Chronic exposure to dim artificial light disrupts the daily rhythm in mitochondrial respiration in mouse suprachiasmatic nucleus.

Circadian rhythms of physiology, behavior, and metabolism have an endogenous 24 h period that synchronizes with environmental cycles of light/dark and food availability. Alterations in light cycles are stressful and disrupt such diurnal oscillations. Recently, we witnessed a sudden rise in studies describing the mechanisms behind the interaction between the key characteristics of mitochondrial functions, peripheral clocks, and stress responses. To our knowledge, there is no study in the suprachiasmatic nuclei (SCN) describing the dysregulated mitochondrial bioenergetics under abnormal lighting conditions, which is common in today's modern world. Thus, we aimed to investigate the existence of daily changes in mitochondrial bioenergetics (respiratory control rate, RCR), mitochondrial abundance (mtDNA/nDNA), plasma corticosterone, and to test whether disturbances in the lighting conditions might influence such rhythms. To confirm this, mice were sacrificed, mitochondria were isolated from the suprachiasmatic nuclei in the brain and blood was collected, every 3 h at various time points zeitgeber time/circadian time, (0, 3, 6, 9, 12, 15, 18, 21, and 24 h) under 12:12 h light-dark (LD, 150 lux L: 0 lux D) cycle and chronic artificial dim lighting (LL, 5 lux: 5lux) conditions, of a 24 h period, respectively. Our results demonstrate the existence of robust daily rhythmicity in RCR, mtDNA/nDNA and plasma CORT under a normal LD cycle. However, these rhythms were significantly disrupted and clock genes expressions were dysregulated under chronic dim LL. Furthermore, mitochondrial abundance was significantly reduced during LL compared to their numbers under LD cycle. Our data demonstrate that the circadian clock regulates mitochondrial functions (RCR, number), essential for accomplishing daily energy demands and supply by the SCN neurons. Abnormal light exposure dysregulates mitochondrial functions in the SCN and may alter metabolism, resulting in obesity, diabetes, and other metabolic disorders. Therefore, properly designing lighting conditions in workplaces is essential to mitigate the adverse consequences of light on humans.