Spinal And Peripheral MK-801 Inhibits Formalin-induced Activation Of Spinal P38 MAPK In Rats

The formalin test is a widely used experimental pain model for understanding inflammatory pain and multiple neurotransmitter and inflammatory mediators including glutamate involved in this inducing pain.P38 MAPK represents a group of enzymes in the MAPK family that phosphorylated and activated by a variety of physical,chemical and biological stimuli. Accumulating evidences new suggests that p38 MPAK play an important role in neural plasticity through regulating cell transcription,pratein synthesis and receptor expression.Previous studies from our lab and other investigators have indicated that NMDA receptors played an important role in formalin-induced inflammation.MK-801,the NMDA receptor antagonist,produced a significant suppression on the formalin-induoed nociceptive behavior.However,it is unclear whether the activation NMDA receptor is involved in the nociceptive signaling at the spinal level by activation of spinal p38 MAPK.Thus,the present study was designed to investigate the effects of spinal and peripheral NMDA receptor antagonist MK-801 on the formalin-induced activation of spinal p38 MAPK by formalin test in rats.Experiments were performed on adult male Wistar rats with two parts of observations.The first part of experiments were to investigate the effect of spinal NMDA receptor antagonist MK-801 on the formalin-induced activation of spinal p38 MAPK in rats.Animals were divided into two guoups:formalin + normal saline(NS) and formalin + MK-801 groups.According to the dosage of MK-801,the formalin + MK-801 group was further divided into three subgroups: 10nmol,50nmol和100nmol(n=6 for each).The nociceptive behavior that liftign and licking towards the injected paw to be observed.Formalin was injected into the plantar aspect of the left hind paw,and the nociceptive response was observed immediately.Western blots were employed to examine the total p38 MAPK and activation or phosphorytated p38 MAPK(P-p38 MAPK) in spinal cord.The results showed that total p38 MAPK constitutively expressed in control group of animals;while phosphorylation of p38 MAPK was in lower level.Injection of formalin into the hind paw resulted in significant increase the phosphorylation of p38 MAPK in spinal cord, started within 5 min,peaked in 20 min,and returned to control in 45 min.Total p38 MAPK, however,was unchanged by peripherical formalin.NS or MK-801(10 nmol,50 nmol,100nmol) was intrathecally injected 15 min prior to the formalin injection,respectively.The second parts of experiments were designed to observe the possible involvement of perpheral NMDA receptors in the activation of p38 MAPK in the spinal cord.Animals were divided into two guoups:formalin + normal saline(NS) and formalin + MK-801 groups. According to the dosage of MK-801,the formalin + MK-801 group was further divided into three subgroups:50nmol,100nmol和150nmol(n=6 for each),and then the animals were tested after injection of formlin by behavior observation and Western blot.The results of experiments stated above showed:in the first part,comparison with control group,MK-801 group produced a significant suppression on the formalin -induced nociceptive behavior response,especially to phase 2.IT treatment with MK-801 results in a reduction of phosphorylation p38 in the spinal cord in comparison to NS-treated rats,with a dose-dependent manner.In the second part,MK-801 produced a limit effect on the formalin-induced nociceptive behavior response and phosphorylation p38 in the spinal cord.These results indicated that:1.Perpheral injection of formalin into plantar aspect of hindpaw produced both nociceptive behavioral responses and P-p38 expression in the spinal cord.2.Formalin-induced nociceptive behavioral responses included two phases,and intrathecal or peripheral pretreatment of MK-801 significantly suppressed the tonic response with little effects on the acute nociceptive response.3.P38 MAPK is constitutively expressed in spinal cord and spinal p38 MAPK is activated following formalin injection.The level of phosphorylated p38(P-p38) expression was increased 5min after the formalin injection,peaked in 20 min,and returned to control in 45 min.4.Intrathecal and peripheral administration of MK-801 produced significant suppression on the activation of p38 in the spinal cord.Conclusion:The spinal and peripheral NMDA receptor is one of the mechanisms for the formalin-induced activation of spinal P38-MAPK in the spinal cord.