MAPK-dependent actin cytoskeletal reorganization underlies BK channel activation by insulin.

Numerous brain regions are enriched with insulin and insulin receptors, and several lines of evidence indicate that insulin is an important modulator of neuronal function. Indeed, recent studies have demonstrated that insulin inhibits hippocampal epileptiform-like activity, in part by activating large-conductance Ca2+-activated potassium (BK) channels. Moreover, the mitogen-activated protein kinase (MAPK) signalling cascade has been found to couple insulin to BK channel activation. However, the cellular events downstream of MAPK that underlie this action of insulin are unknown. Here we demonstrate that in hippocampal neurons, BK channel activation by insulin is blocked by actin filament stabilization, suggesting that this process is dependent on the actin cytoskeleton. Stabilizing actin filaments also markedly attenuated the ability of insulin to inhibit the aberrant hippocampal synaptic activity evoked following Mg2+ removal. Insulin also promoted rapid reorganization of fluorescently labelled polymerized actin filaments; an action that was prevented by inhibitors of MAPK activation. Moreover, in parallel studies, insulin increased the level of phospho-MAPK immunostaining in hippocampal neurons. These data are consistent with BK channel activation by insulin involving MAPK-dependent alterations in actin dynamics. This process may have important implications for the role of insulin in regulating hippocampal excitability.