Mechanism of block by ZD 7288 of the hyperpolarization-activated inward rectifying current in guinea pig substantia nigra neurons in vitro

1995 ◽  
Vol 74 (6) ◽  
pp. 2366-2378 ◽  
Author(s):  
N. C. Harris ◽  
A. Constanti
Keyword(s):  
Substantia Nigra ◽  
Guinea Pig ◽  
Voltage Clamp ◽  
Time Constant ◽  
Slow Inward Current ◽  
Current Clamp ◽  
Inward Current ◽  
Electrotonic Potential ◽  
Zd 7288

1. The effects of the novel bradycardic agent 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD 7288) (Zeneca) were investigated on the hyperpolarization-activated cationic current (Ih) in guinea pig substantia nigra pars compacta neurons in vitro, using a single-microelectrode current-clamp/voltage-clamp technique. 2. Under current-clamp conditions, injection of large negative current pulses (0.1-0.5 nA, 400 ms) evoked a slow depolarizing "sag" in the electrotonic potential due to activation of the slow inward (anomalous) rectifier. In voltage-clamp recordings, hyperpolarizing voltage steps from a holding potential of -60 mV (close to resting potential) elicited slow inward current relaxations with kinetic properties similar to those seen for other neuronal Ihs. 3. ZD 7288 (10-100 microM) produced a consistent abolition of the electrotonic potential sag with no effect on membrane potential or spike properties. Under voltage clamp, Ih amplitude was clearly reduced in a time- and concentration-dependent manner (apparent half-maximum blocking concentration = 2 microM); full block of Ih was typically achieved after 10-15 min of exposure to 50 microM ZD 7288, with no significant recovery observed after 1 h of washing. 4. A similar (although more rapid) block of Ih was seen after application of 3-5 mM Cs+ (partially reversible after 30 min of washing). 5. Partial block of Ih by 10 microM ZD 7288 was accompanied by a reduction in the maximum amplitude of the Ih activation curve, a small negative shift in its position on the voltage axis, and a linearization of the steady-state current-voltage relationship. The estimated Ih reversal potential, however, remained unaffected. 6. In 10 microM ZD 7288, the time course of Ih activation and deactivation was significantly slowed (within the range of -70 to -120 mV for the activation time constant and -70 to -90 mV for the inactivation time constant). 7. Blockade of Ih by ZD 7288 or Cs+ was independent of prior Ih activation (i.e., non-use dependent). 8. Intracellular loading with ZD 7288 also abolished the sag in the electrotonic voltage response and Ih relaxations, suggesting an intracellular site of action. By contrast, intracellular Cs+ had no effect on Ih properties. 9. Block of Ih by ZD 7288 (but not Cs+) was relieved by prolonged cell hyperpolarization, manifested as a slowly developing (half-time approximately 20 s) inward current at a holding potential of -100 mV. 10. We propose that ZD 7288, when applied externally, may behave as a "lipophilic" quaternary cation, capable of passing into the cell interior to block Ih channels in their closed state; this compound may thus prove a useful research tool, in place of Cs+, for studying the properties and significance of Ih currents in controlling neuronal function.

Neuromedin U Depolarizes Rat Hypothalamic Paraventricular Nucleus Neurons In Vitro by Enhancing IH Channel Activity

2003 ◽  
Vol 90 (2) ◽  
pp. 843-850 ◽  
Author(s):  
De-Lai Qiu ◽  
Chun-Ping Chu ◽  
Tetsuro Shirasaka ◽  
Takashi Nabekura ◽  
Takato Kunitake ◽  
...  
Keyword(s):  
Voltage Clamp ◽  
Paraventricular Nucleus ◽  
Dependent Manner ◽  
Inward Current ◽  
Whole Cell Patch Clamp ◽  
Voltage Dependent ◽  
Ih Channel ◽  
Parvocellular Neurons ◽  
Zd 7288

The effect of neuromedin U (NMU) on rat paraventricular nucleus (PVN) neurons was examined using whole cell patch-clamp recordings. Under current-clamp, 31% of PVN parvocellular neurons ( n = 243) were depolarized by 100 nM NMU, but magnocellular neurons were not affected. NMU (10 nM to 1 μM) resulted in increased basal firing rate and depolarization in a dose-dependent manner with an EC50 of 70 nM. NMU-induced depolarization was unaffected by co-perfusion with 0.5 μM TTX + 10 μM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) + 10 μM bicuculline. Extracellular application of 70 μM ZD 7288 completely inhibited NMU-induced depolarization. Under voltage-clamp, 1 μM NMU produced negligible inward current but did increase the hyperpolarization-activated current ( IH) at step potentials less than –80 mV. The effects of NMU on IH were voltage-dependent, and NMU shifted the IH conductance-voltage relationship ( V1/2) by about 10.8 mV and enhanced IH kinetics without changing the slope constant ( k). Extracellular application of 70 μM ZD 7288 or 3 mM Cs+ blocked IH and the effects of NMU in voltage-clamp. These results suggest that NMU selectively depolarizes the subpopulation of PVN parvocellular neurons via enhancement of the hyperpolarization-activated inward current.

Hyperpolarization-Activated Inward Current in Neurons of the Rat's Dorsal Nucleus of the Lateral Lemniscus In Vitro

1997 ◽  
Vol 78 (5) ◽  
pp. 2235-2245 ◽  
Author(s):  
Xiao Wen Fu ◽  
Borys L. Brezden ◽  
Shu Hui Wu
Keyword(s):  
Membrane Potential ◽  
Input Resistance ◽  
Resting Potential ◽  
Extracellular Potassium ◽  
Current Clamp ◽  
Lateral Lemniscus ◽  
Inward Current ◽  
Dorsal Nucleus ◽  
Maximal Activation

Fu, Xiao Wen, Borys L. Brezden, and Shu Hui Wu. Hyperpolarization-activated inward current in neurons of the rat's dorsal nucleus of the lateral lemniscus in vitro. J. Neurophysiol. 78: 2235–2245, 1997. The hyperpolarization-activated current ( I h) underlying inward rectification in neurons of the rat's dorsal nucleus of the lateral lemniscus (DNLL) was investigated using whole cell patch-clamp techniques. Patch recordings were made from DNLL neurons of young rats (21–30 days old) in 400 μm tissue slices. Under current clamp, injection of negative current produced a graded hyperpolarization of the cell membrane, often with a gradual sag in the membrane potential toward the resting value. The rate and magnitude of the sag depended on the amount of hyperpolarizing current. Larger current resulted in a larger and faster decay of the voltage. Under voltage clamp, hyperpolarizing voltage steps elicited a slowly activating inward current that was presumably responsible for the sag observed in the voltage response to a steady hyperpolarizing current recorded under current clamp. Activation of the inward current ( I h) was voltage and time dependent. The current just was seen at a membrane potential of −70 mV and was activated fully at −140 mV. The voltage value of half-maximal activation of I h was −78.0 ± 6.0 (SE) mV. The rate of I h activation was best approximated by a single exponential function with a time constant that was voltage dependent, ranging from 276 ± 27 ms at −100 mV to 186 ± 11 ms at −140 mV. Reversal potential ( E h) of I h current was more positive than the resting potential. Raising the extracellular potassium concentration shifted E h to a more depolarized value, whereas lowering the extracellular sodium concentration shifted E h in a more negative direction. I h was sensitive to extracellular cesium but relatively insensitive to extracellular barium. The current amplitude near maximal-activation (about −140 mV) was reduced to 40% of control by 1 mM cesium but was reduced to only 71% of control by 2 mM barium. When the membrane potential was near the resting potential (about −60 mV), cesium had no effect on the membrane potential, current-evoked firing rate and input resistance but reduced the spontaneous firing. When the membrane potential was more negative than −70 mV, cesium hyperpolarized the cell, decreased current-evoked firing and increased the input resistance. I h in DNLL neurons does not contribute to the normal resting potential but may enhance the extent of excitation, thereby making the DNLL a consistently powerful inhibitory source to upper levels of the auditory system.

The toxic effects of ouabain: A voltage-clamp study

1982 ◽  
Vol 60 (9) ◽  
pp. 1153-1159 ◽  
Author(s):  
Y. Deslauriers ◽  
E. Ruiz-Ceretti ◽  
O. F. Schanne ◽  
M. D. Payet
Keyword(s):  
Action Potential ◽  
Voltage Clamp ◽  
Sodium Current ◽  
Negative Inotropic Effect ◽  
Slow Inward Current ◽  
Inward Current ◽  
Toxic Concentration ◽  
Voltage Curve ◽  
Current Voltage Curve ◽  
High Concentrations

The electrophysiologic effects of a toxic concentration of ouabain (10−5 M) were studied in frog atrial trabeculae. The toxic concentration was determined by the appearance of a negative inotropic effect and an increase in basal tension. Current- and voltage-clamp measurements were performed. Ouabain did not alter the passive electrical properties of the preparation. Under current-clamp conditions the membrane depolarized and the action potential amplitude as well as its maximum rate of rise decreased. The current–voltage curve for the fast inward current was shifted toward more positive potentials and the maximum sodium current decreased. The maximum sodium conductance was also reduced. The process of reactivation of the fast inward current was accelerated. The slow inward current and the maximum slow conductance also decreased under ouabain. These effects could explain the negative inotropic action of high concentrations of glycosides, as well as the action potential changes observed by several investigators. They also help to understand the arrhythmogenic effects of high concentrations of digitalis.

Tl+-Ions: Comparison of the Effects on the Slow Inward Current and Contractility of Ventricular Tissues

1982 ◽  
Vol 37 (10) ◽  
pp. 1015-1022 ◽  
Author(s):  
J. Wiemer ◽  
R. Ziskoven ◽  
C. Achenbach
Keyword(s):  
Voltage Clamp ◽  
Current System ◽  
Slow Inward Current ◽  
Initial Increase ◽  
Purkinje Fibres ◽  
Cardiac Tissues ◽  
Inward Current ◽  
Current Voltage ◽  
Apparent Relationship ◽  
Time Courses

To conclude our investigation of thallium effects on cardiac tissues, we studied the slow inward current of sheep cardiac Purkinje fibres exposed to 10-7 to 10-5 ᴍ Tl+ for extended periods of up to 80 min. Our previous results had suggested a possible involvement of the slow inward current during thallium intoxication: a) the modification of contractility staircases observed during thallium exposure, b) action potential recordings of ventricular muscle, c) changes in spontaneous beating in sino-atrial preparations. The thallium levels chosen were between those yielding strong positive inotropic transients and those producing a marked long­term decay of contraction force.The slow inward current was measured using a conventional two-microelectrode-technique and the standard voltage clamp protocol for this current system. The experimental work was restricted to the determination of d∞, the kinetics of activation of the slow inward current and of īsi, the current voltage relation of the current system. This was necessary since the effects of thallium were known to be short-lived and therefore frequent repeat runs of the voltage clamp program had to be performed in order to obtain the time courses of possible transient changes.The results showed that the slow inward current was first increased and then declined at the low concentration of 10-7 ᴍ Tl+. At 10-5 m Tl+ the initial increase was smaller, whereas the decay of the slow inward current proceeded to lower values. Comparison with contractility measure­ments at the same concentrations of thallium showed a distinct parallelism between changes of the slow inward current and myocardial contractility. Despite this apparent relationship, we do not conclude that the contractile events are primarily a result of changes of the slow inward current, since thallium does not seem to specifically alter the parameters of the slow inward current at the membrane level.

Proarrhythmic and antiarrhythmic actions of ion channel blockers on arrhythmias in the heart: model study

1996 ◽  
Vol 271 (1) ◽  
pp. H329-H356 ◽  
Author(s):  
T. R. Chay
Keyword(s):  
Bifurcation Analysis ◽  
Slow Inward Current ◽  
K Channel ◽  
Inactivation Kinetics ◽  
High Threshold ◽  
Channel Blockers ◽  
Ring Size ◽  
Class Iii ◽  
Inward Current

We explain why 1) some class I and IV antiarrhythmia drugs could exert proarrhythmic action, 2) some class III drugs are effective in controlling reentrant arrhythmias, and 3) cycle length (CL) oscillation is involved in the termination or initiation of reentry. To explain these phenomena, we employ the following three means: bifurcation analysis, simulation, and model construction. Antiarrhythmia drugs are modeled by varying maximal conductances of Na+, Ca2+, and time-dependent delayed rectifying and time-independent inward rectifying K+ channels in the Beeler-Reuter model, where the model cells are arranged in a ring. Bifurcation analysis predicts that there is a critical ring size (CRS) at which infinite ring behavior suddenly breaks down. Channel blockers can affect CRS in different manners: Na+ and Ca2+ blockers shorten CRS, whereas delayed rectifying K+ channel blockers and the inward K+ channel blockers lengthen CRS. This differential explains why some antiarrhythmia drugs are proarrhythmic (i.e., shorten CRS) whereas others are antiarrhythmic (i.e., lengthen CRS). Simulation is then used to investigate how the drugs affect reentrant rhythms in the neighborhood of the CRS. We find that, in this region, CL, conduction velocity, and action potential duration become oscillatory. As ring size shrinks, the pattern of the oscillation becomes more complex. When the ring shrinks to a certain size, reentry can no longer be sustained, and it terminates after a few oscillatory cycles. To explain the basic mechanism involved in CL oscillation, we then construct a minimal model that contains a low-threshold fast inward current and a high-threshold slow inward current. With this model, we show that the two inward currents, with vastly different activation and inactivation kinetics, cause CL oscillations. Our results thus give theoretical explanations for the experimental finding of Frame's group in canine atrial tricuspid ring in vitro that class IC drugs can bring about stable reentry from nonsustained transient reentry, whereas class III drugs transform stable reentry to complex oscillations in CL. Our results also support the result of Frame's group, in that, in "adjustable" tricuspid rings, CL oscillation becomes more complex and its period becomes shorter as an excitable gap is shortened.

Voltage Clamp Studies on the Slow Inward Current during the Excitation ofNitellopsis obtusa

1980 ◽  
Vol 31 (2) ◽  
pp. 589-595 ◽  
Author(s):  
M. GYENES ◽  
A. A. BULYCHEV ◽  
G. A. KURELLA
Keyword(s):  
Voltage Clamp ◽  
Slow Inward Current ◽  
Inward Current

scholarly journals The Effects of Polarizing Current on Nerve Terminal Impulses Recorded from Polymodal and Cold Receptors in the Guinea-pig Cornea

2002 ◽  
Vol 120 (3) ◽  
pp. 395-405 ◽  
Author(s):  
Richard W. Carr ◽  
Svetlana Pianova ◽  
James A. Brock
Keyword(s):  
Guinea Pig ◽  
Nerve Terminal ◽  
Sensory Nerve ◽  
Outward Current ◽  
Nerve Terminals ◽  
Functional Difference ◽  
Inward Current ◽  
Cold Receptor ◽  
Cold Receptors

It was reported recently that action potentials actively invade the sensory nerve terminals of corneal polymodal receptors, whereas corneal cold receptor nerve terminals are passively invaded (Brock, J.A., S. Pianova, and C. Belmonte. 2001. J. Physiol. 533:493–501). The present study investigated whether this functional difference between these two types of receptor was due to an absence of voltage-activated Na+ conductances in cold receptor nerve terminals. To address this question, the study examined the effects of polarizing current on the configuration of nerve terminal impulses recorded extracellularly from single polymodal and cold receptors in guinea-pig cornea isolated in vitro. Polarizing currents were applied through the recording electrode. In both receptor types, hyperpolarizing current (+ve) increased the negative amplitude of nerve terminal impulses. In contrast, depolarizing current (−ve) was without effect on polymodal receptor nerve terminal impulses but increased the positive amplitude of cold receptor nerve terminal impulses. The hyperpolarization-induced increase in the negative amplitude of nerve terminal impulses represents a net increase in inward current. In both types of receptor, this increase in inward current was reduced by local application of low Na+ solution and blocked by lidocaine (10 mM). In addition, tetrodotoxin (1 μM) slowed but did not reduce the hyperpolarization-induced increase in the negative amplitude of polymodal and cold nerve terminal impulses. The depolarization-induced increase in the positive amplitude of cold receptor nerve terminal impulses represents a net increase in outward current. This change was reduced both by lidocaine (10 mM) and the combined application of tetraethylammomium (20 mM) and 4-aminopyridine (1 mM). The interpretation is that both polymodal and cold receptor nerve terminals possess high densities of tetrodotoxin-resistant Na+ channels. This finding suggests that in cold receptors, under normal conditions, the Na+ conductances are rendered inactive because the nerve terminal region is relatively depolarized.

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