Monochromatic Photophase Light Alters Diurnal Profiles of Melatonin Pathway Indoles in the Rat Pineal Gland

Light is a major environmental factor that regulates circadian rhythms and pineal melatonin synthesis. While the effects of nighttime light exposure on melatonin suppression have been extensively studied, much less is known about the impact of photophase light wavelength on pineal function. This study aimed to evaluate the influence of monochromatic light during the photophase on diurnal changes in melatonin-related indoles in the rat pineal gland. Wistar rats were exposed for 7 days to 150 lx of monochromatic blue (463 ± 10 nm), green (523 ± 10 nm), or red (623 ± 10 nm) LED light, or to white fluorescent light (control), under a 12:12 light-dark cycle. Pineal glands were collected every 3 hours over 24 hours and the indole content was analyzed by high-performance liquid chromatography. The results showed that both the timing and course of N-acetylserotonin (NAS) and melatonin (MLT) rhythms were significantly affected by light wavelength. Blue light most effectively preserved the normal rhythmicity observed under full-spectrum white light, whereas green - and particularly red light - delayed nocturnal NAS and MLT synthesis. These changes were accompanied by concurrent alternations in rhythms of serotonin, its precursors and metabolites. The data strongly suggest that spectral light composition during the photophase influences pineal indole metabolism via melanopsin-mediated phototransduction and possibly other retinal mechanisms. These findings may have implications for the design of artificial lighting environments in human life and animal housing.