Modulation of M-current by intracellular Ca2+

rdHSV-CA8 non-opioid analgesic gene therapy decreases somatosensory neuronal excitability by activating Kv7 voltage-gated potassium channels

Chronic pain is common and inadequately treated, making the development of safe and effective analgesics a high priority. Our previous data indicate that carbonic anhydrase-8 (CA8) expression in dorsal root ganglia (DRG) mediates analgesia via inhibition of neuronal ER inositol trisphosphate receptor-1 (ITPR1) via subsequent decrease in ER calcium release and reduction of cytoplasmic free calcium, essential to the regulation of neuronal excitability. This study tested the hypothesis that novel JDNI8 replication-defective herpes simplex-1 viral vectors (rdHSV) carrying a CA8 transgene (vHCA8) reduce primary afferent neuronal excitability. Whole-cell current clamp recordings in small DRG neurons showed that vHCA8 transduction caused prolongation of their afterhyperpolarization (AHP), an essential regulator of neuronal excitability. This AHP prolongation was completely reversed by the specific Kv7 channel inhibitor XE-991. Voltage clamp recordings indicate an effect via Kv7 channels in vHCA8-infected small DRG neurons. These data demonstrate for the first time that vHCA8 produces Kv7 channel activation, which decreases neuronal excitability in nociceptors. This suppression of excitability may translate in vivo as non-opioid dependent behavioral- or clinical analgesia, if proven behaviorally and clinically.

The Molecular Basis for the Calcium-Dependent Slow Afterhyperpolarization in CA1 Hippocampal Pyramidal Neurons

Neuronal signal transmission depends on the frequency, pattern, and timing of spike output, each of which are shaped by spike afterhyperpolarizations (AHPs). There are classically three post-spike AHPs of increasing duration categorized as fast, medium and slow AHPs that hyperpolarize a cell over a range of 10 ms to 30 s. Intensive early work on CA1 hippocampal pyramidal cells revealed that all three AHPs incorporate activation of calcium-gated potassium channels. The ionic basis for a fAHP was rapidly attributed to the actions of big conductance (BK) and the mAHP to small conductance (SK) or Kv7 potassium channels. In stark contrast, the ionic basis for a prominent slow AHP of up to 30 s duration remained an enigma for over 30 years. Recent advances in pharmacological, molecular, and imaging tools have uncovered the expression of a calcium-gated intermediate conductance potassium channel (IK, KCa3.1) in central neurons that proves to contribute to the slow AHP in CA1 hippocampal pyramidal cells. Together the data show that the sAHP arises in part from a core tripartite complex between Cav1.3 (L-type) calcium channels, ryanodine receptors, and IK channels at endoplasmic reticulum-plasma membrane junctions. Work on the sAHP in CA1 pyramidal neurons has again quickened pace, with identified contributions by both IK channels and the Na-K pump providing answers to several mysteries in the pharmacological properties of the sAHP.

Atomistic Insights of Calmodulin Gating of Complete Ion Channels

Intracellular calcium is essential for many physiological processes, from neuronal signaling and exocytosis to muscle contraction and bone formation. Ca²⁺ signaling from the extracellular medium depends both on membrane potential, especially controlled by ion channels selective to K⁺, and direct permeation of this cation through specialized channels. Calmodulin (CaM), through direct binding to these proteins, participates in setting the membrane potential and the overall permeability to Ca²⁺. Over the past years many structures of complete channels in complex with CaM at near atomic resolution have been resolved. In combination with mutagenesis-function, structural information of individual domains and functional studies, different mechanisms employed by CaM to control channel gating are starting to be understood at atomic detail. Here, new insights regarding four types of tetrameric channels with six transmembrane (6TM) architecture, Eag1, SK2/SK4, TRPV5/TRPV6 and KCNQ1–5, and its regulation by CaM are described structurally. Different CaM regions, N-lobe, C-lobe and EF3/EF4-linker play prominent signaling roles in different complexes, emerging the realization of crucial non-canonical interactions between CaM and its target that are only evidenced in the full-channel structure. Different mechanisms to control gating are used, including direct and indirect mechanical actuation over the pore, allosteric control, indirect effect through lipid binding, as well as direct plugging of the pore. Although each CaM lobe engages through apparently similar alpha-helices, they do so using different docking strategies. We discuss how this allows selective action of drugs with great therapeutic potential.

The Role of KV7.3 in Regulating Osteoblast Maturation and Mineralization

KCNQ (KV7) channels are voltage-gated potassium (KV) channels, and the function of KV7 channels in muscles, neurons, and sensory cells is well established. We confirmed that overall blockade of KV channels with tetraethylammonium augmented the mineralization of bone-marrow-derived human mesenchymal stem cells during osteogenic differentiation, and we determined that KV7.3 was expressed in MG-63 and Saos-2 cells at the mRNA and protein levels. In addition, functional KV7 currents were detected in MG-63 cells. Inhibition of KV7.3 by linopirdine or XE991 increased the matrix mineralization during osteoblast differentiation. This was confirmed by alkaline phosphatase, osteocalcin, and osterix in MG-63 cells, whereas the expression of Runx2 showed no significant change. The extracellular glutamate secreted by osteoblasts was also measured to investigate its effect on MG-63 osteoblast differentiation. Blockade of KV7.3 promoted the release of glutamate via the phosphorylation of extracellular signal-regulated kinase 1/2-mediated upregulation of synapsin, and induced the deposition of type 1 collagen. However, activation of KV7.3 by flupirtine did not produce notable changes in matrix mineralization during osteoblast differentiation. These results suggest that KV7.3 could be a novel regulator in osteoblast differentiation.

HIV-1 viral protein R causes peripheral nervous system injury associated with in vivo neuropathic pain

Painful peripheral neuropathy has become the principal neurological disorder in HIV/AIDS patients. Herein, we investigated the effects of a cytotoxic HIV-1 accessory protein, viral protein R (Vpr), on the peripheral nervous system (PNS). Host and viral gene expression was investigated in peripheral nerves from HIV-infected individuals and in HIV-infected human dorsal root ganglion (DRG) cultures by RT-PCR and immunocytochemistry. Cytosolic calcium ([Ca(2+)]) fluxes and neuronal membrane responses were analyzed in cultured DRGs. Neurobehavioral responses and cytokine levels were assessed in a transgenic mouse model in which the vpr transgene was expressed in an immunodeficient background (vpr/RAG1(-/-)). Vpr transcripts and proteins were detected in peripheral nerves and DRGs from HIV-infected patients. Exposure of rat or human cultured DRG neurons to Vpr rapidly increased [Ca(2+)] and action potential frequency while increasing input resistance. HIV infection of human DRG cultures caused neurite retraction (P<0.05), accompanied by induction of interferon-α (IFN-α) transcripts (P<0.05). vpr/RAG1(-/-) mice expressed Vpr together with increased IFN-α (P<0.05) in the PNS and also exhibited mechanical allodynia, unlike their vpr/RAG1(-/-) littermates (P<0.05). Herein, Vpr caused DRG neuronal damage, likely through cytosolic calcium activation and cytokine perturbation, highlighting Vpr's contribution to HIV-associated peripheral neuropathy and ensuing neuropathic pain.

Homeostatic regulation of M-current modulates synaptic integration in secretomotor, but not vasomotor, sympathetic neurons in the bullfrog

We compared how vasomotor C neurons and secretomotor B neurons integrated identical patterns of virtual synaptic activity using dynamic clamp, perforated-patch recordings from dissociated bullfrog sympathetic ganglion cells. The synaptic template modelled one strong nicotinic synapse and nine weak synapses, each firing randomly at 5 Hz, with strength normalized to each cell. B neurons initially fired at 12 Hz, but this declined within seconds, decreasing 27% after 40 s and recovering slowly as evidenced by the threshold synaptic conductance for firing (tau(recovery) = 136 + or - 23 s). C neurons gave an identical initial response that remained steady, declining only 6% after 40 s. The difference resulted from an activity-dependent 379 + or - 65% increase in M-current (I(M)) in B cells (tau(recovery) = 153 + or - 22 s), which was absent in C cells. In addition, action potential afterhyperpolarizations were 2-fold longer in B cells, but this did not produce the differential response to synaptic stimulation. Activity-dependent increases in I(M) were sensitive to 100 microm Cd(2+) and 2.5 microm oxotremorine M (oxo-M), a muscarinic agonist, and fully blocked by zero Ca(2+), 10 microm oxo-M and 2.5 microm oxo-M plus 50 microm wortmannin, a PIP(2) synthesis inhibitor. A leftward shift in voltage-dependent activation could not fully account for the I(M) increase. Firing at 0.5 Hz was sufficient to modulate I(M). Opposing influences of activity and muscarinic excitation thus produce homeostatic I(M) regulation, to stabilize excitability and postsynaptic output in secretomotor sympathetic neurons. Absence of this regulation in vasomotor neurons suggests a different integrative function, where synaptic gain increases in proportion to presynaptic activity.

Coupling of L-type Ca2+ channels to KV7/KCNQ channels creates a novel, activity-dependent, homeostatic intrinsic plasticity

Experience-dependent modification in the electrical properties of central neurons is a form of intrinsic plasticity that occurs during development and has been observed following behavioral learning. We report a novel form of intrinsic plasticity in hippocampal CA1 pyramidal neurons mediated by the KV7/KCNQ and CaV1/L-type Ca2+ channels. Enhancing Ca2+ influx with a conditioning spike train (30 Hz, 3 s) potentiated the KV7/KCNQ channel function and led to a long-lasting, activity-dependent increase in spike frequency adaptation-a gradual reduction in the firing frequency in response to sustained excitation. These effects were abolished by specific blockers for CaV1/L-type Ca2+ channels, KV7/KCNQ channels, and protein kinase A (PKA). Considering the widespread expression of these two channel types, the influence of Ca2+ influx and subsequent activation of PKA on KV7/KCNQ channels may represent a generalized principle in fine tuning the output of central neurons that promotes stability in firing-an example of homeostatic regulation of intrinsic membrane excitability.

Activation of epidermal growth factor receptor inhibits KCNQ2/3 current through two distinct pathways: membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation

KCNQ2/3 currents are the molecular basis of the neuronal M currents that play a critical role in neuron excitability. Many neurotransmitters modulate M/KCNQ currents through their G-protein-coupled receptors. Membrane PtdIns(4,5)P2 hydrolysis and channel phosphorylation are two mechanisms that have been proposed for modulation of KCNQ2/3 currents. In this study, we studied regulation of KCNQ2/3 currents by the epidermal growth factor (EGF) receptor, a member of another family of membrane receptors, receptor tyrosine kinases. We demonstrate here that EGF induces biphasic inhibition of KCNQ2/3 currents in human embryonic kidney 293 cells and in rat superior cervical ganglia neurons, an initial fast inhibition and a later slow inhibition. Additional studies indicate that the early and late inhibitions resulted from PtdIns(4,5)P2 hydrolysis and tyrosine phosphorylation, respectively. We further demonstrate that these two processes are mutually dependent. This study indicates that EGF is a potent modulator of M/KCNQ currents and provides a new dimension to the understanding of the modulation of these channels.

Nonlinear interaction between shunting and adaptation controls a switch between integration and coincidence detection in pyramidal neurons

The membrane conductance of a pyramidal neuron in vivo is substantially increased by background synaptic input. Increased membrane conductance, or shunting, does not simply reduce neuronal excitability. Recordings from hippocampal pyramidal neurons using dynamic clamp revealed that adaptation caused complete cessation of spiking in the high conductance state, whereas repetitive spiking could persist despite adaptation in the low conductance state. This behavior was reproduced in a phase plane model and was explained by a shunting-induced increase in voltage threshold. The increase in threshold allows greater activation of the M current (I(M)) at subthreshold potentials and reduces the minimum adaptation required to stabilize the system; in contrast, activation of the afterhyperpolarization current is unaffected by the increase in threshold and therefore remains unable to stop repetitive spiking. The nonlinear interaction between shunting and I(M) has other important consequences. First, timing of spikes elicited by brief stimuli is more precise when background spikes elicited by sustained input are prohibited, as occurs exclusively with I(M)-mediated adaptation in the high conductance state. Second, activation of I(M) at subthreshold potentials, which is increased in the high conductance state, hyperpolarizes average membrane potential away from voltage threshold, allowing only large, rapid fluctuations to reach threshold and elicit spikes. These results suggest that the shift from a low to high conductance state in a pyramidal neuron is accompanied by a switch from encoding time-averaged input with firing rate to encoding transient inputs with precisely timed spikes, in effect, switching the operational mode from integration to coincidence detection.

Cholinergic suppression of KCNQ channel currents enhances excitability of striatal medium spiny neurons

In response to glutamatergic synaptic drive, striatal medium spiny neurons in vivo transition to a depolarized "up state" near spike threshold. In the up state, medium spiny neurons either depolarize enough to spike or remain below spike threshold and are silent before returning to the hyperpolarized "down state." Previous work has suggested that subthreshold K+ channel currents were responsible for this dichotomous behavior, but the channels giving rise to the current and the factors determining its engagement have been a mystery. To move toward resolution of these questions, perforated-patch recordings from medium spiny neurons in tissue slices were performed. K+ channels with pharmacological and kinetic features of KCNQ channels potently regulated spiking at up-state potentials. Single-cell reverse transcriptase-PCR confirmed the expression of KCNQ2, KCNQ3, and KCNQ5 mRNAs in medium spiny neurons. KCNQ channel currents in these cells were potently reduced by M1 muscarinic receptors, because the effects of carbachol were blocked by M1 receptor antagonists and lost in neurons lacking M1 receptors. Reversal of the modulation was blocked by a phosphoinositol 4-kinase inhibitor, indicating a requirement for phosphotidylinositol 4,5-bisphosphate resynthesis for recovery. Inhibition of protein kinase C reduced the efficacy of the muscarinic modulation. Finally, acceleration of cholinergic interneuron spiking with 4-aminopyridine mimicked the effects of exogenous agonist application. Together, these results show that KCNQ channels are potent regulators of the excitability of medium spiny neurons at up-state potentials, and they are modulated by intrastriatal cholinergic interneurons, providing a mechanistic explanation for variability in spiking during up states seen in vivo.

Bidirectional modulation of GABAergic transmission by cholecystokinin in hippocampal dentate gyrus granule cells of juvenile rats

Cholecystokinin (CCK) interacts with two types of G protein-coupled receptors in the brain: CCK-A and CCK-B receptors. Both CCK and CCK-B receptors are widely distributed in the hippocampal formation, but the functions of CCK there have been poorly understood. In the present study, we initially examined the effects of CCK on GABA(A) receptor-mediated synaptic transmission in the hippocampal formation and then explored the underlying cellular mechanisms by focusing on the dentate gyrus region, where the highest levels of CCK-binding sites have been detected. Our results indicate that activation of CCK-B receptors initially and transiently increased spontaneous IPSC (sIPSC) frequency, followed by a persistent reduction. The effects of CCK were more evident in juvenile rats, suggesting that they are developmentally regulated. Cholecystokinin failed to modulate the miniature IPSCs recorded in the presence of TTX and the amplitude of the evoked IPSCs, but produced a transient increase followed by a reduction in action potential firing frequency recorded from GABAergic interneurons, suggesting that CCK acts by modulating the excitability of the interneurons to regulate GABA release. Cholecystokinin reduced the amplitude of the after-hyperpolarization of the action potentials, and application of paxilline or charybdotoxin considerably reduced CCK-mediated modulation of sIPSC frequency, suggesting that the effects of CCK are related to the inhibition of Ca(2+)-activated K(+) currents (I(K(Ca))). The effects of CCK were independent of the functions of phospholipase C, intracellular Ca(2+) release, protein kinase C or phospholipase A(2), suggesting a direct coupling between the G proteins of CCK-B receptors and I(K(Ca)). Our results provide a novel mechanism underlying CCK-mediated modulation of GABA release.

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.

Structural requirements for differential sensitivity of KCNQ K+ channels to modulation by Ca2+/calmodulin

Calmodulin modulation of ion channels has emerged as a prominent theme in biology. The sensitivity of KCNQ1-5 K+ channels to modulation by Ca2+/calmodulin (CaM) was studied using patch-clamp, Ca2+ imaging, and biochemical and pharmacological approaches. Coexpression of CaM in Chinese hamster ovary (CHO) cells strongly reduced currents of KCNQ2, KCNQ4, and KCNQ5, but not KCNQ1 or KCNQ3. In simultaneous current recording/Ca2+ imaging experiments, CaM conferred Ca2+ sensitivity to KCNQ4 and KCNQ5, but not to KCNQ1, KCNQ3, or KCNQ1/KCNE1 channels. A chimera constructed from the carboxy terminus of KCNQ4 and the rest KCNQ1 displayed Ca2+ sensitivity similar to KCNQ4. Chimeras constructed from different lengths of the KCNQ4 carboxy terminal and the rest KCNQ3 localized a region that confers sensitivity to Ca2+/CaM. Lobe-specific mutations of CaM revealed that its amino-terminal lobe mediates the Ca2+ sensitivity of the KCNQ/CaM complex. The site of CaM action within the channel carboxy terminus overlaps with that of the KCNQ opener N-ethylmaleimide (NEM). We found that CaM overexpression reduced NEM augmentation of KCNQ2, KCNQ4, and KCNQ5, and NEM pretreatment reduced Ca2+/CaM-mediated suppression of M current in sympathetic neurons by bradykinin. We propose that two functionally distinct types of carboxy termini underlie the observed differences among this channel family.

Possible role of phosphatidylinositol 4,5, bisphosphate in luteinizing hormone releasing hormone-mediated M-current inhibition in bullfrog sympathetic neurons

Luteinizing hormone releasing hormone (LHRH) is a physiological modulator of neuronal excitability in bullfrog sympathetic ganglia (BFSG). Actions of LHRH involve suppression of the noninactivating, voltage-dependent M-type K+ channel conductance (gM). We found, using whole-cell recordings from these neurons, that LHRH-induced suppression of gM was attenuated by the phospholipase C (PLC) inhibitor U73122 (10 microM) but not by the inactive isomer U73343 (10 microM). Buffering internal Ca2+ to 117 nM with intracellular 20 mM BAPTA + 8 mM Ca2+ or to < 10 nM with intracellular 20 mM BAPTA + 0.4 mM Ca2+ did not attenuate LHRH-induced gM suppression. Suppression of gM by LHRH was not antagonized by the inositol 1,4,5 trisphosphate (InsP3) receptor antagonist heparin (approximately 300 microM). Preventing phosphatidylinositol-4,5-bisphosphate (PIP2) synthesis by blocking phosphatidylinositol-4-kinase with wortmannin (10 microM) or with the nonhydrolysable ATP analogue AMP-PNP (3 mM) prolonged recovery of LHRH-induced gM suppression. This effect was not produced by blocking phosphatidyl inositol-3-kinase with LY294002 (10 microM). Rundown of gM was attenuated when cells were dialysed with 240 microM di-octanoyl PIP2 or 240 microM di-octanoyl phosphatidylinositol-3,4,5-trisphosphate (PIP3) but not with 240 microM di-octanoyl phosphatidylcholine. LHRH-induced gM suppression was competitively antagonized by dialysis with 240 microM di-octanoyl PIP2, but not with di-octanoyl phosphatidylcholine. These results would be expected if LHRH-induced gM suppression reflects a PLC-mediated decrease in plasma membrane PIP2 levels.

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.

Development and application of caged calcium

Several caged calciums have been synthesized since 1986, and three are commercially available: DM-nitrophen, NP-EGTA, and nitr-5. Each of these caged compounds has uniquely useful properties, making the choice of which cage to use dependent on the specific experiment (i.e., the cell type and divalent cation requirements of the experiments within purview). Significantly, methods have been developed for all three cages that permit their quantitative use inside many cell types, including those with some of the most demanding of requirements for experiments with caged calcium, namely, in relating presynaptic [Ca] to postsynaptic function. The success of such experiments using DM-nitrophen and NP-EGTA suggests that caged calcium is now a mature tool for cellular physiology and neurobiology.

ATP-inhibition of M current in frog sympathetic neurons involves phospholipase C but not Ins P(3), Ca(2+), PKC, or Ras

Suppression of the voltage-activated, noninactivating K(+) conductance (M conductance; g(M)) by muscarinic agonists, P(2Y) agonists or bradykinin increases neuronal excitability. All agonist effects are mediated, at least in part, via the Gq/(11) class of G protein. We found, using whole cell or perforated patch recording from bullfrog sympathetic B neurons that ATP-induced suppression of g(M) was attenuated by the phospholipase C (PLC) inhibitor, U73122 (IC(50) approximately 0.14 microM) but not by the inactive isomer, U73343. The ability of extracellularly applied U73122 to inhibit PLC was confirmed by its antagonism of ATP-induced elevation of intracellular Ca(2+) as measured by fura-2 photometry. ATP-induced g(M) suppression was not antagonized by the protein kinase C (PKC) inhibitor, chelerythrine (5 microM extracellular +10 microM intracellular), by the Ca(2+)-ATPase inhibitor, thapsigargin (5 microM), or by inositol trisphosphate (InsP(3)) receptor antagonists, heparin (approximaterly 300 microM) or xestospongin C (1.8 microM). The effect of ATP on g(M) was thus dependent on PLC yet independent of PKC and of InsP(3)-induced release of intracellular Ca(2+). We therefore tested the involvement of a PKC-independent action of diacylglycerol (DAG) that could occur via activation of Ras. This low-molecular-weight G protein is activated following DAG binding to Ras-GRP, a neuronal Ras-GTP exchange factor. However, impairment of Ras function by culturing neurons with isoprenylation inhibitors (perillic acid, 0.1 mM, or alpha-hydroxyfarnesyl-phosphonic acid, 10 microM) failed to affect ATP-induced g(M) suppression. Inhibition of MEK (mitogen-activated protein kinase), a downstream target of Ras, by using PD 98059 (10 microM) was also ineffective. The transduction mechanism used by ATP to suppress g(M) in frog sympathetic neurons therefore differs from the PLC-independent mechanism used by muscarine and from the PLC and Ca(2+)-dependent mechanism used by bradykinin and UTP in mammalian ganglia. The possibility remains that "lipid-signaling" mechanisms, perhaps involving PLC-induced depletion of phosphatidylinositol bisphosphate, are involved in PLC-mediated inhibition of g(M) by ATP in amphibian sympathetic neurons.

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.

Reconstitution of muscarinic modulation of the KCNQ2/KCNQ3 K(+) channels that underlie the neuronal M current

Channels from KCNQ2 and KCNQ3 genes have been suggested to underlie the neuronal M-type K(+) current. The M current is modulated by muscarinic agonists via G-proteins and an unidentified diffusible cytoplasmic messenger. Using whole-cell clamp, we studied tsA-201 cells in which cloned KCNQ2/KCNQ3 channels were coexpressed with M(1) muscarinic receptors. Heteromeric KCNQ2/KCNQ3 currents were modulated by the muscarinic agonist oxotremorine-M (oxo-M) in a manner having all of the characteristics of modulation of native M current in sympathetic neurons. Oxo-M also produced obvious intracellular Ca(2+) transients, observed by using indo-1 fluorescence. However, modulation of the current remained strong even when Ca(2+) signals were abolished by the combined use of strong intracellular Ca(2+) buffers, an inhibitor of IP(3) receptors, and thapsigargin to deplete Ca(2+) stores. Muscarinic modulation was not blocked by staurosporine, a broad-spectrum protein kinase inhibitor, arguing against involvement of protein kinases. The modulation was not associated with a shift in the voltage dependence of channel activation. Homomeric KCNQ2 and KCNQ3 channels also expressed well and were modulated individually by oxo-M, suggesting that the motifs for modulation are present on both channel subtypes. Homomeric KCNQ2 and KCNQ3 currents were blocked, respectively, at very low and at high concentrations of tetraethylammonium ion. Finally, when KCNQ2 subunits were overexpressed by intranuclear DNA injection in sympathetic neurons, total M current was fully modulated by the endogenous neuronal muscarinic signaling mechanism. Our data further rule out Ca(2+) as the diffusible messenger. The reconstitution of muscarinic modulation of the M current that uses cloned components should facilitate the elucidation of the muscarinic signaling mechanism.

The role of ryanodine receptors in the cyclic ADP ribose modulation of the M-like current in rodent m1 muscarinic receptor-transformed NG108-15 cells

1. The role of cyclic ADP ribose and ryanodine receptors in the inhibition of the M-like current (IK(M,ng)) by acetylcholine was investigated in m1 muscarinic receptor-transformed mouse neuroblastoma-rat glioma hybrid (NG108-15) cells using patch-clamp techniques and calcium microfluorimetry. 2. Acetylcholine (1-100 microM) decreased IK(M,ng) by up to 55 %. Application, via the patch pipette, of the cyclic ADP ribose antagonists 8-amino-cyclic ADP ribose (10-100 microM) and 8-bromo-cyclic ADP ribose (100-1000 microM) reduced this inhibition of IK(M,ng) in a concentration-dependent manner. The half-maximal inhibition concentrations for 8-amino- cyclic ADP ribose and 8-bromo-cyclic ADP ribose were around 40 microM and 1 mM, respectively. 3. Neither of the cyclic ADP ribose antagonists altered the amplitude of IK(M,ng) per se, or the incidence of the concurrent Ca2+-activated K+ current (IIK(Ca)) activation, also mediated by acetylcholine. 4. The ryanodine receptor modulators ryanodine (1-10 microM) and Ruthenium Red (10 microM) did not alter IK(M,ng) amplitude or IK(M,ng) inhibition mediated by acetylcholine. There was a statistically significant increase in the proportion of cells showing outward currents in the presence of Ruthenium Red. 5. Intracellular calcium levels measured with fura-2 microfluorimetry were increased with low concentrations of ryanodine (1 microM), more consistently with caffeine (10 mM), and in almost every case with both bradykinin (300 nM) and acetylcholine (100 microM). Caffeine-, but not bradykinin-evoked responses were abolished by preincubation with ryanodine (10 microM). 6. The fast 'rundown rate' of the M-current recorded in rat superior cervical ganglion cells under whole-cell conditions precluded an investigation of the effects of intracellular dialysis of cyclic ADP ribose. However, when cyclic ADP ribose (5 microM) was applied directly to the cytoplasmic face of inside-out membrane patches excised from rat superior cervical ganglion cells containing M-channels, it had no effect on the main parameters of single channel activity (conductance, mean open time or frequency of opening). 7. These results indicate that cyclic ADP ribose acts on a specific intracellular site to mediate IK(M,ng) inhibition. However, unlike previously established effects of cyclic ADP ribose, the ryanodine receptor is not required, suggesting that another molecular target may be involved. Studies at the single channel level indicate that cyclic ADP ribose may not act directly on the M-channels in inside-out patches.

Differential control of three after-hyperpolarizations in rat hippocampal neurones by intracellular calcium buffering

1. The whole-cell recording technique, combined with internal perfusion, was used to study the effects of intracellular Ca2+ buffering on fast, medium and slow after-hyperpolarizations (fAHP, mAHP and sAHP) in hippocampal CA1 pyramidal neurones in rat brain slices at room temperature. 2. The action potentials and the fAHP were unaffected by 100 microM to 3 mM concentrations of the internally applied fast Ca2+ chelator BAPTA. At higher (10-15 mM) concentrations, BAPTA inhibited the fAHP and prolonged the decay of the action potential, suggesting that the corresponding large-conductance Ca2+-activated K+ channels are located close to the sites of Ca2+ entry during an action potential. Addition of Ca2+ to the BAPTA-containing solution (at a ratio of 4.5 [Ca2+] : 10 [BAPTA]) to maintain the control level of [Ca2+]i did not prevent the effects of high concentrations of BAPTA. 3. The mAHP, activated by a train of action potentials, was inhibited by internally applied BAPTA within the range of concentrations used (100 microM to 15 mM), and this effect could not be reversed or prevented by addition of Ca2+ to the BAPTA-containing solution. The inhibition of the mAHP by BAPTA could also be observed after blockade of the hyperpolarization-activated IQ type mixed Na+-K+ current (also known as Ih) component of the mAHP by bath-applied 3-5 mM Cs+, suggesting that the inhibition of the mAHP by BAPTA is due to inhibition of the depolarization-activated IM (muscarinic) type K+ current. 4. The sAHP, activated by a train of action potentials, was potentiated by 100-300 microM internally applied BAPTA, both with and without added Ca2+. At 1-2 mM or higher concentrations, the potentiation of the sAHP by BAPTA without added Ca2+ was transient and was followed by a fast decrease. With added Ca2+, however, BAPTA caused a persistent potentiation of the sAHP with more than a 10-fold increase in duration for periods exceeding 1 h even at concentrations of the buffer as high as 10-15 mM. Earlier reports showing a blockade of the sAHP by BAPTA, based on experiments without added Ca2+, were apparently due to a sharp reduction in intracellular free [Ca2+] and to a high intracellular concentration of the free buffer. 5. Internally applied BAPTA caused a prolongation of the spike discharge during an 800 ms-long depolarizing current step. At 100-300 microM BAPTA, but not at 1-2 mM or higher concentrations, this effect could be reversed by addition of Ca2+. The effects of BAPTA on the spike discharge occurred in parallel with the changes in the sAHP time course, which was more prolonged at higher concentrations of the buffer. 6. The concentration-dependent differential control of the three types of AHP in hippocampal neurones by BAPTA is related to modulation of intracellular Ca2+ diffusion by a fast acting mobile Ca2+ buffer.

Purine and pyrimidine nucleotides inhibit a noninactivating K+ current and depolarize adrenal cortical cells through a G protein-coupled receptor

Bovine adrenal zona fasciculata (AZF) cells express a noninactivating K+ current (IAC) that sets the resting membrane potential and may mediate depolarization-dependent cortisol secretion. External ATP stimulates cortisol secretion through activation of a nucleotide receptor. In whole-cell patch clamp recordings from bovine AZF cells, we found that ATP selectively inhibited IAC K+ current by a maximum of 75.7 +/- 3% (n = 13) with a 50% inhibitory concentration of 1.3 microM. A rapidly inactivating A-type K+ current was not inhibited by ATP. Other nucleotides, including ADP and the pyrimidines UTP and UDP, also inhibited IAC, whereas 2-methylthio-ATP (2-MeSATP) and CTP were completely ineffective. The rank order of potency for six nucleotides was UTP = ADP > ATP > UDP >> 2-MeSATP = CTP. At maximally effective concentrations, UTP, ADP, and UDP inhibited IAC current by 81.4 +/- 5.2% (n = 7), 70.7 +/- 7.2% (n = 4), and 65.2 +/- 7.9% (n = 5), respectively. Inhibition of IAC by external ATP was reduced from 71. 3 +/- 3.2% to 22.8 +/- 4.5% (n = 18) by substituting guanosine 5'-O-2-(thio) diphosphate for GTP in the patch pipette. Inhibition of IAC by external ATP (10 microM) was markedly suppressed (to 17.3 +/- 5.5%, n = 9) by the nonspecific protein kinase antagonist staurosporine (1 microM) and eliminated by substituting the nonhydrolyzable ATP analog 5-adenylyl-imidodiphosphate or UTP for ATP in the pipette. ATP-mediated inhibition of IAC was not altered by the kinase C antagonist calphostin C, the calmodulin inhibitory peptide, or by buffering the intracellular (pipette) Ca++ with 20 mM 1,2-bis-(2-aminophenoxy)ethane-N, N,N',N'-tetraacetic acid. In current clamp recordings, ATP and UTP (but not CTP) depolarized AZF cells at concentrations that inhibited IAC K+ current. These results demonstrate that bovine AZF cells express a nucleotide receptor with a P2Y3 agonist profile that is coupled to the inhibition of IAC K+ channels through a GTP-binding protein. The inhibition of IAC K+ current and associated membrane depolarization are the first cellular responses demonstrated to be mediated through this receptor. Nucleotide inhibition of IAC proceeds through a pathway that is independent of phospholipase C, but that requires ATP hydrolysis. The identification of a new signaling pathway in AZF cells, whereby activation of a nucleotide receptor is coupled to membrane depolarization through inhibition of a specific K+ channel, suggests a mechanism for ATP-stimulated corticosteroid secretion that depends on depolarization-dependent Ca++ entry. This may be a means of synchronizing the stress-induced secretion of corticosteroids and catecholamines from the adrenal gland.

Ca2+ depolarizes adrenal cortical cells through selective inhibition of an ATP-activated K+ current

Bovine adrenal zona fasciculata cells (AZF) express a noninactivating K+ current (IAC) whose inhibition by adrenocorticotropic hormone and ANG II may be coupled to membrane depolarization and Ca2+-dependent cortisol secretion. We studied IAC inhibition by Ca2+ and the Ca2+ ionophore ionomycin in whole cell and single-channel patch-clamp recordings of AZF. In whole cell recordings with intracellular (pipette) Ca2+ concentration ([Ca2+]i) buffered to 0.02 microM, IAC reached maximum current density of 25.0 +/- 5.1 pA/pF (n = 16); raising [Ca2+]i to 2.0 microM reduced it 76%. In inside-out patches, elevated [Ca2+]i dramatically reduced IAC channel activity. Ionomycin inhibited IAC by 88 +/- 4% (n = 14) without altering rapidly inactivating A-type K+ current. Inhibition of IAC by ionomycin was unaltered by adding calmodulin inhibitory peptide to the pipette or replacing ATP with its nonhydrolyzable analog 5'-adenylylimidodiphosphate. IAC inhibition by ionomycin was associated with membrane depolarization. When [Ca2+]i was buffered to 0.02 microM with 2 and 11 mM 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid (BAPTA), ionomycin inhibited IAC by 89.6 +/- 3.5 and 25.6 +/- 14.6% and depolarized the same AZF by 47 +/- 8 and 8 +/- 3 mV, respectively (n = 4). ANG II inhibited IAC significantly more effectively when pipette BAPTA was reduced from 11 to 2 mM. Raising [Ca2+]i inhibits IAC through a mechanism not requiring calmodulin or protein kinases, suggesting direct interaction with IAC channels. ANG II may inhibit IAC and depolarize AZF by activating parallel signaling pathways, one of which uses Ca2+ as a mediator.

Bradykinin inhibits M current via phospholipase C and Ca2+ release from IP3-sensitive Ca2+ stores in rat sympathetic neurons

A variety of intracellular signaling pathways can modulate the properties of voltage-gated ion channels. Some of them are well characterized. However, the diffusible second messenger mediating suppression of M current via G protein-coupled receptors has not been identified. In superior cervical ganglion neurons, we find that the signaling pathways underlying M current inhibition by B2 bradykinin and M1 muscarinic receptors respond very differently to inhibitors. The bradykinin pathway was suppressed by the phospholipase C inhibitor U-73122, by blocking the IP3 receptor with pentosan polysulfate or heparin, and by buffering intracellular calcium, and it was occluded by allowing IP3 to diffuse into the cytoplasm via a patch pipette. By contrast, the muscarinic pathway was not disrupted by any of these treatments. The addition of bradykinin was accompanied by a [Ca2+]i rise with a similar onset and time to peak as the inhibition of M current. The M current inhibition and the rise of [Ca2+]i were blocked by depletion of Ca2+ internal stores by thapsigargin. We conclude that bradykinin receptors inhibit M current of sympathetic neurons by activating phospholipase C and releasing Ca2+ from IP3-sensitive Ca2+ stores, whereas muscarinic receptors do not use the phospholipase C pathway to inhibit M current channels.

Low concentrations of muscarine potentiate M-current in bullfrog sympathetic B-neurones

The concentration-dependence of the effect of muscarine on M-current (IM) and the underlying M-conductance (gM) in B-cells of bullfrog sympathetic ganglion was examined using whole-cell recording techniques. High concentrations of muscarine (> or = 200 nM) produced the classical suppression and over-recovery of steady-state IM at -30 mV. By contrast, low concentrations of muscarine (< or = 30 nM) shifted the gM activation curve to more negative potentials, increased the activation time constant (tau a) and increased steady-state IM. This effect may reflect muscarine-induced changes in submembrane Ca2+ concentration.

Evidence for the calcium-dependent potentiation of M-current obtained by the ratiometric measurement of the fura-2 fluorescence in bullfrog sympathetic neurons

Intracellular Ca2+ concentration ([Ca]i) was measured following the activation of an inward Ca2+ current and subsequent potentiation of an M-type K+ current (IM) in bullfrog sympathetic neurons. Fura-2 was used as an indicator for [Ca]i. The fluorescence ratio at 340 and 380 nm (F340/F380) was elevated from 0.36 to 1.22 when IM was potentiated by 68% following the Ca2+ current. Based on the in vivo calibration curve obtained from cells permeabilized with digitonin (20 microM), the F340/F380 value of 1.22 was equivalent to a [Ca]i of 0.97 microM. We therefore propose that a rise in [Ca]i into the micromolar range can lead to the potentiation of IM in amphibian autonomic neurons.

Inhibition of M-current in cultured rat superior cervical ganglia by linopirdine: mechanism of action studies

The mechanism involved in the blockade of M-current by linopirdine was studied in cultured rat superior cervical sympathetic ganglia using whole-cell patch clamp recording. The effects of modulators of intracellular signal transduction pathways on muscarine- or linopirdine-induced inhibition of M-current were compared. Intracellular addition of GDP-beta-S (500 microM) attenuated M-current block by muscarine (1 microM) but not that of linopirdine (10 microM). Intracellular dialysis of GTP-gamma)-S (100 microM) enhanced and prolonged muscarine-induced inhibition of M-current but had no effect on the activity of linopirdine. Intracellular BAPTA (20 mM) also inhibited the effects of muscarine without affecting those of linopirdine. Intracellular application of linopirdine had no effect on either basal M-current amplitude or the ability of linopirdine to block M-current when administered extracellularly. These results indicate that M-current inhibition by linopirdine is unlikely to be either G-protein-mediated or calcium-mediated or to involve an intracellular site of action and are, therefore, consistent with a direct block of the M-channel from its extracellular side.

Desensitization, recovery and Ca(2+)-dependent modulation of ATP-gated P2X receptors in nociceptors

We have shown the presence and activity of ATP-gated ion channels (P2X receptors) in nociceptive nerve endings, supporting the theory that these channels mediate some forms of nociception [Cook S.P., Vulchanova L., Hargreaves K. M., Elde R. and McCleskey E. W. (1997) Distinct ATP receptors on pain-sensing and stretch-sensing neurons. Nature 387, 505-508]. The kinetics and pharmacology of ATP-gated currents in nociceptors suggest that the channels are comprised of either homomeric or heteromeric combinations of P2X3 receptors. Consistent with the diverse nature of P2X structure, electrophysiological responses of rat tooth-pulp nociceptors fall into two distinct classes based on desensitization and recovery kinetics. Here, we quantified the dramatic differences in desensitization kinetics of transient and persistent currents. The major component of transient P2X current desensitized with a tau decay = 32 +/- 2 msec, while persistent current desensitized > 100-fold more slowly, tau decay = 4000 +/- 320 msec. Both currents recovered from desensitization in minutes: tau recovery = 4 min for transient current, and tau recovery = 0.7 +/- 0.2 min for persistent current. Persistent current recovery was often accompanied by a current "overrecovery" that averaged ca threefold magnitude prior to desensitization. Comparison of ATP current in elevated Ca2+ext also revealed differences in transient and presistent currents. In 2 mM Ca2+ext medium, decrease of Na+ext resulted in an almost complete reduction of persistent, but not transient, current. Subsequent elevation of Ca2+ext greatly increased the transient, but not persistent, current. Mechanistic explanations for either the increase in transient current magnitude by elevated Ca2+ext, or persistent current overrecovery may reflect endogenous pathways for P2X receptor modulation.

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.

Sulfhydryl modification inhibits K+ (M) current with kinetics close to acetylcholine in rodent NG108-15 cells

The effects of sulfhydryl reagents on M-type voltage-dependent potassium currents (IK(M)) were examined in NG108-15 cells transformed to express ml muscarinic acetylcholine receptors (mAChRs), a NGPM1-27 clone. Focal application of glutathione at millimolar concentrations dissolved in acidic solutions caused a transient inward current in NGPM1-27 cells at holding potentials of -30mV, associated with an inhibition of IK(M). The glutathione-induced response was mimicked by cysteine. These effects were also reproduced by superfusion with micromolar concentrations of HgCl2, AgNO3, N-methylmaleimide and p-chloromercuribenzoic acid (pCMB), agents which target protein thiols. Glutathione, HgCl2, AgNO3 and pCMB inhibited the peak conductance of IK(M) without shifting the half activating voltage (V1/2), which was comparable to the acetylcholine (ACh)-induced response. The voltage dependence of time constants for IK(M) deactivation in sulfhydryl reagent-, ACh- and non-treated cells resembled, but differed from that in Ba(2+)-treated cells. These results reveal that there is an accessible cysteine moiety, but not a disulfide bond, either on the M channel protein itself or on a protein directly involved in agonist-M channel coupling.

M-current suppression in PC12 cells by bradykinin is mediated by a pertussis toxin-insensitive G-protein and modulated by intracellular calcium

Inhibition of the M-current in PC12 cells by bradykinin has been studied under whole-cell recording conditions. In cells dialyzed with GTP-gamma-S, bradykinin produced a total and irreversible M-current suppression, while the inhibition was attenuated in cells dialyzed with GDP-beta-S. Inhibition occurred in cells pretreated with pertussis toxin, while this treatment prevented the modulation of Ca2+ currents by muscarine. The rate and extent of inhibition increased with the level of intracellular Ca2+ from 0 to 70 nM. These results indicate that a pertussis toxin insensitive G-protein mediates the action of bradykinin on the M-current, and some steps in the second messenger cascade are modulated by Ca2+.

Nuclear versus perinuclear and cytoplasmic calcium in osteoclasts

We measured fluorescence from the calcium indicator Fluo-3 in multinucleated osteoclasts. In the initial state, each nucleus is surrounded by a ring of bright fluorescence. Following activation of purinergic receptors by 100 microM ATP there is a pulse of cellular fluorescence increase, and nuclear fluorescence intensity becomes greater than that of the cytoplasm. This is followed by a period during which the fluorescence of the cell decreases below that of the initial state. During the pulsed increase following purinergic receptor activation, the perinuclear fluorescence intensity does not increase as much as that in the nuclear centers and, following this pulse, the perinuclear fluorescence intensity decreases more than that in the nuclear centers, relative to the initial state. Measurements in which Mn2+ was introduced into the cell show that the number of Fluo-3 molecules per unit horizontal area in the nuclear centers is slightly greater than that in the perinuclear regions, and more than twice that in the surrounding cytoplasm. These results show that there is a much higher free calcium concentration in the perinuclear regions than in the nuclear centers in the initial state, with a release of free calcium from the perinuclear regions following activation of the purinergic receptors. These data also provide evidence that the free calcium concentration in the nuclear centers is lower than in the cytoplasm in the initial state.

Calcium-dependent after-hyperpolarization in dissociated bullfrog sympathetic neurons

Whole-cell recordings were made from dissociated bullfrog sympathetic neurons. Tetraethylammonium (30 mM) and apamin (100 nM) were added to the superfusate to eliminate the known calcium-activated potassium currents termed Ic and IAHP. Under these conditions, the action potential carried by calcium ions was followed by a prolonged (10-60 s) after-hyperpolarization. A current component (IAC) underlying the after-hyperpolarization was eliminated by barium (2 mM) and showed voltage-dependence identical to that of a M-type potassium current. I concluded that the after-hyperpolarization is caused not only by IAHP but also by the calcium-dependent potentiation of M-current.

Calcium-dependent potentiation of M-current in bullfrog sympathetic neurons

Whole-cell voltage-clamp recordings were made from cultured bullfrog sympathetic neurons to measure the steady-state activation curve of M-type potassium current. When measured with a calcium-deficient (10 nM) pipette solution M-conductance was 4.8 nS at -35 mV having the 50%-activation voltage at-20 mV. Respective values were 17.2 nS at -35 mV with the 50%-activation voltage at -42 mV when measured with a calcium-rich (1 microM) solution, indicating the hyperpolarizing displacement of the activation curve with high internal calcium. It is suggested that intracellular calcium ions can modulate kinetics of M-current which thereby regulate the number of M-channels being open at given membrane potentials.

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.

Postsynaptic levels of [Ca2+]i needed to trigger LTD and LTP

Long-term potentiation (LTP) and long-term depression (LTD) in CA1 pyramidal neurons are both triggered by a postsynaptic rise in intracellular Ca2+ concentration ([Ca2+]i). We used photolysis of postsynaptic caged Ca2+ compounds to search for differential thresholds for activation of these processes. Long-lasting potentiation (LLP) resembling LTP, and long-lasting depression (LLD) resembling LTD, were evoked by [Ca2+]i elevations of comparable magnitude and duration in different cells. No distinctions in threshold for these processes were detectable. LLP was occluded by tetanically induced LTP and blocked by calmodulin inhibition, and LLD was occluded by electrically induced LTD and blocked by phosphatase inhibition.

A non-inactivating K+ current sensitive to muscarinic receptor activation in rat cultured cerebellar granule neurons

1. Whole-cell recordings were made from cultured cerebellar granule neurons using perforated patch clamp techniques. The primary cultures were prepared using 6- to 9-day-old Sprague-Dawley rats. 2. Neurons in culture for less than 48 h possessed resting membrane potentials of -29 mV. However, neurons in culture for 7 days had much more hyperpolarized resting membrane potentials (-89 mV). Over the same period, these neurons developed an additional component of outward current. 3. This non-inactivating current was activated by depolarization, exhibited outward rectification and reversed close to the potassium equilibrium potential. The kinetics of activation and deactivation were very rapid. 4. Muscarine ((+)-muscarine chloride) reversibly inhibited the current with an EC50 of 0.17 microM. The inhibition by muscarine was unaffected by pre-incubation for 17-20 h with 120 micrograms ml-1 pertussis toxin. 5. The current and its inhibition by muscarine were unaffected by 100 microM Cd2+. In Ca(2+)-free conditions, the current was significantly larger than in 0.5 mM Ca2+, but inhibition by 10 microM muscarine was significantly reduced. 6. The standing outward current was not obviously affected by 50 microM 5-HT, 50 microM noradrenaline, 50 microM 2-chloroadenosine or 5 mM tetraethylammonium. It was reduced by 10 microM La3+, 10 microM Zn2+ and 1 mM Ba2+. 7. Muscarinic agonists increased the input resistance of neurons and shifted the zero current level in the depolarized direction when voltage clamped. This enhanced excitability was evident under current clamp, where 10 microM muscarine depolarized granule neurons such that action potentials became evident.

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+].

Roles of arachidonic acid, lipoxygenases and phosphatases in calcium-dependent modulation of M-current in bullfrog sympathetic neurons

1. M-current (IM) is regulated by intracellular free Ca2+ ([Ca2+]i). Suppression and overrecovery of IM induced by muscarine and luteinizing-hormone releasing hormone (LHRH) are also regulated by [Ca2+]i. The role of the arachidonic acid (AA) pathway in the Ca(2+)-dependent modulation of IM was investigated using whole-cell voltage clamp and intracellular perfusion in dissociated bullfrog sympathetic B neurons. 2. Quinacrine (10-20 microM) and 4-bromophenacyl bromide (4-BPB; 4-10 microM), the inhibitors of phospholipase A2, blocked the enhancement of IM evoked by raising [Ca2+]i. 3. AA (6-120 microM) increased IM by about 50% of the control current in a Ca(2+)-dependent manner. 4. Enhancements of IM by Ca2+ and AA were blocked by the lipoxygenase (LO) inhibitors nordihydroguaiaretic acid (NDGA; 1-5 microM) and 5,8,11-eicosatrynoic acid (ETI; 10 microM). The cyclo-oxygenase inhibitor indomethacin (10 microM) had no effect. 5. Enhancement of IM by Ca2+ was abolished by the selective 12-LO inhibitors baicalein (1-2 microM) and 15(S)-hydroxy-5-cis-8-cis-11-cis-13-trans-eicosatetraenoic acid (15-HETE; 6.5 microM). A 12-LO product, 2(S)-hydroxy-5-cis-8-cis-10-trans-14-cis- eicosatetraenoic acid (12-HETE; 13-20 microM), increased IM without Ca2+ requirement. 6. Enhancement of IM by Ca2+ was not affected by the selective 5-LO inhibitors AA-861 (10 microM), 5,6-dehydroarachidonic acid (5,6-DAA, 10 microM) and L-651,896 (10 microM). The 5-LO metabolites leukotriene C4 (1.5-8 microM) and leukotriene B4 (1.5-5 microM) showed no obvious effect on IM. 7. NDGA alone inhibited IM with an IC50 of 0.73 microM at 120 nM Cai(2+). 8. NDGA did not affect suppression of IM by muscarine or LHRH; however, overrecovery of IM upon removing these agonists was totally eliminated by 1 microM NDGA. 9. Inhibitors of phosphatases, calyculin A (0.1 microM) and okadaic acid (1 microM), completely abolished overrecovery of IM. Calyculin A also blocked the Ca(2+)-induced IM enhancement. 10. It is suggested that Ca2+ enhances IM by stimulating the AA metabolic pathway. Dephosphorylation probably upregulates IM. Overrecovery of IM is probably a result of stimulation of the LO pathway and phosphatases by increased [Ca2+]i.

Losartan-sensitive AII receptors linked to depolarization-dependent cortisol secretion through a novel signaling pathway

In bovine adrenal zona fasciculata (AZF) cells, angiotensin II (AII) may stimulate depolarization-dependent Ca2+ entry and cortisol secretion through inhibition of a novel potassium channel (IAC), which appears to set the resting potential of these cells. Aspects of the signaling pathway, which couples AII receptors to membrane depolarization and secretion, were characterized in patch clamp and membrane potential recordings and in secretion studies. AII-mediated inhibition of IAC, membrane depolarization, and cortisol secretion were all blocked by the AII type I (AT1) receptor antagonist losartan. These responses were unaffected by the AT2 antagonist PD123319. Inhibition of IAC by AII was prevented by intracellular application of guanosine 5'-O-2-(thio)-diphosphate but was not affected by pre-incubation of cells with pertussis toxin. Although mediated through an AT1 receptor, several lines of evidence indicated that AII inhibition of IAC occurred through an unusual phospholipase C (PLC)-independent pathway. Acetylcholine, which activates PLC in AZF cells, did not inhibit IAC. Neither the PLC antagonist neomycin nor PLC-generated second messengers prevented IAC expression or mimicked the inhibition of this current by AII. IAC expression and inhibition by AII were insensitive to variations in intracellular or extracellular Ca2+ concentration. AII-mediated inhibition of IAC was markedly reduced by the non-hydrolyzable ATP analog adenosine 5'-(beta, gamma-imino)triphosphate and by the non-selective protein kinase inhibitor staurosporine. The protein phosphatase antagonist okadaic acid reversibly inhibited IAC in whole cell recordings. These findings indicate that AII-stimulated effects on IAC current, membrane voltage, and cortisol secretion are linked through a common AT1 receptor. Inhibition of IAC in AZF cells appears to occur through a novel signaling pathway, which may include a losartan-sensitive AT1 receptor coupled through a pertussis-insensitive G protein to a staurosporine-sensitive protein kinase. Apparently, the mechanism linking AT1 receptors to IAC inhibition and Ca2+ influx in adrenocortical cells is separate from that involving inositol trisphosphate-stimulated Ca2+ release from intracellular stores. AII-stimulated cortisol secretion may occur through distinct parallel signaling pathways.

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.

Increase in [Ca2+]i by CCh in adult rat sympathetic neurons are not dependent on intracellular Ca2+ pools

We have examined the effects of the muscarinic agonists, carbachol (CCh) and oxotremorine (Oxo), on the intracellular free Ca2+ concentration ([Ca2+]i) in acutely dissociated sympathetic neurons from adult rats using fura 2-based microfluorometry. The drugs increased [Ca2+]i by 86 +/- 7 and 38 +/- 10 nM for CCh and Oxo, respectively (both 10 microM). Basal [Ca2+]i was 52 +/- 3 nM. Depletion of the caffeine-sensitive Ca2+ store or blockade of the Ca(2+)-adenosinetriphosphatase with thapsigargin did not alter the effect of either agonist on the rise in [Ca2+]i. On the other hand, the omission of Ca2+ from the perfusion solution or the use of TA-3090, a Ca2+ channel antagonist, blocked the effects of CCh and Oxo. In whole cell current-clamp recordings, the muscarinic agonists elicited a depolarization and action potential firing, which probably explained the rise in [Ca2+]i observed with microfluorimetric recording. In addition to their direct effects on the [Ca2+]i, muscarinic agonists also reduced the rise in [Ca2+]i induced by a nicotinic agonist. This inhibitory effect, observed in 68% of cells that responded to the nicotinic agonist, was blocked by atropine and pertussis toxin, whereas the muscarinic agonist-induced increase in [Ca2+]i was blocked by atropine but was pertussis toxin insensitive. These results suggest that at least two muscarinic receptors are present on sympathetic neurons and that they mediate opposite effect on the fluctuation of [Ca2+]i.

Effects of muscarine on K(+)-channel currents in the C-cells of bullfrog sympathetic ganglion

The effects of muscarine on small, putative C-cells and large, putative B-cells dissociated from bullfrog paravertebral sympathetic ganglia were studied by whole cell and single channel recording techniques. The dominant action of muscarine was to activate an inwardly-rectifying K+ current (IK(G)) in C-cells and to suppress M-current (IM) in B-cells. However, both IM and IK(G) were affected by muscarine in 5 out of 78 putative C-cells and in 8 others only IM was affected. By contrast, IK(G) was only activated in 1 out of 105 B-cells. This predicts that the muscarinic slow IPSP, which can be evoked by preganglionic stimulation, occurs exclusively in C-cells. 6% of these cells could, however, generate a muscarinic slow EPSP in addition to a slow IPSP and 10% could generate a slow EPSP without a slow IPSP. The rectification associated with IK(G) was neither a direct consequence of the direction of movement of K+ ions nor a simple consequence of channel block by intracellular Mg2+ or Na+ ions. The fit of the activation curve by a Boltzmann equation suggests that the conductance underlying IK(G) is controlled by a voltage-dependent gating charge (valency approximately -2). Muscarine activated no new channels in outside-out or cell-attached patches but increased the opening probability of two types of K+ channels (unitary conductances approximately 20 pS and approximately 55 pS). The possible role of these channels in the generation of IK(G) is discussed.

Changes in potassium channel activity following axotomy of B-cells in bullfrog sympathetic ganglion

1. Whole-cell and microelectrode voltage-clamp techniques were used to investigate the changes in ionic currents and action potential shape that follow axotomy of bullfrog paravertebral sympathetic ganglion B-cells. 2. Axotomy increased M-conductance (gM; muscarine-sensitive, voltage- and time-dependent K+ conductance) by 35% at -30 mV and slowed its deactivation kinetics. 3. The delayed rectifier K+ current (IK; at +50 mV) was reduced in axotomized neurones to 61% of control without any change in activation or deactivation kinetics. Steady-state intracellular Ca2+ levels and leak conductance were unchanged. 4. The fast, voltage-sensitive, Ca(2+)-activated K+ current (IC), evoked from -40 mV, was decreased to about 71% of control (at +30 mV) in axotomized neurones, whereas that evoked from -80 mV was largely unaffected. IC kinetics were also similar in control and axotomized neurones. This suggests that IC channels are not changed after axotomy. 5. In axotomized neurones, commands to +10 from -40 mV had to be extended by 16 ms to evoke voltage-insensitive Ca(2+)-dependent K+ current (IAHP) responses that were similar in magnitude to those observed in control cells. 6. The previously documented, axotomy-induced decrease in Ca2+ current (ICa) due to increased resting inactivation can account for the reduction in IC and IAHP and for the change in the shape of the action potential.