The Impact Of Strong Low-frequency Noise On Spatial Learning Memory And Hippocampus Cytoarchitecture Of Rats

Object:To observe the impact of spatial learning memory and cytoarchitecture of hippocampus in rats exposed to strong low-frequent noise and to explore its injury mechanisms.Methods:48 healthy SD rats were randomly divided into group A (not being exposed to strong low-frequency noise), group B, C, D(exposed to 100Hz,140dB strong low-frequency noise 1h/day for 1 day, 10 days,20 days respectively) and group E, F(keep feeding for 10 days or 20 days regularly after exposure for 20 days). Morris water maze was used to detect spatial learning and memory ability of rats. The morphology and distribution of the hippocampus cells were observed by using HE staining and immunohistochemistry staining and the cell apoptosis in hippocampus were detected by TUNEL method.Results:1. Here were the Morris water maze results in all groups after the exposure of strong low-frequency noise:(1) Incubation period showed:compared with group A, group C, group D, E, F group extended 7s,16.75s,9.5s and 10s respectively; Group C, E, F shorten 9.75s,7.25s, 6.75s respectively, and there was significant difference between each groups. (2) Trajectory figure showed:the way that looking for platform motion in group A and B were mainly the straight line, there was no significant difference about swimming distance; the way in group C, E, F were at random, swimming distance in group C, E, F extended 3.3m, 5.8m and 6.8m respectively compared with group A; however the way in group D which showed the longest swimming distance was marginalized, and compared with group A, B, C, E and F, group D extended 25.9m, 25.3m,22.6m,20.1m,19.1m respectively, and there was significant difference between group A, B, C, E, F and group D. (3) The time during platform quadrant showed:compared with group A, group C, D, E, F were shortened 6s,15.5s,7.75s,8.5s respectively; Compared with group D, group B, C, E and F were extended 14.75s,9.75s,7.75s,7s respectively, and there was significant difference between group B, C, E, F and group D. (4) The times across the platform of all group showed: compared with group A, group C, D, E, F reduced by 2.75,5,3.25,3 times respectively;compared with group D, the group B, C, E and F increased by 4.75,4.75,4.75,2 times, and there was significant difference between group B, C, E, F and group D.2. The hippocampus of rats were stained with HE:cells of the pyramidal layer in group A and B were in neat rows, close together, multiple layer, the group C were turning spars, and those in group D was much more obvious, however, the sparsity of cells of group E, F recovered partly compared with group D; but there had no obvious change after the exposure of strong low-frequency noise in cells of molecular layer and polymorphic layer.3. The apoptosis of hippocampal cell mainly gathered together in the pyramidal stratum after the exposure of strong low-frequency noise.The cell apoptosis rate in group C, D, E, F were 14.08%,31.31%,10.82%, 31.31% respectively, there was significant difference between group C, D, E, F and group A; and there was also significant difference between group C, D, E, F.4. As the extension of time after the exposure of strong low-frequency noise, NSE positive neurons were reduced in pyramidal layer of hippocampus. And after stopping the exposure of strong low-frequency noise,NSE positive neurons had risen.We also calculated the number of NSE positive cells in each group, the results showed A-F group were 57.25,56.25,47.75,33.25,48.25,47.75/field at high magnification; there was also significant difference between group C, D, E, F and group A; there was also significant difference between group B, C, E, F and group D.5. The strong low-frequency noise had no obvious impact on the number of glial cells in the hippocampus.Conclusion:1. The spatial learning and memory function of rats significantly impaired, numbers of neurons in hippocampus was decreased, neuron apoptosis was increased, however, there was no effect on glial cells during long-time exposure (10-20days) to strong low-frequency noise; 2. The spatial learning and memory function of rats could partly improved, numbers of neurons in hippocampus could partly recovered after stopping exposure; 3. It revealed the phenomenon that neuron apoptosis was increased in hippocampus after long-time exposure to strong low-frequency noise, which probably is one of important mechanism of the dysfunction of spatial learning and memory in rats.