The effects of furosemide on the endocochlear potential and auditory-nerve fiber tuning curves in cats.

Furosemide was injected intravenously in anesthetized cats while the endocochlear potential (EP) and tuning curves from fibers of the auditory nerve were simultaneously monitored in the same ear. The characteristic frequencies (CF's) of the fibers studied ranged from 0.25 to 28 kHz. Furosemide administration produced reversible, dose-related decreases in the EP which were accompanied by threshold elevations and alterations in the tuning curves of auditory nerve fibers. There was approximately 1 dB of threshold elevation for every millivolt decrease in the EP. For fibers with CF's above 3 kHz, threshold elevation at CF (in the tip of the tuning curve) was three times that in the tail of the tuning curve. Threshold elevation in the tip was always accompanied by threshold elevation in the tuning curve tail. Threshold elevation was dependent upon the CF of the fiber, with higher CF fibers showing larger threshold shifts than lower CF fibers. Threshold elevation was accompanied by a systematic shift in the CF of the fiber. Fibers with CF above 1 kHz exhibited downward shifts in CF, while those with CF less than 1 kHz generally exhibited upward shifts in CF.