How do individuals cope with stress? Behavioural, physiological and neuronal differences between proactive and reactive coping styles in fish.

Despite the use of fish models to study human mental disorders and dysfunctions, knowledge of regional telencephalic responses in non-mammalian vertebrates expressing alternative stress coping styles is poor. As perception of salient stimuli associated with stress coping in mammals is mainly under forebrain limbic control, we tested region-specific forebrain neural (i.e. mRNA abundance and monoamine neurochemistry) and endocrine responses under basal and acute stress conditions for previously characterised proactive and reactive Atlantic salmon. Reactive fish showed a higher degree of the neurogenesis marker proliferating cell nuclear antigen () and dopamine activity under basal conditions in the proposed hippocampus homologue (Dl) and higher post-stress plasma cortisol levels. Proactive fish displayed higher post-stress serotonergic signalling (i.e. higher serotonergic activity and expression of the 5-HT receptor) in the proposed amygdala homologue (Dm), increased expression of the neuroplasticity marker brain-derived neurotropic factor () in both Dl and the lateral septum homologue (Vv), as well as increased expression of the corticotropin releasing factor 1 ( ) receptor in the Dl, in line with active coping neuro-profiles reported in the mammalian literature. We present novel evidence of proposed functional equivalences in the fish forebrain with mammalian limbic structures.