The effect of recaging into groups or into isolation on the pituitary adrenal response to immobilization of different age groups of C57BL/6J mice

Although there is evidence which suggests that age and social environment are significant variables in all experiments dealing with stress in intact animals, there is relatively little information available on the manner in which these variables interact to influence the pituitary adrenal response to stress. Inbred mice of strain, C57BL/6J, of 3 different age groups (49, 255 and 720 days) were subjected to a brief immobilization stress 24 hours subsequent to regrouping them 1, 2 or 4 per cage. Plasma corticosterone concentration was measured by radioimmunoassay prior to and 15 minutes after immobilization or 60 minutes after treatment with ACTH. It was found that preimmobilization levels of corticosterone and increments in corticosterone in response to ACTH treatment were smaller in 255 day than in 49 or 720 day old mice and that preimmobilization corticosterone levels of control and recaged 49 and 255 day old mice were similar. In 49 day old mice, recaging increased the immobilization evoked increment in corticosterone, but in 255 and 720 day old mice recaging in groups or pairs did not change the immobilization evoked response. However, recaging of 720 day old mice in isolation resulted in a decrease in the immobilization evoked increment. Therefore, it appears that the act of transfer itself increased the pituitary adrenal function in the 49 day old mice while, in the oldest mice, isolation itself reduced pituitary adrenal activity. Finally, it appears that in the 255 day old mice, recaging is only a minor stress since after 24 hours′ there is no evidence of elevated steroid levels.