TRIMETHYLTIN DISRUPTS LOUDNESS RECRUITMENT AND AUDITORY THRESHOLD-SENSITIVITY IN GUINEA-PIGS

Trimethyltin (TMT) impairs auditory thresholds within minutes of syste mic administration. However, there are no data which relate to the out put of the auditory nerve at sound levels above threshold. In this exp eriment, we evaluated the functional effects of TMT on the auditory th reshold by identifying the sound level which just produced a detectabl e compound action potential (CAP). We also assessed outer hair cell fu nction by measuring the cochlear microphonic (CM), a nonpropagated ac potential which is phase-locked to the stimulus. Finally, we measured the growth of the N-1 amplitude as a function of stimulus intensity at levels above threshold and of the summating potential (SP), a de pote ntial which has multiple generators. To isolate cochlear from systemic effects of TMT, the agent was applied directly to the round window, a structure separating the middle and inner ear, of anaesthetized guine a pigs. We show that TMT applied to the round window membrane can disr upt the function of the cochlea. Measurements of auditory function at supra-threshold levels showed clearly that TMT reduced the amplitude o f N-1 while having no measurable effect on the SP. These findings indi cate that TMT blocks the recruitment of neuronal elements by loud soun d. This pattern of impairment differs from that observed with aminogly coside antibiotics, hypothermia, and presbycusis in which loudness rec ruitment has been reported.