Changes in brain and spinal tryptophan and 5-hydroxyindoleacetic acid levels following acute morphine administration in normal and arthritic rats

Brain Res. 1979 Oct 19;175(2):291-301. doi: 10.1016/0006-8993(79)91008-4.


The effects of morphine (10 mg/kg/s.c.) on tryptophan (TRP), 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) levels were studied in normal and arthritic rats. (1) In normal rats morphine induced a discrete but significant increase of 5-HIAA levels in the forebrain and the spinal cord. (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 activation of the ascending pathway previously demonstrated by several authors and confirmed here.

MeSH terms

  • Animals
  • Arthritis / metabolism*
  • Arthritis, Experimental / metabolism*
  • Brain / drug effects*
  • Brain Stem / metabolism
  • Dose-Response Relationship, Drug
  • Hydroxyindoleacetic Acid / metabolism*
  • Hypothalamus / metabolism
  • Male
  • Mesencephalon / metabolism
  • Naloxone / pharmacology
  • Rats
  • Serotonin / metabolism
  • Spinal Cord / drug effects*
  • Spinal Cord / metabolism
  • Tryptophan / metabolism*


  • Serotonin
  • Naloxone
  • Hydroxyindoleacetic Acid
  • Tryptophan