Calcium-dependent potassium currents in neurons from cat sensorimotor cortex

1992 ◽  
Vol 67 (1) ◽  
pp. 216-226 ◽  
Author(s):  
P. C. Schwindt ◽  
W. J. Spain ◽  
W. E. Crill
Keyword(s):  
Membrane Potential ◽  
Voltage Clamp ◽  
Time Constant ◽  
Sensorimotor Cortex ◽  
Pyramidal Neurons ◽  
Outward Current ◽  
Tail Current ◽  
K Current ◽  
The Difference

1. Ca(2+)-dependent K+ currents were studied in large pyramidal neurons (Betz cells) from layer V of cat sensorimotor cortex by use of an in vitro brain slice and single microelectrode voltage clamp. The Ca(2+)-dependent outward current was taken as the difference current obtained before and after blockade of Ca2+ influx. During step depolarizations in the presence of tetrodotoxin (TTX), this current exhibited a fast onset of variable amplitude and a prominent slowly developing component. 2. The Ca(2+)-dependent outward current first appeared when membrane potential was stepped positive to -40 mV. Downsteps from a holding potential of -40 mV revealed little or no time-, voltage-, or Ca(2+)-dependent current. When membrane potential was stepped positive to -40 mV, a prolonged Ca(2+)-dependent outward tail current followed repolarization. The decay of this tail current at -40 mV was best described by a single exponential function having a time constant of 275 +/- 75 (SD) ms. The tail current reversed at 96 +/- 5 mV in 3 mM extracellular K+ concentration ([K+]o) and at more positive potentials when [K+]o was raised, suggesting that it was carried predominantly by K+. 3. The Ca(2+)-dependent K+ current consisted of two pharmacologically separable components. The slowly developing current was insensitive to 1 mM tetraethylammonium (TEA), but a substantial portion was reduced by 100 nM apamin. Most of the remaining current was blocked by the addition of isoproterenol (20-50 microM) or muscarine (10-20 microM). 4. The time courses of the apamin- and transmitter-sensitive components were similar when activated by step depolarizations in voltage clamp, but they were quite different when activated by a train of action potentials. Applying the voltage clamp at the end of a train of 90 spikes (evoked at 100-200 Hz) resulted in an Ca(2+)-dependent K+ current with a prominent rapidly decaying portion (time constant approximately 50 ms at -64 mV) and a smaller slowly decaying portion (time constant approximately 500 ms at -64 mV). The rapidly decaying portion was blocked by apamin (50-200 nM), and the slowly decaying portion was blocked by isoproterenol (20-50 microM). 5. When recorded with microelectrodes containing 2 mM dimethyl-bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (dimethyl-BAPTA), which causes prolonged afterhyperpolarizations, the Ca(2+)-dependent K+ current evoked by step depolarizations had an extremely slow onset and decay. The current recorded after a train of evoked spikes had a similar slow decay.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Influence of anomalous rectifier activation on afterhyperpolarizations of neurons from cat sensorimotor cortex in vitro

1988 ◽  
Vol 59 (2) ◽  
pp. 468-481 ◽  
Author(s):  
P. C. Schwindt ◽  
W. J. Spain ◽  
W. E. Crill
Keyword(s):  
Membrane Potential ◽  
Voltage Clamp ◽  
Sensorimotor Cortex ◽  
Model Parameters ◽  
Fast Decay ◽  
Time Constants ◽  
Anomalous Rectifier ◽  
Betz Cells ◽  
K Currents

1. Large neurons from layer V of cat sensorimotor cortex (Betz cells) were studied to determine the influence of the anomalous rectifier current (IAR) on slow afterhyperpolarizations (AHPs). The neurons were examined using intracellular recording and single-microelectrode voltage clamp in an in vitro brain slice preparation. 2. A faster medium-duration AHP (mAHP) and slower AHP (sAHP) followed repetitive firing (22, 23). The amplitude of the mAHP often increased or remained constant during membrane potential hyperpolarization. The membrane potential trajectory resulting solely from IAR activation was similar to the mAHP. 3. Postrepetitive firing voltage clamp was used to measure directly slowly decaying K+ currents (IK) and IAR at different membrane potentials. IK exhibited both a fast and slow decay. The time constants of the fast decay of IK and IAR activation were similar. IAR increased with hyperpolarization or raised extracellular K+ concentration [( K+]o), whereas both the fast and slow components of IK reversed or nulled near -100 mV and behaved as pure K+ currents in response to raised [K+]o. 4. To determine the precise contribution of IK and IAR to the AHP waveform, theoretical AHPs were computed using a quantitative model based on voltage-clamp measurements. The calculated AHPs were qualitatively similar to measured AHPs. The amplitude of the mAHP showed little change with hyperpolarization because of the increasing dominance of IAR at more negative membrane potentials. The sAHP was little affected by IAR activation. 5. Several model parameters subject to biological variation among Betz cells were varied in the calculations to determine their importance in the AHP waveform. With IK parameters held constant, the amplitude and time course of the mAHP depended on resting potential, membrane time constant, the kinetics of the anomalous rectifier conductance (GAR), and the maximum value of GAR. IAR activation could result in a biphasic AHP even when the fast decay of IK was omitted from the calculations. 6. A wider variation of model parameters revealed behavior that may be relevant to other neurons. Certain values of membrane or IAR activation time constants resulted in a monophasic AHP even when the fast decay of IK was present. The decay of a biphasic AHP could reflect either the onset of IAR or the fast decay of IK, depending on the relative value of their time constants. Procedures are outlined to discriminate between these possibilities using current clamp methods.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Membrane Dysfunction Induced by In Vitro Ischemia in Rat Hippocampal CA1 Pyramidal Neurons

1999 ◽  
Vol 81 (4) ◽  
pp. 1872-1880 ◽  
Author(s):  
E. Tanaka ◽  
S. Yamamoto ◽  
H. Inokuchi ◽  
T. Isagai ◽  
H. Higashi
Keyword(s):  
Membrane Potential ◽  
Voltage Clamp ◽  
Pyramidal Neurons ◽  
Membrane Surface ◽  
Bathing Medium ◽  
Ca1 Pyramidal Neurons ◽  
In Vitro Ischemia ◽  
Hippocampal Ca1 ◽  
Electrode Voltage

Membrane dysfunction induced by in vitro ischemia in rat hippocampal CA1 pyramidal neurons. Intracellular and single-electrode voltage-clamp recordings were made to investigate the process of membrane dysfunction induced by superfusion with oxygen and glucose-deprived (ischemia-simulating) medium in hippocampal CA1 pyramidal neurons of rat tissue slices. To assess correlation between potential change and membrane dysfunction, the recorded neurons were stained intracellularly with biocytin. A rapid depolarization was produced ∼6 min after starting superfusion with ischemia-simulating medium. When oxygen and glucose were reintroduced to the bathing medium immediately after generating the rapid depolarization, the membrane did not repolarize but depolarized further, the potential reaching 0 mV ∼5 min after the reintroduction. In single-electrode voltage-clamp recording, a corresponding rapid inward current was observed when the membrane potential was held at −70 mV. After the reintroduction of oxygen and glucose, the current induced by ischemia-simulating medium partially returned to preexposure levels. These results suggest that the membrane depolarization is involved with the membrane dysfunction. The morphological aspects of biocytin-stained neurons during ischemic exposure were not significantly different from control neurons before the rapid depolarization. On the other hand, small blebs were observed on the surface of the neuron within 0.5 min of generating the rapid depolarization, and blebs increased in size after 1 min. After 3 min, neurons became larger and swollen. The long and transverse axes and area of the cross-sectional cell body were increased significantly 1 and 3 min after the rapid depolarization. When Ca2+-free (0 mM) with Co2+ (2.5 mM)-containing medium including oxygen and glucose was applied within 1 min after the rapid depolarization, the membrane potential was restored completely to the preexposure level in the majority of neurons. In these neurons, the long axis was lengthened without any blebs being apparent on the membrane surface. These results suggest that the membrane dysfunction induced by in vitro ischemia may be due to a Ca2+-dependent process that commences ∼1.5 min after and is completed 3 min after the onset of the rapid depolarization. Because small blebs occurred immediately after the rapid depolarization and large blebs appeared 1.5–3 min after, it is likely that the transformation from small to large blebs may result in the observed irreversible membrane dysfunction.

Anomalous rectification in neurons from cat sensorimotor cortex in vitro

1987 ◽  
Vol 57 (5) ◽  
pp. 1555-1576 ◽  
Author(s):  
W. J. Spain ◽  
P. C. Schwindt ◽  
W. E. Crill
Keyword(s):  
Voltage Clamp ◽  
Time Constant ◽  
Sensorimotor Cortex ◽  
Brain Slices ◽  
Resting Potential ◽  
Extracellular Potassium ◽  
Membrane Hyperpolarization ◽  
Anomalous Rectification ◽  
Voltage Dependent

The ionic mechanisms underlying anomalous rectification in large neurons from layer V of cat sensorimotor cortex were studied in an in vitro brain slice. The anomalous rectification was apparent as an increase of slope conductance during membrane hyperpolarization, and the development of anomalous rectification during a hyperpolarizing current pulse was signaled by a depolarizing sag of membrane potential toward resting potential (RP). Voltage-clamp analysis revealed the time- and voltage-dependent inward current (IAR) that produced anomalous rectification. IAR reversal potential (EAR) was estimated to be approximately -50 mV from extrapolation of linear, instantaneous, current-voltage relations. The conductance underlying IAR (GAR) had a sigmoidal steady-state activation characteristic. GAR increased with hyperpolarization from -55 to -105 mV with half-activation at approximately -82 mV. The time course of both GAR and IAR during a voltage step was described by two exponentials. The faster exponential had a time constant (tau F) of approximately 40 ms; the slow time constant (tau S) was approximately 300 ms. Neither tau F nor tau S changed with voltage in the range -60 mV to -110 mV. The fast component constituted approximately 80% of IAR at each potential. Both IAR and GAR increased in raised extracellular potassium [( K+]o) and EAR shifted positive, but the GAR activation curve did not shift along the voltage axis. Solutions containing an impermeable Na+ substitute caused an initial transient decrease in IAR followed by a slower increase of IAR. Brain slices bathed in Na+-substituted solution developed a gradual increase in [K+]o as measured with K+-sensitive microelectrodes. We conclude that GAR is permeable to both Na+ and K+, but the full contribution of Na+ was masked by the slow increase of [K+]o that occurred in Na+ substituted solutions. Chloride did not appear to contribute significantly to IAR since estimates of EAR were similar in neurons impaled with microelectrodes filled with potassium chloride or methylsulfate, whereas, ECl (estimated from reversal of a GABA-induced ionic current) was approximately 30 mV more positive with the KCl-filled microelectrodes. Extracellular Cs+ caused a reversible dose- and voltage-dependent reduction of GAR, whereas intracellular Cs+ was ineffective. The parameters measured during voltage clamp were used to formulate a quantitative empirical model of IAR.(ABSTRACT TRUNCATED AT 400 WORDS)

Tachykinins modulate multiple ionic conductances in voltage-clamped rat spinal dorsal horn neurons

1989 ◽  
Vol 61 (4) ◽  
pp. 854-865 ◽  
Author(s):  
K. Murase ◽  
P. D. Ryu ◽  
M. Randic
Keyword(s):  
Dorsal Horn ◽  
Outward Current ◽  
K Channel ◽  
Neurokinin A ◽  
Current Response ◽  
Bathing Medium ◽  
Tail Current ◽  
Dorsal Horn Neurons ◽  
K Current

1. The membrane actions of substance P (SP) and a related tachykinin, neurokinin A (NKA), have been investigated by means of a single-electrode, voltage-clamp technique in the immature rat dorsal horn neurons using an in vitro spinal cord slice preparation. 2. When the membrane potential was held at the resting level of between -75 and -55 mV, bath application of SP or NKA (10(-7) to 10(-5) M, for 1-3 min) induced an inward shift in the holding current lasting several minutes. The magnitude of this effect varied between 10 and 400 pA depending on the concentration of the peptides and the holding potential. 3. When a dorsal horn neuron was held at the resting level and subjected to 1-s depolarizing commands to membrane potentials between -60 and -35 mV, slow inward relaxations and inward tail currents, the latter on repolarization to the holding potential, were recorded. During the tachykinin-induced inward shift in the holding current, the inward relaxation and the tail current were augmented in a dose-related manner. 4. The SP-induced augmentation of the slow inward relaxation and the inward tail current is likely to be due to the enhancement of the activation of the Ca2+ current, because the effect was present, and even augmented in a zero-Ca2+, Ba2+-containing solution, it was reduced or completely abolished by zero-Ca2+, Co2+-, or Mg2+-containing solutions and is largely independent of the changes in external Na+, K+, or Cl- ions. Moreover, in the presence of the K+-channel blocker, tetraethylammonium (TEA), the effect is increased. 5. Depolarizing voltage commands to potentials positive to -35 mV evoked a large, outward K+ current response in the dorsal horn neurons, which was in part Ca2+-sensitive. The outward current response was augmented by SP. The SP effect persists, although being reduced in a zero-Ca2+, Ba2+- or Co2+-containing solutions. 6. In a zero-Ca2+ solution containing Co2+ and TEA, the augmentation of the Ca2+ current and the outward K+ current by SP was abolished. However, the SP-induced increase in a Ca2+-sensitive, voltage-insensitive conductance remained, although being reduced, and the response showed a reversal at about -28 mV. This current may be a result of a tachykinin-activated nonspecific increase in cationic permeability of the membrane of dorsal horn neurons, because the current is reduced by more than one-half when Na+ or Ca2+ is removed from the bathing medium.(ABSTRACT TRUNCATED AT 400 WORDS)

scholarly journals Extracellular Hco3− Dependence of Electrogenic Na/Hco3 Cotransporters Cloned from Salamander and Rat Kidney

2000 ◽  
Vol 115 (5) ◽  
pp. 533-546 ◽  
Author(s):  
Irina I. Grichtchenko ◽  
Michael F. Romero ◽  
Walter F. Boron
Keyword(s):  
Membrane Potential ◽  
Voltage Clamp ◽  
Xenopus Oocytes ◽  
Rat Kidney ◽  
Outward Current ◽  
Membrane Vesicles ◽  
Disulfonic Acid ◽  
Ambystoma Tigrinum ◽  
Ph Titration ◽  
Electrode Voltage

We studied the extracellular [HCOabstract 3 −] dependence of two renal clones of the electrogenic Na/HCO3 cotransporter (NBC) heterologously expressed in Xenopus oocytes. We used microelectrodes to measure the change in membrane potential (ΔVm) elicited by the NBC cloned from the kidney of the salamander Ambystoma tigrinum (akNBC) and by the NBC cloned from the kidney of rat (rkNBC). We used a two-electrode voltage clamp to measure the change in current (ΔI) elicited by rkNBC. Briefly exposing an NBC-expressing oocyte to HCOabstract 3 −/CO2 (0.33–99 mM HCOabstract 3−, pHo 7.5) elicited an immediate, DIDS (4,4-diisothiocyanatostilbene-2,2-disulfonic acid)-sensitive and Na+-dependent hyperpolarization (or outward current). In ΔVm experiments, the apparent Km for HCOabstract 3− of akNBC (10.6 mM) and rkNBC (10.8 mM) were similar. However, under voltage-clamp conditions, the apparent Km for HCOabstract 3− of rkNBC was less (6.5 mM). Because it has been reported that SOabstract 3=/HSO abstract 3− stimulates Na/HCO3 cotransport in renal membrane vesicles (a result that supports the existence of a COabstract 3= binding site with which SOabstract 3= interacts), we examined the effect of SOabstract 3=/HSO abstract 3− on rkNBC. In voltage-clamp studies, we found that neither 33 mM SOabstract 4= nor 33 mM SOabstract 3 =/HSOabstract 3− substantially affects the apparent Km for HCO abstract 3−. We also used microelectrodes to monitor intracellular pH (pHi) while exposing rkNBC-expressing oocytes to 3.3 mM HCOabstract 3 −/0.5% CO2. We found that SO abstract 3=/HSOabstract 3 − did not significantly affect the DIDS-sensitive component of the pHi recovery from the initial CO2 -induced acidification. We also monitored the rkNBC current while simultaneously varying [CO2]o, pHo, and [COabstract 3=]o at a fixed [HCOabstract 3−]o of 33 mM. A Michaelis-Menten equation poorly fitted the data expressed as current versus [COabstract 3=]o . However, a pH titration curve nicely fitted the data expressed as current versus pHo. Thus, rkNBC expressed in Xenopus oocytes does not appear to interact with SOabstract 3 =, HSOabstract 3−, or COabstract 3=.

Long-lasting reduction of excitability by a sodium-dependent potassium current in cat neocortical neurons

1989 ◽  
Vol 61 (2) ◽  
pp. 233-244 ◽  
Author(s):  
P. C. Schwindt ◽  
W. J. Spain ◽  
W. E. Crill
Keyword(s):  
Voltage Clamp ◽  
Time Course ◽  
Outward Current ◽  
Repetitive Firing ◽  
K Channel ◽  
Channel Blockers ◽  
Dantrolene Sodium ◽  
Current Time ◽  
Tail Current ◽  
Neocortical Neurons

1. The function and ionic mechanism of a slow outward current were studied in large layer V neurons of cat sensorimotor cortex using an in vitro slice preparation and single microelectrode voltage clamp. 2. With Ca2+ influx blocked, a slow relaxation ("tail") of outward current followed either (1) repetitive firing evoked for 1 s or (2) a small 1-s depolarizing voltage clamp step that activated the persistent Na+ current of neocortical neurons, INaP. When a depolarization that activated INaP was maintained, an outward current gradually developed and increased in amplitude over a period of tens of seconds to several minutes. An outward tail current of similar duration followed repolarization. The slow outward current was abolished by TTX, indicating it depended on Na+ influx. 3. With Ca2+ influx blocked, the onset of the slow Na+-dependent outward current caused spike frequency adaptation during current-evoked repetitive firing. Following the firing, the decay of the Na+-dependent current caused a slow afterhyperpolarization (sAHP) and a long-lasting reduction of excitability. It also was responsible for habituation of the response to repeated identical current pulses. 4. The Na+-dependent tail current had properties expected of a K+ current. Membrane chord conductance increased during the tail, and tail amplitude was reduced or reversed by membrane potential hyperpolarization and raised extracellular K+ concentration [( K+]0). 5. The current tail was reduced reversibly by the K+ channel blockers TEA (5-10 mM), muscarine (5-20 microM), and norepinephrine (100 microM). These agents also resulted in a larger, more sustained inward current during the preceding step depolarization. Comparison of current time course before and after the application of blocking agents suggested that, in spite of its capability for slow buildup and decay, the onset of the Na+-dependent outward current occurs within 100 ms of an adequate step depolarization. 6. With Ca2+ influx blocked, extracellular application of dantrolene sodium (30 microM) had no clear effect on the current tail or the corresponding sAHP.(ABSTRACT TRUNCATED AT 400 WORDS)

Ca-Induced K+-Outward Current in Paramecium Tetraurelia

1980 ◽  
Vol 88 (1) ◽  
pp. 293-304 ◽  
Author(s):  
YOUKO SATOW ◽  
CHING KUNG
Keyword(s):  
Steady State ◽  
Voltage Clamp ◽  
Outward Current ◽  
Paramecium Tetraurelia ◽  
Ca Channels ◽  
Outward Currents ◽  
Ciliary Reversal ◽  
Voltage Dependent ◽  
K Current ◽  
K Outward Current

Late K-outward currents upon membrane depolarization were recorded in Paramecium tetraurelia under a voltage clamp. A Ca-induced K-outward component is demonstrated by subtracting the value of the outward current in a pawn A mutant lacking functional Ca-channels (pwA500). The Ca-induced K-outward current activates slowly, reaching a peak after 100 to 1000 ms. The current then remains steady or reaches the steady state after a decline of several seconds. EGTA2- injection experiments show that the Ca-induced K-outward current is dependent on the internal Ca2+ concentration. The current is shown to depend on the voltage-dependent Ca conductance, by study of the leaky pawn A mutant (pwA132), which has a lowered Ca conductance as well as a lowered Ca-induced K-current. The Ca-induced GK is thus indirectly dependent on the voltage. The maximal GK is about 40 nmho/cell at + 7 mV in 4 mM-K+. The Ca-induced K current is sustained throughout the prolonged depolarization and the prolonged ciliary reversal.

Effect of cromakalim and lemakalim on slow waves and membrane currents in colonic smooth muscle

1991 ◽  
Vol 260 (2) ◽  
pp. C375-C382 ◽  
Author(s):  
J. M. Post ◽  
R. J. Stevens ◽  
K. M. Sanders ◽  
J. R. Hume
Keyword(s):  
Smooth Muscle ◽  
Membrane Potential ◽  
Slow Wave ◽  
Outward Current ◽  
Wave Activity ◽  
Slow Waves ◽  
Slow Wave Activity ◽  
Ca Current ◽  
K Current ◽  
Stimulation Of

The effects of cromakalim (BRL 34915) and its optical isomer lemakalim (BRL 38227) were investigated in intact tissue and freshly dispersed circular muscle cells from canine proximal colon. Cromakalim and lemakalim hyperpolarized resting membrane potential, shortened the duration of slow waves by abolishing the plateau phase, and decreased the frequency of slow waves. Glyburide, a K channel blocker, prevented the effect of cromakalim on slow-wave activity. The mechanisms of these alterations in slow-wave activity were studied in isolated myocytes under voltage-clamp conditions. Cromakalim and lemakalim increased the magnitude of a time-independent outward K current, but cromakalim also reduced the peak outward K current. Glyburide inhibited lemakalim stimulation of the time-independent background current. Nisoldipine also reduced the peak outward current, and in the presence of nisoldipine, cromakalim did not affect the peak outward component of current. This suggested that cromakalim may block a Ca-dependent component of the outward current. Lemakalim did not affect the peak outward current. We tested whether the effects of cromakalim on outward current might be indirect due to an effect on inward Ca current. Cromakalim, but not lemakalim, was found to inhibit L-type Ca channels; however, glyburide did not alter cromakalim inhibition of inward Ca current. We conclude that the effects of cromakalim and lemakalim on membrane potential and slow waves in colonic smooth muscle appear to result primarily from stimulation of a time-independent background K conductance. The effects of these compounds on channel activity may explain the inhibitory effect of these compounds on contractile activity.

Guinea pig visceral C-fiber neurons are diverse with respect to the K+ currents involved in action-potential repolarization

1994 ◽  
Vol 71 (2) ◽  
pp. 561-574 ◽  
Author(s):  
E. P. Christian ◽  
J. Togo ◽  
K. E. Naper
Keyword(s):  
Guinea Pig ◽  
Action Potential ◽  
Outward Current ◽  
Dependent Manner ◽  
Concentration Dependent Manner ◽  
Whole Cell ◽  
C Fiber ◽  
K Current ◽  
Action Potential Repolarization

1. Intracellular recordings were made from C-fiber neurons identified by antidromic conduction velocity in intact guinea pig nodose ganglia maintained in vitro, and whole-cell patch clamp recordings were made from dissociated guinea pig nodose neurons to investigate the contribution of various K+ conductances to action-potential repolarization. 2. The repolarizing phase of the intracellularly recorded action potential was prolonged in a concentration-dependent manner by charybdotoxin (Chtx; EC50 = 39 nM) or iberiatoxin (Ibtx; EC50 = 48 nM) in a subpopulation of 16/36 C-fiber neurons. In a subset of these experiments, removal of extracellular Ca2+ reversibly prolonged action-potential duration (APD) in the same 4/9 intracellularly recorded C-fiber neurons affected by Chtx (> or = 100 nM). These convergent results support that a Ca(2+)-activated K+ current (IC) contributes to action-potential repolarization in a restricted subpopulation of C-fiber neurons. 3. Tetraethylammonium (TEA; 1-10 mM) increased APD considerably further in the presence of 100-250 nM Chtx or Ibtx, or in nominally Ca(2+)-free superfusate in 14/14 intracellularly recorded C-fiber neurons. TEA affected APD similarly in subpopulations of neurons with and without IC, suggesting that a voltage-dependent K+ current (IK) contributes significantly to action-potential repolarization in most nodose C-fiber neurons. 4. Substitution of Mn2+ for Ca2+ reduced outward whole-cell currents elicited by voltage command steps positive to -30 mV (2-25 ms) in a subpopulation of 21/36 dissociated nodose neurons, supporting the heterogeneous expression of IC. The kinetics of outward tail current relaxations (tau s of 1.5-2 ms) measured at the return of 2-3 ms depolarizing steps to -40 mV were indistinguishable in neurons with and without IC, precluding a separation of the nodose IC and IK by a difference in deactivation rates. 5. Chtx (10-250 nM) reduced in a subpopulation of 3/8 C-fiber neurons the total outward current elicited by voltage steps depolarized to -30 mV in single microelectrode voltage-clamp recordings. TEA (5-10 mM) further reduced outward current in the presence of 100-250 nM Chtx in all eight experiments. The Chtx-sensitive current was taken to represent IC, and the TEA-sensitive current, the IK component contributing to action-potential repolarization. 6. Rapidly inactivating current (IA) was implicated in action-potential repolarization in a subpopulation of intracellularly recorded C-fiber neurons. In 4/7 neurons, incremented hyperpolarizing prepulses negative to -50 mV progressively shortened APD.(ABSTRACT TRUNCATED AT 400 WORDS)

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