DIFFERENTIAL-EFFECTS OF GENTAMICIN ON THE DISTRIBUTION OF COCHLEAR FUNCTION IN ALBINO AND PIGMENTED GUINEA-PIGS

It has been suggested that the high affinity of melanin pigment for am inoglycoside antibiotics may cause these drugs to bind preferentially to the pigmented inner ear, producing greater otoxicity than in the am elanotic albino cochlea. However, evidence of greater ototoxicity in a lbinos has led to the hypothesis that melanin inhibits the toxicity of these drugs in the pigmented inner ear. On the other hand, ototoxicit y in the pigmented animals may simply be delayed relative to the albin os, only to become equal or even more severe with time. The present st udy was conducted to determine whether a relatively low dose of gentam icin (68.5 mg/kg) would produce differential ototoxicity between albin o and pigmented guinea pigs which would persist long after drug exposu re had stopped. Nine pigmented and eight albino guinea pigs were given gentamicin sulfate for 14 consecutive days, and were then allowed a t wo-month recovery period before cochlear analysis; 11 pairs of saline- injected or untreated albino and pigmented guinea pigs served as contr ols. The results showed that the gentamicin-treated albinos had signif icantly elevated thresholds for the compound action potential from the auditory nerve (CAP), and significantly lower endocochlear potentials (EP) and cochlear microphonic (CM) input-output voltage functions whe n compared to their respective controls, or to either group of pigment ed guinea pigs. The CAP in drug-treated pigmented animals did not diff er significantly from controls, and the differences in EP and CM were marginally significant. The results indicate that the pigmented cochle a is less susceptible to gentamicin than the albino cochlea, and suppo rt the hypothesis that melanin may inhibit aminoglycoside ototoxicity in the pigmented inner ear.