Exposure to air pollution has previously been implicated in neuroinflammation and oxidative stress but potential impacts on memory and behaviour have yet to be thoroughly investigated. In an article recently published in the Journal of Environmental Health Science and Engineering, researchers use mice to explore the effects of exposure to diesel exhaust particles (DEPs) on behaviour and the hippocampus (Ehsanifar et al, 2021). Prolonged exposure to DEPs, particularly for 2 or more hours a day, resulted in impaired spatial memory and increased anxious behaviour in otherwise healthy mice. Along with altered behaviour, the mice exposed to DEPs also displayed increased neuroinflammation and neuronal loss in the hippocampus. These findings suggest air pollution causes neural damage, impairs memory, and increases anxiety. With increasing air pollution in the world these findings implicate the health of millions living amongst polluted air (Wu et al, 2020).

Background

A significant amount of air pollution comes from motor vehicles, meaning DEPs constitute a large portion of particulate matter in polluted air (Bartra et al, 2007). DEPs are ultrafine and extremely toxic, this allows DEPs to easily enter and travel through the body, causing damage to vital organs. These pollutants have been shown to alter blood-brain barrier function, causing an upregulation of transportation across the barrier along with increasing neuroinflammation (Hartz et al, 2008 and Oppenheim et al, 2013). With passage to the brain through the blood-brain barrier, DEPs have the potential to cause behavioural changes such as learning and memory deficits and increased anxiety. Previous research has indicated an impact on memory after exposure to DEPs (Hougaard et al, 2008). To investigate the cause of DEP exposure-induced memory deficits, the hippocampus must be further analyzed after exposure to DEPs.

Methods

48 healthy, adult male mice were randomly divided into four equal groups: a control group, and three groups exposed to DEPs for 2, 5, and 7 hours a day for 12 weeks. To analyze anxiety in the mice, an elevated plus maze is used. This elevated platform contains 4 arms, two of are enclosed and two of which are open. Mice are placed in the center of the plus maze, facing an open arm, and left to explore for 5 minutes. Spending more time in the closed arms of the maze is considered anxiety-like behaviour while spending time in the open arms constitutes normal, non-anxiety behaviour in the mice (Walf & Frye, 2007). Behaviour is quantified using the amount of time spent in the open arms and the number of entrances into the open arms. The Morris water maze is used to investigate spatial memory and learning (Vorhees & Williams, 2006). This maze involves a pool with four quadrants, one of which has a target platform placed in the center of the quadrant just below the water’s surface. Mice are placed in the quadrant diagonal from the one containing the hidden platform. The mice then swim until finding the platform where they can stand to be rescued from the pool. If mice are unable to find the platform after swimming for 90s the experimenter places the mouse on the platform where the mouse must stand for 10s before being rescued. The water is made opaque using milk powder so the mice must rely on spatial memory to locate the target platform. Mice were trained to find the hidden platform for 4 days with 3 trials per day before the administration of DEPs. This was then repeated (3 trials a day for 4 days) after administration of DEPs. Memory was quantified using the time required to find the hidden platform. On day 5 the hidden platform was removed, and memory was quantified using the time spent in the target quadrant. Following behavioural tests mice were sacrificed and the hippocampi removed for analysis.

Results

Increased exposure to DEPs resulted in a decrease of entries into the open arms, and an extreme decrease in overall time spent in the open arms, of the elevated plus maze. Less time spent in the open arms of the maze indicates increased anxiety in the mice after exposure to DEPs compared to control mice. Prolonged exposure of DEPs for more than 2 hours a day caused a significant decrease in time spent in the target quadrant of the Morris water maze and increased exposure to DEPs resulted in increased time required to find the platform. This indicates impaired spatial memory in the mice corresponding to increased DEP exposure.

Analysis of the hippocampi found increasing MDA levels correlated with increasing DEP exposure. A similar increase is seen in the expression of pro-inflammatory cytokines with increasing exposure to DEPs. This indicates an increase in oxidative stress and neuroinflammation in the hippocampus with increased DEP exposure. Additionally, an overexpression of glutaminase is seen in the hippocampus of mice after DEP exposure which has been previously linked to memory deficits and neuroinflammation (Wang et al, 2017). Nissl staining of the hippocampus also shows loss of CA1 and CA3 neurons with increased exposure to DEPs. This neuronal loss indicates the damage possible from DEP exposure. Surprisingly, expression of NMDA receptor subunits NR2A and NR3B was increased with exposure to DEPs. Normally, increased expression of NMDA receptors corresponds to increased synaptic plasticity and improved memory (Li & Tsien, 2009). This is contradictory to what we see in the mice who exhibit impaired spatial memory after exposure to DEPs. This may be the brain’s way of compensating for the neuronal loss seen with DEP exposure but more research is needed to elucidate this mechanism.

Conclusions

Exposure to DEPs resulted in increased anxiety and impaired spatial memory in the otherwise healthy mice. Further analysis of the hippocampi indicates neuroinflammation and neuronal loss after DEP exposure. The damage seen in the hippocampus provides evidence for possible neural injury due to DEP exposure (Ji et al, 2019). This is relevant to the health and wellbeing of the general population living with polluted air. The serious effects of prolonged DEP exposure highlight the importance of taking care of the environment to take of ourselves. Future research into the mechanism of neuronal death associated with DEP exposure is still needed to fully understand the cellular impact of air pollution in the brain. 

[+] References

[+] Other Work By Alex Jensen