The effect of simulated weightlessness and short-term light-dark cycle on retinoic acid levels in serum and hippocampus of rats

Background: spacecrafts rotate around the Earth every 90 minutes, so the 24-hour cycle turns to 90 minutes. Retinoic acid, an active metabolite of vitamin A, plays a role in regulating the circadian rhythm and its deficiency can impair the biological clock function and consequently impair the circadian rhythm of locomotor activity. The goal of the study was to assay the effects of simulated spaceship conditions on serum and hippocampal retinoic acid levels in rats. Methods: The tail-hanging, hindleg-weightlessness rat model was applied to mimic the microgravity conditions from June to September 2017 in the zoo of the Aja University of Medical Sciences. Thirty-two male Wistar rats weighing 200-250 gr/in an experimental study were randomly assigned into 4 groups (n=8): 1) control animals with 12/12 h day-night rotation, 2) animals with 12/12 h day-night rotation under 14 days unloading, 3) control animals with 45/45 min day-night cycle, and 4) animals with 45/45 min day-night cycle under 14 days unloading. At the termination of the simulation step, rats were anesthetized, and blood samples were collected and the entire brain was removed and each brain was segmented into Hippocampus. ELISA assays were used to detect retinoic acid in the serum and homogenized hippocampus. Data were statistically assayed using two-way ANOVA and Bonferroni as a post hoc test. P<0.05 was considered significant. Results: The circadian cycle of 45/45 min compared to the 12/12 h dark cycle increased the levels of retinoic acid in serum (F=4.901, P=0.037) and hippocampus (F=5.793, P=0.029) in rats. But simulated weightlessness had no effect on retinoic acid content in serum (F=0.149, P=0.703) and hippocampus (F=0.004, P=0.952). Conclusion: It seems that retinoic acid content in serum and hippocampus is affected by circadian cycle change, and weightlessness has not affect.