Ⅰ. Introduction
Recently, particulate matter(PM) has been a major cause of environmental contamination due to carbon dioxide emissions from fossil-fuel combustion, exhaust gas from automobiles, dust from power generation facilities, etc. Ultra-fine dust of size under 2.5 μm contains heavy metals(copper, lead, zinc, chromium, and cadmium) depending on the source of the agent and may affect people’s health depending on their level of exposure to harmful substances, such as nitrates, sulfates, etc[1,2]. Fine dust enters the human body in the respiratory system, causes lung disease after depositing in the pulmonary alveolus, and increases the risk of cardiovascular diseases, such as angina and myocardial infarction, by increasing the viscosity of blood[3,4].
In addition, fine dust entering the respiratory system breaches the blood–brain barrier (BBB) from inflammatory response. Most previous studies to date have focused on the effect of PM on the respiratory system. No study has investigated the effect on cerebrovascular or brain function of fine dust. The aim of this study was to correlate PM exposure with changes in brain blood flow in Sprague Dawley(SD) rats using micro positron emission tomography(micro-PET) and elucidate the possibility of developing cerebrovascular diseases.
Ⅱ. Material and Methods
1. Experimental design
We used a exposure and generation box of fine dust comprising a transparent acrylic box(400(W) × 300(D) × 300(H) mm) and plastic box(200(W) × 200(D) × 200(H) mm).
Only fine dust was introduced through the exposure box with PM10(diameter less than 10 μm) through the filter.
We performed the brain PET scan using a Focus 120 micro-PET system(Concorde Microsystems Inc., Knoxvill, TN)(Fig. 1).
We measured fine dust using the Light Scattering Fine Dust Meter(JSMY-1000, 3M corporation). We used a generation and exposure box of fine dust comprising a transparent acrylic box and plastic box. Only fine dust was introduced through the exposure box with PM10 (diameter less than 10 μm) through the filter (Fig. 2).
We performed the brain PET scan using a Focus 120 micro-PET system. We measured fine dust using the Light Scattering Fine Dust Meter. We used a generation and exposure box of fine dust comprising a transparent acrylic box. Only fine dust was introduced through the exposure box with PM10 through the filter(Fig. 2). We performed the brain PET scan using a Focus 120 micro-PET system. We measured fine dust using the Light Scattering Fine Dust Meter.
2. Animal or Experiment animal
A total of ten 6-week-old SD rats were divided into a 12-h night/day (lighting at 8:00 am) cycle for 1 week. Rats were adapted to the experimental environment by allowing them to freely consume food and water at 21°C ± 1°C temperature and 60% ± 10% humidity.
To maximize the ingestion of 18F-FDG, rats used in the experiment were provided with only water and maintained at an empty stomach for 18 h. The temperature was maintained at 29°C ± 2°C and humidity at 60% ± 10% in a quiet and dark environment to minimize changes in the biodistribution of 18F-FDG depending on the surrounding environment and temperature.
3. 18F-FDG Micro-PET imaging
An 18F-FDG dose of 44.4 MBq was injected into the tail vein[5]. Rats underwent 18F-FDG uptake at room temperature and humidity for 30 min. After 18F-FDG uptake, the inhalational anesthetic Isoflurane was administered, and rats were positioned on a PET table. The anesthetic concentration was maintained at 2% during imaging. We used a Focus 120 micro-PET system with high resolution(0.5~1.0 mm) for small animals and acquired images in the list mode for 30 min after an interval of 40 minutes following the 18F-FDG injection. Attenuation of the skull bones of rats was speculated to be small during image acquisition. The image was reconstructed using the ordered subset expectation maximization(OSEM) algorithm with 12 repetitions of the subset and 18 map iterations without attenuation correction. To reduce the glucose consumption of rats before imaging, their body temperature was maintained at 36°C ± 1°C using a heating pad composed of carbon installed on the inspection table. First, basal scintigraphy was performed on each rat before exposure to fine dust. After exposure to fine dust four times for a total of 6 h, images were acquired in the same manner as in base photography.
4. Quantitative image analysis & Statistical analysis
The experimental results were analyzed using PMOD 3.2v(PMOD Technologies Ltd., Zurich, Switzerland) to measure the standard uptake values of 18F-FDG PET images reconstructed with the OSEM algorithm to quantify the standard uptake values of the entire rat brain area.
Statistical analysis was performed using Kolmogorov– Smirnova and Shapiro–Wilk test with the SPSS statistical program(ver. 14) to test the parametric method with a small sample size with all ten rats participating in the experiment. Normal distribution was confirmed with p<0.200* and p<0.241. The paired t-test was performed to correlate changes in brain blood flow before and after exposure to fine dust in mice. A p-value <0.05 was considered to be statistically significant.
Ⅲ. Results
1. Concentration of fine dust
The average concentration of fine dust exposed to a total of ten rats was 399.8 ± 10.02 μm/m3 at PM10, 206.2 ± 7.74 μm/m3 at PM2.5, and 169.7 ± 7.06 μm/m3 at PM1.0(Table 1).
2. Changes in brain activity before and after exposure to fine dust
Brain PET was performed 40 min after the 18F-FDG injection to determine changes in brain activity pre and post exposure to fine dust, and the following results were obtained.
After the 18F-FDG injection pre and post exposure to fine dust, the average standard intake coefficient for each major part of the rat brain ranged from 5.76 ± 0.90 to 4.78 ± 0.52 g/mL in the cerebral frontal lobe area, 4.71 ± 0.49 to 3.66 ± 0.48 g/mL in the lower area (hippocampus), 5.38 ± 0.59 to 4.10 ± 0.47 g/mL in the midbrain area, and 4.77 ± 0.58 to 3.55±0.46 g/mL in the cerebellum area(Table 2, Fig. 3).
The 18F-FDG standard intake coefficient for the whole brain was 5.21 ± 0.52 g/mL before exposure to fine dust and decreased to 4.22 ± 0.48 g/mL after exposure, indicating a statistically significant difference (Table 3, Fig. 4).
Ⅳ. Discussion
Recently, public interest in fine dust has been increasing, and attention is being paid to the causal relationship between fine dust and disease emergence.
Fine dust of various sizes(PM10 = less than 10 μm; PM2.5 = less than 2.5 μm; PM1.0 = less than 1.0 μm) is generated from the exhaust of automobiles, chimneys of factories or thermal power plants, and construction sites. Most fine dust is artificially generated by humans[6,7].
The effect of the generated fine dust on the human body is already well-known through many several studies[8-10]. In 2013, the World Health Organization (WHO) International Cancer Institute designated it as a group 1 carcinogen because air pollutants and fine dust are associated with a high incidence of cancers, including lung cancer[11]. Fine dust mainly flows through the respiratory system and enters blood vessels, which can worsen symptoms of cardiovascular disease or cause a stroke. The Korea Centers for Disease Control and Prevention and Miller KA et al. reported that long-term exposure to fine dust increases the mortality rate according to the concentration of fine dust entering the body. According to previous studies, when the concentration of fine dust(PM10) increases by 10 μm/m3, the mortality rate of cerebrovascular disease increases by 10%, and when the concentration of fine dust (PM2.5) increases by 10 μm/m3, the risk of death from cerebrovascular disease increases. It was found to have increased by 80% and increased by more than 20% because of stroke[12].
In particular, An experimental study on the capillaries of mice and reported that fine dust induces cytokines and reactive oxygen species, decreasing the expression of tight junction proteins in cerebral blood flow barrier from alteration of the signal of the membrane transporter, thereby altering its function. Fine dust introduced into the systemic circulation through the respiratory tract may eventually enter the brain through the cerebral blood flow barrier[5].
Therefore, the aim of this study was to investigate the changes in the activation of the mouse brain according to short-term exposure to fine dust. As a result, 18F-FDG intake of the entire brain was 5.21 g/mL on average pre exposure but decreased to an average of 4.22 g/mL post exposure. There was a significant difference, indicating an effect of fine dust on brain activation(p<0.05; Tables 1 and 2). Human brain cells are closely linked to the glucose metabolism process that supplies energy; therefore[13], changes in glucose and oxygen metabolisms in a physiological environment are accompanied by changes in blood flow in the brain, unless there is a pathological abnormality in the blood vessels of the brain. Therefore, brain PET can quantitatively evaluate changes related to brain structure and dysfunction by imaging the brain’s perfusion status, glucose metabolism, and physiological substance intake[14].
Jung et al. reported that 18F-FDG, a glucose analogue, is fixed in the form of FDG-6-phosphate by glucose metabolism when injected into the body and is ingested into the brain tissue, which can reflect changes in glucose metabolism in the brain. Changes in brain activity can be observed based on changes in glucose metabolism[15].
According to the results of the overall brain activity of rats, a significant decrease from 5.21 ± 0.52 before to 4.22 ± 1.48 after exposure to fine dust was confirmed(Fig. 4). As such, ultra-fine dust(PM2.5), which has a small particle size compared to other types of fine dust, is the main cause of various diseases, such as changes in the cardiovascular system and brain activity, depending on blood circulation through the alveola, as well as respiratory diseases. Undertaking countermeasures is urgent because it causes extreme stress and depression in modern society.
In our study, exposure to fine dust affected brain activity. Our limitations are that since the brain activity was evaluated after exposure to fine dust over a relatively short period of time, the effect of long-term accumulated fine dust exposure was not considered, and various concentration changes were not evaluated because the average fine dust(PM2.5) concentration was limited to 206.2 ± 7.74 μm/m3. In modern society, which has been exposed to air pollution for a long period of time, research on the relevance of each disease is continuously being conducted. Therefore, it is necessary to study brain diseases caused by exposure to ultra-fine dust for a long time[16].
Ⅴ. Conclusions
Recently, fine dust, which threatens health, has a significant impact on human health in various forms depending on the size of particles, from respiratory diseases through the lungs to cardiovascular diseases to brain diseases.
Until now, there have been reports that fine dust affects stroke and brain nervous system, but there have been difficulties in clinical application due to few previous studies. However, through this study, it was possible to confirm the correlation between fine dust and brain activity through the lungs.
Therefore, since modern society has a population distribution with weak immunity as it ages, the occurrence of fine dust should be lowered as much as possible, and various clinical studies of brain diseases caused by exposure to fine dust should be preceded.
