Adaptive Intrinsic Plasticity in Human Dentate Gyrus Granule Cells during Temporal Lobe Epilepsy

Granule cells in the dentate gyrus are only sparsely active in vivo and survive hippocampal sclerosis (HS) during temporal lobe epilepsy better than neighboring cells. This phenomenon could be related to intrinsic properties specifically adapted to counteract excitation. We studied the mechanisms underlying the excitability of human granule cells using acute hippocampal slices obtained during epilepsy surgery. Patch-clamp recordings were combined with pharmacology, immunocytochemistry, and computer simulations. The input resistance of granule cells correlated negatively with the duration of epilepsy and the degree of HS. Hyperpolarization-activated, ZD7288-sensitive cation (I-H, HCN) currents and highly Ba2+-sensitive, inwardly rectifying K+ (Kir) currents (and HCN1 and Kir2.2 protein) were present somatodendritically and further enhanced in patients with severe HS versus mild HS. The properties and function of I-H were characterized in granule cells. Although I-H depolarized the membrane, it strongly reduced the input resistance and shifted the current-frequency function to higher input values. The shunting influence of HCN and Kir was similar and these conductances correlated. Resonance was not observed. Simulations suggest that the combined upregulation of Kir and HCN conductances attenuates excitatory synaptic input, while stabilizing the membrane potential and responsiveness. Thus, granule cells homeostatically downscale their input-output transfer function during epilepsy.