Toxins affecting calcium channels in neurons

Design Strategies for Cellular Nanosponges as Medical Countermeasures

The interest in using therapeutic nanoparticles to bind with harmful molecules or pathogens and subsequently neutralize their bioactivity has grown tremendously. Among various nanomedicine platforms, cell membrane-coated nanoparticles, namely, "cellular nanosponges," stand out for their broad-spectrum neutralization capability challenging to achieve in traditional countermeasure technologies. Such ability is attributable to their cellular function-based rather than target structure-based working principle. Integrating cellular nanosponges with various synthetic substrates further makes their applications exceptionally versatile and adaptive. This review discusses the latest cellular nanosponge technology focusing on how the structure-function relationship in different designs has led to versatile and potent medical countermeasures. Four design strategies are discussed, including harnessing native cell membrane functions for biological neutralization, functionalizing cell membrane coatings to enhance neutralization capabilities, combining cell membranes and functional cores for multimodal neutralization, and integrating cellular nanosponges with hydrogels for localized applications. Examples in each design strategy are selected, and the discussion is to highlight their structure-function relationships in complex disease settings. The review may inspire additional design strategies for cellular nanosponges and fulfill even broader medical applications.

Animal Venom Peptides as a Treasure Trove for New Therapeutics Against Neurodegenerative Disorders

BACKGROUND

Neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, and cerebral ischemic stroke, impose enormous socio-economic burdens on both patients and health-care systems. However, drugs targeting these diseases remain unsatisfactory, and hence there is an urgent need for the development of novel and potent drug candidates.

METHODS

Animal toxins exhibit rich diversity in both proteins and peptides, which play vital roles in biomedical drug development. As a molecular tool, animal toxin peptides have not only helped clarify many critical physiological processes but also led to the discovery of novel drugs and clinical therapeutics.

RESULTS

Recently, toxin peptides identified from venomous animals, e.g. exenatide, ziconotide, Hi1a, and PcTx1 from spider venom, have been shown to block specific ion channels, alleviate inflammation, decrease protein aggregates, regulate glutamate and neurotransmitter levels, and increase neuroprotective factors.

CONCLUSION

Thus, components of venom hold considerable capacity as drug candidates for the alleviation or reduction of neurodegeneration. This review highlights studies evaluating different animal toxins, especially peptides, as promising therapeutic tools for the treatment of different neurodegenerative diseases and disorders.

Conotoxin MVIIA improves cell viability and antioxidant system after spinal cord injury in rats

This study evaluates whether intrathecal MVIIA injection after spinal cord injury (SCI) elicits neuroprotective effects. The test rats were randomly distributed into six groups— sham, placebo, MVIIA 2.5 μM, MVIIA 5 μM, MVIIA 10 μM, and MVIIA 20 μM—and were administered the treatment four hours after SCI. After the optimal MVIIA dose (MVIIA 10 μM) was defined, the best time for application, one or four hours, was analyzed. Locomotor hind limb function and side effects were assessed. Forty-eight hours after the injury and immediately after euthanasia, spinal cord segments were removed from the test rats. Cell viability, reactive oxygen species, lipid peroxidation, and glutamate release were investigated. To examine the MVIIA mechanism of action, the gene expressions of pro-apoptotic (Bax, nNOS, and caspase-3, -8, -9, -12) and anti-apoptotic (Bcl-xl) factors in the spinal cord tissue samples were determined by real-time PCR, and the activities of antioxidant enzymes were also investigated. Application of intrathecal MVIIA 10 μM four hours after SCI prompted a neuroprotective effect: neuronal death decreased (22.46%), oxidative stress diminished, pro-apoptotic factors (Bax, nNOS, and caspase-3, -8) were expressed to a lesser extent, and mitochondrial viability as well as anti-apoptotic factor (Bcl-xl) expression increased. These results suggested that MVIIA provided neuroprotection through antioxidant effects. Indeed, superoxide dismutase (188.41%), and glutathione peroxidase (199.96%), reductase (193.86%), and transferase (175.93%) expressions increased. Therefore, intrathecal MVIIA (MVIIA 10 μM, 4 h) application has neuroprotective potential, and the possible mechanisms are related to antioxidant agent modulation and to intrinsic and extrinsic apoptotic pathways.

Animal Toxins as Therapeutic Tools to Treat Neurodegenerative Diseases

Neurodegenerative diseases affect millions of individuals worldwide. So far, no disease-modifying drug is available to treat patients, making the search for effective drugs an urgent need. Neurodegeneration is triggered by the activation of several cellular processes, including oxidative stress, mitochondrial impairment, neuroinflammation, aging, aggregate formation, glutamatergic excitotoxicity, and apoptosis. Therefore, many research groups aim to identify drugs that may inhibit one or more of these events leading to neuronal cell death. Venoms are fruitful natural sources of new molecules, which have been relentlessly enhanced by evolution through natural selection. Several studies indicate that venom components can exhibit selectivity and affinity for a wide variety of targets in mammalian systems. For instance, an expressive number of natural peptides identified in venoms from animals, such as snakes, scorpions, bees, and spiders, were shown to lessen inflammation, regulate glutamate release, modify neurotransmitter levels, block ion channel activation, decrease the number of protein aggregates, and increase the levels of neuroprotective factors. Thus, these venom components hold potential as therapeutic tools to slow or even halt neurodegeneration. However, there are many technological issues to overcome, as venom peptides are hard to obtain and characterize and the amount obtained from natural sources is insufficient to perform all the necessary experiments and tests. Fortunately, technological improvements regarding heterologous protein expression, as well as peptide chemical synthesis will help to provide enough quantities and allow chemical and pharmacological enhancements of these natural occurring compounds. Thus, the main focus of this review is to highlight the most promising studies evaluating animal toxins as therapeutic tools to treat a wide variety of neurodegenerative conditions, including Alzheimer’s disease, Parkinson’s disease, brain ischemia, glaucoma, amyotrophic lateral sclerosis, and multiple sclerosis.

TRPM2 Promotes Neurotoxin MPP+/MPTP-Induced Cell Death

In neurons, Ca²⁺ is essential for a variety of physiological processes that regulate gene transcription to neuronal growth and their survival. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and 1-methyl-4-phenylpyridinium ions (MPP⁺) are potent neurotoxins that selectively destroys the dopaminergic (DA) neurons and mimics Parkinson's disease (PD) like symptoms, but the mechanism as how MPP⁺/MPTP effects DA neuron survival is not well-understood. In the present study, we found that MPP⁺ treatment increased the level of reactive oxygen species (ROS) that activates and upregulates the expression and function of melastatin-like transient receptor potential (TRPM) subfamily member, melastatin-like transient receptor potential channel 2 (TRPM2). Correspondingly, TRPM2 expression was also increased in substantia nigra of MPTP-induced PD mouse model and PD patients. ROS-mediated activation of TRPM2 resulted in an increased intracellular Ca²⁺, which in turn promoted cell death in SH-SY5Y cells. Intracellular Ca²⁺ overload caused by MPP⁺-induced ROS also affected calpain activity, followed by increased caspase 3 activities and activation of downstream apoptotic pathway. On the other hand, quenching of H₂O₂ by antioxidants, resveratrol (RSV), or N-acetylcysteine (NAC) effectively blocked TRPM2-mediated Ca²⁺ influx, decreased intracellular Ca²⁺ overload, and increased cell survival. Importantly, pharmacological inhibition of TRPM2 or knockdown of TRPM2 using siRNA, but not control siRNA, showed an increased protection by preventing MPP⁺-induced Ca²⁺ increase and inhibited apoptosis. Taken together, we show here a novel role for TRPM2 expression and function in MPP⁺-induced dopaminergic neuronal cell death.

A spider toxin, ω-agatoxin IV A, binds to fixed as well as living tissues: cytochemical visualization of P/Q-type calcium channels

ω-Agatoxin IV A, a peptidyl toxin from Agelenopsis aperta venom, selectively binds to voltage-gated P/Q-type calcium channels. ω-Agatoxin IV A has been used as a selective tool in pharmacological and electrophysiological studies. Visualization of P/Q-type calcium channels has previously been accomplished using biotin-conjugated ω-Agatoxin IV A in freshly prepared mouse cerebellar and hippocampal slices (Nakanishi et al, J. Neurosci. Res., 41: , 532, 1995). Here biotinylated ω-agatoxin IV A was applied to transcardially fixed brain slices prepared with various fixatives. ω-Agatoxin IV A did not bind to fixed tissues from P/Q-type calcium channel knockout mice, confirming that binding to normal, fixed tissues was not an artifact. Using transmission electron microscopy, locations of biotinylated ω-agatoxin IV A binding sites visualized with gold-conjugated streptavidin showed a similar pattern to those visualized with antibody. The ability of biotinylated ω-agatoxin IV A to bind to fixed tissue provides a new cytochemical technique to study molecular architecture of synapses.

Bio-inspired voltage-dependent calcium channel blockers

Ca²⁺ influx via voltage-dependent Ca_V1/Ca_V2 channels couples electrical signals to biological responses in excitable cells. Ca_V1/Ca_V2 channel blockers have broad biotechnological and therapeutic applications. Here we report a general method for developing novel genetically-encoded calcium channel blockers inspired by Rem, a small G-protein that constitutively inhibits Ca_V1/Ca_V2 channels. We show that diverse cytosolic proteins (Ca_Vβ, 14-3-3, calmodulin, and CaMKII) that bind pore-forming α₁-subunits can be converted into calcium channel blockers with tunable selectivity, kinetics, and potency, simply by anchoring them to the plasma membrane. We term this method “channel inactivation induced by membrane-tethering of an associated protein” (ChIMP). ChIMP is potentially extendable to small-molecule drug discovery, as engineering FK506-binding protein into intracellular sites within Ca_V1.2-α_1C permits heterodimerization-initiated channel inhibition with rapamycin. The results reveal a universal method for developing novel calcium channel blockers that may be extended to develop probes for a broad cohort of unrelated ion channels.

Systemic effects induced by intralesional injection of ω-conotoxin MVIIC after spinal cord injury in rats

Background

Calcium channel blockers such as conotoxins have shown a great potential to reduce brain and spinal cord injury. MVIIC neuroprotective effects analyzed in in vitro models of brain and spinal cord ischemia suggest a potential role of this toxin in preventing injury after spinal cord trauma. However, previous clinical studies with MVIIC demonstrated that clinical side effects might limit the usefulness of this drug and there is no research on its systemic effects. Therefore, the present study aimed to investigate the potential toxic effects of MVIIC on organs and to evaluate clinical and blood profiles of rats submitted to spinal cord injury and treated with this marine toxin. Rats were treated with placebo or MVIIC (at doses of 15, 30, 60 or 120 pmol) intralesionally following spinal cord injury. Seven days after the toxin administration, kidney, brain, lung, heart, liver, adrenal, muscles, pancreas, spleen, stomach, and intestine were histopathologically investigated. In addition, blood samples collected from the rats were tested for any hematologic or biochemical changes.

Results

The clinical, hematologic and biochemical evaluation revealed no significant abnormalities in all groups, even in high doses. There was no significant alteration in organs, except for degenerative changes in kidneys at a dose of 120 pmol.

Conclusions

These findings suggest that MVIIC at 15, 30 and 60 pmol are safe for intralesional administration after spinal cord injury and could be further investigated in relation to its neuroprotective effects. However, 120 pmol doses of MVIIC may provoke adverse effects on kidney tissue.

Neurobiological activity of Parawixin 10, a novel anticonvulsant compound isolated from Parawixia bistriata spider venom (Araneidae: Araneae)

The neurobiological activity of Parawixin 10, isolated from Parawixia bistriata spider venom, was investigated. Cannulas were implanted in the lateral ventricles of Wistar rats (200-250 g, n=6-8 per group) to perform anticonvulsant and behavioral assays, and synaptosomes from cerebral cortices of male Wistar rats were used for neurochemical studies. The results indicate that pretreatment with Parawixin 10 prevents the onset of seizures induced with kainic acid, N-methyl-D-aspartate, and pentylenetetrazole in a dose-response manner. Lower doses of Parawixin 10 significantly increased the latency to onset of kainic acid-, pentylenetetrazole-, and N-methyl-D-aspartate-induced seizures. There were maximum increases of 79% in L-[(3)H]glutamine uptake and 40% in [(3)H]glycine uptake; [(3)H]GABA uptake did not change. The findings demonstrate that this novel compound from P. bistriata venom exerts a pharmacological effect on the glutamatergic and glycinergic systems.

Identification of trichotoxin, a novel chlorinated compound associated with the bloom forming Cyanobacterium, Trichodesmium thiebautii

Trichodesmium is a suspected toxin-producing nonheterocystous cyanobacteria ubiquitous in tropical, subtropical, and temperate seas. The genus is known for its ability to fix nitrogen and form massive blooms. In oligotrophic seas, it can dominate the biomass and be a major component of oceanic primary production and global nitrogen cycling. Numerous reports suggest Trichodesmium-derived toxins are a cause of death of fish, crabs, and bivalves. Laboratory studies have demonstrated neurotoxic effects in T. thiebautii cell extracts and field reports suggest respiratory distress and contact dermatitis of humans at collection sites. However, Trichodesmium toxins have not been identified and characterized. Here, we report the extraction of a lipophilic toxin from field-collected T. thiebautii using a purification method of several chromatographic techniques, nuclear magnetic resonance (NMR), mass spectroscopy (MS), and Fourier transformed-infrared spectroscopy (FT-IR). Trichotoxin has a molecular formula of C(20)H(27)ClO and a mass of 318 m/z and possesses cytotoxic activity against GH(4)C(1) rat pituitary and Neuro-2a mouse neuroblastoma cells. A detection method using liquid chromatography/mass spectrometry (LC/MS) was developed. This compound is the first reported cytotoxic natural product isolated and fully characterized from a Trichodesmium species.

Peptide neurotoxins that affect voltage-gated calcium channels: a close-up on ω-agatoxins

Peptide neurotoxins found in animal venoms have gained great interest in the field of neurotransmission. As they are high affinity ligands for calcium, potassium and sodium channels, they have become useful tools for studying channel structure and activity. Peptide neurotoxins represent the clinical potential of ion-channel modulators across several therapeutic fields, especially in developing new strategies for treatment of ion channel-related diseases. The aim of this review is to overview the latest updates in the domain of peptide neurotoxins that affect voltage-gated calcium channels, with a special focus on ω-agatoxins.

Electrophysiological characterization of a novel small peptide from the venom of Conus californicus that targets voltage-gated neuronal Ca2+ channels

Conus californicus belongs to a genus of marine gastropods with more than 700 extant species. C. californicus has been shown to be distantly related to all Conus species, but showing unusual biological features. We report a novel peptide isolated from C. californicus with a significant inhibitory action over neuronal voltage-gated calcium channels. The new toxin is formed by 13-amino acid residues with two disulfide bonds, whose sequence (NCPAGCRSQGCCM) is strikingly different from regular ω-conotoxins. In the HPLC purification procedure, the venom fraction eluted in the first 10-15 min produced a significant decrease (54% ± 3%) of the Ca(2+) current in Xenopus laevis oocytes transfected with purified rat-brain mRNA. A specific peptide obtained from the elution at 13 min decreased the Ca(2+) current in the adult rat dorsal-root ganglion neurons in a primary culture by 34% ± 2%. The cysteine pattern of this peptide corresponds to the framework XVI described for the M-superfamily of conopeptides and is unprecedented among Conus peptides acting on Ca(2+) channels.

Comparative study between the protective effects of Saudi and Egyptian antivenoms, alone or in combination with ion channel modulators, against deleterious actions of Leiurus quinquestriatus scorpion venom

This study compared efficacy of two polyvalent antivenoms (Saudi Arabian and Egyptian), against lethality and pathophysiological changes of Leiurus quinquestriatus quinquestriatus (LQQ) scorpion venom in mice. Additionally, the study examined whether treatment with selected ion channel modulators, lidocaine, nimodipine or amiodarone would be effective, alone or combined with the antivenoms. The protein concentration of the Saudi antivenom was 1/3 of Egyptian, indicating lesser immunogenicity, while both preservative contents were within limits. In immunodiffusion experiments, both exhibited prominent precipitin bands indicating high concentrations of specific antibodies. Neutralizing capacities (60-70 LD(50)) stated on labels were confirmed. Both antivenoms significantly (P < 0.001) prolonged survival time (from 26.9 +/- 1.18 min, 100% dead with venom to 224-300 min, 0-30% dead) of envenomed mice, whether injected iv before or 5 min after venom. Injection of either antivenom plus ion channel modulators, gave comparable results to that observed in mice treated with antivenoms alone. The Na(+) channel blocker lidocaine and the Ca(2+) channel blocker nimodipine on their own significantly protected the animals (P < 0.05), but to a lesser extent. The two antivenoms, significantly ameliorated the venom-evoked changes in serum LDH (P < 0.001) and CKMB (P < 0.01) plus cardiac TNFalpha and nitrate/nitrite levels (P < 0.001). When combined with lidocaine or nimodipine, the effects were not greater than antivenom alone. Moreover, the antivenoms ameliorated characteristic venom-evoked changes in the isolated perfused Langendorff hearts. Lidocaine and amiodarone were more effective than nimodipine. In Conclusion both Saudi and Egyptian antivenoms protected mice from the pathological and lethal effects of LQQ scorpion. Sodium and calcium channel blockers, lidocaine and nimodipine, may be useful when antivenoms are not available.

Engineering proteins for custom inhibition of Ca(V) channels

The influx of Ca(2+) ions through voltage-dependent calcium (Ca(V)) channels links electrical signals to physiological responses in all excitable cells. Not surprisingly, blocking Ca(V) channel activity is a powerful method to regulate the function of excitable cells, and this is exploited for both physiological and therapeutic benefit. Nevertheless, the full potential for Ca(V) channel inhibition is not being realized by currently available small-molecule blockers or second-messenger modulators due to limitations in targeting them either to defined groups of cells in an organism or to distinct subcellular regions within a single cell. Here, we review early efforts to engineer protein molecule blockers of Ca(V) channels to fill this crucial niche. This technology would greatly expand the toolbox available to physiologists studying the biology of excitable cells at the cellular and systems level.

Calcium channels, neuromuscular synaptic transmission and neurological diseases

Voltage-dependent calcium channels are essential in neuronal signaling and synaptic transmission, and their functional alterations underlie numerous human disorders whether monogenic (e.g., ataxia, migraine, etc.) or autoimmune. We review recent work on Ca(V)2.1 or P/Q channelopathies, mostly using neuromuscular junction preparations, and focus specially on the functional hierarchy among the calcium channels recruited to mediate neurotransmitter release when Ca(V)2.1 channels are mutated or depleted. In either case, synaptic transmission is greatly compromised; evidently, none of the reported functional replacements with other calcium channels compensates fully.

omega-Conotoxin inhibition of excitatory synaptic transmission evoked by dorsal root stimulation in rat superficial dorsal horn

A number of omega-conotoxins are potent and selective antagonists of N-type voltage-gated calcium channels (VGCCs) and are potentially effective as analgesic agents. omega-Conotoxins CVID and CVIB, venom peptides from Conus catus, inhibit N-type and N/P/Q-type VGCCs, respectively, in rat dorsal root ganglion sensory neurons. In the present study, we tested the effects of five different omega-conotoxins, CVID, CVIB, MVIIA, MVIIC and GVIA, on excitatory synaptic transmission between primary afferents and dorsal horn superficial lamina neurons of rat spinal cord. The N-type VGCC antagonists CVID (200nM) and MVIIA (500nM) completely and irreversibly inhibited excitatory postsynaptic currents (EPSCs) in the dorsal horn superficial lamina. The N- and P/Q-type VGCC antagonist CVIB (200nM) reversibly reduced evoked EPSC amplitude an average of 34+/-8%, whereas MVIIC (200nM) had no effect on excitatory synaptic transmission. In neurons receiving polysynaptic input, CVIB reduced both the EPSC amplitude and the "success rate" calculated as the relative number of primary afferent stimulations that resulted in postsynaptic responses. These results indicate that (i) the analgesic action of omega-conotoxins that antagonise N-type VGCCs may be attributed to inhibition of neurotransmission between primary afferents and superficial dorsal horn neurons, (ii) nociceptive synaptic transmission between primary afferents and superficial lamina neurons is mediated predominantly by N-type VGCCs, and (iii) in contrast to the irreversible inhibition by CVID, MVIIA and GVIA, the inhibition of excitatory monosynaptic transmission by CVIB is reversible.

Neurotoxins from invertebrates as anticonvulsants: From basic research to therapeutic application

Invertebrate venoms have attracted considerable interest as a potential source of bioactive substances, especially neurotoxins. These molecules have proved to be extremely useful tools for the understanding of synaptic transmission events, and they have contributed to the design of novel drugs for the treatment of neurological disorders and pain. In this context, as epilepsy involves neuronal substrates, which are sites of action of many neurotoxins; venoms may be particularly useful for antiepileptic drug (AED) research. Epilepsy is a chronic disease whose treatment consists of controlling seizures with antiepileptics that very often induce strong undesirable side effects that may limit treatment. Here, we review the vast, but yet unexplored, world of neurotoxins from invertebrates used as probes in pharmacological screening for novel and less toxic antiepileptics. We briefly review (1) the molecular basis of epilepsy, as well as the sites of action of commonly used anticonvulsants (we bring a comprehensive review of the elements from invertebrate venoms which are mostly studied in neuroscience research and may be useful for drug development); (2) peptides from conus snails; (3) peptides and polyamine toxins from spiders and wasps; and (4) peptides from scorpions.

Effects of the blockade of high voltage-activated calcium channels on in vitro pineal melatonin synthesis

The presence of high voltage-activated calcium channels in the rat pineal gland is well known. However, their role in pineal metabolism is not completely understood and is even controversial. Better to understand this matter, we investigated the effects of L-, N- or P/Q-type calcium channel blockers (nifedipine, omega-conotoxin GVIA, omega-agatoxin IVA, respectively) on melatonin content and arylalkylamine-N-acetyltransferase activity of denervated rat pineal glands kept for 48 h in culture and stimulated with norepinephrine. Melatonin was measured by high performance liquid chromatography with electrochemical detection and arylalkylamine-N-acetyltransferase activity was quantified by radiometric assay. Pre-incubation with any of these high voltage-activated calcium channel blockers reduced the melatonin production induced by norepinephrine although arylalkylamine-N-acetyltransferase activity was reduced only by the N-type calcium channel antagonist, omega-conotoxin GVIA. The results indicate that calcium influx through L-, N- or P/Q-type of high voltage-activated calcium channels is necessary for the full expression of the metabolic process leading to melatonin synthesis in the rat pineal glands. However, the mechanisms involved in this process are different for the L- or P/Q- and N-type calcium channels.

Effects of theparatemnus elongatus pseudoscorpion venom in the uptake and binding of theL-glutamate and GABA from rat cerebral cortex

L-Glu is the most important and widespread excitatory neurotransmitter of the vertebrates. Four types of receptors for L-glu have been described. This neurotransmitter modulates several neuronal processes, and its dysfunction causes chronic and acute diseases. L-Glu action is terminated by five distinct transporters. Antagonists for these receptors and modulators of these transporters have anticonvulsant and neuroprotective potentials, as observed with the acylpoliamines and peptides isolated from spiders, solitary and social wasp venoms. On the other hand, the major inhibitory neurotransmitter in mammalian nervous tissue is the GABA. Drugs that enhance GABA neurotransmission comprise effective approaches to protecting the brain against neuronal injury. Is this study, we demonstrate for the first time the inhibition of the [3H]L-glu binding to its specific sites in synaptosomal membranes from rat cerebral cortex, produced by 0.027 U of Paratemnus elongatus venom (EC50). The venom of P. elongatus changes Km and Vmax into the high affinity uptake of the L-glu and decreases Km and Vmax into the parameters of the GABA uptake from rat synaptosomes. This leads us to speculate on the possible presence of selective and specific compounds in this venom that act in L-glu and GABA dynamics, and therefore, that can serve as tools and new drug models for understanding these neurotransmissions.

Characterization of β-leptinotarsin-h and the effects of calcium flux antagonists on its activity

beta-Leptinotarsin-h, purified from the hemolymph of the beetle Leptinotarsa haldemani, is a potent ( approximately 1 nM) neuroactive protein that rapidly (few seconds) stimulates Ca(2+) influx and neurotransmitter release. Our goals were to further characterize beta-leptinotarsin-h and to test the hypothesis that it stimulates Ca(2+) influx through presynaptic Ca(2+) channels. Analysis of partial amino acid sequences revealed that beta-leptinotarsin-h is a unique protein with significant similarity to only one other protein, the juvenile hormone esterase of Leptinotarsa decemlineata, commonly known as the Colorado potato beetle. We have examined the effect of beta-leptinotarsin-h on Ca(2+) current, Ca(2+) uptake, Ca(2+) levels, and neurotransmitter release in synaptosomes, cell lines, and neuronal systems. We found that its preferred site of action appears to be mammalian presynaptic nerve terminals. We tested antagonists of Ca(2+) flux for their effects on beta-leptinotarsin-h-stimulated Ca(2+) uptake in rat brain synaptosomes. The non-selective Ca(2+) channel blockers flunarizine, Ni(2+), ruthenium red, high-concentration thapsigargin, and SKF 96365 inhibited beta-leptinotarsin-h's activity, but none of the tested selective blockers of voltage-operated Ca(2+) channels (omega-agatoxin IVA, omega-conotoxin GVIA, omega-conotoxin MVIIC, nicardipine, nifedipine, SNX-482) was inhibitory. Selective inhibitors of ligand-operated, store-operated, and transduction-operated channels were also not inhibitory. beta-Leptinotarsin-h did not stimulate Na(+) uptake, ruling out Na(+) channels and many non-selective cation channels as targets. We conclude that beta-leptinotarsin-h stimulated Ca(2+) uptake through presynaptic Ca(2+) channels; which channel is yet to be determined. beta-Leptinotarsin-h may prove to be a useful tool with which to investigate calcium channels and calcium flux.

The biological activity in mammals and insects of the nucleosidic fraction from the spider Parawixia bistriata

Previous work has shown that the crude venom of Parawixia bistriata induces convulsive seizures in rats after intracerebroventricular injection. In this work, the isolation of a bioactive fraction with ultraviolet absorption characteristics of nucleic acid and trace protein or amino acid content is described. NMR analysis demonstrated that the major component of the active fraction is the nucleoside inosine. An analogue of this component (inosine 5'-monophosphate) induced a delayed paralysis effect in termites.

Spider and wasp neurotoxins: pharmacological and biochemical aspects

Venoms from several arthropods are recognized as useful sources of bioactive substances, such as peptides, acylpolyamines, and alkaloids, which show a wide range of pharmacological effects on synaptic transmission. In this work, we summarize and compile several biochemical and pharmacological aspects related to spider and wasp neurotoxins. Their inhibitory and stimulatory actions on ion channels, receptors, and transporters involved in mammalian and insect neurotransmission are considered.

A new sea anemone peptide, APETx2, inhibits ASIC3, a major acid-sensitive channel in sensory neurons

From a systematic screening of animal venoms, we isolated a new toxin (APETx2) from the sea anemone Anthopleura elegantissima, which inhibits ASIC3 homomeric channels and ASIC3-containing heteromeric channels both in heterologous expression systems and in primary cultures of rat sensory neurons. APETx2 is a 42 amino-acid peptide crosslinked by three disulfide bridges, with a structural organization similar to that of other sea anemone toxins that inhibit voltage-sensitive Na+ and K+ channels. APETx2 reversibly inhibits rat ASIC3 (IC50=63 nM), without any effect on ASIC1a, ASIC1b, and ASIC2a. APETx2 directly inhibits the ASIC3 channel by acting at its external side, and it does not modify the channel unitary conductance. APETx2 also inhibits heteromeric ASIC2b+3 current (IC50=117 nM), while it has less affinity for ASIC1b+3 (IC50=0.9 microM), ASIC1a+3 (IC50=2 microM), and no effect on the ASIC2a+3 current. The ASIC3-like current in primary cultured sensory neurons is partly and reversibly inhibited by APETx2 with an IC50 of 216 nM, probably due to the mixed inhibitions of various co-expressed ASIC3-containing channels.

Ca2+Channels and Synaptic Transmission at the Adult, Neonatal, and P/Q-Type Deficient Neuromuscular Junction

Different types of voltage-activated Ca(2+) channels have been established based on their molecular structure and pharmacological and biophysical properties. One of them, the P/Q-type, is the main channel involved in nerve-evoked neurotransmitter release at neuromuscular junctions and the immunological target in Eaton-Lambert Syndrome. At adult neuromuscular junctions, L- and N-type Ca(2+) channels become involved in transmitter release only under certain experimental or pathological conditions. In contrast, at neonatal rat neuromuscular junctions, nerve-evoked synaptic transmission depends jointly on both N- and P/Q-type channels. Synaptic transmission at neuromuscular junctions of the ataxic P/Q-type Ca(2+) channel knockout mice is also dependent on two different types of channels, N- and R-type. At both neonatal and P/Q knockout junctions, the K(+)-evoked increase in miniature endplate potential frequency was not affected by N-type channel blockers, but strongly reduced by both P/Q- and R-type channel blockers. These differences could be accounted for by a differential location of the channels at the release site, being either P/Q- or R-type Ca(2+) channels located closer to the release site than N-type Ca(2+) channels. Thus, Ca(2+) channels may be recruited to mediate neurotransmitter release where P/Q-type channels seem to be the most suited type of Ca(2+) channel to mediate exocytosis at neuromuscular junctions.

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.

Nifedipine-mediated mobilization of intracellular calcium stores increases spontaneous neurotransmitter release at neonatal rat motor nerve terminals

The modulation of spontaneous release of acetylcholine by specific Ca2+ channel blockers was studied at neonatal rat neuromuscular junction. During early postnatal periods (0-4 days), blockers of N- and P/Q-type Ca2+ channels did not affect miniature endplate potential (MEPP) frequency. Unexpectedly, treatment with the L-type Ca2+ channel antagonist nifedipine, although not when treated with isradipine, nitrendipine, or calciseptine, resulted in strong increase in MEPP frequency. The potentiation effect of nifedipine was dose-dependent with a 56-fold maximum effect with 15 microM. The effect decreased during the first two postnatal weeks and disappeared by the third. The effect of nifedipine was not dependent on extracellular Ca2+ and was not altered by the presence of other Ca2+ channel blockers. In contrast, it was abolished by depleting intracellular Ca2+ stores with 2 microM thapsigargin and was partially inhibited by 10 microM ryanodine. In conclusion, we report a new ryanodine receptor-mediated effect of nifedipine on neonatal neuromuscular junction that may indicate the developmental expression of a specific receptor channel that interacts with intracellular Ca2+ stores. This effect of nifedipine should also be considered when using this drug as either a therapeutic or a research tool.

Ontogeny of voltage-sensitive calcium channel alpha(1A) and alpha(1E) subunit expression and synaptic function in rat central nervous system

Immunohistochemical expression in the neocortex, hippocampus and cerebellum of the alpha(1A) or alpha(1E) subunit of the voltage-sensitive Ca(2+) channel was examined in Long-Evans hooded rats on gestational day 18 and postnatal days 1, 4, 7, 10, 14, 21, 90, 360 and 720. On gestational day 18 and postnatal day 1, alpha(1A) immunoreactivity was more dense in the neocortex and hippocampus than the cerebellum. By postnatal day 7, levels of alpha(1A) immunoreactivity increased dramatically in the cerebellum, while in neocortex, alpha(1A) immunoreactivity became more sparse, which approached the more diffuse pattern of cellular staining in the mature brain. Expression of alpha(1E) in the neocortex, hippocampus and cerebellum was much less dense than alpha(1A) between gestational day 18 and postnatal day 4. There was also significant alpha(1E) immunoreactivity in the mossy fibers of the hippocampus and in dendrites of Purkinje cells of the cerebellum. Depolarization-dependent 45Ca(2+) influx was examined in rat brain synaptosomes on postnatal days 4, 7, 10, 14, 21 and >60. In neocortical and hippocampal synaptosomes, 45Ca(2+) influx increased steadily with age and reached adult levels by postnatal day 10. In cerebellar synaptosomes, 45Ca(2+) influx was constant across all ages, except for a spike in activity which was observed on postnatal day 21. In neocortical and hippocampal synaptosomes, 100 nM omega-conotoxin MVIIC significantly inhibited 45Ca(2+) influx on postnatal day 10 and 14, respectively, or after. In cerebellar synaptosomes, influx was inhibited by omega-conotoxin MVIIC only on postnatal day 10 or prior. On postnatal day 7, 45Ca(2+) influx was not inhibited in neocortical and hippocampal synaptosomes by a combination of 10 microM nifedipine, 1 microM omega-conotoxin GVIA and 1 microM omega-conotoxin MVIIC, suggesting that an 'insensitive' flux predominates at this age. Overall, the results suggest that expression of voltage-sensitive Ca(2+) channels during development is dynamic and is important in central nervous system development.

Adenosine inhibits N-type calcium channels at the rat neuromuscular junction

1. In earlier studies, it has been reported that under in vitro conditions transmitter release at the rat neuromuscular junction is normally suppressed due to the effect of adenosine release from the isolated tissue. In the present study we wanted to determine whether this action may involve the inhibition of calcium influx through adenosine-sensitive calcium channels. 2. In order to test this hypothesis, we examined the role of N-type calcium channels in regulating nerve-evoked transmitter release by using the N-type calcium channel-specific blocker omega-conotoxin GVIA (CTX). In order to control the inhibitory action of adenosine, we also used the adenosine A1 receptor antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX). We tested the effect of blocking N-type calcium channels with CTX in the presence and absence of DPCPX. We examined the effects of these drugs on quantal transmitter release in the transected preparation of the phrenic nerve-hemidiaphragm of the rat using intracellular recording techniques. 3. At 10 nmol/L, CTX alone had no effect on nerve-evoked transmitter release; however, in the presence of 0.1 micro mol/L DPCPX, CTX significantly depressed nerve-evoked transmitter release. 4. These data support the view that adenosine inhibits nerve-evoked transmitter release by inhibiting N-type calcium channels on nerve terminals.

Tamapin, a venom peptide from the Indian red scorpion (Mesobuthus tamulus) that targets small conductance Ca2+-activated K+ channels and afterhyperpolarization currents in central neurons

The biophysical properties of small conductance Ca(2+)-activated K(+) (SK) channels are well suited to underlie afterhyperpolarizations (AHPs) shaping the firing patterns of a conspicuous number of central and peripheral neurons. We have identified a new scorpion toxin (tamapin) that binds to SK channels with high affinity and inhibits SK channel-mediated currents in pyramidal neurons of the hippocampus as well as in cell lines expressing distinct SK channel subunits. This toxin distinguished between the SK channels underlying the apamin-sensitive I(AHP) and the Ca(2+)-activated K(+) channels mediating the slow I(AHP) (sI(AHP)) in hippocampal neurons. Compared with related scorpion toxins, tamapin displayed a unique, remarkable selectivity for SK2 versus SK1 ( approximately 1750-fold) and SK3 ( approximately 70-fold) channels and is the most potent SK2 channel blocker characterized so far (IC(50) for SK2 channels = 24 pm). Tamapin will facilitate the characterization of the subunit composition of native SK channels and help determine their involvement in electrical and biochemical signaling.

Differential Ca2+-dependence of transmitter release mediated by P/Q- and N-type calcium channels at neonatal rat neuromuscular junctions

N- and P/Q-type voltage dependent calcium channels (VDCCs) mediate transmitter release at neonatal rat neuromuscular junction (NMJ). Thus the neonatal NMJ allows an examination of the coupling of different subtypes of VDCCs to the release process at a single synapse. We studied calcium dependence of transmitter release mediated by each channel by blocking with omega-conotoxin GVIA the N-type channel or with omega-agatoxin IVA the P/Q-type channel while changing the extracellular calcium concentration ([Ca2+]o). Transmitter release mediated by P/Q-type VDCCs showed steeper calcium dependence than N-type mediated release (average slope 3.6 +/- 0.09 vs. 2.6 +/- 0.03, respectively). Loading the nerve terminals with 10 microm BAPTA-AM in the extracellular solution reduced transmitter release and occluded the blocking effect of omega-conotoxin GVIA (blockade -2 +/- 9%) without affecting the action of omega-agatoxin IVA (blockade 85 +/- 4%). Both VDCC blockers were able to reduce the amount of facilitation produced by double-pulse stimulation. In these conditions facilitation was restored by increasing [Ca2+]o. The facilitation index (fi) was also reduced by loading nerve terminals with 10 microm BAPTA-AM (fi = 1.2 +/- 0.1). The control fi was 2.5 +/- 0.1. These results show that P/Q-type VDCCs were more efficiently coupled to neurotransmitter release than were N-type VDCCs at the neonatal neuromuscular junction. This difference could be accounted for by a differential location of these channels at the release site. In addition, our results indicate that space-time overlapping of calcium domains was required for facilitation.

Insights from mouse models of absence epilepsy into Ca2+ channel physiology and disease etiology

1. Changes in intracellular Ca2+ ([Ca2+]i) levels provide signals that allow neurons to respond to a host of external stimuli. A major mechanism for elevating [Ca2+]i is the influx of extracellular Ca2+ through voltage-gated channels (Ca(V)) in the plasma membrane. Malfunction in Ca(V) due to mutations in genes encoding channel proteins are increasingly being implicated in causing disease conditions, termed channelopathies. 2. Seven spontaneous mutations with cerebellar ataxia and generalized absence epilepsy have been identified in mice (tottering, leaner, rolling Nagoya, rocker, lethargic, ducky, and stargazer), and these overlapping phenotypes are directly related to mutations in genes encoding the four separate subunits that together form the multimeric neuronal Ca(V) complex. 3. The discovery and systematic analysis of these animal models is helping to clarify how different mutations affect channel function and how altered channel function produces disease.

Pharmacology and biochemistry of spider venoms

Spider venoms represent an incredible source of biologically active substances which selectively target a variety of vital physiological functions in both insects and mammals. Many toxins isolated from spider venoms have been invaluable in helping to determine the role and diversity of neuronal ion channels and the process of exocytosis. In addition, there is enormous potential for the use of insect specific toxins from animal sources in agriculture. For these reasons, the past 15-20 years has seen a dramatic increase in studies on the venoms of many animals, particularly scorpions and spiders. This review covers the pharmacological and biochemical activities of spider venoms and the nature of the active components. In particular, it focuses on the wide variety of ion channel toxins, novel non-neurotoxic peptide toxins, enzymes and low molecular weight compounds that have been isolated. It also discusses the intraspecific sex differences in given species of spiders.

Analysis of whole-cell currents by patch clamp of guinea-pig myenteric neurones in intact ganglia

Whole-cell patch-clamp recordings taken from guinea-pig duodenal myenteric neurones within intact ganglia were used to determine the properties of S and AH neurones. Major currents that determine the states of AH neurones were identified and quantified. S neurones had resting potentials of -47 +/- 6 mV and input resistances (R(in)) of 713 +/- 49 MOmega at voltages ranging from -90 to -40 mV. At more negative levels, activation of a time-independent, caesium-sensitive, inward-rectifier current (I(Kir)) decreased R(in) to 103 +/- 10 MOmega. AH neurones had resting potentials of -57 +/- 4 mV and R(in) was 502 +/- 27 MOmega. R(in) fell to 194 +/- 16 MOmega upon hyperpolarization. This decrease was attributable mainly to the activation of a cationic h current, I(h), and to I(Kir). Resting potential and R(in) exhibited a low sensitivity to changes in [K(+)](o) in both AH and S neurones. This indicates that both cells have a low background K(+) permeability. The cationic current, I(h), contributed about 20 % to the resting conductance of AH neurones. It had a half-activation voltage of -72 +/- 2 mV, and a voltage sensitivity of 8.2 +/- 0.7 mV per e-fold change. I(h) has relatively fast, voltage-dependent kinetics, with on and off time constants in the range of 50-350 ms. AH neurones had a previously undescribed, low threshold, slowly inactivating, sodium-dependent current that was poorly sensitive to TTX. In AH neurones, the post-action-potential slow hyperpolarizing current, I(AHP), displayed large variation from cell to cell. I(AHP) appeared to be highly Ca(2+) sensitive, since its activation with either membrane depolarization or caffeine (1 mM) was not prevented by perfusing the cell with 10 mM BAPTA. We determined the identity of the Ca(2+) channels linked to I(AHP). Action potentials of AH neurones that were elongated by TEA (10 mM) were similarly shortened and I(AHP) was suppressed with each of the three omega-conotoxins GVIA, MVIIA and MVIIC (0.3-0.5 microM), but not with omega-agatoxin IVA (0.2 microM). There was no additivity between the effects of the three conotoxins, which indicates the presence of N- but not of P/Q-type Ca(2+) channels. A residual Ca(2+) current, resistant to all toxins, but blocked by 0.5 mM Cd(2+), could not generate I(AHP). This patch-clamp study, performed on intact ganglia, demonstrates that the AH neurones of the guinea-pig duodenum are under the control of four major currents, I(AHP), I(h), an N-type Ca(2+) current and a slowly inactivating Na(+) current.

Contribution of L-type Ca(2+) channels to evoked transmitter release in cultured Xenopus nerve-muscle synapses

1. Simultaneous pre- and postsynaptic patch recordings were obtained from the varicosity synapses formed by Xenopus motoneurons on muscle cells in embryonic cultures, in order to elucidate the contribution of N- and L-type Ca(2+) channels to the varicosity Ca(2+) current (I(Ca)) and evoked transmitter release. 2. Although N-type channels are predominant in the varicosities and generally thought to be responsible for all evoked release, in most synapses a fraction of I(Ca) and release could be reversibly blocked by the L-type channel antagonist nifedipine, and enhanced by the agonist Bay K8644. Up to 50 % (mean, 21 %) of the I(Ca) evoked by a voltage clamp waveform mimicking a normal presynaptic action potential (APWF) is composed of L-type current. 3. Surprisingly, the nifedipine-sensitive (L) channels activated more rapidly (time-constant, 0.46 ms at +30 mV) than the nifedipine-insensitive (N) channels (time constant, 1.42 ms). Thus the L-type current would play a disproportionate role in the I(Ca) linked to a normal action potential. 4. The relationship between I(Ca) and release was the same for nifedipine-sensitive and -resistant components. The N- and L-components of I(Ca) are thus equally potent in evoking release. This may represent an immature stage before N-type channels become predominant. 5. Replacing Ca(2+) in the medium with Ba(2+) strongly enhanced the L-type component, suggesting that L-type channels may be inactivated at Ca(2+) levels close to those at rest. 6. We speculate that populations of L-type channels in different parts of the neuron may be recruited or inactivated by fluctuations of the cytosolic Ca(2+) concentration within the physiological range.

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.

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.

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.

Calcium channels involved in the inhibition of acetylcholine release by presynaptic muscarinic receptors in rat striatum

1. The mechanism of the inhibitory action of presynaptic muscarinic receptors on the release of acetylcholine from striatal cholinergic neurons is not known. We investigated how the electrically stimulated release of [3H]-acetylcholine from superfused rat striatal slices and its inhibition by carbachol are affected by specific inhibitors of voltage-operated calcium channels of the L-type (nifedipine), N-type (omega-conotoxin GVIA) and P/Q-type (omega-agatoxin IVA). 2. The evoked release of [3H]-acetylcholine was not diminished by nifedipine but was lowered by omega-conotoxin GVIA and by omega-agatoxin IVA, indicating that both the N- and the P/Q-type (but not the L-type) channels are involved in the release. The N-type channels were responsible for approximately two thirds of the release. The release was >97% blocked when both omega-toxins acted together. 3. The inhibition of [3H]-acetylcholine release by carbachol was not substantially affected by the blockade of the L- or P/Q-type channels. It was diminished but not eliminated by the blockade of the N-type channels. 4. In experiments on slices in which cholinesterases had been inhibited by paraoxon, inhibition of [3H]-acetylcholine release by endogenous acetylcholine accumulating in the tissue could be demonstrated by the enhancement of the release after the addition of atropine. The inhibition was higher in slices with functional N-type than with functional P/Q-type channels. 5. We conclude that both the N- and the P/Q-type calcium channels contribute to the stimulation-evoked release of acetylcholine in rat striatum, that the quantitative contribution of the N-type channels is higher, and that the inhibitory muscarinic receptors are more closely coupled with the N-type than with the P/Q-type calcium channels.

Astrocytes regulate the developmental appearance of GABAergic and glutamatergic postsynaptic currents in cultured embryonic rat spinal neurons

The effects of astrocytes on the emergence of synaptic transients and excitable membrane properties in cultured, embryonic, rat ventral spinal neurons were studied with electrical and optical recording techniques. Neurons on astrocytes had significantly longer neurites and an accelerated rate of growth in surface membrane during the initial 24 h in culture compared to neurons on poly-D-lysine (PDL). GABAergic (GABA, gamma-aminobutyric acid) and glutamatergic transients appeared spontaneously in co-cultured neurons by 24 h. GABAergic quanta did not appear in neurons on PDL until 4 days in culture, and glutamatergic transients did not emerge until 7 days in culture. Astrocyte-conditioned medium (ACM) partially mimicked the effects of direct astrocytic contact. GABAergic transients appeared by 2 days, and glutamatergic signals by 4 days in neurons on PDL exposed to ACM. All of the spontaneous, synaptic-like transients were eliminated by tetrodotoxin or Ca2+o-free saline, implicating voltage-dependent cation channels in their generation. Astrocytes immediately and significantly increased the density of voltage-dependent Na+ currents compared to neurons on PDL, but by the end of 24 h, Na+ current densities were identical. Electrophysiological and optical recording revealed comparable densities of high-voltage-activated (HVA) Ca2+ currents on both co-cultured neurons and neurons on PDL throughout the first week. However, neurons on astrocytes had significantly greater contributions of P/Q-type currents and lesser contributions of L-type currents beginning at 24 h and continuing for 7 days. The contribution of N-type current was significantly more in co-cultured neurons only at 24 h. Thus, in vitro, astrocytes help to differentiate specific excitable membrane properties in spinal neurons, along with GABAergic and glutamatergic forms of synaptic transmission.

The pharmacology of ion channels: with particular reference to voltage-gated Ca2+ channels

Ion channels are molecular machines that serve as principal integrating and regulatory devices for the control of cellular excitability. They are also major targets for drug action. The basic aspects of ion channel structure and pharmacological control are reviewed and illustrated with specific reference to a major class of therapeutic agents and molecular tools--the clinically available Ca2+ channel antagonists.

N-type voltage-dependent calcium channels mediate the nicotinic enhancement of GABA release in chick brain

The role of voltage-dependent calcium channels (VDCCs) in the nicotinic acetylcholine receptor (nAChR)-mediated enhancement of spontaneous GABAergic inhibitory postsynaptic currents (IPSCs) was investigated in chick brain slices. Whole cell recordings of neurons in the lateral spiriform (SpL) and ventral lateral geniculate (LGNv) nuclei showed that cadmium chloride (CdCl2) blocked the carbachol-induced increase of spontaneous GABAergic IPSCs, indicating that VDCCs might be involved. To conclusively show a role for VDCCs, the presynaptic effect of carbachol on SpL and LGNv neurons was examined in the presence of selective blockers of VDCC subtypes. omega-Conotoxin GVIA, a selective antagonist of N-type channels, significantly reduced the nAChR-mediated enhancement of gamma-aminobutyric acid (GABA) release in the SpL by 78% compared with control responses. Nifedipine, an L-type channel blocker, and omega-Agatoxin-TK, a P/Q-type channel blocker, did not inhibit the enhancement of GABAergic IPSCs. In the LGNv, omega-Conotoxin GVIA also significantly reduced the nAChR-mediated enhancement of GABA release by 71% from control values. Although omega-Agatoxin-TK did not block the nicotinic enhancement, L-type channel blockers showed complex effects on the nAChR-mediated enhancement. These results indicate that the nAChR-mediated enhancement of spontaneous GABAergic IPSCs requires activation of N-type channels in both the SpL and LGNv.

Calcium currents and calcium signaling in rod bipolar cells of rat retinal slices

Combined electrophysiological and imaging techniques were used to study calcium currents (ICa) and their sites of origin at rod bipolar cells in rat retinal slices. We report here for the first time the successful whole-cell patch-clamp recording from presynaptic boutons that were compared with somatic recordings. TTX-resistant inward currents were elicited in response to depolarization. The kinetic and pharmacological properties of ICa were very similar for recordings obtained from the soma and the presynaptic terminals. ICa activated maximally between -30 and -20 mV was enhanced by Bay K 8644 and was blocked by isradipine and nifedipine. Peak amplitude and time to peak were -31.3 +/- 1.2 pA and 3.2 +/- 0.2 msec with somatic recordings (n = 54), whereas the corresponding values were -31.6 +/- 6.1 pA and 3.2 +/- 0.7 msec in recordings obtained directly from terminals (n = 6). ICa showed little inactivation during sustained depolarizations. No T-type ICa was observed with depolarizations from -90 mV. Concomitant with Ca2+ entry, depolarization induced the appearance of transient outward currents that resembled IPSCs and were blocked by GABA and glycine receptor antagonists, suggesting that they arise from activation of amacrine feedback synapses. Upon depolarization, intracellular Ca2+ ([Ca2+]i) rises were restricted to the presynaptic terminals with no somatic or axonal changes and were linearly dependent on pulse duration when using a low-affinity Ca2+ indicator. In cone bipolar cells, ICa inactivated markedly, and [Ca2+]i rises occurred in the axon, as well as in the presynaptic terminals.