A Diphtheria Toxin-Induced Mouse Model of Auditory Neuropathy: From Cochlear Synaptopathy to Neuronal Degeneration.

PURPOSE: Auditory neuropathy (AN) is a hearing disorder that is often overlooked due to its hidden cochlear activity and hearing loss profiles. Diphtheria toxin (DT) is commonly used to induce deafness in conditional gene knockout mouse models, but its ototoxic targets in wild-type animals remain controversial. This study aimed to characterize the pathogenic effects of DT on the cochlea of wild-type mice and establish a reliable model of AN. METHODS: Adult CBA/CaJ mice of both sexes (n = 81) were administered by DT for three consecutive days. AN was confirmed by electrophysiological profiles: abnormal auditory brainstem response (ABR) with preserved distortion product otoacoustic emissions (DPOAE), followed by histopathological evaluation of the cochlear auditory pathway. RESULTS: DT induced a characteristic AN-characteristic hearing loss in mice. It initiated with ribbon synapse degeneration in inner hair cells (IHCs) within 24 h, coinciding with a significant reduction in ABR amplitude peak I. By post-injection day 3, extensive damage was observed, including IHC loss, degeneration of type I spiral ganglion neurons (SGNs), and demyelination of their axons. Despite a minimal loss of OHCs (~ 2.1%), OHC function remained intact as evidenced by DPOAE. A progressive degeneration of SGNs and demyelination of axons persisted over the 21-day observation period. CONCLUSION: Our findings demonstrate that DT selectively targets the IHCs and type I afferent pathway in wild-type mice, resulting in a progressive AN- characteristic pathology. This study establishes a mouse model of AN that can serve as a basis for further investigation into the mechanisms underlying similar pathologies.