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

[Recent Findings on the Mechanism of Cough Hypersensitivity as a Cause of Chronic Cough]

An increasing number of patients complain to medical institutions about a cough that persists for more than 8 weeks, namely chronic cough. The cough observed in patients with chronic cough is not responsive to conventional antitussive agents such as dihydrocodeine and dextromethorphan, and this is a major clinical problem. The most common pathology of chronic cough in Japan is dry cough. Two causes of dry cough are increased sensitivity of cough receptors (cough hypersensitivity) and increased contraction of bronchial smooth muscle. Among these, the mechanisms of cough hypersensitivity are diverse, and understanding these mechanisms is important for the diagnosis and treatment of chronic cough. In this paper I will review the regulatory mechanisms of cough hypersensitivity, especially the regulation of Aδ fiber excitability by C fibers. Furthermore, the central mechanisms involved cough reflex are discussed in relation to central acting antitussives.

Functional significance of nitric oxide in ionomycin-evoked [3H]GABA release from mouse cerebral cortical neurons

We investigated a role of nitric oxide (NO) on ionomycin-evoked [3H]GABA release using mouse cerebral cortical neurons. lonomycin dose-dependently released [3H]GABA up to 1 microM. The extent of the release by 0.1 microM ionomycin was in a range similar to that by 30 mM KCl. The ionomycin (0.1 microM)-evoked [3H]GABA release was dose-dependently inhibited by NO synthase inhibitors and hemoglobin, indicating that the ionomycin-evoked [3H]GABA release is mediated through NO formation. The inhibition of cGMP formation by 1H-[1,2,4] oxodizao [4,3-a] quinoxalin-1-one (ODQ), a selective inhibitor for NO-sensitive guanylate cyclase, showed no affects on the ionomycin-evoked [3H]GABA release. Tetrodotoxin and dibucaine significantly suppressed the ionomycin-evoked [3H]GABA release and ionomycin increased fluorescence intensity of bis-oxonol, suggesting the involvement of membrane depolarization in this release. The ionomycin-evoked [3H]GABA release was maximally reduced by about 50% by GABA uptake inhibitors. The concomitant presence of nifedipine and omega-agatoxin VIA (omega-ATX), inhibitors for L- and P/Q-type voltage-dependent calcium channels, respectively, caused the reduction in the ionomycin-evoked release by about 50%. The simultaneous addition of nifedipine, omega-ATX and nipecotic acid completely abolished the release. Although ionomycin released glutamate, (+)-5-methyl-1-,11-dihydro-5H-dibenzo-[a,d]cycloheptan-5,10-imine (MK-801) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) showed no effects on the ionomycin-induced [3H]GABA release. Based on these results, it is concluded that NO formed by ionomycin plays a critical role in ionomycin-evoked [3H]GABA release from the neurons.

The effects of verapamil and diltiazem on N-, P- and Q-type calcium channels mediating dopamine release in rat striatum

1. The putative inhibitory effects of verapamil and diltiazem on neuronal non-L-type Ca2+ channels were studied by investigating their effects on either K+- or veratridine-evoked [3H]-dopamine ([3H]-DA) release in rat striatal slices. Involvement of N-, P- and Q-type channels was identified by sensitivity of [3H]-DA release to omega-conotoxin GVIA (omega-CTx-GVIA), omega-agatoxin IVA (omega-Aga-IVA) and omega-conotoxin MVIIC (omega-CTx-MVIIC), respectively. 2. KCl (50 mM)-evoked [3H]-DA release was abolished in the absence of Ca2+, and was insensitive to dihydropyridines (up to 30 microM). It was significantly blocked by omega-CTx-GVIA (1 microM), omega-Aga-IVA (30 nM) and was confirmed to be abolished by omega-CTx-MVIIC (3 microM), indicating involvement of N-, P- and Q-type channel subtypes. 3. Verapamil and diltiazem inhibited K+-evoked [3H]-DA release in a concentration-dependent manner. The inhibitory effects of verapamil or diltiazem (each 30 microM) were fully additive to the effect of omega-CTx-GVIA (1 microM), whereas co-application with omega-Aga-IVA (30 nM) produced similar effects to those of omega-Aga-IVA alone. 4. As shown previously, veratridine-evoked [3H]-DA release in Ca2+ containing medium exclusively involves Q-type Ca2+ channels. Here, diltiazem (30 microM) did not inhibit veratridine-evoked [3H]-DA release, whereas verapamil (30 microM) partially inhibited it, indicating possible involvement of Q-type channels in verapamil-induced inhibition. However, verapamil (30 microM) inhibited this release even in the absence of extracellular Ca2+, suggesting that Na+ rather than Q-type Ca2+ channels are involved. 5. Taken together, our results suggest that verapamil can block P- and at higher concentrations possibly N- and Q-type Ca2+ channels linked to [3H]-DA release, whereas diltiazem appears to block P-type Ca2+ channels only.

Control of glutamate release by calcium channels and kappa-opioid receptors in rodent and primate striatum

The modulation of depolarization (4-aminopyridine, 2 mM)-evoked endogenous glutamate release by kappa-opioid receptor activation and blockade of voltage-dependent Ca2+ -channels has been investigated in synaptosomes prepared from rat and marmoset striatum. 4-Aminopyridine (4-AP)-stimulated, Ca2+ -dependent glutamate release was inhibited by enadoline, a selective kappa-opioid receptor agonist, in a concentration-dependent and norbinaltorphimine (nor-BNI, selective kappa-opioid receptor antagonist)-sensitive manner in rat (IC50 = 4.4+/-0.4 microM) and marmoset (IC50 = 2.9+/-0.7 microM) striatal synaptosomes. However, in the marmoset, there was a significant (approximately 23%) nor-BNI-insensitive component. In rat striatal synaptosomes, the Ca2+ -channel antagonists omega-agatoxin-IVA (P/Q-type blocker), omega-conotoxin-MVIIC (N/P/Q-type blocker) and omega-conotoxin-GVIA (N-type blocker) reduced 4-AP-stimulated, Ca2+ -dependent glutamate release in a concentration-dependent manner with IC50 values of 6.5+/-0.9 nM, 75.5+5.9 nM and 106.5+/-8.7 nM, respectively. In marmoset striatal synaptosomes, 4-AP-stimulated, Ca2+ -dependent glutamate release was significantly inhibited by omega-agatoxin-IVA (30 nM, 57.6+/-2.3%, inhibition), omega-conotoxin-MVIIC (300 nM, 57.8+/-3.1%) and omega-conotoxin-GVIA (1 microM, 56.7+/-2%). Studies utilizing combinations of Ca2+ -channel antagonists suggests that in the rat striatum, two relatively distinct pools of glutamate, released by activation of either P or Q-type Ca2+ -channels, exist. In contrast, in the primate there is much overlap between the glutamate released by P and Q-type Ca2+ -channel activation. Studies using combinations of enadoline and the Ca2+ -channel antagonists suggest that enadoline-induced inhibition of glutamate release occurs primarily via reduction of Ca2+ -influx through P-type Ca2+ -channels in the rat but via N-type Ca2+ -channels in the marmoset. In conclusion, the results presented suggest that there are species differences in the control of glutamate release by kappa-opioid receptors and Ca2+ -channels.

Passive transfer of Lambert-Eaton myasthenic syndrome induces dihydropyridine sensitivity of ICa in mouse motor nerve terminals

Mice were injected for 30 days with plasma from three patients with Lambert-Eaton Myasthenic Syndrome (LEMS). Recordings were made from the perineurial sheath of motor axon terminals of triangularis sterni muscle preparations. The objective was to characterize pharmacologically the identity of kinetically distinct, defined potential changes associated with motor nerve terminal Ca2+ currents (ICa) that were affected by LEMS autoantibodies. ICa elicited at 0.01 Hz were significantly reduced in amplitude by approximately 35% of control in LEMS-treated nerve terminals. During 10-Hz stimulation, ICa amplitude was unchanged in LEMS-treated motor nerve terminals, but was depressed in control. During 20- or 100-Hz trains, facilitation of ICa occurred in LEMS-treated nerve terminals whereas in control, no facilitation occurred during the trains at 20 Hz and marked depression occurred at 100 Hz. Saturation for amplitude and duration of ICa in control terminals occurred at 2 and 4-6 mM extracellular Ca2+, respectively; in LEMS-treated terminals, the extracellular Ca2+ concentration had to increase by two to three times of control to cause saturation. Amplitude of the two components of ICa observed when the preparation was exposed to 50 microM 3,4-diaminopyridine and 1 mM tetraethylammonium were both reduced by LEMS plasma treatment. The fast component (ICa,s) was reduced by 35%, whereas the slow component (ICa, s) was reduced by 37%. omega-Agatoxin IVA (omega-Aga-IVA; 0.15 microM) and omega-conotoxin-MVIIC (omega-CTx-MVIIC; 5 microM) completely blocked ICa in control motor nerve terminals. The same concentrations of toxins were 20-30% less effective in blocking ICa in LEMS-treated terminals. The residual ICa remaining after treatment with omega-Aga-IVA or omega-CTx-MVIIC was blocked by 10 microM nifedipine and 10 microM Cd2+. Thus LEMS plasma appears to downregulate omega-Aga-IVA-sensitive (P-type) and/or omega-CTx-MVIIC-sensitive (Q-type) Ca2+ channels in murine motor nerve terminals, whereas dihydropyridine (DHP)-sensitive (L-type) Ca2+ channels are unmasked in these terminals. Acute exposure (90 min) of rat forebrain synaptosomes to LEMS immunoglobulins (Igs; 4 mg/ml) did not alter the binding of [3H]-nitrendipine or [125I]-omega-conotoxin-GVIA (-omega-CgTx GVIA) when compared with synaptosomes incubated with an equivalent concentration of control Igs. Conversely, LEMS Igs significantly decreased the Bmax for [3H]-verapamil to approximately 45% of control. The apparent affinity of verapamil (KD) for the remaining receptors was not significantly altered. Thus acute exposure of isolated central nerve terminals to LEMS Igs does not increase DHP sensitivity, whereas it reduces the number of binding sites for verapamil but not for nitrendipine or omega-CgTx-GVIA. These results suggest that chronic but not acute exposure to LEMS Igs either upregulates or unmasks DHP-sensitive Ca2+ channels in motor nerve endings.

Antinociceptive effects of intrathecal L-type calcium channel blockers on visceral and somatic stimuli in the rat

L-type calcium channels can modulate neuronal transduction in the spinal cord. However, their role in noxious information processing in animals that are physiologically intact has not been elucidated. We evaluated the effects of L-type calcium channel blockers diltiazem and verapamil on somatic and visceral nociception at the level of the spinal cord. Intrathecal catheters were inserted at the L4-5 level in Sprague-Dawley rats. The tail flick (TF) test and colorectal distension (CD) test were used to assess somatic and visceral antinociceptive effects, respectively. Motor function was assessed by posture and muscle tone in the limbs. TF latency and CD threshold were measured before and for 180 min after the intrathecal administration of verapamil (50, 100, 300, and 500 microg), diltiazem (100, 300, 500, and 1000 microg), or isotonic sodium chloride solution. The percent maximal possible effect (%MPE) was calculated by transforming response threshold in TF and CD tests. Intrathecally administered diltiazem or verapamil increased both TF latency and CD threshold in a dose-dependent fashion. Isotonic sodium chloride solution, diltiazem 100 microg, and verapamil 50 microg did not increase %MPE in either test. Diltiazem 300 or 500 microg or verapamil 300 or 500 microg significantly (P < 0.05) increased %MPE, with the peak effects 5 min after administration and short-duration antinociception. %MPE was 100% until 15 min after the administration of diltiazem 1000 microg, and significant antinociception continued until 180 min in the TF test. Motor paralysis was observed after the administration of the larger dose of each drug. We demonstrated that intrathecally administered L-type calcium channel blockers diltiazem or verapamil produced both somatic and visceral antinociception and motor block dose-dependently.

IMPLICATIONS

We examined the effects of intrathecally administered L-type calcium channel blockers diltiazem and verapamil on somatic and visceral nociception in rats. L-type calcium channel blockers produced antinociceptive effects, suggesting a possible clinical application to control pain.

The anticonvulsant actions of sigma receptor ligands in the Mg2+-free model of epileptiform activity in rat hippocampal slices

1. The anticonvulsant potency of a series of structurally-dissimilar compounds which possess nanomolar affinities for high-affinity sigma binding sites was examined in the Mg2+-free model of epileptiform activity in rat hippocampal slices. Extracellular field potential recordings in the CA1 region were employed to examine the effects of test compounds on spontaneous epileptiform activity and multiple population spikes evoked by stimulation of the Schaffer collateral-commissural pathway. 2. Applied at sigma site-selective (i.e. nanomolar) concentrations, dextromethorphan, ditolylguanidine, caramiphen and opipramol failed to modify Mg2+-free epileptiform activity; neither pro- nor anticonvulsant effects were observed. However, applied at micromolar concentrations, these and additional test compounds reversibly inhibited orthodromically-evoked epileptiform field potentials with a rank order potency (IC50 values in microM): dextrorphan (1.5) > ifenprodil (6.3) > dextromethorphan (10) > ditolylguanidine (15) > loperamide (28) > carbetapentane (38) > caramiphen (46) > opipramol (52). Micromolar concentrations of the same compounds also inhibited spontaneous epileptiform bursts recorded during perfusion with Mg2+-free medium. 3. Co-application of ropizine (10 microM), an allosteric modulator of dextromethorphan binding to high-affinity sigma receptors, failed to endow dextromethorphan 10 nM with anticonvulsant properties and did not modify the anticonvulsant potency of 10 microM dextromethorphan. 4. The effects of dextrorphan (10 microM), ifenprodil (20 microM), loperamide (50 microM) and caramiphen (100 microM) were examined in the presence of external Mg2+ on field potential input/output (I/O) relationships and paired-pulse facilitation (PPF) of field excitatory postsynaptic potentials. Only caramiphen elicited effects on these parameters, affecting synaptic transmission at the point of synaptic transfer and depressing PPF ratios to below baseline values. The effects of caramiphen on I/O relationships mimicked those of the established anticonvulsant adenosine: in contrast, adenosine evoked an increase in PPF ratios. 5. Because anticonvulsant activity was observed only at micromolar concentrations of the sigma ligands tested, the results indicate that their anticonvulsant actions should not be ascribed to their occupancy, observed at nanomolar concentrations, of high-affinity sigma binding sites. Rather, anticonvulsant activity more likely reflects functional NMDA receptor antagonism and/or blockade of high voltage-activated Ca2+ channels, effects which are associated with micromolar concentrations of the test compounds. Modulation of GABAergic inhibitory mechanisms may also contribute to the anticonvulsant properties of caramiphen.

Modulation of potassium-evoked [3H]dopamine release from rat striatal slices by voltage-activated calcium channel ligands: effects of omega-conotoxin-MVIIC

We examined the involvement of voltage-activated Ca2+ channels (VACCs) on K+(50 mM)-evoked [3H]dopamine ([3H]DA) release from superfused rat striatal slices. Neither nifedipine nor nitrendipine modified K(+)-evoked [3H]DA release, indicating that L-type VACCs are not involved. K(+)-evoked [3H]DA release was partially inhibited by omega-CTx-GVIA and omega-Aga-IVA, and was abolished by 3 microM omega-CTx-MVIIC (IC50 approximately 128 nM), suggesting the involvement of N-, P-, or Q-type VACCs, respectively. Moreover, even subnanomolar concentrations of omega-CTx-MVIIC (0.1-0.5 nM) inhibited K(+)-evoked [3H]DA release by approximately 25%, suggesting the possible involvement of a still not classified (perhaps O-type?) Ca2+ channel subtype. The effects of omega-CTx-MVIIC (10-100 nM) and omega-CTx-GVIA (1 microM) were additive, suggesting that low nanomolar concentrations of omega-CTx-MVIIC does not interact with N-type VACCs. In conclusion, the K(+)-evoked [3H]DA release from rat striatal slices is mediated by entry of Ca2+ through omega-CTx-GVIA sensitive (N-type) as well as through omega-Aga-IVA (P-type) and omega-CTx-MVIIC (probably Q-type) sensitive VACCs.

Characterization of glutamate and [3H]D-aspartate outflow from various in vitro preparations of the rat hippocampus

The characteristics of high-K+ and electrically evoked endogenous glutamate and [3H]D-aspartate release have been studied in multiple in vitro preparations of the rat hippocampus (transverse slices, granule cells cultures, synaptosomes and mossy fibre synaptosomes) under similar experimental conditions. High external K+ concentrations evoked [3H]D-aspartate and endogenous glutamate overflow in a concentration-dependent manner in all preparations (except it was not possible to measure endogenous glutamate outflow from granule cells). This effect was tetrodotoxin-insensitive but partially calcium-dependent. In slices, field electrical stimulation evoked an overflow of endogenous glutamate, but not of [3H]D-aspartate, in a frequency-dependent manner. This effect was concentration-dependently amplified by the glutamate uptake inhibitor L-trans-pyrrolidine-2,4-dicarboxylic acid (t-PDC). The electrically evoked glutamate overflow in the presence of t-PDC was tetrodotoxin-sensitive and calcium-dependent. In primary dentate gyrus cell cultures, electrical stimulation evoked an overflow of [3H]D-aspartate in a frequency-dependent manner, while endogenous glutamate outflow was not detectable. This effect could be inhibited by tetrodotoxin and by the N-type calcium channel blocker omega-conotoxin GVIA. Finally, the effect of adenosine has been studied in order to assess the pharmacological modulability of [3H]D-aspartate and endogenous glutamate stimulation-induced overflow. Adenosine was found to inhibit 35 mM K(+)- and 20 Hz electrical stimulation-induced [3H]D-aspartate and endogenous glutamate overflow. These effects were all prevented by the A1 receptor antagonist 8-cyclopentyl-1,3-dimethylxanthine (CPT). These data are in line with the hypothesis that reuptake plays a role in regulating glutamate release, and that [3H]D-aspartate represents a valid marker of endogenous glutamate under most (but not all) experimental conditions.

Multiple types of Ca2+ channels in mouse motor nerve terminals

We measured neurotransmitter release and motor nerve terminal currents in mouse phrenic nerve-diaphragm and triangularis sterni preparations, to evaluate the role of Ca2+-channel subtypes in regulating transmitter release. Saturated concentrations of either omega-agatoxin IVA [omega-Aga-IVA (0.3 microM), a blocker of P-type Ca2+ channels] or omega-conotoxin MVIIC [omega-CTx-MVIIC (2 microM), a P- and Q-type Ca2+-channel blocker], inhibited nerve-evoked muscle contractions and the amplitude of endplate potentials respectively. In contrast, combined treatment with nifedipine (50 microM, a blocker of L-type Ca2+ channels) plus omega-conotoxin GVIA [omega-CTx-GVIA (2 microM), a blocker of N-type Ca2+ channels] did not elicit inhibitory effects on nerve-evoked muscle contractions, endplate potentials or nerve terminal waveforms. Because of the non-linear relationship between endplate potentials and Ca2+ signals, a small decrease in presynaptic Ca2+ entry can significantly reduce the amplitude of the endplate potential. Thus, we applied 3,4-diaminopyridine (3,4-DAP, a K+-channel blocker) or high Ca2+ (10 mM) to accelerate and amplify the endplate potentials and Ca2+ currents. The endplate potentials amplified by 3,4-DAP or by high Ca2+ correspondingly proved to be quite resistant to both omega-Aga-IVA and omgea-CTx-MVIIC; omega-Aga-IVA exerted only a partial inhibitory effect on endplate potentials, and the omega-Aga-IVA-resistant component was further inhibited by omega-CTx-MVIIC. The component that was resistant to the two toxins could be completely blocked by the non-selective Ca2+ channel blocker Cd2+ (300 microM). A combination of the two toxins had no significant effects on either spontaneous transmitter release or postsynaptic resting membrane potentials of the diaphragm preparation and the Na+ and K+ waveforms of the triangularis sterni preparations. This finding suggests a preferential inhibitory effect at a presynaptic site. Measuring the Ca2+ currents in the triangularis sterni also revealed partial inhibition by omega-CTx-MVIIC with further incomplete inhibition by omega-Aga-IVA. Cd2+ (300 microM) abolished the toxin-resistant component of the Ca2+ current. In contrast, a combination of nifedipine (50 microM) with omega-CTx-GVIA (2 microM) was without inhibitory effect. We conclude that multiple types of Ca2+ channels, i.e. omega-Aga-IVA-sensitive, omega-CTx-MVIIC-sensitive and toxin-resistant Ca2+ channels, coexist in mouse motor nerve terminals.

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.

Dextromethorphan affects ventilation differently in male and female rats

Subcutaneous administration of aspartic acid results in a long-lasting but reversible depression of ventilation in male but not in female rats. Aspartic acid acts on N-methyl-D-aspartate receptors. The present study tested the hypothesis that a noncompetitive N-methyl-D-aspartate-receptor antagonist, dextromethorphan (Dex), would depress ventilation in female rats and stimulate it in male rats. Moreover, Dex administered prior to aspartic acid should prevent the aspartic acid-induced depression of ventilation in male rats. In female rats, Dex caused a 30% depression of ventilation relative to saline at 5 and 10 mg/kg (P < 0.01) but not at the highest dose (20 mg/kg). In male rats, Dex had no effect on ventilation. At a dose of 20 mg/kg, Dex depressed oxygen consumption to 50% of the saline value at all time points in female rats (P < 0.001) and in male rats 45 and 60 min after administration. The time points when Dex depressed ventilation and oxygen consumption were different in female rats, suggesting that the depression of ventilation was not the result of a depression in oxygen consumption. During a hypercapnic challenge (7% CO2), female rats treated with 5 and 10 mg/kg of Dex exhibited a smaller increase in ventilatory response relative to saline treatment. At a dose of 20 mg/kg, the hypercapnic responsiveness of male rats was markedly stimulated (85.8 +/- 8.95 ml/min) relative to saline (50.6 +/- 9.14 ml/min; P < 0.001). Finally, Dex administered before aspartic acid prevented the aspartic acid-induced depression of ventilation in male rats. Thus, in rats, Dex has gender-specific effects on ventilation and these effects are not associated with changes in oxygen consumption.

Blockade of N- and Q-type Ca2+ channels inhibit K(+)-evoked [3H]acetylcholine release in rat hippocampal slices

In the present study, we examined the contribution of specific Ca2+ channels to K(+)-evoked hippocampal acetylcholine (ACh) release using [3H]choline loaded hippocampal slices. [3H]ACh release was Ca(2+)-dependent, blocked by the nonspecific Ca2+ channel blocker verapamil, but not by blockade of L-type Ca2+ channels. The N-type Ca2+ channel blocker omega-conotoxin GVIA (omega-CgTx GVIA; 250 nM) inhibited [3H]ACh release by 44% and the P/Q-type Ca2+ channel blocker omega-agatoxin IVA (omega-Aga IVA; 400 nM) inhibited [3H]ACh release by 27%, with the combination resulting in a nearly additive 79% inhibition. Four hundred or one thousand nM omega-Aga IVA was necessary to inhibit [3H]ACh release. omega-Conotoxin MVIIC (omega-CTx-MVIIC) was used after first blocking N-type Ca2+ channels with omega-CgTx GVIA (1 microM). Under these conditions, 500 nM omega-CTx-MVIIC led to a nearly maximal inhibition of the omega-CgTx GVIA-insensitive [3H]ACh release. Based on earlier reports about the relative sensitivity of cloned and native Ca2+ channels to these toxins, this study indicates that N- and Q-type Ca2+ channels primarily mediate K(+)-evoked hippocampal [3H]ACh release.

Blockade by sigma site ligands of high voltage-activated Ca2+ channels in rat and mouse cultured hippocampal pyramidal neurones

1. The effects of a series of structurally-dissimilar sigma site ligands were examined on high voltage-activated Ca2+ channel activity in two preparations of cultured hippocampal pyramidal neurones. 2. In mouse hippocampal neurones under whole-cell voltage-clamp, voltage-activated Ca2+ channel currents carried by barium ions (IBa) were reduced with the rank order (IC50 values in microM): 1S,2R-(-)-cis-N-methyl-N-[2-(3,4-dichlorophenyl)ethyl]- 2-(1-pyrrolidinyl)cyclohexylamine (7.8) > rimcazole (13) > haloperidol (16) > ifenprodil (18) > opipramol (32) > carbetapentane (40) = 1-benzylspiro[1,2,3,4-tetrahydronaphthalene-1,4-piperidine] (42) > caramiphen (47) > dextromethorphan (73). At the highest concentrations tested, the compounds almost abolished IBa in the absence of any other pharmacological agent. 3. The current-voltage characteristics of the whole-cell IBa were unaffected by the test compounds. The drug-induced block was rapid in onset and offset, with the exceptions of carbetapentane and caramiphen where full block was achieved only after two to three voltage-activated currents and was associated with an apparent increase in the rate of inactivation of IBa. 4. In rat hippocampal neurones loaded with the Ca(2+)-sensitive dye Fura-2, rises in intracellular free Ca2+ concentration evoked by transient exposure to 50 mM K(+)-containing medium, either in the absence or in the presence of 10 microM nifedipine (to block L-type high voltage-activated Ca2+ channels), were also reversibly attenuated by the sigma ligands. The rank order potencies for the compounds in these experimental paradigms were similar to that observed for blockade of IBa in the electrophysiological studies. 5. These results indicate that, at micromolar concentrations, the compounds tested block multiple subtypes of high voltage-activated Ca2+ channels. These actions, which do not appear to be mediated by high-affinity sigma binding sites, may play a role in some of the functional effects previously described for the compounds.

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

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

Transient brain ischemia in rabbits: the effect of omega-conopeptide MVIIC on hippocampal excitatory amino acids

Neurologic injury that occurs after ischemia results from a cascade of events involving the release of various endogenous neurotoxins. A portion of the release of excitatory neurotransmitters is calcium dependent and may be attenuated by administration of calcium channel blockers. Using an in vivo model of ischemia, we studied the effects of omega-conopeptide MVIIC, a voltage-sensitive calcium channel blocker, and hypothermia (32 degrees C) on hippocampal glutamate and aspartate release in the peri-ischemic period. Thirty-four New Zealand white rabbits of either sex were anesthetized with halothane, intubated, and mechanically ventilated. Monitored variables included blood gases, mean arterial blood pressure, and the electroencephalogram. Microdialysis catheters were transversely inserted through the anterior portion of the dorsal hippocampus and perfused with artificial cerebrospinal fluid at a rate of 2 microliters/min. After stabilization period, animals were randomly assigned to one of the following groups: Control group (n = 8), 10 microM omega-conopeptide MVIIC group (n = 7), 100 microM omega-conopeptide MVIIC group (n = 7), Hypothermia group (n = 6; cranial temperature = 32 degrees C), and omega-conopeptide MVIIC + hypothermia group (n = 6; 100 microM omega-conopeptide MVIIC and cranial temperature 32 degrees C). All the rabbits were subjected to 10 minutes of global cerebral ischemia produced by neck tourniquet inflation combined with hypotension during halothane anesthesia. Conopeptide MVIIC was administered in the artificial cerebrospinal fluid used to perfuse the microdialysis catheter. In control animals, ischemia caused a significant increase in glutamate (9.7 fold) and aspartate (11.3 fold) concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Aminopyridine differentially affects the spontaneous release of radiolabelled transmitters from rat brain slices in vitro

4-Aminopyridine increased the release of [3H]noradrenaline from dorsal hippocampus slices in vitro in a concentration-dependent manner. When the slices were exposed to 4-aminopyridine for 5 min, the overflow of radioactivity returned to pre-exposure values within 20-25 min. When the exposure of the slices was continued, a sustained enhancement of the release of [3H]noradrenaline was observed for the duration of the exposure. 4-Aminopyridine, 10(-4) M, had an effect of similar magnitude, or an even more pronounced effect, on the release of [3H]catecholamine from cortex, septum, periaqueductal gray and striatum slices. The effects of the compound on the release of [3H]5-hydroxytryptamine and [14C]acetylcholine were less pronounced. At this concentration 4-aminopyridine had no effect on the release of [3H]D-aspartate from hippocampus or septum slices, whereas the effect on the release of this transmitter in striatal slices was marginal. The effect of 4-aminopyridine on the release of [3H]noradrenaline in hippocampus slices was largely dependent on the presence of Ca2+ in the superfusion medium. This was also the case for the effect on the release of [3H]noradrenaline from preloaded dorsal hippocampus synaptosomes. In the presence of nitrendipine the effect of 4-aminopyridine was dose-dependently reduced, but the maximal reduction, at a nitrendipine concentration of 10(-4) M, was only 40%. Cd2+ completely abolished the effect of 4-aminopyridine on the release of [3H]noradrenaline. These results confirm that the enhancing effect of 4-aminopyridine on the release of [3H]noradrenaline depends on the entry of extracellular Ca2+ into the nerve terminals.(ABSTRACT TRUNCATED AT 250 WORDS)

NMDA antagonists: antiepileptic-neuroprotective drugs with diversified neuropharmacological profiles

In conclusion, NMDA antagonists as anticonvulsants are especially active in preventing the generalization of the behavioural and electrical seizures and display a typical spectrum of in vitro antiepileptiform activities. In addition, based on in vitro and in vivo limbic kindled studies, the drugs should be regarded more as an antiepileptiform than as an anticonvulsant drugs. As neuroprotective drugs, NMDA antagonists are effective against many types of neuronal injury and show a window of activity which does not exceed 1-2 h, thus suggesting an influence of NMDA receptors in the 'early' or 'acute' mechanisms of brain damage. Among NMDA antagonists, glycine antagonists or the morphinans dextromethorphan and dextrorphan showed a spectrum of antiepileptiform and neuroprotective activities broader than other NMDA antagonists. The primary pharmacological activities of NMDA antagonists are accompanied by some effects including perturbation of many sensory, psychological or motor processes. Typical behavioural and EEG changes were also induced by the drugs. In spite of the side-effects elicited by the drugs, differential effects detected among the various classes of NMDA antagonists (i.e. lack of induction of typical EEG-behavioural effects and of typical cortical neurotoxicity) might render some of these suitable for full clinical application as anticonvulsant-neuroprotective drugs.

Antagonists of the NMDA receptor-channel complex and motor coordination

Many structurally different, centrally active antagonists of the NMDA receptor-channel complex induce phencyclidine-like side effects in mammals which include head weaving, body rolling, sniffing and disturbances of motor coordination. The ability of these compounds to cause disturbances of motor coordination correlates directly with their ability to antagonize the NMDA receptor-channel complex in vivo. Although noncompetitive antagonists increase motility in rodents, whereas competitive antagonists do not, both classes of compounds appear to induce schizophrenia-like psychosis in human beings, and cause similar changes in a variety of different biogenic amine neurotransmitter systems in the limbic and motoric areas of the brain. The complex spectrum of behavioural effects observed after the administration of antagonists of the NMDA receptor-channel complex probably reflects the intricate nature of the interaction with positive and negative feedback loops of the motor circuit. Recent research indicates that the site of integration of this interaction could be the striatal medium spiny GABAergic neuron.

Involvement of P-type calcium channels in high potassium-elicited release of neurotransmitters from rat brain slices

Several types of voltage-dependent calcium channels appear to occur in neurons, although coupling of the particular subtype of calcium channels to the release of neurotransmitter has not been clearly understood. We have examined the effects of subtype-specific inhibitors of the calcium channels on depolarization-induced release of endogenous neurotransmitters from brain slices. High potassium-induced release of glutamate and aspartate from hippocampal and striatal slices was almost completely inhibited by a P-type channel blocker, omega-agatoxin IVA. omega-Agatoxin IVA also completely inhibited the release of serotonin from the hippocampal slices with almost the same potency as in the case of glutamate, whereas the potency in blocking the release of serotonin and dopamine from striatal slices was lower than that from the hippocampal slices. Another calcium channel blocker, omega-agatoxin TK, that was recently found to block P-type channels with very similar selectivity and potency to omega-agatoxin IVA, also inhibited the release of amino acid transmitters and monoamines, though its potency was lower than that of omega-agatoxin IVA. An N-type channel blocker, omega-conotoxin GVIA, partially inhibited the neurotransmitter release, but an L-type channel blocker, nifedipine was ineffective. We propose that the activation of P-type calcium channels makes a major contribution to depolarization-elicited neurotransmitter release in the CNS and that multiple P-type channels sensitive to omega-agatoxin IVA and omega-agatoxin TK modulate the neurotransmitter release.

d-fenfluramine, but not d-norfenfluramine, uses calcium to increase extracellular serotonin

Microdialysis in the hippocampus of freely moving rats was used to assess extracellular serotonin (5-HT) in response to local infusion of d-fenfluramine and its metabolite d-norfenfluramine with and without local calcium depletion. Verapamil (1 mM) in calcium-free Ringer infused via the microdialysis probe increased extracellular 5-HT and prevented the full increase in extracellular 5-HT normally caused by 1 mM d-fenfluramine. The results suggest d-fenfluramine might act in part as a calcium channel agonist favoring a calcium influx that in turn would trigger the exocytotic process in 5-HT terminals. d-norfenfluramine, on the other hand, was capable of releasing 5-HT, in vivo, in spite of depleted Ca levels.

Neuroprotection by N-methyl-D-aspartate antagonists in focal cerebral ischemia is dependent on continued maintenance dosing

While N-methyl-D-aspartate antagonists have been shown to attenuate neuronal damage in focal cerebral ischemia, few studies have examined whether continuous or multiple dose treatment is necessary for maximum efficacy. We studied the effect of a loading dose only or load plus maintenance infusion using several non-competitive N-methyl-D-aspartate antagonists (dextromethorphan, dextrorphan, MK-801) and the levorotatory enantiomer of dextromethorphan (levomethorphan) in a rabbit model of focal cerebral ischemia. Forty-seven anesthetized rabbits underwent occlusion of the left internal carotid, anterior cerebral and middle cerebral arteries for 2 h followed by 4 h of reperfusion. Drugs were administered 10 min after occlusion. Dextromethorphan and dextrorphan protected against ischemic edema only when given as load plus maintenance (29% and 31% reduction, respectively), while both load only and load plus maintenance of MK-801 protected against edema (26% and 31% reduction, respectively). Levomethorphan load plus maintenance also protected against ischemic edema (25% reduction). However, dextromethorphan and dextrorphan both required maintenance infusion to protect against ischemic neuronal damage (24% and 27% reduction in area of ischemic neuronal damage, respectively), while levomethorphan failed to protect against neuronal injury even when given as load plus maintenance. Administration of MK-801 as load plus maintenance reduced ischemic neuronal damage by 23%, but this difference was not quite statistically significant. These results suggest that processes of ischemic damage, such as excitotoxic injury, continue for several hours beyond the initial period of focal ischemia, and that non-competitive N-methyl-D-aspartate antagonists require more prolonged administration to achieve neuroprotection.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurotoxicity of A beta amyloid protein in vitro is not altered by calcium channel blockade

In cortical cultures, A beta protein destabilizes calcium homeostasis, but direct neurotoxicity of A beta is not observed. In hippocampal cultures, we and others find treatment with A beta protein decreases neuronal survival, but the mechanism of neurotoxicity is unknown. We have used low-density, serum-free cultures of hippocampal neurons to determine whether the neurotoxicity of A beta protein in vitro can be altered by voltage- or ligand-gated calcium channel antagonists or cyclic nucleotides. In these cultures, neither omega-conotoxin, nifedipine, verapamil, APV, nor MK-801 altered the survival of neurons exposed to synthetic A beta 1-40. The N-channel antagonist diltiazem decreased A beta 1-40 toxicity repeatedly, but slightly, perhaps by indirectly contributing to increased neuronal viability. Treatment of cultures with dibutyryl cAMP, 8-bromo cAMP, dibutyryl cGMP, and 8-bromo cGMP also failed to alter A beta toxicity. Thus, the toxicity of beta protein in low-density hippocampal cultures was not directly altered either by calcium channel blockers or by the addition of cyclic nucleotides.

Differential inhibition of neuronal calcium entry and [3H]-D-aspartate release by the quaternary derivatives of verapamil and emopamil

1. Verapamil and emopamil are structurally related phenylalkylamine calcium channel/5-HT2 receptor antagonists that differ in their anti-ischaemic properties in experimental studies. The quaternary ammonium derivatives of these compounds were prepared and tested in assays of neuronal voltage-sensitive calcium channel (VSCC) function to determine whether the compounds act at intra- or extracellular sites. 2. The compounds were tested in K(+)-evoked: (1) rat brain synaptosomal 45Ca2+ influx, (2) release of [3H]-D-aspartate from rat hippocampal brain slices and (3) increase of intracellular calcium in rat cortical neurones in primary culture. 3. Verapamil, emopamil and the emopamil quaternary derivative caused concentration-dependent and comparable (IC50 values approximately 30 microM) inhibition of synaptosomal 45Ca2+ influx and [3H]-D-aspartate release. The verapamil quaternary derivative was considerably less active in these assays (IC50 > 300 microM). 4. The evoked increase of intracellular calcium in cortical neurones was inhibited with the following rank order of potency (IC50 value, microM): emopamil (3.6) > verapamil (17) > emopamil quaternary derivative (38) > verapamil quaternary derivative (200). 5. The results suggest that verapamil and emopamil inhibit nerve terminal VSCC function (synaptosomal 45Ca2+ influx and [3H]-D-aspartate release) by acting at distinct intracellular and extracellular sites, respectively. Verapamil and emopamil may inhibit cell body VSCC function (evoked increase of intracellular calcium in neocortical neurones) by acting at both intracellular and extracellular sites. 6. The different 'sidedness' of action of emopamil and verapamil on nerve terminal VSCC function and/or the preferential inhibition of cell body VSCC function by emopamil may at least partially explain the relatively greater neuroprotective efficacy of emopamil in experimental models of ischaemia.

Calcium channel antagonist peptides define several components of transmitter release in the hippocampus

The use of subtype-selective voltage-sensitive calcium channel (VSCC) antagonists has established that neurotransmitter release in mammalian brain is mediated by N-like and P-like VSCCs, and that other subtypes also contribute significantly. To determine the roles presynaptic VSCCs play in nervous system function and to evaluate the therapeutic potential of their selective inhibition, it is necessary to define further the contributions of VSCC subtypes to neurotransmitter release. The novel conopeptide, SNX-230 (omega-conopeptide MVIIC), has revealed a new VSCC subtype, the Q-type, in cerebellar granule cells. We have compared the effects of SNX-230 on release of tritiated D-aspartate ([3H]D-Asp; a non-metabolizable analog of glutamate), gamma-aminobutyric acid ([3H]GABA), and norepinephrine ([3H]NE) from rat hippocampal slices to those of the N-type VSCC blocker, SNX-111 (omega-conopeptide MVIIA), and the P-type blocker, omega-agatoxin-IVA (AgaIVA). SNX-230 blocks both [3H]D-Asp and [3H]GABA release completely, whereas AgaIVA blocks them potently but partially and SNX-111 has no effect. These results suggest that glutamate and GABA release are mediated by two VSCC subtypes, a P-type and another, perhaps Q-like. SNX-111 blocks [3H]NE release potently but partially, while SNX-230 blockade is complete, consisting of one very potent phase and one less potent phase. AgaIVA also blocks [3H]NE release potently but partially. These results suggest that at least two VSCC subtypes, an N-type and a novel non-N-type, mediate NE release. Pair-wise combinations of the three ligands indicate that at least three pharmacologically distinct components comprise [3H]NE release in the hippocampus.

Blockade by ifenprodil of high voltage-activated Ca2+ channels in rat and mouse cultured hippocampal pyramidal neurones: comparison with N-methyl-D-aspartate receptor antagonist actions

1. The block by ifenprodil of voltage-activated Ca2+ channels was investigated in intracellular free calcium concentration ([Ca2+]i) evoked by 50 mM K+ (high-[K+]o) in Fura-2-loaded rat hippocampal pyramidal neurones in culture and on currents carried by Ba2+ ions (IBa) through Ca2+ channels in mouse cultured hippocampal neurones under whole-cell voltage-clamp. The effects of ifenprodil on voltage-activated Ca2+ channels were compared with its antagonist actions on N-methyl-D-aspartate- (NMDA) evoked responses in the same neuronal preparations. 2. Rises in [Ca2+]i evoked by transient exposure to high-[K+]o in our preparation of rat cultured hippocampal pyramidal neurones are mediated predominantly by Ca2+ flux through nifedipine-sensitive Ca2+ channels, with smaller contributions from nifedipine-resistant, omega-conotoxin GVIA-sensitive Ca2+ channels and Ca2+ channels sensitive to crude funnel-web spider venom (Church et al., 1994). Ifenprodil (0.1-200 microM) reversibly attenuated high-[K+]o-evoked rises in [Ca2+]i with an IC50 value of 17 +/- 3 microM, compared with an IC50 value of 0.7 +/- 0.1 microM for the reduction of rises in [Ca2+]i evoked by 20 microM NMDA. Tested in the presence of nifedipine 10 microM, ifenprodil (1-50 microM) produced a concentration-dependent reduction of the dihydropyridine-resistant high-[K+]o-evoked rise in [Ca2+]i with an IC50 value of 13 +/- 4 microM. The results suggest that ifenprodil blocks Ca2+ flux through multiple subtypes of high voltage-activated Ca2+ channels. 3. Application of the polyamine, spermine (0.25-5 mM), produced a concentration-dependent reduction of rises in [Ca2+]i evoked by high-[K+]o. The antagonist effects of ifenprodil 20 micro M on high-[K+]0-evoked rises in [Ca2+]. were attenuated by spermine 0.25 mM but not by putrescine 1 or 5 mM. In contrast,spermine 0.1 mM increased rises in [Ca2+]i evoked by NMDA and enhanced the ifenprodil (5 micro M) block of NMDA-evoked rises in [Ca2+]i.4. Similar results were obtained in mouse cultured hippocampal pyramidal neurones under whole-cell voltage-clamp. Ifenprodil attenuated both the peak and delayed whole-cell IB. with an IC% value of 18 +/- 2 micro M, whilst it attenuated steady-state NMDA-evoked currents with an IC50 of 0.8 +/- 0.2 micro M. Block of IBa by ifenprodil 10 JaM was rapid in onset, fully reversible and occurred without change in thecurrent-voltage characteristics of Ba. The ifenprodil block of IBa was enhanced on membrane depolarization and was weakly dependent on the frequency of current activation. Spermine 0.1 mM potentiated control NMDA-evoked currents but attenuated IB,. In agreement with the microspectrofluorimetric studies, co-application of spermine produced a small enhancement of the inhibitory effect of ifenprodil 10 micro M on NMDA-evoked responses whereas the reduction of I4 by ifenprodil 10 micro M in the presence of spermine was less than expected if the inhibitory effects of ifenprodil and spermine on IBa were simply additive.5. The results indicate that ifenprodil blocks high voltage-activated Ca2+ channels in rat and mouse cultured hippocampal pyramidal neurones. Although the Ca2+ channel blocking actions of ifenprodil are observed at higher concentrations than those associated with NMDA antagonist activity, Ca2+ channel blockade may contribute, at least in part, to the established neuroprotective and anticonvulsant properties of the compound.

Inhibitory effect of imipramine on depolarization-induced increases in intracellular Ca2+ of rat cortical neurons

We examined the effects of imipramine on the increase in intracellular Ca2+ concentration ([Ca2+]i) induced by elevated K+ in cultured neurons of rat cortex. Imipramine (100 nM-200 microM) produced a concentration-dependent inhibition of [Ca2+]i increases induced by 25 mM K+ with an IC50 value of 32 microM. Imipramine had no effect on resting [Ca2+]i levels. When the cells were incubated with imipramine in the presence of a voltage-sensitive Ca2+ channel (VSCC) blocker, either nicardipine (10 microM), verapamil (10 microM), or omega-conotoxin GVIA (1 microM), the combinations of imipramine and each blocker resulted in an additive inhibition of 25 mM K(+)-induced [Ca2+]i increases. The IC50 values were 44, 29 and 24 microM, respectively, which were similar to those found when incubating the cells with imipramine alone. The presence or the absence of imipramine (30 microM) in an incubation with Bay K 8644 (100 nM), a VSCC agonist, showed similar potentiation of the [Ca2+]i increases induced by 15 mM K+ (66 and 52%, respectively). On the other hand, when the cells were incubated with imipramine in the presence of Ni2+ (300 microM) or La3+ (0.3 microM), inorganic Ca(2+)-channel blockers, the IC50 values of inhibition of 25 mM K(+)-induced [Ca2+]i increases were much lower than with imipramine alone (3.2 and 16 microM, respectively). However, incubations with Ni2+ combined with nicardipine or verapamil resulted in an additive inhibition of 25 mM K(+)-induced [Ca2+]i increases.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological identification of a novel Ca2+ channel in chicken brain synaptosomes

Ca2+ influx was measured in rat and chicken brain synaptosomes in the presence of a number of pharmacological tools which have recently been used to define voltage-sensitive Ca(2+)-channel (VSCC) types. In chicken brain synaptosomes. VSCCs which, because of their sensitivity to inhibition by omega-conotoxin (omega-CgTx), are thought to be exclusively N-type, the P-type VSCC polyamine inhibitor FTX (from Agelenopsis aperta venom; 1 microliters/ml), its synthetic analogue, sFTX (1-5 mM) and the polypeptides AgaIVA (IC50 0.29 microM) and omega-CgTx MVIIC (IC50 0.0022 microM) inhibited 70-100% of the measurable K+ stimulated Ca2+ influx. The prototypical N-channel VSCC inhibitor omega-CgTx GVIA (IC50 0.014 microM), Cd2+ (50 microM) and diluted venom from Hololena curta (1:2,500) also caused complete or almost complete, inhibition of Ca2+ influx. In comparable studies using rat brain synaptosomes, sFTX (1-10 mM) caused a dose-dependent reduction of Ca2+ influx, while FTX (1 microliters/ml) and AgaIVA (IC50 0.02 microM) completely inhibited Ca2+ influx. Similar to the findings in chicken synaptosomes, Cd2+ (50 microM) and H. curta (1:2,500 dilution) both inhibited K+ stimulated influx by > 80% whereas omega-CgTx (1 microM) only caused a maximum 25% inhibition. Both sFTX and its congener spermine, inhibited [125I]omega-CgTx binding to rat and chicken synaptosomal membranes. These results strongly implicate P-type channels as the major VSCC in rat brain. The results also clearly demonstrate a heretofore unrecognized, novel, FTX/AgaIVA/omega-CgTx GVIA/omega-CgTx MVIIC-sensitive VSCC in chicken brain.

Effects of diverse omega-conopeptides on the in vivo release of glutamic and gamma-aminobutyric acids

omega-Conopeptides are antagonists of subtypes of neuronal calcium channels. Two omega-conopeptides, SVIB and MVIIC, have recently been identified which have a novel specificity for these ionophores. We have tested the actions these peptides, as well as the more selective MVIIA, on the release of glutamic acid and gamma-aminobutyric acid (GABA) in the hippocampus in vivo. For the assay of peptide effects on release, we used microdialysis to deliver multiple pulses of elevated potassium to the brain extracellular fluid. Peptide effects were quantitated from the decrement of the release with peptide perfused through the probes, in comparison to that in control experiments. Synthetic MVIIC caused a 40-50% decrement in the release of both glutamate and GABA at a probe concentration of about 200 nM. Synthetic SVI-B caused a 50% block at about 20-40 microM, while about 200 microM of MVIIA was required for 50% block. Chromatographic experiments showed that differences in potency between MVIIC and MVIIA were not explained by differential degradation. Blockade of release was also observed in the thalamus. MVIIC provides a tool for investigating the role of calcium mediated release of glutamate and GABA in physiological and pathological processes in the mammalian brain in vivo.

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

Field-potential stimulation of rat dorsal-root ganglion (DRG) neurons evoked action-potential-mediated transient increases in intracellular free calcium concentration ([Ca2+]i) as measured by indo-1-based microfluorimetry. Field-potential-evoked [Ca2+]i transients were abolished by tetrodotoxin, and their dependence on stimulus intensity exhibited an abrupt threshold. omega-Conotoxin GVIA (omega-CgTx, 100 nM) inhibited action-potential-mediated Ca2+ influx by 79%, while nitrendipine (1 microM) had little effect. omega-Grammotoxin SIA (omega-GsTx, 267 nM), a peptide toxin purified from the venom of the tarantula spider, Grammostola spatulata, blocked action-potential-mediated Ca2+ influx as effectively as did omega-CgTx, suggesting that omega-GsTx blocks N-type Ca2+ channels. In contrast to block by omega-CgTx, the block produced by omega-GsTx reversed upon washout of the peptide. omega-GsTx (270 nM) blocked 80%, and omega-CgTx (1 microM) blocked 64%, of whole-cell Ca2+ current (ICa) elicited by step depolarization to 0 mV from a holding potential of -80 mV. omega-GsTx completely occluded inhibition of ICa by omega-CgTx. However, when applied after omega-CgTx, omega-GsTx produced an additional inhibition of 27%, indicating that omega-GsTx also blocked a non-N-type Ca2+ channel. BayK8644 (1 microM) elicited an increase in ICa in the presence of maximally effective concentrations of omega-GsTx, suggesting that omega-GsTx does not block L-type channels. Thus, omega-GsTx displays a selectivity for Ca2+ channel subtypes which should prove useful for studying Ca2+ channels and Ca(2+)-channel-mediated processes.

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.

Effects of non-opioid antitussives on hypoxia-induced electrical changes in rat hippocampal slices: a comparative study with anticonvulsant drugs

1. The effects of the non-opioid antitussives caramiphen and carbetapentane and of the anticonvulsants 5,5-diphenylhydantoin and MK 801 were tested towards hypoxia-induced electrical changes in rat hippocampal slices. 2. The incidence of appearance of hypoxia-induced epileptiform bursting was significantly decreased (P < 0.05) by carbetapentane (50-100 microM), caramiphen (50-100 microM), 5,5-diphenylhydantoin (25-50 microM), and the glutamate antagonist dizocilpine (MK 801, 25-50 microM). 3. The incidence of reappearance of the CA1 population spike after hypoxia was significantly increased (P < 0.05) by carbentapentane (50-100 microM), caramiphen (50-100 microM), 5,5-diphenylhydantoin (25-50 microM), and MK 801 (25-50 microM). 4. The results suggest a useful role for non-opioid antitussives and some anticonvulsants in the treatment of hypoxia-induced functional disturbances.

Depolarization by K+ and glutamate activates different neurotransmitter release mechanisms in GABAergic neurons: vesicular versus non-vesicular release of GABA

Neurotransmitter release and changes in the concentration of intracellular free calcium ([Ca++]i) were studied in cultured GABAergic cerebral cortical neurons, from mice, upon depolarization with either an unphysiologically high potassium concentration (55 mM) or the physiological excitatory neurotransmitter glutamate (100 microM). Both depolarizing stimuli exerted prompt increases in the release of preloaded [3H]GABA as well as in [Ca++]i. However, the basic properties of transmitter release and the increase in [Ca++]i under a variety of conditions were different during stimulation with K+ or glutamate. Potassium-evoked release of [3H]GABA consisted of two phases, a rapid, large and transient phase followed by a smaller, more persistent second phase. The rapid phase was inhibited (60%) by nocodazole which reduced the number of vesicles in the neurites by 80%. This rapid phase of the GABA release was also reduced by organic (verapamil) and inorganic (Co++) Ca++ channel blockers but was insensitive to the GABA transport inhibitor SKF 89976A. In contrast, the second phase was less sensitive to nocodazole and Ca++ channel antagonists but could be inhibited by SKF 89976A. The glutamate-induced [3H]GABA release, which was mainly mediated by N-methyl-D-aspartate receptors, consisted of a single, sustained phase. This was insensitive to nocodazole, partly inhibited by verapamil and could be blocked by Co++ as well as SKF 89976A. The action of Co++ could be attributed to a block of N-methyl-D-aspartate-associated ion channels. These findings strongly suggest that the majority of the K(+)-stimulated GABA release is dependent upon vesicles whereas the glutamate induced release is non-vesicular and mediated by a depolarization-dependent reversal of the direction of high-affinity GABA transport. The basic differences in the mode of action of the two depolarizing stimuli were reflected in the properties of the increase in [Ca++]i elicited by 55 mM K+ and 100 microM glutamate, respectively. The K(+)-induced increase in [Ca++]i was reduced by both verapamil and Ca(++)-free media whereas the corresponding glutamate response was only sensitive to Ca(++)-free conditions. Exposure of the cells to nocodazole or SKF 89976A had no effect on the ability of K+ or glutamate to increase [Ca++]i. Altogether, the results clearly demonstrate that K(+)-induced transmitter release from these GABAergic neurons is vesicular in nature whereas that induced by the neurotransmitter glutamate is not.

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.

AMPA receptor activation potentiates zinc neurotoxicity

Extracellular Zn2+ attenuates NMDA receptor-mediated neurotoxicity and increases AMPA receptor-mediated toxicity. Known electrophysiological effects of Zn2+ predict only the former. We considered the possibility that the latter rather reflects AMPA potentiation of Zn2+ toxicity, perhaps mediated by neuronal depolarization and Zn2+ entry through voltage-gated Ca2+ channels. High K+ or kainate also potentiated Zn2+ toxicity, and AMPA plus Zn2+ toxicity was attenuated by raising extracellular Ca2+, or by Ca2+ channel blockers. AMPA plus Zn2+ exposure induced an increase in fluorescence from neurons loaded with the Zn(2+)-sensitive dye TS-Q and increased subsequent 45Ca2+ accumulation. The ability of AMPA receptor activation to potentiate Zn2+ toxicity may be relevant to neuronal death associated with intense activation of glutamatergic pathways.

Non-opioid antitussives potentiate some behavioural and EEG effects of N-methyl-D-aspartate channel blockers

The effects of the non-opioid oral antitussives dextromethorphan (DM) and caramiphen (CP) were tested against the behavioural and EEG effects elicited by the N-methyl-D-aspartate (NMDA) antagonists dizocilpine (MK 801) and phencyclidine (PCP) in rats and mice. PCP (1.25-10 mg/kg i.p.) induced a dose-dependent increase/decrease of the locomotor/exploratory activity of mice. DM (25-50 mg/kg i.p.) and MK 801 (0.125-0.250 mg/kg i.p.) induced an increase of the locomotor/exploratory activity of mice, while CP (25-50 mg/kg i.p.) did not produce such an effect. CP (12.5 mg/kg i.p.) and DM (12.5 mg/kg i.p.) significantly potentiated the effects of PCP (1.25 mg/kg i.p.) and MK 801 (0.062 mg/kg i.p.) in the open field test in mice. In rats, PCP (1.25-10 mg/kg i.p.) induced three dose-dependent EEG stages: 1) increase of the cortical desynchronization periods; 2) increase of the amplitude of cortical background activity; 3) appearance of cortical slow wave-spike complexes. Even though DM (up to 100 mg/kg i.p.) only induced PCP-like EEG stage 1 by itself, and CP (up to 50 mg/kg i.p.) did not affect basal cortical EEG activity, these drugs, at the doses of 30-50 mg/kg i.p., potentiated all the EEG effects induced by PCP. These data support the view of an interaction between non-opioid antitussives and non-competitive NMDA antagonists.

Calcium channel blocking agents and potassium-stimulated release of glutamate from cerebellar slices

The effects of calcium channel blockers and tetrodotoxin on the potassium-stimulated release of endogenous glutamate from rat cerebellar slices was assessed. Verapamil (10 microM), omega-conotoxin (1 and 10 microM) and cobalt (2 mM) all significantly decreased release. Amiloride (100 microM) and tetrodotoxin (0.5 and 1 microM) had no effect. The results suggest a role for N-type voltage-operated calcium channels in the potassium-stimulated release of glutamate.

Lipophilic amino alcohols with calcium channel blocking activity

A series of novel lipophilic amino alcohols, analogs of the anticholinergic drug vesamicol, were evaluated for Ca2+ channel blocking activity. The effects of these drugs on depolarization-induced intracellular free Ca2+ concentration ([Ca2+]i) transients were examined in single NG108-15 cells and dorsal root ganglion (DRG) neurons in culture. [Ca2+] was recorded with the Ca2+ indicator Indo-1 and a dual emission microfluorimeter. Structure-activity studies indicated that features required for Ca2+ channel blocking activity were distinct from those required for anticholinergic activity. In particular, the Ca2+ channel blocking activity was insensitive to the configuration at the chiral center, whereas the anticholinergic activity was clearly enantioselective. One of the most active compounds, 3-(3-bromophenyl)-2-hydroxy-1-[1-(4-phenylpiperidinyl)]propane (2b), was characterized in more detail. This compound inhibited the dihydropyridine-sensitive Ca2+ channel response in NG108-15 cells, evoked by depolarization with 50 mM K+, with an IC50 of 5 microM. Field potential stimulation of DRG neurons elicited [Ca2+]i transients mediated by at least three Ca2+ channel subtypes; compound 2b inhibited the entire Ca2+ channel response with an IC50 of 1 microM. A key element required for Ca2+ channel blocking activity was the presence of an electron withdrawing substituent on the pendant phenyl ring. Modification of the amino alcohol structure may lead to more potent compounds with broad spectrum Ca2+ channel blocking activity. These structures provide a new chemical starting point for the development of Ca2+ antagonists.

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.

Effect of voltage-sensitive calcium channel antagonists on the release of 5-hydroxytryptamine from rat hippocampus in vivo

The effect of calcium channel antagonists on the release of 5-hydroxytryptamine from the hippocampus of the chloral hydrate-anaesthetised rat was studied using the technique of intracerebral microdialysis. As the basal concentration of 5-hydroxytryptamine was close to the limit of detection of the HPLC method (8 fmol), the 5-hydroxytryptamine reuptake inhibitor, fluoxetine (10 microM), was included in the perfusion fluid. The L-type voltage-sensitive calcium channel antagonists, PN200-110, diltiazem, and verapamil, all passed through the dialysis membrane, giving a recovery of 20-30%. The N-type voltage-sensitive calcium channel antagonist, omega-conotoxin, penetrated less readily (12% recovery). The dihydropyridine, PN200-110, adhered to the probe, resulting in an effective concentration at the membrane 30% of that in the perfusion fluid. The concentration of 5-hydroxytryptamine in the dialysate samples was reduced by 60% in the absence of calcium. The L channel antagonists had little effect on the release of 5-hydroxytryptamine, which was inhibited, in a dose-dependent manner, to a maximum of 40% by omega-conotoxin. It is concluded that, under physiological conditions, the release of 5-hydroxytryptamine from the rat hippocampus is dependent on the entry of calcium through N-type voltage-sensitive calcium channels, although another calcium channel may also be involved.

Omega-conotoxin differentially blocks acetylcholine and adenosine triphosphate releases from Torpedo synaptosomes

We have examined the effect of several blockers of voltage-sensitive calcium channels on the release of acetylcholine and ATP from synaptosomes isolated from Torpedo marmorata electric organ. Depolarization of these nerve terminals with high K(+)-containing solutions resulted in a calcium-dependent release of both molecules. Cadmium ions (10(-6) to 10(-3) M) inhibited similarly both releases whereas nickel ions (10(-4) M) in the external medium did not affect either neurotransmitter or nucleotide release. Both releases were completely resistant to the effect of 1,4-dihydropyridines (antagonists nimodipine, nifedipine and agonist Bay K 8644) and of a related compound (diltiazem) at concentrations up to 10(-5) M. These drugs failed to cause any effect even when synaptosomes were submaximally depolarized during incubation. Omega-conotoxin (10(-8) to 5 x 10(-5) M) showed a differential effect on acetylcholine and ATP releases. Nucleotide release was inhibited 90% at the highest concentration tested (50 microns) while acetylcholine release was only moderately decreased (30%). EC50 values for acetylcholine and ATP were of 167 and 2 microM respectively. The results suggest the implication of different types of calcium channels in the release of these molecules.

Antitussive effects of Ca2+ channel antagonists

The effects of Ca2+ channel antagonists on the capsaicin-induced cough reflex in guinea pigs were studied. Intraperitoneal injection of nifedipine, verapamil and flunarizine in doses that ranged from 0.3 to 3.0 mg/kg decreased the number of coughs in a dose-dependent manner. These Ca2+ channel antagonists exhibited antitussive effects in the following order of potency: flunarizine = verapamil greater than nifedipine. Pretreatment with a low dose of nifedipine (0.3 mg/kg), which by itself had no significant effect on the number of coughs, markedly increased the antitussive effects of morphine, dihydrocodeine and dextromethorphan. These data suggest that Ca2+ channels play an important role in the regulation of the cough reflex.

[3H]D-aspartic acid release in brain slices of adult and aged Fischer 344 rates

Alterations in glutamate content and uptake have been reported to occur in aged animals. The present studies used [3H]D-Aspartic acid [( 3H]-D-ASP) release as a marker for glutamate neurotransmission. Frequency dependent [3H]-D-ASP release was measured in adult (8 month) and aged (28-30 month) Fischer 344 rats. Relatively high stimulation frequencies (greater than 10 Hz) were required to induce [3H]-D-ASP release in both adult and aged F344 rats in temporal cortex and hippocampus. In both brain areas aged animals showed significantly more [3H]-D-ASP release than adult animals. Kainic acid 1 mM failed to induce the release of [3H]-D-ASP in either temporal cortex or hippocampus. Omega conotoxin GVIA (5 x 10(-9) M) a N and L type voltage sensitive calcium channel antagonist failed to inhibit [3H]-D-ASP stimulated release. These results demonstrate an increase in [3H]-D-ASP release in aged compared to adult F344 rats. The data also suggest a novel calcium channel may be involved in [3H]-D-ASP release.

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.