The uptake of methyl mercury in guinea pig cochlea in relation to its ototoxic effect

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.

Exposure to low-dose barium by drinking water causes hearing loss in mice

PURPOSE

We continuously ingest barium as a general element by drinking water and foods in our daily life. Exposure to high-dose barium (>100mg/kg/day) has been shown to cause physiological impairments. Direct administration of barium to inner ears by vascular perfusion has been shown to cause physiological impairments in inner ears. However, the toxic influence of oral exposure to low-dose barium on hearing levels has not been clarified in vivo. We analyzed the toxic influence of oral exposure to low-dose barium on hearing levels and inner ears in mice.

EXPERIMENTAL DESIGN

We orally administered barium at low doses of 0.14 and 1.4 mg/kg/day to wild-type ICR mice by drinking water. The doses are equivalent to and 10-fold higher than the limit level (0.7 mg/l) of WHO health-based guidelines for drinking water, respectively. After 2-week exposure, hearing levels were measured by auditory brain stem responses and inner ears were morphologically analyzed. After 2-month exposure, tissue distribution of barium was measured by inductively coupled plasma mass spectrometry.

RESULTS

Low-dose barium in drinking water caused severe hearing loss in mice. Inner ears including inner and outer hair cells, stria vascularis and spiral ganglion neurons showed severe degeneration. The Barium-administered group showed significantly higher levels of barium in inner ears than those in the control group, while barium levels in bone did not show a significant difference between the two groups. Barium levels in other tissues including the cerebrum, cerebellum, heart, liver and kidney were undetectably low in both groups.

CONCLUSIONS

Our results demonstrate for the first time that low-dose barium administered by drinking water specifically distributes to inner ears resulting in severe ototoxicity with degeneration of inner ears in mice.

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.

An electrophysiological study of the effects of acute methylmercury chloride exposure on the function of the guinea pig cochlea

The inner ear function of methylmercury chloride (MMC)-exposed guinea-pigs was examined in this study. Previous studies which investigated the function of the eighth cranial nerve and Corti-organ using cochlear microphonics (CM), compound action potential (CAP) and measurement of endocochlear potential (EP) reported ototoxicity following experimental exposure to MMC. In this report, the effect of MMC on the cochlea and the eighth cranial nerve were investigated systematically by measuring CM, action potential (AP), EP and K+ ion concentration of the endolymph. Guinea-pigs were injected with 5 mg/kg MMC (using 0.2% solution) twice a week for 1-3 weeks. The maximum output voltage of AP was decreased by injection of MMC (5 mg/kg x 6). A decrease in the CM maximum output voltage and the elevation of CM pseudothreshold was seen after MMC injection. Changes in EP during 3 min anoxia were observed, especially a decrease in the absolute value of the negative potential. The endolymph K+ ion concentration remained unchanged. These findings indicate that the diffusion potentials decreased and at the same time was reduced the maximum output voltage in CM induced by MMC injection (5 mg/kg x 6) in this experiment.

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.

Hearing: the effects of chemicals

Recent studies of human beings exposed to environmental chemicals, as well as experimental animal studies, have identified a number of chemical agents that are commercial products, chemical intermediaries, waste products, or contaminants that are potentially ototoxic. The classes of compounds discussed in this review include organic solvents, asphyxiant gases, and heavy metals that are present in the environment as industrial pollutants or byproducts. Both human and animal investigations are summarized in discussing the actions of these ototoxic compounds. The suggested gaps in our knowledge are highlighted to help direct future research.

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.

Structure side-effect sorting of drugs. VI. Ototoxicities

From a literature survey, over 130 (about 7.8%) drugs and chemicals have been associated with ototoxicities. The major classes are basic aminoglycoside and other antibiotics, anti-inflammatory drugs, antimalarials, beta-blockers, antineoplastic agents, heavy metals, diuretics, some topical agents and various miscellaneous drugs. Possible mechanisms of action are presented and discussed. These include inhibition of protein synthesis, the glycolytic cycle, the TCA cycle, energy utilization, energy generation and the respiratory system within the mitochondria membrane of the hair cell, and also alteration of the permeability of the endolymphatic membrane or alteration of the excretion system for the basic aminoglycosides in the lateral wall of the membranous cochlea. The relative rank order of ototoxicity and reactivity toward mucopolysaccharides of five aminoglycosides is found to be related to the number of basic groups in each molecule.