ELECTRICAL-STIMULATION ACTIVATES 2 DIFFERENT SITES WITHIN THE GUINEA-PIG COCHLEA

This study investigates whether auditory brainstem responses (ABRs) ca n be used to assess the functioning of electrically stimulated cochlea s. Electrically evoked auditory brainstem responses (EABRs) were recor ded in guinea pigs with normal hearing and guinea pigs deafened by ami kacin, a powerful ototoxic antibiotic, combined with diuretic aminooxy acetic acid (AOAA). Two different types of EABRs were observed in norm al animals, depending on the electrical pulse intensity applied to the round window: long-latency brainstem responses were evoked by low sti mulation intensities, short-latency brainstem responses by high intens ities. The absence of effect of strychnine applied intracochlearly rul ed out the possibility of medial efferents being involved in these res ponses. Conversely, an intracochlear application of tetrodotoxin (TTX) , an Na+-channel blocker, resulted in the disappearance of both types of responses, attesting that the sites activated by the electrical sti mulation were located within the cochlea. In AOAA/amikacin poisoned co chleas, in which most of the hair cells were missing with apparently n ormal ganglion neurons, the long-latency brainstem responses evoked by low intensities were completely lacking. These findings suggest that low currents applied to the round window of the guinea pig cochlea pri marily activate the hair cells, the neurons being directly excited at higher intensities.