The Effect Of PM2.5 Short-term Exposure On Depression And Its Related Mechanisms

In recent years,a lot of health problems caused by air pollution,especially particulate matter with a diameter no more than 2.5 μm(PM2.5),have drawn more and more attention.Many in-depth studies focus on the effect of PM2.5 on the respiratory system and cardiovascular system and the related mechanism.PM2.5 exposure may increase the incidence and mortality of cardiopulmonary diseases such as myocardial infarction,stroke,asthma,chronic obstructive pulmonary emphysema and lung cancer.PM2.5 can cause systemic inflammation and oxidative stress by inducing lung inflammation and oxidative stress,or cause the imbalance of autonomic nervous system through the circulatory system or airway neuroreceptor,thus promoting the progression of underlying disease.However,there are few studies in literature pertaining to the effect of PM2.5 exposure on mental disorders,especially on depression.Epidemiological studies have shown that short-term exposure to air pollution can increase the numbers of out-patients with depression and their suicidal ideations,but the mechanism is unknown.The current studies have shown that air pollution can induce inflammation in the central nervous system,and inflammation does play a key role in the pathophysiology of depression.The levels of inflammatory cytokines in depression patients are higher than those in normal people.Meanwhile,in the depression-related brain regions including the hippocampus and hypothalamus,there are a large number of inflammatory factor receptors,which can interfere with neurotransmitter metabolism and transport,affect the plasticity,growth and survival of neurons,and thereby play a role in the pathophysiology of depression.Therefore,we propose a hypothesis that short-term exposure to PM2.5 may aggravate depressive symptoms by inducing central nervous system inflammation.To test this hypothesis,we conducted the experiments in the following three aspects: 1)the effect of short-term exposure to PM2.5 on chronic unpredicted mild stress(CUMS)-induced depression in rats and its mechanism;2)the time sequence of oxidative stress and inflammatory response in lung tissue induced by PM2.5 exposure;3)test at cellular levels and the investigation of related molecular mechanism.Based on the above-mentioned experiments,we can investigate the effect of short-term exposure to PM2.5 on depression and clarify its possible mechanism of action,in order to provide a theoretical reference for the prevention and treatment of depression in susceptible population.Ⅰ The effect of short-term exposure to PM2.5 on CUMS-induced depression in rats and its mechanismIn order to investigate the effect of short-term exposure to PM2.5 on depression and its underlying mechanism,we firstly established a depression model in rats by chronic unpredictable mild stimulation.The bronchial instillation of PM2.5 was given to Sprague-Dawley rats for exposure at a dose of 4 mg/kg,once every two days,for three times.At 12 h after the last exposure,sucrose preference and open-field movement distance were determined in rats.We found that the exposure to PM2.5 reduced sucrose preference and open-field movement distance in rats with CUMS-induced depression,suggesting that short-term exposure to PM2.5 may aggravate depressive symptoms.Subsequently,we analyzed the serum levels of inflammatory cytokines — adrenocorticotrophic hormone(ACTH)and cortisol(CORT)in rats.The results showed that PM2.5 exposure increased the levels of interleukin(IL)-1β,IL-6,and tumor necrosis factor(TNF)-α in depressive rats and induced the elevation of ACTH and CORT concentrations,but presented no significant effect on IL-4 and interferon-γ,indicating that short-term exposure to PM2.5 can induce systemic inflammation in depressive rats,leading to the hyperactivity of the HPA axis.Furthermore,we performed functional magnetic resonance(FMR)analysis in rats at resting state,and found that short-term exposure to PM2.5 reduced the amplitude of low-frequency fluctuation in the hippocampus and lateral nucleus of dorsal thalamus of rats.Next,we used high-performance liquid chromatography to analyze the related neurotransmitters in the two encephalic regions and found that PM2.5 exposure only decreased 5-HT level in the hippocampus and produced no significant effect on other neurotransmitters.Finally,we separated hippocampus tissue and analyzed the expression of inflammatory factors and brain-derived neurotrophic factor(BDNF)by quantitative PCR.The results showed that short-term exposure to PM2.5 could increase the expression of IL-6 and TNF-α in the hippocampus and inhibit the expression of BDNF.The above-mentioned results indicate that short-term exposure to PM2.5 aggravates ethological manifestations in depressive rats,which may be related to the increased expression of inflammatory cytokines and decreased expression of 5-HT and BDNF in the hippocampus induced by short-term exposure to PM2.5.It provides direct experimental evidence for epidemiological reports,suggesting that depressive patients should be included in the population susceptible to air pollutants.Ⅱ The time sequence of oxidative stress and inflammatory response in lung tissue induced by PM2.5 exposureA large number of studies suggest that there is a close association between systemic inflammation and nervous tissue inflammation,and the latter is one of the important factors causing depression.The lung is the direct target organ of PM2.5 exposure and the start site of local and systemic oxidative stress and inflammatory response induced by PM2.5 exposure.In order to understand the relationship between systemic inflammation and nervous tissue inflammation,it is important to analyze the time sequence of oxidative stress and inflammatory response under PM2.5 exposure.In this study,PM2.5 was applied for a single bronchial exposure to observe the time sequence of nitric oxide(NO),inflammation,and oxidative stress in lung tissue.The results showed that PM2.5 exposure first caused an increase in NO level,followed by oxidative stress,and then inflammatory response.It revealed that NO may play an important role in systemic inflammation and oxidative stress induced by PM2.5 exposure.However,NO,as a small molecule that easily passes the blood-brain barrier,may play an important role in the pathophysiology of depression.The results showed that PM2.5 exposure only induced the up-regulation of inducible nitric oxide synthase(iNOS)expression in tissue,but had no significant effect on endothelial nitric oxide synthase and neuronal nitric oxide synthase,suggesting that PM2.5 exposure may mediate the production of NO by inducing up-regulation of iNOS expression.In this section,we found that NO production after PM2.5 exposure preceded oxidative stress and inflammation,which demonstrated the important biological function of NO.Further analysis showed that the elevation of NO level was mediated by the up-regulation of iNOS expression in lung tissue induced by PM2.5 exposure.The results clarify the time sequence of oxidative stress and inflammatory response after PM2.5 exposure,indicate the potential key role of NO in this process,expand the understanding of the association between systemic inflammation and nerve tissue inflammation,and provide a reference for subsequent in-depth studies.Ⅲ The time sequence of oxidative stress,and inflammatory response in Beas-2b cells after PM2.5 exposure and its mechanismIn this section,human bronchial epithelial cells(Beas-2b)were used to verify the results in animal experiment and explore the potential mechanism.At first,we detected the apoptosis of Beas-2b cells after being exposed to different concentrations of PM2.5 by Annexin-V/PI double staining.With the increase in PM2.5 concentration,cell apoptosis was increased.After exposure to 100 μg/mL PM2.5,the levels of NO,IL-6,and reactive oxygen species(ROS)were determined at different time points.It was found that PM2.5 exposure first induced the production of NO and ROS,and then induced the up-regulation of IL-6 expression.These results are consistent with the findings in animal experiment,further suggesting that NO may play an important role in the biological effect induced by PM 2.5 exposure.Furthermore,after exploring the mechanism of NO production,we found that after PM2.5 exposure,the expression of iNOS was specifically elevated,leading to the increased NO production.Noticeably,the results of immunofluorescence staining,Western blot,and electron microscopy all confirmed that the level of cell autophagy was significantly increased during this process.Finally,iNOS siRNA was used to inhibit the expression of iNOS.After blocking the iNOS signaling pathway,the production of NO,as well as the formation of autophagic bodies,was reduced,the expression of autophagy-related proteins was down-regulated,and cell death was also reduced to some degree.The above-mentioned experimental results demonstrated that after Beas-2b cells were exposed to PM2.5,the production of NO and ROS was prior to the expression of inflammatory cytokines.This process was accompanied by the activation of autophagy-related signaling pathways,and iNOS might mediate NO production and cell autophagy after PM2.5 exposure,suggesting that autophagy may be involved in oxidative stress and inflammatory response.In conclusion,short-term exposure to PM2.5 attenuates the Bold signal in the hippocampus and dorsal thalamus,and may aggravate the depressive symptoms by inducing inflammation in the hippocampus and inhibiting the expression of 5-HT and BDNF.Systemic oxidative stress and inflammatory response induced by PM2.5 exposure may be closely related to inflammation in the hippocampus.In the study on the time sequence of PM2.5 exposure-induced oxidative stress and systemic inflammation,we found that the abnormal increase in NO mediated by iNOS first occurred after PM2.5 exposure,followed by oxidative stress and inflammatory response,suggesting that NO plays a key role during the process.At the cellular level,we not only verified the results in animal experiment,but also found that NO produced after PM2.5 exposure mediated the autophagy of cells,suggesting that it may also play a role in subsequent oxidative stress and inflammation.Our study has proved that short-term exposure to PM2.5 may aggravate the progression of depression in rats,which provides a theoretical reference for clarifying the molecular mechanism of PM2.5 exposure aggravating depressive symptoms and preventing the depression related to air pollution.