Naloxone''s effects on operant responding depend upon level of deprivation

Treadmill exercise activates the hypothalamic-pituitary-adrenal axis and evokes metabolic responses proportional to exercise intensity and duration. To determine whether glucocorticoid administration would alter humoral and metabolic regulation during exercise, we administered 4 mg dexamethasone (DEX) or placebo to 11 normal, moderately trained men (19-42 yr old) in a double blinded random fashion 4 h before high intensity intermittent treadmill running. Plasma levels of ACTH, cortisol, arginine vasopressin (AVP), lactate, and glucose were measured before, during, and after exercise. A wide range of ACTH responses were seen in the DEX-treated group and arbitrarily defined as two subsets of individuals according to their responses to dexamethasone: DEX nonsuppressors and DEX suppressors. Exercise-induced increases in heart rate and circulating concentrations of cortisol, AVP, lactate, and glucose were all significantly greater (P < 0.05) in nonsuppressors (n = 4) compared to suppressors (n = 7) after both placebo and DEX administration. Interestingly, heart rate, AVP, and lactate responses were unaltered by DEX alone in both groups. In summary, this study demonstrates that normal individuals exhibit differential neuroendocrine and metabolic responses to exercise and pituitary/adrenal suppression after pretreatment with DEX. These findings reflect marked individual differences in the stress response to exercise that may derive from or lead to differential glucocorticoid negative feedback sensitivity in humans.