BackgroudsATP and P2X receptors play important roles in the modulation of trigeminal neuropathic pain, while the role of G protein-coupled P2Y2 receptors and the underlying mechanisms are less clear. The threshold and frequency of action potentials, fast inactivating transient K+ channels (IA) are important regulators of membrane excitability in sensory neurons because of its vital role in the control of the spike onset. In this study, pain behavior tests, QT-RT-PCR, immunohistochemical staining, and patch-clamp recording, were used to investigate the role of P2Y2 receptors in pain behaviour.ResultsIn control rats: 1) UTP, an agonist of P2Y2/P2Y4 receptors, caused a significant decrease in the mean threshold intensities for evoking action potentials and a striking increase in the mean number of spikes evoked by TG neurons. 2) UTP significantly inhibited IA and the expression of Kv1.4, Kv3.4 and Kv4.2 subunits in TG neurons, which could be reversed by the P2 receptor antagonist suramin and the ERK antagonist U0126. In ION-CCI (chronic constriction injury of infraorbital nerve) rats: 1) mRNA levels of Kv1.4, Kv3.4 and Kv4.2 subunits were significantly decreased, while the protein level of phosphorylated ERK was significantly increased. 2) When blocking P2Y2 receptors by suramin or injection of P2Y2R antisense oligodeoxynucleotides both led to a time- and dose-dependent reverse of allodynia in ION-CCI rats. 3) Injection of P2Y2 receptor antisense oligodeoxynucleotides induced a pronounced decrease in phosphorylated ERK expression and a significant increase in Kv1.4, Kv3.4 and Kv4.2 subunit expression in trigeminal ganglia.ConclusionsOur data suggest that inhibition of P2Y2 receptors leads to down-regulation of ERK-mediated phosphorylation and increase of the expression of IA–related Kv channels in trigeminal ganglion neurons, which might contribute to the clinical treatment of trigeminal neuropathic pain.

Our results indicate the important involvement of mainly ERK and p38 MAPK pathways in modulating BKCa channels in ION-CCI TG neurons. BKCa channels represent a new therapeutic target for the clinical treatment of trigeminal neuropathic pain.

Estrogen has been reported to affect pain perception, although the underlying mechanisms remain unclear. In this investigation, pain behavior testing, patch clamp recording, and immunohistochemistry were used on rats and transgenic mice to determine which estrogen receptors (ERs) and the related signaling pathway are involved in the rapid modulation of estrogen on P2X3 receptor-mediated events. The results showed that 17β-estradiol (E2) rapidly inhibited pain induced by α,β-methylene ATP (α,β-me-ATP), a P2X1 and P2X3 receptor agonist in ovariectomized rats and normal rats in diestrus. The ERα agonist 4,49,499-(4-propyl-[1H]-pyrazole-1,3,5-triyl) trisphenol (PPT) and G protein-coupled receptor 30 (GPR30) agonist G-1 mimicked the estrogen effect, whereas the ERβ agonist diarylpropionitrile (DPN) had no effect. In cultured rat dorsal root ganglion (DRG) neurons, PPT and G-1 but not DPN significantly attenuated α,β-me-ATP-mediated currents, with the dose-response curve of these currents shifted to the right. The inhibitory effect of E2 on P2X3 currents was blocked by G-15, a selective antagonist to the GPR30 estrogen receptor. E2 lacked this effect in DRG neurons from ERα-knockout mice but partly remained in those from ERβ-knockout mice. The P2X3 and GPR30 receptors were coexpressed in the rat DRG neurons. Furthermore, the ERK1/2 inhibitor U0126 reversed the inhibitory effect of E2 on α,β-me-ATP-induced pain and of PPT or G-1 on P2X3 receptor-mediated currents. The cAMP-protein kinase A (PKA) agonist forskolin, but not the PKC agonist phorbol-12-myristate-13-acetate (PMA), mimicked the estrogen-inhibitory effect on P2X3 receptor currents, which was blocked by another ERK1/2 inhibitor, PD98059. These results suggest that estrogen regulates P2X3-mediated peripheral pain by acting on ERα and GPR30 receptors expressed in primary afferent neurons, which probably involves the intracellular cAMP-PKA-ERK1/2 pathway.

Trigeminal neuralgia (TN) is a severe chronic neuropathic pain. Despite numerous available medical interventions, the therapeutic effects are not ideal. To control the pain attacks, the need for more contemporary drugs continues to be a real challenge. Our previous study reported that Ca2+-activated K+ channels (BKCa) channels modulated by mitogen-activated protein kinases (MAPKs) in the trigeminal ganglia (TG) neurons play crucial roles in regulating TN, and some research studies demonstrated that inflammatory cytokine tumor necrosis factor alpha (TNF-α) could promote neuropathic pain. Meanwhile, the trigeminal nucleus caudalis (TNC), the first central site of the trigeminal nociceptive pathway, is responsible for processing sensory and pain signals from the peripheral orofacial area. Thus, this study is aimed to further investigate whether TNF-α and MAPKs phosphorylation in the TNC could mediate the pathogenesis of TN by modulating BKCa channels. The results showed that TNF-α of the TNC region is upregulated significantly in the chronic constriction injury of infraorbital nerve (ION-CCI) rats model, which displayed persistent facial mechanical allodynia. The normal rats with target injection of exogenous TNF-α to the fourth brain ventricle behaved just like the ION-CCI model rats, the orofacial mechanical pain threshold decreased clearly. Meanwhile, the exogenous TNF-α increased the action potential frequency and reduced the BKCa currents of TNC neurons significantly, which could be reversed by U0126 and SB203580, the inhibitors of MAPK. In addition, U0126, SB203580, and another MAPK inhibitor SP600125 could relieve the facial mechanical allodynia by being injected into the fourth brain ventricle of ION-CCI model rats, respectively. Taken together, our work suggests that the upregulation of TNF-α in the TNC region would cause the increase of MAPKs phosphorylation and then the negative regulation of BKCa channels, resulting in the TN.

Substantial evidence has shown that obstructive jaundice can induce vascular hyporesponsiveness. The present study was designed to investigate mechanisms of MaxiK channel and KATP underlying cholestasis-induced vascular dysfunction. The isolated thoracic aorta was used to explore norepinephrine (NE)-induced contraction. The function of MaxiK and KATP channels were investigated using whole-cell patch clamp recording. Compared with Sham group, NE-induced vascular contraction was blunted after bile duct ligation (BDL), which could not be ameliorated significantly after endothelial denudation. Charybdotoxin and glibenclamide induced a more pronounced recovery from vascular hyporesponsiveness to NE in BDL group compared with Sham group. BDL significantly promoted the charybdotoxin sensitive MaxiK current and KATP current in isolated aortic smooth muscle cells. In addition, the expression of auxiliary subunits (MaxiK-β1 and SUR2B) rather pore-forming subunits (MaxiK-α and Kir6.1) was significantly up-regulated after BDL. These findings suggest that MaxiK and KATP channels play an important role in regulating vascular hyporesponsiveness in BDL rats.