Dai Yi, Fukuoka Tetsuo, Wang Hu, Yamanaka Hiroki, Obata Koichi, Tokunaga Atsushi, Noguchi Koichi
Department of Anatomy and Neuroscience, Hyogo College of Medicine, 1-1 Mukogawa-cho, Nishinomiya, Hyogo 663-8501, Japan.
Pain. 2004 Apr;108(3):258-266. doi: 10.1016/j.pain.2003.12.034.
The mechanism of mechanical hyperalgesia in inflammation might involve a 'mechanochemical' process whereby stretch evokes the release of adenosine 5'-triphosphate (ATP) from the damaged tissue that then excites nearby primary sensory nerve terminals. In the present study, phosphorylated extracellular signal-regulated protein kinase (pERK) immunoreactivity was used as a marker indicating functional activation of primary afferent neurons to examine the P2X receptor-mediated noxious response in DRG neurons in a rat model of peripheral inflammation. We found that very few pERK-labeled DRG neurons were detected in normal rats after alpha, beta methylene-ATP (alphabetame-ATP) intraplantar injection. However, a number of DRG neurons were labeled for pERK after alphabetame-ATP injection to the complete Freund's adjuvant (CFA) induced inflamed paw. Seventy-three percent of pERK-labeled DRG neurons co-expressed the P2X3 receptor. After mechanical noxious stimulation to the hind paw of CFA-inflamed rats, we found many more pERK-labeled neurons compared to those in the normal rats. Administration of the P2X3 receptor antagonists, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid or 2'- (or 3')-O-(trinitrophenyl)adenosine 5'-triphosphate (TNP-ATP), significantly decreased the mechanical stimulation-evoked pERK labeling in CFA-inflamed rats, but not in normal rats. We also found the recruitment of neurons with myelinated A fibers labeled for pERK in CFA-inflamed rats, which was reversed by P2X3 receptor antagonists. Moreover, TNP-ATP dose dependently reduced the mechanical hypersensitivity of CFA rats. These data suggest that the P2X receptors in primary afferent neurons increase their activity with enhanced sensitivity of the intracellular ERK signaling pathway during inflammation and then contribute to the hypersensitivity to mechanical noxious stimulation in the inflammatory state.
炎症中机械性痛觉过敏的机制可能涉及一个“机械化学”过程，即牵张刺激可促使受损组织释放5'-三磷酸腺苷（ATP），进而激活附近的初级感觉神经末梢。在本研究中，磷酸化细胞外信号调节蛋白激酶（pERK）免疫反应性被用作指示初级传入神经元功能激活的标志物，以检测大鼠外周炎症模型中背根神经节（DRG）神经元中P2X受体介导的伤害性反应。我们发现，在正常大鼠足底注射α,β-亚甲基ATP（αβmeATP）后，极少能检测到pERK标记的DRG神经元。然而，在向完全弗氏佐剂（CFA）诱导的炎症足注射αβmeATP后，许多DRG神经元被标记上了pERK。73%的pERK标记的DRG神经元共表达P2X3受体。对CFA炎症大鼠的后爪进行机械性伤害性刺激后，我们发现与正常大鼠相比，pERK标记的神经元更多。给予P2X3受体拮抗剂磷酸吡哆醛-6-偶氮苯-2',4'-二磺酸或2'-（或3'）-O-（三硝基苯基）-5'-三磷酸腺苷（TNP-ATP），可显著降低CFA炎症大鼠中机械刺激诱发的pERK标记，但对正常大鼠无此作用。我们还发现在CFA炎症大鼠中，有髓鞘的A纤维标记的pERK的神经元被募集，而P2X3受体拮抗剂可逆转这一现象。此外，TNP-ATP剂量依赖性地降低了CFA大鼠的机械性超敏反应。这些数据表明，初级传入神经元中的P2X受体在炎症过程中通过增强细胞内ERK信号通路的敏感性来增加其活性，进而导致炎症状态下对机械性伤害性刺激的超敏反应。
