Cholinergic excitation of vertebrate neurones is frequently mediated through the action of acetylcholine on muscarinic (atropine-sensitve) receptors. This type of excitation differs substantially from the better known nicotinic excitation. One difference is that, instead of an increased membrane conductance, a decreased conductance (to K+ ions) frequently accompanies muscarinic depolarisation. This has been detected in sympathetic, cortical and hippocampal neurones. Using voltage-clamped frog sympathetic neurones we have now identified a distinctive voltage-sensitive K+-current, separate from the delayed rectifier current, as the prime target for muscarinic agonists. We have termed this current the M-current, IM.

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.

Members of the Kv7 family (Kv7.2-Kv7.5) generate a subthreshold K ؉ current, the M؊ current. This regulates the excitability of many peripheral and central neurons. Recent evidence shows that Kv7.2 and Kv7.3 subunits are targeted to the axon initial segment of hippocampal neurons by association with ankyrin G. Further, spontaneous mutations in these subunits that impair axonal targeting cause human neonatal epilepsy. However, the precise functional significance of their axonal location is unknown. Using electrophysiological techniques together with a peptide that selectively disrupts axonal Kv7 targeting (ankyrin G-binding peptide, or ABP) and other pharmacological tools, we show that axonal Kv7 channels are critically and uniquely required for determining the inherent spontaneous firing of hippocampal CA1 pyramids, independently of alterations in synaptic activity. This action was primarily because of modulation of action potential threshold and resting membrane potential (RMP), amplified by control of intrinsic axosomatic membrane properties. Computer simulations verified these data when the axonal Kv7 density was three to five times that at the soma. The increased firing caused by axosomatic Kv7 channel block backpropagated into distal dendrites affecting their activity, despite these structures having fewer functional Kv7 channels. These results indicate that axonal Kv7 channels, by controlling axonal RMP and action potential threshold, are fundamental for regulating the inherent firing properties of CA1 hippocampal neurons.axon initial segment ͉ CA1 pyramidal neurons ͉ M-current ͉ KCNQ channels N euronal Kv7 (KCNQ) channels form a noninactivating K ϩ current (also known as the MϪ current); this turns on at subthreshold potentials and regulates the excitability of a variety of peripheral and central neurons (1-3). Recent immunohistochemical evidence has shown that the principal subunits forming native M channels, Kv7.2 and Kv7.3 (3,4), are concentrated at the axon initial segment (AIS) and nodes of Ranvier of central and peripheral principal neurons (5-9), where they colocalize with Na ϩ channels. Like Na ϩ channels, they contain an ankyrin G-binding motif that targets them to the AIS (5, 8). They are also expressed at lower densities at the soma and possibly dendrites and synaptic terminals (4,6,7,10,11).Spontaneous mutations in Kv7 subunits cause epilepsy in humans (2) and mice (12). The hippocampus is strongly implicated in epilepsy (13) and accordingly, previous somatic recordings from these neurons have indicated that the Kv7 current is involved in determining several aspects of neuronal excitability, including the resting membrane potential (RMP), spike frequency adaptation, and burst suppression (e.g., refs. 14-16). However, the specific contribution made by Kv7 channels in the AIS to these or other manifestations of excitability has not been determined. This is important, because some human epileptogenic mutations impair axonal Kv7 subunit expression (7).We have used selective pharmacological and mol...