FAST, SLOW, AND STEADY-STATE EFFECTS OF CONTRALATERAL ACOUSTIC ACTIVATION OF THE MEDIAL OLIVOCOCHLEAR EFFERENT SYSTEM IN AWAKE GUINEA-PIGS,ACTION OF GENTAMICIN

The function of the medial olivocochlear efferent system was observed in awake guinea pigs by recording, in the absence of ipsilateral exter nal acoustic stimulation, the ensemble background activity (EBA) of th e VIIIth nerve from an electrode chronically implanted on the round wi ndow of one ear. The EBA was measured by calculating the power value o f the round window signal in the 0.5- to 2.5-kHz band 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 cont ralateral broadband noise (CLBN, 55 dB SPL) induced, via the efferent system, a decrease (suppression) of this EBA. With the use of noise bu rsts 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 +/- 4.2% (mean +/- SD, n = 3), with a laten cy of <10 ms and a decay time constant of 13.1 +/- 1.0 ms (fast effect ). At the offset of the 1-s CLBN, EBA came back to prestimulation valu es with a similar latency and a time constant of 15.5 +/- 2.9 ms. Duri ng longer CLBN stimulation (greater than or equal to 1 min), EBA prese nted, after the fast decrease, an additional, slower decrease of 15.6 +/- 3.1%, with a delay of 9.8 +/- 1.3 s and a decay time constant of 1 6.1 +/- 5.0 s (n = 12, slow effect), and then remained remarkably cons tant for as long as observed, i.e., >2 h (steady state). The average g lobal suppression was thus up to 47.8 +/- 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 del ay and a time constant of 32.1 +/- 8.1 s. Fast and slow changes in EBA power values were observed after a single injection of gentamicin (GM ) at different doses (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 re remained remarkably u nchanged. However, at higher doses both the fast and slow suppressions were totally yet still reversibly blocked. These observations indicat e that the medial olivocochlear efferent system exerts sustained influ ences on outer hair cells and that this effect develops in two differe nt steps that may have different basic cellular mechanisms.