Electrophysiological characteristics of inferior colliculus neurons in mutant mice with hereditary absence of cochlear outer hair cells

Abstract

One strain of homozygous Kit W-v mice (formerly known as W v/W v) lack 98% of the cochlear outer hair cells (OHCs) from birth. Inner hair cells (IHCs) and supporting cells develop normally. Thus, this strain is an attractive model to study the effect of complete OHC absence on central frequency representation. Frequency threshold curves were recorded along the tonotopic axis of inferior colliculus (IC) in mutant and control mice of the genetic background strain (C57BL/6J) and a different outbred strain (NMRI/wild mouse hybrids) known to be free of any cochlear pathology. The average threshold level of neurons in the mutants was 100 dB sound pressure level, 60 dB higher than in C57BL/6J and NMRI mice. Their tuning curves lacked the sharply tuned tip. In the C57BL/6J mice, although younger than four months, abnormal tuning curves were found for about 30% of the neurons, especially in the high frequency range. No abnormal tuning curves were found in the NMRI mice. The bandwidth of the tuning curves, measured at 10 dB above threshold, was on average 1.27 octaves in mutants, 0.62 octaves in C57BL/6J mice, and 0.34 octaves in NMRI mice. The range for the high cut-off frequency of the tuning curves at 10 dB above threshold was 6.4–61.1 kHz in the NMRI and 7–59.5 kHz in C57BL/6J. In the mutants, the range was limited to 11.1–41.7 kHz. The tonotopic gradient based on the cut-off frequency was less steep in the IC of the mutants than in both control groups.