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

Cutaneous C-polymodal fibers lacking TRPV1 are sensitized to heat following inflammation, but fail to drive heat hyperalgesia in the absence of TPV1 containing C-heat fibers

BACKGROUND

Previous studies have shown that the TRPV1 ion channel plays a critical role in the development of heat hyperalgesia after inflammation, as inflamed TRPV1-/- mice develop mechanical allodynia but fail to develop thermal hyperalgesia. In order to further investigate the role of TRPV1, we have used an ex vivo skin/nerve/DRG preparation to examine the effects of CFA-induced-inflammation on the response properties of TRPV1-positive and TRPV1-negative cutaneous nociceptors.

RESULTS

In wildtype mice we found that polymodal C-fibers (CPMs) lacking TRPV1 were sensitized to heat within a day after CFA injection. This sensitization included both a drop in average heat threshold and an increase in firing rate to a heat ramp applied to the skin. No changes were observed in the mechanical response properties of these cells. Conversely, TRPV1-positive mechanically insensitive, heat sensitive fibers (CHs) were not sensitized following inflammation. However, results suggested that some of these fibers may have gained mechanical sensitivity and that some previous silent fibers gained heat sensitivity. In mice lacking TRPV1, inflammation only decreased heat threshold of CPMs but did not sensitize their responses to the heat ramp. No CH-fibers could be identified in naïve nor inflamed TRPV1-/- mice.

CONCLUSIONS

Results obtained here suggest that increased heat sensitivity in TRPV1-negative CPM fibers alone following inflammation is insufficient for the induction of heat hyperalgesia. On the other hand, TRPV1-positive CH fibers appear to play an essential role in this process that may include both afferent and efferent functions.

Affinity for phosphatidylinositol 4,5-bisphosphate determines muscarinic agonist sensitivity of Kv7 K+ channels

Kv7 K⁺-channel subunits differ in their apparent affinity for PIP₂ and are differentially expressed in nerve, muscle, and epithelia in accord with their physiological roles in those tissues. To investigate how PIP₂ affinity affects the response to physiological stimuli such as receptor stimulation, we exposed homomeric and heteromeric Kv7.2, 7.3, and 7.4 channels to a range of concentrations of the muscarinic receptor agonist oxotremorine-M (oxo-M) in a heterologous expression system. Activation of M₁ receptors by oxo-M leads to PIP₂ depletion through G_q and phospholipase C (PLC). Chinese hamster ovary cells were transiently transfected with Kv7 subunits and M₁ receptors and studied under perforated-patch voltage clamp. For Kv7.2/7.3 heteromers, the EC₅₀ for current suppression was 0.44 ± 0.08 µM, and the maximal inhibition (Inhib_max) was 74 ± 3% (n = 5–7). When tonic PIP₂ abundance was increased by overexpression of PIP 5-kinase, the EC₅₀ was shifted threefold to the right (1.2 ± 0.1 µM), but without a significant change in Inhib_max (73 ± 4%, n = 5). To investigate the muscarinic sensitivity of Kv7.3 homomers, we used the A315T pore mutant (Kv7.3^T) that increases whole-cell currents by 30-fold without any change in apparent PIP₂ affinity. Kv7.3^T currents had a slightly right-shifted EC₅₀ as compared with Kv7.2/7.3 heteromers (1.0 ± 0.8 µM) and a strongly reduced Inhib_max (39 ± 3%). In contrast, the dose–response curve of homomeric Kv7.4 channels was shifted considerably to the left (66 ± 8 nM), and Inhib_max was slightly increased (81 ± 6%, n = 3–4). We then studied several Kv7.2 mutants with altered apparent affinities for PIP₂ by coexpressing them with Kv7.3^T subunits to boost current amplitudes. For the lower affinity (Kv7.2 (R463Q)/Kv7.3^T) or higher affinity (Kv7.2 (R463E)/Kv7.3^T) channels, the EC₅₀ and Inhib_max were similar to Kv7.4 or Kv7.3^T homomers (0.12 ± 0.08 µM and 79 ± 6% [n = 3–4] and 0.58 ± 0.07 µM and 27 ± 3% [n = 3–4], respectively). The very low-affinity Kv7.2 (R452E, R459E, and R461E) triple mutant was also coexpressed with Kv7.3^T. The resulting heteromer displayed a very low EC₅₀ for inhibition (32 ± 8 nM) and a slightly increased Inhib_max (83 ± 3%, n = 3–4). We then constructed a cellular model that incorporates PLC activation by oxo-M, PIP₂ hydrolysis, PIP₂ binding to Kv7-channel subunits, and K⁺ current through Kv7 tetramers. We were able to fully reproduce our data and extract a consistent set of PIP₂ affinities.

Phospholipase C-mediated inhibition of the M-potassium current by muscarinic-receptor activation in the vestibular primary-afferent neurons of the rat

The activation of the efferent vestibular system modifies the basal discharge and the dynamic response of primary-afferent neurons to head motion and gravitational stimuli. The efferent input to afferent neurons is mediated primarily by cholinergic synapses that activate both muscarinic and nicotinic receptors. Previously we had shown that the muscarinic-acetylcholine-receptor (mAChR) activation modulates the low-voltage-activated M-type potassium current (I(K,M)) in the vestibular-afferent neurons. In this work we studied the second-messenger system mediating the inhibition of I(K,M) after mAChR activation. For this, voltage and current-clamp recordings were obtained in the cultured vestibular-afferent neurons of the rat. The I(K,M) was measured during its deactivation. Response to current-pulse injection was also studied. The use of the mAChR agonist oxotremorine-M significantly reduced the amplitude of the I(K,M) and modified the discharge response to current pulses from single spike to multiple spiking, reducing the adaptation of the electrical discharge. The intracellular perfusion of the phospholipase C (PLC) inhibitor U73122 significantly attenuated the inhibitory action of the mAChR receptor agonist oxotremorine-M. Its inactive analog U73343 produced no significant action. The use of the phosphatidylinositol 4,5 bis-phosphate (PIP(2)) scavenger poly-l-lysine also led to a significant reduction of the I(K,M). Our results show that the mAChR mediated activation of PLC and subsequent PIP(2) depletion (caused by its hydrolysis), modulates the I(K,M) in the vestibular-afferent neurons, modifying their discharge response dynamics to current-pulse injection.

Signaling pathways from cannabinoid receptor-1 activation to inhibition of N-methyl-D-aspartic acid mediated calcium influx and neurotoxicity in dorsal root ganglion neurons

Although the activation of cannabinoid receptor-1 (CB1) receptors by cannabinoids is known to inhibit neuronal hyperexcitability and reduce excitotoxic cell death, the mechanistic links between these two actions remain elusive. We tested the hypothesis that activation of CB1 receptors inhibits N-methyl-d-aspartic acid (NMDA)-mediated calcium influx and cell death via the inositol triphosphate (IP(3)) signaling pathway in both primary dorsal root ganglia neurons and a cultured neuronal cell line (F-11 cells). These cells were pretreated with the cannabinoid agonist (R)-(+)-[2,3-dihydro-5-methyl-3-(4-morpholinylmethyl)pyrrolo[1,2,3-de)-1,4-benzoxazin-6-yl]-1-napthalenylmethanone (R-(+)-WIN 55,212-2; WIN) before exposure to NMDA. Concentrations of cytosolic calcium were measured with the ratiometric calcium indicator, Fura-2, and cell death was determined by a cell viability test. WIN dose-dependently attenuated both the calcium influx and cell death induced by NMDA. These effects were blocked by selective cannabinoid CB1 receptor antagonists N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (SR141716A) or N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251), but not CB2 receptor antagonist N-[(1S)-endo-1,3,3,-trimethylbicyclo[2.2.1]heptan-2-yl]-5-(4-chloro-3-methylphenyl)-1-(4-methyl-benzyl)-pyrazole-3-carboxamide (SR144528). It is interesting to note that a transient Ca(2+) signal was observed after the acute application of WIN. This Ca(2+) increase was blocked by a CB1 receptor antagonist AM251, IP(3) receptor antagonist 2- aminoethyl diphenylborinate, or by depleting intracellular Ca(2+) stores with the endoplasmic reticulum Ca(2+) pump inhibitor thapsigargin. Removal of extracellular Ca(2+), on the other hand, had no effect on the CB1 receptor-induced Ca(2+) increase. These data suggest that WIN triggers a cascade of events: it activates the CB1 receptor and the IP(3) signaling pathway, stimulates the release of Ca(2+) from intracellular stores, raises the cytosolic Ca(2+) levels, and inhibits the NMDA-mediated Ca(2+) influx and cell death through a process that remains to be determined.

Differential fading of inhibitory and excitatory B2 bradykinin receptor responses in rat sympathetic neurons: a role for protein kinase C

Through inhibitory and excitatory effects on sympathetic neurons, B(2) bradykinin receptors contribute to protective and noxious cardiovascular mechanisms. Presynaptic inhibition of sympathetic transmitter release involves an inhibition of Ca(V)2 channels, neuronal excitation an inhibition of K(V)7 channels. To investigate which of these mechanisms prevail over time, the respective currents were determined. The inhibition of Ca(2+) currents by bradykinin reached a maximum of 50%, started to fade within the first minute, and became attenuated significantly after > or = 4 min. The inhibition of K(+) currents reached a maximum of 85%, started to fade after > 3 min, and became attenuated significantly after > or = 7 min. Blocking Ca(2+)-independent protein kinase C (PKC) enhanced the inhibition of Ca(2+) currents by bradykinin and delayed its fading, left the inhibition of K(+) currents and its fading unaltered, and enhanced the reduction of noradrenaline release and slowed its fading. Conversely, direct activation of PKC abolished the inhibition of noradrenaline release and largely attenuated the inhibition of Ca(2+) currents. These results show that the inhibitory effects of bradykinin in sympathetic neurons are outweighed over time by its excitatory actions because of more rapid, PKC-dependent fading of the inhibitory response.

Enhancing m currents: a way out for neuropathic pain?

Almost three decades ago, the M current was identified and characterized in frog sympathetic neurons (Brown and Adams, 1980). The years following this discovery have seen a huge progress in the understanding of the function and the pharmacology of this current as well as on the structure of the underlying ion channels. Therapies for a number of syndromes involving abnormal levels of excitability in neurons are benefiting from research on M currents. At present, the potential of M current openers as analgesics for neuropathic pain is under discussion. Here we offer a critical view of existing data on the involvement of M currents in pain processing. We believe that enhancement of M currents at the site of injury may become a powerful strategy to alleviate pain in some peripheral neuropathies.

Abnormal bradykinin signalling in fibroblasts deficient in the PIP(2) 5-phosphatase, ocrl1

The oculocerebrorenal syndrome of Lowe (Lowe syndrome) is an X-linked disorder of phosphatidylinositol metabolism characterized by congenital cataracts, renal proximal tubulopathy and neurological deficits. The disorder is due to the deficiency of the phosphatidylinositol 4,5-bisphosphate (PIP(2)) 5-phosphatase, ocrl1. PIP(2) is critical for numerous cellular processes, including cell signalling, actin reorganization and protein trafficking, and is chronically elevated in patients with Lowe syndrome. The elevation of PIP(2) cells of patients with Lowe syndrome provides the unique opportunity to investigate the roles of this phospholipid in fundamental cellular processes. We previously demonstrated that ocrl1 deficiency causes alterations in the actin cytoskeleton. Since actin remodelling is strongly activated by [Ca(+2)], which increases in response to IP(3) production, we hypothesized that altered calcium signalling might contribute to the observed abnormalities in actin organization. Here we report a specific increase in bradykinin-induced Ca(+2) mobilization in Lowe fibroblasts. We show that the abnormal bradykinin signalling occurs in spite of normal total cellular receptor content. These data point to a novel role for ocrl1 in agonist-induced calcium release.

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.

The therapeutic potential of neuronal K V 7 (KCNQ) channel modulators: an update

BACKGROUND

Neuronal KCNQ channels (K(V)7.2-5) represent attractive targets for the development of therapeutics for chronic and neuropathic pain, migraine, epilepsy and other neuronal hyperexcitability disorders, although there has been only modest progress in translating this potential into useful therapeutics.

OBJECTIVE

Compelling evidence of the importance of K(V)7 channels as neuronal regulatory elements, readily amenable to pharmacological modulation, has sustained widespread interest in these channels as drug targets. This review will update readers on key aspects of the characterization of these important ion channel targets, and will discuss possible current barriers to their exploitation for CNS therapeutics.

METHODS

This article is based on a review of recent literature, with a focus on data pertaining to the roles of these channels in neurophysiology. In addition, I review some of the regulatory elements that influence the channels and how these may relate to channel pharmacology, and present a review of recent advances in neuronal K(V)7 channel pharmacology.

CONCLUSIONS

These channels continue to be valid and approachable targets for CNS therapeutics. However, we may need to understand more about the roles of neuronal K(V)7 channels during the development of disease states, as well as to pay more attention to a detailed analysis of the molecular pharmacology of the different channel subfamily members and the modes of interaction of individual modulators, in order to successfully target these channels for therapeutic development.

NGF inhibits M/KCNQ currents and selectively alters neuronal excitability in subsets of sympathetic neurons depending on their M/KCNQ current background

M/KCNQ currents play a critical role in the determination of neuronal excitability. Many neurotransmitters and peptides modulate M/KCNQ current and neuronal excitability through their G protein-coupled receptors. Nerve growth factor (NGF) activates its receptor, a member of receptor tyrosine kinase (RTK) superfamily, and crucially modulates neuronal cell survival, proliferation, and differentiation. In this study, we studied the effect of NGF on the neuronal (rat superior cervical ganglion, SCG) M/KCNQ currents and excitability. As reported before, subpopulation SCG neurons with distinct firing properties could be classified into tonic, phasic-1, and phasic-2 neurons. NGF inhibited M/KCNQ currents by similar proportion in all three classes of SCG neurons but increased the excitability only significantly in tonic SCG neurons. The effect of NGF on excitability correlated with a smaller M-current density in tonic neurons. The present study indicates that NGF is an M/KCNQ channel modulator and the characteristic modulation of the neuronal excitability by NGF may have important physiological implications.

Phosphatidylinositol 4,5-bisphosphate hydrolysis mediates histamine-induced KCNQ/M current inhibition

The M-type potassium channel, of which its molecular basis is constituted by KCNQ2-5 homo- or heteromultimers, plays a key role in regulating neuronal excitability and is modulated by many G protein-coupled receptors. In this study, we demonstrate that histamine inhibits KCNQ2/Q3 currents in human embryonic kidney (HEK)293 cells via phosphatidylinositol 4,5-bisphosphate (PIP(2)) hydrolysis mediated by stimulation of H(1) receptor and phospholipase C (PLC). Histamine inhibited KCNQ2/Q3 currents in HEK293 cells coexpressing H(1) receptor, and this effect was totally abolished by H(1) receptor antagonist mepyramine but not altered by H(2) receptor antagonist cimetidine. The inhibition of KCNQ currents was significantly attenuated by a PLC inhibitor U-73122 but not affected by depletion of internal Ca(2+) stores or intracellular Ca(2+) concentration ([Ca(2+)](i)) buffering via pipette dialyzing BAPTA. Moreover, histamine also concentration dependently inhibited M current in rat superior cervical ganglion (SCG) neurons by a similar mechanism. The inhibitory effect of histamine on KCNQ2/Q3 currents was entirely reversible but became irreversible when the resynthesis of PIP(2) was impaired with phosphatidylinsitol-4-kinase inhibitors. Histamine was capable of producing a reversible translocation of the PIP(2) fluorescence probe PLC(delta1)-PH-GFP from membrane to cytosol in HEK293 cells by activation of H(1) receptor and PLC. We concluded that the inhibition of KCNQ/M currents by histamine in HEK293 cells and SCG neurons is due to the consumption of membrane PIP(2) by PLC.

Calmodulin binding to M-type K+ channels assayed by TIRF/FRET in living cells

Calmodulin (CaM) binds to KCNQ2-4 channels within their carboxy termini, where it regulates channel function. The existing data have not resolved the Ca2+ dependence of the interaction between the channels and CaM. We performed glutathione S-transferase (GST)-pull-down assays between purified KCNQ2-4 carboxy termini and CaM proteins to determine the Ca2+ dependence of the interaction in vitro. The assays showed substantial Ca2+ dependence of the interaction of the channels with wild-type (WT) CaM, but not with dominant-negative (DN) CaM. To demonstrate CaM-channel interactions in individual living cells, we performed fluorescence resonance energy transfer (FRET) between ECFP-tagged KCNQ2-4 channels and EYFP-tagged CaM expressed in CHO cells, performed under total internal reflection fluorescence (TIRF) microscopy, in which excitation light only penetrates several hundred nanometres into the cell, thus isolating membrane events. FRET was assayed between the channels and either WT or DN CaM, performed under conditions of normal [Ca2+]i, low [Ca2+]i or high [Ca2+]i induced by empirically optimized bathing solutions. The FRET data suggest a strong Ca2+ dependence for the interaction between WT CaM and KCNQ2, but less so for KCNQ3 and KCNQ4. FRET between all KCNQ2-4 channels and DN CaM was robust, and not significantly Ca2+ dependent. These data show interactions between CaM and KCNQ channels in living cells, and suggest that the interactions between KCNQ2-4 channels and CaM are likely to have Ca2+-dependent and Ca2+-independent components.

Vascular KCNQ potassium channels as novel targets for the control of mesenteric artery constriction by vasopressin, based on studies in single cells, pressurized arteries, and in vivo measurements of mesenteric vascular resistance

Pressor effects of the vasoconstrictor hormone arginine vasopressin (AVP), observed when systemic AVP concentrations are less than 100 pM, are important for the physiological maintenance of blood pressure, and they are also the basis for therapeutic use of vasopressin to restore blood pressure in hypotensive patients. However, the mechanisms by which circulating AVP induces arterial constriction are unclear. We examined the novel hypothesis that KCNQ potassium channels mediate the physiological vasoconstrictor actions of AVP. Reverse transcriptase polymerase chain reaction revealed expression of KCNQ1, KCNQ4, and KCNQ5 in rat mesenteric artery smooth muscle cells (MASMCs). Whole-cell perforated patch recordings of voltage-sensitive K+ (Kv) currents in freshly isolated MASMCs revealed 1,3-dihydro-1-phenyl-3,3-bis(4-pyridinylmethyl)-2H-indol-2-one (linopirdine)- and 10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone (XE-991)-sensitive KCNQ currents that were electrophysiologically and pharmacologically distinct from other Kv currents. Suppression of KCNQ currents by AVP (100 pM) was associated with significant membrane depolarization, and it was abolished by the protein kinase C (PKC) inhibitor calphostin C (250 nM). The KCNQ channel blocker linopirdine (10 microM) inhibited KCNQ currents in MASMCs, and it induced constriction of isolated rat mesenteric arteries. The vasoconstrictor responses were not additive when combined with 30 pM AVP, and they were prevented by the L-type Ca2+ channel blocker verapamil. Ethyl-N-[2-amino-6-(4-fluorophenylmethylamino)pyridin-3-yl] carbamic acid (flupirtine) significantly enhanced KCNQ currents, and it reversed constrictor responses to 30 pM AVP. In vivo, i.v. administration of linopirdine induced a dose-dependent increase in mesenteric artery resistance and blood pressure, whereas flupirtine had the opposite effects. We conclude that physiological concentrations of AVP induce mesenteric artery constriction via PKC-dependent suppression of KCNQ currents and L-type Ca2+ channel activation in MASMCs.

Regulation of neural KCNQ channels: signalling pathways, structural motifs and functional implications

Neural M-type (KCNQ/Kv7) K(+) channels control somatic excitability, bursting and neurotransmitter release throughout the nervous system. Their activity is regulated by multiple signalling pathways. In superior cervical ganglion sympathetic neurons, muscarinic M(1), angiotensin II AT(1), bradykinin B(2) and purinergic P2Y agonists suppress M current (I(M)). Probes of PLC activity show agonists of all four receptors to induce robust PIP(2) hydrolysis. We have grouped these receptors into two related modes of action. One mode involves depletion of phosphatidylinositol 4,5-bisphosphate (PIP(2)) in the membrane, whose interaction with the channels is thought necessary for their function. The other involves IP(3)-mediated intracellular Ca(2+) signals that stimulate PIP(2) synthesis, preventing its depletion, and suppress I(M) via calmodulin. Carbon-fibre amperometry can evaluate the effect of M channel activity on release of neurotransmitter. Consistent with the dominant role of M current in control of neuronal discharge, M channel openers, or blockers, reduced or augmented the evoked release of noradrenaline neurotransmitter from superior cervical ganglion (SCG) neurons, respectively. We seek to localize the subdomains on the channels critical to their regulation by PIP(2). Based on single-channel recordings from chimeras between high-PIP(2) affinity KCNQ3 and low-PIP(2) affinity KCNQ4 channels, we focus on a 57-residue domain within the carboxy-terminus that is a possible PIP(2) binding site. Homology modelling of this domain using the published structure of IRK1 channels as a template predicts a structure very similar to an analogous region in IRK1 channels, and shows a cluster of basic residues in the KCNQ2 domain to correspond to those implicated in PIP(2) regulation of Kir channels. We discuss some important issues dealing with these topics.

Inositol triphosphate-mediated Ca2+ signals direct purinergic P2Y receptor regulation of neuronal ion channels

Purinergic P2Y receptors are one of four types of G(q/11)-coupled receptors in rat superior cervical ganglia (SCG) sympathetic neurons. In cultured SCG neurons, purinergic and bradykinin suppression of I(M) were similar in magnitude and somewhat less than that by muscarinic agonists. The effects of the P2Y receptor agonist UTP on neuronal excitability and discharge properties were studied. Under current clamp, UTP increased action potential (AP) firing in response to depolarizing current steps, depolarized the resting potential, decreased the threshold current required to fire an AP, and decreased spike-frequency adaptation. These effects were very similar to those resulting from bradykinin stimulation and not as profound as from muscarinic stimulation or full M-current blockade. We then examined the P2Y mechanism of action. Like bradykinin, but unlike muscarinic, purinergic stimulation induced rises in intracellular [Ca(2+)](i). Tests using expression of IP(3)"sponge" or IP(3) phosphatase constructs implicated IP(3) accumulation as necessary for purinergic suppression of I(M). Overexpression of wild-type or dominant-negative calmodulin (CaM) implicated Ca(2+)/CaM in the purinergic action. Both sets of results were similar to bradykinin, and opposite to muscarinic, suppression. We also examined modulation of Ca(2+) channels. As for bradykinin, purinergic stimulation did not suppress I(Ca), unless neuronal calcium sensor-1 (NCS-1) activity was blocked by a dominant-negative NCS-1 construct. Our results indicate that P2Y receptors modulate M-type channels in SCG cells via IP(3)-mediated [Ca(2+)](i) signals in concert with CaM and not by depletion of phosphatidylinositol-4, 5-biphosphate. We group purinergic P2Y and bradykinin B(2) receptors together as having a common mode of action.

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.

Feedback mechanisms in the regulation of intracellular calcium ([Ca2+]i) in the peripheral nociceptive system: role of TRPV-1 and pain related receptors

Multimodal stimuli like heat, cold, bacterial or mechanical events are able to elicit pain, which is necessary to guarantee survival. However, the control of pain is of major clinical importance. The perception and transduction of pain is differentially modulated in the peripheral and central nervous system (CNS): while peripheral structures modulate these signals, the perception of pain occurs in the CNS. In recent years major advances have been made in the understanding of the processes which are involved in pain sensation. For the peripheral pain reception, the importance of specific pain receptors of the transition receptor pore (TRP)-family (e.g. the TRPV-1 receptor) has been analyzed. These receptors/channels are localized at the cell membrane of nociceptive neurones as well as in membranes of intracellular calcium stores like the endoplasmic reticulum. While the associated channel conducts different ions, a major proportion is calcium. Therefore, this review focuses on (1) the modulations of intracellular calcium ([Ca2+]i) initiated by the activation of pain receptors and (2) the consequences of [Ca2+]i changes for the processing of pain signals at the peripheral side. The possible interference of TRPV-1 induced [Ca2+]i modulations to the function of other membrane receptors and channels, like voltage gated calcium, sodium or potassium channels, or co-expressed CB1-receptors will be discussed. The latter interactions are of specific interest since the analgetic properties of endo- and exo-cannabinoids are mediated by CB1 receptors and their activation significantly modulates the calcium induced release of pain related transmitters. Furthermore, multiple cross links between different pain modulating intracellular pathways and their dependence on [Ca2+]i modulations will be illuminated. Overall, this review will summarize new insights resulting in the understanding of the prominent influence of [Ca2+]i for processes which are involved in pain sensation.

Modulation of a delayed-rectifier K+ current by angiotensin II in rat sympathetic neurons

It is well known that angiotensin II (Angio II) mimics most of the muscarinic-mediated excitatory actions of acetylcholine on superior cervical ganglion neurons. For instance, in addition to depolarization and stimulation of norepinephrine release, muscarinic agonists and Angio II modulate the M-type K(+) current and the N-type Ca(2+) current. We recently found that muscarinic receptors modulate the delayed rectifier current I(KV) as well. Therefore a whole cell patch-clamp experiment was carried out in rat cultured sympathetic neurons to assess whether Angio II modulates I(KV). We found that Angio II increased I(KV) by about 30% with a time constant of approximately 30 s. In comparison, inhibition of M-current was faster (tau approximately 8 s) and stronger ( approximately 61%). Modulation of I(KV) was disrupted by the AT(1) receptor-antagonist losartan but not by the AT(2)-antagonist PD123319. I(KV) enhancement was reduced by the G-protein inhibitor GDP-beta-S, whereas current modulation remained unaltered after cell treatment with pertussis toxin. The peptidergic modulation of I(KV) was severely disrupted when internal ATP was replaced by its nonhydrolyzable analogue AMP-PNP. Angio II enhanced I(KV) and further reduced the stimulatory action of a muscarinic agonist on I(KV). Likewise, the muscarinc agonist enhanced I(KV) and occluded the effect of Angio II on I(KV). We have also found that the protein kinase C activator PMA enhanced I(KV), thereby mimicking and further attenuating the action of Angio II on I(KV). These results suggest that AT(1) receptors by coupling to pertussis toxin-insensitive G proteins, stimulate an ATP-dependent and PKC-mediated pathway to modulate I(KV).

PIP(2)-dependent inhibition of M-type (Kv7.2/7.3) potassium channels: direct on-line assessment of PIP(2) depletion by Gq-coupled receptors in single living neurons

The open state of M(Kv7.2/7.3) potassium channels is maintained by membrane phosphatidylinositol-4,5-bisphosphate (PI(4,5)P(2)). They can be closed on stimulating receptors that induce PI(4,5)P(2) hydrolysis. In sympathetic neurons, closure induced by stimulating M1-muscarinic acetylcholine receptors (mAChRs) has been attributed to depletion of PI(4,5)P(2), whereas closure by bradykinin B(2)-receptors (B2-BKRs) appears to result from formation of IP(3) and release of Ca(2+), implying that BKR stimulation does not deplete PI(4,5)P(2). We have used a fluorescently tagged PI(4,5)P(2)-binding construct, the C-domain of the protein tubby, mutated to increase sensitivity to PI(4,5)P(2) changes (tubby-R332H-cYFP), to provide an on-line read-out of PI(4,5)P(2) changes in single living sympathetic neurons after receptor stimulation. We find that the mAChR agonist, oxotremorine-M (oxo-M), produces a near-complete translocation of tubby-R332H-cYFP into the cytoplasm, whereas bradykinin (BK) produced about one third as much translocation. However, translocation by BK was increased to equal that produced by oxo-M when synthesis of PI(4,5)P(2) was inhibited by wortmannin. Further, wortmannin 'rescued' M-current inhibition by BK after Ca(2+)-dependent inhibition was reduced by thapsigargin. These results provide the first direct support for the view that BK accelerates PI(4,5)P(2) synthesis in these neurons, and show that the mechanism of BKR-induced inhibition can be switched from Ca(2+) dependent to PI(4,5)P(2) dependent when PI(4,5)P(2) synthesis is inhibited.

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.

Target-specific PIP(2) signalling: how might it work?

Phosphatidylinositol 4,5-bisphosphate (PIP(2))-mediated signalling is a new and rapidly developing area in the field of cellular signal transduction. With the extensive and growing list of PIP(2)-sensitive membrane proteins (many of which are ion channels and transporters) and multiple signals affecting plasma membrane PIP(2) levels, the question arises as to the cellular mechanisms that confer specificity to PIP(2)-mediated signalling. In this review we critically consider two major hypotheses for such possible mechanisms: (i) clustering of PIP(2) in membrane microdomains with restricted lateral diffusion, a hypothesis providing a mechanism for spatial segregation of PIP(2) signals and (ii) receptor-specific buffering of the global plasma membrane PIP(2) pool via Ca(2+)-mediated stimulation of PIP(2) synthesis or release, a concept allowing for receptor-specific signalling with free lateral diffusion of PIP(2). We also discuss several other technical and conceptual intricacies of PIP(2)-mediated signalling.

Requirement of subunit co-assembly and ankyrin-G for M-channel localization at the axon initial segment

The potassium channel subunits KCNQ2 and KCNQ3 are believed to underlie the M current of hippocampal neurons. The M-type potassium current plays a key role in the regulation of neuronal excitability; however, the subcellular location of the ion channels underlying this regulation has been controversial. We report here that KCNQ2 and KCNQ3 subunits are localized to the axon initial segment of pyramidal neurons of adult rat hippocampus and in cultured hippocampal neurons. We demonstrate that the localization of the KCNQ2/3 channel complex to the axon initial segment is favored by co-expression of the two channel subunits. Deletion of the ankyrin-G-binding motif in both the KCNQ2 and KCNQ3 C-terminals leads to the disappearance of the complex from the axon initial segment, albeit the channel complex remains functional and still reaches the plasma membrane. We further show that although heteromeric assembly of the channel complex favours localization to the axon initial segment, deletion of the ankyrin-G-binding motif in KCNQ2 alone does not alter the subcellular localization of KCNQ2/3 heteromers. By contrast, deletion of the ankyrin-G-binding motif in KCNQ3 significantly reduces AIS enrichment of the complex, implicating KCNQ3 as a major determinant of M channel localization to the AIS.

Inactivation as a new regulatory mechanism for neuronal Kv7 channels

Voltage-gated K(+) channels of the Kv7 (KCNQ) family have important physiological functions in both excitable and nonexcitable tissue. The family encompasses five genes encoding the channel subunits Kv7.1-5. Kv7.1 is found in epithelial and cardiac tissue. Kv7.2-5 channels are predominantly neuronal channels and are important for controlling excitability. Kv7.1 channels have been considered the only Kv7 channels to undergo inactivation upon depolarization. However, here we demonstrate that inactivation is also an intrinsic property of Kv7.4 and Kv7.5 channels, which inactivate to a larger extent than Kv7.1 channels at all potentials. We demonstrate that at least 30% of these channels are inactivated at physiologically relevant potentials. The onset of inactivation is voltage dependent and occurs on the order of seconds. Both time- and voltage-dependent recovery from inactivation was investigated for Kv7.4 channels. A time constant of 1.47 +/- 0.21 s and a voltage constant of 54.9 +/- 3.4 mV were determined. It was further demonstrated that heteromeric Kv7.3/Kv7.4 channels had inactivation properties different from homomeric Kv7.4 channels. Finally, the Kv7 channel activator BMS-204352 was in contrast to retigabine found to abolish inactivation of Kv7.4. In conclusion, this work demonstrates that inactivation is a key regulatory mechanism of Kv7.4 and Kv7.5 channels.

Neuronal two-pore-domain potassium channels and their regulation by G protein-coupled receptors

Leak potassium currents in the nervous system are often carried through two-pore-domain potassium (K2P) channels. These channels are regulated by a number of different G protein-coupled receptor (GPCR) pathways. The TASK subfamily of K2P channels are inhibited following activation of the G protein Galpha(q). The mechanism(s) that transduce this inhibition have yet to be established but there is evidence to support a role of phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis products, depletion of PIP2 itself from the membrane, or a direct action of activated Galpha(q) on TASK channels. It seems possible that more than one pathway may act in parallel to transduce inhibition. By contrast, TRESK channels are stimulated following activation of Galpha(q). This is due to stimulation of the protein phosphatase, calcineurin, which dephosphorylates TRESK channels and enhances their activity. TREK channels are the most widely regulated of the K2P channel subfamilies being inhibited following activation of Galpha(q) and Galpha(s) but enhanced following activation of Galpha(i). The multiple pathways activated and the apparent promiscuous coupling of at least some K2P channel types to different G protein regulatory pathways suggests that the excitability of neurons that express K2P channels will be profoundly sensitive to variations in GPCR activity.

Regulation of TRP channels by PIP(2)

Transient receptor potential (TRP) channels are regulated by a wide variety of physical and chemical factors. Recently, several members of the TRP channel family were reported to be regulated by phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P(2), PIP(2)). This review will summarize the current knowledge on PIP(2) regulation of TRP channels and discuss the possibility that PIP(2) is a common regulator of mammalian TRP channels.

Activation of P2Y1 nucleotide receptors induces inhibition of the M-type K+ current in rat hippocampal pyramidal neurons

We have shown previously that stimulation of heterologously expressed P2Y1 nucleotide receptors inhibits M-type K+ currents in sympathetic neurons. We now report that activation of endogenous P2Y1 receptors induces inhibition of the M-current in rat CA1/CA3 hippocampal pyramidal cells in primary neuron cultures. The P2Y1 agonist adenosine 5'-[beta-thio]diphosphate trilithium salt (ADPbetaS) inhibited M-current by up to 52% with an IC50 of 84 nM. The hydrolyzable agonist ADP (10 microM) produced 32% inhibition, whereas the metabotropic glutamate receptor 1/5 agonist DHPG [(S)-3,5-dihydroxyphenylglycine] (10 microM) inhibited M-current by 44%. The M-channel blocker XE991 [10,10-bis(4-pyridinylmethyl)-9(10H)-anthracenone dihydrochloride] produced 73% inhibition at 3 microM; neither ADPbetaS nor ADP produced additional inhibition in the presence of XE991. The effect of ADPbetaS was prevented by a specific P2Y1 antagonist, MRS 2179 (2'-deoxy-N'-methyladenosine-3',5'-bisphosphate tetra-ammonium salt) (30 microM). Inhibition of the M-current by ADPbetaS was accompanied by increased neuronal firing in response to injected current pulses. The neurons responding to ADPbetaS were judged to be pyramidal cells on the basis of (1) morphology, (2) firing characteristics, and (3) their distinctive staining for the pyramidal cell marker neurogranin. Strong immunostaining for P2Y1 receptors was shown in most cells in these cultures: 74% of the cells were positive for both P2Y1 and neurogranin, whereas 16% were only P2Y1 positive. These results show the presence of functional M-current-inhibitory P2Y1 receptors on hippocampal pyramidal neurons, as predicted from their effects when expressed in sympathetic neurons. However, the mechanism of inhibition in the two cell types seems to differ because, unlike nucleotide-mediated M-current inhibition in sympathetic neurons, that in hippocampal neurons did not appear to result from raised intracellular calcium.

An M2-like muscarinic receptor enhances a delayed rectifier K+ current in rat sympathetic neurones

BACKGROUND AND PURPOSE

Resting superior cervical ganglion (SCG) neurones are phasic cells that switch to a tonic mode of firing upon muscarinic receptor stimulation. This effect is partially due to the muscarinic inhibition of the M-current. Because delayed rectifier K+ channels are essential to sustain tonic firing in central neurones, we asked whether the delayed rectifier current IKV in SCG neurones was modulated by the muscarinic receptors expressed in these cells.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp records of M-current and IKV were done in cultured or acutely dissociated rat SCG neurones. To characterize the receptor that regulates IKV, cells were bathed with muscarinic agonists and antagonists, relatively specific for receptor subtypes.

KEY RESULTS

The muscarinic agonist oxotremorine-M (Oxo-M) enhanced IKV by approximately 46% relative to its basal value. This effect remained unaltered when M-current was suppressed by linopirdine or Ba2+. Enhancement of IKV was insensitive to the M1-antagonist pirenzepine, whereas it was inhibited (approximately 60%) by the M2/4-antagonist himbacine. Further, the relatively specific M2-agonist bethanechol was as potent as Oxo-M in enhancing IKV. The modulation of IKV was insensitive to pertussis toxin (PTX), but was severely attenuated when internal ATP was replaced by its non-hydrolysable analogue AMP-PNP.

CONCLUSIONS AND IMPLICATIONS

These results suggest that an M2-like muscarinic receptor couples to a PTX-insensitive G-protein and to an ATP-dependent pathway to enhance IKV. Modulation of IKV must be taken into consideration in order to understand more precisely how muscarinic receptors acting on different ion channels regulate sympathetic excitability.

Probing the regulation of M (Kv7) potassium channels in intact neurons with membrane-targeted peptides

M-type (Kv7) potassium channels are closed by Gq/11 G-protein-coupled receptors. Several membrane- or channel-associated molecules have been suggested to contribute to this effect, including depletion of phosphatidylinositol-4,5-bisphosphate (PIP2) and activation of Ca2+/calmodulin and protein kinase C. To facilitate further study of these pathways in intact neurons, we have devised novel membrane-targeted probes that can be applied from the outside of the neuron, by attaching a palmitoyl group to site-directed peptides ("palpeptides") (cf. Covic et al., 2002a,b). A palpeptide incorporating the 10-residue C terminus of Galphaq/11 reduced Gq/11-mediated M-current inhibition in sympathetic neurons by the muscarinic acetylcholine receptor (mAChR) agonist oxotremorine-M but not Go-mediated inhibition of the N-type Ca2+ current by norepinephrine. Instead, the latter was inhibited by the corresponding Go palpeptide. A PIP2 palpeptide, based on the putative PIP2 binding domain of the Kv7.2 channel, inhibited M current (IC50 = approximately 1.5 microm) and enhanced inhibition by oxotremorine-M. Inhibition could not be attributed to activation of mAChRs, calcium influx, or block of M channels but was antagonized by intracellular diC8-PIP2 (dioctanoyl-phosphatidylinositol-4,5-bisphosphate), suggesting that it disrupted PIP2-M channel gating. A fluorescently tagged PIP2 palpeptide was highly targeted to the plasma membrane but did not accumulate in the cytoplasm. We suggest that these palpeptides are anchored in the plasma membrane via the palmitoyl group, such that the peptide moiety can interact with target molecules on the inner face of the membrane. The G-protein-replicating palpeptides were sequence specific and probably compete with the receptor for the cognate G-protein. The PIP2 palpeptide was not sequence specific so probably interacts electrostatically with anionic PIP2 head groups.

Angiotensin II regulates neuronal excitability via phosphatidylinositol 4,5-bisphosphate-dependent modulation of Kv7 (M-type) K+ channels

Voltage-gated Kv7 (KCNQ) channels underlie important K+ currents in many different types of cells, including the neuronal M current, which is thought to be modulated by muscarinic stimulation via depletion of membrane phosphatidylinositol 4,5-bisphosphate (PIP2). We studied the role of modulation by angiotensin II (angioII) of M current in controlling discharge properties of superior cervical ganglion (SCG) sympathetic neurons and the mechanism of action of angioII on cloned Kv7 channels in a heterologous expression system. In SCG neurons, which endogenously express angioII AT1 receptors, application of angioII for 2 min produced an increase in neuronal excitability and a decrease in spike-frequency adaptation that partially returned to control values after 10 min of angioII exposure. The increase in excitability could be simulated in a computational model by varying only the amount of M current. Using Chinese hamster ovary (CHO) cells expressing cloned Kv7.2 + 7.3 heteromultimers and AT1 receptors studied under perforated patch clamp, angioII induced a strong suppression of the Kv7.2/7.3 current that returned to near baseline within 10 min of stimulation. The suppression was blocked by the phospholipase C inhibitor edelfosine. Under whole-cell clamp, angioII moderately suppressed the Kv7.2/7.3 current whether or not intracellular Ca2+ was clamped or Ca2+ stores depleted. Co-expression of PI(4)5-kinase in these cells sharply reduced angioII inhibition, but did not augment current amplitudes, whereas co-expression of a PIP2 5'-phosphatase sharply reduced current amplitudes, and also blunted the inhibition. The rebound of the current seen in perforated-patch recordings was blocked by the PI4-kinase inhibitor, wortmannin (50 microM), suggesting that PIP2 re-synthesis is required for current recovery. High-performance liquid chromatographic analysis of anionic phospholipids in CHO cells stably expressing AT1 receptors revealed that PIP2 and phosphatidylinositol 4-phosphate levels are to be strongly depleted after 2 min of stimulation with angioII, with a partial rebound after 10 min. The results of this study establish how angioII modulates M channels, which in turn affects the integrative properties of SCG neurons.

KCNQ1 Assembly and Function Is Blocked by Long-QT Syndrome Mutations That Disrupt Interaction With Calmodulin

Calmodulin (CaM) has been recognized as an obligate subunit for many ion channels in which its function has not been clearly established. Because channel subunits associate early during channel biosynthesis, CaM may provide a mechanism for Ca 2+ -dependent regulation of channel formation. Here we show that CaM is a constitutive component of KCNQ1 K + channels, the most commonly mutated long-QT syndrome (LQTS) locus. CaM not only acts as a regulator of channel gating, relieving inactivation in a Ca 2+ -dependent manner, but it also contributes to control of channel assembly. Formation of functional tetramers requires CaM interaction with the KCNQ1 C-terminus. This CaM-regulated process is essential: LQTS mutants that disrupt CaM interaction prevent functional assembly of channels in a dominant-negative manner. These findings offer a new mechanism for LQTS defects and provide a basis for understanding novel ways that intracellular Ca 2+ and CaM regulate ion channels.

Kinetic, pharmacological and activity-dependent separation of two Ca2+ signalling pathways mediated by type 1 metabotropic glutamate receptors in rat Purkinje neurones

Type 1 metabotropic glutamate receptors (mGluR1) in Purkinje neurones (PNs) are important for motor learning and coordination. Here, two divergent mGluR1 Ca2+-signalling pathways and the associated membrane conductances were distinguished kinetically and pharmacologically after activation by 1-ms photorelease of L-glutamate or by bursts of parallel fibre (PF) stimulation. A new, mGluR1-mediated transient K+ conductance was seen prior to the slow EPSC (sEPSC). It was seen only in PNs previously allowed to fire spontaneously or held at depolarized potentials for several seconds and was slowly inhibited by agatoxin IVA, which blocks P/Q-type Ca2+ channels. It peaked in 148 ms, had well-defined kinetics and, unlike the sEPSC, was abolished by the phospholipase C (PLC) inhibitor U73122. It was blocked by the BK Ca2+-activated K+ channel blocker iberiotoxin and unaffected by apamin, indicating selective activation of BK channels by PLC-dependent store-released Ca2+. The K+ conductance and underlying transient Ca2+ release showed a highly reproducible delay of 99.5 ms following PF burst stimulation, with a precision of 1-2 ms in repeated responses of the same PN, and a subsequent fast rise and fall of Ca2+ concentration. Analysis of Ca2+ signals showed that activation of the K+ conductance by Ca2+ release occurred in small dendrites and subresolution structures, most probably spines. The results show that PF burst stimulation activates two pathways of mGluR1 signalling in PNs. First, transient, PLC-dependent Ca2+ release from stores with precisely reproducible timing and second, slower Ca2+ influx in the cation-permeable sEPSC channel. The priming by prior Ca2+ influx in P/Q-type Ca2+ channels may determine the path of mGluR1 signalling. The precise timing of PLC-mediated store release may be important for interactions of PF mGluR1 signalling with other inputs to the PN.

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.

Potentiation of nicotinic currents by bradykinin in the paratracheal ganglia neurons of rats

The effects of bradykinin on nicotine-induced responses were investigated in neurons dissociated from rat paratracheal ganglia using the nystatin-perforated patch-clamp recording technique. When bradykinin (10(-9) to 10(-8) M) was pretreated and then simultaneously applied with 10(-5) M nicotine, bradykinin potentiated the nicotine-induced currents. The potentiation was mimicked by [Hyp3]-bradykinin and inhibited by HOE-140, pertussis toxin, neomycin and U-73122, but not U-73433. These results suggest that bradykinin potentiates nicotinic currents via bradykinin B2 receptor, pertussis toxin-sensitive G-protein and phospholipase C. Since bradykinin inhibits the M-current via bradykinin B2 receptor and pertussis toxin-insensitive G-protein [Mochidome, T., Ishibashi, H., Takahama, K., 2001. Bradykinin activates airway parasympathetic ganglion neurons by inhibiting M-currents. Neuroscience 105, 785-791.], it seemed that bradykinin B2 receptor activated two distinct signal transduction pathways in the paratracheal ganglia neurons. This effect of bradykinin might cause enhanced synaptic transmission in paratracheal ganglia neurons and contribute to the aggravation of pathological conditions of the lower airway via enhanced acetylcholine release from the postganglionic nerve terminals.

Distinct enzyme combinations in AKAP signalling complexes permit functional diversity

Specificity in cell signalling can be influenced by the targeting of different enzyme combinations to substrates. The A-kinase anchoring protein AKAP79/150 is a multivalent scaffolding protein that coordinates the subcellular localization of second-messenger-regulated enzymes, such as protein kinase A, protein kinase C and protein phosphatase 2B. We developed a new strategy that combines RNA interference of the endogenous protein with a protocol that selects cells that have been rescued with AKAP79/150 forms that are unable to anchor selected enzymes. Using this approach, we show that AKAP79/150 coordinates different enzyme combinations to modulate the activity of two distinct neuronal ion channels: AMPA-type glutamate receptors and M-type potassium channels. Utilization of distinct enzyme combinations in this manner provides a means to expand the repertoire of cellular events that the same AKAP modulates.

Regulation of Kv7 (KCNQ) K+ channel open probability by phosphatidylinositol 4,5-bisphosphate

Voltage-gated Kv7 (KCNQ) channels underlie important K+ currents, including the neuronal M current, and are thought to be sensitive to membrane phosphatidylinositol 4,5-bisphosphate (PIP2) and PIP2 depletion to underlie muscarinic receptor inhibition. We studied regulation of Kv7.2-7.4 channels by PIP2 in Chinese hamster ovary (CHO) cells using single-channel and whole-cell patch clamp and biochemical analysis. Maximal open probabilities (Po) of Kv7.2-Kv7.4 homomultimers and of Kv7.2/7.3 heteromultimers were found to be strongly dependent on the [diC8-PIP2] applied to inside-out patches, with differential apparent affinities that correlate with their maximal Po in on-cell mode. Unitary conductance was not affected by PIP2. Raising tonic [PIP2] by coexpression of phosphatidylinositol (4)5-kinase increased the maximal Po of both Kv7.2 and Kv7.2/7.3 channels studied in on-cell patches and increased whole-cell Kv7.2, but not Kv7.3, current amplitudes. In cells coexpressed with muscarinic M1 receptors, bath application of muscarinic agonist reduced the maximal Po of Kv7.2/7.3 channels isolated in on-cell patches. Coexpression of a PIP2 sequestering construct moderately reduced whole-cell Kv7.2/7.3 currents, and coexpression of a construct containing a PIP2 phosphatase nearly abolished them. Finally, biochemical analysis of anionic phospholipids in CHO cells stably expressing M1 receptors shows that PIP2 and PIP are nearly depleted 1 min after muscarinic stimulation, with an unexpected rebound after 10 min. These results strongly support the direct regulation of Kv7 channels by PIP2 and its depletion as the mechanism of muscarinic suppression of M channels. Divergent apparent affinities of Kv7.2-7.4 channels for PIP2 may underlie their highly differential maximal Po observed in cell-attached patches.

The cannabinoid agonist WIN55,212-2 increases intracellular calcium via CB1 receptor coupling to Gq/11 G proteins

Central nervous system responses to cannabis are primarily mediated by CB(1) receptors, which couple preferentially to G(i/o) G proteins. Here, we used calcium photometry to monitor the effect of CB(1) activation on intracellular calcium concentration. Perfusion with 5 microM CB(1) aminoalkylindole agonist, WIN55,212-2 (WIN), increased intracellular calcium by several hundred nanomolar in human embryonic kidney 293 cells stably expressing CB(1) and in cultured hippocampal neurons. The increase was blocked by coincubation with the CB(1) antagonist, SR141716A, and was absent in nontransfected human embryonic kidney 293 cells. The calcium rise was WIN-specific, being essentially absent in cells treated with other classes of cannabinoid agonists, including Delta(9)-tetrahydrocannabinol, HU-210, CP55,940, 2-arachidonoylglycerol, methanandamide, and cannabidiol. The increase in calcium elicited by WIN was independent of G(i/o), because it was present in pertussis toxin-treated cells. Indeed, pertussis toxin pretreatment enhanced the potency and efficacy of WIN to increase intracellular calcium. The calcium increases appeared to be mediated by G(q) G proteins and phospholipase C, because they were markedly attenuated in cells expressing dominant-negative G(q) or treated with the phospholipase C inhibitors U73122 and ET-18-OCH(3) and were accompanied by an increase in inositol phosphates. The calcium increase was blocked by the sarco/endoplasmic reticulum Ca(2+) pump inhibitor thapsigargin, the inositol trisphosphate receptor inhibitor xestospongin D, and the ryanodine receptor inhibitors dantrolene and 1,1'-diheptyl-4,4'-bipyridinium dibromide, but not by removal of extracellular calcium, showing that WIN releases calcium from intracellular stores. In summary, these results suggest that WIN stabilizes CB(1) receptors in a conformation that enables G(q) signaling, thus shifting the G protein specificity of the receptor.

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.

The antidepressant imipramine inhibits M current by activating a phosphatidylinositol 4,5-bisphosphate (PIP2)-dependent pathway in rat sympathetic neurones

Little is known about the intracellular actions of imipramine (IMI) in the regulation of ion channels. We tested the action of IMI on the intracellular cascade that regulates M current (I(M)) in superior cervical ganglion neurones (SCGs). Dialysis of the cells with GDPbetaS, a G protein signaling blocker, did not disrupt the inhibition of I(M). When we incubated the cells with the phospholipase C (PLC) inhibitor U73122, it prevented the I(M) inhibition by IMI. Also, when we dialyzed the cells with an intracellular Ca2+ chelator, it did not disrupt I(M) inhibition by IMI, as occurs in the M1 cascade. When we incubated the cells with the generic kinase inhibitor wortmannin, it prevented the recovery of I(M) from the inhibition by IMI. Also, when we applied phosphatidylinositol 4,5-bisphosphate (PIP2) intracellularly, it diminished the inhibition of I(M) by IMI. Our findings suggest that PLC is the target for IMI, that recovery of I(M) needs lipid phosphorylation for PIP2 resynthesis, and that IMI inhibits I(M) by activating a PLC-dependent pathway, likely by decreasing the concentration of PIP2.

Phosphoinositide lipid second messengers: new paradigms for calcium channel modulation

Neuronal Ca2+ channels are key transducers coupling excitability to cellular function. As such, they are tightly regulated by multiple G protein-signaling pathways that finely tune their activity. In addition to fast, direct G(beta)gamma modulation of Ca2+ channels, a slower Galpha(q/11)-mediated mechanism has remained enigmatic despite intensive study. Recent work suggests that membrane phosphoinositides are crucial determinants of Ca2+ channel activity. Here, we discuss their role in Ca2+ channel modulation and the leading theories that seek to elucidate the underlying molecular details of the so-called "mysterious" G(q/11)-mediated signal.

Group I mGluR-induced epileptogenesis: distinct and overlapping roles of mGluR1 and mGluR5 and implications for antiepileptic drug design

The group I metabotropic glutamate receptor subtypes, mGluR1 and mGluR5, have both distinct and overlapping actions in epileptogenesis. Data are reviewed revealing how activation of these receptor subtypes participates in the induction and maintenance of the long-lasting epileptiform discharges elicited in the hippocampal circuit. Differences in the cellular actions and regional distributions of mGluR1 and mGluR5 provide hints regarding the potential usefulness and limitations of subtype-specific antagonists as antiepileptic agents.

Local Bradykinin Formation Is Controlled by Glycosaminoglycans

Bradykinin is a potent inflammatory mediator that induces vasodilation, vascular leakage, and pain sensations. This short-lived peptide hormone is liberated from its large precursor protein high molecular weight kininogen (HK) through the contact system cascade involving coagulation factor XII and plasma kallikrein. Although bradykinin release is well established in vitro, the factors and mechanisms controlling bradykinin generation in vivo are still incompletely understood. In this study we demonstrate that binding of HK to glycosaminoglycans (GAGs) of the heparan and chondroitin sulfate type efficiently interferes with bradykinin release in plasma and on endothelial surfaces. Proteolytic bradykinin production on endothelial cells is restored following degradation of cell surface GAG through heparinase. Alternatively, application of HK fragments D3 or light chain, which compete with uncleaved HK for cell binding, promote kininogen proteolysis and bradykinin release. Intravital microscopy revealed that HK fragments increase bradykinin-mediated mesentery microvascular leakage. Topical application of D3 or light chain enhanced bradykinin generation and edema formation in the mouse skin. Our results demonstrate that bradykinin formation is controlled by HK binding to and detachment from GAGs. Separation of the precursor from cell surfaces is a prerequisite for its efficient proteolytic processing. By this means, fragments arising from HK processing propagate bradykinin generation, revealing a novel regulatory level for the kallikrein-kinin system.

Presynaptic inhibition via a phospholipase C- and phosphatidylinositol bisphosphate-dependent regulation of neuronal Ca2+ channels

Presynaptic inhibition of transmitter release is commonly mediated by a direct interaction between G protein betagamma subunits and voltage-activated Ca2+ channels. To search for an alternative pathway, the mechanisms by which presynaptic bradykinin receptors mediate an inhibition of noradrenaline release from rat superior cervical ganglion neurons were investigated. The peptide reduced noradrenaline release triggered by K+-depolarization but not that evoked by ATP, with Ca2+ channels being blocked by Cd2+. Bradykinin also reduced Ca2+ current amplitudes measured at neuronal somata, and this effect was pertussis toxin-insensitive, voltage-independent, and developed slowly within 1 min. The inhibition of Ca2+ currents was abolished by a phospholipase C inhibitor, but it was not altered by a phospholipase A2 inhibitor, by the depletion of intracellular Ca2+ stores, or by the inactivation of protein kinase C or Rho proteins. In whole-cell recordings, the reduction of Ca2+ currents was irreversible but became reversible when 4 mM ATP or 0.2 mM dioctanoyl phosphatidylinositol-4,5-bisphosphate was included in the pipette solution. In contrast, the effect of bradykinin was entirely reversible in perforated-patch recordings but became irreversible when the resynthesis of phosphatidylinositol-4,5-bisphosphate was blocked. Thus, the inhibition of Ca2+ currents by bradykinin involved a consumption of phosphatidylinositol-4,5-bisphosphate by phospholipase C but no downstream effectors of this enzyme. The reduction of noradrenaline release by bradykinin was also abolished by the inhibition of phospholipase C or of the resynthesis of phosphatidylinositol-4,5-bisphosphate. These results show that the presynaptic inhibition was mediated by a closure of voltage-gated Ca2+ channels through depletion of membrane phosphatidylinositol bisphosphates via phospholipase C.

Phosphatidylinositol [correction] 4,5-bisphosphate signals underlie receptor-specific Gq/11-mediated modulation of N-type Ca2+ channels

Modulation of voltage-gated Ca2+ channels via G-protein-coupled receptors is a prime mechanism regulating neurotransmitter release and synaptic plasticity. Despite extensive studies, the molecular mechanism underlying Gq/11-mediated modulation remains unclear. We found cloned and native N-type Ca2+ channels to be regulated by phosphatidylinositol [correction] 4,5-bisphosphate (PIP2). In inside-out oocyte patches, PIP2 greatly attenuated or reversed the observed rundown of expressed channels. In sympathetic neurons, muscarinic M1 ACh receptor suppression of the Ca2+ current (ICa) was temporally correlated with PIP2 hydrolysis, blunted by PIP2 in whole-cell pipettes, attenuated by expression of PIP2-sequestering proteins, and became irreversible when PIP2 synthesis was blocked. We also probed mechanisms of receptor specificity. Although bradykinin also induced PIP2 hydrolysis, it did not inhibit ICa. However, bradykinin receptors became nearly as effective as M1 receptors when PIP2 synthesis, IP3 receptors, or the activity of neuronal Ca2+ sensor-1 were blocked, suggesting that bradykinin receptor-induced intracellular Ca2+ increases stimulate PIP2 synthesis, compensating for PIP2 hydrolysis. We suggest that differential use of PIP2 signals underlies specificity of Gq/11-coupled receptor actions on the channels

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.

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.

Receptor-induced depletion of phosphatidylinositol 4,5-bisphosphate inhibits inwardly rectifying K+ channels in a receptor-specific manner

Phosphatidylionsitol 4,5-bisphosphate (PIP(2)), a substrate of phospholipase C, has recently been recognized to regulate membrane-associated proteins and act as a signal molecule in phospholipase C-linked Gq-coupled receptor (GqPCR) pathways. However, it is not known whether PIP(2) depletion induced by GqPCRs can act as receptor-specific signals in native cells. We investigated this issue in cardiomyocytes where PIP(2)-dependent ion channels, G protein-gated inwardly rectifying K(+) (GIRK) and inwardly rectifying background K(+) (IRK) channels, and various GqPCRs are present. The GIRK current was recorded by using the patch-clamp technique during the application of 10 microM acetylcholine. The extent of receptor-mediated inhibition was estimated as the current decrease over 4 min while taking the GIRK current (I(GIRK)) value during a previous stimulation as the control. Each GqPCR agonist inhibited I(GIRK) with different potencies and kinetics. The extents of inhibition induced by phenylephrine, angiotensin II, endothelin-1, prostaglandin F2alpha, and bradykinin at supramaximal concentrations were (mean +/- SE) 32.1 +/- 0.6%, 21.9 +/- 1.4%, 86.4 +/- 1.6%, 63.7 +/- 4.9%, and 5.7 +/- 1.9%, respectively. GqPCR-induced inhibitions of I(GIRK) were not affected by protein kinase C inhibitor (calphostin C) but potentiated and became irreversible when the replenishment of PIP(2) was blocked by wortmannin (phosphatidylinositol kinase inhibitor). Loading the cells with PIP(2) significantly reduced endothelin-1 and prostaglandin F2alpha-induced inhibition of I(GIRK). On the contrary, GqPCR-mediated inhibitions of inwardly rectifying background K(+) currents were observed only when GqPCR agonists were applied with wortmannin, and the effects were not parallel with those on I(GIRK). These results indicate that GqPCR-induced inhibition of ion channels by means of PIP(2) depletion occurs in a receptor-specific manner.

Regulation of KCNQ2/KCNQ3 current by G protein cycling: the kinetics of receptor-mediated signaling by Gq

Receptor-mediated modulation of KCNQ channels regulates neuronal excitability. This study concerns the kinetics and mechanism of M₁ muscarinic receptor–mediated regulation of the cloned neuronal M channel, KCNQ2/KCNQ3 (Kv7.2/Kv7.3). Receptors, channels, various mutated G-protein subunits, and an optical probe for phosphatidylinositol 4,5-bisphosphate (PIP₂) were coexpressed by transfection in tsA-201 cells, and the cells were studied by whole-cell patch clamp and by confocal microscopy. Constitutively active forms of Gα_q and Gα₁₁, but not Gα₁₃, caused a loss of the plasma membrane PIP₂ and a total tonic inhibition of the KCNQ current. There were no further changes upon addition of the muscarinic agonist oxotremorine-M (oxo-M). Expression of the regulator of G-protein signaling, RGS2, blocked PIP₂ hydrolysis and current suppression by muscarinic stimulation, confirming that the G_q family of G-proteins is necessary. Dialysis with the competitive inhibitor GDPβS (1 mM) lengthened the time constant of inhibition sixfold, decreased the suppression of current, and decreased agonist sensitivity. Removal of intracellular Mg²⁺ slowed both the development and the recovery from muscarinic suppression. When combined with GDPβS, low intracellular Mg²⁺ nearly eliminated muscarinic inhibition. With nonhydrolyzable GTP analogs, current suppression developed spontaneously and muscarinic inhibition was enhanced. Such spontaneous suppression was antagonized by GDPβS or GTP or by expression of RGS2. These observations were successfully described by a kinetic model representing biochemical steps of the signaling cascade using published rate constants where available. The model supports the following sequence of events for this G_q-coupled signaling: A classical G-protein cycle, including competition for nucleotide-free G-protein by all nucleotide forms and an activation step requiring Mg²⁺, followed by G-protein–stimulated phospholipase C and hydrolysis of PIP₂, and finally PIP₂ dissociation from binding sites for inositol lipid on the channels so that KCNQ current was suppressed. Further experiments will be needed to refine some untested assumptions.