Tityustoxin-mediated Na+ influx is more efficient than KCl depolarisation in promoting Ca(2+)-dependent glutamate release from synaptosomes

Analysis of neuroimmune interactions by an in vitro coculture approach

Nerve fibers innervate every organ of the body and are involved in monitoring changes of the external and internal environment. Innervation directly controls a variety of physiological responses in an adaptive manner. Today, many lines of research indicate that also the immunological response is influenced by the nervous system and that nerve and immune cells directly interact through intercellular signal transduction by cytokines, neurotransmitters, and neuropeptides. For instance, mast cells are often found in close proximity of nerve fibers containing substance P and calcitonin gene-related peptide, two widely studied sensory neuropeptides, in a variety of tissues. To investigate the molecular mechanism of the direct functional interplay between nerve and immune cells, we have studied their communication using an in vitro coculture system and confocal microscopy. Here, we introduce methods for the in vitro coculture of nerve and immune cells and the imaging analysis of cellular activation, and discuss soluble mediators and adhesion molecules involved in the neuroimmune interaction. Improvement of our understanding of neuropeptide functions on these issues would lead to new therapeutic modalities for diseases based on neuroimmune interaction such as neurogenic inflammation, intestinal bowel diseases, asthma, and autoimmune disorders.

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 voltage-gated Na+ channel toxins from Tityus serrulatus venom on rat arterial blood pressure and plasma catecholamines

Scorpion toxins interact with ionic channels of excitable cells, leading to a massive release of neurotransmitters. Voltage-gated Na+ channel toxins are mainly responsible for the toxic effects of scorpion envenoming and can be classified into two classes: alpha- and beta-neurotoxins. TsTX-V and TsTX-I from Tityus serrulatus venom (TsV) are, respectively, examples of these toxins. In this work, we compared the effects of these toxins on mean arterial pressure (MAP) and catecholamines release in rats. Toxins were isolated by ion exchange chromatography (TsTX-I) followed by RP-HPLC (TsTX-V). All experiments were performed on conscious unrestrained rats previously catheterised. The toxins (15 and 30 microg/kg) and TsV (50 and 100 microg/kg) were injected intravenously. MAP was continuously monitored through femoral catheter. Epinephrine (E) and norepinephrine (NE) levels were determined by RP-HPLC with electrochemical detection, at 10 min before and 2.5, 30 and 90 min after treatments. Maximal pressor effects were observed at 2.5-3.5 min. TsV induced intense long lasting increase in MAP, as did TsTX-I. TsTX-V showed the lowest pressor effects. TsV showed the highest effects on catecholamines release, followed by TsTX-I and TsTX-V with maximal effect at 2.5 min, followed by a gradual reduction, however remaining higher than controls. Although both toxins act on Na+ channels, TsTX-I displayed significant and more intense effects on catecholamines release and blood pressure than TsTX-V. It seems that the toxicity of TsTX-V is not related only with its ability to release catecholamines, indicating that other neurotransmitters, may be involved in its toxicity.

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.

Modulation of Na+-channels by neurotoxins produces different effects on [3H]ACh release with mobilization of distinct Ca2+-channels

1. Voltage-gated Na+ channels are responsible for initiation and conduction of action potentials. The arrival of an action potential at nerve terminal increases intracellular Na+ and Ca2+ concentrations. Calcium entry into neurons through voltage-dependent calcium channels is associated with a variety of intracellular processes. Scorpion neurotoxins have been used as tools to investigate mechanisms involved in neurotransmitter release. Tityustoxin (TsTX) is an alpha-type toxin that delays Na+-channel inactivation. Toxin-gamma (TiTX-gamma) is a beta-type toxin that induces Na+-channel activation at resting potentials. 2. In the present work, we describe the effects of both toxins on [3H]acetylcholine ([3H]ACh) release from rat cerebrocortical synaptosomes, in the presence or absence of the calcium channels blockers: omega-conotoxin-GVIA (omega-CgTx), 1 microM; omega-agatoxin-IVA (omega-Aga), 30 nM; omega-conotoxin-MVIIC (omega-MVIIC), 1 microM; or verapamil, 1 microM. 3. TsTX evokes [3H]ACh release in a concentration-dependent manner with a gradual increase up to saturation at concentrations of 500 nM. However, release of ACh evoked by TiTX-gamma was not linear regarding the toxin concentration. The [3H]-ACh release evoked by TsTX or TiTX-gamma was partially inhibited by omega-CgTx or omega-Aga, and blocked with omega-MVIIC. Verapamil (1 microM) had no effect. Tetrodotoxin blocked [3H]ACh release evoked by both toxins. 4. These results show that different actions on Na+-channels produce different effects on [3H]ACh release with involvement of distinct presynaptic Ca2+-channels, which supports the idea that sodium channels may modulate neurotransmitter release.

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

The modulation of neurotransmitter release by calcium channels is well established, yet, sodium channels were regarded mainly as charge carriers. Many lines of evidence suggest a more fine-tuning role played by sodium channels. Using rat cerebrocortical isolated nerve endings (synaptosomes) and two toxins that have separate sites of action on sodium channels and provoke distinct changes in channel kinetics, we were able to show that depending on the rate of increase in channel conductance, the outcome in terms of neurotransmitter release and calcium channel types coupled to that event are different. Mainly, our study focused on veratridine, an alkaloid from lilaceous plants that binds to sodium channel toxin site 2, and tityustoxin, a toxin purified from the venom of the Brazilian yellow scorpion Tityus serrulatus that binds to site 3. Veratridine induces a slower increase in intrasynaptosomal sodium and calcium concentrations, slower depolarization, delayed exocytosis and a slower and predominantly calcium-independent glutamate release, when compared to tityustoxin.Thus, we have used these two toxins to investigate the events that start with sodium entry and culminate with the release of glutamate in isolated nerve endings (synaptosomes) from rat cerebral cortex. With that in mind we measured intrasynaptosomal free sodium concentration [Na(+)](i), intrasynaptosomal free calcium concentration [Ca(2+)](i), membrane potential, exocytosis and glutamate release using fluorescent probes.

Spider neurotoxins block the beta scorpion toxin-induced calcium uptake in rat brain cortical synaptosomes

In this paper we describe the effects of the beta scorpion toxin Tityus gamma (TiTX gamma) and spider neurotoxins Tx3-3 and Tx3-4 in the (45)Ca(2+) uptake in synaptosomes. The TiTX gamma-stimulatory effect on (45)Ca(2+) uptake in synaptosomes was inhibited omega-Conotoxin MVIIC (omega-CgTX MVIIC) (0.1 microM) and omega-Agatoxin IVA (0.1 microM) by 70% and 41%, respectively. omega-CgTX MVIIC (1.0 microM) almost completely blocked the TiTX gamma-induced (45)Ca(2+) uptake in synaptosomes. Verapamil (1.0 microM) and omega-Conotoxin GVIA (0.1 microM) had no effect in the scorpion toxin-induced (45)Ca(2+) influx. The spider neurotoxins Tx3-3 and Tx3-4 inhibited the TiTX gamma-induced calcium uptake with an IC(50) of 10.0 and 30.0 nM, respectively. It is suggested that spider neurotoxins Tx3-3 and Tx3-4 blocking effect in the TiTX gamma-induced calcium uptake involves P/Q-type calcium channels.

Calcium channels coupled to depolarization-evoked glutamate release in the myenteric plexus of guinea-pig ileum

Glutamate is the major excitatory neurotransmitter in the CNS. The recent characterization of glutamate as a neurotransmitter in the enteric nervous system opened a new line of investigation concerning the role of glutamate in that system. The present study aimed to further characterize the enteric glutamate release and the calcium channels coupled to it. For this study the myenteric plexus-longitudinal muscle of guinea-pig ileum was stimulated with potassium chloride or with electrical pulses. The released glutamate was detected by spectrofluorimetry. Laser scanning confocal microscopy was used for analysis of immunolabeled enteric tissue for co-localization studies of calcium channels (N- and P/Q-type) and glutamate transporters (EAAC1). Here we report the effects of known Ca(2+)-channel blockers on glutamate release evoked by KCl-depolarization or electrical stimulation in the myenteric plexus. We find that N-type Ca(2+) channels control a major portion of evoked glutamate release from this system, with a very small contribution from L-type Ca(2+) channels. Moreover, alpha(1A)-like (P-type Ca(2+) channel) and alpha(1B)-like (N-type Ca(2+ )channel) immunoreactivity co-localized with glutamate transporters in the myenteric plexus. In addition, KCl-evoked or electrically stimulated glutamate release was sensitive to omega-agatoxin IVA, in a frequency-dependent manner, suggesting that P-type channels are also coupled to the release of glutamate. We, thus, conclude that both N-type and P-type Ca(2+) channels control most of the evoked glutamate release from the enteric nervous system, as also occurs in some parts of the CNS.

Pharmacological evidence that neuropeptides mediate part of the actions of scorpion venom on the guinea pig ileum

Severe human scorpion envenoming is characterised by instability of several physiological systems and death. These manifestations are explained by the ability of the venom toxins to activate sodium channels in nerve terminals with the subsequent release of neurotransmitters, specially acetylcholine and noradrenaline. However, there is evidence to suggest that other neurotransmitters are also released. We now have sought evidence for a role of the substance P receptor, the tachykinin NK1 receptor, in mediating part of the contractile actions of Tityus serrulatus venom on the isolated guinea pig ileum. Scorpion venom induced a significant elevation of baseline tension with frequent and periodic superimposed contractions on the elevated baseline. Pretreatment with atropine partially blocked the elevation in baseline and in the number of superimposed contractions. These responses were also partially inhibited by the tachykinin NK1 receptor antagonist, CP96,345 (the dihydrochloride salt of (2S,3S)-cis-2-(diphenylmethyl)-N-((2-methoxyphenyl)methyl)-1-az abicycol[2.2.2]octan-3-amine), but not by its inactive enantiomer, CP96,344 (the 2R-3R enantiomer of CP96,345). Pretreatment with the combination of atropine and CP96,345 completely inhibited the effects of the venom. Moreover, pretreatment with the combined drugs abolished the effects of toxin gamma, a toxin purified from the venom. Finally, another tachykinin NK1 receptor antagonist, RP67,580 ((3aR, 7ar)-7,7-diphenyl-2-[1-imino-2-(2-methoxy-phenyl)ethyl]perhydro isoindol-4-one), significantly inhibited the venom-induced contractions. These results demonstrate an important role for NK1 receptors in mediating part of the contractile effects of the venom on guinea pig ileum. The release of neuropeptides may play an important role in the systemic manifestations of severe envenoming.

Alpha- and beta-scorpion toxins evoke glutamate release from rat cortical synaptosomes with different effects on [Na+]i and [Ca2+]i

Scorpion toxins have long been used as tools in the investigation of neurotransmitter release mechanisms. We have used rat cortical synaptosomes to study the effects of a beta-type scorpion toxin (TiTX-gamma) on the release of glutamate and on the concentrations of free sodium and calcium ions inside the synaptosomes. The effects are compared with those of an alpha-type scorpion toxin (TsTX), on which there have been more studies. TsTX increased overall internal sodium and calcium ion concentrations and glutamate release in an incremental, dose dependent manner. TiTX-gamma similarly evoked glutamate release in an incremental, dose dependent manner. However, TiTX-gamma caused little increase in the overall internal sodium and calcium ion concentrations at low doses that evoked a significant release of glutamate and a maximal increase in these ions at somewhat higher doses. The results suggest that TiTX-gamma preferentially binds sodium channels close to the active zones for glutamate release and indicates that modifications of the activation or inactivation of the Na+-channel can lead to very different changes in the cytosolic concentrations of free Na+and Ca2+, with consequences for neurotransmission. This provides an interesting perspective concerning modulation of neurotransmitter release via pharmacological manipulation of Na+-channel properties, that may lead to a better comprehension of its physiological and pathological roles.

Tityustoxin-induced release of ATP from rat brain cortical synaptosomes

Tityustoxin, a scorpion toxin that alters the Na+ channel activity, induces release of ATP from rat brain cortical synaptosomes. The effect of tityustoxin is dependent on its concentration and incubation time. Continuously or cumulative release of ATP evoked by tityustoxin was calcium-dependent and interestingly only partially inhibited by tetrodotoxin. We suggest that tityustoxin mainly releases ATP from the vesicular pool but other pools may also be involved.

Action of New World scorpion venom and its neurotoxins in secretion

New World scorpion venom contains protein toxins specific for ion channels in the plasmalemma of excitable cells. The effects were examined of whole venoms from Tityus serrulatus, T. bahiensis and T. stigmurus, and some purified toxins in isolated nerve endings (synaptosomes) and pancreatic acinar cells. Both systems initiated exocytosis in a dose-dependent response to the venom or its bioactive protein toxins. Actions differed, however, such that pancreatic acinar cells required Ca2+ while cerebrocortical synaptosomes responded by a Ca(2+)-dependent mechanism, except in the case of one toxin, IV-5, that elicited a Ca(2+)-independent response. Membrane depolarization caused by scorpion venom toxins was measured via radioisotopic discharge of tetra[3H]phenylphosphonium bromide. The role of protein kinase C in second-messenger coupling in pancreatic acinar cells is favored over ion-exclusive routes characteristic of synaptosomes.

Modulation of Ca(2+)-stimulated glutamate release from synaptosomes by Na+ entry through tetrodotoxin-sensitive channels

Tityustoxin (TsTX), a toxin obtained from the venom of the Brazilian scorpion Tityus serrulatus, stimulates Na+ influx through tetrodotoxin (TTX)-sensitive Na+ channels which, in turn, promotes both Ca(2+)-dependent and Ca(2+)-independent release of glutamate from rat cerebrocortical synaptosomes. The level of Ca(2+)-dependent glutamate release after addition of 0.5 microM TsTX is greater than that produced by a maximally depolarizing concentration of KCl. This effect of TsTX, which is entirely dependent on Na+ entry, suggests that Na+ has a role in modulating Ca2+ entry and glutamate release that is not simply related to membrane depolarization. In order to investigate possible modulatory role(s) of Na+ on Ca(2+)-dependent glutamate release, we compared the effects of TsTX with those of KCl and the Na+ ionophore gramicidin D. When used alone, 100 nM gramicidin D produced a larger increase in intrasynaptosomal free Na+ than did 0.5 microM TsTX, and a similar rise in intrasynaptosomal free Ca2+, but was much less effective in promoting glutamate release. Even the combination of membrane depolarization (by 33 mM KCl) and elevation of intrasynaptosomal free Na+ (by 100 nM gramicidin) was still less effective than TsTX at causing Ca(2+)-dependent glutamate release. These data suggest that localized Na+ entry, through TTX-sensitive Na+ channels, exerts a modulatory role on Ca(2+)-dependent glutamate release from nerve endings in the cerebral cortex.

The use of gadolinium to investigate the relationship between Ca2+ influx and glutamate release in rat cerebrocortical synaptosomes

Gadolinium chloride is a potent blocker of voltage-sensitive Ca2+ channels. When added to fura-2 loaded rat cortical synaptosomes 1 min before depolarization with 33 mM KCl, it causes a dose-dependent inhibition of the resulting rise in intrasynaptosomal free Ca2+ with an IC50 of approximately 10 microM. The effect of GdCl3 on intrasynaptosomal free Ca2+ is not accompanied by an equivalent effect on Ca(2+)-dependent glutamate release. In the presence of 100 microM GdCl3, 33 mM KCl does not produce a detectable change in the fura-2 signal but Ca(2+)-dependent glutamate release is only reduced by around 12%. Using BaCl2, which is less effectively buffered in synaptosomes than Ca2+, we have shown that there is residual KCl-stimulated divalent cation entry into synaptosomes in the presence of 100 microM GdCl3. These data, combined with those from other laboratories, strengthen the argument for localized Ca2+ entry through Ca2+ channels linked to neurotransmitter release from synaptosomes and add to the evidence that the channels may exhibit considerable neurotransmitter specificity.