Evidence for distinct sites coupled to high affinity omega-conotoxin receptors in rat brain synaptic plasma membrane vesicles

Regulation of transforming growth factor-β1-dependent integrin β6 expression by p38 mitogen-activated protein kinase in bile duct epithelial cells

Bile duct epithelial cells (BDECs) contribute to liver fibrosis by expressing αVβ6 integrin, a critical activator of latent transforming growth factor β (TGF-β). β6 integrin (Itgβ6) mRNA induction and αVβ6 integrin expression in BDECs are partially TGF-β-dependent. However, the signaling pathways required for TGF-β-dependent Itgβ6 mRNA induction in BDECs are not known. We tested the hypothesis that the p38 mitogen-activated protein kinase (MAPK) signaling pathway contributes to TGF-β1 induction of Itgβ6 mRNA by activating SMAD and activator protein 1 (AP-1) transcription factors. Pretreatment of transformed human BDECs (MMNK-1 cells) with two different p38 MAPK inhibitors, but not a control compound, inhibited TGF-β1 induction of Itgβ6 mRNA. Inhibition of p38 also reduced TGF-β1 activation of a SMAD-dependent reporter construct. Expression of a dominant-negative SMAD3 (SMAD3ΔC) significantly reduced TGF-β1-induced Itgβ6 mRNA expression. Expression of JunB mRNA, but not other AP-1 proteins, increased in TGF-β1-treated MMNK-1 cells, and induction of JunB expression was p38-dependent. Consistent with a requirement for de novo induction of JunB protein, cycloheximide pretreatment inhibited TGF-β1 induction of Itgβ6 mRNA. Expression of a dominant-negative AP-1 mutant (TAM67) also inhibited TGF-β1 induction of Itgβ6 mRNA. Overall, the results suggest that p38 contributes to TGF-β1-induced Itgβ6 mRNA expression in MMNK-1 cells by regulating activation of both SMAD and AP-1 transcription factors.

Analgesic effects of a substituted N-triazole oxindole (TROX-1), a state-dependent, voltage-gated calcium channel 2 blocker

Voltage-gated calcium channel (Ca(v))2.2 (N-type calcium channels) are key components in nociceptive transmission pathways. Ziconotide, a state-independent peptide inhibitor of Ca(v)2.2 channels, is efficacious in treating refractory pain but exhibits a narrow therapeutic window and must be administered intrathecally. We have discovered an N-triazole oxindole, (3R)-5-(3-chloro-4-fluorophenyl)-3-methyl-3-(pyrimidin-5-ylmethyl)-1-(1H-1,2,4-triazol-3-yl)-1,3-dihydro-2H-indol-2-one (TROX-1), as a small-molecule, state-dependent blocker of Ca(v)2 channels, and we investigated the therapeutic advantages of this compound for analgesia. TROX-1 preferentially inhibited potassium-triggered calcium influx through recombinant Ca(v)2.2 channels under depolarized conditions (IC(50) = 0.27 microM) compared with hyperpolarized conditions (IC(50) > 20 microM). In rat dorsal root ganglion (DRG) neurons, TROX-1 inhibited omega-conotoxin GVIA-sensitive calcium currents (Ca(v)2.2 channel currents), with greater potency under depolarized conditions (IC(50) = 0.4 microM) than under hyperpolarized conditions (IC(50) = 2.6 microM), indicating state-dependent Ca(v)2.2 channel block of native as well as recombinant channels. TROX-1 fully blocked calcium influx mediated by a mixture of Ca(v)2 channels in calcium imaging experiments in rat DRG neurons, indicating additional block of all Ca(v)2 family channels. TROX-1 reversed inflammatory-induced hyperalgesia with maximal effects equivalent to nonsteroidal anti-inflammatory drugs, and it reversed nerve injury-induced allodynia to the same extent as pregabalin and duloxetine. In contrast, no significant reversal of hyperalgesia was observed in Ca(v)2.2 gene-deleted mice. Mild impairment of motor function in the Rotarod test and cardiovascular functions were observed at 20- to 40-fold higher plasma concentrations than required for analgesic activities. TROX-1 demonstrates that an orally available state-dependent Ca(v)2 channel blocker may achieve a therapeutic window suitable for the treatment of chronic pain.

Association with the auxiliary subunit PEX5R/Trip8b controls responsiveness of HCN channels to cAMP and adrenergic stimulation

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are key modulators of neuronal activity by providing the depolarizing cation current I(h) involved in rhythmogenesis, dendritic integration, and synaptic transmission. These tasks critically depend on the availability of HCN channels, which is dynamically regulated by intracellular cAMP; the range of this regulation, however, largely differs among neurons in the mammalian brain. Using affinity purification and high-resolution mass spectrometry, we identify the PEX5R/Trip8b protein as the beta subunit of HCN channels in the mammalian brain. Coassembly of PEX5R/Trip8b affects HCN channel gating in a subtype-dependent and mode-specific way: activation of HCN2 and HCN4 by cAMP is largely impaired, while gating by phosphoinositides and basal voltage-dependence remain unaffected. De novo expression of PEX5R/Trip8b in cardiomyocytes abolishes beta-adrenergic stimulation of HCN channels. These results demonstrate that PEX5R/Trip8b is an intrinsic auxiliary subunit of brain HCN channels and establish HCN-PEX5R/Trip8b coassembly as a mechanism to control the channels' responsiveness to cyclic nucleotide signaling.

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.

Antigen selectivity characteristic of polyclonal antibodies against omega-conotoxin GVIA and N-type voltage-dependent calcium channels

The antibodies against omega-conotoxin GVIA (omega-CTX GVIA; N-type voltage-dependent calcium channel [VDCC] blocker) and B1Nt (N-terminal segment [residues 1-13] of BI alpha1 subunits of VDCCs) were prepared, and the selectivity for each antigen omega-CTX GVIA and B1Nt was investigated. For the antigen selectivity of anti-omega-CTX GVIA antibody against omega-CTX GVIA, ELISA, and immunoprecipitation were used. The reactions for ELISA and immunoprecipitation were observed except when antibody IgG purified by Protein A-Sepharose CL-4B from nonimmunized serum (purified NI-Ab) was used. The specific reactions were inhibited by 10 nM omega-CTX GVIA, but not by omega-CTX SVIB (N-type VDCC blocker), omega-CTX MVIIC (N- and P-type VDCC blocker), or omega-Aga IVA (P-type VDCC blocker). For the antigen selectivity of the anti-B1Nt antibody, analyses by ELISA, immunoprecipitation, and Western blotting were conducted. The reactions were observed except when NI-Ab was used. The ELISA and immunoprecipitation reactions were inhibited by the antigen peptide B1Nt, and the IC50 values were about 1.2 x 10(-8) and 1.3 x 10(-8) M, respectively. The bands of 210 and 190 kD by Western blotting of crude membranes from chick brain were also inhibited by 1 microM B1Nt. These results suggest that the antibodies prepared against omega-CTX GVIA and B1Nt in this work have high selectivity for their antigen. Therefore we assume that the antibodies against omega-CTX GVIA and B1Nt are useful tools for the analyses of the function and distribution of N-type VDCCs. The anti omega-CTX GVIA antibody must also be useful for the radioimmunoassay of omega-CTX GVIA.

Characteristics of the inhibitory effect of calmodulin on specific [125i]omega-conotoxin GVIA binding to crude membranes from chick brain

The characteristics of the inhibitory effect of calcium ion (Ca2+)/calmodulin (CaM) on specific [125I]-omega-conotoxin GVIA (125I-omega-CTX) binding and on the labeling of 125I-omega-CTX to crude membranes from chick brain were investigated. The inhibitory effect of Ca2+/CaM depended on the concentrations of free Ca2+ and CaM. The IC50 values for free Ca2+ and CaM were about 2.0 x 10(-8) M and 3.0 microg protein/ml, respectively. The inhibitory effect of Ca2+/CaM was attenuated by the CaM antagonists W-7, prenylamine and CaM-kinase II fragment (290-309), but not by the calcineurin inhibitor FK506. Ca2+/CaM also inhibited the labeling of a 135-kDa band (which was considered to be part of N-type Ca2+ channel alpha1 subunits) with 125I-omega-CTX using a cross-linker. These results suggest that Ca2+/CaM affects specific 125I-omega-CTX binding sites, probably N-type Ca2+ channel alpha1 subunits, in crude membranes from chick whole brain.

The spider toxin omega-Aga IIIA defines a high affinity site on neuronal high voltage-activated calcium channels

The spider toxin omega-agatoxin IIIA (omega-Aga-IIIA) is a potent inhibitor of high voltage-activated calcium currents in the mammalian brain. To establish the biochemical parameters governing its action, we radiolabeled the toxin and examined its binding to native and recombinant calcium channels. In experiments with purified rat synaptosomal membranes, both kinetic and equilibrium data demonstrate one-to-one binding of omega-Aga-IIIA to a single population of high affinity sites, with K(d) = approximately 9 pm and B(max) = approximately 1.4 pmol/mg protein. Partial inhibition of omega-Aga-IIIA binding by omega-conotoxins GVIA, MVIIA, and MVIIC identifies N and P/Q channels as components of this population. omega-Aga-IIIA binds to recombinant alpha(1B) and alpha(1E) calcium channels with a similar high affinity (K(d) = approximately 5-9 pm) in apparent one-to-one fashion. Results from recombinant alpha(1B) binding experiments demonstrate virtually identical B(max) values for omega-Aga-IIIA and omega-conotoxin MVIIA, providing further evidence for a one-to-one stoichiometry of agatoxin binding to calcium channels. The combined evidence suggests that omega-Aga-IIIA defines a unique, high affinity binding site on N-, P/Q-, and R-type calcium channels.

High affinity interaction of mibefradil with voltage-gated calcium and sodium channels

Mibefradil is a novel Ca(2+) antagonist which blocks both high-voltage activated and low voltage-activated Ca(2+) channels. Although L-type Ca(2+) channel block was demonstrated in functional experiments its molecular interaction with the channel has not yet been studied. We therefore investigated the binding of [(3)H]-mibefradil and a series of mibefradil analogues to L-type Ca(2+) channels in different tissues. [(3)H]-Mibefradil labelled a single class of high affinity sites on skeletal muscle L-type Ca(2+) channels (K(D) of 2.5+/-0.4 nM, B(max)=56.4+/-2.3 pmol mg(-1) of protein). Mibefradil (and a series of analogues) partially inhibited (+)-[(3)H]-isradipine binding to skeletal muscle membranes but stimulated binding to brain L-type Ca(2+) channels and alpha1C-subunits expressed in tsA201 cells indicating a tissue-specific, non-competitive interaction between the dihydropyridine and mibefradil binding domain. [(3)H]-Mibefradil also labelled a heterogenous population of high affinity sites in rabbit brain which was inhibited by a series of nonspecific Ca(2+) and Na(+)-channel blockers. Mibefradil and its analogue RO40-6040 had high affinity for neuronal voltage-gated Na(+)-channels as confirmed in binding (apparent K(i) values of 17 and 1.0 nM, respectively) and functional experiments (40% use-dependent inhibition of Na(+)-channel current by 1 microM mibefradil in GH3 cells). Our data demonstrate that mibefradil binds to voltage-gated L-type Ca(2+) channels with very high affinity and is also a potent blocker of voltage-gated neuronal Na(+)-channels. More lipophilic mibefradil analogues may possess neuroprotective properties like other nonselective Ca(2+)-/Na(+)-channel blockers.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Neuroprotective action of a novel compound--M50463--in primary cultured neurons

The neuroprotective effects of a novel synthetic compound, M50463, have been determined by using embryonic rat neocortical neurons in various culture conditions. M50463 was initially characterized as a potent specific ligand for a voltage-dependent sodium channel by radioligand binding studies. In fact, M50463 inhibited neuronal cell death induced by veratrine and inhibited an increase of the intracellular calcium level in neurons evoked by veratrine. In addition to such expected effects, M50463 had the ability to prevent glutamate neurotoxicity, to promote the neuronal survival in serum-deprived medium and to prevent nitric oxide-induced neurotoxicity. These results suggested that M50463 is not a simple sodium channel blocker, but a neuroprotective agent which has some crucial mechanism of action on neuronal death occurring in various situations, and it is a novel, innovative candidate for neuroprotective therapy for various neurodegenerative disorders.

Effects of drugs interfering with sodium channels and calcium channels on the release of endogenous dopamine from superfused substantia nigra slices

The importance of voltage-dependent sodium channels and different types of voltage-sensitive calcium channels for depolarisation-induced release of endogenous dopamine from dendrites and cell bodies in superfused guinea pig substantia nigra slices was investigated. The stimulatory effect of veratridine (10 microM) on dopamine release was only marginally attenuated in Ca(2+)-free medium but was completely blocked by tetrodotoxin (1 microM) and by the dopamine reuptake inhibitor GBR 12909 (10 microM). Low extracellular concentration of Na+ stimulated the dopamine release. Potassium-evoked dopamine release was completely Ca(2+)-dependent, not blocked by GBR 12909 and partially blocked by tetrodotoxin. Nifedipine (20 microM), omega-conotoxin GVIA (0.5 microM), penfluridol (5 microM), and Ni2+ (20 microM) had no effect, amiloride (1 mM) attenuated and neomycin (350 microM), and omega-agatoxin IVA (1 microM) almost totally blocked the potassium-induced dopamine release. The results suggest that veratridine released dopamine mostly by reversing the dopamine transporter. High concentrations of potassium induced release of nigral dopamine by opening of voltage-sensitive calcium channels of P/Q type but not L-type, N-type and probably not T-type. The depolarisation evoked by high concentrations of potassium seems to open voltage-sensitive calcium channels both by the depolarisation induced by potassium per se and by the secondary depolarisation induced by opening of voltage-dependent sodium channels.

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.

Relationship between specific binding of 125I-omega-conotoxin GVIA and GTP binding protein: effects of the GTP analogues, mastoparan and A1F4-

We investigated whether the specific binding or labeling of 125I-omega-CgTX on crude membranes from chick whole brain was affected when endogenous GTP binding protein (G protein) was activated by GTP analogues, mastoparan (MP) and aluminum fluoride (AIF4-; AICl3 + NaF). Both GTPgammaS and Gpp(NH)p attenuated the inhibitory effect of selective N-type Ca channel inhibitors such as aminoglycoside antibiotics (AGs) or dynorphine (1-13)(Dyn) on specific 125I-omega-CgTX binding in a dose-dependent manner. On the other hand, the inhibitory effects of the divalent metal cations Cd2+, Co2+, Mg2+ and Mn2- on such binding were not attenuated by GTPgammaS. MP and AIF4- also attenuated the inhibitory effect of Neo on this binding similar to GTPgammaS. The attenuating effect of MP was enhanced by the presence of Mg2+ in a dose-dependent manner. However, GTP analogues, MP and AIF4-, did not affect binding or labeling without AGs or Dyn. GTPgammaS, MP and AIF4- also attenuated the specific labeling of a 215-kDa band in crude membranes with 125I-omega-CgTX using the cross-linker DSS (non-reduced condition) in the presence of Neo. These results indicate that there are direct or indirect relationships between N-type Ca channels and G proteins via binding sites for AGs or MP.

Different effects of reducing agents on omega-conotoxin GVIA inhibition of [3H]-acetylcholine release from rat cortical slices and guinea-pig myenteric plexus

1. The effect of reducing reagents on omega-conotoxin GVIA (omega-CgTX) inhibition of the release of [3H]-acetylcholine ([3H]-ACh) induced by tityustoxin, K+ 50 mM and electrical stimulation was investigated in rat brain cortical slices. 2. In cortical slices the inhibition of tityustoxin or electrically-stimulated [3H]-ACh release by omega-CgTX was dramatically increased by reducing reagents ascorbate or beta-mercaptoethanol. Dehydroascorbic acid did not substitute for ascorbate. 3. Depolarization induced by K+ 50 mM caused [3H]-ACh release from cortical slices which was not inhibited by omega-CgTX, even in the presence of ascorbate. 4. In the guinea-pig myenteric plexus, omega-CgTX inhibition of the tityustoxin induced release of [3H]-ACh was independent of ascorbate. 5. It is suggested that N-type-like calcium channels in guinea-pigs myenteric plexus may have pharmacological/biochemical diversity from similar channels of rat cerebral cortex.

Block of P/Q-type calcium channels by therapeutic concentrations of aminoglycoside antibiotics

Aminoglycoside antibiotics can cause neuromuscular block by inhibiting Ca2+ influx into motor nerve terminals. P/Q-type Ca2+ channels, which are formed by alpha 1A subunits, are mainly responsible for depolarization-dependent presynaptic Ca2+ entry in motor neurons. We therefore investigated the possibility that aminoglycosides function as P/Q-type channel blockers. They inhibited [125I]-omega-CTx-MVIIC binding to P/Q-type channels in guinea pig cerebellum membranes with nanomolar IC50 values (e.g., 8 nM for neomycin). Divalent cations decreased the apparent affinity of neomycin. Barium inward currents through alpha 1A subunits expressed in Xenopus oocytes were partially blocked by therapeutic concentrations of aminoglycosides. This explains that therapeutically relevant concentrations of these drugs decrease the reserve of neuromuscular transmission, which can lead to neuromuscular block. We conclude that micromolar concentrations of aminoglycosides block not only N-type but also P/Q-type channels in mammalian neurons.

Binding and labeling of omega-conotoxin GVIA in crude membranes from subfractionated fractions and various areas of chick brain

Specific binding and specific labeling of 125I-omega-CgTX were investigated in crude membranes from both subfractionated fractions and various brain areas in chick whole brain. The specific activities of the marker enzymes 2',3'-cyclic nucleotide 3'-phosphorylase, Na/K ATPase and succinic dehydrogenase in the subfractionated fractions were three- to five-fold higher than those in the P2 fraction. However, the amount of specific [125I] omega-CgTX binding in the fractions of synaptosomes and synaptic plasma membranes was only about 1.2-times higher than that in the P2 fraction. The characteristics of specific 125I-omega-CgTX labeling with disuccinimidyl suberate to the 135-kDa band were generally comparable to those of specific [125I] omega-CgTX binding sites. These results suggest that the specific binding sites of [125I] omega-CgTX were not localized the synaptosomes and synaptic plasma membranes fractions, although each fraction was well isolated from the others from which were decided by the strength of specific activity for marker enzymes.

Characteristics of [125I]omega-conotoxin labeling using bifunctional cross linker DSP in crude membranes from chick brain

Characteristic of [125I]omega-conotoxin (omega-CgTX) labeling using bifunctional cross linker (dithio bis[succinimidyl propionate]:DSP) was systematically investigated in crude membranes from chick whole brain. [125I]omega-CgTX specifically labeled 216 kDa as a main and 236 kDa as a minor bands in the crude membranes under non-reduced condition, but not labeled under reduced condition. We investigated the effect of various Ca channel antagonists on [125I]omega-CgTX labeling with DSP in detail, and found that there is a strong correlation between the effects of Ca channel antagonists on [125I]omega-CgTX labeling of the 216 kDa band and specific [125I]omega-CgTX binding. These results suggest that labeling of the 216 kDa band under non-reduced condition with [125I]omega-CgTX using DSP involves the specific binding sites of [125I]omega-CgTX, perhaps including one of the neuronal N-type Ca channel subunits in the crude membranes.

Characteristics of specific 125I-omega-conotoxin GVIA binding and 125I-omega-conotoxin GVIA labeling using bifunctional crosslinkers in crude membranes from chick whole brain

Characteristics of specific 125I-omega-conotoxin GVIA (125I-omega-CgTX) binding and 125I-omega-CgTX labeling using bifunctional crosslinkers were systematically investigated in crude membranes from chick whole brain. Aminoglycosides and dynorphine A (1-13) inhibited the specific binding of 125I-omega-CgTX, but not that of the L-type calcium ion channel antagonist [3H](+)PN200-110. It seems likely that the inhibitory effect of dynorphine A (1-13) does not involve kappa-opiate receptors, based on results with the opiate receptor antagonist naloxone and the kappa-opiate receptor agonist U50488H. Spider venom, Cd2+ and La3+ inhibited the specific binding of 125I-omega-CgTX, as well as that of [3H](+)PN200-110. Various L-type Ca2+ channel antagonists did not affect the specific binding of 125I-omega-CgTX. 125I-omega-CgTX specifically labeled 135 kDa and 215 kDa bands in crude membranes under reduced and non-reduced conditions, respectively. The crosslinker disuccinimidyl suberate (DSS) yielded better 125I-omega-CgTX labeling than the other two crosslinkers tested. We investigated the effect of various Ca2+ channel antagonists on 125I-omega-CgTX labeling with DSS in detail, and found that there is a strong correlation between the effects of Ca2+ channel antagonists on 125I-omega-CgTX labeling of the 135 kDa band and specific 125I-omega-CgTX binding. These results suggest that aminoglycosides and dynorphine A (1-13) are specific inhibitors of specific 125I-omega-CgTX binding, and that labeling of the 135 kDa band with 125I-omega-CgTX using DSS involves the specific binding sites of 125I-omega-CgTX, perhaps including one of the neuronal N-type Ca2+ channel subunits in the crude membranes.

Type III omega-agatoxins: a family of probes for similar binding sites on L- and N-type calcium channels

The peptide omega-agatoxin-IIIA (omega-Aga-IIIA) from venom of the funnel web spider Agelenopsis aperta is the only known agent that blocks L-type and N-type Ca channels with equal high potency (IC50 < or = 1 nM). From the same venom, we have purified and sequenced a family of peptides which are homologous to omega-Aga-IIIA but vary over 100-fold in their relative affinity for L-type versus N-type Ca channels. One of these, omega-Aga-IIIB, is 76 amino acids long and identical to omega-Aga-IIIA in 66 positions. We identified two other similar peptides, omega-Aga-IIIC and omega-Aga-IIID, as well as one single amino acid variant of omega-Aga-IIIA and two of omega-Aga-IIIB. The type III omega-agatoxins exhibit similar but distinct activities on voltage-gated Ca channels. omega-Aga-IIIA, omega-Aga-IIIB, and omega-Aga-IIID are nearly indistinguishable in their actions at the insect neuromuscular junction (no effect at 0.1 microM), on atrial T-type Ca channels (no effect at 0.5 microM), and in two assays for synaptosomal Ca channels: they are nearly equipotent inhibitors of 125I-omega-conotoxin GVIA binding to rat brain synaptic membranes (IC50 = 0.17-0.33 nM) and blockers of the K(+)-induced 45Ca2+ influx into chick brain synaptosomes (omega-Aga-IIIB, 1.2 nM; omega-Aga-IIIA, 2.4 nM). In contrast, omega-Aga-IIIA is a better blocker of locust Ca channels (IC50 approximately 10-50 nM) than is omega-Aga-IIIB. Finally, although omega-Aga-IIIA, omega-Aga-IIIB, and omega-Aga-IIID all block atrial L-type Ca channels, omega-Aga-IIIA is over 100-fold more potent. Thus, although type III omega-agatoxins appear to recognize a binding site common to L- and N-type Ca channels, omega-Aga-IIIB and omega-Aga-IIID identify differences between the two channels.

A toxin from the venom of the predator snail Conus textile modulates ionic currents in Aplysia bursting pacemaker neuron

Conus textile crude venom and a peptide component ('King Kong' toxin) purified from this venom, alter membrane excitability of Aplysia neurons. Venom, applied to the medium bathing an abdominal ganglion, changes dramatically the electrical activity of bursting pacemaker neuron. The effects on bursting neuron R15 was examined in current-clamp and voltage-clamp modes. A dual phase effect of both the venom and the purified toxin were observed. The first phase starts immediately after venom or toxin application and is observed as an increase in membrane excitability, resulting in an enhancement of bursting. The second phase begins about 15 min later and consists of a long-lasting hyperpolarization. The dual phase effect of the venom and the toxin persists even when synaptic input is eliminated either by axotomy, or by recording from freshly dissociated neurons or from neurons in primary cell culture. The ionic currents affected are an inward current, INSR, which is activated upon depolarization and an anomalously rectifying potassium current, IR, which is activated upon hyperpolarization. In the first phase of toxin action INSR is increased. In the second phase both the venom and the toxin block INSR and increase IR. The toxin effects may be due to complex alteration of one or more second messenger cascades rather than a direct action on ion channels.

Neuropharmacological characterization of voltage-sensitive calcium channels: possible existence of neomycin-sensitive, omega-conotoxin GVIA- and dihydropyridines-resistant calcium channels in the rat brain

We attempted to characterize the functional roles of subtypes of voltage-sensitive calcium channels in the brain. The maximal number of [125I]omega-conotoxin GVIA (omega-CTX) binding sites in rat brain associated with N-type calcium channels (N-channels) was approximately 10 times more than that of [3H]-PN200-110 associated with L-type calcium channels (L-channels). [125I]omega-CTX binding was inhibited by aminoglycoside antibiotics, neomycin and dynorphin A(1-13), but not by various classes of L-channel antagonists. A 6-hydroxydopamine-induced lesion of the striatum resulted in a marked reduction of both [125I]-omega-CTX and [3H]PN200-110 binding. Kainic acid-induced lesion of the striatum reduced [3H]PN200-110 binding by 57%, but did not reduce [125I]omega-CTX binding. Omega-CTX produced a small (18%) but significant reduction of potassium-stimulated Ca2+ influx into rat brain synaptosomes, although it produced a concentration-dependent inhibition in chick brain synaptosomes. Neomycin inhibited Ca2+ influx in both preparations in a concentration-dependent manner. Both omega-CTX and neomycin inhibited potassium-stimulated [3H]dopamine (DA) release from rat striatal slices. The L-channel antagonists had no effect on either Ca2+ influx or [3H]DA release. These results suggest that DA release in the striatum is regulated by Ca2+ influx through N-channels located in presynaptic nerve terminals, and that the most of the Ca2+ influx in rat brain appears to be governed by neomycin-sensitive, omega-CTX- and DHP-resistant calcium channels.

Characteristics of specific 125I-omega-conotoxin GVIA binding in rat whole brain

Characteristics of specific 125I-omega-conotoxin (omega-CgTX) binding were systematically investigated in crude membranes from rat whole brain. Kd and Bmax Values for the binding were 49.7 pM and 181.5 fmol/mg of protein, respectively. The effects of various types of Ca channel antagonists on the binding were investigated. Dynorphin A (1-13), in particular, specifically inhibited 125I-omega-CgTX binding, but not that of [3H](+)PN200-110. Spider venom from Plectreurys tristes did not specifically inhibit specific binding of 125I-omega-CgTX, because the venom also inhibited the binding of [3H](+)PN200-110 to a similar degree. The amount of specific binding of 125I-omega-CgTX was less in the cerebellum than that in any other area of whole brain. The cross-linker disuccinimidyl suberate did not label with 125I-omega-CgTX and its binding sites in rat whole brain, although it did in chick whole brain, which was used as a positive control. These findings suggested that dynorphine A (1-13) was a selective blocker of omega-CgTX-sensitive Ca channels in crude membranes from rat whole brain and that omega-CgTX-sensitive Ca channels were mainly present a rat brain except cerebellum.

Characterization of the purified N-type Ca2+ channel and the cation sensitivity of omega-conotoxin GVIA binding

A functional N-type Ca2+ channel (omega-conotoxin GVIA receptor) has been purified from rabbit brain and shown to be composed of four subunits of molecular weights 230 K (alpha 1B), 160 K (alpha 2 delta), 95 K and 57 K (beta 3) [Witcher D. R., De Waard M., Sakamoto J., Franzini-Armstrong C., Pragnell M., Kahl S.D. and Campbell K. D. (1993) Science 261: 486-489]. These four subunits migrate on sucrose density gradients as a single complex and are identified by subunit specific polyclonal antibodies. Polyclonal antibodies against the purified receptor complex immunoprecipitate greater than 90% of the [125I]omega-conotoxin GVIA (omega-CgTx) binding sites in solubilized crude rabbit brain membranes. Furthermore, polyclonal antibodies affinity-purified against unique GST fusion proteins from two of the cloned subunits in the complex (alpha 1B and beta 3) specifically immunoprecipitated [125I]omega-CgTx binding sites and not [3H]PN200-110 binding sites. Analysis of [125I]omega-CgTx binding to the purified N-type Ca2+ channel demonstrated that the equilibrium binding was sensitive to increasing cation concentration. The IC50 for calcium and barium was 2.5 and 5 mM, respectively. [125I]omega-CgTx binding was not significantly reduced within 15 min after the addition of 50 mM barium. However, single channel analysis of the purified N-type Ca2+ channel preincubated with 10 microM omega-CgTx demonstrated that in the presence of 50 mM barium and 0.5 microM omega-CgTx, channel activity was detected but at a low open state probability (P < 0.10). These data suggest that the Ca2+ binding site(s) allosterically regulates the omega-CgTx binding site. Since the channel gating persisted in the presence of omega-CgTx, the omega-CgTx binding site may not be located within the pore of the channel and may be different from intra-pore Ca2+ binding sites.

Pharmacological characterisation of the calcium channels coupled to the plateau phase of KCl-induced intracellular free Ca2+ elevation in chicken and rat synaptosomes

The effect of various blockers of voltage operated calcium channels (VOCCs) was studied on the non-inactivating, plateau phase of KCl-induced intracellular free Ca2+ ([Ca2+]i) elevation in rat cortical and chicken forebrain synaptosomes. In chicken synaptosomes, omega-CgTx GVIA (0.1 nM to 1 microM) and omega-CgTx MVIIA (0.1 nM to 1 microM), both selective blockers of N-type Ca2+ channels, produced a concentration-dependent inhibition of the plateau phase of [Ca2+]i elevation. omega-CgTx GVIA (IC50 value 28 nM) was more potent than omega-CgTx MVIIA (IC50 value 78 nM), but at submaximal concentrations, took longer to reach its maximum effect (20 min for omega-CgTx GVIA; 10 min for omega-CgTx MVIIA). At 1 microM, the highest concentration tested, each toxin blocked > 85% of [Ca2+]i elevation. The effect of omega-CgTx GVIA on the extent and time-course of inhibition of [Ca2+]i elevation was maintained in a Na(+)-free, choline substituted, medium. omega-Aga IVA (300 nM), a selective blocker of P-type calcium channels, inhibited 28 +/- 5% of [Ca2+]i elevation. The effect of a combination of submaximal inhibitory concentrations of omega-CgTx GVIA (100 nM) and omega-Aga IVA (300 nM) was less than additive. In rat synaptosomes, omega-CgTx GVIA (1 microM) and omega-CgTx MVIIA (1 microM), blocked only 18 +/- 5% and 17 +/- 4% of the plateau phase of free Ca2+ elevation, respectively. omega-Aga IVA produced a concentration-dependent inhibition of [Ca2+]i elevation in this preparation. Threshold inhibition was observed at 1 nM, and maximum inhibition (64 +/- 8%) at 1 microM.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence of mammalian Ca2+ channel inhibitors in venom of the spider Plectreurys tristis

Plectreurys tristis venom inhibited K(+)-stimulated Ca2+ influx in a concentration-dependent manner in rat (0.5-4.0 micrograms venom protein/ml) and chicken (1.0-64.0 micrograms venom protein/ml) brain synaptosomes. In contrast to Hololena curta venom or omega conotoxin GVlA which both show selectivity for avian synaptosomes, inhibition of Ca2+ influx by the venom appeared to be relatively selective for rat synaptosomes. Plectreurys tristis venom also inhibited K(+)-evoked release of [3H](-)-noradrenaline from labeled rat cortical synaptosomes. Responses to electric field stimulation of the sympathetically innervated rat vas deferens in vitro were inhibited by Plectreurys tristis venom at dilutions similar to those which inhibited Ca2+ influx in synaptosomes. Inhibition persisted following washout of the venom. K(+)-evoked contractions of rat aortic rings were relaxed by the dihydropyridine antagonist (-)-202-791, but not by Plectreurys tristis venom, thus precluding an effect on K(+)-depolarized smooth muscle L-type channels. Contractions to exogenous (-)-noradrenaline in rat aorta were not inhibited by Plectreurys tristis venom, ruling out an effect on alpha 1-adrenergic receptors, and further suggesting a prejunctional site of action. The results suggest that this venom inhibits N-type Ca2+ channels, as well as unclassified Ca2+ channels, which are neither N- nor L-type.

Identification of bis(agmatine)oxalamide in venom from the primitive hunting spider, Plectreurys tristis (Simon)

N, N'-bis(4-guanidinobutyl)oxalamide, a novel bis(agmatine)oxalamide, is identified as a major component (8 micrograms/microliters) and the predominant acylpolyamine in venom from the primitive hunting spider, Plectreurys tristis. The function of this compound is unknown since it does not confer insecticidal or fungicidal activity in the systems examined.

Action and binding of omega-conotoxin on the putative calcium channel of synaptosomal plasma membrane from electric organ of Japanese electric ray, Narke japonica

Actions of omega-conotoxin GVIA on synaptosomes isolated from a Japanese electric ray, Narke japonica, were investigated. omega-Conotoxin inhibited, in a dose-dependent manner, both increases in free calcium concentration in, and acetylcholine release from synaptosomes depolarized with a high concentration of potassium ions. The concentrations of omega-conotoxin required for half-maximal inhibition (IC50) of increase in intrasynaptosomal Ca and acetylcholine release were 8 and 7 microM, respectively. Assay using radioiodinated toxin derivative revealed a specific binding site with a dissociation constant (KD) of 2.8 microM and a density (Bmax) of 45 pmol/mg protein of synaptosome. Binding assay with synaptosomal plasma membrane showed a KD = 7 microM and a Bmax = 200 pmol/mg protein. Autoradiography with sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis after covalent cross-linking of the toxin, using disuccinimidyl suberate, revealed the 170,000 mol. wt peptide to be an omega-conotoxin receptor. The present study has directly and clearly shown that omega-conotoxin inhibits acetylcholine release by blocking Ca influx into nerve terminals. The 170,000 mol. wt peptide identified as a receptor of the toxin exists in high density in the plasma membrane of the presynaptic nerve terminal and is likely to be a component of a voltage-dependent Ca channel responsible for the neurotransmitter release.

Actions of neomycin on neuronal L-, N-, and non-L/non-N-type voltage-sensitive calcium channel responses

The effects of neomycin on neuronal voltage-sensitive calcium channel (VSCC) responses were investigated by evaluating its effects on calcium-dependent neuronal responses that are sensitive and insensitive to the N-type voltage-sensitive calcium channel antagonist omega-conotoxin GVIA and the L-type VSCC antagonist nitrendipine. Chick synaptosomal 45Ca2+ influx and K(+)-evoked release of [3H]norepinephrine from chick cortical brain slices were omega-conotoxin GVIA sensitive and nitrendipine insensitive, suggesting that these responses are mediated predominantly by N-type VSCC. The K(+)-evoked increase of intracellular calcium in cortical neurons and the K(+)-evoked release of [3H]norepinephrine from rat brain cortical slices was partially sensitive to omega-conotoxin GVIA and nitrendipine, suggesting that these responses are mediated by N-, L- and non-L/non-N-type VSCC. Rat synaptosomal 45Ca2+ influx and the K(+)-evoked release of [3H]D-aspartate from rat hippocampal slices were completely insensitive to omega-conotoxin GVIA and nitrendipine, suggesting that these responses were mediated predominantly by non-L/non-N-type VSCC. Neomycin caused a concentration-dependent and virtually complete inhibition of all response parameters, with IC50 values ranging from 90 to 400 microM. The results suggest that neomycin is a nonselective inhibitor of neuronal responses mediated by L-, N-, and non-L/non-N-type VSCC.

P-type calcium channels blocked by the spider toxin omega-Aga-IVA

Voltage-dependent calcium channels mediate calcium entry into neurons, which is crucial for many processes in the brain including synaptic transmission, dendritic spiking, gene expression and cell death. Many types of calcium channels exist in mammalian brains, but high-affinity blockers are available for only two types, L-type channels (targeted by nimodipine and other dihydropyridine channel blockers) and N-type channels (targeted by omega-conotoxin). In a search for new channel blockers, we have identified a peptide toxin from funnel web spider venom, omega-Aga-IVA, which is a potent inhibitor of both calcium entry into rat brain synaptosomes and of 'P-type' calcium channels in rat Purkinje neurons. omega-Aga-IVA will facilitate characterization of brain calcium channels resistant to existing channel blockers and may assist in the design of neuroprotective drugs.

N-type and L-type calcium channels are present in nerve growth cones. Numbers increase on synaptogenesis

We have demonstrated the presence of both N- and L-type calcium channels in growth cone and other subcellular fractions of fetal rat brain, using the ligands omega-conotoxin GVIA for N-type channels and nitrendipine for L-type channels. The N-type channels seem to be distributed evenly throughout the perikaryon, neurite shaft and growing tip of the neurons. In contrast, the L-type channels appear to have a lower density in the growth cone than on the rest of the neuron. These observations apply at least within the limitations of cell fractionation technology. We have also studied both calcium channel subtypes in rat brain synaptosomal membranes. In both adult and fetal fractions there are approximately 6 times more N-type than L-type channels. Synaptosomal membranes contain more N- and L-type channels than any of the fetal subfractions, indicating that there is a substantial increase in calcium channel numbers upon synaptogenesis.

The inhibition of [125I]omega-conotoxin GVIA binding to neuronal membranes by neomycin may be mediated by a GTP-binding protein

omega-Conotoxin GVIA (omega-CT) has been reported to block calcium currents at the L- and N-type calcium channels. In neuronal membranes omega-CT, and the aminoglycoside antibiotic neomycin, have been shown to inhibit [125I]omega-CT binding, presumably acting at the N-type calcium channel. We demonstrate here that the concentration curve for neomycin sulfate inhibition of [125I]omega-CT binding is shifted to the right by GTP analogues or fluoride, increasing the IC50 for neomycin. [125I]omega-CT binding is unaffected by these agents and in competition studies the potency of omega-CT, Ca2+, or La3+ is not modulated by GTP analogues or fluoride. These results indicate that the inhibition of [125I]omega-CT binding by neomycin may be mediated by a GTP binding protein.

A monoclonal antibody to conotoxin reveals the distribution of a subset of calcium channels in the rat cerebellar cortex

Voltage-sensitive calcium channels (VSCC) are a family of ionophores having different electrical and pharmacological properties. The omega-conotoxin GVIA (omega-CgTX) is a specific blocker of one subset of VSCCs. Because of the specificity of this toxin, a monoclonal anti-omega-CgTX antibody was generated against a omega-CgTX-key hole limpet hemocyanin conjugate and used as a specific marker to study VSCC distributions. This mab was shown to recognize omega-CgTX on Western blots and to display omega-CgTX-dependent immunoperoxidase staining of rat cerebellum. Incubation of fresh, unfixed sections of adult rat cerebellum in omega-CgTX followed by light fixation and peroxidase immunocytochemistry with mab anti-omega-CgTX revealed a specific pattern of labelling. All principal classes of cerebellar neurons were immunoreactive, but in general glial cells were not stained. Most interestingly, strong focal immunoreactivity was encountered at branching points of Purkinje cell dendrites. This characteristic staining pattern implies that a subset of VSCC is specifically concentrated in these regions and suggests that these channels may play a role in the functional integration of dendritic signals.

Classification of presynaptic calcium channels at the squid giant synapse: neither T-, L- nor N-type

We examined both pharmacological and functional characteristics of the calcium channels which trigger transmitter secretion from giant nerve terminals of squid. These calcium channels are insensitive to organic calcium channel blockers such as dihydropyridines and omega-conotoxin GVIA, moderately sensitive to cadmium, activated by very small depolarizations and slowly inactivating. We conclude that the characteristics of these presynaptic channels do not correspond to the properties of T-, L-, or N-type calcium channels found in other cells.

The polyamine spermine affects omega-conotoxin binding and function at N-type voltage-sensitive calcium channels

1. The effects of the polyamines, spermine and spermidine on neuronal N-type voltage-sensitive calcium channels were investigated using the binding and function of the ligand omega-conotoxin GVIA (omega-CT). 2. Spermine and spermidine enhanced (EC50 approximately 0.16 and 0.45 microM) and, at higher concentrations, inhibited (IC50 of 9 and 240 microM) the binding of [125I]omega-CT to rat hippocampal synaptosomes. 3. Spermine and, less potently, spermidine inhibited the neurotransmitter-mediated, omega-CT-sensitive, electrical-field-stimulated contractile responses of the rat vas deferens. 4. The polyamines also inhibited the phenylephrine-evoked contractile responses of the vas deferens with the same rank order, consistent with a postsynaptic mechanism of inhibition. 5. However, pre-exposure to spermine prevented the irreversible inhibition of vas deferens twitch responses by omega-CT (previously found to be presynaptic). The prevention of inhibition by omega-CT demonstrates that the neuronal binding of spermine and omega-CT is mutually exclusive. Thus spermine (and presumably spermidine at higher concentrations) appears to modulate the actions of omega-CT at N-type voltage-sensitive calcium channels.