Modulation of stress hormones in rainbow trout by means of anesthesia, sensory deprivation and receptor blockade.

Sympathetic activation leading to increased levels of blood catecholamines, and stimulation of the hypothalamic-pituitary inter-renal axis leading to increased cortisol, are difficult to avoid when handling animals. Yet, in research on effects of acute stress, elicitation of such responses must be minimized in the control groups. The work examines means to achieve a minimally disturbed state in rainbow trout (Oncorhynchus mykiss). Level of arousal was determined by adrenaline and cortisol concentrations in plasma, and by the spleen:somatic index. Fish were prepared for bleeding by rapid capture and concussion, by infusion of anesthetic into the undisturbed home tank, by confinement in black boxes, or by being fed alpha- and beta-receptor antagonists. Even when done quickly, netting and concussion yielded fish with ca. 200-pmol adrenaline/ml plasma. Cortisol was elevated (to > 10 ng/ml) within 30 s of stress initiation. Surreptitious infusion of anesthetic (2-phenoxyethanol, PE) into tanks yielded fish with lower adrenaline levels (means 19.34 and 19.58 pmols/ml in home tank and black boxes, respectively). Among fish given phentolamine and propranolol, spleen:somatic indices and plasma adrenaline were higher than in diet controls, whether undisturbed or stressed, indicative of successful receptor blockade. Since careful infusion of 2-PE yielded the lowest adrenaline levels, and requires no special apparatus, it is the method of choice for obtaining minimally stressed fish.