Dequalinium: a potent inhibitor of apamin-sensitive K+ channels in hepatocytes and of nicotinic responses in skeletal muscle

Dequalinium chloride is an antagonists of α7 nicotinic acetylcholine receptors

Dequalinium chloride has been used primarily as antiseptic compounds, but recently has been investigated for its effects on specific targets, including muscarinic acetylcholine receptors. Here we investigated dequalinium chloride as an antagonist to α7 nicotinic acetylcholine receptors. The pharmacological properties of dequalinium were established using cell lines stably co-transfected with the calcium-permeable human α7 nicotinic acetylcholine receptors and its chaperone NACHO, calcium dye fluorescent measurements or a calcium-sensitive protein reporter, and patch clamp recording of ionic currents. Using calcium dye fluorescence plate reader measurements, we find dequalinium chloride is an antagonist of α7 nicotinic acetylcholine receptors with an IC₅₀ of 672 nM in response to activation with 500 μM acetylcholine chloride and positive allosteric modulator PNU-120596. However, using a membrane-tethered GCAMP7s calcium reporter allowed detection of α7-mediated calcium flux in the absence of PNU-120596. Using this approach revealed an IC₅₀ of 157 nM for dequalinium on 300 μM acetylcholine-evoked currents. Using patch clamp recordings with 300 μM acetylcholine chloride and 10 μM PNU-120596, we find lower concentrations are sufficient to block ionic currents, with IC₅₀ of 120 nM for dequalinium chloride and 54 nM for the related UCL 1684 compound. In summary, we find that dequalinium chloride and UCL1684, which are generally used to block SK-type potassium channels, are also highly effective antagonists of α7 nicotinic acetylcholine receptors. This finding, in combination with previous studies of muscarinic acetylcholine receptors, clearly establishes dequalinium compounds within the class of general anti-cholinergic antagonists.

Effects of Sublethal Organophosphate Toxicity and Anti-cholinergics on Electroencephalogram and Respiratory Mechanics in Mice

Organophosphates are used in agriculture as insecticides but are potentially toxic to humans when exposed at high concentrations. The mechanism of toxicity is through antagonism of acetylcholinesterase, which secondarily causes excess activation of cholinergic receptors leading to seizures, tremors, respiratory depression, and other physiological consequences. Here we investigated two of the major pathophysiological effects, seizures and respiratory depression, using subcutaneous injection into mice of the organophosphate diisopropylfluorophosphate (DFP) at sublethal concentrations (2.1 mg/Kg) alone and co-injected with current therapeutics atropine (50 mg/Kg) or acetylcholinesterase reactivator HI6 (3 mg/Kg). We also tested a non-specific cholinergic antagonist dequalinium chloride (2 mg/Kg) as a novel treatment for organophosphate toxicity. Electroencephalogram (EEG) recordings revealed that DFP causes focal delta frequency (average 1.4 Hz) tonic spikes in the parietal region that occur transiently (lasting an average of 171 ± 33 min) and a more sustained generalized theta frequency depression in both parietal and frontal electrode that did not recover the following 24 h. DFP also caused behavioral tremors that partially recovered the following 24 h. Using whole body plethysmography, DFP revealed acute respiratory depression, including reduced breathing rates and tidal volumes, that partially recover the following day. Among therapeutic treatments, dequalinium chloride had the most potent effect on all physiological parameters by reducing acute EEG abnormalities and promoting a full recovery after 24 h from tremors and respiratory depression. Atropine and HI6 had distinct effects on EEGs. Co-treatment with atropine converted the acute 1.4 Hz tonic spikes to 3 Hz tonic spikes in the parietal electrode and promoted a partial recovery after 24 h from theta frequency and respiratory depression. HI6 fully removed the parietal delta spike increase and promoted a full recovery in theta frequency and respiratory depression. In summary, while all anticholinergic treatments promoted survival and moderated symptoms of DFP toxicity, the non-selective anti-cholinergic dequalinium chloride had the most potent therapeutic effects in reducing EEG abnormalities, moderating tremors and reducing respiratory depression.

Medicinal applications and molecular targets of dequalinium chloride

For more than 60 years dequalinium chloride (DQ) has been used as anti-infective drug, mainly to treat local infections. It is a standard drug to treat bacterial vaginosis and an active ingredient of sore-throat lozenges. As a lipophilic bis-quaternary ammonium molecule, the drug displays membrane effects and selectively targets mitochondria to deplete DNA and to block energy production in cells. But beyond its mitochondriotropic property, DQ can interfere with the correct functioning of diverse proteins. A dozen of DQ protein targets have been identified and their implication in the antibacterial, antiviral, antifungal, antiparasitic and anticancer properties of the drug is discussed here. The anticancer effects of DQ combine a mitochondrial action, a selective inhibition of kinases (PKC-α/β, Cdc7/Dbf4), and a modulation of Ca²⁺-activated K⁺ channels. At the bacterial level, DQ interacts with different multidrug transporters (QacR, AcrB, EmrE) and with the transcriptional regulator RamR. Other proteins implicated in the antiviral (MPER domain of gp41 HIV-1) and antiparasitic (chitinase A from Vibrio harveyi) activities have been identified. DQ also targets α -synuclein oligomers to restrict protofibrils formation implicated in some neurodegenerative disorders. In addition, DQ is a typical bolaamphiphile molecule, well suited to form liposomes and nanoparticules useful for drug entrapment and delivery (DQAsomes and others). Altogether, the review highlights the many pharmacological properties and therapeutic benefits of this old 'multi-talented' drug, which may be exploited further. Its multiple sites of actions in cells should be kept in mind when using DQ in experimental research.

Bis-Quinolinium Cyclophane Blockers of SK Potassium Channels Are Antagonists of M3 Muscarinic Acetylcholine Receptors

Dequalinium is used as an antimicrobial compound for oral health and other microbial infections. Derivatives of dequalinium, the bis-quinolinium cyclophanes UCL 1684 and UCL 1848, are high affinity SK potassium channel antagonists. Here we investigated these compounds as M3 muscarinic receptor (mACHR) antagonists. We used the R-CEPIAer endoplasmic reticulum calcium reporter to functionally assay for Gq-coupled receptor signaling, and investigated the bis-quinolinium cyclophanes as antagonists of M3 mACHR activation in transfected CHO cells. Given mACHR roles in airway smooth muscle (ASM) contractility, we also tested the ability of UCL 1684 to relax ASM. We find that these compounds antagonized M3 mACHRs with an IC₅₀ of 0.27 μM for dequalinium chloride, 1.5 μM for UCL 1684 and 1.0 μM for UCL 1848. UCL 1684 also antagonized M1 (IC₅₀ 0.12 μM) and M5 (IC₅₀ 0.52 μM) mACHR responses. UCL 1684 was determined to be a competitive antagonist at M3 receptors as it increased the EC₅₀ for carbachol without a reduction in the maximum response. The Ki for UCL1684 determined from competition binding experiments was 909 nM. UCL 1684 reduced carbachol-evoked ASM contractions (>90%, IC₅₀ 0.43 μM), and calcium mobilization in rodent and human lung ASM cells. We conclude that dequalinium and bis-quinolinium cyclophanes antagonized M3 mACHR activation at sub- to low micromolar concentrations, with UCL 1684 acting as an ASM relaxant. Caution should be taken when using these compounds to block SK potassium channels, as inhibition of mACHRs may be a side-effect if excessive concentrations are used.

Pharmacology of Small- and Intermediate-Conductance Calcium-Activated Potassium Channels

The three small-conductance calcium-activated potassium (K_Ca2) channels and the related intermediate-conductance K_Ca3.1 channel are voltage-independent K⁺ channels that mediate calcium-induced membrane hyperpolarization. When intracellular calcium increases in the channel vicinity, it calcifies the flexible N lobe of the channel-bound calmodulin, which then swings over to the S4-S5 linker and opens the channel. K_Ca2 and K_Ca3.1 channels are highly druggable and offer multiple binding sites for venom peptides and small-molecule blockers as well as for positive- and negative-gating modulators. In this review, we briefly summarize the physiological role of K_Ca channels and then discuss the pharmacophores and the mechanism of action of the most commonly used peptidic and small-molecule K_Ca2 and K_Ca3.1 modulators. Finally, we describe the progress that has been made in advancing K_Ca3.1 blockers and K_Ca2.2 negative- and positive-gating modulators toward the clinic for neurological and cardiovascular diseases and discuss the remaining challenges.

Pharmacological modulation of mitochondrial ion channels

The field of mitochondrial ion channels has undergone a rapid development during the last three decades, due to the molecular identification of some of the channels residing in the outer and inner membranes. Relevant information about the function of these channels in physiological and pathological settings was gained thanks to genetic models for a few, mitochondria-specific channels. However, many ion channels have multiple localizations within the cell, hampering a clear-cut determination of their function by pharmacological means. The present review summarizes our current knowledge about the ins and outs of mitochondrial ion channels, with special focus on the channels that have received much attention in recent years, namely, the voltage-dependent anion channels, the permeability transition pore (also called mitochondrial megachannel), the mitochondrial calcium uniporter and some of the inner membrane-located potassium channels. In addition, possible strategies to overcome the difficulties of specifically targeting mitochondrial channels versus their counterparts active in other membranes are discussed, as well as the possibilities of modulating channel function by small peptides that compete for binding with protein interacting partners. Altogether, these promising tools along with large-scale chemical screenings set up to identify new, specific channel modulators will hopefully allow us to pinpoint the actual function of most mitochondrial ion channels in the near future and to pharmacologically affect important pathologies in which they are involved, such as neurodegeneration, ischaemic damage and cancer. LINKED ARTICLES: This article is part of a themed section on Mitochondrial Pharmacology: Featured Mechanisms and Approaches for Therapy Translation. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.22/issuetoc.

Evolving mechanisms of vascular smooth muscle contraction highlight key targets in vascular disease

Vascular smooth muscle (VSM) plays an important role in the regulation of vascular function. Identifying the mechanisms of VSM contraction has been a major research goal in order to determine the causes of vascular dysfunction and exaggerated vasoconstriction in vascular disease. Major discoveries over several decades have helped to better understand the mechanisms of VSM contraction. Ca2+ has been established as a major regulator of VSM contraction, and its sources, cytosolic levels, homeostatic mechanisms and subcellular distribution have been defined. Biochemical studies have also suggested that stimulation of Gq protein-coupled membrane receptors activates phospholipase C and promotes the hydrolysis of membrane phospholipids into inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). IP3 stimulates initial Ca2+ release from the sarcoplasmic reticulum, and is buttressed by Ca2+ influx through voltage-dependent, receptor-operated, transient receptor potential and store-operated channels. In order to prevent large increases in cytosolic Ca2+ concentration ([Ca2+]c), Ca2+ removal mechanisms promote Ca2+ extrusion via the plasmalemmal Ca2+ pump and Na+/Ca2+ exchanger, and Ca2+ uptake by the sarcoplasmic reticulum and mitochondria, and the coordinated activities of these Ca2+ handling mechanisms help to create subplasmalemmal Ca2+ domains. Threshold increases in [Ca2+]c form a Ca2+-calmodulin complex, which activates myosin light chain (MLC) kinase, and causes MLC phosphorylation, actin-myosin interaction, and VSM contraction. Dissociations in the relationships between [Ca2+]c, MLC phosphorylation, and force have suggested additional Ca2+ sensitization mechanisms. DAG activates protein kinase C (PKC) isoforms, which directly or indirectly via mitogen-activated protein kinase phosphorylate the actin-binding proteins calponin and caldesmon and thereby enhance the myofilaments force sensitivity to Ca2+. PKC-mediated phosphorylation of PKC-potentiated phosphatase inhibitor protein-17 (CPI-17), and RhoA-mediated activation of Rho-kinase (ROCK) inhibit MLC phosphatase and in turn increase MLC phosphorylation and VSM contraction. Abnormalities in the Ca2+ handling mechanisms and PKC and ROCK activity have been associated with vascular dysfunction in multiple vascular disorders. Modulators of [Ca2+]c, PKC and ROCK activity could be useful in mitigating the increased vasoconstriction associated with vascular disease.

Protein Kinase C as Regulator of Vascular Smooth Muscle Function and Potential Target in Vascular Disorders

Vascular smooth muscle (VSM) plays an important role in maintaining vascular tone. In addition to Ca2+-dependent myosin light chain (MLC) phosphorylation, protein kinase C (PKC) is a major regulator of VSM function. PKC is a family of conventional Ca2+-dependent α, β, and γ, novel Ca2+-independent δ, ɛ, θ, and η, and atypical ξ, and ι/λ isoforms. Inactive PKC is mainly cytosolic, and upon activation it undergoes phosphorylation, maturation, and translocation to the surface membrane, the nucleus, endoplasmic reticulum, and other cell organelles; a process facilitated by scaffold proteins such as RACKs. Activated PKC phosphorylates different substrates including ion channels, pumps, and nuclear proteins. PKC also phosphorylates CPI-17 leading to inhibition of MLC phosphatase, increased MLC phosphorylation, and enhanced VSM contraction. PKC could also initiate a cascade of protein kinases leading to phosphorylation of the actin-binding proteins calponin and caldesmon, increased actin-myosin interaction, and VSM contraction. Increased PKC activity has been associated with vascular disorders including ischemia-reperfusion injury, coronary artery disease, hypertension, and diabetic vasculopathy. PKC inhibitors could test the role of PKC in different systems and could reduce PKC hyperactivity in vascular disorders. First-generation PKC inhibitors such as staurosporine and chelerythrine are not very specific. Isoform-specific PKC inhibitors such as ruboxistaurin have been tested in clinical trials. Target delivery of PKC pseudosubstrate inhibitory peptides and PKC siRNA may be useful in localized vascular disease. Further studies of PKC and its role in VSM should help design isoform-specific PKC modulators that are experimentally potent and clinically safe to target PKC in vascular disease.

Further studies on bis-charged tetraazacyclophanes as potent inhibitors of small conductance Ca(2+)-activated K+ channels

Previously, quinolinium-based tetraazacyclophanes, such as UCL 1684 and UCL 1848, have been shown to be extraordinarily sensitive to changes in chemical structure (especially to the size of the cyclophane system) with respect to activity as potent non-peptidic blockers of the small conductance Ca(2+)-activated K(+) ion channels (SKCa). The present work has sought to optimize the structure of the linking chains in UCL 1848. We report the synthesis and SKCa channel-blocking activity of 29 analogues of UCL 1848 in which the central CH2 of UCL 1848 is replaced by other groups X or Y = O, S, CF2, CO, CHOH, CC, CHCH, CHMe to explore whether subtle changes in bond length or flexibility can improve potency still further. The possibility of improving potency by introducing ring substituents has also been explored by synthesizing and testing 25 analogues of UCL 1684 and UCL 1848 with substituents (NO2, NH2, CF3, F, Cl, CH3, OCH3, OCF3, OH) in the 5, 6 or 7 positions of the aminoquinolinium rings. As in our earlier work, each compound was assayed for inhibition of the afterhyperpolarization (AHP) in rat sympathetic neurons, an action mediated by the SK3 subtype of the SKCa channel. One of the new compounds (39, R(7) = Cl, UCL 2053) is twice as potent as UCL 1848 and UCL 1684: seven are comparable in activity.

Trafficking of intermediate (KCa3.1) and small (KCa2.x) conductance, Ca(2+)-activated K(+) channels: a novel target for medicinal chemistry efforts?

Ca(2+)-activated K(+) (KCa) channels play a pivotal role in the physiology of a wide variety of tissues and disease states, including vascular endothelia, secretory epithelia, certain cancers, red blood cells (RBC), neurons, and immune cells. Such widespread involvement has generated an intense interest in elucidating the function and regulation of these channels, with the goal of developing pharmacological strategies aimed at selective modulation of KCa channels in various disease states. Herein we give an overview of the molecular and functional properties of these channels and their therapeutic importance. We discuss the achievements made in designing pharmacological tools that control the function of KCa channels by modulating their gating properties. Moreover, this review discusses the recent advances in our understanding of KCa channel assembly and anterograde trafficking toward the plasma membrane, the micro-domains in which these channels are expressed within the cell, and finally the retrograde trafficking routes these channels take following endocytosis. As the regulation of intracellular trafficking by agonists as well as the protein-protein interactions that modify these events continue to be explored, we anticipate this will open new therapeutic avenues for the targeting of these channels based on the pharmacological modulation of KCa channel density at the plasma membrane.

Identification of novel telomeric G-quadruplex-targeting chemical scaffolds through screening of three NCI libraries

Thirteen compounds with diverse chemical structures have been identified as selective telomeric G-quadruplex-binding ligands through screening the NCI Diversity Set II, the NCI Natural Products Set II and the NCI Mechanistic Diversity Set libraries containing a total of 2307 members against a human telomeric G-quadruplex using a FRET-based DNA melting assay. These compounds show significant selectivity towards a telomeric G-quadruplex compared to duplex DNA, fall within a molecular weight range of 327-533, and are generally consistent with the Lipinski Rule of Five for drug-likeness. Thus they provide new chemical scaffolds for the development of novel classes of G-quadruplex-targeting agents.

High throughput chemical screening supports the involvement of Ca2+ in cyclic nucleotide-gated ion channel-mediated programmed cell death in Arabidopsis

Recently, we reported the role of Arabidopsis cyclic nucleotide-gated ion channel (AtCNGC) 11 and 12 in Ca2+-dependent physiological responses. AtCNGC11 and 12 have been reported to be involved in plant immunity, but whether these channels play additional physiological roles was not clear before. Using single and double knockout mutants, we have found that these channels play significant roles in Ca2+ signaling, which mediates several physiological processes, such as gravitropic bending and senescence. Here, we conducted a high throughput, non-biased chemical screen using the gain-of-function mutant of AtCNGC11 and 12, cpr22. Our data presented here indicates that Ca2+ but not K+ channel blockers suppress AtCNGC11/12-induced lethality. Our data further suggest that AtCNGC11 and 12 are involved in Ca2+-dependent, but not K+-dependent physiological responses in planta.

Allosteric block of KCa2 channels by apamin

Activation of small conductance calcium-activated potassium (K(Ca)2) channels can regulate neuronal firing and synaptic plasticity. They are characterized by their high sensitivity to the bee venom toxin apamin, but the mechanism of block is not understood. For example, apamin binds to both K(Ca)2.2 and K(Ca)2.3 with the same high affinity (K(D) approximately 5 pM for both subtypes) but requires significantly higher concentrations to block functional current (IC(50) values of approximately 100 pM and approximately 5 nM, respectively). This suggests that steps beyond binding are needed for channel block to occur. We have combined patch clamp and binding experiments on cell lines with molecular modeling and mutagenesis to gain more insight into the mechanism of action of the toxin. An outer pore histidine residue common to both subtypes was found to be critical for both binding and block by the toxin but not for block by tetraethylammonium (TEA) ions. These data indicated that apamin blocks K(Ca)2 channels by binding to a site distinct from that used by TEA, supported by a finding that the onset of block by apamin was not affected by the presence of TEA. Structural modeling of ligand-channel interaction indicated that TEA binds deep within the channel pore, which contrasted with apamin being modeled to interact with the channel outer pore by utilizing the outer pore histidine residue. This multidisciplinary approach suggested that apamin does not behave as a classical pore blocker but blocks using an allosteric mechanism that is consistent with observed differences between binding affinity and potency of block.

Identification of a pharmacophore of SKCa channel blockers

Small conductance calcium-activated potassium channels (SK) are widely expressed throughout the central nervous system (CNS) and the periphery. Three subtypes of SK channels have so far been identified in different parts of the brain. Activation of the SK channels by a rise in intracellular calcium leads to the hyperpolarisation of the membrane, reducing cell excitability. Blocking the SK channels might be beneficial in the treatment of depression, Parkinson's disease and cognitive disorders. However, few blockers of SK channels have been characterized. In this study, a pharmacophoric model of SK channels blockers is presented. It is based on a series of nonpeptidic compounds and apamin, a peptidic blocker. To create the pharmacophore model, the conformational space of nonpeptidic blockers was investigated to generate a series of distance constraints applied to a simulated annealing study of apamin. The resulting conformation was superimposed with the nonpeptidic blockers to give a pharmacophore.

Synthesis and pharmacological testing of polyaminoquinolines as blockers of the apamin-sensitive Ca2+-activated K+ channel (SK(Ca))

The synthesis and pharmacological testing of a series of non-peptidic blockers of the SK(Ca) (SK-3) channel is described. Target compounds were designed to mimic the spatial relationships of selected key residues in the energy-minimised structure of the octadecapeptide apamin, which are a highly potent blocker of this channel. Structures consist of a central unit, either a fumaric acid or an aromatic ring, to which are attached two alkylguanidine or two to four alkylaminoquinoline substituents. Potency was tested by the ability to inhibit the SK(Ca) channel-mediated after-hyperpolarization (AHP) in cultured rat sympathetic neurones. It was found that bis-aminoquinoline derivatives are significantly more potent as channel blockers than are the corresponding guanidines. This adds to the earlier evidence that delocalisation of positive charge through the more extensive aminoquinolinium ring system is important for effective channel binding. It was also found that an increase in activity can be gained by the addition of a third aminoquinoline residue to give non-quaternized amines which have submicromolar potencies (IC(50)=0.13-0.36 microM). Extension to four aminoquinoline residues increased the potency to IC(50)=93 nM.

Synthesis, antifungal and haemolytic activity of a series of bis(pyridinium)alkanes

A series of bis(pyridinium)alkanes have been prepared and their antifungal activity, haemolytic activity and ability to inhibit fungal phospholipase B1 have been investigated, together with those of the commercially available antiseptics octenidine and dequalinium. Removal of the amino substituents from the pyridinium rings resulted in a significant decrease in antifungal activity. However, shortening or removing the alkyl chains attached to the amino groups had little effect on antifungal activity and significantly reduced haemolytic activity. Only octenidine was a strong inhibitor of fungal phospholipase B1.

Baclofen, an agonist at peripheral GABAB receptors, induces antinociception via activation of TEA-sensitive potassium channels

BACKGROUND AND PURPOSE

Central anti-nociceptive actions of baclofen involve activation of K+ channels. Here we assessed what types of K+ channel might participate in the peripheral anti-nociception induced by baclofen.

EXPERIMENTAL APPROACH

Nociceptive thresholds to mechanical stimulation in rat paws treated with intraplantar prostaglandin E2.(PGE2) to induce hyperalgesia were measured 3 h after PGE2 injection. Other agents were also given by intraplantar injection.

KEY RESULTS

Baclofen elicited a dose-dependent (15 - 240 microg per paw) anti-nociceptive effect. An intermediate dose of baclofen (60 microg) did not produce antinociception in the contralateral paw, showing its peripheral site of action. The GABAB receptor antagonist saclofen (12.5 - 100 microg per paw) antagonized, in a dose-dependent manner, peripheral antinociception induced by baclofen (60 microg), suggesting a specific effect. This antinociceptive action of baclofen was unaffected by bicuculline, GABAA receptor antagonist (80 microg per paw), or by (1,2,5,6 tetrahydropyridin-4-yl) methylphosphinic acid, GABAC receptor antagonist (20 microg per paw). The peripheral antinociception induced by baclofen (60 microg) was reversed, in a dose-dependent manner, by the voltage-dependent K+ channel blockers tetraethylammonium (7.5 - 30 microg per paw) and 4-aminopyridine (2.5 - 10 microg per paw). The blockers of other K+ channels, glibenclamide (160 microg), tolbutamide (320 microg), charybdotoxin (2 microg), dequalinium (50 microg) and caesium (500 microg) had no effect.

CONCLUSIONS AND IMPLICATIONS

This study provides evidence that the peripheral antinociceptive effect of the GABAB receptor agonist baclofen results from the activation of tetraethylammonium-sensitive K+ channels. Other K+ channels appear not to be involved.

Inhibitory Gating Modulation of Small Conductance Ca2+-Activated K+Channels by the Synthetic Compound (R)-N-(Benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) Reduces Afterhyperpolarizing Current in Hippocampal CA1 Neurons

SK channels are small conductance Ca(2+)-activated K(+) channels important for the control of neuronal excitability, the fine tuning of firing patterns, and the regulation of synaptic mechanisms. The classic SK channel pharmacology has largely focused on the peptide apamin, which acts extracellularly by a pore-blocking mechanism. 1-Ethyl-2-benzimidazolinone (1-EBIO) and 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) have been identified as positive gating modulators that increase the apparent Ca(2+) sensitivity of SK channels. In the present study, we describe inhibitory gating modulation as a novel principle for selective inhibition of SK channels. In whole-cell patch-clamp experiments, the compound (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) reversibly inhibited recombinant SK3-mediated currents (human SK3 and rat SK3) with potencies around 100 nM. However, in contrast to known pore blockers, NS8593 did not inhibit (125)I-apamin binding. Using excised patches, it was demonstrated that NS8593 decreased the Ca(2+) sensitivity by shifting the activation curve for Ca(2+) to the right, only slightly affecting the maximal Ca(2+)-activated SK current. NS8593 inhibited all the SK1-3 subtypes Ca(2+)-dependently (K(d) = 0.42, 0.60, and 0.73 microM, respectively, at 0.5 microM Ca(2+)), whereas the compound did not affect the Ca(2+)-activated K(+) channels of intermediate and large conductance (hIK and hBK channels, respectively). The site of action was accessible from both sides of the membrane, and the NS8593-mediated inhibition was prevented in the presence of a high concentration of the positive modulator NS309. NS8593 was further tested on mouse CA1 neurons in hippocampal slices and shown to inhibit the apaminand tubocurarine-sensitive SK-mediated afterhyperpolarizing current, at a concentration of 3 microM.

Delta-opioid receptor agonist SNC80 induces peripheral antinociception via activation of ATP-sensitive K+ channels

We investigated the effect of several K+ channel blockers on the antinociception induced by delta-opioid receptor agonist SNC80 using the paw pressure test, in which pain sensitivity is increased by an intraplantar injection (2 microg) of prostaglandin E2 (PGE2). Administration of SNC80 (20, 40 and 80 microg/paw) caused a decrease in the hyperalgesia induced by PGE2, in a dose-dependent manner. The possibility of higher dose of SNC80 (80 microg) causing a central or systemic effect was excluded since administration of the drug into the contralateral paw did not elicit antinociception in the right paw. Specific blockers of ATP-sensitive K+ channels, glibenclamide (20, 40 and 80 microg/paw) and tolbutamide (40, 80 and 160 microg/paw), antagonized the peripheral antinociception induced by SNC80 (80 microg). On the other hand, charybdotoxin (2 microg/paw), a large-conductance blocker of Ca(2+)-activated K+ channels, and dequalinium (50 microg/paw), a small conductance selective blocker of Ca(2+)-activated K+ channels, did not modify the effect of SNC80. This effect also remained unaffected by intraplantar administration of the voltage-dependent K+ channel blockers tetraethylammonium (30 microg/paw) and 4-aminopyridine (10 microg/paw), and of a non-specific K+ channel blocker, cesium (500 microg/paw). This study provides evidence that the peripheral antinociceptive effect of SNC80 result from the activation of ATP-sensitive K+ channels, and the other K+ channels are not involved.

Mutation of the pore glutamate affects both cytoplasmic and external dequalinium block in the rat olfactory CNGA2 channel

Dequalinium has recently been reported to block CNGA1 and CNGA2 channels expressed in Xenopus laevis. Using the inside-out configuration of the patch-clamp technique, we examined the effects of dequalinium on rat olfactory CNGA2 channels expressed in human embryonic kidney (HEK293) cells and studied aspects of its molecular mechanism of action. We found that cytoplasmic dequalinium blocked wild-type (WT) CNGA2 channels in a voltage-dependent manner with an IC(50) of approximately 1.3 muM at a V(m) of + 60 mV, and an effective fractional charge, zdelta, of +0.8 (z=2, delta=+0.4), suggesting that cytoplasmic dequalinium interacts with a binding site that is about two fifths of the way along the membrane electric field (from the intracellular side). Neutralizing the negatively charged pore lining glutamate acid residue (E342Q) still allows effective channel block by cytoplasmic dequalinium with an IC(50) of approximately 2.2 muM at a V(m) of +60 mV but now having a zdelta of +0.1 (delta=+0.05), indicating a profoundly decreased level of voltage-dependence. In addition, by comparing the extent of block under different levels of channel activation, we show that the block by cytoplasmic dequalinium displayed clear state-dependence in WT channels by interacting predominantly with the closed channel, whereas the block in E342Q channels was state-independent. Application of dequalinium to the external membrane surface also blocked currents through WT channels and the E342Q mutation significantly increased the IC(50) for external block approximately fivefold. These results confirm dequalinium as a potent, voltage-dependent and state-dependent blocker of cyclic-nucleotide-gated channels, and show that neutralization of the E342 residue profoundly affects the block by both cytoplasmic and external application of dequalinium.

Inhibition of the antigen-induced activation of RBL-2H3 cells by charybdotoxin and cetiedil

Quinidine and Ba(2+), non-selective K(+)-channel blockers, have previously been shown to inhibit antigen-induced mediator (beta-hexosaminidase) release from RBL-2H3 cells, a mucosal-type mast cell line. We therefore used selective blockers of Ca(2+)-activated and other K(+) channels to determine if there was a role for these channels in antigen-induced mediator release. Charybdotoxin and cetiedil dose-dependently inhibited beta-hexosaminidase release with IC(50) values of 133 nM and 84 microM, respectively. Charybdotoxin also inhibited the repolarization phase of the antigen-induced biphasic change in the membrane potential (IC(50) 84 nM), antigen-stimulated 86Rb(+)-efflux and increase in free intracellular calcium, [Ca(2+)](i). Iberiotoxin, margatoxin, apamin and tetraethylammonium had no effect on beta-hexosaminidase release. These results suggest that K(+) conductances play a significant role in mediator release from RBL-2H3, that these conductances are of the intermediate conductance Ca(2+)-activated K(+) channel (IK(Ca)) type, and that they are somewhat similar to those which have been described in red blood cells, though they are much less sensitive to clotrimazole.

Bis-quinolinium cyclophanes: toward a pharmacophore model for the blockade of apamin-sensitive SKCa channels in sympathetic neurons

The synthesis, pharmacological evaluation, and molecular modeling studies of unsymmetrical bis-alkylene bis-quinolinium cyclophanes and xylylene-alkylene bis-quinolinium cyclophanes is described. Two important structural features of the pharmacophore for SK(Ca) channel blockade have been identified. These are (i) an optimum distance of ca. 5.8A between the centroids of the pyridinium rings of the two quinolinium groups and (ii) a preference for conformations having the quinolinium groups in a synperiplanar orientation.

Defining determinant molecular properties for the blockade of the apamin-sensitive SKCa channel in guinea-pig hepatocytes: the influence of polarizability and molecular geometry

QSAR studies of a series of blockers of the SK(Ca) channel in guinea-pig hepatocytes suggests that the polarizability of the blocker is an important factor controlling the binding to the channel. It is suggested that, upon binding, an ion-pair is formed, a process that is promoted by the reorganization of the water molecules. The polarizability is not adequate to describe the potency of the most potent blockers with a good stereochemical fit to the channel, presumably due to more specific interactions taking place.

Small-conductance calcium-activated K+ channels are expressed in pancreatic islets and regulate glucose responses

Glucose-stimulated insulin secretion is associated with transients of intracellular Ca(2+) concentration [Ca(2+)](i) in the pancreatic beta-cell. We identified the expression and function of specific small-conductance Ca(2+)-activated K(+) (SK) channel genes in insulin-secreting cells. The presence of mRNA for SK1, -2, -3, and -4 (intermediate-conductance Ca(2+)-activated K(+) 1 [IK1]) channels was demonstrated by RT-PCR in rodent islets and insulinoma cells. SK2 and -3 proteins in mouse islets were detected by immunoblot and immunocytochemistry. In the tTA-SK3 tet-off mouse, a normal amount of SK3 protein was present in islets, but it became undetectable after exposure to doxycycline (DOX), which inhibits the transcription of the tTA-SK3 gene. The SK/IK channel-blockers apamin, dequalinium, and charybdotoxin caused increases in average [Ca(2+)](i) levels and in frequency of [Ca(2+)](i) oscillations in wild-type mouse islets. In SK3-tTA tet-off mice, the addition of apamin with glucose and tetraethylammonium (TEA) caused a similar elevation in [Ca(2+)](i), which was greatly diminished after DOX suppression of SK3 expression. We conclude that SK1, -2, -3, and IK1 (SK4) are expressed in islet cells and insulin-secreting cells and are able to influence glucose-induced calcium responses, thereby regulating insulin secretion.

Pharmacological and molecular characterisation of SK3 channels in the TE671 human medulloblastoma cell line

The expression of the small conductance calcium-activated potassium channels SK1, SK2 and SK3 was investigated in the TE671 human medulloblastoma cell line using RT-PCR and transcripts were detected only for SK3. Immunodetection experiments confirmed this result, demonstrating the presence of the SK3 protein. This potassium channel was characterised in TE671 cells using whole-cell patch-clamp recordings. Voltage steps to -100 mV from a holding potential of 0 mV in equimolar 140 mM intra- and extracellular K(+) (K(+)(in/out)) elicited an inward current. The reversal potential of this current shifted 56.6 mV per 10-fold increase in K(+)(out) thus suggesting K(+) selectivity. This current was dependent on the concentration of Ca(2+)(in) with an EC(50) of 104.2 nM. A pharmacological characterisation of this current revealed that it was not blocked by 1 microM charybdotoxin (ChTX), 0.3 microM iberiotoxin (IbTX) or 10 microM clotrimazole (CLT) and only modestly inhibited (<50%) by 30 nM scyllatoxin (ScTX), 200 microM dequalinium chloride (Deq) or 300 microM d-tubocurarine (d-TC). The non-selective SK blocker d-TC blocked the current with an IC(50) of 43.2 microM while apamin blocked the current to a much greater extent (87.8% at 1 microM) with an IC(50) of 4.3 nM. Furthermore, the current was significantly increased (132.6+/-5.2%, n=7) by 500 microM 1-ethyl-2-benzimidazolinone (EBIO). Collectively, these data demonstrate the presence of an endogenous SK3 channel in human TE671 cells.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

Suramin prevents cerebellar granule cell-death induced by dequalinium

In this study, we demonstrated that an anticancer drug, dequalinium, a bisquaternary ammonium compound, is a potent neurotoxicant with IC(50) of 0.46 microM on the cultured cerebellar granule neurons. Its selective neurotoxicity revealed by 100-fold more toxic than the other two analogs, pancuronium and vecuronium. The mechanisms underlying dequalinium (DQ)-induced neurotoxicity were explored and found to be associated with decreased mitochondrial membrane potential, increased free radical production and ATP depletion. Suramin (a nonselective purinergic P(2) receptor antagonist and an anticancer drug) but not the glutamate receptor antagonists, MK-801, NBQX (1,2,3,4 tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulfonamide disodium), and DNQX (6,7-dinitroquinoxaline-2,3-dione) significantly prevents the DQ-induced neurotoxicity. By means of microfluorometric image-processing technique using the fluorescent probes, fluorescein diacetate/propidium iodide and Hoechst 33258, respectively, we showed that 1 microM DQ for 24 h induced about 53.5% of apoptosis and 37.5% of necrosis. All of these effects of DQ can be completely prevented by suramin. From these results, we conclude that DQ-induced neurotoxicity was not mediated by glutamate receptor, but by increasing free radical productions and cell energy depletion. Suramin with its beneficial antagonistic effects on DQ-induced neurotoxicity may provide an effective approach for neurodegeneration.

Compounds that block both intermediate-conductance (IK(Ca)) and small-conductance (SK(Ca)) calcium-activated potassium channels

1. Nine bis-quinolinyl and bis-quinolinium compounds related to dequalinium, and previously shown to block apamin-sensitive small conductance Ca(2+)-activated K(+) channels (SK(Ca)), have been tested for their inhibitory effects on actions mediated by intermediate conductance Ca(2+)-activated K(+) channels (IK(Ca)) in rabbit blood cells. 2. In most experiments, a K(+)-sensitive electrode was employed to monitor the IK(Ca)-mediated net loss of cell K(+) that followed the addition of the Ca(2+) ionophore A23187 (2 microM) to red cells suspended at an haematocrit of 1% in a low K(+) (0.12 - 0.17 mM) solution. The remainder used an optical method based on measuring the reduction in light transmission that occurred on applying A23187 (0.4 or 2 microM) to a very dilute suspension of red cells (haematocrit 0.02%). 3. Of the compounds tested, the most potent IK(Ca) blocker was 1,12 bis[(2-methylquinolin-4-yl)amino]dodecane (UCL 1407) which had an IC(50) of 0.85+/-0.06 microM (mean+/-s.d. mean). 4. The inhibitory action of UCL 1407 and its three most active congeners was characterized by (i) a Hill slope greater than unity, (ii) sensitivity to an increase in external [K(+)], and (iii) a time course of onset that suggested use-dependence. Also, the potency of the nonquaternary compounds tested increased with their predicted lipophilicity. These findings suggested that the IK(Ca) blocking action resembles that of cetiedil rather than of clotrimazole. 5. Some quaternized members of the series were also active. The most potent was the monoquaternary UCL 1440 ((1-[N-[1-(3, 5-dimethoxybenzyl)-2-methylquinolinium-4-yl]amino]-10-[N'-(2-me thylqu inolinium-4yl)amino] decane (trifluoroacetate) which had an IC(50) of 1.8+/-0.1 microM. The corresponding bisquaternary UCL 1438 (1, 10-bis[N-[1-(3,5-dimethoxybenzyl)-2-methylquinolinium-4-yl]amino] decane bis(trifluoroacetate) was almost as active (IC(50) 2.7+/-0.3 microM). 6. A bis-aminoquinolium cyclophane (UCL 1684) had little IK(Ca) blocking action despite its great potency at SK(Ca) channels (IC(50) 4.1+/-0.2 nM). 7. The main outcome is the identification of new intermediate-conductance Ca(2+)-activated K(+) channel blockers with a wide range of IK(Ca)/SK(Ca) selectivities.

Pharmacological characterization of small-conductance Ca(2+)-activated K(+) channels stably expressed in HEK 293 cells

Three genes encode the small-conductance Ca(2+)-activated K(+) channels (SK channels). We have stably expressed hSK1 and rSK2 in HEK 293 cells and addressed the pharmacology of these subtypes using whole-cell patch clamp recordings. The bee venom peptide apamin blocked hSK1 as well as rSK2 with IC(50) values of 3.3 nM and 83 pM, respectively. The pharmacological separation between the subtypes was even more prominent when applying the scorpion peptide blocker scyllatoxin, which blocked hSK1 with an IC(50) value of 80 nM and rSK2 at 287 pM. The potent small molecule blockers showed little differentiation between the channel subtypes. The bis-quinolinium cyclophane UCL 1684 blocked hSK1 with an IC(50) value of 762 pM and rSK2 at 364 pM. The antiseptic compound dequalinium chloride blocked hSK1 and rSK2 with IC(50) values of 444 nM and 162 nM, respectively. The nicotinic acetylcholine receptor antagonist d-tubocurarine was found to block hSK1 and rSK2 with IC(50) values of 27 microM and 17 microM when measured at +80 mV. The inhibition by d-tubocurarine was voltage-dependent with increasing affinities at more hyperpolarized potentials. The GABA(A) receptor antagonist bicuculline methiodide also blocked hSK1 and rSK2 in a voltage-dependent manner with IC(50) values of 15 and 25 microM when measured at +80 mV. In conclusion, the pharmacological separation between SK channel subtypes expressed in mammalian cells is too small to support the notion that the apamin-insensitive afterhyperpolarization of neurones is mediated by hSK1.

Synthesis, molecular modeling, and pharmacological testing of bis-quinolinium cyclophanes: potent, non-peptidic blockers of the apamin-sensitive Ca(2+)-activated K(+) channel

The synthesis and pharmacological testing of two series of novel bis-quinolinium cyclophanes as blockers of the apamin-sensitive Ca(2+)-activated K(+) (SK(Ca)) channel are presented. In these cyclophanes the two 4-aminoquinolinium groups are joined at the ring N atoms (linker L) and at the exocyclic N atoms (linker A). In those cases where A and L contain two or more aromatic rings each, the activity of the compound is not critically dependent upon the nature of the linkers. When A and L each have only one benzene ring, the blocking potency changes dramatically with simple structural variations in the linkers. One of these smaller cyclophanes having A = benzene-1,4-diylbis(methylene) and L = benzene-1, 3-diylbis(methylene) (3j, 6,10-diaza-1,5(1,4)-diquinolina-3(1,3),8(1, 4)-dibenzenacyclodecaphanedium tritrifluoroacetate, UCL 1684) has an IC(50) of 3 nM and is the most potent non-peptidic SK(Ca) channel blocker described to date. Conformational analysis on the smaller cyclophanes using molecular modeling techniques suggests that the differences in the blocking potencies of the compounds may be attributable to their different conformational preferences.

The pharmacology of hSK1 Ca2+-activated K+ channels expressed in mammalian cell lines

The pharmacology of hSK1, a small conductance calcium-activated potassium channel, was studied in mammalian cell lines (HEK293 and COS-7). In these cell types, hSK1 forms an apamin-sensitive channel with an IC(50) for apamin of 8 nM in HEK293 cells and 12 nM in COS-7 cells. The currents in HEK293 cells were also sensitive to tubocurarine (IC(50)=23 microM), dequalinium (IC(50)=0.4 microM), and the novel dequalinium analogue, UCL1848 (IC(50)=1 nM). These results are very different from the pharmacology of hSK1 channels expressed in Xenopus oocytes and suggest the properties of the channel may depend on the expression system. Our findings also raise questions about the role of SK1 channels in generating the apamin-insensitive slow afterhyperpolarization observed in central neurones.

Regulation of the rebound depolarization and spontaneous firing patterns of deep nuclear neurons in slices of rat cerebellum

Current-clamp recordings were made from the deep cerebellar nuclei (DCN) of 12- to 15-day-old rats to understand the factors that mediate intrinsic spontaneous firing patterns. All of the cells recorded were spontaneously active with spiking patterns ranging continuously from regular spiking to spontaneous bursting with the former predominating. A robust rebound depolarization (RD) leading to a Na(+) spike burst was elicited after the offset of hyperpolarizing current injection. The voltage and time dependence of the RD was consistent with mediation by low-threshold voltage-gated Ca(2+) channels. In addition, induction of a RD also may be affected by activation of a hyperpolarization-activated cation current, I(h). A RD could be evoked efficiently after brief high-frequency bursts of inhibitory postsynaptic potentials (IPSPs) induced by stimulation of Purkinje cell axons. IPSP-driven RDs were typically much larger and longer than those elicited by direct hyperpolarizing pulses of approximately matched amplitude and duration. Intracellular perfusion of the Ca(2+) buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) dramatically enhanced the RD and its associated spiking, sometimes leading to a plateau potential that lasted several hundred milliseconds. The effects of BAPTA could be mimicked partly by application of apamin, a blocker of small conductance Ca(2+)-gated K(+) channels, but not by paxilline, which blocks large conductance Ca(2+)-gated K(+) channels. Application of both BAPTA and apamin, but not paxilline, caused cells that were regularly spiking to burst spontaneously. Taken together, our data suggest that there is a strong relationship between the ability of DCN cells to elicit a RD and their tendency burst spontaneously. The RD can be triggered by the opening of T-type Ca(2+) channels with an additional contribution of hyperpolarization-activated current I(h). RD duration is regulated by small-conductance Ca(2+)-gated K(+) channels. The RD also is modulated tonically by inhibitory inputs. All of these factors are in turn subject to alteration by extrinsic modulatory neurotransmitters and are, at least in part, responsible for determining the firing modes of DCN neurons.

Dequalinium vesicles form stable complexes with plasmid DNA which are protected from DNase attack

Upon sonication, the antimicrobial and antineoplastic compound dequalinium forms vesicles (DQAsomes, Weissig et al., 1998). Dequalinium (1,1'-(1,10-decamethylene-bis-[aminoquinaldinium])-chloride) was shown to be a fluorophore with an emission maximum at 366 nm. Addition of DNA results in a characteristic quenching of its intrinsic fluorescence. After density gradient centrifugation a band of dequalinium (DQA) tightly associated with DNA is located between the DNA and DQA bands. DQA/DNA-complexes containing plasmid DNA at a molar ratio of DQA/DNA 6:1 are completely protected against DNase activity. Addition of negatively-charged lipids release intact DNA in the same manner as from cationic lipid/DNA complexes. As regards biological effects, DQAsomes show a differential cytotoxicity for normal and sarcoma cell lines. In vitro incubation with fluorescein-labeled oligodeoxynucleotides (5'-fluorescein-[GATC]5) showed an increased uptake of the tagged oligodeoxynucleotide if complexed with dequalinium. We hypothesize that the DQA/DNA complexes are well-suited for 'DQAsomal gene transfer' in vitro and in vivo. Noteworthy, they display an intrinsic antitumor activity manifested by differential cytotoxicity for normal and sarcoma cells.

UCL 1684: a potent blocker of Ca2+ -activated K+ channels in rat adrenal chromaffin cells in culture

The novel K+ channel blocker 6,10-diaza-3(1,3)8,(1,4)-dibenzena-1,5(1,4)-diquinolinacy clodecaphane (UCL 1684) has been tested for its ability to inhibit Ca2+ -activated K+ currents in cultured rat chromaffin cells. Low nanomolar concentrations of UCL 1684 produced a rapid and reversible inhibition of the slow, apamin-sensitive, tail current activated by a depolarizing voltage command. This compound also inhibited the muscarine activated outward current with an IC50 of 6 nM. These results confirm UCL 1684 to be the most potent non-peptidic blocker of the apamin-sensitive Ca2+ -activated K+ channel so far described.

Characterization of the cloned human intermediate-conductance Ca2+-activated K+ channel

The human intermediate-conductance, Ca2+-activated K+ channel (hIK) was identified by searching the expressed sequence tag database. hIK was found to be identical to two recently cloned K+ channels, hSK4 and hIK1. RNA dot blot analysis showed a widespread tissue expression, with the highest levels in salivary gland, placenta, trachea, and lung. With use of fluorescent in situ hybridization and radiation hybrid mapping, hIK mapped to chromosome 19q13.2 in the same region as the disease Diamond-Blackfan anemia. Stable expression of hIK in HEK-293 cells revealed single Ca2+-activated K+ channels exhibiting weak inward rectification (30 and 11 pS at -100 and +100 mV, respectively). Whole cell recordings showed a noninactivating, inwardly rectifying K+ conductance. Ionic selectivity estimated from bi-ionic reversal potentials gave the permeability (PK/PX) sequence K+ = Rb+ (1.0) > Cs+ (10.4) >> Na+, Li+, N-methyl-D-glucamine (>51). NH+4 blocked the channel completely. hIK was blocked by the classical inhibitors of the Gardos channel charybdotoxin (IC50 28 nM) and clotrimazole (IC50 153 nM) as well as by nitrendipine (IC50 27 nM), Stichodactyla toxin (IC50 291 nM), margatoxin (IC50 459 nM), miconazole (IC50 785 nM), econazole (IC50 2.4 microM), and cetiedil (IC50 79 microM). Finally, 1-ethyl-2-benzimidazolinone, an opener of the T84 cell IK channel, activated hIK with an EC50 of 74 microM.

Hexamethonium- and methyllycaconitine-induced changes in acetylcholine release from rat motor nerve terminals

1. The neuronal nicotinic receptor antagonists hexamethonium and methyllycaconitine (MLA) have been used to study the putative prejunctional nicotinic ACh receptors (AChRs) mediating a negative-feedback control of ACh release from motor nerve terminals in voltage-clamped rat phrenic nerve/ hemidiaphragm preparations. 2. Hexamethonium (200 microM), but not MLA (0.4-2.0 microM), decreased the time constant of decay of both endplate currents (e.p.cs) and miniature endplate currents (m.e.p.cs), indicating endplate ion channel block with hexamethonium. However, driving function analysis and reconvolution of e.p.cs and m.e.p.cs indicated that this ion channel block did not compromise the analysis of e.p.c. quantal content. 3. At low frequencies of stimulation (0.5-2 Hz), hexamethonium (200 microM) and MLA (2.0 microM) increased e.p.c. quantal content by 30-40%. At high frequencies (50-150 Hz) neither compound affected e.p.c. quantal content. All effects on quantal content were paralleled by changes in the size of the pool of quanta available for release. 4. The low frequency augmentation of e.p.c. quantal content by hexamethonium was absent when extracellular [Ca2+] was lowered from 2.0 to 0.5 mM. 5. At the concentrations studied, MLA and hexamethonium produced a small (10-20%) decrease in the peak amplitude of m.e.p.cs. 6. Neither apamin (100 nM) nor charybdotoxin (80 nM) had effects on spontaneous or nerve evoked current amplitudes at any frequency of stimulation. Thus the ability of nicotinic antagonists to augment e.p.c. quantal content is not due to inhibition of Ca(2+)-activated K(+)-channels. 7. We suggest that hexamethonium and MLA increase evoked ACh release by blocking prejunctional nicotinic AChRs. These receptors exert a negative feedback control over evoked ACh release and are probably of the alpha-bungarotoxin-insensitive neuronal type.

Evidence for a non-GABAergic action of quaternary salts of bicuculline on dopaminergic neurones

Intracellular recordings were made from neurones, presumed to be dopaminergic, in the rat midbrain slice preparation. Bicuculline methiodide (BMI) and methochloride (BMC) reversibly blocked the slow, apamin-sensitive component of the afterhyperpolarization in these cells. The IC50 for this effect was about 26 microM. In comparison, BMC antagonized the input resistance decrease evoked by muscimol (3 microM) with an IC50 of 7 microM. The base of bicuculline was less potent in blocking the slow afterhyperpolarization. SR95531 (2-[carboxy-3'-propyl]-3-amino-6-paramethoxy-phenyl-pyridaziniu m bromide), another competitive GABA(A) antagonist, and picrotoxin, a non-competitive GABA(A) antagonist, also blocked the action of muscimol (IC50s: 2 and 12 microM respectively), but had no effect on the afterhyperpolarization at a concentration of up to 100 microM. Moreover, 100 microM SR95531 did not affect the blockade of the afterhyperpolarization by BMC. This blockade persisted in the presence of tetrodotoxin and was apparently not due to a reduction of calcium entry, suggesting that it involved a direct action on the channels which mediate this afterhyperpolarization. These results strongly suggest that quaternary salts of bicuculline act on more than one target in the central nervous system. Thus, the involvement of GABA(A) receptors in a given effect cannot be proven solely on the basis of its blockade by these agents.

On the concept of a bivalent pharmacophore for SKCa channel blockers: synthesis, pharmacological testing, and radioligand binding studies on mono-, bis-, and tris-quinolinium compounds

The dissociation equilibrium constants (Kd values) of dequalinium (2) and the monoquinolinium compounds 1a and 1b have been determined from competition equilibrium radioligand binding with [125I]apamin on rat brain synaptic plasma membranes (SPMs). Dequalinium binds to the channel with 2 orders of magnitude higher affinity than 1a or 1b, suggesting that both quinolinium groups are needed for potent and selective SKCa channel blockade. The trisquinolinium compound 3 (1,1'-[5-[4-(4- aminoquinolinium-1-yl)but-1-yl]non-4-en-1,9-diyl]-bis-(4- aminoquinolinium)) has been synthesized and tested for inhibition of the afterhyperpolarization of rat sympathetic neurones and on the binding assay. Compound 3 shows approximately one order of magnitude higher potency than 2, being the most potent non-peptidic SKCa channel blocker reported so far (Kd approximately 30 nM). The higher affinity of 3 compared with 2 may be due to direct binding of the third quinolinium group to the channel or may arise from a reduction of the unfavorable entropy of binding via an increase of the "local concentration" of quinolinium groups.

Synthesis, molecular modeling, and K+ channel-blocking activity of dequalinium analogues having semirigid linkers

Dequalinium [1,1'-(decane-1, 10-diyl)bis(2-methyl-4-aminoquinolinium)] is an effective blocker of the small conductance Ca2(+)-activated K+ channel. It has been shown that the number of methylene groups in the alkyl chain linking the two quinolinium rings of this type of molecule is not critical for activity. To further investigate the role of the linker, analogues of dequalinium have been synthesized, in which the alkyl chain has been replaced by CH2XCH2 where X is a rigid or semirigid group containing aromatic rings. The compounds have been tested for blockade of the slow after-hyperpolarization on rat sympathetic neurons. The most potent compounds have X = phenanthryl, fluorenyl, cis-stilbene, and C6H4(CH2)nC6H4, where n = 0-4. The conformational preferences of the compounds were investigated using the XED/COSMIC molecular modeling system. Although there is some dependence of the potency of the analogue on the conformational properties of the linker (X), overall, X groups having substantial structural differences are tolerated. It seems that X provides a support for the two quinolinium groups and does not interact with the channel directly. The intramolecular separation between the quinolinium rings, which is provided by rigid groups X, is not critical for activity; this may be attributed to the residual conformational mobility of the heterocycles and to the extensive delocalization of the positive charge. These two factors may permit favorable contacts between the quinolinium groups and the channel over a range of intramolecular separations.