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Reduced excitability in the dentate gyrus network of βIV‐spectrin mutant mice in vivo

The submembrane cytoskeletal meshwork of the axon contains the scaffolding protein βIV‐spectrin. It provides mechanical support for the axon and anchors membrane proteins. Quivering (qv3j) mice lack functional βIV‐spectrin and have reduced voltage‐gated sodium channel (VGSC) immunoreactivity at the axon initial segment and nodes of Ranvier. Because VGSCs are critically involved in action potential generation and conduction, we hypothesized that qv3j mice should also show functional deficits at the network level. To test this hypothesis, we investigated granule cell function in the dentate gyrus of anesthetized qv3j mice after electrical stimulation of the perforant path in vivo. This revealed an impaired input‐output relationship between stimulus intensity and granule cell population spikes and an enhanced paired‐pulse inhibition of population spikes, indicating a reduced ability of granule cells to generate action potentials and decreased network excitability. In contrast, the input‐output curve for evoked field excitatory postsynaptic potentials (fEPSPs) and paired‐pulse facilitation of fEPSPs were unchanged, suggesting normal excitatory synaptic transmission at perforant path‐granule cell synapses in qv3j mutants. To corroborate our findings, we analyzed the influence of VGSC density reduction on dentate network activity using an established computational model of the dentate gyrus network. This in silico approach confirmed that the loss of VGSCs is sufficient to explain the electrophysiological changes observed in qv3j mice. Taken together, our findings demonstrate that βIV‐spectrin is required for normal granule cell firing and for physiological levels of network excitability in the mouse dentate gyrus in vivo. © 2009 Wiley‐Liss, Inc.