[Effect of intravenous injection of aspirin on the cochlea]

It is not yet well understood how aspirin acts on the auditory system. This study was aimed at elucidating the effects of aspirin on the cochlear hair cells and cochlear nerve of guinea pigs by recording (1) spontaneous activity of cochlear nerve fibers, (2) compound action potentials (CAPs) evoked by electrical pulses applied to the cochlea through the round window membrane, (3) acoustically evoked action potentials (APs) through the round window membrane and (4) acoustic emissions (sound pressure near the tympanic membrane) evoked by electrical pulses applied to the cochlea. The following results were obtained. (1) After guinea pigs were given aspirin (200 mg/kg) intravenously, a transient reduction in spontaneous activity of cochlear nerve fibers and elevation over the subsequent 10-20 minutes were observed in three of four fibers recorded for more than 30 minutes. The mean spontaneous discharge rate in 102 fibers after aspirin administration (200 mg/kg) was not significantly higher than the control values from 30 to 120 minutes after aspirin administration, while the rate in 112 fibers after aspirin administration (400 mg/kg) was significantly higher. (2) After injecting of 100 mg/kg, 200 mg/kg and 400 mg/kg of aspirin, the amplitude of electrically evoked CAPs decreased significantly, while their latency increased significantly only after injection of 400 mg/kg of aspirin. (3) Changes in the amplitude of acoustically evoked APs and electrically evoked CAPs were compared before and after intravenous injections of 400 mg/kg of aspirin. The AP suppression ratio at low sound intensity was significantly greater than that of CAPs at low and high current levels. (4) After injection of 400 mg/kg of aspirin, the waves of electrically evoked acoustic emissions and their peaks in the frequency analysis disappeared either reversibly or irreversibly. The observed changes in spontaneous activity of cochlear nerve fibers in response to aspirin administration to guinea pigs may represent a tinnitus-like phenomenon. The detection of electrically evoked CAPs suggests that aspirin acts on the cochlear nerve and causes a decrease in its excitability, and the discovery of acoustically evoked APs and electrically evoked acoustic emissions is interpreted as indicating that aspirin acts not only on the cochlear nerve but also on cochlear hair cells.