Abstracts

Rationale: Targeted disruption of the Kcna1 gene encoding the voltage-gated potassium channel alpha-subunit Kv1.1 in mice leads to frequent spontaneous seizures early in postnatal development. Importantly, Kcna1-null (KO) mice exhibit hallmark features of human temporal lobe epilepsy (TLE). Although previous studies have shown increased excitability of both hippocampal and neocortical pyramidal cells in KO animals, far less is known about interneurons, especially in hippocampus. As interneurons are known to regulate network excitability and patterned neuronal oscillatory activity, a greater delineation of their role could reveal much about the pathogenesis of seizures and epilepsy. Here, we investigated the electrophysiological properties and localization of hippocampal interneurons in both KO and wild-type (WT) C3H control mice. We also tested whether a ketogenic diet (KD) metabolite, beta-hydroxybutyrate (BHB), alters the firing pattern of hippocampal interneurons.Methods: Whole-cell patch clamp and immunocytochemical plus confocal imaging techniques were used. The effects of BHB (2 mM) on interneuron firing were also studied.Results: Interneuron morphological sub-types were identified in stratum oriens (horizontal basket cells, oriens lacunosum-moleculare and oriens-bistratified cells) and stratum radiatum (multipolar basket cell and bistratified cells). The density of these cells in slices from KO animals appeared reduced. Also, interneurons in CA1 stratum oriens were more excitable than WT controls, and the number of action potentials elicited by suprathreshold pulses (100 pA) increased from 20 in WT to 30 in KO (p<0.001) while the threshold current amplitude (the current required to elicit one action potential) decreased from 33 to 27 pA (p<0.05). Similarly, we found that stratum radiatum interneurons were also more excitable in KO mice compared to controls, as action potential number increased from 12 in control to 17 in KO (p<0.05), and the threshold current decreased from 40 to 29 pA (p<0.05). Acute application of 2mM BHB in KO slices altered the firing in 8 of 15 stratum oriens interneurons but did not change the spiking pattern in 7 of 15 cells. In BHB-sensitive interneurons, there was a trend towards decreased excitability as the number of action potentials elicited by 100 pA suprathreshold depolarizing pulse decreased from 32 to 21 (p=0.087).Conclusions: Our results indicate that Kv1.1 plays an important role in CA1 interneuron excitability by limiting spontaneous action potential firing. The changes in interneuron activity seen in KO mice might perturb the timing and oscillatory rhythms of the CA1 hippocampal network, effects that might contribute to their epileptic phenotype. Further, BHB effects on a subpopulation of interneurons might represent a homeostatic mechanism through which the KD could attenuate the increases in interneuron firing seen in Kcna1-null mice. Importantly, this cell-type selectivity might provide a therapeutic rationale for targeting specific interneuron populations to block seizure activity.