An impaired neocortical I_h is associated with enhanced excitability and absence epilepsy

Abstract

Neuronal subthreshold excitability and firing behaviour are markedly influenced by the activation and deactivation of the somato-dendritic hyperpolarization-activated cation current (Ih). Here, we evaluated possible contributions of Ih to hyperexcitability in an animal model of absence seizures (WAG/Rij rats). We investigated pyramidal neurons of the somatosensory neocortex, the site of generation of spike-wave discharges. Ih-mediated functions in neurons from WAG/Rij rats, Wistar rats (sharing the same genetic background with WAG/Rij, but less epilepsy-prone) and ACI rats (an inbred strain, virtually free of seizures) were compared. We complemented whole-cell recordings from layer 2-3 pyramidal neurons with immunohistochemistry, Western blot and RT-PCR analysis of the h-channel subunits HCN1-4. The fast component of Ih activation in WAG/Rij neurons was significantly reduced (50% reduction in the h-current density) and four times slower than in neurons from nonepileptic Wistar or ACI rats. The results showing decreases in currents corresponded to a 34% reduction in HCN1 protein in the WAG/Rij compared to the Wistar neocortex, but HCN1 mRNA showed stable expression. The other three Ih subunit mRNAs and proteins (HCN2-4) were not affected. The alterations in Ih magnitude and kinetics of gating in WAG/Rij neurons may contribute to augmented excitatory postsynaptic potentials, the increase in their temporal summation and the facilitation of burst firing of these neurons because each of these effects could be mimicked by the selective Ih antagonist ZD 7288. We suggest that the deficit in Ih-mediated functions may contribute to the development and onset of spontaneously occurring hyperexcitability in a rat model of absence seizures.

Upholding WAG/Rij rats as a model of absence epileptogenesis: Hidden mechanisms and a new theory on seizure development

“…Therefore, despite an effect of the treatment on the changes observed in the expression of Na + channels and hyperpolarization-activated cation channels (HCN1), which accompanies the development of SWDs in this strain (see below Section 3.3) (Klein et al, 2004;Strauss et al, 2004), ETH treatment did not seem to influence the function of the circuit in the maintenance of a single seizure once it is started, but only their generation at the focal site(s).…”

“…This loss of neocortical HCN1 function may contribute to an increased cortical excitability since there are substantially few HCN1 subunits in the combined complex of the I h channel in the cortical zone containing the focal region in WAG/Rij rats in comparison to non-epileptic rats. Indeed, the density of HCN1 channels in the apical dendrites of the subgranular layers in the cortical onset zone determines the somatodendritic excitability and a decrease of these dendritic HCN1 channels is accompanied by an age-dependent increase of SWDs (Bazyan and van Luijtelaar, 2013;Strauss et al, 2004). Recently, it was demonstrated that HCN1 channels from the thalamus only (not in genomic DNA) of WAG/Rij rats have an N-terminal deletion of 37 amino acids; these channels expressed in Xenopus oocytes indicated a gain-of-function of about 2 fold which was further accompanied by a suppression in HCN2 and HCN4 currents (Wemhoner et al, 2015).…”

“…In agreement with this, it has recently been demonstrated that the removal of only both foci completely abolishes SWDs (Scicchitano et al, 2015) while fMRI (functional magnetic resonance imaging) studies have confirmed the presence of bilateral activated regions in the somatosensory cortex . Based on this background, we can then suppose that these focal epileptic areas may be genetically predetermined to be hyperexcitable (D'Antuono et al, 2006;Kole et al, 2007;Luttjohann et al, 2011;Strauss et al, 2004) from a certain developmental age onwards. These areas might then trigger neuroadaptive changes into the cortico-thalamo-cortical network, which will then become responsive to the hyperexcitable stimulus generating seizures.…”

Molecular Targets for Antiepileptic Drug Development

“…There is also evidence in this model of a reduction in HCN1 protein and I h currents in pyramidal neurons in the somatosensory cortex. 164 It has been noted previously that lamotrigine enhances I h , so that it could theoretically reverse this reduction. 170 Lamotrigine is the only classical Na ϩ channel-blocking AED with efficacy in the treatment of absence epilepsy; its unique activity could be due to specific biophysical differences in its action on Na ϩ channels or to additional actions, such as the effect on I h .…”

“…In mice, deletion of the HCN2 subunit produces animals with 5-Hz spike-and-wave discharges and absence-like seizures. 163 The WAG/Rij rat model of ab-sence epilepsy shows a loss of HCN1 function in the cortex 164 (possibly linked to the origin of cortical spikeand-wave discharges) but enhanced HCN1 expression in the thalamus. 165 In addition, changes in I h and in HCN subunit expression have been observed in epileptogenesis.…”