Ontogeny of Daily Rhythmicity in Plasma Corticosterone and Variation in Sensitivity of the Corticosterone Response in Turkey Poults

Two trials were conducted with turkey poults to examine the ontogeny of daily rhythmicity of plasma corticosterone (CS), and to determine the plasma CS response to adrenocorticotropic hormone (ACTH) injections and temperature stress at different times during a light-dark cycle. In Trial 1, CS levels were determined at 3-h intervals over a 24-h period in 3, 10, and 14-day-old tom and hen poults maintained on a 12 h light:12 h dark (12L:12D) light-dark cycle. Plasma CS levels of tom and hen poults were similar. A robust daily rhythm in plasma CS occurred by 10 days of age, with maximal levels of CS occurring during the scotophase. In Trial 2, daily variation in plasma CS sensitivity to cold treatment or ACTH in 2-wk-old toms was examined. Tom poults were exposed to cold water immersion (5 C for 1 min), porcine ACTH injection (1 IU/kg), or saline injection at 1100 h in midphotophase and at 2300 h in midscotophase. Treatments significantly increased plasma CS in both the photophase and scotophase, with the levels being significantly higher in the scotophase. At midphotophase cold or ACTH treatment induced, respectively, a 489 and 546% increase in plasma CS above that of controls. During midscotophase, 746 and 695% increases in CS above levels for controls were induced by cold stress or ACTH, respectively. Differences between percent changes for times of day were not significantly different. It was concluded that a daily rhythm in plasma CS in turkey poults grown under a 12L:12D light-dark cycle occurs between 3 and 10 days of age, with maximum levels occurring during the scotophase. A daily variation in the adrenal cortical response to exogenous ACTH or cold water treatment in excess of the normal plasma CS rhythm was not apparent.