Bright daytime light enhances circadian amplitude in a diurnal mammal

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.

Daily electrical activity in the master circadian clock of a diurnal mammal

“…Recordings were performed at room temperature (~ 23°C). A portion of the data appearing in this study also contributed to the investigation of the impact of daytime light intensity on the neurophysiological activity and circadian amplitude in the R. pumilio SCN ( Bano-Otalora et al, 2021 ).…”

“…In summary, our whole-cell recordings and computational modeling highlight the potential importance of I A and cellular membrane resistance in tuning excitability in the R. pumilio SCN. This may be an important step in accommodating SCN activity to diurnal living while maintaining the high day/night contrast in electrical activity (RMP and firing rate) necessary to sustain the robustness of the clock, a contributing factor to promote health and wellbeing ( Ramkisoensing and Meijer, 2015 ; Bano-Otalora et al, 2021 ).…”

“…Adult R. pumilio (male and female, age 3–9 months) were housed under a 12:12 hr light dark cycle (14.80 Log Effective photon flux/cm 2 /s for melanopsin or Melanopic EDI (equivalent daylight illuminance) of 1941.7 lx) and 22°C ambient temperature in light tight cabinets. Lighting conditions were aimed to reproduce the R. pumilio experience of natural daylight by approximating the relative activation for melanopsin, rod opsin, and cone opsins ( Bano-Otalora et al, 2021 ). Food and water were available ad libitum.…”

“…Rhabdomys pumilio (the four striped mouse) represents an excellent opportunity to address this question. This species is primarily day-active ( Dewsbury and Dawson, 1979 ; Schumann et al, 2005 ; Mallarino et al, 2018 ; Bano-Otalora et al, 2021 ), and is a murid rodent, facilitating comparison with established findings from closely related nocturnal species (mice and rats). The designation of R. pumilio as a reliably day-active species is supported by other aspects of its biology, particularly its visual system which has several adaptations (cone rich retina, UV blocking lens) that are typical of animals relying on daytime vision ( Allen et al, 2020 ).…”

A Growing Link between Circadian Rhythms, Type 2 Diabetes Mellitus and Alzheimer’s Disease

“…Besides the genetic models, circadian misalignment caused by environmental/behavioral changes also contributed to glucose metabolism disorders in animal and human research [ 69 ]. The environmental changes often referred to altered light/dark (LD) cycles, and both the time/period and intensity of light would exert a significant effect on circadian rhythms as the SCN receives light signals [ 70 , 71 ]. Nankivell et al reported that short photoperiod exposure led to impaired glucose tolerance in Psammomys obesus [ 72 ].…”