Altered hippocampal activation in seizure-prone CACNA2D2 knockout mice

The voltage-gated calcium channel subunit 2{delta}-2 controls calcium-dependent signaling in neurons, and loss of this subunit causes epilepsy in both mice and humans. To determine whether mice without 2{delta}-2 demonstrate hippocampal activation or histopathological changes associated with seizure activity, we measured expression of the activity-dependent gene c-fos and various histopathological correlates of temporal lobe epilepsy in hippocampal tissue from wildtype (WT) and 2{delta}-2 knockout (CACNA2D2 KO) mice using immunohistochemical staining and confocal microscopy. Both genotypes demonstrated similarly sparse c-fos expression within the hippocampal dentate granule cell layer (GCL) at baseline, consistent with no difference in basal activity of granule cells between genotypes. Surprisingly, when mice were assayed 1 hour after handling-associated convulsions, KO mice had fewer c-fos-positive cells in the dentate gyrus, indicating that activity in the dentate gyrus actually decreased. However, the dentate was significantly more active in KO mice compared to WT after administration of a subthreshold pentylenetetrazole dose, consistent with increased susceptibility to proconvulsant stimuli. Other histopathological markers of temporal lobe epilepsy in these mice, including markers of neurogenesis, glial activation, and mossy fiber sprouting, were similar in WT and KO mice, apart from a small but significant increase in hilar mossy cell density, opposite to what is typically found in mice with temporal lobe epilepsy. This suggests that the differences in seizure-associated hippocampal function in the absence of 2{delta}-2 protein are likely due to altered functional properties of the network without associated structural changes in the hippocampus at the typical age of seizure onset. Significance StatementCalcium channel 2{delta} subunits play important roles in controlling neuronal circuit structure and function, and mutation of the 2{delta}-2 isoform of this protein is associated with spontaneous seizures. In this study, we find that seizures in 2{delta}-2 mutant mice involve altered hippocampal activation without substantial histopathological changes in hippocampal structure. This suggests that the differences in seizure-associated hippocampal function are likely due to altered functional properties of the network as well as to the contribution of additional brain regions to seizures in mice lacking this protein.