Effect of MK- 801 on acute and tonic pain in male rats

Excitatory amino acid receptors (EAAs) play vast roles of which a possible application has been suggested as nociceptive neurotransmitters. Although studies on the role played by EAAs eg. N- methyl- D- aspartic acid (NMDA) in acute and tonic pain has shown controversial results most studies attribute a critical role to NMDA receptor in the development of persistent pain and secondary hyperalgesia following nerve and tissue injury. The aim of the present study was to evaluate the effects of MK-801 on acute and tonic pain in rats. In this experimental study formalin was used to produce acute and tonic pain. In order to examine the effects of MK-801 as a non- competitive antagonist of NMDA receptor on formalin test male rats weighing 250-300gr were used. Each group contained 6 rats. Twenty minutes before formalin test, the rats were injected with different concentrations of MK-801 (0.075, 0.1, 0.15, 0.3, 0.45 mg/kg) i. P the control group was injected with saline 0.9%. Reaction against pain was recorded as 0-3. Data were analyzed using ANOVA and Tukey’s tests. It was shown that MK-801 at doses more than 0.3 mg/kg significantly reduced pain behavior during the first phase (P<0.001) and MK-801 at doses more than 0.1 mg/kg significantly reduced formalin induced pain behavior during the late phase (P<0.05) Mk-801 at 0.45 mg/kg had the most significant effect (P<0.001). Showing significant differences with other doses (P<0.05) moreover high doses of MK-801 (0.45 mg/kg) increased motivation in rats. These results suggested that MK-801 the non- competitive antagonist of N- methyl D- Aspartic acid receptor (NMDA), could decrease acute and tonic pain. Therefore NMDA receptor plays an important role in pain.