EFFERENT-MEDIATED PROTECTION FROM ACOUSTIC OVEREXPOSURE - RELATION TO SLOW EFFECTS OF OLIVOCOCHLEAR STIMULATION

1. The present study attempts to resolve discrepancies in the reported role of olivocochlear (OC) efferent activation in protecting the inner ear from acoustic overstimulation: in previous studies, activating the OC system in guinea pigs reduced the threshold shift caused by 1 min monaural exposure to a 10-kHz tone: whereas unilateral OC activation in cats had no effect on threshold shifts following binaural exposure to a 10 min 6-kHz tone. 2. In this study, anesthetized and curarized guinea pigs were exposed either monaurally or binaurally to tones of different duration (1-5 min), frequency (6 to 10 kHz) and intensity (105-118 dB SPL). For each exposure condition, threshold shifts were compared among ears with different levers of OC activation: in some cases, the OC bundle (OCB) was electrically stimulated during (and/or before) the acoustic overexposure: in others, the OCB was cut before the exposure; in control cases. the OCB was neither cut nor electrically stimulated. 3. Electrical stimulation of the OCB delivered simultaneously with acoustic overstimulation produced significant reductions in threshold shift only for acoustic exposures at higher frequencies (8 and 10 kHz) and shorter durations( 1 and 2 min). The protective effects on 1-min exposures could be extinguished by prior stimulation of the OCB, i.e., if the OC stimulation was turned on 1 min before the acoustic overexposure. 4. These observations provide circumstantial evidence that protection is related to the newly discovered ''slow'' effects of OC activation rather than the classic ''fast'' effects because, in contrast to fast effects, slow effects 1) are maximal in frequency regions affected by 10-kHz exposures and are minimal in regions affected by 6-kHz exposures and 2) can only be maintained for 1-2 min in the face of continuous OC stimulation.