Excitotoxic glutamate causes neuronal insulin resistance by inhibiting insulin receptor/Akt/mTOR pathway

An impaired biological response to insulin in the brain, known as central insulin resistance, was identified during stroke and traumatic brain injury, for which glutamate excitotoxicity is a common pathogenic factor. The exact molecular link between excitotoxicity and central insulin resistance remains unclear. To explore this issue, the present study aimed to investigate the effects of glutamate-evoked increases in intracellular free Ca²⁺ concentrations [Ca²⁺]_i and mitochondrial depolarisations, two key factors associated with excitotoxicity, on the insulin-induced activation of the insulin receptor (IR) and components of the Akt/ mammalian target of rapamycin (mTOR) pathway in primary cultures of rat cortical neurons.

Changes in [Ca²⁺]_i and mitochondrial inner membrane potentials (ΔΨ_m) were monitored in rat cultured cortical neurons, using the fluorescent indicators Fura-FF and Rhodamine 123, respectively. The levels of active, phosphorylated signalling molecules associated with the IR/Akt/mTOR pathway were measured with the multiplex fluorescent immunoassay.

When significant mitochondrial depolarisations occurred due to glutamate-evoked massive influxes of Ca²⁺ into the cells, insulin induced 48% less activation of the IR (assessed by IR tyrosine phosphorylation, pY^1150/1151), 72% less activation of Akt (assessed by Akt serine phosphorylation, pS⁴⁷³), 44% less activation of mTOR (assessed by mTOR pS²⁴⁴⁸), and 38% less inhibition of glycogen synthase kinase β (GSK3β) (assessed by GSK3β pS⁹) compared with respective controls. These results suggested that excitotoxic glutamate inhibits signalling via the IR/Akt/mTOR pathway at multiple levels, including the IR, resulting in the development of acute neuronal insulin resistance within minutes, as an early pathological event associated with excitotoxicity.