Auditory-nerve responses in mice with noise-induced cochlear synaptopathy

Area of Science:

• Auditory Neuroscience
• Ototoxicology
• Sensory Biology

Background:

• Cochlear synaptopathy involves synapse loss between inner hair cells (IHCs) and auditory-nerve fibers (ANFs) due to noise or aging.
• Previous studies in guinea pigs suggested noise selectively damages high-threshold, low-spontaneous rate (SR) ANFs.

Purpose of the Study:

• To investigate noise-induced cochlear synaptopathy in CBA/CaJ mice.
• To compare ANF vulnerability and functional recovery following noise exposure in mice versus guinea pigs.

Main Methods:

• CBA/CaJ mice were exposed to octave-band noise.
• Single ANF activity (sound-evoked and spontaneous) was recorded post-exposure.
• Cochlear synapses were analyzed using immunostaining for pre- and postsynaptic markers.

Main Results:

• Noise exposure caused 40%-50% loss of ANF synapses in the basal cochlea.
• Contrary to findings in guinea pigs, both low- and high-SR ANF groups were affected.
• ANF thresholds and tuning recovered, but surviving fibers exhibited increased firing rates and decreased jitter, indicating a gain-of-function.

Conclusions:

• The nature of noise-induced cochlear synaptopathy differs significantly between mice and guinea pigs.
• In mice, noise exposure leads to a paradoxical hyperexcitability at the auditory nerve level, not just in central auditory circuits.
• This gain-of-function may enhance auditory perception robustness under noisy conditions.