Use-Dependent Inhibition of the N-Methyl-D-aspartate Currents by Felbamate: a Gating Modifier with Selective Binding to the Desensitized Channels

Abstract

Felbamate (FBM) is a potent nonsedative anticonvulsant whose clinical effect may be related to the inhibition of N-methyl-Daspartate (NMDA) currents, but the exact molecular action remains unclear. Using whole-cell patch-clamp recording in rat hippocampal neurons, we found that submillimolar FBM effectively modifies the gating process of NMDA channels. During a single high-concentration (1 mM) NMDA pulse, FBM significantly inhibits the late sustained current but not the early peak current. However, if the 1 mM NMDA pulse is preceded by a low-concentration (10 M) NMDA prepulse, then FBM significantly inhibits both the peak and the sustained currents in the 1 mM pulse. In sharp contrast, the NMDA currents elicited by micromolar NMDA are only negligibly inhibited or even enhanced by FBM. These findings indicate that the inhibitory effect of FBM on NMDA currents is stronger with both higher NMDA concentration and longer NMDA exposure, and is thus "use-dependent". FBM also slows recovery of the desensitized NMDA channel, and quantitative analyses of FBM effects on the activation kinetics and the desensitization curve of the NMDA currents further disclose dissociation constants of ϳ200, ϳ110, and ϳ55 M for FBM binding to the resting, activated, and desensitized NMDA channels, respectively. We conclude that therapeutic concentrations (50 -300 M) of FBM could bind to and modify a significant proportion of the resting NMDA channel even when NMDA or other glutamatergic ligand is not present and then decrease the NMDA currents at subsequent NMDA pulses by stabilization of the desensitized channels. Because the inhibitory effect is apparent only when there is excessive NMDA exposure, FBM may effectively inhibit many seizure discharges but preserve most normal neuronal firings.Felbamate (FBM; 2-phenyl-1,3-propanediol dicarbamate) is a potent new-generation anticonvulsant that is effective against many different types of epilepsy (Pellock and Brodie, 1997). In addition to clinical cases, the broad-spectrum antiepileptic effect is also well documented in experimental seizures. For example, FBM is effective against both supramaximal extension seizures induced by maximal electroshock and threshold seizures induced by pentylenetetrazol (Swinyard et al., 1986). Although serious complications such as aplastic anemia and hepatotoxicity have limited its use, FBM is an anticonvulsant which is too important to discard. With informed consent of the patients, FBM has remained as a useful anticonvulsant for Lennox-Gastaut syndrome in children and for a variety of complex partial seizures that are refractory to the other anticonvulsants in adults (Kaufman et al., 1997;Pellock, 1999).The intriguing pharmacological profile of FBM implies a unique mechanism of action. Just as for the other nonsedative anticonvulsants, any proposed mechanism underlying FBM action preferably should explain why seizure discharges are effectively inhibited but normal neuronal firings are relatively preserved. FBM has been reported to have multiple pharmac...

The Activation Gate and Gating Mechanism of the NMDA Receptor

“…Double-mutant cycle analysis of A652(NR1) and A651(NR2B) mutations consistent with the ideas that the foregoing residues are located in the external vestibule of the pore and are closely related to the activation gating control of the channel (see Introduction). In this regard, it is interesting to note that felbamate, the only approved anticonvulsant with significant gating-modification effects on NMDA receptors (Kuo et al, 2004), also binds to this region and acts as an "opportunistic" pore blocker with the "entrance" of its binding site undergoing a large gating conformational change Kuo, 2007a,b, 2008). Moreover, MT-SEA and MTSET modification of M3c (NR2B) is ϳ500-to 1000-fold faster in the activated than in the closed channels for the positions below A648 -A651 (Fig.…”

Molecular Targets for Antiepileptic Drug Development

“…272,273 The drug acts as an allosteric modulator of channel gating, 274 with a preferential action to stabilize the inactivated state so as to produce a usedependent block. 275 This use-dependent action may selectively inhibit NMDA receptors that are excessively activated as is believed to occur during seizure discharges.…”

Diverse mechanisms of antiepileptic drugs in the development pipeline

“…However, this effect appears to be specific for GABA A receptors containing α1 or α2 subunits along with β2 or β3 subunits (Simeone and McClellan, 2000). In addition, felbamate at a concentration of 100 μM has been found to block NMDA receptor mediated synaptic responses (Pugliese et al, 1996) and to inhibit NMDA receptor currents in isolated neurons (K d , ~1 mM; Rho et al, 1994;Subramaniam et al, 1995;Kuo et al, 2004). Studies with recombinant NMDA receptor subunit combinations have indicated that felbamate selectively bocks NMDA receptors composed of NR2B subunits at lower concentrations (IC 50 , 0.5 mM), than other subunit combinations Harty and Rogawski, 2000).…”