Evidence for a role of Na_v1.6 in facilitating increases in neuronal hyperexcitability during epileptogenesis

Abstract

During epileptogenesis a series of molecular and cellular events occur, culminating in an increase in neuronal excitability, leading to seizure initiation. The entorhinal cortex has been implicated in the generation of epileptic seizures in both humans and animal models of temporal lobe epilepsy. This hyperexcitability is due, in part, to proexcitatory changes in ion channel activity. Sodium channels play an important role in controlling neuronal excitability, and alterations in their activity could facilitate seizure initiation. We sought to investigate whether medial entorhinal cortex (mEC) layer II neurons become hyperexcitable and display proexcitatory behavior of Na channels during epileptogenesis. Experiments were conducted 7 days after electrical induction of status epilepticus (SE), a time point during the latent period of epileptogenesis and before the onset of seizures. mEC layer II stellate neurons from post-SE animals were hyperexcitable, eliciting action potentials at higher frequencies compared with control neurons. Na channel currents recorded from post-SE neurons revealed increases in Na current amplitudes, particularly persistent and resurgent currents, as well as depolarized shifts in inactivation parameters. Immunocytochemical studies revealed increases in voltage-gated Na (Nav) 1.6 isoform levels. The toxin 4,9-anhydro-tetrodotoxin, which has greater selectivity for Nav1.6 over other Na channel isoforms, suppressed neuronal hyperexcitability, reduced macroscopic Na currents, persistent and resurgent Na current densities, and abolished depolarized shifts in inactivation parameters in post-SE neurons. These studies support a potential role for Nav1.6 in facilitating the hyperexcitability of mEC layer II neurons during epileptogenesis.

Differential binding of tetrodotoxin and its derivatives to voltage‐sensitive sodium channel subtypes (Nav1.1 to Nav1.7)

“…Indeed, 300 nM 4,9-anhydroTTX (4) preferentially inhibited Na v 1.6 over that of Na v 1.4 and Na v 1.5 (Supporting Information Table S1). Because the voltage-dependent block of Na v 1.6 by 4,9-anhydroTTX (4) was assumed from the previous studies (Hargus et al, 2013;Rogers et al, 2016), optimization of the Figure 4 An evaluation of the molecular determinants for the reduced binding of CHTX (2) to Na v 1.7. (A) An alignment of the Na v sequences at the TTXbinding sites from domains I to IV; the letters represent the amino acid residues.…”

Activation of murine pre-proglucagon–producing neurons reduces food intake and body weight

“…Coronal brain slices (300 μm) containing the NTS were prepared from AAV hM3Dq-DREADD-injected mice (n = 4). Recordings were performed as described previously (67).…”

Pathogenic mechanism of recurrent mutations of SCN8A in epileptic encephalopathy

“…Currents were recorded using the whole‐cell configuration of the patch clamp recording technique as described previously 24. The intracellular recording solution contained (in mmol/L): 140 CsF, 2 MgCl 2 , 1 EGTA, 10 HEPES, 4 Na 2 ATP, 0.3 NaGTP (pH adjusted to 7.3 with CsOH, osmolarity adjusted to 290 mOsm with sucrose).…”

“…The current–voltage relationship was determined using a 100‐msec voltage pulse from −80 to +70 mV in steps of 5 mV from a holding potential of −120 mV at 2 sec intervals. Conductance as a function of voltage was derived from the current–voltage relationship and fitted by a Boltzmann function as described previously 24. Decays of macroscopic currents were fitted to a single exponential function and time constants were determined 25.…”