Non-specific effect of beettahistine on cochlear electrophysiology in guinea pig

Beettahistine hydrochloride (BH) is reported to be beneficial in the treatment of certain type of vestibulo-cochlear disorders. Experimentally is shown that BH inhibits vestibular firing rate and improves cochlear blood flow by dilatating the larger feeding vessels. Both these effects are weak, and hardly can explain the clinical benefits reported for the drug. We investigated the effect of BH on the organ of Corti electrophysiology in vivo. Two microliters of 1, 2, 5 and 10% saline solutions of BH was applied (in increasing order) to the round window (RW) membrane in an anesthetized guinea pig (n = 5). Summating potential (SP), action potential (AP), and cochlear microphonics (CM) were recorded at 16 kHz frequency using RW Ag wire electrode. The repeated applications of BH with increasing concentrations produced a dose-dependent threshold shift in CAP. The I/O function for 20, 30, 40, 50, 60, 70, and 80 dB at 16 kHz indicated that there was a saturation before complete blockade of the responsiveness in all measured parameters. SP and CM changes nearly followed CAP changes. The changes recover during 2 hours follow-up period. Our findings indicate that BH affects the organ of Corti electrophysiology. That the drug effect saturates before a complete blockade suggests non-specific effect of BH on the organ of Corti structures and cochlear nerve.