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Hypoxia preconditioning may protect focal cerebral ischemia in rats by downregulation of glycogen synthase kinase 3β, phosphorylated signal transducer and activator of transcription 3

Objective 
To investigate the roles of phosphorylated glycogen synthase kinase 3β (pGSK3β) and phosphorylated signal transducer and activator of transcription 3 (pSTAT3) in hypoxic preconditioning-induced neuroprotection against ischemic brain injury in rats. 
 
 
Methods 
Sixty SD rats were randomly divided into a sham operation group, a cerebral ischemia group, and a hypoxia preconditioning group (n=20 in each group). A model of middle cerebral artery occlusion (MCAO) was induced by the modified suture method. Before the preparation of MCAO model, the rats in the hypoxia preconditioning group were put into a hypobaric oxygen chamber at a simulated altitude of 5 000 m (pressure: 0.53 × 105 kPa; partial pressure of oxygen: 81 mmHg; 1 mmHg=0.133 kPa), 3 h a day for 5 days. At 24 h after MCAO modeling, the rats were subjected to neurobehavioral score (n=6) and cerebral infarction volume measurement (n=6). Immunohistochemical staining was used to detect the expression levels of neuronal nuclei (NeuN) and pGSK3β (Ser9) (n=7). Western blot was used to detect the expression levels of pGSK3β (Ser9) and pSTAT3 (Tyr705) in the ischemic cortex (n=7). 
 
 
Results 
The neurological deficit score (1.833±0.408 vs. 2.667±0.516; t=3.101, P=0.011) and cerebral infarction volume (18.137%±0.801% vs. 24.125%±0.694%; t=13.840, P<0.001) in the hypoxia preconditioning group were significantly lower or smaller than those in the cerebral ischemia group. Immunohistochemical staining showed that the numbers of NeuN positive cells in the cerebral ischemia group and the hypoxia preconditioning group were significantly less than that in the sham operation group (48.000±1.414/high power field [HPF], 124.833±3.061/HPF, and 213.500±2.429/HPF; F=7 150.550, P<0.001), the hypoxia preconditioning group was significantly more than the ischemia group (P<0.001); the numbers of pSTAT3 positive cells in the cerebral ischemia group and the hypoxia preconditioning group were significantly higher than that in the sham operation group (57.667±1.366/HPF, 29.167±1.941/HPF and 3.500±1.049/HPF; F=1 962.649, P<0.001), and the hypoxia preconditioning group was significantly less than the ischemia group (P<0.001). Western blot analysis showed that the expression levels of ischemic cortical pGSK3β and pSTAT3 in the cerebral ischemia group and the hypoxia preconditioning group were significantly higher than those of the sham operation group (pGSK3β: 2.336±0.102, 0.876±0.196 and 0.440±0.012; F=1 610.826, P<0.001; pSTAT3 : 8.368±0.230, 4.883±0.123 and 0.595±0.138; F=4 018.051, P<0.001), the hypoxia preconditioning group were significantly lower than the ischemia group (all P<0.001). 
 
 
Conclusions 
Hypoxia preconditioning has neuroprotective effect for ischemic brain injury in rats. It may be associated with the down-regulation of the expressions of pGSK3 and pSTAT3. 
 
 
Key words: 
Brain Ischemia; Ischemic Preconditioning; Hypoxia, Brain; Glycogen Synthase Kinase 3; STAT3 Transcription Factor; Disease Models, Animal; Rats