Extracellular calcium ions are required for muscarine-sensitive potassium current in bullfrog sympathetic neurons

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.

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.

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

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

Inhibition by wortmannin of M-current in bullfrog sympathetic neurones

1. The actions of wortmannin, an inhibitor of myosin light chain kinase (MLCK), on M-type potassium current of dissociated bullfrog sympathetic neurones have been examined. 2. The amplitude of M-current was measured by whole cell recordings from cells pretreated with wortmannin (0.01-10 microM) or the wortmannin vehicle, dimethylsulphoxide (0.0001-0.1 vol%), for 30 min. Internal (recording pipette) solutions having three different pCa values (6, 7 and 8) were used for the measurements. 3. Irrespective of the pCa, M-current was not detectable when the cells were pretreated with 10 microM wortmannin. Wortmannin, 3 microM, produced 85-95% inhibition of the M-current. Pretreatment with 10-30 nM wortmannin was without effect on M-current. 4. The M-current inhibition by wortmannin at concentrations of 0.1-1 microM depended on the pCa of the internal solution. Inhibition occurred only when the calcium-rich (pCa = 6) internal solution was used. 5. Pre-treatment of the cells with wortmannin (10 microM) did not affect rapidly-inactivating A-type or delayed rectifier-type potassium currents not did it alter inwardly rectifying sodium-potassium current (IH). 6. These observations show that M-current inhibition by wortmannin has two pharmacological profiles. One is calcium-dependent and occurs at lower concentrations (0.1-1 microM), and is attributed to inhibition of MLCK by wortmannin. At higher concentrations (3-10 microM), wortmannin has an additional, calcium-independent action, inhibiting the M-current by an unknown mechanism.

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.

Myosin light chain kinase occurs in bullfrog sympathetic neurons and may modulate voltage-dependent potassium currents

A polyclonal antibody against myosin light chain kinase (MLCK) of chicken gizzard recognized a 130 kd peptide of bullfrog sympathetic ganglia as MLCK. MLCK immunoreactivity was confined to the neuronal cell body. A synthetic peptide corresponding to an inhibitory domain of MLCK (Ala783-Gly804) was applied intracellularly to isolated sympathetic neurons during whole-cell recordings of ionic currents. The peptide inhibitor reversibly decreased M-type potassium current (IM) while not affecting A-type of delayed rectifier-type potassium currents. Intracellular application of an active fragment of MLCK enhanced IM, whereas application of an inactive MLCK fragment did not. The results suggest that IM can be modulated by MLCK-catalyzed phosphorylation.

Kinetic and pharmacological properties of the M-current in rodent neuroblastoma x glioma hybrid cells

1. The M-like current IK(M,ng) in differentiated NG108-15 mouse neuroblastoma x rat glioma hybrid cells has been studied using tight-seal, whole-cell patch-clamp recording. 2. When calculated from steady-state current-voltage curves, the conductance underlying IK(M,ng) showed a Boltzmann dependence on voltage with half-activation voltage Vo = -44 mV (in 3 mM [K+]) and slope factor (a) = 8.1 mV/e-fold increase in conductance. In 12 mM [K+] Vo = -38 mV and a = 6.9 mV. The deactivation reciprocal time constant accelerated with hyperpolarization with slope factor 17 mV/e-fold voltage change. 3. The reversal potential for deactivation tail currents varied with external [K+] as if PNa/PK were 0.005. 4. Steady-state current was increased on removing external Ca2+. In the presence of external Ca2+, reactivation of IK(M, ng) after a hyperpolarizing step was delayed. This delay was preceded by an inward Ca2+ current, and coincided with an increase in intracellular [Ca2+] as measured with Indo-1 fluorescence. Elevation of intracellular [Ca2+] with caffeine also reduced IK(M, ng). 5. IK(M, ng) was inhibited by external divalent cations in decreasing order of potency (mM IC50 in parentheses): Zn2+ (0.011) greater than Cu2+ (0.018) greater than Cd2+ (0.070) greater than Ni2+ (0.44) greater than Ba2+ (0.47) greater than Fe2+ (0.69) greater than Mn2+ (0.86) greater than Co2+ (0.92) greater than Ca2+ (5.6) greater than Mg2+ (16) greater than Sr2+ (33). This was not secondary to inhibition of ICa since: (i) inhibition persisted in Ca(2+)-free solution; (ii) La3+ did not inhibit IK(M, ng) at concentrations which inhibited ICa; and (iii) organic Ca2+ channel blockers were ineffective. Inhibition comprised both depression of the maximum conductance and a positive shift of the activation curve. Addition of Ca2+ (10 microM free [Ca2+]) or Ba2+ (1 mM total [Ba2+]) to the pipette solution did not significantly change IK(M, ng). 6. IK(M, ng) was reduced by 9-amino-1,2,3,4-tetrahydroacridine (IC50 8 microM) and quinine (30 microM) but was insensitive to tetraethylammonium (IC50 greater than 30 mM), 4-aminopyridine (greater than 10 mM), apamin (greater than 3 microM) or dendrotoxin (greater than 100 nM). 7. IK(M, ng) was inhibited by bradykinin (1-10 microM) or angiotensin II (1-10 microM), but not by the following other receptor agonists: acetylcholine (10 mM), muscarine (10 microM), noradrenaline (100 microM), adrenaline (100 microM), dopamine (100 microM), histamine (100 microM), 5-hydroxytryptamine (10 microM), Met-enkephalin (1 microM), glycine (100 microM), gamma-aminobutyric acid (100 microM) or baclofen (500 microM).(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of muscarine and adrenaline on patch-clamped frog cardiac parasympathetic neurones

1. The whole-cell patch-clamp technique was used to record membrane currents from neurones which were acutely dissociated from the intra-atrial parasympathetic ganglia of Rana pipiens. The effects of muscarine and adrenaline were observed at a holding potential of -30 mV. Extracellular potassium concentration ([K+]o) was 2, 6 or 20 mM. 2. Muscarine (10 microM) produced inward current in thirteen cells, outward current in eighteen cells and seven cells were unaffected. Inward currents were observed in six out of ten neurones in which the intracellular solution contained adenosine triphosphate (ATP; 100 microM) and outward currents were seen in eleven out of fourteen neurones which contained adenosine 3',5'-cyclic monophosphate (cyclic AMP; 100 microM). 3. In five out of nine cells tested, the inward current produced by muscarine was attributable to a 30% depression of a voltage-dependent current which resembled the M-current (IM). Muscarine-induced inward current in the other four cells involved a steady-state conductance increase that reached a null potential at -10 mV. Modest IM suppression also contributed to the response in three of these four cells. 4. Adrenaline (10 or 100 microM) produced inward currents in twelve cells, outward current in ten cells and three cells were unaffected. Outward currents were only seen in cells which contained ATP or cyclic AMP (ten out of sixteen cells) whereas inward currents were seen in eight out of nine cells which did not contain adenosine nucleotides. These inward currents were always attributable to IM suppression. 5. The outward currents induced by muscarine and adrenaline resulted from an increase in a potassium conductance (GK) that exhibited inward rectification.

Excitatory responses to serotonin (5-HT) in neurons of the rat piriform cortex: evidence for mediation by 5-HT1C receptors in pyramidal cells and 5-HT2 receptors in interneurons

As a prerequisite to pharmacological analysis of the excitatory effects of serotonin (5-HT) on piriform pyramidal cells and interneurons, this study first examined the physiological characteristics of these two cell types. Intracellular recordings confirmed that the subpopulation of 5-HT-activated cells located at the border of layers II and III are indeed interneurons. Voltage clamp recordings in pyramidal cells showed that the increase in excitability produced by 5-HT in these cells was the result of voltage- and Ca(2+)-dependent outward currents with the characteristics of IM and IAHP. Pharmacological studies were designed to discriminate 5-HT2 from 5-HT1C responses in interneurons and pyramidal cells of piriform cortex. The 5-HT antagonist spiperone, which has a much higher affinity for 5-HT2 receptors than for 5-HT1C receptors, blocked the excitatory effect of 5-HT at lower concentrations in interneurons (IC50 = 31 nM) than in pyramidal cells (IC50 = 2.1 microM). Similarly, ritanserin, a drug which also has a higher affinity for 5-HT2 than 5-HT1C receptors, blocked the effect of 5-HT at lower concentrations in interneurons (IC50 = 400 nM) than in pyramidal cells (IC50 = 8.1 microM). In contrast, LY 53857, an antagonist with higher affinity for 5-HT1C than for 5-HT2 receptors, blocked the effect of 5-HT at lower concentrations in pyramidal cells (IC50 = 26 nM) than in interneurons (IC50 = 364 nM). The 5-HT1C partial agonist/5-HT2 antagonist mCPP produced agonist-like effects in only 66% of pyramidal cells tested indicating that not all pyramidal cells may express 5-HT1C receptors. In that both spiperone and ritanserin have higher affinity for 5-HT2 receptors than for 5-HT1C receptors and LY 53857 has a higher affinity for 5-HT1C receptors than for 5-HT2 receptors, these data suggest that in piriform cortex excitatory effects of 5-HT are mediated by 5-HT1C receptors in pyramidal cells an by 5-HT2 receptors in interneurons.

Muscarinic suppression of the M-current is mediated by a rise in internal Ca2+ concentration

The role of intracellular Ca2+ in the muscarinic suppression of M-current was examined. Intracellular injection of Ca2+ buffer into cells in the intact ganglion reduced the response to muscarinic agonist. In similar experiments on isolated cells, Ca2+ buffer was introduced into the cytoplasm using a perfused recording pipette. Ca2+ buffer (20 mM) with the free Ca2+ concentration set to normal resting levels produced a reversible reduction of the muscarinic response. In a second line of investigation, it was found that pharmacological procedures designed to deplete internal stores of Ca2+ produced a decrease in the muscarinic response. These results, taken together with previous work, support the hypothesis that the muscarinic suppression of M-current is mediated by the release of Ca2+ from intracellular stores.

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).

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.