Role Of Protease-activated Receptor 2(PAR2) In Rat CFA Inflammatory Pain And Morphine-induced Analgesia

BackgroundPAR2 is distributed on the surface of most immune cells,and it is also expressed in the dorsal horn of the spinal cord and the superficial layer of the spinal cord,especially in the central end of peripheral afferent nerve fibers,microglia,and astrocytes.PAR2 can participate in inflammatory responses either by affecting immune cells or directly through neuronal activation.PAR2 is also involved in nociceptive signaling,and its activation can activate nociceptors and induce hyperalgesia.PAR2 has also been found to be involved in pruritus studies such as atopic dermatitis.Due to the important role of PAR2 in inflammatory diseases and pain,this experiment used an inflammatory pain model to study the effect of PAR2 on pain and the analgesic effect of analgesic drugs.MethodFirst the CFA inflammatory pain model was established,and the pain threshold changes after different drug treatments were recorded.SD(Sprague-Dawley)male rats were selected.The CFA model group received CFA reagent injection on the left hind foot,the Sham group received saline injection in equal volume,and the Na?ve group received no intervention.The mechanical pain and thermal pain thresholds of the three groups of rats were measured continuously for 14 days,the characteristics of CFA-induced inflammatory pain were observed,and the subsequent drug intervention time points were selected.On the 1st/3rd/7th/14th day of modeling,the left L4-6 spinal cords of the rats in the CFA group,the Sham group and the Na?ve group were collected for western blot analysis of the expression levels of related proteins.On the 7th day after the establishment of the CFA model,4.0 mg/kg morphine was used for analgesia,and the changes in analgesic effects caused by PAR2 antagonist FSLLRY alone,PAR2 agonist 2fly alone and morphine combined with the above two drugs were observed.ResultsThis experiment successfully established the CFA inflammatory pain model in SD rats.In terms of behavior,compared with the Sham group(n=6),the CFA group(n=8)had a significantly lower left plantar mechanical pain threshold from the 4th day of modeling,and lasted till the 14th day of modeling,and on the 8th day the threshold reached the lowest point(p<0.0001).The change trend of the mechanical pain threshold of the right foot was similar to that of the left foot.Compared with the Sham group,the left foot thermal pain threshold did not change significantly within 14 days.Western blot results showed that on day 1/3/7/14,there was no significant difference in the expression of PAR2 protein and p38,p-p38,NF-κB in the left L4-6 spinal cord of the CFA group(p>0.05).The single application of FSLLRY(2 mg/kg)and 2fly(0.2 mg/kg)alone had no significant effect on the mechanical pain threshold of the left foot of CFA rats,but the mechanical pain threshold of the left foot of the rats was significantly increased 30 minutes after the single application of morphine(4.0 mg/kg and 8.0 mg/kg)(p<0.001),while low-dose morphine(1.0 mg/kg and 2.0 mg/kg)did not significantly improve mechanical pain threshold.When FSLLRY was combined with 4.0 mg/kg morphine,the mechanical pain threshold did not increase after 30 minutes.When 2fly was combined with 4.0 mg/kg morphine,the mechanical pain threshold was increased at 30 and 60 minutes(p<0.05).Conclusionsa)There is no significant correlation between the expression of PAR2 signal in spinal cord of CFA model rats and pain behavior;b)The activation and inhibition of PAR2 had no effect on pain behavior in CFA rats;c)PAR2 agonists can prolong the analgesic time of morphine,while PAR2 antagonists can attenuate the analgesic effect of morphine.