Effects of chlordiazepoxide on depolarization-induced calcium influx into synaptosomes

Nantenine alkaloid presents anticonvulsant effect on two classical animal models

The present study investigated the anticonvulsant and convulsant profiles of nantenine, an aporphine alkaloid found in several vegetal species. At lower doses (20-50 mg/kg, i.p.) the alkaloid proved to be effective in inhibiting pentylenotetrazol- (PTZ 100 mg/kg, s.c.) and maximal electroshock-induced seizures (80 mA, 50 pulses/s, 0.2 s), suggesting its potential as an anticonvulsant drug. However, at higher doses (> or = 75 mg/kg, i.p.) a convulsant activity was observed. Comparing the present in vivo nantenine effects on seizures with previous in vitro biphasic action on Na+, K+-ATPase activity, the convulsant effect appears to be related to inhibition of these phosphatase at high doses whereas anticonvulsant effect, observed at low doses, seems attributable to its stimulation and the resultant decrease of Ca2+-influx into the cell.

Effects of flunarizine and diltiazem on physical dependence on barbital in rats

The effects of flunarizine and diltiazem both on development of physical dependence on barbital and on barbital withdrawal signs in rats were examined using the drug-admixed food (DAF) method. Rats were chronically treated with barbital or barbital in combination with flunarizine (fixed at 1.5 mg/g of food) or diltiazem (fixed at 0.75 mg/g of food)-admixed food on the schedule of gradually increasing doses of barbital. Motor incoordination during the treatment was potentiated by coadministration of flunarizine, but not by coadministration of diltiazem. After the termination of drug treatment, the body weight loss and withdrawal scores were significantly suppressed in the group coadministered flunarizine, but not in that coadministered diltiazem. There were no significant differences in plasma barbital levels after the withdrawal between groups. In the substitution test, flunarizine (20 and 40 mg/kg, IP) significantly suppressed the body weight loss and withdrawal scores after the withdrawal, but diltiazem (20 mg/kg, IP) did not. These results indicated that flunarizine suppressed both the development of physical dependence on barbital and barbital withdrawal signs, mainly according to the suppression of convulsions, but not diltiazem, which is known to poorly penetrate into the brain. Therefore, the present findings suggest that central calcium channels may be involved in both the development of physical dependence on barbital and the appearance of barbital withdrawal signs.

Modulation of noradrenaline release from hippocampal slices by hexachlorocyclohexane isomers. Effects of GABAergic compounds

The effects of hexachlorocyclohexane (HCH) isomers and some GABAergic compounds on [3H]noradrenaline (NA) release from rat hippocampal slices prelabelled with 80 nM [3H]NA were determined. The convulsant gamma-HCH isomer facilitated (EC50 = 21 microM) and the depressant delta-HCH isomer reduced (EC50 = 48 microM) the Ca(2+)-dependent K(+)-evoked release of [3H]NA, whereas alpha- and beta-HCH isomers did not show any effect. Moreover, alpha- and delta-HCH isomers antagonized the facilitation of evoked [3H]NA release induced by the gamma-HCH isomer. The GABAergic convulsant drugs, bicuculline, picrotoxin and pentylenetetrazol, did not cause any modification of the evoked [3H]NA release even at high concentrations. Neither bicuculline nor picrotoxin blocked the effects of HCH isomers on K(+)-evoked release of [3H]NA. Exposure of slices to diazepam reduced the K(+)-evoked release of [3H]NA (EC50 = 33 microM) in a manner similar to that of the delta-HCH isomer. In addition, diazepam (50 microM) blocked the gamma-HCH effect and caused an additive inhibitory response with the delta-HCH isomer. On the other hand, diazepam and delta-HCH induced a time-dependent Ca(2+)-independent enhancement of basal [3H]NA release. The results suggest that modulation of [3H]NA release in the hippocampus by HCH isomers may be involved in the central actions of these compounds, and that sites other than the classic GABAA receptor may underlie their presynaptic mechanisms of action.

Effects of Ca2+ channel blockers on physical dependence on diazepam in mice

The effects of Ca2+ channel blockers on the development of physical dependence on diazepam were examined in mice. Co-administration of flunarizine (T-type Ca2+ channel sensitive blocker), but not of either nifedipine or diltiazem (L-type Ca2+ channel sensitive blockers), with diazepam significantly suppressed the hypersensitivity to FG 7142 following chronic treatment with diazepam. The hypersensitivity to FG 7142 may reflect benzodiazepine withdrawal convulsions. These results suggest that flunarizine, but not nifedipine or diltiazem, may suppress the development of physical dependence on diazepam, and that T-type Ca2+ channels in the brain, rather than L-type Ca2+ channels, may be involved in the development of physical dependence on diazepam.

Concurrent treatment with verapamil prevents diazepam withdrawal-induced anxiety, in the absence of altered calcium flux in cortical synaptosomes

Rats, chronically treated with diazepam (4 mg/kg/day) for 28 days, displayed increased anxiety when tested in the elevated plus-maze, 42 hr after the last dose. This anxiogenic withdrawal response was entirely prevented by the concurrent administration of the calcium channel antagonist, verapamil. No anxiolytic effect of chronic administration of verapamil was observed in vehicle-treated rats. To investigate the possibility that increased calcium function in nerve terminals might underlie diazepam withdrawal-induced anxiety, the uptake by cortical synaptosomes of 45Ca2+ was studied. Both fast (3-sec) and slow (60-sec) phase uptake were measured. No changes in basal (5 mM), potassium-stimulated (55 mM) or net uptake were observed during either fast or slow phase uptake. It is concluded that increased calcium influx in nerve terminals in the cortex does not underlie the anxiogenic effect of withdrawal of the benzodiazepine but that further studies must be carried out in other regions of the brain.

Neuroadaptive responses to chronic ethanol

Cellular responses of neuronal tissue to chronic ethanol exposure are reviewed. Evidence for adaptive responses to the acute actions of ethanol is available for five systems: GABA-activated chloride channels, voltage-sensitive calcium channels, NMDA-activated cation channels, receptors coupled through stimulatory guanine nucleotide binding proteins, and membrane lipid order. We suggest that at least some of these adaptive responses occur because of ethanol actions at the level of gene expression.

Diazepam enhances intrasynaptosomal free calcium concentrations

Synaptosomes prepared from brains of rats were incubated in different concentrations of diazepam under conditions designed to reduce the action of a reversed Na+/Ca2+ exchanger. In synaptosomes depolarized in the presence of added Ca2+, doses of diazepam ranging from 0.1 to 100 microM were found to significantly enhance Ca2+ levels measured with the fluorescent dye fura-2, compared to control incubations without drug. Furthermore, doses of diazepam as low as 1 microM significantly increased the concentration of Ca2+ in non-depolarized synaptosomes without added Ca2+ in the medium. The effects of depolarization and diazepam treatment were synergistic in increasing the levels of intrasynaptosomal Ca2+.

Changes in benzodiazepine receptors of rat brain after long-term treatment with the Ca(2+)-antagonists nifedipine, verapamil, flunarizine and with the calmodulin antagonist trifluoperazine

1. The binding of [3H]flunitrazepam to benzodiazepine receptors in the cerebral cortex and hippocampus (membrane fraction P2) was studied after 13-day oral treatment of male Wistar rats with the Ca(2+)-antagonists nifedipine (20 mg/kg), verapamil (50 mg/kg), flunarizine (10 mg/kg) and with the calmodulin (CaM)-antagonist trifluoperazine (TFP) (3 mg/kg). 2. The changes in the binding characteristics of the benzodiazepine receptors in the frontal cortex were studied in vitro after the addition of nifedipine (10(-6) and 10(-5) M) and verapamil (10(-6) and 10(-5) M). 3. A significant decrease of the binding capacity (Bmax) of [3H]flunitrazepam was established after in vivo treatment with the three Ca(2+)-antagonists as well as after TFP, the decrease being much more pronounced in the hippocampus. 4. Changes in the affinity values (Kd) of [3H]flunitrazepam binding were found in neither of the groups. 5. No data for a direct interaction of nifedipine and verapamil with the brain benzodiazepine receptors were obtained in in vitro experiments.

Adenosine analogs attenuate tolerance-dependence on alprazolam

1. Tolerance to and physical dependence on alprazolam were induced in mice by administering two doses of a slow release preparation. 2. Physical dependence was evaluated by the abstinence syndrome induced by flumazenil. Tolerance was studied by measuring the motor incoordination induced by a test dose of alprazolam. 3. The intensity of tolerance was decreased by the administration of L-phenylisopropyl adenosine (L-PIA), cyclopentyl adenosine (CPA), cyclohexyl adenosine (CHA), N-ethylcarboxamide adenosine (NECA), 8-phenyltheophylline (8-PTP) and theophylline (TP). 4. The intensity of the abstinence syndrome induced by flumazenil was attenuated by L-PIA, CPA NECA, TP and 8-PTP. 5. The results suggest that benzodiazepines may exert, at least in part, their effects by involving adenosine in the central nervous system.

The role of neuronal calcium channels in dependence on ethanol and other sedatives/hypnotics

This review discusses the importance of neuronal calcium currents in dependence on ethanol, barbiturates, benzodiazepines and opiates. The main sections describe the actions of ethanol on control of intracellular calcium and on calcium and calcium-dependent conductance mechanisms. In particular, the effects of both acute and chronic ethanol treatment on dihydropyridine-sensitive, voltage-dependent, calcium channels are described. The later sections cover the effects of barbiturates, benzodiazepines and opiates on these systems. The conclusions suggest that dihydropyridine calcium channel antagonists may offer a new therapeutic approach to the treatment of ethanol and opiate dependence.

Chronic ethanol treatment alters omega-conotoxin and Bay K 8644 sensitive calcium channels in rat striatal synaptosomes

Two groups of adult Sprague-Dawley rats were maintained on a nutritionally complete liquid diet. In one group, 37% of the calories normally provided by dextrin were replaced with ethanol. Animals were maintained on this diet for eight weeks. The addition of ethanol (200 mM) in vitro significantly inhibited both calcium influx and dopamine release from control synaptosomes but did not alter calcium influx or dopamine release from synaptosomes isolated from ethanol-treated rats. The dihydropyridine calcium channel agonist Bay K 8644 (1 nM) significantly increased both calcium entry and dopamine release from control synaptosomes depolarized with 15 mM KCl. Bay K 8644 (1 nM) had no significant effect on either calcium entry or dopamine release in synaptosomes isolated from ethanol-treated animals. This loss of functional effect was accompanied by a slight (15%) but statistically insignificant increase in the binding of 3H-nitrendipine to striatal membranes from ethanol-treated rats as compared to control. The calcium-channel blocker, omega-conotoxin (500 nM) had no effect on voltage-dependent calcium uptake into synaptosomes prepared from control or ethanol-treated rats. Conotoxin (500 nM) inhibited the voltage-dependent release of endogenous dopamine from synaptosomes isolated from both groups by 36-44%. Ethanol (200 mM) added in vitro to control synaptosomes did not alter conotoxin's inhibition of dopamine release but completely abolished the omega-conotoxin-induced inhibition of dopamine release in synaptosomes isolated from ethanol-treated animals. These results suggest that DHP-sensitive and omega-conotoxin-sensitive calcium channels in rat brain respond differentially to chronic exposure to ethanol.

Antiepileptic drug actions

Antiepileptic drugs (AEDs) vary in their efficacy against generalized tonic-clonic, myoclonic, and absence seizures, suggesting different mechanisms of action. Phenytoin (PHT), carbamazepine (CBZ), and valproate (VPA) reduced the ability of mouse central neurons to sustain high-frequency repetitive firing of action potentials (SRF) at therapeutic free serum concentrations. Phenobarbital (PB) and the benzodiazepines (BZDs), diazepam (DZP), clonazepam (CZP), and lorazepam (LZP), also reduced SRF, but only at supratherapeutic free serum concentrations achieved in treatment of generalized tonic-clonic status epilepticus. These AEDs interact with sodium channels to slow the rate of recovery of the channels from inactivation. The BZDs and PB enhanced gamma-aminobutyric acid (GABA) responses evoked on mouse central neurons by binding to two different sites on the GABAA receptor channel. BZDs increased the frequency of GABA receptor channel openings. In contrast, barbiturates increased the open duration of these channels. VPA enhanced brain GABA concentration and may enhance release of GABA from nerve terminals. Ethosuximide (ESM) reduced a small transient calcium current which has been shown to be involved in slow rhythmic firing of certain neurons. Reduction of SRF, enhancement of GABA-ergic inhibition, and reduction of calcium current may be, in part, the bases for AED action against generalized tonic-clonic, myoclonic, and absence seizures, respectively.

Mechanisms of action of anticonvulsant drugs

Understanding the mechanisms of action of anticonvulsant drugs has been a major research effort of neuroscientists for the last three decades. Numerous biochemical and electrophysiological processes have been shown to be modulated by anticonvulsant drugs. However, clear correlations of these effects with anticonvulsant activity are often difficult to determine. Over the last 5 years, several major research areas have developed that shed new light on the mechanisms of action of anticonvulsant drugs. Although a complete discussion of all aspects of anticonvulsant research are not possible in the scope of this article, a description of benzodiazepine receptors, mechanisms involved in neuronal sustaining repetitive firing, and calcium regulation of neuronal function have direct bearing on neuronal excitability and ultimately on anticonvulsant drug effect. Each of these major research areas has opened new insights into our understanding of molecular mechanisms of how anticonvulsant drugs work.

Genetic selection for benzodiazepine ataxia produces functional changes in the gamma-aminobutyric acid receptor chloride channel complex

The gamma-aminobutyric acid (GABA) receptor-operated chloride channel complex was evaluated in mice selected for differential sensitivity to the ataxic effects of diazepam (diazepam-sensitive (DS) and diazepam-resistant (DR) lines). The ataxic effects of several drugs purported to produce some of their actions through the benzodiazepine-GABA receptor complex were examined using the rotarod test. The duration of impairment produced by diazepam, ethanol, 4,5,6,7-tetrahydroisoxazol[5,4-C]pyridine-3-ol (THIP) and phenobarbital was greater in the diazepam-sensitive than in the diazepam-resistant mice. In contrast, pentobarbital produced an equivalent duration of ataxia in the two lines. Muscimol-stimulated 36Cl- influx and the binding of [35S]t-butylbicyclophosphorothionate (TBPS) and [3H]flunitrazepam were measured using isolated brain membrane vesicles (microsacs). Depolarization-dependent 45Ca2+ uptake was measured in whole brain synaptosomes. Muscimol was a more potent stimulator of 36Cl- flux in the DS compared to the DR mice, although no difference between the lines was found in muscimol-stimulation of [3H]flunitrazepam binding. Flunitrazepam augmented the muscimol-stimulated 36Cl- uptake in the DS but not in the DR mice. However, no differences between the lines of mice were found in either density or affinity of [3H]flunitrazepam binding sites. Similarly, no differences in either the density or affinity of [35S]TBPS binding sites was found. Ethanol (10-45 mM) potentiated the muscimol-stimulation of 36Cl- in DS, with no effect in DR mice. However, ethanol inhibition of [35S]TBPS binding was equivalent in the two lines of mice. Pentobarbital produced an equal potentiation of the muscimol-stimulated 36Cl- flux in the two lines, but phenobarbital potentiated the muscimol-induced 36Cl- influx slightly more in DS mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Field-potential assay of antiepileptic drugs in the hippocampal slice

The effects of nine clinically active antiepileptic drugs and the NMDA antagonist 2-amino-7-phosphonoheptanoic acid (2-APH) were examined in three models in the in vitro hippocampal slice. In the "low Mg2+" model, removal of Mg2+ from the perfusion fluid increased excitatory neurotransmission and led to epileptogenic field potentials. In the "low Ca2+" model, decrease of Ca2+ and increase of Mg2+ and K+ in the perfusion fluid induced spontaneous "bursts" in the absence of synaptic transmission. Paired-pulse stimulation was used to estimate the strength of recurrent inhibition in the "inhibition" model. The rank order of the potency of the compounds to antagonize the second epileptogenic population spike in the low Mg2+ model was 2-APH greater than pentobarbital greater than midazolam greater than phenytoin greater than carbamazepine greater than chlordiazepoxide greater than phenobarbital = flurazepam. Ethosuximide and valproate were inactive. In the low Ca2+ model, the rank order of the potency of the drugs to antagonize spontaneous epileptogenic bursts was phenytoin greater than carbamazepine greater than midazolam greater than pentobarbital greater than chlordiazepoxide greater than flurazepam greater than phenobarbital. 2-APH, ethosuximide, and valproate were inactive. Only pentobarbital was active in the inhibition model. These experiments demonstrate the potential of in vitro tests in the hippocampus to reveal profiles of anticonvulsant activity.

Lack of effect of nitrendipine on the pharmacokinetics and pharmacodynamics of midazolam during steady state

1. The possible interaction (as indicated by rat experiments) between calcium channel blocking agents and benzodiazepines has been evaluated in nine healthy subjects. 2. Subsequently to an intravenous loading dose (0.07 mg kg-1) midazolam was infused for 6 h (0.035 mg kg-1 h-1) and steady state plasma levels between 54 to 114 micrograms l-1 were achieved. Two hours after the bolus of midazolam a solution of 20 mg nitrendipine or placebo was administered in a randomized, double-blind crossover fashion. 3. The marked sedative-hypnotic effects of midazolam as assessed by visual analogue scales (about four fold increase in the sedation index) and choice reaction time (100% prolongation) indicated some form of adaptation or tolerance towards the end of the infusion. However, the midazolam-induced impairments were not affected by nitrendipine. 4. EEG-data indicated stabile benzodiazepine-like effects during the complete infusion period of midazolam (e.g. decrease in alpha activity, increase in sigma, delta 2 and beta 1 activity). Again, these alterations were not modified by nitrendipine. 5. There was also no pharmacokinetic interaction between both agents, since elimination of midazolam (t 1/2 = 2.5 +/- 0.8 h; CL = 548 +/- 143 ml min-1) was in close agreement with control values (t 1/2 = 2.4 +/- 0.6 h; CL = 512 +/- 102 ml min-1). Likewise, plasma levels of nitrendipine were comparable to literature data. 6. Thus, it could be concluded that nitrendipine does not affect the action of midazolam and therefore a direct involvement of calcium at the benzodiazepine receptor site is unlikely under our clinical conditions.

A molecular approach to the development of anticonvulsants

Anticonvulsants are neuronal stabilizing compounds that exhibit multiple clinical effects, including anticonvulsant, anxiolytic, sedative, and muscle-relaxant properties. This complex therapeutic picture complicates the treatment of seizure disorders in individuals with mental and developmental disorders, and frequently impairs the routine integration into society for these individuals. In order to improve the therapeutic effectiveness of these compounds, it is necessary to identify their precise molecular actions on the neuronal membrane and their effects on neuronal function. We have identified two major classes of low-affinity BZ binding sites that seem to function as generalized anticonvulsant receptors and that may mediate the anticonvulsant and sedative effects produced by these compounds. The identification of these binding sites and their anticonvulsant binding profile may clarify the complex picture of anticonvulsant mechanisms and elucidate the site(s) at which anticonvulsants produce their inhibition of MES-induced seizures and sedative effects. We will continue to examine the physiological changes induced by anticonvulsant binding at these BZ binding sites that may be a foundation for understanding the molecular basis of sedation and MES-induced seizure inhibition. Specifically, we will investigate the specific membrane components associated with the inhibition of Ca2+ channels, Na+ channel rectification, and CaM kinase II. If these goals can be achieved, then model systems could be developed to screen potential anticonvulsant or sedative compounds in the search for more effective therapeutic drugs.

5-(2-Cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB): correlation of hypnotic and convulsant properties with alterations of synaptosomal 45Ca2+ influx

Male ICR mice (20-35 g) were given either 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB) alone (10-15 mg/kg i.p.) or CHEB (25-75 mg/kg i.p.) after a 1 h pretreatment with phenobarbital (75 mg/kg i.p.). CHEB alone (10 mg/kg) produced excitatory behavior but not convulsive seizures. Higher doses (11-15 mg/kg) produced convulsive seizures resulting in death. Pretreatment with phenobarbital prevented seizure activity. Following phenobarbital pretreatment, CHEB in doses of 50 and 75, but not 25 mg/kg, resulted in hypnosis of 53 +/- 16 and 64 +/- 9 min duration, respectively. In vitro, CHEB (10-200 microM) significantly inhibited 'fast-phase' (3 s) K+-stimulated 45Ca2+ uptake into cerebrocortical synaptosomes. CHEB (10 and 100 microM) also significantly increased basal 45Ca2+ uptake. The addition of CHEB (50 and 100 microM) or pentobarbital (100 microM) to striatal synaptosomes inhibited 'fast-phase' K+-stimulated 45Ca2+ uptake and endogenous dopamine release. CHEB (10-200 microM), but not pentobarbital (100 microM), produced a time- and dose-dependent increase in the resting release of endogenous dopamine from striatal synaptosomes. The results of this study show that CHEB possesses hypnotic activity if its lethal convulsant actions are blocked. The hypnotic actions of CHEB appear to correlate with inhibition of voltage-dependent calcium channels in brain synaptosomes.

Correlation of the hypnotic potency of benzodiazepines with inhibition of voltage-dependent calcium uptake into mouse brain synaptosomes

Nine benzodiazepines were tested for their ability to inhibit 45Ca2+ uptake into mouse whole brain synaptosomes and for hypnotic activity as indicated by their ability to produce loss of the righting reflex. Eight of the benzodiazepines significantly inhibited fast-phase voltage-dependent 45Ca2+ uptake and five exhibited hypnotic activity. There was a direct correlation between the hypnotic potency of these five benzodiazepines and their ability to inhibit 45Ca2+ uptake. There does not appear to be a correlation between the anticonvulsant potency of the benzodiazepines and their potency for inhibiting 45Ca2+ uptake. These results support previous findings with other sedative/hypnotic drugs and suggest that inhibition of presynaptic calcium uptake may be linked with hypnotic but not anticonvulsant actions of benzodiazepines.

Inhibition of fast phase calcium uptake and endogenous norepinephrine release in rat brain region synaptosomes by ethanol

The effects of ethanol on fast phase calcium (Ca2+) uptake and endogenous norepinephrine release were assessed simultaneously in KCl-depolarized synaptosomes isolated from rat hypothalamus, brainstem and cerebellum. Incubation of brain regional synaptosomes with ethanol resulted in a concentration-dependent inhibition of Ca2+ uptake after 1 s of depolarization. Hypothalamic synaptosomes were most sensitive to the inhibitory effect of ethanol on voltage-dependent Ca2+ uptake and brainstem synaptosomes were least sensitive. Endogenous norepinephrine release from synaptosomes was not altered by addition of ethanol in vitro at any of the concentrations examined (25-200 mM). Chronic ethanol administration resulted in an adaptation to the inhibitory effect of ethanol on Ca2+ uptake into hypothalamic synaptosomes but did not alter the inhibitory effect of ethanol on Ca2+ uptake into brainstem or cerebellar synaptosomes. Fast phase, voltage-dependent norepinephrine release was inhibited by ethanol added in vitro but only in synaptosomes isolated from hypothalami and cerebella of chronically treated animals. Brain regional norepinephrine concentrations were unaltered by chronic ethanol administration. These results suggest that chronic ethanol treatment may alter the coupling of Ca2+ entry with norepinephrine release in some noradrenergic neurons. Effects of ethanol on synaptosomal Ca2+ entry and norepinephrine release differ depending on the brain region.

Chronic ethanol treatment uncouples striatal calcium entry and endogenous dopamine release

Chronic ethanol treatment resulted in a marked reduction in the release of dopamine from striatal synaptosomes in response to depolarization. Calcium entry into the same synaptosomal preparations was not altered. Calculation of the ratio of calcium entry vs dopamine release showed that, under control conditions, approximately 15 calcium ions were required to cause the release of 1 dopamine molecule. Chronic ethanol treatment increased this ratio to more than 80:1, suggesting that chronic ethanol administration altered the coupling between calcium entry and dopamine release. Addition of ethanol in vitro to synaptosomes isolated from chronic ethanol-treated rats returned this ratio to approximately 20:1. These results suggest that chronic ethanol treatment results in dependence which is reflected biochemically in striatum through changes in the coupling between voltage-dependent calcium entry into nerve endings and subsequent neurotransmitter release.

Effects of calcium antagonists on KCl-evoked calcium uptake by rat cortical synaptosomes

A series of calcium antagonists were used to study their blocking effect on high potassium-induced calcium uptake into rat cortical synaptosomes; these antagonists were classified into five groups: dihydropyridine group (i.e. nifedipine and nitrendipine), benzothiazepine group (i.e. diltiazem), phenylalkylamine group (i.e. verapamil and D600), phenothiazine group (i.e. trifluoperazine) and diphenylpiperazine group (i.e. flunarizine and cinnarizine). Voltage-dependent 45Ca2+-uptake into this fraction was measured after 20 sec KCl-induced depolarization. The ID30 values of the above-mentioned antagonists affecting 45Ca2+-uptake were calculated to be nitrendipine (80 microM), nifedipine (100 microM), verapamil (50 microM), D600 (15 microM), diltiazem (70 microM), trifluoperazine (7 microM), cinnarizine (1.2 microM) and flunarizine (0.7 microM). Our results reveal that in rat brain synaptosomal fractions, calcium influx via the voltage-gated calcium channel appears to be more sensitive to diphenylpiperazine and phenothiazine groups; whereas, phenylalkylamine, benzothiazepine and dihydropyridine groups were relatively insensitive. This contrasts with the well known data obtained from vascular smooth muscle, in which the dihydropyridine group is the most sensitive of all the groups studied. Our results suggest that calcium channels in neuronal tissue are most likely different from those in non-neuronal tissue.

Dihydropyridine and peripheral type benzodiazepine binding sites: subcellular distribution and molecular size determination

Electrophysiological and pharmacological studies have shown that peripheral-type benzodiazepine receptors modulate voltage-sensitive calcium channels in the heart. We have compared these binding sites with binding sites for [3H]dihydropyridines, which are believed to label such channels. Although no direct or allosteric interaction could be demonstrated between the two sites, their subcellular distribution--sarcolemma and ryanodine-sensitive sarcoplasmic reticulum--was parallel. Size determination of the two sites suggests that the receptors for these two classes of compounds are separate molecules packaged in the same membrane compartment.

Inhibition of Ca2+ conductance in identified leech neurons by benzodiazepines

Benzodiazepines (BZs) in micromolar concentrations inhibit Mn2+- and Co2+-sensitive regenerative divalent cation potentials, which are revealed in the presence of tetraethylammonium ion, in leech nociceptive neurons (N cells). This BZ effect is reversible and dose-dependent. The BZs, like Mn2+ and Co2+, inhibit the maximum rate of depolarization (Vmax) and duration of divalent cation potentials at concentrations that do not significantly affect resting membrane potential or Vmax of the Na+-dependent action potential. Ultraviolet-induced BZ binding to micromolar-affinity sites in ganglia and isolated cells irreversibly blocks Ca2+ conductance in neurons without significantly affecting resting membrane potentials. BZ binding studies with leech neuronal membrane show saturable, specific binding in the micromolar concentration range that was similar to BZ binding to synaptosomal membrane fractions. The apparent Kd obtained from the micromolar-affinity BZ binding curve for leech ganglionic membrane preparations agrees well with the apparent Ki estimated from the dose-response curve measuring BZ inhibition of Vmax of the divalent cation potentials. These findings indicate that BZs act like Ca2+-channel antagonists in intact neuronal preparations and are consistent with the hypothesis that BZ binding to micromolar-affinity receptors modulates voltage-gated Ca2+ channels.

Diazepam and its anomalous p-chloro-derivative Ro 5-4864: comparative effects on mouse neurons in cell culture

The actions of diazepam and its p-chloro-derivative Ro 5-4864 were compared on mouse spinal cord and dorsal root ganglion neurons in cell culture. Diazepam enhanced but Ro 5-4864 reduced iontophoretic GABA responses in a concentration-dependent manner. Both diazepam and Ro 5-4864 limited sustained, high frequency repetitive firing of spinal cord neurons but diazepam was more potent. Ro 5-4864 was, however, more potent than diazepam in inhibiting spontaneous neuronal activity of spinal cord neurons and reducing the duration of calcium-dependent action potentials of dorsal root ganglion neurons. The differing actions of diazepam and Ro 5-4864 may account for the contrasting pharmacological spectra of the two benzodiazepines.

Benzodiazepine receptor ligand actions on GABA responses. Benzodiazepines, CL 218872, zopiclone

The effects on GABA (4-aminobutyric acid) responses of several benzodiazepine and nonbenzodiazepine benzodiazepine receptor ligands were examined using mouse spinal cord neurons in dissociated cell culture. Diazepam, clonazepam and nitrazepam enhanced GABA responses potently at low nanomolar concentrations. Diazepam and clonazepam were most potent with significant enhancement at 1 nM and peak enhancement of 80.7 and 50.2% at 10 nM respectively. Nitrazepam was least potent with no significant enhancement at 1 nM and enhancement of only 20.7% at 10 nM. The benzodiazepine antagonist, Ro 15-1788, blocked enhancement by diazepam but also weakly enhanced GABA responses at low micromolar concentrations, suggesting partial agonist activity. The convulsant benzodiazepine, Ro 5-4864, did not enhance GABA responses at any concentration tested but antagonized GABA responses at 1 microM and above. Diazepam shifted GABA dose-response curves to the left by decreasing the apparent KD but without altering the apparent Vmax (Lineweaver-Burk analysis). Two nonbenzodiazepine anxiolytic/anticonvulsants, CL 218872 and zopiclone, were weak enhancers of GABA responses at high nanomolar concentrations. These results with benzodiazepines, CL 218872 and zopiclone are consistent with their anxiolytic and anticonvulsant profile in vivo and with studies of their effects upon low affinity GABA binding in vitro.

Micromolar-affinity benzodiazepine receptors regulate voltage-sensitive calcium channels in nerve terminal preparations

Benzodiazepines in micromolar concentrations significantly inhibit depolarization-sensitive Ca2+ uptake in intact nerve-terminal preparations. Benzodiazepine inhibition of Ca2+ uptake is concentration dependent and stereospecific. Micromolar-affinity benzodiazepine receptors have been identified and characterized in brain membrane and shown to be distinct from nanomolar-affinity benzodiazepine receptors. Evidence is presented that micromolar, and not nanomolar, benzodiazepine binding sites mediate benzodiazepine inhibition of Ca2+ uptake. Irreversible binding to micromolar benzodiazepine binding sites also irreversibly blocked depolarization-dependent Ca2+ uptake in synaptosomes, indicating that these compounds may represent a useful marker for identifying the molecular components of Ca2+ channels in brain. Characterization of benzodiazepine inhibition of Ca2+ uptake demonstrates that these drugs function as Ca2+ channel antagonists, because benzodiazepines effectively blocked voltage-sensitive Ca2+ uptake inhibited by Mn2+, Co2+, verapamil, nitrendipine, and nimodipine. These results indicate that micromolar benzodiazepine binding sites regulate voltage-sensitive Ca2+ channels in brain membrane and suggest that some of the neuronal stabilizing effects of micromolar benzodiazepine receptors may be mediated by the regulation of Ca2+ conductance.

The effects of chlordiazepoxide on synaptic transmission and amino acid neurotransmitter release in slices of rat olfactory cortex

The rat olfactory cortex slice has been used to investigate the effects of chlordiazepoxide on evoked field potentials and the release of endogenous amino acid neurotransmitters (aspartate, glutamate, GABA and possibly taurine) which accompany electrical stimulation of the lateral olfactory tract. When single, low frequency stimuli were employed, chlordiazepoxide (2 microM-1 mM) depressed the amplitude of the field potential correlate of the depolarizing actions of the lateral olfactory tract excitatory transmitter (aspartate?) although aspartate release was unaffected. The field potential correlate of GABA-mediated presynaptic inhibition (late N-wave) was also depressed in amplitude but low drug concentrations (between approximately 2 and 50 microM) increased its peak duration . Effects of chlordiazepoxide on evoked inhibition were analyzed by giving paired stimuli such that the second stimulus occurred during the field potentials evoked by the first stimulus. Chlordiazepoxide (1-20 microM) increased the depression in amplitudes of the presynaptic massed action potential and late N-wave evoked by the second of a pair of stimuli compared with those evoked by the first stimulus suggesting that presynaptic inhibition was potentiated. These effects of chlordiazepoxide were accompanied by a significant reduction in aspartate release from the lateral olfactory tract terminals. Moreover, the drug effects on presynaptic inhibition and aspartate release were antagonized by picrotoxin (5 microM). On the other hand, chlordiazepoxide (1-50 microM) had no significant effect on postsynaptic inhibition. The results are discussed in terms of both the sites (presynaptic or postsynaptic) and mechanisms of action of chlordiazepoxide.