An excitatory role for 5-HT in spinal inflammatory nociceptive transmission; state-dependent actions via dorsal horn 5-HT(3) receptors in the anaesthetized rat.

The involvement of 5-HT(3) receptor mediated modulation of formalin and carrageenan induced inflammatory transmission was investigated. The effects of the selective 5-HT(3) receptor antagonist ondansetron on the electrically evoked responses of dorsal horn neurones in normal animals were compared to those following carrageenan. The effect of pre-treatment on the formalin response was also studied. Ondansetron had no significant effect on the electrically evoked responses of dorsal horn neurones in normal animals or following carrageenan induced inflammation, but significantly inhibited both phases of the formalin response. Our results suggest that 5-HT(3) receptors in the spinal cord have no significant role under normal conditions. However, during formalin (but not carrageenan) induced inflammation this system is activated, maintaining the response of nociceptive spinal neurones to peripheral formalin.