Molecular correlates of the M-current in cultured rat hippocampal neurons

Meclofenamic acid and diclofenac, novel templates of KCNQ2/Q3 potassium channel openers, depress cortical neuron activity and exhibit anticonvulsant properties

The voltage-dependent M-type potassium current (M-current) plays a major role in controlling brain excitability by stabilizing the membrane potential and acting as a brake for neuronal firing. The KCNQ2/Q3 heteromeric channel complex was identified as the molecular correlate of the M-current. Furthermore, the KCNQ2 and KCNQ3 channel alpha subunits are mutated in families with benign familial neonatal convulsions, a neonatal form of epilepsy. Enhancement of KCNQ2/Q3 potassium currents may provide an important target for antiepileptic drug development. Here, we show that meclofenamic acid (meclofenamate) and diclofenac, two related molecules previously used as anti-inflammatory drugs, act as novel KCNQ2/Q3 channel openers. Extracellular application of meclofenamate (EC(50) = 25 microM) and diclofenac (EC(50) = 2.6 microM) resulted in the activation of KCNQ2/Q3 K(+) currents, heterologously expressed in Chinese hamster ovary cells. Both openers activated KCNQ2/Q3 channels by causing a hyperpolarizing shift of the voltage activation curve (-23 and -15 mV, respectively) and by markedly slowing the deactivation kinetics. The effects of the drugs were stronger on KCNQ2 than on KCNQ3 channel alpha subunits. In contrast, they did not enhance KCNQ1 K(+) currents. Both openers increased KCNQ2/Q3 current amplitude at physiologically relevant potentials and led to hyperpolarization of the resting membrane potential. In cultured cortical neurons, meclofenamate and diclofenac enhanced the M-current and reduced evoked and spontaneous action potentials, whereas in vivo diclofenac exhibited an anticonvulsant activity (ED(50) = 43 mg/kg). These compounds potentially constitute novel drug templates for the treatment of neuronal hyperexcitability including epilepsy, migraine, or neuropathic pain.

c-Fos expression in rat brain stem and spinal cord in response to activation of cardiac ischemia-sensitive afferent neurons and electrostimulatory modulation

The purpose of this study was to identify central neuronal sites activated by stimulation of cardiac ischemia-sensitive afferent neurons and determine whether electrical stimulation of left vagal afferent fibers modified the pattern of neuronal activation. Fos-like immunoreactivity (Fos-LI) was used as an index of neuronal activation in selected levels of cervical and thoracic spinal cord and brain stem. Adult Sprague-Dawley rats were anesthetized with urethane and underwent intrapericardial infusion of an “inflammatory exudate solution” (IES) containing algogenic substances that are released during ischemia (10 mM adenosine, bradykinin, prostaglandin E2, and 5-hydroxytryptamine) or occlusion of the left anterior descending coronary artery (CoAO) to activate cardiac ischemia-sensitive (nociceptive) afferent fibers. IES and CoAO increased Fos-LI above resting levels in dorsal horns in laminae I–V at C2 and T4 and in the caudal nucleus tractus solitarius. Dorsal rhizotomy virtually eliminated Fos-LI in the spinal cord as well as the brain stem. Neuromodulation of the ischemic signal by electrical stimulation of the central end of the left thoracic vagus excited neurons at the cervical and brain stem level but inhibited neurons at the thoracic spinal cord during IES or CoAO. These results suggest that stimulation of the left thoracic vagus excites descending inhibitory pathways. Inhibition at the thoracic spinal level that suppresses the ischemic (nociceptive) input signal may occur by a short-loop descending pathway via signals from cervical propriospinal circuits and/or a longer-loop descending pathway via signals from the nucleus tractus solitarius.

Differences between the negatively activating potassium conductances of Mammalian cochlear and vestibular hair cells

Cochlear and type I vestibular hair cells of mammals express negatively activating potassium (K(+)) conductances, called g(K,n) and g(K,L) respectively, which are important in setting the hair cells' resting potentials and input conductances. It has been suggested that the channels underlying both conductances include KCNQ4 subunits from the KCNQ family of K(+) channels. In whole-cell recordings from rat hair cells, we found substantial differences between g(K,n) and g(K,L) in voltage dependence, kinetics, ionic permeability, and stability during whole-cell recording. Relative to g(K,L), g(K,n) had a significantly broader and more negative voltage range of activation and activated with less delay and faster principal time constants over the negative part of the activation range. Deactivation of g(K,n) had an unusual sigmoidal time course, while g(K,L) deactivated with a double-exponential decay. g(K,L), but not g(K,n), had appreciable permeability to Cs(+). Unlike g(K,L), g(K,n)'s properties did not change ("wash out") during the replacement of cytoplasmic solution with pipette solution during ruptured-patch recordings. These differences in the functional expression of g(K,n) and g(K,L) channels suggest that there are substantial differences in their molecular structure as well.

Dual phosphorylations underlie modulation of unitary KCNQ K(+) channels by Src tyrosine kinase

Src tyrosine kinase suppresses KCNQ (M-type) K(+) channels in a subunit-specific manner representing a mode of modulation distinct from that involving G protein-coupled receptors. We probed the molecular and biophysical mechanisms of this modulation using mutagenesis, biochemistry, and both whole-cell and single channel modes of patch clamp recording. Immunoprecipitation assays showed that Src associates with KCNQ2-5 subunits but phosphorylates only KCNQ3-5. Using KCNQ3 as a background, we found that mutation of a tyrosine in the amino terminus (Tyr-67) or one in the carboxyl terminus (Tyr-349) abolished Src-dependent modulation of heterologously expressed KCNQ2/3 heteromultimers. The tyrosine phosphorylation was much weaker for either the KCNQ3-Y67F or KCNQ3-Y349F mutants and wholly absent in the KCNQ3-Y67F/Y349F double mutant. Biotinylation assays showed that Src activity does not alter the membrane abundance of channels in the plasma membrane. In recordings from cell-attached patches containing a single KCNQ2/3 channel, we found that Src inhibits the open probability of the channels. Kinetic analysis was consistent with the channels having two discrete open times and three closed times. Src activity reduced the durations of the longest open time and lengthened the longest closed time of the channels. The implications for the mechanisms of channel regulation by the dual phosphorylations on both channel termini are discussed.

Mechanisms underlying modulation of neuronal KCNQ2/KCNQ3 potassium channels by extracellular protons

Changes in extracellular pH occur during both physiological neuronal activity and pathological conditions such as epilepsy and stroke. Such pH changes are known to exert profound effects on neuronal activity and survival. Heteromeric KCNQ2/3 potassium channels constitute a potential target for modulation by H⁺ ions as they are expressed widely within the CNS and have been proposed to underlie the M-current, an important determinant of excitability in neuronal cells. Whole-cell and single-channel recordings demonstrated a modulation of heterologously expressed KCNQ2/3 channels by extracellular H⁺ ions. KCNQ2/3 current was inhibited by H⁺ ions with an IC₅₀ of 52 nM (pH 7.3) at −60 mV, rising to 2 μM (pH 5.7) at −10 mV. Neuronal M-current exhibited a similar sensitivity. Extracellular H⁺ ions affected two distinct properties of KCNQ2/3 current: the maximum current attainable upon depolarization (I_max) and the voltage dependence of steady-state activation. Reduction of I_max was antagonized by extracellular K⁺ ions and affected by mutations within the outer-pore turret, indicating an outer-pore based process. This reduction of I_max was shown to be due primarily to a decrease in the maximum open-probability of single KCNQ2/3 channels. Single-channel open times were shortened by acidosis (pH 5.9), while closed times were increased. Acidosis also recruited a longer-lasting closed state, and caused a switch of single-channel activity from the full-conductance state (∼8 pS) to a subconductance state (∼5 pS). A depolarizing shift in the activation curve of macroscopic KCNQ2/3 currents and single KCNQ2/3 channels was caused by acidosis, while alkalosis caused a hyperpolarizing shift. Activation and deactivation kinetics were slowed by acidosis, indicating specific effects of H⁺ ions on elements involved in gating. Contrasting modulation of homomeric KCNQ2 and KCNQ3 currents revealed that high sensitivity to H⁺ ions was conferred by the KCNQ3 subunit.

Single-channel analysis of KCNQ K+ channels reveals the mechanism of augmentation by a cysteine-modifying reagent

The cysteine-modifying reagent N-ethylmaleimide (NEM) is known to augment currents from native M-channels in sympathetic neurons and cloned KCNQ2 channels. As a probe for channel function, we investigated the mechanism of NEM action and subunit specificity of cloned KCNQ2-5 channels expressed in Chinese hamster ovary cells at the whole-cell and single-channel levels. Biotinylation assays and total internal reflection fluorescence microscopy indicated that NEM action is not caused by increased trafficking of channels to the membrane. At saturating voltages, whole-cell currents of KCNQ2, KCNQ4, and KCNQ5 but not KCNQ3 were augmented threefold to fourfold by 50 microm NEM, and their voltage dependencies were negatively shifted by 10-20 mV. Unitary conductances of KCNQ2 and KCNQ3 (6.2 and 8.5 pS, respectively) were much higher that those of KCNQ4 and KCNQ5 (2.1 and 2.2 pS, respectively). Surprisingly, the maximal open probability (P(o)) of KCNQ3 was near unity, much higher than that of KCNQ2, KCNQ4, and KCNQ5. NEM increased the P(o) of KCNQ2, KCNQ4, and KCNQ5 by threefold to fourfold but had no effect on their unitary conductances, suggesting that the increase in macroscopic currents can be accounted for by increases in P(o). Analysis of KCNQ3/4 chimeras determined the C terminus to be responsible for the differential maximal P(o), channel expression, and NEM action between the two channels. To further localize the site of NEM action, we mutated three cysteine residues in the C terminus of KCNQ4. The C519A mutation alone ablated most of the augmentation by NEM, suggesting that NEM acts via alkylation of this residue.

KCNQ/M channels control spike afterdepolarization and burst generation in hippocampal neurons

KCNQ channel subunits are widely expressed in peripheral and central neurons, where they give rise to a muscarinic-sensitive, subthreshold, and noninactivating K+ current (M-current). It is generally agreed that activation of KCNQ/M channels contributes to spike frequency adaptation during sustained depolarizations but is too slow to influence the repolarization of solitary spikes. This concept, however, is based mainly on experiments with muscarinic agonists, the multiple effects on membrane conductances of which may overshadow the distinctive effects of KCNQ/M channel block. Here, we have used selective modulators of KCNQ/M channels to investigate their role in spike electrogenesis in CA1 pyramidal cells. Solitary spikes were evoked by brief depolarizing current pulses injected into the neurons. The KCNQ/M channel blockers linopirdine and XE991 markedly enhanced the spike afterdepolarization (ADP) and, in most neurons, converted solitary ("simple") spikes to high-frequency bursts of three to seven spikes ("complex" spikes). Conversely, the KCNQ/M channel opener retigabine reduced the spike ADP and induced regular firing in bursting neurons. Selective block of BK or SK channels had no effect on the spike ADP or firing mode in these neurons. We conclude that KCNQ/M channels activate during the spike ADP and limit its duration, thereby precluding its escalation to a burst. Consequently, down-modulation of KCNQ/M channels converts the neuronal firing pattern from simple to complex spiking, whereas up-modulation of these channels exerts the opposite effect.

KCNQ2 is a nodal K+ channel

Mutations in the gene encoding the K+ channel KCNQ2 cause neonatal epilepsy and myokymia, indicating that KCNQ2 regulates the excitability of CNS neurons and motor axons, respectively. We show here that KCNQ2 channels are functional components of axon initial segments and nodes of Ranvier, colocalizing with ankyrin-G and voltage-dependent Na+ channels throughout the CNS and PNS. Retigabine, which opens KCNQ channels, diminishes axonal excitability. Linopirdine, which blocks KCNQ channels, prolongs the repolarization of the action potential in neonatal nerves. The clustering of KCNQ2 at nodes and initial segments lags that of ankyrin-G during development, and both ankyrin-G and KCNQ2 can be coimmunoprecipitated in the brain. KCNQ3 is also a component of some initial segments and nodes in the brain. The diminished activity of mutant KCNQ2 channels accounts for neonatal epilepsy and myokymia; the cellular locus of these effects may be axonal initial segments and nodes.

M channels containing KCNQ2 subunits modulate norepinephrine, aspartate, and GABA release from hippocampal nerve terminals

KCNQ subunits encode for the M current (I(KM)), a neuron-specific voltage-dependent K+ current with a well established role in the control of neuronal excitability. In this study, by means of a combined biochemical, pharmacological, and electrophysiological approach, the role of presynaptic I(KM) in the release of previously taken up tritiated norepineprine (NE), GABA, and d-aspartate (d-ASP) from hippocampal nerve terminals (synaptosomes) has been evaluated. Retigabine (RT) (0.01-30 microm), a specific activator of I(KM), inhibited [3H]NE, [3H]d-ASP, and [3H]GABA release evoked by 9 mm extracellular K+ ([K+]e). RT-induced inhibition of [3H]NE release was prevented by synaptosomal entrapment of polyclonal antibodies directed against KCNQ2 subunits, an effect that was abolished by antibody preabsorption with the KCNQ2 immunizing peptide; antibodies against KCNQ3 subunits were ineffective. Flupirtine (FP), a structural analog of RT, also inhibited 9 mm [K+]e-induced [3H]NE release, although its maximal inhibition was lower than that of RT. Electrophysiological studies in KCNQ2-transfected Chinese hamster ovary cells revealed that RT and FP (10 microm) caused a -19 and -9 mV hyperpolarizing shift, respectively, in the voltage dependence of activation of KCNQ2 K+ channels. In the same cells, the cognition enhancer 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE-991) (10 microm) blocked KCNQ2 channels and prevented their activation by RT (1-10 microm). Finally, both XE-991 (10-100 microm) and tetraethylammonium ions (100 microm) abolished the inhibitory effect of RT (1 microm) on [3H]NE release. These findings provide novel evidence for a major regulatory role of KCNQ2 K+ channel subunits in neurotransmitter release from rat hippocampal nerve endings.

Localization of KCNQ5 in the normal and epileptic human temporal neocortex and hippocampal formation

The KCNQ family of voltage-dependent non-inactivating K+ channels is composed of five members, four of which (KCNQ2-5) are expressed in the CNS and are responsible for the M-current. Mutations in either KCNQ2 or KCNQ3 lead to a hereditary form of dominant generalized epilepsy. Using specific antisera to the KCNQ2, KCNQ3 and KCNQ5 subunits, we found that KCNQ3 co-immunoprecipitated with KCNQ2 and KCNQ5 subunits, but no association was detected between KCNQ2 and KCNQ5. Intense KCNQ5 immunoreactivity was found to be widely distributed throughout the temporal neocortex and the hippocampal formation. In these structures, both pyramidal and non-pyramidal neurons and a population of glial cells in the white matter expressed the KCNQ5 subunit. In the sclerotic areas of the CA fields of epileptic patients, a marked loss of KCNQ5 immunoreactive pyramidal neurons was found in relation with the loss of neurons in these regions. However, in the regions adjacent to the sclerotic areas, the distribution and intensity of KCNQ5 immunostaining was apparently normal. The widespread distribution of KCNQ5 subunits, its persistence in pharmacoresistant epilepsy, along with the significant role of the M-current in the control of neuronal excitability, makes this protein a possible target for the development of anticonvulsant drugs.

KCNQ/M currents in sensory neurons: significance for pain therapy

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.

Functional coupling between heterologously expressed dopamine D(2) receptors and KCNQ channels

Activation of KCNQ potassium channels by stimulation of co-expressed dopamine D(2) receptors was studied electrophysiologically in Xenopus laevis oocytes and in mammalian cells. To address the specificity of the interaction between D(2)-like receptors and KCNQ channels, combinations of KCNQ1-5 channels and D(2)-like receptors (D(2L), D(3), and D(4)) were investigated in Xenopus oocytes. Activation of either receptor with the selective D(2)-like receptor agonist quinpirole (100 nM) stimulated all the KCNQ currents, independently of the subunit combination, indicating a common pathway of receptor-channel interaction. The KCNQ4 current was investigated in further detail and was increased by 19.9+/-1.6% ( n=20) by D(2L) receptor stimulation. The effect could be mimicked by injection of GTPgammaS and prevented by injection of Bordetella pertussis toxin, indicating that channel stimulation was mediated via a G protein of the G(alphai/o) subtype. Cells of the human neuroblastoma line SH-SY5Y were co-transfected transiently with KCNQ4 and D(2L) receptors. Stimulation of D(2L) receptors increased the KCNQ4 current ( n=6) as determined in whole-cell patch-clamp recordings. The specificity of the dopaminergic activation of the KCNQ channels was confirmed by co-expression of other neuronal K(+) channels (BK, K(V)1.1, and K(V)4.3) with the D(2L) receptor in Xenopus oocytes. None of these K(+) channels responded to stimulation of the D(2L) receptor. In the mammalian brain, dopamine D(2) receptors and KCNQ channels co-localise postsynaptically in several brain regions, so modulation of neuronal excitability by dopamine release could in part be mediated via an effect on KCNQ channels.

KCNQ/M currents in sensory neurons: significance for pain therapy

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.

Stoichiometry of expressed KCNQ2/KCNQ3 potassium channels and subunit composition of native ganglionic M channels deduced from block by tetraethylammonium

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.

Calmodulin mediates Ca2+-dependent modulation of M-type K+ channels

To quantify the modulation of KCNQ2/3 current by [Ca2+]i and to test if calmodulin (CaM) mediates this action, simultaneous whole-cell recording and Ca2+ imaging was performed on CHO cells expressing KCNQ2/3 channels, either alone, or together with wild-type (wt) CaM, or dominant-negative (DN) CaM. We varied [Ca2+]i from <10 to >400 nM with ionomycin (5 microM) added to either a 2 mM Ca2+, or EGTA-buffered Ca2+-free, solution. Coexpression of wt CaM made KCNQ2/3 currents highly sensitive to [Ca2+]i (IC50 70 +/- 20 nM, max inhibition 73%, n = 10). However, coexpression of DN CaM rendered KCNQ2/3 currents largely [Ca2+]i insensitive (max inhibition 8 +/- 3%, n = 10). In cells without cotransfected CaM, the Ca2+ sensitivity was variable but generally weak. [Ca2+]i modulation of M current in superior cervical ganglion (SCG) neurons followed the same pattern as in CHO cells expressed with KCNQ2/3 and wt CaM, suggesting that endogenous M current is also highly sensitive to [Ca2+]i. Coimmunoprecipitations showed binding of CaM to KCNQ2-5 that was similar in the presence of 5 mM Ca2+ or 5 mM EGTA. Gel-shift analyses suggested Ca2+-dependent CaM binding to an "IQ-like" motif present in the carboxy terminus of KCNQ2-5. We tested whether bradykinin modulation of M current in SCG neurons uses CaM. Wt or DN CaM was exogenously expressed in SCG cells using pseudovirions or the biolistic "gene gun." Using both methods, expression of both wt CaM and DN CaM strongly reduced bradykinin inhibition of M current, but for all groups muscarinic inhibition was unaffected. Cells expressed with wt CaM had strongly reduced tonic current amplitudes as well. We observed similar [Ca2+]i rises by bradykinin in all the groups of cells, indicating that CaM did not affect Ca2+ release from stores. We conclude that M-type currents are highly sensitive to [Ca2+]i and that calmodulin acts as their Ca2+ sensor.

Stoichiometry of expressed KCNQ2/KCNQ3 potassium channels and subunit composition of native ganglionic M channels deduced from block by tetraethylammonium

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.

Effects of varying impulse number on cotransmitter contributions to sympathetic vasoconstriction in rat tail artery

We examined the contributions of the cotransmitters norepinephrine (NE), ATP, and neuropeptide Y (NPY) to sympathetically evoked vasoconstriction in the rat tail artery in isolated vascular rings by using 1-100 stimulation impulses at 20 Hz. Phentolamine (2 microM), the alpha-adrenoceptor antagonist, markedly reduced responses to all stimuli, although responses to lower impulse numbers were reduced less than responses to longer trains. The purinergic receptor antagonist suramin (100 microM) reduced all responses, but to a much greater extent with few impulse trains. Responses were further reduced or abolished by addition of the second antagonist. Any remaining responses were abolished by the NPY-Y(1) receptor antagonist BIBP-3226 (75 nM). NPY had a direct agonist action and potentiated sympathetically mediated responses. NPY (75 nM) potentiated responses and BIBP-3226 decreased responses to 2- and 20-impulse trains. Both affected responses from 2 impulses to >20 impulses, but there was no preferential effect on purinergic contributions to responses because neurally released NPY potentiated both "pure" NE and ATP responses equally. We conclude that all three cotransmitters contribute significantly to vascular responses and their contribution varies markedly with impulse numbers. There is considerable synergy between cotransmitters, especially with lower impulse numbers where NPY contributions are greater than expected.

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

Selective expression of a persistent tetrodotoxin-resistant Na+ current and NaV1.9 subunit in myenteric sensory neurons

Voltage-gated Na(+) currents play critical roles in shaping electrogenesis in neurons. Here, we have identified a TTX-resistant Na(+) current (TTX-R I(Na)) in duodenum myenteric neurons of guinea pig and rat and have sought evidence regarding the molecular identity of the channel producing this current from the expression of Na(+) channel alpha subunits and the biophysical and pharmacological properties of TTX-R I(Na). Whole-cell patch-clamp recording from in situ neurons revealed the presence of a voltage-gated Na(+) current that was highly resistant to TTX (IC(50), approximately 200 microm) and selectively distributed in myenteric sensory neurons but not in interneurons and motor neurons. TTX-R I(Na) activated slowly in response to depolarization and exhibited a threshold for activation at -50 mV. V(1/2) values of activation and steady-state inactivation were -32 and -31 mV in the absence of fluoride, respectively, which, as predicted from the window current, generated persistent currents. TTX-R I(Na) also had prominent ultraslow inactivation, which turns off 50% of the conductance at rest (-60 mV). Substituting CsF for CsCl in the intracellular solution shifted the voltage-dependent parameters of TTX-R I(Na) leftward by approximately 20 mV. Under these conditions, TTX-R I(Na) had voltage-dependent properties similar to those reported previously for NaN/Na(V)1.9 in dorsal root ganglion neurons. Consistent with this, reverse transcription-PCR, single-cell profiling, and immunostaining experiments indicated that Na(V)1.9 transcripts and subunits, but not Na(V)1.8, were expressed in the enteric nervous system and restricted to myenteric sensory neurons. TTX-R I(Na) may play an important role in regulating subthreshold electrogenesis and boosting synaptic stimuli, thereby conferring distinct integrative properties to myenteric sensory neurons.

Two forms of electrical resonance at theta frequencies, generated by M-current, h-current and persistent Na+ current in rat hippocampal pyramidal cells

Coherent network oscillations in the brain are correlated with different behavioural states. Intrinsic resonance properties of neurons provide a basis for such oscillations. In the hippocampus, CA1 pyramidal neurons show resonance at theta (theta) frequencies (2-7 Hz). To study the mechanisms underlying theta-resonance, we performed whole-cell recordings from CA1 pyramidal cells (n = 73) in rat hippocampal slices. Oscillating current injections at different frequencies (ZAP protocol), revealed clear resonance with peak impedance at 2-5 Hz at approximately 33 degrees C (increasing to approximately 7 Hz at approximately 38 degrees C). The theta-resonance showed a U-shaped voltage dependence, being strong at subthreshold, depolarized (approximately -60 mV) and hyperpolarized (approximately -80 mV) potentials, but weaker near the resting potential (-72 mV). Voltage clamp experiments revealed three non-inactivating currents operating in the subthreshold voltage range: (1) M-current (I(M)), which activated positive to -65 mV and was blocked by the M/KCNQ channel blocker XE991 (10 microM); (2) h-current (I(h)), which activated negative to -65 mV and was blocked by the h/HCN channel blocker ZD7288 (10 microM); and (3) a persistent Na(+) current (I(NaP)), which activated positive to -65 mV and was blocked by tetrodotoxin (TTX, 1 microM). In current clamp, XE991 or TTX suppressed the resonance at depolarized, but not hyperpolarized membrane potentials, whereas ZD7288 abolished the resonance only at hyperpolarized potentials. We conclude that these cells show two forms of theta-resonance: "M-resonance" generated by the M-current and persistent Na(+) current in depolarized cells, and "H-resonance" generated by the h-current in hyperpolarized cells. Computer simulations supported this interpretation. These results suggest a novel function for M/KCNQ channels in the brain: to facilitate neuronal resonance and network oscillations in cortical neurons, thus providing a basis for an oscillation-based neural code.