Investigation of the modulation of glutamate release by sodium channels using neurotoxins

Mast Cells Modulate the Immune Response and Redox Status of the Gastrointestinal Tract in Induced Venom Pathogenesis

The pathogenesis of Androctonus autralis hector (Aah) scorpion venom involved cellular and molecular mechanisms resulting in multi-organ dysfunction. However, little is reported about the effects of venom on the gastrointestinal axis. Mast cells (MCs) are known to play a crucial role in modulating immune response of the gut. This study aims to investigate the involvement of this cell type in venom-induced gastric and intestinal disorders in a time course (3 and 24h). The obtained results revealed that Aah scorpion venom induced inflammatory cell infiltration as shown by the increase of the myeloperoxidase and eosinophil peroxidase activities. Overexpression of the c-kit receptor (CD117) severely imbalanced the redox status with depletion of antioxidant systemic accompanied by gastrointestinal tissue damage. Moreover, an increased level of lactate dehydrogenase in the serum was correlated with tissue injuries. Pharmacological inhibition of MCs targeting tyrosine kinase (TK) reduces the generation of reactive oxygen species and normalizes catalase, and gluthation S-transferase activities to their physiological levels. In addition, histopathological alterations were restored after pretreatment with c-kit receptor inhibitor associated with a considerable reduction of MC density. Interestingly, obtained results indicate that MCs might be involved in gastric modulation and intestinal inflammation through c-kit signaling following sub-cutaneous Aah venom injection.

Protective effects of Astragaloside IV against oxidative injury and apoptosis in cultured astrocytes by regulating Nrf2/JNK signaling

Ischemic stroke is a worldwide complex brain disease that results in numerous disabilities and deaths. It leads to the deprivation of oxygen and glucose, which causes energy failure and neuronal death. The activation of astrocytes contributes to neuronal damage or repair after brain ischemia/reperfusion, although astrocytes get little attention as potential drug targets. This study investigated the protective effects of Astragaloside IV (AS-IV) on oxygen glucose deprivation/reoxygenation (OGD/R)-induced damage in rat primary cultured astrocytes and the underlying molecular mechanism. The results showed that compared with the control group, astrocytes under OGD/R exposure significantly decreased cell viability and increased the number of apoptotic cells, whereas AS-IV evidently protected the astrocytes against OGD/R-induced cell damage. In addition, low and medium concentrations of AS-IV can promote the increase of intracellular superoxide dismutase (SOD) level, as well as restored the morphological changes caused by OGD/R exposure. Supplementation with AS-IV after OGD/R exposure promoted the expression of oxidation and apoptosis indexes and further study demonstrated that AS-IV inhibited CXCR4 receptor and downregulated the activation of p-JNK/JNK pathway, which suppressed the expression of Bax/Bcl-2, and finally uprising Nrf2/Keap1 signaling. In conclusion, these findings revealed that AS-IV protected against OGD/R-induced astrocytes through inhibiting oxidative stress and apoptotic pathways.

Activation of voltage-gated sodium channels by BmK NT1 augments NMDA receptor function through Src family kinase signaling pathway in primary cerebellar granule cell cultures

Voltage-gated sodium channels (VGSCs) are responsible for the generation and propagation of action potentials in excitable cells and are the molecular targets of an array of neurotoxins. BmK NT1, an α-scorpion toxin obtained from the scorpion Buthus martensii Karsch (BmK), produces neurotoxicity that is associated with extracellular Ca²⁺ influx through Na⁺-Ca²⁺ exchangers, N-methyl-d-aspartic acid (NMDA) receptors, and L-type Ca²⁺ channels in cultured cerebellar granule cells (CGCs). In the present study, we demonstrated that BmK NT1 triggered concentration-dependent release of excitatory neurotransmitters, glutamate and aspartate; both effects were eliminated by VGSC blocker, tetrodotoxin. More importantly, we demonstrated that a threshold concentration of BmK NT1 that produced marginal Ca²⁺ influx and neuronal death augmented glutamate-induced Ca²⁺ elevation and neuronal death in CGCs. BmK NT1-augmented glutamate-induced Ca²⁺ influx and neuronal death were suppressed by tetrodotoxin and MK-801 suggesting that the augmentation was through activation of VGSCs and NMDA receptors. Consistently, BmK NT1 also enhanced NMDA-induced Ca²⁺ influx. Further mechanistic investigations demonstrated that BmK NT1 increased the expression level of NMDA receptors on the plasma membrane and increased the phosphorylation level of NR2B at Tyr1472. Src family kinase inhibitor, 1-tert-butyl-3-(4-chlorophenyl)pyrazolo[3,4-d]pyrimidin-4-yl]amine (PP2), but not the inactive analogue, 4-amino-1-phenylpyrazolo[3,4-d]pyrimidine (PP3), eliminated BmK NT1-triggered NR2B phosphorylation, NMDA receptor trafficking, as well as BmK NT1-augmented NMDA Ca²⁺ response and neuronal death. Considered together, these data demonstrated that both presynaptic (excitatory amino acid release) and postsynaptic mechanisms (augmentation of NMDA receptor function) are critical for VGSC activation-induced neurotoxicity in primary CGC cultures.

Effect of Tityus serrulatus scorpion venom on isolated jejunum: A very useful tool to study the interaction between neurons in the enteric nervous system

Scorpion envenomation is a public health problem in tropical and subtropical areas. In Brazil, Tityus serrulatus is the biggest cause of accidents with venomous animals. Tityus serrulatus venom causes symptoms related to a great activation of the autonomic system attributed to a massive release of sympathetic and parasympathetic mediators. This effect is attributed to the presence of toxins acting in Na⁺ and K⁺ ion channels, leading to an increase in cell excitability. Although gastrointestinal symptoms, like diarrhoea and sialorrhea, is observed in moderate to severe cases, little attention is given in clinical reports. Gastrointestinal motility is controlled by the enteric nervous system which is composed of a wide variety of interconnected neurons that are influenced by the sympathetic and parasympathetic nervous systems. Thus, this work aimed to characterize the effects of Tityus serrulatus venom on sympathetic and parasympathetic neurotransmission of rat jejunum, as well as to investigate possibles effects on other neurons of the enteric nervous system. To this, we verify the effects of Tityus serrulatus venom on the contractility of isolated rat jejunum through organ-bath experiments. We observed that venom can induce both contraction and relaxation. The contraction was partially inhibited by atropine (1 μM) and by suramin (0.1 mM) through tetrodotoxin-resistant and sensitive mechanisms. The relaxation was completely inhibited by 3 μM propranolol and partially inhibited by 1 μM phentolamine. Suramin induced a slowing of relaxation curve. Tetrodotoxin completely inhibits the relaxation induced by Tityus serrulatus venom, but the contraction curves were only partially reduced in their initial portion. The final part of the curve was largely enhanced by Tetrodotoxin. Atropine blocks almost completely the contraction curve in the presence of Tetrodotoxin. These results indicate that Tityus serrulatus venom induces the release of both excitatory (predominantly acetylcholine) and inhibitory (mainly noradrenaline) neurotransmitters. The effects of Tityus serrulatus venom on organ contractility was quite complex and seem to derive from a diffuse and nonspecific release of mediators from autonomic and enteric nervous systems. Further investigation of venom action and its isolated toxins can reveal important aspects to deepen our knowledge about the enteric nervous system transmission and the interaction between excitatory and inhibitory mediators as well as the physiological role of Na⁺ and K⁺ ion channels in gut motility.

Activation of sodium channels by α-scorpion toxin, BmK NT1, produced neurotoxicity in cerebellar granule cells: an association with intracellular Ca2+ overloading

Voltage-gated sodium channels (VGSCs) are responsible for the action potential generation in excitable cells including neurons and involved in many physiological and pathological processes. Scorpion toxins are invaluable tools to explore the structure and function of ion channels. BmK NT1, a scorpion toxin from Buthus martensii Karsch, stimulates sodium influx in cerebellar granule cells (CGCs). In this study, we characterized the mode of action of BmK NT1 on the VGSCs and explored the cellular response in CGC cultures. BmK NT1 delayed the fast inactivation of VGSCs, increased the Na⁺ currents, and shifted the steady-state activation and inactivation to more hyperpolarized membrane potential, which was similar to the mode of action of α-scorpion toxins. BmK NT1 stimulated neuron death (EC₅₀ = 0.68 µM) and produced massive intracellular Ca²⁺ overloading (EC₅₀ = 0.98 µM). TTX abrogated these responses, suggesting that both responses were subsequent to the activation of VGSCs. The Ca²⁺ response of BmK NT1 was primary through extracellular Ca²⁺ influx since reducing the extracellular Ca²⁺ concentration suppressed the Ca²⁺ response. Further pharmacological evaluation demonstrated that BmK NT1-induced Ca²⁺ influx and neurotoxicity were partially blocked either by MK-801, an NMDA receptor blocker, or by KB-R7943, an inhibitor of Na⁺/Ca²⁺ exchangers. Nifedipine, an L-type Ca²⁺ channel inhibitor, slightly suppressed both Ca²⁺ response and neurotoxicity. A combination of these three inhibitors abrogated both responses. Considered together, these data ambiguously demonstrated that activation of VGSCs by an α-scorpion toxin was sufficient to produce neurotoxicity which was associated with intracellular Ca²⁺ overloading through both NMDA receptor- and Na⁺/Ca²⁺ exchanger-mediated Ca²⁺ influx.

Neuroprotective effect of Buyang Huanwu decoction against focal cerebral ischemia/reperfusion injury in rats--time window and mechanism

ETHNOPHARMACOLOGICAL RELEVANCE

Buyang Huanwu Decoction, a traditional Chinese medicine, consists of different herbal medicines, and has been traditionally used for centuries to treat paralysis and stroke. However, its optimal therapeutic time window and the mechanism are still unclear.

AIM OF THE STUDY

This study was designed to explore the therapeutic time window and mechanism of Buyang Huanwu Decoction on transient focal cerebral ischemia/reperfusion injury.

MATERIALS AND METHODS

Middle cerebral artery occlusion was conducted in male Sprague-Dawley rats, and 40g/kg of Buyang Huanwu Decoction was intragastrically infused at different time points, and the same dose was infused every 24h for 3 days. The level of glutamate in cerebrospinal fluid and the expression of metabotropic glutamate receptor-1 RNA in striatum were detected before, during, and after ischemia/reperfusion. Neurological deficit scores and brain infarction volumes were measured at 72h after reperfusion.

RESULT

Cerebral ischemia/reperfusion resulted in significant neurological deficit and extensive cerebral infarct volume, associated with a large amount of glutamate in cerebrospinal fluid and elevation of metabotropic glutamate receptor-1 RNA expression. Buyang Huanwu Decoction significantly suppressed the release of glutamate, and reduced the expression of metabotropic glutamate receptor-1 RNA. The neurological defect score and infarction volume were significantly improved by administration of Buyang Huanwu Decoction, when compared with the Ischemia group.

CONCLUSIONS

Administration of Buyang Huanwu Decoction, within 4h of post-transient focal stroke, reduced significant cerebral ischemia/reperfusion damage. The neuroprotective mechanism of Buyang Huanwu Decoction is, in part, associated with the down-regulation of metabotropic glutamate receptor-1 RNA and inhibition of glutamate release resulting from cerebral ischemia.

Regional differences in the effects of isoflurane on neurotransmitter release

Stimulus evoked neurotransmitter release requires that Na(+) channel-dependent nerve terminal depolarization be transduced into synaptic vesicle exocytosis. Inhaled anesthetics block presynaptic Na(+) channels and selectively inhibit glutamate over GABA release from isolated nerve terminals, indicating mechanistic differences between excitatory and inhibitory transmitter release. We compared the effects of isoflurane on depolarization-evoked [(3)H]glutamate and [(14)C]GABA release from isolated nerve terminals prepared from four regions of rat CNS evoked by 4-aminopyridine (4AP), veratridine (VTD), or elevated K(+). These mechanistically distinct secretegogues distinguished between Na(+) channel- and/or Ca(2+) channel-mediated presynaptic effects. Isoflurane completely inhibited total 4AP-evoked glutamate release (IC(50) = 0.42 ± 0.03 mM) more potently than GABA release (IC(50) = 0.56 ± 0.02 mM) from cerebral cortex (1.3-fold greater potency), hippocampus and striatum, but inhibited glutamate and GABA release from spinal cord terminals equipotently. Na(+) channel-specific VTD-evoked glutamate release from cortex was also significantly more sensitive to inhibition by isoflurane than was GABA release. Na(+) channel-independent K(+)-evoked release was insensitive to isoflurane at clinical concentrations in all four regions, consistent with a target upstream of Ca(2+) entry. Isoflurane inhibited Na(+) channel-mediated (tetrodotoxin-sensitive) 4AP-evoked glutamate release (IC(50) = 0.30 ± 0.03 mM) more potently than GABA release (IC(50) = 0.67 ± 0.04 mM) from cortex (2.2-fold greater potency). The magnitude of inhibition of Na(+) channel-mediated 4AP-evoked release by a single clinical concentration of isoflurane (0.35 mM) varied by region and transmitter: Inhibition of glutamate release from spinal cord was greater than from the three brain regions and greater than GABA release for each CNS region. These findings indicate that isoflurane selectively inhibits glutamate release compared to GABA release via Na(+) channel-mediated transduction in the four CNS regions tested, and that differences in presynaptic Na(+) channel involvement determine differences in anesthetic pharmacology.

Intrahippocampal injection of TsTX-I, a beta-scorpion toxin, causes alterations in electroencephalographic recording and behavior in rats

AIMS

TsTX-I scorpion toxin, also known as γ-toxin, is a β-toxin which binds to site 4 of the sodium channel, shifting its activation potential. There are few studies about its pharmacological action in the central nervous system. The objective of this work was to determine the electroencephalographic, behavioral and histopathological effects of intrahippocampal injection of TsTX-I.

MAIN METHODS

Rats were anesthetized and fitted with cannulae for injection into the hippocampus and with electrodes for cerebral recording. The animals were treated with Ringer solution, some doses of TsTX-I, DMSO 0.1% or veratridine. Behavioral and electrographic recordings were observed for 4 hours after the injection. After 7 days, the rats were perfused, and their brains removed for histological analysis.

KEY FINDINGS

Increasing doses of the toxin evoked epileptic-like discharges, wet dog shakes, and in some cases hind limb paralysis and intense respiratory difficulty followed by death. The histopathological analysis demonstrated no cell loss. Animals injected with veratridine developed epileptiform activity in the electrographic recording and neuronal loss.

SIGNIFICANCE

The results suggest that TsTX-I toxin may be responsible, at least in part, for the epileptic and behavioral effects observed with the crude venom, and although veratridine and TsTX-I act on Na-channel, the differences between them are remarkable, demonstrating that toxins can have different functional effects depending on the site of action in the channel. Thus, animal neurotoxins are often highly selective and may be useful for the identification of the sequence of events underlying neurotransmission.

Treadmill pre-training suppresses the release of glutamate resulting from cerebral ischemia in rats

This study was designed to investigate the neuroprotective effect of treadmill pre-training against the over-release of glutamate resulting from cerebral ischemia. Sprague-Dawley rats underwent 2 weeks of treadmill run-training before cerebral ischemia was performed by middle cerebral artery occlusion. The level of glutamate in brain extracellular fluid was detected before, during and after ischemia/reperfusion. The expression of metabotropic glutamate receptor-1 (mGluR1) mRNA in striatum was examined after ischemia for 80 min and reperfusion for 240 min. Neurological defect score and brain infarction volumes were measured. The treadmill pre-training significantly suppressed the release of glutamate, and reduced the expression of mGluR1 mRNA at 59% (P < 0.01) and 62% (P < 0.05), respectively, as compared with the ischemia group. The neurological defect score and infarction volume were significantly improved by 75% (P < 0.01) and 74% (P < 0.01), respectively, in the pre-training group, as compared to the ischemia group. Treadmill pre-training has a significant neuroprotective function against ischemia/reperfusion injury, by suppressing glutamate release resulting from cerebral ischemia, and this effect may be mediated by downregulation of mGluR1.

Diazepam and pentobarbital protect against scorpion venom toxin-induced epilepsy

We have characterized earlier the long-term behavioural, electroencephalographic and histopatologic features after a single TsTx microinjection, consisting of a neuropeptide isolated from the Tityus serrulatus scorpion venom, into the hippocampus of rats. TsTx was able to induce status epilepticus (SE) and developed later epilepsy. The present study was designed to investigate the outcomes of diazepam plus pentobarbital administered at 30 min, 1, 2 or 6h after the beginning of TsTx-induced SE, on the development of spontaneous recurrent motor seizures (SRMSs), mossy fibre sprouting and hippocampal neurodegeneration in rats. The administration of diazepam (DZ)+pentobarbital (PB) 30 min after the beginning of the TsTx-induced SE was able to markedly reduce the frequency of the SRMSs and prevent the development of mossy fibres sprouting and hippocampal lesion. In the other groups the augment of the extent of hipocampal neurodegeneration, the frequency of SRMSs and degree of aberrant mossy fibre sprouting was directly proportional to the time that the animals were subjected to TsTx-induced SE. In conclusion, our results point out that the early blockade of the TsTx-induced SE with diazepam plus pentobarbital, was effective treatment against later epilepsy development. The effectiveness of this treatment depends on the time that the animals were subjected to the SE. Furthermore, the TsTx model could be a useful tool to study antiepileptogenic drugs in chronic epileptic animals, neuronal degeneration, as well as for the mechanisms underlying epilepsy.

Rat epileptic seizures evoked by BmK αIV and its possible mechanisms involved in sodium channels

This study showed that rat unilateral intracerebroventricular injection of BmK alphaIV, a sodium channel modulator derived from scorpion Buthus martensi Karsch, induced clusters of spikes, epileptic discharges and convulsion-related behavioral changes. BmK alphaIV potently promoted the release of endogenous glutamate from rat cerebrocortical synaptosomes. In vitro examination of the effect of BmK alphaIV on intrasynaptosomal free calcium concentration [Ca(2+)](i) and sodium concentration [Na(+)](i) revealed that BmK alphaIV-evoked glutamate release from synaptosomes was associated with an increase in Ca(2+) and Na(+) influx. Moreover, BmK alphaIV-mediated glutamate release and ion influx was completely blocked by tetrodotoxin, a blocker of sodium channel. Together, these results suggest that the induction of BmK alphaIV-evoked epileptic seizures may be involved in the modulation of BmK alphaIV on tetrodotoxin-sensitive sodium channels located on the nerve terminal, which subsequently enhances the Ca(2+) influx to cause an increase of glutamate release. These findings may provide some insight regarding the mechanism of neuronal action of BmK alphaIV in the central nervous system for understanding epileptogenesis involved in sodium channels.

Effects of Tityus serrulatus scorpion venom and its toxin TsTX-V on neurotransmitter uptake in vitro

Scorpion neurotoxins targeting the Na(v) channel can be classified into two classes: alpha- and beta-neurotoxins and are reported as highly active in mammalian brain. In this work, we evaluate the effects of Tityus serrulatus venom (Ts venom) and its alpha-neurotoxin TsTX-V on gamma-aminobutyric acid (GABA), dopamine (DA) and glutamate (Glu) uptake in isolated rat brain synaptosomes. TsTX-V was isolated from Ts venom by ion exchange chromatography followed by reverse-phase (C18) high-performance liquid chromatography. Neither Ts venom nor TsTX-V was able to affect (3)H-Glu uptake. On the other hand, Ts venom (0.13 microg/mg) significantly inhibited both (3)H-GABA and (3)H-DA uptake ( approximately 50%). TsTX-V showed IC(50) values of 9.37 microM and 22.2 microM for the inhibition of (3)H-GABA and (3)H-DA uptake, respectively. These effects were abolished by pre-treatment with tetrodotoxin (TTX, 1 microM), indicating the involvement of voltage-gated Na(+) channels in this process. In the absence of Ca(2+), and at low Ts venom concentrations, the reduction of (3)H-GABA uptake was not as marked as in the presence of Ca(2+). TsTX-V did not reduce (3)H-GABA uptake in COS-7 cells expressing the GABA transporters GAT-1 and GAT-3, suggesting that this toxin indirectly reduces the transport. The reduced (3)H-GABA uptake by synaptosomes might be due to rapid cell depolarization as revealed by confocal microscopy of C6 glioma cells. Thus, TsTX-V causes a reduction of (3)H-GABA and (3)H-DA uptake in a Ca(2+)-dependent manner, not directly affecting GABA transporters, but, in consequence of depolarization, involving voltage-gated Na(+) channels.

Simultaneous detection of L-glutamate and nitric oxide from adherently growing cells at known distance using disk shaped dual electrodes

An ex vivo system for simultaneous detection of nitric oxide (NO) and L-glutamate using integrated dual 250 microm platinum disk electrodes modified individually with suitable sensing chemistries has been developed. One of the sensors was coated with an electrocatalytic layer of Ni tetrasulfonate phthalocyanine tetrasodium salt (Ni-TSPc) covered by second layer of Nafion, which stabilises on the one hand the primary oxidation product NO(+) and prevents interferences from negatively charged compounds such as NO(2)(-). For glutamate determination, the second electrode was modified with a crosslinked redox hydrogel consisting of Os complex modified poly(vinylimidazol), glutamate oxidase and peroxidase. A manual x-y-z micromanipulator on top of an inverted optical microscope was used to position the dual electrode sensor at a defined distance of 5 microm from a cell population under visual control. C6 glioma cells were stimulated simultaneously with bradykinin or VEGF to release NO while KCl was used to invoke glutamate release. For evaluation of the glutamate sensors, in some experiments HN10 cells were used. To investigate the sensitivity and reliability of the system, several drugs were applied to the cells, e.g. Ca(2+)-channel inhibitors for testing Ca(2+)-dependence of the release of NO and glutamate, rotenone for inducing oxidative stress and glutamate antagonists for analysing glutamate release. With these drugs the NO and glutamate release was modulated in a similar way then expected from previously described systems or even in-vivo measurements. We therefore conclude that our system is suitable to analyse stress-induced mechanisms in cell lines.

Neurochemical Characterization of a Neuroprotective Compound fromParawixia bistriataSpider Venom That Inhibits Synaptosomal Uptake of GABA and Glycine

The major contribution of this work is the isolation of a neuroprotective compound referred to as 2-amino-5-ureidopentanamide (FrPbAII) (M(r) = 174) from Parawixia bistriata spider venom and an investigation of its mode of action. FrPbAII inhibits synaptosomal GABA uptake in a dose-dependent manner and probably does not act on Na(+), K(+), and Ca(2+) channels, GABA(B) receptors, or gamma-aminobutyrate:alpha-ketoglutarate aminotransferase enzyme; therefore, it is not directly dependent on these structures for its action. Direct increase of GABA release and reverse transport are also ruled out as mechanisms of FrPbAII activities as well as unspecific actions on pore membrane formation. Moreover, FrPbAII is selective for GABA and glycine transporters, having slight or no effect on monoamines or glutamate transporters. According to our experimental glaucoma data in rat retina, FrPbAII is able to cross the blood-retina barrier and promote effective protection of retinal layers submitted to ischemic conditions. These studies are of relevance by providing a better understanding of neurochemical mechanisms involved in brain function and for possible development of new neuropharmacological and therapeutic tools.

Dopamine release evoked by beta scorpion toxin, tityus gamma, in prefrontal cortical slices is mediated by intracellular calcium stores

1. We have investigated the effect of tityus gamma (TiTX gamma) scorpion toxin on the release of [3H]dopamine in rat brain prefrontal cortical slices. The stimulatory effect of TiTX gamma on the release of [3H]dopamine was dose/time-dependent with an EC50 of 0.01 microM. 2. Tetrodotoxin blocked the TiTX gamma-induced release of [3H]dopamine, indicating the dependency for Na+ channels. 3. EGTA had no effect on the TiTX gamma-induced release of [3H]dopamine, indicating the process is independent of extracellular calcium. Release of [3H]dopamine evoked by TiTX gamma was inhibited by 57% by BAPTA, a chelator of intracellular calcium. 4. Xestospongin and 2-APB, putative blockers of IP3-sensitive release of intracellular calcium stores, caused an equal and significant inhibition of 24% of the TiTX gamma-induced release of [3H]dopamine, while the slight inhibition evoked by dantrolene, a putative blocker of ryanodine-sensitive calcium store was not significant. 5. Nomifensine and ascorbic acid, blockers of dopamine transporter (DAT), caused an inhibition of 27 and 29%, respectively, on the toxin-induced release of [3H]dopamine suggesting that most of the TiTX gamma-induced release of dopamine is not due to the reversal of Na+ gradient. 6. In conclusion the majority of the TiTX gamma-induced release of [3H]dopamine is exocytotic and mobilizes calcium from the intracellular IP3-sensitive calcium stores.

Effects of toxic environmental contaminants on voltage-gated calcium channel function: from past to present

Voltage-gated Ca2+ channels are targets of the number of naturally occurring toxins, therapeutic agents as well as environmental toxicants. Because of similarities of their chemical structure to Ca2+ in terms of hydrated ionic radius, electron orbital configuration, or other chemical properties, polyvalent cations from aluminum to zinc variously interact with multiple types of voltage-gated Ca2+ channels. These nonphysiological metals have been used to study the structure and function of the Ca2+ channel, especially its permeability characteristics. Two nonphysiological cations, Pb2+ and Hg2+, as well as their organic derivatives, are environmental neurotoxicants which are highly potent Ca2+ channel blockers. These metals also apparently gain intracellular access in part by permeating through Ca2+ channels. In this review the history of Ca2+ channel block produced by Pb2+ and Hg2+ as well as other nonphysiological cations is traced. In particular the characteristics of Ca2+ channel block induced by these environmental neurotoxic metals and the consequences of this action for neuronal function are discussed.

Dihydropiridines mechanism of action in striatal isolated nerve endings: comparison with omega-agatoxin IVA

The relative contribution of Ca2+ and Na+ channels to the mechanism underlying the action of the dihydropiridines (DHPs), nimodipine, nitrendipine and nifedipine was investigated in rat striatum synaptosomes. The rise in internal Ca2+ (Ca(i), as determined with fura-2) induced by high K+ was unchanged by the DHPs, which like tetrodotoxin (TTX) inhibited both the rise in internal Na+ (Na(i), as determined with the Na+ selective indicator dye, SBFI) and the rise in Ca(i) induced by veratridine. Nimodipine and nitrendipine were much more potent than nifedipine. Oppositely to TTX and to the DHPs, the P/Q type Ca2+ channel blocker, omega-agatoxin IVA did not inhibit the rise in Ca(i) induced by veratridine, but inhibited the rise in Ca(i) induced by high K+. Veratridine-evoked release of dopamine, GABA, Glu, and Asp (detected by HPLC) was inhibited by nimodipine, nitrendipine, and TTX, while high K+-evoked release was unchanged by the DHPs or TTX. It is concluded that the reduction in presynaptic Na+ channel permeability might contribute to the cerebral effects of DHPs.

Characterization of the actions of AvTx 7 isolated fromAgelaia vicina (Hymenoptera: Vespidae) wasp venom on synaptosomal glutamate uptake and release

It has previously been shown that the denatured crude extract of Agelaia vicina wasp venom inhibits glutamate and GABA uptake in rat cerebral cortex synaptosomes. To identify the components responsible for these effects, the neurotoxin AvTx 7 (molecular weight of 1210 Da) was isolated from A. vicina venom and its effects on glutamate neurotransmission investigated. AvTx 7 inhibits glutamate uptake in a dose-dependent and uncompetitive manner. AvTx 7 was found to stimulate the glutamate release in the presence of calcium and sodium channel blockers, suggesting that its action is not mediated through these channels. AvTx 7 potentiates glutamate release in the presence of K(+) channel blockers tetraethylammonium and 4-aminopyridine, indicating that the toxin may act through these drugs-sensible K(+) channels. We suggest that AvTx 7 can be a valuable tool to enhance our understanding of K(+) channels' involvement in the release of glutamate.

Correction of image instability in confocal microscopy using image realignment. Effects on the analysis of intracellular calcium

Using confocal microscopy, we have examined the increases in [Ca(2+)](i) evoked by sodium channel toxins in cells labelled with the fluorescent dye INDO-1. We describe a new image analysis method that improves the detection of region-specific, toxin-induced patterns of change of intracellular calcium. This method is based on correction of global image motion followed by calculation of the strength of correlation between calcium changes in "seed" or reference pixels chosen to represent different regions of cells and those in other regions of the image. When the selected "seed" pixel was chosen to be in either varicosities or neurites, correlations were detected in the same regions of other cells as well as in the soma, indicating specific but spatially distinct patterns of behaviour. Control images (without changes in [Ca(2+)](i)) did not reveal significant interpixel correlations. The ability to recognize correlated patterns of calcium change in different regions of cells was greatly improved by correction for global motion.

TSII toxin isolated from Tityus serrulatus scorpion venom: behavioral, electroencephalographic, and histopathologic studies

We have reported earlier that intrahippocampal administration of the C-pool from Tityus serrulatus scorpion venom induces convulsions in rats. Here we report the effects of seven toxins isolated from the C-pool. The strongest effects were seen after toxin 5C, which was sequenced and identified as TSII, a beta-type toxin that affects Na+ channel activation. Unilateral injection of TSII in the rat hippocampus (1.7 microg/microl) induced clusters of spikes and epileptic discharges of mainly moderate intensity, convulsion-related behavioral changes (wet dog shakes, staring, masticatory jaw movements, facial automatisms, orofacial movements, intense sniffing, blinking, and forelimb clonus with rearing and falling) and a massive neuronal loss of pyramidal cells in the ipsilateral CA1, CA3, and CA4 subfields and of granulate cells of the ipsilateral dentate gyrus. Toxins C3, C4, and C6 induced weaker changes in the EEG and behavioral changes and failed to induce cell death, and toxins C1, C2, and C7 had no effects. The similarities in the effects of TsTx, a alpha-type toxin that affects Na+ channel, suggest that the loss of modulation of activation of the sodium channel caused by TSII increases glutamate release, leading to long-lasting increases in intracellular Ca2+ and cell death.

Sodium channel toxins and neurotransmitter release

Voltage-dependent sodium channels (VDSC) are an important class of ion channels in excitable cells, where they are responsible for the generation and conduction of action potential. In addition, the release of neurotransmitters from nerve terminals is influenced by sodium channel activity. The function of VDSC is subject to modulation by various neurotoxins, such as scorpion toxins, which have long been used as tools in the investigation of neurotransmitter release. This opens an interesting perspective concerning modulation of neurotransmission via pharmacological manipulation of sodium channel properties, which can lead to a better understanding of their physiological and pathological roles. Here we briefly review the studies of neurotoxins acting on sodium channels, focusing primarily on the view of the mechanisms of neurotransmitter release.