SULFHYDRYL-MODIFYING REAGENTS ALTER OTOTOXIN BLOCK OF MUSCARINIC RECEPTOR-LINKED PHOSPHOINOSITIDE TURNOVER IN THE COCHLEA

In the 12-day-old rat cochlea, the synthesis of inositol phosphates (I Ps) can be activated via M3 cholinoceptors. This stimulation is blocke d by ototoxins (mercury, ethacrynate, cisplatin, neomycin), drugs with side effects that lead to damage of hair cells and strial cells. As t hese toxic effects can be reversed in vivo by thiol molecules, we inve stigated whether modifications of thiol compounds could be involved in ototoxin-induced inhibition of the IP turnover in the cochlea. For th is purpose, we assessed whether the sulphhydryl-modifying reagents N-e thylmaleimide and cadmium modify the carbachol-stimulated formation of IPs in the 12-day-old rat cochlea. Both molecules inhibit the carbach ol effect on a dose-dependent way without altering the basal metabolis m of IPs. As cadmium may block some calcium channels, the effect of ve rapamil, another calcium channel antagonist, was tested, Verapamil (1- 50 muM) does not alter carbachol-evoked IP formation, suggesting that the inhibitory effect of cadmium is not due to a calcium influx block. Binding experiments with the muscarinic ligand quinuclidinyl benzylat e (QNB) showed that the sulphhydryl-modifying reagents do not displace QNB from binding sites. Combining ototoxins and reagents shows that N -ethylmaleimide acts synergistically with all ototoxins but ethacrynat e while cadmium does so only with mercury. Both N-ethylmaleimide and c admium have additive effects with ethacrynate. As a supplement, disulp hide bond-modifying agents do not alter the carbachol-enhanced metabol ism of IPs. These results suggest that molecules having thiol-modifyin g properties inhibit the carbachol-induced turnover of IPs without act ing at the muscarinic sites. Since thiol modifiers and ethacrynate sha re similar features in both QNB binding and IP response it is hypothes ized that they strike common targets, possibly G proteins.