Voltage-dependent Potassium Currents Research Articles

Heterogeneity of motorneuron driver potential properties along the anterior-posterior axis of the lobster cardiac ganglion

Endogenous long-duration burst-organizing potentials (driver potentials, DPs) generated by neurons in the lobster cardiac ganglion play a critical role in the ability of the system to generate rhythmic bursts of nerve impulses. The DPs are normally terminated by a voltage-dependent potassium current, but when this is suppressed by tetraethylammonium ion (TEA), the five motorneurons in the system show heterogeneity in properties. Perfusion with TEA increases DP amplitude in all cells in a similar fashion, but it increases DP duration disproportionately in the most anterior motorneurons. Substitution of barium ions for calcium also prolongs DPs in all of the motorneurons, but the effect of this treatment is not different along the anterior-posterior axis of the ganglion. The results suggest that the different behavior of the neurons reflects a difference in either the extent of calcium inactivation of the calcium current responsible for the DP, or in the kinetics or magnitude of a calcium-activated potassium conductance which contributes to membrane repolarization. In contrast to previously reported results, reduction in extracellular sodium ion concentration decreases driver potential amplitude and/or duration. This effect is not differentially expressed in different motorneurons.

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles in cardiac ventricular myocytes. Effects via modification of surface potential.

Modulation of voltage-dependent sodium and potassium currents by charged amphiphiles was investigated in cardiac ventricular myocytes using the patch-clamp technique. Negatively charged sodium dodecylsulfate (SDS) increased amplitude of INa, whereas positively charged dodecyltrimethylammonium (DDTMA) decreased INa. Furthermore, SDS shifted the steady-state activation and inactivation of INa in the negative direction, whereas DDTMA shifted the curves in the opposite direction. These shifts provided an explanation for the changes in current amplitude. Activation and inactivation kinetics of INa were accelerated by SDS but slowed by DDTMA. These changes in both steady-state gating and kinetics of INa are consistent with a decrease of the intramembrane field by SDS and an increase of the field by DDTMA due to an alteration of surface potential after their insertion into the outer monolayer of the sarcolemma. The effect of SDS on the steady-state inactivation of INa was concentration dependent and partially reversed by screening surface charges with increased extracellular [Ca2+]. These amphiphiles also altered the activation of the delayed rectifier K+ current (IK,del), producing a shift in the negative direction by SDS but in the positive direction by DDTMA. These results suggest that the insertion of charged amphiphiles into the cell membrane alters the behavior of voltage-dependent INa and IK,del by changing the surface charge density, and consequently the surface potential and implies, although indirectly, that the lipid surface charges are important to the voltage-dependent gating of these channels.

Electrophysiologic and molecular properties of cultured enteric glia

Enteric glia, the support cells of myenteric ganglia, have been widely studied with respect to their morphology and immunohistochemical phenotype, but little is known about their functional properties. We developed a method for the amplification of enteric glia from newborn guinea pigs to further characterize these cells. Treatment with a combination of basic fibroblast growth factor and the adenylate cyclase activator, cholera toxin, permitted expansion of enteric glial cultures to confluence and serial passage for up to 8 months. The long-term cultured cells retained expression of 1) S100 protein, 2) GD3 ganglioside recognized by the monoclonal antibody LB1, and 3) the gene encoding glutamine synthetase. The electrophysiologic properties of cultured enteric glia were studied under whole-cell patch clamp conditions. Most cells expressed "delayed rectifier"-type potassium currents, and some also demonstrated tetrodotoxin-sensitive sodium currents. Other subsets of voltage-dependent potassium currents, calcium currents, and glutamate-gated currents were not demonstrable.

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

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

Cooperative interactions among subunits of a voltage-dependent potassium channel. Evidence from expression of concatenated cDNAs.

Four copies of the coding sequence for a voltage-dependent potassium channel (RBK1, rat Kv1.1) were ligated contiguously and transcribed in vitro. The resulting RNA encodes four covalently linked subunit domains ([4]RBK1). Injection of this RNA into Xenopus oocytes resulted in the expression of voltage-dependent potassium currents. A single amino acid substitution, Tyr-->Val, located within the outer mouth of the pore, introduced into the equivalent position of any of the four domains, reduced affinity for external tetraethylammonium by approximately the same amount. In constructs containing 0, 1, 2, 3, or 4 Tyr residues the free energy of binding tetraethylammonium was linearly related to the number of Tyr residues. A different amino acid substitution, Leu-->Ile, located in the S4 region, was made in the equivalent position of one, two, three, or four domains. The depolarization required for channel activation increased approximately linearly with the number of Ile residues, whereas models of independent gating of each domain predict marked nonlinearity. Expression of this concatenated channel provides direct evidence that voltage-dependent potassium channels have four subunits positioned symmetrically around a central permeation pathway and that these subunits interact cooperatively during channel activation.

Elevation of cytosolic calcium by cholinoceptor agonists in SH-SY5Y human neuroblastoma cells: estimation of the contribution of voltage-dependent currents.

1. Muscarinic but not nicotinic receptor stimulation in SH-SY5Y human neuroblastoma cells induces a concentration-dependent increase in [3H]-inositol phosphate formation and a biphasic increase in [Ca2+]i. The latter involves release from both an intracellular store and Ca2+ entry across the plasma membrane. Here we examine the possibility that this agonist-stimulated Ca2+ entry occurs indirectly, as a consequence of depolarization. 2. Electrophysiological characterization, by whole cell patch-clamp techniques revealed that SH-SY5Y cells possess a tetrodotoxin-sensitive inward sodium current, a dihydropyridine-insensitive calcium current and an outward potassium current which was blocked by tetraethylammonium, 4-aminopyridine and intracellular caesium ions. The outward potassium current showed voltage-dependent activation and inactivation, similar to that seen for A-currents. 3. Application of nicotinic agonists evoked an inward current in cells voltage-clamped at negative holding potentials, but this current rectified, resulting in little or no outward current flow at positive potentials. The mean amplitude at a holding potential of -60 mV was -1.14 nA. Extrapolation of the current-voltage relation gave a reversal potential of +8 mV, indicative of a non-specific cationic permeability. 4. Application of muscarinic agonists had no detectable effect in most of the cells tested. However, in one third of cells studied, a small slowly activating inward current was observed. The mean amplitude of this current at a holding potential of -60 mV was -8.3 pA.5. This study confirms that SH-SY5Y cells possess voltage-dependent sodium, potassium and calcium currents. In addition, these cells are strongly depolarized by nicotinic agonists, which produce little change in [Ca2t]1. On the other hand, muscarinic agonists produce profound changes in [Ca2+1J with only a small inward current (depolarization). The contrasting effects of these two cholinoceptor agonists strongly implies that the Ca2+ entry after muscarinic receptor activation is not primarily due to activation of voltage-dependent calcium channels.

Multiple kinetic states underlying macroscopic M-currents in bullfrog sympathetic neurons.

M-current is a time- and voltage-dependent potassium current which is suppressible by muscarinic receptor activation. We have used curve fitting and noise analysis to determine if macroscopic M-currents deviate from a previously predicted simple two-state kinetic scheme. The M-current was best described by three kinetically distinct components: 'fast' (tau 0), 'intermediate' (tau 1) and 'slow' (tau 2) time constants. The 'fast' (tau 0) and 'intermediate' (tau 1) components were identified from the spectra of M-current noise at potentials positive to the cells' resting membrane potential. The 'intermediate' (tau 1) and 'slow' (tau 2) components were seen by curve fitting M-current deactivation currents. The 'intermediate' (tau 1) time constant was voltage dependent (decreasing e-fold in 23 mV), but voltage dependence of the 'fast' (tau 0) and 'slow' (tau 2) components was not obvious. All kinetic components were sensitive to muscarine, with the 'intermediate' (tau 1) and 'slow' (tau 2) being equally so. These data suggest that all components may derive from the same channel population, and that the M-channel may have at least four kinetic states.

APB suppresses synaptic input to retinal horizontal cells in fish: a direct action on horizontal cells modulated by intracellular pH.

1. Membrane potentials and cone-driven light responses were recorded from the H1-type horizontal cells in isolated retinas. Membrane potentials and intracellular pH were recorded also in enzymatically dissociated solitary horizontal cells. 2. In isolated retinas the glutamate analogue 2-amino-4-phosphonobutyrate (APB) hyperpolarized horizontal cells and reduced their light responses in a dose-dependent manner (5-200 microM). 3. The action of APB depended on the formulation of the saline; APB was effective in L-15 saline buffered with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid (HEPES) but not in a commonly used, nominally CO2-free bicarbonate/Tris-buffered saline. 4. The major factor controlling the potency of APB was intracellular pH. APB was ineffective during retinal perfusion with NH4Cl-containing or CO2-free bicarbonate saline, both of which are known to alkalinize cells. In contrast, APB was effective in salines formulated to acidify the retinal neurons. These included both HEPES and Tris-buffered salines containing a weak acid and bicarbonate/Tris-buffered saline gassed with CO2. 5. APB reduced the size of glutamate-evoked depolarizations in solitary horizontal cells but had no independent action in the absence of glutamate. This reduction of glutamate-induced depolarization was observed in salines formulated to block voltage-dependent calcium and potassium currents. 6. The magnitude of APB's antagonistic action on solitary horizontal cells increased in a dose-dependent manner from 10 to 200 microM. The antagonism was increased by intracellular acidification and was reduced or eliminated by alkalinization. 7. We conclude that APB can reduce glutamate-evoked and, by inference, the photoreceptor neurotransmitter-evoked depolarization of horizontal cells by acting directly on the horizontal cells. This effect of APB is modulated by intracellular pH.

Expression of the H-ras oncogene induces potassium conductance and neuron-specific potassium channel mRNAs in the AtT20 cell line

Expression of the EJ-ras oncogene in the AtT20 cell line results in several changes in their properties that correspond to a switch of these anterior pituitary-derived cells to a more neuronlike phenotype. The width of action potentials following transfection with ras is reduced 20-fold from over 200 msec in control AtT20 cells to less than 10 msec in ras-transfected cells. This is associated with a two- to threefold increase in the density of voltage-dependent potassium currents. In addition, the rate of inactivation of these currents is decreased approximately twofold in ras-transfected cells. At least part of the change in potassium current may be due to differential expression of potassium channel mRNAs. In the ras-transfected cells, mRNA species were detected using a probe for the voltage-dependent potassium channels, Kv4, a species that appears to be uniquely expressed in the nervous system, and NGK2, an alternatively spliced product transcribed from the same gene. These mRNAs are not detected in control AtT20 cells. The results suggest that the ras protein modulates the phenotype of excitable cells by influencing the expression of specific potassium channels and thereby altering the density and types of channels in the plasma membrane.

Cellular metabolism regulating H and M currents in bullfrog sympathetic ganglia.

Much evidence has accumulated suggesting that neurons in autonomic and dorsal root ganglia possess voltage-dependent currents that link with transmitter receptors through intracellular signal transduction systems. The M current (IM), a voltage-dependent potassium current, was activated at potentials more positive than -65 mV, while the H current (IH), a voltage-dependent nonselective cationic current, was activated at potentials more negative than -50 mV. The hydrolyzable form of ATP was required to activate IM and IH. Intracellular application of calmodulin enhanced the amplitude of IM in a calcium-dependent manner. IM was reduced by W-7, a calmodulin antagonist, and by ML-9, an inhibitor of calmodulin-dependent protein kinase. IH was enhanced by intracellular loading with cyclic adenosine monophosphate (AMP) or bath application of forskolin and membrane-permeable cyclic AMP analogues. Isobutylmethylxanthine also increased the maximal conductance of IH. IH was depressed by H-8 but not by phorbol ester. It is concluded that the resting membrane conductance of these ganglion cells can be regulated by basal activities of calmodulin-dependent protein kinase and A kinase.

Dendrotoxin homologues block voltage-dependent potassium currents in cultured rat dorsal root ganglion cells by different mechanisms: A. Hall, J. Stow, O. Dolly and D. Owen (Department of Biochemistry, Imperial College, London and Wyeth Research (U.K.), Huntercombe Lane South, Taplow, Berks, SL6 0PH, U.K.).

Dendrotoxin homologues block voltage-dependent potassium currents in cultured rat dorsal root ganglion cells by different mechanisms: A. Hall, J. Stow, O. Dolly and D. Owen (Department of Biochemistry, Imperial College, London and Wyeth Research (U.K.), Huntercombe Lane South, Taplow, Berks, SL6 0PH, U.K.).

Regulation by second messengers of the slowly activating, voltage-dependent potassium current expressed in Xenopus oocytes.

1. Voltage-clamp recordings of membrane current were made from Xenopus oocytes that had been injected with RNA which had been transcribed in vitro from a cloned complementary DNA. 2. Depolarization from -80 mV evoked outward potassium currents that developed very slowly. At -20 mV the time constant for activation was about 50 s, and at +40 mV about 6 s. 3. The potassium current was increased by the calcium ionophore A23187 or by intracellular injection of inositol 1,4,5-trisphosphate (IP3), each of which should increase the intracellular calcium concentration ([Ca2+]i). The current was decreased by injection of BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid). The current was also reduced by phorbol esters; this effect was blocked by staurosporine. 4. In oocytes that had also been injected with RNA encoding the 5-hydroxytryptamine (5-HT2) receptor, 5-HT increased the potassium current. After caffeine pretreatment, to block the release of intracellular calcium, 5-HT decreased the current; this decrease was prevented by staurosporine. 5. It is concluded that the slowly activating, voltage-dependent potassium current expressed in Xenopus oocytes is increased by increases in [Ca2+]i and is decreased by activation of protein kinase C. Stimulation of 5-HT2 receptors can have both these effects, but the former normally predominates.

Characterization and functional expression of genomic DNA encoding the human lymphocyte type n potassium channel.

Voltage-gated potassium channels play important functional roles in the development and maintenance of human lymphocyte functions. One such channel, known as the type n channel, has been well defined in human T cells and exhibits unique functional properties that distinguish it from other species of potassium channels. We report the characterization of a human genomic DNA clone, HGK5, encoding a 523-amino-acid potassium channel protein encoded by an open reading frame on a single exon. RNA transcribed in vitro from HGK5 genomic DNA directs expression of functional voltage-dependent potassium currents in Xenopus oocytes. The functional characteristics of the expressed channels are strikingly similar to those of the type n channel on human T lymphocytes. This, together with the presence of significant levels of HGK5 mRNA in human T lymphocytes, supports the notion that HGK5 encodes the human type n voltage-gated potassium channel. The effects of concanavalin A treatment on HGK5 mRNA levels in cultured human T lymphocytes was also examined. Mitogenic concentrations of concanavalin A induced a time-dependent decrease in HGK5 mRNA levels, suggesting that previously observed increases in potassium current density following concanavalin A treatment of human T lymphocytes are not due to increased transcriptional activity of the type n potassium channel gene.

Serotonin reduces a voltage-dependent transient outward potassium current and enhances excitability of cerebellar Purkinje cells.

Utilizing current-clamp and single electrode voltage-clamp techniques we have studied the actions of serotonin (5-HT) on a transient outward voltage-dependent potassium current in adult rat Purkinje cells (PCs) in vitro. Under voltage-clamp, bath-applied 5-HT (10 nM-100 microM) reversibly depressed a transient outward potassium current in a dose-dependent manner in 24 of 26 neurons. This depression was apparent at all test command voltages. In current clamp, with normal bathing medium, 5-HT decreased the latency to first spike firing and increased the number of spikes in response to depolarization. Similar effects were evident when 4-aminopyridine was applied. In bathing medium containing TTX, Cs+ and Ni2+ to block voltage-activated currents, 5-HT increased the trajectory of the electrotonic membrane response elicited by depolarizing current injection. Enhanced responsiveness of PCs by 5-HT was not related to changes in membrane potential or input resistance. These experiments indicate that one mechanism by which 5-HT can increase the excitability of PCs is via reduction of a transient outward current.

Modulation by cAMP of a slowly activating potassium channel expressed in Xenopus oocytes

When expressed in the Xenopus oocyte, the minK protein induces a slowly activating voltage-dependent potassium current (Isk). We studied the modulation of this current by altering intracellular cAMP levels and found that the amplitude of Isk is dramatically increased by treatments that raise cAMP levels and decreased by agents that lower cAMP levels. Preinjection of a protein inhibitor of the cAMP-dependent protein kinase blocked the effects of increased cAMP levels. There were no changes in the voltage dependence or kinetics of Isk. Mutations that eliminate a potential phosphorylation site on the minK protein did not block the effects of activating the kinase. In addition, the membrane capacitance of the oocyte increased and decreased in parallel with Isk. Our results fit a mechanism in which channel proteins are selectively inserted into and removed from the plasma membrane in response to changes in kinase activity.

A transient voltage-dependent outward potassium current in mammalian cerebellar Purkinje cells

Utilizing the single electrode voltage clamp technique applied to rat Purkinje cells (PCs), we have recorded a transient outward voltage-dependent potassium current, I to. Half maximal values for inactivation and activation were −65 mV and −39 mV, respectively. I to decays as a single exponential with a time constant of 95 ms and is reduced by 4-aminopyridine. Based on criteria of voltage dependency of activation and inactivation, kinetics of inactivation, ionic selectivity and pharmacologic sensitivity, we have verified a strong resemblance between the typical A current in other neurons, and I to in PCs. As in other cells, I to may be important in shaping PC output by modulating intrinsic and extrinsic factors that govern PC firing.

Effect of metabolic inhibition on the excitability of isolated hippocampal CA1 neurons: developmental aspects.

1. The effects of brief exposures to hypoxia on the membrane currents of isolated hippocampal CA1 neurons were studied with the use of the whole-cell variation of the patch-clamp technique. Neurons were acutely dissociated from immature (day 2-7) and mature (day 21-43) rats. 2. In the current-clamp mode, Na-cyanide (CN) hyperpolarized both mature and immature neurons. In the voltage-clamp mode, CN decreased the magnitude of the hyperpolarizing holding current in both age groups. 3. CN did not have a consistent effect on the voltage-dependent calcium and potassium currents of immature and mature CA1 neurons but decreased the voltage-dependent inward current of neurons at both ages. This effect was age dependent: the inward current of immature neurons decreased by only 10%, but that of mature neurons decreased by approximately 40%. 4. The decrease in the magnitude of the hyperpolarizing holding current and the depression of the voltage-dependent inward current of mature neurons were observed during brief exposure to N2 (PO2 = 0), indicating that the electroresponses observed with CN were the result of blocking oxidative respiration. 5. The hypoxia-sensitive inward current was blocked by tetrodotoxin (TTX) but was not blocked by cadmium or cesium + tetraethylammonium (TEA). Therefore this current was identified as the voltage-dependent, fast-inactivating sodium current (INa). 6. The isolated sodium current was studied with the use of cadmium to block calcium and TEA + cesium to block potassium currents. In mature neurons, CN left-shifted the steady-state inactivation curve for INa and slowed the deactivation kinetics of INa. CN caused little or no change in INa activation, fast inactivation, recovery from inactivation, or current-voltage (I-V) relationship. 7. We conclude that brief exposures to CN and hypoxia alter the intrinsic excitability of CA1 neurons by at least two mechanisms: 1) alterations in leakage currents and 2) alterations in the fast Na+ conductance that are maturationally dependent. We propose that the alterations in the Na+ conductance may play an adaptive role by reducing O2 demands and thus possibly delaying neuronal injury.

Interaction of 4-aminopyridine with normal and chloramine-T-modified K channels of neuroblastoma cells

The steady-state effects and rate of action of 4-aminopyridine (4-AP) on normal and chloramine-T (CL-T)-modified voltage-dependent potassium (K) currents were studied in neuroblastoma cells with the whole-cell voltage-clamp current recording technique. 4-AP apparently slows both the activation and inactivation of the normal current but does not modify the time course of the CL-T-modified current. These differential effects of 4-AP are interpreted as resulting from the existence of two types of K channels with different 4-AP sensitivities under normal conditions and similar 4-AP sensitivities after CL-T, which furthermore slows their inactivation [8, 9]. While the onset of 4-AP action on the normal current is delayed and can be described by the difference of two exponentials, the onset of 4-AP action on CL-T-modified current starts immediately after the external application of the drug and can be described by the sum of two exponentials. The 4-AP-induced block of the normal current exhibits use-dependent features and is relieved by long conditioning depolarizations. In contrast, the block of the CL-T-modified current is not use-dependent. At high 4-AP concentrations (1-10 mM), the steady-state block of the normal current reaches a saturating value of 95%, while the steady-state block of the CL-T-modified current and the "unblocked" normal current only reaches a saturating value of 35%. The results suggest that CL-T inhibits a channel or membrane constituent which contributes to the inactivation of channels and increases their apparent affinity for 4-AP when they are in closed or open states.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a delayed rectifier-like potassium current in the clonal rat pituitary cell line GH3

Whole cell patch-clamp techniques were used to investigate voltage-dependent potassium currents in the clonal rat pituitary cell line GH3. Inactivation of the voltage-dependent potassium current was best fit by two time constants (50-80 ms and 2-3 s) plus a sustained value. These components of inactivation could be separated based on their voltage-dependent properties and pharmacological sensitivity to 10 mM tetraethylammonium (TEA) and 5 mM 4-aminopyridine (4-AP). The fast component begins to activate around -50 mV, is half-maximally activated at -19 mV, is 50% inactivated at -55 mV, and is sensitive to 4-AP but insensitive to TEA. The slow component begins to activate at around -10 mV, is half-maximally activated at +4 mV, is 50% inactivated at -23 mV, and is sensitive to both TEA and 4-AP. The sustained component is apparent by 0 mV but has not yet reached half-maximal activation at +57 mV. It is somewhat sensitive to TEA but relatively resistant to 4-AP. In the presence of TEA it was found that the fast-inactivating component actually inactivated in a biphasic manner with time constants of approximately 50 and 500 ms. From the properties of these components it is concluded that at least three distinct voltage-dependent potassium channel types exist in GH3 cells as follows: an A-like current (fast-inactivating component), a delayed rectifier-like current (slow-inactivating component), and the voltage-dependent properties of calcium-dependent potassium channels (the sustained component).

Mast Cell Degranulating Peptide Increases the Frequency of Spontaneous Miniature Postsynaptic Currents in CA3 Rat Hippocampal Neurons.

Mast cell degranulating peptide (MCDP) is a neurotoxic agent isolated from bee venom. It produces a long-term potentiation in the hippocampus. We now report that MCDP, at nanomolar concentrations, induces a reduction of a transient voltage-dependent potassium current (ID) in CA3 rat pyramidal neurons and a persistent (>30 min) enhancement of the frequency of spontaneous miniature excitatory and inhibitory postsynaptic currents (m.e.p.s.c.s. and m.i.p.s.c.s.). M.e.p.s.c.s. and m.i.p.s.c.s. were recorded in the presence of bicuculline (30 microM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), respectively. The increased frequency of m.e.p.s.c.s. (408 +/- 60%) and m.i.p.s.c.s. (583 +/- 553%) was independent of the reduction of ID because 4-aminopyridine (4-AP, 30 microM - 2 mM) blocked ID but had no effects on m.e.p.s.c.s. and m.i.p.s.c.s. In the presence of the calcium channel blocker manganese (3 mM), MCDP still enhanced the frequency of m.e.p.s.c.s. (326 +/- 162%). It is concluded that MCDP augments the release of excitatory and inhibitory transmitter by an action, which is independent of calcium influx, through voltage-dependent channels.