Plateau Hypoxia Influences Mouse Circadian Behavior by Remodeling the Hypothalamic Transcriptional Landscape

Background: Hypoxia signaling and the circadian clock exhibit extensive molecular crosstalk, but how endogenous circadian timing influences behavioral responses during high-altitude exposure remains incompletely understood. Methods: Male C57BL/6J mice were exposed to hypobaric hypoxia equivalent to an altitude of 6000 m. Wheel-running activity was continuously monitored under 12 h light/12 h dark (LD) cycle and constant-dark conditions to evaluate locomotor activity, rhythm amplitude, circadian phase, and free-running period. After 48 h of exposure, whole-hypothalamus samples were collected across the circadian cycle for time-series RNA sequencing, rhythmicity and pathway-enrichment analyses, and qPCR validation. In a separate experiment, hypoxia was initiated at ZT2 or ZT14 to assess circadian-phase-dependent behavioral responses. Results: High-altitude exposure acutely reduced locomotor activity and rhythm amplitude and subsequently delayed behavioral circadian phase under a LD cycle, whereas the free-running period under constant darkness remained unchanged. Transcriptomic analysis revealed extensive remodeling of rhythmic gene expression in the whole hypothalamus, involving multiple signaling and metabolic pathways. Relaxin-related signaling emerged as a candidate pathway associated with hypoxia-induced temporal transcriptional remodeling, and selected core clock genes and relaxin pathway-associated genes showed altered temporal expression profiles. Furthermore, exposure initiated at ZT14 produced greater locomotor suppression than exposure initiated at ZT2. Conclusions: High-altitude exposure disrupts behavioral circadian organization and remodels hypothalamic rhythmic transcription. These findings suggest that endogenous circadian timing modulates behavioral sensitivity to hypoxic stress.