Differential effects of calcium channel antagonists (omega-conotoxin GVIA, nifedipine, verapamil) on the electrically-evoked release of [3H]acetylcholine from the myenteric plexus, phrenic nerve and neocortex of rats

Intestinal distension orchestrates neuronal activity in the enteric nervous system of adult mice

The enteric nervous system (ENS) regulates the motor, secretory and defensive functions of the gastrointestinal tract. Enteric neurons integrate mechanical and chemical inputs from the gut lumen to generate complex motor outputs. How intact enteric neural circuits respond to changes in the gut lumen is not well understood. We recorded intracellular calcium in live-cell confocal recordings in neurons from intact segments of mouse intestine in order to investigate neuronal response to luminal mechanical and chemical stimuli. Wnt1-, ChAT- and Calb1-GCaMP6 mice were used to record neurons from the jejunum and colon. We measured neuronal calcium response to KCl (75 mM), veratridine (10 μM), 1,1-dimethyl-4-phenylpiperazinium (DMPP; 100 μM) or luminal nutrients (Ensure®), in the presence or absence of intraluminal distension. In the jejunum and colon, distension generated by the presence of luminal content (chyme and faecal pellets, respectively) renders the underlying enteric circuit unresponsive to depolarizing stimuli. In the distal colon, high levels of distension inhibit neuronal response to KCl, while intermediate levels of distension reorganize Ca2+ response in circumferentially propagating slow waves. Mechanosensitive channel inhibition suppresses distension-induced Ca2+ elevations, and calcium-activated potassium channel inhibition restores neuronal response to KCl, but not DMPP in the distended colon. In the jejunum, distension prevents a previously unknown tetrodotoxin-resistant neuronal response to luminal nutrient stimulation. Our results demonstrate that intestinal distension regulates the excitability of ENS circuits via mechanosensitive channels. Physiological levels of distension locally silence or synchronize neurons, dynamically regulating the excitability of enteric neural circuits based on the content of the intestinal lumen. KEY POINTS: How the enteric nervous system of the gastrointestinal tract responds to luminal distension remains to be fully elucidated. Here it is shown that intestinal distension modifies intracellular calcium levels in the underlying enteric neuronal network, locally and reversibly silencing neurons in the distended regions. In the distal colon, luminal distension is integrated by specific mechanosensitive channels and coordinates the dynamics of neuronal activation within the enteric network. In the jejunum, distension suppresses the neuronal calcium responses induced by luminal nutrients. Physiological levels of distension dynamically regulate the excitability of enteric neuronal circuits.

Detection of Ca2+-induced acetylcholine released from leukemic T-cells using an amperometric microfluidic sensor

A microfluidic structured-dual electrodes sensor comprising of a pair of screen printed carbon electrodes was fabricated to detect acetylcholine, where one of them was used for an enzyme reaction and another for a detection electrode. The former was coated with gold nanoparticles and the latter with a porous gold layer, followed by electropolymerization of 2, 2:5,2-terthiophene-3-(p-benzoic acid) (pTTBA) on both the electrodes. Then, acetylcholinesterase was covalently attached onto the reaction electrode, and hydrazine and choline oxidase were co-immobilized on the detection electrode. The layers of both modified electrodes were characterized employing voltammetry, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, and quartz crystal microscopy. After the modifications of both electrode surfaces, they were precisely faced each other to form a microfluidic channel structure, where H₂O₂ produced from the sequential enzymatic reactions was reduced by hydrazine to obtain the analytical signal which was analyzed by the detection electrode. The microfluidic sensor at the optimized experimental conditions exhibited a wide dynamic range from 0.7nM to 1500μM with the detection limit of 0.6 ± 0.1nM based on 3s (S/N = 3). The biomedical application of the proposed sensor was evaluated by detecting acetylcholine in human plasma samples. Moreover, the Ca²⁺-induced acetylcholine released in leukemic T-cells was also investigated to show the in vitro detection ability of the designed microfluidic sensor. Interference due to the real component matrix were also studied and long term stability of the designed sensor was evaluated. The analytical performance of the designed sensor was also compared with commercially available ACh detection kit.

The organ-protective effect of N-type Ca(2+) channel blockade

The six subtypes of voltage-dependent Ca(2+) channels (VDCCs) mediate a wide range of physiological responses. N-type VDCCs (NCCs) were originally identified as a high voltage-activated Ca(2+) channel selectively blocked by omega-conotoxin (ω-CTX)-GVIA. Predominantly localized in the nervous system, NCCs are key regulators of neurotransmitter release. Both pharmacological blockade with ω-CTX-GVIA and, more recently, mice lacking CNCNA1B, encoding the α1B subunit of NCC, have been used to assess the physiological and pathophysiological functions of NCCs, revealing in part their significant roles in sympathetic nerve activation and nociceptive transmission. The evidence now available indicates that NCCs are a potentially useful therapeutic target for the treatment of several pathological conditions. Efforts are therefore being made to develop effective NCC blockers, including both synthetic ω-CTX-GVIA derivatives and small-molecule inhibitors. Cilnidipine, for example, is a dihydropyridine L-type VDCC blocking agent that also possesses significant NCC blocking ability. As over-activation of the sympathetic nervous system appears to contribute to the pathological processes underlying cardiovascular, renal and metabolic diseases, NCC blockade could be a useful approach to treating these ailments. In this review article, we provide an overview of what is currently known about the physiological and pathophysiological activities of NCCs and the potentially beneficial effects of NCC blockade in several disease conditions, in particular cardiovascular diseases.

Behavioral and neurochemical characterization of mice deficient in the N-type Ca2+ channel alpha1B subunit

N-type voltage-dependent calcium channels (VDCCs) play an important role in neurotransmission, synaptic plasticity, and brain development. They are composed of several subunits named alpha(1), alpha(2), delta, beta and gamma. The alpha(1) subunit is essential for channel functions and determines fundamental channel properties. Since N-type VDCC are critically involved in the release of neurotransmitters and clinical relevance, we predicted that alpha(1) subunit KO mice would show several alterations in behavior. In the present study, we investigated neuronal functions in mice lacking the alpha(1B) (Ca(V)2.2) subunit of the N-type calcium channels. Ca(V)2.2(-/-) mice exhibited a significant increase in locomotion on an activity wheel during the dark phase. Furthermore, when challenged with apomorphine, mutant mice showed enhanced locomotor activity. Cognitive functions were examined using a Y-maze task for short-term memory and a passive avoidance task for long-term memory. The Y-maze revealed no differences in spontaneous alternation behavior between mutant and wild-type mice. The passive avoidance test revealed that the latency time in mutant mice was significantly decreased. The mutant mice showed prepulse inhibition deficits reminiscent of the sensorimotor gating deficits observed in a large majority of schizophrenic patients. Decreases in baseline levels of dopamine and serotonin within the striata and frontal cortices of mutant mice were also observed. These results suggest that Ca(2+) in the central nervous system modulates various neurophysiological functions, such as locomotor activity, long-term memory, and sensorimotor gating through the alpha(1B) subunit of the N-type calcium channels.

Dual purinergic synaptic transmission in the human enteric nervous system

Based on findings in rodents, we sought to test the hypothesis that purinergic modulation of synaptic transmission occurs in the human intestine. Time series analysis of intraneuronal free Ca(2+) levels in submucosal plexus (SMP) from Roux-en-Y specimens was done using Zeiss LSM laser-scanning confocal fluo-4 AM Ca(2+) imaging. A 3-s fiber tract stimulation (FTS) was used to elicit a synaptic Ca(2+) response. Short-circuit current (I(sc) = chloride secretion) was recorded in mucosa-SMP in flux chambers. A distension reflex or electrical field stimulation was used to study I(sc) responses. Ca(2+) imaging was done in 1,222 neurons responding to high-K(+) depolarization from 61 surgical cases. FTS evoked synaptic Ca(2+) responses in 62% of recorded neurons. FTS caused frequency-dependent Ca(2+) responses (0.1-100 Hz). FTS Ca(2+) responses were inhibited by Omega-conotoxin (70%), hexamethonium (50%), TTX, high Mg(2+)/low Ca(2+) (< or = 100%), or capsaicin (25%). A P2Y(1) receptor (P2Y(1)R) antagonist, MRS-2179 or PLC inhibitor U-73122, blocked FTS responses (75-90%). P2Y(1)R-immunoreactivity occurred in 39% of vasoactive intestinal peptide-positive neurons. The selective adenosine A(3) receptor (AdoA(3)R) agonist 2-chloro-N(6)-(3-iodobenzyl)adenosine-5'-N-methylcarboxamide (2-Cl-IBMECA) caused concentration- and frequency-dependent inhibition of FTS Ca(2+) responses (IC(50) = 8.5 x 10(-8) M). The AdoA(3)R antagonist MRS-1220 augmented such Ca(2+) responses; 2-Cl-IBMECA competed with MRS-1220. Knockdown of AdoA(1)R with 8-cyclopentyl-3-N-(3-{[3-(4-fluorosulphonyl)benzoyl]-oxy}-propyl)-1-N-propyl-xanthine did not prevent 2-Cl-IBMECA effects. MRS-1220 caused 31% augmentation of TTX-sensitive distension I(sc) responses. The SMP from Roux-en-Y patients is a suitable model to study synaptic transmission in human enteric nervous system (huENS). The P2Y(1)/Galphaq/PLC/inositol 1,3,5-trisphosphate/Ca(2+) signaling pathway, N-type Ca(2+) channels, nicotinic receptors, and extrinsic nerves contribute to neurotransmission in huENS. Inhibitory AdoA(3)R inhibit nucleotide or cholinergic transmission in the huENS.

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.

N-type calcium channel alpha1B subunit (Cav2.2) knock-out mice display hyperactivity and vigilance state differences

Differential properties of voltage-dependent Ca2+ channels have been primarily ascribed to the alpha1 subunit, of which 10 different subtypes are currently known. For example, channels that conduct the N-type Ca2+ current possess the alpha1B subunit (Cav2.2), which has been localized, inter alia, to the piriform cortex, hippocampus, hypothalamus, locus coeruleus, dorsal raphe, thalamic nuclei, and granular layer of the cortex. Some of these regions have been previously implicated in metabolic and vigilance state control, and selective block of the N-type Ca2+ channel causes circadian rhythm disruption. In this study of Cav2.2-/- knock-out mice, we examined potential differences in feeding behavior, spontaneous locomotion, and the sleep-wake cycle. Cav2.2-/- mice did not display an overt metabolic phenotype but were hyperactive, demonstrating a 20% increase in activity under novel conditions and a 95% increase in activity under habituated conditions during the dark phase, compared with wild-type littermates. Cav2.2-/- mice also displayed vigilance state differences during the light phase, including increased consolidation of rapid-eye movement (REM) sleep and increased intervals between non-REM (NREM) and wakefulness episodes. EEG spectral power was increased during wakefulness and REM sleep and was decreased during NREM sleep in Cav2.2-/- mice. These results indicate a role of the N-type Ca2+ channel in activity and vigilance state control, which we interpret in terms of effects on neurotransmitter release.

Electrical stimulation reveals complex neuronal input and activation patterns in single myenteric guinea pig ganglia

The myenteric plexus plays a key role in the control of gastrointestinal motility. We used confocal calcium imaging to study responses to electrical train stimulation (ETS) of interganglionic fiber tracts in entire myenteric ganglia of the guinea pig small intestine. ETS induced calcium transients in a subset of neurons: 52.2% responded to oral ETS, 65.4% to aboral ETS, and 71.7% to simultaneous oral and aboral ETS. A total of 41.3% of the neurons displayed convergence of oral and aboral ETS-induced responses. Responses could be reversibly blocked with TTX (10(-)6 M), demonstrating involvement of neuronal conduction, and by removal of extracellular calcium. omega-Conotoxin (5 x 10(-7) M) blocked the majority of responses and reduced the amplitude of residual responses by 45%, indicating the involvement of N-type calcium channels. Staining for calbindin and calretinin did not reveal different response patterns in these immunohistochemically identified neurons. We conclude that, at least for ETS close to a ganglion, confocal calcium imaging reveals complex oral and aboral input to individual myenteric neurons rather than a polarization in spread of activity.

Characterizing voltage-dependent Ca(2+) channels coupled to VIP release and NO synthesis in enteric synaptosomes

In enteric synaptosomes of the rat, the role of voltage-dependent Ca(2+) channels in K(+)-induced VIP release and nitric oxide (NO) synthesis was investigated. Basal VIP release was 39 +/- 4 pg/mg, and cofactor-substituted NO synthase activity was 7.0 +/- 0.8 fmol. mg(-1). min(-1). K(+) depolarization (65 mM) stimulated VIP release Ca(2+) dependently (basal, 100%; K(+), 172.2 +/- 16.2%; P < 0.05, n = 5). K(+)-stimulated VIP release was reduced by blockers of the P-type (omega-agatoxin-IVA, 3 x 10(-8) M) and N-type (omega-conotoxin-GVIA, 10(-6) M) Ca(2+) channels by ~50 and 25%, respectively, but not by blockers of the L-type (isradipine, 10(-8) M), Q-type (omega-conotoxin-MVIIC, 10(-6) M), or T-type (Ni(2+), 10(-6) M) Ca(2+) channels. In contrast, NO synthesis was suppressed by omega-agatoxin-IVA, omega-conotoxin-GVIA, and isradipine by ~79, 70, and 70%, respectively, whereas Ni(2+) and omega-conotoxin-MVIIC had no effect. These findings are suggestive of a coupling of depolarization-induced VIP release primarily to the P- and N-type Ca(2+) channels, whereas NO synthesis is presumably dependent on Ca(2+) influx not only via the P- and N- but also via the L-type Ca(2+) channel. In contrast, none of the Ca(2+) channel blockers affected VIP release evoked by exogenous NO, suggesting that NO induces VIP secretion by a different mechanism, presumably involving intracellular Ca(2+) stores.

Vagal afferent responses to fatty acids of different chain length in the rat

The role of cholecystokinin (CCK) in the effect of dietary lipid on proximal gastrointestinal function and satiety is controversial. Recent work suggests that fatty acid chain length may be a determining factor. We investigated the mechanism by which long- and short-chain fatty acids activate jejunal afferent nerves in rats. Whole mesenteric afferent nerve discharge was recorded in anaesthetized male Wistar rats during luminal perfusion of saline, sodium oleate, and sodium butyrate (both 10 mM). Both fatty acids evoked characteristic afferent nerve responses, distinct from the mechanical response to saline, that were abolished in rats following chronic subdiaphragmatic vagotomy. The effect of oleate was abolished by the CCK-A receptor antagonist Devazepide (0.5 mg/kg), whereas the effect of butyrate persisted despite pretreatment with either Devazepide or a combination of the calcium channel inhibitors nifedipine (1 mg/kg) and the omega-conotoxins GVIA and SVIB (each 25 microg/kg). In summary, long- and short-chain fatty acids activate intestinal vagal afferents by different mechanisms; oleate acts via a CCK-mediated mechanism and butyrate appears to have a direct effect on afferent terminals.

Functional disorders of the sympathetic nervous system in mice lacking the alpha 1B subunit (Cav 2.2) of N-type calcium channels

N-type voltage-dependent Ca(2+) channels (VDCCs), predominantly localized in the nervous system, have been considered to play an essential role in a variety of neuronal functions, including neurotransmitter release at sympathetic nerve terminals. As a direct approach to elucidating the physiological significance of N-type VDCCs, we have generated mice genetically deficient in the alpha(1B) subunit (Ca(v) 2.2). The alpha(1B)-deficient null mice, surprisingly, have a normal life span and are free from apparent behavioral defects. A complete and selective elimination of N-type currents, sensitive to omega-conotoxin GVIA, was observed without significant changes in the activity of other VDCC types in neuronal preparations of mutant mice. The baroreflex response, mediated by the sympathetic nervous system, was markedly reduced after bilateral carotid occlusion. In isolated left atria prepared from N-type-deficient mice, the positive inotropic responses to electrical sympathetic neuronal stimulation were dramatically decreased compared with those of normal mice. In contrast, parasympathetic nervous activity in the mutant mice was nearly identical to that of wild-type mice. Interestingly, the mutant mice showed sustained elevation of heart rate and blood pressure. These results provide direct evidence that N-type VDCCs are indispensable for the function of the sympathetic nervous system in circulatory regulation and indicate that N-type VDCC-deficient mice will be a useful model for studying disorders attributable to sympathetic nerve dysfunction.

Calcium channels involved in the inhibition of acetylcholine release by presynaptic muscarinic receptors in rat striatum

1. The mechanism of the inhibitory action of presynaptic muscarinic receptors on the release of acetylcholine from striatal cholinergic neurons is not known. We investigated how the electrically stimulated release of [3H]-acetylcholine from superfused rat striatal slices and its inhibition by carbachol are affected by specific inhibitors of voltage-operated calcium channels of the L-type (nifedipine), N-type (omega-conotoxin GVIA) and P/Q-type (omega-agatoxin IVA). 2. The evoked release of [3H]-acetylcholine was not diminished by nifedipine but was lowered by omega-conotoxin GVIA and by omega-agatoxin IVA, indicating that both the N- and the P/Q-type (but not the L-type) channels are involved in the release. The N-type channels were responsible for approximately two thirds of the release. The release was >97% blocked when both omega-toxins acted together. 3. The inhibition of [3H]-acetylcholine release by carbachol was not substantially affected by the blockade of the L- or P/Q-type channels. It was diminished but not eliminated by the blockade of the N-type channels. 4. In experiments on slices in which cholinesterases had been inhibited by paraoxon, inhibition of [3H]-acetylcholine release by endogenous acetylcholine accumulating in the tissue could be demonstrated by the enhancement of the release after the addition of atropine. The inhibition was higher in slices with functional N-type than with functional P/Q-type channels. 5. We conclude that both the N- and the P/Q-type calcium channels contribute to the stimulation-evoked release of acetylcholine in rat striatum, that the quantitative contribution of the N-type channels is higher, and that the inhibitory muscarinic receptors are more closely coupled with the N-type than with the P/Q-type calcium channels.

Role of cAMP and neuronal K+ channels on alpha 2-AR-induced inhibition of ACh release in equine trachea

To investigate the effects of changes in intracellular cAMP on alpha 2-adrenoceptor (AR)-induced inhibition of airway acetylcholine (ACh) release, we examined the effects of the alpha 2-AR agonist clonidine on electrical field stimulation-evoked ACh release from equine tracheal parasympathetic nerves before and after treatment with 8-bromo-cAMP or forskolin. We also tested whether charybdotoxin (ChTX)- or iberiotoxin (IBTX)-sensitive Ca(2+)-activated K+ channels mediate alpha 2-AR-induced inhibition by examining the effect of clonidine in the absence and presence of ChTX or IBTX on ACh release. The amount of released ACh was measured by HPLC coupled with electrochemical detection. Clonidine (10(-7) to 10(-5) M) dose dependently inhibited ACh release before and after treatment with 8-bromo-cAMP (10(-3) M) or forskolin (3 x 10(-5) M). ChTX and IBTX, both at the concentration of 5 x 10(-7) M, significantly increased ACh release; however, they did not alter the magnitude of clonidine-induced inhibition. These results indicated that in equine tracheal parasympathetic nerves, alpha 2-AR-induced inhibition of ACh release is via an intracellular cAMP-independent pathway. Activation of both ChTX- and IBTX-sensitive Ca(2+)-activated K+ channels inhibits the electrical field stimulation-evoked ACh release, but these channels are not involved in the alpha 2-AR-induced inhibition of ACh release.

The effect of water soluble cyanotoxin(s) produced by two species of Anabaena on the release of acetylcholine from the peripheral cholinergic nervous system of the rat airway

A water extract of the lyophilised fresh-water alga Anabaena flos-aquae enhanced substantially the release of [(3)H]acetylcholine ([(3)H]acetylcholine and [(3)H]choline) from cholinergic nerves of rat bronchi. Parallel experiments performed with the related species Anabaena lemmermannii did not demonstrate this effect. The effect on the release of [(3)H]acetylcholine by A. flos-aquae extract was concentration dependent. The A. flos-aquae induced [(3)H]acetylcholine release was not reduced by exposure to a low concentration of Ca(2+), but ω-conotoxin GVIA (1.0 μM), a blocker of N-type Ca(2+) channels reduced the release of [(3)H]acetylcholine induced by the A. flos-aquae extract. Addition of verapamil in a concentration (1.0 μM) specific for inhibition of L-type Ca(2+) channels had no effect on the neurotransmitter release. A reduction in the release was, moreover, observed with the intracellular Ca(2+) chelator BAPTA/AM (30 μM) and with the Na(+) channel blocker tetrodotoxin (3.0 μM). During patch-clamp studies of GH(4)C(1) neuronal cells, which have L- and T-type Ca(2+) channels, but no Na(+) channels, it was shown that a water extract of A. flos-aquae depolarised these cells and reduced, rather than enhanced, the influx of Ca(2+). Such an effect was not seen following exposure of GH(4)C(1) cells to water extracts of A. lemmermannii. In addition to its presynaptic activity, the water extract of A. flos-aquae showed an antimuscarinic effect by displacing [(3)H]QNB binding from muscarinic receptors in homogenates of rat bronchi. A similar but more potent effect was observed during experiments with water extract of A. lemmermannii. None of the respective water extracts showed any effects on cholinesterase activities in rat bronchial smooth muscle. The present observations suggest, therefore, that water extracts of A. flos-aquae may depolarise cells by activation of mono and divalent cation channels in cholinergic nerve cells. These channels are probably Na(+) channels and N-type, but not L- or T-type Ca(2+) channels. L- and T-type Ca(2+) channels were blocked in experiments with GH(4)C(1) cells and high concentrations of Ca(2+) channel blockers were necessary to reduce the effects of A. flos-aquae extract in cholinergic nerves in the airways. Furthermore, A. flos-aquae extract may also mobilise Ca(2+) from intracellular compartments. A. lemmermannii, on the other hand, does not contain components which alter mono and divalent cation-fluxes across cell membranes, but may rather have substances with more potent antagonistic effects on muscarinic cholinergic receptors than what is observed in experiments with A. flos-aquae.

Effect of subtype-specific Ca(2+)-antagonists and Ca(2+)-free media on the field stimulation-evoked release of ATP and [3H]acetylcholine from rat habenula slices

The involvement of different subtypes of voltage-sensitive (Ca2+ channels in the initiation of field stimulation-induced endogenous adenosine triphosphate (ATP) and [3H]acetylcholine ([3H]ACh) release was investigated in the superfused rat habenula slices. ATP, measured by the luciferin-luciferase assay, and [3H]ACH were released simultaneously from the tissue in response to low frequency electrical stimulation (2 Hz, 2.5 msec, 360 shocks). The N-type Ca(2+)-channel blocker omega-conotoxin GVIA (omega-CgTX, 0.01-1 microM) reduced the stimulation-evoked release of ATP and [3H]ACh in a dose-dependent manner. Similarly, the P-type Ca2+ channel antagonist omega-agatoxin IVA (omega-Aga IVA) (0.05 microM) and the inorganic Ca(2+)-channel blocker Ca2+ (0.2 mM) inhibited the outflow of both transmitters, while Ni2+ (0.1 mM) was without significant effect. A high correlation was observed between the percent inhibition of ATP release and percent inhibition of ACh release caused by the different Ca2+ antagonists. Long-term perfusion (i.e., 90 min) with Ca(2+)-free solution inhibited the evoked-release of ATP and [3H]ACh. In contrast, perfusion of slices with the same media for a shorter time (i.e., 20 min) did not reduce the release of [3H]ACh and ATP but even increased the evoked-release of ATP about fourfold. The breakdown of extracellular ATP was not blocked under low [Ca2+]0 condition, measured by the creatine phosphokinase assay and HPLC-UV technique. Application of extra- or intracellular Ca2+ chelators, and dipyridamole (2 microM), the nucleoside transporter inhibitor, did not reduce the excess release of ATP after short-term perfusion with Ca(2+)-free media. Tetrodotoxin (TTX, 1 microM), while inhibiting the majority of ATP release under normal conditions, was also unable to reduce release under low [Ca2+]0 conditions. In summary, we showed that both N- and P-type Ca2+ channels are involved in the initiation of electrical stimulation-evoked release of ATP and [3H]ACh in the rat habenula under normal extracellular calcium concentration. Under low [CA2+]0 conditions an additional release of ATP occurs, which is not associated with action potential propagation.

Role of L- and N-type Ca2+ channels in muscarinic receptor-mediated facilitation of ACh and noradrenaline release in the rat urinary bladder

1. 3H-Noradrenaline (NA) and 14C-acetylcholine (ACh) released by electrical field stimulation were measured simultaneously in strips from the body of rat urinary bladder. 2. omega-Conotoxin GVIA (omega-CgTX; 20-100 nM) suppressed the non-facilitated transmitter release evoked by intermittent stimulation (IS), whereas nifedipine (1 microM) did not affect release. 3. Continuous electrical stimulation (CS) facilitated NA and ACh release via an atropine-sensitive mechanism. omega-CgTX reduced the facilitated release of NA (44% depression) but did not affect ACh release. Nifedipine depressed ACh release (43%) but not NA release. Combined administration of nifedipine and omega-CgTX (20 nM) produced a greater suppression of NA and ACh release (86 and 91%, respectively). 4. Maximal muscarinic facilitation of NA (5-fold) and ACh (17-fold) release occurred following administration of eserine, an anticholinesterase agent. Release of both NA and ACh was depressed by nifedipine (70 and 83%, respectively) but not by omega-CgTX. Combined application of omega-CgTX and nifedipine elicited a further depression of NA (95%) but not ACh release. 5. When NA and ACh release was facilitated with phorbol dibutyrate (0.5 microM), nifedipine inhibited ACh (67%) but not NA release, whereas omega-CgTX inhibited NA (73%) but not ACh release. Combined administration of both Ca2+ channel blockers did not elicit greater inhibition. 6. Bay K 8644, the L-type Ca2+ channel activator, increased ACh release in a dose-dependent manner (up to 5-fold) but did not significantly change NA release. 7. Both omega-CgTX (20-100 nM) and nifedipine (100 nM-1 microM) significantly decreased (50-80%) the neurally evoked contractions of the bladder strips. 8. It is concluded that L-type Ca2+ channels play a major role in muscarinic facilitation of NA and ACh release in the urinary bladder but are not essential for non-facilitated release. Other types of Ca2+ channels, including N-type, are involved to varying degrees in non-facilitated and facilitated release under different experimental conditions.

Mode and mechanism of neurotensin action in rat proximal colon

This study examined the mechanism of action of neurotensin on intraluminal pressure in rat proximal colon. The direct and indirect contractile response to neurotensin (100 nM) was abolished in Ca(2+)-free solution, and was antagonized by nifedipine (1-5-10 nM) and potentiated by Bay K 8644 (methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)- pyridine-5-carboxylate) (10-100-1000 nM). Neurotensin, in the presence of nifedipine (10 nM) and atropine (1 microM), induced a tetrodotoxin-insensitive inhibitory effect, which was antagonized by SR 48692 (2[(1-(7-chloro-4-quinolinyl)-5-(2,6-dimethoxy-phenyl)pyrazol-3-yl) carbonyl amino]tricyclo (3.3.1.1.(3.7)) decan-2-carboxylic acid) (300 nM) or apamin (0.1 microM). The results demonstrate that the neurotensin response is dependent on the influx of Ca2+ via L-type channels and results from summation of excitatory and inhibitory effects.

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.

Inhibition of neuromuscular transmission in the myenteric plexus of guinea-pig ileum by omega-conotoxins GVIA, MVIIA, MVIIC and SVIB

1. The effects of a number of Ca2+ channel blockers on the transmural electrical stimulation or receptor agonist-elicited contractile responses of guinea-pig ileum were compared. 2. omega-Conotoxins (MVIIA, GVIA, SVIB and MVIIC), but not omega-agatoxin IVA, completely blocked the twitch responses evoked by low frequency (0.1 Hz) transmural stimulation without inhibition of the contractures evoked by exogenous acetylcholine. The concentration-inhibition curves were shifted by changes of external Ca2+. 3. The tetanic contractures produced by a high frequency (30 Hz) train of stimulation were inhibited by omega-conotoxins by only 25-30%, except for omega-conotoxin MVIIC, which produced about 55% inhibition, all significantly less than that produced by atropine (about 70%) or tetrodotoxin (about 85%). Combinations of omega-conotoxins did not produce additive inhibitory effects. 4. The four omega-conotoxins as well as atropine produced similar partial inhibition (53-62%) of the contractures evoked by dimethylphenylpiperazinium, while tetrodotoxin inhibited the contracture completely. 5. Nifedipine and Ni2+ depressed the nerve stimulation-evoked twitch response and tetanic contracture as well as acetylcholine contracture. 6. These observations suggest that, in the myenteric plexus, a subset of N-type Ca2+ channel dominates under low frequency stimulation, while high frequency stimulation may recruit additional channels and non-cholinergic pathways.

Non-adrenergic, non-cholinergic inhibitory junction potential in rat colonic circular muscle is partly sensitive to omega-conotoxin GVIA and resistant to L-, P- or Q-type calcium channel blockers

The effects of several Ca2+ channel blockers were evaluated on inhibitory junction potential (IJP) evoked in rat colonic circular muscle by electrical field stimulation (EFS). Glass microelecrodes were used to record membrane potential of smooth muscle cells. IJPs were tetrodotoxin-sensitive (1 microM) and disappeared in Ca(2+)-free solution. L-type calcium channels blockers, such as nifedipine (1 microM) or verapamil (1 microM), did not affect IJPs. IJPs were significantly reduced by omega-conotoxin GVIA (300 nM), an N-type Ca2+ channel blocker. IJPs were resistant to omega-agatoxin IVA (50 nM), a P-type Ca2+ channel blocker, and omega-conotoxin MVIIC (1 microM), which blocks both N- and Q-type Ca2+ channels at micromolar concentrations. We conclude that the release of NANC neurotransmitter-mediating IJPs in the rat colon evoked by EFS involves N-type Ca2+ channels. The fact that omega-conotoxin GVIA does not abolish the IJPs suggests a putative role for L-, P- or Q-type Ca2+ channels.

Calcium channel subtypes for the sympathetic and parasympathetic nerves of guinea-pig atria

1. The Ca2+ channel subtypes of the autonomic nerves of guinea-pig atria were elucidated by monitoring the effects of specific Ca2+ channel blockers on the negative and positive inotropic responses associated respectively, with stimulation of the parasympathetic and sympathetic nerves. 2. In left atria paced at 2-4 Hz, the negative inotropic effect induced by field stimulation of parasympathetic nerves (in the presence of propranolol) was abolished by omega-conotoxin MVIIC, a blocker of N-type and OPQ subfamily Ca2+ channels. omega-Conotoxin GVIA (an N-type blocker), omega-agatoxin IVA (a P-type blocker), nifedipine (an L-type blocker) and Ni2+ (a T- and R-type blocker) were much less effective. 3. The positive inotropic response resulting from field stimulation of the sympathetic nerves (in the presence of atropine) was abolished by both omega-conotoxins, while omega-agatoxin IVA, nifedipine and Ni2+ were ineffective. 4. In the spontaneously beating right atria, the early negative inotropic effect produced by 1,1-dimethyl-4-phenylpiperazinium was abolished by omega-conotoxin MVIIC, whereas the late positive inotropic effect was partially reduced, but not abolished, by a high concentration of omega-conotoxin GVIA. 5. None of the peptide toxins affected the chronotropic and the inotropic responses evoked by carbachol and isoprenaline. 6. These results suggested that, under physiological conditions, the release of acetylcholine from parasympathetic nerves is dominated by an OPQ subfamily Ca2+ channel while that of noradrenaline from sympathetic nerves is controlled by an N-type Ca2+ channel. Ligand-induced noradrenaline release appeared to recruit additional type(s) of Ca2+ channel.

A role for Q type Ca2+ channels in neurotransmission in the rat urinary bladder

1. In isolated bladder strips of the rat, a substantial component (46%) of the Ca(2+)-dependent contractile response to electrical field stimulation (5 Hz) was resistant to combined block of both N and P type Ca2+ channels by omega-conotoxin-GVIA (300 nM) and omega-agatoxin-IVA (100 nM) respectively. 2. The resistant portion (non-N, non-P) was sensitive to omega-conotoxin-MVIIC (3 microM), which in addition to N and P also blocks Q type channels at this concentration. omega-Conotoxin-MVIIC administered alone, inhibited the neurogenic response to the same degree as that observed in the combined presence of omega-agatoxin-IVA, omega-conotoxin-GVIA and omega-conotoxin-MVIIC. 3. omega-Agatoxin-IVA (100 nM), a concentration that fully inhibits P type channels, had a negligible effect on the neurogenic response. Following blockade of N type Ca2+ channels with omega-conotoxin-GVIA (300 nM), omega-agatoxin-IVA (3 microM) (a concentration well above that used to block P channels, inhibits Q type channels, but spares N type channels), inhibited the residual response to the same degree as omega-conotoxin-MVIIC alone. 4. Results suggest that neurotransmission in rat urinary bladder is supported by both N and Q type Ca2+ channels.

The effect of PhTx3 on the release of 3H-acetylcholine induced by tityustoxin and potassium in brain cortical slices and myenteric plexus

The venom of the Brazilian spider Phoneutria nigriventer possesses several neurotoxic polypeptidic fractions. Previous work has established that one of the toxic components, PhTx3, inhibited Ca(2+)-dependent glutamate release and the increase in cytosolic free Ca2+ in response to membrane depolarization. In the present work, we investigated the effect of PhTx3 on the release of acetylcholine (ACh) from brain and peripheral neurons. PhTx3 decreased the release of [3H]-ACh induced by tityustoxin and KCl in brain cortical slices and myenteric plexus. The inhibitory effect of myenteric plexus had the same magnitude as that obtained in the absence of extracellular Ca2+. However, in brain PhTx3 was less efficient at decreasing the evoked release of ACh. These experiments suggest that the target of PhTx3 is coupled to the process of release of ACh in brain and autonomic nervous system.

P-type Ca2+ channels trigger stimulus-evoked [3H]acetylcholine release from mammalian motor endplates

In the present experiments it was tested whether omega-agatoxin-IVA, a peptide blocking P-type voltage-dependent Ca2+ channels, inhibits the evoked release of newly synthesized [3H]acetylcholine from the rat phrenic nerve. Release of [3H]acetylcholine was evoked by electrical stimulation of the isolated phrenic nerve (100 or 750 pulses at 5 Hz). omega-Agatoxin-IVA inhibited evoked [3H]acetylcholine release in a concentration-related manner; inhibition started at a concentration of 30 nM with complete block occurring at 500 nM. In conclusion, the present experiments demonstrate that omega-agatoxin-IVA-sensitive P-type Ca2+ channels are critically involved in the regulation of stimulus-induced transmitter release at mammalian motor endplates.

Properties of the voltage-gated calcium channels mediating dopamine and acetylcholine release from the isolated rat retina

We examined the properties of voltage-gated calcium channels mediating endogenous dopamine (DA) and acetylcholine (ACh) release in the isolated rat retina. Application of 30 mM KCl elicited the release of DA and ACh, and these releases were abolished in Ca(2+)-free medium. The high K(+)-evoked DA release was largely blocked by both of omega-agatoxin IVA and omega-conotoxin MVIIC, P- and Q-type calcium channel antagonists, and partly blocked by isradipine, and L-type calcium channel antagonist, and omega-conotoxin GVIA, an N-type calcium channel antagonist. omega-Agatoxin IVA at a small dose, sufficient to block P-type channels alone, was however without effect. On the other hand, the high K(+)-evoked ACh release was partly blocked by omega-agatoxin IVA and omega-conotoxin MVIIC, but was resistant to isradipine and omega-conotoxin GVIA. Flunarizine, a non-selective T-type calcium channel antagonist, did not inhibit the release of DA and ACh. Cd2+ markedly blocked the release of both DA and ACh, Co2+ and Ni2+ slightly blocked the release of DA, and the release of ACh was not blocked by these two divalent cations. These results suggest that the high K(+)-evoked release of retinal DA is largely mediated by omega-agatoxin IVA and omega-conotoxin MVIIC sensitive calcium channels (probably Q-type channels), while the release of retinal ACh is largely mediated by as yet uncharacterized Cd2+ sensitive calcium channels. The properties of voltage-gated calcium channels involved in the release of ACh in the rat retina differ from those of DA.

Inhibition of low- and high-threshold Ca2+ channels of human neuroblastoma IMR32 cells by Lambert-Eaton myasthenic syndrome (LEMS) IgGs

IgGs from two LEMS patients applied to human neuroblastoma IMR32 cells reduced the density of low- (LVA; T) and high-threshold (HVA; L and N) Ba2+ currents by different percentages: 36% (LVA) and 56% (HVA) for one and 48% and 45% for the other. A pharmacological assay of IgGs action based on the block of L-type channel by nifedipine and on the delayed activation of N-type channel by noradrenaline, indicated a preferential inhibition of the N-type current in IMR32 cells (55% and 47% for the two patients). The L-type current, contributing to approximately one-third of the total, was also depressed by LEMS IgGs but to a minor degree (49% and 30%). Except for an increase of single N-type channel inactivation, LEMS antibodies preserved the elementary properties of single HVA channels, suggesting that the macroscopic current reduction after IgGs treatment is likely due to a decrease in the number of active HVA Ca2+ channels.

Effect of omega-agatoxin-IVA on autonomic neurotransmission

omega-Agatoxin-IVA, a peptide from the venom of the funnel-web spider Agelenopsis aperta and a P type Ca2+ channel inhibitor, was examined for effects on responses to nerve stimulation in isolated autonomic neuroeffector preparations from the rabbit, guinea-pig and rat. Ca(2+)-dependent, tetrodotoxin sensitive, noradrenergic excitatory responses of rabbit pulmonary artery, rat vas deferens, and anococcygeus muscles, and cholinergic guinea-pig myenteric plexus preparations (all highly sensitive to the N type Ca2+ channel inhibitor omega-conotoxin-GVIA) were unaffected by omega-agatoxin-IVA (100 nM). Similarly, the neurogenic response of rat bladder, which has cholinergic, and non-adrenergic non-cholinergic (NANC) excitatory components, and the NANC inhibitory response of rat jejunum (atropine 0.5 microM- and guanethidine 5.0 microM-treated), which are partially sensitive and insensitive to omega-conotoxin-GVIA, respectively, were unaffected by omega-agatoxin-IVA (100 nM). Neurogenic NANC inhibitory responses of the guinea-pig taenia caecum, and rat anococcygeus muscles (atropine- and guanethidine-treated, and tone raised with prostaglandin F2 alpha), were also insensitive to omega-agatoxin-IVA. These results suggest that P type Ca2+ channels, if present, play an insignificant role in supplying the Ca2+ necessary for neurotransmitter release in the peripheral autonomic nervous system.

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

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

Calcium-channel blockers and gastrointestinal motility: basic and clinical aspects

Several calcium-channel blockers currently in use for the treatment of cardiovascular disorders have recently been tested for their effects on gastrointestinal motility. The rationale for this approach centers on the concept that calcium-channel blockers are at least as potent in inhibiting intestinal smooth muscle as in relaxing vascular smooth muscle. This review will give an outline of the most recent findings on the role of calcium and calcium channels in smooth muscle and neuronal function in the digestive system. It will also consider the mechanisms by which calcium-channel blockers may affect gastrointestinal motility and assess potential clinical applications in gastroenterology. The main goal for researchers in this field will be the development of gut-selective agents, with no cardiovascular side effects.

K(+)-stimulated 45Ca2+ flux into rat neocortical mini-slices is blocked by omega-Aga-IVA and the dual Na+/Ca2+ channel blockers lidoflazine and flunarizine

High-threshold neuronal voltage-sensitive Ca2+ channels (VSCCs) have been classified into at least three subtypes, including L, N, and P, based on biophysical and pharmacological criteria. We examined K(+)-induced 45Ca2+ flux into rat neocortical mini-slices to determine which of these subtype(s) might be involved in this phenomenon. Neither the L-type Ca2+ channel antagonist isradipine at 10 microM nor the N-type antagonist omega-conotoxin GVIA at 1 microM were effective antagonists of 45Ca2+ flux in this model. However, the P-type Ca2+ channel antagonist, omega-Aga-IVA, blocked 70% of flux at 200 nM, with an IC50 of 17 nM, strongly implicating P-type Ca2+ channel involvement in K(+)-stimulated Ca2+ entry into mammalian nerve terminals. About 30% of the flux response was resistant to the action of omega-Aga-IVA, suggesting that a still uncharacterized subtype of VSCC is involved in Ca2+ entry into mammalian nerve terminals. Both the omega-Aga-IVA sensitive and insensitive components of 45Ca2+ flux were blocked by the diphenylalkylpiperazines, lidoflazine and flunarizine (IC50 = 6.4 microM and 11 microM, respectively), which have dual Na+/Ca2+ channel blocking actions.

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

The presence of P-type calcium channels in synaptosomes prepared from electric organ of Torpedo marmorata was investigated by using the venom of Agelenopsis aperta, a toxin purified from it, FTX, and its synthetic analog. We analysed the action of these agents on acetylcholine release which was continuously followed using a chemiluminescent assay. Agelenopsis aperta venom, FTX and synthetic FTX inhibit acetylcholine release from synaptosomes induced by a presynaptic membrane depolarization with 60 mM KCl. A stronger inhibition of acetylcholine release was observed with the venom than with FTX (70 and 50%, respectively). Another way of triggering acetylcholine release from Torpedo synaptosomes is to insert in the presynaptic membrane a calcium ionophore A23187 which allows the bypass of the natural calcium channels. The venom of Agelenopsis aperta inhibits A23187-evoked acetylcholine release. Purified and synthetic FTX does not possess this property, suggesting that this inhibition of acetylcholine release was due to other toxins of the venom. Another type of pharmacological sensitivity of Torpedo calcium channels was also demonstrated using omega-conotoxin GVIA. At a concentration of 20 microM, this toxin was able to inhibit about 35% of KCl-evoked acetylcholine release. When FTX + omega-conotoxin GVIA were applied together, the inhibitory effect on KCl-evoked acetylcholine release was not significantly increased in comparison with the one observed with FTX alone. In conclusion, we examined the effect of different agents on acetylcholine release from Torpedo marmorata electric organ synaptosomes; acetylcholine release was elicited with KCl depolarization and followed continuously with a chemiluminescent assay.(ABSTRACT TRUNCATED AT 250 WORDS)

Calcium channels at the adrenergic neuroeffector junction in the rabbit ear artery

Neurotransmitter release is dependent on influx of Ca2+ through voltage-operated calcium channels (VOCCs). These channels may be divided into L, N, T and P subtypes. To investigate the subtypes of VOCC involved in transmitter release from adrenergic nerves in the isolated rabbit ear artery, the effects of some subtype selective VOCC antagonists were examined on contractile responses induced by electrical field stimulation (EFS), and exposure to an isosmolar (low Na+, normal Cl- content) or a hyperosmolar (normal Na+, high Cl- content) 60 mM K+ solution. Tetrodotoxin (TTX) and the L channel blocker nimodipine were present in the latter experiments to inhibit sodium-dependent action potential discharge and the direct contractile effect of K+ depolarization on the smooth muscle cells. Prazosin abolished the contractile effect of EFS, indicating that the response was elicited by activation of adrenergic nerves. The EFS-induced contractions were concentration-dependently inhibited by the N channel blocker omega-conotoxin (pIC50 = 9.0) and the proposed L channel blocker T-cadinol (pIC50 = 4.5), while nimodipine and the T channel blocker tetramethrin had no effect. The isosmolar and hyperosmolar K+ solutions induced a prazosin-sensitive contraction, amounting to 46% and 10% of the response to 10(-5) M noradrenaline (NA), respectively. omega-Conotoxin inhibited the contractile response to the hyperosmolar K+ solution, but not that to the isosmolar K+ solution. T-cadinol preferentially inhibited the response to the hyperosmolar K+ solution. Tetramethrin had no effect on contractions induced by either type of K+ solution.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of endogenous acetylcholine release by blockade of voltage-dependent calcium channels in enteric neurons of the guinea-pig colon

The effects on acetylcholine release from the guinea-pig colon of the N-type calcium channel blocker omega-conotoxin GVIA (omega-conotoxin), the L-type calcium channel blocker nifedipine and the putative blocker of T-type channels, flunarizine, have been investigated. Endogenous basal acetylcholine release and electrically (1 Hz, 1 ms, 450 mA)-evoked overflow in the presence of cholinesterase inhibitor were studied. omega-Conotoxin (1-10 nM) and nifedipine (0.03-3 microM) dose-dependently inhibited basal and electrically-evoked acetylcholine release. Maximal inhibition of basal or electrically-evoked acetylcholine release was about 40% for nifedipine and about 75% for omega-conotoxin. The potency of nifedipine was inversely related to the external calcium concentration: its EC50 value in low-calcium medium (0.5 mM) was as low as 12 nM. Flunarizine inhibited acetylcholine release only at concentrations higher than 0.2 microM. Our results are consistent with an involvement of N- and L-type calcium channels in the control of the endogenous acetylcholine release from the guinea-pig colon.

Transmitter release and calcium currents at an Aplysia buccal ganglion synapse--I. Characterization

The Ca2+ current recorded in the presynaptic neuron (B4/B5) of an identified Aplysia synapse was characterized in terms of its activation, voltage sensitivity, Ca2+ dependence of inactivation and pharmacology. It was compared to that recorded in left upper quadrant abdominal ganglion neurons which, unlike B4/B5, display Ca2+ action potentials. The two Ca2+ currents could not be distinguished in terms of their activation threshold or voltage sensitivity. The Ca2+ current recorded in left upper quadrant neurons, however, displayed more important Ca(2+)-dependent inactivation. The peak Ca2+ current in B4/B5 neurons was significantly reduced (30-40%) by the dihydropyridine Ca2+ channel antagonist, nifedipine, while it was increased (15-20%) by the dihydropyridine Ca2+ channel agonist, BAY K8644, although none of these agents had any effect on transmitter release from B4/B5. omega-Conotoxin similarly reduced the Ca2+ current by 30-40%, but unlike nifedipine, it also caused a 50-60% reduction in B4/B5 transmitter release. The pharmacological properties of the Ca2+ current present in left upper quadrant neurons were somewhat different, as this current was unaffected by either BAY K8644 or omega-conotoxin and moderately suppressed (20%) by nifedipine.

Pharmacological characterization of voltage-sensitive Ca2+ channels in autonomic nerves

Hololena curta venom a potent inhibitor of voltage sensitive Ca2+ channels and neurotransmitter release in mammalian brain, and synthetic funnel web spider toxin an inhibitor of P channels, were examined for their activity on autonomic nerves. Hololena curta (0.5 to 5.0 micrograms venom protein/ml) potently inhibited motor responses of the cholinergic guinea pig ileum myenteric plexus and the adrenergic rat anococcygeus muscle. Synthetic funnel web spider toxin was inactive at concentrations up to 100 microM. Hololena curta inhibited K+, and electrically evoked release of tritium from labeled superfused tissues. Furthermore, K(+)-contracted rat aorta was not relaxed by Hololena curta thereby precluding effects of Hololena curta on postjunctional L type smooth muscle Ca2+ channels. The pattern of effects of Hololena curta on peripheral autonomic nerves was similar to the N channel inhibitor omega-conotoxin GVIA. These results suggest that Hololena curta venom constituents block Ca2+ channels in peripheral autonomic nerves. The study failed to establish the presence of functional P type Ca2+ channels on these peripheral autonomic nerves and further suggests that N type channels may be exclusively responsible for supplying the Ca2+ necessary for neurotransmitter release in these nerves.

Effects of stimulus intensity on the inhibition by omega-conotoxin GVIA and neomycin of K(+_-evoked [3H]norepinephrine release from hippocampal brain slices and synaptosomal calcium influx

The effects of various K+ concentrations on the inhibition of [3H]norepinephrine release from rat hippocampal brain slices and evoked synaptosomal 45Ca2+ influx by omega-conotoxin GVIA (omega-CgTx) and neomycin were examined. K+ (15-75 mM) caused a concentration-dependent release of [3H]norepinephrine that was greater than 90% dependent on extracellular calcium. The ability of omega-CgTx to inhibit [3H]norepinephrine release was optimal at 25 mM K+ and was reduced substantially at higher concentrations of K+. omega-CgTx maximally inhibited [3H]norepinephrine release by 49% (15 mM K+), 58% (25 mM K+), 22% (50 mM K+), and 12% (75 mM K+). In contrast, neomycin caused a concentration-dependent and virtually complete inhibition of [3H]norepinephrine release at all concentrations of K+, with IC50 values of 210 microM (15 mM K+), 150 microM (25 mM K+), 450 microM (50 mM K+), and 1500 microM (75 mM K+). omega-CgTx (1 microM) had little effect (10% or less inhibition) on hippocampal synaptosomal 45Ca2+ influx at any concentration of K+, whereas 3 mM neomycin caused at least 75% inhibition of 45Ca2+ influx, with the largest inhibition (96%) occurring at 25 mM K+. The results suggest that increasing stimulus intensity decreases the contribution of N-type voltage-sensitive calcium channels (VSCC) in mediating K(+)-evoked release of [3H]norepinephrine. The comparative absence of omega-CgTx-sensitive synaptosomal 45Ca(2+)-influx sites suggests that N-type calcium channels are a small subset of channels in rat hippocampal synaptosomes. The demonstration that neomycin can inhibit omega-CgTx-sensitive and -insensitive neurotransmitter release and calcium influx suggests that neomycin may block N-type VSCC as well as non-N-type VSCC.

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

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

Vasoactive intestinal polypeptide induces neurogenic contraction of guinea-pig ileum. Involvement of acetylcholine and substance P

The effect and mode of action of vasoactive intestinal polypeptide (VIP), a peptidergic neuromodulator in the gastrointestinal nervous system, were investigated in isolated muscle strips of the guinea-pig ileum. VIP induced concentration-dependent (20 nM-1 microM) contractions of longitudinal ileal strips. TTX (1 microM), a mixture of atropine (3 microM) and spantide (30 microM), a mixture of atropine (3 microM) and omega-conotoxin GVIA (100 nM), somatostatin (60 nM) and dynorphin (100 nM) abolished the effect of VIP. In most cases a small relaxation became evident. Desensitization to substance P in the presence of atropine prevented VIP-induced contraction. A partial inhibition was observed in the presence of atropine (3 microM), spantide (30 microM), omega-conotoxin GVIA (100 nM), beta-endorphin (265 nM), met-enkephalin (1100 nM) and a mixture of spantide (30 microM) and omega-conotoxin GVIA (100 nM). The action of VIP was not significantly modified by guanethidine (3 microM) or hexamethonium (150 microM). In circular ileal strips VIP (10-300 nM) caused concentration-dependent relaxations through a direct myogenic effect. These results indicate that the VIP produced contractions of the guinea-pig ileum are exclusively neurally mediated and involve a cholinergic as well as a noncholinergic-nonadrenergic (NANC) pathway. It is concluded that besides acetylcholine (Ach) VIP releases the peptidergic transmitter substance P from postganglionic nerve fibers of myenteric plexus. Opioid peptides and somatostatin modulate the activity of cholinergic and peptidegic nerves in the guinea-pig ileum. The release of substance P appears to depend completely on N-type voltage sensitive calcium channels.

Smithkline Beecham Prize for Young Psychopharmacologists: A review of the relationship between calcium channels and psychiatric disorders

The symptoms and etiology of most major psychiatric disorders probably represent an underlying disturbance of neurotransmitter function. Understanding the mechanisms which control neurotransmitter function, and in particular neurotransmitter release, is therefore of considerable importance in determining the appropriate pharmacological treatment for these disorders. Calcium entry into neurons triggers the release of a wide range of neurotransmitters and recently our understanding of the mechanisms which control neuronal calcium entry has increased considerably. Neuronal calcium entry occurs through either voltage-sensitive or receptor-operated calcium channels. This article reviews the different subtypes of calcium channel, with particular reference to their structure; drugs which act upon them; and the possible function of the subtypes identified to date. In addition, it reviews the potential role of calcium channel antagonists in the treatment of a wide range of psychiatric disorders, and concludes that these drugs may have an increasing therapeutic role particularly in the treatment of drug dependence, mood disorders and Alzheimer's disease.

Inhibition of K(+)-evoked [3H]D-aspartate release and neuronal calcium influx by verapamil, diltiazem and dextromethorphan: evidence for non-L/non-N voltage-sensitive calcium channels

The effects of inhibitors of voltage-sensitive calcium channels (VSCC) on K(+)-evoked [3H]D-aspartate release from rat hippocampal slices and the K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture were examined. K+ caused a concentration-dependent release of [3H]D-aspartate that was approximately 85% dependent on the presence of extracellular calcium. Neither the marine snail toxin, omega-conotoxin GVIA, nor the dihydropyridine VSCC antagonist, nitrendipine, had any effect on K(+)-evoked release of [3H]D-aspartate. omega-Conotoxin GVIA and nitrendipine caused a relatively small (20-30%) inhibition of K(+)-evoked increase in intracellular calcium in neocortical neurons in primary culture. This suggests that K(+)-evoked [3H]D-aspartate release is not dependent on L- or N-type VSCC, whereas K(+)-evoked neuronal calcium influx was only partially dependent on L- and N-type VSCC. Verapamil, dextromethorphan and diltiazem caused a concentration-dependent inhibition of K(+)-evoked release of [3H]D-aspartate with IC50 values of 30, 100 and 120 microM, respectively. The K(+)-evoked increase in intracellular calcium was inhibited with essentially the same rank order of potency, but with slightly greater potencies (IC50 values for verapamil, diltiazem and dextromethorphan were 20, 50 and 50 microM, respectively). At 300 microM, neither verapamil, diltiazem nor dextromethorphan inhibited [3H]D-aspartate release evoked by the calcium ionophore ionomycin, suggesting that these compounds are not acting intracellularly to inhibit the ability of free cytosolic calcium to evoke release.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential sensitivities of avian and mammalian neuromuscular junctions to inhibition of cholinergic transmission by omega-conotoxin GVIA

Nerve stimulation-induced contractions of the chick biventer cervicis muscle were slowly reduced by omega-conotoxin. However, omega-conotoxin had no effect on skeletal muscle function after i.v. injection in mice or on nerve stimulation-induced contractions of focally innervated muscle of the rat diaphragm or the rabbit proximal oesophagus, or the multiply innervated extra-ocular rectus muscle from rabbit. The lack of effect of omega-conotoxin on mammalian neuromuscular junctions was not due to the high safety factor in transmission or to a high local concentration of Ca2+ originating from the muscle, and could not be accounted for in terms of the operation of facilitatory or inhibitory feedback modulation of transmitter release from motoneurone terminals. It is concluded that the Ca2+ channels of mammalian motoneurone terminals differ from those of avian motoneurone terminals and other omega-conotoxin-sensitive nerve terminals.