HAIR CELL REGENERATION IN THE EUROPEAN STARLING (STURNUS-VULGARIS) - RECOVERY OF PURE-TONE DETECTION THRESHOLDS

Behavioral detection thresholds were obtained from four starlings befo re, during, and after 11 days of subcutaneous injections of kanamycin. Birds were operantly conditioned to respond to pure-tones ranging in frequency from 0.25 kHz to 7 kHz using the method of constant stimuli and were tested daily for 141 days after the first injection of aminog lycoside. All four birds sustained hearing losses greater than 60 dB a t frequencies from 4 kHz to 7 kHz by the end of the 11 day injection s chedule. Two birds had a slight shift in threshold at 3 kHz. No change in threshold occurred for any of the birds at lower frequencies. Reco very of detection thresholds began soon after the injections ceased an d continued for approximately 50 days. In all four birds there was som e degree of permanent hearing loss: 5 dB to 15 dB at frequencies betwe en 4 kHz and 6 kHz, and approximately 25 dB at 7 kHz. Scanning electro n microscopy (SEM) was performed at 0 and 5 days post-injection in a s eparate group of starlings given the same injection schedule. Hair cel l loss and damage was observed across the basal 34% to 36% of the basi lar papilla. SEM in two behaviorally tested birds sacrificed 142 days after the first injection showed that there was regeneration of hair c ells to populate the previously damaged region, but that disorientatio n of stereocilia bundles in the basal third of the basilar papilla was common. The other two behaviorally tested birds were treated with kan amycin again for 16 days beginning at 142 days after the first injecti on. Thresholds shifted again, but less than during the first dosing pe riod. SEM of these birds' basilar papillae showed less hair cell loss than observed in the birds given only a single, 11 day dosing of kanam ycin. This result suggests that birds may be less susceptible to the o totoxic effects of kanamycin in repeated treatments. In all four birds , the degree and position of damage observed with SEM corresponded wit h the extent and frequency of hearing loss.