Does individual variation in stress responsiveness and behavior reflect divergent stress coping strategies in rainbow trout?

In rainbow trout (Oncorhynchus mykiss) the magnitude of the cortisol response to stress shows both consistency over time and a high degree of heritability, and high responding (HR) and low responding (LR) lines of rainbow trout has been generated by individual selection for consistently high or low post-stress cortisol values (Pottinger and Carrick, 1999). In addition to divergent post-stress plasma levels of cortisol, HR and LR trout also differ in other neuroendocrine parameters. For instance, LR fish show higher brain serotonin (5-Hydroxytryptamine, 5-HT) turnover (Øverli et al., 2001), and respond to acute stress with a larger increase in plasma catecholamines, than HR fish do. Moreover, a link between neuroendocrine stress responses and behavioural traits is evident from observations on these lines of rainbow trout. In particular, LR fish are socially dominant over HR fish when paired (Pottinger and Carrick, 2001). LR trout also resume feeding faster than HR trout when transferred to social isolation, and display a reduced locomotor response in a territorial intrusion test (Øverli et al., 2002a). When kept in groups LR fish show higher locomotor activity than HR fish, a difference that could be related to higher levels of aggression in groups of LR fish. A higher level of aggression in LR fish is also suggested by the fact that LR fish grew faster than HR fish when kept in mixed groups. The combination of these results suggests that LR and HR trout are displaying divergent stress coping styles, LR fish being proactive and HR fish reactive (Koolhaas et al., 1999). Recent results from similar studies using a non-selected hatchery population of rainbow trout supports the suggestion that divergent stress coping strategies, akin to what has been described as proactive and reactive stress coping in rodents (Koolhaas et al., 1999), exists in rainbow trout. A reciprocal relationship between the brain and endocrine system appear to exist, linking cortisol levels during stress to behavioural traits such as the outcome of agonistic interactions. Data from other studies suggest that the brain 5-HT system could provide such a link between stress responses and behaviour. In salmonid fish, as in many other vertebrates, social subordination results in a drastic but reversible inhibition of behavioural responsiveness along with a sustained activation of the brain 5-HT system. Elevated brain 5-HT activity appears important in mediating several behavioural and physiological responses to social subordination, including inhibition of aggressive behaviour, lowered feed intake, suppression of locomotor activity and elevated plasma cortisol levels (Winberg and Nilsson, 1993). However, cortisol may also have behavioural effects, as well as effects on the synthesis and release of 5-HT and other monoaminergic neurotransmitters. We have shown that short-term cortisol treatment increases activity and aggression whereas long-term treatment with cortisol has the opposite effect, decreasing activity and aggression in rainbow trout (Øverli et al., 2002b). Thus, cortisol appears to have time- and context-dependent effects on behaviour, which could either be mediated by interaction with 5-HT or other neurotransmitter systems, or reflect a direct behavioural role for cortisol receptors in fish.