Gabapentin potentiation of the antiepileptic efficacy of vigabatrin in an in vitro model of epilepsy

A Candidate Gliotransmitter, L-β-Aminoisobutyrate, Contributes to Weight Gain and Metabolic Complication Induced by Atypical Antipsychotics

Lurasidone and quetiapine are effective atypical mood-stabilizing antipsychotics, but lurasidone and quetiapine are listed as lower-risk and high-risk for weight gain/metabolic complications, respectively. The pathophysiology of the discrepancy of metabolic adverse reactions between these antipsychotics remains to be clarified. The GABA isomer, β-aminoisobutyric acid (BAIBA) enantiomer, was recently re-discovered as myokine via an AMP-activated protein kinase activator (AMPK) enhancer and inhibitory gliotransmitter. Notably, activation of AMPK in peripheral organs improves, but in the hypothalamus, it aggravates metabolic disturbances. Therefore, we determined effects of chronic administration of lurasidone and quetiapine on intracellular and extracellular levels of the BAIBA enantiomer. L-BAIBA is a major BAIBA enantiomer in the hypothalamus and astrocytes, whereas L-BAIBA only accounted for about 5% of total plasma BAIBA enantiomers. Chronic lurasidone administration did not affect body weight but decreased the L-BAIBA level in hypothalamus and cultured astrocytes, whereas chronic quetiapine administration increased body weight and the L-BAIBA level in hypothalamus and astrocytes. Contrary, neither lurasidone nor quetiapine affected total plasma levels of the BAIBA enantiomer since D-BAIBA levels were not affected by these antipsychotics. These results suggest that activation of intracellular L-BAIBA signaling is, at least partially, involved in the pathophysiology of metabolic adverse reaction of quetiapine. Furthermore, this study also demonstrated that lurasidone and quetiapine suppressed and enhanced astroglial L-BAIBA release induced by ripple-burst stimulation (which physiologically contributes to cognitive memory integration during sleep), respectively. Therefore, L-BAIBA probably contributes to the pathophysiology of not only metabolic adverse reactions, but also a part of clinical action of lurasidone or quetiapine.

Opposing effects of clozapine and brexpiprazole on β-aminoisobutyric acid: Pathophysiology of antipsychotics-induced weight gain

Clozapine is one of the most effective antipsychotics and has the highest risk of weight gain and metabolic complications; however, the detailed pathophysiology of its clinical action and adverse reactions remains to be clarified. Therefore, the present study determined the chronic effects of clozapine (high risk of weight gain) and brexpiprazole (relatively low risk of weight gain) on intracellular and extracellular levels of β-aminoisobutyric acid (BAIBA) enantiomers, which are endogenous activators of AMP-activated protein kinase (AMPK). L-BAIBA is the dominant BAIBA enantiomer in the rat hypothalamus and cultured astrocytes, whereas L-BAIBA accounts for only approximately 5% of the total plasma BAIBA enantiomers. L-BAIBA displayed GABAB receptor agonistic action in the extracellular space and was released through activated astroglial hemichannels, whereas in the intracellular space, L-BAIBA activated AMPK signalling. Chronic administration of the effective doses of clozapine increased intracellular and extracellular levels of L-BAIBA in the hypothalamus and cultured astrocytes, whereas that of brexpiprazole decreased them. These results suggest that enhancing hypothalamic AMPK signalling by increasing intracellular L-BAIBA levels is, at least partially, involved in the pathophysiology of clozapine-induced weight gain and metabolic complications.

Pregabalin: latest safety evidence and clinical implications for the management of neuropathic pain

Used mainly for the management of neuropathic pain, pregabalin is a gabapentinoid or anticonvulsant that was initially developed as an antiepileptic agent. After more than a decade of experience with pregabalin, experience and studies have shown that the adverse effect profile of pregabalin is well tolerated for the management of neuropathic pain and other conditions. Its use is associated with benign central nervous system and systemic adverse effects, and there are very limited metabolic, idiosyncratic or known teratogenic adverse effects. Along with its efficacy in particular neuropathic pain conditions, pregabalin's safety led it to be one of the first pharmacotherapies considered for the management of neuropathic pain. This review discusses the use of pregabalin as well as its potential adverse effects, including the most commonly noted features of sedation, dizziness, peripheral edema and dry mouth. Although other adverse effects may occur, these appear to be uncommon. The review also discusses the clinical implications of pregabalin's use for the clinician.

New in vitro phenotypic assay for epilepsy: fluorescent measurement of synchronized neuronal calcium oscillations

Research in the epilepsy field is moving from a primary focus on controlling seizures to addressing disease pathophysiology. This requires the adoption of resource- and time-consuming animal models of chronic epilepsy which are no longer able to sustain the testing of even moderate numbers of compounds. Therefore, new in vitro functional assays of epilepsy are needed that are able to provide a medium throughput while still preserving sufficient biological context to allow for the identification of compounds with new modes of action. Here we describe a robust and simple fluorescence-based calcium assay to measure epileptiform network activity using rat primary cortical cultures in a 96-well format. The assay measures synchronized intracellular calcium oscillations occurring in the population of primary neurons and is amenable to medium throughput screening. We have adapted this assay format to the low magnesium and the 4-aminopyridine epilepsy models and confirmed the contribution of voltage-gated ion channels and AMPA, NMDA and GABA receptors to epileptiform activity in both models. We have also evaluated its translatability using a panel of antiepileptic drugs with a variety of modes of action. Given its throughput and translatability, the calcium oscillations assay bridges the gap between simplified target-based screenings and compound testing in animal models of epilepsy. This phenotypic assay also has the potential to be used directly as a functional screen to help identify novel antiepileptic compounds with new modes of action, as well as pathways with previously unknown contribution to disease pathophysiology.

Isobolographic and behavioral characterizations of interactions between vigabatrin and gabapentin in two experimental models of epilepsy

The aim of this study was to characterize the pharmacodynamic, pharmacokinetic and adverse-effect profiles of vigabatrin and gabapentin. Isobolographic analysis was used in two mouse experimental models of epilepsy: the maximal electroshock seizure threshold test and pentylenetetrazole-induced seizures. In the maximal electroshock seizure threshold test, electroconvulsions were produced by a current with various intensities whilst in the pentylenetetrazole test a CD(97) dose (100 mg/kg) was used. Potential adverse-effect profiles of interactions of vigabatrin with gabapentin at three fixed-ratios of 1:3, 1:1 and 3:1 from both seizure tests were evaluated in the chimney (motor performance) and grip-strength (skeletal muscular strength) tests. Vigabatrin and gabapentin total brain concentrations were determined with high performance liquid chromatography. Vigabatrin and gabapentin administered singly increased the electroconvulsive threshold (TID(20) - 226.2 and 70.0 mg/kg, respectively). With isobolography, the combination of vigabatrin with gabapentin at the fixed-ratio of 1:3 exerted supra-additive (synergistic) interactions whilst at 1:1 and 3:1 additivity occurred. Similarly, vigabatrin and gabapentin administered singly suppressed the pentylenetetrazole-induced seizures (ED(50) values - 622.5 and 201.1 mg/kg, respectively). Isobolography revealed that vigabatrin with gabapentin in combination at the fixed-ratio of 1:1 produced supra-additive (synergistic) interaction whilst at 1:3 and 3:1 additivity occurred. In combination neither motor coordination nor skeletal muscular strength was affected. Total vigabatrin and gabapentin brain concentrations revealed that neither drug affected the pharmacokinetics of the other. Vigabatrin and gabapentin have a favorable pharmacodynamic interaction in animal seizure models in the absence of acute adverse effects or concurrent pharmacokinetic changes.

Effects of anticonvulsants on soman-induced epileptiform activity in the guinea-pig in vitro hippocampus

Seizures arising from acetylcholinesterase inhibition are a feature of organophosphate anticholinesterase intoxication. Although benzodiazepines are effective against these seizures, alternative anticonvulsant drugs may possess greater efficacy and fewer side-effects. We have investigated in the guinea-pig hippocampal slice preparation the ability of a series of anticonvulsants to suppress epileptiform bursting induced by the irreversible organophosphate anticholinesterase, soman (100 nM). Carbamazepine (300 microM), phenytoin (100 microM), topiramate (100-300 microM) and retigabine (1-30 microM) reduced the frequency of bursting but only carbamazepine and phenytoin induced a concurrent reduction in burst duration. Felbamate (100-500 microM) and clomethiazole (100-300 microM) had no effect on burst frequency but decreased burst duration. Clozapine (3-30 microM) reduced the frequency but did not influence burst duration. Levetiracetam (100-300 microM) and gabapentin (100-300 microM) were without effect. These data suggest that several compounds, in particular clomethiazole, clozapine, felbamate, topiramate and retigabine, merit further evaluation as possible treatments for organophosphate poisoning.

Antiepileptic action induced by a combination of vigabatrin and tiagabine

Vigabatrin, an inhibitor of GABA breakdown by GABA transaminase and of GABA transporter isoform 1 (GAT1), and tiagabine, a highly specific inhibitor of GAT1, have successfully been applied in the treatment of epilepsy. We investigated the effects of individual and combined application of these drugs on GAT1 expressed in Xenopus oocytes, and examined the effects on epileptiform discharges in the CA3 area of brain slices of genetically epileptic El and control ddY mice, and on the occurrence of seizures in El mice. Simultaneous application of vigabatrin and tiagabine inhibited epileptiform discharges induced by high-K+ solution in the brain slices in an antagonistic fashion. The degree of inhibition by tiagabine after pre-treatment with vigabatrin was additive in ddY mice and synergistic in El mice. In Mg2+-free solution, co-treatment by the two drugs produced additive inhibition in slices from both mouse strains, but pre-treatment with vigabatrin produced synergistic inhibition in slices only from ddY mice. In the slices from El mice, a combination of drugs resulted in additive effects in both co- and pre-treatment by the drugs. Although these drugs are also effective in vivo at suppressing seizure occurrence in El mice, the combined application does not show synergistic effects, but rather is antagonistic under the experimental conditions in this particular variant of epilepsy. The synergistic inhibition of epileptiform discharges in brain slices may, in part, have originated from the complex interaction with GAT1. In experiments on the GAT1 expressed in oocytes it could be demonstrated that synergistic inhibition occurs only at low concentration (0.1 nM) of vigabatrin. This illustrates that the oocytes may form a powerful test system for drug screening and investigation of complex drug interactions. These results present a novel interpretation of synergistic inhibition of certain epileptic discharges using vigabatrin and another drug, and that for successful synergistic treatment of epilepsies carefully designed timed dosage regimens are essential.

Phenylethylidenehydrazine, a novel GABA-transaminase inhibitor, reduces epileptiform activity in rat hippocampal slices

Phenylethylidenehydrazine (PEH), an analog of the monoamine oxidase inhibitor, beta-phenylethylhydrazine (phenelzine), inhibits the gamma-aminobutyric acid (GABA) catabolic enzyme GABA-transaminase and increases brain levels of GABA. GABA is the predominant fast inhibitory transmitter counteracting glutamatergic excitation, and increased neural GABA could influence a wide range of synaptic and circuit properties under both physiologic and pathophysiologic conditions. To examine the scope of these effects, we applied PEH (or vehicle) to rat hippocampal slices and measured basal glutamatergic transmission, synaptic plasticity, and epileptiform activity using extracellular field and whole cell patch clamp recordings. In vitro pre-treatment with PEH (100 microM) increased the GABA content of hippocampal slices by approximately 60% over vehicle-treated controls, but it had no effect on basal field excitatory postsynaptic potentials, tonic GABA currents, paired-pulse facilitation, or long-term potentiation. In contrast, pre-incubation with PEH caused a dose- and time-dependent reduction in epileptiform burst frequency induced by superfusion with Mg2+-free or high-K+ artificial cerebrospinal fluid. Thus, the inhibitory effects of PEH are state-dependent: hyper-excitation during epileptiform bursting was reduced, whereas synaptic transmission and plasticity were unaffected.

Diurnal actions of melatonin on epileptic activity in hippocampal slices of rats

Since melatonin receptors have been found in the hippocampus of mammals it has been suggested that melatonin can modulate neuronal functions of hippocampal cells. The effect of melatonin (10 nM/l and 1 microM/l) on frequency and amplitude of epileptiform field potentials (EFP) elicited by low Mg(2+) or by bicuculline was tested in the CA1 region of hippocampal slices of rats. In the low Mg(2+) model, melatonin, applied in a near physiological concentration of 10 nM/l, exerts no effect on EFP in slices prepared at night or during the day. In a concentration of 1 microM/l, however, melatonin enhances the frequency of EFP to approximately 140% in slices prepared during the day. This effect was suppressed through simultaneous administration of the melatonin receptor antagonist luzindole (10 microM/l). In contrast, melatonin did not affect epileptic activity in slices prepared at night. Epileptiform discharges elicited by blocking the GABAergic inhibition (bicuculline model) were not affected by melatonin, either during the day or at night. The results indicate that melatonin affects epileptic activity in a diurnal manner and that the action of melatonin is different in relation to the epilepsy model.

Tiagabine synergistically interacts with gabapentin in the electroconvulsive threshold test in mice

Polytherapy, based on the rational combining of antiepileptic drugs (AEDs), is required for patients with drug-resistant epilepsy. In such cases, the combinations of AEDs usually offer a significant enhancement of their protective effects against seizures. There has appeared a hypothesis that combining two AEDs, influencing the same neurotransmitter system, results in the potentialization of their anticonvulsant effects. For corroborating this hypothesis, a pharmacological character of interaction between tiagabine (TGB) and gabapentin (GBP)-two novel AEDs affecting the GABA-ergic system, in the maximal electroshock seizure threshold (MEST)-test in mice was evaluated. TGB at the dose of 4 mg/kg and GBP at 75 mg/kg significantly raised the electroconvulsive threshold. Further, using the isobolographic calculations, TGB was coadministered with GBP at three fixed-ratios (1 : 3, 1 : 1, and 3 : 1) of their respective protective drug doses. All examined combinations of TGB with GBP exerted supra-additive (synergistic) interactions against MEST-induced seizures in mice. The interaction index, describing the strength and magnitude of interaction, ranged between 0.25 and 0.50 indicating supra-additivity. Adverse (neurotoxic) effects were evaluated in the chimney (motor performance) and the step-through, light-dark passive avoidance (long-term memory) tests in mice. The examined combinations of TGB with GBP did not affect the motor coordination, except for the fixed-ratio of 1 : 1, at which significant impairment of motor performance was observed. Moreover, all combinations selectively impaired the acquisition of the task in the passive avoidance test, having no impact on consolidation and retrieval in the long-term memory test. The pain threshold test revealed that the observed disturbances in the passive avoidance testing resulted presumably from the antinociceptive activity of these AEDs in combinations. After lengthening the exposing time to the direct current stimulus in the passive avoidance test from 2 to 6 s, the acquisition of the task, in animals receiving the combinations of TGB and GBP was not impaired. Neither the plasma, nor brain concentrations of GBP were affected by TGB application, so pharmacokinetic events that might negatively influence the observed effects are not probable. Results of this study clearly indicate that the activation of the same neurotransmitter system (GABA-ergic) leads to a synergistic interaction. The pain threshold test is a very good paradigm for screening the antinociceptive properties of AEDs, which may disturb the long-term memory testing in animals. Combinations of TGB with GBP (very promising from a preclinical point of view) should be clinically verified for elaborating the most effective treatment regimen in patients with intractable seizures.

Pre- rather than co-application of vigabatrin increases the efficacy of tiagabine in hippocampal slices

PURPOSE

The antiepileptic drug vigabatrin (VGB) increases intracellular availability of the inhibitory transmitter gamma-aminobutyric acid (GABA) by inhibition of GABA-transaminase. A blockade of the GABA uptake is the main mechanism of action of tiagabine (TGB). Based on this, the two antiepileptic drugs (AEDs) can be speculated to act synergistically so that their combined antiepileptic efficacy is supraadditive.

METHODS

To test this, experiments were performed on hippocampal slices of guinea-pigs. As an epilepsy model, epileptiform field potentials (EFPs) were induced by omission of Mg2+ from the bath solution and recorded in stratum pyramidale of the CA3 region. VGB (7.5 microM) and TGB (0.75 microM) were added to the superfusate.

RESULTS

VGB, given alone, failed to decrease the repetition rate of EFPs. Similarly, TGB applied alone only transiently led to a nonsignificant reduction of the EFP frequency. Combining VGB and TGB, their suppressive efficacy increased, yielding a significant reduction of EFP frequency, which, however, again did not persist. Pretreatment of the preparations with VGB for 2 h, followed by additional application of TGB, or TGB alone, drastically and persistently potentiated the effects.

CONCLUSIONS

These results demonstrate that VGB and TGB show favorable pharmacodynamic interactions, provided VGB is allowed to block intracellular GABA degradation before GABA uptake block by TGB.

Modulation of the in vivo effects of gabapentin by vigabatrin and SKF89976A

Gabapentin (GBP) has been shown to reduce paired-pulse inhibition in the dentate gyrus of the urethane-anesthetized rat, which is a proconvulsant effect, and to shorten the afterdischarge duration, which is an antiepileptic effect. The mechanism by which GBP exerts these effects is not known, but a number of possibilities have been proposed. Here we tested the ability of vigabatrin (VGB), a GABA transaminase inhibitor, and SKF89976A, a selective GAT-1 blocker, to alter the effectiveness of GBP in the dentate gyrus in urethane-anesthetized adult Sprague-Dawley rats. VGB, alone at 100 mg/kg, had no effect on the evoked potentials or paired-pulse inhibition in the dentate gyrus, but did block lengthening of the afterdischarge. Pretreatment with VGB had no effect on the ability of GBP to reduce paired-pulse inhibition, but blocked the effect of GBP on seizure duration. SKF89976A, alone at 10 mg/kg, increased paired-pulse inhibition and blocked the lengthening of the afterdischarge in the seizure model. Pretreatment with SKF89976A had no effect on the actions of GBP on either paired-pulse inhibition or seizure duration. These results suggest that the action of GBP is not mediated through an inhibition of the GAT-1 transporter and probably not through an increase in basal levels of GABA. The data also suggest that the combination of VGB and GBP may be clinically less effective than the use of GBP alone.

Effect of levetiracetam on epileptiform discharges in human neocortical slices

PURPOSE

The anticonvulsant effects of the novel antiepileptic drug (AED) levetiracetam (LEV) were tested in neocortical slice preparations from 23 patients who underwent surgery for the treatment of refractory epilepsy.

METHODS

Slices were used to evaluate the effects of LEV on two different models of epilepsy: low-Mg2+-induced untriggered and bicuculline-evoked stimulus-triggered epileptiform burst discharges and spontaneously appearing rhythmic sharp waves.

RESULTS

LEV (0.1-1 mM) did not influence spontaneously appearing rhythmic sharp waves or Mg2+-free aCSF-induced epileptiform field potentials. LEV affected neither the amplitudes or duration nor the repetition rates of burst discharges in these epilepsy models. However, LEV (100-500 microM) significantly suppressed the ictal-like discharges elicited by the gamma-aminobutyric acid subtype A (GABAA)-receptor antagonist bicuculline. A marked reduction of the amplitude and duration of bicuculline-evoked field response in the presence of LEV was observed.

CONCLUSIONS

The results indicate the potential for LEV to inhibit epileptiform burst discharges in human neocortical tissue, which is consistent with its effects in animal models of epilepsy. These results also support the seizure reduction observed in clinical trials and support that this may, in part, be related to the ability of LEV to inhibit epileptiform discharges.

Melatonin receptors in rat hippocampus: molecular and functional investigations

Since binding sites for melatonin have been found in the hippocampus of several mammals, it has been suggested that the pineal hormone melatonin is able to modulate neuronal functions of hippocampal cells. In order to get more insight into the role of melatonin for the functions of hippocampal cells, the following experiments were performed: male rats, maintained under a 12/12-h light-dark cycle, were sacrificed by decapitation at zeitgeber times (h) ZT2, ZT8, and ZT15 (ZT0 = lights on); for experiment 1, gene expression for melatonin receptors was detected in the hippocampus and in hippocampal subfields by means of the RT-PCR technique; for experiment 2, electrophysiological and pharmacological properties of melatonin receptors heterologously expressed in Xenopus oocytes after injection of mRNA from the hippocampus were analyzed by means of voltage clamp technique; and for experiment 3, effects of melatonin on the spontaneous firing rate of action potentials in the CA1 regions of hippocampal slices were analyzed by means of extracellular recordings. The RT-PCR data revealed that transcripts for both the MT1 and MT2 melatonin receptors are present in the dentate gyrus, CA3, and CA1 regions, and the subiculum of the hippocampus. Injection of mRNA from rat hippocampus into the Xenopus oocytes led to the functional reconstitution of melatonin-sensitive receptors, which activates calcium-dependent chloride inward currents. The melatonin responses were abolished by simultaneous administration of the antagonists 2-phenylmelatonin and luzindole, and were unaffected by the MT2 antagonist 4-phenyl-2-propionamidotetralin. Bath-applied melatonin (1 micromol/l) enhances the firing rate of neurons in the CA1 region. The effect was small in experiments performed at ZT8 (<2 times the initial level) and large in experiments performed at ZT15 (>6 times). The changes of neuronal firing rate induced by melatonin were completely suppressed with simultaneous administration of the melatonin receptor antagonist luzindole (10 micromol/l). The results indicate that melatonin may play an important role in modulating neuronal excitability in the hippocampus.

Vigabatrin reduces epileptiform activity in brain slices from pharmacoresistant epilepsy patients

Human neocortical temporal lobe tissue resected for treatment of pharmacoresistant epilepsy was investigated. In slices prepared from this tissue, epileptiform field potentials (EFP) were induced by omission of magnesium from the artificial cerebrospinal fluid (ACSF). The effects of the gamma-aminobutyric acid transaminase inhibitor vigabatrin on EFP were tested. Vigabatrin exerted a dose-dependent reduction of the repetition rate of EFP: after 3 h of administration of vigabatrin in concentrations of 100 and 200 micromol/l, the repetition rate of EFP was reduced to 35% and 18% of the initial values, respectively. This effect was not reversible. In control experiments with neocortical slices from rats, vigabatrin reduced EFP in a comparable range. The results demonstrate a strong antiepileptic effect of vigabatrin on EFP in tissues from pharmacoresistant epilepsy patients.

Acute effects of gamma-vinyl-GABA on low-magnesium evoked epileptiform activity in vitro

Vigabatrin (gamma-vinyl-GABA, VGB) is a gamma-aminobutyric acid (GABA) derivative designed to boost synaptic inhibition by inhibiting the degradation of GABA in brain tissue. Indeed, VGB shows potent anti-convulsant activity in animal models of epilepsy and in humans with complex partial seizures. However, details of the mechanism of action of VGB are not well understood and the systemic effects include possible pro-convulsant actions. We therefore analysed the effects of VGB in rat brain slices in the low-Mg(2+) model in vitro. VGB at 100 microM-5 mM showed a concentration- and time-dependent reduction of interictal-like events in the hippocampal CA1 region. Likewise, VGB suppressed epileptiform discharges in the medial entorhinal cortex (mEC), which are known to resist conventional anti-convulsants. In contrast, evoked population spikes in CA1 (which became repetitive after washout Mg(2+)) were not altered by VGB. Our data show that VGB is efficient against epileptiform discharges in temporal structures including pharmacoresistant patterns of activity. The waveform of evoked population spikes in this in vitro model is no indicator for the anti-convulsant properties of drugs.

The anticonvulsant retigabine potently suppresses epileptiform discharges in the low Ca ++ and low Mg++ model in the hippocampal slice preparation

Retigabine (N-(2-amino-4-(4-fluorobenzylamino)-phenyl) carbamic acid ethyl ester, D-23129) is a broad acting anticonvulsant currently undergoing phase II clinical trials. An opening effect on leakage conductance K+ channels, potentiation of GABA induced currents and a weak blocking effect on Na+ and Ca++ channels were previously reported. The goal of this study was to investigate whether retigabine is capable of blocking epileptiform discharges in the low Ca++ and low Mg++ model in the hippocampal slice preparations and whether the anti-burst activity can be related to the K+ channel opening effect. In the low Ca++ model, synaptic transmission is blocked and discharges evolve from ephaptically-coupled neurons. Compounds which directly interfere with the threshold for action potential induction via alteration of ion channel function (i.e. Na+ channel blocker) may alter the discharges, while compounds interfering with synaptic transmission are not active. Retigabine suppressed the discharges in a concentration-dependent manner. A significant reduction in frequency without effect on amplitude was observed after application of 1 microM, and a full block of all discharges after application of 25 microM. The opener of the ATP sensitive K+ channels cromakalim was also active. Application of 300 microM cromakalim yielded to a lower frequency with no effects on the amplitude of discharges. Treatment with phenytoin and carbamazepine resulted in a marked reduction in amplitude accompanied by a rise in frequency; only at higher concentrations was a full block observed. The effect of retigabine therefore differs from sodium channel blockers and can be related to the K+ channel opening effect. In the low Mg++ model, excitatory neurotransmission is augmented by reducing the Mg++ block of NMDA channels. This results in development of interictal-like epileptiform activity in area CA1 in isolated hippocampal slices. Treatment with retigabine 10 microM resulted in a significant reduction of the discharges, and discharges were fully blocked after application of 25 microM. Qualitatively similar effects were observed with cromakalim and valproate, albeit at higher concentrations. The data indicate that retigabine exerts potent broad spectrum activity making it an interesting candidate for treatment of drug resistant patients.

Inhibition of uptake, steady-state currents, and transient charge movements generated by the neuronal GABA transporter by various anticonvulsant drugs

1. We have expressed the GABA transporter (GAT1) of mouse brain in Xenopus oocytes and have investigated the effects of four antiepileptic drugs, tiagabine (TGB), vigabatrin (VGB), gabapentin (GBP) and valproate (VAL), on GAT1 transporter function by measurements of 3H-labelled GABA uptake and GAT1-mediated currents. 2. Not only TGB, a well-known inhibitor of GAT1-mediated transport, but also the other drugs efficiently inhibit the uptake of [3H]-GABA by GAT1. Inhibition at 50% is obtained for VGB, TGB, GBP, and VAL at concentrations of about 1 nM, 1 microM, 50 microM and 100 microM, respectively. 3. However, GAT1-mediated steady-state and transient currents are nearly unaffected by VGB, GBP, and VAL at even five times higher concentrations. Only TGB blocks the uptake and steady-state and transient currents at micromolar concentrations. 4. VGB exhibits a complex interaction with GAT1; at concentrations about 1 nM, the inhibition of uptake is released, but at millimolar concentrations the uptake is inhibited again, and also the GAT1-mediated current is finally inhibited at these concentrations with a KI value of 0.5 mM. The concentration dependency of inhibition of uptake can be explained by two interaction sites with different affinities, a blocking site and a transport site. 5. The differences in effects of VAL, GBP, and VGB on uptake and currents can be attributed to the fact that GAT1 has the capability to operate in an electrogenic mode without uptake of GABA. We suggest that inhibition occurs only when GAT1 operates in the GABA-uptake mode. 6. The inhibition of GABA uptake by these four drugs will result in an elevation of the GABA concentration in the synaptic cleft, which will enhance synaptic inhibition and thereby contribute to their antiepileptic effects.