SOCIAL EFFECTS AND CIRCADIAN-RHYTHMS IN SQUIRREL-MONKEY PITUITARY-ADRENAL ACTIVITY

Simultaneous measures of plasma cortisol and ACTH were collected at th e morning peak (AM) and evening nadir (PM) of the circadian rhythm in group-housed and individually housed squirrel monkeys (Saimiri sciureu s). Pronounced AM-PM differences in cortisol were evident in both cond itions, but morning measures of cortisol in monkeys housed without com panions were 32% higher than baseline control values observed when the same monkeys were sampled in groups. Consistent AM-PM differences in cortisol were not associated with consistent AM-PM differences in ACTH , and for monkeys housed without companions, plasma ACTH concentration s were consistently and significantly reduced (23% lower in the mornin g, 42% lower in the evening). All monkeys were subsequently pretreated overnight with dexamethasone to temporarily suppress the secretion of endogenous ACTH and then challenged the following morning with a bolu s injection of synthetic ACTH. Monkeys housed without companions respo nded to the challenge with greater, more prolonged elevations in corti sol relative to monkeys housed in groups. These observations together suggest that squirrel monkeys housed without companions hypersecrete c ortisol at the morning peak of the rhythm because adrenal responsivene ss to ACTH is enhanced. Low circulating ACTH levels in turn are mainta ined by robust glucocorticoid feedback mechanisms that inhibit the syn thesis or release of pituitary ACTH. (C) 1995 Academic Press, Inc.