EVIDENCE THAT INHIBITION OF SPINAL NITRIC OXIDE PRODUCTION CONTRIBUTES TO THE ANTINOCICEPTIVE EFFECTS OF EMULSIFIED ISOFLURANE ON FORMALIN-INDUCED PAIN IN RATS

Abstract

The aim of the present study was to investigate the contribution of spinal nitric oxide (NO) to the antinociceptive effects of emulsified isofluane in rats. The formalin test was used to assess nociceptive responses. Immunocytochemistry and histochemistry were performed to determine the effects of emulsified isoflurane on formalin-induced changes in Fos-like immunoreactive (Fos-LI)- and nicotinamide adenine dinucleotide phosphatediaphorase (NADPH-d)-positive neurons, respectively. The results showed that emulsified isofluane, administered intraperitoneally, significantly decreased the formalin-induced paw licking time and that this was attenuated by pretreatment with intrathecal injection of the NO precursor L-arginine. Furthermore, Fos-LI- and NADPH-d-positive neurons were mainly found in the ipsilateral dorsal horn after injection of formalin, some of which were Fos-LI/NADPH-d double-labelled neurons. Administration of emulsified isofluane significantly decreased Fos-LI- and NADPH-d-positive, as well as Fos-LI/NADPH-d double-labelled, neurons. Finally, emulsified isofluane produced a significant reduction of NOS activity and a decrease of NO production in the spinal cord of formalin-treated rats. In conclusion, the results suggest that inhibition of spinal NO production contributes to the antinociceptive effects of emulsified isofluane on formalin-induced pain in rats.