TRIMETHYLTIN DISRUPTS AUDITORY FUNCTION AND COCHLEAR MORPHOLOGY IN PIGMENTED RATS

Trimethyltin (TMT) produces auditory deficits, presumably of cochlear origin, in rats. The present study identified pathological changes in the cochlea following treatment with TMT and correlated them with auditory threshold changes. Thresholds were determined by reflex-modulation audiometry, before and after treatment with TMT or with saline vehicle. Animals were then perfused and their cochleas embedded for examination as block-surface preparations or radial sections. In the first week following treatment, all TMT-treated rats showed threshold shifts of 40 to 60 dB at 40 kHz, and smaller threshold shifts (10-25 dB) at 2.5 and 10 kHz. At 3 weeks they showed threshold shifts similar to those identified one week following treatment, but with some recovery at 10 kHz. At 10 weeks, one animal showed complete recovery and three showed recovery of function at 10 but not at 40 kHz. TMT-treated animals showed losses of outer hair cells (OHC) in the basal turn of the cochlea as early as 48 hours following exposure. Comparable OHC pathology was seen at 9 days, along with some losses of inner hair cells. More extensive pathology occurred at longer survival times including the loss of type 1 spiral ganglion cells. The loss of auditory sensitivity at high frequencies was closely related to the loss of outer hair cells in the basal turn of the cochlea.