Abstracts

Rationale: AMPA receptors, which play a critical role in the generation and spread of epileptic activity, are tetrameric combinations of four subunits (GluR1–4). The GluR2 subunit controls Ca2+ permeability, and AMPA receptors lacking GluR2(R) (the Q-to-R RNA-edited variant), are Ca2+ permeable. In normal brains, GluR2(R)-containing AMPA receptors are predominant, while Ca2+-permeable AMPA receptors are upregulated under pathological conditions such as status epilepticus. PER, a selective noncompetitive AMPA receptor antagonist, has been reported to exert equipotent inhibition of Ca2+-permeable and -impermeable AMPA receptors (Barygin OI, Neurosci Lett. 2016;633;146-151). Its subtype selectivity, however, is still not understood. In addition to PER, some other antiepileptic drugs (AEDs) such as topiramate (TPM) have also been reported to inhibit AMPA receptors (Sills GJ. “Mechanism of action in of AEDs” in Epilepsy 2011: From Science to Society. A Practical Guide to Epilepsy. ILAE, National Society for Epilepsy, London, UK, 2011). However, it is not clear whether these AEDs truly inhibit AMPA receptors directly; for example, in the case of TPM, there are reports suggesting that it affects AMPA receptors indirectly (Angehagen M, et al. J Neurochem. 2004;88;320-325). To investigate these points, we performed electrophysiological assessments using AMPA, NMDA and kainate receptor-expressing cells. Methods: We established AMPA receptor-expressing cells (hGluR1–4, with two kinds of Q/R RNA-edited variants of hGluR2), an NMDA receptor-expressing cell (hNR1/hNR2B), and a kainate receptor-expressing cell (hGluK2). We evaluated the effects of PER and other AEDs, including TPM, phenobarbital (PB), lamotrigine (LTG), gabapentin (GBP), carbamazepine (CBZ), valproate (VPA), levetiracetam (LEV), and lacosamide (LCM), on agonist-induced current amplitude in the cells, by automated patch-clamp technique with QPatch HTX system. AMPA current was elicited by AMPA in the presence of the receptor-desensitization inhibitor, cyclothiazide. NMDA with glycine and glutamate was used as a stimulant for the NMDA and kainate receptor assays, respectively. Results: PER inhibited AMPA-induced current amplitude in cells expressing each AMPA receptor subtype, in a concentration-dependent manner, but did not inhibit NMDA and kainate currents. Among the other AEDs tested, only PB at a high concentration showed an inhibitory effect on hGluR2(R). In addition, high concentrations of LTG and CBZ had inhibitory effects on hNR1/hNR2B. The 50% inhibitory concentration (IC₅₀) values for each AED are summarized in Table 1. Conclusions: In the present study, we demonstrated for the first time that PER had potent inhibitory effects on all the AMPA receptor subtypes, and confirmed that it did not inhibit NMDA and kainate receptors. In contrast, such potent inhibition was not observed with the other AEDs tested, suggesting that PER is the only AED that affects AMPA receptor functions selectively and directly. Funding: Eisai Co., Ltd.