Effects of 5-HT2 and 5-HT3 receptors on the modulation of nociceptive transmission in rat spinal cord according to the formalin test

We used the formalin test to clarify the 5-hydroxytryptamine (5-HT) recepto r subtypes involved in the modulation of spinal nociceptive transmission in rats. Intrathecal administration of a 5-HT1A receptor agonist, 8-hydroxy-2 -(di-n-propylamino)-tetraline (8-OH-DPAT; 1. 10, and 30 mug), or a 5-HT1B r eceptor agonist, 1. 4-dihydro-3-(1, 2, 3, 6-tetrahydro-4-pyridinyl)-5H-pyrr ol (3, 2-b) pyridin-5-one (CP 93129; 1 and 10 mug), produced no significant change in the number of flinches. A 5-HT, receptor agonist, (+/-)-2, 5-dim ethoxy-4-iodoamphetamine (DOI; 10, 30, and 100 mug), and a 5-HT, receptor a gonist, 2-methyl-5-HT (100 and 300 pg), produced dose-dependent decreases i n the number of flinches in phases 1 (1 to 6 thin) and 2 (10 to 61 min) of the test. The antinociceptive effects of DOI and 2-methyl-5-HT were antagon ized by intrathecal pretreatment with a 5-HT, receptor antagonist, ketanser in, and a 5-HT3 receptor antagonist, 3-tropanyl-3, 5-dichlorobenzoate (MDL- 72222), respectively. These results suggest that 5-HT, and 5-HT, receptors in the spinal cord mediate antinociception to chemical stimuli. (C) 2001 Pu blished by Elsevier Science B.V.