Role of P2X7 receptor in ventrolateral periaqueductal gray in tramadol-induced reduction of neuropathic pain in rats

Objective To evaluate the role of P2X7 receptor in the ventrolateral periaqueductal gray(vlPAG)in tramadol-induced reduction of neuropathic pain(NP)in rats. Methods Fifty-four male clean-grade healthy Sprague-Dawley rats, aged 7 days, weighing 190-230 g, were studied.NP was induced by chronic constrictive injury(CCI)to sciatic nerve.Experiment Ⅰ Thirty-six rats were divided into 3 groups(n=12 each)using a random number table method: sham operation group(group S), group NP1 and NP plus tramadol group(group NP1+ T). Tramadol 15 mg/kg was intraperitoneally injected once a day from day 7 to day 14 after CCI in group NP1+ T.The mechanical and thermal pain thresholds of the nerve-injured hindlimb were measured before CCI and on 1, 5, 7, 10, 12 and 14 days after CCI.Rats were sacrificed after measurement of pain threshold on day 14 after CCI, and the expression of P2X7 receptor in vlPAG was detected by immunohistochemistry and Western blot assay.Experiment Ⅱ Eighteen rats were divided into 3 groups(n=6 each)using a random number table method: group NP2, NP plus tramadol group(group NP2+ T)and NP plus tramadol plus a specific P2X7 receptor antagonist A-438079 group(group NP2+ T+ A). In NP2+ T+ A group, a catheter was implanted in vlPAG, and the NP model was established on 5th day after successful catheterization.Tramadol 15 mg/kg was intraperitoneally injected once a day from day 7 to day 14 after CCI in group NP2+ T.In group NP2+ T+ A, tramadol 15 mg/kg was intraperitoneally injected once a day from day 7 to day 14 after CCI, followed by a microinjection of A-438079 100 pmol(0.3 μl)via vlPAG before giving tramadol on day 14.The mechanical and thermal pain thresholds were measured at the end of the last tramadol administration and within 1 h after the end of the last tramadol administration. Results Experiment Ⅰ Compared with group S, the mechanical and thermal pain thresholds were significantly decreased at each time point after CCI, the number of P2X7 receptor positive cells was increased, and the expression of P2X7 receptor was up-regulated in the other two groups(P<0.01). Compared with group NP1, the mechanical and thermal pain thresholds were significantly increased at days 7-14 after CCI, the number of P2X7 receptor positive cells was increased, and the expression of P2X7 receptor was up-regulated in group NP1+ T(P<0.01). Experiment Ⅱ Compared with group NP2, the mechanical and thermal pain threshold were significantly increased at each time point after CCI in NP2+ T and NP2+ T+ A groups(P<0.01). Compared with group NP2+ T, the mechanical and thermal pain thresholds were significantly decreased at each time point after CCI in group NP2+ T+ A(P< 0.01). Conclusion The mechanism by which tramadol mitigates NP is partially related to enhanced function of P2X7 receptors in vlPAG of rats. Key words: Tramadol; Neuralgia; Receptors, purinergic P2; Periaqueductal gray