Distinct effect of orphanin FQ in nucleus raphe magnus and nucleus reticularis gigantocellularis on the rat tail flick reflex

The aim of the present study is to investigate the effects of orphanin FQ ( OFQ) microinjected into the nucleus raphe magnus (NRM) and the nucleus reti cularis gigantocellularis (NGC) on pain modulation. The tail-flick latency (TFL) was used as a behavioral index of nociceptive responsiveness. The res ult showed microinjection of OFQ into the NRM significantly increased the T FL, whereas microinjection of OFQ into the NGC decreased the TFL, suggestin g the analgesic effect of OFQ in the NRM and the hyperalgesic effect of OFQ in the NGC. As there are three classes of putative pain modulating neurons in the rostral ventromedial medulla (RVM), the hyperalgesic or analgesic e ffect of OFQ in the RVM might depend upon the different class of the neuron s being acted. (C) 2001 Published by Elsevier Science Ireland Ltd.