Abstract

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 Ca_V channel activity in superior cervical ganglion neurons (SCGNs). In parallel, we investigated the effects of the inactive LEV R-enantiomer, (R)-α-ethyl-2-oxo-1-pyrrolidine acetamide (UCB L060). LEV but not UCB L060 (each at 100 μM) 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.