CONICET | Buscador de Institutos y Recursos Humanos

Abstract: A role for vitamin A has been established in phenomena related to cognitive function in the adult mouse and rat brain. On the other hand, some evidence points out retinoids as regulators of clock genes activity through their retinoid nuclear receptors (RARs and RXRs). RARa and RXRb nuclear receptors have been detected in the rat hippocampus. The brain-derived neurotrophic factor (BDNF), a molecular marker of synaptic plasticity related to learning and memory, contains fourteen clock responsive, E-box, sites along its gene promoter region. The objectives of this study were to investigate whether BDNF display a circadian expression pattern in the rat hippocampus, and evaluate to which extent vitamin A deficiency could modify the daily expression (mRNA and protein) of Bmal1, Per1 and BDNF. Holtzman rats were weaned at 21 days and immediately assigned to either the experimental diet, devoid of vitamin A (vitamin A-deficient group) or the same diet with 4000 IU of vitamin A per Kg of diet (control group) during 3 months. Hippocampus samples were taken every 5 hours at zeitgeber times (ZT): 2, 7, 12, 17 and 22, with ZT0 when light is on. Total RNA was extracted using the Trizol reagent and following manufacturer?s instructions. Transcript levels of Bmal1, Per1 and BDNF were determined by RT-PCR and normalized to b-actin as endogenous control. Relative quantification by Real-time PCR was performed to measure mRNA levels of RARa and RXRb using SYBR Green I fluorescent dye. Protein levels were determined by immunobloting for the detecting antibodies. Statistical analysis of daily rhythms was performed by one-way ANOVA followed by Tukey´s post-hoc test. A P value of less than 0.05 was considered to be significant. We found mRNA levels of BDNF display a robust daily rhythmicity in the hippocampus of control rats with a peak at ZT2 (P<0.01). RXRb mRNA levels were significantly lower in the vitamin A-deficient rats compared to controls (P<0,025). Daily rhythms and peaks of Bmal1 and BDNF expression (at ZT17 and ZT2, respectively) were 10-12-h phase shifted in the vitamin A-deficient group, while Per1 oscillating expression was attenuated by the vitamin deficiency. Thus, vitamin A deficiency modifies the circadian expression of clock (BMAL1 and PER1) and putative clock-controlled genes (BDNF), probably by reducing the availability of retinoid nuclear receptors such as RXRb. Above observations would suggest that nutritional vitamin A deficiency might affect the daily cognitive performance and raise the possibility that nutritional factors might be essential to maintain the circadian expression of clock and clock-controlled genes in peripheral clocks such as the hippocampus. :

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