The effect of two substances (alpha-bungarotoxin, alpha-BGTX, a small
protein, and the local anesthetic bupivacaine hydrochloride) with an a
ssumed effect on outer hair cell (OHC) motility were analyzed after ex
posing the cochlea via the round window membrane. Electrophysiological
measurements were performed with a very narrow frequency-specific gat
ing(+/- 100 Hz) technique to determine auditory brainstem response (AB
R) thresholds, including ABR-based frequency tuning curves. Exposure t
o alpha-BGTX gave a minor improvement in thresholds, interpreted as a
facilitation of OHCs, i.e. releasing their efferent inhibitory control
, whereas exposure to bupivacaine hydrochloride impaired ABR threshold
s, possibly due to immobilization of OHC motility via the lateral cell
membrane. Our results are consistent with the hypothesis that efferen
t influence on the cochlea may be linked with a modulation of the mech
anical function of OHCs. We can now postulate that there is in vivo ev
idence that acetylcholine exerts its effect at the OHCs via an alpha-B
GTX binding acetylcholine receptor.