KCNQ genes encode five Kv7 K + channel subunits (Kv7.1-Kv7.5). Four of these (Kv7.2-Kv7.5) are expressed in the nervous system. Kv7.2 and Kv7.3 are the principal molecular components of the slow voltage-gated M-channel, which widely regulates neuronal excitability, although other subunits may contribute to M-like currents in some locations. M-channels are closed by receptors coupled to Gq such as M1 and M3 muscarinic receptors; this increases neuronal excitability and underlies some forms of cholinergic excitation. Muscarinic closure results from activation of phospholipase C and consequent hydrolysis and depletion of membrane phosphatidylinositol-4,5-bisphosphate, which is required for channel opening. Some effects of M-channel closure, determined from transmitter action, selective blocking drugs (linopirdine and XE991) and KCNQ2 gene disruption or manipulation, are as follows: (i) in sympathetic neurons: facilitation of repetitive discharges and conversion from phasic to tonic firing; (ii) in sensory nociceptive systems: facilitation of A-delta peripheral sensory fibre responses to noxious heat; and (iii) in hippocampal pyramidal neurons: facilitation of repetitive discharges, enhanced after-depolarization and burst-firing, and induction of spontaneous firing through a reduction of action potential threshold at the axon initial segment. Several drugs including flupirtine and retigabine enhance neural Kv7/M-channel activity, principally through a hyperpolarizing shift in their voltage gating. In consequence they reduce neural excitability and can inhibit nociceptive stimulation and transmission. Flupirtine is in use as a central analgesic; retigabine is under clinical trial as a broad-spectrum anticonvulsant and is an effective analgesic in animal models of chronic inflammatory and neuropathic pain.

Neuronal hyperexcitability is a feature of epilepsy and both inflammatory and neuropathic pain. M currents [IK(M)] play a key role in regulating neuronal excitability, and mutations in neuronal KCNQ2/3 subunits, the molecular correlates of IK(M), have previously been linked to benign familial neonatal epilepsy. Here, we demonstrate that KCNQ/M channels are also present in nociceptive sensory systems. IK(M) was identified, on the basis of biophysical and pharmacological properties, in cultured neurons isolated from dorsal root ganglia (DRGs) from 17-d-old rats. Currents were inhibited by the M-channel blockers linopirdine (IC50, 2.1 microm) and XE991 (IC50, 0.26 microm) and enhanced by retigabine (10 microm). The expression of neuronal KCNQ subunits in DRG neurons was confirmed using reverse transcription-PCR and single-cell PCR analysis and by immunofluorescence. Retigabine, applied to the dorsal spinal cord, inhibited C and Adelta fiber-mediated responses of dorsal horn neurons evoked by natural or electrical afferent stimulation and the progressive "windup" discharge with repetitive stimulation in normal rats and in rats subjected to spinal nerve ligation. Retigabine also inhibited responses to intrapaw application of carrageenan in a rat model of chronic pain; this was reversed by XE991. It is suggested that IK(M) plays a key role in controlling the excitability of nociceptors and may represent a novel analgesic target.

Human acute and inflammatory pain requires the expression of voltage-gated sodium channel Nav1.7 but its significance for neuropathic pain is unknown. Here we show that Nav1.7 expression in different sets of mouse sensory and sympathetic neurons underlies distinct types of pain sensation. Ablating Nav1.7 gene (SCN9A) expression in all sensory neurons using Advillin-Cre abolishes mechanical pain, inflammatory pain and reflex withdrawal responses to heat. In contrast, heat-evoked pain is retained when SCN9A is deleted only in Nav1.8-positive nociceptors. Surprisingly, responses to the hotplate test, as well as neuropathic pain, are unaffected when SCN9A is deleted in all sensory neurons. However, deleting SCN9A in both sensory and sympathetic neurons abolishes these pain sensations and recapitulates the pain-free phenotype seen in humans with SCN9A loss-of-function mutations. These observations demonstrate an important role for Nav1.7 in sympathetic neurons in neuropathic pain, and provide possible insights into the mechanisms that underlie gain-of-function Nav1.7-dependent pain conditions.

Recent studies have demonstrated the importance of local protein synthesis for neuronal plasticity. In particular, local mRNA translation through the mammalian target of rapamycin (mTOR) has been shown to play a key role in regulating dendrite excitability and modulating long-term synaptic plasticity associated with learning and memory. There is also increased evidence to suggest that intact adult mammalian axons have a functional requirement for local protein synthesis in vivo. Here we show that the translational machinery is present in some myelinated sensory fibers and that active mTOR-dependent pathways participate in maintaining the sensitivity of a subpopulation of fast-conducting nociceptors in vivo. Phosphorylated mTOR together with other downstream components of the translational machinery were localized to a subset of myelinated sensory fibers in rat cutaneous tissue. We then showed with electromyographic studies that the mTOR inhibitor rapamycin reduced the sensitivity of a population of myelinated nociceptors known to be important for the increased mechanical sensitivity that follows injury. Behavioural studies confirmed that local treatment with rapamycin significantly attenuated persistent pain that follows tissue injury, but not acute pain. Specifically, we found that rapamycin blunted the heightened response to mechanical stimulation that develops around a site of injury and reduced the long-term mechanical hypersensitivity that follows partial peripheral nerve damage - a widely used model of chronic pain. Our results show that the sensitivity of a subset of sensory fibers is maintained by ongoing mTOR-mediated local protein synthesis and uncover a novel target for the control of long-term pain states.

KCNQ2 and KCNQ3 potassium-channel subunits can form both homomeric and heteromeric channels; the latter are thought to constitute native ganglionic M channels. We have tried to deduce the stoichiometric contributions of KCNQ2 and KCNQ3 subunits to currents generated by the coexpression of KCNQ2 and KCNQ3 cDNA plasmids in Chinese hamster ovary (CHO) cells, and to native M currents in dissociated rat superior cervical ganglion (SCG) neurons, by comparing the block of these currents produced by tetraethylammonium (TEA) with the block of currents generated by a tandem KCNQ3/2 construct. TEA concentration-inhibition curves against coexpressed KCNQ2 plus KCNQ3 currents, and against native M currents in SCG neurons from 6-week-old [postnatal day 45 (P45)] rats, were indistinguishable from those for the expressed tandem construct, and fully accorded with a 1:1 stoichiometry. Inhibition curves in neurons from younger (P17) rats could be better fitted assuming an additional small proportion of current carried by KCNQ2 homomultimers. Single-cell PCR yielded signals for KCNQ2, KCNQ3, and KCNQ5 mRNAs in all SCG neurons tested from both P17 and P45 rats. Quantitative PCR of whole-ganglion mRNA revealed stable levels of KCNQ2 and KCNQ5 mRNA between P7 and P45, but excess and incrementing levels of KCNQ3 mRNA. Increasing levels of KCNQ3 protein between P17 and P45 were confirmed by immunocytochemistry. We conclude that coexpressed KCNQ2 plus KCNQ3 cDNAs generate channels with 1:1 (KCNQ2:KCNQ3) stoichiometry in CHO cells and that native M channels in SCG neurons adopt the same conformation during development, assisted by the increased expression of KCNQ3 mRNA and protein.