Spider toxins as tools for dissecting elements of excitatory amino acid transmission

Bioinformation Systems with Detectors and Signal Coding Capabilities

Introduction. The integration of computer technologies into various fields of science allows the development of new methodologies, hybrid information systems with advanced capabilities, such as EcoIS bioinformation system for monitoring the environment with the use of biological data detectors.Problem Statement. The development of innovation bioinformation systems with biological data detectors is a very important task, as they have numerous advantages: allow rapid diagnostics and testing of chemicals in thefirst moments of their action, may be incorporated easily into electronic registration systems, may serve as elementary analytical units with data coding capabilities, etc.Purpose. The purpose of this research is to make a comprehensive analysis of different types of biological data detectors to develop a physical model of a biosensor capable of encoding signals and a bioinformation system with such detectors.Materials and Methods. The comparative analysis of information systems with functions of ecomonitoring and different types of biosensors have been used; the data are taken from electrophysiological experiments on registration of chemosensitive transmembrane electric currents in voltage clamp and patch clamp modes.Results. The physical model of biosensor has been developed and tested. The integration of the developed biosensors into the electronic bioinformation system by the example of EcoIS authors’ system has been demonstrated. Neuron-like biosensor has been considered an abstraction in the unity of its functions: signal receiver — filter — analyzer — encoder/decoder, where the input information is obtained in the form of chemical structures or electrical signals, after the conversion (recoding) of information it is registered as electrical signals with changed characteristics. The prospects for developing the cutting-edge methods for information protection in systems with biosensors have been shown. Conclusions. This development may be used for creating a bioinformation system for environmental moni toring with integrated biosensor system and data protection based on the principles and achievements of contemporary biophysics.

Homeostatic plasticity at the Drosophila neuromuscular junction

In biology, homeostasis refers to how cells maintain appropriate levels of activity. This concept underlies a balancing act in the nervous system. Synapses require flexibility (i.e. plasticity) to adjust to environmental challenges. Yet there must also exist regulatory mechanisms that constrain activity within appropriate physiological ranges. An abundance of evidence suggests that homeostatic regulation is critical in this regard. In recent years, important progress has been made toward identifying molecules and signaling processes required for homeostatic forms of neuroplasticity. The Drosophila melanogaster third instar larval neuromuscular junction (NMJ) has been an important experimental system in this effort. Drosophila neuroscientists combine genetics, pharmacology, electrophysiology, imaging, and a variety of molecular techniques to understand how homeostatic signaling mechanisms take shape at the synapse. At the NMJ, homeostatic signaling mechanisms couple retrograde (muscle-to-nerve) signaling with changes in presynaptic calcium influx, changes in the dynamics of the readily releasable vesicle pool, and ultimately, changes in presynaptic neurotransmitter release. Roles in these processes have been demonstrated for several molecules and signaling systems discussed here. This review focuses primarily on electrophysiological studies or data. In particular, attention is devoted to understanding what happens when NMJ function is challenged (usually through glutamate receptor inhibition) and the resulting homeostatic responses. A significant area of study not covered in this review, for the sake of simplicity, is the homeostatic control of synapse growth, which naturally, could also impinge upon synapse function in myriad ways. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Kainate-induced calcium overload of cortical neurons in vitro: Dependence on expression of AMPAR GluA2-subunit and down-regulation by subnanomolar ouabain

Whereas kainate (KA)-induced neurodegeneration has been intensively investigated, the contribution of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) in neuronal Ca2+ overload ([Ca2+]i) is still controversial. Using Ca2+ imaging and patch-clamp techniques, we found different types of Ca2+ entry in cultured rat cortical neurons. The presence of Ca2+ in the extracellular solution was required to generate the [Ca2+]i responses to 30 μM N-methyl-d-aspartate (NMDA) or KA. The dynamics of NMDA-induced [Ca2+]i responses were fast, while KA-induced responses developed slower reaching high [Ca2+]i. Ifenprodil, a specific inhibitor of the GluN2B subunit of NMDARs, reduced NMDA-induced [Ca2+]i responses suggesting expression of GluN1/GluN2B receptors. Using IEM-1460, a selective blocker of Ca(2+)-permeable GluA2-subunit lacking AMPARs, we found three neuronal responses to KA: (i) IEM-1460 resistant neurons which are similar to pyramidal neurons expressing Ca(2+)-impermeable GluA2-rich AMPARs; (ii) Neurons exhibiting nearly complete block of both KA-induced currents and [Ca2+]i signals by IEM-1460 may represent interneurons expressing GluA2-lacking AMPARs and (iii) neurons with moderate sensitivity to IEM-1460. Ouabain at 1 nM prevented the neuronal Ca2+ overload induced by KA. The data suggest, that cultured rat cortical neurons maintain functional phenotypes of the adult brain cortex, and demonstrate the key contribution of the Na/K-ATPase in neuroprotection against KA excitotoxicity.

Small molecules from spiders used as chemical probes

Spiders are important species in ecological systems and as major predators of insects they are endowed with a plethora of low-molecular-weight natural products having intriguing biological activities. The isolation and biological characterization of these entities are well established, however, only very recently have these compounds been used as templates for the design, synthesis, and biological evaluation of synthetic analogues. In contrast, the investigation of compounds responsible for chemical communication between spiders is far less developed, but recently new light has been shed onto the area of pheromones and allomones from spiders. Herein, we recapitulate these recent results, put them into perspective with previous findings, and provide an outlook for future studies of these chemotypes.

Anthraquinone polyamines: novel channel blockers of N-methyl-d-aspartate receptors

Polyamines, in particular spermine, as well as some natural and synthetic polyamine derivatives have been found to be blockers of N-methyl-D-aspartate receptors. We developed novel, polyamine-based channel blockers to analyze the structure of NMDA receptors. Anthraquinone polyamines block NMDA receptors with some selectivity compared to other glutamate receptors. Results using mutant NR1 and NR2 subunits identified amino acid residues that influence blockade by anthraquinone polyamines. The head group (anthraquinone) may be positioned at the selectivity filter/narrowest constriction of the channel and the polyamine tail penetrates this constriction into the inner vestibule below the level of the selectivity filter. The results are consistent with other work showing that NR1 (Asn616) and NR2B (Asn616), but not NR2B (Asn615), make the narrowest constriction of NMDA channel, and that the M3 segments from the two subunits, which form the outer vestibule, are likely staggered relative to each other in the vertical axis of the channel.

Nanoanalysis of the arthropod neuro-toxins

Many kinds of venomous principles modulate physiological responses of mammalian signal transduction systems, on which they act selectively as enhancers, inhibitors or some other kind of effectors. These toxins become useful tools for physiological research. We have employed and characterized paralyzing toxins from the venom of spiders, insects and scorpions with a limited supply. We have developed rapid and sensitive mass spectrometric technology and applied for the identification of these toxins. Venom profiles are screened by MALDI-TOF fingerprinting analysis prior to purification of venomous components, then marked target toxins of small molecular mass (1000-5000) are characterized directly by means of mass spectrometric techniques such as Frit-FAB MS/MS, CID/PSD-TOF MS, Capil.-HPLC/Q-TOF MS/MS etc.

Structure Determination of a Tetrahydro-?-carboline of Arthropod Origin: A Novel Alkaloid-Toxin Subclass from the Web of SpiderNephila clavipes

The orb-web spiders are polyphagous animals in which the web plays a very important role in the capture of preys; oily droplets usually cover the capture-web of the spider Nephila clavipes and seem to be of great importance for prey capture. The knowledge of the chemical composition of these droplets is necessary to understand the function of this adhesive material in web mechanics and prey capture. A novel subclass of spider toxins, tetrahydro-beta-carboline, was identified among the weaponry of compounds present inside of oily droplets. This type of alkaloid is not common among the natural compounds of spider toxins. Apparently, when the prey arthropods get caught by the spider web, their bodies are covered with many adhesive oily droplets, which disrupt delivering the tetrahydro-beta-carboline to the direct contact with the prey integument. Toxicity assays demonstrated a potent lethal effect of the alkaloid toxin to the spider preys; topical applications of the tetrahydro-beta-carboline at first caused clear signs of neurotoxicity, followed by the death of preys. The structure of the major component, a tetrahydro-beta-carboline, among the alkaloid toxins was elucidated by means of UV spectrophotometry, ESI mass spectrometry, 1H-NMR spectroscopy, and high-resolution mass spectrometry. The structure of the natural toxin was determined as 1-(2-guanidinoethyl)-1,2,3,4-tetrahydro-6-hydroxymethyl)-beta-carboline; the investigation of the pharmacological properties and neurotoxic actions of this compound may be used in the future as reference for the development of new drugs to be applied at level of pest control in agriculture.

Modeling noncompetitive antagonism of a nicotinic acetylcholine receptor

Models of closed and open channel pores of a muscle-type nicotinic acetylcholine receptor (nAChR) channel comprising M1 and M2 segments are presented. A model of the closed channel is proposed in which hydrophobic residues of the Equatorial Leucine ring screen the oxygen domain formed by the Serine ring, thereby preventing ion flux without completely occluding the pore. This model demonstrates a high similarity with the structure derived from a recent electron microscopy study. We propose that hydrophobic residues of the Equatorial Leucine ring are retracted when the pore is open. Our models provide a possible resolution of the nAChR gate controversy. We have also obtained explanations for the complex mechanisms underlying inhibition of nAChR by philanthotoxins (PhTXs). PhTX-343, containing a spermine moiety with a charge of +3, binds deep in the pore near the Serine ring where classical open channel blockers of nAChR bind. In contrast, PhTX-(12), which has a single charged amino group is unable to reach deeply located rings because of steric restrictions. Both philanthotoxins may bind to a hydrophobic site located close to the external entrance of the pore in a region that includes residues associated with the regulation of desensitization.

Design and Synthesis of a Novel Water-Soluble NMDA Receptor Antagonist with a 1,4,7,10-Tetraazacyclododecane Group

Polyamines, especially spermine, inhibit N-methyl-D-aspartate (NMDA) receptors as open channel blockers. Two types of water-soluble NMDA receptor antagonist, ACCn (1) and TGCn (2), with a 1,4,7,10-tetraazacyclododecane cyclic polyamine group, were synthesized and the effects of both compounds on NMDA receptors were studied using voltage-clamp recordings of recombinant NMDA receptors expressed in Xenopus oocytes. These compounds inhibited macroscopic currents in both NR1/NR2A and NR1/NR2B receptor subtypes in oocytes voltage-clamped at -70 mV. Inhibition by the compounds of NR1/NR2A receptors were more prominent than that of NR1/NR2B receptors. The inhibitory effects of ACCn (1) on both NMDA receptors were more potent than those of TGCn (2).

Targeting ionotropic receptors with polyamine-containing toxins☆

This review summarises current knowledge of polyamine-containing spider toxins and their interactions with ionotropic receptors of invertebrate and vertebrate excitable cells. Their diverse actions on ionotropic glutamate and acetylcholine receptors, which include potentiation, closed channel block and open channel block, are discussed in the context of toxin and target structures. Factors that complicate attempts to identify and pharmacologically characterise the binding sites for these toxins include their ability to permeate channels of some ionotropic receptors and their apparent accumulation in a cellular compartment, possibly the membrane bilayer.

Anthraquinone Polyamines: Novel Channel Blockers to StudyN-Methyl-d-Aspartate Receptors

The effects of various anthraquinone polyamines (AQP) were studied at recombinant N-methyl-d-aspartate (NMDA) receptors expressed in Xenopus laevis oocytes. The AQP derivatives had different numbers of methylene groups between the NH(2) (or NH) groups in their spermidine-like tail. Thus, we termed these derivatives AQ33, AQ34, etc. All AQP derivatives inhibited responses of NR1/NR2 receptors in oocytes voltage-clamped at -70 mV, with IC(50) values between 4 and 22 microM. The block was strongly voltage-dependent. AQ34 and AQ33b inhibited responses of NR1/NR2 receptors but did not inhibit responses of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors expressed from GluR1 or GluR2(Q), indicating that AQ34 and AQ33b are preferential NMDA antagonists. Results of experiments using mutant NR1 and NR2 subunits identified residues that influence block by AQ34 and AQ33b. These residues are located in the outer vestibule at the selectivity filter/narrowest constriction of the channel and in the inner vestibule below the level of the selectivity filter. The results with mutant NR1 and NR2 subunits are consistent with the idea that NR1(Asn616) and NR2B(Asn616), but not NR2B(Asn615), make the narrowest constriction of NMDA channel.

Contrasting actions of philanthotoxin-343 and philanthotoxin-(12) on human muscle nicotinic acetylcholine receptors

Whole-cell recordings and outside-out patch recordings from TE671 cells were made to investigate antagonism of human muscle nicotinic acetylcholine receptors (nAChR) by the philanthotoxins, PhTX-343 and PhTX-(12). When coapplied with acetylcholine (ACh), PhTX-343 caused activation-dependent, noncompetitive inhibition (IC50 = 17 microM at -100 mV) of whole-cell currents that was strongly voltage-dependent. However, preapplication of PhTX-343 unveiled a voltage-independent antagonism that also required receptor activation, which is suggestive of desensitization enhancement. In single-channel studies, 10 microM PhTX-343 significantly reduced the mean open time of channel openings evoked by 1 microM ACh from 4.42 +/- 0.44 to 1.58 +/- 0.10 ms with a minor increase (1.26-fold) in mean closed time. These data indicate that PhTX-343 predominantly blocks the open channel gated by ACh. In contrast, PhTX-(12) caused potent (IC50 = 0.77 microM at-100 mV), activation-dependent, noncompetitive inhibition of ACh-induced whole-cell currents that was only weakly voltage-dependent and suggestive of desensitization enhancement. It caused only a small decrease (7.5%) in the mean open time of channel openings induced by 1 microM ACh, whereas the mean closed time was significantly increased from 200 +/- 45 ms to 586 +/- 145 ms. The different voltage-dependencies of the two modes of action of these philanthotoxins suggest two binding sites, one deep in the nAChR pore, the other near the extracellular entrance to the pore.

Purification of a neuroprotective component of Parawixia bistriata spider venom that enhances glutamate uptake

(1) In this study, we examined the effects of crude venom from the spider Parawixia bistriata on glutamate and GABA uptake into synaptosomes prepared from rat cerebral cortex. Addition of venom to cortical synaptosomes stimulated glutamate uptake and inhibited GABA uptake in a concentration-dependent manner. (2) The venom was fractionated using reverse-phase high-performance liquid chromatography on a preparative column. The fraction that retained glutamate uptake-stimulating activity was further purified on a reverse-phase analytical column followed by ion-exchange chromatography. (3) The active fraction, referred to as PbTx1.2.3, stimulated glutamate uptake in synaptosomes without changing the K(M) value, and did not affect GABA uptake. Additional experiments showed that the enhancement of glutamate uptake by PbTx1.2.3 occurs when ionotropic glutamate receptors or voltage-gated sodium and calcium channels are completely inhibited or when GABA receptors and potassium channels are activated, indicating that the compound may have a direct action on the transporters. (4) In an experimental model for glaucoma in which rat retinas are subjected to ischemia followed by reperfusion, PbTx1.2.3 protected neurons from excitotoxic death in both outer and inner nuclear layers, and ganglion cell layers. (5) This active spider venom component may serve as a basis for designing therapeutic drugs that increase glutamate clearance and limit neurodegeneration.

Synaptic activity modifies the levels of Dorsal and Cactus at the neuromuscular junction of Drosophila

The Drosophila Rel transcription factor Dorsal and its inhibitor Cactus participate in a signal transduction pathway involved in several biologic processes, including embryonic pattern formation, immunity, and muscle development. In contrast with embryonic muscle, where Dorsal is reportedly absent, this protein and Cactus accumulates in the neuromuscular junctions in the muscle of both larvae and adults. The phenotype of homozygous dorsal mutant larvae suggested that Dorsal and Cactus maybe necessary for normal function and maintenance of the neuromuscular system. Here we investigate if these proteins can respond to synaptic activity. Using larval body wall preparations and antibodies specific for Dorsal or Cactus we show that the amount of these proteins at the neuromuscular junction is substantially decreased after electrical stimulation of the nerves or incubation in glutamate, the principal transmitter in this type of synapse. The specificity of the response was tested with a glutamate receptor antagonist (argiotoxin 636). Because the effect can be reproduced using a calcium ionophore (ionomycin treatment) as well as blocked by the inhibition of the muscle ryanodine receptor (tetracaine treatment), the involvement of calcium in this process seems likely. We also observed that the inhibition of the calcium dependent protein phosphatase calcineurin prevents the effect of glutamate on the fluorescence for Dorsal and Cactus, suggesting its participation in a signal transduction cascade that may activate Dorsal in the muscle independently of Toll. Our results are consistent with a novel function of the Rel factor Dorsal in a molecular pathway turned on by neural activity and/or contractile activity.

Effect of two polyamine toxins on the bacterial porin OmpF

Spermine, a polyamine based on a 12-carbon motif, is an effective inhibitor of E. coli OmpF porin. Here we study the inhibition of porin by two polyamine toxins commonly used as modulators of polyamine-sensitive channels: Philanthotoxin-433 (PhTX) from wasp venom and Joro spider toxin (JSTX). Both are highly asymmetric molecules, with at one end a 12-carbon chain polyamine targeting the molecule to the porin constriction zone, and at the other end large aromatic groups conferring to this extremity a size in the order of the OmpF constriction zone. Here we report that PhTX, but not Joro toxin, induces a high degree of flickering in the OmpF-mediated current. The effect is concentration and voltage-dependent, and greatly diminished in a mutant lacking D113 on the constriction loop, a residue previously shown to be required for spermine sensitivity. Possible reasons for the distinct sensitivity of OmpF to PhTX and Joro toxin are discussed.

A comparative neuroethological study of limbic seizures induced by Parawixia bistriata venom and kainic acid injections in rats

A number of neurotoxins derived from arthropod venoms are known to show highly selective effects on nervous tissue. These neurotoxins have been proved to be extremely useful tools to investigate either convulsive or anticonvulsive mechanisms in the nervous system. In the present work, intracerebroventricular injection of the crude venom from the spider Parawixia bistriata (Araneae, Araneidae) in rats induced convulsive limbic seizures (head and forelimb myoclonus, as well as rearing and falling). Neuroethological analysis showed that the limbic seizures induced by the venom were different from those induced by kainic acid. Intravenous injection of the same venom did not induce seizures, but the neuroethological analysis showed an intensification of grooming behavior similar to a displaced activity. In conclusion, our experiments point that crude venom of P. bistriata may contain convulsant neurotoxins probably acting in limbic system structures. The mechanism of action of these neurotoxins may be different from simple activation of glutamatergic kainate receptors, as evidenced by a comparative neuroethological analysis of seizures induced by either venom or kainic acid.

Purification and amino acid sequence of a highly insecticidal toxin from the venom of the brazilian spider Phoneutria nigriventer which inhibits NMDA-evoked currents in rat hippocampal neurones

A new insecticidal toxin Tx4(5-5) was isolated from the fraction PhTx4 of the venom of the spider Phoneutria nigriventer by reverse phase high performance liquid chromatography (HPLC) and anion exchange HPLC. The complete amino acid sequence determined by automated Edman degradation showed that Tx4(5-5) is a single chain polypeptide composed of 47 amino acid residues, including 10 cysteines, with a calculated molecular mass of 5175 Da. Tx4(5-5) shows 64% of sequence identity with Tx4(6-1), another insecticidal toxin from the same venom. Tx4(5-5) was highly toxic to house fly (Musca domestica), cockroach (Periplaneta americana) and cricket (Acheta domesticus ), producing neurotoxic effects (knock-down, trembling with uncoordinated movements) at doses as low as 50 ng/g (house fly), 250 ng/g (cockroach) and 150 ng/g (cricket). In contrast, intracerebroventricular injections (30 microg) into mice induced no behavioural effects. Preliminary electrophysiological studies carried out on whole-cell voltage-clamped rat hippocampal neurones indicated that Tx4(5-5) (at 1 microM) reversibly inhibited the N-methyl-D-aspartate-subtype of ionotropic glutamate receptor, while having little or no effect on kainate-, alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid- or gamma-aminobutyric acid-activated currents.

Long-term potentiation in the presence of NMDA receptor antagonist arylalkylamine spider toxins

The role of the NMDA receptor (NMDAR) in long-term potentiation (LTP) is now well established. All potent NMDAR antagonists known to date inhibit the induction of LTP at the Schaffer collateral-CA1 pyramidal cell synapse in rat hippocampus, regardless of their site and mechanism of action. Arylalkylamine toxins are noncompetitive NMDAR antagonists in the mammalian central nervous system (CNS). The synthetic toxins argiotoxin-636 (Arg-636), Joro spider toxin (JSTX-3), alpha-agatoxin-489 and -505 (Agel-489 and Agel-505) and philanthotoxin-433 (delta-PhTX) were found in the present study to have no effect on the induction of LTP in the Schaffer collateral-CA1 pyramidal cell pathway in rat hippocampal slices maintained in vitro. Arylalkylamine toxins represent a class of potent NMDAR antagonists that fail to affect hippocampal LTP, and thus provide novel structural leads for the development of NMDAR antagonists that do not impair cognition.

Effects of the crude venom of the social wasp Agelaia vicina on gamma-aminobutyric acid and glutamate uptake in synaptosomes from rat cerebral cortex

Glutamate (L-glu) is the most important excitatory neurotransmitter in the mammalian central nervous system. Its action is terminated by transporters located in the plasma membrane of neurons and glial cells, which have a critical role in preventing glutamate excitotoxicity under normal conditions. The neurotransmitter gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the mammalian central nervous system. Venoms of solitary wasps and orb-spiders are composed of large proteins, medium-size peptides, polyamine amides (PAs), and other neuroactive components that are highly selective to nervous tissues. The abnormal operation of uptake systems is involved in several failures. Several studies indicate alterations in extracellular GABA and glutamate concentrations in epilepsy conditions that may relate to transporter functions. The effects of the crude and boiled venom of the social wasp Agelaia vicina, "cassununga," on GABA and L-glu uptake in rat cerebral cortex synaptosomes are related. The venom uncompetitively inhibited high- and low-affinity GABA uptake by 91.2% and by 76%, respectively. This kind of inhibition was also found to affect high- (99.6%) and low-affinity (90%) uptake of L-glu. These results suggest that the effects observed in these experiments indicate the venom of A. vicina to be a useful tool to further characterize GABA- and L-glu-uptake systems.

Neuroethological analysis of the effects of spider venom from Scaptocosa raptoria (Lycosidae: Araneae) microinjected in the lateral ventricle of Wistar rats

The inhibition of excitatory mechanisms by components of low molecular weight, isolated from spiders and solitary wasps, such as, the acyl-polyamines, has demonstrated, on the one hand, neuroprotection potential, and on the other hand, it is well known that some arthropod venom components have convulsant activity. While many sophisticated experiments are conducted to determine the mechanisms and effects of arthropod venoms, relatively little attention has been paid to the behavioral changes that occur in mammals after being administered given doses of them. The precise detection of these behavioral changes can be used as a sensitive indicator of central nervous system dysfunction. This study investigated the behavioral effects of crude venom from the spider Scaptocosa raptoria after intracerebroventricular injection in male Wistar rats. The venom induced behavioral changes quantified using a neuroethological method, which allows the evaluation of the following parameters: frequency, duration, and strength of statistical association (chi-square) between pairs of behaviors. The rats exhibited a period of freezing, which was always followed by procursive-type seizures (running, gyrating, atonic falling, and jumping).

Natural and synthetic polyamines: modulators of signalling proteins

Ionotropic receptors are modulated allosterically by natural polyamines, such as spermine, and by polyamine derivatives, such as polyamine amides (e.g. philanthotoxin-343) and polymethylene tetraamines (e.g. methoctramine. Modulation can be either positive (potentiation) or negative (non-competitive antagonism of either open or closed channel receptor conformation). Photoaffinity labelling studies have identified a site close to the channel lumen on the nicotinic acetylcholine receptor Torpedo electroplax that is probably the allosteric site responsible for antagonism of the closed channel conformation of this receptor.

Solid phase synthesis and biological evaluation of enantiomerically pure wasp toxin analogues PhTX-343 and PhTX-12

PhTX-343 and PhTX-12, analogues of the natural polyamine wasp toxin PhTX-433, were synthesised in 40-60% yields as pure enantiomers using solid phase synthesis techniques. Capillary electrophoresis procedures were developed for chiral separation and determination of enantiomeric purity (ee) of the enantiomers of PhTX-343 and PhTX-12. The methods were optimised with respect to chiral selector, buffer pH, and temperature around the capillary. Thus, rac-PhTX-343 was resolved using a separation buffer containing 30 mM heptakis-(2, 6-di-O-methyl)-beta-cyclodextrin in 50 mM 6-aminocarproic acid (pH 4. 0) at 15 degrees C. rac-PhTX-12 was not resolvable in this system, but could be resolved using a separation buffer containing 10% w/v of dextrin 10, a linear maltodextrin, in 50 mM 6-aminocaproic acid (pH 4.0) at 15 degrees C. Using these methods, the optical purity of the synthetic enantiomers was determined to be ee > 99%. The enantiomers were also characterised by chiroptical methods. The antagonist potency of the enantiomers was tested on nicotinic acetylcholine receptors (human muscle-type nAChR) expressed in TE671 cells, ionotropic glutamate receptors in Xenopus laevis oocytes (expressing recombinant GluR1flop receptors), and locust muscle ionotropic glutamate receptors sensitive to quisqualate (qGluR). The potencies of each pair of enantiomers were similar (eudismic ratio close to 1).

Blockade of ion channels as an approach to studying AMPA receptor subtypes

This article reviews current progress in studies of the relationship between the molecular structure of different subtypes of AMPA receptors and their functional properties. Differences in the subunit composition of AMPA receptors involved in glutamatergic synaptic inputs to efferent (main) neurons and interneurons are discussed with reference to neurons isolated from the hippocampus, striatum, and cerebellum. Data on the possibility of selective pharmacological actions on the ion channels of different AMPA receptor subtypes are presented; this allows these receptors to be identified and their functions to be studied in greater depth in normal and pathological conditions.

Voltage-dependent block of native AMPA receptor channels by dicationic compounds

1. The kinetics of open channel block of GluR2-containing and GluR2-lacking AMPA receptors (AMPAR) by dicationic compounds (IEM-1460, IEM-1754, and IEM-1925) have been studied in rat hippocampal neurones using whole-cell patch clamp recording and concentration-jump techniques. Neurones were isolated from hippocampal slices by vibrodissociation. 2. The dicationic compounds were approximately 100 - 200 times more potent as blockers of GluR2-lacking AMPAR than as blockers of GluR2-containing AMPAR. The subunit specificity of channel block is determined by the blocking rate constant of a dicationic compound, whereas differences in unblocking rate constants account for differences in potency. 3. Hyperpolarization may decrease the block produced by IEM-1460 and IEM-1754 block due to the voltage-dependence of the unblocking rate constants for these compounds. This suggests that dicationic compounds permeate the AMPAR channel at negative membrane potentials. The effect was particularly apparent for GluR2-lacking AMPAR. These findings indicate that the presence of GluR2-subunit(s) in AMPAR hinders the binding of the cationic compounds and their permeation through the channel. 4. The most potent compound tested was IEM-1925. The presence of a phenylcyclohexyl moiety instead of an adamantane moiety, as in IEM-1460 and IEM1754, is probably responsible for the higher potency of IEM-1925. Dicationic compounds are important not only as pharmacological tools, but also as templates for the synthesis of new selective AMPAR blockers which may be potential therapeutic agents.

Paralizing activity of the Parawixia bistriata crude venom in termites: a new bioassay

Spider venoms have high specificity to neuronal elements. Therefore, the use of venom has been important in the characterisation of mammal and insect nervous systems. The evaluation of insect paralysis has been an important tool for distinguishing the biological effects of venom. In this study we describe the paralysing effect of a spider crude venom (Parawixia bistriata) in termites, utilising a new bioassay. The crude venom of P. bistriata caused an irreversible and dose-dependent paralysis in the animals in the following doses: 2.10(-5) U; 2.10(-4) U; 2.10(-3) U; 2.10(-2) U and 0.12 U (1 U = 1 gland). This bioassay will allow for easy and direct evaluation of biological effects from different venoms and purified fractions.

Long-term depression in hippocampal interneurons: joint requirement for pre- and postsynaptic events

Long-term depression (LTD) is a well-known form of synaptic plasticity of principal neurons in the mammalian brain. Whether such changes occur in interneurons is still controversial. CA3 hippocampal interneurons expressing Ca2+-permeable AMPA receptors exhibited LTD after tetanic stimulation of CA3 excitatory inputs. LTD was independent of NMDA receptors and required both Ca2+ influx through postsynaptic AMPA receptors and activation of presynaptic mGluR7-like receptors. These results point to the capability of interneurons to undergo plastic changes of synaptic strength through joint activation of pre- and postsynaptic glutamate receptors.

PhTx4, a new class of toxins from Phoneutria nigriventer spider venom, inhibits the glutamate uptake in rat brain synaptosomes

We report the characterization of a new class of glutamate uptake inhibitors isolated from Phoneutria nigriventer venom. Glutamate transport activity was assayed in rat cerebrocortical synaptosomes by using [(3)H]-L-glutamate. PhTx4 inhibited glutamate uptake in a dose dependent manner. The IC(50) value obtained was 2.35+/-0.9 microg/ml which is in the observed range reported for glutamate uptake blockers. Tx4-7, one of PhTx4 toxins, showed the strongest inhibitory activity (50.3+/-0.69%, n=3).

Glutamate receptor expression regulates quantal size and quantal content at the Drosophila neuromuscular junction

At the Drosophila glutamatergic neuromuscular junction, the postsynaptic cell can regulate synaptic strength by both changing its sensitivity to neurotransmitter and generating a retrograde signal that regulates presynaptic transmitter release. To investigate the molecular mechanisms underlying these forms of plasticity, we have undertaken a genetic analysis of two postsynaptic glutamate receptors that are expressed at this synapse. Deletion of both genes results in embryonic lethality that can be rescued by transgenic expression of either receptor. Although these receptors are redundant for viability, they have important differences. By transgenically rescuing the double mutant, we have investigated the relationship of receptor gene dosage and composition to synaptic function. We find that the receptor subunit composition regulates quantal size, Argiotoxin sensitivity, and receptor desensitization kinetics. Finally, we show that the activity of the receptor can regulate the retrograde signal functioning at this synapse. Thus, the diversity of receptors expressed at this synapse provides the cell with mechanisms for generating synaptic plasticity.

The role of excitotoxicity in neurodegenerative disease: implications for therapy

Glutamic acid is the principal excitatory neurotransmitter in the mammalian central nervous system. Glutamic acid binds to a variety of excitatory amino acid receptors, which are ligand-gated ion channels. It is activation of these receptors that leads to depolarisation and neuronal excitation. In normal synaptic functioning, activation of excitatory amino acid receptors is transitory. However, if, for any reason, receptor activation becomes excessive or prolonged, the target neurones become damaged and eventually die. This process of neuronal death is called excitotoxicity and appears to involve sustained elevations of intracellular calcium levels. Impairment of neuronal energy metabolism may sensitise neurones to excitotoxic cell death. The principle of excitotoxicity has been well-established experimentally, both in in vitro systems and in vivo, following administration of excitatory amino acids into the nervous system. A role for excitotoxicity in the aetiology or progression of several human neurodegenerative diseases has been proposed, which has stimulated much research recently. This has led to the hope that compounds that interfere with glutamatergic neurotransmission may be of clinical benefit in treating such diseases. However, except in the case of a few very rare conditions, direct evidence for a pathogenic role for excitotoxicity in neurological disease is missing. Much attention has been directed at obtaining evidence for a role for excitotoxicity in the neurological sequelae of stroke, and there now seems to be little doubt that such a process is indeed a determining factor in the extent of the lesions observed. Several clinical trials have evaluated the potential of antiglutamate drugs to improve outcome following acute ischaemic stroke, but to date, the results of these have been disappointing. In amyotrophic lateral sclerosis, neurolathyrism, and human immunodeficiency virus dementia complex, several lines of circumstantial evidence suggest that excitotoxicity may contribute to the pathogenic process. An antiglutamate drug, riluzole, recently has been shown to provide some therapeutic benefit in the treatment of amyotrophic lateral sclerosis. Parkinson's disease and Huntington's disease are examples of neurodegenerative diseases where mitochondrial dysfunction may sensitise specific populations of neurones to excitotoxicity from synaptic glutamic acid. The first clinical trials aimed at providing neuroprotection with antiglutamate drugs are currently in progress for these two diseases.

Distribution of functional glutamate receptors in cultured embryonic Drosophila myotubes revealed using focal release of L-glutamate from caged compound by laser

During the formation of neuromuscular junctions in Drosophila embryos, glutamate receptors undergo a drastic change in distribution. To study the underlying mechanism of this developmental process, it is desirable to map the distribution of functional receptors with accurate spatial resolution. Since glutamate receptors desensitize within several milliseconds, the agonist must be applied rapidly. To fulfil these requirements we used laser stimulation of a caged compound to release L-glutamate at a focal spot. Since the glutamate receptor channel is permeable to Ca2+, we assayed the change in internal Ca2+ concentration using a Ca2+ indicator, fluo-3. Using this approach, we mapped the distribution of functional glutamate receptors in cultured embryonic Drosophila myotubes and myoblasts. Consistent with previous immunofluorescence studies using an antibody against a glutamate receptor subunit, a large increase of internal Ca2+ concentration was observed when laser stimulation was located close to some nuclei in the myotube. No change was detected when the laser stimulus was applied over any regions of the myoblasts. No increase of the internal Ca2+ concentration in myotubes was observed when the external solution contained either glutamate at a desensitizing concentration (1 mM) or a glutamate receptor channel blocker, argiotoxin (1 microg/ml). These results indicate that a rise in intracellular Ca2+ concentration can be used to show the distribution of the functional receptor on the muscle surface membrane.

Glutamate receptors in the mammalian central nervous system

Glutamate receptors (GluRs) mediate most of the excitatory neurotransmission in the mammalian central nervous system (CNS). In addition, they are involved in plastic changes in synaptic transmission as well as excitotoxic neuronal cell death that occurs in a variety of acute and chronic neurological disorders. The GluRs are divided into two distinct groups, ionotropic and metabotropic receptors. The ionotropic receptors (iGluRs) are further subdivided into three groups: alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA), kainate and N-methyl-D-aspartate (NMDA) receptor channels. The metabotropic receptors (mGluRs) are coupled to GTP-binding proteins (G-proteins), and regulate the production of intracellular messengers. The application of molecular cloning technology has greatly advanced our understanding of the GluR system. To date, at least 14 cDNAs of subunit proteins constituting iGluRs and 8 cDNAs of proteins constituting mGluRs have been cloned in the mammalian CNS, and the molecular structure, distribution and developmental change in the CNS, functional and pharmacological properties of each receptor subunit have been elucidated. Furthermore, the obtained clones have provided valuable tools for conducting studies to clarify the physiological and pathophysiological significances of each subunit. For example, the generation of gene knockout mice has disclosed critical roles of some GluR subunits in brain functions. In this article, we review recent progress in the research for GluRs with special emphasis on the molecular diversity of the GluR system and its implications for physiology and pathology of the CNS.

Potent and voltage-dependent block by philanthotoxin-343 of neuronal nicotinic receptor/channels in PC12 cells

1. Block by philanthotoxin-343 (PhTX-343), a neurotoxin from wasps, of ionic currents mediated through neuronal nicotinic acetylcholine (ACh) receptor/channels was characterized in rat phaeochromocytoma PC12 cells, by use of whole cell voltage-clamp techniques. 2. In the cells held at -60 mV, PhTX-343 at 0.1 and 1 microM inhibited an inward current activated by 100 microM ACh. The current inhibition was relieved by depolarizing steps, and augmented at negative potentials, suggesting that PhTX-343 blocks the channel in a voltage-dependent manner. Joro spider toxin-3 (JSTX-3) also exerted voltage-dependent inhibition of ACh-activated currents in a similar concentration range, but argiotoxin636 did not affect the currents. 3. Analysis of the current decay during hyperpolarizing steps indicated that the current inhibition by 100 nM PhTX-343 develops in an order of several hundres of milliseconds. On the other hand, the recovery from the current inhibition during depolarizing steps developed in an order of about 100 ms. 4. The results suggest that PhTX-343 blocks neuronal nicotinic receptor channels in PC12 cells at concentrations lower than those required for channel block in non-mammalian cells, and the block exhibits clear voltage-dependence. Estimated from the voltage-dependence, the binding site of PhTX-343 may be located near the outer mouth of the channel.

Interactions of polyamines with ion channels

Endogenous polyamines, in particular spermine, have been found to cause block and modulation of a number of types of ion channel. Intracellular spermine is responsible for intrinsic gating and rectification of strong inward rectifier K+ channels by directly plugging the ion channel pore. These K+ channels control the resting membrane potential in both excitable and non-excitable cells, and control the excitability threshold in neurons and muscle cells. Intracellular spermine causes inward rectification at some subtypes of Ca2+-permeable glutamate receptors in the central nervous system, again by plugging the receptor channel pore, and spermine can even permeate the ion channel of these receptors. Extracellular spermine has multiple effects at the N-methyl-d-aspartate (NMDA) subtype of glutamate receptor, including stimulation that increases the size of NMDA receptor currents, and voltage-dependent block. A number of polyamine-conjugated arthropod toxins and synthetic polyamine analogues are potent antagonists of glutamate receptors, and represent new tools with which to study these receptors. Interactions of polyamines with other types of cation channels have been reported. This area of research represents a new biology and a new pharmacology of polyamines.

N1-dansyl-spermine and N1-(n-octanesulfonyl)-spermine, novel glutamate receptor antagonists: block and permeation of N-methyl-D-aspartate receptors

The effects of several N-sulfonyl-polyamines, including N1-dansyl-spermine (N1-DnsSpm) and N1-(n-octanesulfonyl)-spermine (N1-OsSpm), were studied at recombinant N-methyl-D-aspartate (NMDA) receptors expressed in Xenopus laevis oocytes. N1-DnsSpm and N1-OsSpm inhibited NMDA receptors and were approximately 1000-fold more potent than spermine in oocytes voltage-clamped at -70 mV. Block by N1-DnsSpm and N1-OsSpm was strongly voltage dependent, being more pronounced at hyperpolarized membrane potentials. With the Woodhull model of voltage-dependent channel block, the values of Kd(0) were 779 microM, 882 microM, and 7.4 mM and those of z delta were 2.58, 2.57, and 1.07 for N1-DnsSpm, N1-OsSpm, and spermine, respectively. This suggests that an increase in the voltage dependence of block together with an increase in affinity contributes to the increased potencies of N1-DnsSpm and N1-OsSpm compared with spermine. Sensitivity to N1-DnsSpm was reduced by mutation NR1(N616Q) and was increased by mutations NR1(N616G) and NR2A(N615G). The NR1(N616G) and NR2A(N615G) mutations decreased the Kd(0) value of N1-DnsSpm without affecting z delta, whereas the NR1(N616Q) mutation reduced z delta. These mutations may alter the accessibility of part of the polyamine binding site within the channel pore or directly alter the properties of that site. Block by N1-DnsSpm (0.3 microM) was almost complete at -100 mV, and there was no relief of block at extreme negative membrane potentials (-100 to -200 mV) at wild-type NR1/NR2A channels. In contrast, block by N1-DnsSpm was partially relieved at extreme negative potentials at receptors containing NR1(N616G) or NR2A(N615G), suggesting that N1-DnsSpm can permeate these mutant channels but not wild-type NR1/NR2A channels. This is hypothesized to be due to an increase in the pore size of channels containing NR1(N616G) or NR2A(N615G), which allows passage of the bulky head group of N1-DnsSpm. In contrast to N1-DnsSpm, N1-OsSpm could easily permeate wild-type NR1/NR2A channels, presumably because the head group of N1-OsSpm can pass through the narrowest part of the channel pore. N-Sulfonyl-polyamines such as N1-DnsSpm and N1-OsSpm represent a new class of polyamine antagonists with which to study glutamate receptor ion channels.

Voltage-dependent blockage of Ca(2+)-permeable AMPA receptors by joro spider toxin in cultured rat hippocampal neurones

1. The effect of synthetic joro spider toxin (JSTX-3) on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor channels in cultured rat hippocampal neurones was investigated using the whole-cell patch-clamp technique. 2. A population of cultured neurones had AMPA receptors with strong inward rectification and substantial Ca2+ permeability (type II neurones), whereas most neurones (type I neurones) had slight outward rectification and little Ca2+ permeability. JSTX-3 selectively suppressed the inwardly rectifying and Ca(2+)-permeable AMPA receptors expressed in type II neurones without affecting AMPA receptors in type I neurones. 3. The effect of JSTX-3 on the Ca(2+)-permeable AMPA receptors was use and voltage dependent. In the steady state, current responses induced by ionophoretic applications of kainate (a non-desensitizing agonist of AMPA receptors) were suppressed by the toxin in a dose-dependent manner at negative potentials (IC50 = 56 nM at -60 mV). 4. At the standard membrane potential (-60 mV), recovery from the blockage by JSTX-3 was very slow. Even after washout for more than 7 min, the recovery was only partial. However, the blockage was completely removed immediately after application of a +60 mV voltage pulse for 5 s in conjunction with a single ionophoretic application of kainate.

Molecular pharmacology of voltage-dependent calcium channels

Voltage-dependent Ca2+ channels serve as the only link to transduce membrane depolarization into cellular Ca(2+)-dependent reactions. A wide variety of chemical substances that have the ability to modulate Ca2+ channels have been demonstrated both for their clinic utility and for importance in elucidating the molecular basis of various biological responses. Recently, introduction of molecular biology to pharmacology has brought a great deal of information about the molecular basis of drug action in Ca2+ channels. In this review, we attempt to overview recent progress in understanding the interactions between Ca2+ channels and their blockers, namely Ca2+ antagonists, from a molecular and structural point of view.

Polyamine spider toxins and mammalian N-methyl-D-aspartate receptors. Structural basis for channel blocking and binding of argiotoxin636

Recombinant N-methyl-D-aspartate receptors composed of NR1/NR2A subunits were expressed in Xenopus oocytes to analyse the voltage-dependent and use-dependent channel blocking activity of argiotoxin636. Functional assays demonstrate that the toxin competes with other open channel blockers such as Mg2+ and MK-801. Direct binding or competition assays using radiolabeled ligands and isolated rat brain membranes, in contrast, reveal no specific binding or yield binding constants which differ by orders of magnitude from the IC50 values of the functional assays. One explanation is that argiotoxin636 does not bind with high affinity to the inhibitory site in the N-methyl-D-aspartate-receptor channel under in vitro conditions when membranes are depolarised. The structure of argiotoxin636 was investigated by NMR spectroscopy. In solution the positively charged argiotoxin636 acquires an extended conformation and its dimensions might allow permeation deep into the channel. In the absence of direct structural information on the channel protein, the detailed analysis of blockade in conjunction with structural information, as provided here, may be of aid in the deduction of structural features of glutamate-receptor channel ion pores.

Polyamine amides are neuroprotective in cerebellar granule cell cultures challenged with excitatory amino acids

Primary cultures of rat cerebellar granule cells have been used to assess the potential neuroprotective effects of philanthotoxins and argiotoxin-636 (ArgTX-636). These polyamine amides are potent antagonists of ionotropic L-glutamate (L-Glu) receptors. In granule cells loaded with fluo-3, ArgTX-636 and philanthotoxin-343 (PhTX-343) antagonised increases of intracellular free calcium concentration ([Ca2+]i) that were stimulated by N-methyl-D-aspartate (NMDA). The antagonism was use-dependent. Antagonism by PhTX-343 was fully reversible, but recovery following antagonism by ArgTX-636 was slow and only partial during the time-course of an experiment. Neither compound inhibited K(+)-induced increases in [Ca2+]i. In excitotoxicity studies with cerebellar granule cells, the release of lactate dehydrogenase (LDH) and morphological observations were used to assess cell death. A 20-30 min exposure to 500 microM NMDA, 100 microM L-Glu or 500 microM kainate was sufficient to kill > 90% of the cells after 18-20 h. When added 5 min prior to, and during agonist exposure, PhTX-343 and ArgTX-636 provided total neuroprotection. ArgTX-636 was about 20-30 fold more potent than PhTX-343 against NMDA, but was approximately equipotent with PhTX-343 against a kainate challenge. Neither of the toxins showed any inherent toxicity even at 400 microM and 100 microM respectively. Some analogues of PhTX-343 are more potent, both in terms of antagonism of NMDA-stimulated increases of [Ca2+]i and neuroprotection, than PhTX-343 and ArgTX-636.

Antagonism of glutamate receptors by a chromatographic fraction from the exudate of the sea anemone Phyllactis flosculifera

In the search for new glutamate antagonists it seems promising to characterize the effects of venom from invertebrates that prey mainly on crustaceans. In this work, the exudate of the sea anemone Phyllactis flosculifera was used as a source of this type of compound. The action of chromatographic fraction D from P. flosculifera was tested upon microion-tophoretically evoked glutamate responses in intracellular recordings from central neurons of the land snail Zachrysia guanensis. Bath application of fraction D (2-8 mg/ml, n = 13) diminished both the excitatory and the inhibitory components of glutamate agonists in Z. guanensis neurons; this action was dose-dependent and partially reversible. Fraction D actions were also tested in the multiunit spontaneous and mechanically evoked responses of the glutamatergic junction between hair cells and afferent neurons of the axolotl Ambystoma tigrinum. Pressure ejection of fraction D in concentrations ranging from 0.5 to 2 mg/ml (n = 9) decreased the spontaneous and mechanically evoked activity of semicircular canal afferent neurons and the responses evoked by kainic acid and alpha-amino-3-hydroxy-5-methylisoxasole-4-propionic acid. This action was also dose-dependent and partially reversible. These results indicate that fraction D acts as a glutamate receptor antagonist in snail and amphibian neurons. Further studies are required to characterize the active compounds responsible for this action and its specificity upon the subtypes of glutamate receptors.

Multiple actions of arylalkylamine arthropod toxins on the N-methyl-D-aspartate receptor

The effects of the arylalkyamine arthropod toxins argiotoxin 636 and philanthotoxin 343 were studied on N-methyl-D-aspartate receptor currents in cultured rat hippocampal neurons using whole-cell recording techniques. Argiotoxin 636 and philanthotoxin 343 blocked 10 microM N-methyl-D-aspartate (+10 microM glycine) currents in a concentration-dependent fashion (steady-state IC50 values, 0.9 and 56 microM, respectively). The onset and recovery from argiotoxin 636 block occurred slowly (forward and reverse rate constants, 7.5 x 10(3) s-1 M-1 and 6.9 x 10(-3) s-1, respectively) whereas the philanthotoxin 343 block was more rapid (forward and reverse rate constants, 1.1 x 10(5) s-1 M-1 and 0.1 s-1). A portion, but not all, of the block by the two toxins could be reversed by depolarization to positive holding potentials, indicating that there are voltage-dependent and non-voltage-dependent components of the block. The long-lasting argiotoxin 636 block at -60 mV occurred in a use-dependent fashion and could be substantially reduced by co-application with 10 mM Mg2+, providing evidence that the toxin has a channel blocking action. In contrast to the use dependence of the voltage-dependent argiotoxin 636 block, the non-voltage-dependent component of block (at +60 mV) did not require agonist gating of the channel. The non-voltage-dependent block by argiotoxin 636 was unaffected by increasing the glycine concentration, but was reversed by increasing the N-methyl-D-aspartate concentration, suggesting that the toxin may act as a competitive antagonist at the N-methyl-D-aspartate recognition site. This mechanism was further supported by the near identity of the time constant for argiotoxin 636 block with the time constant for agonist dissociation, irrespective of whether the rapidly dissociating agonist N-methyl-D-aspartate or the more slowly dissociating agonist glutamate was used. With high concentrations of N-methyl-D-aspartate (> or = 100 microM), argiotoxin 636 produced a potentiation of the peak N-methyl-D-aspartate response (at +60 mV) that was accompanied by a slowing in the rate of current desensitization and an increase in the affinity for glycine. We conclude that the arylalkylamine toxins antagonize N-methyl-D-aspartate receptor currents by two distinct mechanisms: use-dependent and voltage-dependent open channel block and competitive antagonism at the N-methyl-D-aspartate recognition site. In addition, argiotoxin 636 exerts a polyamine-like facilitation of N-methyl-D-aspartate receptor currents. This facilitation occurs via both the glycine-dependent and glycine-independent mechanisms, whereby spermine is known to potentiate N-methyl-D-aspartate receptor responses.