Effects of infrasound at 8 Hz 90 dB/130 dB on NMDAR1 expression and changes in intracellular calcium ion concentration in the hippocampus of rats.

In the present study, we investigated the effect of infrasound on the expression of N-methyl-D-aspartate (NMDAR)1 as well as changes in intracellular calcium ion concentration [(Ca2+](i)) in the hippocampus of rats. Sprague-Dawley (SD) rats were exposed for 2 h daily to infrasound at 8 Hz 90 dB or 130 dB, and NMDAR1 expression was examined on days I, 7, 14, 21 and 28. The expression of NMDAR1 in the rat hippocampus upon exposure to infrasound at 8 Hz 90 dB sound pressure level (SPL) showed an initial decrease on day 1, an increase on days 7 and 14, a further decrease on day 21, and a return to normal levels on day 28. The peak level was observed on day 14 in every examined subregion of the hippocampus. By contrast, exposure to infrasound at 8 Hz 130 dB SPL had opposite effects, showing an increase on day 1, a decrease on day 7, a decrease to the lowest point on days 14, another increase on day 21 and a return to normal levels on day 28. The lowest expression of NMDAR1 was found in the CA1 and CA3 regions on day 14 and in the DG region on day 7 with exposure at 130 dB. There were significant differences in [Ca2+](i) concentration on days 14 and 21 with infrasonic exposure at both 8 Hz 90 dB and 130 dB, but no significant differences in [Ca2+](i) concentration on days 1, 7 and 28 compared to the control group. The highest [Ca2+] (i) level was noted on day 14 with infrasound exposure at 8 Hz 130 dB. These changes suggest that 8 Hz 90 dB/130 dB infrasound exposure induced certain reversible changes in NMDAR I expression and [Ca2+](i) concentration in hippocampal cells, which may influence mnemonic functions related to the hippocampus.