Using a computerized video-image analysis system, spontaneous locomotor activity was measured in dominant and subordinate individuals of Arctic charr (Salvelinus alpinus) and in individuals treated with inhibitors of serotonin (5-HT) synthesis and re-uptake. Arctic charr were put together in pairs. After 1 week, subordinate individuals were found to have elevated brain levels of 5-hydroxyindoleacetic acid, a major 5-HT metabolite, suggesting an increase in serotonergic activity. The subordinate individuals had significantly lower spontaneous locomotor activity than the dominant fish. Similarly, Arctic charr displayed a significantly reduced locomotor activity when their serotonergic activity was stimulated by the specific 5- HT re-uptake inhibitor zimeldine. In contrast, fish treated with the 5-HT synthesis inhibitor p-chlorophenylalanine showed a significant increase in locomotor activity. Dominant, subordinate and pharmacologically treated fish all had very similar activity rhythms for the 18 h test period. Thus, neither the previous social experience nor the pharmacological treatment seemed to affect the diurnal activity rhythm per se. Taken together, these results suggest that the brain serotonergic system inhibits locomotor activity and support the possibility that 5-HT is involved in the inhibition of locomotor activity displayed by subordinate fish.

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