Agonistic interactions affect brain serotonergic activity in an acanthopterygiian fish: the bicolor damselfish (Pomacentrus partitus).

Bicolor damselfish were allowed to interact in pairs for 15 min a day during a five-day period. Agonistic behaviour was quantified, and at the end of the experimental series, 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 (an index of 5-HT activity) were calculated in the telencephalon, hypothalamus and brain stem. Socially interacting fish, dominant as well as subordinate, showed higher telencephalic 5-HIAA/5-HT ratios than isolated controls. Social interaction also decreased telencephalic TRP concentrations in subordinate fish but did not affect 5-HT concentrations in any of the brain parts. In subordinate fish, 5-HIAA/5-HT ratios in the telencephalon were positively correlated with the number of aggressive acts received. Moreover, in dominant fish 5-HIAA/5-HT ratios in the hypothalamus were positively correlated with the number of aggressive acts performed. These results indicate that the brain serotonergic system is involved in intraspecific aggression and/or stress reactions in bicolor damselfish.