On the transduction mechanism for muscarine-induced inhibition of M-current in cultured rat sympathetic neurones

Muscarinic acetylcholine response in pyramidal neurones of rat cerebral cortex

1. The effects of acetylcholine (ACh) on pyramidal neurons acutely dissociated from the rat cerebral cortex were studied in the whole-cell mode, by use of the nystatin-perforated patch recording configuration. 2. ACh induced a net inward current (IACh) accompanied by a membrane conductance decrease at a holding potential (VH) of -40 mV. IACh increased in a concentration-dependent manner with a half-maximum concentration (EC50) of 8.7 x 10(-7) M. 3. IACh mainly resulted from the suppression of the voltage- and time-dependent K+ current (M-current). 4. Muscarine and muscarinic agonists such as McN-A-343, oxotremorine and oxotremorine-M mimicked the ACh response. The potency was in the order of oxotremorine-M > McN-A-343 > or = muscarine > oxotremorine. 5. Pirenzepine shifted the concentration-response curve for ACh to the right and the corresponding Schild plot yielded a pA2 value of 7.81. Other muscarinic antagonists also reversibly blocked IACh in a concentration-dependent manner. The inhibitory potency was in the order of atropine > 4-DAMP > pirenzepine > AF-DX-116. 6. IACh could be induced normally even after pre-incubation of dissociated neurones in external solution with 200 ng ml-1 pertussis toxin (PTX) for 8 h, whereas the inhibitory effect of ACh on high-voltage-activated Ca2+ channels was completely abolished by the PTX treatment.

Muscarinic M-current inhibition via G alpha q/11 and alpha-adrenoceptor inhibition of Ca2+ current via G alpha o in rat sympathetic neurones

1. Microinjection of selective antibodies into superior cervical ganglion (SCG) neurones has identified the G-protein alpha-subunits mediating muscarinic receptor inhibition of M-type K+ current (IK(M)) and alpha-adrenoceptor inhibition of Ca2+ current (ICa). 2. Antibodies specific for G alpha q/11, but not those for G alpha o, reduced M-current inhibition by the muscarinic agonist oxotremorine-M, whereas anti-G alpha o antibodies, but not anti-G alpha q/11 or anti-G alpha i1-3 antibodies, reduced calcium current inhibition by noradrenaline. 3. Immunoblots with specific anti-G-protein antibodies demonstrated the presence of both G alpha q and G alpha 11, while G alpha o1 (but virtually no G alpha o2) was present. 4. We conclude that M1 muscarinic receptor inhibition of IK(M) is transduced by G alpha q and/or G alpha 11, and that G alpha o transduces alpha-adrenoceptor inhibition of ICa.

Muscarinic inhibition of two potassium currents in guinea-pig prevertebral neurons: differentiation by extracellular cesium

Muscarinic responses were studied in dissociated guinea-pig celiac ganglion neurons using the whole-cell voltage-clamp technique. Muscarine (0.025-1 mM; EC50 = 95 microM) administered to cells for 1.5 s evoked inward shifts in holding current in 53 of 74 cells. The amplitude of the inward current transients decreased with hyperpolarization and the null potential averaged -71 +/- 3.4 mV (n = 11). The currents that underlie the responses to muscarine were examined with hyperpolarizing voltage stepping protocols to -100 mV from a holding potential of -30 mV. Eighty-one per cent of cells displayed voltage-dependent current relaxations characteristic of the M-potassium current. Twenty per cent of responding cells displayed no M-current but only a voltage-independent current consistent with a leak current. In the latter type of cells, the muscarine-evoked inward currents reversed near EK and became outward at more hyperpolarized potentials. Analysis of steady state I-V relationships before and after bath application of muscarine showed that the two muscarine-sensitive potassium currents were distributed differently among three types of cells: (i) with M-current (18%); (ii) with leak current (18%); and (iii) with M-current and with leak current (64%). Cesium and barium were used to differentiate the M-current and the muscarine-sensitive leak current. Barium (2 mM) reduced the M-current and the leak potassium current, whereas cesium (2 mM) reduced the M-current but did not affect leak current. Thus, barium reduced the amplitude of muscarinic responses by 79% but cesium reduced them by only 14%. We conclude that muscarinic responses in guinea-pig celiac neurons are produced by suppression of two K+ currents: the M-current and a muscarine-sensitive leak current. These two currents are differentially susceptible to the potassium channel blockers barium and cesium.

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.

Muscarine modulation by a G-protein alpha-subunit of delayed rectifier K+ current in rat ventromedial hypothalamic neurones

1. Rat cultured ventromedial hypothalamic (VMH) neurones obtained from embryonic hypothalamus were used to study the muscarinic (carbachol) modulation of voltage-gated K+ currents with the whole-cell patch-clamp technique. 2. Carbachol produced a potent and concentration-dependent (100 fM to 100 microM) decrease of the outward delayed rectifier K+ current (IK) with an IC50 of 44 pM and a Hill coefficient of 0.4. The carbachol-induced depression of IK was reduced by pirenzepine (1-10 microM) and atropine (1 microM). Carbachol had no effect on the transient outward K+ current (IA). 3. Intracellular dialysis with guanosine 5'-O-(2-thiodiophosphate) (GDP-beta-S, 500 microM) significantly diminished the carbachol-induced depression of IK, suggesting GTP-binding protein (G-protein) involvement. Pre-incubation of VMH neurones with pertussis toxin (200-400 ng ml-1) or cholera toxin (1 microgram ml-1) for 24-48 h had no effect on the carbachol-induced depression of IK. This suggested that the G alpha o, G alpha i, and G alpha s G-protein alpha-subunits were not involved in mediating the carbachol-induced depression of IK in VMH neurones. 4. Treatment (24-48 h) of VMH neurones with antisense phosphothio-oligodeoxynucleotides to the G alpha 11 G-protein subunit (10 microM) significantly diminished the carbachol-induced depression of IK. Treatment with 10 microM of either G alpha 11 sense or antisense to G alpha q had no effect. 5. These results demonstrate a novel and potent muscarinic depression of IK in VMN neurones, and that this depression is specifically mediated by the G alpha 11 G-protein subunit.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of membrane potential and muscarine on potassium M-channel kinetics in rat sympathetic neurones

1. Using cell-attached patch pipettes, sustained activity of single potassium M-channels was recorded from dissociated rat superior cervical ganglion neurones. Previous results indicated that this activity, consisting of three main levels of open-channel conductance (congruent to 7, congruent to 12 and congruent to 19 pS) was activated by membrane depolarization and inhibited by muscarine added outside the patch. Consequently, a kinetic analysis was undertaken in order to identify M-channel states sensitive to muscarine and membrane potential. 2. Channel activity recorded at 30 mV positive to the resting membrane potential level (congruent to -60 mV) showed three shut and two open times. Mean shut times were: tau s1 = 8.0 +/- 2.2 ms; tau s2 = 71.3 +/- 8.6 ms and tau s3 = 740 +/- 220 ms. Mean open times were: tau o1 = 10.6 +/- 1.9 ms and tau o2 = 59.3 +/- 8.7 ms. When bursts of channel openings were determined as those including tau s1, two exponential components were evident in burst duration distributions (tau b1 = 11.0 +/- 0.9 ms and tau b2 = 80.4 +/- 11.0 ms). 3. Membrane hyperpolarization significantly lengthened all three shut times and shortened both open times. It also slightly enhanced the relative contribution of high-conductance channels and decreased the relative contribution of low-conductance channels to overall activity. 4. All three shut times of the M-channels were lengthened by 10 microM muscarine without significantly affecting their open times. 5. It is concluded that both open and shut states of the M-channel are voltage sensitive while only shut states are sensitive to muscarine.

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

M-current is widespread in the nervous system. It stabilizes cell excitability, and its suppression by muscarinic receptor activation underlies slow synaptic transmission in sympathetic neurons. Suppression of M-current was one of the first examples of neuromodulation of a potassium current, but the mechanism is not understood. Single-channel recording was used to study this issue. An M-channel with two conductance states, which exhibited appropriate voltage-dependent kinetics with two modes of gating, has been resolved. Mode 1 comprises short open time, low open probability events, and mode 2 openings represent long open time, high open probability behavior. Muscarine decreased M-channel activity by selectively reducing mode 2 M-channel gating through a diffusible second messenger. It is suggested that control of modal gating may be a widespread mechanism for neuromodulation.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

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.

Closure of potassium M-channels by muscarinic acetylcholine-receptor stimulants requires a diffusible messenger

The M-current (IK(M)) is a slow voltage-gated K+ current which can be inhibited by muscarinic acetylcholine-receptor (mAChR) agonists. In the present experiments we have tested whether this inhibition results from a local (membrane-delimited) interaction between the receptor and adjacent channels, or whether channel closure is mediated by a diffusible messenger. To do this, single KM(+)-channel currents were recorded from membrane patches in dissociated rat superior cervical sympathetic neurons by using cell-attached patch electrodes. Channel activity was inhibited when muscarine was applied to the cell membrane outside the patch but persisted when channels were exposed to muscarine added to the pipette solution. We conclude that a diffusible molecule (or molecules) is (are) required to induce intrapatch channel closure following activation of extra-patch receptors.

Muscarinic receptor subtypes: modulation of ion channels

The discovery of five muscarinic receptor subtypes by molecular genetic techniques has resulted in new approaches to understanding their function. This involves the expression of the individual genes encoding each receptor subtype in isolation, such that their effects and mechanisms of action can be studied. The coupling of the receptors with G-proteins and ion channels is the subject of this review and emphasis is placed upon the assignment of genetically defined receptor subtypes with a given physiological function. Activation of inwardly rectifying potassium conductances by m2 and m4 and inhibition by m1, as well as stimulation of calcium-dependent conductances by m1, m3 and m5 are discussed.

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.

Dissection of bradykinin-evoked responses by buffering intracellular Ca2+ in neuroblastoma x glioma hybrid NG108-15 cells

Signal transduction pathways from bradykinin (BK) receptors were investigated in NG108-15 neuroblastoma x glioma hybrid cells by buffering the intracellular calcium (Ca2+) with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a Ca2+ chelator. BK increased inositol-1,4,5-trisphosphate (Ins(1, 4,5)P3) formation at the same rate in the control and in BAPTA-acetoxy methyl ester (AM)-treated NG108-15 cells. However, a transient increase of intracellular Ca2+ concentrations in response to BK was significantly suppressed in Ca(2+)-buffered hybrid cells. Accordingly the BK-induced outward current was inhibited in BAPTA-AM-treated hybrid cells, while the subsequent inward current associated with a fall in membrane conductance was apparently increased. The initial phase of acetylcholine release from NG108-15 cells in response to BK was markedly inhibited in BAPTA-AM-treated coculture dishes when detected as miniature end-plate potentials of myotubes, though the late phase of acetylcholine secretion was observed. These results indicate that BK induces two distinct responses in NG108-15 cells: Ins(1,4,5)P3-dependent intracellular Ca2+ rise-sensitive and -insensitive components.

Characterization of muscarinic receptor subtypes inhibiting Ca2+ current and M current in rat sympathetic neurons

Muscarinic receptors mediating suppression of Ca2+ current and of M-type K+ current in rat superior cervical ganglion neurons were subclassified pharmacologically by using the muscarinic receptor antagonists pirenzepine and himbacine. Our voltage clamp experiments previously distinguished fast and slow intracellular signaling pathways coupling muscarinic receptors to calcium channels. We now establish that the fast, pertussis toxin-sensitive suppression of Ca2+ current is mediated primarily by muscarinic receptors of the M4 subtype, whereas the slow, bis(2-aminophenoxy)-ethane-N,N,N',N'-tetraacetate (BAPTA)-sensitive suppression of Ca2+ current is mediated primarily by muscarinic receptors of the M1 subtype. Both actions on Ca2+ current are blocked by guanosine 5'-[beta-thio]diphosphate. Muscarinic suppression of M current is slow, BAPTA-sensitive, and mediated by receptors of the M1 subtype. Hence the two muscarinic pathways use different receptors and different guanine nucleotide binding proteins to produce different actions on channels.

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.

Kinetics of G protein-mediated modulation of the potassium M-current in bullfrog sympathetic neurons

The inhibition of the voltage-dependent, K+ M-current (IM) following receptor-independent G protein activation with controlled intracellular perfusion of nonhydrolyzable GTP analogs had an exponential time course, with rates hyperbolically dependent on GTP analog concentration, and a limiting value of 0.53 min-1. The inhibitory agonist muscarine caused a concentration-dependent acceleration of the rate of nucleotide-induced inhibition, with a plateau of about 20 min-1 and an exponential time course. In neurons not treated with nucleotide analogs the IM recovery rate following agonist removal was 3-7 min-1. It is proposed that the overall kinetics of the transduction pathway for IM modulation is governed by the agonist-dependent kinetics of nucleotide interaction with G proteins. A simple model of IM modulation based on G proteins' kinetics has been developed. These data suggest a possible cellular process responsible for the time course of slow synaptic potentials caused by IM inhibition in sympathetic neurons.

Pertussis toxin and voltage dependence distinguish multiple pathways modulating calcium channels of rat sympathetic neurons

Agonist-induced suppression of current in voltage-gated Ca2+ channels was studied in rat sympathetic neurons. We have previously distinguished two intracellular signaling pathways used by muscarinic agonists to suppress neuronal Ca2+ current-one fast and membrane delimited, the other slow and acting via a diffusible second messenger. We now show that the fast pathway is sensitive mainly to pertussis toxin and shifts the gating of Ca2+ channels to more positive voltages (voltage dependent). The slow pathway is pertussis toxin insensitive and depresses currents at all test potentials (voltage independent). Muscarinic agonists may also activate a pertussis toxin-insensitive fast pathway. alpha-Adrenergic agonists use the fast pertussis toxin-sensitive and the fast insensitive pathways, but not the slow one.

An ion channel locus for the protein kinase C potentiation of transmitter glutamate release from guinea pig cerebrocortical synaptosomes

The mechanism by which protein kinase C (PKC) activates transmitter release from guinea pig cerebrocortical synaptosomes was investigated by employing parallel fluorescent assays of glutamate release, cytoplasmic free Ca2+, and plasma membrane potential. 4 beta-Phorbol dibutyrate (4 beta-PDBu) enhances the Ca(2+)-dependent, 4-aminopyridine (4AP)-evoked release of glutamate from synaptosomes, the 4AP-evoked elevation of cytoplasmic free Ca2+, and the 4AP-evoked depolarization of the plasma membrane. 4 beta-PDBu itself causes a slow depolarization, which may underlie the small effect of 4 beta-PDBu on spontaneous, KCl-evoked, and Ca(2+)-independent/4AP-evoked glutamate release. Because 4AP (but not KCl) generates spontaneous, tetrodotoxin-sensitive action potentials in synaptosomes, a major locus of presynaptic PKC action is to enhance these action potentials, perhaps by inhibiting delayed rectifier K+ channels.

Pharmacological properties of cloned muscarinic receptors expressed in A9 L cells; comparison with in vitro models

The effects of a series of muscarinic agonists and antagonists at cloned m1 and m3 muscarinic receptors expressed in mouse fibroblast A9 L cells have been compared with their effects in in vitro models of M1 (rat superior cervical ganglion) and M3 (guinea-pig ileum) muscarinic receptors. A good correlation existed between the potencies of muscarinic agonists at cloned m1 muscarinic receptors and the M1 sites in rat ganglion (r = 0.80) as well as at cloned m3 receptors and guinea-pig ileum M3 receptors (r = 0.87). However, cross correlations of potencies in rat ganglion and cloned m3 receptors as well as in guinea-pig ileum and in cloned m1 receptors also yielded relatively high correlation coefficients (0.71 and 0.91, respectively). Low correlation coefficients were found for the maximal responses of muscarinic agonists in rat ganglion and cloned m1 receptors (0.53) and in guinea-pig ileum and cloned m3 receptors (0.36). A high correlation between pA2 values of muscarinic antagonists at cloned m1 receptors and in rat ganglion (r = 0.97) and between cloned m3 receptors and guinea-pig ileum (r = 0.98) was found. Cross correlation of pA2 values in rat ganglion and cloned m3 receptors and in guinea-pig ileum and cloned m1 receptors yielded correlation coefficients of 0.82 and 0.72, respectively. The data indicate that the cloned muscarinic receptor sites seem similar to the corresponding endogenous sites. The good correlations in corresponding but also non-corresponding receptor models reflect the relatively low selectivity of the majority of the compounds investigated.

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.

Inhibition of the M current in NG 108-15 neuroblastoma x glioma hybrid cells

The M current, IM, a voltage-dependent non-inactivating K current, was recorded in NG108-15 neuroblastoma x glioma hybrid cells, using the whole-cell mode of the patch-clamp technique. We studied inhibition of the M current by bradykinin, phorbol dibutyrate (PDBu), an activator of protein kinase C (PKC), and methylxanthines. Focal application of 0.1-5 microM bradykinin inhibited IM by about 60%; 5 nM bradykinin inhibited by about 40%. Bath application of 0.1 microM and 1 microM PDBu diminished IM to about half of the control value. Staurosporine, a PKC inhibitor, applied for 35-43 min in a concentration of 0.3 microM significantly reduced the effect of 1 microM PDBu. M current blockage by PDBu could be partly reversed by bath application of H-7 (51-64 microM), another PKC inhibitor. These observations suggest that the PDBu effect is really due to activation of PKC. The findings are compatible with the view [Brown DA, Higashida H (1988) J Physiol (Lond) 397:185-207] that the bradykinin effect on IM is mediated by PKC. However, three further observations suggest that this is only true for part of the bradykinin effect. When the suppression of IM by 1 microM PDBu was fully developed, 0.1 microM bradykinin produced a further inhibition of IM. Down-regulation of PKC by long-term treatment with PDBu reduced the effect of 0.1 microM bradykinin significantly but did not abolish it. Staurosporine (0.3 microM, applied for 31-46 min) failed to reduce the effect of 5 nM bradykinin significantly. The M current could be reversibly blocked by methylxanthines (caffeine, isobutyl-methylxanthine, theophylline) in the millimolar range, probably because of a direct action on the M channels.

Modulation of M-current by intracellular Ca2+

IM is a voltage- and time-dependent K+ current that is suppressed by muscarinic receptor activation. IM augmentation following agonist washout was blocked by heavily buffering [Ca2+]i using BAPTA. Although IM is not primarily Ca2+ dependent, small increases in [Ca2+]i by photolysis of the "caged" Ca2+ chelator nitr-5 or by evoking action potentials augmented, while larger increases inhibited, IM. Raising [Ca2+]i for prolonged periods, by nitr-5 photolysis, reduced its sensitivity to agonist, leaving a poorly reversible response. These results suggest that IM can be regulated by physiologically relevant changes in [Ca2+]i, placing IM in a unique position to modulate cell excitability.

Slowly inactivating potassium current in cultured bull-frog primary afferent and sympathetic neurones

1. Cultured bull-frog dorsal root ganglion cells were voltage clamped in the whole-cell configuration. The cells were superfused with a nominally calcium-free Ringer solution containing tetrodotoxin (3 microM), magnesium (10 mM), cobalt (1 mM), barium (2 mM), 4-aminopyridine (3 mM) and caesium (2 mM). 2. Step depolarizations (10-40 mV, 100-300 ms) from a holding potential close to the rest (typically -70 mV) evoked an outward current (IK) followed by an outward tail current. The peak amplitude of the current was reduced to less than 10% by tetraethylammonium (30 mM). 3. IK developed to its peak in 200 ms at -30 mV. Tail currents reversed at potentials that changed according to the logarithm of the extracellular potassium concentrations. 4. Tail currents declined to the baseline according to an exponential function of time (tau congruent to 40 ms at -60 mV) and its reciprocal time constant increased e-fold with a 13 mV hyperpolarization. 5. The current inactivated during sustained (1-20 s) depolarizing pulses according to a single exponential function (tau congruent to 3 s). 6. The peak amplitude of IK at -30 mV was progressively increased as the holding potential was made more negative than -70 mV reaching the maximum with step depolarizations from -120 mV. Reversed phenomenon was observed as the holding potential was made less negative than -70 mV. 7. The removal of the steady-state inactivation occurred along with a single exponential function and the time constant was decreased from 70 ms at -70 mV to 10 ms at -120 mV. 8. It is suggested that a slowly inactivating potassium current which we called IK in amphibian sensory neurones could be a class of a 'delayed' rectifier potassium current. A potassium current with properties indistinguishable from those which have been described for the sensory IK also occurred in cultured bull-frog sympathetic neurones. 9. Forskolin (1-30 microM) and 1,9-dideoxy forskolin (10 microM) reduced the amplitude of IK by up to 85% but these actions were not mimicked by any of 8-bromo-cyclic AMP (1 mM), dibutyryl cyclic AMP (1 mM) and 3-isobutyl-1-methylxanthine (1 mM). A hydrophilic forskolin analogue, 7-O-hemisuccinyl-7-deacetyl forskolin (10 microM), was about one-tenth as potent as forskolin (10 microM).

Intracellular Ca2+ buffers disrupt muscarinic suppression of Ca2+ current and M current in rat sympathetic neurons

The role of intracellular Ca2+ concentration ([Ca2+]i) in the muscarinic suppression of Ca2+ current and M-type K+ current has been investigated in isolated rat sympathetic neurons using the whole-cell patch-clamp technique and fura-2 fluorescence measurements. Muscarinic stimulation suppressed currents without raising [Ca2+]i. Nonetheless, intracellular bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetate (BAPTA) (11-12 mM), a Ca2+ chelator, reduced Ca2(+)-current suppression from 82 to 15%. For the latter, we explain the BAPTA action by a requirement for a certain minimum [Ca2+]i for continued operation of the pathway coupling muscarinic receptors to M-type K+ channels. The pathway coupling muscarinic receptors to Ca channels also showed some dependence on [Ca2+]i, but there may also be a blocking action of BAPTA that is independent of Ca2+ chelation.

Carbachol and bradykinin elevate cyclic AMP and rapidly deplete ATP in cultured rat sympathetic neurons

The agonists carbachol (CCh) and bradykinin (BK) and 54 mM KCl (high K+) were among the most potent stimulants of cyclic AMP (cAMP) production in cultured rat sympathetic neurons, measured with the use of a high-fidelity assay developed for small samples. The rise in cAMP evoked by CCh (through muscarinic receptors), BK, and high K+ was inhibited in Ca2(+)-depleted medium (1.3 mM Ca2+ and 2 mM BAPTA or EGTA), which also prevented the sustained rise in [Ca2+]i evoked by each of these stimuli, showing that elevation of cAMP requires extracellular Ca2+ and, possibly, Ca2+ influx. Preliminary results obtained with the novel calmodulin inhibitor CGS 9343B, which blocked the elevation of cAMP, and with the cyclogenase inhibitor indomethacin, which partially blocked the actions of the agonists but not those of high K+, suggest that calmodulin and arachidonate metabolites may be two components of the signaling pathway. In addition to their effects on cAMP metabolism, CCh, muscarine, and BK, but not nicotine, caused a 30-40% decrease in ATP levels. This effect was much greater than that evoked by high K+ and was largely inhibited by CGS 9343B but slightly enhanced in the Ca(+)-depleted medium, showing that agonists are still active in the absence of [Ca2+]o. Thus, agonists that activate phosphoinositide metabolism can also increase cAMP production and substantially deplete cells of ATP. These novel actions may have to be taken into account when the mechanisms by which such agonists regulate cell function are being considered.

M-current noise and putative M-channels in cultured rat sympathetic ganglion cells

1. Whole-cell recordings of M-currents and single-channel recordings have been made in cultured rat sympathetic ganglion (SCG) neurones using the patch clamp technique. 2. Muscarine caused a reduction in macroscopic M-current relaxations, induced by voltage steps, and a concomitant reduction in whole-cell current noise. Power spectra of the muscarine-sensitive component of current noise were fitted with two Lorentzian components corresponding, on average, to 162 and 15 ms. The longer time constant was very similar to that of deactivation tail currents measured at the same potential. 3. The single-channel conductance at -30 mV was estimated from power density spectra and whole-cell current-variance relationships to be 1-2 pS. 4. Putative single M-channels, activated by depolarization, were identified in cell-attached and outside-out patches from cultured SCG neurones. In particular, the ensemble average of a small amplitude channel (estimated to be ca4 pS in physiological [K+]) in a cell-attached patch, exhibited a similar time dependence to whole-cell M-current.

Muscarinic actions in hippocampus are probably not mediated by cyclic GMP

It has been proposed that, in a variety of tissues, guanosine 3':5'-monophosphate (cyclic GMP) is the intracellular mediator of muscarinic effects. This hypothesis was tested in the CA1 region of the hippocampus, in urethane-anaesthetized rats, by studying extracellularly muscarinic disinhibition of disfacilitation and the effect of dibutyryl cyclic GMP, muscarinic agents and an inhibitor of cyclic nucleotide-dependent kinase (H-8), all applied by microiontophoresis. The main findings were: (a) cyclic GMP analogues do not mimic disfacilitation or disinhibition produced by muscarinic agents; (b) N-(2-(methylamino)ethyl)-5-isoquinoline sulfonamide (H-8) does not prevent the excitatory actions of muscarinic agents; and (c) H-8 alone does not change the field responses. In conclusion, cyclic nucleotide-dependent kinases do not seem to play a major role in the on-going modulation of excitability in the hippocampus and cyclic GMP is unlikely to be a major intracellular messenger mediating directly or indirectly the excitatory actions of acetylcholine.

Arachidonic acid metabolites as mediators of somatostatin-induced increase of neuronal M-current

The M-current (IM) is a time- and voltage-dependent K+ current that persists at slightly depolarized membrane potentials. IM is reduced by muscarinic cholinergic agonists and certain peptides, and is thought to be responsible in part for the slow and late slow excitatory postsynaptic potentials in sympathetic neurons. Recently, we reported that IM in hippocampal neurons was also augmented by somatostatin-14 and -28 suggesting that two different receptors reciprocally regulate one neuronal channel type. Muscarinic effects on IM may be mediated by various components of the phosphatidylinositol phosphate pathway. We now report the involvement of a different second messenger pathway, that generated by phospholipase A2, in the somatostatin-induced augmentation of IM in hippocampal cells. This pathway generates arachidonic acid from which leukotrienes can be produced by lipoxygenases. We find that the IM-augmenting effects of somatostatin are abolished by two substances that can inhibit phospholipase A2, quinacrine and 4-bromophenacyl bromide, and that both arachidonic acid and leukotriene C4 mimic the effects of somatostatin-14 on hippocampal pyramidal neurons in vitro. Arachidonic and somatostatin effects are blocked by a lipoxygenase inhibitor, implicating an arachidonic acid metabolite, perhaps a leukotriene, in the somatostatin effect.

ATP regulates muscarine-sensitive potassium current in dissociated bull-frog primary afferent neurones

1. Bull-frog dorsal root ganglion cells in primary culture were voltage clamped in the whole-cell configuration. The pipette solution contained ATP (5 mM). 2. Step depolarizations (5-70 mV, 0.1-1 s) from a holding potential close to the resting potential (range, -64 to -79 mV) evoked a non-inactivating potassium current with properties indistinguishable from those which have been reported for the M-current of bull-frog sympathetic neurones. 3. An unhydrolysable ATP analogue APP(NH)P (5 mM), substitute with ATP in the pipette solution, did not support the M-current activation. 4. Bath application of ATP (30 nM-30 microM) reduced the amplitude of the M-current in a concentration-dependent manner, congruent to 50% inhibition of the current occurring with 1 microM-ATP. The main effect of ATP was to reduce the maximum M-conductance without changing the activation and deactivation kinetics of the M-current. 5. Essentially the same results were obtained with ADP (0.1-30 microM) and alpha, beta-methylene-ATP (10-30 microM). AMP (10-100 microM) and adenosine (10-30 microM) were without effect on the M-current. 6. The ATP-induced inhibition of the M-current was irreversible when an unhydrolysable GTP analogue GTP-gamma-S (10-30 microM) was present in the pipette solution. ATP (3 microM) reduced the amplitude of the M-current only by about 10% when GDP-beta-S (100 microM) was present in the pipette solution. Pre-treatment of the cells with pertussis toxin (IAP; 500 ng ml-1) for 24 h at 24 degrees C did not prevent the ATP-induced M-current inhibition. 7. Phorbol 12-myristate 13-acetate (PMA; 1-3 microM) reduced the amplitude of the M-current to about 50%. A reduction in the M-current amplitude by PMA (3 microM) and ATP (10 microM) was attenuated when staurosporine (200 nM) was present in the pipette solution. Forskolin (10 microM) was without effect on the M-current. 8. It is concluded that ATP acting at P2 receptors, associated with an IAP-insensitive GTP-binding protein, inhibits the M-current in amphibian primary afferent neurones.

Effects of muscarine and adrenaline on neurones from Rana pipiens sympathetic ganglia

1. Neurones dissociated from Rana pipiens paravertebral sympathetic ganglia were studied by means of the whole-cell patch-clamp technique. Responses to agonists were best recorded when cyclic AMP was included in the patch pipette. 2. Two populations of cells were identified on the basis of size (input capacitance, Cin) and the presence or absence of a fast, transient outward current (A-current, IA). This current was usually present in the 'large' cells (Cin = 40.5 +/- 1.5 pF, n = 66) but absent from 'small' cells (Cin = 21.0 +/- 0.8 pF, n = 70). 3. Both cell types exhibited a slowly activating, non-inactivating K+ current (M-current, IM) which was suppressed by luteinizing hormone-releasing hormone (LHRH, 10-100 microM). Threshold for activation of IM was about -75 mV, half-maximal activation was at -50 mV and the M-conductance GM increased e-fold for at 7 mV change in membrane potential. The maximum value for IM studied in large cells by patch-clamp procedures was less than 0.2 nA. More M-channels were available per unit membrane area in the small cells (GM = 1495 microS cm-2) than in the large cells (GM = 1034 microS cm-2). Time constants for IM deactivation at -70 mV were faster in the large cells (37.2 +/- 4.6 ms, n = 16) than in the small cells (66.1 +/- 5.9 ms, n = 9). 4. Muscarine (10 microM) produced inward current in the large cells as a result of IM suppression. In 40% of the large cells, some of the M-channels were also sensitive to adrenaline (10-100 microM). In a few large cells (less than 10%) adrenaline produced outward current by increasing IM. 5. Muscarine failed to effect IM in the small cells and instead produced an inwardly rectifying K+ current which activated within 5 ms at -110 mV. The outward current produced in twenty out of thirty-seven small cells by adrenaline was occluded by that produced by muscarine, suggesting that both agonists affect the same K+ channels. 6. Inclusion of the protein kinase inhibitors, 1-(5-isoquinolinyl-sulphonyl)-2-methyl piperazine (H-7, 50 microM) or gold sodium thiomalate (GST, 50 microM) in the pipette solution failed to antagonize either muscarine-induced current. Both currents were prolonged when the 'internal solution' contained GTP-gamma-S (50 microM). 7. Phorbol-12-myristate-13-acetate (PMA, 2-5 microM) produced an inward current as a result of IM suppression in both small and large cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of protein kinase C prevents phorbol ester- but not muscarine-induced depolarizations in the rat superior cervical ganglion

The role of protein kinase C (PKC) activation in mediating muscarinic depolarization was assessed in the rat superior cervical ganglion. Staurosporine, an inhibitor of PKC, abolished a depolarization elicited by the direct PKC activator beta-phorbol 12,13-dibutyrate, but had little effect on the response to muscarine. Thus, activation of PKC may not be an obligatory transduction step between muscarinic receptor stimulation and depolarization.

Inhibition of Ca2+ and K+ channels in sympathetic neurons by neuropeptides and other ganglionic transmitters

Neuropeptides are known to modulate the excitability of frog sympathetic neurons by inhibiting the M-current and increasing the leak current, but their effects on Ca2+ channels are poorly understood. We compared effects of LHRH, substance P, epinephrine, and muscarine on Ca2+, K+, and leak currents in dissociated frog sympathetic neurons. At concentrations that inhibit M-current, LHRH and substance P strongly reduced N-type Ca2+ current and induced a leak conductance that may contribute to slow EPSPs. In contrast, muscarine produced little reduction of Ca2+ current, even in cells in which it strongly suppressed the M-current. We find that peptidergic inhibition of Ca2+ channels involves G proteins, but does not require protein kinases. In addition, it leads to reductions in Ca2(+)-activated K+ current and catecholamine release.

Bradykinin inhibits a potassium M-like current in rat pheochromocytoma PC12 cells

We studied the action of bradykinin (BK) on ionic currents in fused pheochromocytoma PC12 cells under voltage-clamp in whole-cell mode, and on intracellular calcium using fura-2 BK induced the development of an outward current associated with an increase in intracellular calcium, followed by inhibition of an M-like current. The outward current was blocked by (+)-tubocurarine, and prevented when the calcium BAPTA or high concentrations of inositol 1,4,5-triphosphate were introduced into the cell, whereas the M-like current and its inhibition by BK remained unaffected. The protein kinase activator phorbol 12,13 dibutyrate partially reduced the M-current. M-current density did not substantially change after prolonged treatment with nerve growth factor.