Abstracts

Rationale: Previous studies have reported that the antiepileptic drug levetiracetam (LEV) bound to the SV2A protein located on synaptic vesicles (Lynch BA et al., 2004). However, the ubiquitous CNS expression of SV2A in all synaptic vesicles is inconsistent with the discrete binding regions seen observed for [³H]LEV binding (Fuks, B et al., 2003). Also, the SV2A protein is present in neuroendocrine cells and neuromuscular junctions (Dong, M et al., 2006), but [³H]LEV does not bind to these sites (Fuks, B et al., 2003). Protein tomography showed no conformational change in the SV2A protein when the protein was incubated with LEV (Lynch, BA et al., 2008). SV2A allelic variants showed no influence on predisposition to epilepsy or clinical outcomes (Lynch, JM et al., 2009). Collectively, these studies suggest that LEV produces its antiepileptic effects by binding to a protein other than the SV2A protein. Methods: Receptor binding assays were performed with rat brain membranes and [³H]LEV in 50 mM Tris-HCl, pH 7.5, at 4°C for 4 hr. For some experiments, anions, which stimulated [³H]LEV binding were included. Competition experiments were performed with many CNS drugs. Results: Anions increased [³H]LEV binding proportionally relative to their molecular weight. This anionic stimulation of binding has been observed with AMPA glutamate receptor modulators. Five allosteric AMPA modulators (BCP-1, CX 546, cyclothiazide, doxycycline, IDRA21) inhibited [³H]LEV binding in a concentration-dependent manner. AMPA receptor and kainate receptor agonists and antagonists had no effect on [³H]LEV binding. Conclusions: These results demonstrate that LEV binds to an allosteric modulator site on the AMPA glutamate receptor family. Modulating the function of AMPA receptors provides a more rational explanation of the antiepileptic effects observed with LEV than the SV2A protein does.