Maturation of the Circadian Rhythm of Plasma Corticosterone in the Rat

The maturation of certain features of the hypothalamic-pituitary adrenocortical system was studied in rats from birth to the 40th postnatal day. Increases in plasma corticosterone concentration were observed following exposure to ether and after exogenous ACTH in animals 2 weeks before diurnal variation in plasma corticosterone appeared. A definite, though slight, plasma B response to exogenous ACTH was present even at birth but did not begin to approach adult responsiveness until after the 15th day. The plasma B response to ether began on day 15. The afternoon plasma B was higher by the 25th day than on any previous afternoon of the study, but a definite circadian variation was not present until day 30–32. These data suggest that maturation of the central nervous system’s ability to regulate rhythmic ACTH secretion is delayed well beyond the time when the pituitary is capable of secreting ACTH and the adrenal cortex is capable of secreting corticosterone. (Endocrinology80: 926, 1967)