Constant darkness disrupt daily rhythm of adrenocorticotrophin in horses

Cortisol and other adrenal steroids are typically secreted in a pulsate fashion and plasma concentrations can vary widely during a 24-hour period. To investigate daily rhythmicity of plasma adrenocorticotrophin (ACTH) concentration, as well as rectal temperature (RT), sample collections were performed in five Quarter Horses housed in individual boxes under natural light/dark (L/D) cycle followed by constant darkness. The two variables exhibited 24-h rhythmicity under the L/D cycle. Whereas rhythmicity of RT persisted in constant darkness, rhythmicity of ACTH concentration did not. These findings strongly suggested that ACTH secretion in the horse is not under circadian control and is modulated only by environmental light.