To investigate the effects of extremely low frequency magnetic fields on spatial memory and learning in mice as well as its underlying molecular mechanisms.
In previous studies, the authors reported that mice exposed to extremely low frequency magnetic fields showed an increased neurogenesis in the hippocampus (e.g. Podda et al., 2014). Further understanding of this phenomen could help to develop novel therapeutic approaches in the treatment of neurological disorders.
For an in vivo experiment, mice (n=38) were randomly divided into an exposure and a sham exposure group. Different examinations were performed:
1.) For immunohistochemical analysis, 7 mice from each group received daily injections of 100 mg bromodeoxyuridine per kg body weight before each exposure (12 days). Mice were killed 1 day (n=4) or 40 days (n=3) after the last exposure or sham exposure session and the brain was taken for examinations.
2.) 30 days after the last exposure (n=16) or sham exposure (n=14) session, the Morris water maze was performed.
3.) 36 days after the last exposure or sham exposure session, the novel recognition test was performed (each n=8).
For an in vitro experiment, neural stem cells were cultivated in an incubator while exposed or sham exposed. Partially, the calcium channel blocker nifedipine was added to the cell cultures. Different examinations were performed:
1.) For proliferation and immunohistochemical analysis, cells were exposed or sham exposed for 6 or 24 hours while cultivated in proliferation medium.
2.) Cells were exposed or sham exposed for up to 10 days in differentiation medium. Gene expression was examined during the first 2 days (after 0, 6, 12, 24, and 48 hours) and protein expression on the first exposure day as well as from day 6 to 10.
| Exposure | Parameters |
|---|---|
|
Exposure 1:
50 Hz
Exposure duration:
3.5 hours per day for 12 days
|
|
|
Exposure 2:
50 Hz
Exposure duration:
6, 12 or 24 hours per day for up to 10 days
|
|
| Frequency | 50 Hz |
|---|---|
| Type | |
| Waveform | |
| Exposure duration | 3.5 hours per day for 12 days |
| Additional info | in vivo |
| Exposure source | |
|---|---|
| Chamber | 3 to 4 animals in a plastic cage (33 Π15 Π13 cm) which was put in the solenoid |
| Setup | solenoid was made of copper wire wrapped around a Plexiglas cylinder (diameter 20 cm; length 42 cm) with open extremities in which the cage was positioned; geometry of the system assured field uniformity within the entire length of the cage |
| Sham exposure | A sham exposure was conducted. |
| Measurand | Value | Type | Method | Mass | Remarks |
|---|---|---|---|---|---|
| magnetic flux density | 1 mT | - | - | - | - |
| Frequency | 50 Hz |
|---|---|
| Type | |
| Waveform | |
| Exposure duration | 6, 12 or 24 hours per day for up to 10 days |
| Additional info | in vitro |
| Exposure source |
|
|---|---|
| Setup | solenoid was placed inside an CO2 incubator |
| Sham exposure | A sham exposure was conducted. |
| Additional info | control cell cultures were put in another incubator; maximum temperature increase due to exposure was 0.4°C ± 0.1°C |
| Measurand | Value | Type | Method | Mass | Remarks |
|---|---|---|---|---|---|
| magnetic flux density | 1 mT | - | - | - | - |
In the in vivo experiment, exposed mice showed significantly enhanced memory and learning abilities compared to sham exposed mice. Additionally, immunohistochemistry revealed that the cell proliferation and cell differentiation were significantly increased in the exposed mice in comparison to the sham exposed ones.
Also in the in vitro experiment, in exposed cell cultures, the cell proliferation was significantly higher and the cell differentiation was significantly more enhanced than in sham exposed cell cultures. Additionally, exposure partially led to significant changes in the gene expression of Hes1 (increased after 0 and 24 hours, decreased after 6 and 12 hours), NeuroD1 (increased after 24 hours), Neurogenin1 (increased after 24 and 48 hours) and Mash1 (increased after 0 and 6 hours, decreased after 12 and 48 hours). Furthermore, in a exposed cell cultures, the acetylation of histone and the protein expression of phosphorylated CREB were significantly increased compared to the sham exposure. An addition of nifedipine prevented the exposure-induced effects.
The authors conclude that extremely low frequency magnetic fields could enhance hippocampal neurogenesis and improve learning and memory abilities of mice.
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