Are dihydropyridine binding sites voltage sensitive calcium channels?

Effects of changes in extracellular potassium, magnesium and calcium concentration on synaptic transmission in area CA1 and the dentate gyrus of rat hippocampal slices

The dependence of stimulus-induced synaptic potentials on changes of extracellular ionic concentrations of potassium ([K+]o 3, 5, 8 mM), magnesium ([Mg2+]o 2, 4, 8 mM) and calcium [Ca2+]o (2 mM and continuous lowering by washing with Ca2(+)-free solutions) was investigated in area CA1 and dentate gyrus of rat hippocampal slices. Field potentials (fps), [K+]o and [Ca2+]o were measured with double-barreled ion selective/reference microelectrodes. Paired pulse stimulation (interval 50-ms) was applied either to the lateral perforant path or to the Schaffer collaterals. Elevation of [K+]o from 5 to 8 mM and of [Mg2+]o from 2 to 8 mM depressed the rise of excitatory postsynaptic potentials, as well as the amplitude of population spikes. With elevation of [K+]o, the effect was stronger in the dentate gyrus, while with elevation of [Mg2+]o, the reduction was more pronounced in area CA1. During washout of Ca2+, synaptic potentials became reduced and finally depressed. The [Ca2+]o at which synaptic transmission was blocked increased with higher [Mg2+]o and decreased with a change of [K+]o from 3 to 5 mM, whereas with an elevation of [K+]o from 5 to 8 mM, it rose in area CA1 but was reduced in dentate gyrus. All ionic changes also affected frequency habituation and potentiation in paired pulse experiments. In dentate gyrus, frequency habituation was reversed to frequency potentiation with moderate lowering of [Ca2+]o and with elevation of [Mg2+]o and [K+]o. In contrast, in area CA1 frequency potentiation was reduced upon elevation of [K+]o.

Effects of calcium channel inhibitors upon the efficacy of common antiepileptic drugs

Diltiazem and nifedipine (both 1.25 mg/kg) markedly potentiated the protective action of carbamazepine and diphenylhydantoin against maximal electroshock-induced seizures in mice. These calcium channel inhibitors retained their activity at lower doses. Diltiazem and nifedipine (2.5 mg/kg) also moderately potentiated the efficacy of phenobarbital and valproate. Verapamil (up to 10 mg/kg) was not effective against the action carbamazepine, diphenylhydantoin, phenobarbital, and valproate. None of the calcium channel inhibitors used (up to 40 mg/kg) influenced aminophylline-induced convulsions and mortality. Moreover, the anti-aminophylline activity of valproate and phenobarbital was not potentiated by the calcium channel inhibitors in doses up to 10 mg/kg. Further, combination of carbamazepine, ethosuximide, and trimethadione with the calcium channel inhibitors (up to 10 mg/kg) did not offer any protection against aminophylline-induced convulsions. It can be concluded that calcium channel inhibitors enhance the protective efficacy of some antiepileptics against electroconvulsions. A pharmacokinetic interaction does not seem to be responsible for this effect.

The search for the hypnogenic center

1. Electrophysiological and lesion studies have suggested that a number of specific sites in the brainstem and basal forebrain may be involved in the regulation of sleep and waking. In contrast, a study of glucose consumption as measured by the 2-deoxyglucose technique reported a generalized decrease in nonREM sleep compared to waking. The rate of protein synthesis was relatively unchanged in nonREM sleep. 2. Another approach to understanding sleep regulation is to study the mechanism by which hypnotic drugs affect the nervous system. This may be done at both a molecular and neuroanatomic level. Studies with B-carbolines, inverse agonists of benzodiazepines (BZs), indicate that sleep induction by BZs is mediated by binding at the BZ recognition site of the BZ receptor complex. Binding at this site by a long-acting B-carboline parallels the time course of its arousing effects. A study with an enantiomeric BZ indicates that the effects on sleep are stereospecific. It is conceivable that some inverse agonists or enantiomeric benzodiazepines might be developed for clinical use as analeptics. 3. Microinjection of a BZ into the dorsal raphe nucleus acutely increases wakefulness, while administration into the medial preoptic area of the hypothalamus enhances sleep maintenance.

Effects of dihydropyridine calcium channel antagonists in ethanol withdrawal; doses required, stereospecificity and actions of Bay K 8644

The effects of dihydropyridine calcium channel antagonists, and the calcium channel activator, Bay K 8644, were examined on the convulsive behaviour induced by handling in mice following withdrawal from chronic ethanol inhalation. Nimodipine and nitrendipine and PN 200-110 significantly decreased the convulsive behaviour, after intraperitoneal doses of the same order of magnitude as have been found by others to be required for displacement of radiolabelled dihydropyridine in the CNS. The (+) isomer of PN 200-110 was effective, but the (-) isomer, which is ineffective in vitro, had no significant action. Bay K 8644 prevented the actions of nimodipine against the ethanol withdrawal syndrome. The behavioural ratings after nimodipine plus Bay K 8644 were significantly higher than after vehicle treatment. Bay K 8644 alone, when given to naive mice, caused convulsive behaviour resembling that seen in withdrawal from chronic ethanol treatment, but when given during ethanol withdrawal did not significantly increase the behavioural signs.

Central neurochemical effects of dihydropyridine calcium antagonists

Recent advances in central dihydropyridine (DHP)-binding sites are reviewed. DHP-binding sites are pre-synaptically and post-synaptically localized in the brain. The functional role of post-synaptic sites is still unknown, whereas pre-synaptic sites seem to contribute to the control of calcium uptake and of neurotransmitter release. DHP-binding sites may be modualated in physiological (age, sex) and pathological events (hypertension, ischaemia, neurological diseases) or after drug treatments (alcohol, morphine, etc.). The reviewed data suggest new therapeutic implications of DHP calcium channel antagonists in the treatment of other diseases and of drug withdrawal syndrome.

Activators and inactivators of calcium channels: effects in the central nervous system

The interactions of calcium antagonists or channel activators with the different classes of calcium channel are reviewed with particular emphasis on interactions with neuronal tissue; reasons for the failure of calcium antagonists to inhibit neurotransmitter release under normal circumstances are outlined. Calcium antagonists may be protective in several pathological situations and the possibilities of protection against ischaemic damage in the central nervous system are evaluated.

Dihydropyridines modulate K+-evoked amino acid and adenosine release from cerebellar neuronal cultures

Partial depolarization of primary cerebellar neuronal cultures with K+ evoked the release of aspartate, glutamate, adenosine, serine, taurine, gamma-aminobutyric acid (GABA), alanine and proline. The dihydropyridine calcium channel agonist, BAY K 8644, significantly augmented the K+-induced release of adenosine, aspartate, glutamate and GABA, but not that of serine, taurine, alanine or proline. However, in all cases the dihydropyridine antagonist nifedipine decreased this BAY K 8644-enhanced, K+-evoked efflux to below control levels. Neither BAY K 8644 nor nifedipine alone affected basal efflux levels. The phenylalkylamine calcium channel antagonist, verapamil, was ineffective in antagonizing K+-evoked amino acid release except at very high concentration (100 microM). These findings suggest that L-type Ca2+ channels are present in both excitatory (glutamatergic granule cells) and inhibitory (GABAergic stellate and basket cells) neurons in these cultures, and that they appear to be involved in regulating the release of not only neuroactive amino acids, but also some neutral amino acids and adenosine.

High affinity binding of the calcium channel blocker, (+)-[methyl-3H]PN200-110(3HPN) in rat brain

PN200-110 is a recently introduced 1,4-dihydropyridine which has been demonstrated to be a potent calcium channel blocker. 3HPN has been shown to bind in a specific saturable manner to P2 fractions obtained from brain homogenates from male Sprague-Dawley rats. 3HPN binding was found to be temperature-dependent. Specific 3HPN binding was maximal at 25 degrees C; binding decreased at 2 degrees C and 37 degrees C. The KD calculated from Scatchard analysis was 0.0943 +/- 0.0038 nM while the Bmax was found to be 109.1 +/- 2.3 fmol/mg protein. A concentration dependent inhibition of 3HPN binding by various cations was determined and found to be as follows: ZN2+ greater than La3+ greater than Rh3+, Al3+ greater than Co2+, Ni2+, Mn2+ greater than Ca2+, Mg2+ greater than Ba2+ greater than Sr2+. These results provide evidence for the existence of central high affinity dihydropyridine receptor sites in rat brain.

Diltiazem enhances and flunarizine inhibits nimodipine's antiseizure effects

The dihydropyridine calcium channel antagonist, nimodipine has antiepileptic and anticonvulsive properties that are thought to be mediated through neuronal calcium channel blockade. The dihydropyridine binding site can be positively and negatively allosterically regulated by the benzothiazepines and the phenylalkylamines/piperazines, respectively. We investigated this binding interaction at the physiologic level by examining the effects of diltiazem (a benzothiazepine) and flunarizine (a piperazine) on the antiseizure activity of nimodipine. Seizures were induced with pentylenetetrazole in awake rats with chronically implanted EEG electrodes. Calcium channel antagonists were administered intracerebroventricularly 30 min after pentylenetetrazole at doses given at 15 min intervals. Diltiazem and flunarizine alone lacked antiseizure properties. The calculated ED50 values for nimodipine were: nimodipine alone = 135 micrograms; nimodipine + diltiazem (100 micrograms) = 67 micrograms. Nimodipine + flunarizine (10 micrograms) completely suppressed nimodipine's antiseizure activity. These findings may reflect the interaction observed among these agents at binding sites associated with the calcium channel and supports the idea that dihydropyridines mediate their antiseizure actions through neuronal calcium channel antagonism.

Calcium channels that are required for secretion from intact nerve terminals of vertebrates are sensitive to omega-conotoxin and relatively insensitive to dihydropyridines. Optical studies with and without voltage-sensitive dyes

Extrinsic absorption changes exhibited by potentiometric dyes have established the ionic basis of the action potential in synchronously activated populations of nerve terminals in the intact neurohypophyses of amphibia and mammals (Salzberg et al., 1983; Obaid et al., 1983, 1985b). Also, large and rapid changes in light scattering, measured as transparency, have been shown to follow membrane depolarization and to be intimately associated with the release of neuropeptides from the nerve terminals of the mouse neurohypophysis (Salzberg et al., 1985; Gainer et al., 1986). We report some experiments that help to define the pharmacological profile of the calcium channels present in intact neurosecretory terminals of vertebrates. For these, we used the peptide toxin omega-conotoxin GVIA (1-5 microM) and the dihydropyridine compounds Bay-K 8644 and nifedipine (2-5 microM), together with the after-hyperpolarization of the nerve terminal action potential. This undershoot depends upon the activation of a calcium-mediated potassium channel, as suggested by its sensitivity to [Ca++]o and charybdotoxin. omega-conotoxin GVIA substantially reduced the after-hyperpolarization in neurosecretory terminals of Xenopus, while neither of the dihydropyridine compounds had any effect under conditions that mimic natural stimulation. The effects of these calcium channel modifiers on the action potential recorded optically from the terminals of the Xenopus neurohypophysis were faithfully reflected in the behavior of the light-scattering changes observed in the neurohypophysis of the CD-1 mouse. omega-conotoxin GVIA (5 microM) reduced the size of the intrinsic optical signal associated with secretion by 50%, while the dihydropyridines had little effect. These observations suggest that the type of calcium channel that dominates the secretory behavior of intact vertebrate nerve terminals is at least partially blocked by omega-conotoxin GVIA and is insensitive, under normal conditions, to dihydropyridines.

Central and peripheral effects of the dihydropyridine calcium channel activator BAY K 8644 in the rat

Following intraperitoneal (i.p.) administration BAY K 8644 (0.5-4 mg/kg) induced an increase in blood pressure associated with bradycardia, increased tail-flick latency in response to radiant heat, decreased locomotion, induced muscle contraction, postural changes and also reduced reflex activity. Only the postural changes and reduced locomotion were seen after intracerebroventricular administration (5-20 micrograms/kg), suggesting that the other effects are mediated peripherally. All the above effects were antagonised by the calcium channel blocker nifedipine. BAY K 8644 (4 mg/kg i.p.) also significantly increased homovanillic acid and 3,4-dihydroxyphenylacetic acid concentrations in the cortex and striatum, an effect which could also be reversed by nifedipine. Apart from inducing hypotension and tachycardia, nifedipine alone had no effect on any of the above parameters. The analgesic-like activity of BAY K 8644 observed in the tail-flick test appears to be related to its vasoconstrictor effects as the peripherally acting vasodilator phenylephrine had similar analgesic activity. These results show that both central and peripheral dihydropyridine-sensitive calcium channels mediate the effects of BAY K 8644. Although a physiological role for the dihydropyridine-sensitive voltage-operated calcium channel in the CNS remains to be demonstrated, activation of these channels can clearly have functional effects.

Stimulus-coupled taurine efflux from cerebellar neuronal cultures: on the roles of Ca++ and Na+

Primary cultures of cerebellar neurons obtained from 7-9-day-old rats and grown 7-9 days in vitro (DIV) were used to study the effects of Na+ and Ca++ on K+-evoked taurine release. These cultures, made up largely of granule neurons (90%) and inhibitory interneurons (5-7%), produced a dose-dependent, depolarization-evoked taurine release that was Ca++-dependent at 40 mM K+, and Ca++-independent at K+ concentrations above 40 mM. The dihydropyridine Ca++ channel agonist BAY K 8644 (1 microM) augmented 30 mM K+-evoked release, while the antagonist nifedipine (5 microM) abolished both the BAY K 8644- and K+-enhanced release. Depolarization with the Na+ channel agonist veratridine (50 microM) stimulated taurine efflux, which was completely blocked by pretreatment with tetrodotoxin (2 microM). However, 50 mM K+-evoked taurine release was not affected by tetrodotoxin pretreatment. Substitution of choline Cl for NaCl partially antagonized 50 mM K+-evoked release, and by itself, the Na+ ionophore monensin (50 microM) stimulated release. These results suggest that both K+-evoked and basal taurine release from primary cerebellar neuronal cultures are sensitive to the levels of both intracellular and extracellular Na+ and Ca++. In contrast to previous findings using cerebellar astrocytes, neuronal L-type Ca++ channels, but not voltage-dependent Na+ channels, also appear to be necessary. The implications of these results on taurine's status as a putative neurotransmitter are discussed.

Potassium-stimulated calcium uptake in astrocytes and its potent inhibition by nimodipine

Elevation of the extracellular potassium concentration above its "resting" level of 5.4 mM stimulated uptake of 45Ca2+ in primary cultures of astrocytes. This effect was only observed when cells were exposed to excess potassium shortly after their exposure to 45Ca2+ and was potently inhibited (IC50 congruent to 3 nM) by the calcium channel blocker nimodipine. In contrast, nimodipine exerted little effect on unstimulated basal uptake of 45Ca2+. These findings suggest that the therapeutic benefit of calcium channel blockers in epilepsy may result in part from the ability of these drugs to prevent calcium entry into astrocytes during seizures when the extracellular potassium is elevated four- to fivefold above normal.

Ca antagonistic and non-specific effects of diltiazem on the rat phrenic nerve diaphragm preparation

The calcium antagonist diltiazem (2.8 X 10(-4) M) blocked the twitches of a rat phrenic nerve diaphragm preparation after a period of twitch potentiation. Its ability to block twitches was greater during indirect than direct stimulation. Experiments on the isolated phrenic nerve indicated that the excitability of the nerve was blocked. Diltiazem (2.3-9.0 X 10(-5) M) caused a similar inhibition of indirectly and directly elicited tetanic contractions and EMG. Experiments with d-tubocurarine and lowered temperature disclosed a separate inhibition at the neuromuscular junction. High Ca2+ did not reverse the diltiazem-affected twitch or tetanic contractions, which suggests that they are non-specific effects. KCl (100 mM)-induced contractures were antagonized at low (2.3-4.5 X 10(-5) M) but not at high (1 mM) concentrations of diltiazem. Diltiazem depressed the initial phase of the two-phasic caffeine (10 mM) contracture and increased and accelerated the slow phase. Diltiazem greatly reduced the amplitude and duration of the caffeine-potentiated KCl contracture, and reduced and delayed the slow phase of the KCl-potentiated caffeine contracture. The effects on the combined contractures (caffeine-induced, KCl-potentiated) were partly antagonized by a high Ca2+ (2.2 X 10(-5) M) solution, which suggests that diltiazem has calcium antagonistic effects.

K+-evoked taurine efflux from cerebellar astrocytes: on the roles of Ca2+ and Na+

The ionic requirements for K+-evoked efflux of endogenous taurine from primary cerebellar astrocyte cultures were studied. The Ca2+ ionophore A23187 evoked taurine efflux in a dose-dependent fashion with a time-course identical to that of K+-induced efflux. The Ca2+-channel antagonist nifedipine had no effect upon efflux induced by 10 or 50 mM K+. In addition, verapamil did not antagonize 50 mM K+-evoked efflux except at high, non-pharmacological concentrations (greater than 100 microM), and preincubation with 2 microM omega-conotoxin had no effect on 50 mM K+-evoked efflux. Similarly, preincubation with 1 mM ouabain had no effect on the amount of taurine released by K+ stimulation, but did accelerate the onset of efflux by 2-4 min. Although 2 microM tetrodotoxin had no effect on K+-evoked release, replacing Na+ with choline abolished the taurine efflux seen in response to K+ stimulation. Together, these findings suggest that neuronal N- and L-type Ca2+- and voltage-dependent Na+-channels are not involved in the influx of Ca2+ which appears to be necessary for K+-evoked taurine efflux, and that in addition to Ca2+, extracellular Na+ is also required.

Stimulation of D2 dopamine receptors decreases intracellular calcium levels in rat anterior pituitary cells but not striatal synaptosomes: a flow cytometric study using indo-1

An important question is whether all D2 dopamine (DA) receptors employ the same signal transduction mechanisms. Anterior pituitary cells and striatal synaptosomes, which possess pharmacologically similar D2 DA receptors, were compared with respect to the effect of D2 DA receptor stimulation on free intracellular Ca2+ levels [( Ca2+]i). Flow cytometry, in combination with either the fluorescent calcium indicator indo-1 or fluorescent voltage-sensitive dyes, was used to measure [Ca2+]i and to detect changes in membrane potential. In subpopulations of anterior pituitary cells, increases in [Ca2+]i were produced by elevated K+, veratridine, thyrotropin-releasing hormone, and BAY K 8644. These increases were blocked by nifedipine, suggesting the involvement of L-type voltage-sensitive calcium channels (VSCC's). In 10-15% of the cells, D2 agonists decreased resting [Ca2+]i, reversed stimulus-induced increases in [Ca2+]i, and caused a hyperpolarization. In striatal synaptosomes, elevated K+ and veratridine also increased [Ca2+]i. However, the K+-induced increase was eliminated if choline was substituted for Na+ in the medium, suggesting that Ca2+ entry in response to sustained K+ depolarization resulted from reversal of Na+/Ca2+ exchange. Nifedipine and verapamil inhibited K+-induced increases in [Ca2+]i only at concentrations greater than 10 microM, while omega-conotoxin had no effect. D2 agonists had no effect on resting or stimulated [Ca2+]i but did hyperpolarize 10-20% of the synaptosomes, indicating that D2 DA receptors are functional in this preparation. The ability of pituitary but not striatal D2 DA receptors to modulate [Ca2+]i may reflect the fact that the two systems differ with respect to pathways for Ca2+ influx.

Inhibition of noradrenaline release by omega-conotoxin GVIA in the rat tail artery

1. The perivascular nerves of isolated tail arteries from Wistar rats were stimulated with field pulses (1 Hz, 2 pulses, every 2 min). omega-Conotoxin 10 nmol l-1 depressed neurogenically mediated contractions, but did not influence the contractions to noradrenaline 0.1-0.3 mumol l-1. 2. The inhibitory effect of omega-conotoxin was concentration-dependent (IC50 = 3.8 nmol l-1). It did not reach a steady-state during 30 min incubation and could not be reversed upon subsequent washout for another 60 min. 3. A gradual increase in the Ca2+ concentration of the medium from 1.25 mmol l-1 to 10 mmol l-1 enhanced vasoconstriction and attenuated the action of omega-conotoxin 10 nmol l-1. When a low stimulation intensity (120 mA) was used at high external Ca2+ (10 mmol l-1), similar contractile responses were obtained as under normal conditions (200 mA current, 2.5 mmol l-1 Ca2+). However, the inverse relationship between the effect of the toxin and external Ca2+ remained unchanged. 4. The time-course and degree of the inhibition by omega-conotoxin 3 nmol l-1 was identical in tail arteries of spontaneously hypertensive rats (SHR) and their normotensive controls (WKY). 5. When tail arteries of Wistar rats were preincubated with [3H]-noradrenaline, field stimulation (0.4 Hz, 24 pulses, every 16 min) evoked tritium overflow and vasoconstriction. omega-Conotoxin 30 nmol l-1 inhibited both responses to a similar extent. 6. Our results suggest that omega-conotoxin selectively blocks Ca2+ channels in the terminals of perivascular nerves and thereby reduces the release, but not the contractile effect of the sympathetic transmitter.

Calcium uptake studies of 1,4-dihydropyridine agonists into rabbit aortic smooth muscle cells in culture

The effects of the three dihydropyridine calcium channel agonists (+/-)BAY K 8644, (+)202-791 and (+/-)CGP 28392 on 45Ca++ uptake were studied in cultures of rabbit aortic smooth muscle cells. At 10(-7) M each agonist enhanced 45Ca++ uptake in 15-50 mM K+ but had no effect on the basal 45Ca++ uptake at 5 mM K+. At the uptake threshold of 15 mM K+ each agonist potentiated 45Ca++ uptake in a dose-dependent manner with half maximal effects at 2.4 nM for (+/-)BAY K 8644, 22 nM for (+)202-791 and 18 nM for (+/-)CGP 28392. The agonists showed no significant antagonistic activity. Responses were antagonized competitively by nifedipine and non-competitively by (+/-)D-600. The 45Ca++ uptake dose-response curves and the half maximal effects of the three agonists were over the same range of concentrations as their inhibition of [3H]nitrendipine binding to rat ventricular receptor membrane preparations. The data suggest that these cells mimic the calcium uptake by the intact aorta better than commercial vascular smooth muscle lines or cardiac cells.

Intracellular doxorubicin concentrations and drug-induced DNA damage in a human colon adenocarcinoma cell line and in a drug-resistant subline

The mechanisms of resistance to doxorubicin (DX) were investigated using a human colon adenocarcinoma cell line (LoVo) and a subline approximately 30 times less sensitive to doxorubicin. LoVo and LoVo/DX were similar in terms of DNA and protein content, cell volume, duration of S phase and the generation time, and proportion of cycling cells. LoVo/DX showed cross-resistance to other anthracyclines, to vinca alkaloids, epipodophyllotoxin derivatives, 4'-(9-acridinylamino-methanesulfon-m-aniside) and actinomycin D. LoVo/DX was equally sensitive to melphalan and showed collateral sensitivity to cis-platinum and 1-beta-D-arabinofuranosylcytosine. On exposing LoVo and LoVo/DX to 1.25 and 40 micrograms/ml DX respectively, for 4 hr, similar DX intracellular concentrations were reached in the two cell lines. In these treatment conditions protein associated DNA-single strand breaks or DNA-double strand breaks, assessed by alkaline elution methods were only slightly less in LoVo/DX than in LoVo cells. In LoVo/DX cells, however, DNA breaks disappeared very quickly after drug removal whereas they persisted longer in LoVo cells. This persistance is probably related to the much slower DX efflux from LoVo than LoVo/DX. When verapamil was combined with DX it inhibited the rapid DX efflux from LoVo/DX and reversed the resistance in this cell line, but it had no significant activity on LoVo cells. Verapamil also increased DX-induced DNA-single strand breaks and DNA-double strand breaks in LoVo/DX cells, but not in LoVo cells.

Enantiomer selectivity and the development of tolerance to the behavioral effects of the calcium channel activator BAY K 8644

The putative behavioral effects of the enantiomers of BAY K 8644 and the behavioral responses to (+/-)-BAY K 8644 following chronic injection were assessed on motor function in mice. The interaction of the enantiomers of BAY K 8644 with mouse brain dihydropyridine binding sites was also evaluated. The calcium channel activating enantiomer (-)-S-BAY K 8644 impaired rotarod and motor activity with an ED50 value of 0.5 mg/kg. The calcium channel blocker enantiomer (+)-R-BAY K 8644 neither affected rotarod nor motor activity. (+)-R-BAY K 8644, and the structurally related dihydropyridine calcium channel blockers nifedipine and (-)-202-791 inhibited the impairment of rotarod activity by (-)-S-BAY K 8644 in a dose-dependent manner. (+/-)-BAY K 8644 produced convulsions in mice with a CD50 of 5 mg/kg. Chronic injection of (+/-)-BAY K 8644 (8 mg/kg IP once each day for four days) resulted in a significant tolerance to, and increase in recovery from, the motor deficits produced by (+/-)-BAY K 8644. Furthermore, chronic treatment with (+/-)-BAY K 8644 increased the onset time, but did not reduce the number of mice having convulsions to (+/-)-BAY K 8644. Chronic injection of nifedipine did not affect the motor deficit and convulsive activity of (+/-)-BAY K 8644. The behavioral effects of (+/-)-BAY K 8644 were observed at significant brain levels of drug. [3H]Nitrendipine binding to mouse brain dihydropyridine binding sites was unchanged in mice chronically injected with either (+/-)-BAY K 8644 or nifedipine.(ABSTRACT TRUNCATED AT 250 WORDS)

Verapamil blocks the afterhyperpolarization but not the spike frequency accommodation of rat CA1 pyramidal cells in vitro

The effects of prolonged periods (up to 4 h) of perfusion with verapamil (100 microM) or D600 (100 microM) on synaptically and directly evoked responses of rat CA1 pyramidal cells were determined in vitro. The slow depolarization underlying burst generation and the slow afterhyperpolarization following directly evoked repetitive firing were blocked, but spike frequency accommodation was not. There was an increase in threshold for evoking synaptic responses and the amplitude of evoked inhibitory postsynaptic potentials (IPSPs) was decreased slightly. The results suggest that verapamil can partially block voltage dependent Ca influx into CA1 cells and that currents underlying accommodation and the slow afterhyperpolarization (AHP) may differ.

Characteristics of [3H]nimodipine binding to sarcolemmal membranes from rat vas deferens and its regulation by guanine nucleotide

The binding properties of a 1,4-dihydropyridine (DHP) calcium entry blocker, [3H]nimodipine, to a microsomal fraction from rat vas deferens was characterized. The specific binding was saturable, rapid and reversible. Scatchard analysis of the binding revealed a single binding site, and the dissociation constant and the maximum number of binding sites were 0.31 +/- 0.02 nM and 97.0 +/- 7.19 fmol/mg protein, respectively. Both the Kd value obtained from the kinetic study and the IC50 value from relaxation of the K+-depolarized organ were approximately 0.4 nM, indicating that the binding site is closely related to the functional Ca2+ channel. The specific [3H]nimodipine binding was displaced by DHP derivatives at low concentration and by verapamil at high concentration, but diltiazem had no effect on the binding. Calcium chelating agents decreased the [3H]nimodipine binding which was restored by adding Ca2+. 5'-Guanylylimidodiphosphate caused a rightward shift of the displacement curve for Bay K 8644 but not for nimodipine, suggesting the involvement of guanine nucleotide binding protein in the signal transduction between the DHP binding site and the Ca2+ channel.

The effects of calcium channel agonists and antagonists on the release of endogenous glutamate from cerebellar slices

The effects of compounds acting at the calcium channel on neurotransmitter release are equivocal. We report here the effects of the antagonists, verapamil, diltiazem and nifedipine; the agonists, bay K8644 and the calcium ionophore, A23187 on the release of endogenous glutamate from rat cerebellar slices. Of these compounds, only verapamil and diltiazem modified glutamate release and these were effective at relatively high concentrations (greater than 1 x 10(-5) M). It is suggested that the high-affinity binding sites found in neuronal tissue for the dihydropyridine-like compounds are not involved in neurotransmitter release.

Use of tissue culture models to study neuronal regulatory trophic and toxic factors in the aged brain

Dementia is believed to result from the loss of selective neurons within the brain, but approaches for systematic study of that degenerative process are hampered by the complexity of the neuronal milieu. Tissue culture models provide a means to reduce dramatically the variables inherent in the study of neuronal plasticity. Three levels of complexity can be described: cellular and molecular diversity; primary and secondary interconnections; and finally, the dynamics influenced by age. The following review discusses the advantages and disadvantages of tissue culture models for the detailed study of neuronal trophic and toxic factors. Our selection of factors is broadened to include ions, intermediate metabolites, antioxidants, steroids, neuropeptides, gangliosides, metals, neurotransmitters, brain extracts, and protein molecules. Most of these factors have been shown to be altered in the aged brain, to have a significant effect on cultured neurons, or both. This multilevel analysis provides the reader with an overview of the events regulating neuronal survival, differentiation and death. An understanding of these basic questions is necessary to sequence the molecular events resulting in neuronal death.

Involvement of DHP voltage-sensitive calcium channels and protein kinase C in thyroliberin (TRH) release by developing hypothalamic neurons in culture

The intracellular mechanisms regulating the process of thyroliberin (TRH) release were studied using fetal hypothalamic neurons grown in serum-free medium. In particular, we compared the effects of dihydropyridine (DHP) derivatives, omega-conotoxin and phorbol esters on basal and K+-evoked TRH release from 12 days in vitro (DIV) neurons. BAY K 8644, a DHP calcium channel agonist increased in a dose-related manner basal and K+-evoked TRH release. PN 200-110, an antagonist of DHP-sensitive calcium channels, completely suppressed the effect of BAY K 8644, whatever the extracellular K+ concentration, but did not modify basal or K+-evoked TRH release. In contrast, omega-conotoxin partially inhibited the two latter processes. The active phorbol ester 12-O-tetradecanoyl-phorbol-beta-acetate (TPA), and to a lesser extent Sn-1,2-dioctanoylglycerol (DAG), triggered TRH release. This effect was specific, time and dose dependent and only partly dependent on extracellular calcium. Simultaneous addition of BAY K 8644 and TPA to the cells displayed a synergistic effect. The same compounds were studied on younger neurons (6-DIV cultures): BAY K 8644 stimulated TRH release whereas neither 60 mM K+ nor TPA did. These results suggest that TRH release can be mediated at least by two intracellular routes: (i) increase of intracellular calcium mediated by the opening of different types of voltage sensitive calcium channels, and (ii) activation of protein kinase C (PKC). The asynchrony in the maturation of the intracellular mechanisms underlying TRH release may be explained by different subcellular localizations of these mechanisms in neurons and is discussed in relation to synapse differentiation.

Effects of calcium channel antagonists on the depolarization-evoked release of norepinephrine in the rabbit iris-ciliary body

The effects of several representative calcium channel antagonists on depolarization-evoked release of [3H]-norepinephrine were investigated in isolated, superfused rabbit iris-ciliary bodies. Potassium (50 mM)-evoked neurosecretion was blocked by 5 mM CoCl2 and partially inhibited by 10(-6) M nitrendipine or verapamil. Electrically-evoked neurosecretion was similarly blocked by CoCl2, but was unaffected by nitrendipine or verapamil. It is concluded from these results that sympathetic terminals in the rabbit iris-ciliary body contain dihydropyridine- and verapamil-sensitive calcium channels which contribute, under conditions of prolonged depolarization, to neurotransmitter release.

Effect of various calcium channel blockers on three different models of limbic seizures in rats

Voltage-dependent calcium channel-blockers were studied for their ability to modulate limbic seizures induced in rats by injection of quinolinic acid and kainic acid into the hippocampus or by hippocampal kindling. Flunarizine, at 40 mg/kg (but not 20 mg/kg), reduced the total number of seizures and total time spent in seizures induced by quinolinic acid by 75%; at 60 mg/kg, both parameters were reduced more than 90%, while at 80 mg/kg seizures induced by kainic acid were not affected. Forty and 60 mg/kg of flunarizine protected hippocampal-kindled rats from fully developed convulsions (Stage 5). Nifedipine, at 20 and 40 mg/kg, was ineffective on seizures induced by both quinolinate and kainate. However, at 20 mg/kg, 57% of the kindled animals were protected from Stage 5 and total protection was achieved at 40 mg/kg. Verapamil, at 40 mg/kg, reduced by respectively, 88% and 78%, the total number of seizures and the total time spent in seizures induced by quinolinic acid, but had no effect on seizures induced by kainate and Stage 5 seizures. The results suggest that, while seizures induced by kainic acid were refractory to all voltage-dependent calcium channel blockers, binding sites affected by flunarizine and verapamil in the brain may selectively facilitate ictal activity induced by quinolinic acid. Binding sites for dihydropyridine might contribute to the increased hippocampal excitability in kindled animals. The role of calcium entry through voltage-dependent calcium channels in the occurrence of seizures in these models of limbic epilepsy is discussed.

Functional dihydropyridine binding site associated with slow calcium channel in rat cultured neurones

There is a high affinity binding of [3H]PN 200-110 (Kd = 0.21 nM) to slow calcium channels in cultured neurones. Several calcium antagonists, which recognize the [3H]PN 200-110 binding site, did not affect the K+-induced calcium uptake. The calcium channel activator BAY K 8644 increased the calcium uptake in depolarizing conditions and this effect was antagonized by pharmacological concentrations of calcium entry blockers. We conclude that the dihydropyridine binding site is involved in the modulation of calcium entry through the voltage-sensitive channel in depolarized cultured neurones.

Amiloride selectively blocks the low threshold (T) calcium channel

More than one type of voltage-gated calcium channel has been identified in muscle cells and neurons. Many specific organic and inorganic blockers of the conventional, slowly inactivating high threshold (L) calcium channel have been reported. No specific blockers of the low threshold (T) channel have been as yet identified. Amiloride, a potassium sparing diuretic, has now been shown to selectively block the low threshold calcium channel in mouse neuroblastoma and chick dorsal root ganglion neurons. The selective blockade of the T-type calcium channel will allow identification of this channel in different tissues and characterization of its specific physiological role.

Dihydropyridine binding sites regulate calcium influx through specific voltage-sensitive calcium channels in cerebellar granule cells

In primary cultures of cerebellar granule cells, [3H]nitrendipine binds with high affinity to a single site (KD 1 nM and Bmax 20 fmol/mg protein). The 1,4-dihydropyridine (DHP) class of compounds such as nitrendipine, nifedipine, and BAY K 8644 displace [3H]nitrendipine binding at nanomolar concentrations. Verapamil partially inhibits whereas diltiazem slightly increases the [3H]nitrendipine binding. In these cells, the calcium influx that is induced by depolarization is very rapid and is blocked by micromolar concentrations of inorganic calcium blockers such as cadmium, cobalt, and manganese. The calcium influx resulting from cell depolarization is potentiated by BAY K 8644 and partially inhibited (approximately 40%) by nitrendipine and nifedipine. Other non-DHP voltage-sensitive calcium channel (VSCC) antagonists, such as verapamil and diltiazem, completely blocked the depolarization-induced calcium influx. This suggested that nitrendipine and nifedipine block only a certain population of VSCCs. In contrast, verapamil and diltiazem do not appear to be selective and block all of VSCCs. Perhaps some VSCCs can be allosterically modulated by the binding site for the DHPs, whereas verapamil and diltiazem may block completely the function of all VSCCs by occupying a site that differs from the DHP binding site.

Sites of action of dihydropyridine drugs in the mouse hemidiaphragm muscle

The effects of nifedipine and BAY K8644 on directly evoked isometric twitch and potassium (K+)- and caffeine-induced contractures were investigated in mouse hemidiaphragm preparations in which neuromuscular transmission had been irreversibly blocked. Both drugs caused initial potentiation of twitch which at high concentrations (greater than 3 x 10(-5) M) was followed by blockade. A simultaneous slow contracture was seen with nifedipine but not BAY K8644. Control K+ contractures were triphasic. The initial fast and slow phases of this contracture were potentiated by BAY K8644 at all times and concentrations. Both phases were potentiated by nifedipine at low concentrations but, during prolonged exposure to high concentrations, potentiation was replaced by an inhibition. The time course of activation and inactivation of the slow phase was also accelerated by all concentrations of nifedipine. The initial phase of caffeine-induced contracture was potentiated and resolved into two components. From these results at least three sites of action were postulated. Conventional binding to t-tubular Ca2+ channels was linked to effects on the slow phase of K+ contracture. An effect on Ca2+ release from the sarcoplasmic reticulum and an inhibition of Ca2+ transfer from uptake to release compartments in the sarcoplasmic reticulum are also postulated.

Modulation of neurotransmitter metabolism by dihydropyridine calcium channel ligands in mouse brain

The regional concentrations of dopamine, serotonin, dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindole acetic acid were measured in mouse brain following administration of the dihydropyridine calcium channel activator BAY K 8644, and antagonist, nifedipine. BAY K 8644 (1-8 mg/kg) produced dose- and time-dependent increases in dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindoleacetic acid concentrations in the caudate, without altering dopamine and serotonin levels. No changes in 5-hydroxyindoleacetic acid concentration were observed in the raphe nuclei, hypothalamus, hippocampus and frontal cortex. Nifedipine (4 mg/kg) blocked BAY K 8644- (2 mg/kg) elicited increases in dihydroxyphenylacetic acid in the caudate. Furthermore, a higher dose of nifedipine (8 mg/kg) decreased dihydroxyphenylacetic acid and homovanillic acid, but did not affect dopamine, serotonin or 5-hydroxyindoleacetic acid concentrations, while a lower dose of nifedipine (2 mg/kg) significantly increased serotonin, 5-hydroxyindoleacetic acid and homovanillic acid, but did not affect dopamine and dihydroxyphenylacetic acid concentrations. The findings that both BAY K 8644 and nifedipine affect neurotransmitter metabolism in vivo in a dose-, time- and brain region-dependent manner, suggest that high-affinity dihydropyridine calcium channel binding sites play an important role in regulating neurotransmitter turnover in the central nervous system.

Pitfalls in the use of brain slices

In vitro brain slices are the preparation of choice for the detailed examination of local circuit properties in mammalian brain. However it is the investigator's responsibility to verify that the circuits under investigation are indeed confined within the boundaries of the functional region of the slice used. The medium in which the slice is maintained is under the full control of the investigator. This places the burden on the investigator to ensure that: (1) the properties of the medium are fully under control; (2) the effects of the medium on the slice are known; (3) the conditions under which the slice is being maintained bear some reasonable relation to those it enjoys (or endures) in vivo. Generalizations to in vivo conditions must be made with caution. If at all possible, similar studies (perhaps less extensive, due to the greater technical difficulties) should be done in vivo to provide a basis for comparison. Investigators using drugs should be aware of, and respect, the basic pharmacological principles cited in the text. In particular, the substantial freedom the investigator has in defining the extracellular medium should not be abused.

Regulation of luteinizing hormone secretion by neuropeptide Y in rats: hypothalamic and pituitary actions

Previous studies have suggested that the stimulatory effect of neuropeptide Y (NPY) on the release of luteinizing hormone (LH) in rats may be due to a central action of the peptide that promotes the release of LH-releasing hormone (LHRH) from the hypothalamus, and to an action in the pituitary gland, to potentiate the release of LH induced by LHRH. The objectives of the present experiments were to test 1) whether NPY stimulation of LHRH release requires extracellular Ca++, and 2) whether NPY can exert direct stimulatory effects on the release of LH from anterior pituitary cells. The in vitro release of LHRH from medial basal hypothalamic fragments induced by KCl depolarization (56 mM), but not the basal release, was blocked by omission of Ca++ and addition of 0.1 mM EGTA to the incubation medium and also by cobalt (1 mM). Depolarization-induced release of the peptide was unaffected by nifedipine, diltiazem, or lanthanum. However, the stimulation of LHRH release by NPY (1 microM) still occurred in Ca++ free/EGTA medium. In a second set of experiments, 10 min pulses of NPY (1-100 nM) alone were ineffective in stimulating the release of LH from dispersed, perifused anterior pituitary cells obtained from ovariectomized, untreated or ovariectomized, estrogen-treated rats, under conditions where pulses of LHRH (0.1-10 nM) were consistently effective. A brief increase in LH release was observed during a 30 min exposure to 100 nM NPY in estrogen-pretreated cells, but not from untreated cells, and the effect was not as marked as that produced by LHRH.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrically coupled but chemically isolated synapses: dendritic spines and calcium in a rule for synaptic modification

An influential model of learning assumes synaptic enhancement occurs when there is pre- and post-synaptic conjunction of neuronal activity, as proposed by Hebb (1949) and studied in the form of long-term potentiation (LTP). There is evidence that LTP has a post-synaptic locus of control and is triggered by an elevation of intracellular calcium ion concentration, [Ca2+]i. Since synapses which undergo LTP are usually situated on dendritic spines, three effects of spine morphology on this system should be considered: (i) synapses on spines are chemically isolated by the barrier to Ca2+ diffusion due to the spine neck dimensions; (ii) the resistance of the spine neck permits a given synaptic current to bring about greater depolarization (of the spine head membrane) than the same current into a dendrite; while (iii) the spine neck resistance does not significantly attenuate current flow (in the dendrite to spine direction) because of the relatively high impedance of the spine head, and this permits electrical coupling via the dendritic tree. The specificity of LTP to activated synapses on depolarized cells has recently been attributed to special properties of the receptor-linked channel specifically activated by N-methyl-D-aspartate (NMDA). This admits calcium and other ions only when there is both depolarization and receptor activation. However, consideration of point (ii) suggests that, for spines with high resistance necks, the current through a synapse on the spine head will cause sufficient depolarization to unblock the NMDA channel. Thus, the properties of the NMDA channel do not account for the requirement for conjunction of pre- and post-synaptic activity, if these channels are located on the spine head. This suggests that additional mechanisms are required to explain why it is necessary to depolarize the post-synaptic cell in order to induce LTP. As an alternative, it is postulated that there exist voltage-sensitive calcium channels (VSCCs) on the spine head membrane, of a type which require greater membrane depolarization for activation. To generate the greater depolarization required, both pre- and post-synaptic activation would be necessary. If so, the role of dendritic or somatically located NMDA channels may be to "prime" neurons for LTP by enchancing voltage-dependent responses. A corollary is that spine resistance may regulate the threshold number of synapses required to produce LTP. It is predicted that, on spines with very high neck resistance (say, greater than 600 M omega), synaptic current alone may produce sufficient depolarization to activate VSCCs.(ABSTRACT TRUNCATED AT 400 WORDS)

On the possible central effects of calcium antagonists

The present paper discusses the possible central effects of calcium antagonists on the central nervous system in the light of present knowledge of the role of Ca2+ in physiological and pathophysiological processes.

Different localization of receptors for omega-conotoxin and nitrendipine in rat brain

The bindings of radioiodinated omega-conotoxin GVIA and [3H]-nitrendipine to subcellular fractions of rat brain were examined. The results indicated that omega-conotoxin binding site was mainly present in the mitochondrial fraction, whereas nitrendipine binding site was rich in the mitochondrial but also present in the post-mitochondrial fraction. Fractionation of the mitochondrial fraction on a sucrose density gradient centrifugation showed that the both binding sites were localized in the heavy synaptosomal fraction. These results strongly suggest that the N- and L-type voltage-sensitive calcium channels have different localizations.

Regional distribution of calcium channel ligand (1,4-dihydropyridine) binding sites and 45Ca2+ uptake processes in rat brain

The binding of nimodipine, a 1,4-dihydropyridine Ca2+ channel antagonist, and of Bay K 8644, a Ca2+ channel activator, was measured in several regions of rat brain and compared to the distribution of K+ depolarization-induced 45Ca2+ uptake into synaptosomes. The maximum binding densities (Bmax) of [3H]nimodipine and [3H]Bay K 8644 were not significantly different one from the other, but differed according to brain region with binding being highest in the olfactory bulb and hippocampus, intermediate in the caudate nucleus and cerebral cortex (various regions), and lowest in the cerebellum [563 to 107 fmol/mg protein (mean)]. The KD values, [3H]nimodipine = 1.8 X 10(-10) M (mean) and [3H]Bay K 8644 = 1.4 X 10(-9) M (mean), did not differ according to region. Depolarization-induced 45Ca2+ uptake in synaptosomes occurred as fast (1 sec) and slow (10 sec) components distinguished by their selective occurrence in choline-containing and pre-depolarized preparations respectively. Distribution of the fast component of uptake paralleled that of [3H]nimodipine binding, being least in the cerebellum and greatest in the hippocampus and cortex, but the magnitude of the slow phase of 45Ca2+ uptake did not vary in the three brain regions studied.

Identification and affinity labeling of very high affinity binding sites for the phenylalkylamine series of Ca+ channel blockers in the Drosophila nervous system

The interaction of putative Ca2+ channels of Drosophila head membranes with molecules of the phenylalkylamine series was studied from binding experiments using (-)-[3H]D888 and (+/-)-[3H]verapamil. These ligands recognize a single class (Kd = 0.1-0.4 nM; Bmax = 1600-1800 fmol/mg of protein) of very high affinity binding sites. The most potent molecule in the phenylalkylamine series was (-)-verapamil with a Kd value as exceptionally low as 4.7 pM. Molecules in the benzothiazepine and diphenylbutylpiperidine series of Ca2+ channel blockers as well as bepridil inhibited (-)-[3H]D888 binding in a competitive way with Kd values between 12 and 190 nM, suggesting a close correlation, as in the mammalian system, between these receptor sites and those recognizing phenylalkylamines. A tritiated (arylazido)phenylalkylamine with high affinity for the Drosophila head membranes, phenylalkylamine receptor Kd = 0.24 nM), was used in photoaffinity experiments. A protein of Mr 135,000 +/- 5,000 was specifically labeled after ultraviolet irradiation.

Absence of (-) [3H]desmethoxyverapamil binding sites on human platelets and lack of evidence for voltage-dependent calcium channels

The two major pathways for Ca2+ entry into cells are potential-sensitive channels and receptor-operated channels. The main object of this investigation was to identify which mechanism regulates Ca2+ entry into human platelets. Platelet stimulation with thrombin, adenosine diphosphate, platelet activating factor and arachidonic acid resulted in a concentration-dependent 2.5-3-fold increase in cytoplasmic free calcium concentration over the basal levels (140 +/- 32 nM or 104 +/- 21 respectively) as measured with the fluorescent dyes Quin-2 and Fura-2. Adrenaline and collagen had no effect in promoting intracellular Ca2+ increase as measured with Quin-2 and little effect when measured with Fura-2. Incubation of Quin-2-loaded platelets with the calcium antagonists verapamil and diltiazem, which are known to inhibit Ca2+ entry from voltage-gated channels in many types of cells, over the concentration range 10(-8) - 10(-4) M did not alter significantly either the resting or the cytoplasmic free Ca2+ after stimulation of platelets by several agonists. Moreover, the calcium antagonists exhibited little or no effect on aggregation and 5-hydroxytryptamine secretion induced by platelet activating factor, adenosine diphosphate, collagen or arachidonic acid in whole blood, platelet-rich plasma or washed platelets when employed at concentration ranges as above. Similar results were obtained in washed thrombin-stimulated platelets. High doses of verapamil (but not diltiazem) inhibited platelet aggregation and secretion in response to adrenaline. Direct radioligand binding studies with (-)[3H]desmethoxyverapamil showed that platelet membranes have no receptors for this drug, suggesting that Ca2+ entry occurs in human platelets via a pathway different from potential-sensitive Ca2+ channels.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ channels in the apical membrane of the toad urinary bladder

Previously (Van Driessche et al. 1987) we showed that small inward (mucosa towards serosa) oriented short-circuit currents (Isc) were recorded through the toad urinary bladder when the mucosal side was exposed to Ca2+ free solutions containing K+, Na+ (+ amiloride), Cs+ or Rb+ as main cation. This current component is inhibitable by micromolar concentrations of mucosal La3+ and divalent cations (Ca2+, Cd2+) and is considerably elevated by oxytocin (0.1 U/ml). The present study demonstrates that the addition of 50 nmol/l Ag+ to the mucosal medium during oxytocin treatment caused an additional large increase of the La3+-sensitive Isc component. The power density spectrum of the fluctuation in current contained a Lorentzian component which was enhanced by oxytocin treatment. The Lorentzian component disappeared as a consequence of the administration of mucosal Ag+. In experiments with Ca2+, Ba2+ or Mg2+ as principal mucosal cation, the La3+-sensitive Isc component was negligible under control conditions and during oxytocin treatment. Mucosal Ag+ (40 nmol/l) elicited a large inward oriented current which was blockable by the calcium channel blockers, La3+ and Cd2+. Also the organic calcium entry blockers, nicardipine and verapamil (10 mumol/l) depressed the inward current considerably. Noise analysis of the currents carried by divalent cations showed a La3+-sensitive noise component. Oxytocin-Ag+ activated currents could not be recorded in the absence of the divalent cations or small inorganic cations, e.g. with solutions which contained N-methyl D-glucamine (NMDG) as main mucosal cation.

Inhibition of central neurotransmitter release by omega-conotoxin GVIA, a peptide modulator of the N-type voltage-sensitive calcium channel

Tritium overflows, evoked by electrical stimulation (2 min; 2 ms, 3 Hz, 5 V/cm, and 24 mA) of [3H]-dopamine-, [3H]-noradrenaline-, [3H]-5-hydroxytryptamine, and [3H]-acetylcholine-labeled slices prepared from discrete regions of the rabbit central nervous system, were inhibited 39-50% by omega-conotoxin GVIA (omega-CT; 5 nmol/l), a peptide modulator of the N-type voltage-sensitive calcium channel (N-VSCC). Additional experiments using omega-CT (5 nmol/l) and [3H]-noradrenaline-labeled hippocampal slices indicated the time dependence of omega-CT-induced inhibition, the competitive antagonism between buffer calcium concentration and omega-CT, and the lack of effect of prolonged electrical stimulation (15 min; 0.4 Hz) on omega-CT-induced inhibition. These various results suggest that 1) omega-CT competes with calcium for the N-VSCC, 2) the inhibitory effects of omega-CT are independent of the gating state of the N-VSCC, and 3) the molecular nature of the N-VSCC may be similar or identical across central neurotransmitter systems. omega-CT appears to be a useful pharmacological tool in studying the involvement of the N-VSCC in neurotransmitter release.

Characterization of the dihydropyridine binding sites of rat neocortical synaptosomes and microvessels

The dihydropyridine binding sites associated with rat neocortical synaptosomes and microvessels were compared using an in vitro [3H]PN 200-110 [(+)-[methyl-3H]-isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5- methoxycarbonylpyridine-3-carboxylate] binding assay. Saturation experiments yielded similar KD values (approximately 70 pM) and Bmax values (approximately 400 fmol/mg of protein) for the two membrane preparations. Interaction experiments with [3H]PN 200-110 and various calcium-modulating substances provided further evidence for the practically identical nature of the synaptosomal and microvascular dihydropyridine binding sites. These findings predict that lipophilic dihydropyridines, simultaneously occupying the two central binding sites, have the dual effect of altering neuronal function and local blood flow.