The Synaptic Vesicle Glycoprotein 2A Ligand Levetiracetam Inhibits Presynaptic Ca2+ Channels through an Intracellular Pathway

Levetiracetam (LEV) is a prominent antiepileptic drug that binds to neuronal synaptic vesicle glycoprotein 2A protein and has reported effects on ion channels, but with a poorly defined mechanism of action. We investigated inhibition of voltage-dependent Ca2+ (Ca-V) channels as a potential mechanism through which LEV exerts effects on neuronal activity. We used electrophysiological methods to investigate the effects of LEV on cholinergic synaptic transmission and CaV channel activity in superior cervical ganglion neurons (SCGNs). In parallel, we investigated the effects of the inactive LEV R-enantiomer, (R)-alpha-ethyl-2-oxo-1-pyrrolidine acetamide (UCB L060). LEV but not UCB L060 (each at 100 mu M) inhibited synaptic transmission between SCGNs in long-term culture in a time-dependent manner, significantly reducing excitatory postsynaptic potentials after a >= 30-min application. In isolated SCGNs, LEV pretreatment (>= 1 h) but not short-term application (5 min) significantly inhibited whole-cell Ba2+ current (I-Ba) amplitude. In current-clamp recordings, LEV reduced the amplitude of the afterhyperpolarizing potential in a Ca2+-dependent manner but also increased the action potential latency in a Ca2+-independent manner, which suggests additional mechanisms associated with reduced excitability. Intracellular LEV application (4-5 min) caused rapid inhibition of I-Ba amplitude, to an extent comparable to that seen with extracellular LEV pretreatment (>= 1 h). Neither pretreatment nor intracellular application of UCB L060 produced any inhibitory effects on I-Ba amplitude. These results identify a stereospecific intracellular pathway through which LEV inhibits presynaptic Ca-V channels; resultant reductions in neuronal excitability are proposed to contribute to the anticonvulsant effects of LEV.