Psychophysical and anatomical evidence for hidden hearing loss in laboratory mice

Exposure to high intensity sound can lead to temporary or permanent threshold shifts. Noise exposures that do not cause long-term hearing deficits, however, can induce extensive afferent ribbon synapse loss, while hair cells and spiral ganglion neurons remain mostly intact (Kujawa and Liberman, 2009). This loss of synapses despite normal hearing thresholds is referred to as hidden hearing loss (HHL). We examined the development of HHL in laboratory mice using operant conditioning with positive reinforcement. After exposing mice to 8–16 kHz narrowband noise at 100 dB SPL for 2 h, hearing thresholds temporarily shifted for both pure tone and ultrasonic vocalization stimuli; however, post-exposure thresholds and threshold shifts varied by sex and age. Immunohistochemistry and transmission electron microscopy were conducted to quantify peripheral damage and central synaptic reorganization once behavioral testing was complete. Brains were collected, sectioned, and labeled against VGLUT1 or GAD65 and labeling was quantified in the ventral cochlear nucleus. Cochleas were also collected, dissected, and labeled for myosin6 to label hair cells and either CTBP2 or SV2 to identify afferent or efferent terminals, respectively. Our findings show that mice are able to behaviorally recover hearing following non-traumatic noise exposure despite changes in peripheral and central auditory structures.