Role of c-Jun N-terminal kinase in activation of astrocytes in midbrain periaqueductal gray of rats with neuropathic pain

Abstract

Objective To evaluate the role of c-Jun N-terminal kinase(JNK)in activation of astrocytes in midbrain periaqueductal gray(PAG)of rats with neuropathic pain. Methods A total of 72 pathogen-free male Sprague-Dawley rats, aged 9 weeks, weighing 160-200 g, were divided into 4 groups using a random number table: control group(group C, n=8), neuropathic pain group(group NP, n=40), dimethyl sulfoxide control group(group DS, n=12)and JNK inhibitor SP600125 group(group SP, n=12). Neuropathic pain was produced by chronic constriction injury(CCI). At 14 days after CCI, 10 nmol JNK inhibitor SP600125 0.5 μl was intraperitoneally injected into the PAG in group SP, and 10% dimethyl sulfoxide 0.5 μl was given instead in group DS.Eight rats were selected in group C, before CCI and at 3, 7, 14 and 21 days after CCI in group NP, and in DS and SP groups, and the mechanical pain threshold was measured before CCI, before administration on 14 days after CCI and at 30, 45, 60, 75 and 90 min after administration.The rats in group C were sacrificed after the end of measurement of the mechanical pain threshold, and brains were removed for determination of phosphorylated JNK(p-JNK)and glial fibrillary acidic protein expression(by Western blot)in PAG region.The rats in group NP were sacrificed after the end of measurement of the mechanical pain threshold at each time point, and brains were removed for detection of p-JNK expression in PAG region.Four rats in DS and SP groups were sacrificed after the last measurement of the mechanical pain threshold at 45 min after administration, and brains were removed for determination of glial fibrillary acidic protein expression in PAG region. Results Compared with group C, the mechanical pain threshold was significantly decreased at each time point after CCI, and the expression of p-JNK was up-regulated at 7-21 days after CCI in group NP(P<0.01). Compared with group DS, the mechanical pain threshold was significantly increased at 30 min after administration, and GFAP expression was down-regulated at 45 min after administration in group SP(P<0.01). The mechanical pain threshold was significantly higher at 30-75 min after administration than before administration in group SP(P<0.01). Conclusion The mechanism underlying activation of astrocytes in PAG is related to activating JNK in the rats with neuropathic pain. Key words: Astrocytes; JNK mitogen-activated protein kinases; Cerebral aqueduct; Periaqueductal gray; Neuralgia