scholarly journals Analysis of Mechanisms of Spiking in Normally ‘Non-Spiking’ Motoneurone Somata in Crayfish

1989 ◽  
Vol 147 (1) ◽  
pp. 91-110
Author(s):  
GERARD CZTERNASTY ◽  
RAYMOND T. KADO ◽  
JAN BRUNER
Keyword(s):  
Voltage Clamp ◽  
Sodium Channels ◽  
Outward Current ◽  
Motor Neurone ◽  
Current Injection ◽  
Membrane Current ◽  
Calcium Currents ◽  
Resting Potential ◽  
Soma Membrane

The soma membrane of the abdominal giant motor neurone (MoG) of the crayfish was studied by use of current injection and by measurements of total and local membrane current under voltage-clamp. Depolarization of the soma from the recorded resting potential (about −70 mV) did not produce a regenerative potential. Sodium and calcium currents were observed under voltage-clamp. It was shown that the sodium spike cannot develop at the usual resting potential since sodium channels are already partially inactivated; the spike can appear after the removal of inactivation by a hyperpolarizing prepulse. It was also shown that an early outward current prevents the Ca2+; regenerative potential; the potential can appear after inactivation of this outward current by a depolarizing prepulse.

scholarly journals Ionic Basis of Membrane Potential and of Acetylcholine-evduced Currents in the Cell Body of the Cockroach Fast Coxal Depressor Motor Neurone

1990 ◽  
Vol 151 (1) ◽  
pp. 21-39 ◽  
Author(s):  
JONATHAN A. DAVID ◽  
DAVID B. SATTELLE
Keyword(s):  
Cell Body ◽  
Outward Current ◽  
Motor Neurone ◽  
Resting Potential ◽  
Inward Current ◽  
Current Voltage ◽  
Low K ◽  
Current Voltage Relationship ◽  
Ionic Basis ◽  
Voltage Relationship

The ionic basis of the resting potential and of the response to acetylcholine (ACh) has been investigated in the cell body membrane of the fast coxal depressor motor neurone in the metathoracic ganglion of the cockroach Periplaneta americana. By means of ion-sensitive microelectrodes, intracellular concentrations of three ion species were estimated (mmoll−1): [K+]i, 1443; [Na+]i, 9±1; [Cl−], 7±1. The resting potential of continuously superfused cells was −75.6±1.9mV at 22° C. A change in resting potential of 42.0±2.5mV accompanied a decade change in [K+]o. Experiments with (10−4moll−1) ouabain, Na+ injection, low temperature (10°C) and non-superfused cells indicated the presence of an electrogenic sodium pump. Under current-clamp, the cell body membrane was depolarized by sequentially applied, ionophoretic pulses (500ms duration) of ACh. Under voltage-clamp, such doses of ACh resulted in an inward current which was abolished in low-Na+ saline. Ion-sensitive electrodes revealed an increase in [Na+]i but no change in [Cl−1]j in response to externally applied ACh. The ACh-induced current-voltage relationship was shifted in a negative direction by low-K+ saline. The AChinduced inward current was usually followed by a delayed outward current which reversed at Ek. Low-K+ saline had the same effect on this outward component as depolarizing the membrane. This suggests that the outward current component is carried by K+. The ACh-induced inward current and the delayed outward current were potentiated either when [Ca2+]i was lowered by injecting the calcium chelator BAPTA or by exposure of the cell to low-Ca2+ saline. High-Ca2+ saline reduced the inward component of the response and produced a negative shift in the AChinduced current-voltage relationship. The amplitude of the delayed outward

Calcium-dependent current generating the afterhyperpolarization of hippocampal neurons

1986 ◽  
Vol 55 (6) ◽  
pp. 1268-1282 ◽  
Author(s):  
B. Lancaster ◽  
P. R. Adams
Keyword(s):  
Voltage Clamp ◽  
Hippocampal Neurons ◽  
Hippocampal Slices ◽  
Outward Current ◽  
Pyramidal Cells ◽  
Resting Potential ◽  
Tail Current ◽  
Calcium Dependent ◽  
K Current

A single-electrode voltage-clamp technique was employed on in vitro hippocampal slices to examine the membrane current responsible for the slow afterhyperpolarization (AHP) in CA1 pyramidal cells. This was achieved by using conventional procedures to evoke an AHP in current clamp, followed rapidly by a switch into voltage clamp (hybrid clamp). The AHP current showed a dependence on extracellular K+, which was close to that predicted for a K+ current by the Nernst equation. The AHP current could be blocked by Cd2+ or norepinephrine. Although the AHP current showed a requirement for voltage-dependent Ca2+ entry, the current did not show any clear intrinsic voltage dependence. Once activated, AHP current is not turned off by hyperpolarizing the membrane potential. The effects of norepinephrine, Cd2+, and tetraethylammonium (TEA) were used to identify an AHP current component to the outward current evoked by depolarizing voltage commands from holding potentials that approximate to the resting potential for these cells. The AHP current can contribute significantly to the outward current during the depolarizing command. Upon repolarization it is evident as a slow outward tail current. This slow tail current had the same time constant as AHP currents evoked by hybrid clamp. Fast components to the tail currents were also observed. These were sensitive to Cd2+ and TEA. They probably represent a voltage-sensitive gKCa, sometimes termed C-current. The strong sensitivity to voltage and TEA displayed by the conventionally described gKCa (IC) are properties inconsistent with the AHP. It seems likely that the AHP current (IAHP) represents a Ca2+-activated K+ current separate from IC and that these two currents coexist in the same cell.

Contribution of Ca and Ca-activated Cl channels to regenerative depolarization and membrane bistability of cone photoreceptors

1992 ◽  
Vol 68 (3) ◽  
pp. 745-755 ◽  
Author(s):  
S. Barnes ◽  
M. C. Deschenes
Keyword(s):  
Voltage Clamp ◽  
Current Injection ◽  
Resting Potential ◽  
Negative Slope ◽  
Equilibrium Potential ◽  
Ca Channels ◽  
Cone Photoreceptors ◽  
Membrane Hyperpolarization ◽  
Cl Channels

1. Cone photoreceptors in several vertebrate species generate Ca-dependent regenerative depolarizations (e.g., Ca spikes lasting up to 2 s) in response to current injection or surround illumination and may remain in a state of prolonged depolarization (e.g., a permanent plateau near 0 mV) after these stimuli. This paper, while confirming the role of Ca channels in the regenerative depolarization, demonstrates that Ca-activated Cl channels either enhance or hinder prolonged depolarization, depending on the value of the chloride equilibrium potential (ECl). 2. Current- and voltage-clamp recordings obtained with the whole-cell patch-clamp technique were compared in 158 isolated tiger salamander cones to determine the contribution of specific ion channel types to the two forms of depolarizing response. Cones dialyzed with CsCl or KCl intracellular solution (such that ECl = 0 mV) that had sustained negative slope regions in their current-voltage (I-V) relations recorded under voltage clamp, were, under current clamp, bistable with respect to their resting potential. Injection of approximately 20-pA steps of depolarizing current resulted in transitions from the negative stable membrane potential (near -50 mV) to a long lasting plateau around 0 mV. Injection of 200–300 pA of hyperpolarizing current could then force a return to the negative stable resting potential, although once repolarization occurred, current injection had to be reduced or terminated to prevent damaging hyperpolarization of the cell. 3. The inward currents accounting for the negative slope region of the I-V relation were carried in Ca and Ca-activated Cl channels. Specific block of Ca-activated Cl current (ICl(Ca)) by 100 microM niflumic acid (NFA) eliminated the prolonged depolarization, even though the negative slope conductance region in the I-V persisted and the cone could still produce the briefer Ca-dependent regenerative depolarizations. Application of 100 microM Cd2+ blocked both forms of depolarization. 4. Substitution of Ba2+, which among other actions did not activate ICl(Ca), usually supported regenerative depolarizations of shortened duration, demonstrating the role of Ca channels in the initial phase of these responses. 5. A difference was observed in the regenerative depolarization when ECl was shifted away from 0 mV, where it had been in the experiments described above. With ECl set to -40 or -60 mV by reduction of [Cl-] in the pipette, steady-state membrane bistability was eliminated and prolonged depolarization did not occur. Under these conditions, application of the Cl channel blocker NFA showed that ICl(Ca) contributes to membrane hyperpolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Potassium currents contributing to action potential repolarization and the afterhyperpolarization in rat vagal motoneurons

1992 ◽  
Vol 68 (5) ◽  
pp. 1834-1841 ◽  
Author(s):  
P. Sah ◽  
E. M. McLachlan
Keyword(s):  
Action Potential ◽  
Time Constant ◽  
Action Potentials ◽  
Potassium Current ◽  
Outward Current ◽  
Potassium Currents ◽  
Membrane Potentials ◽  
Calcium Currents ◽  
Resting Potential ◽  
Action Potential Repolarization

1. Intracellular recordings were made from neurons in the dorsal motor nucleus of the vagus (DMV) in transverse slices of rat medulla maintained in vitro at 30 degrees C. Neurons had a resting potential of -59.8 +/- 1.4 (SE) mV (n = 39) and input resistance of 293 +/- 23 M omega (n = 44). 2. Depolarization elicited overshooting action potentials that were blocked by tetrodotoxin (TTX; 1 microM). In the presence of TTX, two types of action potentials having low and high thresholds could be elicited. The action potentials were blocked by cobalt (2 mM) indicating they were mediated by calcium currents. 3. Under voltage clamp, depolarization of the cell from membrane potentials negative of the resting potential activated a transient potassium current. This current was selectively blocked by 4-aminopyridine (4-AP) (5 mM) and catechol (5 mM) indicating that it is an A-type current. This current inactivated with a time constant of 420 ms and recovered from inactivation with a time constant of 26 ms. 4. When calcium currents were blocked by cadmium or cobalt, the rate of action potential repolarization was slower. In the presence of tetraethylammonium (TEA; 200-400 microM) or charybdotoxin (CTX; 30 nM) a small "hump" appeared on the repolarizing phase of the action potential that was abolished by addition of cadmium. These results indicate that a calcium-activated potassium current (IC) contributes to action potential repolarization. 5. Actions potentials elicited from hyperpolarized membrane potentials repolarized faster than those elicited from resting membrane potential. This effect could be blocked by catechol, indicating that voltage-dependent potassium currents (IA) can also contribute to action-potential repolarization. In the presence of catechol and calcium channel blockers, action potentials still had a significant early afterhyperpolarization suggesting that another calcium independent outward current is also active during repolarization. This fast afterhyperpolarizations (AHP) was not blocked by TEA. 6. Action potentials were followed by prolonged AHPs, which had two phases. The initial part of the AHP was blocked by apamin (100 nM) indicating that it results from activation of SK type calcium-activated potassium channels. The slow phase was selectively blocked by catechol suggesting that it is due to activation of IA. 7. It is concluded that a TTX-sensitive sodium current and two calcium currents contribute to the action potential in rat DMV neurons. At least three different currents contribute to action-potential repolarization: IC, IA, and a third unidentified calcium-insensitive outward current.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Membrane current following activity in canine cardiac Purkinje fibers.

1984 ◽  
Vol 83 (5) ◽  
pp. 771-799 ◽  
Author(s):  
R T Falk ◽  
I S Cohen
Keyword(s):  
Voltage Clamp ◽  
Time Constant ◽  
Outward Current ◽  
Repetitive Stimulation ◽  
Membrane Current ◽  
Drive Current ◽  
Purkinje Fibers ◽  
Cardiac Purkinje Fibers ◽  
Rapid Stimulation ◽  
Mean Time

Membrane current following prolonged periods of rapid stimulation was examined in short (less than 1.5 mm) canine cardiac Purkinje fibers of radius less than 0.15 mm. The Purkinje fibers were repetitively stimulated by delivering trains of depolarizing voltage clamp pulses at rapid frequencies. The slowly decaying outward current following repetitive stimulation ("post-drive" current) is eliminated by the addition of 10(-5) M dihydro-ouabain. The post-drive current is attributed to enhanced Na/K exchange caused by Na loading during the overdrive. Depolarizing voltage clamp pulses initiated from negative (-80 mV) or depolarized (-50 mV) holding potentials can give rise to post-drive current because of activation of tetrodotoxin-sensitive or D600-sensitive channels. The magnitude of the post-drive current depends on the frequency of voltage clamp pulses, the duration of each pulse, and the duration of the repetitive stimulation. The time constant of decay of the post-drive current depends on extracellular [K] in accordance with Michaelis-Menten kinetics. The Km is 1.2 mM bulk [K], [K]B. The mean time constant in 4 mM [K]B is 83 s. Epinephrine (10(-5) M) decreases the time constant by 20%. The time constant is increased by lowering [Ca]o between 4 and 1 mM. Lowering [Ca]o further, to 0.1 mM, eliminates post-drive current following repetitive stimulation initiated from depolarized potentials. The latter result suggests that slow inward Ca2+ current may increase [Na]i via Na/Ca exchange.

Voltage-clamp analysis of the effects of classical conditioning on the hippocampus

1991 ◽  
Vol 65 (4) ◽  
pp. 796-807 ◽  
Author(s):  
J. V. Sanchez-Andres ◽  
D. L. Alkon
Keyword(s):  
Voltage Clamp ◽  
Outward Current ◽  
Pyramidal Cells ◽  
Linear Increase ◽  
Resting Potential ◽  
Equilibrium Potential ◽  
Training Experience ◽  
Ca1 Pyramidal Cells ◽  
Cholinergic Agents ◽  
K Currents

1. Effects of nictitating membrane conditioning on K+ currents of CA1 pyramidal cells of rabbit hippocampus were studied by the use of the single-electrode voltage-clamp (SEVC) technique. 2. IQ, IM, IC, and IAHP were recorded in slices from control animals, showing behavior similar to that previously described for other preparations. IQ developed as an inward current during hyperpolarizing steps to potentials more negative than the K+ equilibrium potential. IM appeared as an inward inactivating relaxation during hyperpolarizing pulses, from potentials slightly more positive than the resting potential (approximately -40 mV). Such depolarization is thought to activate the IM, IC was recorded during long depolarizing pulses as a slow outward current. IAHP appeared during short depolarizing pulses as an outward current peaking at approximately 200 ms after the pulse. Progressively more positive pulses were accompanied by a linear increase of the peak IAHP value. The slope of the IAHP-voltage relation was used for comparison of cells between groups of animals that had different training experience. 3. Responses of control cells to cholinergic agents were similar to those previously characterized in other preparations. Specifically, cholinergic agonists blocked IM and IAHP, partially reduced IC, and did not affect IQ. 4. Conditioning did not affect IQ, IM, and IC but reduced the slope values of the IAHP-voltage relation. This change is consistent with the conditioning-specific afterhyperpolarization (AHP) reduction previously reported. 5. The effect of conditioning on the IAHP but not on the IC, both Ca(2+)-dependent K+ currents, suggests a direct effect on the former, rather than a reduction of ICa2+ or a change in the levels of Cai2+.

Currents under voltage clamp of burst-forming neurons of the cardiac ganglion of the lobster (Homarus americanus)

1986 ◽  
Vol 56 (6) ◽  
pp. 1739-1762 ◽  
Author(s):  
K. Tazaki ◽  
I. M. Cooke
Keyword(s):  
Voltage Clamp ◽  
Action Potentials ◽  
Reversal Potential ◽  
Outward Current ◽  
Homarus Americanus ◽  
Resting Potential ◽  
Molluscan Neurons ◽  
Cardiac Ganglion ◽  
Inward Current ◽  
Tail Current

Crustacean cardiac ganglion neuronal somata, although incapable of generating action potentials, produce regenerative, slow (greater than 200 ms) depolarizing potentials reaching -20 mV (from -50 mV) in response to depolarizing stimuli. These potentials initiate a burst of action potentials in the axon and are thus termed driver potentials. The somata of the anterior-most neurons (cells 1 or 2) were isolated by ligaturing for study of their membrane currents with a two-electrode voltage clamp. Inward current is attributed to Ca2+ by reason of dependence of driver potential amplitude on [Ca2+]0, independence of [Na+]0, resistance to tetrodotoxin, and inhibition by Cd (0.2 mM) and Mn (4 mM). Ca-mediated current (ICa) is present at -40 mV. It is optimally activated by a holding potential (Vh) of -50 to -60 mV and by clamps (command potential, Vc) to -10 mV. Time to peak (10-30 ms) and amplitude are strongly voltage dependent. Maximum tail-current amplitudes observed at -70 to -85 mV are ca. 100 nA. Inward tail peaks may not be resolved by our clamp (settling time, 2 ms). Tails relax with a time constant (tau) of approximately equal to 12 ms (at -70 to -85 mV). ICa exhibits inactivation in double pulse regimes. Recovery has a tau of approximately equal to 0.7 s. Tail current analyses indicate an exponential decline (tau approximately equal to 23 ms at -20 mV) toward a maintained amplitude of inward current tails. Analysis of outward currents indicates the presence of three conductance mechanisms having voltage dependences, time courses, and pharmacology similar to those of early outward current (IA), delayed outward current (IK), and outward current (IC) of molluscan neurons. Analysis of tail currents indicates a reversal potential for each of these near -75 mV, indicating that they are K currents. Early outward current, IA, shows a peak at 5 ms followed by rapid decline. Response to a second clamp given within 0.4 s is reduced; recovery is exponential, with a tau of approximately equal to 200 ms (at Vh = -50 mV). The amplitude of IA tested at 0 mV shows activation or deactivation by subthreshold shifts of Vh. The extent and rate of these changes shows voltage dependence (tau approximately equal to 100-500 ms for subthreshold prepulses). At the normal cell resting potential of -50 mV the amplitude of IA is 25% of that tested from -80 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Roles of Ca2+ and Na+ in the inward current and action potentials of guinea pig ureteral myocytes

1997 ◽  
Vol 272 (2) ◽  
pp. C535-C542 ◽  
Author(s):  
J. L. Sui ◽  
C. Y. Kao
Keyword(s):  
Guinea Pig ◽  
Action Potentials ◽  
Outward Current ◽  
Membrane Conductance ◽  
Membrane Current ◽  
Resting Potential ◽  
Inward Current ◽  
K Current ◽  
Free Media ◽  
Ca2 Channels

Physiological roles of Ca2+ vs. Na+ in membrane currents and action potentials of ureteral myocytes were investigated on freshly dissociated guinea pig ureteral myocytes with the patch-clamp method. The myocytes are spindle shaped, with cell volume of 2,473 microm3, surface area of 2,014 microm2, capacitance of 48.2 pF, resting potential of -47.9 mV, and membrane conductance of 840 pS. The membrane current consists of a slow inward Ca2+ current (ICa) conducted by L-type Ca2+ channels and an actively fluctuating Ca2+-activated K+ current [IK(Ca)] conducted by Ca2+-activated maxi-K+ channels. ICa dominates the membrane current by being long lasting and more active at less depolarized potentials than IK(Ca) and by regulating IK(Ca). Ca2+-free media, Co2+, and nifedipine reduce or block ICa, whereas high extracellular Ca2+ concentration and BAY K 8644 enhance it. Action potential amplitudes and plateaus are regulated correspondingly. Related changes are also seen in IK(Ca) In contrast, no fast inward current attributable to Na+ was found. Replacing extracellular Na+ with tris(hydroxymethyl)aminomethane had no apparent effects on the inward or outward current or on the action potentials.

scholarly journals Two Fast Transient Current Components during Voltage Clamp on Snail Neurons

1971 ◽  
Vol 58 (1) ◽  
pp. 36-53 ◽  
Author(s):  
Erwin Neher
Keyword(s):  
Voltage Clamp ◽  
Ganglion Cells ◽  
Helix Pomatia ◽  
Outward Current ◽  
Giant Cells ◽  
Resting Potential ◽  
Transient Outward Current ◽  
Pacemaker Activity ◽  
Inward Currents ◽  
Fast Transient

Voltage clamp currents from medium sized ganglion cells of Helix pomatia have a fast transient outward current component in addition to the usually observed inward and outward currents. This component is inactivated at normal resting potential. The current, which is carried by K+ ions, may surpass leakage currents by a factor of 100 after inactivation has been removed by hyperpolarizing conditioning pulses. Its kinetics are similar to those of the inward current, except that it has a longer time constant of inactivation. It has a threshold close to resting potential. This additional component is also present in giant cells, where however, it is less prominent. Pacemaker activity is controlled by this current. It was found that inward currents have a slow inactivating process in addition to a fast, Hodgkin-Huxley type inactivation. The time constants of the slow process are similar to those of slow outward current inactivation.

Passively propagated spikes from the soma of cells in pineal gland of guinea pigs

1989 ◽  
Vol 257 (4) ◽  
pp. C802-C809 ◽  
Author(s):  
H. C. Parkington ◽  
H. A. Coleman ◽  
I. McCance
Keyword(s):  
Pineal Gland ◽  
Voltage Clamp ◽  
Nerve Stimulation ◽  
Guinea Pigs ◽  
Outward Current ◽  
Membrane Properties ◽  
Resting Potential ◽  
Slow Inward Current ◽  
Alpha Adrenoceptors ◽  
Current Steps

The membrane properties of cells within the pineal gland of guinea pigs were studied using intracellular electrophysiological techniques. The electrotonic responses to intracellular current injection decayed with a single exponential in approximately 60% of cells but was preceded by a quicker component in the remainder. The membrane time constant was 2.8 ms. Depolarization beyond -29 mV activated an outward current that reversed at around the value of the resting potential. Hyperpolarization activated a slow inward current. Spikes occurred in response to activation of alpha-adrenoceptors. They were resistant to tetrodotoxin but were abolished by nifedipine and verapamil, suggesting that calcium carries the current during their upstroke. Spikes could not be evoked by depolarizing current pulses of 1-ms to 2-s duration. The responses to hyperpolarizing current steps or voltage-clamp steps applied during the peak of spikes evoked by nerve stimulation were indistinguishable from the responses to those applied between spikes. During nerve stimulation, fluctuations were observed in the current trace of cells under voltage clamp, indicating that the spikes could not be voltage clamped successfully. It is concluded that the spikes occurring in response to nerve stimulation are generated on the processes of the pinealocytes and are passive in the soma.

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