Endocochlear potentials and compound action potential recovery: functions in the C57BL/6J mouse

Area of Science:

• Neuroscience
• Otolaryngology
• Auditory Physiology

Background:

• The C57BL/6J mouse is a common model for studying age-related hearing loss (presbycusis) due to its early-onset cochlear degeneration.
• Understanding the physiological changes in the auditory system with age is crucial for developing effective interventions.

Purpose of the Study:

• To investigate age-related changes in auditory nerve function in C57BL/6J mice.
• To assess the endocochlear potential (EP) and compound action potential (CAP) responses across different age groups.

Main Methods:

• EP and CAP measurements were performed on C57BL/6J mice at ages one, four, 12, and 24 months.
• CAP assessments included thresholds, input/output functions, and recovery functions to conditioning tones.
• Auditory nerve fiber activity was analyzed using recovery functions.

Main Results:

• EP values remained stable across all age groups and cochlear turns.
• CAP thresholds significantly increased in four-month-old mice compared to one-month-old controls at high frequencies.
• CAP responses were absent in 12- and 24-month-old mice, despite normal EP.
• Auditory nerve recovery functions showed two components (fast and slow) in one- and four-month-old mice, suggesting distinct auditory neuron classes.
• Recovery functions were unchanged with age, indicating preserved low-spontaneous rate fiber activity when EP is stable.

Conclusions:

• Age-related hearing loss in C57BL/6J mice involves significant changes in auditory nerve excitability and response generation.
• The stability of EP and auditory nerve recovery functions in older mice suggests a unique mechanism of presbycusis compared to other models.
• The findings highlight the presence of different spontaneous rate (SR) auditory neuron classes and their differential preservation with age.