EFFECTS OF INNER HAIR CELL LOSS ON INFERIOR COLLICULUS EVOKED-POTENTIAL THRESHOLDS, AMPLITUDES AND FORWARD MASKING FUNCTIONS IN CHINCHILLAS

The effects of outer hair cell (OHC) loss on evoked potential (EVP) th resholds, amplitudes and forward masking (FWM) functions have been fai rly well characterized. In contrast, the effects of inner hair cell (I HC) losses are largely unknown, primarily due to the difficulty of pro ducing selective IHC lesions. Recent studies have shown that IHCs of t he chinchilla are preferentially damaged by the anticancer drug: carbo platin. In this study, we administered a single 100 mg/kg dose of carb oplatin to four chinchillas, to examine the effects of IHC lesions on EVPs measured from the inferior colliculus (IC-EVPs). Thresholds and a mplitude functions were measured for 0.25-16 kHz tone bursts, and FWM functions were measured at 1, 2 and 4 kHz, using masker-probe interval s of 2, 5, 10, 20, 40 and 80 ms, before and 1-2 months after carboplat in treatment. Histology revealed IHC lesions ranging from approximatel y 15 to 90%, with virtually no loss of OHCs. Surprisingly, even massiv e IHC lesions were not associated with elevations of IC-EVP thresholds . IC-EVP amplitudes at suprathreshold levels were sometimes depressed, sometimes enhanced, and in some cases unchanged. IHC lesions increase d susceptibility to FWM, particularly at intermediate (10-20 ms) maske r-probe intervals, without significantly changing the overall time cou rse of FWM. The results provide new perspectives on the contribution o f IHCs to FWM, and on the ability of the central auditory system to ad apt to a significant reduction of neural input from the cochlea. (C) 1 998 Elsevier Science B.V. All rights reserved.