Role of hippocampal sodium channel Nav1.6 in kindling epileptogenesis

Department of Neurology, Yale University School of Medicine, New Haven, Connecticut, USA.
Epilepsia (Impact Factor: 4.57). 01/2009; 50(1):44-55. DOI: 10.1111/j.1528-1167.2008.01710.x
Source: PubMed


Central nervous system plasticity is essential for normal function, but can also reinforce abnormal network behavior, leading to epilepsy and other disorders. The role of altered ion channel expression in abnormal plasticity has not been thoroughly investigated. Nav1.6 is the most abundantly expressed sodium channel in the nervous system. Because of its distribution in the cell body and axon initial segment, Nav1.6 is crucial for action potential generation. The goal of the present study was to investigate the possible role of changes in Nav1.6 expression in abnormal, activity-dependent plasticity of hippocampal circuits.
We studied kindling, a form of abnormal activity-dependent facilitation. We investigated: (1) sodium channel protein expression by immunocytochemistry and sodium channel messenger RNA (mRNA) by in situ hybridization, (2) sodium current by patch clamp recordings, and (3) rate of kindling by analysis of seizure behavior. The initiation, development, and expression of kindling in wild-type mice were compared to Nav1.6 +/-med(tg) mice, which have reduced expression of Nav1.6.
We found that kindling was associated with increased expression of Nav1.6 protein and mRNA, which occurred selectively in hippocampal CA3 neurons. Hippocampal CA3 neurons also showed increased persistent sodium current in kindled animals compared to sham-kindled controls. Conversely, Nav1.6 +/-med(tg) mice resisted the initiation and development of kindling.
These findings suggest an important mechanism for enhanced excitability, in which Nav1.6 may participate in a self-reinforcing cycle of activity-dependent facilitation in the hippocampus. This mechanism could contribute to both normal hippocampal function and to epilepsy and other common nervous system disorders.

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Available from: Angelika Lampert, Mar 31, 2015
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    • "Heterozygous Scn8a null mice exhibit spike-wave discharges and absence seizures (Papale et al., 2009), suggesting that loss-of-function mutations may also cause some forms of epilepsy, possibly due to a differential impact on inhibitory neurons where Na v 1.6 is also expressed (Lorincz and Nusser, 2008). However, reduced abundance of Na v 1.6 in heterozygous null mice is also protective against kainate-induced epilepsy and reduces seizures in compound heterozygotes carrying an SCNA1 Dravet mutation (Blumenfeld et al., 2009; Martin et al., 2007; Papale et al., 2009). The pro-seizure effects of increased Na v 1.6 activity and the protective effect of haploinsufficiency both support the pathogenicity of gain-of-function mutations of human Na v 1.6 that increase channel activity in excitatory pyramidal neurons. "
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    ABSTRACT: Rare de novo mutations of sodium channels are thought to be an important cause of sporadic epilepsy. The well established role of de novo mutations of sodium channel SCN1A in Dravet Syndrome supports this view, but the etiology of many cases of epileptic encephalopathy remains unknown. We sought to identify the genetic cause in a patient with early onset epileptic encephalopathy by whole exome sequencing of genomic DNA. The heterozygous mutation c. 2003C > T in SCN8A, the gene encoding sodium channel Nav1.6, was detected in the patient but was not present in either parent. The resulting missense substitution, p.Thr767Ile, alters an evolutionarily conserved residue in the first transmembrane segment of channel domain II. The electrophysiological effects of this mutation were assessed in neuronal cells transfected with mutant or wildtype cDNA. The mutation causes enhanced channel activation, with a 10 mV depolarizing shift in voltage dependence of activation as well as increased ramp current. In addition, pyramidal hippocampal neurons expressing the mutant channel exhibit increased spontaneous firing with PDS-like complexes as well as increased frequency of evoked action potentials. The identification of this new gain-of-function mutation of Nav1.6 supports the inclusion of SCN8A as a causative gene in infantile epilepsy, demonstrates a novel mechanism for hyperactivity of Nav1.6, and further expands the role of de novo mutations in severe epilepsy.
    Full-text · Article · Sep 2014 · Neurobiology of Disease
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    • "c o m / l o c a t e / y n b d i 2011; Martin et al., 2007). In addition, Scn8a-deficient mice are resistant to amygdala kindling, while there is increased Scn8a expression in the CA3 region of the hippocampus of amygdala-kindled rats (Blumenfeld et al., 2009). Together, these studies raise the possibility that reduced hippocampal Scn8a expression may contribute to the seizure protection observed in Scn8a med/+ mutants and that the selective targeting of Scn8a may be efficacious in some forms of epilepsy. "
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    ABSTRACT: SCN1A mutations are the main cause of the epilepsy disorders Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS +). Mutations that reduce the activity of the mouse Scn8a gene, in contrast, are found to confer seizure resistance and extend the lifespan of mouse models of DS and GEFS +. To investigate the mechanism by which reduced Scn8a expression confers seizure resistance, we induced interictal-like burst discharges in hippocampal slices of heterozygous Scn8a null mice (Scn8amed/+) with elevated extracellular potassium. Scn8amed/+mutants exhibited reduced epileptiform burst discharge activity after P20, indicating an age-dependent increased threshold for induction of epileptiform discharges. Scn8a deficiency also reduced the occurrence of burst discharges in a GEFS + mouse model (Scn1aR1648H/+). There was no detectable change in the expression levels of Scn1a (Nav1.1) or Scn2a (Nav1.2) in the hippocampus of adult Scn8amed/+mutants. To determine whether the increased seizure resistance associated with reduced Scn8a expression was due to alterations that occurred during development, we examined the effect of deleting Scn8a in adult mice. Global Cre-mediated deletion of a heterozygous floxed Scn8a allele in adult mice was found to increase thresholds to chemically and electrically induced seizures. Finally, knockdown of Scn8a gene expression in the adult hippocampus via lentiviral Cre injection resulted in a reduction in the number of EEG-confirmed seizures following the administration of picrotoxin. Our results identify the hippocampus as an important structure in the mediation of Scn8a-dependent seizure protection and suggest that selective targeting of Scn8a activity might be efficacious in patients with epilepsy.
    Full-text · Article · Aug 2014 · Neurobiology of Disease
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    • "In this respect, decreased expression of accessory Na + channel β1 and β2 subunits following experimentally induced status epilepticus appears to be a consistent finding (Gastaldi, et al. 1998). Indeed, Blumenfeld et al., has shown that the kindling process per se was associated with an increased expression of the Nav1.6 subunit of sodium channels and increased persistent sodium currents in hippocampal CA3 neurons (Blumenfeld, et al. 2009). Kindling is an ongoing activity-dependent plasticity phenomenon. "
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    ABSTRACT: Purpose: Kindling is a well-established model of secondarily generalized partial seizures that is widely employed in the search for novel antiseizure drugs. During the kindling and postkindling acquisition phase, an active process of neuronal remodeling occurs. We tested the hypothesis that exposure to the voltage-gated sodium channel blockers lamotrigine (LTG) and carbamazepine (CBZ) during the period of active remodeling will lead to a diminished therapeutic effect. Methods: Two days after the last kindling stimulation, fully kindled rats were randomized to receive either 0.5% methyl cellulose (MC), LTG (30 mg/kg), or CBZ (40 mg/kg). The effect of LTG and CBZ on behavioral seizure severity and electrographic afterdischarge duration (ADD) was recorded. One week after this treatment, rats in both groups were rechallenged with LTG 30 or CBZ 40 mg/kg and their seizure score and ADD recorded. In vitro efficacy of LTG on neuronal action potentials was also evaluated using whole cell current clamp recording in hippocampal brain slices obtained from kindled control rats, LTG-sensitive kindled rats, and LTG-resistant kindled rats. Key findings: When acutely administered 48 h after the last kindling stimulation, LTG and CBZ blocked the expression of behavioral seizures and reduced the ADD. In contrast, a second challenge dose of LTG or CBZ administered after a 7-day "no drug, no stimulation" period did not result in reduction of either the seizure score or the ADD. Interestingly, the potassium channel opener, ezogabine, also known as retigabine (EZG; 40 mg/kg), blocked the expression of behavioral seizures at both time points evaluated (i.e., 2 days and 9 days after last stimulation). In vivo resistance to LTG was associated with a similar reduction in the ability of LTG to limit action potential firing in CA1 neurons. LTG (50 μm) significantly decreased the number of action potentials generated by a depolarizing current pulse in neurons recorded from slices obtained from kindled control and LTG-sensitive rats, but not in slices obtained from LTG-resistant rats. Significance: Collectively, results obtained from both in vivo and in vitro studies demonstrate that even a single exposure to the sodium channel blockers LTG, or CBZ, during the postkindling remodeling phase leads to an altered pharmacologic response to these two ASDs, but not to EZG. The LTG- and CBZ-resistant amygdala kindled rats may serve as a useful model of therapy-resistant epilepsy.
    Full-text · Article · Jun 2013 · Epilepsia
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