NON-STEROIDAL ANTI-INFLAMMATORY DRUGS'S ANTINOCICEPTION MEDIATED BY THE OPIOID MECHANISM IN THE NUCLEUS RAPHE MAGNUS

It has been established that the midbrain periaqueductal gray matter (PAG) and rostral ventro-medial medulla (RVM) are involved in the descending pain control system. The latter involves the midline nucleus raphe magnus (NRM) and adjacent reticular formation. These brain structures are is one of important parts of CNS circuit that controls nociceptive transmission at the level of spinal cord. Here we report that microinjection of commonly used non-steroidal anti-inflammatory drugs (NSAIDs), diclofenac, ketorolac, metamizol, and xefocam into the NRM produces strong antinociception which is mediated by the opioid mechanism. The experiments were carried out on experimental and control (saline) white albino male rats. Animals were implanted with a guide cannula in the NRM and tested for antinociception following microinjection of NSAIDs into the NRM in the tail flick (TF) and hot plate (HP) tests. The analysis of variance (ANOVA) with post-hoc Tukey-Kramer multiple comparison tests were used for statistical evaluation. The obtained data show that microinjection of these NSAIDs into the NRM produced antinociception as revealed by a latency increase in the tail-flick (TF) and hot plate (HP) latencies compared to the saline control microinjected into the same nucleus. Furthermore, we definitely showed that pre-treatment with opioid antagonist naloxone in the NRM diminishes NSAID-induced antinociception expressing in significant decrease in TF and HP latencies (P<0.001). The present findings support the concept that antinociceptive effects of NSAIDs are mediated via an endogenous opioid system possibly involving the descending pain modulatory circuit.