Binding of batrachotoxinin A 20-alpha-benzoate to a receptor site associated with sodium channels in synaptic nerve ending particles

Dual receptor-sites reveal the structural basis for hyperactivation of sodium channels by poison-dart toxin batrachotoxin

The poison dart toxin batrachotoxin is exceptional for its high potency and toxicity, and for its multifaceted modification of the function of voltage-gated sodium channels. By using cryogenic electron microscopy, we identify two homologous, but nonidentical receptor sites that simultaneously bind two molecules of toxin, one at the interface between Domains I and IV, and the other at the interface between Domains III and IV of the cardiac sodium channel. Together, these two bound toxin molecules stabilize α/π helical conformation in the S6 segments that gate the pore, and one of the bound BTX-B molecules interacts with the crucial Lys1421 residue that is essential for sodium conductance and selectivity via an apparent water-bridged hydrogen bond. Overall, our structure provides insight into batrachotoxin's potency, efficacy, and multifaceted functional effects on voltage-gated sodium channels via a dual receptor site mechanism.

Differential effects of modified batrachotoxins on voltage-gated sodium channel fast and slow inactivation

Voltage-gated sodium channels (NaVs) are targets for a number of acute poisons. Many of these agents act as allosteric modulators of channel activity and serve as powerful chemical tools for understanding channel function. Herein, we detail studies with batrachotoxin (BTX), a potent steroidal amine, and three ester derivatives prepared through de novo synthesis against recombinant NaV subtypes (rNaV1.4 and hNaV1.5). Two of these compounds, BTX-B and BTX-cHx, are functionally equivalent to BTX, hyperpolarizing channel activation and blocking both fast and slow inactivation. BTX-yne-a C20-n-heptynoate ester-is a conspicuous outlier, eliminating fast but not slow inactivation. This property differentiates BTX-yne among other NaV modulators as a unique reagent that separates inactivation processes. These findings are supported by functional studies with bacterial NaVs (BacNaVs) that lack a fast inactivation gate. The availability of BTX-yne should advance future efforts aimed at understanding NaV gating mechanisms and designing allosteric regulators of NaV activity.

New insights on the arylpiperazinylalkyl pyridazinone ET1 as potent antinociceptive and anti-inflammatory agent

Pyridazine derivatives, such as arylpiperazinylalkyl pyridazinones, display antinociceptive effects to thermal and chemical stimuli. Here, we extended our previous knowledge on the pharmacological profile of 4-amino-6-methyl-2-(3-(4-(4-methylcyclohexa-1,3-dien-1-yl)piperazin-1-yl)propyl)-5-vinylpyridazin-3(2H)-one, here referred as ET1, paving the way for the comprehension of its complete mechanism of action. To this aim, we have evaluated the mouse behavioural responses in several animal models of pain, the effect of ET1 in the murine model of zymosan-induced paw oedema and air-pouch, assessing the cytokines and the cellular phenotype and finally, an in vitro radioligand binding study was performed on a panel of 30 different receptors. In the formalin test, ET1 reduced both neurogenic and inflammatory phase of nociception induced by the aldehyde. Similarly, ET1 strongly reduced paw licking response in the capsaicin test, the abdominal stretching in the writhing test and the carrageenan-induced thermal hyperalgesia. ET1 also evoked a long-lasting reduction of thermal hyperalgesia. Furthermore, ET1 produced a long-lasting anti-inflammatory effect in the zymosan-induced mouse paw oedema and air-pouch through the selective inhibition of inflammatory monocytes recruitment and the modulation of IL-1β, IL-6, TNF-α and MCP-1. Binding experiments confirmed an inhibitory effect on adrenergic α_1A, α_1B and α_2A receptors subtypes and, for the first time, a moderate affinity was observed for the following receptors: histamine H₁, imidazoline I₂, sigma non-opioid intracellular receptor 1 and σ2. These results prompt ET1 as a potent analgesic and anti-inflammatory agent, and support the possibility that it may be suitable for clinical applications in a wide-range of inflammatory-based diseases.

Asymmetric synthesis of batrachotoxin: Enantiomeric toxins show functional divergence against NaV

The steroidal neurotoxin (-)-batrachotoxin functions as a potent agonist of voltage-gated sodium ion channels (Na_Vs). Here we report concise asymmetric syntheses of the natural (-) and non-natural (+) antipodes of batrachotoxin, as well both enantiomers of a C-20 benzoate-modified derivative. Electrophysiological characterization of these molecules against Na_V subtypes establishes the non-natural toxin enantiomer as a reversible antagonist of channel function, markedly different in activity from (-)-batrachotoxin. Protein mutagenesis experiments implicate a shared binding side for the enantiomers in the inner pore cavity of Na_V These findings motivate and enable subsequent studies aimed at revealing how small molecules that target the channel inner pore modulate Na_V dynamics.

Inhibition of Sodium Ion Channel Function with Truncated Forms of Batrachotoxin

A novel family of small molecule inhibitors of voltage-gated sodium channels (Na_Vs) based on the structure of batrachotoxin (BTX), a well-known channel agonist, is described. Protein mutagenesis and electrophysiology experiments reveal the binding site as the inner pore region of the channel, analogous to BTX, alkaloid toxins, and local anesthetics. Homology modeling of the eukaryotic channel based on recent crystallographic analyses of bacterial Na_Vs suggests a mechanism of action for ion conduction block.

Lipid Regulation of Sodium Channels

The lipid landscapes of cellular membranes are complex and dynamic, are tissue dependent, and can change with the age and the development of a variety of diseases. Researchers are now gaining new appreciation for the regulation of ion channel proteins by the membrane lipids in which they are embedded. Thus, as membrane lipids change, for example, during the development of disease, it is likely that the ionic currents that conduct through the ion channels embedded in these membranes will also be altered. This chapter provides an overview of the complex regulation of prokaryotic and eukaryotic voltage-dependent sodium (Nav) channels by fatty acids, sterols, glycerophospholipids, sphingolipids, and cannabinoids. The impact of lipid regulation on channel gating kinetics, voltage-dependence, trafficking, toxin binding, and structure are explored for Nav channels that have been examined in heterologous expression systems, native tissue, and reconstituted into artificial membranes. Putative mechanisms for Nav regulation by lipids are also discussed.

Preclinical development of Ramizol, an antibiotic belonging to a new class, for the treatment of Clostridium difficile colitis

Antibiotic-resistant bacteria is a major threat to human health and is predicted to become the leading cause of death from disease by 2050. Despite the recent resurgence of research and development in the area, few antibiotics have reached the market, with most of the recently approved antibiotics corresponding to new uses for old antibiotics, or structurally similar derivatives thereof. We have recently reported an in silico approach that led to the design of an entirely new class of antibiotics for the bacteria-specific mechanosensitive ion channel of large conductance: MscL. Here, we present the preclinical development of one such antibiotic, Ramizol, a first generation antibiotic belonging to that class. We present the lack of interaction between Ramizol and other mammalian channels adding credibility to its MscL selectivity. We determine the pharmacokinetic profile in a rat model and show <0.1% of Ramizol is absorbed systemically. We show this non-systemic nature of the antibiotic translates to over 70% survival of hamsters in a Clostridium difficile colitis model. Lastly, initial in vitro data indicate that resistance to Ramizol occurs at a low frequency. In conclusion, we establish the potential of Ramizol as an effective new treatment for C. difficile associated disease.

Synthesis and anticonvulsant activity of new N-phenyl-2-(4-phenylpiperazin-1-yl)acetamide derivatives

Twenty-two new N-phenyl-2-(4-phenylpiperazin-1-yl)acetamide derivatives have been synthesized and evaluated for their anticonvulsant activity in animal models of epilepsy. These molecules have been designed as analogs of previously obtained anticonvulsant active pyrrolidine-2,5-diones in which heterocyclic imide ring has been changed into chain amide bound. The final compounds were synthesized in the alkylation reaction of the corresponding amines with the previously obtained alkylating reagents 2-chloro-1-(3-chlorophenyl)ethanone (1) or 2-chloro-1-[3-(trifluoromethyl)phenyl]ethanone (2). Initial anticonvulsant screening was performed using standard maximal electroshock (MES) and subcutaneous pentylenetetrazole screens in mice and/or rats. Several compounds were tested additionally in the psychomotor seizures (6-Hz model). The acute neurological toxicity was determined applying the rotarod test. The results of pharmacological studies showed activity exclusively in the MES seizures especially for 3-(trifluoromethyl)anilide derivatives, whereas majority of 3-chloroanilide analogs were inactive. It should be emphasize that several molecules showed also activity in the 6-Hz screen which is an animal model of human partial and therapy-resistant epilepsy. In the in vitro studies, the most potent derivative 20 was observed as moderate binder to the neuronal voltage-sensitive sodium channels (site 2). The SAR studies for anticonvulsant activity confirmed the crucial role of pyrrolidine-2,5-dione core fragment for anticonvulsant activity.

Synthesis and evaluation of a 125I-labeled iminodihydroquinoline-derived tracer for imaging of voltage-gated sodium channels

In vivo imaging of voltage-gated sodium channels (VGSCs) can potentially provide insights into the activation of neuronal pathways and aid the diagnosis of a number of neurological diseases. The iminodihydroquinoline WIN17317-3 is one of the most potent sodium channel blockers reported to date and binds with high affinity to VGSCs throughout the rat brain. We have synthesized a ¹²⁵I-labeled analogue of WIN17317-3 and evaluated the potential of the tracer for imaging of VGSCs with SPECT. Automated patch clamp studies with CHO cells expressing the Na_v1.2 isoform and displacement studies with [³H]BTX yielded comparable results for the non-radioactive iodinated iminodihydroquinoline and WIN17317-3. However, the ¹²⁵I-labeled tracer was rapidly metabolized in vivo, and suffered from low brain uptake and high accumulation of radioactivity in the intestines. The results suggest that iminodihydroquinolines are poorly suited for tracer development.

Evaluation of a 125I-labelled benzazepinone derived voltage-gated sodium channel blocker for imaging with SPECT

Voltage-gated sodium channels (VGSCs) are a family of transmembrane proteins that mediate fast neurotransmission, and are integral to sustain physiological conditions and higher cognitive functions. Imaging of VGSCs in vivo holds promise as a tool to elucidate operational functions in the brain and to aid the treatment of a wide range of neurological diseases. To assess the suitability of 1-benzazepin-2-one derived VGSC blockers for imaging, we have prepared a (125)I-labelled analogue of BNZA and evaluated the tracer in vivo. In an automated patch-clamp assay, a diastereomeric mixture of the non-radioactive compound blocked the Na(v)1.2 and Na(v)1.7 VGSC isoforms with IC(50) values of 4.1 ± 1.5 μM and 0.25 ± 0.07 μM, respectively. [(3)H]BTX displacement studies revealed a three-fold difference in affinity between the two diastereomers. Iodo-destannylation of a tin precursor with iodine-125 afforded the two diastereomerically pure tracers, which were used to assess binding to VGSCs in vivo by comparing their tissue distributions in mice. Whilst the results point to a lack of VGSC binding in vivo, SPECT imaging revealed highly localized uptake in the interscapular region, an area typically associated with brown adipose tissue, which in addition to high metabolic stability of the iodinated tracer, demonstrate the potential of 1-benzazepin-2-ones for in vivo imaging.

RDX binds to the GABA(A) receptor-convulsant site and blocks GABA(A) receptor-mediated currents in the amygdala: a mechanism for RDX-induced seizures

BACKGROUND

Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) is a high-energy, trinitrated cyclic compound that has been used worldwide since World War II as an explosive in both military and civilian applications. RDX can be released in the environment by way of waste streams generated during the manufacture, use, and disposal of RDX-containing munitions and can leach into groundwater from unexploded munitions found on training ranges. For > 60 years, it has been known that exposure to high doses of RDX causes generalized seizures, but the mechanism has remained unknown.

OBJECTIVE

We investigated the mechanism by which RDX induces seizures.

METHODS AND RESULTS

By screening the affinity of RDX for a number of neurotransmitter receptors, we found that RDX binds exclusively to the picrotoxin convulsant site of the γ-aminobutyric acid type A (GABA(A)) ionophore. Whole-cell in vitro recordings in the rat basolateral amygdala (BLA) showed that RDX reduces the frequency and amplitude of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic currents and the amplitude of GABA-evoked postsynaptic currents. In extracellular field recordings from the BLA, RDX induced prolonged, seizure-like neuronal discharges.

CONCLUSIONS

These results suggest that binding to the GABA(A) receptor convulsant site is the primary mechanism of seizure induction by RDX and that reduction of GABAergic inhibitory transmission in the amygdala is involved in the generation of RDX-induced seizures. Knowledge of the molecular site and the mechanism of RDX action with respect to seizure induction can guide therapeutic strategies, allow more accurate development of safe thresholds for exposures, and help prevent the development of new explosives or other munitions that could pose similar health risks.

Antillatoxin is a sodium channel activator that displays unique efficacy in heterologously expressed rNav1.2, rNav1.4 and rNav1.5 α subunits

BACKGROUND

Antillatoxin (ATX) is a structurally unique lipopeptide produced by the marine cyanobacterium Lyngbya majuscula. ATX activates voltage-gated sodium channel α-subunits at an undefined recognition site and stimulates sodium influx in neurons. However, the pharmacological properties and selectivity of ATX on the sodium channel α-subunits were not fully characterized.

RESULTS

In this study, we characterized the pharmacological properties and selectivity of ATX in cells heterologously expressing rNa(v)1.2, rNa(v)1.4 or rNa(v)1.5 α-subunits by using the Na(+) selective fluorescent dye, sodium-binding benzofuran isophthalate. ATX produced sodium influx in cells expressing each sodium channel α-subunit, whereas two other sodium channel activators, veratridine and brevetoxin-2, were without effect. The ATX potency at rNa(v)1.2, rNa(v)1.4 and rNa(v)1.5 did not differ significantly. Similarly, there were no significant differences in the efficacy for ATX-induced sodium influx between rNa(v)1.2, rNa(v)1.4 and rNa(v)1.5 α-subunits. ATX also produced robust Ca²(+) influx relative to other sodium channel activators in the calcium-permeable DEAA mutant of rNa(v)1.4 α-subunit. Finally, we demonstrated that the 8-demethyl-8,9-dihydro-antillatoxin analog was less efficacious and less potent in stimulating sodium influx.

CONCLUSIONS

ATX displayed a unique efficacy with respect to stimulation of sodium influx in cells expressing rNa(v)1.2, rNa(v)1.4 and rNa(v)1.5 α-subunits. The efficacy of ATX was distinctive inasmuch as it was not shared by activators of neurotoxin sites 2 and 5 on VGSC α-subunits. Given the unique pharmacological properties of ATX interaction with sodium channel α-subunits, decoding the molecular determinants and mechanism of action of antillatoxin may provide further insight into sodium channel gating mechanisms.

Hoiamide a, a sodium channel activator of unusual architecture from a consortium of two papua new Guinea cyanobacteria

Hoiamide A, a novel bioactive cyclic depsipeptide, was isolated from an environmental assemblage of the marine cyanobacteria Lyngbya majuscula and Phormidium gracile collected in Papua New Guinea. This stereochemically complex metabolite possesses a highly unusual structure, which likely derives from a mixed peptide-polyketide biogenetic origin, and includes a peptidic section featuring an acetate extended and S-adenosyl methionine modified isoleucine moiety, a triheterocyclic fragment bearing two alpha-methylated thiazolines and one thiazole, and a highly oxygenated and methylated C15-polyketide substructure. Pure hoiamide A potently inhibited [(3)H]batrachotoxin binding to voltage-gated sodium channels (IC(50) = 92.8 nM), activated sodium influx (EC(50) = 2.31 microM) in mouse neocortical neurons, and exhibited modest cytotoxicity to cancer cells. Further investigation revealed that hoiamide A is a partial agonist of site 2 on the voltage-gated sodium channel.

The cannabinoid receptor agonist CP-55,940 and ethyl arachidonate interfere with [(3)H]batrachotoxinin A 20 alpha-benzoate binding to sodium channels and inhibit sodium channel function

Recent investigations in our laboratory showed that voltage-gated sodium channels (VGSCs) in brain are sensitive to inhibition by various synthetic cannabinoids and endocannabinoids. The present experiments examined the effects of the cannabinoid-1 (CB1) receptor agonist CP-55,940 and ethyl arachidonate on [(3)H]batrachotoxinin A 20 alpha-benzoate ([(3)H]BTX-B]) binding and VGSC-dependent depolarization of the nerve membrane in synaptoneurosomes isolated from mouse whole brain. CP-55,940 acted as a full inhibitor of [(3)H]BTX-B binding and its IC(50) was established at 22.3 microM. At its maximum effect concentration, ethyl arachidonate achieved partial (approximately 70%) inhibition and was less effective than CP-55,940 as an inhibitor of binding (IC(50)=262.7 microM). The potent CB1 receptor antagonist AM251 (2 microM) had no significant effect on the displacement of [(3)H]BTX-B by either compound (P>0.05). Scatchard analyses showed that CP-55,940 and ethyl arachidonate reduce the binding of [(3)H]BTX-B by lowering its B(max) but ethyl arachidonate also increased the K(d) of radioligand binding. In kinetic experiments, CP-55,940 and ethyl arachidonate were found to boost the dissociation of [(3)H]BTX-B from VGSCs to rates that exceed the maximum velocity achievable by veratridine, indicating they operate as allosteric inhibitors of [(3)H]BTX-B binding. Neither compound was effective at changing the initial rate of association of [(3)H]BTX-B with sodium channels. CP-55,940 and ethyl arachidonate inhibited veratridine-dependent (TTX-suppressible) depolarization of the plasma membrane of synaptoneurosomes with IC(50)s of 3.2 and 50.1 microM respectively. These inhibitory effects were again not influenced by 2 microM AM251. Our data demonstrate that the potent cannabinoid receptor agonist CP-55,940 and the ethyl ester of arachidonic acid have the ability to associate with VGSCs and inhibit their function independently of effects on CB1 receptors. Binding data comparisons using mouse brain preparations indicate CP-55,940 is approximately 10,000 times more potent as a CB1 receptor ligand than a sodium channel ligand while ethyl arachidonate shows a much smaller differential. Ethyl arachidonate has been shown previously to be the principal metabolite of ethanol in the brains of intoxicated individuals and effects of this ester on VGSCs and CB1 receptors may contribute to the depressant effects of alcohol.

Influence of lipid-soluble gating modifier toxins on sodium influx in neocortical neurons

The electrical signals of neurons are fundamentally dependent on voltage-gated sodium channels (VGSCs), which are responsible for the rising phase of the action potential. An array of naturally occurring and synthetic neurotoxins have been identified that modify the gating properties of VGSCs. Using murine neocortical neurons in primary culture, we have compared the ability of VGSC gating modifiers to evoke Na+ influx. Intracellular sodium concentration ([Na+](i)) was monitored using the Na+-sensitive fluorescent dye, sodium-binding benzofuran isophthalate. All sodium channel gating modifier compounds tested produced a rapid and concentration-dependent elevation in neuronal [Na+](i). The increment in [Na+](i) exceeded 40 mM at high concentrations of brevetoxins, batrachotoxin, and the novel lipopeptide, antillatoxin. The maximal increments in neuronal [Na+](i) produced by neurotoxin site 2 alkaloids, veratridine and aconitine, and the pyrethroid deltamethrin were somewhat lower with maximal [Na+](i) increments of less than 40 mM. The rank order of efficacy of sodium channel gating modifiers was brevetoxin (PbTx)-1 > PbTx-desoxydioxolane > batrachotoxin > antillatoxin > PbTx-2 = PbTx-3 > PbTx-3alpha-naphthoate > veratridine > deltamethrin > aconitine > gambierol. These data demonstrate that the ability of sodium channel gating modifiers to act as partial agonists is shared by compounds acting at both neurotoxin sites 2 and 5. The concentration-dependent increases in [Na+](i) produced by PbTx-2, antillatoxin, veratridine, deltamethrin, aconitine, and gambierol were all abrogated by tetrodotoxin, indicating that VGSCs represent the sole pathway of Na+ entry after exposure to gating modifier neurotoxins.

Inhibition of [3H]batrachotoxinin A-20alpha-benzoate binding to sodium channels and sodium channel function by endocannabinoids

A number of putative endocannabinoids were found to modify the binding of [(3)H]batrachotoxinin A-20alpha-benzoate ([(3)H]BTX-B) to site 2 on voltage-gated sodium channels of mouse brain and achieve functional inhibition of sodium channels in vitro. 2-Arachidonoyl-glycerol (2-AG), arachidonoyl glycerol ether (AGE), N-arachidonoyl-dopamine (NADA) gave almost complete inhibition of [(3)H]BTX-B binding with IC(50) values of 90.4, 51.2 and 20.7 microM, respectively. The CB1 receptor antagonist AM251 (2 microM) had no effect on the displacement of radioligand by these endocanabinoids. Arachidonoyl-glycine (A-Gly) and arachidonoyl-GABA (A-GABA) were apparently less effective inhibitors of [(3)H]BTX-B binding giving 14.8+/-2.2 and 23.9+/-4.8% inhibition at 100 microM. Phenylmethanesulphonylfluoride (PMSF) did not alter the inhibitory effects of 2-AG, AGE, NADA and A-Gly on binding, but the efficacy of 100 microM A-GABA was increased by 60.3+/-6.3% (P<0.05). Scatchard analyses showed that 2-AG, AGE and NADA reduce the binding of [(3)H]BTX-B by lowering B(max) although increases in K(D) were also evident for AGE and NADA. Our kinetic experiments found that 2-AG, AGE and NADA increase the dissociation velocity of radioligand from site 2 on sodium channels demonstrating that these endocannabinoids operate as allosteric inhibitors of [(3)H]BTX-B binding. 2-AG, AGE and NADA inhibited veratridine-dependent (TTX-suppressible) depolarization of the plasma membrane of synaptoneurosomes at low micromolar concentrations and again the capacities of A-Gly and A-GABA to inhibit this response were less pronounced. The three most effective endocannabinoids (2-AG, AGE and NADA) were then examined in a synaptosomal transmitter release assay where they were observed to inhibit sodium channel- (veratridine-dependent) release of l-glutamate and GABA in the low micromolar range. These effects also occurred through a mechanism that was not influenced by 2 microM AM251. It is concluded that direct inhibition of sodium channel function leading to reduced neuronal excitation and depression of presynaptic release of amino acid transmitters is a property shared by several endocannabinoids.

Vanilloid (subtype 1) receptor-modulatory drugs inhibit [3H]batrachotoxinin-A 20-alpha-benzoate binding to Na+ channels

This investigation was conducted to provide further insight into the effects of vanilloid (subtype 1) receptor (VR1) drugs at voltage-gated sodium channels and examine the potential of this interaction to influence release of neurotransmitters from synaptosomes prepared from mammalian brain. The VR1 modulatory drugs capsaicin, olvanil and capsazepine inhibited the binding of batrachotoxinin-A 20-alpha-benzoate ([(3)H]BTX-B) to receptor site 2 of voltage-gated sodium channels. All drugs reduced the affinity of radioligand for sodium channels, and capsazepine also decreased the number of [(3)H]BTX-B binding sites. In kinetic experiments, no reduction in radioligand association rate was found, but capsaicin, olvanil and capsazepine all enhanced the dissociation rate of [(3)H]BTX-B. All drugs inhibited veratridine-evoked release of L-glutamic acid, gamma-amino butyric acid and L-aspartic acid from synaptosomes; however, their inhibitory effects on transmitter release were much weaker when 35 mM potassium chloride was used to depolarize synaptosomes. The study compounds, in common with other central nervous system depressants, interact with a region on the voltage-gated sodium channel that permits negative allosteric coupling with receptor site 2 and this mechanism likely accounts for blockade of sodium channel-activated transmitter release.

Potent analgesic effects of a putative sodium channel blocker M58373 on formalin-induced and neuropathic pain in rats

M58373, 4-[2-(4-hydroxy-4-{[N-(4-isopropoxyphenyl)-N-methylamino]methyl}piperidin-1-yl)ethyl]benzonitrile monohydrochloride, is a novel compound, which has an inhibitory activity on neurotoxin binding to the site 2 of voltage-gated sodium channels. In this study, we investigated the effects of M58373 on substance P release from sensory neurons in vitro and pain behaviors/responses in rats, compared with mexiletine. M58373 (1-10 microM) inhibited veratridine-induced release of substance P from dorsal root ganglion cells. In the formalin test, oral M58373 (0.3-10 mg/kg) reduced the time spent in nociceptive behaviors only in the late phase. In the neuropathic pain model, oral M58373 (1-10 mg/kg) attenuated mechanical allodynia and heat hyperalgesia in the nerve-injured paw without affecting normal responses in the uninjured paw. In contrast, oral mexiletine (10-100 mg/kg) had a narrow therapeutic dose range in both models because of the adverse effects on the central nervous system. These results suggest that M58373 is a favorable prototype for novel anti-neuropathic pain agents.

Pharmacological Profiles of the Novel Analgesic M58996 in Rat Models of Persistent and Neuropathic Pain

We investigated the effects of 4-(N-{1-[2-(4-cyanophenyl)ethyl]-4-hydroxypiperidin-4-ylmethyl}-N-methylamino)benzoic acid monohydrochloride (M58996), a novel analgesic, on persistent and neuropathic pain in rats. In the formalin test, oral M58996 (0.3 - 10 mg/kg) reduced nociceptive behaviors only in the late phase. In the neuropathic pain model, oral M58996 (1 - 10 mg/kg) attenuated mechanical allodynia and heat hyperalgesia in the nerve-injured paw without affecting normal responses of the uninjured paw. High doses (10 - 100 mg/kg) of oral M58996 did not influence normal motor function. Thus, M58996 had a wide dose range showing antinociceptive, antiallodynic, and antihyperalgesic effects without motor dysfunction. In addition, we studied the possible mechanisms involved in the M58996-induced antinociception. The antinociceptive effect of M58996 was reversed by intrathecal pertussis toxin, an inhibitor of the inhibitory- and other-GTP-binding protein (G(i/o) protein), but not by subcutaneous naloxone, an opioid-receptor antagonist. This effect was also reversed by intracerebroventricular or intrathecal tropisetron, a 5-hydroxytryptamine(3) (5-HT(3))-receptor antagonist, and intraperitoneal bicuculline, a gamma-aminobutyric acid(A) (GABA(A))-receptor antagonist. These results suggest that M58996 produces its antinociceptive effect by a pertussis toxin-sensitive G protein mechanism. In addition, the GABA released by the activation of supraspinal and/or spinal 5-HT(3) receptors is likely to contribute to the M58996-induced antinociception.

Natural products from ginseng inhibit [3H]batrachotoxinin A 20-alpha-benzoate binding to Na+ channels in mammalian brain

A [(3)H]batrachotoxinin A-20alpha-benzoate ([(3)H]BTX-B) binding assay was used to investigate the interaction of two ginseng aglycones (20(S)protopanaxadiol and 20(S)protopanaxatriol) and Rh(2) (a monoglucoside of 20(S)protopanaxadiol) with voltage-gated sodium channels in mouse brain. All compounds inhibited the binding of [(3)H]BTX-B and IC(50)s were established at 42 microM (20(S)protopanaxadiol), 79 microM (20(S)protopanaxatriol) and 162 microM (Rh(2)). Scatchard analysis confirmed that 20(S)protopanaxadiol and Rh-2 reduced the B(max) of [(3)H]BTX-B binding while Rh(2) also increased the K(d). At IC(50) concentrations and above, 20(S)protopanaxadiol and Rh(2) increased the dissociation of the [(3)H]BTX-B:sodium channel complex above that produced by a saturating concentration of veratridine, but failed to reduce the rate of association of [(3)H]BTX-B with sodium channels. Reversal of the inhibition of [(3)H]BTX-B binding by 20(S)protopanaxadiol and Rh(2) occurred slowly. We conclude that the 20(S)protopanaxadiol and the less potent inhibitor Rh(2) destabilize BTX-B-activated sodium channels through non-covalent allosteric modification of neurotoxin binding site 2.

Tolperisone-type drugs inhibit spinal reflexes via blockade of voltage-gated sodium and calcium channels

The spinal reflex depressant mechanism of tolperisone and some of its structural analogs with central muscle relaxant action was investigated. Tolperisone (50-400 microM), eperisone, lanperisone, inaperisone, and silperisone (25-200 microM) dose dependently depressed the ventral root potential of isolated hemisected spinal cord of 6-day-old rats. The local anesthetic lidocaine (100-800 microM) produced qualitatively similar depression of spinal functions in the hemicord preparation, whereas its blocking effect on afferent nerve conduction was clearly stronger. In vivo, tolperisone and silperisone as well as lidocaine (10 mg/kg intravenously) depressed ventral root reflexes and excitability of motoneurons. However, in contrast with lidocaine, the muscle relaxant drugs seemed to have a more pronounced action on the synaptic responses than on the excitability of motoneurons. Whole-cell measurements in dorsal root ganglion cells revealed that tolperisone and silperisone depressed voltage-gated sodium channel conductance at concentrations that inhibited spinal reflexes. Results obtained with tolperisone and its analogs in the [3H]batrachotoxinin A 20-alpha-benzoate binding in cortical neurons and in a fluorimetric membrane potential assay in cerebellar neurons further supported the view that blockade of sodium channels may be a major component of the action of tolperisone-type centrally acting muscle relaxant drugs. Furthermore, tolperisone, eperisone, and especially silperisone had a marked effect on voltage-gated calcium channels, whereas calcium currents were hardly influenced by lidocaine. These data suggest that tolperisone-type muscle relaxants exert their spinal reflex inhibitory action predominantly via a presynaptic inhibition of the transmitter release from the primary afferent endings via a combined action on voltage-gated sodium and calcium channels.

Voltage-gated sodium and calcium channels in nerve, muscle, and heart

Voltage-gated ion channels are membrane proteins which underlie rapid electrical signals among neurons and the spread of excitation in skeletal muscle and heart. We outline some recent advances in the study of voltage-sensitive sodium and calcium channels. Investigations are providing insight into the changes in molecular conformation associated with open-closed gating of the channels, the mechanisms by which they allow only specific ion species to pass through and carry an electric current, and the pathological consequences of small perturbations in channel structure which result from genetic mutations. Determination of three-dimensional structures, coupled with molecular manipulations by site-directed mutagenesis, and parallel electrophysiological analyses of currents through the ion channels, are providing an understanding of the roles and function of these channels at an unprecedented level of molecular detail. Crucial to these advances are studies of bacterial homologues of ion channels from man and other eukaryotes, and the use of naturally occurring peptide toxins which target different ion channel types with exquisite specificity.

Three structurally related, highly potent, peptides from the venom of Parabuthus transvaalicus possess divergent biological activity

The venom of South African scorpion Parabuthus transvaalicus contains a novel group of peptide toxins. These peptides resemble the long chain neurotoxins (LCN) of 60-70 residues with four disulfide bridges; however they are 58 residues long and have only three disulfide bridges constituting a new family of peptide toxins. Here we report the isolation and characterization of three new members of this mammal specific group of toxins. Dortoxin is a lethal peptide, bestoxin causes writhing in mice and altitoxin is a highly depressant peptide. Binding ability of these peptides to rat brain synaptosomes is tested. While the crude venom of P. transvaalicus enhances the binding of [(3)H] BTX to rat brain synaptosomes none of these individual toxins had a positive effect on binding. Although the primary structures of these toxins are very similar to birtoxin, their 3D models indicate significant differences. Dortoxin, bestoxin and altitoxin cumulatively constitute at least 20% of the peptide contained in the venom of P. transvaalicus and contribute very significantly to the toxicity of the venom of this medically important scorpion species. Therefore the amino acid sequences presented here can be used to make more specific and effective antivenins. Possible approaches to a systematic nomenclature of toxins are suggested.

A disubstituted succinamide is a potent sodium channel blocker with efficacy in a rat pain model

Sodium channel blockers are used clinically to treat a number of neuropathic pain conditions, but more potent and selective agents should improve on the therapeutic index of currently used drugs. In a high-throughput functional assay, a novel sodium channel (Na(V)) blocker, N-[[2'-(aminosulfonyl)biphenyl-4-yl]methyl]-N'-(2,2'-bithien-5-ylmethyl)succinamide (BPBTS), was discovered. BPBTS is 2 orders of magnitude more potent than anticonvulsant and antiarrhythmic sodium channel blockers currently used to treat neuropathic pain. Resembling block by these agents, block of Na(V)1.2, Na(V)1.5, and Na(V)1.7 by BPBTS was found to be voltage- and use-dependent. BPBTS appeared to bind preferentially to open and inactivated states and caused a dose-dependent hyperpolarizing shift in the steady-state availability curves for all sodium channel subtypes tested. The affinity of BPBTS for the resting and inactivated states of Na(V)1.2 was 1.2 and 0.14 microM, respectively. BPBTS blocked Na(V)1.7 and Na(V)1.2 with similar potency, whereas block of Na(V)1.5 was slightly more potent. The slow tetrodotoxin-resistant Na(+) current in small-diameter DRG neurons was also potently blocked by BPBTS. [(3)H]BPBTS bound with high affinity to a single class of sites present in rat brain synaptosomal membranes (K(d) = 6.1 nM), and in membranes derived from HEK cells stably expressing Na(V)1.5 (K(d) = 0.9 nM). BPBTS dose-dependently attenuated nociceptive behavior in the formalin test, a rat model of tonic pain. On the basis of these findings, BPBTS represents a structurally novel and potent sodium channel blocker that may be used as a template for the development of analgesic agents.

Sodium channel blocking activity of AM-36 and sipatrigine (BW619C89): in vitro and in vivo evidence

Sodium channel blockers are neuroprotective against cerebral ischemia in animal models. A novel neuroprotective compound AM-36, when screened for activity at the most common receptor and ion channel binding sites, revealed activity at site 2 Na+ channels. Studies then investigated this Na+ channel blocking activity in vitro and in vivo relative to other Na+ channel blockers, including the neuroprotective agent sipatrigine (BW619C89). AM-36 inhibited batrachotoxinin (BTX)-sensitive Na+ channel binding in rat brain homogenates with an IC50 of 0.28 microM. Veratridine (100 microM)-induced neurotoxicity in murine cerebellar granule cells was completely inhibited by AM-36 (1.7 microM) compared to only partial inhibition by sipatrigine (26 microM). Veratridine-stimulated glutamate release, as measured through a microdialysis probe in the cortex of anesthetised rats, was inhibited by 90% by superfusion of AM-36 (1000 microM). In the endothelin-1 (ET-1) model of middle cerebral artery occlusion (MCAo) in conscious rats, both AM-36 (6 mg/kg i.p.) and sipatrigine (10 mg/kg i.p.) 30 min post-MCAo significantly reduced cortical, but not striatal infarct volume. As the refractiveness of the striatum is likely to be dependent on the route and time of drug administration, AM-36 (1 mg/kg i.v.) was administered 3 or 5 h after MCAo and significantly reduced both cortical and striatal infarct volumes. The present studies demonstrate Na+ channel blocking activity of AM-36 both in vitro and in vivo, together with significant neuroprotection when administration is delayed up to 5 h following experimental stroke.

Characterization of the preferred stereochemistry for the neuropharmacologic actions of antillatoxin

Antillatoxin is a potent ichthyotoxin and cytotoxin previously discovered from the marine cyanobacterium Lyngbya majuscula. Ensuing studies of its mechanism of action showed it to activate the mammalian voltage-gated sodium channel at a pharmacological site that is distinct from any previously described. The structure of antillatoxin, initially formulated from spectroscopic information, was subsequently corrected at one stereocenter (C-4) as a result of synthesis of four different antillatoxin stereoisomers (all possible C-4 and C-5 diastereomers). In the current study these four stereoisomers, (4R,5R)-, (4S,5R)-, (4S,5S)-, and (4R,5S)-antillatoxin, were characterized in five different biological assay systems: ichthyotoxicity to goldfish, microphysiometry using cerebellar granule cells (CGCs), lactose dehydrogenase efflux from CGCs, monitoring of intracellular Ca(2+) concentrations in CGCs, and cytotoxicity to Neuro 2a cells. Across these various biological measures there was great consistency in that the natural antillatoxin (the 4R,5R-isomer) was greater than 25-fold more potent than any of the other stereoisomers. Detailed NMR studies provided a number of torsion and distance constraints that were modeled using the MM2 force field to yield predicted solution structures of the four antillatoxin stereoisomers. The macrocycle and side chain of natural (4R,5R)-antillatoxin present an overall "L-shaped" topology with an accumulation of polar substituents on the external surface of the macrocycle and a hydrogen bond between N(H)-7' and the C(O)-1 carbonyl. The decreased potency of the three non-naturally occurring antillatoxin stereoisomers is certainly a result of their dramatically altered overall molecular topologies.

Inhibition of voltage-sensitive sodium channels by the cannabinoid 1 receptor antagonist AM 251 in mammalian brain

The cannabinoid 1 receptor antagonist AM 251 is known to block the inhibitory effects of endocannabinoids and synthetic cannabinoid agonists on transmitter release through an action at presynaptic cannabinoid 1 receptors in brain. We examined the ability of AM 251 to inhibit sodium channel-dependent functions and the binding of [3H]batrachotoxinin A 20-alpha-benzoate to sodium channels in mouse brain synaptic preparations. Depolarization of synaptoneurosomes by the sodium channel site 2-specific neurotoxin veratridine, which is abolished by tetrodotoxin, was found to be inhibited in a concentration-dependent fashion by AM 251 (IC50=8.9 microM). Veratridine-dependent (tetrodotoxin suppressible) release, of L-glutamic acid and GABA from synaptosomes was also reduced by AM 251 [IC50s=8.5 microM (L-glutamic acid), 9.2 microM (GABA)]. The binding of the radioligand [3H]batrachotoxinin A 20-alpha-benzoate to site 2 on sodium channels was displaced by AM 251 (IC50=11.2 microM). Scatchard analysis of binding showed that at its IC50, AM 251 increased (by 2.3 times) the KD of radioligand without altering Bmax, suggesting a competitive mechanism of inhibition by AM 251. Kinetic experiments indicated that AM 251 inhibits equilibrium binding by allosterically accelerating the dissociation of the [3H]-batrachotoxinin A 20-alpha-benzoate:sodium channel complex. Our data suggest that micromolar concentrations of AM 251 are capable of reducing neuronal excitability and inhibiting release of excitatory and inhibitory transmitters through blockade of voltage-sensitive sodium channels in brain.

Principles of electrophysiology: an overview

This overview gives brief accounts of the mechanism of nerve excitation, receptor/channel functions, cable properties of neurons, and voltage clamp and patch clamp principles. Among various aspects of electrophysiology, emphasis is placed on the mechanistic studies of receptors/channels. A few sample records of membrane currents are illustrated to aid reader's comprehension. Finally some useful reference books are given for more comprehensive information.

Sodium channel inhibition by anandamide and synthetic cannabimimetics in brain

Anandamide is a prominent member of the endocannabinoids, a group of diffusible lipid molecules which influences neuronal excitability. In this context, endocannabinoids are known to modulate certain presynaptic Ca(2+) and K(+) channels, either through cannabinoid (CB1) receptor stimulation and second messenger pathway activation or by direct action. We investigated the susceptibility of voltage-sensitive sodium channels to anandamide and other cannibimimetics using both biochemical and electrophysiological approaches. Here we report that anandamide, AM 404 and WIN 55,212-2 inhibit veratridine-dependent depolarization of synaptoneurosomes (IC(50)s, respectively 21.8, 9.3 and 21.1 microM) and veratridine-dependent release of L-glutamic acid and GABA from purified synaptosomes [IC(50)s: 5.1 microM (L-glu) and 16.5 microM (GABA) for anandamide; 1.6 microM (L-glu) and 3.3 microM (GABA) for AM 404, and 12.2 (L-glu) and 14.4 microM (GABA) for WIN 55,212-2]. The binding of [3H]batrachotoxinin A 20-alpha-benzoate to voltage-sensitive sodium channels was also inhibited by low to mid micromolar concentrations of anandamide, AM 404 and WIN 55,212-2. In addition, anandamide (10 microM), AM 404 (10 microM) and WIN 55,212-2 (1 microM) were found to markedly block TTX-sensitive sustained repetitive firing in cortical neurones without altering primary spikes, consistent with a state-dependent mechanism. None of the inhibitory effects we demonstrate on voltage-sensitive sodium channels are attenuated by the potent CB1 antagonist AM 251 (1-2 microM). Anandamide's action is reversible and its effects are enhanced by fatty acid amidohydrolase inhibition. We propose that voltage-sensitive sodium channels may participate in a novel signaling pathway involving anandamide. This mechanism has potential to depress synaptic transmission in brain by damping neuronal capacity to support action potentials and reducing evoked release of both excitatory and inhibitory transmitters.

Differential interaction of anaesthetics and antiepileptic drugs with neuronal Na+ channels, Ca2+ channels, and GABA(A) receptors

BACKGROUND

Current theories favour multiple agent-specific neuronal actions for both general anaesthetics and antiepileptic drugs, but the pharmacological properties that distinguish them are poorly understood. We compared the interactions of representative agents from each class on their putative targets using well-characterized radioligand binding assays.

METHODS

Synaptosomes or membranes prepared from rat cerebral cortex were used to analyse drug effects on [(35)S]t-butyl bicyclophosphorothionate ([(35)S]TBPS) binding to the picrotoxinin site of GABA(A) receptors, [(3)H]batrachotoxinin A 20-alpha benzoate ([(3)H]BTX-B) binding to site 2 of voltage-gated Na(+) channels, (+)-[methyl-(3)H]isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-5-methoxycarboxyl-2,6-dimethyl-3-pyridinecarboxylate ([(3)H]PN200-110; isradipine) binding to L-type Ca(2+) channels, and [cyclohexyl-2,3-(3)H](N)glibenclamide ([(3)H]GB) binding to K(ATP) channels.

RESULTS

I.V. anaesthetics other than ketamine preferentially inhibited [(35)S]TBPS binding (etomidate approximately equal alphaxalone > propofol > thiopental > pentobarbital). Volatile anaesthetics inhibited both [(35)S]TBPS and [(3)H]BTX-B binding with comparable potencies (halothane approximately equal isoflurane approximately equal enflurane). Antiepileptic drugs preferentially antagonized either [(35)S]TBPS (diazepam > phenobarbital) or [(3)H]BTX-B (phenytoin > carbamazepine) binding. Local anaesthetics (lidocaine, tertracaine) selectively antagonized [(3)H]BTX-B binding. None of the drugs tested were potent antagonists of [(3)H]PN200-110 or [(3)H]GB binding.

CONCLUSIONS

Comparative radioligand binding assays identified distinct classes of general anaesthetic and antiepileptic drugs based on their relative specificities for a defined target set. I.V. anaesthetics interacted preferentially with GABA(A) receptors, while volatile anaesthetics were essentially equipotent at Na(+) channels and GABA(A) receptors. Antiepileptic drugs could be classified by preferential actions at either Na(+) channels or GABA(A) receptors. Anaesthetics and antiepileptic drugs have agent-specific effects on radioligand binding. Both general anaesthetics and antiepileptic drugs interact with Na(+) channels and GABA(A) receptors at therapeutic concentrations, in most cases with little selectivity.

Brevetoxin derivatives act as partial agonists at neurotoxin site 5 on the voltage-gated Na+ channel

Brevetoxins (PbTx-1 to PbTx-10) are potent lipid-soluble polyether neurotoxins produced by the marine dinoflagellate Karina brevis, an organism associated with 'red tide' blooms in the Gulf of Mexico. Ingestion of shellfish contaminated with K. brevis produces neurotoxic shellfish poisoning (NSP) in humans. NSP symptoms emanate from brevetoxin activation of neurotoxin site 5 on voltage-gated sodium channels (VGSC) [Toxicon 20 (1982) 457]. In primary cultures of rat cerebellar granule neurons (CGN), brevetoxins produce acute neuronal injury and death. The ability of a series of naturally occurring and synthetic brevetoxins to trigger Ca(2+) influx in CGN was explored in the present study. Intracellular Ca(2+) concentration was monitored in fluo-3-loaded CGN using a fluorescent laser imaging plate reader. The naturally occurring derivatives PbTx-1, PbTx-2 and PbTx-3 all produced a rapid and concentration-dependent increase in cytosolic [Ca(2+)]. The maximum response to PbTx-1 was approximately two-fold greater than that of either PbTx-2 or PbTx-3. Two synthetic derivatives of PbTx-3, alpha-naphthoyl-PbTx-3 and beta-naphthoyl-PbTx-3, were also tested. Both alpha- and beta-naphthoyl-PbTx-3 stimulated a rapid and concentration-dependent Ca(2+) influx that was, however, less efficacious than that of PbTx-3. These data indicate that, analogous to neurotoxin site 2 ligands, activators of neurotoxin site 5 display a range of efficacies, with PbTx-1 being a full agonist and other derivatives acting as partial agonists.

Hydantoin-substituted 4,6-dichloroindole-2-carboxylic acids as ligands with high affinity for the glycine binding site of the NMDA receptor

A novel series of C-3 substituted 4,6-dichloroindole-2-carboxylic acids was synthesized to investigate the influence of different hydrogen-bond donor and acceptor groups at this specific position on the affinity to the glycine site of the NMDA receptor. These novel 3-indolylmethyl derivatives with ring-open (amines, sulfonamides, amides, ureas) and cyclic substituents (imidazolidin-2-ones, (thio)hydantoins) led to the discovery that compounds bearing a hydantoin substituent at the C-3 position of the indole nucleus are the most promising ones. In this series the hydantoins, ureas, and imidazolidin-2-ones were identified as very potent inhibitors of the binding of the glycine site specific ligand [(3)H]MDL 105,519 to pig cortical brain membranes. Since the hydantoins can be produced via a versatile synthetic approach, further amendments of the hydantoin-substituted compounds were conducted to elucidate the influence of aromatic and aliphatic moieties at position 3 of the hydantoin as well as of sterically hindered compounds (5-substituted hydantoins). On the basis of the pharmacological data obtained in displacement experiments with [(3)H]MDL 105,519 and the emerging structure-activity relationships, we confirm the existing pharmacophore model that suggests a hydrogen-bond acceptor and an aromatic substituent at position 3 of the indole as the key features for high affinity. Log P values indicate brain permeability and selected compounds showed anticonvulsant activity in vivo. Binding studies for the sodium channel (site 2) were also performed on some selected compounds.

Synthesis and structure-activity relationships of 6,7-benzomorphan derivatives as use-dependent sodium channel blockers for the treatment of stroke

We have synthesized a series of 6,7-benzomorphan derivatives and determined their ability to bind to voltage-dependent sodium channels. We have also compared the functional consequences of this blockade in vitro and in vivo. The ability of the compounds to displace [(3)H]batrachotoxin from voltage-dependent sodium channels was compared with their ability to inhibit [(3)H]glutamate release in rat brain slices and block convulsions in the maximal electroshock test in mice. We found that the hydroxyl function in the 4'-position is crucial for improving the sodium channel blocking properties. Moreover, the stereochemistry and the topology of the N-linked side chain also influence this interaction. Indeed, the affinity is improved by an aromatic substitution in the side chain. By modifying the N substituent and the substitution pattern of the hydroxyl function, we were able to discover (2R)-[2alpha,3(S),6alpha]-1,2,3,4,5,6-hexahydro-6,11,11-tri-methyl-3-[2-(phenylmethoxy)propyl]-2,6-methano-3-benzazocin-10-ol hydrochloride. This compound was chosen as the best candidate for further pharmacological investigations.

Soluble guanylyl cyclase activator YC-1 protects white matter axons from nitric oxide toxicity and metabolic stress, probably through Na(+) channel inhibition

In the rat isolated optic nerve, nitric oxide (NO) activates soluble guanylyl cyclase (sGC), resulting in a selective accumulation of cGMP in the axons. The axons are also selectively vulnerable to NO toxicity. The experiments initially aimed to determine any causative link between these two effects. It was shown, using a NONOate donor, that NO-induced axonal damage occurred independently of cGMP. Unexpectedly, however, the compound YC-1, which is an allosteric activator of sGC, potently inhibited NO-induced axonopathy (IC(50) = 3 microM). This effect was not attributable to increased cGMP accumulation. YC-1 (30 microM) also protected the axons against damage by simulated ischemia, which (like NO toxicity) is sensitive to Na(+) channel inhibition. Although chemically unrelated to any known Na(+) channel inhibitor, YC-1 was effective in two biochemical assays for activity on Na(+) channels in synaptosomes. Electrophysiological recording from hippocampal neurons showed that YC-1 inhibited Na(+) currents in a voltage-dependent manner. At a concentration giving maximal protection of optic nerve axons from NO toxicity (30 microM), YC-1 did not affect normal axon conduction. It is concluded that the powerful axonoprotective action of YC-1 is unrelated to its activity on sGC but is explained by a novel action on voltage-dependent Na(+) channels. The unusual ability of YC-1 to protect axons so effectively without interfering with their normal function suggests that the molecule could serve as a prototype for the development of more selective Na(+) channel inhibitors with potential utility in neurological and neurodegenerative disorders.

Antillatoxin is a marine cyanobacterial toxin that potently activates voltage-gated sodium channels

Antillatoxin (ATX) is a lipopeptide derived from the pantropical marine cyanobacterium Lyngbya majuscula. ATX is neurotoxic in primary cultures of rat cerebellar granule cells, and this neuronal death is prevented by either N-methyl-d-aspartate (NMDA) receptor antagonists or tetrodotoxin. To further explore the potential interaction of ATX with voltage-gated sodium channels, we assessed the influence of tetrodotoxin on ATX-induced Ca2+ influx in cerebellar granule cells. The rapid increase in intracellular Ca2+ produced by ATX (100 nM) was antagonized in a concentration-dependent manner by tetrodotoxin. Additional, more direct, evidence for an interaction with voltage-gated sodium channels was derived from the ATX-induced allosteric enhancement of [3H]batrachotoxin binding to neurotoxin site 2 of the alpha subunit of the sodium channel. ATX, moreover, produced a strong synergistic stimulation of [3H]batrachotoxin binding in combination with brevetoxin, which is a ligand for neurotoxin site 5 on the voltage-gated sodium channel. Positive allosteric interactions were not observed between ATX and either alpha-scorpion toxin or the pyrethroid deltamethrin. That ATX interaction with voltage-gated sodium channels produces a gain of function was demonstrated by the concentration-dependent and tetrodotoxin-sensitive stimulation of 22Na+ influx in cerebellar granule cells exposed to ATX. Together these results demonstrate that the lipopeptide ATX is an activator of voltage-gated sodium channels. The neurotoxic actions of ATX therefore resemble those of brevetoxins that produce neural insult through depolarization-evoked Na+ load, glutamate release, relief of Mg2+ block of NMDA receptors, and Ca2+ influx.

Interaction of the novel anticonvulsant, BIA 2-093, with voltage-gated sodium channels: comparison with carbamazepine

PURPOSE

BIA 2-093 [(S)-(-)-10-acetoxy-10,11-dihydro-5H-dibenz/b,f/azepine-5-carboxamide] is endowed with an anticonvulsant potency similar to that of carbamazepine (CBZ), but produces less cognitive and motor impairment. This study evaluated whether voltage-gated sodium channels (VGSCs) are a primary locus for the action of BIA 2-093.

METHODS

We used the whole-cell voltage-clamp technique in the mouse neuroblastoma cell line N1E-115 to investigate the effects of BIA 2-093 and CBZ on VGSCs, displacement of [3H]-batrachotoxinin A 20-alpha-benzoate ([3H]-BTX), and [3H]-saxitoxin to define their relative potency to bind to rat brain sodium channels, and inhibition of uptake of 22Na by rat brain cortical synaptosomes stimulated by veratridine as a measure of sodium entry.

RESULTS

The inhibitory potencies of BIA 2-093 and CBZ increased as the holding potential was made less negative (-100, -90, -80, and -70 mV) with median inhibitory concentration (IC50) values (in microM) of, respectively, 4,337, 618, 238, and 139 for BIA 2-093, and 1,506, 594, 194, and 101 for CBZ. BIA 2-093 displayed a similar potency in displacing [3H]-BTX (IC50 values, 222 vs. 361 microM; p > 0.05) and inhibiting the uptake of 22Na (IC50 values, 36 vs. 138 microM; p > 0.05). Both drugs failed to displace [3H]-saxitoxin in concentrations up to 300 microM.

CONCLUSIONS

BIA 2-093, like CBZ, inhibits sodium currents in a voltage-dependent way by an interaction predominantly with the inactivated state of the channel and interacts with neurotoxin receptor site 2, but not with receptor site 1. BIA 2-093 displayed a potency blocking VGSCs similar to that of CBZ.

SM-20550, a new Na+/H+ exchange inhibitor and its cardioprotective effect in ischemic/reperfused isolated rat hearts by preventing Ca2+-overload

We investigated the effect of a newly synthesized compound, SM-20550 [N-(aminoiminomethyl)-1,4-dimethyl-1H-indole-2-carboxamide methanesulfonic acid] on Na+/H+ or Na+/Ca2+ exchange activity in rat cardiomyocytes, and on radioligand binding with several channels or receptors in membrane preparations, and ischemia/reperfusion injury in isolated perfused rat hearts. In myocytes, SM-20550 concentration-dependently inhibited the recovery from acidosis induced by an NH4Cl prepulse in HCO3(-)-free solution. Its IC50 was 10(-8) M, which was 10 times lower than that of ethylisopropyl amiloride (EIPA). SM-20550 (10(-6) M) did not affect the Na+-dependent Ca2+ influx (Na+/Ca2+ exchange activity) in cardiomyocytes. In the radioligand binding assay, SM-20550 did not have affinity for K+ channel, beta-adrenoceptor, adenosine, angiotensin, or endothelin receptors, and had low affinity for Na+ and Ca2+ channels and alpha-adrenoceptors, only at the concentrations of 10(-6)-10(-5) M. In perfused hearts exposed to 40 min of global ischemia and 20 min of reperfusion, SM-20550 (10(-8)-10(-7) M) significantly reduced the elevation of left ventricular end-diastolic pressure during reperfusion, improved the postischemic recovery of developed pressure, and prevented coronary perfusion pressure increase after reperfusion. Furthermore, SM-20550 reduced creatine phosphokinase release during reperfusion and prevented the abnormal gain of tissue Na+ and Ca2+ at the end of reperfusion. These results suggest that SM-20550 is a potent, highly specific Na+/H+ exchange inhibitor, which exerts a protective effect against myocardial ischemia/reperfusion injury. In addition, our data strongly support the hypothesis that Na+/H+ exchange plays an important role in the development of postischemic cardiac dysfunction, most likely by inducing Na+ and Ca2+ overload.

Potent blockade of sodium channels and protection of brain tissue from ischemia by BIII 890 CL

We have synthesized a new benzomorphan derivative, 2R-[2alpha,3(S*), 6alpha]-1,2,3,4,5,6-hexahydro-6,11, 11-trimethyl-3-[2-(phenylmethoxy)propyl]-2, 6-methano-3-benzazocin-10-ol hydrochloride (BIII 890 CL), which displaced [(3)H]batrachotoxinin A-20alpha-benzoate from neurotoxin receptor site 2 of the Na(+) channel in rat brain synaptosomes (IC(50) = 49 nM), but exhibited only low affinity for 65 other receptors and ion channels. BIII 890 CL inhibited Na(+) channels in cells transfected with type IIA Na(+) channel alpha subunits and shifted steady-state inactivation curves to more negative potentials. The IC(50) value for the inactivated Na(+) channel was much lower (77 nM) than for Na(+) channels in the resting state (18 microM). Point mutations F1764A and Y1771A in transmembrane segment S6 in domain IV of the alpha subunit reduced the voltage- and frequency-dependent block, findings which suggest that BIII 890 CL binds to the local anesthetic receptor site in the pore. BIII 890 CL inhibited veratridine-induced glutamate release in brain slices, as well as glutamate release and neurotoxicity in cultured cortical neurons. BIII 890 CL (3-30 mg/kg s.c.) reduced lesion size in mice and rats when administered 5 min after permanent focal cerebral ischemia at doses that did not impair motor coordination. In contrast to many other agents, BIII 890 CL was neuroprotective in both cortical and subcortical regions of the rat brain. Our results demonstrate that BIII 890 CL is a potent, selective, and highly use-dependent Na(+) channel blocker that protects brain tissue from the deleterious effects of focal cerebral ischemia in rodents.

Sodium channel activity and sigma binding of 2-aminopropanamide anticonvulsants

Sodium channel blocking, anticonvulsant activity, and sigma (sigma) binding of selected leads in a series of alpha-amino amide anticonvulsants were examined. While anticonvulsant compounds were always endowed with low micromolar sodium (Na+) channel site-2 binding, compounds with low site-2 Na+ channel affinity failed to control seizures. No correlation could be drawn with sigma1 binding. Both anticonvulsant and Na+ channel blocking activities were independent of stereochemistry, while sigma1 binding seems to be favoured by an S-configuration on the aminoamide moiety.

WIN 17317-3, a new high-affinity probe for voltage-gated sodium channels

The iminodihydroquinoline WIN 17317-3 was previously shown to inhibit selectively the voltage-gated potassium channels, K(v)1.3 and K(v)1.4 [Hill, R. J., et al. (1995) Mol. Pharmacol. 48, 98-104; Nguyen, A., et al. (1996) Mol. Pharmacol. 50, 1672-1679]. Since these channels are found in brain, radiolabeled WIN 17317-3 was synthesized to probe neuronal K(v)1 channels. In rat brain synaptic membranes, [(3)H]WIN 17317-3 binds reversibly and saturably to a single class of high-affinity sites (K(d) 2.2 +/- 0.3 nM; B(max) 5.4 +/- 0.2 pmol/mg of protein). However, the interaction of [(3)H]WIN 17317-3 with brain membranes is not sensitive to any of several well-characterized potassium channel ligands. Rather, binding is modulated by numerous structurally unrelated sodium channel effectors (e.g., channel toxins, local anesthetics, antiarrhythmics, and cardiotonics). The potency and rank order of effectiveness of these agents in affecting [(3)H]WIN 17317-3 binding is consistent with their known abilities to modify sodium channel activity. Autoradiograms of rat brain sections indicate that the distribution of [(3)H]WIN 17317-3 binding sites is in excellent agreement with that of sodium channels. Furthermore, WIN 17317-3 inhibits sodium currents in CHO cells stably transfected with the rat brain IIA sodium channel with high affinity (K(i) 9 nM), as well as agonist-stimulated (22)Na uptake in this cell line. WIN 17317-3 interacts similarly with skeletal muscle sodium channels but is a weaker inhibitor of the cardiac sodium channel. Together, these results demonstrate that WIN 17317-3 is a new, high-affinity, subtype-selective ligand for sodium channels and is a potent blocker of brain IIA sodium channels.

1-(3-Cyanobenzylpiperidin-4-yl)-5-methyl-4-phenyl-1, 3-dihydroimidazol-2-one: a selective high-affinity antagonist for the human dopamine D(4) receptor with excellent selectivity over ion channels

After the requirement of pseudocycle formation in the ureas 3 and 7 for hD(4) binding and selectivity was confirmed, structural hybridization with the known hD(4) ligand 2 led to the design and identification of the lead 4-(2-oxo-1, 3-dihydroimidazol-2-yl)piperidine 8. Optimization studies were carried out on 8 with the aim of achieving 1000-fold selectivity for hD(4) over all other receptors while retaining the good pharmacokinetic properties of the lead. After initial preparation of 8 as a minor component in a low-yielding reaction, a novel and regioselective "four-step/one-pot" procedure was developed which proved to be applicable to rapid investigation of the SAR of the 1, 3-dihydroimidazol-2-one ring. Various changes to substituents attached to the 3-, 4-, or 5-position of the 1, 3-dihydroimidazol-2-one core of 8 did not significantly improve selectivity for hD(4) over hD(2) and hD(3). Greater selectivity (>1000-fold) was ultimately achieved by meta substitution of the benzyl group of 8 with various substituents. Compounds 28, 31, and 32 all possess the required selectivity for hD(4) over the other dopamine subtypes, but only 32 has >1000-fold selectivity over all the key counterscreens we tested against. Compound 32 is an antagonist at hD(4) and has a good pharmacokinetic profile in the rat, with excellent estimated in vivo receptor occupancy, thus making it a potentially useful pharmacological tool to investigate the role of the D(4) receptor.

Synthesis and preliminary biological evaluation of new alpha-amino amide anticonvulsants incorporating a dextromethorphan moiety

Dextromethorphan 1 is an effective neuroprotectant in animal models of epilepsy and ischemia but showed side-effects during clinical trials limiting its potential use in a clinical setting. Here we describe the enantioselective and enantiospecific syntheses and the initial in vitro and in vivo biological evaluation of new hybrid structures between 1 and a previously disclosed alpha-amino amide anticonvulsant (3).

Neuroprotective action of a novel compound--M50463--in primary cultured neurons

The neuroprotective effects of a novel synthetic compound, M50463, have been determined by using embryonic rat neocortical neurons in various culture conditions. M50463 was initially characterized as a potent specific ligand for a voltage-dependent sodium channel by radioligand binding studies. In fact, M50463 inhibited neuronal cell death induced by veratrine and inhibited an increase of the intracellular calcium level in neurons evoked by veratrine. In addition to such expected effects, M50463 had the ability to prevent glutamate neurotoxicity, to promote the neuronal survival in serum-deprived medium and to prevent nitric oxide-induced neurotoxicity. These results suggested that M50463 is not a simple sodium channel blocker, but a neuroprotective agent which has some crucial mechanism of action on neuronal death occurring in various situations, and it is a novel, innovative candidate for neuroprotective therapy for various neurodegenerative disorders.

Delta-atracotoxins from Australian funnel-web spiders compete with scorpion alpha-toxin binding on both rat brain and insect sodium channels

Atracotoxins are novel peptide toxins from the venom of Australian funnel-web spiders that slow sodium current inactivation in a similar manner to scorpion alpha-toxins. To analyse their interaction with known sodium channel neurotoxin receptor sites we determined their effect on scorpion toxin, batrachotoxin and saxitoxin binding. Nanomolar concentrations of delta-atracotoxin-Hv1 and delta-atracotoxin-Ar1 completely inhibited the binding of the scorpion alpha-toxin AaH II to rat brain synaptosomes as well as the binding of LqhalphaIT, a scorpion alpha-toxin highly active on insects, to cockroach neuronal membranes. Moreover, delta-atracotoxin-Hv1 cooperatively enhanced batrachotoxin binding to rat brain synaptosomes in an analogous fashion to scorpion alpha-toxins. Thus the delta-atracotoxins represent a new class of toxins which bind to both mammalian and insect sodium channels at sites similar to, or partially overlapping with, the receptor binding sites of scorpion alpha-toxins.

Interaction of batrachotoxin with the local anesthetic receptor site in transmembrane segment IVS6 of the voltage-gated sodium channel

The voltage-gated sodium channel is the site of action of more than six classes of neurotoxins and drugs that alter its function by interaction with distinct, allosterically coupled receptor sites. Batrachotoxin (BTX) is a steroidal alkaloid that binds to neurotoxin receptor site 2 and causes persistent activation. BTX binding is inhibited allosterically by local anesthetics. We have investigated the interaction of BTX with amino acid residues I1760, F1764, and Y1771, which form part of local anesthetic receptor site in transmembrane segment IVS6 of type IIA sodium channels. Alanine substitution for F1764 (mutant F1764A) reduces tritiated BTX-A-20-alpha-benzoate binding affinity, causing a 60-fold increase in Kd. Alanine substitution for I1760, which is adjacent to F1764 in the predicted IVS6 transmembrane alpha helix, causes only a 4-fold increase in Kd. In contrast, mutant Y1771A shows no change in BTX binding affinity. For wild-type and mutant Y1771A, BTX shifted the voltage for half-maximal activation approximately 40 mV in the hyperpolarizing direction and increased the percentage of noninactivating sodium current to approximately 60%. In contrast, these BTX effects were eliminated completely for the F1764A mutant and were reduced substantially for mutant I1760A. Our data suggest that the BTX receptor site shares overlapping but nonidentical molecular determinants with the local anesthetic receptor site in transmembrane segment IVS6 as well as having unique molecular determinants in transmembrane segment IS6, as demonstrated in previous work. Evidently, BTX conforms to a domain-interface allosteric model of ligand binding and action, as previously proposed for calcium agonist and antagonist drugs acting on L-type calcium channels.