Effect of mild hypothermia on ischemic brain edema and expression of aquaporin-4 in rats

Lei YAN; Ruiguo DONG; Yinming ZENG; Deqin GENG

ABSTRACT

BACKGROUND:

Mild hypothermia (28-35 ℃) is becoming one of the promising methods in treating acute ischemic stroke. Hypothermia can effectively lessen brain edema, which is one of its neuroprotective roles.

OBJECTIVE:

To investigate the effect of mild hypothermia on brain water content and aquaporin-4 (AQP4) expression level following global cerebral ischemia-reperfusion injury in rats, so as to study the neuroprotective mechanisms of mild hypothermia.

DESIGN:

A randomized controlled trial.

SETTING:

Neurobiological Laboratory of Xuzhou Medical College.MATERIALS 110 healthy male SD rats with body mass 250-300 g, provided by the Animal Center of Xuzhou Medical College, No. SYNK (Jiangsu) 2002-0079, were selected and randomly divided into 3 groups by SPSS 11.0software ①sham-operated group (n=10);②normothermiagroup (n=50); ③mild hypothermia group (33 ℃, n=50). Normothermia group and mild hypothermia group were subdivided into five reperfusion subgroups for 6 hours, 1, 2, 3 and 7 days, respectively with 10 rats in each subgroup,in which 5 rats were used for measurement of brain water content, and others for HE staining and immunohistochemistry staining.

METHODS:

The models of global cerebral ischemia were established in the normothermia group and mild hypothermia group by four-vessel occlusion (4-VO) method with ischemia for 15 minutes as Pulsinelli described.The rats in the sham-operated group were only underwent the electrocauterization of bilateral vertebral arteries and the isolation of common carotid arteries except for occlusion of common carotid arteries. Twenty-four hours later, the rats were decapitated to take out the brains. The brains of rats in the normothermia group and mild hypothermia group were taken out to make sections for HE staining and immunohistochemistry staining, and the dynamic change of pathology of the brain tissue and AQP4 expression level were observed after reperfusion for 6 hours, 1, 2, 3 and 7 days, respectively. The brain wet-to-dry weight measurement was used to measure the brain water content of the rats at each time point of each group.MAIN OUTCOME

MEASURES:

①The pathologic changes of brain tissues of rats in the normothermia group and mild hypothermia group. ②The brain water content and the AQP4 expression level of all rats at each time point.

RESULTS:

①After 6 hours of reperfusion, brain edema appeared in the normothermia group including amplification of periyascular spaces and intercellular spaces, rarefaction of brain tissues, etc, which got worst after 2 days of reperfusion; the phenomenon of brain edema of the rats in the mild hypothermia group at each time point was relatively lighter than the normothermia group. ②Brain water content of the normothermia group and mild hypothermia group was increased after 6 hours of reperfusion and reached peak at the 2nd day; Although decreased at the 7th day, it was still higher than the sham-operated group. The brain water content of the mild hypothermia group at each time point was less than the normothermia group (values after 6 hour and 7 day, P ＜ 0.01, the rest groups P ＜ 0.05).③AQP4 expression level of the normothermia group and mild hypothermia group was increased after 6 hours of reperfusion and reached peak at the 2nd day. Although decreased at the 7th day, it was still higher than the sham-operated group.The AQP4 expression level of the mild hypothermia group at each time point was lower than the normothermia group (P ＜ 0.01).

CONCLUSION:

The change tendency of AQP4 level is parallel to that of brain water content after ischemia reperfusion, which indicates that the upregulation of AQP4 expression is one of molecular mechanisms for the for mation of ischemicbrain edema. Mild hypothermia can release ischemic brain edema by inhibiting AQP4 expression, which is one of its mechanisms.