Effects Of Intramuscular Heat Stimulation On Endogenous Descending Modulation Of Pain And Its Mechanisms In Spinal Cord Injury Rats

Purpose:The aims of present study are to observe the changes in endogenous modulation of pain following spinal cord injury（SCI）,to explore the possible effect of intramuscular heat stimulation on the endogenous modulation of pain in SCI rats,to explore thalamic MD and VM nuclei P2X3 receptor associated mechanisms in effects of intramuscular heat stimulation on endogenous descending modulation of pain in SCI rats.Methods:1.Behavioral assessment:Basso,Beattie&Bresnahan（BBB）scores were performed before and 8-21 days after SCI,paw withdrawal mechanical threshold and paw withdrawal thermal latency of forelimbs and hind limbs were measured.2.SCI modeling:The spinal cord was exposed by laminectomy at the 10th thoracic vertebra（T10）in rats,and a moderate contusion model was prepared by spinal cord percussion.Sham rats only underwent laminectomy.3.Intramuscular heat stimulation:on the 8^th day of SCI,under respiratory anesthesia,a single needle was inserted into the gastrocnemius muscle of the right hind limb at a depth of 0.5cm,and the temperature was set as 43 and 46℃respectively,the duration of heat stimulation was 30min.Paw withdrawal mechanical threshold and paw withdrawal thermal latency were observed on forelimb and hind limb bilaterally,0.5-3h and 1-6d following intramuscular heat stimulation.4.Intracerebral microinjection:0.5-4.5nmol of P2X3 receptor antagonist A-317491were microinjected into thalamic MD and VM nuclei,paw withdrawal mechanical threshold and paw withdrawal thermal latency were observed 0.5-4h before and after injection.The mechanisms of heat stimulation at 43℃and 46℃on the endogenous regulation of pain in SCI rats were explored.Experimental results:1.BBB score,mechanical and heat withdrawal reflexes following SCI Before spinal cord injury model was established,there were no statistically significant differences in BBB score,paw withdrawal mechanical threshold and paw withdrawal thermal latency（P>0.05）.After SCI modeling,compared with Naive and Sham groups,BBB score of SCI group was significantly lower（P<0.05）;no significant changes in paw withdrawal thermal latency of forelimbs were observed（P>0.05）.Heat hyperalgesia was observed in bilateral hind limbs and Lasted at least 21 days（P<0.05）.2.BBB score of SCI rats did not change significantly after needle stimulation without heating（P>0.05）.There were no significant changes in paw withdrawal mechanical threshold and paw withdrawal thermal latency of forelimbs and hind limbs（P>0.05）.3.Changes in BBB score,mechanical and heat withdrawal reflexes following intramuscular heat stimulation at 43℃in SCI rats There was no significant changes in BBB score following intramuscular heat stimulation at 43℃for 30min（P>0.05）,no significant changes in paw withdrawal mechanical threshold of forelimbs and hind limbs were found 0.5-3h and 1-4d after heat stimulation（P>0.05）;paw withdrawal thermal latencies of forelimbs and hind limbs were prolonged significantly（P<0.05）.4.Changes in BBB score,mechanical and heat withdrawal reflexes following intramuscular heat stimulation at 46℃in SCI rats There was no significant change in BBB score of SCI rats treated with heating-needle stimulation at 46℃for 30min（P>0.05）,paw withdrawal mechanical threshold of forelimbs and hind limbs decreased significantly 0.5h-3h and 1-4d after the treatment（P<0.05）;paw withdrawal thermal latencies of forelimbs and hind limbs were increased following heat stimulation（P<0.05）.5.Microinjection of P2X3 receptor antagonist A-317491 into thalamic MD nucleus can dose-dependently reverse the reduction of paw withdrawal mechanical threshold of forelimb and hind limb induced by 46℃heat stimulation（P<0.05）.However,there was no significant influence in changes in paw withdrawal thermal latency of forelimb and hindlimb induced by 46℃heat stimulation（P>0.05）.6.Microinjection of A-317491 into thalamic VM nucleus had no significant effect on changes in paw withdrawal mechanical threshold of forelimb and hindlimb induced by 43℃and 46℃heat stimulation（P>0.05）,However,it could dose-dependently reverse the prolonged paw withdrawal thermal latency of forelimb and hindlimb induced by heat stimulation at 43℃and 46℃（P<0.05）.Conclusions:1.In SCI rats,the endogenous facilitation of pain was enhanced and the endogenous inhibition was weakened.2.43℃heat stimulation could enhance the descending inhibition of pain;46℃heat stimulation could enhance both the descending facilitation and inhibition of pain.3.Thalamic P2X3 receptor in the MD nucleus play roles in the enhanced descending facilitation induced by 46℃heat stimulation in SCI rats;P2X3 receptor in the VM nucleus of thalamus participates in the enhancement of descending inhibition on pain induced by 46℃and 43℃heat stimulation in SCI rats.