Abstract: Objective To explore the effects of delayed mild hypothermia (MHT) in different time windows on the expressions of Bcl-2, Bax and Caspase-3 in brain tissue of model rats with traumatic brain injury (TBI). Methods Thirtysix clean adult male SD rats were randomly divided into NT group (normal temperature), MHT 15 min group, MHT 2 h group and MHT 4 h group. TBI rat model was established by electronical controlled cortical injury device. The rats in the NT group were treated with normothermia (37 ℃) and the rats in the three hypothermia groups were implemented with low temperature (33.0±1.0) ℃ at 15 min, 2 h and 4 h for 6 h respectively after establishment of TBI model. The modified neurological senerity scores (mNSS), morphological changes in hippocampal CA1 areas, immunohistochemical staining and Western blot assay for Bcl-2, Bax and Caspase-3 were compared 3 days after TBI between the four groups. Results The neurological behavioral deficits were found in each group. Compared with the NT group, the mNSS were decreased in the three hypothermia groups (P < 0.01). The results of HE staining showed that the structure of neurons was regular and arranged neatly, and the number of neurons decreased with alleviated nuclear fragmentation and dissolution in hypothermia groups. Compared with the NT group, the expression of Bcl- 2 was upregulated, and the expressions of Bax and Caspase- 3 were downregulated in three hypothermia groups (P < 0.05). The above experimental results were superior in MHT15 min group to MHT 2 h group, and the therapeutic effect in MHT 2 h group was similar to MHT 4 h group. Conclusion The proper delayed mild hypothermia treatment could inhibit neuronal apoptosis and alleviate brain damage.

Key words: craniocerebral trauma, proto- oncogene proteins c- bcl- 2, bcl- 2- associated X protein, caspase 3, low temperature, time window