ELEVATION OF INTRACELLULAR CALCIUM LEVELS IN SPIRAL GANGLION-CELLS BYTRIMETHYLTIN

The neurotoxicant, trimethyltin (TMT) produces cochlear impairment at far lower dose levels and far more rapidly than it does central nervou s system effects. The initial effects of TMT in the cochlea, in vivo, are consistent with disruption of the inner hair cell type-1 spiral ga nglion cell synapse although it is uncertain whether the effect is on presynaptic and/or postsynaptic units. This synapse is believed to be an excitatory glutamatergic one, providing the possibility that TMT co uld induce an excitotoxic process resulting in elevations in intracell ular calcium ([Ca2+](i)). The objective of this study was to determine whether TMT had direct toxic effects on the postsynaptic spiral gangl ion cells studied in primary culture and to identify the role of extra cellular calcium in such an effect. The marker of interest was the eff ect of this agent on [Ca2+](i) levels as determined using quantitation of the fluorescent calcium dye, Fura-2. TMT did induce a marked and s ustained elevation in [Ca2+](i) level in the spiral ganglion cells tha t appeared to have a rapid initial phase and a slower saturating phase . Studies performed using calcium-free medium showed that elevation of [Ca2+](i) in spiral ganglion cells by TMT was attenuated but not enti rely blocked. Further, the L-type calcium channel blocker, nifedipine, was able to inhibit the initial increase in [Ca2+](i), suggesting tha t at least this phase of the TMT effect was mediated by calcium channe ls, although nifedipine had no significant effect on the time to reach the maximal [Ca2+](i) level. Parallel control experiments performed u sing application of exogenous glutamate and depolarizing K+ concentrat ions also produced elevation in [Ca2+](i) levels. The data indicate th at TMT elevates [Ca2+](i) in isolated spiral ganglion cells both by in creasing extracellular uptake via Ca2+ channels and also by releasing Ca2+ from intracellular stores. Thus TMT ototoxicity appears to includ e a direct postsynaptic toxic event.