Excitoprotective effect of felbamate in cultured cortical neurons

Characterization of the gating conformational changes in the felbamate binding site in NMDA channels

The anticonvulsant effect of felbamate (FBM) is ascribable to inhibition of N-methyl-d-aspartate (NMDA) currents. Using electrophysiological studies in rat hippocampal neurons to examine the kinetics of FBM binding to and unbinding from the NMDA channel, we show that FBM modifies NMDA channel gating via a one-to-one binding stoichiometry and has quantitatively the same enhancement effect on NMDA and glycine binding to the NMDA channel. Moreover, the binding rates of FBM to the closed and the open/desensitized NMDA channels are 187.5 and 4.6 x 10(4) M(-1) s(-1), respectively. The unbinding rates of FBM from the closed and the open/desensitized NMDA channels are approximately 6.2 x 10(-2) and approximately 3.1 s(-1), respectively. From the binding and unbinding rate constants, apparent dissociation constants of approximately 300 and approximately 70 microM could be calculated for FBM binding to the closed and the open/desensitized NMDA channels, respectively. The slight (approximately fourfold) difference in FBM binding affinity to the closed and to the open/desensitized NMDA channels thus is composed of much larger differences in the binding and unbinding kinetics (approximately 250- and approximately 60-fold difference, respectively). These findings suggest that the effects of NMDA and glycine binding coalesce or are interrelated before or at the actual activation gate, and FBM binding seems to modulate NMDA channel gating at or after this coalescing point. Moreover, the entrance zone of the FBM binding site very likely undergoes a much larger conformational change along the gating process than that in the binding region(s) of the binding site. In other words, the FBM binding site becomes much more accessible to FBM with NMDA channel activation, although the spatial configurations of the binding ligand(s) for FBM themselves are not altered so much along the gating process.

Suppression of caspase-3-dependent proteolytic activation of protein kinase C delta by small interfering RNA prevents MPP+-induced dopaminergic degeneration

The cellular mechanisms underlying the neurodegenerative process in Parkinson's disease are not well understood. Using RNA interference (RNAi), we demonstrate that caspase-3-dependent proteolytic activation of protein kinase Cdelta (PKCdelta) contributes to the degenerative process in dopaminergic neurons. The Parkinsonian toxin MPP(+) activated caspase-3 and proteolytically cleaved PKCdelta into catalytic and regulatory subunits, resulting in persistent kinase activation in mesencephalic dopaminergic neuronal cells. The caspase-3 inhibitor Z-DEVD-FMK and the caspase-9 inhibitor Z-LEHD-FMK effectively blocked MPP(+)-induced PKCdelta proteolytic activation. To characterize the functional role of PKCdelta activation in MPP(+)-induced dopaminergic cell death, RNAi-mediated gene knockdown was performed. Among four siRNAs designed against PKCdelta, two specifically suppressed PKCdelta expression. The application of siRNA abolished the MPP(+)-induced PKCdelta activation, DNA fragmentation, and tyrosine hydroxylase (TH)-positive neuronal loss. Together, these results suggest that proteolytic activation of PKCdelta may be a critical downstream event in the degenerative process of Parkinson's disease.

Use-dependent inhibition of the N-methyl-D-aspartate currents by felbamate: a gating modifier with selective binding to the desensitized channels

Felbamate (FBM) is a potent nonsedative anticonvulsant whose clinical effect may be related to the inhibition of N-methyl-D-aspartate (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 microM) 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 approximately 200, approximately 110, and approximately 55 microM for FBM binding to the resting, activated, and desensitized NMDA channels, respectively. We conclude that therapeutic concentrations (50-300 microM) 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.

Advancing the medical management of epilepsy: disease modification and pharmacogenetics

Despite the recent development of new antiepilepsy drugs, a significant number of children are still unable to achieve seizure freedom without side effects. Understanding the factors behind individual variability in antiepilepsy drug tolerability and dose response and incorporating these factors into a treatment plan would represent an important advance in epilepsy pharmacotherapy. A more thorough understanding of the epileptogenic process may allow clinicians to select antiepilepsy drugs that interrupt or modify various steps in the epileptogenic progression (ie, disease modification). Additionally, advances in the understanding of human genetics may allow for selection of antiepilepsy drugs and dosage regimens based on a patient's clinical characteristics and genotype (ie, pharmacogenetics). This article focuses on these two areas of potential improvement in the medical treatment of patients with epilepsy. Such methods of tailoring antiepilepsy drug therapy would be preferable to the trial-and-error system that is currently used.

Positive and negative modulation of the GABA(A) receptor and outcome after traumatic brain injury in rats

Glutamate-mediated excitotoxicity has been shown to contribute to cellular dysfunction following traumatic brain injury (TBI). Increasing inhibitory function through stimulation of gamma-aminobutyric acid (GABA(A)) receptors may attenuate excitotoxic effects and improve outcome. The present experiment examined the effects of diazepam, a positive modulator at the GABA(A) receptor, on survival and cognitive performance in traumatically brain-injured animals. In experiment 1, 15 min prior to central fluid percussion brain injury, rats (n=8 per group) were injected (i.p.) with saline or diazepam (5 mg/kg or 10 mg/kg). Additional rats (n=8) were surgically prepared but not injured (sham-injury). Rats pre-treated with the 5 mg/kg dose of diazepam had significantly lower mortality (0%) than injured, saline-treated rats (53%). Also, diazepam-treated (5 mg/kg) rats had significantly shorter latencies to reach the goal platform in the Morris water maze test performed 11-15 days post-injury. In experiment 2, at 15 min post-injury, rats were given either saline (n=5) or 5 mg/kg diazepam (n=6). Rats treated with diazepam did not differ in mortality from injured rats treated with vehicle. However, rats treated with diazepam at 15 min post-injury had significantly shorter latencies to reach the goal platform in the Morris water maze than injured, vehicle-treated rats. In experiment 3, the post-injury administration of bicuculline (1.5 mg/kg, n=8), a GABA(A) antagonist, increased Morris water maze goal latencies compared to injured animals treated with saline (n=8). These results suggest that enhancing inhibitory function during the acute post-injury period produces beneficial effects on both survival and outcome following experimental TBI.

Felbamate block of recombinant N-methyl-D-aspartate receptors: selectivity for the NR2B subunit

The anticonvulsant felbamate blocks N-methyl-D-asparate (NMDA) receptors but fails to exhibit the neurobehavioral toxicity characteristic of other NMDA receptor antagonists. To investigate the possibility that felbamate's favorable toxicity profile could be related to NMDA receptor subtype selectivity, we examined the specificity of felbamate block of recombinant NMDA receptors composed of the NR1a subunit and various NR2 subunits. Felbamate produced a rapid, concentration-dependent block of currents evoked by 50 microM NMDA and 10 microM glycine in human embryonic kidney 293 cells expressing the rat NR1a subunit, and either the NR2A, NR2B or NR2C subunits; the IC50 values for block were 2.6, 0.52 and 2.4 mM, respectively (holding potential, - 60 mV). The Hill coefficient values were < 1 and, in kinetic analyses, onset and recovery from block were well fit by double exponential functions, indicating binding to more than one blocking site on the NMDA receptor channel complex. The higher affinity of felbamate block of NMDA receptors containing the NR2B subunit could be accounted for by more rapid association and slower dissociation from these sites. We conclude that felbamate exhibits modest selectivity for NMDA receptors composed of NR1a/NR2B subunits. This selectivity could, in part, account for the more favorable clinical profile of felbamate in comparison with NMDA receptor antagonists that do not show subunit selectivity.

Electrophysiological effects of felbamate

Felbamate is a broad spectrum antiepileptic drug recently introduced into clinical practice for controlling seizures in patients affected by Lennox-Gastaut epilepsy, complex partial seizures or otherwise intractable epilepsies. However, the cellular mechanisms by which the drug exerts its anticonvulsant actions are not fully understood. The aim of the present article is to outline the possible mechanisms of action of felbamate as suggested by findings obtained with electrophysiological approaches.

Neuroprotective effects of the strychnine-insensitive glycine site NMDA antagonist (R)-HA-966 in an experimental model of Parkinson's disease

The neuroprotective effects of (R)-HA-966 and (S)-HA-966 (3-amino-1-hydroxy-2-pyrrolidinone) were examined in an MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine)-induced animal model of Parkinson's disease. Systemic pretreatment of C57 black mice with the strychnine-insensitive glycine site antagonist, (R)-HA-966 (3-30 mg/kg, i.p.), dose-dependently attenuated MPTP-induced depletion of striatal dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC). Pretreatment with (R)-HA-966 also significantly protected the degeneration of tyrosine hydroxylase-positive neurons in the substantia nigra of mice treated with MPTP and alleviated the acute behavioral changes caused by the neurotoxin. In contrast, the other racemic form, (S)-HA-966, neither prevented the neurochemical depletions nor the neuronal injury caused by MPTP. These results indicate that excitatory mechanisms of neurodegeneration are involved in the pathophysiology of Parkinson's disease, and that strychnine-insensitive glycine site NMDA antagonists may serve as dopaminoprotective agents which intervene in the progressive neurodegeneration in Parkinson's disease.