Circadian rhythms of corticosterone and nucleic acids in the rat adrenal in relation to age

For 3 successive days, at 0600, 1200, 1800 and 0000, the circadian variations of corticosterone, RNA and DNA were followed up in the adrenals of 3 lots of prebuertal (30 days old), adult (120 days old) and old (28 months old) white Wistar male rats. There is a correlation between the circadian rhythm of corticosterone secretion and the circadian rhythms of RNA and DNA synthesis all along ontogenesis. These rhythms appear to be formed at 30 days of life, but they still differ from the ones in the adult rats by the levels recorded in the second half of the nocturnal period. In old rats the adrenal circadian variations have a considerably smaller amplitude, their curve taking a plateau form. The results support the hypothesis of the circadian rhythm ontogenesis in the rat adrenal.