An endogenous time-keeping mechanism controls circadian biological rhythms in mammals. Previously, we showed that the vitamin A deficiency modifies clock BMAL1 and PER1 as well as BDNF and Neurogranin daily rhythmicity in the rat hippocampus, when animals are maintained under 12h-light:12h-dark conditions. Retinoic acid nuclear receptors, RARs and RXRs, have been detected in the same brain area. Our objectives were 1) to analyze whether RARα, RARβ and RXRβ exhibit a circadian variation in the rat hippocampus, 2) to investigate the effect of a vitamin A-deficient diet on the circadian expression of BMAL1, PER1 and retinoic acid receptors (RARs and RXRβ) genes. Holtzman male rats from control and vitamin A-deficient groups were maintained under 12h-light:12h-dark or 12h-dark:12h-dark during the last week of treatment. RARα, RARβ, RXRβ, BMAL1 and PER1 transcript and protein levels were determined in hippocampus samples isolated every 4h in a 24h period. Regulatory regions of RARs and RXRβ genes were scanned for clock-responsive sites while BMAL1 and PER1 promoters were analyzed for RAREs and RXREs. E-box and RORE sites were found on regulatory regions of retinoid receptors genes, which display an endogenously-controlled circadian expression in the rat hippocampus. Those temporal profiles were modified when animals were fed with a vitamin A-deficient diet. Similarly, the nutritional vitamin A deficiency phase shifted BMAL1 and abolished PER1 circadian expression at both, mRNA and protein levels. Our data suggests that the vitamin A deficiency may affect the circadian expression in the hippocampus by modifying the rhythmic profiles of retinoic acid receptors.

Examples of hormonal phase-shifting of circadian gene expression began to emerge a few years ago. Vitamin A fulfills a hormonal function by binding of retinoic acid to its nuclear receptors, RARs and RXRs. We found retinoid- as well as clock-responsive sites on regulatory regions of Glutathione reductase (GR) and Glutathione peroxidase (GPx) genes. Interestingly, we observed retinoid receptors, as well as GSH, GR and GPx, display daily oscillating patterns in the rat liver. We also found that feeding animals with a vitamin A-free diet, dampened daily rhythms of RARα and RXRβ mRNA, GR expression and activity, GSH, BMAL1 protein levels and locomotor activity. Differently, day-night oscillations of RXRα, GPx mRNA levels and activity and PER1 protein levels, were phase-shifted in the liver of vitamin A-deficient rats. These observations would emphasize the importance of micronutrient vitamin A in the modulation of biological rhythms of GSH and cellular redox state in liver.

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