1,3-DI-O-TOLYLGUANIDINE (DTG) DIFFERENTIALLY AFFECTS ACUTE AND TONIC FORMALIN PAIN - ANTAGONISM BY RIMCAZOLE

The role of the sigma receptor in prolonged pain was examined by asses sing the effects of 1,3,di-o-tolylguanidine (DTG), a selective sigma r eceptor ligand, on the formalin test in mice. Formalin injected subcut aneously into the hindpaw produces a biphasic pain response: an acute phase of short duration followed by a longer-lasting tonic phase. DTG (10 mg/kg, IP) potently reduced pain behavior in the acute phase but i ncreased pain behavior in the tonic phase. Rimcazole (5 and 10 mg/kg, IP), a selective sigma receptor antagonist, blocked both the DTG-induc ed decrease and increase in pain behavior observed in the acute and to nic phases, respectively. These data support previous findings indicat ing a modulatory role for the sigma receptor in nociceptive processes, and suggest that this receptor differentially modulates acute vs. ton ic pain.