Role of peripheral and spinal 5-HT6 receptors according to the rat formalin test

The present study assessed the possible pronociceptive role of peripheral and spinal 5-HT6 receptors in the formalin test. For this, local peripheral administration of selective 5-HT6 receptor antagonists N-[3,5-dichloro-2-(methoxy)phenyl]-4-(methoxy)-3-(1-piperazinyl)-benzenesulphonamide (SB-399885) (0.01–1 nmol/paw) and 4-iodo-N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]benzene-sulfonamide hydrochloride (SB-258585) (0.001–0.1 nmol/paw) significantly reduced formalin-induced flinching. Local peripheral serotonin (5-HT) (10–100 nmol/paw) or 5-chloro-2-methyl-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-indole hydrochloride (EMD-386088) (0.01–0.1 nmol/paw; a selective 5-HT6 receptor agonist) augmented 0.5% formalin-induced nociceptive behavior. The local pronociceptive effect of 5-HT (100 nmol/paw) or EMD-386088 (0.1 nmol/paw) was significantly reduced by SB-399885 or SB-258585 (0.1 nmol/paw). In contrast to peripheral administration, intrathecal injection of 5-HT6 receptor antagonists SB-399885 and SB-258585 (0.1–10 nmol/rat) did not modify 1% formalin-induced nociceptive behavior. Spinal 5-HT (50–200 nmol/rat) significantly reduced formalin-induced flinching behavior during phases 1 and 2. Contrariwise, intrathecal EMD-386088 (0.1–10 nmol/rat) dose-dependently increased flinching during phase 2. The spinal pronociceptive effect of EMD-386088 (1 nmol/rat) was reduced by SB-399885 (1 nmol/rat) and SB-258585 (0.1 nmol/rat). Our results suggest that 5-HT6 receptors play a pronociceptive role in peripheral as well as spinal sites in the rat formalin test. Thus, 5-HT6 receptors could be a target to develop analgesic drugs.