Centrally administered tryptophan suppresses food intake in free fed chicks through the serotonergic system

The effects of intracerebroventricular injection of L-tryptophan on feeding behavior and the levels of brain neurotransmitters (amino acids or monoamines) were investigated in ad libitum chicks. The tryptophan treatment (3 or 6 mu mol) significantly inhibited food intake in chicks at 30min postinjection. The levels of serotonin (5-HT) and its metabolite, 5-dihydroxyindolacetic acid, in chicks treated with tryptophan were significantly higher than those with saline at 15 min postinjection. However, there were no differences in the levels of catecholamines (adrenaline, noradrenaline and dopamine) and amino acid neurotransmitters (e.g., gamma-aminobutyric acid, glycine and glutamic acid). The tryptophan-induced anorexia tended to be attenuated by the 5-HT2A receptor antagonist ketanserin (10 mu g). These results suggest that the administration of tryptophan into the chick brain produces the anorexic effect, and that the change in brain 5-HT content may be involved in this anorexia.