The effect of L-tryptophan on motor activity and its prevention by an extracerebral decarboxylase inhibitor and by 5-HT receptor blockers.

L-Tryptophan at moderately low dosage (20 mg/kg) reduced the activity of rats taken during a dark period (red light) and put into an open field illuminated by bright white light. Activity was not altered when the field was illuminated by red light. Tryptophan did not cause significant hypoactivity in rats pretreated with the 5-hydroxytryptamine (5-HT) receptor antagonists methysergide, cyproheptadine and metergoline. However, tryptophan did not alter brain 5-HT concentration and only increased 5-hydroxyindoleacetic acid (5-HIAA) slightly in rats killed shortly after behavioural observation. A further indication that the behavioural effect of tryptophan was not due to increased brain 5-HT was its prevention by R04-4602 at a dose sufficient to block peripheral but not central L-aromatic amino acid decarboxylase. The results suggest that the above behavioural effect of L-tryptophan is peripherally mediated. A number of potential mechanisms are discussed.