The Up-regulation of TNF-α Maintains Trigeminal Neuralgia by Modulating MAPKs Phosphorylation and BKCa Channels in Trigeminal Nucleus Caudalis.

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 (BK Ca ) 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 BK Ca 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 BK Ca 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 BK Ca channels, resulting in the TN.

Overexcited MaxiK and KATP channels underlie obstructive jaundice-induced vasoconstrictor hyporeactivity of arterial smooth muscle.

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-ß1 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.

The role of large-conductance, calcium-activated potassium channels in a rat model of trigeminal neuropathic pain.

BACKGROUND: Trigeminal neuralgia is a disorder of paroxysmal and severely disabling facial pain and continues to be a real therapeutic challenge. At present there are few effective drugs. Here the aim of this study was to investigate the role of BKCa channels in trigeminal neuropathic pain. METHODS: Rats were divided into two groups: a sham and a chronic constriction injury of infraorbital branch of trigeminal nerve (ION-CCI) group. Nociceptive behavior testing, immunohistochemistry, RT-PCR, Western blotting and whole-cell patch clamp recording were used. RESULTS: Relative to the sham group, rats with ION-CCI consistently displayed lower mechanical pain thresholds in the vibrissal pad region from day 6 to 42 after ION-CCI operation. ION-CCI induced a significant down-regulation of BKCa channels both in mRNA and protein levels in the ipsilateral trigeminal ganglion (TG), a lower threshold intensity of action potential, and decreased total BKCa currents in cultured TG neurons. TG target injection of NS1619 (20-100 µg), an opener of BKCa channels, dose-dependently increased the mechanical pain threshold, which was blocked by the BKCa channel inhibitor iberiotoxin (IbTX, 20 µg). NS1619 (10 µM) significantly increased the mean threshold intensities of action potentials in ION-CCI rats, while failing to affect those in the sham rats. The levels of phosphorylated extracellular signal-regulated kinase (ERK), p38 and c-Jun N-terminal kinases (JNK) in TG were significantly increased after ION-CCI operation. The ERK1/2 antagonist U0126, p38 antagonist SB203580 and JNK antagonist SP600125 significantly reversed the facial mechanical allodynia in ION-CCI rats. However, the ERK1/2 antagonist U0126, p38 antagonist SB203580 but not JNK antagonist SP600125 significantly increased BKCa currents in ION-CCI TG neurons. CONCLUSIONS: 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.

Inhibition of G protein-coupled P2Y2 receptor induced analgesia in a rat model of trigeminal neuropathic pain.

BACKGROUNDS: ATP 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. RESULTS: In 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. CONCLUSIONS: Our data suggest that inhibition of P2Y2 receptors leads to down-regulation of ERK-mediated phosphorylation and increase of the expression of I(A)-related Kv channels in trigeminal ganglion neurons, which might contribute to the clinical treatment of trigeminal neuropathic pain.

17ß-estradiol rapidly attenuates P2X3 receptor-mediated peripheral pain signal transduction via ERα and GPR30.

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 17ß-estradiol (E2) rapidly inhibited pain induced by α,ß-methylene ATP (α,ß-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 ERß agonist diarylpropionitrile (DPN) had no effect. In cultured rat dorsal root ganglion (DRG) neurons, PPT and G-1 but not DPN significantly attenuated α,ß-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 ERß-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,ß-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.