The Effect of Transient Global Ischemia Followed by Reperfusion and Intranasal Insulin Administration on Metabolic and Functional Changes in the Rat Brain

Transient global brain ischemia can occur as a result of cardiac arrest, surgery, or systemic hypotension. Although neuronal death may not become apparent until 3 days after ischemia, metabolic disturbances can be detected within the first hour after reperfusion. A single intranasal administration of insulin (0.5 IU/rat) 1 hour prior to ischemia caused by occlusion of carotid arteries in combination with hypotension can prevent progression of these disturbances. Such a rapid effect may be due to insulin’s ability to modulate the activity of key signaling pathways and transcription factors, which aim to increase the stress resistance of the brain. During the study, phosphorylation levels of Akt, GSK3β, p38MAPK, and ERK1/2 protein kinases were measured in the cerebral cortex, hippocampus, striatum, and hypothalamus of sham-operated rats and ischemic rats after insulin administration using Western blotting. The expression of the genes for the transcription factors FOXO3 and HIF-1α, as well as genes involved in mitochondrial biogenesis (NRF1, NRF2α, TFAM, including the transcription co-activator PGC-1α), was assessed using PCR-RT. The alterations observed in the study, including an increase in the levels of ph-Akt (Ser473)/Akt and ph-GSK3β(Ser9)/GSK3β in animals exposed to ischemia/reperfusion, indicate that the direction of these changes markedly differs from that described in focal ischemia. At the same time, a single administration of insulin before the ischemic episode returned the phosphorylation levels of Akt and GSK3β to those of the sham-operated group. The development of inflammatory responses in transient global brain ischemia and subsequent reperfusion is mediated by both ERK1/2 and p38MAPK. Among the examined transcription factors, alterations in gene expression following ischemia/reperfusion were detected for FOXO3 exclusively in the cerebral cortex and hippocampus of rats, whereas for HIF1α, these changes were observed in the hypothalamus. A decrease in the expression of all biogenesis genes in the hypothalamus, in the absence of changes in other brain structures, under conditions of ischemia/reperfusion, is more likely to be a response to extreme stress. In this case, the hypothalamus may switch to an “energy-saving” mode in order to cope with the stress.