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Jassar BS, Pennefather PS, Smith PA. Changes in potassium channel activity following axotomy of B-cells in bullfrog sympathetic ganglion. J Physiol 1994; 479 ( Pt 3):353-70. [PMID: 7837094 PMCID: PMC1155755 DOI: 10.1113/jphysiol.1994.sp020301] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023] Open
Abstract
1. Whole-cell and microelectrode voltage-clamp techniques were used to investigate the changes in ionic currents and action potential shape that follow axotomy of bullfrog paravertebral sympathetic ganglion B-cells. 2. Axotomy increased M-conductance (gM; muscarine-sensitive, voltage- and time-dependent K+ conductance) by 35% at -30 mV and slowed its deactivation kinetics. 3. The delayed rectifier K+ current (IK; at +50 mV) was reduced in axotomized neurones to 61% of control without any change in activation or deactivation kinetics. Steady-state intracellular Ca2+ levels and leak conductance were unchanged. 4. The fast, voltage-sensitive, Ca(2+)-activated K+ current (IC), evoked from -40 mV, was decreased to about 71% of control (at +30 mV) in axotomized neurones, whereas that evoked from -80 mV was largely unaffected. IC kinetics were also similar in control and axotomized neurones. This suggests that IC channels are not changed after axotomy. 5. In axotomized neurones, commands to +10 from -40 mV had to be extended by 16 ms to evoke voltage-insensitive Ca(2+)-dependent K+ current (IAHP) responses that were similar in magnitude to those observed in control cells. 6. The previously documented, axotomy-induced decrease in Ca2+ current (ICa) due to increased resting inactivation can account for the reduction in IC and IAHP and for the change in the shape of the action potential.
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Affiliation(s)
- B S Jassar
- Department of Pharmacology, University of Alberta, Edmonton, Canada
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52
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Affiliation(s)
- P A Smith
- Department of Pharmacology, University of Alberta, Edmonton, Canada
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53
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Marrion NV. M-current suppression by agonist and phorbol ester in bullfrog sympathetic neurons. Pflugers Arch 1994; 426:296-303. [PMID: 8183640 DOI: 10.1007/bf00374785] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
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.
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Affiliation(s)
- N V Marrion
- Howard Hughes Medical Institute, SUNY at Stony Brook 11794
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54
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Affiliation(s)
- R Zucker
- Molecular and Cell Biology Department, University of California at Berkeley 94720
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55
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Hadley RW, Kirby MS, Lederer WJ, Kao JP. Does the use of DM-nitrophen, nitr-5, or diazo-2 interfere with the measurement of indo-1 fluorescence? Biophys J 1993; 65:2537-46. [PMID: 8312490 PMCID: PMC1225996 DOI: 10.1016/s0006-3495(93)81328-6] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
Emission spectra of the photolabile Ca2+ chelators DM-nitrophen, nitr-5, and diazo-2 were studied alone, and in the presence of indo-1, to investigate potential interactions that would make the simultaneous manipulation and ratiometric measurement of the intracellular Ca2+ concentration difficult. Neither diazo-2 nor its photoproduct were found to be significantly fluorescent, and consequently concentrations of diazo-2 up to 20 times that of indo-1 did not distort the emission spectra of indo-1. DM-nitrophen was scarcely fluorescent, but its fluorescence did increase upon photolysis. In contrast to diazo-2 and DM-nitrophen, nitr-5 itself was found to be quite fluorescent, and this fluorescence was significantly increased upon photolysis. Thus, combined use of nitr-5 and indo-1 poses the most difficulty. The emission spectra of all the investigated compounds were used to define experimental conditions and calibration procedures that make possible simultaneous measurement and manipulation of the intracellular Ca2+ concentration.
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Affiliation(s)
- R W Hadley
- Department of Physiology, University of Maryland School of Medicine, Baltimore 21201
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56
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Akasu T, Ito M, Nakano T, Schneider CR, Simmons MA, Tanaka T, Tokimasa T, Yoshida M. Myosin light chain kinase occurs in bullfrog sympathetic neurons and may modulate voltage-dependent potassium currents. Neuron 1993; 11:1133-45. [PMID: 7903859 DOI: 10.1016/0896-6273(93)90226-h] [Citation(s) in RCA: 40] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/27/2023]
Abstract
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.
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Affiliation(s)
- T Akasu
- Department of Physiology, Kurume University School of Medicine, Japan
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57
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Munakata M, Akaike N. Theophylline affects three different potassium currents in dissociated rat cortical neurones. J Physiol 1993; 471:599-616. [PMID: 8120824 PMCID: PMC1143979 DOI: 10.1113/jphysiol.1993.sp019918] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023] Open
Abstract
1. The effects of theophylline in pyramidal neurones acutely dissociated from the rat frontal cortex were investigated in the whole-cell configuration, using the nystatin-perforated patch-clamp technique. 2. Ten millimolar theophylline evoked triphasic responses: a small slow outward current (Iso), then a large transient outward current (Ito) and finally a slow sustained inward current (Isi). The reversal potentials of the three current components shifted 56-58 mV for a 10-fold change in extracellular K+ concentration, thereby indicating that all these current components were predominantly carried by K+. 3. Iso had no voltage dependence, whereas Ito showed a steep outward rectification. Iso was relatively resistant to tetraethylammonium (TEA) with an IC50 of 10 mM. Ito was susceptible to submillimolar TEA with an IC50 of 0.8 mM. 4. Isi was a net inward current mainly resulting from suppression of the M-current (IM). 5. These three current components had a distinct concentration dependence; in particular, Isi was evoked at a relatively lower concentration range. 6. Ito was not observed when the intracellular Ca2+ was chelated by 1,2-bis(O-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA) of 10 mM, using the conventional whole-cell recording configuration, whereas both Iso and Isi were retained but gradually diminished. 7. In Ca(2+)-free external solution, these responses were fully elicited by the first application of theophylline. However, Ito disappeared during successive applications and Iso, but not Isi, also decreased. Similar results were obtained in the presence of ryanodine. 8. Theophylline apparently affects three different kinds of K+ currents in rat cortical neurones. Both Iso and Ito depend on internal calcium mobilized from an intracellular Ca2+ store by theophylline, while Isi was not primarily mediated by a change in [Ca2+]i.
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Affiliation(s)
- M Munakata
- Department of Neurophysiology, Tohoku University School of Medicine, Sendai, Japan
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58
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Chen H, Kurenny DE, Smith PA. Heparin prevents M-current over-recovery but not M-current suppression in bullfrog sympathetic ganglion neurones. Brain Res 1993; 625:323-7. [PMID: 8275314 DOI: 10.1016/0006-8993(93)91075-4] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023]
Abstract
The excitatory actions of G-protein-coupled agonists on amphibian sympathetic ganglion cells involve suppression of a voltage and time dependent, non-inactivating K(+)-current called the M-current. Suppression of this current by muscarine or peptides is followed by a phase of 'over-recovery' during which the M-current exceeds its original level. Whilst it has been suggested that release of intracellular Ca2+ following the agonist-induced liberation of inositol 1,4,5-trisphosphate is involved in the suppression phase of the response, another hypothesis suggests that the agonist-induced elevation of intracellular Ca2+ may account for M-current 'over-recovery'. The present study supports the latter hypothesis because intracellular application of the inositol 1,4,5-trisphosphate antagonist, heparin (150 microM), had little or no effect on muscarine-induced M-current suppression whilst the 'over-recovery' phase of the response was markedly attenuated.
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Affiliation(s)
- H Chen
- Department of Pharmacology, University of Alberta, Edmonton, Canada
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59
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Robbins J, Marsh SJ, Brown DA. On the mechanism of M-current inhibition by muscarinic m1 receptors in DNA-transfected rodent neuroblastoma x glioma cells. J Physiol 1993; 469:153-78. [PMID: 8271196 PMCID: PMC1143866 DOI: 10.1113/jphysiol.1993.sp019809] [Citation(s) in RCA: 48] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/29/2023] Open
Abstract
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)
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Affiliation(s)
- J Robbins
- Department of Pharmacology, University College London
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60
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Abstract
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.
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Affiliation(s)
- N V Marrion
- Department of Neurobiology and Behavior, SUNY, Stony Brook 11794
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61
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Abstract
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.
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Affiliation(s)
- M P Caulfield
- Department of Pharmacology, University College London, U.K
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62
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Schlichter R, Mouginot D, Ciranna L, Feltz P. Modulation of GABAA receptor channels and calcium currents in cultured porcine melanotrophs. Ann N Y Acad Sci 1993; 680:199-210. [PMID: 7685574 DOI: 10.1111/j.1749-6632.1993.tb19685.x] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- R Schlichter
- Institut de Physiologie, Systèmes Endocrines (URA 1446 CNRS), Université Louis Pasteur, Strasbourg, France
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63
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Affiliation(s)
- K Krnjević
- Anaesthesia Research Department, McGill University, Montréal, Québec, Canada
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64
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Hara S, Kuba K. Mechanical modulation of a voltage-dependent non-inactivating K+ current in cultured bullfrog sympathetic neurones. Pflugers Arch 1993; 422:305-15. [PMID: 8437884 DOI: 10.1007/bf00374285] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023]
Abstract
Cultured bullfrog sympathetic ganglion cells were voltage-clamped with a whole-cell patch-clamp technique. Local flow of a solution (identical to the bathing solution) from a micropipette to a cell, but not other mechanical stimuli, produced a non-inactivating outward (in 34 cells out of 141) or inward (in 70 cells) current [I(f)(out) or I(f)(in), respectively] depending on cells. Both I(f)(out) and I(f)(in) appeared at voltages more positive than -60 mV. The mechanism, however, was activated even at -70 mV, as I(f)(out) or I(f)(in) appeared on shifting membrane potential to -30 mV immediately after the local flow. I(f)(out) and I(f)(in) were accompanied by increases and decreases, respectively, in the membrane conductance and current relaxation to a voltage jump between -30 mV and -55 mV without a change in its time constant (whose value was similar to that of a voltage-dependent non-inactivating K+ current, IM), and reversed at a membrane potential close to the equilibrium potential for K+. Both I(f)(out) and I(f)(in) were blocked by Ba2+ (4-8 mM), a blocker of IM, and by muscarine (10 microM), which produced either an "apparent inward" or outward current. A transient outward current activated by a voltage jump from -85 mV (or -75 mV) to -30 mV was little affected by a local flow of a solution which produced I(f)(out) or I(f)(in). These results suggest that the local solution flow produced I(f)(in) or I(f)(out) by deactivating or activating IM, respectively.
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Affiliation(s)
- S Hara
- Department of Physiology, Saga Medical School, Japan
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65
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Kimura Y, Higashida H. Dissection of bradykinin-evoked responses by buffering intracellular Ca2+ in neuroblastoma x glioma hybrid NG108-15 cells. Neurosci Res 1992; 15:213-20. [PMID: 1336834 DOI: 10.1016/0168-0102(92)90007-y] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
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.
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Affiliation(s)
- Y Kimura
- Department of Biophysics, Kanazawa University School of Medicine, Japan
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66
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Church J. A change from HCO3(-)-CO2- to hepes-buffered medium modifies membrane properties of rat CA1 pyramidal neurones in vitro. J Physiol 1992; 455:51-71. [PMID: 1336555 PMCID: PMC1175633 DOI: 10.1113/jphysiol.1992.sp019290] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
1. Intracellular recordings were obtained from CA1 pyramidal neurones in rat hippocampal slices. Perfusion with a HCO3(-)-CO2-free, HEPES-buffered medium at pH 7.4 produced a wide variety of reversible effects on neuronal excitability, compared to responses obtained under standard (21 mM-HCO3-, 5% CO2, pH 7.4) conditions. 2. Introduction of HCO3(-)-CO2-free medium most commonly elicited, within 5-20 min, a fall in resting membrane potential (Vm), a rise in threshold for Na(+)-dependent action potential generation, and a reduction in input resistance. Anomalous inward rectification in the hyperpolarizing direction and subthreshold inward rectification were commonly reduced in HEPES-buffered medium. More prolonged exposure (> or = 25 min) to HCO3(-)-CO2-free medium produced, on occasion, Na+ spike inactivation. 3. The amplitudes of the fast and medium after-hyperpolarizations (AHPs) following a single depolarizing current-evoked action potential were attenuated during perfusion with HEPES-buffered medium at pH 7.4, as was the composite AHP following a train of action potentials. 4. Perfusion with HEPES-buffered medium at pH 7.4 reduced the degree of spike frequency adaptation and abolished depolarizing current-evoked burst-firing behaviour when this was present under standard conditions. 5. In tetrodotoxin (TTX)- and tetraethylammonium (TEA)-poisoned neurones, perfusion with HCO3(-)-CO2-free medium at pH 7.4 slightly raised the threshold for activation of Ca(2+)-dependent potentials and slightly reduced their duration, compared to responses obtained in HCO3(-)-CO2-buffered medium at the same pH. The AHP following the Ca2+ spike was, however, markedly attenuated. 6. Perfusion with a low-pH HCO3(-)-CO2-buffered medium (7 mM-HCO3-, 5% CO2, pH 6.9) produced changes qualitatively similar to those observed during perfusion with HEPES-buffered medium at pH 7.4. Raising the pH of the HEPES-buffered medium to 7.8 or 7.9 reversed inconsistently and then only in part the changes noted on the transition from a HCO3(-)-CO2- to a HEPES-buffered medium at the same pH (7.4). 7. The effects noted are unlikely to be due to a direct action of HEPES itself on neuronal membrane conductances. Rather, I suggest that they are likely to be caused by intracellular acidosis consequent upon the omission of HCO3- and CO2 from the extracellular medium.
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Affiliation(s)
- J Church
- Department of Physiology, University of British Columbia, Vancouver, Canada
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67
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Konishi S, Song SY, Saito K. An after-depolarization following action potentials and its modulation by substance P in rat sympathetic neurons. Neurosci Lett 1992; 142:245-8. [PMID: 1280796 DOI: 10.1016/0304-3940(92)90383-i] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
Sympathetic neurons in rat coeliac-superior mesenteric ganglia (C-SMG) displayed an after-depolarization (ADP) following action potentials and after-hyperpolarization. The ADP had amplitudes of 3-10 mV and lasted for 2-6 s, during which membrane resistance and excitability of C-SMG neurons increased. The reversal potential of ADP was dependent on external K+ concentrations. The ADP was suppressed in a solution containing Cd2+ or zero Ca2+. The ADP thus appears to be produced by inhibition of certain K+ channels via a Ca(2+)-dependent process. Substances P (SP) depolarized ganglion cells and increased the ADP, leading to a long-lasting increase in membrane excitability of rat C-SMG neurons.
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Affiliation(s)
- S Konishi
- Department of Neuroscience, Mitsubishi Kasei Institute of Life Sciences, Tokyo, Japan
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68
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Chen H, Smith PA. M-currents in frog sympathetic ganglion cells: manipulation of membrane phosphorylation. Br J Pharmacol 1992; 105:329-34. [PMID: 1373098 PMCID: PMC1908666 DOI: 10.1111/j.1476-5381.1992.tb14254.x] [Citation(s) in RCA: 18] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022] Open
Abstract
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.
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Affiliation(s)
- H Chen
- Department of Pharmacology, University of Alberta, Edmonton, Canada
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69
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Kirkwood A, Lisman JE. Action potentials produce a long-term enhancement of M-current in frog sympathetic ganglion. Brain Res 1992; 580:281-7. [PMID: 1354558 DOI: 10.1016/0006-8993(92)90955-9] [Citation(s) in RCA: 12] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/17/2022]
Abstract
M-current is a voltage-gated K+ current that can be turned off by the muscarinic action of acetylcholine. We examined the effects of postsynaptic action potential firing on the level of M-current in B-cells of the bullfrog sympathetic ganglion. High frequency stimulation of action potentials induced an approximately two-fold increase in the level of the M-current that could last up to 35 min. The 'enhanced' M-current was similar to the 'resting' one in its time-dependence, voltage-dependence and sensitivity to neurotransmitters. Experiments were undertaken to examine the functional consequences of the enhanced M-current. Following high frequency stimulation the number of spikes evoked by depolarizing current was reduced. In addition, the excitatory postsynaptic potential (EPSP) evoked by maximal input became subthreshold, thereby blocking information flow through the ganglion cell. These results indicate that the enhancement of M-current by spikes provides a negative feedback mechanism for the control of excitability. It has been reported that postsynaptic stimulation of ganglion cells also produces a long-term increase in the nicotinic EPSP, but we were unable to confirm this observation.
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Affiliation(s)
- A Kirkwood
- Department of Biology, Brandeis University, Waltham, MA 02154
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70
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Goh JW, Sanchez-Vives MV, Sanches-Vives MV, Pennefather PS. Influence of Na/Ca exchange and mobilization of intracellular calcium on the time course of the slow afterhyperpolarization current (IAHP) in bullfrog sympathetic ganglion neurons. Neurosci Lett 1992; 138:123-7. [PMID: 1328955 DOI: 10.1016/0304-3940(92)90487-r] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022]
Abstract
IAHP is a calcium dependent potassium current that underlies slow afterhyperpolarizations following action potentials in bullfrog sympathetic ganglion neurons. The decay rate of IAHP increases with increasing calcium loads. This effect was found not to be due to mobilization on intracellular calcium from ryanodine and caffeine sensitive stores. The relation is not affected by ryanodine at concentrations that block mobilization in the presence of caffeine, a drug that enhances mobilization of those stores. Nor does the relation seem to be due to a reduction of the driving force of the Na/Ca exchange process. The relation between decay rate and calcium load persists when Na+ is replaced by Li+. Our results suggest that Na/Ca exchange and mobilization of intracellular calcium normally have little influence in determining the time course of IAHP in these neurons.
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Affiliation(s)
- J W Goh
- Department of Pharmacology and Toxicology, Queen's University, Kingston, Ont., Canada
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Lopez HS. Kinetics of G protein-mediated modulation of the potassium M-current in bullfrog sympathetic neurons. Neuron 1992; 8:725-36. [PMID: 1567621 DOI: 10.1016/0896-6273(92)90093-s] [Citation(s) in RCA: 22] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
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.
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Affiliation(s)
- H S Lopez
- Howard Hughes Medical Institute, Department of Neurobiology and Behavior, SUNY, Stony Brook, New York 11794
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Robbins J, Trouslard J, Marsh SJ, Brown DA. Kinetic and pharmacological properties of the M-current in rodent neuroblastoma x glioma hybrid cells. J Physiol 1992; 451:159-85. [PMID: 1403809 PMCID: PMC1176156 DOI: 10.1113/jphysiol.1992.sp019159] [Citation(s) in RCA: 75] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/26/2022] Open
Abstract
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)
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Affiliation(s)
- J Robbins
- Department of Pharmacology, University College London
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Abstract
The fluorescence properties of the calcium indicators Fura-2 and Fluo-3 have been investigated in the presence of the 'caged calcium' photolabile chelators Nitr-5 and DM-nitrophen. The excitation spectra of dilute solutions of these indicators was distorted by the presence of photolabile chelators, owing to differential absorbance of excitation light by the chelators, as well as calcium-dependent fluorescence of the chelators themselves. This distortion was altered on partial photolysis of the chelators, due to changes in their absorbance and fluorescence. At high concentrations of indicators (100 microM) and photolabile chelators (10 mM), similar to those used experimentally, DM-nitrophen quenched the fluorescence of Fluo-3 at low calcium concentrations. The results suggest that Fura-2 may be used with either chelator, and Fluo-3 with Nitr-5, to measure calcium released on photolysis of the caged compounds, but that careful calibration of the chelator-indicator mixture after the appropriate degree of photolysis is necessary.
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Affiliation(s)
- R S Zucker
- Department of Molecular and Cell Biology, University of California, Berkeley
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74
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Affiliation(s)
- P A Smith
- Department of Pharmacology, University of Alberta, Edmonton, Canada
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75
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Affiliation(s)
- R J Miller
- Department of Pharmacological and Physiological Sciences, University of Chicago, IL 60637
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