Threshold Shift, Hair Cell Loss, and Hair Bundle Stiffness Following Exposure to 120 and 125 dB Pure Tones in the Neonatal Chick

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Abstract
One-day old chicks were exposed to one of two pure tone stimuli (0.9 kHz at 120 or 125 dB SPL) for 48 h. Three major results arose from a variety of tests that assessed the structural and functional consequences of the exposure on the peripheral auditory system at either 0 days or 12 to 15 days recovery. First, brainstem response data showed that the 120 and 125 dB groups had maximum evoked potential threshold shifts of 57 and 71 dB immediately after removal from the sound. Fifteen days post-exposure, the thresholds in the 120 dB group returned to near-normal levels, while in the 125 dB group, recovery was within 19 dB of control thresholds. Second, scanning electron microscopic measurements of hair cell density within the lesion showed that at 0 days recovery, the 120 and 125 dB groups had a 30% and 59% short hair cell loss, respectively, but by 15 days no differences could be identified between the exposed and control animals, regardless of exposure level. Finally, at 0 days of recovery, micromechanical stimulation data did not reveal any significant difference in stiffness between the control and surviving hair cells in the lesion area. Although the more intense exposure induced greater structural and functional damage in the chick cochlea, the birds retained or even enhanced their ability to replace lost hair cells and had partial hearing recovery by 15 days post-exposure.