Behavioural and Neuroendocrine Effects of Stress in Salmonid Fish

Stress can affect several behavioural patterns, such as food intake and the general activity level of an animal. The central monoamine neurotransmitters serotonin, dopamine, and norepinephrine are important in the mediation of both behavioural and neuroendocrine stress effects. This thesis describes studies of two salmonid fish model systems: Fish that become socially dominant or subordinate when reared in pairs, and rainbow trout (Oncorhynchus mykiss) genetically selected for high (HR) and low (LR) stress responsiveness, in terms of stress induced cortisol release. Socially subordinate individuals are often subject to chronic stress, and it was found that plasma cortisol and brain monoaminergic activity rapidly increased in subordinate fish during the initial 24 h period following fights for social dominance in pairs of rainbow trout. In pairs of Arctic char (Salvelinus alpinus), subordinate individuals were characterised by an inhibition of food intake and aggression, and low spontaneous locomotion. Appetite inhibition in subordinate fish was reversed by subsequent rearing in isolation, and this effect was probably related to a concomitant decrease in brain serotonergic activity. Furthermore, differential stress responsiveness in HR and LR rainbow trout was associated with differences in behaviour, as well as changes in brain monoaminergic activity. HR fish displayed higher locomotor activity when challenged by a conspecific intruder. This response was probably related to a larger stress induced activation of brain dopaminergic systems in these fish. Finally it was shown that the steroid 'stress-hormone' cortisol has dose- and context-dependent behavioural effects in fish, as has been described in mammals. Specifically, short- term cortisol treatment elevated the behavioural response to a territorial intruder, while long-term treatment, like chronic stress, had the opposite effect, inhibiting locomotor activity and aggression. It is concluded that the signalling systems involved in behavioural and neuroendocrine control during stress display extensive similarities between teleost fishes and mammals