No abstract

Neuronal excitability is critically determined by the properties of voltage-gated Na ϩ currents. Fast transient Na ϩ currents (I NaT ) mediate the fast upstroke of action potentials, whereas low-voltage-activated persistent Na ϩ currents (I NaP ) contribute to subthreshold excitation. Na ϩ channels are composed of a pore-forming ␣ subunit and ␤ subunits, which modify the biophysical properties of ␣ subunits. We have examined the idea that the presence of ␤ subunits also modifies the pharmacological properties of the Na ϩ channel complex using mice lacking either the ␤ 1 (Scn1b) or ␤ 2 (Scn2b) subunit. Classical effects of the anticonvulsant carbamazepine (CBZ), such as the use-dependent reduction of I NaT and effects on I NaT voltage dependence of inactivation, were unaltered in mice lacking ␤ subunits. Surprisingly, CBZ induced a small but significant shift of the voltage dependence of activation of I NaT and I NaP to more hyperpolarized potentials. This novel CBZ effect on I NaP was strongly enhanced in Scn1b null mice, leading to a pronounced increase of I NaP within the subthreshold potential range, in particular at low CBZ concentrations of 10 -30 M. A combination of current-clamp and computational modeling studies revealed that this effect causes a complete loss of CBZ efficacy in reducing repetitive firing. Thus, ␤ subunits modify not only the biophysical but also the pharmacological properties of Na ϩ channels, in particular with respect to I NaP . Consequently, altered expression of ␤ subunits in other neurological disorders may cause altered neuronal sensitivity to drugs targeting Na ϩ channels.

In human epilepsy, pharmacoresistance to antiepileptic drug therapy is a major problem affecting a substantial fraction of patients. Many of the currently available antiepileptic drugs target voltage-gated sodium channels, leading to a rate-dependent suppression of neuronal discharge. A loss of use-dependent block has emerged as a potential cellular mechanism of pharmacoresistance for anticonvulsants acting on voltage-gated sodium channels. There is a need both for compounds that overcome this resistance mechanism and for novel drugs that inhibit the process of epileptogenesis. We show that eslicarbazepine acetate, a once-daily antiepileptic drug, may constitute a candidate compound that addresses both issues. Eslicarbazepine acetate is converted extensively to eslicarbazepine after oral administration. We have first tested using patch-clamp recording in human and rat hippocampal slices if eslicarbazepine, the major active metabolite of eslicarbazepine acetate, shows maintained activity in chronically epileptic tissue. We show that eslicarbazepine exhibits maintained use-dependent blocking effects both in human and experimental epilepsy with significant add-on effects to carbamazepine in human epilepsy. Second, we show that eslicarbazepine acetate also inhibits Cav3.2 T-type Ca(2+) channels, which have been shown to be key mediators of epileptogenesis. We then examined if transitory administration of eslicarbazepine acetate (once daily for 6 weeks, 150 mg/kg or 300 mg/kg) after induction of epilepsy in mice has an effect on the development of chronic seizures and neuropathological correlates of chronic epilepsy. We found that eslicarbazepine acetate exhibits strong antiepileptogenic effects in experimental epilepsy. EEG monitoring showed that transitory eslicarbazepine acetate treatment resulted in a significant decrease in seizure activity at the chronic state, 8 weeks after the end of treatment. Moreover, eslicarbazepine acetate treatment resulted in a significant decrease in mossy fibre sprouting into the inner molecular layer of pilocarpine-injected mice, as detected by Timm staining. In addition, epileptic animals treated with 150 mg/kg, but not those that received 300 mg/kg eslicarbazepine acetate showed an attenuated neuronal loss. These results indicate that eslicarbazepine potentially overcomes a cellular resistance mechanism to conventional antiepileptic drugs and at the same time constitutes a potent antiepileptogenic agent.

The modulation of synaptic plasticity by NMDA receptor (NMDAR)-mediated processes is essential for many forms of learning and memory. Activation of NMDARs by glutamate requires the binding of a coagonist to a regulatory site of the receptor. In many forebrain regions, this coagonist is D-serine. Here, we show that experimental epilepsy in rats is associated with a reduction in the CNS levels of D-serine, which leads to a desaturation of the coagonist binding site of synaptic and extrasynaptic NMDARs. In addition, the subunit composition of synaptic NMDARs changes in chronic epilepsy. The desaturation of NMDARs causes a deficit in hippocampal long-term potentiation, which can be rescued with exogenously supplied D-serine. Importantly, exogenous D-serine improves spatial learning in epileptic animals. These results strongly suggest that D-serine deficiency is important in the amnestic symptoms of temporal lobe epilepsy. Our results point to a possible clinical utility of D-serine to alleviate these disease manifestations.

T-type Ca2+ channels give rise to low-threshold inward currents that are central determinants of neuronal excitability. The availability of T-type Ca 2+ channels is strongly influenced by voltage-dependent inactivation and recovery from inactivation. Here, we show that native and cloned T-type Ca 2+ channel subunits selectively encode specific aspects of prior membrane potential changes via a powerful modulation of the rates with which these channels recover from inactivation. Increasing the duration of subthreshold (−70 to −55 mV) conditioning depolarizations caused a pronounced slowing of subsequent recovery from inactivation of both cloned (Ca v 3.1-3.3) and native T-type channels (thalamic neurones). The scaling of recovery rates with increasing duration of conditioning depolarizations could be well described by a power law function. Different T-type channel isoforms exhibited overlapping but complementary ranges of recovery rates. Intriguingly, scaling of recovery rates was dramatically reduced in Ca v 3.2 and Ca v 3.3, but not Ca v 3.1 subunits, when mock action potentials were superimposed on conditioning depolarizations. Our results suggest that different T-type channel subunits exhibit dramatic differences in scaling relationships, in addition to well-described differences in other biophysical properties. Furthermore, the availability of T-type channels is powerfully modulated over time, depending on the patterns of prior activity that these channels have encountered. These data provide a novel mechanism for cellular short-term plasticity on the millisecond to second time scale that relies on biophysical properties of specific T-type Ca 2+ channel subunits.