Potassium currents in the inner segment of single retinal cone photoreceptors

1. The K+ currents of cone inner segments isolated from the retina of a lizard were studied with the use of tight-seal electrodes in the whole cell configuration. To conduct these studies other identified currents in the cell were blocked. Co2+ blocked a voltage-dependent Ca2+ current and a Ca2(+)-dependent Cl- current, and Cs+ blocked an inward-rectifying current partially carried by K+. 2. The cells sustained a voltage-dependent K+ current that was blocked by tetraethylammonium (TEA)+ and had characteristics typical of the delayed rectifier. However, we found no evidence for the existence of "A"-type K+ currents or Ca2(+)-dependent K+ currents. 3. The delayed-rectifier current was nearly ideally selective for K+. Increasing external K+ concentration 10-fold shifted the reversal potential by 55 mV. 4. Analysis of the voltage dependence of the activation of the delayed-rectifier current revealed the existence of two distinct subclasses of this current. We referred to them as IdrL and IdrH for low and high threshold of voltage activation. 5. IdrL activated at voltages above -70 mV. Its dependence on voltage was described by Boltzmann's function with average half-maximum activation at -51 mV and steepness factor k = 7.5 mV. IdrH activated at voltages above -50 mV. Its dependence on voltage was described by Boltzmann's function with average half-maximum activation at -4.6 mV and steepness factor k = 17.1 mV. 6. Of nine cells analyzed in detail, one demonstrated IdrH alone, whereas the remaining had a variable mixture of the two current subtypes. At maximum activation the current through IdrL ranged between 0.3 and 0.5 of the total delayed-rectifier current. 7. The kinetics of activation of the total delayed-rectifier current were described by the sum of two exponentials the amplitudes and time constants of which were voltage dependent. However, the kinetics of the current subtypes were not resolved individually. The current inactivated slowly with a single-exponential time course that was voltage dependent. 8. The voltage dependence of the delayed-rectifier current indicates the current is active in a cone photoreceptor in the dark. The current is 20-30 pA in amplitude at the dark-membrane potential and outwardly directed. 9. IdrL may generate a rapid relaxation of photovoltages activated by dim lights--those that hyperpolarize the membrane by only a few millivolts. The delayed-rectifier currents help shape the action potentials that can be generated in isolated cone photoreceptors.     

Anomalous rectification in neurons from cat sensorimotor cortex in vitro

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)     

Membrane rectification in single smooth muscle cells from the rat tail artery

Membrane rectification to depolarization was studied by voltage recording with patch electrodes in freshly isolated cells from the rat tail artery. Injection of depolarizing currents elicited electrotonic potentials that developed with a single-exponential time course (time constant of 94.8 ms). When the cell was depolarized beyond –30 mV, delayed rectification was observed. A second type of rectification, characterized by oscillations, was observed when the cell was depolarized positive to + 30 mV. The threshold of this rectification and the oscillations were sensitive to changes in intracellular Ca²⁺. Delayed rectification was more sensitive to 4-aminopyridine but more resistant to tetraethylammonium and charybdotoxin than the Ca²⁺-sensitive rectification. A 4-aminopyridine-sensitive outward current (I _K,dr) with a threshold of around –30 mV and a second Ca²⁺-sensitive outward current (I _K,Ca) activated at around + 30 mV were observed from whole-cell voltage clamp recordings. I _K,Ca was blocked by tetraethylammonium and charybdotoxin. An 11-pS and a 122-pS channel, having characteristics similar to I _K,dr and I _K,Ca respectively, were identified from single-channel recordings. These observations showed how membrane depolarization of vascular smooth-muscle cells was regulated by these two populations of K⁺ channels under various conditions.     

Na(+) entry through AMPA receptors results in voltage-gated k(+) channel blockade in cultured rat spinal cord motoneurons

alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor currents, evoked with the agonist kainate, were studied with the gramicidin perforated-patch-clamp technique in cultured rat spinal cord motoneurons. Kainate-induced currents could be blocked by the AMPA receptor antagonist LY 300164 and displayed an apparent strong inward rectification. This inward rectification was not a genuine property of AMPA receptor currents but was a result of a concomitant decrease in outward current at potentials positive to -40.5 +/- 1.3 mV. The AMPA receptor current itself was nearly linear (rectification index 0.91). The kainate-inhibited outward current had a reversal potential close to the estimated K(+) equilibrium potential and was blocked by 30 mM tetraethylammonium. When voltage steps were applied, it was found that kainate inhibited both the delayed rectifier K(+) current K(V) and the transient outward K(+) current, K(A). The kainate-induced inhibition of K(+) currents was dependent on ion flux through the AMPA receptor, because no change in the membrane conductance was noticed in the presence of LY 300164. Removing extracellular Ca(2+) had no effect, whereas replacing extracellular Na(+) or clamping the membrane close to the estimated Na(+) equilibrium potential during kainate application attenuated the inhibition of the K(+) current. Sustained Na(+) influx induced by application of the Na(+) ionophore monensin could mimic the effect of kainate on K(+) conductance. These findings demonstrate that Na(+) influx through AMPA receptors results in blockade of voltage-gated K(+) channels.     

AMRP peptides modulate a novel K(+) current in pleural sensory neurons of Aplysia

Modulation of Aplysia mechanosensory neurons is thought to underlie plasticity of defensive behaviors that are mediated by these neurons. In the past, identification of modulators that act on the sensory neurons and characterization of their actions has been instrumental in providing insight into the functional role of the sensory neurons in the defensive behaviors. Motivated by this precedent and a recent report of the presence of Aplysia Mytilus inhibitory peptide-related (AMRP) neuropeptides in the neuropile and neurons of the pleural ganglia, we sought to determine whether and how pleural sensory neurons respond to the AMRPs. In cultured pleural sensory neurons under voltage clamp, AMRPs elicited a relatively rapidly developing, then partially desensitizing, outward current. The current exhibited outward rectification; in normal 10 mM K(+), it was outward at membrane potentials more positive than -80 mV but disappeared without reversing at more negative potentials. When external K(+) was elevated to 100 mM, the AMRP-elicited current reversed around -25 mV; the shift in reversal potential was as expected for a current carried primarily by K(+). In the high-K(+) solution, the reversed current began to decrease at potentials more negative than -60 mV, creating a region of negative slope resistance in the I-V relationship. The AMRP-elicited K(+) current was blocked by extremely low concentrations of 4-aminopyridine (4-AP; IC(50) = 1.7 x 10(-7) M) but was not very sensitive to TEA. In cell-attached patches, AMRPs applied outside the patch-thus presumably through a diffusible messenger-increased the activity of a K(+) channel that very likely underlies the macroscopic current. The single-channel current exhibited outward rectification, and the open probability of the channel decreased with hyperpolarization; together, these two factors accounted for the outward rectification of the macroscopic current. Submicromolar 4-AP included in the patch pipette blocked the channel by reducing its open probability without altering the single-channel current. Based on the characteristics of the AMRP-modulated K(+) current, we conclude that it is a novel current that has not been previously described in Aplysia mechanosensory neurons. In addition to this current, two other AMRP-elicited currents, a slow, 4-AP-resistant outward current and a Na(+)-dependent inward current, were occasionally observed in the cultured sensory neurons. Responses consistent with all three currents were observed in sensory neurons in situ in intact pleural ganglia.     

Ih without Kir in adult rat retinal ganglion cells

Antisera directed against hyperpolarization-activated mixed-cation ("I(h)") and K(+) ("K(ir)") channels bind to some somata in the ganglion cell layer of rat and rabbit retina. Additionally, the termination of hyperpolarizing current injections can trigger spikes in some cat retinal ganglion cells, suggesting a rebound depolarization arising from activation of I(h). However, patch-clamp studies showed that rat ganglion cells lack inward rectification or present an inwardly rectifying K(+) current. We therefore tested whether hyperpolarization activates I(h) in dissociated, adult rat retinal ganglion cell somata. We report here that, although we found no inward rectification in some cells, and a K(ir)-like current in a few cells, hyperpolarization activated I(h) in roughly 75% of the cells we recorded from in voltage clamp. We show that this current is blocked by Cs(+) or ZD7288 and only slightly reduced by Ba(2+), that the current amplitude and reversal potential are sensitive to extracellular Na(+) and K(+), and that we found no evidence of K(ir) in cells presenting I(h). In current clamp, injecting hyperpolarizing current induced a slowly relaxing membrane hyperpolarization that rebounded to a few action potentials when the hyperpolarizing current was stopped; both the membrane potential relaxation and rebound spikes were blocked by ZD7288. These results provide the first measurement of I(h) in mammalian retinal ganglion cells and indicate that the ion channels of rat retinal ganglion cells may vary in ways not expected from previous voltage and current recordings.     

Differential inhibition of glial K(+) currents by 4-AP

Spinal cord astrocytes express four biophysically and pharmacologically distinct voltage-activated potassium (K(+)) channel types. The K(+) channel blocker 4-aminopyridine (4-AP) exhibited differential and concentration-dependent block of all of these currents. Specifically, 100 microM 4-AP selectively inhibited a slowly inactivating outward current (K(SI)) that was insensitive to dendrototoxin (< or = 10 microM) and that activated at -50 mV. At 2 mM, 4-AP inhibited fast-inactivating, low-threshold (-70 mV) A-type currents (K(A)) and sustained, TEA-sensitive noninactivating delayed-rectifier-type currents (K(DR)). At an even higher concentration (8 mM), 4-AP additionally blocked inwardly rectifying, Cs(+)- and Ba(2+)-sensitive K(+) currents (K(IR)). Current injection into current-clamped astrocytes in culture or in acute spinal cord slices induced an overshooting voltage response reminiscent of slow neuronal action potentials. Increasing concentrations of 4-AP selectively modulated different phases in the repolarization of these glial spikes, suggesting that all four K(+) currents serve different roles in stabilization and repolarization of the astrocytic membrane potential. Our data suggest that 4-AP is an useful, dose-dependent inhibitor of all four astrocytic K(+) channels. We show that the slowly inactivating astrocytic K(+) currents, which had not been described as separate current entities in astrocytes, contribute to the resting K(+) conductance and may thus be involved in K(+) homeostatic functions of astrocytes. The high sensitivity of these currents to micromolar 4-AP suggests that application of 4-AP to inhibit neuronal A-currents or to induce epileptiform discharges in brain slices also may influence astrocytic K(+) buffering.     

Ca2+-channel current and its modification by the dihydropyridine agonist BAY k 8644 in isolated smooth muscle cells

The electrophysiological properties of single smooth muscle cells isolated from the longitudinal layer of the guinea-pig ileum were studied with the whole-cell patch-clamp technique. The finding of resting potentials between -45 and -50 mV and the occurrence of spontaneous electrical activity when K+ was the predominant intracellular cation indicated that the cells were not leaky or hyperpermeable. The existence of an inward Ca2+ current overlapping in time with an outward rectifying K+ current was demonstrated. The latter could be selectively blocked by replacing internal K+ with Cs+ and external Ca2+ with Ba2+. Depolarizations to potentials between -40 and +50 mV evoked time-dependent inward currents, with a maximum peak value between -20 and 0 mV. For depolarizations beyond +50 mV time-dependent outward currents appeared. These currents were inhibited by 0.1 mM CdCl2. The activation of the inward current showed a sigmoidal time course, and the rate of onset of the current increased at more positive potentials. Inactivation could be described by two exponentials. The threshold for activation was about -40 mV, and full activation was reached at 0 mV. Inactivation was complete near 0 mV, whereas the channels were fully available at -80 mV. The fully-activated Ca2+-channel current was strongly voltage dependent. The conductance decreased for potentials close to the reversal potential, and showed rectification for hyperpolarizing potentials. The Ca2+ agonist BAY k 8644 enhanced the Ca2+-channel current without a significant effect on its kinetics. The fully-activated current and the steady-state activation were enhanced in a rather voltage-independent way.     

Properties of the inwardly rectifying K+ conductance in the toad retinal pigment epithelium.

An inwardly rectifying K+ current was analysed in isolated toad retinal pigment epithelial (RPE) cells using the perforated-patch clamp technique. The zero-current potential (Vo) of RPE cells averaged -71 mV when the extracellular K+ concentration ([K+]o) was 2 mM. Increasing [K+]o from 0.5 to 5 mM shifted V0 by +43 mV, indicating a relative K+ conductance (TK) of 0.74. At [K+]o greater than 5 mM, TK decreased to 0.53. Currents were larger in response to hyperpolarizing voltage pulses than depolarizing pulses, indicating an inwardly rectifying conductance. Currents were time independent except in response to voltage pulses to potentials positive to 0 mV, where the outward current decayed with an exponential time course. Both the inwardly rectifying current and the transient outward current were eliminated by the addition of 0.5 mM Ba2+, 5 mM Cs+ or 2 mM Rb+ to the extracellular solution. The current blocked by these ions reversed near the K+ equilibrium potential (EK) over a wide range of [K+]o, indicating a highly selective K+ channel. The current-voltage relationship of the isolated K+ current exhibited mild inward rectification at voltages negative to -20 mV and a negative slope conductance at voltages positive to -20 mV. The Cs(+)- and Ba(2+)-induced blocks of the K+ current were concentration dependent but voltage independent. The apparent dissociation constants were 0.8 mM for Cs+ and 40 microM for Ba2+. The K+ conductance decreased when extracellular Na+ was removed. Increasing [K+]o decreased the K+ chord conductance (gK) at negative membrane potentials. In the physiological voltage range, increasing [K+]o from 2 to 5 mM caused gK to decrease by approximately 25%. We conclude that the inwardly rectifying K+ conductance represents the resting K+ conductance of the toad RPE apical membrane. The unusual properties of this conductance may enhance the ability of the RPE to buffer [K+]o changes that take place in the subretinal space at the transition between dark and light.     

Potassium channels of myenteric neurons in guinea-pig small intestine

Patch-clamp recording was used to study rectifying K+ currents in myenteric neurons in short-term culture. In conditions that suppressed Ca2+ -activated K+ current, three kinds of voltage-activated K+ currents were identified by their voltage range of activation, inactivation, kinetics and pharmacology. These were A-type current, delayed outwardly rectifying current (I(K),dr) and inwardly rectifying current (I(K),ir). I(K),ir consisted of an instantaneous component followed by a time-dependent current that rapidly increased at potentials negative to -80 mV. Time-constant of activation was voltage-dependent with an e-fold decrease for a 31-mV hyperpolarization amounting to a decrease from 800 to 145 ms between -80 and -100 mV. I(K),ir did not inactivate. I(K),ir was abolished in K+ -free solution. Increases in external K+ increased I(K),ir conductance in direct relation to the square root of external K+ concentration. Activation kinetics were accelerated and the activation range shifted to more positive K+ equilibrium potentials. I(K),ir was suppressed by external Cs+ and Ba2+ in a concentration-dependent manner. Ca2+ and Mg+ were less effective than Ba2+. I(K),ir was unaffected by tetraethylammonium ions. I(K),dr was activated at membrane potentials positive to - 30 mV with an e-fold decrease in time-constant of activation from 145 to 16 ms between -20 and 30 mV. It was half-activated at 5 mV and fully activated at 50 mV. Inactivation was indiscernible during 2.5 s test pulses. I(K),dr was suppressed in a concentration-, but not voltage-dependent manner by either tetraethylammonium or 4-aminopyridine and was insensitive to Cs+. The results suggest that I(K),ir may be important in maintaining the high resting membrane potentials found in afterhyperpolarization-type enteric neurons. They also suggest importance of I(K),ir channels in augmentation of the large hyperpolarizing after-potentials in afterhyperpolarization-type neurons and the hyperpolarization associated with inhibitory postsynaptic potentials. I(K),dr in afterhyperpolarization-type enteric neurons has overall kinetics and voltage behaviour like delayed rectifier currents in other excitable cells where the currents can also be distinguished from A-type and Ca2+ -activated K+ current.     

A voltage- and time-dependent rectification in rat dorsal spinal root axons.

1. Rat dorsal spinal roots were studied by the use of whole-nerve sucrose gap and intra-axonal recording techniques. A prominent time-dependent conductance increase as evidenced by a relaxation or "sag" in membrane potential toward resting potential was elicited in dorsal spinal roots by constant hyperpolarizing current pulses. The relaxation, or sag, indicative of inward rectification, reached a maximal level and then decayed during the current pulse. 2. The time-dependent sag elicited by hyperpolarization was reduced when Na+ or K+ was removed from the normal bath solution but was abolished with the removal of both Na+ and K+. Tetrodotoxin (TTX), tetraethylammonium (TEA), and 4-aminopyridine (4-AP) did not affect the depolarization sag, suggesting that conventional voltage-dependent sodium and potassium channels do not underlie the inward rectification. 3. Cs+ in low concentrations completely abolished the inward rectification, whereas Ba2+ induced a partial block. 4. Current-voltage curves indicate that the magnitude of the depolarizing sag increases monotonically with increasing hyperpolarization. The time required to reach peak hyperpolarization, maximal sag potential, and the time between peak hyperpolarization and sag membrane potentials decreases with increasing levels of hyperpolarization. 5. The inward rectification is refractory to further stimulation during its decay phase, as revealed by paired-pulse protocols. This decay in inward rectification is both time and voltage dependent and is observed on a single axon level by the use of intra-axonal recording techniques as well as from whole-root recordings in the sucrose gap. 6. It is concluded that rat dorsal root fibers display a prominent time-dependent conductance increase in response to hyperpolarization that depends on both Na+ and K+ permeability and is blocked by Cs+. This rectification displays a decay phase that has not been previously described for similar conductances. It is argued that the Na+ component of this conductance is primarily responsible for stabilizing membrane potential near resting potential during periods of hyperpolarization.     

Single channel analysis of the inward rectifier K current in the rabbit ventricular cells

The inward rectifier K channel in rabbit ventricular cells was studied by the patch-clamp method. Single channel currents were recorded in giga-sealed cell-attached patches with 150 mM K+ in the pipette. The slope conductance in the membrane potential range from -140 to -40 mV was 46.6 +/- 6.7 pS (mean +/- S.D., n = 16), and was reduced by decreasing [K+] in the pipette (20 or 50 mM). The channel was blocked by an application of Cs+ or Ba2+ (0.04-1 mM) in the pipette. Outwardly directed current, recorded with 50 mM K+ in the pipette, revealed the inward rectification of the single channel current. The probability of the channel being open was 0.33 +/- 0.05 (n = 10) at the resting potential (RP=-81.7 +/- 1.7 mV, n = 16) with 150 mM K+ in the pipette, and it decreased with hyperpolarization. The mean open time of the channel was 178 +/- 25 msec (n = 6) at RP. The closed time of the channel seemed to have two exponential components, with time constants of 11.0 +/- 2.0 msec and 1.92 +/- 0.52 sec (n = 6) at RP. The slower time constant was increased with hyperpolarization. The averaged patch current recorded upon hyperpolarizing pulses demonstrated a time-dependent current decay as expected from the single channel kinetics. The results indicated that the inward rectifier K+ current has time- and voltage-dependent kinetics.     

A voltage-dependent transient K(+) current in rat dental pulp cells

We characterized a voltage-dependent transient K(+) current in dental pulp fibroblasts on dental pulp slice preparations by using a nystatin perforated-patch recording configuration. The mean resting membrane potential of dental pulp fibroblasts was -53 mV. Depolarizing voltage steps to +60 mV from a holding potential of -80 mV evoked transient outward currents that are activated rapidly and subsequently inactivated during pulses. The activation threshold of the transient outward current was -40 mV. The reversal potential of the current closely followed the K(+) equilibrium potential, indicating that the current was selective for K(+). The steady-state inactivation of the peak outward K(+) currents described by a Boltzmann function with half-inactivation occurred at -47 mV. The K(+) current exhibited rapid activation, and the time to peak amplitude of the current was dependent on the membrane potentials. The inactivation process of the current was well fitted with a single exponential function, and the current exhibited slow inactivating kinetics (the time constants of decay ranged from 353 ms at -20 mV to 217 ms at +60 mV). The K(+) current was sensitive to intracellular Cs(+) and to extracellular 4-aminopyridine in a concentration-dependent manner, but it was not sensitive to tetraethylammonium, mast cell degranulating peptide, and dendrotoxin-I. The blood depressing substance-I failed to block the K(+) current. These results indicated that dental pulp fibroblasts expressed a slow-inactivating transient K(+) current.     

Calcium currents in respiratory neurons of the cat in vivo

Under in vivo conditions, periodic burst discharges of medullary respiratory neurons of mature cat typically start with a rebound depolarization when inhibition through antagonistic neurons stops. This rebound can be blocked by ionophoretically applied extracellular Cd2+. A similar Cd(2+)-sensitive rebound depolarization is triggered by hyperpolarizing current pulses even in the presence of extracellular tetrodotoxin (TTX) and tetraethylammonium (TEA). In current-clamp mode, the current/voltage (I/V) curves rectify outwardly at positive voltages, and this rectification is blocked by Cd2+. Intracellular injection of the L-type Ca(2+)-channel blocker methoxy-verapamil changes the spontaneous activity patterns of neurons. In those neurons that typically show augmenting patterns, the membrane depolarization is slowed down, while in those neurons that have a declining pattern, voltage changes become augmenting. Voltage-clamp measurements reveal a transient, low-voltage-activated T-type Ca2+ current. The current is deinactivated at -100 mV and almost completely inactivated at -60 mV. Depolarizing voltage commands starting from more positive holding potentials evoke sustained Ca2+ currents that reach a maximum at 0 mV. The sustained L-type Ca2+ currents are completely blocked by extracellular Cd2+. We conclude that low- and high-voltage-activated Ca2+ currents are expressed in all types of respiratory neurons and play an essential role in rhythm generation and pattern formation in adult cats in vivo.     

Membrane properties of guinea pig cingulate cortical neurons in vitro

1. The passive and active membrane properties of guinea pig cingulate cortical neurons were studied in vitro using the slice preparation. Results were reported for intracellular recordings made from neurons that were penetrated in layers V/VI of the anterior cingulate cortex areas 1 and 3. 2. The neurons had an average resting potential of -71 mV, an input resistance of 71 M omega, a spike amplitude of 93 mV, and a spike duration of 1.6 ms. The firing occurred regularly at an average rate of 13 spikes/s at the membrane potential of -55 mV, suggesting that they are probably regular spiking pyramidal cells. 3. The voltage decay following a hyperpolarizing current pulse could always be fitted by two exponentials in most cells. The slope of the charging function was analyzed to estimate the two cable theory parameters of the neurons, based on a simple Rall model: the electrotonic length (LN) of the equivalent dendritic cylinder and the conductance ratio (rho) of the dendrites to that of the soma. There were no significant differences in the LN (0.9-1.1) and the rho (2.8-3.0) of neurons in normal media and solutions containing tetrodotoxin (TTX), Cs+ and low Ca2+, indicating that the neurons may be electrically compact. 4. In most cells the steady-state current-voltage (I-V) relationship revealed three distinct types of rectification: an anomalous inward rectification in the hyperpolarizing direction, a subthreshold inward rectification, and a delayed outward rectification in the depolarizing direction. 5. The anomalous rectification was increased in high K+ solutions and was decreased in low K+ solutions. Analysis of the Ba2+ and Cs+ sensitivity confirmed that the anterior cingulate neurons had two distinct types of anomalous rectification, one that was time dependent and Ba2+ insensitive and the other that was fast and Ba2+ sensitive. Ionic analyses indicated that the time-dependent anomalous rectification was due to an increased permeability to both Na+ and K+, whereas the fast, Ba(2+)-sensitive rectification was probably only K+ dependent. 6. The subthreshold inward rectification was depressed by TTX, lidocaine, or Co2+, as well as the reduction of extracellular Na+, whereas it was augmented by extracellular Ba2+. This persistent Na(+)-Ca2+ conductance triggered the generation of Na(+)-dependent action potentials. 7. The delayed outward rectification was recorded in the potential range between -65 and -20 mV.(ABSTRACT TRUNCATED AT 400 WORDS)     

Synchronous and asynchronous exocytosis induced by subthreshold high K+ at Cs(+)-loaded terminals of rat hippocampal neurons

Transmitter release at Cs(+)-loaded autaptic terminals was selectively activated by the subthreshold concentration of external K+, and Ca(2+) channel types and transmitter pools involved in synchronous and asynchronous exocytosis were studied. When a neuron was depolarized to +30 mV by applying a current through a pipette containing Cs(+) for >30 s, a rapid external K+ jump to 3.75-10 mM, otherwise ineffective, produced an outward current (K10 response). K10 responses were initially graded (type-1) and then became a spike and plateau-shape with (type-2) or without a latency (type-3). On repolarization to -60 mV, a high K+ jump induced inward currents (called also K10 response) similar to those at +30 mV, whose shape changed from that of type-3, then type-2 and finally type-1 over 30 min. During a period favorable for inducing a type-3 response, a current similar to this response was generated by a voltage pulse (+ 80 or 90 mV, 20 or 30 ms) to the cell soma. Currents similar to K10 responses were rarely induced by a high K+ jump without a conditioning depolarization except for some cells, but consistently produced when 3 mM Cs(+) and 50 microM 4-aminopyridine were externally applied for tens of minutes. Picrotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione with 3-[(RS)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid or Cd(2+) in, or Ca(2+) removal from, a high-K+ solution blocked all the K10 responses, while a plateau remaining after a high K+ jump was not blocked by Ca(2+) removal immediately after the K+ jump. Thus Cs(+) loading and decreased K+ concentration in autaptic terminals by a conditioning depolarizing current selectively sensitize the terminals to a subthreshold high K+ jump for depolarization to activate synchronous or asynchronous transmitter release. Nicardipine (5-10 microM) blocked type-1 and -2 responses but not type-3 responses, while omega-conotoxin (10 microM) blocked all the types of K10 response in the presence of nicardipine. Increasing the interval of high K+ jumps biphasically increased the magnitude of K10 response, preferentially in the postjump fraction reflecting purely the asynchronous activation of exocytotic machinery, and decreased the reduction of miniature postsynaptic current frequency after a K10 response. These results suggest the roles of N(P/Q)-type Ca(2+) channels in synchronous exocytosis at the terminals, L-type Ca(2+) channels in initiating a Ca(2+) action potential at the parent axon and both types in asynchronous exocytosis and also suggest the different releasable pools of transmitter for two modes of exocytosis in cultured hippocampal neurons.     

视锥光感受器内向整流电流特性和功能

应用牛蛙视网膜薄片标本，在红外可视条件下，运用全细胞记录技术研究了视锥内向整流电流的特性。内向整流电流可被Ｃｓ＋ 完全阻断。对内向整流电流门控特性分析显示，该电流的翻转电位约为 ３５ ｍ Ｖ，ＰＮａ／ＰＫ 约为０．３３，不显示去活（ｉｎａｃｔｉｖａ ｔｉｏｎ），最大半激活电压为 ８８ ｍ Ｖ，陡度约１１ ｍ Ｖ；动力学分析揭示，内向整流电流激活（ａｃｔｉｖａｔｉｏｎ）相可用极性相反的双指数函数很好地拟合，内向整流电流失活（ｄｅａｃｔｉｖａｔｉｏｎ）相则符合单指数函数规律。进而，视锥在超极化至 ６０～ ７０ ｍ Ｖ 时产生的电压瞬变成分可为Ｃｓ＋ 阻断，表明该反应成分系由内向整流电流介导的。上述结果提示，内向整流电流对视锥强光反应具有重要的整形功能。     

cGMP activates a pH-sensitive leak K+ current in the presumed cholinergic neuron of basal forebrain

In an earlier study, we demonstrated that nitric oxide (NO) causes the long-lasting membrane hyperpolarization in the presumed basal forebrain cholinergic (BFC) neurons by cGMP-PKG-dependent activation of leak K+ currents in slice preparations. In the present study, we investigated the ionic mechanisms underlying the long-lasting membrane hyperpolarization with special interest in the pH sensitivity because 8-Br-cGMP-induced K+ current displayed Goldman-Hodgkin-Katz rectification characteristic of TWIK-related acid-sensitive K+ (TASK) channels. When examined with the ramp command pulse depolarizing from -130 to -40 mV, the presumed BFC neurons displayed a pH-sensitive leak K+ current that was larger in response to pH decrease from 8.3 to 7.3 than in response to pH decrease from 7.3 to 6.3. This K+ current was similar to TASK1 current in its pH sensitivity, whereas it was highly sensitive to Ba(2+), unlike TASK1 current. The 8-Br-cGMP-induced K+ currents in the presumed BFC neurons were almost completely inhibited by lowering external pH to 6.3 as well as by bath application of 100 microM Ba(2+), consistent with the nature of the leak K+ current expressed in the presumed BFC neurons. After 8-Br-cGMP application, the K+ current obtained by pH decrease from 7.3 to 6.3 was larger than that obtained by pH decrease from pH 8.3 to 7.3, contrary to the case seen in the control condition. These observations strongly suggest that 8-Br-cGMP activates a pH- and Ba(2+)-sensitive leak K+ current expressed in the presumed BFC neurons by modulating its pH sensitivity.     

Block by the neuropeptide TRH of an apparently novel K+ conductance of rat motoneurones

Using a single electrode voltage clamp technique the actions of rapidly superfused thyrotropin-releasing hormone (TRH, 1 microM) on lumbar motoneurones of the isolated neonatal rat spinal cord were investigated. TRH induced a slowly developing inward current (associated with an input conductance fall) with slow recovery on washout. In the presence of TRH the normally linear current-voltage relations displayed strong inward rectification up to about -40 mV. The TRH-induced current peaked at -50 mV, reversed at -120 mV and was not blocked by Cs+, tetraethylammonium, 4-aminopyridine, Cd2+, or low Na+. Its reversal potential was sensitive to changes in extracellular K+. Ba2+ (0.2-1.5 mM) depressed the effects of TRH. It is suggested that in rat motoneurones TRH blocked an apparently novel K+ conductance (IK(T)) active at resting membrane potential.     

Whole-cell K+ currents in isolated rabbit corneal epithelial cells.

Membrane currents were recorded from enzymatically isolated cells from basal layers of rabbit corneal epithelium by the whole-cell clamp technique. Pipettes contained 140.4 mM KCl and extracellular K+ concentration was varied. The membrane currents on step voltage changes were rectangular currents with some fluctuations. The fluctuations disappeared near the zero-current potential. The reversal potential in normal Tyrode's solution with 5.4 mM K+ was -57.8 +/- 6.2 mV (mean +/- S.D., n = 10). Increasing [K+]o from 5.4 to 140.4 mM shifted the reversal potentials in the positive direction with a slope of 41.0 mV/decade. Concomitant depolarization of the resting potential was observed on increasing [K+]o. The whole-cell currents were blocked by Cs+ or Ba2+. These suggest that the major current component in the corneal epithelial cells in K+.