Selective reduction of one mode of M-channel gating by muscarine in sympathetic neurons

Modulation of KV7 Channel Deactivation by PI(4,5)P2

The activity of KV7 channels critically contributes to the regulation of cellular electrical excitability in many cell types. In the central nervous system, the heteromeric KV7.2/KV7.3 channel is thought to be the chief molecular entity giving rise to M-currents. These K+-currents as so called because they are inhibited by the activation of Gq protein-coupled muscarinic receptors. In general, activation of Gq protein-coupled receptors (GqPCRs) decreases the concentration of the phosphoinositide PI(4,5)P2 which is required for KV7 channel activity. It has been recently reported that the deactivation rate of KV7.2/KV7.3 channels decreases as a function of activation. This suggests that the activated/open channel stabilizes as activation persists. This property has been regarded as evidence for the existence of modal behavior in the activity of these channels. In particular, it has been proposed that the heteromeric KV7.2/KV7.3 channel has at least two modes of activity that can be distinguished by both their deactivation kinetics and sensitivity to Retigabine. The current study was aimed at understanding the effect of PI(4,5)P2 depletion on the modal behavior of KV7.2/KV7.3 channels. Here, it was hypothesized that depleting the membrane of P(4,5)P2 would hamper the stabilization of the activated/open channel, resulting in higher rates of deactivation of the heteromeric KV7.2/KV7.3 channel. In addressing this question, it was found that the activity-dependent slowdown of the deactivation was not as prominent when channels were co-expressed with the chimeric phosphoinositide-phosphatase Ci-VS-TPIP or when cells were treated with the phosphoinositide kinase inhibitor Wortmannin. Further, it was observed that either of these approaches to deplete PI(4,5)P2 had a higher impact on the kinetic of deactivation following prolonged activation, while having little or no effect when activation was short-lived. Furthermore, it was observed that the action of either Ci-VS-TPIP or Wortmannin reduced the effect of Retigabine on the kinetics of deactivation, having a higher impact when activation was prolonged. These combined observations led to the conclusion that the deactivation kinetic of KV7.2/KV7.3 channels was sensitive to PI(4,5)P2 depletion in an activation-dependent manner, displaying a stronger effect on deactivation following prolonged activation.

Neurons, Receptors, and Channels

Here, I recount some adventures that I and my colleagues have had over some 60 years since 1957 studying the effects of drugs and neurotransmitters on neuronal excitability and ion channel function, largely, but not exclusively, using sympathetic neurons as test objects. Studies include effects of centrally active drugs on sympathetic transmission; neuronal action and neuroglial uptake of GABA in the ganglia and brain; the action of muscarinic agonists on sympathetic neurons; the action of bradykinin on neuroblastoma-derived cells; and the identification of M-current as a target for muscarinic action, including experiments to determine its distribution, molecular composition, neurotransmitter sensitivity, and intracellular regulation by phospholipids and their hydrolysis products. Techniques used include electrophysiological recording (extracellular, intracellular microelectrode, whole-cell, and single-channel patch-clamp), autoradiography, messenger RNA and complementary DNA expression, antibody injection, antisense knockdown, and membrane-targeted lipidated peptides. I finish with some recollections about my scientific career, funding, and changes in laboratory life and pharmacology research over the past 60 years.

AKAP79/150 signal complexes in G-protein modulation of neuronal ion channels

Voltage-gated M-type (KCNQ) K+ channels play critical roles in regulation of neuronal excitability. Previous work showed A-kinase-anchoring protein (AKAP)79/150-mediated protein kinase C (PKC) phosphorylation of M channels to be involved in M current (I(M)) suppression by muscarinic M1, but not bradykinin B2, receptors. In this study, we first explored whether purinergic and angiotensin suppression of I(M) in superior cervical ganglion (SCG) sympathetic neurons involves AKAP79/150. Transfection into rat SCG neurons of ΔA-AKAP79, which lacks the A domain necessary for PKC binding, or the absence of AKAP150 in AKAP150(-/-) mice, did not affect I(M) suppression by purinergic agonist or by bradykinin, but reduced I(M) suppression by muscarinic agonist and angiotensin II. Transfection of AKAP79, but not ΔA-AKAP79 or AKAP15, rescued suppression of I(M) by muscarinic receptors in AKAP150(-/-) neurons. We also tested association of AKAP79 with M(1), B(2), P2Y(6), and AT(1) receptors, and KCNQ2 and KCNQ3 channels, via Förster resonance energy transfer (FRET) on Chinese hamster ovary cells under total internal refection fluorescence microscopy, which revealed substantial FRET between AKAP79 and M1 or AT1 receptors, and with the channels, but only weak FRET with P2Y(6) or B2 receptors. The involvement of AKAP79/150 in G(q/11)-coupled muscarinic regulation of N- and L-type Ca2+) channels and by cAMP/protein kinase A was also studied. We found AKAP79/150 to not play a role in the former, but to be necessary for forskolin-induced upregulation of L-current. Thus, AKAP79/150 action correlates with the PIP(2) (phosphatidylinositol 4,5-bisphosphate)-depletion mode of I(M) suppression, but does not generalize to G(q/11)-mediated inhibition of N- or L-type Ca2+ channels.

Stationary gating of GluN1/GluN2B receptors in intact membrane patches

NMDA receptors are heteromeric glutamate-gated channels composed of GluN1 and GluN2 subunits. Receptor isoforms that differ in their GluN2-subunit type (A-D) are expressed differentially throughout the central nervous system and have distinct kinetic properties in recombinant systems. How specific receptor isoforms contribute to the functions generally attributed to NMDA receptors remains unknown, due in part to the incomplete functional characterization of individual receptor types and unclear molecular composition of native receptors. We examined the stationary gating kinetics of individual rat recombinant GluN1/GluN2B receptors in cell-attached patches of transiently transfected HEK293 cells and used kinetic analyses and modeling to describe the full range of this receptor's gating behaviors. We found that, like GluN1/GluN2A receptors, GluN1/GluN2B receptors have three gating modes that are distinguishable by their mean open durations. However, for GluN1/GluN2B receptors, the modes also differed markedly in their mean closed durations and thus generated a broader range of open probabilities. We also found that regardless of gating mode, glutamate dissociation occurred approximately 4-fold more slowly (k(-) = 15 s(-1)) compared to that observed in GluN1/GluN2A receptors. On the basis of these results, we suggest that slow glutamate dissociation and modal gating underlie the long heterogeneous activations of GluN1/GluN2B receptors.

Direct activation of transient receptor potential vanilloid 1(TRPV1) by diacylglycerol (DAG)

The capsaicin receptor, known as transient receptor potential channel vanilloid subtype 1 (TRPV1), is activated by a wide range of noxious stimulants and putative ligands such as capsaicin, heat, pH, anandamide, and phosphorylation by protein kinase C (PKC). However, the identity of endogenous activators for TRPV1 under physiological condition is still debated. Here, we report that diacylglycerol (DAG) directly activates TRPV1 channel in a membrane-delimited manner in rat dorsal root ganglion (DRG) neurons. 1-oleoyl-2-acetyl-sn-glycerol (OAG), a membrane-permeable DAG analog, elicited intracellular Ca²⁺ transients, cationic currents and cobalt uptake that were blocked by TRPV1-selective antagonists, but not by inhibitors of PKC and DAG lipase in rat DRG neurons or HEK 293 cells heterologously expressing TRPV1. OAG induced responses were about one fifth of capsaicin induced signals, suggesting that OAG displays partial agonism. We also found that endogenously produced DAG can activate rat TRPV1 channels. Mutagenesis of rat TRPV1 revealed that DAG-binding site is at Y511, the same site for capsaicin binding, and PtdIns(4,5)P₂binding site may not be critical for the activation of rat TRPV1 by DAG in heterologous system. We propose that DAG serves as an endogenous ligand for rat TRPV1, acting as an integrator of G_q/11-coupled receptors and receptor tyrosine kinases that are linked to phospholipase C.

Regulation of M(Kv7.2/7.3) channels in neurons by PIP(2) and products of PIP(2) hydrolysis: significance for receptor-mediated inhibition

M-channels are voltage-gated K+ channels that regulate the excitability of many neurons. They are composed of Kv7 (KCNQ) family subunits, usually Kv7.2 + Kv7.3. Native M-channels and expressed Kv7.2 + 7.3 channels are inhibited by stimulating G(q/11)-coupled receptors - prototypically the M1 muscarinic acetylcholine receptor (M1-mAChR). The channels require membrane phosphatidylinositol-4,5-bisphosphate (PIP(2)) to open and the effects of mAChR stimulation result primarily from the reduction in membrane PIP(2) levels following G(q)/phospholipase C-catalysed PIP(2) hydrolysis. However, in sympathetic neurons, M-current inhibition by bradykinin appears to be mediated through the release and action of intracellular Ca(2)+ by inositol-1,4,5-trisphosphate (IP(3)), a product of PIP(2) hydrolysis, rather than by PIP(2) depletion. We have therefore compared the effects of bradykinin and oxotremorine-M (a muscarinic agonist) on membrane PIP(2) in sympathetic neurons using a fluorescently tagged mutated C-domain of the PIP(2) binding probe, 'tubby'. In concentrations producing equal M-current inhibition, bradykinin produced about one-quarter of the reduction in PIP(2) produced by oxotremorine-M, but equal reduction when PIP(2) synthesis was blocked with wortmannin. Likewise, wortmannin restored bradykinin-induced M-current inhibition when Ca(2)+ release was prevented with thapsigargin. Thus, inhibition by bradykinin can use product (IP(3)/Ca(2)+)-dependent or substrate (PIP(2)) dependent mechanisms, depending on Ca(2)+ availability and PIP(2) synthesis rates.

Pathways modulating neural KCNQ/M (Kv7) potassium channels

K(+) channels play a crucial role in regulating the excitability of neurons. Many K(+) channels are, in turn, regulated by neurotransmitters. One of the first neurotransmitter-regulated channels to be identified, some 25 years ago, was the M channel. This was categorized as such because its activity was inhibited through stimulation of muscarinic acetylcholine receptors. M channels are now known to be composed of subunits of the Kv7 (KCNQ) K(+) channel family. However, until recently, the link between the receptors and the channels has remained elusive. Here, we summarize recent developments that have begun to clarify this link and discuss their implications for physiology and medicine.

Low mobility of phosphatidylinositol 4,5-bisphosphate underlies receptor specificity of Gq-mediated ion channel regulation in atrial myocytes

We have shown previously that cardiac G protein-gated inwardly rectifying K+ (GIRK) channels are inhibited by Gq protein-coupled receptors (GqPCRs) via phosphatidylinositol 4,5-bisphosphate (PIP2) depletion in a receptor-specific manner. To investigate the mechanism of receptor specificity, we examined whether the activation of GqPCRs induces localized PIP2 depletion. When we applied endothelin-1 to the bath, GIRK channel activities recorded in cell-attached patches were not changed, implying that PIP2 signal is not diffusible but is a localized signal. To test this possibility, we directly measured lateral diffusion by introducing fluorescence-labeled phosphoinositides to a small area of the membrane with patch pipettes. After pipettes were attached, phosphatidylinositol 4-monophosphate or phosphatidylinositol diffused rapidly to the entire membrane, whereas PIP2 was confined to the membrane patch inside the pipette. The confinement of PIP2 was disrupted after cytochalasin D treatment, suggesting that the cytoskeleton is responsible for the low mobility of PIP2. The diffusion coefficient (D) of PIP2 in the plasma membrane measured with the fluorescence recovery after photobleaching technique was 0.00039 microm2/s (n = 6), which is markedly lower than D of phosphatidylinositol (5.8 microm2/s, n = 5). Simulation of PIP2 concentration profiles by the diffusion model confirms that when D is small, the kinetics of PIP2 depletion at different distances from phospholipase C becomes similar to the characteristic kinetics of GIRK inhibition by different agonists. These results imply that PIP2 depletion is localized adjacent to GqPCRs because of its low mobility, and that spatial proximity of GqPCR and the target protein underlies the receptor specificity of PIP2-mediated signaling.

Relationship between membrane phosphatidylinositol-4,5-bisphosphate and receptor-mediated inhibition of native neuronal M channels

The relationship between receptor-induced membrane phosphatidylinositol-4'5'-bisphosphate (PIP2) hydrolysis and M-current inhibition was assessed in single-dissociated rat sympathetic neurons by simultaneous or parallel recording of membrane current and membrane-to-cytosol translocation of the fluorescent PIP2/inositol 1,4,5-trisphosphate (IP3)-binding peptide green fluorescent protein-tagged pleckstrin homology domain of phospholipase C (GFP-PLCdelta-PH). The muscarinic receptor agonist oxotremorine-M produced parallel time- and concentration-dependent M-current inhibition and GFP-PLCdelta-PH translocation; bradykinin also produced parallel time-dependent inhibition and translocation. Phosphatidylinositol-4-phosphate-5-kinase (PI5-K) overexpression reduced both M-current inhibition and GFP-PLCdelta-PH translocation by both oxotremorine-M and bradykinin. These effects were partly reversed by wortmannin, which inhibits phosphatidylinositol-4-kinase (PI4-K). PI5-K overexpression also reduced the inhibitory action of oxotremorine-M on PIP2-gated G-protein-gated inward rectifier (Kir3.1/3.2) channels; bradykinin did not inhibit these channels. Overexpression of neuronal calcium sensor-1 protein (NCS-1), which increases PI4-K activity, did not affect responses to oxotremorine-M but reduced both fluorescence translocation and M-current inhibition by bradykinin. Using an intracellular IP3 membrane fluorescence-displacement assay, initial mean concentrations of membrane [PIP2] were estimated at 261 microm (95% confidence limit; 192-381 microm), rising to 693 microm (417-1153 microm) in neurons overexpressing PI5-K. Changes in membrane [PIP2] during application of oxotremorine-M were calculated from fluorescence data. The results, taken in conjunction with previous data for KCNQ2/3 (Kv7.2/Kv7.3) channel gating by PIP2 (Zhang et al., 2003), accorded with the hypothesis that the inhibitory action of oxotremorine-M on M current resulted from depletion of PIP2. The effects of bradykinin require additional components of action, which might involve IP3-induced Ca2+ release and consequent M-channel inhibition (as proposed previously) and stimulation of PIP2 synthesis by Ca2+-dependent activation of NCS-1.

Kinetic diversity of single-channel burst openings underlying persistent Na(+) current in entorhinal cortex neurons

The kinetic diversity of burst openings responsible for the persistent Na(+) current (I(NaP)) in entorhinal cortex neurons was examined by separately analyzing single bursts. Although remarkable kinetic variability was observed among bursts in terms of intraburst opening probability and mean open and closed times, the values of time constants describing intraburst open times (tau(o(b))s) and closed times (tau(c(b))s) were distributed around well-defined peaks. At -40 mV, tau(o(b)) peaks were found at approximately 0.34 (tau(o(b))1) and 0.77 (tau(o(b))2) ms, and major tau(c(b)) peaks were found at approximately 0.24 (tau(c(b))1) and 0.54 (tau(c(b))2) ms. In approximately 80% of the bursts two preferential gating modes were found that consisted of a combination of either tau(o(b))1 and tau(c(b))2 ("intraburst mode 1"), or tau(o(b))2 and tau(c(b))1 ("intraburst mode 2"). Individual channels could switch between different gating modalities, but normally tended to maintain a specific gating mode for long periods. Mean burst duration also displayed considerable variability. At least three time constants were found to describe burst duration, and the frequencies at which each of the corresponding "bursting states" occurred varied in different channels. Short-lasting bursting states were preferentially associated with intraburst mode 1, whereas very-long-lasting bursts tended to gate according to mode 2 only or other modes that included considerably longer mean open times. These results show that I(NaP) channels can generate multiple intraburst open and closed states and bursting states, but these different kinetic states tend to combine in definite ways to produce a limited number of prevalent, well-defined gating modalities. Modulation of distinct gating modalities in individual Na(+) channels may be a powerful form of plasticity to influence neuronal excitability and function.

Neurosteroids shift partial agonist activation of GABA(A) receptor channels from low- to high-efficacy gating patterns

Although GABA activates synaptic (alphabetagamma) GABA(A) receptors with high efficacy, partial agonist activation of alphabetagamma isoforms and GABA activation of the primary extrasynaptic (alphabetadelta) GABA(A) receptors are limited to low-efficacy activity, characterized by minimal desensitization and brief openings. The unusual sensitivity of alphabetadelta receptor channels to neurosteroid modulation prompted investigation of whether this high sensitivity was dependent on the delta subunit or the low-efficacy channel function that it confers. We show that the isoform specificity (alphabetadelta > alphabetagamma) of neurosteroid modulation could be reversed by conditions that reversed isoform-specific activity modes, including the use of beta-alanine to achieve increased efficacy with alphabetadelta receptors and taurine to render alphabetagamma receptors low efficacy. We suggest that neurosteroids preferentially enhance low-efficacy GABA(A) receptor activity independent of subunit composition. Allosteric conversion of partial to full agonism may be a general mechanism for reversibly scaling the efficacy of GABA(A) receptors to endogenous partial agonists.

PIP(2) activates KCNQ channels, and its hydrolysis underlies receptor-mediated inhibition of M currents

KCNQ channels belong to a family of potassium ion channels with crucial roles in physiology and disease. Heteromers of KCNQ2/3 subunits constitute the neuronal M channels. Inhibition of M currents, by pathways that stimulate phospholipase C activity, controls excitability throughout the nervous system. Here we show that a common feature of all KCNQ channels is their activation by the signaling membrane phospholipid phosphatidylinositol-bis-phosphate (PIP(2)). We show that wortmannin, at concentrations that prevent recovery from receptor-mediated inhibition of M currents, blocks PIP(2) replenishment to the cell surface. Moreover, we identify a C-terminal histidine residue, immediately proximal to the plasma membrane, mutation of which renders M channels less sensitive to PIP(2) and more sensitive to receptor-mediated inhibition. Finally, native or recombinant channels inhibited by muscarinic agonists can be activated by PIP(2). Our data strongly suggest that PIP(2) acts as a membrane-diffusible second messenger to regulate directly the activity of KCNQ currents.

Activation of muscarinic m5 receptors inhibits recombinant KCNQ2/KCNQ3 K+ channels expressed in HEK293T cells

A variety of G-protein-coupled receptors regulate membrane excitability via M-type K(+) current (M-current) modulation. Muscarinic m1 and m3 acetylcholine receptors have both been implicated in the modulation of M-current. The muscarinic m5 receptor, like muscarinic m1 and m3 receptors, couples to phospholipase C via a pertussis toxin-insensitive G protein. Since a number of other receptors which activate phospholipase C also modulate M-current, we investigated if muscarinic m5 receptors could modulate recombinant M-type (KCNQ2/KCNQ3) K(+) channels after heterologous expression in human embryonic kidney (HEK) 293T cells. Application of Oxo-tremorine M to HEK293T cells expressing muscarinic m1, m3, or m5 receptors produced a similar robust inhibition of M-current, whereas muscarinic m2 and m4 receptor stimulation was without effect. Muscarinic m1, m3, or m5 receptor stimulation decreased the deactivation time constants of M-current at -50 mV. The inhibition of M-current by stimulation of muscarinic m1, m3, or m5 receptors was insensitive to overnight treatment with pertussis toxin or cholera toxin, which interfere with G(i/o) and G(s) G-protein signaling. These data suggest that muscarinic m1, m3, and m5 receptors inhibit M-channels via the activation of a common G protein.

Activation of a PTX-insensitive G protein is involved in histamine-induced recombinant M-channel modulation

The M-type potassium current (I(M)) plays a dominant role in regulating membrane excitability and is modulated by many neurotransmitters. However, except in the case of bradykinin, the signal transduction pathways involved in M-channel modulation have not been fully elucidated. The channels underlying I(M) are produced by the coassembly of KCNQ2 and KCNQ3 channel subunits and can be expressed in heterologous systems where they can be modulated by several neurotransmitter receptors including histamine H(1) receptors. In HEK293T cells, histamine acting via transiently expressed H(1)R produced a strong inhibition of recombinant M-channels but had no overt effects on the voltage dependence or voltage range of I(M) activation. In addition, the modulation of I(M) by histamine was not voltage sensitive, whereas channel gating, particularly deactivation, was accelerated by histamine. Non-hydrolysable guanine nucleotide analogues (GDP-beta-S and GTP-gamma-S) and pertussis toxin (PTX) treatment demonstrated the involvement of a PTX-insensitive G protein in the signal transduction pathway mediating histamine-induced I(M) modulation. Abrogation of the histamine-induced modulation of I(M) by expression of a C-terminal construct of phospholipase C (PLC-beta1-ct), which buffers activated Galpha(q/11) subunits, implicates this G protein alpha subunit in the modulatory pathway. On the other hand, abrogation of the histamine-induced modulation of I(M) by expression of two constructs which buffer free betagamma subunits, transducin (Galphat) and a C-terminal construct of a G protein receptor kinase (MAS-GRK2-ct), implicates betagamma dimers in the modulatory pathway. These findings demonstrate that histamine modulates recombinant M-channels in HEK293T cells via a PTX-insensitive G protein, probably Galpha(q/11), in a similar manner to a number of other G protein-coupled receptors. However, histamine-induced I(M) modulation in HEK293T cells is novel in that betagamma subunits in addition to Galpha(q/11) subunits appear to be involved in the modulation of KCNQ2/3 channel currents.

Recovery from muscarinic modulation of M current channels requires phosphatidylinositol 4,5-bisphosphate synthesis

Suppression of M current channels by muscarinic receptors enhances neuronal excitability. Little is known about the molecular mechanism of this inhibition except the requirement for a specific G protein and the involvement of an unidentified diffusible second messenger. We demonstrate here that intracellular ATP is required for recovery of KCNQ2/KCNQ3 current from muscarinic suppression, with an EC(50) of approximately 0.5 mM. Substitution of nonhydrolyzable ATP analogs for ATP slowed or prevented recovery. ADPbetaS but not ADP also prevented the recovery. Receptor-mediated inhibition was irreversible when recycling of agonist-sensitive pools of phosphatidylinositol-4,5-bisphosphate (PIP(2)) was blocked by lipid kinase inhibitors. Lipid phosphorylation by PI 4-kinase is required for recovery from muscarinic modulation of M current.

KCNQ4 channels expressed in mammalian cells: functional characteristics and pharmacology

Human cloned KCNQ4 channels were stably expressed in HEK-293 cells and characterized with respect to function and pharmacology. Patch-clamp measurements showed that the KCNQ4 channels conducted slowly activating currents at potentials more positive than -60 mV. From the Boltzmann function fitted to the activation curve, a half-activation potential of -32 mV and an equivalent gating charge of 1.4 elementary charges was determined. The instantaneous current-voltage relationship revealed strong inward rectification. The KCNQ4 channels were blocked in a voltage-independent manner by the memory-enhancing M current blockers XE-991 and linopirdine with IC(50) values of 5.5 and 14 microM, respectively. The antiarrhythmic KCNQ1 channel blocker bepridil inhibited KCNQ4 with an IC(50) value of 9.4 microM, whereas clofilium was without significant effect at 100 microM. The KCNQ4-expressing cells exhibited average resting membrane potentials of -56 mV in contrast to -12 mV recorded in the nontransfected cells. In conclusion, the activation and pharmacology of KCNQ4 channels resemble those of M currents, and it is likely that the function of the KCNQ4 channel is to regulate the subthreshold electrical activity of excitable cells.

Modulation and genetic identification of the M channel

Potassium channels constitute a superfamily of the most diversified ion channels, acting in delicate and accurate ways to control or modify many physiological and pathological functions including membrane excitability, transmitter release, cell proliferation and cell degeneration. The M-type channel is a unique ligand-regulated and voltage-gated K(+) channel showing distinct physiological and pharmacological characteristics. This review will cover some important progress in the study of M channel modulation, particularly focusing on membrane transduction mechanisms. The K(+) channel genes corresponding to the M channel have been identified and will be reviewed in detail. It has been a long journey since the discovery of M current in 1980 to our present understanding of the mysterious mechanisms for M channel modulation; a journey which exemplifies tremendous achievements in ion channel research and exciting discoveries of elaborate modulatory systems linked to these channels. While substantial evidence has accumulated, challenging questions remain to be answered.

Modulation of M-channel conductance by adenosine 5' triphosphate in bullfrog sympathetic B-neurones

1. Adenosine 5' triphosphate (ATP) (0.5-500 microM) or muscarine (0.1-1.0 microM) suppressed M-current/conductance (IM/gM) in B-cells of bullfrog sympathetic ganglion. Both agonists suppressed steady-state M-conductance (gM) at -30 mV and there was either no change or a slight increase in the time constants for gM activation (tau(a) at -30 mV) and deactivation (tau(d) at -50 mV). 2. It has previously been shown that experimental elevation of intracellular Ca2+ concentration ([Ca2+]i) suppresses gM and this is associated with decreases in both tau(a) and tau(d). As these changes in kinetics differ from those we observe with agonist application, our results cast doubt on the hypothesis that elevation of [Ca2+]i is involved in the transduction mechanism for ATP- or muscarine-induced gM suppression.

M-type K+ currents in rat cultured thoracolumbar sympathetic neurones and their role in uracil nucleotide-evoked noradrenaline release

Cultured sympathetic neurones are depolarized and release noradrenaline in response to extracellular ATP, UDP and UTP. We examined the possibility that, in neurones cultured from rat thoracolumbar sympathetic ganglia, inhibition of the M-type potassium current might underlie the effects of UDP and UTP. Reverse transcriptase-polymerase chain reaction indicated that the cultured cells contained mRNA for P2Y(2)-, P2Y(4)- and P2Y(6)-receptors as well as for the KCNQ2- and KCNQ3-subunits which have been suggested to assemble into M-channels. In cultures of neurones taken from newborn as well as from 10 day-old rats, oxotremorine, the M-channel blocker Ba(2+) and UDP all released previously stored [(3)H]-noradrenaline. The neurones possessed M-currents, the kinetic properties of which were similar in neurones from newborn and 9 - 12 day-old rats. UDP, UTP and ATP had no effect on M-currents in neurones prepared from newborn rats. Oxotremorine and Ba(2+) substantially inhibited the current. ATP also had no effect on the M-current in neurones prepared from 9 - 12 day-old rats. Oxotremorine and Ba(2+) again caused marked inhibition. In contrast to cultures from newborn animals, UDP and UTP attenuated the M-current in neurones from 9 - 12 day-old rats; however, the maximal inhibition was less than 30%. The results indicate that inhibition of the M-current is not involved in uracil nucleotide-induced transmitter release from rat cultured sympathetic neurones during early development. M-current inhibition may contribute to release at later stages, but only to a minor extent. The mechanism leading to noradrenaline release by UDP and UTP remains unknown.

M-channel gating and simulation

Single potassium M-channels in rat sympathetic neurons have multiple voltage-dependent kinetic components in their activity: short, medium, and long closed times (tau(CS), tau(CM), and tau(CL)) and short and long open times (tau(OS) and tau(OL)). All five components can be detected in cell-attached patches, but only four of them (tau(CS), tau(CM), tau(OS), and tau(OL)) in excised patches (, J. Physiol. (Lond.). 472:711-724; 1996, Neuron. 16:151-162; 1996, Neuropharmacology. 35:933-947). Analysis of the burst structure of activity recorded from cell-attached and excised inside-out patches showed it to be consistent with the sequential kinetic scheme C(L) left arrow over right arrow O(S) left arrow over right arrow C(M) left arrow over right arrow O(L) left arrow over right arrow C(S). Using this scheme and experimentally determined kinetic parameters, we successfully simulated the activity of M-channels both under steady-state conditions and during depolarizing voltage steps. Consistent with the characteristic behavior of macroscopic M-current, ensemble currents constructed from simulated M-channels had exponential activation and deactivation, with no delays, when tested in the range between -50 and -20 mV.

Multiple G-protein-coupled pathways inhibit N-type Ca channels of neurons

Muscarinic receptors depress Ca2+ currents in superior cervical ganglion neurons by two signaling pathways. One is sensitive to pertussis toxin and acts rapidly by a membrane-delimited pathway on the channels. The other is not sensitive to pertussis toxin and acts more slowly through an unknown second messenger. These pathways are shared with several other agonists.

Gating of recombinant small-conductance Ca-activated K+ channels by calcium

Small-conductance Ca-activated K+ channels play an important role in modulating excitability in many cell types. These channels are activated by submicromolar concentrations of intracellular Ca2+, but little is known about the gating kinetics upon activation by Ca2+. In this study, single channel currents were recorded from Xenopus oocytes expressing the apamin-sensitive clone rSK2. Channel activity was detectable in 0.2 micro M Ca2+ and was maximal above 2 micro M Ca2+. Analysis of stationary currents revealed two open times and three closed times, with only the longest closed time being Ca dependent, decreasing with increasing Ca2+ concentrations. In addition, elevated Ca2+ concentrations resulted in a larger percentage of long openings and short closures. Membrane voltage did not have significant effects on either open or closed times. The open probability was approximately 0.6 in 1 micro M free Ca2+. A lower open probability of approximately 0.05 in 1 micro M Ca2+ was also observed, and channels switched spontaneously between behaviors. The occurrence of these switches and the amount of time channels spent displaying high open probability behavior was Ca2+ dependent. The two behaviors shared many features including the open times and the short and intermediate closed times, but the low open probability behavior was characterized by a different, long Ca2+-dependent closed time in the range of hundreds of milliseconds to seconds. Small-conductance Ca- activated K+ channel gating was modeled by a gating scheme consisting of four closed and two open states. This model yielded a close representation of the single channel data and predicted a macroscopic activation time course similar to that observed upon fast application of Ca2+ to excised inside-out patches.

Contribution of outward currents to spike-frequency adaptation in hypoglossal motoneurons of the rat

Contribution of outward currents to spike-frequency adaptation in hypoglossal motoneurons of the rat. J. Neurophysiol. 78: 2246-2253, 1997. Spike-frequency adaptation has been attributed to the actions of several different membrane currents. In this study, we assess the contributions of two of these currents: the net outward current generated by the electrogenic Na+-K+ pump and the outward current that flows through Ca2+-activated K+ channels. In recordings made from hypoglossal motoneurons in slices of rat brain stem, we found that bath application of a 4-20 microM ouabain solution produced a partial block of Na+-K+ pump activity as evidenced by a marked reduction in the postdischarge hyperpolarization that follows a period of sustained discharge. However, we observed no significant change in either the initial, early, or late phases of spike-frequency adaptation in the presence of ouabain. Adaptation also has been related to increases in the duration and magnitude of the medium-duration afterhyperpolarization (mAHP) mediated by Ca2+-activated K+ channels. When we replaced the 2 mM Ca2+ in the bathing solution with Mn2+, there was a significant decrease in the amplitude of the mAHP after a spike. The decrease in mAHP amplitude resulted in a decrease in the magnitude of the initial phase of spike-frequency adaptation as has been reported previously by others. However, quite unexpectedly we also found that reducing the mAHP resulted in a dramatic increase in the magnitude of both the early and late phases of adaptation. These changes could be reversed by restoring the normal Ca2+ concentration in the bath. Our results with ouabain indicate that the Na+-K+ pump plays little, if any, role in the three phases of adaptation in rat hypoglossal motoneurons. Our results with Ca2+ channel blockade support the hypothesis that initial adaptation is, in part, controlled by conductances underlying the mAHP. However, our failure to eliminate initial adaptation completely by blocking Ca2+ channels suggests that other membrane mechanisms also contribute. Finally, the increase in both the early and late phases of adaptation in the presence of Mn2+ block of Ca2+ channels lends further support to the hypothesis that the initial and later (i.e., early and late) phases of spike-frequency adaptation are mediated by different cellular mechanisms.

Nonstationary noise analysis of M currents simulated and recorded in PC12 cells

M current relaxations recorded in PC12 cells were subjected to nonstationary noise analysis (NSNA) to obtain estimates of single-channel current (i), channel number (N), and open probability (Po) for the channels responsible for M current. The analysis was constrained such that N and single-channel conductance were the same at two potentials. The relation between variance and current indicated that the fraction of channels open was 0.58 +/- 0.06 (mean +/- SD) and 0.05 +/- 0.04 (mean +/- SD: n = 9) at -33 and -63 mV, respectively. The single M channel conductance was 4.0 pS, and a density of 1 functional M channel per 4 microm2 was estimated. Monte Carlo simulations of a two-state model of M channels were used to obtain sets of simulated macroscopic M currents that were subjected to the same NSNA procedure so as to evaluate the accuracy of M channel parameters obtained with this method. The influence of current rundown and filter frequency on estimates of i, N, and Po were evaluated. The single-channel parameters estimated from the simulations differed by < 10% from actual values at any level of current rundown, N, or Po. The dispersion in the estimation of N and Po increased as Po decreased. Decreasing filter frequency caused an underestimation of i, paralleled by an overestimation of N. The estimation of Po was relatively immune to the filter frequency, especially for data simulated with Po = 0.77.

Exotoxin-insensitive G proteins mediate synaptically evoked muscarinic sodium current in rabbit sympathetic neurones

1. The involvement of G proteins in the transduction pathway that links muscarinic receptors to the low-threshold voltage-dependent sodium current (INa,M) was studied in neurones from intact sympathetic prevertebral ganglia using the whole-cell configuration of the patch-clamp technique. Experiments were performed in the presence of the nicotinic receptor antagonists hexamethonium (50 microM) and d-tubocurarine (50 microM). 2. INa,M was activated by either bath-applying muscarinic agonists or stimulating the preganglionic splanchnic nerves. Synaptically and agonist-mediated INa,M did not display significant run-down or changes in their properties in cells tested, irrespective of whether the pipette solutions contained GTP. 3. Dialysis of sympathetic neurones with GDP beta S (500-750 microM) decreased the amplitude of INa,M by approximately 65% compared with control neurones within 30 min. 4. In the absence of muscarinic receptor stimulation, intracellular dialysis with GTP gamma S (500 microM) for 10 min slowly and slightly (20-25%) activated INa,M. GTP gamma S dialysis markedly slowed down the decay of INa,M after its transient activation with carbachol pulses (10-20 s) or nerve stimulation (3-5 s). The INa,M activation became fully irreversible 2.9 min after the start of GTP gamma S dialysis. Dialysing cells with the G protein activator AIF4-led to a rapid but transient activation of INa,M. 5. Synaptically and agonist-evoked INa,M were not affected in ganglia treated with 0.5-1 microgram ml-1 pertussis toxin (PTX) for 7-24 h at 37 degrees C. Control experiments showed that this treatment severely reduced the PTX-sensitive inhibition of N-type calcium currents induced by carbachol (CCh) and noradrenaline. Application of NEM (N-ethylmaleimide) for 2 min depressed the INa,M evoked in response to bath-applied CCh by only 27%. 6. Incubating ganglia with 5-10 micrograms ml-1 of cholera toxin for 7 h had no effect on the carbachol-induced INa,M but greatly potentiated (approximately 250%) the synaptically evoked INa,M, presumably via a presynaptic mechanism. 7. These results show that the coupling between muscarinic receptors and NaM channels is mediated by pertussis toxin- and cholera toxin-insensitive G proteins, possibly of the Gq/11 or G12 class.

Control of M-current

M-current is a non-inactivating potassium current found in many neuronal cell types. In each cell type, it is dominant in controlling membrane excitability by being the only sustained current in the range of action potential initiation. It can be modulated by a large array of receptor types, and the modulation can occur either by suppression or enhancement. Modulation of M-current has dramatic effects on neuronal excitability. This review discusses the numerous second messenger pathways that converge on regulation of this current: in particular, two forms of regulation of the M-current, receptor-mediated modulation and the control of macroscopic current amplitude by intracellular calcium. Both types of regulation are discussed with reference to the modulation of single-channel gating properties.

Muscarinic mechanisms in nerve cells

The receptor subtype and transduction mechanisms involved in the regulation of various neuronal ionic currents are reviewed, with some recent observations on sympathetic neurons, hippocampal cell membranes and basal forebrain cells.

Ion permeation and block of M-type and delayed rectifier potassium channels. Whole-cell recordings from bullfrog sympathetic neurons

Ion permeation and conduction were studied using whole-cell recordings of the M-current (I(M)) and delayed rectifier (IDR), two K+ currents that differ greatly in kinetics and modulation. Currents were recorded from isolated bullfrog sympathetic neurons with 88 mM [K+]i and various external cations. Selectivity for extracellular monovalent cations was assessed from permeability ratios calculated from reversal potentials and from chord conductances for inward current. PRb/PK was near 1.0 for both channels, and GRb/GK was 0.87 +/- 0.01 for IDR but only 0.35 +/- 0.01 for I(M) (15 mM [Rb+]o or [K+]o). The permeability sequences were generally similar for I(M) and IDR: K+ approximately Rb+ > NH4+ > Cs+, with no measurable permeability to Li+ or CH3NH3+. However, Na+ carried detectable inward current for IDR but not I(M). Nao+ also blocked inward K+ current for IDR (but not IM), at an apparent electrical distance (delta) approximately 0.4, with extrapolated dissociation constant (KD) approximately 1 M at 0 mV. Much of the instantaneous rectification of IDR in physiologic ionic conditions resulted from block by Nao+. Extracellular Cs+ carried detectable inward current for both channel types, and blocked I(M) with higher affinity (KD = 97 mM at 0 mV for I(M), KD) approximately 0.2 M at 0 mV for IDR), with delta approximately 0.9 for both. IDR showed several characteristics reflecting a multi-ion pore, including a small anomalous mole fraction effect for PRb/PK, concentration-dependent GRb/GK, and concentration-dependent apparent KD's and delta's for block by Nao+ and Cso+. I(M) showed no clear evidence of multi-ion pore behavior. For I(M), a two-barrier one-site model could describe permeation of K+ and Rb+ and block by Cso+, whereas for IDR even a three-barrier, two-site model was not fully adequate.

Hyperpolarizing shift of the M-current activation curve after washout of muscarine in bullfrog sympathetic neurons

The mechanism underlying the over-recovery of an M-type potassium current following the washout of muscarine (20 microM) has been examined. Whole-cell recordings were made from single neurons dissociated from bullfrog sympathetic ganglia. During over-recovery, the maximum M-conductance decreased by about 2.8 nS while the steady-state M-current activation curve was displaced in the hyperpolarizing direction by about 13 mV. These data suggest that a hyperpolarizing shift in the kinetics of M-current causes over-recovery in amphibian autonomic neurons.

Calcineurin regulates M channel modal gating in sympathetic neurons

The M current regulates neuronal excitability, with its amplitude resulting from high open probability modal M channel behavior. The M current is affected by changing intracellular calcium levels. It is proposed that internal calcium acts by regulating M channel modal gating. Intracellular application of a preactivated form of the calcium-dependent phosphatase calcineurin (CaN420) inhibited the macroscopic M current, while its application to excised inside-out patches reduced high open probability M channel activity. Addition of ATP reversed the action of CaN420 on excised patches. The change in M channel gating induced by CaN420 was different from the effect of muscarine. A kinetic model supports the proposition that shifts in channel gating induced by calcium-dependent phosphorylation and dephosphorylation control M current amplitude.

Intracellular calcium directly inhibits potassium M channels in excised membrane patches from rat sympathetic neurons

Complex effects of altering intracellular [Ca2+] on M-type K+ currents have previously been reported using whole-cell current recording. To study the direct effect of Ca2+ on M-channel activity, we have applied Ca2+ to the inside face of membrane patches excised from rat superior cervical sympathetic ganglion cells. Ca2+ rapidly and reversibly inhibited M-channel activity in 28/44 patches by up to 87%, with a mean IC50 of 100 nM. This effect persisted in the absence of ATP, implying that it was not due to phosphorylation/dephosphorylation. A similar effect was observed in 13/13 cell-attached patches when cells were transiently "Ca(2+)-loaded" by adding 2 mM Ca2+ to a 25 mM K+ solution bathing the extrapatch cell membrane. These observations provide new evidence that Ca2+ can directly inhibit M channels, so supporting the view that Ca2+ might mediate M current inhibition following muscarinic receptor activation.

Regulation of M-type potassium channels in mammalian sympathetic neurons: action of intracellular calcium on single channel currents

Currents through single M-type potassium channels were recorded in membrane patches excised from rat superior cervical sympathetic neurons. Application of Ca+ to the internal face of inside-out patches produce two forms of M-channel inhibition: a slow, all-or-nothing suppression of activity; and a fast block associated with a concentration-dependent shortening of open times compatible with open-channel block. Both forms of block were enhanced by patch depolarization. Neither was replicated or affected by Mg2+, and both could be recorded in the absence of intracellular ATP, implying that they did not involve phosphorylation. Since the block was reversible in the absence of ATP and since alkaline phosphatase did not reduce channel activity, block was unlikely to have resulted from dephosphorylation. In cell-attached patch recording, M-channel activity increased during exposure of the cell to Ca2(+)-free solution and was rapidly reduced on applying 2mM Ca2+ to the extra-patch solution. This suggests that M-channel activity in these neurons may be tonically regulated by variations in resting intracellular [Ca2+].

Methods for studying neurotransmitter transduction mechanisms

This review describes the methodologies used to study the transduction mechanisms that are activated in excitable cells by G-protein-coupled agonists. In view of the complexity of second-messenger systems, it is no longer relevant to ask, "What is the transduction mechanism involved in the action of a given neuromodulator?" because, in many cases, a variety of transduction mechanisms and physiological responses are invoked following receptor activation. This means that a single aspect of the physiological response must be selected for study in order to address the question of transduction mechanism. This review is therefore concerned with a description the use of patch- and voltage-clamp procedures to study transduction mechanism because they are designed to isolate one aspect of the physiological response: the change in activity of a single type of membrane ion channel.

Effects of thyrotropin-releasing hormone on rat motoneurons are mediated by G proteins

Numerous transmitter receptors are linked via GTP-binding proteins (G proteins) to membrane phosphoinositide metabolism by phospholipase C (PLC) and generation of second messengers such as activated protein kinase C (PKC), inositol trisphosphate (IP3) and/or elevations in intracellular calcium. In many cases, these same receptors also inhibit a resting ('leak') potassium current (IK(L)), thereby depolarizing neurons. It is unclear if activation of this PLC pathway mediates inhibition of IK(L) by neurotransmitter receptors. Therefore, we tested the contribution of this pathway to the TRH-induced inhibition of IK(L) in rat hypoglossal motoneurons (HMs) using conventional intracellular recording in brainstem slices. When HMs were recorded with electrodes containing 3 M KCl or 30 mM GTP (in KCl), TRH induced a depolarization that recovered quickly (within 8-10 min) and could be repeated with only modest tachyphylaxis (< 20%). However, with electrodes containing the non-hydrolyzable G protein activator, GTP gamma S (10 mM), the TRH-induced depolarization was long lasting (up to 1 h); with electrodes containing the G protein inhibitor, GDP beta S (20 mM) the tachyphylaxis with repeated TRH application was exaggerated (approximately 60%). Activation of PKC by phorbol dibutyrate (10 microM in perfusate) neither mimicked nor occluded the effects of TRH. There were no effects on membrane potential, input resistance (RN) or the response to TRH in HMs during long recordings with electrodes containing high concentrations of IP3 (60 mM).(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of ion-channel function by G-protein-coupled receptors

Neurotransmitters acting through G-protein-coupled receptors change the electrical excitability of neurons. Activation of receptors can affect the voltage dependence, the speed of gating, and the probability of opening of various ion channels, thus changing the computational state and outputs of a neuron. Each cell expresses many kinds of receptors, and uses several intracellular signaling pathways to modulate channel function in different ways. It has become possible to dissect these pathways by combining pharmacological and biophysical experiments. Recent patch-clamp work in sympathetic neurons will be summarized to illustrate the mechanisms underlying modulation and its significance.

Phorbol ester-induced M-current suppression in bull-frog sympathetic ganglion cells: insensitivity to kinase inhibitors

1. The effects of 1-oleoyl-2-acetyl-sn-glycerol (OAG), phorbol 12-myristate 13-acetate (PMA), 4-alpha-phorbol and muscarine on B-neurones from bull-frog sympathetic ganglion were studied by means of whole-cell patch-clamp recording. With the exception of 4-alpha-phorbol, all of these agonists reduced the steady-state outward current recorded at -30 mV as a result of suppression of a voltage-dependent, non-inactivating K(+)-current, the M-current, (IM). 2. Of the cells tested, 34% displayed bona fide responses to OAG (20 microM). The chance of recording a response was not decreased when the protein kinase inhibitor, 1-(5-isoquinolinylsulphonyl)-2-methyl-piperazine (H-7; 50 or 75 microM) was included simultaneously in the extracellular solution and in the pipette solution. 3. The presence of 50 microM H-7 on both sides of the membrane or 500 nM staurosporine in the pipette solution did not prevent responses to brief (1-2 min) or prolonged (> 20 min) applications of PMA. 4. Brief (1-2 min) extracellular application of H-7 (300 microM) suppressed IM by about 29%. 5. The most likely explanation of these data is that PMA and OAG modulate IM via a mechanism that is independent of protein kinase C (PKC). The availability of such a mechanism poses new questions as to the mechanism of muscarine-induced IM suppression.