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

Neuronal subthreshold excitability and firing behaviour are markedly influenced by the activation and deactivation of the somatodendritic hyperpolarization-activated cation current (I_h). Here, we evaluated possible contributions of I_h 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. I _h-mediated functions in neurons from WAG/Rij rats, Wistar rats (sharing the same genetic background with WAG/Rij, but less epilepsy-prone) and ACl 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 I_h 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 ACl 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 I_h subunit mRNAs and proteins (HCN2-4) were not affected. The alterations in I_h 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 I_h antagonist ZD 7288. We suggest that the deficit in I_h-mediated functions may contribute to the development and onset of spontaneously occurring hyperexcitability in a rat model of absence seizures.