Noise-induced hearing loss enhances Ca ²⁺ -dependent spontaneous bursting activity in lateral cochlear efferents

Exposure to loud and/or prolonged noise damages cochlear hair cells and triggers downstream changes in synaptic and electrical activity in multiple brain regions, resulting in hearing loss and altered speech comprehension. It remains unclear however whether or not noise exposure also compromises the cochlear efferent system, a feedback pathway in the brain that fine-tunes hearing sensitivity in the cochlea. We examined the effects of noise-induced hearing loss on the spontaneous action potential (AP) firing pattern in mouse lateral olivocochlear (LOC) neurons. This spontaneous firing exhibits a characteristic burst pattern dependent on Ca ²⁺ channels, and we therefore also examined the effects of noise-induced hearing loss on the function of these and other ion channels. The burst pattern was sustained by an interaction between inactivating Ca ²⁺ currents contributed largely by L-type channels, and steady outward currents mediated by Ba ²⁺ -sensitive inwardly-rectifying and two-pore domain K ⁺ channels. One week following exposure to loud broadband noise, hearing thresholds were significantly elevated, and the duration of AP bursts was increased, likely as a result of an enhanced Ca ²⁺ current. Additional effects of noise-induced hearing loss included alteration of Ca ²⁺ -dependent inactivation of Ca ²⁺ currents and a small elevation of outward K ⁺ currents. We propose that noise-induced hearing loss enhances efferent activity and may thus amplify the release of neurotransmitters and neuromodulators (i.e., neuropeptides), potentially altering sensory coding within the damaged cochlea.