Inflammatory pain is thought to be mediated in part through the action of inflammatory mediators on membrane receptors of peripheral nerve terminals, however, the downstream signalling events which lead to pain are poorly understood. In this study we investigated the nociceptive pathways induced by activation of protease-activated receptor 2 (PAR-2) in damage sensing (nociceptive) neurons from rat dorsal root ganglion (DRG). We found that activation of PAR-2 in these cells strongly inhibited M-type potassium currents (conducted by Kv7 potassium channels). Such inhibition caused depolarization of the neuronal resting membrane potential leading, ultimately, to nociception. Consistent with this mechanism, injection of the specific M channel blocker XE991 into rat paw induced nociception in a concentration-dependent manner. Injection of a PAR-2 agonist peptide also induced nociception but co-injection of XE991 and the PAR-2 agonist did not result in summation of nociception, suggesting that the action of both agents may share a similar mechanism. We also studied the signalling pathway of M current inhibition by PAR-2 using patch clamp and fluorescence imaging of DRG neurons. These experiments revealed that the PAR-2 effect was mediated by phospholipase C (PLC). Further experiments demonstrated that M current inhibition required concurrent rises in cytosolic Ca2+ concentration and depletion of membrane phosphatidylinositol 4,5-bisphosphate (PIP2). We propose that PLC- and Ca2+/PIP2- mediated inhibition of M current in sensory neurons may represent one of the general mechanisms underlying pain produced by inflammatory mediators, and may therefore open up a new therapeutic window for treatment of this major clinical problem.

Neuropathic pain is a severe health problem for which there is a lack of effective therapy. A frequent underlying condition of neuropathic pain is a sustained overexcitability of pain-sensing (nociceptive) sensory fibres. Therefore, the identification of mechanisms for such abnormal neuronal excitability is of utmost importance for understanding neuropathic pain. Despite much effort, an inclusive model explaining peripheral overexcitability is missing. We investigated transcriptional regulation of the Kcnq2 gene, which encodes the Kv7.2 subunit of membrane potential-stabilizing M channel, in peripheral sensory neurons in a model of neuropathic pain—partial sciatic nerve ligation (PSNL). We show that Kcnq2 is the major Kcnq gene transcript in dorsal root ganglion (DRG); immunostaining and patch-clamp recordings from acute ganglionic slices verified functional expression of Kv7.2 in small-diameter nociceptive DRG neurons. Neuropathic injury induced substantial downregulation of Kv7.2 expression. Levels of repressor element 1–silencing transcription factor (REST), which is known to suppress Kcnq2 expression, were upregulated in response to neuropathic injury identifying the likely mechanism of Kcnq2 regulation. Behavioural experiments demonstrated that neuropathic hyperalgesia following PSNL developed faster than the downregulation of Kcnq2 expression could be detected, suggesting that this transcriptional mechanism may contribute to the maintenance rather than the initiation of neuropathic pain. Importantly, the decrease in the peripheral M channel abundance could be functionally compensated by peripherally applied M channel opener flupirtine, which alleviated neuropathic hyperalgesia. Our work suggests a novel mechanism for neuropathic overexcitability and brings focus on M channels and REST as peripheral targets for the treatment of neuropathic pain.Neuropathic injury induces transcriptional downregulation of the Kcnq2 potassium channel gene by the transcriptional suppressor repressor element 1–silencing transcription factor; this mechanism contributes to peripheral sensitization of the afferent fibres.

Inflammatory pain results from the increased excitability of peripheral nociceptive sensory fibres produced by the action of inflammatory mediators. This excitatory effect, in turn, is a result of the altered activity of ion channels within affected sensory fibres. This review will consider the molecular consequences of inflammation within the peripheral nerves with particular focus on the effects of different inflammatory mediators on the ion channels in sensory neurons. We will discuss the main signalling pathways triggered in neurons by inflammatory mediators; the ionic mechanisms underlying inflammatory hyperalgesia and spontaneous inflammatory pain and finally will briefly consider ion channels underlying pain in chronic inflammation.

It has been established that CaV3.2 T-type voltage-gated calcium channels (T-channels) play a key role in the sensitized (hyperexcitable) state of nociceptive sensory neurons (nociceptors) in response to hyperglycemia associated with diabetes, which in turn can be a basis for painful symptoms of peripheral diabetic neuropathy (PDN). Unfortunately, current treatment for painful PDN has been limited by nonspecific systemic drugs with significant side effects or potential for abuse. We studied in vitro and in vivo mechanisms of plasticity of CaV3.2 T-channel in a leptin-deficient (ob/ob) mouse model of PDN. We demonstrate that posttranslational glycosylation of specific extracellular asparagine residues in CaV3.2 channels accelerates current kinetics, increases current density, and augments channel membrane expression. Importantly, deglycosylation treatment with neuraminidase inhibits native T-currents in nociceptors and in so doing completely and selectively reverses hyperalgesia in diabetic ob/ob mice without altering baseline pain responses in healthy mice. Our study describes a new mechanism for the regulation of CaV3.2 activity and suggests that modulating the glycosylation state of T-channels in nociceptors may provide a way to suppress peripheral sensitization. Understanding the details of this regulatory pathway could facilitate the development of novel specific therapies for the treatment of painful PDN.