In the recent past, the importance of the fish brain monoaminergic system in aggression, mating and feeding has been documented. There are several apparent similarities between the functioning of the fish and mammalian monoaminergic systems. In fish, the hypermetabolism of catecholamines (norepinephrine and dopamine) and indoleamine (serotonin, 5-hydroxytryptamine, 5-HT) has been found to be associated with stressful conditions. In contrast to the situation in mammals, these monoamines can pass through the blood­brain barrier in teleost fishes, contributing to the high levels of biogenic amines in the periphery. Hence, high levels of serotonin in the peripheral circulation, during different stressful conditions, may influence the functioning of other physiological systems, notably the immune system. Serotonin is also stored in considerable amounts by mast cells and platelets, and can be synthetized by chromaffin cells. In mammals, it has been established that 5-HT modulates immune function at a variety of levels. However, little is known about the role of serotonin in the functioning of the immune system in fish. In this perspective article, we will discuss our results and the findings of other laboratories, although meagre on this subject, on the possible role of serotonin in the functioning of immunocompetent cells in fish.

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