Suppression of <i>in vitro</i> epileptiform activity by GABA transaminase inhibition is dependent on GABA _B receptors and GABA transporter type 1 (GAT1)

Many antiepileptic drugs act by modulation of ionotropic GABA_A receptors or effects on extracellular GABA concentrations. Metabotropic GABA_B receptors also affect neuronal excitability, but are not recognized as a target of antiepileptic drugs. We investigated effects of GABA_B receptors and antiepileptic drug vigabatrin on epileptiform discharges induced by 4-aminopyridine (4AP) in hippocampal brain slices from male mice. The GABA_B receptor agonist baclofen caused dose-dependent reduction in frequency of 4AP-induced epileptiform discharges, but did not affect amplitude or duration of discharges. The inhibitory effects of baclofen were blocked by GABA_B receptor antagonist CGP55845 (CGP) and the K⁺ channel blocker Ba²⁺, indicating that baclofen was acting on GABA_B receptors and activating GIRK channels. Baclofen effects were independent of GABA_A receptors. Vigabatrin inhibits GABA transaminase, thereby increasing GABA concentrations. Pretreatment of brain slices with vigabatrin suppressed 4AP-induced discharges, significantly prolonging the latency to onset of spontaneous activity and reducing frequency of discharges. Similar to baclofen, vigabatrin effects on latency and frequency were reversed by GABA_B antagonist CGP and the GIRK channel blocker Ba²⁺ Vigabatrin had no effect on 4AP-induced activity in GABA transporter type 1 knockout mice (GAT1KO), suggesting that vigabatrin effects are dependent on GAT1 function. Our results indicate for the first time that a clinically-used antiepileptic drug (vigabatrin) suppresses <i>in vitro</i> epileptiform activity via activation of GABA_B receptors and GIRK channels. Data are consistent with a model that depicts altered thermodynamic equilibrium of GAT1 after GABA transaminase inhibition that leads to elevated extracellular GABA concentration and activation of GABA_B receptors.<b>Significance Statement</b> GABA receptor subtypes are ubiquitously expressed in central nervous system neurons and are important therapeutic targets for neuropsychiatric disease. Therapeutic mechanisms include modulation of ionotropic GABA_A receptors for treatment of anxiety and seizures and activation of GABA_B receptors for treatment of spasticity. Although GABA_B receptors have potent effects on neuronal excitability, they are not recognized targets of antiepileptic drugs. Our data demonstrate that suppression of <i>in vitro</i> epileptiform activity (4-aminopyridine model) after GABA transaminase inhibition with the antiepileptic drug vigabatrin is dependent on GABA_B receptors and GIRK channels. GABA_B receptor activation is hypothesized to contribute to vigabatrin's clinical efficacy as an antiepileptic drug.