Downregulation of Spermine Augments Dendritic Persistent Sodium Currents and Synaptic Integration after Status Epilepticus.

Dendritic voltage-gated ion channels profoundly shape the integrative properties of neuronal dendrites. In epilepsy, numerous changes in dendritic ion channels have been described, all of them due to either their altered transcription or phosphorylation. In pilocarpine-treated chronically epileptic rats, we describe a novel mechanism that causes an increased proximal dendritic persistent Na(+) current (INaP). We demonstrate using a combination of electrophysiology and molecular approaches that the upregulation of dendritic INaP is due to a relief from polyamine-dependent inhibition. The polyamine deficit in hippocampal neurons is likely caused by an upregulation of the degrading enzyme spermidine/spermine acetyltransferase. Multiphoton glutamate uncaging experiments revealed that the increase in dendritic INaP causes augmented dendritic summation of excitatory inputs. These results establish a novel post-transcriptional modification of ion channels in chronic epilepsy and may provide a novel avenue for treatment of temporal lobe epilepsy. SIGNIFICANCE STATEMENT: In this paper, we describe a novel mechanism that causes increased dendritic persistent Na(+) current. We demonstrate using a combination of electrophysiology and molecular approaches that the upregulation of persistent Na(+) currents is due to a relief from polyamine-dependent inhibition. The polyamine deficit in hippocampal neurons is likely caused by an upregulation of the degrading enzyme spermidine/spermine acetyltransferase. Multiphoton glutamate uncaging experiments revealed that the increase in dendritic persistent Na current causes augmented dendritic summation of excitatory inputs. We believe that these results establish a novel post-transcriptional modification of ion channels in chronic epilepsy.

Efficacy loss of the anticonvulsant carbamazepine in mice lacking sodium channel beta subunits via paradoxical effects on persistent sodium currents.

Neuronal excitability is critically determined by the properties of voltage-gated Na(+) currents. Fast transient Na(+) currents (I(NaT)) mediate the fast upstroke of action potentials, whereas low-voltage-activated persistent Na(+) currents (I(NaP)) contribute to subthreshold excitation. Na(+) channels are composed of a pore-forming alpha subunit and beta subunits, which modify the biophysical properties of alpha subunits. We have examined the idea that the presence of beta subunits also modifies the pharmacological properties of the Na(+) channel complex using mice lacking either the beta(1) (Scn1b) or beta(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 beta 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 microm. 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, beta subunits modify not only the biophysical but also the pharmacological properties of Na(+) channels, in particular with respect to I(NaP). Consequently, altered expression of beta subunits in other neurological disorders may cause altered neuronal sensitivity to drugs targeting Na(+) channels.

An increase in persistent sodium current contributes to intrinsic neuronal bursting after status epilepticus.

Brain damage causes multiple changes in synaptic function and intrinsic properties of surviving neurons, leading to the development of chronic epilepsy. In the widely used pilocarpine-status epilepticus (SE) rat model of temporal lobe epilepsy (TLE), a major alteration is the marked increase in the fraction of intrinsically bursting CA1 pyramidal cells. Here we have differentiated between two types of bursting phenotypes: 1) bursting in response to threshold-straddling excitatory current pulses (low-threshold bursting) and 2) bursting only in response to suprathreshold stimuli (high-threshold bursting). Low-threshold bursting prevailed in 46.5% of SE-experienced neurons sampled 1-4 wk after pilocarpine-SE, but was rarely seen in control neurons (1.9%). As previously shown, it appeared to be driven predominantly by a T-type Ca(2+) current (I(CaT)) in the apical dendrites. After blocking low-threshold bursting with Ni(2+), the same neurons still manifested a high-threshold bursting phenotype. Another 40.1% of SE-experienced neurons displayed only a high-threshold bursting phenotype and the remaining 13.4% of these neurons were nonbursters. Altogether, high-threshold bursting prevailed in 86.6% of SE-experienced neurons, but only in 33.0% of control neurons. Several lines of evidence indicated that high-threshold bursting is driven by persistent Na(+) current (I(NaP)) at or near the soma. Congruently, I(NaP) was 1.5-fold larger in SE-experienced versus control neurons. We conclude that an increase in I(NaP), conjointly with an increase in I(CaT), strongly contributes to the predominance of bursting phenotypes in CA1 pyramidal cells early after pilocarpine-SE and thus likely plays a role in the development of a chronic epileptic condition in this TLE model.

Axon initial segment dysfunction in a mouse model of genetic epilepsy with febrile seizures plus.

Febrile seizures are a common childhood seizure disorder and a defining feature of genetic epilepsy with febrile seizures plus (GEFS+), a syndrome frequently associated with Na+ channel mutations. Here, we describe the creation of a knockin mouse heterozygous for the C121W mutation of the beta1 Na+ channel accessory subunit seen in patients with GEFS+. Heterozygous mice with increased core temperature displayed behavioral arrest and were more susceptible to thermal challenge than wild-type mice. Wild-type beta1 was most concentrated in the membrane of axon initial segments (AIS) of pyramidal neurons, while the beta1(C121W) mutant subunit was excluded from AIS membranes. In addition, AIS function, an indicator of neuronal excitability, was substantially enhanced in hippocampal pyramidal neurons of the heterozygous mouse specifically at higher temperatures. Computational modeling predicted that this enhanced excitability was caused by hyperpolarized voltage activation of AIS Na+ channels. This heat-sensitive increased neuronal excitability presumably contributed to the heightened thermal seizure susceptibility and epileptiform discharges seen in patients and mice with beta1(C121W) subunits. We therefore conclude that Na+ channel beta1 subunits modulate AIS excitability and that epilepsy can arise if this modulation is impaired.

Role of axonal NaV1.6 sodium channels in action potential initiation of CA1 pyramidal neurons.

In many neuron types, the axon initial segment (AIS) has the lowest threshold for action potential generation. Its active properties are determined by the targeted expression of specific voltage-gated channel subunits. We show that the Na+ channel NaV1.6 displays a striking aggregation at the AIS of cortical neurons. To assess the functional role of this subunit, we used Scn8amed mice that are deficient for NaV1.6 subunits but still display prominent Na+ channel aggregation at the AIS. In CA1 pyramidal cells from Scn8amed mice, we found a depolarizing shift in the voltage dependence of activation of the transient Na+ current (INaT), indicating that NaV1.6 subunits activate at more negative voltages than other NaV subunits. Additionally, persistent and resurgent Na+ currents were significantly reduced. Current-clamp recordings revealed a significant elevation of spike threshold in Scn8amed mice as well as a shortening of the estimated delay between spike initiation at the AIS and its arrival at the soma. In combination with simulations using a realistic computer model of a CA1 pyramidal cell, our results imply that a hyperpolarized voltage dependence of activation of AIS NaV1.6 channels is important both in determining spike threshold and localizing spike initiation to the AIS. In addition to altered spike initiation, Scn8amed mice also showed a strongly reduced spike gain as expected with combined changes in persistent and resurgent currents and spike threshold. These results suggest that NaV1.6 subunits at the AIS contribute significantly to its role as spike trigger zone and shape repetitive discharge properties of CA1 neurons.