Dietary L-Tryptophan Suppresses Aggression and Stress Responsiveness in Juvenile Rainbow Trout (Oncorhynchus mykiss) The Role of Brain Serotonin and Plasma Melatonin

Abstract: Juvenile rainbow trout (Oncorhynchus mykiss) fed L-tryptophan (TRP) supplemented feed for 7 days show a suppression of aggressive behaviour along with a reduction in post-stress plasma cortisol levels. However, 3 days of elevated dietary TRP does not have any effects on either aggressive behaviour or post-stress plasma cortisol levels. The effects of TRP on aggression and stress responsiveness could be related to elevated brain serotonergic activity, TRP being the precursor of serotonin (5-HT). However, 5-HT is, in turn, the precursor of melatonin (ME) and we found that elevated dietary intake of TRP results in an increase in daytime plasma levels of ME. Elevated plasma ME concentrations in rainbow trout fed TRP supplemented feed appear to be of gastrointestinal origin. ME has been suggested to have inhibitory effects on aggressive behaviour and to act as a glucocorticoid antisecretagogue. However, we found that exogenuos ME did not affect aggressive behaviour or post-stress plasma cortisol in rainbow trout. Instead, the effects of TRP on aggressive behaviour and plasma cortisol were mimicked by treatment with the selective serotonin re-uptake inhibitor, citalopram, suggesting that the effects of TRP are mediated by a stimulation of brain 5-HT activity and not by increased ME synthesis and secretion. However, the slow time-course of the effects of TRP on aggressive behaviour and plasma cortisol suggests that effects on 5-HT receptors mechanism are likely to be involved.