Evaluation of Neonatal Deafening Protocols: Limitations in Achieving Complete Auditory Deprivation in Rats

Congenital deafness disrupts the early critical phases of auditory development. Animal models mimicking congenital deafness are essential for studying auditory system maturation, necessitating neonatal deafening protocols achieving complete deprivation before hearing onset. This study investigated whether kanamycin alone or in combination with furosemide induces complete hearing loss in neonatal rats during the prehearing period, with the expectation that the combination would produce faster and more profound deprivation.

Sprague-Dawley rat pups received kanamycin (n=9) or kanamycin-furosemide (n=9) from postnatal day (P) 5 to 11. Controls (n=3)received saline. Auditory brainstem responses (ABRs) to tone pips (4-32 kHz) and clicks were recorded daily from P12 to P21, and at 8 weeks.

Both ototoxic protocols elevated ABR thresholds across all tested frequencies. However, neither regimen induced complete deafness by P12, as all animals retained low-frequency hearing (4-8 kHz). The combined treatment group showed significantly higher thresholds at low frequencies on P14 compared to the kanamycin-only group, but the effect diminished by P17. Auditory brainstem response thresholds remained stable between P21 and 8 weeks, indicating no further progression of hearing loss. Click-evoked responses did not reliably reflect low-frequency hearing during early development, leading to high false-negative rates when used alone as the deafness criterion.

Although both protocols elevated ABR thresholds, neither fully eliminated auditory input during the prehearing period. The combined treatment transiently enhanced ototoxicity but did not achieve complete deafness. These findings suggest that current protocols may allow residual high-threshold hearing, which should be considered when interpreting developmental outcomes.