Cochlear aging after synaptopathic noise: age-noise interactions in hair cell loss and axonal degeneration

Despite many studies of cochlear synaptic loss and/or recovery after acoustic injury, none has followed degeneration of auditory-nerve fiber (ANF) peripheral axons - a critical knowledge gap in the context of regenerative strategies. We exposed CBA/CaJ mice to a synaptopathic 8-16 kHz noise stimulus producing an immediate 50% loss of inner hair cell synapses, with minimal post-exposure recovery. Noise-exposed and unexposed animals were allowed to survive for 1 month, 6 months, 1 year or 2 years before tissue harvest. Using immunostaining and machine-learning pipelines, we quantified hair cells, synaptic ribbons, ANF peripheral axons, efferent fibers, ANF myelination and nodal/heminodal structures. The expected 50% ribbon loss showed no recovery from 1 month to 1 year, with modest additional losses at 2 years. Peripheral axon loss was markedly delayed and did not exceed age-matched controls until 6 months. Even at the longest survival, axonal loss remained only half that of ribbon loss. Heminode locations were unaffected by noise or aging, and counts suggested all surviving ANFs retained a spike generator despite loss of synaptic contact. Outer hair cells showed early loss in basal-most regions, with minimal noise-age interaction at later survivals. Inner hair cells showed no losses at 1 month, whereas, at later ages, noise-exposed ears showed enhanced degeneration in the extreme base. Non-nodal myelin gaps in the peripheral axons were increasingly prevalent in both groups, suggesting a novel age-related pathology that could degrade cross-fiber synchrony. Nevertheless, most ANF axons survive long after synaptic disconnection, suggesting an extended window for therapeutic interventions.