Food intake and spontaneous swimming activity in Arctic char (Salvelinus alpinus): role of brain serotonergic activity and social interactions

We investigated the relationship between social interactions, brain seroton ergic activity, and two behavioural patterns in juvenile Arctic char (Salve linus alpinus): feeding and spontaneous swimming activity. Dominant and sub ordinate individuals were observed during rearing in pairs, followed by rea ring in isolation. Throughout the experiment, levels of both food intake an d swimming activity remained high in dominant fish. When they were in pairs , food intake was completely inhibited in subordinate fish; thus, dominant fish were able to monopolise food. At the same time, brain serotonergic act ivity, as indexed by the ratio of 5-hydroxyindoleacetic acid (5-HIAA) to se rotonin (5-hydroxytryptamine, 5-HT) was elevated in the hypothalamus and br ain stem of subordinate fish compared with dominants. During subsequent rea ring in isolation, food intake, but not spontaneous locomotor activity, gra dually increased in previously subordinate fish, while serotonergic activit y fell to near that of dominants. Thus, appetite inhibition in subordinate fish can be reversed by rearing in isolation, an effect that may be related to the reversal of a stress-induced activation of brain serotonergic neuro nes accompanying social subordination. Reduced swimming activity is either a long-lasting response to social subordination or reflects permanently dif ferent behavioural strategies of subordinate and dominant individuals.