Proceedings of The Physiological Society

Future Physiology (Leeds, UK) (2017) Proc Physiol Soc 39, PC28

Poster Communications

Electrophysiology of mammalian cochlear hair cells in aging mice

J. Jeng1, W. Marcotti1

1. University of Sheffield, Sheffield, United Kingdom.

Age-related hearing loss (ARHL) is affecting millions of people worldwide with degeneration of auditory hair cells as one of the pathologies. In mammals, auditory hair cells are responsible for detecting mechanical sound stimuli and transducing them into electrical signals. Both inner and outer hair cells (IHCs and OHCs) express cadherin 23 (CDH23), a protein crucial for the function of mechanically sensitive stereociliary bundles positioned at their apical pole. C57BL/6J and C57BL/6N mice, which are a model for early-onset ARHL, have a mutation in CDH23. Currently, it is not known if the CDH23 mutation changes hair cell physiology with age or how it contributes to the early ARHL in these mice. To investigate the possible effect of the CDH23 mutation in C57BL/6N and C57BL/6J mice, we compared them with mice in which the mutation has been repaired with CRISPR/Cas9 (Repaired mice). Whole-cell patch clamp recordings were performed from IHCs and OHCs in the low-frequency region of the acutely dissected cochlea from male mice at 1 and 6 months of age (number of cells at 1 month: C57BL/6N n = 6, C57BL/6J n = 9, Repaired n = 5; at 6 month: C57BL/6N n = 5, C57BL/6J n = 7, Repaired n = 8). Hearing thresholds were determined in male mice at 12 months (number of animals: C57BL/6N n = 12, C57BL/6J n = 11, Repaired n = 9). The animals were anaesthetised with a mixture of ketamine (100 mg kg-1) and xylazine (10 mg kg-1, both i.p.), and auditory brain-stem responses were recorded during different sound stimulus intensities. Values are means ± S.E.M. We have defined an electrophysiological baseline for IHCs and OHCs in male and female mice from each strain at 1 month of age. At this age, the total outward currents in OHCs from male repaired mice were larger than those in C57BL/6N mice (t-test, p<0.05). This difference was not seen at 6 months, the age at which increased hearing thresholds have been observed in C57BL/6N mice but not in Repaired mice (Li & Borg, 1991; Mianné et al., 2016). In contrast, the total current at 0 mV in IHCs from C57BL/6N and C57BL/6J mice increased from 1 month to 6 months (12.2±0.8 to 14.9±2.6 nA for C57BL/6N and 10.7±1.2 to 14.2±1.6 nA for C57BL/6J). Surprisingly, we found a large variability in the hearing thresholds of C57BL/6N and C57BL/6J mice at 12 months, which was not present in Repaired mice. Our results suggest that the CDH23 mutation in C57BL/6N and C57BL/6J mice may influence the function of OHCs and IHCs differently. IHCs, but not OHCs, show larger total currents at older ages in those mice, suggesting that IHCs might become less responsive to sound stimuli with age. Moreover, the CDH23 mutation causes differences in the progression of ARHL in those mice, possibly making them more vulnerable to other unknown ARHL factors.

Where applicable, experiments conform with Society ethical requirements