Seven-day hypobaric hypoxia induces early basal cochlear presynaptic ribbon injury and oxidative stress in mice before overt cellular loss.

BACKGROUND/OBJECTIVES: Acute hypobaric hypoxia at high altitude may impair hearing, but the earliest cochlear lesion site and associated biological responses remain unclear. We aimed to characterize the early cochlear phenotype after 7 days of hypobaric hypoxia in adult C57BL/6J mice. METHODS: Mice were exposed to simulated hypobaric hypoxia equivalent to 5500 m altitude for 7 days. Auditory brainstem response (ABR), electrocochleography (ECochG), cochlear immunofluorescence, presynaptic ribbon quantification, oxidative-stress marker analysis, and bulk RNA sequencing were used to assess early functional, structural, and molecular changes. RESULTS: Hypoxia induced predominantly high-frequency auditory dysfunction, including elevated ABR thresholds at 16 kHz, reduced suprathreshold ABR wave I amplitude, and an increased SP/AP area ratio on ECochG. Gross hair-cell and spiral-ganglion-cell counts were preserved, whereas CtBP2-positive presynaptic ribbons were reduced, with the most prominent change in the basal turn. Oxidative-stress-associated immunolabeling was increased, as shown by elevated 4-HNE and 8-OHdG-related signals in hair cells and increased 4-HNE in spiral-ganglion tissue. Transcriptomic analysis identified 676 differentially expressed genes enriched in synaptic transmission, neurotransmitter transport, postsynaptic membrane organization, ion-channel activity, and calcium signaling. CONCLUSIONS: Seven-day hypobaric hypoxia induces an early cochlear injury state characterized by basal-turn-predominant presynaptic ribbon injury, oxidative stress, and reduced suprathreshold auditory-nerve output before overt cellular loss. Transcriptomic findings support glutamatergic and calcium-signaling remodeling as mechanistic directions but do not establish individual hub genes as validated causal drivers.