Study On The Analgesic Effects Of Paeoniflorin On Inflammatory Pain And Neuropathic Pain In Rats And The Underlying Mechanism

Objective:This subject studied the analgesic effect of intraplantar injection of paeoniflorin（PF）on inflammatory pain and neuropathic pain in rats,and explored whether adenosine A₁ receptors participate in the analgesic effect of paeoniflorin,which will provide a new scientific basis for clinical application of paeoniflorin as an analgesic and for the development of analgesic drugs targeting adenosine A₁ receptors.Methods:Wistar adult male rats were used in this experiment,which were divided into two parts.（1）The first part of the experiment is the analgesic effect and mechanism of paeoniflorin on carrageenan（Carr）-induced inflammatory pain in rats.First,in order to observe whether intraplantar injection of paeoniflorin has an analgesic effect on rats with carrageenan-induced inflammatory pain,the animals were randomly divided into5 groups:Carr,Carr+low-dose PF,Carr+medium-dose PF,Carr+high-dose PF,and the control,the thermal pain,mechanical pain and foot weight bearing were measured.Next,the adenosine A₁ receptor was blocked with 1,3-dipropyl-8-cyclopentylxanthine（DPCPX）,a selective adenosine A₁ receptor antagonist to observe whether A₁ receptor is involved in the analgesic effect of paeoniflorin on carrageenan-induced inflammatory pain in rats,the animals were randomly divided into 4 groups:Carr,Carr+DPCPX,Carr+DPCPX+high-dose PF,and Carr+high-dose PF,and then thermal pain,mechanical pain,and foot weight bearing were measured.Finally,fluorescent immunohistochemical method was used to observe whether the adenosine A₁ receptor,TRPV1（Transient receptor potential vanilloid 1）and CGRP（Calcitonin gene related peptide）coexisted in the skin,dorsal root ganglia and dorsal horn of the spinal cord of normal rats.The expression levels of adenosine A₁ receptors in the spinal cord dorsal horn of rats in each group were detected by immunoblotting.（2）The second part of the experiment is the analgesic effect and mechanism of paeoniflorin on sciatic nerve chronic compression injury（CCI）rats.First of all,in order to observe whether paeoniflorin had analgesic effect on CCI rats,the animals were randomly divided into5 groups:CCI,CCI+low-dose PF,CCI+medium-dose PF,CCI+high-dose PF,and sham,and then thermal and mechanical pain were measured.Next,DPCPX was used to block the adenosine A₁ receptor to observe whether the adenosine A₁ receptor was involved in the analgesic effect of paeoniflorin on CCI rats,the animals were randomly divided into 4 groups:CCI,CCI+DPCPX,CCI+DPCPX+high-dose PF,CCI+high-dose PF,and then thermal pain and mechanical pain were measured.Finally,the expression levels of adenosine A₁ receptors in the spinal cord dorsal horn of rats in each group were detected by immunoblotting.Results:（1）Intraplantar injection of carrageenan can reduce thermal pain threshold,mechanical pain threshold and foot weight bearing.This effect reaches a peak at the 4th hour,and the effect lasts for 11-12 hours,indicating that injection of carrageenan in rats can produce thermal hyperalgesia,mechanical hyperalgesia,and spontaneous pain.Paeoniflorin was injected intraplantarily 3 hours after carrageenan injection,and high doses of paeoniflorin can effectively elevate the thermal pain threshold,mechanical pain threshold and foot weight bearing,indicating that paeoniflorin has analgesic effects on carrageenan-induced thermal hyperalgesia,mechanical hyperalgesia,and spontaneous pain in rats,and the effects were dose-dependent.DPCPX was injected intraplantarily 10 minutes before the injection of paeoniflorin,and it was observed that DPCPX can effectively lower paeoniflorin-indued increases in thermal pain threshold,mechanical pain threshold,and foot weight bearing in rats with carrageenan-indued inflammatory pain,indicating that DPCPX can effectively relieve analgesic effects of paeoniflorin on carrageenan-induced thermal hyperalgesia,mechanical hyperalgesia,and spontaneous pain.IHC detected the coexistence of A₁ receptor,TRPV1 and CGRP in some neurons or nerve endings in the skin,dorsal root ganglion and dorsal horn of the spinal cord.WB detected that paeoniflorin could increase the expression level of adenosine A₁ receptor in the spinal cord dorsal horn of rats with carrageenan-induced inflammatory pain.The above results indicated that:adenosine A₁ receptor may be involved in the analgesic effect of paeoniflorin on carrageenan-induced inflammatory pain in rats.（2）After CCI surgery of the left hind leg in rats,the thermal pain threshold and the mechanical pain threshold decreased on the 3rd day,and the thermal pain threshold and the mechanical pain threshold reached the minimum on the 7th to 10th day,and this effect lasted for more than 14 days.It shows that CCI rats exibited thermal hyperalgesia and mechanical hyperalgesia.On the 7th day after CCI in rats,paeoniflorin was injected intraplantarily.High doses of paeoniflorin can effectively increase the thermal pain threshold and mechanical pain threshold of CCI rats,indicating that paeoniflorin can produce analgesia on CCI-induced thermal and mechanical hyperalgesia,and the effect was dose-dependent.DPCPX was injected intraplantarily 10 minutes before the injection of paeoniflorin,and it was observed that DPCPX could effectively lower paeoniflorin-indued increases in thermal pain threshold and mechanical pain threshold in CCI rats,indicating that DPCPX can effectively relieve analgesic effects of paeoniflorin on CCI-induced thermal hyperalgesia and mechanical hyperalgesia.WB detected that paeoniflorin could increase the expression level of adenosine A₁ receptor in the spinal cord dorsal horn of CCI rats.The above results indicated that adenosine A₁ receptor may be involved in the analgesic effect of paeoniflorin on CCI rats.Conclusions:（1）Intraplantar injection of paeoniflorin in rats with carrageenan-induced inflammatory pain produced an analgesic effect on evoked pain（thermal and mechanical pain）and spontaneous pain which may be mediated by adenosine A₁receptor.（2）Intraplantar injection of paeoniflorin in rats with CCI-induced neuropathic pain produced an analgesic effect on evoked pain（thermal and mechanical pain）which may be mediated by adenosine A₁ receptor.