To investigate whether mild hypothermia can promote neurogenesis in the dentate gyrus of hippocampus and cognitive function recovery after traumatic brain injury (TBI) through inhibiting apoptosis of hippocampal neurons.

A total of 66 healthy adult Sprague-Dawley rats were randomly divided into sham group, TBI group and TBI+ hypothermia group, with 22 rats in each group. The rat TBI model was established using the fluid percussion device. The rats in TBI+ hypothermia group received 4-hour hypothermia therapy immediately after injury, with the target temperature of 33.5℃. Bromodeoxyuridine (BrdU) was injected into the rats' abdominal cavity to label the mitotic cells. The test of Morris water maze was used to evaluate the rats' spatial learning and memory capabilities. Immunofluorescence staining was used to observe the expression levels of BrdU, doublecortin (DCX), neuron specific nuclear protein (NeuN), cysteinyl aspartate specific proteinase 3 (caspase-3) and cleaved caspase-3 expressions in dentate gyrus of hippocampus at 7 days and 28 days after injury. Expressions apoptosis-related proteins including the factor associated suicide (FAS)/factor associated suicide ligand (FASL), B-cell lymphoma-2 (Bcl-2), caspase-3 and cleaved caspase-3 expressions were detected by Western blot assay.

The water maze tests at 28 days after injury showed that compared with TBI group, the escape latency in TBI+ hypothermia group was significantly shorter [(24.2±5.9)s∶(18±4.1)s], and both the time in the target quadrant and the number of platform crossing were increased significantly [(24.9±6.5)s∶(31.7±5.2)s; (1.9±0.8) times∶(3.5±1.2)times](P<0.05). Compared with the sham group, in TBI group and TBI+ hypothermia group, the BrdU⁺ new-born cells in the dentate gyrus of hippocampus were significantly increased at 7 days after injury [(9.4±4.1)∶(33.4±3.8); (9.4±4.1)∶(45.8±5.6)], the BrdU⁺ /DCX⁺ new-born neurons were increased at 7 days after injury [(2.0±0.6)∶(9.6±1.6); (2.0±0.6)∶(19.2±3.7)], and the BrdU⁺ /NeuN⁺ mature neurons were increased at 28 days after injury [(2.6±1.0)∶(17.2±3.9); (2.6±1.0)∶(33.6±9.1)] (P<0.01). TBI group showed more obvious increase than the TBI+ hypothermia group (P<0.01). Moreover, compared with 7 days after injury, the number of BrdU⁺ cells at 28 days after injury was further increased in TBI+ hypothermia group but decreased in TBI group [(45.8±5.6)∶(58.8±9.2); (33.4±3.8)∶(22.0±3.5)](P<0.05 or <0.01). Compared with the sham group, the caspase-3⁺ NeuN⁺ and caspase-3⁺ NeuN⁺ apoptotic neurons were significantly increased at 7 days after injury in TBI group [(2.0±0.9)∶(11.6±2.6); (2.6±1.0)∶(10.2±2.9)] (P<0.05). Compared with the TBI group, the cleaved caspase-3⁺ NeuN⁺ apoptotic neurons were decreased in TBI+ hypothermia group [(6.6±2.0)∶(11.6±2.6)](P<0.05). Furthermore, compared with the TBI group, mild hypothermia might down-regulate the expression of FAS, FASL, cleaved caspase-3 and caspase-3 and up-regulate the expression of Bcl-2 in the hippocampus [(1.54±0.15) ∶(1.14±0.12); (1.06±0.04)∶(0.80±0.09); (0.84±0.03)∶(0.62±0.08); (0.93±0.06)∶(0.86±0.09); (0.71±0.01)∶(1.58±0.18)](P<0.05).

Mild hypothermia might inhibit apoptosis of hippocampal neurons through cleaved caspase-3, FAS/FASL and Bcl-2 pathways, thus improving the neurogenesis and maturation of neurons in the dentate gyrus of hippocampus and facilitating cognitive function recovery in rats. It indicates that the function of hypothermia in anti-apoptosis and neurogenesis and maturity of hippocampal neurons may have a potential role in predicting the prognosis of TBI patients.

Hypothermia; Brain injuries; Hippocampus; Neuve regeneration; Apoptosis