Mechanisms Of Infrasound-Induced Hippocampal Neurogenesis Suppression

Infrasound, which is usually defined as sound with frequency below20Hz,is generated by mechanical vibration and ubiquitous in nature and socialenvironment. Due to its properties of low frequncy, strong penetration, littleattenuation during long distance propagation, and difficulty in protection,infrasound at high intensity has become a new public health hazard. There aremany sources in our environment that produce infrasound, which play animportant part in noise pollution. A lot of studies about infrasound have foundthat it can do harm to many body systems, especially for CNS, because of thesimilar range of natural frequency between brain and infrasound. Among theseinjuries, the learning and memory impairments draw our great attention.Learning and memory in adult mammals have great relation to theneurogenesis of dentate gyrus in the brain, where granule neurons are generatedthroughout life from a population of NSCs. NSCs in the SGZ of dentate gyrusproliferate, migrate, differentiate into newborn neurons, integrate into theexisting circuitry and receive functional input. Previous research has shown thatinfrasound of a certain exposure parameter can down-regulate the number ofNSCs in SGZ, but the mechanism of function is still unknown.The regulation of hippocampal neurogenesis is complicated and usually affected by both intracellular and extracellular factors simultaneously. Amongnumerous extracellular negative regulators, the inflammation induced bymicroglia and astrocyte plays an important part. Previous studies haveinvestigated that the microglia and astrocyte are both activated immediately andtheir numbers increase significantly after infrasound exposure, so we wonder ifthe activated glia can do some adverse effects to NSCs. Besides, intracellularpathway is another important regulator of neurogenesis, and infrasound mayalso exert its function by this way.Objective:(1) To identify the roles of microglia and astrocyte in the infrasound-induced neurogenesis suppression, and speculate the possible mechanism offunction.(2) To observe the effects of infrasound on the survival, proliferationand differentiation of NSCs in vitro.Methods:(1) Adult rats were put into infrasound chamber with the parameter of16Hz and130dB for7d. After that, the proliferation, migration and differentiationof NSCs in SGZ were examined.（2）We used minocycline and fluorocitrate toinhibit the activation of microglia and astrocyte respectively, and observed thechange of the number of NSCs in SGZ.(3) Microglia and astrocyte werecultured and received infrasound exposure respectively. Then the activatedconditioned medium of them were used to interfere cultured NSCs and thechange of NSCs was detected.(4) To investigate the possible inflammatorymechanism of infrasound-induced neurogenesis reduction, we tested theconcentration of several proinflammatory factors of hippocampal tissue in vivoand observed the expression of NF-κB in both microglia and astrocyte afterinfrasound in vitro.(5) We cultured NSCs and observed the effects of infrasoundon the survival, proliferation and differentiation of NSCs. Result:(1) After the7-day exposure to infrasound, the numbers of NSCs andnewborn neurons in the SGZ were reduced sharply compared with the controlgroup, but the migration and differentiation of NSCs did not changesignificantly.(2) By immunohistostaining, we found that the NSCs and newbornneurons recovered greatly in the group received minocycline or fluorocitratetreatment versus to the infrasound group.(3) MTT assay and immunostaining ofBrdU and Ki67proved that, both the supernatant of microglia and astrocytecollected8h，12h and24h after infrasound inhibited the survival andproliferation of NSCs significantly.(4) The result of liquid protein chip testshowed that, the concentration IL-1β increased a lot in hippocampal tissue ofinfrasound group. Using the method of Western blot, we found that, theexpressions of NF-κB p65in both microglia and astrocyte were increasedstrikingly.(5) After infrasound intervention, the number and size of neurospheredecreased markedly. The survival rate and number of NSCs were also decreasedsignificantly, which was detected by the MTT assay and immunostaining ofBrdU and Ki67. Beside, infrasound had no affect on the differentiation of NSCs.After the infrasound exposure, the percentage of neuron, astrocyte oroligodendrocyte was unaffected.Conclusion:These findings indicate that infrasound can affect NSCs through both theintracellular and extracellular factors. Inflammation may be one of the mostimportant extracellular negative regulators after infrasound, which can activatemicroglia and astrocyte, modulate the NF-κB pathway and increase theexpression of IL-1β to interfere the process of neurogenesis. Moreover，infrasound can also exert its inhibitory effect on the survival and proliferation ofNSCs through intracellular pathway.