were administered edaravone once a day for 7 days after AMPA infusion. In the topical application group, edaravone-soaked gelfoam was placed on the round window membrane just after, 12 h after or 24 h after AMPA infusion. Spontaneous nystagmus was observed after AMPA infusion. Immunohistochemistry for 4-hydroxy-2-nonenal (4-HNE), a marker of free radical-induced lipid peroxidation, was performed 24 h after AMPA infusion. In addition, caloric tests were performed to evaluate vestibular function 1 week after AMPA infusion. Animals in both groups showed decreased spontaneous nystagmus, but results were not significant. Animals treated topically with edaravone within 12 h of AMPA infusion showed normal morphology of the ampullar sensory epithelia of the lateral semicircular canals and showed a good response to the caloric tests. 4-HNE immunoreactivity in the sensory epithelia was very low in these animals. In contrast, untreated animals and animals treated with edaravone systemically or topically 24 h after AMPA infusion showed morphologic hair cell damage, reduced caloric response and remarkable 4-HNE immunoreactivity in the sensory epithelia. These results indicate that topical application of edaravone within 12 h after damage protects the vestibular periphery from free radical-induced toxicity in response to intracochlear infusion of AMPA.

NAC for noise: from the bench top to the clinic

Noise-induced hearing loss (NIHL) is an important etiology of deafness worldwide. Hearing conservation programs are in place and have reduced the prevalence of NIHL, but this disorder is still far too common. Occupational and recreational pursuits expose people to loud noise and ten million persons in the US have some degree of noise-induced hearing impairment. It is estimated that 50 million in the US and 600 million people worldwide are exposed to noise hazards occupationally. Noise deafness is still an important and frequent cause of battlefield injury in the US military. A mainstay of hearing conservation programs is personal mechanical hearing protection devices which are helpful but have inherent limitations. Research has shown that oxidative stress plays an important role in noise-induced cochlear injury resulting in the discovery that a number of antioxidant and cell death inhibiting compounds can ameliorate deafness associated with acoustic trauma. This article reviews one such compound, N-acetylcysteine (NAC), in terms of its efficacy in reducing hearing loss in a variety of animal models of acute acoustic trauma and hypothesizes what its therapeutic mechanisms of action might be based on the known actions of NAC. Early clinical trials with NAC are mentioned.

CGRP Expression in the Vestibular Periphery after Transient Blockage of Bilateral Vestibular Input

This study aimed to establish an animal model of reversible bilateral vestibular disorders that is suitable for examining the mechanisms of vestibular plasticity, and to observe the changes in the plasticity of vestibular efferent systems. Tetrodotoxin (TTX) was infused continuously for 7 days into the bilateral perilymph of guinea pig cochlea. We assessed the vestibulo-ocular reflex (VOR) for evaluating the vestibular function. We also investigated the changes in calcitonin gene-related peptide (CGRP) immunoreactivity in vestibular end organs to observe the changes in the plasticity of vestibular systems. The VOR was completely eliminated by TTX administration and returned to the preoperative levels within 120 h after TTX discontinuation. An obvious increase in the number of CGRP-immunoreactive fibers was observed within the neurosensory epithelia of the maculae and cristae. An animal model of reversible bilateral vestibular disorders was established and used for investigating the plasticity of the vestibular nervous system.

Compounds for the prevention and treatment of noise-induced hearing loss

Noise-induced hearing loss (NIHL) is the leading occupational disease and a major contributor to the development of age-related hearing loss. The pharmacological prevention and treatment of NIHL has been under preclinical investigation for the past 20 years. Promising treatments have now been identified and entered into clinical development. Within the next five years, safe and effective drugs could be approved as the first generation of otoprotectants. This review covers strategies that are under investigation for NIHL. Drugs that effectively prevent and treat NIHL will have a significant impact on medical costs, disability compensation and several issues affecting the quality of life.

Post-exposure administration of edaravone attenuates noise-induced hearing loss

We investigated the effects of the antioxidant edaravone against acoustic trauma in guinea pigs. Edaravone (1.722 x 10(-2) M) was infused into the right ear by an osmotic pump, and the left ear was untreated for control. Animals received edaravone 9 h before (-9 h group, n = 7) and 9 h (+9 h group, n = 8), 21 h (+21 h group, n = 7) and 33 h (+33 h group, n = 4) after 3-h exposure to 130-dB noise. Seven days after noise exposure, we examined the shift in auditory brainstem response thresholds and histopathologic characteristics of the sensory epithelia. The smallest shift in auditory brainstem response threshold and smallest proportion of missing outer hair cells were observed in the +9 h group. This result was supported by immunohistochemical analysis of 4-hydroxy-2-nonenal. Our data suggest that edaravone may be clinically effective in the treatment of acoustic trauma, especially if given within 21 h of noise exposure.

Inhibition of caspases alleviates gentamicin-induced cochlear damage in guinea pigs

The efficacy of caspase inhibitors for protecting the cochlea was evaluated in an in vivo study using guinea pigs, as the animal model system. Gentamicin (12 mg/ml) was delivered via an osmotic pump into the cochlear perilymphatic space of guinea pigs at 0.5 microl/h for 14 days. Additional animals were given either z-Val-Ala-Asp (Ome)-fluoromethyl ketone (z-VAD-FMK) or z-Leu-Glu-His-Asp-FMK (z-LEHD-FMK), a general caspase inhibitor and a caspase 9 inhibitor, respectively, in addition to gentamicin. The elevation in auditory brain stem response thresholds, at 4, 7, and 14 days following gentamicin administration, were decreased in animals that received both z-VAD-FMK and z-LEHD-FMK. Cochlear sensory hair cells survived in greater numbers in animals that received caspase inhibitors in addition to gentamicin, whereas sensory hair cells in animals that received gentamicin only were severely damaged. These results suggest that auditory cell death induced by gentamicin is closely related to the activation of caspases in vivo.

Peripheral vestibular disorder induced by (+/-)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA)

An intracochlear infusion of (+/-)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA) was done in guinea pigs with a syringe pump and peripheral vestibular disorder was induced. Spontaneous nystagmus toward the intact side reached a peak 9 h after the infusion and disappeared within 18 h. As a control, artificial perilymph was infused and animals had no nystagmus. The nystagmus frequency was decreased by simultaneous infusion of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) in a dose-dependent manner. In the animals treated with AMPA or AMPA + CNQX, caloric tests performed 1 week after treatment revealed a partial dysfunction of vestibular periphery. These results indicate that the nystagmus observed is induced by AMPA via AMPA receptors and that AMPA-induced vestibular disorder is partial. This animal model may be a candidate for pharmacological study of inner ear diseases induced by glutamate excitotoxicity.

The clinical free radical scavenger, edaravone, protects cochlear hair cells from acoustic trauma

It is known that reactive oxygen species have toxicity to the cochlea. We investigated the effect of edaravone, a free radical scavenger for clinical use, on the cochleae of guinea pigs subjected to acoustic trauma. We assessed auditory brainstem response (ABR) thresholds to evaluate cochlear function and observed the sensory epithelium. After noise exposure (130 dB SPL, 3 h), we observed that the auditory brainstem response threshold shift in edaravone-treated ears was significantly less than that in untreated ears. This result suggests that edaravone protected the cochleae from acoustic trauma.

Cochlear administration of adenosine triphosphate facilitates recovery from acoustic trauma (temporary threshold shift)

BACKGROUND

Adenosine triphosphate (ATP) has often been used in the treatment of acoustic trauma although evidence supporting its clinical use was lacking. The aim of this study was to evaluate the chronic effects of ATP on acoustic trauma in guinea pigs.

METHODS

We infused ATP into the perilymph of the guinea pig cochlea concurrently with intense noise exposure to investigate the effect of ATP on the process of recovery after acoustic trauma. We assessed auditory brainstem response (ABR) thresholds to evaluate cochlear function.

RESULTS

After noise exposure (120 dB SPL, 5 h), ABR thresholds showed an increase of approximately 50 dB SPL that returned to normal after 14 days. Cochlear function in ATP-treated ears recovered more quickly than in control ears. The effect of ATP was inhibited by the administration of the ATP receptor antagonist: pyridoxal- phosphate-6-azophenyl-2',4'-disulfonic acid.

CONCLUSION

These results suggest that ATP mitigates the effects of noise trauma through the ATP receptor.

Effect of Edaravone on Streptomycin-Induced Vestibulotoxicity in the Guinea Pig

OBJECTIVES/HYPOTHESIS

The effect of topical administration of edaravone to the inner ear was investigated in guinea pigs with streptomycin-induced vestibulotoxicity.

METHODS

Vestibulotoxicity was induced in 20 animals by delivery of streptomycin into the inner ear through osmotic pump for 24 hours. Edaravone (n = 8, systemic administration group) or saline (n = 6, control group) was injected intraperitoneally once a day for 7 days or edaravone-soaked Gelfoam was placed on the round window before wound closure (n = 6, topical administration group).

RESULTS

Yaw head tilt and spontaneous nystagmus were observed in all animals after the operation. The number of spontaneous nystagmus beats in the topical administration group was statistically less than that in other two groups at 12, 18, and 24 hours after the operation.

CONCLUSION

The study results suggest that topical administration of edaravone better suppresses streptomycin-induced vestibulotoxicity than systemic administration.

Inner ear changes with intracochlear gentamicin administration in Guinea pigs

OBJECTIVES/HYPOTHESIS

Transtympanic administration of gentamicin is reported to be a useful treatment for vertigo in such conditions as Meniere's disease, and determining appropriate clinical dosage of gentamicin is difficult. The authors examined the relation between gentamicin dosages and inner ear function in guinea pigs.

STUDY DESIGN

This study is a basic science project designed to examine cochlear and vestibular function in animal models.

METHODS

Various concentrations of gentamicin solution were infused into the right inner ear of guinea pigs by osmotic pumps. Caloric nystagmus as a marker of vestibular function and the change in auditory brainstem response (ABR) threshold as a marker of cochlear function were observed.

RESULTS

After 14 days of treatment, high gentamicin concentrations of 40 mg/mL caused canal paralysis and a rapid shift in ABR threshold. Animals exposed to low gentamicin concentrations of 4 mg/mL showed no obvious change in either vestibular or cochlear function. Animals exposed to moderate gentamicin concentrations of 12 mg/mL showed a moderate shift in ABR threshold and caloric malfunction. Histopathological examination revealed that after 14 days of treatment with 40 mg/mL gentamicin, severe cytoplasmic damage occurred in both vestibular and cochlear end organs. In animals treated with 12 mg/mL gentamicin, hair cells remained in the cochlear third turn and ampulla of the lateral semicircular canal.

CONCLUSION

The authors established an animal model that showed the moderate damage of inner ear with moderate-dose gentamicin. The study results indicated that the appropriate administration of gentamicin could establish a stable effect on the inner ear. It may be important to select the protocol that delivers a stable dosage of gentamicin to treat patients with Meniere's disease safely and effectively.

Heat shock transcription factor HSF1 is required for survival of sensory hair cells against acoustic overexposure

To analyze the role of heat shock response in the cochleae, we induced major heat shock proteins, Hsp70, Hsp90, and Hsp27 by perfusion of hot saline into the middle ear cavity (called 'local heat shock') in guinea pigs. Hsps were induced in almost all of the cochlear cells including the sensory hair cells in the organ of Corti. We showed that loss of both the sensory hair cells and the auditory function induced by acoustic overexposure was inhibited by pretreatment of the inner ear with local heat shock. To examine the role of heat shock transcription factor 1(HSF), which activates heat shock genes in response to heat shock, in the protection of sensory hair cells, we analyzed acoustic injury in HSF1-null mice. We found that the loss of sensory hair cells was more significant in HSF1-null mice compared with that of wild-type mice when mice were subjected to acoustic overexposure. These results indicate that HSF1 is required for survival of the sensory hair cells against acoustic overexposure.

Protective effect of edaravone against streptomycin-induced vestibulotoxicity in the guinea pig

This study investigated alleviation of streptomycin-induced vestibulotoxicity by edaravone in guinea pigs. Edaravone, a free radical scavenger, has potent free radical quenching action and is used in clinical practice to treat cerebral infarction. Streptomycin was administered to the inner ear by osmotic pump for 24 h, and edaravone (n=8) or saline (n=6) was intraperitoneally injected once a day for 7 days. We observed horizontal vestibulo-ocular reflex as a marker of postoperative vestibular function. Animals injected with saline showed statistically smaller gains than those injected with edaravone. These results suggest that edaravone suppresses streptomycin-induced vestibulotoxicity.

Fibroblast growth factors (FGFs) in the cochlear nucleus of the adult mouse following acoustic overstimulation

To see if fibroblast growth factors (FGFs) might function in the central changes following auditory overstimulation we tracked immunostaining in the cochlear nucleus of adult mice with monoclonal antibodies to FGFs (FGF-1, FGF-2) and FGF receptor. After exposure nearly all outer hair cells died, while inner hair cell and fiber loss were restricted to a region midway along the cochlear spiral. FGFs staining in the cochlear nucleus appeared in hypertrophied astrocytes in the regions of nerve fiber degeneration only. For normal-sized astrocytes there was an increase in the number stained and the intensity of staining across all frequency domains, but not in neurons. The increases were modest at 3-7 days, pronounced at 14 days, modest again by 30 days, and back to control levels by 60 days. FGF receptor staining of neurons occurred equally in all mice, exposed or not. The findings suggest that astrocytes play a role in the central responses to acoustic overstimulation and cochlear damage, involving FGFs, possibly regulating the activity of intrinsic neurons or signaling axonal growth. Not limited to regions of cochlear nerve fiber and inner hair cell loss, the changes in FGFs may represent a reaction to outer hair cell damage which spreads broadly across the central pathways.