PROTECTIVE AND ANTIOXIDANT EFFECTS OF INSULIN ON RAT BRAIN CORTICAL NEURONS IN A MODEL OF OXYGEN AND GLUCOSE DEPRIVATION IN VITRO

Intranasal insulin is one of the most promising protectors in the treatment of neurodegenerative and other diseases associated with brain injuries. In these diseases, insulin levels in the brain (in contrast to its blood levels) are as a rule heavily reduced, which, along with the development of insulin resistance, leads to impaired insulin signaling in neurons. The aim of this work was to study the protective effect of insulin on cultured rat cortical neurons using an in vitro oxygen–glucose deprivation (OGD) model of ischemia–reperfusion brain injury followed by a resumption of oxygen and glucose supply to neurons. OGD exposure for 1 or 3 h with subsequent incubation of cultured rat cortical neurons in complete (oxygen- and glucose-containing) growth medium decreased neuronal viability and increased the production of reactive oxygen species, while the preincubation of neurons with insulin at micromolar concentrations had protective and antioxidant effects. One-hour OGD followed by incubation in complete growth medium led to downregulation of protein kinase B/Akt (decreased pAkt(Ser⁴⁷³)/Akt ratio) and upregulation of glycogen synthase kinase-3beta (GSK-3beta), one of the main Akt targets (decreased pGSK-3beta(Ser⁹)/GSK-3beta ratio). In contrast, preincubation with insulin activated Akt and inactivated GSK-3beta. Apparently, these effects of insulin significantly contribute to its neuroprotective action, because GSK-3beta activation leads to mitochondrial dysfunction and neuronal death. Insulin was shown to increase the neuronal activity of protein kinase regulated by extracellular signals (ERK1/2), which was diminished by OGD and subsequent exposure to growth medium containing glucose and oxygen.