Inhibition of amino acid transmitter release from rat brain slices by phenytoin and related anticonvulsants

Ethosuximide modifies network excitability in the rat entorhinal cortex via an increase in GABA release

Ethosuximide is the drug of choice for treating generalized absence seizures, but its mechanism of action is still a matter of debate. It has long been thought to act by disrupting a thalamic focus via blockade of T-type channels and, thus, generation of spike-wave activity in thalamocortical pathways. However, there is now good evidence that generalized absence seizures may be initiated at a cortical focus and that ethosuximide may target this focus. In the present study we have looked at the effect ethosuximide on glutamate and GABA release at synapses in the rat entorhinal cortex in vitro, using two experimental approaches. Whole-cell patch-clamp studies revealed an increase in spontaneous GABA release by ethosuximide concurrent with no change in glutamate release. This was reflected in studies that estimated global background inhibition and excitation from intracellularly recorded membrane potential fluctuations, where there was a substantial rise in the ratio of network inhibition to excitation, and a concurrent decrease in excitability of neurones embedded in this network. These studies suggest that, in addition to well-characterised effects on ion channels, ethosuximide may directly elevate synaptic inhibition in the cortex and that this could contribute to its anti-absence effects. This article is part of a Special Issue entitled 'Post-Traumatic Stress Disorder'.

Chronic stress-induced hippocampal dendritic retraction requires CA3 NMDA receptors

Chronic stress induces dendritic retraction in the hippocampal CA3 subregion, but the mechanisms responsible for this retraction and its impact on neural circuitry are not well understood. To determine the role of NMDA (N-methyl-d-aspartic acid) receptor (NMDAR)-mediated signaling in this process, we compared the effects of chronic immobilization stress (CIS) on hippocampal dendritic morphology, hypothalamic-pituitary-adrenal (HPA) axis activation, and anxiety-related and hippocampus-dependent behaviors, in transgenic male mice in which the NMDAR had been selectively deleted in CA3 pyramidal cells and in non-mutant littermates. We found that CIS exposure for 10 consecutive days in non-mutant mice effectively induces HPA axis activation and dendritic retraction of CA3 short-shaft pyramidal neurons, but not CA3 long-shaft pyramidal neurons, suggesting a differential cellular stress response in this region. Dendritic reorganization of short-shaft neurons occurred throughout the longitudinal axis of the hippocampus and, in particular, in the ventral pole of this structure. We also observed a robust retraction of dendrites in dorsal CA1 pyramidal neurons in the non-mutant C57BL/6 mouse strain. Strikingly, chronic stress-induced dendritic retraction was not evident in any of the neurons in either CA3 or CA1 in the mutant mice that had a functional lack of NMDARs restricted to CA3 pyramidal neurons. Interestingly, the prevention of dendritic retraction in the mutant mice had a minimal effect on HPA axis activation and behavioral alterations that were induced by chronic stress. These data support a role for NMDAR-dependent glutamatergic signaling in CA3 in the cell-type specific induction of dendritic retraction in two hippocampal subregions following chronic stress.

What is the functional significance of chronic stress-induced CA3 dendritic retraction within the hippocampus?

Chronic stress produces consistent and reversible changes within the dendritic arbors of CA3 hippocampal neurons, characterized by decreased dendritic length and reduced branch number. This chronic stress-induced dendritic retraction has traditionally corresponded to hippocampus-dependent spatial memory deficits. However, anomalous findings have raised doubts as to whether a CA3 dendritic retraction is sufficient to compromise hippocampal function. The purpose of this review is to outline the mechanism underlying chronic stress-induced CA3 dendritic retraction and to explain why CA3 dendritic retraction has been thought to mediate spatial memory. The anomalous findings provide support for a modified hypothesis, in which chronic stress is proposed to induce CA3 dendritic retraction, which then disrupts hypothalamic-pituitary-adrenal axis activity, leading to dysregulated glucocorticoid release. The combination of hippocampal CA3 dendritic retraction and elevated glucocorticoid release contributes to impaired spatial memory. These findings are presented in the context of clinical conditions associated with elevated glucocorticoids.

Sulthiame but not levetiracetam exerts neurotoxic effect in the developing rat brain

Antiepileptic drugs (AEDs) used to treat seizures in pregnant women, infants, and young children can cause cognitive impairment. One mechanism implicated in the development of neurocognitive deficits is a pathologic enhancement of physiologically occurring apoptotic neuronal death in the developing brain. We investigated whether the newer antiepileptic drug levetiracetam (LEV) and the older antiepileptic drug sulthiame (SUL) have neurotoxic properties in the developing rat brain. SUL significantly enhanced neuronal death in the brains of rat pups ages 0 to 7 days at doses of 100 mg/kg and above, whereas LEV did not show this neurotoxic effect. Dosages of both drugs used in the context of this study comply with an effective anticonvulsant dose range applied in rodent seizure models. Thus, LEV is an AED which lacks neurotoxicity in the developing rat brain and should be considered in the treatment of epilepsy in pregnant women, infants, and toddlers once general safety issues have been properly addressed.

Effects of anticonvulsants on veratridine- and KCl-evoked glutamate release from rat cortical synaptosomes

We compared the effects of three conventional (phenytoin, carbamazepine and phenobarbital) and three novel (BW 1003C87, lamotrigine and riluzole) anticonvulsants on evoked glutamate release from rat cortical synaptosomes. Glutamate release was evoked by either 20 microM veratridine (which requires both Na+ and Ca2+ channel activation) or 30 mM KCl (which requires Ca2+ channel, but not Na+ channel, activation) to assess the involvement of Na+ and/or Ca2+ channels in the presynaptic actions of these anticonvulsants. All six compounds inhibited veratridine-evoked glutamate release; BW 1003C87 (IC50 = 2.0 microM) was the most potent and phenobarbital (IC50 = 3.2 mM) was the least potent inhibitor. Only phenobarbital (IC50 = 6.4 mM), riluzole (IC50 > 50 microM) and phenytoin (IC50 > 800 microM) significantly inhibited KCl-evoked glutamate release. These results suggest that therapeutic concentrations of BW 1003C87, lamotrigine, phenytoin, carbamazepine and riluzole, but not phenobarbital, inhibit synaptic glutamate release by preferentially blocking presynaptic Na+ channels. Presynaptic Na+ channel blockade with inhibition of the release of glutamate, and possibly other transmitters, may contribute to their anticonvulsant and/or neuroprotective effects.

Chronic psychosocial stress induces morphological alterations in hippocampal pyramidal neurons of the tree shrew

The effect of sustained psychosocial stress on the morphology of hippocampal pyramidal neurons was analysed in male tree shrews after 14, 20, and 28 days of social confrontation. A variety of physiological changes such as constantly elevated levels of urinary cortisol and norepinephrine and reduced body weight, which are indicative of chronic stress were observed in the subordinate, but not in the dominant males. Light microscopic analysis of Nissl-stained hippocampal sections showed that the staining intensity of the nucleoplasm in the CA1 and CA3 pyramidal neurons was increased after prolonged psychosocial stress, indicating a change in the nuclear chromatin structure. These alterations were observed only in subordinate animals and increased in a time dependent manner in accordance with the length of the stress period. There was, however, neither a reduction in density nor a degeneration of pyramidal neurons in chronically stressed animals. Mechanisms which may possibly account for the observed alterations are discussed.

An in vitro investigation of the action of lamotrigine on neuronal voltage-activated sodium channels

Lamotrigine (LTG), 6-(2,3-dichlorophenyl)-1,2,4-triazine-3,5-diamine, is a novel antiepieptic drug structurally unrelated to the major anticonvulsants in current use. Previous studies of LTG in rodents revealed efficacy in maximal electroshock test, pentylenetetrazol test and kindling models of seizures suggesting potential utility in the treatment of partial and generalized (tonic-clonic) seizures. In the present study, LTG was found to block sustained repetitive firing of sodium-dependent action potentials in mouse spinal cord cultured neurons and inhibit [3H]batrachotoxinin A 20-alpha-benzoate binding in rat brain synaptosomes suggesting a direct interaction with voltage-activated sodium channels.

Phenytoin prevents stress- and corticosterone-induced atrophy of CA3 pyramidal neurons

Repeated daily restraint stress and daily corticosterone administration to adult male Sprague-Dawley rats leads to decreases in the number of branch points and length of dendrites of CA3 pyramidal neurons of the hippocampal formation. This decrease is prevented by daily administration of the antiepileptic drug phenytoin (Dilantin), which is known to interfere with excitatory amino acid release and actions. Phenytoin had no obvious effect on behavior during and after stress and failed to prevent stress-induced reduction of body weight gain and stress-induced increases of adrenal weight relative to body weight; it also failed to attenuate glucocorticoid-induced diminution of the size of the thymus gland, indicating that it does not directly antagonize glucocorticoid actions. Stress- and corticosterone-induced effects on dendritic length and branch point number are more pronounced on the apical, as opposed to the basal, CA3 dendrites that receive the largest mossy fiber input from the dentate gyrus. Because phenytoin is also known to prevent ischemic damage, these results are consistent with a model in which stress- and corticosterone-induced CA3 dendritic atrophy is produced by excitatory amino acids released from the mossy fibers.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition of N6-[3H]cyclohexyladenosine binding by carbamazepine

The mechanism of action of carbamazepine (CBZ) (Tegretol), despite widespread use in the management of partial and tonic-clonic seizures in adults, is not completely understood. In animals, adenosine and adenosine analogues have anticonvulsant effects that may be due to interactions with central A1 adenosine receptors. CBZ (at therapeutically relevant concentrations) inhibits the binding of agonists and antagonists to brain A1 adenosine receptors, but whether as an agonist/antagonist is not clear. The adenosine agonist, N6-[3H]cyclohexyladenosine ([3H]CHA), binds to membranes from rat cortex and hippocampus at two nanomolar binding sites or states. To clarify the actions of carbamazepine at the A1 adenosine receptor, its inhibitory actions were compared with those of known adenosine agonists and xanthine antagonists using 0.1 nM[3H]CHA, in which almost all binding is to the higher affinity state, or 10 nM [3H]CHA, in which there is a substantial contribution of binding from both states. The ratios of the IC50 values (concentration that inhibits specific binding by 50%) at 10 nM [3H]CHA to the IC50 values at 0.1 nM [3H]CHA were 18-31 for the agonists and 4-10 for the xanthine antagonists. CBZ had a ratio of 3. The inhibitory effects of GTP on [3H]CHA binding were less in the presence of the adenosine agonist, 2-chloroadenosine than were inhibitory effects in the presence of the xanthine antagonist theophylline or CBZ in both cortex and hippocampus. These in vitro studies indicate that CBZ is an antagonist at A1 adenosine receptors in cerebral cortical and hippocampal membranes from rat brain. Agonist activity at A1 adenosine receptors would have been compatible with the sedative anticonvulsant effects of CBZ, but these data do not support a role of the anticonvulsant action of carbamazepine on A1 adenosine receptors in cerebral cortex or hippocampus.

Antiepileptic drugs and cerebral glucose metabolism

Using PET with [18-F]-2-deoxyglucose (FDG), we studied the effects of antiepileptic drugs on cerebral glucose metabolism. Serial scans were performed before and after the test drug was added to or removed from the patient's regimen. At least 3 weeks elapsed after achieving steady-state plasma levels when drugs were added, or after plasma levels were undetectable when drugs were tapered, before repeat scans were obtained. Only a single drug was changed between scans. In the phenobarbital (PB) study, the "on-drug" scan was performed first in each case. In this instance, a mean of 14 weeks elapsed between the time blood levels were undetectable and repeat scanning in order to avoid the possibility of withdrawal effects. Scanning in each group was performed 30 min after injection of 5 mCi of FDG, with EEG monitoring to exclude ictal activity. Regional glucose metabolic rates were calculated in 8 to 20 regions of interest. PB reduced LCMRglu in seven of eight regions studied, with a mean reduction over all regions of 37 +/- 3%. Phenytoin (PHT) reduced LCMRglu in only two of 10 regions (mean = 13%). We studied the effect of PHT on cerebellar metabolism in 42 patients using unpaired scans. Cerebellar LCMRglu was lower when patients were taking PHT at the time of scan, as well as in those who were taking PHT for 5 years or more, but the differences were not significant. There was a weak inverse correlation between PHT serum level and cerebellar LCMRglu in patients taking the drug at the time of scan (r = -0.36; 0.05 less than p less than 0.1).(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions of carbamazepine, chlormethiazole and pentobarbitone with adenosine on hippocampal slices

1. The effects of the anticonvulsants carbamazepine, chlormethiazole and pentobarbitone and their interactions with adenosine have been studied in rat hippocampal slices. 2. Neither chlormethiazole nor pentobarbitone changed the evoked population spike in CA1 stratum pyramidale, while carbamazepine produced a gradual decline of this potential at concentrations of 100 microM or greater. 3. This effect was not prevented by 8-phenyltheophylline. 4. Only carbamazepine (greater than 20 microM) produced any change of adenosine responses: a slowly developing reduction of the inhibitory action. 5. None of the anticonvulsants altered responses to the amino acid excitant N-methyl-aspartate, or kynurenic acid.

Mechanisms of anticonvulsant drug action. I. Drugs primarily used for generalized tonic-clonic and partial epilepsies

The mechanisms by which the clinically effective anticonvulsant drugs act include effects on neurotransmitter action, effects on repetitive neuronal firing mechanisms, effects on neuronal networks, and effects on neuronal ionic transport. The combination of effects possessed by each individual agent along with its pharmacokinetic properties determine the usefulness of each agent. Phenytoin, carbamazepine and phenobarbital are effective in generalized tonic-clonic and partial epilepsies. Phenytoin exerts important effects on neuronal sodium and calcium ion transport, reduces repetitive firing, reduces excitation in neuronal networks of the brainstem reticular formation, and produces some decrease in the effect of the inhibitory transmitter, gamma-aminobutyric acid (GABA). Carbamazepine blocks repetitive firing mechanisms, reduces excitation in neuronal networks with some effect on sodium and potassium ion transport, and has effects on the actions of norepinephrine, adenosine and perhaps acetylcholine. Phenobarbital enhances the action of GABA with some reduction of repetitive firing and reduces excitation in reticular formation neuronal networks.

Pharmacological studies on lamotrigine, a novel potential antiepileptic drug: II. Neurochemical studies on the mechanism of action

Lamotrigine (LTG) [3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazine] is a novel anticonvulsant chemically unrelated to current antiepileptic drugs and with a pharmacological profile similar to that of phenytoin. The effect of LTG has been compared with that of phenytoin, on the release of endogenous amino acids and radiolabelled acetylcholine evoked by veratrine or potassium, from slices of rat cerebral cortex in vitro. Both veratrine and potassium evoked a marked release of glutamate and gamma-aminobutyric acid (GABA), with a more moderate release of aspartate. LTG inhibited veratrine-evoked release of glutamate and aspartate, with ED50 values of 21 microM for both amino acids, but LTG was less potent in the inhibition of GABA release (ED50 = 44 microM). At concentrations up to 300 microM, LTG had no effect on potassium-evoked amino acid release or on spontaneous release. Also, LTG was some five times less potent in the inhibition of veratrine-evoked [3H]acetylcholine release (ED50 = 100 microM) than in glutamate or aspartate release. The total lack of effect of LTG on potassium-evoked release and the potent effect on veratrine-evoked release (at concentrations found in rat brain after anticonvulsant doses) strongly suggest that LTG acts at voltage-sensitive sodium channels to stabilise neuronal membranes and inhibit transmitter release, principally glutamate. The role of glutamate in the aetiology of epilepsy is discussed.

The effect of phenytoin on glutamate and GABA transport

Phenytoin was observed to inhibit competitively the sodium dependent high affinity synaptosomal transport of both glutamate (Glu) and gamma-aminobutyric acid (GABA) with Ki values of 66 +/- 10 and 185 +/- 65 microM, respectively. This contrasted with a previous report that the uptakes of Glu and GABA were enhanced by phenytoin. The degree of inhibition is dependent on the concentrations of the competing drug and substrate present. Taking the therapeutic levels of phenytoin and the overall brain Glu and GABA contents, the degrees of inhibition obtainable appear to be negligible. However, as most of the high levels of Glu and GABA in the brain are intracellular, Glu, and GABA concentrations in the microenvironment of the uptake sites may be sufficiently small so that the ability of phenytoin to inhibit Glu and GABA transport may contribute significantly to the anticonvulsant property of this drug.

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.

Electrophysiological and neurochemical investigations on the action of carbamazepine on the rat hippocampus

Carbamazepine moderately depressed the input fiber volley resulting in attenuation of the dendritic epsp and the population spike in CA1 of rat hippocampal slices with a threshold concentration of 20 microM. The depressant effect on the population spike was not antagonized by the adenosine receptor blocker caffeine. Paired-pulse inhibition was not affected by carbamazepine (40 microM). Epileptic-like rhythmic discharge of CA1 neurons in medium containing low Ca2+/high Mg2+ was attenuated at even lower concentrations of carbamazepine (8 microM) indicating that there was also a postsynaptic site of action. Imipramine being significantly more potent than carbamazepine in the rabbit corneal test for local anaesthetic activity had no effect on the population spike (20 microM). In neurochemical studies, carbamazepine reduced the [22Na]- and [3H]L-glutamate efflux induced by potassium and veratridine from hippocampal slices with a threshold concentration of 10 microM. The drug (400 microM) failed to affect Na+-dependent binding of [3H]L-glutamate to hippocampal synaptic membranes. In conclusion, the present findings demonstrate pre- and postsynaptic depressant actions of carbamazepine in CAI of hippocampus.

Muscle relaxant action of excitatory amino acid antagonists

Antagonists of neuronal excitation induced by dicarboxylic amino acids were tested in genetically spastic rats of the Han-Wistar strain. These animals exhibit an increased muscle tone which can be measured as a spontaneous tonic activity in the electromyogram of the gastrocnemius-soleus muscle. Compounds that block excitation due to N-methyl-D-aspartic acid reduced the spontaneous activity measured in the electromyogram in a dose-related manner. The most potent compounds, 2-amino-7-phosphonoheptanoic and kynurenic acids were effective muscle relaxants when given either intraperitoneally or intracerebroventricularly. 2-Amino-5-phosphonopentanoic acid possessed much weaker muscle relaxant activity, while L-glutamic acid diethylester was inactive by either route. The results suggest that blockade of N-methyl-D-aspartic acid receptors results in a myorelaxant effect. Specific antagonists of excitation at N-methyl-D-aspartic acid receptors may provide a new class of muscle relaxants.

Interactions of the anticonvulsants diphenylhydantoin and carbamazepine with adenosine on cerebral cortical neurons

Diphenylhydantoin, administered either by iontophoresis from a multibarreled pipette or intraperitoneally, prolonged the duration of adenosine-evoked depressions of the spontaneous firing of rat cerebral cortical neurons. In larger amounts, iontophoretically applied diphenylhydantoin depressed the firing of cortical neurons. The depressant actions of both adenosine and diphenylhydantoin were antagonized by caffeine (20 mg/kg). These results support a previous suggestion that diphenylhydantoin may exert its central effects by inhibiting adenosine uptake, thus potentiating the levels of extracellular adenosine. Carbamazepine failed to potentiate the actions of iontophoretically applied adenosine on cerebral cortical neurons, and at higher doses it reduced the duration of adenosine-elicited depressions. This finding is consistent with suggestions that carbamazepine may act as an antagonist at adenosine receptors.

Interactions of the anticonvulsant carbamazepine with adenosine receptors. 1. Neurochemical studies

At therapeutic concentrations the tricyclic anticonvulsant carbamazepine inhibited the binding of the adenosine analogue [3H]L-N6-phenylisopropyladenosine ([3H]PIA) to rat brain membranes (Ki = 46 microM) in vitro. Carbamazepine interacted much less potently with muscarinic cholinergic, beta-adrenergic, gamma-aminobutyric acid, or L-glutamate binding sites. Carbamazepine was of lower potency (Ki = 112 microM) as an inhibitor of the binding of the putative A2 adenosine agonist [3H]5'-N-ethylcarboxamidoadenosine. GTP greatly reduced the potencies of purine agonists, but not antagonists, as inhibitors of [3H]PIA. The potency of carbamazepine, like that of the antagonist theophylline, was not reduced by GTP. Studies on the adenosine-stimulated adenylate cyclase activity in guinea pig brain slices also revealed theophyllinelike activity of carbamazepine. The possible relevance of agonist and antagonist interactions with adenosine receptors to the anticonvulsant action of carbamazepine is discussed.