Strain-specific differences in the development of neuronal excitability in the mouse ventral nucleus of the trapezoid body.

This investigation compared the development of neuronal excitability in the ventral nucleus of the trapezoid body (VNTB) between two strains of mice with differing progression rates for age-related hearing loss. In contrast to CBA/Ca (CBA) mice, the C57BL/6J (C57) strain are subject to hearing loss from a younger age and are more prone to damage from sound over-exposure. Higher firing rates in the medial olivocochlear system (MOC) are associated with protection from loud sounds and these cells are located in the VNTB. We postulated that reduced neuronal firing of the MOC in C57 mice could contribute to hearing loss in this strain by reducing efferent protection. Whole cell patch clamp was used to compare the electrical properties of VNTB neurons from the two strains initially in two age groups: before and after hearing onset at ∼ P9 and ∼P16, respectively. Prior to hearing onset VNTB neurons electrophysiological properties were identical in both strains, but started to diverge after hearing onset. One week after hearing onset VNTB neurons of C57 mice had larger amplitude action potentials but in contrast to CBA mice, their waveform failed to accelerate with increasing age, consistent with the faster inactivation of voltage-gated potassium currents in C57 VNTB neurons. The lower frequency action potential firing of C57 VNTB neurons at P16 was maintained to P28, indicating that this change was not a developmental delay. We conclude that C57 VNTB neurons fire at lower frequencies than in the CBA strain, supporting the hypothesis that reduced MOC firing could contribute to the greater hearing loss of the C57 strain.