Trimethyltin disrupts auditory function and cochlear morphology in pigmented rats

Neurotoxicity of trimethyltin in rat cochlear organotypic cultures

Trimethyltin (TMT), which has a variety of applications in industry and agricultural, is a neurotoxin that is known to affect the auditory system as well as central nervous system of humans and experimental animals. However, the mechanisms underlying TMT-induced auditory dysfunction are poorly understood. To gain insights into the neurotoxic effect of TMT on the peripheral auditory system, we treated cochlear organotypic cultures with concentrations of TMT ranging from 5 to 100 μM for 24 h. Interestingly, TMT preferentially damaged auditory nerve fibers and spiral ganglion neurons in a dose-dependent manner, but had no noticeable effects on the sensory hair cells at the doses employed. TMT-induced damage to auditory neurons was associated with significant soma shrinkage, nuclear condensation, and activation of caspase-3, biomarkers indicative of apoptotic cell death. Our findings show that TMT is exclusively neurotoxicity in rat cochlear organotypic culture and that TMT-induced auditory neuron death occurs through a caspase-mediated apoptotic pathway.

Calpain immunoreactivity and morphological damage in chinchilla inner ears after carboplatin

Carboplatin produces an unusual pattern of damage in the chinchilla inner ear, characterized by early destruction of type I afferent fibers and preferential loss of type I hair cells in the vestibular end organs and inner hair cells (IHCs) in the cochlea. In the present study, we investigated a potential role of calpains, a family of calcium-activated proteases, in carboplatin ototoxicity. Chinchillas received carboplatin (100 mg/kg IP) and were sacrificed 12, 24, 48, or 72 h later for morphological evaluation or immunocytochemistry. Nerve fibers and myelin were the initial sites of increased calpain immunoreactivity (IR) and morphological damage. At 12 h, granular immunoreactive puncta were present within nerve fibers and their myelin sheaths in the spiral ganglion. In the habenula perforata, dense reaction product was present in large vacuoles in the myelin surrounding the nerve fibers. At 24 h, nerve fibers and myelin were destroyed in the habenula, and those in the spiral ganglion showed increased calpain IR and morphological damage. At 72 h, nerve fibers and myelin were completely destroyed. Calpain IR was not a prominent feature of IHCs, type I vestibular hair cells, or ganglion cells at any time after carboplatin. The results show a correlation between calpain IR and carboplatin-induced axon and myelin degeneration. We propose that calpain-induced axonopathy and myelinopathy are primary features of carboplatin ototoxicity in chinchilla.

The effect of combined administration of cadmium and furosemide on auditory function in the rat

A number of heavy metals have been associated with toxic effects to the peripheral or central auditory system. These include lead, arsenic, mercury, platinum and organic tin compounds. In addition, the ototoxic effects of some metals may be potentiated by other factors. However, the auditory effects of cadmium have not previously been reported. The purpose of the present study was to investigate the potential ototoxic effects of cadmium from an acute dosage, and its potentiation by furosemide. Auditory brainstem response (ABR) thresholds were measured in adult Sprague-Dawley rats. Rats received either cadmium chloride (5 mg/kg, i.p.) followed by saline (4 ml/kg, i.p.). cadmium chloride followed by furosemide (200 mg/kg, i.p.), or furosemide alone. Follow-up ABRs were carried out 7 days post-treatment and threshold changes were compared between each treatment group. No significant threshold change was seen for the cadmium chloride plus saline treated or the furosemide treated animals. However, significant threshold elevations were observed in animals receiving cadmium chloride plus furosemide. In addition, scanning electron microscopy revealed extensive hair cell loss in animals treated with cadmium chloride and furosemide. Although functional auditory changes were not seen after the administration of cadmium alone, the potentiation of threshold changes by furosemide suggests that cadmium may be ototoxic under certain conditions.

The ototoxicity of 3,3'-iminodipropionitrile: functional and morphological evidence of cochlear damage

Previous reports have suggested that IDPN may be ototoxic (Wolff et al., 1977; Crofton and Knight, 1991). The purpose of this research was to investigate the ototoxicity of IDPN using behavioral, physiological and morphological approaches. Three groups of adult rats were exposed to IDPN (0-400 mg/kg/day) for three consecutive days. In the first group, at 9-10 weeks post-exposure, thresholds for hearing of 5.3- and 38-kHz filtered clicks were measured electrophysiologically and brainstem auditory evoked responses (BAERs) were also recorded to a suprathreshold broadband click stimulus. A second set of animals was tested at 9 weeks for behavioral hearing thresholds (0.5- to 40-kHz tones) and at 11-12 weeks post-exposure for BAER thresholds (5- to 80-kHz filtered clicks). A third group of animals was exposed (as above), and killed at 12-14 weeks post-exposure for histological assessment. Kanamycin sulfate was used as a positive control for high-frequency selective hearing loss. Surface preparations of the organ of Corti were prepared in order to assess hair cells, and mid-modiolar sections of the cochlea were used to examine Rosenthal's canal and the stria vascularis. Functional data demonstrate a broad-spectrum hearing loss ranging from 0.5 kHz (30 dB deficit) to 80 kHz (40 dB deficit), as compared to a hearing deficit in kanamycin-exposed animals that was only apparent at frequencies greater than 5 kHz. Surface preparations revealed IDPN-induced hair cell loss in all turns of the organ of Corti, with a basal-to-apical gradient (more damage in the basal turns) at the lower dosages. At higher dosages there was complete destruction of the organ of Corti. There was also a dosage-related loss of spiral ganglion cells in all turns of the cochlea, again with a basal-to-apical gradient at the lower dosages. These data demonstrate that IDPN exposure in the rat results in extensive hearing loss and loss of neural structures in the cochlea.

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.

Rapid disruption of cochlear function and structure by trimethyltin in the guinea pig

Trimethyltin (TMT) is a potent ototoxicant which acutely disrupts generation of the action potential evoked by a broad range of tone frequencies and subsequently produces selective high frequency impairment and outer hair cell (OHC) damage in the extreme basal turn of the cochlea. We investigated the development of TMT ototoxicity in the guinea pig 6-48 h following treatment using the compound action potential (CAP), cochlear microphonic (CM), endocochlear potential (EP) and light and electron microscopic examinations. At all time intervals studied, TMT reduced CAP sensitivity and CM amplitude. The effect was relatively broad across test frequencies at 6 h and subsequently became restricted to higher frequencies. No disruption of the EP was observed between 6 and 24 h following TMT. OHC pathology in the basal turn of the cochlea 12 h following TMT consisted of vacuolization in the supranuclear region and disruption of the cuticular plate; some mitochondria exhibited dark inclusions. Type 1 spiral ganglion cells appeared swollen at 24 h with separation of myelin from the cell bodies. No pathological changes were observed in the inner hair cells (IHC). The present data identify the OHC as targets responsible for the loss of CM sensitivity after TMT as the EP was unaffected. These data suggest that CAP and CM recovery at low and middle frequencies following acute TMT administration is accompanied by recovery of neurotransmission at the IHC or Type 1 SGC level and OHC recovery at apical regions of the cochlea.

Acute ototoxicity of trialkyltins in the guinea pig

Two trialkyltin compounds, trimethyltin chloride (TMT) and triethyltin bromide (TET) were evaluated for their acute effects on cochlear function in pigmented guinea pigs. Compound action potential (CAP) thresholds and 1 microV RMS cochlear microphonic (CM) isopotential curves were generated for 25 subjects following ip injection of TMT (2 mg/kg), TET (12 or 24 mg/kg) or inert vehicle (0.9% saline or 15% ethanol). The CAP is generated by the release of neurotransmitters from the inner hair cells and the subsequent depolarization of spiral ganglion cells. However, the sensitivity of the CAP is influenced by other cochlear structures including the outer hair cells which are thought to influence sensitivity of the inner hair cells. By contrast, CM reflects electromechanical function of the outer hair cells. CAP function was severely disrupted by organotin exposure while CM was unaffected by these agents. TMT administration impaired CAP thresholds at all frequencies within 30 min of administration. Thresholds deteriorated slightly more between 30 and 60 min. TET also reduced the sensitivity of the CAP to all frequencies. At the lower dose moderate impairments of function were observed at 30 min which became more noticeable at 60 min. Following 24 mg/kg TET injection, CAP sensitivity was markedly impaired even at 30 min. The CM isopotential values were not significantly altered 30 min or 60 min after either TMT or TET treatment at any of the 11 frequencies tested. These data document far more rapid toxic effects of TMT and TET than have been seen in most intact neuronal systems. They indicate that both organotins initially disrupt the functional integrity of either inner hair cells or spiral ganglion cells within the cochlea such that depolarization occurs only following a significant increase in stimulus intensity.