ACTH does not mediate divergent stress responsiveness in rainbow trout

Abstract

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.