Abstract
Mammalian circadian rhythms are orchestrated by a master pacemaker in the hypothalamic suprachiasmatic nuclei (SCN), which receives information about the 24 h light-dark cycle from the retina. The accepted function of this light signal is to reset circadian phase in order to ensure appropriate synchronization with the celestial day. Here, we ask whether light also impacts another key property of the circadian oscillation, its amplitude. To this end, we measured circadian rhythms in behavioral activity, body temperature, and SCN electrophysiological activity in the diurnal murid rodent Rhabdomys pumilio following stable entrainment to 12:12 light-dark cycles at four different daytime intensities (ranging from 18 to 1,900 lx melanopic equivalent daylight illuminance). R. pumilio showed strongly diurnal activity and body temperature rhythms in all conditions, but measures of rhythm robustness were positively correlated with daytime irradiance under both entrainment and subsequent free run. Whole-cell and extracellular recordings of electrophysiological activity in ex vivo SCN revealed substantial differences in electrophysiological activity between dim and bright light conditions. At lower daytime irradiance, daytime peaks in SCN spontaneous firing rate and membrane depolarization were substantially depressed, leading to an overall marked reduction in the amplitude of circadian rhythms in spontaneous activity. Our data reveal a previously unappreciated impact of daytime light intensity on SCN physiology and the amplitude of circadian rhythms and highlight the potential importance of daytime light exposure for circadian health.
Highlights
In mammals, near-24-h rhythms in physiology and behavior are orchestrated by a master clock located in the hypothalamic suprachiasmatic nuclei (SCN) [1, 2]
It is well established that the amplitude and reliability of 24-h rhythms in some aspects of physiology and behavior can be enhanced by increasing daytime light exposure [10,11,12,13,14,15,16,17,18,19,20]
We first set out to determine the impact of increasing daytime light intensity on circadian rhythms in behavior and physiology (Tb) under stable entrainment to a 12:12 LD cycle
Summary
Near-24-h (circadian) rhythms in physiology and behavior are orchestrated by a master clock located in the hypothalamic suprachiasmatic nuclei (SCN) [1, 2]. Using a diurnal rodent species and carefully calibrated light stimuli, we show that increasing the intensity of daytime light enhances the reproducibility and robustness of behavioral and physiological rhythms and increases the amplitude of circadian rhythms in electrical activity in the central brain pacemaker. These findings reveal an impact of light on circadian amplitude and highlight the potential importance of daytime light exposure for circadian health
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