Abstracts

Rationale: OV329 is a highly potent GABA-amino transferase (GABA-AT) inhibitor currently in clinical stages, showing potential as an antiepileptic drug with a mechanism of action akin to vigabatrin. GABA, the primary inhibitory neurotransmitter in the adult central nervous system (CNS), is broken down by GABA-AT. Inhibiting GABA-AT can increase GABA levels, potentially reducing the neuronal hyperexcitability linked to seizures and epilepsy. Therefore, it's crucial to expand our understanding of the functional consequences of GABA-AT inhibition and the significance of elevated GABA at the cellular level. Here, we contrast the GABA-AT inhibition resulting from acute treatment with OV329 and vigabatrin in ex vivo brain slices of mice and their effects on both phasic and tonic GABAergic transmission in the hippocampal dentate gyrus, a brain region closely associated with temporal lobe epilepsy. Mechanistic insights from this research could guide the clinical application and optimal use of OV329.


Methods: We investigated the effects of OV329 and vigabatrin on both forms of ionotropic GABAergic inhibition using whole-cell patch-clamp electrophysiology in the Dentate Gyrus Granule Cell (DGGC) layer of six- to eight-week-old male C57Bl6/J mice. This study employed acute hippocampal brain slices incubated for one hour with either PBS (vehicle), 0.01, 0.25, 1, 10 and 100 μM OV329, or 1, 10, 100, 1000, and 10000 μM vigabatrin to determine GABA-AT enzyme inhibition. Electrophysiological recordings were performed in DGGCs following a one-hour incubation in vehicle, OV329 or vigabatrin in a subset of the above concentrations, capturing spontaneous inhibitory postsynaptic currents (sIPSCs) before applying picrotoxin to block GABA_A receptors. This allowed us to measure the shift in holding current due to the blockade of extrasynaptic GABA_A receptors.


Results: Acute inhibition of GABA-AT by OV329 and vigabatrin resulted in an IC₅₀ of 205 nM for OV329 and 454 μM for vigabatrin. Following a one-hour incubation, we show OV329 significantly enhances tonic inhibition at 5 and 10 μM, while tonic inhibition is elevated at 10 μM and significantly increased at 100 μM with vigabatrin. Both vigabatrin and OV329 induce a significant left-shift in the cumulative distribution of the sIPSC interevent-intervals at the above concentrations, however only OV329 at the 10 μM concentration induces a right-shift in the cumulative distribution of sIPSC amplitudes.


Conclusions: Results from this study suggest GABA-AT inhibition is a mechanism by which both phasic and tonic inhibition can be altered. Interestingly, we observed acute enhancement of tonic inhibition from vigabatrin at concentrations lower than our calculated IC₅₀, suggesting there may be some non-GABA-AT based mechanism by which vigabatrin may be acutely acting. In contrast, OV329 shaped phasic and tonic GABA signaling only at concentrations higher than our calculated IC₅₀.

Funding: SJM is supported by NINDS grants NS087662, NS081986, NS108378, NS101888, NS103865, and NS111338