Acute hypoxia induces transient olfactory dysfunction through olfactory epithelial degeneration and bulbar mitochondrial stress in zebrafish

Hypoxic-ischemic injury is a major cause of olfactory dysfunction, yet the cellular and morphological mechanisms underlying this sensory loss remain poorly understood. Here, we investigated the structural, cellular, and functional effects of acute hypoxic exposure on the olfactory system of adult zebrafish (Danio rerio) of both sexes, a model organism with remarkable neuroregenerative capacity. Fish were subjected to 15 minutes of acute severe hypoxia (0.8 mg/L dissolved oxygen) and assessed at 1 and 5 days post-hypoxia (dph). We evaluated olfactory function by means of cadaverine-evoked aversive behavioral assays. Structural and morphological integrity and inflammation of the olfactory epithelium (OE) and olfactory bulb (OB) were characterized using immunohistochemistry, histological stainings, and a 2,3,5-triphenyltetrazolium chloride (TTC) colorimetric assay. Acute hypoxic exposure impaired olfactory-mediated behaviors without affecting locomotion or exploratory behavior. In the peripheral OE, hypoxia caused neurodegeneration, disruption of the nasal mucus layer, and robust leukocytic infiltration. We observed reduced mitochondrial dehydrogenase activity in the olfactory bulb (OB) along with reactive astrogliosis. Olfactory function recovered by 5 days, coinciding with full restoration of OE morphology, and supported by a strong proliferative response. These findings reveal a coordinated degenerative and regenerative response to hypoxia across the olfactory axis, with implications for understanding hypoxia-induced sensory loss and neural repair. SIGNIFICANCEThis work addresses an important gap in knowledge regarding the mechanisms linking hypoxic insult and olfactory dysfunction. By using adult zebrafish, an extraordinarily regenerative vertebrate, it also provides insight into neuronal repair and regenerative processes supporting olfactory recovery. The novelty of our study resides in that, to our knowledge, there are no studies that provide a comprehensive characterization of the effects of hypoxia in the olfactory system across molecular, histological, and functional levels. These findings advance our understanding of hypoxia-induced sensory neurodegeneration and regeneration, and highlight the zebrafish olfactory system as a powerful model for investigating neural repair mechanisms relevant to hypoxic-ischemic brain injury.