omega-Grammotoxin blocks action-potential-induced Ca2+ influx and whole-cell Ca2+ current in rat dorsal-root ganglion neurons

ω-Grammotoxin-SIA inhibits voltage-gated Na+ channel currents

ω-Grammotoxin-SIA (GrTX-SIA) was originally isolated from the venom of the Chilean rose tarantula and demonstrated to function as a gating modifier of voltage-gated Ca2+ (CaV) channels. Later experiments revealed that GrTX-SIA could also inhibit voltage-gated K+ (KV) channel currents via a similar mechanism of action that involved binding to a conserved S3-S4 region in the voltage-sensing domains (VSDs). Since voltage-gated Na+ (NaV) channels contain homologous structural motifs, we hypothesized that GrTX-SIA could inhibit members of this ion channel family as well. Here, we show that GrTX-SIA can indeed impede the gating process of multiple NaV channel subtypes with NaV1.6 being the most susceptible target. Moreover, molecular docking of GrTX-SIA onto NaV1.6, supported by a p.E1607K mutation, revealed the voltage sensor in domain IV (VSDIV) as being a primary site of action. The biphasic manner in which current inhibition appeared to occur suggested a second, possibly lower-sensitivity binding locus, which was identified as VSDII by using KV2.1/NaV1.6 chimeric voltage-sensor constructs. Subsequently, the NaV1.6p.E782K/p.E838K (VSDII), NaV1.6p.E1607K (VSDIV), and particularly the combined VSDII/VSDIV mutant lost virtually all susceptibility to GrTX-SIA. Together with existing literature, our data suggest that GrTX-SIA recognizes modules in NaV channel VSDs that are conserved among ion channel families, thereby allowing it to act as a comprehensive ion channel gating modifier peptide.

Block of voltage-gated calcium channels by peptide toxins

Venoms from various predatory species, such as fish hunting molluscs scorpions, snakes and arachnids contain a large spectrum of toxins that include blockers of voltage-gated calcium channels. These peptide blockers act by two principal manners - physical occlusion of the pore and prevention of activation gating. Many of the calcium channel-blocking peptides have evolved to tightly occupy their binding pocket on the principal pore forming subunit of the channel, often rendering block poorly reversible. Moreover, several of the best characterized blocking peptides have developed a high degree of channel subtype selectivity. Here we give an overview of different types of calcium channel-blocking toxins, their mechanism of action, channel subtype specificity, and potential use as therapeutic agents. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Interleukin-1β activates an Src family kinase to stimulate the plasma membrane Ca2+ pump in hippocampal neurons

The plasma membrane Ca(2+) ATPase (PMCA) plays a major role in clearing Ca(2+) from the neuronal cytoplasm. The cytoplasmic Ca(2+) clearance rate affects neuronal excitability, synaptic plasticity, and neurotransmission. Here, we examined the modulation of PMCA activity by PTKs in hippocampal neurons. PMCA-mediated Ca(2+) clearance slowed in the presence of pyrazolopyrimidine 2, an inhibitor of Src family kinases (SFKs), and accelerated in the presence of C2-ceramide, an activator of PTKs. Ca(2+) clearance kinetics were attenuated in cells expressing a dominant-negative Src mutant, suggesting that the pump is tonically stimulated by a PTK. Tonic stimulation was reduced in hippocampal neurons expressing short hairpin (sh)RNA directed to mRNA for Yes. shRNA-mediated knockdown of PMCA isoform 1 (PMCA1) removed tonic stimulation of Ca(2+) clearance, indicating that the kinase stimulates PMCA1. IL-1β accelerated Ca(2+) clearance in a manner blocked by an IL-1β receptor antagonist or by an inhibitor of neutral sphingomyelinase, the enzyme that produces ceramide. Thus IL-1β activates an SFK to stimulate the plasma membrane Ca(2+) pump, decreasing the duration of Ca(2+) transients in hippocampal neurons.

Mechanisms of conotoxin inhibition of N-type (Ca(v)2.2) calcium channels

N-type (Ca(v)2.2) voltage-gated calcium channels (VGCC) transduce electrical activity into other cellular functions, regulate calcium homeostasis and play a major role in processing pain information. Although the distribution and function of these channels vary widely among different classes of neurons, they are predominantly expressed in nerve terminals, where they control neurotransmitter release. To date, genetic and pharmacological studies have identified that high-threshold, N-type VGCCs are important for pain sensation in disease models. This suggests that N-type VGCC inhibitors or modulators could be developed into useful drugs to treat neuropathic pain. This review discusses the role of N-type (Ca(v)2.2) VGCCs in nociception and pain transmission through primary sensory dorsal root ganglion (DRG) neurons (nociceptors). It also outlines the potent and selective inhibition of N-type VGCCs by conotoxins, small disulfide-rich peptides isolated from the venom of marine cone snails. Of these conotoxins, ω-conotoxins are selective N-type VGCC antagonists that preferentially block nociception in inflammatory pain models, and allodynia and/or hyperalgesia in neuropathic pain models. Another conotoxin family, α-conotoxins, were initially proposed as competitive antagonists of muscle and neuronal nicotinic acetylcholine receptors (nAChR). Surprisingly, however, α-conotoxins Vc1.1 and RgIA, also potently inhibit N-type VGCC currents in the sensory DRG neurons of rodents and α9 nAChR knockout mice, via intracellular signaling mediated by G protein-coupled GABAB receptors. Understanding how conotoxins inhibit VGCCs is critical for developing these peptides into analgesics and may result in better pain management. This article is part of a Special Issue entitled: Calcium channels.

Homer proteins accelerate Ca2+ clearance mediated by the plasma membrane Ca2+ pump in hippocampal neurons

The plasma membrane Ca(2+) ATPase (PMCA) is responsible for maintaining basal intracellular Ca(2+) concentration ([Ca(2+)](i)) and returning small increases in [Ca(2+)](i) back to resting levels. The carboxyl terminus of some PMCA splice variants bind Homer proteins; how binding affects PMCA function is unknown. Here, we examined the effects of altered expression of Homer proteins on PMCA-mediated Ca(2+) clearance from rat hippocampal neurons in culture. The kinetics of PMCA-mediated recovery from the [Ca(2+)](i) increase evoked by a brief train of action potentials was determined in the soma of single neurons using indo-1-based photometry. Exogenous expression of Homer 1a, Homer 1c or Homer 2a did not affect PMCA function. However, shRNA mediated knockdown of Homer 1 slowed PMCA mediated Ca(2+) clearance by 28% relative to cells expressing non-silencing shRNA. The slowed recovery rate in cells expressing Homer 1 shRNA was reversed by expression of a short Homer 2 truncation mutant. These results indicate that constitutively expressed Homer proteins tonically stimulate PMCA function in hippocampal neurons. We propose a model in which binding of short or long Homer proteins to the carboxyl terminus of the PMCA stimulates Ca(2+) clearance rate. PMCA-mediated Ca(2+) clearance may be stimulated following incorporation of the pump into Homer organized signaling domains and following induction of the Homer 1a immediate early gene.

Inhibition of the plasma membrane Ca2+ pump by CD44 receptor activation of tyrosine kinases increases the action potential afterhyperpolarization in sensory neurons

The cytoplasmic Ca(2+) clearance rate affects neuronal excitability, plasticity, and synaptic transmission. Here, we examined the modulation of the plasma membrane Ca(2+) ATPase (PMCA) by tyrosine kinases. In rat sensory neurons grown in culture, the PMCA was under tonic inhibition by a member of the Src family of tyrosine kinases (SFKs). Ca(2+) clearance accelerated in the presence of selective tyrosine kinase inhibitors. Tonic inhibition of the PMCA was attenuated in cells expressing a dominant-negative construct or shRNA directed to message for the SFKs Lck or Fyn, but not Src. SFKs did not appear to phosphorylate the PMCA directly but instead activated focal adhesion kinase (FAK). Expression of constitutively active FAK enhanced and dominant-negative or shRNA knockdown of FAK attenuated tonic inhibition. Antisense knockdown of PMCA isoform 4 removed tonic inhibition of Ca(2+) clearance, indicating that FAK acts on PMCA4. The hyaluronan receptor CD44 activates SFK-FAK signaling cascades and is expressed in sensory neurons. Treating neurons with a CD44-blocking antibody or short hyaluronan oligosaccharides, which are produced during injury and displace macromolecular hyaluronan from CD44, attenuated tonic PMCA inhibition. Ca(2+)-activated K(+) channels mediate a slow afterhyperpolarization in sensory neurons that was inhibited by tyrosine kinase inhibitors and enhanced by knockdown of PMCA4. Thus, we describe a novel kinase cascade in sensory neurons that enables the extracellular matrix to alter Ca(2+) signals by modulating PMCA-mediated Ca(2+) clearance. This signaling pathway may influence the excitability of sensory neurons following injury.

Ca2+-dependent, stimulus-specific modulation of the plasma membrane Ca2+ pump in hippocampal neurons

The plasma membrane Ca(2+) ATPase (PMCA) plays a major role in restoring Ca(2+) to basal levels following transient elevation by neuronal activity. Here we examined the effects of various stimuli that increase [Ca(2+)](i) on PMCA-mediated Ca(2+) clearance from hippocampal neurons. We used indo-1-based microfluorimetry in the presence of cyclopiazonic acid to study the rate of PMCA-mediated recovery of Ca(2+) elevated by a brief train of action potentials. [Ca(2+)](i) recovery was described by an exponential decay and the time constant provided an index of PMCA-mediated Ca(2+) clearance. PMCA function was assessed before and for >or=60 min following a 10-min priming stimulus of either 100 microM N-methyl-d-aspartate (NMDA), 0.1 mM Mg(2+) (reduced extracellular Mg(2+) induces intense excitatory synaptic activity), 30 mM K(+), or control buffer. Recovery kinetics slowed progressively following priming with NMDA or 0.1 mM Mg(2+); in contrast, Ca(2+) clearance initially accelerated and then slowly returned to initial rates following priming with 30 mM K(+)-induced depolarization. Treatment with 10 muM calpeptin, an inhibitor of the Ca(2+) activated protease calpain, prevented the slowing of kinetics observed following treatment with NMDA but had no affect on the recovery kinetics of control cells. Calpeptin also blocked the rapid acceleration of Ca(2+) clearance following depolarization. In calpeptin-treated cells, 0.1 mM Mg(2+) induced a graded acceleration of Ca(2+) clearance. Thus in spite of producing comparable increases in [Ca(2+)](i), activation of NMDA receptors, depolarization-induced activation of voltage-gated Ca(2+) channels and excitatory synaptic activity each uniquely affected Ca(2+) clearance kinetics mediated by the PMCA.

Contribution of calcium channel subtypes to the intracellular calcium signal in sensory neurons: the effect of injury

BACKGROUND

Although the activation-induced intracellular Ca signal is disrupted by sensory neuron injury, the contribution of specific Ca channel subtypes is unknown.

METHODS

Transients in dissociated rat dorsal root ganglion neurons were recorded using fura-2 microfluorometry. Neurons from control rats and from neuropathic animals after spinal nerve ligation were activated either by elevated bath K or by field stimulation. Transients were compared before and after application of selective blockers of voltage-activated Ca channel subtypes.

RESULTS

Transient amplitude and area were decreased by blockade of the L-type channel, particularly during sustained K stimulation. Significant contributions to the Ca transient are attributable to the N-, P/Q-, and R-type channels, especially in small neurons. Results for T-type blockade varied widely between cells. After injury, transients lost sensitivity to N-type and R-type blockers in axotomized small neurons, whereas adjacent small neurons showed decreased responses to blockers of R-type channels. Axotomized large neurons were less sensitive to blockade of N- and P/Q-type channels. After injury, neurons adjacent to axotomy show decreased sensitivity of K-induced transients to L-type blockade but increased sensitivity during field stimulation.

CONCLUSIONS

All high-voltage-activated Ca current subtypes contribute to Ca transients in sensory neurons, although the L-type channel contributes predominantly during prolonged activation. Injury shifts the relative contribution of various Ca channel subtypes to the intracellular Ca transient induced by neuronal activation. Because this effect is cell-size specific, selective therapies might potentially be devised to differentially alter excitability of nociceptive and low-threshold sensory neurons.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

?-Conotoxin CVIB differentially inhibits native and recombinant N- and P/Q-type calcium channels

Omega-conotoxins are routinely used as selective inhibitors of different classes of voltage-gated calcium channels (VGCCs) in excitable cells. In the present study, we examined the potent N-type VGCC antagonist omega-conotoxin CVID and non-selective N- and P/Q-type antagonist CVIB for their ability to block native VGCCs in rat dorsal root ganglion (DRG) neurons and recombinant VGCCs expressed in Xenopus oocytes. Omega-conotoxins CVID and CVIB inhibited depolarization-activated whole-cell VGCC currents in DRG neurons with pIC50 values of 8.12 +/- 0.05 and 7.64 +/- 0.08, respectively. Inhibition of Ba2+ currents in DRG neurons by CVID (approximately 66% of total) appeared to be irreversible for > 30 min washout, whereas Ba2+ currents exhibited rapid recovery from block by CVIB (> or = 80% within 3 min). The recoverable component of the Ba2+ current inhibited by CVIB was mediated by the N-type VGCC, whereas the irreversibly blocked current (approximately 22% of total) was attributable to P/Q-type VGCCs. Omega-conotoxin CVIB reversibly inhibited Ba2+ currents mediated by N- (Ca(V)2.2) and P/Q- (Ca(V)2.1), but not R- (Ca(V)2.3) type VGCCs expressed in Xenopus oocytes. The alpha2delta1 auxiliary subunit co-expressed with Ca(V)2.2 and Ca(V)2.1 reduced the sensitivity of VGCCs to CVIB but had no effect on reversibility of block. Determination of the NMR structure of CVIB identified structural differences to CVID that may underlie differences in selectivity of these closely related conotoxins. Omega-conotoxins CVIB and CVID may be useful as antagonists of N- and P/Q-type VGCCs, particularly in sensory neurons involved in processing primary nociceptive information.

Glutamate-induced protease-mediated loss of plasma membrane Ca2+ pump activity in rat hippocampal neurons

Ca2+ dysregulation is a hallmark of excitotoxicity, a process that underlies multiple neurodegenerative disorders. The plasma membrane Ca2+ ATPase (PMCA) plays a major role in clearing Ca2+ from the neuronal cytoplasm. Here, we show that the rate of PMCA-mediated Ca2+ efflux from rat hippocampal neurons decreased following treatment with an excitotoxic concentration of glutamate. PMCA-mediated Ca2+ extrusion following a brief train of action potentials exhibited an exponential decay with a mean time constant (tau) of 8.8 +/- 0.2 s. Four hours following the start of a 30 min treatment with 200 microm glutamate, a second population of cells emerged with slowed recovery kinetics (tau = 16.5 +/- 0.3 s). Confocal imaging of cells expressing an enhanced green fluorescent protein (EGFP)-PMCA4b fusion protein revealed that glutamate treatment internalized EGFP and that cells with reduced plasma membrane fluorescence had impaired Ca2+ clearance. Treatment with inhibitors of the Ca2+-activated protease calpain protected PMCA function and prevented EGFP-PMCA internalization. PMCA internalization was triggered by activation of NMDA receptors and was less pronounced for a non-toxic concentration of glutamate relative to one that produces excitotoxicity. PMCA isoform 2 also internalized following exposure to glutamate, although the Na+/K+ ATPase did not. These data suggest that glutamate exposure initiated protease-mediated internalization of PMCAs with a corresponding loss of function that may contribute to the Ca2+ dysregulation that accompanies excitotoxicity.

Calcium Signaling and Exocytosis in Adrenal Chromaffin Cells

At a given cytosolic domain of a chromaffin cell, the rate and amplitude of the Ca2+concentration ([Ca2+]c) depends on at least four efficient regulatory systems: 1) plasmalemmal calcium channels, 2) endoplasmic reticulum, 3) mitochondria, and 4) chromaffin vesicles. Different mammalian species express different levels of the L, N, P/Q, and R subtypes of high-voltage-activated calcium channels; in bovine and humans, P/Q channels predominate, whereas in felines and murine species, L-type channels predominate. The calcium channels in chromaffin cells are regulated by G proteins coupled to purinergic and opiate receptors, as well as by voltage and the local changes of [Ca2+]c. Chromaffin cells have been particularly useful in studying calcium channel current autoregulation by materials coreleased with catecholamines, such as ATP and opiates. Depending on the preparation (cultured cells, adrenal slices) and the stimulation pattern (action potentials, depolarizing pulses, high K+, acetylcholine), the role of each calcium channel in controlling catecholamine release can change drastically. Targeted aequorin and confocal microscopy shows that Ca2+entry through calcium channels can refill the endoplasmic reticulum (ER) to nearly millimolar concentrations, and causes the release of Ca2+(CICR). Depending on its degree of filling, the ER may act as a sink or source of Ca2+that modulates catecholamine release. Targeted aequorins with different Ca2+affinities show that mitochondria undergo surprisingly rapid millimolar Ca2+transients, upon stimulation of chromaffin cells with ACh, high K+, or caffeine. Physiological stimuli generate [Ca2+]cmicrodomains in which the local subplasmalemmal [Ca2+]crises abruptly from 0.1 to ∼50 μM, triggering CICR, mitochondrial Ca2+uptake, and exocytosis at nearby secretory active sites. The fact that protonophores abolish mitochondrial Ca2+uptake, and increase catecholamine release three- to fivefold, support the earlier observation. This increase is probably due to acceleration of vesicle transport from a reserve pool to a ready-release vesicle pool; this transport might be controlled by Ca2+redistribution to the cytoskeleton, through CICR, and/or mitochondrial Ca2+release. We propose that chromaffin cells have developed functional triads that are formed by calcium channels, the ER, and the mitochondria and locally control the [Ca2+]cthat regulate the early and late steps of exocytosis.

Mitochondrial modulation of Ca2+ -induced Ca2+ -release in rat sensory neurons

Ca2+ -induced Ca2+ -release (CICR) from ryanodine-sensitive Ca2+ stores provides a mechanism to amplify and propagate a transient increase in intracellular calcium concentration ([Ca2+]i). A subset of rat dorsal root ganglion neurons in culture exhibited regenerative CICR when sensitized by caffeine. [Ca2+]i oscillated in the maintained presence of 5 mM caffeine and 25 mM K+. Here, CICR oscillations were used to study the complex interplay between Ca2+ regulatory mechanisms at the cellular level. Oscillations depended on Ca2+ uptake and release from the endoplasmic reticulum (ER) and Ca2+ influx across the plasma membrane because cyclopiazonic acid, ryanodine, and removal of extracellular Ca2+ terminated oscillations. Increasing caffeine concentration decreased the threshold for action potential-evoked CICR and increased oscillation frequency. Mitochondria regulated CICR by providing ATP and buffering [Ca2+]i. Treatment with the ATP synthase inhibitor, oligomycin B, decreased oscillation frequency. When ATP concentration was held constant by recording in the whole cell patch-clamp configuration, oligomycin no longer affected oscillation frequency. Aerobically derived ATP modulated CICR by regulating the rate of Ca2+ sequestration by the ER Ca2+ pump. Neither CICR threshold nor Ca2+ clearance by the plasma membrane Ca2+ pump were affected by inhibition of aerobic metabolism. Uncoupling electron transport with carbonyl cyanide p-trifluoromethoxy-phenyl-hydrazone or inhibiting mitochondrial Na+/Ca2+ exchange with CGP37157 revealed that mitochondrial buffering of [Ca2+]i slowed oscillation frequency, decreased spike amplitude, and increased spike width. These findings illustrate the interdependence of energy metabolism and Ca2+ signaling that results from the complex interaction between the mitochondrion and the ER in sensory neurons.

Activation of protein kinase C in sensory neurons accelerates Ca2+ uptake into the endoplasmic reticulum

The rate of Ca2+ clearance from the neuronal cytoplasm affects the amplitude, duration, and localization of Ca2+ signals and influences a variety of Ca2+-dependent functions. We reported previously that activation of protein kinase C (PKC) accelerates Ca2+ efflux in rat sensory neurons mediated by the plasma membrane Ca2+-ATPase isoform 4 (PMCA4). Here we show that sarco-endoplasmic reticulum Ca2+-ATPase (SERCA)-mediated Ca2+ uptake into intracellular stores is also accelerated by PKC activation. The rate of intracellular Ca2+ concentration ([Ca2+]i) clearance was studied after small (<350 nM) action potential-induced Ca2+ loads in rat dorsal root ganglion neurons. Under these conditions, mitochondrial Ca2+ uptake and Na+/Ca2+ exchange do not significantly influence [Ca2+]i recovery. Phorbol dibutyrate (PDBu) increased the rate of [Ca2+]i clearance by 71% in a manner sensitive to the selective PKC inhibitors GF109203x (2-[1-(3-dimethylaminopropyl)-1H-indol-3-yl]-3-(1H-indol-3-yl)maleimide) and calphostin. PKC-dependent acceleration was still observed (approximately 39%) when the PKC-sensitive PMCA isoform was knocked down by expression of an antisense PMCA4 cDNA (AS4). Direct measurement of Ca2+ in the endoplasmic reticulum (ER) lumen revealed that PKC increased the rate of store refilling more than twofold after depletion by treatment with cyclopiazonic acid. ER refilling was less complete in PDBu-treated cells, although, in AS4-expressing cells, PDBu accelerated the rate without reducing the ER capacity, suggesting that PMCA and SERCA compete for Ca2+. Thus, activation of PKC accelerates the clearance of Ca2+ from the cytoplasm by the concerted stimulation of Ca2+ sequestration and Ca2+ efflux.

Anandamide transport inhibitor AM404 and structurally related compounds inhibit synaptic transmission between rat hippocampal neurons in culture independent of cannabinoid CB1 receptors

N-(hydroxyphenyl)-arachidonamide (AM404) is an inhibitor of endocannabinoid transport. We examined the effects of AM404 on glutamatergic synaptic transmission using network-driven increases in intracellular Ca2+ concentration ([Ca2+] spikes) as an assay. At a concentration of 1 microM AM404 inhibited [Ca2+]i spiking by 73+/-8%. The cannabinoid CB1 receptor antagonist N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A), the vanilloid VR1 receptor antagonist capsazepine (CPZ), and treatment with pertussis toxin failed to block AM404-mediated inhibition. AM404 (3 microM) inhibited action-potential-evoked Ca2+ influx by 58+/-3% but failed to affect calcium influx evoked by depolarization with 30 mM K+, suggesting that the inhibition of electrically evoked [Ca2+]i increases and that [Ca2+]i spiking was due to inhibition of Na+ channels. Palmitoylethanolamide (PMEA), capsaicin (CAP) and (5Z,8Z,11Z,14Z)-N-(4-hydroxy-2-methylphenyl)-5,8,11,14-eicosatetraenamide (VDM11), compounds structurally similar to AM404, inhibited [Ca2+]i spiking by 34+/-10%, 42+/-18% and 67+/-12%, respectively. Thus, AM404 and related compounds inhibit depolarization-induced Ca2+ influx independent of cannabinoid receptors, suggesting caution when using these agents as pharmacological probes to study synaptic transmission.

Molecular pharmacology of high voltage-activated calcium channels

Voltage-gated calcium channels are key sources of calcium entry into the cytosol of many excitable tissues. A number of different types of calcium channels have been identified and shown to mediate specialized cellular functions. Because of their fundamental nature, they are important targets for therapeutic intervention in disorders such as hypertension, pain, stroke, and epilepsy. Calcium channel antagonists fall into one of the following three groups: small inorganic ions, large peptide blockers, and small organic molecules. Inorganic ions nonselectively inhibit calcium entry by physical pore occlusion and are of little therapeutic value. Calcium-channel-blocking peptides isolated from various predatory animals such as spiders and cone snails are often highly selective blockers of individual types of calcium channels, either by preventing calcium flux through the pore or by antagonizing channel activation. There are many structure-activity-relation classes of small organic molecules that interact with various sites on the calcium channel protein, with actions ranging from selective high affinity block to relatively nondiscriminatory action on multiple calcium channel isoforms. Detailed interactions with the calcium channel protein are well understood for the dihydropyridine and phenylalkylamine drug classes, whereas we are only beginning to understand the molecular actions of some of the more recently discovered calcium channel blockers. Here, we provide a comprehensive review of pharmacology of high voltage-activated calcium channels.

Tarantulas: eight-legged pharmacists and combinatorial chemists

Tarantula venoms represent a cornucopia of novel ligands for a variety of cell receptors and ion channels. The diversity of peptide toxin pharmacology has been barely explored as indicated by pharmacological, toxicological and mass spectrometry investigations on more than 55 tarantula venoms. MALDI-TOF MS analysis reveals that the pharmacological diversity is based on relatively small size peptides, which seem to fall into a limited number of structural patterns. Properties and biological activities of the 33 known peptide toxins from tarantula venoms are described. Most known toxins conform to the Inhibitory Cystine Knot (ICK) motif, with differences in the length of intercysteine loops. Recently described peptides show that tarantula toxins can fold according to an elaboration of the Disulfide-Directed beta-Hairpin (DDH) motif which is also the canonical motif for the ICK fold. The ICK fold itself offers many variations leading to differing toxin properties. Examination of pharmacological data gives insights on the possible conserved site of action of toxins acting on voltage-gated ion channels and other toxins acting by a pore-blocking mechanism. Structure-activity data shows the versatility of the toxin scaffolds and the importance of surface features in the selectivity and specificity of these toxins. Tarantulas appear to be a good model for the discovery of novel compounds with important therapeutic potential, and for the study of the molecular evolution of peptide toxins.

Transient rise in intracellular calcium produces a long-lasting increase in plasma membrane calcium pump activity in rat sensory neurons

The plasma membrane Ca2+ ATPase (PMCA) plays a major role in clearing Ca2+ from the neuronal cytoplasm. Calmodulin stimulates PMCA activity and for some isoforms this activation persists following clearance of Ca2+ owing to the slow dissociation of calmodulin. We tested the hypothesis that PMCA-mediated Ca2+ efflux from rat dorsal root ganglion (DRG) neurons in culture would remain stimulated following increases in intracellular Ca2+ concentration ([Ca2+]i). PMCA-mediated Ca2+ extrusion was recorded following brief trains of action potentials using indo-1-based photometry in the presence of cyclopiazonic acid. A priming stimulus that increased [Ca2+]i to 506 +/- 28 nm (>15 min) increased the rate constant for [Ca2+]i recovery by 47 +/- 3%. Ca2+ clearance from subsequent test stimuli remained accelerated for up to an hour despite removal of the priming stimulus and a return to basal [Ca2+]i. The acceleration depended on the magnitude and duration of the priming [Ca2+]i increase, but was independent of the source of Ca2+. Increases in [Ca2+]i evoked by prolonged depolarization, sustained trains of action potentials or activation of vanilloid receptors all accelerated Ca2+ efflux. We conclude that PMCA-mediated Ca2+ efflux in DRG neurons is a dynamic process in which intense stimuli prime the pump for the next Ca2+ challenge.

Modulation of voltage-activated calcium currents by mechanical stimulation in rat sensory neurons

We examined the effects of mechanical stress, induced by a stream of bath solution, on evoked action potentials, electrical excitability, and Ca2+ currents in rat dorsal root ganglion neurons in culture with the use of the whole cell patch-clamp technique. Action-potential duration was altered reversibly by flow in 39% of the 51 neurons tested, but membrane potential and excitability were unaffected. The flow-induced increases and decreases in action-potential duration were consistent with the different effects of flow on two types of Ca2+ channel, determined by voltage-clamp recordings of Ba2+ currents. Current through omega-conotoxin-sensitive (N-type) Ca2+ channels increased by an estimated 74% with flow, corresponding to 23% increase in the total high voltage-activated current, whereas current through low-threshold voltage-activated (T-type) channels decreased by 14%. We conclude that modulation of voltage-activated Ca2+ currents constitutes a route by which mechanical events can regulate Ca2+ influx in sensory neurons.

Omega-conotoxin GVIA-resistant neurotransmitter release from postganglionic sympathetic nerves in the guinea-pig vas deferens and its modulation by presynaptic receptors

1 Intracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals in the guinea-pig isolated vas deferens. 2 Recently, a component of action potential-evoked release which is insensitive to high concentrations of the selective N-type calcium channel blocker omega-conotoxin GVIA termed 'residual release' has been described. Under these conditions, release of the neurotransmitter ATP evoked by trains of low frequency stimuli is abolished, but at higher frequencies a substantial component of release is revealed. 3 'Residual release' was studied with trains of 5 or 10 stimuli at stimulation frequencies of 10, 20 and 50 Hz. The alpha2-adrenoceptor agonist clonidine (30-100 nM) inhibited 'residual release', the degree of inhibition being most marked at the beginning of a train. 4 The alpha2-adrenoceptor antagonist yohimbine (1 microM) induced a marked increase in 'residual release' which was dependent on both the frequency of stimulation and the number of stimuli in a train. 5 Prostaglandin E2 (30 nM) and neuropeptide Y (100 nM) caused a rapid inhibition of 'residual release' at all stimulation frequencies examined. 6 4-Aminopyridine (100 microM) induced a powerful potential of 'residual release' and could reverse the inhibition of omega-conotoxin GVIA. 7 'Residual release' was modulated through presynaptic alpha2-adrenoceptors suggesting that (i) residual release of ATP is subject to alpha-autoinhibition through the co-release of noradrenaline, (ii) noradrenaline release can be triggered by calcium channels other than the N-type and (iii) when presynaptic receptors are activated, inhibition of transmitter release can occur by mechanisms other than modulation of calcium-entry through N-type calcium channels in postganglionic sympathetic nerves. Prostaglandin E2 and neuropeptide Y also modulated neurotransmitter release.

Voltage-dependent inhibition of N- and P-type calcium channels by the peptide toxin omega-grammotoxin-SIA

We studied the mechanism by which the peptide omega-grammotoxin-SIA inhibits voltage-dependent calcium channels. Grammotoxin at concentrations of > 50 nM completely inhibited inward current carried by 2 mM barium through P-type channels in rat cerebellar Purkinje neurons when current was elicited by depolarizations up to +40 mV. However, outward current (carried by internal cesium) elicited by depolarizations to > +100 mV was either unaffected or enhanced in the presence of toxin. Tail current activation curves showed that grammotoxin shifted the steady state voltage dependence of channel activation by approximately +40 mV. Activation in the presence of toxin was far slower in addition to having altered voltage dependence. Grammotoxin also inhibited N-type calcium channels in rat and frog sympathetic neurons, with changes in channel voltage dependence and kinetics nearly identical to those of P-type channels. Experiments with monovalent ions as the only charge carriers showed that toxin effects on channel activation and kinetics depended on voltage, not on direction of current flow or on the current-carrying ion. Repeated trains of large depolarizations relieved toxin inhibition, as if toxin affinity for activated channels were low. The effects of grammotoxin on gating of P-type channels are very similar to those of omega-Aga-IVA, but combined application of the two toxins showed that grammotoxin binding is not prevented by saturating binding of omega-Aga-IVA. We conclude that grammotoxin potently inhibits both P-type and N-type channels by impeding channel gating and that grammotoxin binds to distinct or additional sites on P-type channels compared with omega-Aga-IVA.

All-or-none Ca2+ release from intracellular stores triggered by Ca2+ influx through voltage-gated Ca2+ channels in rat sensory neurons

Ca2+-induced Ca2+ release (CICR) from intracellular stores amplifies the Ca2+ signal that results from depolarization. In neurons, the amplification has been described as a graded process. Here we show that regenerative CICR develops as an all-or-none event in cultured rat dorsal root ganglion neurons in which ryanodine receptors have been sensitized to Ca2+ by caffeine. We used indo-1-based microfluorimetry in combination with whole-cell patch-clamp recording to characterize the relationship between Ca2+ influx and Ca2+ release. Regenerative release of Ca2+ was triggered when action potential-induced Ca2+ influx increased the intracellular Ca2+ concentration ([Ca2+]i) above threshold. The threshold was modulated by caffeine and intraluminal Ca2+. A relative refractory period followed CICR. The pharmacological profile of the response was consistent with Ca2+ influx through voltage-gated Ca2+ channels triggering release from ryanodine-sensitive stores. The activation of a suprathreshold response increased more than fivefold the amplitude and duration of the [Ca2+]i transient. The switch to a suprathreshold response was regulated very precisely in that addition of a single action potential to the stimulus train was sufficient for this transformation. Confocal imaging experiments showed that CICR facilitated propagation of the Ca2+ signal from the plasmalemma to the nucleus. This all-or-none reaction may serve as a switch that determines whether a given electrical signal will be transduced into a local or widespread increase in [Ca2+]i.

Multiple calcium channels control neurotransmitter release from rat postganglionic sympathetic nerve terminals

1. Intracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals of the rat isolated anococcygeus muscle. 2. Low concentrations of the N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTX GVIA) irreversibly abolished excitatory junction potentials (EJPs) evoked by trains of < or = five stimuli at 10 Hz. When the frequency of stimulation was increased (10-50 Hz) trains of stimuli evoked EJPs even in the presence of 1 microM omega-CgTX GVIA. We have termed this omega-CgTX GVIA-resistant release 'residual release'. EJP amplitude in the presence of omega-CgTX GVIA depended on both the frequency and number of stimuli in a train. 3. Residual release was inhibited by the P-type calcium channel blocker omega-agatoxin IVA (100 nM). However, even in the presence of both toxins, longer trains of stimuli could still evoke neurotransmitter release. 4. Residual release was abolished by omega-conotoxin MVIIC and by the non-specific calcium channel antagonist omega-grammotoxin SIA. Therefore, it would appear that a heterogeneous population of calcium channels is involved in mediating neurotransmitter release from these sympathetic nerve terminals.

Ryanodine-sensitive calcium stores involved in neurotransmitter release from sympathetic nerve terminals of the guinea-pig

1. Intracellular and focal extracellular recording techniques were used to study neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals in the guinea-pig isolated vas deferens. 2. High concentrations of the selective N-type calcium channel blocker omega-conotoxin GVIA abolished the release of the neurotransmitter ATP evoked by trains of low-frequency stimuli. However, in the presence of high concentrations of the blocker, a 'residual release' persisted at higher frequencies. 3. Residual release was dependent on calcium entry through a pharmacologically distinct voltage-dependent calcium channel. 4. Residual release was inhibited by ryanodine in a use- and time-dependent manner and this inhibitory effect was potentiated by caffeine. The inhibitory effect of ryanodine on residual release was reversed by 4-aminopyridine. 5. These findings indicate that calcium-induced calcium released from intraneuronal stores plays an important role in action potential-evoked neurotransmitter release mechanisms in postganglionic sympathetic nerve terminals.

Omega-conotoxin GVIA-resistant neurotransmitter release in postganglionic sympathetic nerve terminals

Intracellular recording techniques were used to study neurotransmitter release in the guinea-pig isolated vas deferens. Low concentrations of the irreversible and selective N-type calcium channel blocker omega-conotoxin GVIA have previously been shown to block excitatory junction potentials evoked by low frequencies (< or = 1 Hz) of nerve stimulation. Here we report a component of action potential-evoked release which is insensitive to high concentrations of omega-conotoxin GVIA. We have termed this component "residual release" and show (i) it is positively frequency-dependent, (ii) its magnitude is dependent on both the train length and interval between trains, (iii) "residual release" can be modulated through prejunctional alpha 2-adrenoceptors and (iv) "residual release" is insensitive to many calcium entry blockers but abolished by omega-grammotoxin SIA and cadmium ions. Although noradrenaline is released by nerve action potentials, residual excitatory junction potentials were abolished by alpha,beta-methylene-ATP and therefore resulted entirely from the actions of neuronally released ATP acting through postjunctional P2x-purinoceptors. The results suggest that calcium entry through a novel, pharmacologically uncharacterized voltage-dependent calcium channel is responsible for "residual release" in sympathetic nerve terminals. It seems that in response to single or short trains of nerve action potentials, N-type calcium channels dominate the release process. However, at higher frequencies other voltage-dependent calcium channels are recruited and these may have an important role to play in triggering the mechanisms underlying frequency-dependent facilitation.

Complete and reversible block by omega-grammotoxin SIA of glutamatergic synaptic transmission between cultured rat hippocampal neurons

omega-Grammotoxin SIA (omega-GsTx SIA), a peptide isolated from tarantula venom, inhibits synaptosomal Ca2+ influx and neurotransmitter release, and blocks N-, P-, and Q-type voltage-gated Ca2+ channels. The whole-cell patch-clamp was used to record glutamatergic excitatory post-synaptic currents (EPSCs) evoked by extracellular stimulation of presynaptic neurons in primary rat hippocampal cultures. EPSCs displayed rapid kinetics and were blocked by CNQX. omega-Conotoxin (1 microM) GVIA inhibited EPSCs by 46%, while 30 nM and 1 microM omega-agatoxin IVA produced 12% and 69% inhibition, respectively, consistent with coupling of N-, P- and Q-type Ca2+ channels to glutamatergic synaptic transmission. omega-GsTx SIA (1 microM) rapidly, completely, and reversibly blocked glutamatergic EPSCs, but did not affect currents evoked by bath application of kainate. Thus, omega-GsTx SIA blocks glutamatergic synaptic transmission by blocking presynaptic voltage-gated Ca2+ channels. omega-GsTx SIA is the only agent that blocks selectively and reversibly the Ca2+ channels coupled to glutamate release. omega-GsTx SIA provides a unique and powerful tool for experiments requiring recovery of function following presynaptic block of synaptic transmission.

Comparative actions of synthetic omega-grammotoxin SIA and synthetic omega-Aga-IVA on neuronal calcium entry and evoked release of neurotransmitters in vitro and in vivo

The effects of synthetic omega-grammotoxin SIA (omega-GsTxSIA) and synthetic omega-Aga-IVA were tested in in vitro and in vivo neurochemical assays that are reflective of voltage-sensitive calcium channel function. Synthetic omega-GsTx SIA inhibited K(+)-evoked rat and chick synaptosomal 45Ca2+ flux, K(+)-evoked release of [3H]D-aspartate and [3H]norepinephrine from rat hippocampal brain slices and K(+)-evoked release of [3H]norepinephrine from chick cortical brain slices with potency values that were comparable to those found previously with omega-GsTx SIA purified from the venom of the tarantula spider Grammostola spatulata. These results indicate that trace contaminants do not account for the pharmacology of purified omega-GsTx SIA. omega-GsTx SIA caused a complete inhibition of rat synaptosomal 45Ca2+ flux and hippocampal slice [3H]D-aspartate release, whereas omega-Aga-IVA caused a maximal inhibition of approx 75%. omega-GsTx SIA and omega-Aga-IVA caused an identical partial inhibition of K(+)-evoked increases of intracellular calcium in cortical neurons in primary culture. The addition of nitrendipine to either omega-GsTx SIA or omega-Aga-IVA resulted in an additive and virtually complete inhibition of the cortical neuron intracellular calcium response. In in vivo microdialysis studies, the K(+)-evoked release of glutamate from hippocampus of awake freely moving rats was inhibited with the following rank order of potency: omega-conotoxin GVIA > omega-GsTx SIA > omega-Aga-IVA. Complete inhibition of K(+)-evoked hippocampal glutamate release was observed with 300 nM omega-conotoxin GVIA and 3 microM omega-GsTx SIA. In urethane anesthetized rats, omega-CgTx GVIA caused a partial inhibition, whereas omega-GsTx SIA caused a concentration-dependent and complete inhibition, of basal serotonin release in the hippocampus. Therefore, omega-GsTx SIA was shown to inhibit responses that are sensitive to omega-conotoxin GVIA, omega-Aga-IVA and omega-conotoxin MVIIC, consistent with the notion that omega-GsTx SIA inhibits N-, P- and Q-type high threshold voltage-sensitive calcium channels.