Tryptophan-morphine interactions and postoperative pain

Patients undergoing abdominal surgery were infused with saline or the 5-hydroxytryptamine (5-HT) precursor tryptophan starting in the operating room and continuing for three hours in the recovery room. There was a nonsignificant trend for patients who received tryptophan to have higher pain scores. In the saline-treated patients, plasma tryptophan was below the range for normal healthy subjects, and there was a strong positive relationship between plasma tryptophan and morphine requirements. These data, taken together with animal data obtained using the formalin pain test, suggest that a 5-HT system in the brain can antagonize the dissociative state produced by morphine, which helps patients to tolerate pain. When plasma tryptophan falls below normal levels, brain 5-HT falls and morphine requirements are reduced. While tryptophan may potentiate spinal 5-HT function to decrease nociceptive afference in some circumstances, there may be clinical conditions in which the use of tryptophan is contraindicated.