Circadian variations in the rat serum total antioxidant status: correlation with melatonin levels

In this paper, we show for the first time, a nyctohemeral rhythm in serum total antioxidant status (TAS) in rats which parallels the 24-H melatonin cycle. Both TAS and melatonin in rat serum exhibited 24 hr variations with nocturnal peak values at 05.00 hr and low basal values during the day. When rats were maintained under light exposure (>500 lux) from 20.00 h to 05.00 hr, serum TAS was significantly reduced when compared with control rat killed in darkness. Moreover, when animals were maintained under continuous light exposure for 5 days and killed at 05.00 hr, serum TAS exhibited an additional decrease when compared with control rats. Since administering exogenous melatonin also increased TAS in the rat serum, results suggest that melatonin may be relevant in terms of participating in the antioxidative capacity of the rat serum.