CIRCADIAN RHYTHM AND AMBIENT LIGHT SHAPE DEFENSIVE BEHAVIOR TO VISUAL THREAT IN MICE

Innate aversive behaviors like predator avoidance must be rapid, reliable, and adaptable. For instance, time of day and ambient light signal varying predation risks, with light serving as both a temporal cue and dynamic environmental factor. However, how time and light interact to modulate behavior remains unclear. Here, we examined how circadian timing and ambient light affect mouse escape responses to overhead visual threats and the underlying neural circuitry.
We monitored mouse avoidance behavior in response to visual looming stimuli across day and night. Under dim light (~0.1 lux), mice exhibited robust escape responses at both circadian phases, yet escape latency and vigor were significantly stronger during the subjective night compared to during the day, revealing an intrinsic circadian modulation of threat responses. Brighter light (~100 lux) during the subjective day further decreased escape probability and vigor, indicating an additional ambient light suppression effect. Notably, artificial light at night (ALAN) selectively delayed escape latency without altering vigor, suggesting that circadian and ambient light signals converge to shape defensive behavior through distinct mechanisms.
To investigate the neural basis of circadian-dependent regulation, brain-wide c-Fos mapping revealed circadian modulation in key midbrain defense hubs, including the superior colliculus and periaqueductal gray, which strongly correlated with escape vigor. Crucially, optogenetic activation of the suprachiasmatic nucleus (SCN) induced a 'day-like' state that suppressed escape, identifying the SCN as a state-dependent gate for midbrain-mediated defensive behaviors. These findings imply that the endogenous circadian clock can affect essential evoked behaviors partially independent of external light cues.