Exposure to chronic noise is believed to negatively impact the gut microbiome, which in turn may increase the risk of Alzheimer ’s disease.
Alzheimer’s is a chronic disease that causes brain cells to degenerate and eventually die, leading to a decline in cognitive function. Patients often suffer from symptoms such as long-term memory loss, mood swings and difficulty in communicating. Unfortunately, there is currently no cure for the disease.
Scientists are still unclear about the exact cause of Alzheimer’s disease. Genetics, along with aging, appear to be the most significant risk factors for the disease. The composition of an individual’s gut microbiome, the community of bacteria present in the gut, also appears to be important because these bacteria are known to produce chemicals that communicate with the brain. Animal studies have also suggested that environmental factors such as noise pollution (from cars, trains, and airplanes) can cause symptoms consistent with Alzheimer’s disease and alter the gut microbiome. However, a definitive link between these factors remained elusive until a recent study.
In this study, a team of Chinese scientists sought to better clarify the role of the gut microbiome in Alzheimer’s disease. Specifically, the team from the Tianjin Institute of Environmental and Operational Medicine wanted to uncover the relationship between chronic noise exposure, the microbiome, and aging in the onset of Alzheimer’s.
To investigate this link, the team used a special genetically modified mouse strain that is prone to accelerated aging and exposed the young mice to loud or soft white noise for 4 hours each day for 30 consecutive days. They then performed a variety of cognitive and laboratory tests to see if the mice showed any signs of Alzheimer’s symptoms in response to the noise.
In the first test, the team wanted to determine if chronic noise exposure in mice led to cognitive impairments similar to those seen in human Alzheimer’s patients. To assess their spatial learning ability, the team trained mice to swim to a platform hidden beneath the water from one of four different starting positions. Importantly, the pool was placed in a room with numerous visual cues to allow the mice to orient themselves towards the platform from their starting position. After three days of training, the researchers then recorded the time it took for the mice to find the hidden platform. They found that mice that had been exposed to loud chronic noise and aging mice took significantly longer to find the platform compared to the younger mice. The effect was less pronounced in mice that were exposed to lower volumes indicating that higher noise volume might contribute to more severe effects on cognition.
Perhaps of most interests, Alzheimer’s patients exhibit plaques in their brains that are caused by the accumulation of a protein called beta-amyloid. When researchers in this study examined the brains of the noise-exposed and older mice, they found similarly high levels of the beta-amyloid protein — which suggests that chronic noise exposure, like aging, can lead to symptoms consistent with Alzheimer’s disease.
Furthermore, as they had hypothesized, the researchers were able to determine that noise exposure did indeed cause significant changes in the bacterial community of the gut, reducing the overall diversity of species present in the gut with a marked increase in a particular bacterial species associated with inflammation.
This study is a major step forward in helping us better understand how chronic noise exposure and aging impact the gut microbiome which, in turn, accelerates the development of Alzheimer’s disease.