[Show abstract][Hide abstract] ABSTRACT: Action potential (AP) responses of dentate gyrus granule (DG) cells have to be tightly regulated to maintain hippocampal function. However, which ion channels control the response delay of DG cells is not known. In some neuron types, spike latency is influenced by a dendrotoxin (DTX)-sensitive delay current (ID) mediated by unidentified combinations of voltage-gated K(+) (Kv) channels of the Kv1 family Kv1.1-6. In DG cells, the ID has not been characterized and its molecular basis is unknown. The response phenotype of mature DG cells is usually considered homogenous but intrinsic plasticity likely occurs in particular in conditions of hyperexcitability, for example during temporal lobe epilepsy (TLE). In this study, we examined response delays of DG cells and underlying ion channel molecules by employing a combination of gramicidin-perforated patch-clamp recordings in acute brain slices and single-cell reverse transcriptase quantitative polymerase chain reaction (SC RT-qPCR) experiments. An in vivo mouse model of TLE consisting of intrahippocampal kainate (KA) injection was used to examine epilepsy-related plasticity. Response delays of DG cells were DTX-sensitive and strongly increased in KA-injected hippocampi; Kv1.1 mRNA was elevated 10-fold, and the response delays correlated with Kv1.1 mRNA abundance on the single cell level. Other Kv1 subunits did not show overt changes in mRNA levels. Kv1.1 immunolabeling was enhanced in KA DG cells. The biophysical properties of ID and a delay heterogeneity within the DG cell population was characterized. Using organotypic hippocampal slice cultures (OHCs), where KA incubation also induced ID upregulation, the homeostatic reversibility and neuroprotective potential for DG cells were tested. In summary, the AP timing of DG cells is effectively controlled via scaling of Kv1.1 subunit transcription. With this antiepileptic mechanism, DG cells delay their responses during hyperexcitation.
[Show abstract][Hide abstract] ABSTRACT: 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 Ba(2+)-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.
[Show abstract][Hide abstract] ABSTRACT: Granule cells very effectively sparsen excitatory input from the entorhinal cortex to the hippocampus. This filtering is important for the function of the hippocampus, and likely relates to the intrinsic properties of granule cells. However, the mechanisms underlying granule cell function are unclear. During temporal lobe epilepsy (TLE), granule cell properties change, partly due to an upregulation of inward rectifier K+ (Kir) channels [Stegen et al., 2009; Young et al., 2009]. Also, the expression of hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels alters during TLE; predominantly resulting in hyperexcitability of the respective cell types [Dyhrfjeld-Johnsen et al., 2009]. The functional role of HCN channels in granule cells and the consequence of HCN changes in the dentate gyrus are unknown [Bender et al., 2003].
To investigate the functional consequences of ion channel changes in granule cells during TLE we constructed detailed conductance-based models from recordings and morphologies of human dentate granule cells. As conductances were elevated in sections with severe hippocampal sclerosis (sHS) versus those only mildly affected (mHS), two representative models were build. HCN currents depolarized granule cells, but due to the shunting influence, increases in this current reduced the excitability and shifted the input-output transfer function of granule cells. Our computational investigations show that the simultaneous upregulation of HCN and KIR conductances observed in TLE is well suited to decrease input resistivity while balancing the resting potential. The model also indicates that a low ratio of HCN to KIR prevents theta frequency resonance oscillations in granule cells. Thus, co-regulation of HCN and Kir conductances might be a mechanism of intrinsic plasticity by which granule cells homeostatically scale their output; a property important for the filter function of granule cells, and likely for their survival in epilepsy.