The goal of this review is to discuss and clarify some of the issues currently being debated regarding the acute humoral adrenergic stress response in fish. The afferent limb of the response, consisting of catecholamine secretion into the circulation, occurs under conditions of extreme physiological impairment. During mild or moderate stress, however, circulating catecholamine levels generally do not change and thus are unlikely to play a role in the mediation of physiological/metabolic responses at such times. The mechanisms leading to catecholamine secretion during severe stress involve the classical sympathetic pre-ganglionic neuronal cholinergic pathway in addition to a multitude of non-cholinergic pathways of neuronal and humoral origin. With respect to the efferent limb of the acute stress response, there is considerable controversy surrounding the importance of circulating catecholamines on influencing physiological function. For example, the control of ventilation in fishes may be less dependent on circulating catecholamines than previously thought. On the other hand, the levels of catecholamines achieved in the circulation of trout during severe stress are indeed sufficient to markedly influence red blood cell and cardiovascular function.
