Total and free serum tryptophan levels and brain 5-hydroxytryptamine metabolism in arthritic rats

In rats suffering from experimentally induced arthritis produced by Freund's adjuvant, there is a marked decrease in total serum tryptophan levels and a marked increase in plasma-free tryptophan levels at both 15 and 21 days after the administration of the adjuvant. Tryptophan, 5-hydroxytryptamine and 5-hydroxyindoleacetic acid levels are increased in the brain and the spinal cord at 15 days. However, 21 days after administration of the adjuvant these levels returned to normal values in the brain, but remained increased in the spinal cord. These results are in agreement with investigations suggesting the possible involvement of the raphe-spinal system in response to pain stimuli but are contrary to the observation that there is a large decrease in plasma-free tryptophan levels in arthritic human patients. These opposite results still question the hypothesis of a relationship between changes in plasma-free tryptophan levels and the severity of pain.