Agelenopsis aperta venom and FTX, a purified toxin, inhibit acetylcholine release in Torpedo synaptosomes

Calcium-dependent acetylcholine release from Xenopus oocytes: simultaneous ionic currents and acetylcholine release recordings

The fusion of synaptic vesicles with presynaptic membranes is controlled by a complex network of protein-protein and protein-lipid interactions. SNAP-25, syntaxin and synaptobrevin (SNARE complex) are thought to participate in the formation of the core of the membrane fusion machine but the molecular basis of SNARE interactions is not completely understood. Thus, it would be interesting to design experiments to test those relationships in a new model. Xenopus laevis oocytes are valuable tools for studying the molecular structure and function of ionic channels and neurotransmitter receptors. Here we show that SNARE proteins are present in native Xenopus oocytes and that those oocytes injected with acetylcholine and presynaptic plasma membranes extracted from the electric organ of Torpedo marmorata assume some of the functions of a cholinergic nerve terminal. Neurotransmitter release and macroscopic currents were recorded and analysed simultaneously in a single oocyte electrically depolarized: acetylcholine release was detected using a chemiluminiscent method and calcium entry was measured by exploiting the endogenous Ca2+-activated chloride current of the oocyte with a two-electrode voltage-clamp system. Neurotransmitter release was calcium- and voltage-dependent and partially reduced in the presence of several calcium channel blockers. Clostridial neurotoxins, both holotoxin and injected light-chain forms, also inhibited acetylcholine release. We also studied the role of the SNARE complex in synaptic transmission and membrane currents by using monoclonal antibodies against SNAP-25, syntaxin or VAMP/synaptobrevin. The use of antibodies against VAMP/synaptobrevin, SNAP-25 and syntaxin inhibited acetylcholine release, as did clostridial toxins. However, macroscopic currents were only modified either by syntaxin antibody or by Botulinium-C1 neurotoxin. This model constitutes a new approach for understanding the vesicle exocytosis processes.

Chick cerebellar Purkinje cells express omega-conotoxin GVIA-sensitive rather than funnel-web spider toxin-sensitive calcium channels

Voltage-gated calcium channels form a complex family of distinct molecular entities which participate in multiple neuronal functions. In cerebellar Purkinje cells these channels contribute to the characteristic electrophysiological pattern of complex spikes, first described in birds and later in mammals. A specific calcium channel, the P-type channel, has been shown to mediate the majority of the voltage-gated calcium flux in mammalian Purkinje cells. P-type channels play an essential role in synaptic transmission of mammalian cerebellum. It is unclear whether the P-type calcium channel is present in birds. Studies in chick synaptosomal preparations show that the pharmacological profile of calcium channels is complex and suggest a minimal expression of the P-type channel in avian central nervous system. In the present work, we studied voltage-gated calcium channels in dissociated chick cerebellar Purkinje cells to examine the presence of different calcium channel types. Purkinje cells were used because, in mammals, they express predominantly P-type channels and because the morphology of these cells is thought to be phylogenetically conserved. We found that omega-conotoxin GVIA (omega-CgTx GVIA), a specific antagonist of N-type calcium channel, rather than the synthetic funnel-web spider toxin (sFTX), a P-type channel antagonist, blocks the majority of the barium current flowing through calcium channels in chick Purkinje neurons.

Activation of adenosine A1 and A2 receptors differentially affects acetylcholine release from electric organ synaptosomes by modulating calcium channels

Adenosine inhibited the release of acetylcholine (ACh) evoked by high K+ depolarization from synaptosomes isolated from the electric organ of the Japanese electric ray Narke japonica. The adenosine A1 receptor agonist N6-cyclohexyladenosine was an effective inhibitor. Conversely, in the presence of an A1 receptor antagonist, 8-cyclopentyltheophylline, adenosine potentiated the release of ACh. The A2 receptor agonist N6-[2-(3,5-dimethoxyphenyl)-2-(2-methylphenyl)ethyl] adenosine also facilitated the evoked ACh release. Thus, adenosine inhibits the evoked release of ACh via the A1 receptor while it facilitates the release via the A2 receptor. The EC50 for inhibition and facilitation by adenosine was about 1 and 41 microM, respectively. There are three known types of calcium channels (N-, P/Q- and L-type) in synaptosomes. The effects of Ca2+ channel type-specific blockers on the modulation of ACh release by adenosine A1 or A2 receptor activation revealed that inhibition by A1 receptor activation was caused via inhibition of N-type calcium channels and the facilitative effects by A2 receptor activation was mediated by potentiation of P-type calcium channels.

Nitric oxide and peroxynitrite affect differently acetylcholine release, choline acetyltransferase activity, synthesis, and compartmentation of newly formed acetylcholine in Torpedo marmorata synaptosomes

Recent reports proposed that nitric oxide was a modulator of cholinergic transmission. Here, we examined the role of NO on cholinergic metabolism in a model of the peripheral cholinergic nervous synapse: synaptosomes from Torpedo electric organ. The presence of NO synthase was immunodetected in the cell bodies, in the nerve ending area of nerve-electroplate tissue and in the electroplates. Exogenous source of NO was provided from SIN1, a donor of NO and O2-., and an end-derivative peroxynitrite (ONOO-). SIN1 increased calcium-dependent acetylcholine (ACh) release induced by KCl depolarization or a calcium ionophore A23187. The formation of ONOO- was continuously followed by a new chemiluminescent assay. The addition of superoxide dismutase, that decreases the formation of ONOO-, did not impair the stimulation of ACh release, suggesting that NO itself was the main stimulating agent. When the endogenous source of NO was blocked by proadifen, an inhibitor of cytochrome P450 activity of NO synthase, both KCl- and A23187-induced ACh release were abolished; nevertheless, the inhibitor Ng-monomethyl-L-arginine did not modify ACh release when applied in a short time duration of action. Both NO synthase inhibitors reduced the synthesis of ACh from the radioactive precursor acetate and its incorporation into synaptic vesicles as did ONOO- chemically synthesized or formed from SIN1. In addition, choline acetyltransferase activity was strongly inhibited by ONOO- and SIN1 but not by the NO donors SNAP and SNP or, by NO synthase inhibitors. Altogether these results indicate that NO and ONOO modulate presynaptic cholinergic metabolism in the micromolar range, NO (up to 100 microM) being a stimulating agent of ACh release and ONOO- being an inhibitor of ACh synthesis and choline acetyltransferase activity.

Toxins affecting calcium channels in neurons

Calcium enters the cytoplasm mainly via voltage-activated calcium channels (VACC), and this represents a key step in the regulation of a variety of cellular processes. Advances in the fields of molecular biology, pharmacology and electrophysiology have led to the identification of several types of VACC (referred to as T-, N-, L-, P/Q- and R-types). In addition to possessing distinctive structural and functional characteristics, many of these types of calcium channels exhibit differential sensitivities to pharmacological agents. In recent years a large number of toxins, mainly small peptides, have been purified from the venom of predatory marine cone snails and spiders. Many of these toxins have specific actions on ion channels and neurotransmitter receptors, and the toxins have been used as powerful tools in neuroscience research. Some of them (omega-conotoxins, omega-agatoxins) specifically recognize and block certain types of VACC. They have common structural backbones and some been synthesized with identical potency as the natural ones. Natural, synthetic and labeled calcium channel toxins have contributed to the understanding of the diversity of the neuronal calcium channels and their function. In particular, the toxins have been useful in the study of the role of different types of calcium channels on the process of neurotransmitter release. Neuronal calcium channel toxins may develop into powerful tools for diagnosis and treatment of neurological diseases.

Polyamine FTX-3.3 and polyamine amide sFTX-3.3 inhibit presynaptic calcium currents and acetylcholine release at mouse motor nerve terminals

FTX-3.3 is the proposed structure of a calcium-channel blocking toxin that has been isolated from the funnel web spider (Agelenopsis aperta). The effects of FTX-3.3 and one of its analogues, sFTX-3.3, on acetylcholine release, on presynaptic currents at mouse motor nerve terminals and on whole-cell sodium currents in SK.N.SH cells (a human neuroblastoma cell line) have been studied. FTX-3.3 (10-30 microM) and sFTX-3.3 (100-300 microM) reversibly reduced release of acetylcholine by approximately 70-90% and 40-60%, respectively. FTX-3.3 (10 microM) blocked the fast component of presynaptic calcium currents by approximately 60%. sFTX-3.3 (100 microM) reduced the duration of the slow component of presynaptic calcium currents by about 50% of the control and also reduced presynaptic sodium current by approximately 20% of the control. sFTX-3.3 (100 microM) reduced whole-cell sodium current recorded from SK.N.SH cells by approximately 15%, whereas FTX-3.3, even at 200 microM, did not affect this current. Since the only difference in chemical structures of these toxins is that sFTX-3.3 has an amide function which is absent in FTX-3.3, the amide function may be responsible for the reduced potency and selectivity of sFTX-3.3. This study also provides further support for the existence of P-type calcium channels at mouse motor nerve terminals.

Ca(2+)-dependent changes of acetylcholine release and IP3 mass in Torpedo cholinergic synaptosomes

The aim of the present study was to investigate possible changes of inositol 1,4,5-trisphosphate (IP3) mass in Torpedo cholinergic synaptosomes in conditions promoting stimulated acetylcholine (ACh) release. For this purpose, we used a radioreceptor IP3 mass assay and a chemiluminescent method for ACh detection. Torpedo cholinergic synaptosomes have consistent IP3 mass levels under resting conditions. The IP3 mass was neither modified by changes in external Ca2+ nor by a Ca(2+)-free medium containing EGTA. IP3 mass and ACh release, measured in the same conditions and in parallel, were increased by depolarization with high K+ and by the ionophores A-23/87 and gramicidin-D in a manner dependent on external Ca2+ emphasizing that Ca2+ entry, independently of the influx mechanism involved, leads to an IP3 increase. The phospholipase C beta inhibitors U-73122 and U-73343 reduced K(+)-stimulated IP3 levels while K(+)-evoked ACh release was almost completely blocked suggesting an additional effect of these drugs on depolarization-neurotransmitter secretion coupling. The effect reported showing an increase of IP3 by agents that stimulate ACh release may suggest a possible link between IP3 metabolism and the neurotransmitter release mechanism. However, such a link is probably not a direct one as implied by the results obtained with the inhibitors of phospholipase C.

Differential inhibition of Ca2+ channels in mature rat cerebellar Purkinje cells by sFTX-3.3 and FTX-3.3

Synthetic funnel web spider toxin (sFTX-3.3) is a polyamine amide analogue of FTX, a toxin fraction isolated from the venom of the funnel web spider, Agelenopsis aperta, that blocks P-type Ca2+ channels. The structures of these polyamine containing compounds are not identical: sFTX-3.3 contains an amide carbonyl oxygen that is absent from the predicted structure of native FTX. Recently, a compound called FTX-3.3 was synthesized with the structure predicted for native FTX. We have compared the effects of polyamine amide sFTX-3.3 and polyamine FTX-3.3, on Ca2+ channel currents in the soma of mature rat cerebellar Purkinje neurons, in which the predominant Ca2+ channels are defined as P-type. Differential inhibition by sFTX-3.3 and FTX-3.3 revealed three populations of Ca2+ channels. One group, mediating approximately 66% of the current, was blocked by sFTX-3.3 with an IC50 (concentration producing half maximal inhibition) of 33 nM or by FTX-3.3 with an IC50 of 55 pM. A second population (5-25% of the total current) was inhibited by sFTX-3.3 with an IC50 of 33 nM, but was insensitive to FTX-3.3, while a third (10-30%) was blocked by FTX-3.3 with an IC50 of 125 nM and was resistant to sFTX-3.3. These channels also showed distinctive current-voltage relationships. Our results suggest that P-type Ca2+ channels in mature rat cerebellar Purkinje cells may be subdivided according to pharmacological and biophysical properties.

Inhibition of acetylcholine release from presynaptic terminals of skate electric organ by calcium channel antagonists: a detailed pharmacological study

Release of acetylcholine (ACh) from the presynaptic terminals in skate electric organ was tested for its sensitivity to calcium channel antagonists. A pharmacological profile was established by measuring inhibition of K(+)-stimulated release of [3H]ACh from prelabelled tissue slices. Peptide antagonists of N-type (omega-conotoxins GVIA and MVIIA) and P-type (omega-agatoxin-IVA) channels had no effect, whereas both omega-conotoxins MVIIC and SVIB produced concentration-dependent inhibition and could completely block ACh release. omega-Conotoxin GVIA and omega-agatoxin IVA did not attenuate the block by omega-conotoxin MVIIC. The inorganic ions, Cd2+ and Ni2+, also produced a full inhibition of release (Cd2+ > > Ni2+) and Gd3+ a partial one. Drugs targeting L-type channels (diltiazem, nifedipine and verapamil) at low microM concentrations and a synthetic analogue of the polyamine toxin from funnel web spider venom (sFTX) at 1 mM were all non-inhibitory. Inhibition by omega-conotoxins MVIIC (IC50 25 nM) and SVIB (IC50 500 nM) was reversible and modulated by external concentrations of Ca2+. Inhibitory potency was increased by lowering and decreased by elevating external Ca2+. This "antagonistic" effect of Ca2+ was also seen with Cd2+ inhibition. The inhibitory potency of omega-conotoxin MVIIC was unaffected by predepolarisation. End plate potentials generated by release of endogenous ACh in electrically-stimulated slices were also reversibly blocked by Cd2+ and omega-conotoxins MVIIC and SVIB but were unaffected by omega-conotoxin GVIA and omega-agatoxin IVA. It is concluded that ACh release in skate electric organ depends on presynaptic calcium channels which have different pharmacological properties from established sub-types.

Different calcium channels mediate transmitter release evoked by transient or sustained depolarization at mammalian sympathetic ganglia

We have compared the effect of calcium channel blockers on the potassium-evoked release of tritium-labeled acetylcholine and on preganglionic spike-evoked synaptic transmission in the rat superior cervical ganglion. Transmitter release at the nerve terminals is mediated by the influx of calcium through voltage-gated calcium channels. While four types of voltage-gated calcium channels (T, L, N and P) have been identified in neurons, it is not clear which may actually be involved in excitation-secretion coupling. Release of tritiated acetylcholine evoked by sustained depolarization in high (40 mM) extracellular potassium decreased markedly in the absence of calcium or the presence of cadmium. High potassium-evoked release was substantially inhibited by the P-type channel blockers, purified from funnel-web spider toxin, and omega-agatoxin-IVA, and by the N-type channel blocker omega-conotoxin-GVIA, but was unaffected by the L-type channel blocker nitrendipine. In contrast, postganglionic compound action potentials synaptically triggered by preganglionic stimulation were strongly blocked by funnel-web spider toxin and slightly blocked by a high concentration of omega-agatoxin-IVA, but were unaffected by either omega-conotoxin-GVIA, nitrendipine or a low concentration of omega-agatoxin-IVA. Thus, at the superior cervical ganglion, funnel-web spider toxin-sensitive calcium channels play a dominant role in transmitter release evoked by transient, spike-mediated depolarization, but other types of voltage-gated calcium channels in addition to the funnel-web spider toxin-sensitive channel mediate the transmitter release that is evoked by sustained high potassium depolarization.

Presynaptic mechanisms regulating Ca2+ concentration triggering acetylcholine release at an identified neuro-neuronal synapse of Aplysia

We have used an identified cholinergic neuro-neuronal synapse in the buccal ganglion of Aplysia to determine which types of Ca2+ channels are involved in triggering transmitter release. omega-Conotoxin as well as funnel web spider toxin partially reduced acetylcholine release indicating that both N- and P-type Ca2+ channels are involved. Nifedipine-sensitive L-type Ca2+ channels are also present but they are not directly implicated in acetylcholine release. We have identified presynaptic receptors to two peptides. FMRFamide and buccalin and to the neurotransmitter histamine. FMRFamide facilitates acetylcholine release by increasing the presynaptic Ca2+ influx whereas buccalin and histamine have an opposite effect. These neuromodulators control only the influx of Ca2+ through N-type Ca2+ channels since their action on transmitter release can be prevented by omega-conotoxin but not by funnel web spider toxin. FMRFamide and histamine, respectively, increased and decreased Ca2+ influx by shifting in opposite ways the voltage sensitivity to activation of the channels. Buccalin reduced Ca2+ influx by decreasing the number of available channels. 2,5-Diterbutyl 1,4-benzohydroquinone, a blocker of the reticulum Ca2+ pump, increased evoked transmitter release by increasing the intracellular concentration of Ca2+ without affecting the presynaptic Ca2+ influx. It is suggested that a reticulum-like Ca2+ buffer, in close proximity to N- and P-type Ca2+ channels, controls the intracellular concentrations of Ca2+ actually triggering acetylcholine release.

Involvement of P-type Ca2+ channels in the K(+)- and d-fenfluramine-induced [3H]5-HT release from rat hippocampal synaptosomes

The Ca2(+)-dependent [3H]5-HT release induced by depolarization or by 0.5 microM d-fenfluramine in rat hippocampal synaptosomes, was significantly reduced (35-42%) by three different P-type Ca2+ channels blockers (omega-Agatoxin-IVA, 100 nM, funnel-web spider toxin, FTX, 0.05 microliters/ml, and its synthetic analogue, sFTX, 1 mM), indicating the major role of these channels in the Ca2+ influx preceding neurotransmitter release.

N- and P-type Ca2+ channels are involved in acetylcholine release at a neuroneuronal synapse: only the N-type channel is the target of neuromodulators

Cholinergic transmission in an identified neuro-neuronal synapse of the Aplysia buccal ganglion was depressed by application of a partially purified extract of the funnel-web-spider venom (FTx) or of its synthetic analog (sFTx). This specific blocker of voltage-dependent P-type Ca2+ channels did not interfere with the effect of the N-type Ca2+ channel blocker omega-conotoxin, which could further decrease synaptic transmission after a previous application of FTx. Similar results were obtained when the reversal order of application of these two Ca2+ channel blockers was used. Both P- and N-type Ca2+ currents trigger acetylcholine release in the presynaptic neuron. The neuromodulatory effects of FMRF-amide, histamine, and buccalin on transmitter release disappeared after the blockade of the N-type Ca2+ channels but remained still effective in the presence of FTx. These results indicate that only N-type Ca2+ channels appear to be sensitive to the neuromodulators we have identified.

Effects of calcium channel blockers on the kinetics of voltage-dependent changes in synaptosomal calcium concentrations

Synaptosomal preparations from rat cerebral cortex have been used in stopped-flow fluorescence studies to measure rapid changes in intrasynaptosomal calcium concentrations upon depolarization. Synaptosomes were loaded with the fluorescent calcium chelating dye, Fura-2, by incubation with the membrane permeant acetoxymethyl ester derivative. Depolarization by elevated external K+ concentration resulted in a rapid increase in cytoplasmic Ca2+ as measured by a quench in Fura-2 fluorescence when excited at 390 nm. The fluorescence change could be reasonably fit by a single exponential process with an apparent rate of 10-15 s-1 and the magnitude of the response was voltage-dependent, increasing with increasing external K+ over the range of 5-30 mM. The observed quench was blocked by micromolar concentrations of the inorganic calcium channel blockers, Cd2+, Co2+ and La3+. Nimodipine, a dihydropyridine which blocks L-type calcium channels, inhibited only 10-15% of the flux response while nitrendipine had no consistent effect. omega-Conotoxin GVIA, a blocker of N-type channels in many species, had only a small inhibitory effect at high (1-10 microM) concentrations. The response was, however, inhibited by pre-incubation of the synaptosomes with venom of the funnel web spider. Agelenopsis aperta (0.1-300 micrograms/ml). Inhibition was observed with both a purified polyamine fraction (FTX) from the venom (IC50 = 4 nl/ml) and a purified peptide toxin, omega-AgaIVA (IC50 = 30 nM). These results indicate that voltage-dependent Ca2+ uptake by mammalian nerve terminals is mediated primarily by channels that are insensitive to dihydropyridines and omega-conotoxin GVIA but are sensitive to components of funnel web spider venom.

Role of different types of Ca2+ channels and a reticulum-like Ca2+ pump in neurotransmitter release

The factors controlling the Ca2+ concentration directly responsible for triggering acetylcholine (ACh) release were investigated at an identified neuro-neuronal synapse of the Aplysia buccal ganglion. The types of presynaptic voltage-gated Ca2+ channels associated with transmitter release were determined by using selective blockers such as nifedipine, omega-conotoxin and a partially purified extract from the venom of a funnel web spider (FTx). L-type, N-type and P-type Ca2+ channels are present in the presynaptic neuron. The influx of Ca2+ through both N- and P-types induces the release of ACh whereas Ca2+ flowing through L-type channels modulates the duration of the presynaptic action potential by controlling the Ca(2+)-dependent K+ current. tBuBHQ, a blocker of the reticulum Ca2+ pump, induces a potentiation of evoked release without modifying the presynaptic Ca2+ influx. This seems to indicate that a part of the Ca2+ entering the presynaptic terminal through N- and P-type Ca2+ channels is sequestered in a presynaptic reticulum-like Ca2+ buffer preventing these ions from contributing to ACh release. To exert its control, this Ca2+ buffer must be located close to both the presynaptic Ca2+ channels and the transmitter release mechanism.