Cochlear histopathology in macaques after noise-induced temporary threshold shifts.

Noise exposures causing transient hearing loss were previously considered benign. However, recent work has revealed that temporary noise-induced threshold shifts may be associated with long-lasting cochlear histopathology. One such effect is cochlear synaptopathy, i.e. changes to the afferent synapse between inner hair cells and auditory nerve fibers. Noise-induced synaptopathy has been extensively characterized in several rodent models, and temporal bone studies suggest similar age-related changes in humans. However, it remains unclear how noise-induced temporary threshold shifts affect cochlear structures in humans and nonhuman primates, which show greater resistance to noise exposure than other animals. Additionally, the long-term sequelae of temporary threshold shifts are largely unknown. Here, we characterized the effects of a noise exposure causing temporary threshold shifts on cochlear histopathology in macaque monkeys at long post-exposure survival times. Overall, cochlear histopathology was variable across subjects, similar to the variable susceptibility observed in humans. At 2 and 10 months post-exposure, macaques had no significant loss of hair cells, inner hair cell synapses, or cholinergic efferent innervation. However, both inner and outer hair cell ribbons showed significant enlargement. Together, these findings provide insight into the cochlear effects of single-exposure temporary threshold shifts in nonhuman primates.