Extreme posture elevates corticosterone in a forced ambulation model of chronic stress in rats

The adrenocorticotropin (ACTH), corticosterone, and corticosterone-binding-globulin (CBG) levels in rats stressed chronically by a method designed to induce biomechanical stress were investigated in order to determine the relationship between degree of behavioral stress and evoked change in posture during forced ambulation and pituitary-adrenocortical hormone secretion. The method consists of forcing rats to ambulate and compares rats forced to ambulate in a normal (control group) and extreme (experimental) posture. In this method an extreme posture of relative extension was assumed by rats forced to ambulate inside a rotating cylinder. The numbers of faecal boli produced during forced ambulation were determined as a behavioral measure of stress. Rats forced to ambulate in cylinders produced more faecal boli while they ambulated than rats forced to ambulate on a flat surface. Rats forced to ambulate in cylinders had higher resting levels of corticosterone and lower levels of CBG than rats forced to ambulate on a flat surface. The higher corticosterone levels were not associated with increased ACTH levels. Because adaptation to forced ambulation resulted in higher corticosterone and lower CBG levels when exercise is performed in an extreme posture, it is suggested that extreme posture is stressful.