DISCHARGE PATTERNS OF CHICKEN COCHLEAR GANGLION NEURONS FOLLOWING KANAMYCIN-INDUCED HAIR CELL LOSS AND REGENERATION

Hair cells in the basal, high frequency region (> 1100 Hz) of the chic ken cochlea were destroyed with kanamycin (400 mg/kg/d x 10 d) and all owed to regenerate. Afterwards, single unit recordings were made from cochlear ganglion neurons at various times post-treatment. During the first few weeks post-treatment, only neurons with low characteristic f requencies(< 1100 Hz) responded to sound. Despite the fact that the lo w frequency region of the cochlea was not destroyed, neurons with low characteristic frequencies had elevated thresholds, abnormally broad U -shaped or W-shaped tuning curves and low spontaneous discharge rates. At 2 days post-treatment, the spontaneous discharge rates of some aco ustically unresponsive units fluctuated in a rhythmical manner. As rec overy time increased, thresholds decreased, tuning curves narrowed and developed a symmetrical V-shape, spontaneous rate increased and neuro ns with higher characteristic frequencies began to respond to sound. I n addition, the proportion of interspike interval histograms with regu larly spaced peaks increased. These improvements progressed along a lo w-to-high characteristic frequency gradient. By 10-20 weeks post-treat ment, the thresholds and tuning curves of neurons with characteristic frequencies below 2000 Hz were within normal limits; however, the spon taneous discharge rates of the neurons were still significantly lower than those from normal animals.