Arylamine spider toxins antagonize NMDA receptor-mediated synaptic transmission in rat hippocampal slices

Spider Neurotoxins as Modulators of NMDA Receptor Signaling

Molecules that selectively act on N-methyl-D-aspartate (NMDA) receptors may have a multidirectional effect by modulating the activity of NMDARs, affecting their active sites as well as by changing the composition of their subunits. The results of the clinical trials conducted so far in mood disorders and schizophrenia indicate that such agents may become new effective drugs for the treatment of these diseases. Number of spider neurotoxins e.g. ctenitoxins extracted from Phoneutria sp. venom act as potent and selective NMDAR blockers that do not disturb cortical and hippocampal glutamate signaling, LTP generation and synaptic neurochemistry. Possibly this intriguing kind of promising neuroregulatory peptides and polyamines can be clinically applicable in a wide spectrum of neuropsychiatric disorders, including epilepsy, neurotrauma and ischemic injuries. These novel medications can potentially be helpful in the future treatment of stroke and several neurodegenerative diseases.

Can neurophysiologic measures serve as biomarkers for the efficacy of repetitive transcranial magnetic stimulation treatment of major depressive disorder?

Repetitive transcranial magnetic stimulation (rTMS) is an effective treatment for Major Depressive Disorder (MDD). There are clinical data that support the efficacy of many different approaches to rTMS treatment, and it remains unclear what combination of stimulation parameters is optimal to relieve depressive symptoms. Because of the costs and complexity of studies that would be necessary to explore and compare the large number of combinations of rTMS treatment parameters, it would be useful to establish reliable surrogate biomarkers of treatment efficacy that could be used to compare different approaches to treatment. This study reviews the evidence that neurophysiologic measures of cortical excitability could be used as biomarkers for screening different rTMS treatment paradigms. It examines evidence that: (1) changes in excitability are related to the mechanism of action of rTMS; (2) rTMS has consistent effects on measures of excitability that could constitute reliable biomarkers; and (3) changes in excitability are related to the outcomes of rTMS treatment of MDD. An increasing body of evidence indicates that these neurophysiologic measures have the potential to serve as reliable biomarkers for screening different approaches to rTMS treatment of MDD.

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.

Distinct forms of Gq-receptor-dependent plasticity of excitatory transmission in the BNST are differentially affected by stress

Long-term depression (LTD) is an important synaptic mechanism for limiting excitatory influence over circuits subserving cognitive and emotional behavior. A major means of LTD induction is through the recruitment of signaling via G(q)-linked receptors activated by norepinephrine (NE), acetylcholine, and glutamate. Receptors from these transmitter families have been proposed to converge on a common postsynaptic LTD maintenance mechanism, such that hetero- and homosynaptic induction produce similar alterations in glutamate synapse efficacy. We report that in the dorsolateral and ventrolateral bed nucleus of the stria terminalis (BNST), recruitment of G(q)-linked receptors by glutamate or NE initiates mechanistically distinct forms of postsynaptically maintained LTD and these LTDs are differentially regulated by stress exposure. In particular, we show that although both mGluR5- and alpha(1)-adrenergic receptor (AR)-dependent LTDs involve postsynaptic endocytosis, the alpha(1)-AR-initiated LTD exclusively involves modulation of signaling through calcium-permeable AMPA receptors. Further, alpha(1)-AR- but not mGluR5- dependent LTD is disrupted by restraint stress. alpha(1)-AR LTD is also impaired in mice chronically exposed to ethanol. These data thus suggest that in the BNST, NE- and glutamate-activated G(q)-linked signaling pathways differentially tune glutamate synapse efficacy in response to stress.

Neuroprotection and peptide toxins

Neurodegeneration induced by excitatory neurotransmitter glutamate is considered to be of particular relevance in several types of acute and chronic neurological impairments ranging from cerebral ischaemia to neuropathological conditions such as motor neuron disease, Alzheimer's, Parkinson's disease and epilepsy. The hyperexcitation of glutamate receptors coupled with calcium overload can be prevented or modulated by using well-established competitive and non-competitive antagonists targeting ion/receptor channels. The exponentially increasing body of pharmacological evidence over the years indicates potential applications of peptide toxins, due to their exquisite subtype selectivity on ion channels and receptors, as lead structures for the development of drugs for the treatment of wide variety of neurological disorders. This review comprehensively highlights the overview of the diversity in the molecular as well as neurobiological mechanisms of different peptide toxins derived from venomous animals with particular reference to neuroprotection. In addition, the potential applications of peptide toxins in the diagnosis and treatment of neurological disorders such as neuromuscular disorders, epilepsy, Alzheimer's and Parkinson's diseases, gliomas and ischaemic stroke and their future prospects in the diagnosis as well as in the therapy are addressed.

AMPA receptor ligands: Synthetic and pharmacological studies of polyamines and polyamine toxins

Alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors (AMPAR), subtype of the ionotropic glutamate receptors (IGRs), mediate fast synaptic transmission in the central nervous system (CNS), and are involved in many neurological disorders, as well as being a key player in the formation of memory. Hence, ligands affecting AMPARs are highly important for the study of the structure and function of this receptor, and in this regard polyamine-based ligands, particularly polyamine toxins, are unique as they selectively block Ca2+ -permeable AMPARs. Indeed, endogenous intracellular polyamines are known to modulate the function of these receptors in vivo. In this study, recent developments in the medicinal chemistry of polyamine-based ligands are given, particularly focusing on the use of solid-phase synthesis (SPS) as a tool for the facile generation of libraries of polyamine toxin analogues. Moreover, the recent development of highly potent and very selective AMPAR ligands is described. Additionally, we provide a detailed account on the mechanism and site of action of AMPAR blockade by polyamine-based ligands, including examples of how these ligands are used as tools to study AMPAR, and a comparison with their action on other ionotropic receptors.

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.

Agatoxins: ion channel specific toxins from the american funnel web spider, Agelenopsis aperta

Agatoxins from Agelenopsis aperta venom target three classes of ion channels, including transmitter-activated cation channels, voltage-activated sodium channels, and voltage-activated calcium channels. The alpha-agatoxins are non-competitive, use-dependent antagonists of glutamate receptor channels, and produce rapid but reversible paralysis in insect prey. Their actions are facilitated by the micro-agatoxins, which shift voltage-dependent activation of neuronal sodium channels to more negative potentials, causing spontaneous transmitter release and repetitive action potentials. The omega-agatoxins target neuronal calcium channels, modifying their properties in distinct ways, either through gating modification (omega-Aga-IVA) or by reduction of unitary current (omega-Aga-IIIA). The alpha-agatoxins and omega-agatoxins modify both insect and vertebrate ion channels, while the micro-agatoxins are selective for insect channels. Agatoxins have been used as selective pharmacological probes for characterization of ion channels in the brain and heart, and have been evaluated as candidate biopesticides.

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.

Design, synthesis, and biological evaluation of spider toxin (argiotoxin-636) analogs as NMDA receptor antagonists

PURPOSE

Twelve synthetic spider toxin analogs were prepared in an effort to better understand the structure-activity relationships of the polyamine portion of argiotoxin-636 (Arg-636), a noncompetitive NMDA receptor (NMDAR) antagonist.

METHODS

The 1,13-diamino-4,8-diazatridecane portion of the side chain of Arg-636 was systematically modified in an effort to further our knowledge of the structural requirements for the alkyl linker spacing between the amine nitrogens. Systematic isosteric replacement of each of the amine nitrogens in the polyamine moiety with either oxygen or carbon provided a series of compounds which were evaluated in vitro for NMDAR antagonist activity.

RESULTS

One-half of the heteroatoms found in Arg-636 were removed to provide analogs which maintained in vitro potency below 1 microM. However, these simplified analogs produced similar or more pronounced effects on the cardiovascular system than Arg-636 in vivo.

CONCLUSIONS

In this set of analogs, a minimum of three basic nitrogens in the side chain was required for maximum potency as NMDAR antagonists. Isosteric nitrogen substitutions in the polyamine chain reduced the in vitro potency of these analogs. An analog binding-conformation model was proposed to rationalize the inactivity of these isosterically substituted analogs.

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.

Argiotoxin detects molecular differences in AMPA receptor channels

Argiotoxin, a component of the spider venom from Argiope lobata, blocks AMPA receptor channels expressed in homomeric and heteromeric configuration in Xenopus oocytes. Argiotoxin acts as an open channel blocker in a voltage-dependent manner and discriminates between the functionally diverse AMPA receptors. Importantly, a transmembrane region 2 determinant for divalent cation permeability also determines argiotoxin sensitivity. Subunit-specific differences in the time courses of block and recovery demonstrate that heteromeric AMPA receptors can assemble in variable ratios. Thus, argiotoxin can be used as a tool in analyzing the subunit composition of AMPA receptors in native membranes.

Interactions of polyamines with neuronal ion channels

Polyamines, a group of aliphatic amines, exert selective and complex actions on a variety of ion channels. Polyamines are found endogenously, as normal metabolic intermediates, and also in the venoms of several invertebrates, where they act as potent neurotoxins. In addition, evidence suggests that polyamines may mediate or potentiate excitotoxic mechanisms responsible for neuronal damage during ischaemia. Now that the structures and functions of various polyamines are beginning to be deduced, and synthetic analogues become available, these compounds are of importance, not only as pharmacological tools to study specific receptor/ion channel complexes, but also as templates on which to base drugs designed for neuroprotective effects in a number of neurodegenerative disorders.

NBQX is a selective non-NMDA receptor antagonist in rat hippocampal slice

The effects of NBQX and DNQX on synaptic transmission in rat hippocampal slice were investigated. Both agents produced dose-dependent blockade of field potentials evoked by low frequency stimulation of Schaffer collateral-commissural fibers recorded in medium containing 4 mM Mg2+ (non-NMDA mediated transmission), with half-maximal effects at about 0.15 microM for NBQX and 1.0 microM for DNQX. When the studies were conducted in Mg(2+)-free medium (predominantly NMDA mediated transmission), 100 microM NBQX failed to block transmission; however, the response could be completely blocked by the addition of 10 microM of the competitive NMDA antagonist CPP. In contrast, 47 microM DNQX completely blocked secondary field potentials recorded in Mg(2+)-free medium and this effect could be reversed by the addition of 200 microM of the glycine agonist D-serine. Thus, NBQX exhibited selective blockade of non-NMDA mediated synaptic transmission whereas DNQX had effects at both non-NMDA and NMDA receptor sites, the latter effect via an interaction with the glycine site on the NMDA receptor complex.