Lateral olivocochlear neurons modulate cochlear responses to noise exposure

Abstract

The sense of hearing originates in the cochlea, which detects sounds across dynamic sensory environments. Like other peripheral organs, the cochlea is subjected to environmental insults, including loud, damage-inducing sounds. In response to internal and external stimuli, the central nervous system directly modulates cochlear function through olivocochlear neurons (OCNs), which are located in the brainstem and innervate the cochlear sensory epithelium. One population of OCNs, the lateral olivocochlear (LOC) neurons, target spiral ganglion neurons (SGNs), the primary sensory neurons of the ear. LOCs alter their transmitter expression for days to weeks in response to noise exposure (NE), suggesting that they are well-positioned to tune SGN excitability over long time periods in response to auditory experience. To examine how LOCs affect auditory function after NE, we characterized the transcriptional profiles of OCNs and found that LOCs exhibit transient changes in gene expression after NE, including upregulation of multiple neuropeptide-encoding genes. Next, by generating intersectional mouse lines that selectively target LOCs, we chemogenetically ablated LOC neurons and assayed auditory responses at baseline and after NE. Compared to controls, mice lacking LOCs showed stronger NE-induced functional deficits one day later and had worse auditory function after a two-week recovery period. The number of remaining presynaptic puncta at the SGN synapse with inner hair cells did not differ between control and LOC-ablated animals, suggesting that the primary role of LOCs after NE is likely not one of protection, but one of compensation, ensuring that SGN function is enhanced during periods of need.