An excitatory role for 5-HT in spinal inflammatory nociceptive transmission; state-dependent actions via dorsal horn 5-HT3 receptors in the anaesthetized rat

The involvement of 5-HT3 receptor mediated modulation of formalin and carra geenan induced inflammatory transmission was investigated. The effects of t he selective 5-HT3 receptor antagonist ondansetron on the electrically evok ed responses of dorsal horn neurones in normal animals were compared to tho se following carrageenan. The effect of pre-treatment on the formalin respo nse was also studied. Ondansetron had no significant effect on the electric ally evoked responses of dorsal horn neurones in normal animals or followin g carrageenan induced inflammation, but significantly inhibited both phases of the formalin response. Our results suggest that 5-HT3 receptors in the spinal cord have no significant role under normal conditions. However, duri ng formalin (but not carrageenan) induced inflammation this system is activ ated, maintaining the response of nociceptive spinal neurones to peripheral formalin. (C) 2000 international Association for the Study of Pain. Publis hed by Elsevier Science B.V. All rights reserved.