Protective effect and mechanism of heat acclimation on hippocampus neuron injury in mice after exposure to electromagnetic field

To investigate the protective effect and mechanism of heat acclimation (HA) on electromagnetic field (EMF) induced hippocampus neuron injury in mice.

Forty healthy BALB/c male mice (18~22 g, 7 weeks old) were randomly divided into 4 groups (n=10): Control group (Con), HA group (34 ℃, 30 d), EMF group (2450 MHz, 20 min/d, 4 weeks) and HA+EMF group (HA preconditioning+EMF). Sucrose preference test was performed to evaluate sucrose preference levels of mice in each group. Tail suspension test and forced swimming test were utilized to observe the immobility time. Morris water maze test was conducted to determine the learning and memory capabilities. Pathological changes in the hippocampus were observed with HE staining. Immunohistochemical assay for Iba1 (marker of microglia), CD68 (marker of pro-inflammatory phenotype) and CD206 (marker of anti-inflammatory phenotype) were used to detect the number and activation phenotype of microglia in the hippocampus. ELISA was applied to measure the levels of TNF-α, IL-1β, TGF-β and IL-10 in the hippocampus of each group. Western blotting was performed to determine the protein levels of HSP70 in the hippocampus.

As compared with the Con group, the EMF group showed a decreased preference for sucrose (P<0. 05), prolonged immobile time in the tail suspension test (P<0. 01) as well as in the forced swimming test (P<0. 01), extented escape latency on the 7th day (P<0. 01), and a decreased time of crossing the platform (P<0. 05). EMF exposure resulted in that the hippocampal neurons were in disordered arrangement, loose structure and irregular morphology, with swollen cytoplasm and condensed nuclei, swollen and more microglial cells in the hippocampus (P<0. 01), and enhanced relative fluorescence intensity of CD68 (P<0. 01), but not in CD206 fluorescence intensity (P=0. 885). All these findings suggested that activated microglia predominantly exhibited a pro-inflammatory M1phenotype during this phase. In the hippocampus, the levels of TNF-α and IL-1β were significantly increased, while the levels of IL-10 and TGF-β were significantly decreased (P<0. 01). HA treatment reversed the conditions induced by EMF exposure, including better preference for sucrose (P<0. 01), shorten immobile time in tail suspension test (P<0. 05) and forced swimming test (P<0. 01), less escape latency on the 7th day (P<0. 01), and improved hippocampal cell injuries. Compared with the Con group, there were more microglial cells in the hippocampus in the HA+EMF group, with increased relative fluorescence intensity of M2phenotype marker CD206 (P<0. 01) and decreased CD68 fluorescence intensity (P<0. 01). HA treatment also significantly decreased the expression of TNF-α and IL-1β levels (P<0. 01), increased the expression of IL-10and TGF-β (P<0. 01), and elevated the protein level of HSP70 (P<0. 01) when compared with the EMF group.

HA may ameliorate EMF-induced hippocampus neurons injury in mice by altering the phenotype of activated microglia and inhibiting inflammatory responses.