REGENERATED HAIR-CELLS IN THE EUROPEAN STARLING - ARE THEY MORE RESISTANT TO KANAMYCIN OTOTOXICITY THAN ORIGINAL HAIR-CELLS

Previous work from our laboratory [Marean et al. (1993) Hear. Res. 71, 125-136] has shown that a 10 day dose of 200 mg/kg/day kanamycin prod uced damage to the basal 34% of the starling basilar papilla. We also observed that repeating the dosing schedule following a 4 month surviv al period resulted in significantly less damage to the regenerated aud itory epithelium. The present study investigated whether or not this a pparent resistance was the result of a tendency for regenerated hair c ells to be less susceptible to kanamycin ototoxicity, or if other, sys temic factors may be involved. Eight European starlings were given sub cutaneous injections of 200 mg/kg/day kanamycin for 10 days. Serum lev els of kanamycin were measured at the time of sacrifice for all birds, and the basilar papillae of all birds were examined by scanning elect ron microscopy (SEM). Two of these birds (Group 1) were sacrificed imm ediately following the dosing period. Two of the birds were allowed to survive for 60 days (Group 2). Two of the birds were redosed with 200 mg/kg/day for 10 days after 60 days survival (Group 3). Finally, two birds were redosed with 250 mg/kg/day until serum levels of kanamycin were the same as Group 1 when sacrificed(> 9 mu g/ml). The SEM results showed that the regenerated auditory epthelium of the birds dosed a s econd time sustained less damage compared to previously untreated ears , even though the dosing regimen was the same (Group 3 versus Group 1) . The regenerated auditory epethelium of birds dosed a second time sus tained the same damage as previously untreated animals when the dosage was increased to attain similar serum levels (Group 4 versus Group 1) . These results suggest metabolic changes occur in the starling in res ponse to the initial dose of kanamycin which do not necessarily involv e changes in hair cell resistance to ototoxicity.