EFFECT OF SOCIAL RANK ON BRAIN MONOAMINERGIC ACTIVITY IN A CICHLID FISH

In Haplochromis burtoni, an African cichlid fish, male sexual maturati on is regulated via social interactions, and these effects are mediate d by gonadotropin-releasing-hormone (GnRH)-containing neurons in the p reoptic area of the brain. Since brain monoaminergic systems are known to be involved in the regulation of GnRH release, and the activity of these systems is influenced by agonistic interactions, we analyzed th e effect of social status on brain monoaminergic activity in H. burton i. Animals were either (1) in normal social groups consisting of two m ales and four females or (2) in groups of one male and five females. Q uantitative behavioral observations were made on each group of animals and, following sacrifice several physiological measurements were made . Concentrations of serotonin (5-HT), 5-hydroxyindoleacetic acid (5-HI AA, the main 5-HT metabolite) and tryptophan (TRP, the amino acid prec ursor of 5-HT), dopamine (DA) and 3, 4-dihydroxyphenylacetic acid (DOP AC, the main DA metabolite) were measured. The 5-HIAA/5-HT and DOPAC/D A ratios were calculated and used as indexes of 5-HT and DA activity, respectively. In addition, the gonadosomatic index was calculated from body and gonadal weights and used as an index of reproductive status. Concentrations of 5-HIAA as well as 5-HIAA/5-HT ratios were significa ntly higher in the brainstem of nan-territorial males than in that of territorial males, and similar trends were seen in the telencephalon a nd hypothalamus. Moreover, TRP concentrations in the telencephalon and brainstem were significantly lower in non-territorial males. In this species, sexual maturation in females is not socially regulated, and t here was no significant correlation between measured antagonistic beha vior and biochemical indices. These results suggest a fundamental diff erence in the neurochemical responses between male and female H. burto ni.