Protein kinase C is not necessary for peptide-induced suppression of M current or for desensitization of the peptide receptors

Involvement of Ca2+ in Signaling Mechanisms Mediating Muscarinic Inhibition of M Currents in Sympathetic Neurons

Acetylcholine can excite neurons by suppressing M-type (KCNQ) potassium channels. This effect is mediated by M1 muscarinic receptors coupled to the Gq protein. Although PIP2 depletion and PKC activation have been strongly suggested to contribute to muscarinic inhibition of M currents (IM), direct evidence is lacking. We investigated the mechanism involved in muscarinic inhibition of IM with Ca2+ measurement and electrophysiological studies in both neuronal (rat sympathetic neurons) and heterologous (HEK cells expressing KCNQ2/KCNQ3) preparations. We found that muscarinic inhibition of IM was not blocked either by PIP2 or by calphostin C, a PKC inhibitor. We then examined whether muscarinic inhibition of IM uses multiple signaling pathways by blocking both PIP2 depletion and PKC activation. This maneuver, however, did not block muscarinic inhibition of IM. Additionally, muscarinic inhibition of IM was not prevented either by sequestering of G-protein βγ subunits from Gα-transducin or anti-Gβγ antibody or by preventing intracellular trafficking of channel proteins with blebbistatin, a class-II myosin inhibitor. Finally, we re-examined the role of Ca2+ signals in muscarinic inhibition of IM. Ca2+ measurements showed that muscarinic stimulation increased intracellular Ca2+ and was comparable to the Ca2+ mobilizing effect of bradykinin. Accordingly, 20-mM of BAPTA significantly suppressed muscarinic inhibition of IM. In contrast, muscarinic inhibition of IM was completely insensitive to 20-mM EGTA. Taken together, these data suggest a role of Ca2+ signaling in muscarinic modulation of IM. The differential effects of EGTA and BAPTA imply that Ca2+ microdomains or spatially local Ca2+ signals contribute to inhibition of IM.

M-Current Suppression, Seizures and Lipid Metabolism: A Potential Link Between Neuronal Kv7 Channel Regulation and Dietary Therapies for Epilepsy

Neuronal Kv7 channel generates a low voltage-activated potassium current known as the M-current. The M-current can be suppressed by various neurotransmitters that activate Gq-coupled receptors. Because the M-current stabilizes membrane potential at the resting membrane potential, its suppression transiently increase neuronal excitability. However, its physiological and pathological roles in vivo is not well understood to date. This review summarizes the molecular mechanism underlying M-current suppression, and why it remained elusive for many years. I also summarize how regulation of neuronal Kv7 channel contributes to anti-seizure action of valproic acid through inhibition of palmitoylation of a Kv7 channel binding protein, and discuss about a potential link with anti-seizure mechanisms of medium chain triglyceride ketogenic diet.

Intrinsic disorder within AKAP79 fine-tunes anchored phosphatase activity toward substrates and drug sensitivity

Scaffolding the calcium/calmodulin-dependent phosphatase 2B (PP2B, calcineurin) focuses and insulates termination of local second messenger responses. Conformational flexibility in regions of intrinsic disorder within A-kinase anchoring protein 79 (AKAP79) delineates PP2B access to phosphoproteins. Structural analysis by negative-stain electron microscopy (EM) reveals an ensemble of dormant AKAP79-PP2B configurations varying in particle length from 160 to 240 Å. A short-linear interaction motif between residues 337–343 of AKAP79 is the sole PP2B-anchoring determinant sustaining these diverse topologies. Activation with Ca2⁺/calmodulin engages additional interactive surfaces and condenses these conformational variants into a uniform population with mean length 178 ± 17 Å. This includes a Leu-Lys-Ile-Pro sequence (residues 125–128 of AKAP79) that occupies a binding pocket on PP2B utilized by the immunosuppressive drug cyclosporin. Live-cell imaging with fluorescent activity-sensors infers that this region fine-tunes calcium responsiveness and drug sensitivity of the anchored phosphatase.

Signaling molecules such as the hormone epinephrine (also known as adrenaline) activate a range of responses inside cells. The responses often involve proteins being chemically modified to change how active they are, which in turn controls specific processes happening inside the cell. One type of modification involves certain enzymes adding or removing molecules known as phosphate groups from specific proteins. For example, an enzyme called PP2B (also known as calcineurin) is able to remove phosphate groups from a variety of proteins.

PP2B plays crucial roles in many different processes in animals including immune responses, nerve cell signaling and heart activity, and is the target of several medicinal drugs that suppress the immune system. Since PP2B plays so many roles in the body, these drugs often have unintended side effects. Therefore, studying how the body regulates this enzyme may help us to understand what causes these side effects.

Previous studies have shown that PP2B is activated by calcium ions, which can act as signals in many different situations inside cells. A protein called AKAP79 anchors PP2B to specific locations in the cell so that it only operates where it is needed. Some evidence suggests that calcium ions affect how AKAP79 and PP2B interact, but it is not known how this works. Nygren et al. investigated how the PP2B enzyme and AKAP79 protein interact inside human cells and in cell-free systems.

The experiments showed that short regions within the AKAP79 protein are responsible for binding to PP2B. These regions and the flexible structure of the entire AKAP79 protein work together to fine-tune how PP2B responds to calcium ions. In the presence of higher levels of calcium ions, another ‘auxiliary’ region of AKAP79 also binds to PP2B. This auxiliary region binds to a site on the enzyme where an immunosuppressive drug called cyclosporine can also bind. This suggests that AKAP79 binding to PP2B may affect the sensitivity of the PP2B enzyme to cyclosporine.

This study demonstrates that the activity of PP2B can be precisely controlled by interactions with proteins such as AKAP79. Further work on these interactions may help develop more effective drugs that cause fewer side effects in patients.

Modulation of Kv7 channels and excitability in the brain

Neuronal Kv7 channels underlie a voltage-gated non-inactivating potassium current known as the M-current. Due to its particular characteristics, Kv7 channels show pronounced control over the excitability of neurons. We will discuss various factors that have been shown to drastically alter the activity of this channel such as protein and phospholipid interactions, phosphorylation, calcium, and numerous neurotransmitters. Kv7 channels locate to key areas for the control of action potential initiation and propagation. Moreover, we will explore the dynamic surface expression of the channel modulated by neurotransmitters and neural activity. We will also focus on known principle functions of neural Kv7 channels: control of resting membrane potential and spiking threshold, setting the firing frequency, afterhyperpolarization after burst firing, theta resonance, and transient hyperexcitability from neurotransmitter-induced suppression of the M-current. Finally, we will discuss the contribution of altered Kv7 activity to pathologies such as epilepsy and cognitive deficits.

Coordinated signal integration at the M-type potassium channel upon muscarinic stimulation

Several neurotransmitters, including acetylcholine, regulate neuronal tone by suppressing a non-inactivating low-threshold voltage-gated potassium current generated by the M-channel. Agonist dependent control of the M-channel is mediated by calmodulin, activation of anchored protein kinase C (PKC), and depletion of the phospholipid messenger phosphatidylinositol 4,5-bisphosphate (PIP2). In this report, we show how this trio of second messenger responsive events acts synergistically and in a stepwise manner to suppress activity of the M-current. PKC phosphorylation of the KCNQ2 channel subunit induces dissociation of calmodulin from the M-channel complex. The calmodulin-deficient channel has a reduced affinity towards PIP2. This pathway enhances the effect of concomitant reduction of PIP2, which leads to disruption of the M-channel function. These findings clarify how a common lipid cofactor, such as PIP2, can selectively regulate ion channels.

Reactive oxygen species are second messengers of neurokinin signaling in peripheral sensory neurons

Substance P (SP) is a prominent neuromodulator, which is produced and released by peripheral damage-sensing (nociceptive) neurons; these neurons also express SP receptors. However, the mechanisms of peripheral SP signaling are poorly understood. We report a signaling pathway of SP in nociceptive neurons: Acting predominantly through NK1 receptors and G(i/o) proteins, SP stimulates increased release of reactive oxygen species from the mitochondrial electron transport chain. Reactive oxygen species, functioning as second messengers, induce oxidative modification and augment M-type potassium channels, thereby suppressing excitability. This signaling cascade requires activation of phospholipase C but is largely uncoupled from the inositol 1,4,5-trisphosphate sensitive Ca(2+) stores. In rats SP causes sensitization of TRPV1 and produces thermal hyperalgesia. However, the lack of coupling between SP signaling and inositol 1,4,5-trisphosphate sensitive Ca(2+) stores, together with the augmenting effect on M channels, renders the SP pathway ineffective to excite nociceptors acutely and produce spontaneous pain. Our study describes a mechanism for neurokinin signaling in sensory neurons and provides evidence that spontaneous pain and hyperalgesia can have distinct underlying mechanisms within a single nociceptive neuron.

ATP competitive protein kinase C inhibitors demonstrate distinct state-dependent inhibition

We previously reported that some ATP competitive protein kinase C (PKC) inhibitors are either competitive or uncompetitive inhibitors with respect to substrate peptides. In this report, we demonstrate how the interactions between PKC and inhibitors change PKC activation kinetics. A substrate competitive inhibitor, bisindolylmaleimide I, targets activated PKC and stabilizes PKC in the activated conformation. This leads to transient activation and prolonged deactivation of PKC in the presence of bisindolylmaleimide I. In contrast, an uncompetitive substrate inhibitor, bisindolylmaleimide IV, targets quiescent PKC and stabilizes PKC in the quiescent conformation, which generates slower activation and suppressed translocation upon activation of PKC.

Interaction with AKAP79 modifies the cellular pharmacology of PKC

A-kinase anchoring proteins (AKAPs) coordinate cell signaling events. AKAP79 brings together different combinations of enzyme binding partners to customize the regulation of effector proteins. In neurons, muscarinic agonists mobilize an AKAP79-anchored pool of PKC that phosphorylates the KCNQ2 subunit of the M channel. This inhibits potassium permeability to enhance neuronal excitability. Using a dual fluorescent imaging/patch-clamp technique, we visualized AKAP79-anchored PKC phosphorylation of the kinase activity reporter CKAR concurrently with electrophysiological changes in KCNQ2 channels to show that AKAP79 synchronizes both signaling events to optimize the attenuation of M currents. AKAP79 also protects PKC from certain ATP-competitive inhibitors. Related studies suggest that context-dependent protein-protein interactions alter the susceptibility of another protein kinase, PDK1, to ATP analog inhibitors. This implies that intracellular binding partners not only couple individual molecular events in a cell signaling process but can also change the pharmacological profile of certain protein kinases.

The case of the disappearing drug target

In this issue of Molecular Cell, Hoshi et al. (2010) report two examples in which small molecule inhibitors are rendered ineffective when their kinase targets are involved in protein-protein interactions, highlighting differences between in vivo and in vitro inhibition kinetics.

Specific inhibitory effect of amyloid-beta on presynaptic muscarinic receptor subtypes modulating neurotransmitter release in the rat nucleus accumbens

In this study we investigate on the effect of amyloid-beta1-40 (A beta 1-40) on the oxotremorine (OXO)-induced release of [(3)H] dopamine (DA), [(3)H]GABA and [(3)H]acetylcholine (ACh) from synaptosomes in the rat nucleus accumbens (NAc). OXO in presence of himbacine (HIMBA) was able to increase the basal release of [(3)H]GABA. The OXO-elicited [(3)H]GABA overflow was significantly antagonized by atropine (A; 94%), by the M3 antagonists DAU5884 (96%) and 4-DAMP (70%), and by A beta 1-40 (65%). Exposure of NAc synaptosomes to OXO produced a dose-dependent increase of [(3)H]DA overflow which was antagonized by A, partially inhibited by A beta 1-40 (100 nM) but unaffected by DAU5884 and 4-DAMP. The K(+)-evoked [(3)H]ACh overflow was inhibited by OXO. This effect was counteracted by the M2 antagonist AFDX-116 but not by the selective M4 antagonist mamba toxin 3 (MT3). The K(+)-evoked [(3)H]GABA overflow was also inhibited by OXO but conversely, this effect was counteracted by MT3 and not by AFDX-116. A beta 1-40 (100 nM) did not modify the inhibitory effect of OXO both on the K(+)-evoked [(3)H]ACh and [(3)H]GABA overflow. The results show that in the rat NAc, A beta 1-40 selectively inhibits the function of the muscarinic subtypes which stimulate neurotransmitter release and not those which modulate negatively the stimulated release.

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.

Does diacylglycerol regulate KCNQ channels?

Some ion channels are regulated by inositol phospholipids and by the products of cleavage by phospholipase C (PLC). KCNQ channels (Kv7) require membrane phosphatidylinositol 4,5-bisphosphate (PIP(2)) and are turned off when muscarinic receptors stimulate cleavage of PIP(2) by PLC. We test whether diacylglycerols are also important in the regulation of KCNQ2/KCNQ3 channels using electrophysiology and fluorescent translocation probes as indicators for PIP(2) and diacylglycerol in tsA cells. The cells are transfected with M(1) muscarinic receptors, channel subunits, and translocation probes. Although they cause translocation of a fluorescent probe with a diacylglycerol-binding C1 domain, exogenously applied diacylglycerol (oleoyl-acetyl-glycerol and dioctanoyl glycerol) and phorbol ester do not mimic or occlude the suppression of KCNQ current by muscarinic agonist. Blocking the metabolism of endogenous diacylglycerol by inhibiting diacylglycerol kinase with R59022 or R59949 slows the decay of diacylglycerol twofold but does not mimic or occlude muscarinic regulation and recovery of current. Blocking diacylglycerol lipase with RHC-80267 also does not occlude muscarinic modulation of current. We conclude that the diacylglycerol produced during activation of PLC, any activation of protein kinase C that it may stimulate, and downstream products of its metabolism are not essential players in the acute muscarinic modulation of KCNQ channels.

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.

Role of gonadotropin-releasing hormone II in the mammalian nervous system

Gonadotropin-releasing hormone (GnRH) is a small neuropeptide of which there are multiple structural variants. The first variant identified in mammals, GnRH I, controls the release of pituitary gonadotropins. More recently, a second isoform, GnRH II, first isolated in the bird, was identified in the mammalian brain and periphery. Although it is unlikely to be a primary regulator of gonadotropin release, GnRH II appears to have a wide array of physiological and behavioral functions. GnRH II-containing fibers are present in several nuclei known to regulate reproduction and/or feeding, and its concentration in several of these areas fluctuates in response to changes in food availability, and thus energetic status. In musk shrews, GnRH II acts as a permissive regulator of female reproductive behavior based on energy status, as well as an inhibitor of short-term food intake. In this regard, GnRH II is similar to leptin, neuropeptide Y and several other neurotransmitters that regulate both feeding and reproduction. At least two GnRH receptors are present in the mammalian brain, and increasing evidence suggests that the behavioral effects of GnRH II are mediated by receptor subtypes distinct from the type-1 GnRH receptor (which mediates GnRH I action); the most probable candidate is the type-2 GnRH receptor. GnRH II also regulates the density and/or activity of calcium and potassium channels in the nervous systems of amphibians and fish, a function that may also exist in mammalian neurons. It is likely that the highly conserved GnRH II system has been co-opted over evolutionary time to possess multiple regulatory functions in a broad range of neurobiological aspects.

Protein kinase C shifts the voltage dependence of KCNQ/M channels expressed in Xenopus oocytes

It is well established that stimulation of G(q)-coupled receptors such as the M1 muscarinic acetylcholine receptor inhibits KCNQ/M currents. While it is generally accepted that this muscarinic inhibition is mainly caused by the breakdown of PIP(2), the role of the subsequent activation of protein kinase C (PKC) is not well understood. By reconstituting M currents in Xenopus oocytes, we observed that stimulation of coexpressed M1 receptors with 10 microm oxotremorine M (oxo-M) induces a positive shift (4-30 mV, depending on which KCNQ channels are expressed) in the conductance-voltage relationship (G-V) of KCNQ channels. When we applied phorbol 12-myristate 13-acetate (PMA), a potent PKC activator, we observed a large positive shift (17.8 +/- 1.6 mV) in the G-V curve for KCNQ2, while chelerythrine, a PKC inhibitor, attenuated the shift caused by the stimulation of M1 receptors. By contrast, reducing PIP(2) had little effect on the G-V curve for KCNQ2 channels; although pretreating cells with 10 mum wortmannin for 30 min reduced KCNQ2 current amplitude by 80%, the G-V curve was shifted only slightly (5 mV). Apparently, the shift induced by muscarinic stimulation in Xenopus oocytes was mainly caused by PKC activation. When KCNQ2/3 channels were expressed in HEK 293T cells, the G-V curve seemed already to be shifted in a positive direction, even before activation of PKC, and PMA failed to shift the curve any further. That alkaline phosphatase in the patch pipette shifted the G-V curve in a negative direction suggests KCNQ2/3 channels are constitutively phosphorylated in HEK 293T cells.

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.

AKAP signalling complexes: focal points in space and time

Multiprotein signalling networks create focal points of enzyme activity that disseminate the intracellular action of many hormones and neurotransmitters. Accordingly, the spatio-temporal activation of protein kinases and phosphatases is an important factor in controlling where and when phosphorylation events occur. Anchoring proteins provide a molecular framework that orients these enzymes towards selected substrates. A-kinase anchoring proteins (AKAPs) are signal-organizing molecules that compartmentalize various enzymes that are regulated by second messengers.

Effects of muscarinic acetylcholine receptor activation on membrane currents and intracellular messengers in medium spiny neurones of the rat striatum

Acetylcholine, acting through muscarinic receptors, modulates the excitability of striatal medium spiny neurones. However, the underlying membrane conductances and intracellular signalling pathways have not been fully determined. Our aim was to characterize excitatory effects mediated by M1 muscarinic acetylcholine receptors in these neurones using whole-cell patch-clamp recordings in brain slices of postnatal rats. Under voltage-clamp, muscarine evoked an inward current associated with an increase in cell membrane resistance. The current, which reversed at -85 mV, was sensitive to the M1 receptor antagonist pirenzepine. Blocking the potassium conductance attenuated the response and the residual current was further reduced by ruthenium red (50 microm) and reversed at +15 mV. Simultaneous recordings from cholinergic interneurones and medium spiny neurones in conjunction with spike-triggered averaging revealed small unitary excitatory postsynaptic currents in four of 39 cell pairs tested. The muscarine-induced inward current was attenuated by a phospholipase C (PLC) inhibitor, U73122, but not by a protein kinase C inhibitor, chelerythrine, or by the intracellular calcium chelator 1,2-bis(2-aminophenoxy) ethane-N,N,N',N'-tetra-acetic acid, suggesting that the current was associated with PLC in a protein kinase C- and Ca2+ -independent manner. The phosphatidylinositol 4-kinase inhibitor wortmannin (10 microm) reduced the recovery of the inward current, indicating that the recovery process was dependent on the removal of diacylglycerol and/or inositol 1,4,5 triphosphate or resynthesis of phospholipid phosphatidylinositol 4,5-bisphophate. Ratiometric measurement of intracellular calcium after cell loading with fura-2 demonstrated a muscarine-induced increase in calcium signal that originated mainly from intracellular stores. Thus, the cholinergic excitatory effect in striatal medium spiny neurones, which is important in motor disorders associated with altered cholinergic transmission in the striatum such as Parkinson's disease, is mediated through M1 receptors and the PLC-dependent pathway.

Gonadotropin releasing hormone (GnRH) modulates odorant responses in the peripheral olfactory system of axolotls

Peripheral signal modulation plays an important role in sensory processing. Activity in the vertebrate olfactory epithelium may be modulated by peptides released from the terminal nerve, such as gonadotropin releasing hormone (GnRH). Here, we demonstrate that GnRH modulates odorant responses in aquatic salamanders (axolotls, Ambystoma mexicanum). We recorded electrical field potentials (electro-olfactograms, or EOGs) in response to stimulation with four different amino acid odorants, L-lysine, L-methionine, L-cysteine, and L-glutamic acid. EOG responses were recorded from the main olfactory epithelium before, during, and after application of 10 microM GnRH. This protocol was repeated for a total of three trials with 60-80 min between trials. The effect of GnRH on EOG responses was broadly similar across odorants and across trials. In general, EOG responses were reduced to 79% of the initial magnitude during application of GnRH; in some trials in which glutamic acid served as the odorant, EOG responses were enhanced during the wash period. Although the 4-min inter-stimulus interval did not lead to adaptation of EOG responses during the first trial, we frequently observed evidence of adaptation during the second and third trials. In addition, we found that lower concentrations of GnRH produced a smaller effect. These results demonstrate that GnRH can modulate odorant responses in the peripheral olfactory system.

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

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

Tachykinin peptide-induced activation and desensitization of neurokinin 1 receptors

The actions of four tachykinins on inhibition and desensitization of the M-current of bullfrog sympathetic neurons have been characterized. Radioligand binding parameters of the tachykinins were determined at a neurokinin receptor in a heterologous expression system. The correlation between binding, signaling and receptor regulation was investigated. A correlation between receptor binding and signaling was found between the peptides; however, their ability to produce desensitization was not correlated with binding and signaling. These results show that the ability of a tachykinin peptide to induce signal activation is not indicative of its ability to induce receptor regulation.

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

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

Evidence Against a Role for Protein Kinase C in the Inhibition of the Calcium-activated Potassium Current IAHP by Muscarinic Stimulants in Rat Hippocampal Neurons

The possible role of protein kinase C activation in the inhibitory action of cholinergic transmitters on the slow Ca-dependent afterhyperpolarizing current (IAHP) in hippocampal CA3 pyramidal neurons was investigated using hippocampal slice cultures. IAHP was inhibited reversibly by methacholine (100 - 600 nM) and irreversibly by the protein kinase C activator, phorbol-12,13-dibutyrate (PDBu, 10 nM to 1 microM). The inhibitory action of PDBu was antagonized by prior (15 - 60 min) exposure to staurosporin (1 microM). In contrast, the inhibitory effect of methacholine on IAHP was not reduced after up to 3 h of exposure to this compound. In addition, methacholine produced a reversible inward current at the holding potential, which was augmented by staurosporin. However, prior exposure to PDBu reduced the effect of methacholine on IAHP and occluded the methacholine-induced inward current. This effect of PDBu was also observed in the presence of staurosporin, suggesting that it might be exerted through a protein kinase C-independent pathway. Noradrenalin (2 - 5 microM) and 8-bromo cyclic adenosine 3',5'monophosphate (8-Br-cAMP, 1 mM) also produced a reversible block of IAHP. Their action was antagonized by staurosporin, probably via its effect on protein kinase A. Thus the present experiments suggest that the action of muscarinic agonists on IAHP cannot be explained by an effect on protein kinase C, but support a role for protein kinase A in mediating the action of noradrenalin.

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.

The antidepressant imipramine inhibits the M-type K+ current in rat sympathetic neurons

This study aimed to assess the effects of the antidepressant drug imipramine (IMI) on the neuronal M-type K+ current (IK(M)). We show that IMI reversibly reduces IK(M) with an IC50 of 7 microM. The V0.5 and slope factor of the steady state activation curve remained unchanged after IMI, indicating a mode of action that is voltage insensitive for blocking the M-channel. Patch pipette application of IMI elicits same inhibitory response suggesting a binding site on the M-channel accessible from both sides of the cell membrane. Accordingly, the inhibitory effect of IMI is larger by rising external pH near to the pKa of the drug. Therefore, we propose that a neutral form of IMI binds more efficiently to M-channels to exert its inhibitory action by a voltage-independent mechanism.

Differential Expression of Functionally Identified and Immunohistochemically Identified NK1 Receptors on Sympathetic Neurons

We have used multiple-labeling immunohistochemistry, intracellular dye-filling, and intracellular microelectrode recordings to characterize the distribution of tachykinin receptors and substance P boutons on subpopulations of neurons within the guinea pig celiac ganglion. Superfusion of substance P (SP, 1 μM for 1 min) depolarized 42% of tonic neurons and inhibited afterhyperpolarizations in 66% of long afterhyperpolarizing (LAH) neurons without significant desensitization. Twenty-one percent of tonic neurons and 24% of LAH neurons responded to the NK3 agonist senktide but did not respond to SP, indicating SP did not activate NK3 receptors at this concentration. All effects of SP were abolished by the selective NK1 receptor antagonist, SR140333, but not by the selective NK3 receptor antagonist, SR142801, suggesting that exogenous SP activated a receptor with NK1 pharmacology. No dye-filled LAH neuron and only 50% of tonic neurons responding to SP expressed NK1 receptor immunoreactivity (NK1-IR). All neurons responding to SP had SP immunoreactive fibers within one cell diameter, indicating good spatial matching between SP release sites and target neurons. These results indicate that SP may act via a receptor with NK1-like pharmacology that has a C terminus not recognized by antibodies to the intracellular domain of the conventional NK1 receptor. Inward currents evoked by SP acting on this NK1-like receptor or senktide acting through NK3 receptors had identical current-voltage relationships. In LAH neurons, both agonists suppressed I sAHP without reducing I AHP. Responses evoked by SP and senktide were resistant to PKC inhibitors, suggesting that the transduction mechanisms for the NK1-like receptor and the NK3 receptor may be similar.

Changes of the responses of single sympathetic ganglionic neurones to substance P following desensitization

1. The neuropeptide substance P (SP) exerts an excitatory effect on sympathetic neurones by inhibiting a time- and voltage-dependent potassium current. During prolonged application of SP, the response desensitizes. The changes in kinetics of the SP response in single neurones after desensitization have been studied in an attempt to gain some insight as to the molecular mechanism of desensitization in live, functioning neurones. 2. Desensitization to SP resulted in subsequent SP responses being smaller, but the time course was unchanged in desensitized cells compared with non-desensitized cells. 3. Experimental manipulations were performed to decrease receptor and G protein function for comparison to desensitization. Intracellular application of GDPbetaS, to decrease G protein function, led to successive responses to agonist becoming smaller and slower. When functional muscarinic receptors were decreased by extracellular application of propylbenzilylcholine mustard (PrBCM), the response to muscarine became smaller, but the time course was unchanged compared with the change in time course produced by PrBCM vehicle alone. 4. The results have also been compared with simulations from a mathematical model of drug-receptor-G protein interactions. Under a constrained set of conditions, the model predicts that decreasing the size of the G protein pool will decrease both the magnitude and the time course of the response to agonist. Decreasing receptor levels results in a more efficient decrease in the magnitude of the response but no change in the time course of the response. 5. These data provide evidence that desensitization of the response to SP in single neurones results from a decrease in functional receptors.

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.

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.

Regulation of M-like K+ current, IKx, by Ca(2+)-dependent phosphorylation in rod photoreceptors

An M-like K+ current (IKx) helps set the rod photoreceptor resting potential and accelerates the response to dim light. Recorded with ruptured-patch whole cell techniques, the amplitude of IKx diminished, and activation occurred at increasingly negative potentials as a function of time. In contrast, IKx was stable during nystatin perforated-patch recording. Stability during ruptured-patch recording could be induced by raising the intracellular Ca2+ concentration ([Ca2+]i) or by including caffeine or D-myo-inositol 1,4,5-trisphosphate in the pipette. This Ca(2+)-induced stability of IKx was blocked by inhibitors of Ca2+/calmodulin-dependent protein kinases, such as W-7, KN-62, chelerythrine, or H-7. Okadaic acid, an inhibitor of protein phosphatases, maintained IKx stability even at low [Ca2+]i. The requirement for phosphorylation was demonstrated by depleting MgATP or by providing 5'-adenylylimidophosphate, a nonhydrolyzable analog of ATP, either of which blocked the Ca(2+)-induced stability of IKx. These observations show that phosphorylation regulates IKx and that a stimulus controlling this action is [Ca2+]i. Should [Ca2+]i change during light adaptation, changes in IKx might alter the resting potential and temporal response properties of rod photoreceptors.

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.

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.

Delayed onset and slow time course of the non-M-type muscarinic current in bullfrog sympathetic neurons

The onset and time course of the muscarinic currents induced by brief applications of acetylcholine (ACh) were examined in voltage-clamped neurons of bullfrog sympathetic ganglia bathed in a solution containing d-tubocurarine. At a potential of -40 mV, the ACh-induced current (IACh) appeared within 1.2 s and rapidly increased to its peak with a half-activation time of 2.2 s. This initial current was termed the fast IACh and was blocked by 4 mM Ba2+. At a potential more negative than -60 mV, the fast IACh disappeared and the remaining IACh activated with a delay of 3.9 s and slowly increased to its peak with a half-activation time of 8.2 s. This delayed current was termed the slow IACh and is thought to be associated with inhibition of a K+ current, or IM, as well as activation of an inward current through non-M-type muscarinic cation channels. The slow IACh was not inhibited by Ba2+, but its amplitude was reduced with depolarization (the extrapolated reversal potential was +3 mV). In Na(+)-free solution, the amplitude of the slow IACh reduced, but its polarity did not reverse in the voltage region examined (-30 to -100 mV). The slow excitatory postsynaptic current was also recorded, and was shown to have a similar delay in onset and slow time course. The results demonstrate that ACh activates the non-M-type muscarinic current three times more slowly than it inhibits IM.

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.

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

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

Rapid down-regulation of substance P binding to guinea-pig pancreatic acinar cells during homologous desensitization

Binding of 125I-labelled peptides, cytoplasmic Ca2+ concentration ([Ca2+]i) and amylase release were studied in guinea-pig pancreatic acinar cells during exposure to substance P (SP), and cholecystokinin octapeptide (CCK-8). Pre-incubation of cells at 22 degrees C with 0.03 nM to 1 microM SP for 10 min or at 37 degrees C for 5 min followed by acid or neutral washes reduced subsequent binding of 125I-Bolton-Hunter reagent-labelled SP (125I-BH-SP) in a biphasic manner by up to 95%. Incubation at 4 degrees C eliminated high-affinity binding of 125I-BH-SP and concentrations of SP above 1 nM were required for inhibition of subsequent tracer binding. Pre-incubation of cells at 37 degrees C with 1 nM to 1 microM CCK-8 for 10 min followed by neutral washes reduced subsequent binding of 125I-BH-CCK-8 by up to 65%. In cell suspensions, the [Ca2+]i response to SP was gradually reduced by pre-exposure to increasing agonist concentrations from 0.2 to 20 nM. Pre-incubation with high SP concentrations for 10 min caused profound reduction of subsequent amylase responses to SP, whereas secretion was little affected in corresponding experiments with CCK-8. Down-regulation of receptor binding is not important during short exposure to CCK-8, but it is a pronounced and rapid phenomenon during SP exposure, which explains tachyphylaxis of [Ca2+]i and amylase responses.

M-current suppression by agonist and phorbol ester in bullfrog sympathetic neurons

Activation of protein kinase C (PKC) by phorbol esters is known to suppress M-current. 4-beta-Phorbol 12,13-dibutyrate (PDBu) irreversibly suppressed M-current in a concentration-dependent manner (Ki 38 nM). Inhibitors of PKC, the pseudo-substrate peptide PKCI (19-31), staurosporine and 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) antagonized PDBu-mediated suppression of M-current. Suppression of M-current by muscarine and luteinizing hormone-releasing hormone (LHRH) was unaffected by PKCI (19-31) and H7, but was antagonized by staurosporine. The balance of data suggests that suppression of M-current by agonists is probably not mediated by activation of PKC. Addition and subsequent removal of PDBu to M-current suppressed by muscarine prevented the action of PDBu, while closing M-channels by voltage or blocking by barium did not. This suggests that M-channel closure by muscarine protects those channels from the effects of PDBu. Partial suppression of M-current by low concentrations of muscarine antagonized the response to PDBu, with the magnitude of suppression equivalent to that seen with PDBu alone. It is suggested that two interconvertable populations of M-channels exist, one that is sensitive to both agonist and PDBu and another that can only be suppressed by agonist.

On the mechanism of M-current inhibition by muscarinic m1 receptors in DNA-transfected rodent neuroblastoma x glioma cells

1. Acetylcholine (ACh) produces two membrane current changes when applied to NG108-15 mouse neuroblastoma x rat glioma hybrid cells transformed (by DNA transfection) to express m1 muscarinic receptors: it activates a Ca(2+)-dependent K+ conductance, producing an outward current, and it inhibits a voltage-dependent K+ conductance (the M conductance), thus diminishing the M-type voltage-dependent K+ current (IK(M)) and producing an inward current. The present experiments were undertaken to find out how far inhibition of IK(M) might be secondary to stimulation of phospholipase C, by recording membrane currents and intracellular Ca2+ changes with indo-1 using whole-cell patch-clamp methods. 2. Bath application of 100 microM ACh reversibly inhibited IK(M) by 47.3 +/- 3.2% (n = 23). Following pressure-application of 1 mM ACh, the mean latency to inhibition was 420 ms at 35 degrees C and 1.79 s at 23 degrees C. Latencies to inhibition by Ba2+ ions were 148 ms at 35 degrees C and 92 ms at 23 degrees C. 3. The involvement of a G-protein was tested by adding 0.5 mM GTP-gamma-S or 10 mM potassium fluoride to the pipette solution. These slowly reduced IK(M), with half-times of about 30 and 20 min respectively, and rendered the effect of superimposed ACh irreversible. Effects of ACh were not significantly changed after pretreatment for 24 h with 500 ng ml-1 pertussis toxin or on adding up to 10 mM GDP-beta-S to the pipette solution. 4. The role of phospholipase C and its products was tested using neomycin (to inhibit phospholipase C), inositol 1,4,5-trisphosphate (InsP3) and inositol 1,3,4,5-tetrakisphosphate (InsP4), heparin, and phorbol dibutyrate (PDBu) and staurosporin (to activate and inhibit protein kinase C respectively). Both neomycin (1 mM external) and InsP3 (100 microM intrapipette) inhibited the ACh-induced outward current and/or intracellular Ca2+ transient but did not block ACh-induced inhibition of IK(M). Intrapipette heparin (1 mM) blocked activation of IK(Ca) and reduced Ach-induced inhibitions of IK(M), but also reduced inhibition of ICa via endogeneous m4 receptors. PDBu (with or without intrapipette ATP) and staurosporin had no significant effects.(ABSTRACT TRUNCATED AT 400 WORDS)

Suppression of the slow K+ current by cholinergic agonists in cultured chick cochlear ganglion neurones

1. Effects of cholinergic agonists on the cultured chick cochlear ganglion (CG) neurone were examined using the whole-cell patch-clamp method. 2. Acetylcholine (ACh, 0.1-100 microM) and its non-hydrolysable form, carbamylcholine (CCh, 0.1-300 microM), suppressed the outward current. The CCh-sensitive current was activated at membrane potentials more positive than -70 mV. 3. The CCh-sensitive current slowly activated after step depolarization with a time constant from 20 to 150 ms. The activation time constant decreased monotonically with depolarization of the membrane. 4. The reversal potential of CCh-sensitive current changed as a function of the external K+ concentration (-79, -65 and -44 mV in 5, 10 and 25 mM, respectively) and was approximately equal to the potassium equilibrium potential (-89, -71 and -48 mV in 5, 10 and 25 mM, respectively). The CCh-sensitive current is concluded to be K+ selective. 5. The CCh-sensitive current showed a sigmoid log dose vs. response relationship with an apparent dissociation constant (KD) of 1.4 microM and a Hill coefficient of 1.0. When ACh was applied, an apparent KD of 1.8 microM and a Hill coefficient of 1.0 was measured. 6. The suppression of K+ current by CCh was blocked by atropine (3 microM) and pirenzepine (3 microM), suggesting that the current is mediated by an M1 muscarinic receptor. 7. The CCh suppression of the K+ current was enhanced by GTP-gamma-S (0.1 mM), suggesting that a GTP-binding protein is involved. 8. The CCh suppression of the K+ current was mimicked by protein kinase C activators, 1-oleoyl-2-acetyl-sn-glycerol (OAG, 100 microM), phorbol dibutyrate (PDBu, 2 microM) and phorbol 12-myristate 13-acetate (PMA, 1 microM). The protein kinase inhibitor, staurosporine (0.2 microM) applied internally blocked the CCh suppression of the K+ current which suggests an involvement of protein kinase C.

Transduction mechanism for glutamate-induced potassium current in neurones of the mollusc Planorbarius corneus

1. The potassium currents evoked by glutamate agonists on isolated and identified neurones of molluscan pedal ganglia were investigated using the voltage clamp technique. 2. Glutamate responses were not modified by increasing intracellular cyclic nucleotide concentrations (treatment with 8-Br-cAMP, 8-Br-cGMP, forskolin and/or the phosphodiesterase inhibitor isobutylmethylxantine, IBMX), whereas inward-going currents induced by the nucleotides were observed. It follows that glutamate currents are independent of intracellular cyclic nucleotide control. 3. Protein kinase C activation with phorbol esters or oleoylacetylglycerol induced a slowly developing outward current and reduced glutamate response amplitude. Staurosporine itself did not affect the glutamate responses but completely prevented the effects of phorbol esters and oleoylacetylglycerol. This indicated that protein kinase C was not involved in the transduction mechanism for the potassium component of the glutamate response. 4. The possible involvement of inositol-1,4,5-trisphosphate seems to be improbable because the glutamate responses were independent of intracellular calcium concentration. Intracellular injection of calcium buffer BAPTA, failed to affect any of the glutamate currents, although it effectively blocked the after-hyperpolarization following directly evoked action potentials. 5. Nordihydroguaiaretic acid (NDGA) and indomethacin, inhibitors of the lipoxygenase and cyclo-oxygenase pathways of arachidonic acid metabolism, correspondingly, did not change the glutamate responses of these neurones. 6. The failure to demonstrate the involvement of any known secondary messenger systems in glutamate response transduction favours two assumptions: (1) the receptor-G protein complex controls the potassium channel directly; or (2) some still unknown transduction system is used.

Protein kinase C modulates glutamate receptor inhibition of Ca2+ channels and synaptic transmission

Fast synaptic transmission in the central nervous system can be modulated by neurotransmitters and second-messenger pathways. For example, transmission at glutamatergic synapses can be depressed by the metabotropic glutamate receptor, providing autoreceptor-mediated negative feedback. Metabotropic glutamate receptor inhibition of Ca2+ channels may contribute to this pathway. In contrast, stimulation of protein kinase C can enhance excitatory synaptic transmission, whereas both depression and enhancement of Ca2+ current have been reported. Here we show that in hippocampal CA3 and cortical pyramidal neurons, activation of protein kinase C enhances current through N-type Ca2+ channels and, in addition, dramatically reduces G protein-dependent inhibition of these same channels by the metabotropic glutamate receptor. In parallel experiments on fast excitatory transmission at corticostriatal synapses, kinase C activators were similarly found to reduce the inhibitory effect produced by stimulation of the metabotropic glutamate receptor. The results show that second-to-second control of Ca2+ channels by the metabotropic glutamate receptor can itself be modulated on a slower timescale by protein kinase C. These mechanisms may be used in the control of fast excitatory synaptic transmission.

Role for diacylglycerol in mediating the actions of ACh on M-current in gastric smooth muscle cells

The role of the second messenger diacylglycerol (DAG) in mediating muscarinic suppression of M-current, a type of a voltage-gated K+ current that is suppressed by acetylcholine (ACh), was examined in freshly isolated smooth muscle cells from toad stomach. Currents were recorded using a single electrode voltage clamp employing conventional microelectrodes. Extracellular application of 1,2-dioctanoyl-sn-glycerol (DiC8), a synthetic DAG that is a potent activator of protein kinase C (PKC), reversibly suppressed M-current. Current relaxations, representing the voltage-dependent closure of K+ channels underlying M-current, were also decreased by DiC8, although suppression was not always as complete as it was with ACh. In contrast, another DAG analogue, 1,2-dioctanoyl-3-thioglycerol, which has a structure closely related to DiC8 but does not activate PKC, failed to inhibit M-current. Furthermore, M-current induced by the beta-agonist isoproterenol, by a mechanism apparently mediated by adenosine 3',5'-cyclic monophosphate (S. M. Sims, L. H. Clapp, J. V. Walsh, Jr., and J. J. Singer. Pflugers Arch. 417: 291, 1990), was also suppressed by DiC8. Both ACh and DiC8 were found to suppress endogenous and isoproterenol-induced M-current without altering the time course of M-current deactivation, suggesting that these agents act by decreasing the number of channels available to be opened. These results provide evidence that muscarinic regulation of M-current is mediated by DAG.

G-protein mediation in nociceptive signal transduction: an investigation into the excitatory action of bradykinin in a subpopulation of cultured rat sensory neurons

Bradykinin is one of several pro-inflammatory, pain-inducing substances produced during inflammation--the body's response to injury. In previous work we have shown that bradykinin and guanosine-5'-O-3-thiotriphosphate increase excitability in a subpopulation of cultured neonatal rat dorsal root ganglion neurons. We now describe experiments in which the mechanism underlying the stimulatory action of these two substances has been examined in more detail. Using the whole-cell voltage-clamp technique, bradykinin-sensitive cells were distinguished by their response to a 1-s depolarizing voltage-pulse which evoked more than one inward current during the step command. The secondary inward currents are likely to represent action potentials generated at the poorly clamped neurites of these cells. Bradykinin- and guanosine-5'-O-3-thiotriphosphate-induced changes in excitability were measured indirectly by a change in the number of inward currents recorded during the 1-s depolarizing voltage-step. The effect of activators and inhibitors of protein kinase C, arachidonic acid metabolism, G-protein activation and release of intracellular Ca2+ were examined on this response. In the presence of extracellular staurosporine (1.0 microM) or nordihydroguaiaretic acid (10 microM), these excitatory effects were reduced but not abolished, whilst indomethacin (20 microM) had no effect. Intracellular application of guanosine-5'-O-2-thiodiphosphate (10 mM) or ryanodine (100 microM) substantially reduced the effect of bradykinin. The excitatory effect of internal guanosine-5'-O-3-thiotriphosphate (500 microM) occurred gradually over time, and this was mimicked by internal application of myo-inositol 1,4,5-trisphosphorothioate (1.0 microM). From the results, it is proposed that G-protein activation is an essential component of the bradykinin response, which may also require a Ca(2+)-activated conductance modulated by protein kinase C and lipoxygenase metabolites of arachidonic acid.

M-currents in frog sympathetic ganglion cells: manipulation of membrane phosphorylation

1. The inward current and the M-current (IM) suppression produced when muscarine is applied to frog sympathetic ganglion cells was recorded by means of the whole-cell patch-clamp technique. The holding potential was -30 mV and [K+]o was 6 mM. 2. The steady-state IM was maintained for at least 20 min when the patch pipette contained neither adenosine 5'-triphosphate (ATP) nor adenosine 3':5'-cyclic monophosphate (cyclic AMP). Inclusion of these substances or the ATP antagonist, beta,gamma-methyleneadenosine 5'-triphosphate (beta,gamma-MethATP; 1 or 2 nM) (failed to alter the rate of IM 'run down'. By contrast, inclusion of adenosine-5'-O-(3-thiotriphosphate) (ATP-gamma-S, 1 or 2 mM) resulted in a 60% reduction of the current within 18 min. 3. Despite the inability of ATP-gamma-S to maintain steady-state IM, it had no effect on the ability of muscarine (2-100 microM) to suppress a constant fraction of the available current. ATP-gamma-S and beta,gamma-MethATP increased the rise time and duration of the response to muscarine. 4. Inclusion of a phosphatase inhibitor, diphosphoglyceric acid (DPG, 1-2.5 mM) or alkaline phosphatase (100 micrograms ml-1) failed to affect the amplitude of muscarinic responses. 5. These results question the role of the phosphorylation and/or dephosphorylation reactions in the transduction mechanism for muscarine-induced IM suppression but are consistent with the possibility that M-channels are 'directly coupled' via G-protein to the muscarinic receptor.

Protein kinase C-mediated enhancement of NMDA currents by metabotropic glutamate receptors in Xenopus oocytes

1. N-Methyl-D-aspartate (NMDA) receptors were expressed in Xenopus oocytes injected with rat brain RNA. The modulation of NMDA-induced currents was examined by activating protein kinase C (PKC) either directly (using phorbol esters) or indirectly (via metabotropic glutamate agonists). 2. Bath application of the PKC activator, 4-beta-phorbol-12,13-dibutyrate (PDBu) resulted in a two-fold increase in the NMDA-evoked current at all holding potentials examined (-80 to 0 mV). The inactive (alpha) stereoisomer of phorbol ester was ineffective. 3. The increase was observed under conditions that eliminate the oocyte's endogenous calcium-dependent chloride current, which often contributes to the NMDA response in oocytes. 4. The PDBu effect was specific to the NMDA subclass of glutamate receptors in that no increase was observed in the responses to two other glutamate agonists, kainate and AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid). 5. Stimulation of PKC by activation of metabotropic receptors via either quisqualate or trans-ACPD (trans-1-aminocyclopentane-1,3-dicarboxylic acid) also led to an increase in NMDA currents. 6. Both methods of enhancement induced transient effects. PDBu effects lasted 10-45 min, depending upon both dose and length of application. Quisqualate and trans-ACPD effects were shorter, lasting less than 10 min under these conditions of application. 7. Both methods of enhancement were blocked by the PKC inhibitor, staurosporine. In addition, the phorbol ester-induced enhancement of NMDA responses occluded further enhancement by quisqualate. 8. The results suggest a role for metabotropic glutamate receptors in modulation of NMDA-mediated processes.

Time course of receptor-channel coupling in frog sympathetic neurons

The M-type potassium current and the N-type calcium current are inhibited by several different neurotransmitters in frog sympathetic neurons. These effects seem to be mediated via G proteins, but it is not clear whether diffusible second messengers are involved. Using a rapid (approximately 100 ms) flow tube perfusion system to apply agonists, the inhibition of calcium current develops and recovers rapidly but not instantaneously (t1/2 = 1-2 s). M-current inhibition is considerably slower, with t1/2 approximately 30 s for recovery from inhibition. At least for M-current inhibition, there appears to be sufficient time for involvement of an enzymatic cascade in receptor-channel coupling.