Cochlear pathology following exposure to mercury

Experimental animal models of drug-induced sensorineural hearing loss: a narrative review

Objective

This narrative review describes experimental animal models of sensorineural hearing loss (SNHL) caused by ototoxic agents.

Background

SNHL primarily results from damage to the sensory organ within the inner ear or the vestibulocochlear nerve (cranial nerve VIII). The main etiology of SNHL includes genetic diseases, presbycusis, ototoxic agents, infection, and noise exposure. Animal models with functional and anatomic damage to the sensory organ within the inner ear or the vestibulocochlear nerve mimicking the damage seen in humans are employed to explore the mechanism and potential treatment of SNHL. These animal models of SNHL are commonly established using ototoxic agents.

Methods

A literature search of PubMed, Embase, and Web of Science was performed for research articles on hearing loss and ototoxic agents in animal models of hearing loss.

Conclusions

Common ototoxic medications such as aminoglycoside antibiotics (AABs) and platinum antitumor drugs are extensively used to induce SNHL in experimental animals. The effect of ototoxic agents in vivo is influenced by the chemical mechanisms of the ototoxic agents, the species of animal, routes of administration of the ototoxic agents, and the dosage of ototoxic agents. Animal models of drug-induced SNHL contribute to understanding the hearing mechanism and reveal the function of different parts of the auditory system in humans.

Neurophysiologic measures of auditory function in fish consumers: associations with long chain polyunsaturated fatty acids and methylmercury

BACKGROUND

Determining if associations exist between child neurodevelopment and environmental exposures, especially low level or background ones, is challenging and dependent upon being able to measure specific and sensitive endpoints. Psychometric or behavioral measures of CNS function have traditionally been used in such studies, but do have some limitations. Auditory neurophysiologic measures examine different nervous system structures and mechanisms, have fewer limitations, can more easily be quantified, and might be helpful additions to testing. To date, their use in human epidemiological studies has been limited. We reviewed the use of auditory brainstem responses (ABR) and otoacoustic emissions (OAE) in studies designed to determine the relationship of exposures to methyl mercury (MeHg) and nutrients from fish consumption with neurological development. We included studies of experimental animals and humans in an effort to better understand the possible benefits and risks of fish consumption.

OBJECTIVES

We reviewed the literature on the use of ABR and OAE to measure associations with environmental exposures that result from consuming a diet high in fish. We focused specifically on long chain polyunsaturated fatty acids (LCPUFA) and MeHg.

METHODS

We performed a comprehensive review of relevant studies using web-based search tools and appropriate search terms.

RESULTS

Gestational exposure to both LCPUFA and MeHg has been reported to influence the developing auditory system. In experimental studies supplemental LCPUFA is reported to prolong ABR latencies and human studies also suggest an association. Experimental studies of acute and gestational MeHg exposure are reported to prolong ABR latencies and impair hair cell function. In humans, MeHg exposure is reported to prolong ABR latencies, but the impact on hair cell function is unknown.

CONCLUSION

The auditory system can provide objective measures and may be useful in studying exposures to nutrients and toxicants and whether they are associated with children's neurodevelopment.

A systematic review of mercury ototoxicity

Mercury is neurotoxic, and numerous studies have confirmed its ototoxic effect. However, the diagnosis and follow-up of mercury exposure require understanding the pathophysiology of the chemical substance. Based on a systematic literature review, this study aimed to demonstrate whether mercury is ototoxic and to analyze its mechanism of action on the peripheral and central auditory system, in order to contribute to the diagnosis and follow-up of exposure. This was a systematic review of studies published on the effects of mercury exposure on the auditory system. The full text of the studies and their methodological quality were analyzed. The review identified 108 studies published on the theme, of which 28 met the inclusion criteria. All the articles in the analysis showed that mercury exposure is ototoxic and produces peripheral and/or central damage. Acute and long-term exposure produces irreversible damage to the central auditory system. Biomarkers were unable to predict the relationship between degree of mercury poisoning and degree of lesion in the auditory system.

Acoustic stapedius muscle reflex in mercury-exposed Andean children and adults

CONCLUSION

The results suggested mercury (Hg)-induced anomalies in the brainstem-mediated acoustic stapedius muscle reflex in children.

OBJECTIVES

Hg exposure has been associated with hearing impairment and brainstem anomalies. Acoustic stapedius reflex (ASR) thresholds, growth functions, decay/adaptation times, and behavioral auditory thresholds were used to screen Andean children and adults for Hg-induced auditory brainstem and facial nerve impairment.

METHODS

Fifty-one participants, which included 22 children (aged 6-17 years) and 29 adults (aged 19-83 years) living in gold mining areas of Ecuador where Hg is widely used in amalgamation, were screened using ASR immittance procedures.

RESULTS

Mean blood mercury (HgB) level in the children was 15.6 μg/L (SD, 21.3; median, 7 μg/L; range, 2.0-89 μg/L), and in the adults 8.5 μg/L (SD, 7.1; median, 6 μg/L; range, 2.0-32 μg/L). Mean contralateral ASR thresholds (ASRT) for the screening frequency of 2000 Hz in the children (39 ears) was 92.9 dB HL (SD, 6.1; range, 80-105 dB HL), and in the adults (53 ears) 90.0 dB HL (SD, 6.4; range, 65-105 dB HL). The ASRT in the children increased significantly with HgB level (rho = 0.433; p = 0.008).

Alterações auditivas em trabalhadores expostos a mercúrio

OBJETIVO: identificar possíveis alterações auditivas periféricas e centrais, em trabalhadores de uma indústria de reciclagem de lâmpadas fluorescentes, que tiveram exposição ao mercúrio metálico. MÉTODOS: foram realizados os testes de audiometria tonal (via aérea e via óssea), audiometria vocal e imitanciometria (timpanometria e pesquisa do reflexo estapediano) em 13 trabalhadores, todos do sexo masculino, com idade média de 38,5 anos e tempo médio de exposição de seis anos. Após foram realizados os teste de processamento auditivo (dicótico de dígitos e PPS) em nove trabalhadores. RESULTADOS: dos 13 sujeitos que realizaram a audiometria e a imitanciometria, seis (46%) apresentaram perda auditiva sensório-neural e sete (54%) apresentaram alteração no reflexo estapediano. Dos nove sujeitos que realizaram o teste de processamento auditivo, três (33,5%) apresentaram resultados alterados tanto no teste dicótico de dígitos, quanto no PPS. CONCLUSÕES: foram identificadas tanto alterações auditivas periféricas quanto centrais.

Mercury (Hg2+) suppression of potassium currents of outer hair cells

The heavy metal mercury (Hg(2+)) is an insidious environmental pollutant that causes toxic effects on sensory systems. It is well known that the group IIB divalent cation Hg(2+) is an inhibitor of the group I monovalent potassium (K(+)) cation pore-forming channel in several biological preparations. Here, we used the whole cell patch clamp technique on freshly isolated outer hair cells (OHCs) of the guinea pig cochlea to record outward K(+) currents and inward K(+) currents treated with mercuric chloride (HgCl(2)). HgCl(2) affected K(+) currents in a voltage- and dose-dependent manner. The effects of HgCl(2) at 1.0-100 microM are more pronounced on onset peak current than on steady-state end current. HgCl(2) depolarized also the resting membrane potential. Although the effect of HgCl(2) at 1.0 microM was partially washed out over several minutes, the effects at 10 and 100 microM were irreversible to washout. Since K(+) channels of OHCs are targets for HgCl(2) ototoxicity, this may lead to auditory transduction problems, including a loss in hearing sensitivity. A better understanding of fundamental mechanisms underlying K(+) channelopathies in OHCs due to HgCl(2) poisoning may lead to better preventive or therapeutic agents.

Inhibition of the carbachol-evoked synthesis of inositol phosphates by ototoxic drugs in the rat cochlea

The ability of amikacin, neomycin, ethacrynate, mercuric chloride and cisplatin to alter the inositol phosphate (IP) signalling pathway was assessed in the 12-day-old rat cochlea, where the turnover of IPs is coupled to muscarinic receptors. This study was motivated by: (1) the demonstration of neomycin binding to phosphatidylinositol 4,5-biphosphate, the precursor of IPs, and (2) the fact that ototoxic drugs induce some common symptoms in outer hair cells. At concentrations below 1 mM, none of the compounds changed the control 3H-IP formation. Mercuric chloride, cisplatin and ethacrynate inhibited the carbachol-induced formation of IPs in a dose-dependent manner with IC50 values of 74,340 and 430 microM, respectively. The aminoglycosides were less efficient in reducing the carbachol-stimulated accumulation of IPs, since neither amikacin nor neomycin, both at 1 mM, had any significant effect. However, neomycin applied at 15 and 30 microM induced 29% and 43% of inhibition of the stimulated IP response. Finally, additive effects are obtained between some of the toxic drugs. The results suggest that a block of the IP transduction system, associated with the cholinergic efferent innervation of the organ of Corti, is a feature that may be involved in some types of ototoxicity. The inefficiency of aminoglycosides and the putative targets of the ototoxic agents are discussed.

Exposure to methyl mercury from birth to adulthood impairs high-frequency hearing in monkeys

Hearing deficits are a frequent consequence of both developmental and adult methyl mercury exposure in humans. However, a detailed characterization of these deficits has not been performed in either humans or animals. Cynomolgus monkeys (Macaca fascicularis) were dosed from birth to 7 years of age with 50 micrograms/kg/day of mercury as methyl mercuric chloride. Steady-state blood mercury levels during dosing were 0.6-0.9 ppm. When monkeys were 14 years old, pure tone detection thresholds were determined by a psychophysical procedure. Control monkeys exhibited thresholds at frequencies between 125 and 31,500 Hz comparable to previously published values for macaques. One methyl mercury-treated monkey exhibited normal detection thresholds at all frequencies. Three treated monkeys were impaired at the second highest frequency tested (25,000 Hz) and therefore were not tested at 31,500 Hz. The fifth treated monkey displayed severely elevated thresholds at middle frequencies (10,000-12,500 Hz), precluding testing at higher frequencies. These results indicate a selective high-frequency deficit in monkeys exposed to methyl mercury from birth to adulthood and not exposed to methyl mercury in the 7 intervening years before auditory testing. These findings extend previous results in this group of monkeys in which deficits in spatial and temporal visual function were observed.

Animal models of inner ear vascular disturbances

Animal models of vascular disorders are identified or developed for the evaluation of functional deficits and morphologic alterations. This information will serve a useful purpose for a better understanding of sudden deafness, Meniere's disease, and presbycusis. The study of microcirculation of the inner ear vessels reveals that their responses to various stimuli, such as anoxia, sympathetic nerve stimulation, hypothermia, and drugs, are different from those of the middle ear vessels. In sudden occlusion of the major vascular supply to the inner ear, the cochlea is found to be more vulnerable than the vestibular labyrinth; outer and inner hair cells and stria vascularis are most often affected. Animal models for Meniere's disease are also described, and the importance of vascularity at the endolymphatic duct and sac is discussed from an etiologic viewpoint. In presbycusis, animal models show sensory cell and spiral ganglion cell atrophies in different locations than in human cochleas, and the relationship between these atrophies and vascular impairments is not clear at the present time.

Cutaneous and auditory function in rats following methyl mercury poisoning

Rats were given a total dose of 50 mg/kg (Exp. 1), 13.3 or 40 mg/kg (Exp. 2), or 40 mg/kg (Exp. 3) of methyl mercury chloride subcutaneously over a course of 5 days. At varying times after the toxic exposure, up to 1 year, their sensory functioning was assessed by reflex modulation methods: stimuli of interest were presented just before an intense tone which elicited the startle reflex, and stimulus reception was measured by the inhibitory control of the stimuli over the amplitude of the reflex. In Experiment 1 cutaneous prestimuli (electric shock to the tail) and brief acoustic transients (silent periods in noise) were less effective inhibitors of reflex activity in poisoned animals, compared to controls, indicating that the poisoned animals had impairments in cutaneous sensitivity and audition. In Experiment 2 the time course of sensory loss and subsequent recovery was studied. Impaired auditory function was shown further by a deficit in the effectiveness of weak noise pulses, and, in addition, the cutaneous deficit for weak tail shocks was accompanied by an exaggerated or hyperpathic response to more intense tail shocks. Experiment 3 confirmed the finding that the loss of sensitivity to weak shock was accompanied by an enhancement of the response to more intense shock. These data were related to peripheral neuropathy and shown to be analogous to certain clinical symptoms of Minamata disease reported in humans.

Methyl mercury ototoxicity in mice determined by auditory brainstem responses

Mice of the C57 B1/6 strain were used to assess auditory deficits due to methyl mercury chloride intoxication. Auditory Brainstem Responses were obtained to pure-tone stimuli from 4 to 78 kHz. Physiologic thresholds were compared with behavioral methods for determining audibility in mice. Two dosage levels were studied; 4 and 8 mg/kg. Recordings were taken weekly, for each mouse, for 3 consecutive weeks after initial injection. Analysis of physiologic threshold and latency indicated that methyl mercury chloride causes auditory deficits at all frequencies tested with the greater effect in the higher frequencies. Additionally, latencies indicated nerve conduction hypersensitivity in the brainstem.

Problems and pitfalls in ototoxicity testing

A local anaesthetic drug under development, intended for use in ear surgery, was tested with regard to adverse, ototoxic effects. Inner ear damage was not obvious. However, in the guinea pig a massive formation of new bone occurred in the middle ear (bulla tympanica) within 5 weeks after a single local administration of the drug.