The fast, slow and steady state effects of contralateral acoustic activation of the medial olivocochlear efferent system in awake guinea pigs. action of gentamicin. Deise Lima da Costa, Anne CHIBOIS, Jean-Paul Erre, Christophe Blanchet, Renaud Charlet de Sauvage, Jean-Marie Aran. Laboratoire d'Audiologie Exp‚rimentale et Clinique, UPR-ES Universit‚ de Bordeaux II, H“pital Pellegrin, 33076 - Bordeaux (France).
APStracts 4:0099N, 1997.
ABSTRACT
The function of the medial olivocochlear efferent system was observed in awake guinea pigs by recording, in the absence of ipsilateral external acoustic stimulation, the ensemble background activity (EBA) of the VIIIth nerve from an electrode chronically implanted on the round window of one ear. The EBA was measured by calculating the power value of the round window signal in the band 0.5-2.5 kHz after digital or analog (active) filtering. This EBA was compared with and without the addition of a low-level, broadband noise to the opposite ear. The contralateral broadband noise (CLBN, 55 dB SPL) induced, via the efferent system, a decrease (suppression) of this EBA. Using noise bursts of different durations, two components in this suppression could be observed. After the onset of a 1-s CLBN, the power value of the EBA decreased rapidly by 38.0 ñ 4.2 % (mean ñ SD, n = 3), with a latency of less than 10 ms and a decay time constant of a 13.1 ñ 1.0 ms (fast effect). At the offset of the 1-s CLBN, EBA came back to pre-stimulation values with a similar latency and a time constant of 15.5 ñ 2.9 ms. During longer CLBN stimulation ( ò 1 minute), EBA presented, after the fast decrease, an additional, slower decrease of 15.6 ñ 3.1 %, with a delay of 9.8 ñ 1.3 s and a decay time constant of 16.1 ñ 5.0 s (n = 12) (slow effect), and then remained remarkably constant for as long as was observed, i.e. over 2 hours (steady state). The average global suppression was thus up to 47.8 ñ 5.8 % of the basal, pre-CLBN stimulation EBA value. At the offset of the CLBN, EBA returned to pre-CLBN level with fast and slow phases, with, for the slow phase, no delay and a time constant of 32.1 ñ 8.1 s. Fast and slow changes in EBA power values were observed after a single injection of gentamicin (GM) at different doses of 150, 200 and 250 mg/kg. At 150 and 200 mg/kg GM progressively- and reversibly blocked the rapid effect, but the slow component of the efferent medial suppression remained remarkably unchanged. However at higher doses both the fast and slow suppressions were totally, still reversibly blocked. These observations indicate that the medial olivocochlear efferent system exerts sustained influences on outer hair cells, and that this effect develops in two different steps which may have different basic cellular mechanisms. 

Received 24 February 1997; accepted in final form 13 June 1997.
APS Manuscript Number J158-7.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1997 The American Physiological Society.
Published in APStracts on 15 July 1997