CHANGES IN BRAIN AND SPINAL TRYPTOPHAN AND 5-HYDROXYINDOLEACETIC ACID LEVELS FOLLOWING ACUTE MORPHINE ADMINISTRATION IN NORMAL AND ARTHRITIC RATS

The effects of morphine (10 mg/kg/s.c.) on tryptophan (TRP), 5-hydroxy-tryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels were studied in normal and arthritic rats. 1. (1) In normal rats morphine induced a discrete but significant increase of 5-HIAA levels in the forebrain and the spinal cord. 2. (2) By contrast, in rats suffering from experimentally induced arthritis large modifications were apparent. The basal levels of TRP, 5-HT and 5-HIAA were significantly higher than in normal rats. Morphine induced clear increases of 5-HIAA and TRP in the forebrain, the brain stem and the spinal cord, without any modification of 5-HT. The effects were dose-dependent and suppressed by naloxone (1 mg/kg/ i.m.). Statistical analysis clearly revealed that arthritic rats were much more sensitive to morphine. The results support the hypothesis of an activation of a 5-HT descending pathway by morphine which parallels the activationn of theascending pathway previously demonstrated by several authors and confirmed here. © 1979.