Effect of charybdotoxin and leiurotoxin I on potassium currents in bullfrog sympathetic ganglion and hippocampal neurons

Mechanism of an Intrinsic Oscillation in Rat Geniculate Interneurons

Depolarizing current injections produced a rhythmic bursting of action potentials - a bursting oscillation - in a set of local interneurons in the lateral geniculate nucleus (LGN) of rats. The current dynamics underlying this firing pattern have not been determined, though this cell type constitutes an important cellular component of thalamocortical circuitry, and contributes to both pathologic and non-pathologic brain states. We thus investigated the source of the bursting oscillation using pharmacological manipulations in LGN slices in vitro and in silico. 1. Selective blockade of calcium channel subtypes revealed that high-threshold calcium currentsI L andI P contributed strongly to the oscillation. 2. Increased extracellular K+ concentration (decreased K+currents) eliminated the oscillation. 3. Selective blockade of K+ channel subtypes demonstrated that the calcium-sensitive potassium current (I A H P ) was of primary importance. A morphologically simplified, multicompartment model of the thalamic interneuron characterized the oscillation as follows: 1. The low-threshold calcium currentI T provided the strong initial burst characteristic of the oscillation. 2. Alternating fluxes through high-threshold calcium channels andI A H P then provided the continuing oscillation's burst and interburst periods respectively. This interplay betweenI L andI A H P contrasts with the current dynamics underlying oscillations in thalamocortical and reticularis neurons, which primarily involveI T andI H , orI T andI A H P respectively. These findings thus point to a novel electrophysiological mechanism for generating intrinsic oscillations in a major thalamic cell type. Because local interneurons can sculpt the behavior of thalamocortical circuits, these results suggest new targets for the manipulation of ascending thalamocortical network activity.

Modulation of afterpotentials and firing pattern in guinea pig CA3 neurones by group I metabotropic glutamate receptors

Activation of group I metabotropic glutamate receptors (mGluRs) alters the firing patterns of individual CA3 pyramidal cells in guinea pig hippocampal slices. Following addition of the selective group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) to the bathing solution, pyramidal cells initially firing regular, single action potentials switched to firing in brief bursts. This change in firing pattern resulted from modulation by mGluRs of three afterpotentials. The medium and slow afterhyperpolarizations (m and sAHPs) were blocked by mGluR activation. In addition, a voltage-dependent after depolarization (ADP) was induced. Recordings from mutant mice lacking phospholipase C(beta1) (PLC(beta1)) showed that mGluR block of the mAHP, as well as induction of the ADP, depended on the phosphoinositide hydrolysis pathway. Block of the sAHP, however, was partly spared in the absence of PLC(beta1). Optical recordings of post spike intracellular Ca(2+) rises showed that mGluR block of the AHP was not mediated by alterations of action potential-associated Ca(2+) increases (Ca(2+) transients). The mGluR induction of an ADP was also independent of any changes in the Ca(2+) transient. The mGluR-induced change in the firing pattern of hippocampal pyramidal cells is thus the result of multiple mechanisms, including suppression of both m and sAHPs and activation of an ADP, that act together to produce a specific excitatory effect, namely an increased likelihood that a single action potential will lead immediately to one or more following action potentials.

SK3 is an important component of K(+) channels mediating the afterhyperpolarization in cultured rat SCG neurones

1. Our aim was to identify the small-conductance Ca(2+)-activated K(+) channel(s) (SK) underlying the apamin-sensitive afterhyperpolarization (AHP) in rat superior cervical ganglion (SCG) neurones. 2. Degenerate oligonucleotide primers designed to the putative calmodulin-binding domain conserved in all mammalian SK channel sequences were employed to detect SK DNA in a cDNA library from rat SCG. Only a single band, corresponding to a fragment of the rSK3 gene, was amplified. 3. Northern blot analysis employing a PCR-generated rSK3 fragment showed the presence of mRNA coding for SK3 in SCG as well in other rat peripheral tissues including adrenal gland and liver. 4. The same rSK3 fragment enabled the isolation of a full-length rSK3 cDNA from the library. Its sequence was closely similar to, but not identical with, that of the previously reported rSK3 gene. 5. Expression of the rSK3 gene in mammalian cell lines (CHO, HEK cells) caused the appearance of a K(+) conductance with SK channel properties. 6. The application of selective SK blocking agents (including apamin, scyllatoxin and newer non-peptidic compounds) showed these homomeric SK3 channels to have essentially the same pharmacological characteristics as the SCG afterhyperpolarization, but to differ from those of homomeric SK1 and SK2 channels. 7. Immunohistochemistry using a rSK3 antipeptide antibody revealed the presence of SK3 protein in the cell bodies and processes of cultured SCG neurones. 8. Taken together, these results identify SK3 as a major component of the SK channels responsible for the afterhyperpolarization of cultured rat SCG neurones.

Modulation and genetic identification of the M channel

Potassium channels constitute a superfamily of the most diversified ion channels, acting in delicate and accurate ways to control or modify many physiological and pathological functions including membrane excitability, transmitter release, cell proliferation and cell degeneration. The M-type channel is a unique ligand-regulated and voltage-gated K(+) channel showing distinct physiological and pharmacological characteristics. This review will cover some important progress in the study of M channel modulation, particularly focusing on membrane transduction mechanisms. The K(+) channel genes corresponding to the M channel have been identified and will be reviewed in detail. It has been a long journey since the discovery of M current in 1980 to our present understanding of the mysterious mechanisms for M channel modulation; a journey which exemplifies tremendous achievements in ion channel research and exciting discoveries of elaborate modulatory systems linked to these channels. While substantial evidence has accumulated, challenging questions remain to be answered.

Effects of inhibitors of small- and intermediate-conductance calcium-activated potassium channels, inwardly-rectifying potassium channels and Na(+)/K(+) ATPase on EDHF relaxations in the rat hepatic artery

1. In the rat hepatic artery, the SK(Ca) inhibitors UCL 1684 (300 nM) completely blocked, and scyllatoxin (1 microM) and d-tubocurarine (100 microM) partially inhibited EDHF relaxations when each of them was combined with charybdotoxin (300 nM). 2. The IK(Ca) inhibitors clotrimazole (3 microM) and 2-chlorophenyl-bisphenyl-methanol (3 microM) strongly depressed EDHF relaxations when each of them was combined with apamin (300 nM). The cytochrome P450 mono-oxygenase inhibitor ketoconazole (10 microM) had no effect in the presence of apamin. 3. Ciclazindol (10 microM), which abolishes EDHF relaxations in the presence of apamin, almost completely prevented the calcium ionophore (A23187) stimulated (86)Rb(+) influx via the Gardos channel (IK(Ca)) in human erythrocytes. 4. The Na(+)/K(+) ATPase inhibitor ouabain (500 microM) and the K(IR) blocker Ba(2+) (30 microM) neither alone nor in combination inhibited EDHF relaxations. Ba(2+) was also without effect in the presence of either apamin or charybdotoxin. 5. In contrast to EDHF, an increase in extracellular [K(+)] from 4.6 mM to 9.6, 14.6 and 19.6 mM inconsistently relaxed arteries. In K(+)-free physiological salt solution, re-admission of K(+) always caused complete and sustained relaxations which were abolished by ouabain but unaffected by Ba(2+). 6. The present study provides pharmacological evidence for the involvement of SK(Ca) and IK(Ca) in the action of EDHF in the rat hepatic artery. Our results are not consistent with the idea that EDHF is K(+) activating Na(+)/K(+) ATPase and K(IR) in this blood vessel.

Activation kinetics of the delayed rectifier potassium current of bullfrog sympathetic neurons

We examined the activation kinetics of the delayed rectifier K+ current of bullfrog sympathetic neurons, primarily using whole cell recording. On depolarization, currents activated with a sigmoid delay but did not show a Cole-Moore shift. The time course of activation differed systematically from an exponential raised to a power. At most voltages, a power of 2 gave the best overall fit but a power of 3 better described the initial delay. After the delay, the time course could be fitted by a single exponential. Time constants were 15-20 ms at 0 mV and decreased to a limiting tau = 7 ms at +50 to +100 mV. Tail currents were well fitted by single exponential functions and accelerated with hyperpolarization, from tau = 15-20 ms at 0 mV to tau = 2 ms at -110 mV (e-fold for 40 mV). Eleven kinetic models were evaluated for their ability to describe the activation kinetics of the delayed rectifier. Hodgkin-Huxley-like models did not fit the data well. A linear model where voltage sensor movement is followed by a distinct channel opening step, allosteric models based on the Monod-Wyman--Changeux model, and an unconstrained C-C-C-O model could describe whole cell data from -100 to +40 mV. After including whole cell data at +60 and +80 mV, and a maximal p(open) of 0.8 from noise analysis of cell-attached patches, an allosteric model fit the data best, as the other models had difficulty describing qualitative features of the data. However, some more complex schemes (with additional free parameters) cannot be excluded. We propose the allosteric model as an empirical description of macroscopic ionic currents, and as a model worth considering in future studies on the molecular mechanism of potassium channel gating.

Role of K+ channels in adrenal catecholamine secretion in anesthetized dogs

We examined the role of K+ channels in the secretion of adrenal catecholamine (CA) in response to splanchnic nerve stimulation (SNS), acetylcholine (ACh), 1,1-dimethyl-4-phenyl-piperazinium (DMPP), and muscarine in anesthetized dogs. K+ channel blockers and the cholinergic agonists were infused and injected, respectively, into the adrenal gland. The voltage-dependent K+ channel (KA type) blocker mast cell degranulating (MCD) peptide infusion (10-100 ng/min) enhanced increases in CA output induced by SNS (1-3 Hz), but it did not affect increases in CA output induced by ACh (0.75-3 micrograms), DMPP (0.1-0.4 microgram), or muscarine (0.5-2 micrograms). The small-conductance Ca(2+)-activated K+ (SKCa) channel blocker scyllatoxin infusion (10-100 ng/min) enhanced the ACh-, DMPP-, and muscarine-induced increases in CA output, but it did not affect the SNS-induced increases in CA output. These results suggest that KA channels may play an inhibitory role in the regulation of adrenal CA secretion in response to SNS and that SKCa channels may play the same role in the secretion in response to exogenously applied cholinergic agonists.

Developmental changes of potassium currents of embryonic leech ganglion cells in primary culture

The expression of calcium-activated potassium currents (IK(Ca)), delayed outward rectifier potassium currents (IK(slow)), and transient outward currents (IA) was studied during the development of the nervous system of the leech using the whole-cell patch-clamp recording technique. Dissociated cells were isolated from leech embryos between stage E7 and E16 and maintained in primary culture. K+ currents were recorded at E7, when only few anterior ganglia had formed beneath the primordial mouth. IK(slow) was present in all cells tested, while IK(Ca) was expressed in only 67% of the cells studied. Even as early as E7, different types of IK(Ca) have been found. Neither frequency of occurrence nor the charge density of IK(Ca) showed significant changes between E7 and E16. The density of IK(slow), however, increased by a factor of two between E7 and E8, which resulted in a significant increase in the total K+ current of these cells. This rise in potassium outward current developed in parallel with the appearance of Na+ and Ca2+ inward currents (Schirrmacher and Deitmer: J Exp Biol 155:435-453, 1991) during early development, shaping the electrical excitability in embryonic leech neurones. I(A) could be separated by its voltage-dependence and pharmacological properties. The current was detected at stage E9, when all 32 ganglia are formed in the embryo. The frequency of occurrence of I(A) increased from 16% at E9 to 70% at E15. The channel density, steady state inactivation, and kinetics showed no significant changes during development.

Vasopressin receptor subtypes differentially modulate calcium-activated potassium currents in the horizontal limb of the diagonal band of Broca

The actions of vasopressin on acutely dissociated neurons within the rat horizontal limb of the diagonal band of Broca were examined using the whole-cell patch-clamp technique. Vasopressin elicited two distinct responses in 45 of 62 neurons. In one group of cells, 300 nM vasopressin decreased voltage-activated outward currents (26/45 cells) whereas in a second group, vasopressin increased outward currents (19/45 cells). The vasopressin-mediated decrease in outward currents was blocked by 1 microM Manning compound, a V1 receptor antagonist, suggesting that this response was mediated via V1 receptors. In contrast, the vasopressin-induced increase in outward current was blocked by 1 microM d(CH2)5)1,D-Ile2,Ile4,Arg8,Ala9, a V2 receptor antagonist, indicating that V2 receptor activation underlies this second response. When cells were perfused with 0 Ca2+/50 microM Cd2+, application of vasopressin did not cause any change in voltage-activated outward currents, suggesting that vasopressin modulates a calcium-dependent conductance. In the presence of 25 nM charybdotoxin, an Ic channel antagonist, vasopressin application did not influence outward currents, indicating that vasopressin modulates Ic. Currents through voltage-gated calcium channels which are responsible for activation of Ic were unaffected by vasopressin, suggesting a direct effect of vasopressin on Ic channels. These observations indicate a differential modulation of Ic channels by vasopressin via V1 and V2 receptors in the horizontal limb of the diagonal band of Broca. Our data also demonstrate the ionic mechanisms whereby vasopressin may act at V1 for V2 receptors to influence the excitability of the horizontal limb of the diagonal band of Broca neurons.

Enhanced fast synaptic transmission and a delayed depolarization induced by transient potassium current blockade in rat hippocampal slice as studied by optical recording

In hippocampal neurons, a slowly inactivating aminopyridine-sensitive transient potassium current, D-current, influences the time course of action potential repolarization and therefore activity-dependent Ca2+ entry. We used high-speed optical recording techniques to study the effects of selectively inhibiting D-current with 4-AP (40 microM) on transmission at the Schaffer collateral (CA3)-CA1 synapse in rat hippocampal slices stained with the voltage-sensitive dye RH-155. We observed that addition of 4-AP to the bathing solution resulted in (1) augmentation of a fast component of the optical signal corresponding to the postsynaptic EPSP and action potential, and (2) the appearance of a delayed depolarization of CA1 neurons and other adjacent cells. 4-AP appeared to alter the presynaptic action potential and the dynamics of synaptic transmission to both reduce the sensitivity of the postsynaptic EPSP and action potential to omega-toxin calcium channel blockers (omega-conotoxin GVIA and omega-agatoxin IVA) and the Ca(2+)-dependent potassium channel blocker charybdotoxin, and to increase sensitivity to the dihydropyridine nifedipine, the NMDA receptor blocker aminophosphonopentanoic acid, and the intracellular Ca2+ release inhibitor thapsigargin. The delayed depolarization induced by 4-AP was inhibited in hyperosmotic extracellular solution, suggesting that enhanced transmitter release resulted in increased accumulation of K+ in the extracellular space. Because 4-AP is a convulsant at concentrations similar to those used here, we suggest that the 4-AP-targeted channel(s) carrying D-current may contribute to the hyperexcitability associated with epilepsy.

Functional colocalization of calcium and calcium-gated potassium channels in control of transmitter release

We examined, using physiological and morphological techniques, the distribution of Ca(2+)-gated K+ (gKca) channels relative to the location of Ca2+ channels and transmitter release sites at the frog neuromuscular junction (NM). Charybdotoxin (ChTx) and iberiotoxin, blockers of gKca channels with large conductances, increase transmitter release at the frog NMJ. Intracellular Ca2+ buffers with rapid binding kinetics, dimethyl BAPTA and BAPTA, prevented the effect of ChTx, but EGTA, a Ca2+ buffer with similar affinity for Ca2+ but slower binding kinetics, did not. Dimethyl BAPTA and BAPTA, but not EGTA, caused a temporary increase in transmitter release. Labeling of gKca channels with ChTx-biotin revealed a series of bands located at the sites of Ca2+ channels, but this labeling did not occur in denervated preparations. Cross sections of NMJs revealed that gKca channels are clustered in the presynaptic membrane facing the postsynaptic membrane. We conclude that gKca channels are strategically clustered at the neurotransmitter release sites, where they can be quickly activated by Ca2+ entering the terminal.