Calcium uptake mechanisms affected by some convulsant and anticonvulsant drugs

Effects of antiepileptic drugs on induced epileptiform activity in a rat model of dysplasia

Seizure activity associated with cortical dysplasia (CD) is often resistant to standard pharmacologic treatments. Although several animal models exhibit CD, virtually nothing is known about antiepileptic drug (AED) responses in these animals. Here we have used rats exposed to methylazoxymethanol acetate (MAM) in utero, an animal model featuring nodular heterotopia, to investigate the effects of AEDs in the dysplastic brain. 4-aminopyridine (100 microM), a K(+) channel blocker, was used to induce interictal epileptiform bursting in acute hippocampal slices from MAM-exposed and age-matched vehicle-injected control animals. Extracellular field recordings were used to monitor seizure activity in vitro. Five commonly used AEDs were tested: phenobarbital, 25-400 microM; carbamazepine, 25-200 microM; valproate (VPA), 0.19-4 mM; ethosuximide (ESM), 0.5-8 mM; and lamotrigine (LTG), 49-390 microM. 4-AP-induced bursting occurred with shorter latencies in slices from MAM-exposed rats in comparison with slices from controls, confirming the intrinsic hyperexcitability of dysplastic tissue. Each AED tested demonstrated significant burst suppression in control slices, but interictal epileptiform bursting in MAM-exposed slices was resistant to these treatments. Even at the highest concentrations, VPA, ESM and LTG had no effect on burst amplitude in slices from MAM-exposed rats. Pharmaco-resistance was further tested by measuring seizure latencies in awake, freely-moving rats after kainate administration (15 mg/kg, i.p.) with and without pre-treatment with VPA (400 mg/kg i.p.). Pre-treatment with VPA prolonged seizure latency in control rats, but had no effect in MAM-exposed animals. These results suggest MAM-exposed rats exhibit a dramatically reduced sensitivity to commonly prescribed AEDs.

Effects of AWD 140-190 on stimulus-induced field potentials and on different patterns of epileptiform activity induced by low calcium or low magnesium in rat entorhinal cortex hippocampal slices

AWD 140-190 a potent new anticonvulsant was tested on several types of epileptiform activities in entorhinal cortex hippocampal slices. AWD 140-190 suppressed completely and in a dose-dependent manner spontaneous seizure-like events induced by lowering extracellular Ca2+. In the low magnesium model, AWD 140-190 applied with 200 microM reduced recurrent short discharges in area CA1 by 48.1 +/- 14.7%, while in the entorhinal cortex seizure-like events were not depressed. Late recurrent discharges were increased in frequency to 213.8 +/- 78.1 and reduced in amplitude by 50.1 +/- 14.4%. Responses to paired pulse stimuli with intervals ranging from 20 to 150 ms were reduced both with alvear and stratum radiatum stimulation. Decreases in [Ca2+]0 and associated slow field potentials evoked by repetitive stimulation of stratum radiatum were also depressed in a dose-dependent manner. AWD 140-190 also reduced stimulus-induced rises in [K+]0. AWD 140-190 200 microM diminished the amplitude of slow field potentials observed during high K(+)-induced spreading depression by about 17% in CA1 and 34% in entorhinal cortex without any significant effect on SD-associated rises in [K+]0. These results suggest that AWD 140-190 has an anticonvulsant effect presumably by interfering with repetitive generation of action potentials. AWD 140-190 may also possess modulatory effects on glial cells as suggested by the strong depression of SD-associated slow negative potential shifts.

Basis of the antiseizure action of phenytoin

1. Phenytoin has been used with much clinical success against all types of epileptiform seizures, except petit mal epilepsy, for over 50 years. Its mechanism of action, however, is still open to interpretation. 2. Several potential targets for phenytoin action have been identified within the central nervous system. These include the Na-K-ATPase, the GABAA receptor complex, ionotropic glutamate receptors, calcium channels and sigma binding sites. 3. To date, though, the best evidence hinges on the inhibition of voltage-sensitive Na+ channels in the plasma membrane of neurons undergoing seizure activity. Quieter nerve cells are far less affected. Moreover, the fact that phenytoin also has important cardiac antiarrhythymic effects and can inhibit Na+ influx into cardiac cells supports the idea that the primary target of phenytoin is, indeed, the Na+ channel.

CHEB, a convulsant barbiturate, evokes calcium-dependent spontaneous glutamate release from rat cerebrocortical synaptosomes

CHEB [5-(2-cyclohexylidene-ethyl)-5-ethyl barbituric acid] is a potent convulsant barbiturate that causes direct neuronal excitation by an unknown mechanism. We have analyzed the effects of CHEB on the release of endogenous glutamate from rat cerebrocortical synaptosomes using an on-line enzyme-coupled fluorimetric assay. CHEB evoked spontaneous Ca(2+)-dependent glutamate release with an EC50 = 14.2 microM and an Emax = 3.2 mumol/min/mg. The non-convulsant barbiturates pentobarbital and phenobarbital evoked significantly less glutamate release at high concentrations. CHEB (30 microM) increased intrasynaptosomal [Ca2+] by 58 +/- 4 nM (p < 0.01; n = 4) above baseline compared to an increase of 5 +/- 4 nM (NS; n = 4) produced by pentobarbital (30 microM). CHEB-evoked glutamate release was inhibited by pentobarbital, phenobarbital, EGTA, CoCl2/CdCl2 and flunarizine, but not by local anesthetics, tetrodotoxin, nitrendipine or omega-conotoxin GVIA. These results demonstrate that CHEB acts as a potent and effective secretogogue for glutamate by a pre-synaptic mechanism that does not require activation of Na+ channels or of L-type or N-type Ca2+ channels. Stimulation of spontaneous glutamate release may contribute to the convulsant properties of CHEB.

Nimodipine as an add-on therapy for intractable epilepsy

OBJECTIVE

To analyze the effect of nimodipine in patients with intractable epilepsy.

DESIGN

We conducted a double-blind placebo-controlled crossover study in 95 patients.

MATERIAL AND METHODS

The dihydropyridine calcium antagonist nimodipine was used as add-on therapy (60 mg four times a day) in a 1-year placebo-controlled crossover study in 71 patients with localization-related epilepsy and 24 with generalized seizure disorders. Of the 95 patients, 81 were receiving two or more antiepileptic drugs. Patients diaries were used to record the number of seizures and any side effects.

RESULTS

Nimodipine seemed to be well tolerated during the study; only two patients were unable to complete the study because of probable adverse effects. The trial demonstrated no significant crossover effect and no significant effect of nimodipine on either the mean or the median number of seizures or seizure days. The peak median serum nimodipine level was less than 5 ng/mL in the 78 patients who completed the study.

CONCLUSION

This clinical trial found no beneficial effect with use of nimodipine as add-on therapy for intractable epilepsy. Potential reasons for the absence of efficacy of nimodipine may be the inclusion of patients with very refractory seizure disorders or the relatively low serum nimodipine concentrations related to the pharmacokinetic effect of concurrent antiepileptic medication.

Electrophysiological and metabolic effects of a convulsant barbiturate on dissociated mouse primary sensory neurons

1. The convulsant barbiturate 5-(2-cyclohexylidene-ethyl)-5-ethyl barbituric acid (CHEB) depolarizes dorsal root ganglion (DRG) neurons. We have applied microfluorimetric and whole-cell patch clamp techniques to investigate the mechanisms underlying this response in freshly dissociated mouse DRG cells. 2. Application of CHEB (2-200 microM) raised cytosolic calcium concentration ([Ca2+]i) rapidly and reversibly in 55% of eighty-three neurons tested. This population did not correlate with other classifications of sensory neurons based on either cell size or the expression of membrane currents. 3. The response was dependent on external calcium and was reduced by 81 +/- 22% by Ruthenium Red. A rise in [Ca2+]i was still seen with the membrane potential clamped at -70 mV, excluding membrane depolarization and activation of voltage-dependent Ca2+ channels as the principal mechanism for the response. 4. The rise in [Ca2+]i was associated with an increase in membrane conductance and a current, ICHEB, which was inward at -70 mV. Both the rise in [Ca2+]i and the current showed 'run-down' under whole-cell recording conditions. When K+ conductances were blocked, the reversal potential of ICHEB was close to 0 mV. This was independent of the Cl- reversal potential, suggesting that ICHEB is carried as a non-specific cation current. 5. In contrast to the change in [Ca2+]i, ICHEB was not dependent on external Ca2+ and the current was still seen when [Ca2+]i as strongly buffered by the pipette filling solution. These data suggest that CHEB opens a non-selective cation channel permeant to Ca2+, raising [Ca2+]i and further depolarizing the cell membrane potential. The exact nature of this conductance remains unknown. These actions could readily account for the convulsant actions of the drug, depolarizing neurons and increasing transmitter release. 6. It was also noted that CHEB increases autofluorescence derived from mitochondrial NAD(P)H. Further examination of this phenomenon using the dye rhodamine 123 to follow changes in mitochondrial potential (psi m) suggested that CHEB is a potent inhibitor of mitochondrial respiration, probably acting at complex I. These effects appeared to be quite distinct from the action of CHEB at the level of the plasma membrane.

Differential up-regulation of voltage-dependent Na+ channels induced by phenytoin in brains of genetically seizure-susceptible (E1) and control (ddY) mice

We investigated the effect of in vivo administration of an antiepileptic drug, phenytoin, on the saxitoxin binding capacity of receptor site 1 of the Na+ channel alpha-subunit, and the expression activity of the channel messenger RNA in epileptic El mouse brains, as compared with parental ddY mice. Subchronic treatment with phenytoin (25 mg/kg per day) for 14 days increased the [3H]saxitoxin binding to brain-derived synaptic membranes of both El and control ddY mice in a time dependent manner. This increase plateaued at 21 +/- 4% in El mice and 28 +/- 3% in ddY control mice after administration of phenytoin for seven days. After cessation of treatment with phenytoin, [3H]saxitoxin binding capacity returned to the basal level within two weeks in both ddY and El brains. Scatchard plot analysis revealed that the phenytoin treatment caused a 20-30% increase in maximum binding capacity of [3H]saxitoxin binding without any change in equilibrium dissociation constant in the brain cortical synaptic membranes of both epileptic El and control ddY mice. A single injection of phenytoin (25 mg/kg) elevated the level of Na+ channel messenger RNA within 1 h in ddY mouse brains. The increase in Na+ channel messenger RNA reached a peak (about 80% increase) after 5 h of phenytoin administration in a concentration-dependent manner (6.25-50 mg/kg). On the other hand, in El mouse brains, Na+ channel messenger RNA was not elevated until more than 5 h after phenytoin injection, and was increased by only about 33%.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of some calcium antagonists upon the activity of common antiepileptic compounds on sound-induced seizures in DBA/2 mice

1. Flunarizine (2.65 mumol/kg, i.p.) and nimodipine (5.25 mumol/kg, i.p.) potentiated the anticonvulsant properties of phenytoin, phenobarbital and valproate against audiogenic seizures in DBA/2 mice. 2. Diltiazem (5.25 mumol/kg, i.p.) was able to potentiate the antiseizure activity of phenytoin but was not effective against the anticonvulsant action of phenobarbital and valproate. 3. Verapamil (5.25 mumol/kg, i.p.) was unable to potentiate the anticonvulsant properties of all antiepileptic drugs studied. 4. Bay K 8644 (1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluorophenyl)-pyridine- 5-carboxylic acid), a calcium agonist at a dose of 2.65 mumol/kg, i.p., induced a reduction of anticonvulsant potency of phenytoin, phenobarbital and valproate. 5. None of the calcium antagonists used significantly increased the plasma levels of antiepileptic compounds or significantly affected the body temperature changes induced by anticonvulsant drugs. 6. It may be concluded that some calcium antagonists enhance the anticonvulsant properties of some antiepileptic drugs against audiogenic seizures. A pharmacokinetic interaction does not seem responsible for these effects.

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 effect of nimodipine on picrotoxin-induced seizures

The effects of the calcium channel antagonist, nimodipine, on picrotoxin-induced myoclonic (MYO) and generalized tonic-clonic (GTC) seizures were investigated in male and female rats. In males, a dose-response study of nimodipine's effects on seizures induced by different doses of picrotoxin was conducted. In a second experiment, female rats were tested for latency to and incidence of MYO and GTC seizures after being pretreated with nimodipine 2 hr, 24 hr, or 72 hr prior to seizure testing. The results showed that, in males, various doses of nimodipine significantly increased the mean latencies to MYO and GTC seizures and significantly reduced the incidence of GTC seizures. In females, nimodipine significantly reduced the incidence and/or increased the latency of GTC seizures when given 24 hr of 72 hr prior to administration. In addition to the anticonvulsant effects, nimodopine significantly increased survival after seizures in both males and females even when it had no significant effects on seizure incidence or latency. The results of this study support the hypothesis of calcium involvement in seizure induction. However, the sex- and time-dependent nature of the nimodipine effects as well as the effects of nimodipine on survival after seizures suggest that the relationship between calcium and seizure activity is complex.

Calcium-dependent actions of the convulsant barbiturate, CHEB, on transmitter release at the rat neuromuscular junction

1. The effect of convulsant barbiturates on spontaneous and evoked acetylcholine release was studied at the rat neuromuscular junction in vitro. 2. The convulsant barbiturates (+)-5-(1,3-dimethylbutyl)-5-ethyl barbituric acid [(+)-DMBB], 5-(2-cyclohexylideneethyl)-5-ethyl barbituric acid (CHEB), 5-ethyl-5-(3-methylbut-2'-enyl) barbituric acid (3M2B) and 5-ethyl-5-(1,3-dimethylbut-1'-enyl) barbituric acid (1,3M1B) all produced a concentration-dependent increase in miniature end-plate potential (MEPP) frequency. 3. With CHEB (100 microM) this increase in MEPP frequency was found to be dependent on the [Ca2+]o. CHEB in 0.5 mM [Ca2+]o did not alter MEPP amplitude, but in 1.3 and 2.5 mM [Ca2+]o CHEB significantly reduced the amplitude. 4. At a [Ca2+]o of 0.5 mM, CHEB produced an increase in both EPP amplitude and quantal content, while at 1.3 mM [Ca2+]o CHEB did not alter EPP amplitude or quantal content. 5. The plot of log quantal content vs log [Ca2+]o showed a parallel shift to the left in the presence of 100 microM CHEB. This change occurred without any alteration in the maximum quantal content. This suggests that the enhancement of transmitter release may be mediated by an effect on calcium fluxes in the pre-junctional nerve terminal.

Calcium, neuronal hyperexcitability and ischemic injury

Due to tight regulatory controls, a 10,000-fold concentration gradient exists between intracellular and extracellular free Ca2+ concentrations. With appropriate stimulus Ca2+ will rapidly flow into neurons through various types of membrane channels including voltage-dependent and receptor-operated channels. Intracellular Ca2+ concentrations are then quickly restored primarily through Ca2+-ATPase, Na+/Ca2+ exchange, and endoplasmic reticulum sequestration. It is well-known that Ca2+ is essential for neurotransmitter release. More recent investigations indicate that Ca2+ influx is essential for neuronal excitability independent from synaptic function. In fact, abnormal Ca2+ metabolism may play a dominant role in both the initiation and propagation of seizure discharge. Accordingly, Ca2+ channel blockers may represent a new therapeutic modality to treat epilepsy. Analyzed in this article are the major mechanisms by which neurons control Ca2+ fluxes and the evidence supporting the role of Ca2+ in seizure phenomena. Thereafter, an integrative theory for the role of calcium in neuronal hyperexcitability and ischemic cell death is constructed.

The effects of phenytoin on calcium uptake in osteoblastic cells

The effects of phenytoin (diphenylhydantoin, DPH) on calcium uptake in osteoblastic cells were studied to elucidate the potential mechanism of action of this antiepileptic drug on bone metabolism. Preincubation of the human osteoblastic osteosarcomal cell line, SaOS-2, and normal rat osteoblastic cells with DPH decreased basal calcium uptake. This inhibition occurred at DPH doses from 0.1 to 50 micrograms/ml. Parathyroid hormone (PTH) and prostaglandin E2 (PGE2) increased calcium uptake in the SaOS-2 cell line. Following preincubation with DPH, calcium uptake in cells treated with PTH or PGE2 did not exceed control levels. However, significant increases in the PTH- or PGE2-treated + DPH-pretreated cells compared to DPH pretreatment alone were still observed. These studies indicate that DPH induces decreases in osteoblastic calcium influx and they add further information on the possible mode of action of this drug on bone.

Effects of chronic diphenylhydantoin on cerebral metabolism in the adult rat

Diphenylhydantoin is one of the most widely used anticonvulsant agents in humans. To examine its effects on brain metabolism, we used the quantitative autoradiographic [14C]deoxyglucose method to measure local cerebral glucose utilization in adult rats receiving 50 mg/kg/day diphenylhydantoin for 1 week. Thirty-three brain structures were analyzed to determine whether the drug has global or site-specific effects on cerebral metabolism. Chronic administration produced statistically significant decreases in 23 neocortical and subcortical structures compared with those in vehicle-injected control animals. Therefore, our data support the concept that diphenylhydantoin has widespread depressant effects on brain metabolism.

Suppression of pentylenetetrazole seizures by oral administration of a dihydropyridine Ca2+ antagonist

We previously demonstrated that the dihydropyridine calcium channel blocker, nimodipine, is an effective anticonvulsant in experimental seizures when administered parentally. Reported now are the results for the oral administration of nimodipine in pentylenetetrazole (PTZ)-induced seizures in the rabbit. Twenty rabbits were randomly assigned into 10 controls and 10 treated with nimodipine 5 mg/kg/day orally for 5 days. All animals received increasing doses of the convulsant PTZ intravenously (i.v). The epileptogenecity of this agent was assessed in all animals (mg/kg) by four electrocorticographic criteria: first seizure greater than 5 s, two seizures within 5 min, epileptiform activity for 1 h, and status epilepticus. In all four categories, nimodipine increased the seizure threshold by 50-60%. The dose of PTZ required to produce the first seizure was 27.0 +/- 5.4 mg/kg in controls and 49.6 +/- 9.9 mg/kg in treated animals (p less than 0.001). Similar values were obtained for the other three electrocorticographic categories. There were no observable adverse side effects. The results confirm our previous findings that calcium influx is critical for seizure induction, and that selective central nervous system (CNS) calcium channel blockers may emerge as a new class of anticonvulsants.

Common anticonvulsants inhibit Ca2+ uptake and amino acid neurotransmitter release in vitro

Phenytoin (PHT), phenobarbital (PB), and carbamazepine (CBZ) dose-dependently inhibited veratrine-stimulated calcium influx and evoked amino acid neurotransmitter release in rat cortical slices at relatively low concentrations. The action on Ca2+ influx was in the clinical effective dose range for these anticonvulsant drugs, whereas the action on amino acid release was mostly well above this range. Neither ethosuximide (ESM) nor sodium valproate (VPA) had any effect on the veratrine-stimulated Ca2+ influx. Stimulated amino acid release was not affected by ESM, whereas VPA specifically inhibited the release of aspartate in preference to glutamate and GABA at concentrations well within the clinically effective dose range. The actions of VPA and ESM on other parameters measured were detectable only at very high concentrations, which are likely to be irrelevant in defining their clinical mode of action.

Induction of seizures in mice by intracerebroventricular administration of the calcium channel agonist BAY k 8644

The calcium channel agonist BAY k 8644 was used to investigate the role of the calcium ion (Ca2+) in epileptogenesis. Intracerebroventricular administration of the compound induced murine seizures that were reversed by calcium channel inhibitors (CCIs) but not by anticonvulsants such as carbamazepine, pentobarbital, and diazepam. The seizures were exacerbated by phenytoin and valproic acid. Chronic administration of CCI's, previously shown to produce down-regulation of the binding of the CCI [3H]nitrendipine, resulted in augmentation of BAY k 8644-induced seizures.

Inhibition of electrically induced seizures by a dihydropyridine calcium channel blocker

Nimodipine, a calcium channel blocker with high affinity for central dihydropyridine Ca2+ channels, produced a dose-dependent suppression of electrically induced seizures in the rabbit. Verapamil, a diphenylalkylamine which acts at peripheral Ca2+ channels, was ineffective. Phenytoin was less effective than nimodipine. These results suggest that calcium flux into neurons may be a biochemical precipitant for seizure genesis. Centrally acting calcium channel blockers may prove to be a new class of anticonvulsants.

Phenytoin: mechanisms of its anticonvulsant action

Phenytoin is a major anticonvulsant drug that is very effective in controlling a wide variety of seizure disorders while impairing neurological function little, if at all. Early work suggested the hypothesis that the drug's effects were due to a selective block of high-frequency neuronal activity. This theory is reevaluated in the light of accumulated observations on the effects of phenytoin in many neuronal and synaptic preparations. Most of these observations can be explained by a use- and frequency-dependent suppression of the sodium action potential by phenytoin, with a consequent filtering out of sustained high-frequency neuronal discharges and synaptic activity. The molecular mechanism for this is a voltage-dependent blockade of membrane sodium channels responsible for the action potential. Through this action, phenytoin obstructs the positive feedback that underlies the development of maximal seizure activity, while normal brain activity, proceeding at lower neuronal firing rates, is spared its depressant action. Other mechanisms of action that may contribute to the drug's efficacy and selectivity are also discussed.

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.

Barbiturates block divalent cation action potentials in leech nociceptive cells

Phenobarbital (PNB), pentobarbital (PTB) and methohexital (MTX) decreased the maximum rate of depolarization Vmax and duration of divalent cation action potentials elicited in leech nociceptive neurons in Na+-free solutions containing the K+-channel blocker TEA, without significantly affecting resting membrane potential or conductance. The block of the divalent cation action potentials was reversible and dose-dependent, ED50 for inhibition of Vmax being 560 microM for MTX, 800 microM for PTB and 3000 microM for PNB. This order of potency correlated well with the ratio of unchanged/charged form of the drugs at physiological pH suggesting that in leech, as in other preparations, the non-ionized form was the active one. In Na+-containing Ringer, the 3 barbiturates depolarized and decreased membrane resistance in the lateral nociceptive cells, but not the medial nociceptive cells. These results provide additional information regarding the newly described pharmacological differences among closely related neurons. These membrane actions may be related to some of the excitatory properties described for other barbiturates in invertebrate and mammalian preparations.

Effect of phenytoin on [3H]norepinephrine release from synaptosomes

Phenytoin reduces depolarization-linked [3H]norepinephrine release from rat brain synaptosomes. When choline chloride was substituted for NaCl the phenytoin effect was attenuated but still significant. This is consistent with the theory that phenytoin reduces both Na and Ca influx during depolarization.

Effect of phenytoin on intracellular calcium and intracellular protein changes during pentylenetetrazole-induced bursting activity in snail neurons

Effects of phenytoin (PHT) on the intracellular calcium and intracellular protein changes during pentylenetetrazole (PTZ)-induced bursting activity in the neurons of the Japanese land snail Euhadra peliomphala were examined. In the examination with a computer controlled electron probe X-ray microanalyzer, PHT clearly inhibited the intracellular calcium shift induced by PTZ as well as the calcium binding state change near the cell membrane. PHT also clearly inhibited the intracellular protein changes induced by PTZ. PHT, however, did not show any change in the transmembrane ionic currents such as the sodium current, calcium current and potassium current. These findings suggest that one of the main sites of anticonvulsant action of PHT is pathologically changed intracellular calcium movement and intracellular protein changes during seizure discharge.

Phenytoin interacts with calcium channels in brain membranes

Phenytoin at concentrations of between 30 and 300 microM inhibited binding of the calcium antagonist [3H] nitrendipine to voltage-dependent calcium channels in brain membranes. Other anticonvulsants (phenobarbital, carbamazepine, valproic acid, and clonazepam) failed to inhibit binding or did so only at concentrations much higher than occur clinically. Calcium channel blockade may be important in the clinical actions of phenytoin, including certain of the adverse effects of the drug.

Acetylcholine release from synaptosomes and phenytoin action

The release of labeled acetylcholine from synaptosomes loaded with methyl-[3H]choline has been measured in Krebs-Ringer-Bicarbonate (KRB) media containing either 5.6 or 56 mM KCl. Experiments have been performed in media containing either 1.0 mM Ca or 0 Ca with 1 mM EGTA. Phenytoin, 2 X 10(-4) M, reduced the depolarization-dependent release of acetylcholine in media containing 1.0 mM Ca and 56 mM KCl. It also significantly increased the release of acetylcholine from undepolarized samples in 5.6 mM KCl irrespective of the Ca concentration. The drug did not affect release from synaptosomes depolarized in Ca-free media. These results confirm the hypothesis that phenytoin has a dual effect on transmitter release.

Contrasting effects of a convulsant (CHEB) and an anticonvulsant barbiturate (phenobarbitone) on amino acid release from rat brain slices

The effects of a convulsant barbiturate, 5(2-cyclohexylidine-ethyl)-5-ethyl barbituric acid (CHEB), and phenobarbitone (PhB) on the release of exogenous D-aspartate and GABA from slices of rat cerebral cortex were investigated. While PhB inhibited potassium-evoked release of D-aspartate more so than that of GABA, CHEB potently inhibited potassium-evoked GABA release and stimulated evoked D-aspartate release, in a concentration-dependent manner. These actions are consistent with the observed in vivo convulsant and anticonvulsant properties of these barbiturates. CHEB, but not PhB also elevated spontaneous efflux of both amino acids. The actions of these barbiturates were further studied in calcium- and sodium-free media, and in the presence of tetrodotoxin and ruthenium red, agents known to alter ion flux across neuronal membranes. The results obtained indicate that different ionic mechanisms may be involved in the release of excitatory and inhibitory amino acid transmitters.

Effects of pentobarbitone on 45Ca2+ transport by rat brain mitochondria

The effects of pentobarbitone on the transport of 45Ca2+ by rat brain mitochondria were studied, using the Ruthenium Red-EGTA quench technique. In the presence of succinate and inorganic phosphate, mitochondria rapidly accumulate 45Ca2+. Pentobarbitone (0.1-1.0 mM) stimulates the initial rate of Ca2+ transport. In contrast, pentobarbitone (1 mM) did not affect the NaCl (50 mM)-induced efflux of 45Ca2+ from mitochondria. Dibucaine (60 micro M), a clinically used local anaesthetic, inhibits both 45Ca2+ uptake an efflux. The results suggest that barbiturate stimulation of mitochondrial Ca2+ uptake may, in combination with effects on other Ca2+ sequestering processes, contribute to the inhibitor of transmitter release observed at a number of synapses.