Changes and significance of occludin expression in rats with blood-brain barrier injury induced by microwave radiation

Objective To observe the changes in occludin expression in hippocampus of rats injured by long-term microwave exposure at low dosage,and explore the mechanism of the effect of occludin on microwave-induced blood-brain barrier(BBB) injury.Methods A total of 156 male Wistar rats were assigned into 4 groups(39 each) according to the microwave dosage they received(0,2.5,5 and 10 mW/cm2).Irradiation was given 5 times/week and 6 minutes for each exposure for one month.Five rats of each group were sacrificed at each time point(6h and day 7,14,30 and 60 after irradiation),the structural changes in hippocampus tissues BBB were observed by light and electron microscopy.Another 3 rats of each group were sacrificed after an intravenous injection of 2% Evans blue(EB) at the time points of 6h and day 7 and 30 after irradiation,and the change in BBB permeability was examined by laser scanning confocal microscopy.Western blotting,real-time PCR and image analysis were used to detect the gene and protein expression of occludin in hippocampus of rats.Results After microwave irradiation in doses of 2.5,5 and 10 mW/cm2,respectively,on day 7,14 and 30,the astrocytes were found to be swollen,the capillary gaps of hippocampus were broadened and tight junction(TJ) structure was inconspicuous or widened.In sham-group(0 mW/cm2),the red fluorescence of EB was limited in lumens of blood vessels,while in 5 and 10 mW/cm2 groups,Evans blue was dispersed surrounding the blood vessels at 6h and day 7 till day 30 in an irradiation dose-dependent manner.In 5 and 10 mW/cm2 groups,the expression of occludin protein in hippocampus decreased at day 14(P0.05) and day 7(P0.01),and a remarkable decrease appeared at day 30 in both groups(P0.01),followed by a recovery to the sham-group level at day 60(P0.05).In the 2.5,5 and 10 mW/cm2 groups,mRNA expression of occludin began to significantly decrease at day 7 and 14 and it lowered to the nadir at day 30(P0.01),followed by a recovery to the level of the sham-group at day 60(P0.05).Conclusions A long-term microwave irradiation with dosage of 2.5 to 10 mW/cm2 may destroy the structure of BBB of rats' hippocampus and increase permeability in a radiation-dose-dependent manner.The results suggest that the decrease of occludin expression may play a part in the changes in BBB permeability induced by microwave irradiation.