The effects of stress on plasma ACTH, α-MSH, and cortisol levels in salmonid fishes

ACTH does not mediate divergent stress responsiveness in rainbow trout

“…Plasma levels of ACTH both before and during confinement in the present study were comparable with those reported elsewhere for rainbow trout (Sumpter et al, 1986;Balm and Pottinger, 1995). Increases in plasma ACTH precede changes in plasma cortisol levels but this relative difference in the rate of change of the two hormones is most pronounced during the early phase of the stress response (2 -10 mins; Sumpter et al, 1986) and was therefore not evident in the present study in which the first sample was collected 30 mins after the start of confinement.…”

“…Increases in plasma ACTH precede changes in plasma cortisol levels but this relative difference in the rate of change of the two hormones is most pronounced during the early phase of the stress response (2 -10 mins; Sumpter et al, 1986) and was therefore not evident in the present study in which the first sample was collected 30 mins after the start of confinement. No statistically significant differences in plasma ACTH levels between the HR and LR fish were apparent at any point during the time-course.…”

Sexual Maturity Modifies the Responsiveness of the Pituitary-Interrenal Axis to Stress in Male Rainbow Trout

“…This stress-induced decline in α-MSH levels is consistent with recent studies on immature rainbow trout (Balm and Pottinger, 1995) and tilapia but at odds with previous work on brown (Sumpter et al, 1985) and rainbow trout (Sumpter et al, 1986) in which no α-MSH response to a moderate stress was observed; rather, a significant increase in plasma α-MSH levels was observed after exposure to severe stress. Again, as discussed with regard to the N-acetyl-β-END response, there is a minimal amount of information available on these peptides in fish; until further data are available we can only speculate that species and procedural differences may account for the inconsistencies.…”

“…A significant correlation between MSH and END is observed throughout, which excludes the assays as a possible source of variation. Instead, these data highlight the possibility that the melanotrope response to stress may be less fundamental than that of the corticotropes, but that under certain circumstances it is of adaptive value to increase melanotrope activity (Sumpter et al, 1985;1986), to sustain melanotrope activity at a constant level (Sumpter et al, 1985;1986) or to reduce the output of the melanotropes (this study; Balm and Pottinger, 1995). The functional significance of these responses is unclear but melanotropes may exert a potent corticotropic signal (Tran et al, 1989; and melanotrope POMC-derived products other than MSH and END may play a role in the regulation of interrenal cortisol production (Takahashi et al, 1985).…”

“…While increased metabolic clearance of cortisol from the blood may contribute to a reduction of stress-induced levels in mature male fish, the data obtained in Experiment 3 suggest that a change in pituitary responsiveness to stress is largely responsible for the difference. In rainbow trout, experimental evidence indicates that interrenal cortisol production during confinement stress is predominantly controlled by ACTH (Sumpter et al, 1986;Balm and Pottinger, submitted) and the results of Experiment 3 demonstrate that plasma ACTH levels in mature male trout during stress are significantly lower than those in immature fish. This interpretation of the results is supported by earlier work which indicated that sexual maturation did not cause a significant change in interrenal responsiveness to exogenous ACTH in sockeye salmon (Fagerlund, 1970) although it contrasts with the reduced responsiveness to ACTH observed in mature chinook salmon (Hane et al, 1966).…”