Juvenile rainbow trout Salmo gairdnen were either fed cortisol or subjected to an acute stress daily for 10 wk to determine the long-term effects of these factors on growth, physiological conditions, and stress responses. In fish fed cortisol, growth and condition factor, liver glycogen, and circulating lymphocytes were reduced and resting plasma glucose and hematocrit were increased. In fish stressed daily, all condtions were similar to those in controls except that lymphocyte numbers were lower. Continuous feedng of cortisol to fish for 10 wk completely eliminated the plasma cortisol elevation after acute handling, but the magnitude of the stress-induced glucose increase was unchanged. In fish stressed daily for 10 wk, a reduction in post-stress levels of both plasma cortisol and glucose after handling demonstrated the effect of habituation on these stress responses. We concluded that continuously elevated plasma cortisol from exogenous feedng had a profound long-term effect on juvenile rainbow trout, but that d d y stress-induced acute elevations of endogenous cortisol did not, except for a suppression of circulating lymphocytes. The occurrence of changes commonly observed in chronically stressed fish that are smilar to those in the cortisol-fed trout in our experiment supports the view that long-term detrimental effects of stress in fish are largely medated by cortisol. The results also indicated that continuous negative feedback of cortisol on the hypothalamic-pituitary axis may compromise the abihty of fish to cope with additional stress factors by limiting their capacity to elicit an interrenal response to a stimulus.

The regulation of the interrenal of teleostean fishes is reviewed from the perspective of non-classical control mechanisms and new evidence is presented suggesting gonadotropic control of the interrenal. Cortisol secretion by the interrenal, in addition to regulation by ACTH, appears to be mediated by other hormones. Physiologically relevant, direct control of interrenal function by hydromineral factors is unclear.In vitro experiments with interrenals of coho salmon (Oncorhynchus kisutch) indicate that salmon gonadotropin is extremely corticotropic and both ACTH and gonadotropin stimulate the secretion of large quantities of androstenedione from the interrenal.

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