DUAL EFFECT OF SEROTONIN ON FORMALIN-INDUCED NOCICEPTION IN THE RAT SPINAL-CORD

To examine the role of the descending serotonergic system in the regul ation of spinal nociceptive processing, the effects of serotonin (5-HT ) and selective ligands for 5-HT receptor subtypes on persistent nocic eption were investigated. Formalin (5% formaldehyde) injected into the plantar region of the rat hindpaw induced two phases of aversive resp onses such as licking and biting. Intrathecal administration of select ive 5-HT3 receptor antagonists, granisetron (0.1-100 pmol/rat) and ond ansetron (1-1000 pmol/rat), reduced the second phase of the formalin-i nduced aversive responses without affecting the first one. The antinoc iceptive effect of granisetron (100 pmol/rat) was abolished when 5-HT was depleted from the lumbar cord by pretreatment with 5,7-dihydroxytr yptamine (5,7-DHT). In the 5,7-DHT-treated rats, intrathecal administr ation of 1-(m-chlorophenyl)-biguanide, a selective 5-HT3 receptor agon ist, facilitated the aversive responses in the second phase whereas th at of 8-OH-DPAT, a selective 5-HT1A receptor agonist, suppressed them. Intrathecal administration of 5-HT showed a dual effect on the second phase of the aversive responses in the 5,7-DHT-treated rats; 5-HT inh ibited the aversive responses when administered at a low dose (0.1 nmo l/rat) but facilitated them at a high dose (1 nmol/rat). In addition, the inhibitory and facilitatory effects of intrathecal 5-HT were block ed by its co-administration with NAN190, a 5-HT1A receptor antagonist, and granisetron, respectively. These results suggest that 5-HT suppre sses formalin-induced nociception in the spinal cord via the 5-HT1A re ceptor and facilitates it via the 5-HT3 receptor.