Absence of nyctohemeral variation in stress-induced ACTH secretion in the rat

Changes in plasma ACTH concentrations were measured in the adult male rat following a stressor in order to ascertain if there is a nyctohemeral variation in ACTH secretion following stress. The immuno-reactive ACTH concentrations obtained following a 2.5-min hind-leg tourniquet at 0400 or 1600 hours in pentobarbital-anesthetized animals were compared to those obtained in anesthetized and unanesthetized, unstressed controls. Neither absolute nor increment change in plasma ACTH concentrations following morning or afternoon tourniquet stress were significantly different after either 20 or 40 min anesthesia. Moreover, plasma ACTH concentrations from unstressed, unanesthetized rats showed a slight but significant difference (p smaller than 0.05) at 0400 hours [118 plus or minus 18 pg/ml (mean plus or minus SE)] compared to 1600 hours (64 plus or minus 12 pg/ml) indicating a reversal of the expected ACTH rhythm. Plasma corticosterone concentrations in these same rats showed the predicted lower-morning and higher-afternoon corticosterone pattern suggesting a possible nyctohemeral difference in the adrenal sensitivity to circulating levels of ACTH.