The Acute Effects of Gentamycin Ototoxicity on Bobwhite Quail Basilar Papilla and Auditory Brainstem Responses
The present study aimed to investigate the effectiveness of a single dose of gentamycin in elevating Auditory Brainstem Response (ABR) thresholds and causing inner ear hair cell loss or damage. A total of twenty-two Bobwhite quail were used (n=7 control, n=15 experimental); experimental group quail were injected with a 200 mg/kg dose of gentamycin within 24 hours of hatch (P0). Three to four days post-hatch or injection the quail underwent ABR testing. Thresholds were measured using a 1,000 Hz, 2,000 Hz, and 4,000 Hz tone pip and a click beginning at 90 dB nominalSPL. Resulting waveforms were analyzed using MATLAB (R2007) at each intensity level. A subset of experimental and control group Bobwhite quail inner ears or basilar papillae (BP) were also analyzed histologically. Within 24 hours following ABR testing, basilar papilla were fixed by intralabyrinthine perfusion of 4% paraformaldehyde in 0.4M phosphate buffer. Photomicrographs were taken at overlapping intervals and combined to make montages of the entire basilar papilla. Montages were measured for an average length and divided into deciles for analysis of estimated hair cell loss (100% hair cell loss, >50% hair cell loss, <50% hair cell loss, 0% hair cell loss). Electrophysiologic results showed absent or elevated thresholds in all experimental group quail. All of the quail in the subset analyzed histologically had total hair cell loss in the basal half of the basilar papilla; 83.3% (five out of six) had total hair cell loss across 60% or more of the length of the basilar papilla. Previous studies in altricial and precocial birds have shown hair cell loss and threshold shift following a single dose of aminoglycoside antibiotics. However, no previous studies have shown consistent hair cell loss over an area greater than the basal one-third of the basilar papilla. In this study, hair cells were reliably lost over the basal half of the basilar papilla and a subsequent profound hearing loss for frequencies above 1,000 Hz was measured. The Bobwhite minimal audibility curve indicates best hearing is between 1,000-3,000 Hz; therefore, we can conclude that the hearing loss induced represents a substantial disruption in the typical auditory world of these birds during early development.