Activity-dependent enhancement of hyperpolarizing and depolarizing gamma-aminobutyric acid (GABA) synaptic responses following inhibition of GABA uptake by tiagabine

Recording of pig neuronal activity in the comparative context of the awake human brain

Gyriform mammals display neurophysiological and neural network activity that other species exhibit only in rudimentary or dissimilar form. However, neural recordings from large mammals such as the pig can be anatomically hindered and pharmacologically suppressed by anesthetics. This curtails comparative inferences. To mitigate these limitations, we set out to modify electrocorticography, intracerebral depth and intracortical recording methods to study the anesthetized pig. In the process, we found that common forms of infused anesthesia such as pentobarbital or midazolam can be neurophysiologic suppressants acting in dose-independent fashion relative to anesthetic dose or brain concentration. Further, we corroborated that standard laboratory conditions may impose electrical interference with specific neural signals. We thus aimed to safeguard neural network integrity and recording fidelity by developing surgical, anesthesia and noise reduction methods and by working inside a newly designed Faraday cage, and evaluated this from the point of view of neurophysiological power spectral density and coherence analyses. We also utilized novel silicon carbide electrodes to minimize mechanical disruption of single-neuron activity. These methods allowed for the preservation of native neurophysiological activity for several hours. Pig electrocorticography recordings were essentially indistinguishable from awake human recordings except for the small segment of electrical activity associated with vision in conscious persons. In addition, single-neuron and paired-pulse stimulation recordings were feasible simultaneously with electrocorticography and depth electrode recordings. The spontaneous and stimulus-elicited neuronal activities thus surveyed can be recorded with a degree of precision similar to that achievable in rodent or any other animal studies and prove as informative as unperturbed human electrocorticography.

Tiagabine-related status epilepticus: a case report and systematic literature review

Tiagabine-related status epilepticus (SE) is an uncommon complication of tiagabine use. We aimed to detail the features and outcomes in a patient with tiagabine poisoning and review the relevant literature. We describe a case of tiagabine-related SE and literature review based on a 1995-2019 PubMed search. We report the case of a 30-year-old man with super-refractory SE after tiagabine poisoning. He fully recovered after 72 h of general anesthesia and was discharged from the ICU on day 16. A literature review showed distinct features among patients with tiagabine-related SE. Tiagabine side effects were characterized by non-convulsive SE after a slight increase in tiagabine dose and a rapid favorable evolution after benzodiazepine and early tiagabine withdrawal. Generalized convulsive SE was a complication of voluntary or involuntary tiagabine poisoning and was particularly refractory. Both presentations are characterized by a return to baseline after prompt and adequate management. Tiagabine-related SE electroclinical features vary according to the underlying pathophysiological mechanism and can be life threatening. Recovery is the rule after tiagabine withdrawal and SE management with progressive therapeutic escalation guided by response to prior anticonvulsant treatments.

Effects of antiepileptic drugs on cortical excitability in humans: A TMS-EMG and TMS-EEG study

Brain responses to transcranial magnetic stimulation (TMS) recorded by electroencephalography (EEG) are emergent noninvasive markers of neuronal excitability and effective connectivity in humans. However, the underlying physiology of these TMS-evoked EEG potentials (TEPs) is still heavily underexplored, impeding a broad application of TEPs to study pathology in neuropsychiatric disorders. Here we tested the effects of a single oral dose of three antiepileptic drugs with specific modes of action (carbamazepine, a voltage-gated sodium channel (VGSC) blocker; brivaracetam, a ligand to the presynaptic vesicle protein VSA2; tiagabine, a gamma-aminobutyric acid (GABA) reuptake inhibitor) on TEP amplitudes in 15 healthy adults in a double-blinded randomized placebo-controlled crossover design. We found that carbamazepine decreased the P25 and P180 TEP components, and brivaracetam the N100 amplitude in the nonstimulated hemisphere, while tiagabine had no effect. Findings corroborate the view that the P25 represents axonal excitability of the corticospinal system, the N100 in the nonstimulated hemisphere propagated activity suppressed by inhibition of presynaptic neurotransmitter release, and the P180 late activity particularly sensitive to VGSC blockade. Pharmaco-physiological characterization of TEPs will facilitate utilization of TMS-EEG in neuropsychiatric disorders with altered excitability and/or network connectivity.

Divergent effects of levetiracetam and tiagabine against spontaneous seizures in adult rats following neonatal hypoxia

Animal models are valuable tools for screening novel therapies for patients who suffer from epilepsy. However, a wide array of models are necessary to cover the diversity of human epilepsies. In humans, neonatal hypoxia (or hypoxia-ischemia) is one of the most common causes of epilepsy early in life. Hypoxia-induced seizures (HS) during the neonatal period can also lead to spontaneous seizures in adulthood. This phenomenon, i.e., early-life hypoxia leading to adult epilepsy - is also seen in experimental models, including rats. However, it is not known which anti-seizure medications are most effective at managing adult epilepsy resulting from neonatal HS. Here, we examined the efficacy of three anti-seizure medications against spontaneous seizures in adult rats with a history of neonatal HS: (1) phenobarbital (PHB), the oldest epilepsy medicine still in use today; (2) levetiracetam (LEV); and (3) tiagabine (TGB). Both LEV and TGB are relatively new anticonvulsant drugs that are ineffective in traditional seizure models, but strikingly effective in other models. We found that PHB and LEV decreased seizures in adult rats with a history of HS, whereas TGB exacerbated seizures. These divergent drug effects indicate that the HS model may be useful for differentiating the clinical efficacy of putative epilepsy therapies.

Impairment of GABA transporter GAT-1 terminates cortical recurrent network activity via enhanced phasic inhibition

In the central nervous system, GABA transporters (GATs) very efficiently clear synaptically released GABA from the extracellular space, and thus exert a tight control on GABAergic inhibition. In neocortex, GABAergic inhibition is heavily recruited during recurrent phases of spontaneous action potential activity which alternate with neuronally quiet periods. Therefore, such activity should be quite sensitive to minute alterations of GAT function. Here, we explored the effects of a gradual impairment of GAT-1 and GAT-2/3 on spontaneous recurrent network activity – termed network bursts and silent periods – in organotypic slice cultures of rat neocortex. The GAT-1 specific antagonist NO-711 depressed activity already at nanomolar concentrations (IC₅₀ for depression of spontaneous multiunit firing rate of 42 nM), reaching a level of 80% at 500–1000 nM. By contrast, the GAT-2/3 preferring antagonist SNAP-5114 had weaker and less consistent effects. Several lines of evidence pointed toward an enhancement of phasic GABAergic inhibition as the dominant activity-depressing mechanism: network bursts were drastically shortened, phasic GABAergic currents decayed slower, and neuronal excitability during ongoing activity was diminished. In silent periods, NO-711 had little effect on neuronal excitability or membrane resistance, quite in contrast to the effects of muscimol, a GABA mimetic which activates GABA_A receptors tonically. Our results suggest that an enhancement of phasic GABAergic inhibition efficiently curtails cortical recurrent activity and may mediate antiepileptic effects of therapeutically relevant concentrations of GAT-1 antagonists.

Synaptic activation of GABA(B) receptors regulates neuronal network activity and entrainment

In the mammalian central nervous system, GABA(B) receptors mediate slow pre- and postsynaptic inhibition. Using rat hippocampal slices we investigated the role of synaptic GABA(B) receptors in regulating kainate-induced subthreshold neuronal network oscillations in the gamma frequency range (25-80 Hz). The GABA(B) receptor agonist baclofen largely eliminated gamma oscillations. The GABA(B) receptor antagonist CGP55845 reversed this action of baclofen but alone did not alter the power or frequency of ongoing oscillations. To examine the role of synaptically released GABA on network activity, we electrically stimulated stratum radiatum of CA3 whilst recording gamma oscillations from stratum pyramidale. Single stimuli produced a pronounced transient (up to 1 s in duration) inhibition of gamma frequency oscillations. This stimulus-induced shutdown of network activity was enhanced by the GABA uptake inhibitor tiagabine and largely inhibited by CGP55845. Multiple stimuli delivered at frequencies of 1-3 Hz resulted in an activity-dependent fatigue of the inhibition of gamma activity, such that, after a number of stimuli, oscillations could be detected tens of milliseconds after the stimulus. Interestingly, this activity-dependent fatigue of inhibition uncovered a stimulus-dependent temporal entrainment of the gamma oscillations. Furthermore, the amount of repetitive synaptic input that was required to cause this entrainment was dramatically reduced by GABA(B) receptor antagonism such that it was evident within just a few stimuli. These data suggest that convergent afferent synaptic activity can alter the precise temporal arrangement of neuronal network activity. Furthermore, the flow of such information into a functioning neuronal network is highly regulated by GABA(B) receptor-mediated synaptic inhibition.

Inhibition of gamma-aminobutyric acid uptake: anatomy, physiology and effects against epileptic seizures

The transport of gamma-aminobutyric (GABA) limits the overspill from the synaptic cleft and serves to maintain a constant extracellular level of GABA. Two transporters, GABA transporter-1 (GAT-1) and GAT-3, are the most likely candidates for regulating GABA transport in the brain. Drugs acting either selectively or nonselectively at GATs exert distinct anticonvulsant effects, presumably because of distinct regions of action. Here I shall give a brief review of the localization and physiology of GATs and describe effects of selective and nonselective inhibitors thereof in different animal models of epilepsy.

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.

Unaltered control of extracellular GABA-concentration through GAT-1 in the hippocampus of rats after pilocarpine-induced status epilepticus

The uptake of the inhibitory transmitter GABA (gamma-aminobutyric acid) limits the efficacy of synaptic and tonic inhibition in brain tissue. It has been reported that GABA-uptake is down-regulated in temporal lobe epilepsy. This down-regulation may increase the inhibitory action of GABA but may also limit the anticonvulsant activity of GABA-uptake blockers. We have directly compared the function of GABA-uptake in hippocampal slices from normal and chronically epileptic rats. We raised the global extracellular concentration of GABA by bath-application of the agonist in the absence and presence of the GABA-uptake blocker tiagabine. GABA-induced currents were measured in dentate granule cells and CA1 pyramidal neurons in hippocampal slices. The potentiation of currents by tiagabine was taken as a measure for the efficacy of GABA-uptake in the hippocampal tissue. There was no difference between cells from control- or pilocarpine-treated animals in the response to GABA or in the conductance increase following application of tiagabine. Our data show that in the chronic phase of the pilocarpine-model GABA-uptake maintains its ability to control the extracellular background concentration of GABA.

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 release of endogenous gamma-aminobutyric acid by cannabinoids in the guinea pig ileum

Interactions between cannabinoid CB(1) and GABA receptors and ligands were investigated in the myenteric plexus-longitudinal muscle of the guinea pig ileum. Electrically evoked contractions of the myenteric plexus-longitudinal muscle were inhibited by the cannabinoid receptor agonist CP55,940 ((-)-cis-3-[2-Hydroxy-4-(1,1-dimethylheptyl) phenyl]-trans-4-(3-hydroxypropyl) cyclohexanol), the GABA(B) receptor agonist, baclofen (4-amino-3-(chlorophenyl) butanoic acid), or exogenous GABA. Electrically evoked contractions of the myenteric plexus-longitudinal muscle were also inhibited by the addition of the GABA releasing agent ethylenediamine. CP55,940 (1 nM) or the endogenous cannabinoid anandamide (arachidonyl ethanolamide, 1 microM) reduced the inhibition produced by ethylenediamine, while in contrast, anandamide (10 microM) significantly increased the inhibition produced by ethylenediamine. The results suggest that while there is no interaction between cannabinoid CB(1) and GABA(B) receptors in the myenteric plexus-longitudinal muscle of the guinea pig, cannabinoid CB(1) receptor stimulation reduces the ethylenediamine-evoked GABA release. In addition, anandamide at higher concentrations also potentiates the inhibitory effect of ethylenediamine at least partly by stimulating vanilloid receptors.

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.

Seizures and neurodegeneration induced by 4-aminopyridine in rat hippocampus in vivo: role of glutamate- and GABA-mediated neurotransmission and of ion channels

Infusion of the K(+) channel blocker 4-aminopyridine in the hippocampus induces the release of glutamate, as well as seizures and neurodegeneration. Since an imbalance between excitation and inhibition, as well as alterations of ion channels, may be involved in these effects of 4-aminopyridine, we have studied whether they are modified by drugs that block glutamatergic transmission or ion channels, or drugs that potentiate GABA-mediated transmission. The drugs were administered to anesthetized rats subjected to intrahippocampal infusion of 4-aminopyridine through microdialysis probes, with simultaneous collection of dialysis perfusates and recording of the electroencephalogram, and subsequent histological analysis. Ionotropic glutamate receptor antagonists clearly diminished the intensity of seizures and prevented the neuronal damage, but did not alter substantially the enhancement of extracellular glutamate induced by 4-aminopyridine. None of the drugs facilitating GABA-mediated transmission, including uptake blockers, GABA-transaminase inhibitors and agonists of the A-type receptor, was able to reduce the glutamate release, seizures or neuronal damage produced by 4-aminopyridine. In contrast, nipecotate, which notably increased extracellular levels of the amino acid, potentiated the intensity of seizures and the neurodegeneration. GABA(A) receptor antagonists partially reduced the extracellular accumulation of glutamate induced by 4-aminopyridine, but did not exert any protective action. Tetrodotoxin largely prevented the increase of extracellular glutamate, the electroencephalographic epileptic discharges and the neuronal death in the CA1 and CA3 hippocampal regions. Valproate and carbamazepine, also Na(+) channel blockers that possess general anticonvulsant action, failed to modify the three effects of 4-aminopyridine studied. The N-type Ca(2+) channel blocker omega-conotoxin, the K(+) channel opener diazoxide, and the non-specific ion channel blocker riluzole diminished the enhancement of extracellular glutamate and slightly protected against the neurodegeneration. However, the two former compounds did not antagonize the 4-aminopyridine-induced epileptiform discharges, and riluzole instead markedly increased the intensity and duration of the disharges. Moreover, at the highest dose tested (8mg/kg, i.p.), riluzole caused a 75% mortality of the rats. We conclude that 4-aminopyridine stimulates the release of glutamate from nerve endings and that the resultant augmented extracellular glutamate is directly related to the neurodegeneration and is involved in the generation of epileptiform discharges through the concomitant overactivation of glutamate receptors. Under these conditions, a facilitated GABA-mediated transmission may paradoxically boost neuronal hyperexcitation. Riluzole, a drug used to treat amyotrophic lateral sclerosis, seems to be toxic when combined with neuronal hyperexcitation.

Frontal nonconvulsive status epilepticus associated with high-dose tiagabine therapy in a child with familial bilateral perisylvian polymicrogyria

PURPOSE

Antiepileptic drugs are known to exacerbate absence and myoclonic seizures, especially in patients with idiopathic generalized epilepsies. Exacerbation of nonconvulsive generalized seizures in patients with partial epilepsy is less common. Recently, however, a number of cases of putative generalized nonconvulsive status epilepticus (NCSE) or NCSE without further specification have been reported in patients with chronic partial epilepsy treated with the gamma-aminobutyric acid reuptake inhibitor tiagabine. Although complex partial status epilepticus during tiagabine therapy has also been reported, possible precipitation of NCSE specifically associated with frontal lobe discharges does not appear to have been recognized. In this communication, we describe the case of a boy with familial bilateral perisylvian polymicrogyria who developed frontal NCSE after being stabilized on high-dose tiagabine

METHODS

A 12-year-old boy with familial bilateral perisylvian polymicrogyria, mental retardation, and refractory partial seizures was administered tiagabine in addition to sodium valproate. The tiagabine dosage was increased gradually up to 10 mg t.i.d. (1 mg/kg per day), resulting in complete seizure control.

RESULTS

After 1 week on maintenance treatment, seizures were completely controlled, but the child developed hypoactivity, decreased reactivity, and affective detachment. An EEG recording revealed subcontinuous sharp-wave discharges with irregular runs of atypical spike-wave complexes over the anterior regions of both hemispheres, consistent with a diagnosis of frontal NCSE. A reduction in tiagabine dosage to 15 mg/day led to complete regression of the behavioral and affective changes and to disappearance of the subcontinuous EEG discharges.

CONCLUSIONS

Although tiagabine-induced NCSE has been described previously, particularly in patients with preexisting spike-wave abnormalities, this is the first report that identifies its potential role in the precipitation of frontal NCSE.

Flumazenil prevents diazepam-elicited anticonvulsant action and concomitant attenuation of glutamate overflow

Systemic administration of diazepam (5 mg/kg, i.p.) produced a prompt anticonvulsant effect in pilocarpine-induced seizures in freely moving rats. The anticonvulsant effect was associated with significant attenuation of pilocarpine-evoked increases in extracellular hippocampal glutamate levels to below the baseline levels. The purpose of the present microdialysis study, therefore, was to investigate if the effect of diazepam on glutamate release was mediated at the level of the benzodiazepine gamma-aminobutyric acid(A) (GABA(A)) receptor complex to preclude any non-GABAergic mechanisms. Systemic administration of the specific benzodiazepine-receptor antagonist flumazenil (10 mg/kg, i.p. )-elicited complete reversal of diazepam-evoked anticonvulsant action and concomitant attenuation of extracellular glutamate efflux below the baseline levels. This provides evidence that under the given experimental conditions, diazepam-evoked alterations in glutamate overflow associated with the anticonvulsant action were indeed mediated at the level of benzodiazepine-GABA(A) receptor complex, possibly involving the modulation of both pre- and post-synaptic sites of the receptor complex.