title: Elevation of intracellular calcium levels in spiral ganglion cells by trimethyltin.
abstract: The neurotoxicant, trimethyltin (TMT) produces cochlear impairment at far lower dose levels and far more rapidly than it does central nervous system effects. The initial effects of TMT in the cochlea, in vivo, are consistent with disruption of the inner hair cell type-1 spiral ganglion 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 could induce an excitotoxic process resulting in elevations in intracellular calcium ([Ca2+]i). The objective of this study was to determine whether TMT had direct toxic effects on the postsynaptic spiral ganglion cells studied in primary culture and to identify the role of extracellular calcium in such an effect. The marker of interest was the effect of this agent on [Ca2+]i levels as determined using quantitation of the fluorescent calcium dye, Fura-2. TMT did induce a marked and sustained elevation in [Ca2+]i level in the spiral ganglion cells that 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 entirely blocked. Further, the L-type calcium channel blocker, nifedipine, was able to inhibit the initial increase in [Ca2+]i, suggesting that at least this phase of the TMT effect was mediated by calcium channels, although nifedipine had no significant effect on the time to reach the maximal [Ca2+]i level. Parallel control experiments performed using application of exogenous glutamate and depolarizing K+ concentrations also produced elevation in [Ca2+]i levels. The data indicate that TMT elevates [Ca2+]i in isolated spiral ganglion cells both by increasing extracellular uptake via Ca2+ channels and also by releasing Ca2+ from intracellular stores. Thus TMT ototoxicity appears to include a direct postsynaptic toxic event.
authors.affiliation: Toxicology Program, College of Pharmacy, University of Oklahoma Health Sciences Center, Oklahoma City 73190, USA.
authors.forename: L D
authors.initials: LD
authors.lastname: Fechter
authors.forename: Y
authors.initials: Y
authors.lastname: Liu
mesh_terms: D000704:Analysis of Variance; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects; D002478:Cells, Cultured; D004305:Dose-Response Relationship, Drug; D005456:Fluorescent Dyes; D016257:Fura-2 / Q000737:chemistry; D006168:Guinea Pigs; D009543:Nifedipine / Q000494:pharmacology; D011188:Potassium / Q000378:metabolism; D013136:Spiral Ganglion / Q000166:cytology / Q000187:drug effects* / Q000378:metabolism; D014298:Trimethyltin Compounds / Q000008:administration & dosage / Q000633:toxicity*
chemical_list: D002121:Calcium Channel Blockers; D015220:Calcium Channels; D005456:Fluorescent Dyes; D014298:Trimethyltin Compounds; C046488:trimethyltin; D009543:Nifedipine; D011188:Potassium; D002118:Calcium; D016257:Fura-2