Potentiation of the analgesic effects of tryptophan by allopurinol in rats.

The administration to rats of tryptophan (CAS 73-22-3) in high dosage causes a significant increase in pain threshold values. The analgesic effects of tryptophan are potentiated by allopurinol (CAS 315-30-0). The analgesic effects shown by tryptophan injection are associated with increased levels of serotonin and 5-hydroxyindoleacetic acid in some areas of the brain. The combined allopurinol and tryptophan treatment elevates the serotonin levels furtherly when they are compared with the values observed in animals receiving tryptophan only. The analgesia caused by tryptophan administration has been attributed to an increased activity of the serotoninergic system involved in the control of pain transmission.