Effect of dexmedetomidine and small dose of ketamine on expression of P2X4 receptor mRNA and P2X7 receptor mRNA in dorsal root ganglion of rats with neuropathic pain

Abstract

Objective To evaluate the effect of dexmedetomidine and small dose of ketamine on the expression of P2X4 receptor (P2X4 R) mRNA and P2X7 receptor (P2X7R) mRNA in the dorsal root ganglion of rats with neuropathic pain.Methods Ninety male Sprague-Dawley rats,aged 6-9 weeks,weighing 180-220 g,were randomly divided into 5 groups (n =18 each):sham group (group S),chronic constrictive injury group (group CCI),dexmedetomidine group (group D),ketamine group (group K) and dexmedetomidine + ketamine group (group DK).The animals were anesthetized with intraperitoneal 10％ chloral hydrate 400 mg/kg.Neuropathic pain was induced by CCI in CCI,D,K and DK groups.The sciatic nerve was exposed and 4 loose ligatures were placed on the sciatic nerve at 1mmintervals with 4-0 silk thread.In group S,the sciatic nerves were only exposed but not ligated.In D,K and DK groups,dexmedetomidine 50μg/kg,ketamine 10 mg/kg and dexmedetomidine 25μg/kg + ketamine 5 mg/kg were injected intraperitoneally,respectively,while the equal volume of normal saline was injected in S and CCI groups,once a day for 14 consecutive days after CCI.Mechanical paw withdrawal threshold (MWT) and thermal paw withdrawal latency (TWL) were measured at 1 day before CCI,and 3,7 and 14 days after CCI.Six animals were sacrificed after measurement of pain threshold at 3,7 and 14 days after CCI and the lumbar segments (L4-6) of the dorsal root ganglion were removed for determination of P2X4 R mRNA and P2X7 R mRNA expression by RT-PCR.Results Compared with group S,MWT and TWL were significantly decreased at 3,7 and 14 days after CCI in groups CCI,D,K and DK,the expression of P2X4R mRNA and P2X7R mRNA was up-regulated at 3,7 and 14 days after CCI in groups CCI,D and K,and the expression of P2X4 R mRNA and P2X7 R mRNA was up-regulated at 3 and 7 days after CCI in group DK (P ＜ 0.05).Compared with group CCI,TWL and MWT were significantly increased and the expression of P2X4 R mRNA and P2X7 R mRNA was down-regulated at 3,7 and 14 days after CCI in groups D,K and DK (P ＜ 0.05).Compared with D and K groups,TWL and MWT were significantly increased and the expression of P2X4 R mRNA and P2X7 R mRNA was down-regulated at 3,7 and 14 days after CCI in group DK (P ＜ 0.05).Conclusion The mechanism by which the combination of dexmedetomidine and small dose of ketamine produces a synergistic antinociception in rats with neuropathic pain may be related to down-regulation of the expression of P2X4 R mRNA and P2X7 R mRNA. Key words: Dexmedetomidine ; Ketamine ; Neuralgia ; Receptors, purinergic P2 ; Ganglia, spinal