Concentrations of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HIAA) and tryptophan (TRP, the amino acid precursor of 5-HT) were measured, and 5-HIAA/5-HT ratios calculated, in the telencephalon, hypothalamus and brain stem of Arctic charr (Salvelinus alpinus) with 1–21 days experience of a dominant or subordinate position in a pair. Brain 5-HIAA levels and 5-HIAA/5-HT ratios (an index of serotonergic activity) increased rapidly in all three areas of the brain in subordinate fish and remained high for up to 21 days. The brain stem 5-HIAA concentration in dominant fish showed a temporary increase after 1 day of social interaction, but returned to the control level 2 days later. The social interactions did not affect 5-HT concentrations in any of the brain regions. An initial, but temporary, increase in brain TRP concentration was seen in both subordinate and dominant fish. After 1–3 days of social interaction, brain TRP levels declined. This decline was most pronounced in subordinate individuals which, after 7 and 21 days, had hypothalamic TRP concentrations significantly lower than those of controls. Moreover, TRP levels in the telencephalon after 21 days, and in the hypothalamus after 7 days, were significantly lower in subordinate individuals than in dominant fish. These results show that subordinate experience rapidly causes a sustained increase in brain 5-HT metabolism which does not correlate with changes in brain TRP levels. Thus, the increases in brain 5-HIAA concentration and in brain 5-HIAA/5-HT ratios probably reflect an increase in functional 5-HT release, a phenomenon that appears to have a wide distribution in the brain.

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