Baby mice exposed to air particles seek instant rewards, a behaviour linked to addiction

A new study, published recently in the journal Environmental Health Perspectives, has found that baby mice that breathed tiny particles found in urban air were more impatient when they grew up than mice that breathed filtered air. As adults, 43 percent more of the mice exposed to the ultrafine particles repeatedly hit a lever for a treat rather than waiting for one to fall. In people, the need for an immediate reward is related to alcohol addiction and other cognitive and behavioural disorders. Ultrafine particles are widespread pollutants emitted by diesel engines and other sources of combustion. The exposed mice in a sense lacked self-control. They were less patient in their quest for the sugary treats offered to them during the study. The exposed mice feverishly pushed a lever for the reward 43 percent more times than the mice that breathed clean air, instead of waiting for one to fall without any effort on their part. The ability to wait for a reward is a self-control measure important in decision-making. The inability to wait – or acting impulsively – is a form of self-gratification. Increased self-gratification in people is linked to cognitive and behaviour disorders such as addictions, obesity, attention deficits and rule breaking. Human and animal research has linked exposure to particulate air pollution with cognitive impairment and behaviour disorders, including ADHD and substance abuse. This study is notable because the mice were exposed early in life, but the behaviour effects were not observed until they were adults. Mice that were only exposed as adults did not show the behaviour changes, suggesting that damage to the developing brain was critical for the observed behavioural effects. The research contributes to a growing number of studies that show early life exposure can result in effects much later in life. Another unique aspect of the study was that the mice were exposed to real air particulates that had been filtered and concentrated from the road. Some studies use air particles that are made in the lab and may not be as environmentally relevant. Exactly why the ultrafine particles could influence response behaviour was not determined. The researchers suggest the particles may cause inflammation and oxidative damage in the developing mouse brain, an idea supported by other studies. Ultrafine particles are the smallest type of particulate matter studied in air pollution. They measure less than 100 nanometers – 1,000 times smaller than the average human hair of 100,000 nanometers. These tiny particles are created during combustion and released from cars and trucks as well as industry and incineration. Exposure varies regionally and can be higher near freeways and heavily travelled roads. Indoor sources may include cooking, office equipment and wood fires. Scientists are concerned about ultrafine particles because they can penetrate deep into the lungs, pass into the blood and lodge in the heart, brain and other parts of the body. The small bits also can carry other pollutants with them. Exposure in people can damage lungs, affect cardiovascular health and lead to respiratory problems such as asthma. Researchers exposed 4-day-old baby mice either to filtered air or air polluted with ultrafine particles at concentrations 10 times greater than ambient air. The particles were collected from outdoor air in a major urban region and then concentrated. The mice were exposed to the polluted air for four hours a day for four days, then clean air for two days and then the polluted air for an additional four days. Some of the adult mice were further exposed to four days of ultrafine particles. All the mice were tested for their response behaviour – technically called delay discounting. The mice had been trained to press a lever 25 times to receive a reward – a small sugar pellet – but the reward system was altered during the nine testing sessions. After the initial reward, subsequent pellets were released at different intervals – either 5, 7.5, 10 or 12.5 seconds – in the sessions. The mice were rewarded with pellets for patiently waiting. Scientists call this “fixed ratio waiting-for-reward.” If a mouse pressed the lever before the next sugar pellet was released, the system would reset, and the mouse would need to press the lever 25 times before another pellet was delivered. The mice that breathed the particulate pollution reset the system up to 43 percent more times than those that were not exposed. The higher reset frequency means the exposed mice have increased tendencies for immediate rewards and self-gratification. The increased activity to get the reward shows the mice prefer shorter delays even if they have to work harder and expend more energy pressing the lever more times to get the treats. A next step could look at cumulative exposures and their long-term effects on response behaviour, the researchers suggest.