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.