ACTH does not mediate divergent stress responsiveness in rainbow trout.

Two lines of rainbow trout selected for high (HR) and low (LR) responsiveness to a standardised confinement stressor displayed a sustained divergence in plasma cortisol levels during a 3-h period of confinement (max.: HR: 167+/-13 ng ml(-1); LR: 103+/-8 ng ml(-1); P<0.001). However, no significant difference in plasma ACTH levels was evident (max: HR: 153+/-9 pg ml(-1); LR: 142+/-7 pg ml(-1)). Dexamethasone (DEX) was administered to HR and LR fish to block endogenous adrenocorticotropin (ACTH) release. Administration of a weight-adjusted dose of ACTH to the DEX-blocked fish elevated plasma cortisol levels to a significantly greater extent in HR (233+/-24 ng ml(-1)) than LR (122+/-14 ng ml(-1)) fish (P<0.001). Plasma cortisol levels in DEX-blocked HR and LR fish after sham injection were low but also significantly different (HR: 6.7+/-1 ng ml(-1); LR: 2.2+/-0.2 ng ml(-1); P<0.001). These results indicate that modulation of cortisol responsiveness to stressors in HR and LR fish resides, at least in part, downstream of the hypothalamic-pituitary axis.