Aggression and monoamines: effects of sex and social rank in zebrafish (Danio rerio).

Social defeat is a common model for studies on depression. However, such models are most often used to study aggression in males and sex differences in depression may therefore be overseen. This study investigated the potential of the zebrafish (Danio rerio) as a model for male and female aggression. In addition, effects on the brain serotonergic and dopaminergic neurotransmitter systems after agonistic interaction are well studied in many species, but not in zebrafish. We wanted to explore whether the zebrafish follows the same patterns as many other species. Therefore, the effects of agonistic interaction on brain monoaminergic activity were studied in adult male and female wild-type zebrafish. The fish interacted in pairs with one of the same sex for five days during which agonistic behaviour was quantified daily. Clear dominant/subordinate relationships developed in all pairs, both in males and females. The frequency of aggressive acts increased over time but did not differ between male and female pairs. Further, we found that dyadic agonistic interaction resulted in elevated brain serotonergic activity in subordinate zebrafish, as indicated by elevated hindbrain 5-hydroxyindoleacetic acid to serotonin ratios (5-hydroxyindolacetic acid (5-HIAA)/5-hydroxytryptamine (5-HT) ratios). We also observed a sex difference in forebrain dopamine levels and forebrain 5-HIAA/5-HT ratios, with females displaying higher concentrations of dopamine but lower 5-HIAA/5-HT ratios than males. These results suggest that zebrafish is a suitable model for studies on female aggression and sex differences in brain monoaminergic neurotransmission.