Objective To investigate the relationship between edema and aquaporin 4 (AQP4) within traumatic penumbra (TP) of rats with brain trauma.Methods Eighty-eight healthy adult Wistar rats were randomly divided into control group (n =11) and trauma group (n =77),according to the random number table.Trauma group were further subdivided into seven time points (1 h,6 h,12 h,24 h,48 h,72 h and 7 d) of 11 animals each.Brain tissue samples from the moderate brain models were collected to evaluate brain edema with histological observation,blood brain barrier (BBB) permeability with semiquantitative immunohistohemical staining of IgG,and AQP4 expression with immunofluorescence and Western blotting.Results In control group pathology and IgG staining revealed no abnormalities and expression of AQP4 was few.In trauma group light edema zone was visualized at 1 h,began widening,reached the peak at 12 h [(1.589 ±0.020)mm],and then began narrowing.There were significances in width of the edematous band at each time point except for the comparison at 24 h vs.48 h and 72 h vs.7 d (P ＜ 0.05).After trauma,vasogenic edema was found in edema zone at 1 h,intracellular edema was found at 6 h,both aggravated at 12 h and alleviated slightly at 24 h,and intracellular edema predominated at 48 h.IgG showed intensively positive staining at 1,12 and 48 h,and weak staining at 6 h,24 h,72 h and 7 d.After trauma,expression of AQP4 decreased at 1 h (0.659 ± 0.021),returned slightly at 6 h (1.257 ±0.058),peaked at 12 h (2.499 ±0.136),declined again at 24 h (2.267 ± 0.068),re-raised at 72 h (2.078 ± 0.065),and returned to the baseline at 7 d (1.280 ± 0.065).There were significant differences in level of AQP4 at each time point except for the comparison at6h vs.7 d,24 h vs.72 h and 24 h vs.72 h (P＜0.05).Conclusions In the early phase vasogenic edema characterized by BBB damage is significant within TP,which leads to decreased expression of AQP4.However,the subsequent up-regulation of AQP4 results in intracellular edema,which accelerate the spreading of TP.AQP4 may involve in body's defense reaction.