Insulin inhibits rat hippocampal neurones via activation of ATP-sensitive K⁺ and large conductance Ca²⁺-activated K⁺ channels

In this study, we have used a combination of immunocytochemical and Ca²⁺ imaging techniques to determine the functional localisation of insulin receptors as well as the potential role for insulin in modulating hippocampal synaptic activity. Comparison of insulin receptor and MAP2 labelling demonstrated extensive insulin receptor immunoreactivity on the soma and dendrites of cultured hippocampal neurones. Dual labelling with synapsin 1 also showed punctate insulin receptor labelling associated with synapses. In functional studies, insulin inhibited spontaneous Ca²⁺ oscillations evoked in cultured hippocampal neurones following Mg²⁺ removal. This action of insulin was mimicked by the ATP-sensitive K⁺ (K_ATP) channel opener diazoxide or the large conductance Ca²⁺-activated K⁺ (BK) channel activator NS-1619. Furthermore, application of the K_ATP channel blocker glybenclamide or the BK channel inhibitors iberiotoxin or charybdotoxin attenuated the actions of insulin, whereas prior incubation with a combination of glybenclamide and iberiotoxin completely blocked insulin action. The ability of insulin to modulate the Ca²⁺ oscillations was reduced by the inhibitors of MAPK activation PD 98059 and U0126, but not by the PI 3-kinase inhibitors LY 294002 or wortmannin, indicating that a MAPK-driven process underlies insulin action. In conclusion, insulin inhibits spontaneous Ca²⁺ oscillations via a process involving MAPK-driven activation of BK and K_ATP channels. This process may be a useful therapeutic target for the treatment of epilepsy and certain neurodegenerative diseases.