Effects of cholesterol levels on the excitability of rat hippocampal neurons

Changes in the cholesterol levels dynamically alter the microenvironment of the plasma membrane and have been shown to modify functions of ion channels. However, the cellular effect of these modifications is largely unknown. In this report, we demonstrate that cholesterol levels modulate neuronal excitability in rat hippocampal neurons. Reduction of cholesterol levels shortened the duration and increased the firing frequency and peak amplitude of action potentials, while enrichment of cholesterol reversed the effect. Furthermore, we showed that reduction of cholesterol levels increased, while enrichment of cholesterol decreased the amplitude of the delayed rectifier I(K) currents. On the other hand, reduction of cholesterol levels slowed down the inactivation of the fast transient I(A) currents, but enrichment of cholesterol had no significant effect on the I(A) currents. Besides, alteration in cholesterol levels had no significant effect on the action potential in the presence of blockers for both I(K) and I(A) currents. These observations demonstrate that cholesterol levels bi-directionally regulate the neuronal excitability mainly through modifications of the I(K) and I(A) currents, suggesting an optimum level of cholesterol for the optimum excitability of neurons. Alterations in the neuronal cholesterol levels have been associated with aging, cognitive decline, neurodegenerative diseases, etc. Therefore, our findings are important for a deeper understanding of the relationship between the cholesterol level and dysfunctions of the brain at the molecular level.     

Inhibitory Effects of Honokiol on the Voltage-Gated Potassium Channels in Freshly Isolated Mouse Dorsal Root Ganglion Neurons

Voltage-gated potassium (K_V) currents, subdivided into rapidly inactivating A-type currents (I _A) and slowly inactivating delayed rectifier currents (I _K), play a fundamental role in modulating pain by controlling neuronal excitability. The effects of Honokiol (Hon), a natural biphenolic compound derived from Magnolia officinalis, on K_V currents were investigated in freshly isolated mouse dorsal root ganglion neurons using the whole-cell patch clamp technique. Results showed that Hon inhibited I _A and I _K in concentration-dependent manner. The IC₅₀ values for block of I _A and I _K were 30.5 and 25.7 µM, respectively. Hon (30 µM) shifted the steady-state activation curves of I _A and I _K to positive potentials by 17.6 and 16.7 mV, whereas inactivation and recovery from the inactivated state of I _A were unaffected. These results suggest that Hon preferentially interacts with the active states of the I _A and I _K channels, and has no effect on the resting state and inactivated state of the I _A channel. Blockade on K⁺ channels by Hon may contribute to its antinociceptive effect, especially anti-inflammatory pain.

     

Kv 1.1 is associated with neuronal apoptosis and modulated by protein kinase C in the rat cerebellar granule cell

Previously, we reported that apoptosis of cerebellar granular neurons induced by low‐K⁺ and serum‐free (LK‐S) was associated with an increase in the A‐type K⁺ channel current (I_A), and an elevated expression of main α‐subunit of the I_A channel, which is known as Kv4.2 and Kv4.3. Here, we show, as assessed by quantitative RT‐PCR and whole‐cell recording, that besides Kv4.2 and Kv4.3, Kv1.1 is very important for I_A channel. The expression of Kv1.1 was elevated in the apoptotic neurons, while silencing Kv1.1 expression by siRNA reduced the I_A amplitude of the apoptotic neuron, and increased neuron viability. Inhibiting Kv1.1 current by dendrotoxin‐K evoked a similar effect of reduction of I_A amplitude and protection of neurons. Applying a protein kinase C (PKC) activator, phorbol ester acetate A (PMA) mimicked the LK‐S‐induced neuronal apoptotic effect, enhanced the I_A amplitude and reduced the granule cell viability. The PKC inhibitor, bisindolylmaleimide I and Gö6976 protected the cell against apoptosis induced by LK‐S. After silencing the Kv1.1 gene, the effect of PMA on the residual K⁺ current was reduced significantly. Quantitative RT‐PCR and Western immunoblot techniques revealed that LK‐S treatment and PMA increased the level of the expression of Kv1.1, in contrast, bisindolylmaleimide I inhibited Kv1.1 expression. In addition, the activation of the PKC isoform was identified in apoptotic neurons. We thus conclude that in the rat cerebellar granule cell, the I_A channel associated with apoptotic neurons is encoded mainly by the Kv1.1 gene, and that the PKC pathway promotes neuronal apoptosis by a brief modulation of the I_A amplitude and a permanent increase in the levels of expression of the Kv1.1 α‐subunit.     

Developmental temperature selectively regulates a voltage-activated potassium current in Drosophila

Ionic currents are regulated by many conditions including disease states, ageing, learning and memory, and chronic drug treatment. Here we describe a novel phenomenon of regulation of ionic currents by developmental temperature. Raising Drosophila larvae at 28°C instead of 18°C increased one of the two voltage-activated K⁺- currents, the delayed sustained I_K, in their muscles by up to 3.5-fold, with little effect on the early transient current, I_A. Consistent with this increase in I_K, the amplitude and the duration of the action potentials were reduced. The major increase in I_K occurred between a rather abrupt interval from 25° to 28°C. The activation curve of the increased current was shifted towards hyperpolarizing potentials. There was no change in activation kinetics. This phenomenon has mechanistic implications for activity-dependent neuronal plasticity, expression of ion channels in cultured cells and heterologus systems, phototransduction, and behavior. The specificity of the regulation suggests a discrete mechanism geared to affect excitability such that it can respond to altered external stimuli such as temperature. 1994 John Wiley & Sons, Inc.     

Computational study of enhanced excitability in Hermissenda: membrane conductances modulated by 5-HT

Serotonin (5-HT) applied to the exposed but otherwise intact nervous system results in enhanced excitability of Hermissenda type-B photoreceptors. Several ion currents in the type-B photoreceptors are modulated by 5-HT, including the A-type K⁺ current (I_K,A), sustained Ca²⁺ current (I_Ca,S), Ca-dependent K⁺ current (I_K,Ca), and a hyperpolarization-activated inward rectifier current (I_h). In this study, we developed a computational model that reproduces physiological characteristics of type B photoreceptors, e.g. resting membrane potential, dark-adapted spike activity, spike width, and the amplitude difference between somatic and axonal spikes. We then used the model to investigate the contribution of different ion currents modulated by 5-HT to the magnitudes of enhanced excitability produced by 5-HT. Ion currents were systematically varied within limits observed experimentally, both individually and in combinations. A reduction of I_K,A or I_K,Ca, or an increase in I_h enhanced excitability by 20–50%. Decreasing I_Ca,S produced a dramatic decrease in excitability. Reductions of I_K,V produced only minimal increases in excitability, suggesting that I_K,V probably plays a minor role in 5-HT induced enhanced excitability. Combinations of changes in I_K,A, I_K,Ca, I_h and I_Ca,S produced increases in excitability comparable to experimental observations. After 5-HT application, the cell's depolarization force is shifted from the I_h–I_Ca,S combination to predominantly I_h.     

Effects of Estradiol on Voltage-Gated Potassium Channels in Mouse Dorsal Root Ganglion Neurons

Voltage-gated potassium channels are regulators of membrane potentials, action potential shape, firing adaptation, and neuronal excitability in excitable tissues including in the primary sensory neurons of dorsal root ganglion (DRG). In this study, using the whole-cell patch-clamp technique, the effect of estradiol (E2) on voltage-gated total outward potassium currents, the component currents transient “A-type” current (I _A) currents, and “delayed rectifier type” (I _KDR) currents in isolated mouse DRG neurons was examined. We found that the extracellularly applied 17β-E2 inhibited voltage-gated total outward potassium currents; the effects were rapid, reversible, and concentration-dependent. Moreover, the membrane impermeable E2-BSA was as efficacious as 17β-E2, whereas 17α-E2 had no effect. 17β-E2-stimulated decrease in the potassium current was unaffected by treatment with ICI 182780 (classic estrogen receptor antagonist), actinomycin D (RNA synthesis inhibitor), or cycloheximide (protein synthesis inhibitor). We also found that I _A and I _KDR were decreased after 17β-E2 application. 17β-E2 significantly shifted the activation curve for I _A and I _KDR channels in the hyperpolarizing direction. In conclusion, our results demonstrate that E2 inhibited voltage-gated K⁺ channels in mouse DRG neurons through a membrane ER-activated non-genomic pathway.     

Shal/K(v)4 channels are required for maintaining excitability during repetitive firing and normal locomotion in Drosophila

Background

Rhythmic behaviors, such as walking and breathing, involve the coordinated activity of central pattern generators in the CNS, sensory feedback from the PNS, to motoneuron output to muscles. Unraveling the intrinsic electrical properties of these cellular components is essential to understanding this coordinated activity. Here, we examine the significance of the transient A-type K⁺ current (I_A), encoded by the highly conserved Shal/K_v4 gene, in neuronal firing patterns and repetitive behaviors. While I_A is present in nearly all neurons across species, elimination of I_A has been complicated in mammals because of multiple genes underlying I_A, and/or electrical remodeling that occurs in response to affecting one gene.

Methodology/Principal Findings

In Drosophila, the single Shal/K_v4 gene encodes the predominant I_A current in many neuronal cell bodies. Using a transgenically expressed dominant-negative subunit (DNK_v4), we show that I_A is completely eliminated from cell bodies, with no effect on other currents. Most notably, DNK_v4 neurons display multiple defects during prolonged stimuli. DNK_v4 neurons display shortened latency to firing, a lower threshold for repetitive firing, and a progressive decrement in AP amplitude to an adapted state. We record from identified motoneurons and show that Shal/K_v4 channels are similarly required for maintaining excitability during repetitive firing. We then examine larval crawling, and adult climbing and grooming, all behaviors that rely on repetitive firing. We show that all are defective in the absence of Shal/K_v4 function. Further, knock-out of Shal/K_v4 function specifically in motoneurons significantly affects the locomotion behaviors tested.

Conclusions/Significance

Based on our results, Shal/K_v4 channels regulate the initiation of firing, enable neurons to continuously fire throughout a prolonged stimulus, and also influence firing frequency. This study shows that Shal/K_v4 channels play a key role in repetitively firing neurons during prolonged input/output, and suggests that their function and regulation are important for rhythmic behaviors.     

Inhibition of NMDA-induced outward currents by interleukin-1beta in hippocampal neurons

There is increasing evidence that a functional interaction exists between interleukin-1β (IL-1β) and N-methyl-d-aspartate (NMDA) receptors. The present study attempted to elucidate the effect of IL-1β on the NMDA-induced outward currents in mechanically dissociated hippocampal neurons using a perforated patch recording technique. IL-1β (30-100 ng/ml) inhibited the mean amplitude of the NMDA-induced outward currents that were mediated by charybdotoxin (ChTX)-sensitive Ca²⁺-activated K⁺ (K_Ca) channels. IL-1β (100 ng/ml) also significantly increased the mean ratio of the NMDA-induced inward current amplitudes measured at the end to the beginning of a 20-s application of NMDA. In hippocampal neurons from acute slice preparations, IL-1β significantly inhibited ChTX-sensitive K_Ca currents induced by a depolarizing voltage-step. IL-1 receptor antagonist antagonized effects of IL-1β. These results strongly suggest that IL-1β increases the neuronal excitability by inhibition of ChTX-sensitive K_Ca channels activated by Ca²⁺ influx through both NMDA receptors and voltage-gated Ca²⁺ channels.     

Sodium Metabisulfite Modulation of Potassium Channels in Pain-Sensing Dorsal Root Ganglion Neurons

The effects of sodium metabisulfite (SMB), a general food preservative, on potassium currents in rat dorsal root ganglion (DRG) neurons were investigated using the whole-cell patch-clamp technique. SMB increased the amplitudes of both transient outward potassium currents and delayed rectifier potassium current in concentration- and voltage-dependent manner. The transient outward potassium currents (TOCs) include a fast inactivating (A-current or I _A) current and a slow inactivating (D-current or I _D) current. SMB majorly increased I_A, and I_D was little affected. SMB did not affect the activation process of transient outward currents (TOCs), but the inactivation curve of TOCs was shifted to more positive potentials. The inactivation time constants of TOCs were also increased by SMB. For delayed rectifier potassium current (I _K), SMB shifted the activation curve to hyperpolarizing direction. SMB differently affected TOCs and I _K, its effects major on A-type K⁺ channels, which play a role in adjusting pain sensitivity in response to peripheral redox conditions. SMB did not increase TOCs and I _K when adding DTT in pipette solution. These results suggested that SMB might oxidize potassium channels, which relate to adjusting pain sensitivity in pain-sensing DRG neurons.     

Current Transients Associated with BK Channels in Human Glioma Cells

We have previously demonstrated the expression of BK channels in human glioma cells. There was a curious feature to the whole-cell currents of glioma cells seen during whole-cell patch-clamp: large, outward current transients accompanied repolarization of the cell membrane following an activating voltage step. This transient current, I _transient, activated and inactivated rapidly (1 ms). The I-V relationship of I _transient had features that were inconsistent with simple ionic current through open ion channels: (i) I _transient amplitude peaked with a –80 mV voltage change and was invariant over a 200 mV range, and (ii) I _transient remained large and outward at –140 mV. We provide evidence for a direct relationship of I _transient to glioma BK currents. They had an identical time course of activation, identical pharmacology, identical voltage-dependence, and small, random variations in the amplitude of the steady-state BK current and I _transient seen over time were often perfectly in phase. Substituting intracellular K⁺ with Cs⁺, Li⁺, or Na ⁺ ions reversibly reduced I _transient and BK currents. I _transient was not observed in recordings of other BK currents (hbr5 expressed in HEK cells and BK currents in rat neurons), suggesting I _transient is unique to BK currents in human glioma cells. We conclude that I _transient is generated by a mechanism related to the deactivation, and level of prior activation, of glioma BK channels. To account for these findings we propose that K⁺ ions are trapped within glioma BK channels during deactivation and are forced to exit to the extracellular side in a manner independent of membrane potential.     

Contribution of BKCa-Channel Activity in Human Cardiac Fibroblasts to Electrical Coupling of Cardiomyocytes-Fibroblasts

Cardiac fibroblasts are involved in the maintenance of myocardial tissue structure. However, little is known about ion currents in human cardiac fibroblasts. It has been recently reported that cardiac fibroblasts can interact electrically with cardiomyocytes through gap junctions. Ca²⁺-activated K⁺ currents (I _K[Ca]) of cultured human cardiac fibroblasts were characterized in this study. In whole-cell configuration, depolarizing pulses evoked I _K(Ca) in an outward rectification in these cells, the amplitude of which was suppressed by paxilline (1 μM) or iberiotoxin (200 nM). A large-conductance, Ca²⁺-activated K⁺ (BK_Ca) channel with single-channel conductance of 162 ± 8 pS was also observed in human cardiac fibroblasts. Western blot analysis revealed the presence of α-subunit of BK_Ca channels. The dynamic Luo-Rudy model was applied to predict cell behavior during direct electrical coupling of cardiomyocytes and cardiac fibroblasts. In the simulation, electrically coupled cardiac fibroblasts also exhibited action potential; however, they were electrically inert with no gap-junctional coupling. The simulation predicts that changes in gap junction coupling conductance can influence the configuration of cardiac action potential and cardiomyocyte excitability. I _k(Ca) can be elicited by simulated action potential waveforms of cardiac fibroblasts when they are electrically coupled to cardiomyocytes. This study demonstrates that a BK_Ca channel is functionally expressed in human cardiac fibroblasts. The activity of these BK_Ca channels present in human cardiac fibroblasts may contribute to the functional activities of heart cells through transfer of electrical signals between these two cell types.     

Transient K<Superscript>+</Superscript> Current is Blocked by Lanthanum in <Emphasis Type="Italic">Drosophila</Emphasis> Neurons

The potassium A-current (IK_A) is important in regulating the membrane potential between action potentials. The whole-cell patch-clamp technique was applied to cultured Drosophila neurons derived from embryonic neuroblasts. IK_A was measured from neurons before and after application of 0.1 mM lanthanum to the external saline. IK_A was smaller in the lanthanum-containing saline (7±1 pA) than in the control saline (34±6 pA). Activation and inactivation of IK_A were unchanged by lanthanum. These results suggest that lanthanum neurotoxicity may lead to increased neuronal excitability. Moreover, given this inhibition of IK_A, lanthanum should not be used to block calcium current in studies of K⁺ currents.     

Whole-cell currents in macrophages: II. Alveolar macrophages

Summary Although an outwardly rectifying K⁺-conductance has been described in murine peritoneal macrophages and a murine macrophage cell line, the expression of this conductance in human monocyte-derived macrophages (HMDMs) is rare. Whole-cell current recordings in this study were obtained from HMDMs differentiated in adherent culture for varying periods of time following isolation and compared to currents obtained in human alveolar macrophages (HAMs) obtained from bronchoalveolar lavage. These studies were undertaken to compare ionic current expression in the in vitro differentiated macrophage to that of a human tissue macrophage. HAMs are the major population of immune and inflammatory cells in the normal lung and are the most readily available source of human tissue macrophages. Of the 974 HMDMs in the study obtained from a total of 36 donors, we were able to observe the presence of the inactivating outward current (I _A ) which exhibited voltage-dependent availability in only 49 (or 5%) of the cells. In contrast, whole-cell current recordings from HAMs, revealed a significantly higher frequency ofI _A expression (50% in a total of 160 cells from 26 donors). In the alveolar cell, there was no correlation observed between cell size and peakI _A amplitude, nor was there a relationship between peakI _A amplitude and time in culture. The current in both cell types was K⁺ selective and 4-aminopyridine (4-AP) sensitive.I _A in both cell types inactivated with a time course which was weakly voltage-dependent and which exhibited a time constant of recovery from inactivation of approximately 30 sec. The time course of current inactivation was dependent upon the external K⁺ concentration. An increase in the time constant describing current decay was observed in elevated K⁺. Current activation was half-maximal at approximately –18 mV in normal bathing solution. Steady-state inactivation was half-maximal at approximately –44 mV. The presence of the outwardly rectifying K⁺ conductance may alter the potential of the mononuclear phagocyte to respond to extracellular signals mediating chemotaxis, phagocytosis, and tumoricidal functions.     

Membrane Potential Dependent Duration of Action Potentials in Cultured Rat Hippocampal Neurons

(1) Fluctuations of the membrane potential states are essential for the brain functions from the response of individual neurons to the cognitive function of the brain. It has been reported in slice preparations that the action potential duration is dependent on the membrane potential states. (2) In order to examine whether dependence of action potential duration on the membrane potential could happen in isolated individual neurons that have no network connections, we studied the membrane potential dependence of the action potential duration by artificially setting the membrane potentials to different states in individual cultured rat hippocampal neurons using patch-clamp technique. (3) We showed that the action potential of individual neurons generated from depolarized membrane potentials had broader durations than those generated from hyperpolarized membrane potentials. (4) Furthermore, the membrane potential dependence of the action potential duration was significantly reduced in the presence of voltage-gated K⁺ channel blockers, TEA, and 4-AP, suggesting involvement of both delayed rectifier I _K and transient I _A current in the membrane potential dependence of the action potential duration. (5) These results indicated that the dependence of action potential duration on the membrane potential states could be an intrinsic property of individual neurons. Bo Gong and Mingna Liu contributed equally to this work.     

Role of cell-cell interactions in the developmental regulation of Ca2+-activated K+ currents in vertebrate neurons

The functional expression of the Ca²⁺-activated K⁺ current (I_K[Ca]) is dependent on cell-cell interactions in developing chick autonomic neurons. In chick ciliary ganglion (CG) neurons, expression of macroscopic I_K[Ca] coincides with the formation of synapses with target tissues. CG neurons that develop in vivo in the absence of normal target tissues fail to express functional I_K[Ca], although voltage-activated Ca²⁺ currents and most other ionic currents are expressed at normal amplitudes and densities. CG neurons placed in cell culture prior to formation of synapses with target tissues also fail to express macroscopic I_K[Ca]. However, CG neurons cultured in the presence of a heat- and trypsin-sensitive extract of target tissues express I_K[Ca] at normal levels. Similarly, interactions with target tissue appear to regulate the expression of whole-cell I_K[Ca] in developing chick sympathetic ganglion neurons, although the relevant trophic factors appear to be different from those required by CG neurons. In addition to target tissue interactions, an intact preganglionic innervation is required for the normal in vivo development of I_K[Ca] in chick CG neurons. The trophic effects of the afferent innervation do not require synaptic activation of the CG neurons, indicating secretion of a trophic factor, possibly an isoform of β-neuregulin. The results are consistent with the hypothesis that target- and nerve terminal-derived trophic factors interact at a posttranslational level in the regulation of a functional I_K[Ca]. Together, this body of data demonstrates an essential role for cell-cell interactions in the differentiation of neuronal excitability. © 1998 John Wiley & Sons, Inc. J Neurobiol 37: 23–36, 1998     

Properties of the charibdotoxin-sensitive component of Ca2+-dependent K+ current in smooth muscle cells of the guinea pigTaenia Coli

Using the voltage-clamp technique, we investigated transmembrane ion currents in isolated smooth muscle cells of the guinea pigtaenia coli. In our study, we identified and studied a charibdotoxin-sensitive component of Ca²⁺-dependent K⁺ current carried through the channels of high conductance (in most publications called “big conductance,”I _BK(Ca)). This component was completely blocked by 100 nM charibdotoxin and by tetraethylammonium in concentrations as low as 1 mM.I _BK(Ca) demonstrated fast kinetics of inactivation, which nearly coincided with that of Ca²⁺ current. In addition to the dependence on Ca²⁺ concentration, this current also showed voltage-dependent properties: with a rise in the level of depolarization its amplitude increased. In many cells, depolarizing shifts in the membrane potential evoke spontaneous outward currents. Such currents probably represent the secondary effect of cyclic Ca²⁺ release from the caffeine-sensitive intracellular stores that result in short-term activation of charibdotoxin-sensitive Ca²⁺-dependent K⁺ channels.     

Contribution of slowly inactivating potassium current to delayed firing of action potentials in NG108-15 neuronal cells: experimental and theoretical studies

The properties of slowly inactivating delayed-rectifier K⁺ current (I_Kdr) were investigated in NG108-15 neuronal cells differentiated with long-term exposure to dibutyryl cyclic AMP. Slowly inactivating I_Kdr could be elicited by prolonged depolarizations from −50 to +50 mV. These outward K⁺ currents were found to decay at potentials above −20 mV, and the decay became faster with greater depolarization. Cell exposure to aconitine resulted in the reduction of I_Kdr amplitude along with an accelerated decay of current inactivation. Under current-clamp recordings, a delay in the initiation of action potentials (APs) in response to prolonged current stimuli was observed in these cells. Application of aconitine shortened the AP initiation in combination with an increase in both width of spike discharge and firing frequency. The computer model, in which state-dependent inactivation of I_Kdr was incorporated, was also implemented to predict the firing behavior present in NG108-15 cells. As the inactivation rate constant of I_Kdr was elevated, the firing frequency was progressively increased along with a shortening of the latency for AP appearance. Our theoretical work and the experimental results led us to propose a pivotal role of slowly inactivating I_Kdr in delayed firing of APs in NG108-15 cells. The results also suggest that aconitine modulation of I_Kdr gating is an important molecular mechanism through which it can contribute to neuronal firing.     

Role of the inward K rectifier in the repetitive activity at the depolarized level in single ventricular myocytes

The role of the inward K⁺ rectifier in the repetitive activity at depolarized levels was studied in guinea pig single ventricular myocytes by voltage- and current-clamp methods. In action potentials arrested at the plateau by a depolarizing current, small superimposed hyperpolarizing currents caused much larger voltage displacements than at the resting potential and sometimes induced a regenerative repolarization. Around –20 mV, sub- and suprathreshold repetitive inward currents were found. In the same voltage range, small hyperpolarizing currents reversed their polarity. During depolarizing voltage-clamp ramps, around –20 mV there was a sudden decrease in the outward current (I_ns: current underlying the negative slope in the inward K⁺ rectifier steady state I–V relation). During repolarizing ramps, the reincrease in outward current was smaller and slower. During depolarizing and repolarizing current ramps, sudden voltage displacements showed a similar asymmetry. Repetitive I_ns could continue as long as the potential was kept at the level at which they appeared. Depolarizing voltage-clamp steps also caused repetitive I_ns and depolarizing current steps induced repetitive slow responses. Cadmium and verapamil reduced I_ns amplitude during the depolarizing ramp. BRL 34915 (cromakalim), an opener of the ATP-sensitive K⁺ channel, eliminated the negative slope and I_ns, whereas barium increased I_ns frequency (an effect abolished by adding BRL). Depolarization-induced slow responses persisted in an NaCl-Ca-free solution. Thus, the mechanism of repetitive activity at the depolarized level appears to be related to the presence of the negative slope in the inward K⁺ rectifier I–V relation.     

Properties of voltage-gated potassium currents of microglia differentiated with granulocyte/macrophage colony-stimulating factor

Voltage-gated whole-cell currents were recorded from cultured microglial cells which had been developed in the presence of the macrophage/microglial growth factor granulocyte/macrophage colony-stimulating factor. Outward K⁺ currents (I _K) were most prominent in these cells. I _Kcould be activated at potentials more positive than –40 mV. Half-maximal activation of I _Kwas achieved at –13.8 mV and half-maximal inactivation of I _Kwas determined at –33.8 mV. The recovery of I _Kfrom inactivation was described by a time constant of 7.9 sec. For a tenfold change in extracellular K⁺ concentration the reversal potential of I _Kshifted by 54 mV.Extracellularly applied 10 mm tetraethylammonium chloride reduced I _K by about 50%, while 5 mm 4-aminopyridine almost completely abolished I _K. Several divalent cations (Ba²⁺, Cd²⁺, Co²⁺, Zn²⁺) reduced current amplitudes and shifted the activation curve of I _Kto more positive values. Charybdotoxin (IC₅₀ = 1.14 nm) and noxiustoxin (IC₅₀=0.89 nm) blocked I _Kin a concentration-dependent manner, whereas dendrotoxin and mast cell degranulating peptide had no effect on the current amplitudes.