The release of serotonin in rat spinal dorsal horn and periaqueductal gray following carrageenan inflammation.

The level of serotonin (5-HT) and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) in dorsal horn of spinal cord and periaqueductal gray (PAG) were measured using in vivo microdialysis coupled with high performance liquid chromatography and electrochemical detection. Intraplantar injection of carrageenan was used to evoke release. Extracellular concentrations of 5-HT and 5-HIAA in spinal dorsal horn and PAG significantly increased following carrageenan inflammation. The peak occurred at 3h and then gradually returned to baseline at 5-7h. The carrageenan-induced release of 5-HT and 5-HIAA in spinal cord rather than in PAG was decreased by intracerebroventricular (i.c.v.) injection of naloxone, and both in spinal dorsal horn and PAG, the release was increased by i.c.v. injection of bicuculine. These results suggest that activity of 5-HT in spinal cord and PAG increases with carrageenan inflammation; the carrageenan-induced release of 5-HT and 5-HIAA in spinal cord may be tonically modulated by supraspinal opioid and GABA systems, whereas the release in PAG may only be tonically modulated by endogenous GABA system in supraspinal level.