Antagonism of ORLI receptor produces an algesic effect in the rat formalin test

The authors investigated the role of endogenously released nociceptin (also known as orphanin FQ) spinal and supraspinal nociceptive transmission during the rat formalin test by examining the effect of intrathecal and intracerebroventricular injection of J-113397, a non-peptidyl ORL1 receptor selective antagonist. When J-113397 was injected intrathecally or intracerebroventricularly 10 min before the formalin injection, it enhanced the agitation behavior induced by paw formalin injection. This suggested that paw formalin injection induced nociceptin release in the spinal cord and the supraspinal brain sites, that this endogenously released nociceptin produced an analgesic effect and that J-113397 antagonized this analgesic effect of nociceptin and produced an algesic effect in the rat formalin test.