Quercetin relieved diabetic neuropathic pain by inhibiting upregulated P2X4 receptor in dorsal root ganglia

The upregulation of nociceptive ion channels expressed in dorsal root ganglia (DRG) contributes to the development and retaining of diabetic pain symptoms. The flavonoid quercetin (3,3 ',4 ',5,7-pentahydroxyflavone) is a component extracted from various fruits and vegetables and exerts anti-inflammatory, analgesic, anticarcinogenic, antiulcer, and antihypertensive effects. However, the exact mechanism underlying quercetin's analgesic action remains poorly understood. The aim of this study was to investigate the effects of quercetin on diabetic neuropathic pain related to the P2X(4) receptor in the DRG of type 2 diabetic rat model. Our data showed that both mechanical withdrawal threshold and thermal withdrawal latency in diabetic rats treated with quercetin were higher compared with those in untreated diabetic rats. The expression levels of P2X(4) messenger RNA and protein in the DRG of diabetic rats were increased compared with the control rats, while quercetin treatment significantly inhibited such enhanced P2X(4) expression in diabetic rats. The satellite glial cells (SGCs) enwrap the neuronal soma in the DRG. Quercetin treatment also lowered the elevated coexpression of P2X(4) and glial fibrillary acidic protein (a marker of SGCs) and decreased the upregulation of phosphorylated p38 mitogen-activated protein kinase (p38MAPK) in the DRG of diabetic rats. Quercetin significantly reduced the P2X(4) agonist adenosine triphosphate-activated currents in HEK293 cells transfected with P2X(4) receptors. Thus, our data demonstrate that quercetin may decrease the upregulation of the P2X(4) receptor in DRG SGCs, and consequently inhibit P2X(4) receptor-mediated p38MAPK activation to relieve the mechanical and thermal hyperalgesia in diabetic rats.