Attention Deficit Hyperactivity Disorder, more commonly known as ADHD, is a well-known mental disorder typically associated with overactive children. Although it is the most commonly diagnosed mental disorder in children, it can also present in adolescents and even adults. ADHD is characterised by difficulties in paying attention or remaining still, racing thoughts, and impulsivity.
Recognising and diagnosing ADHD has become fairly easy for doctors and scientists, however little is known about what causes the disorder and how the brain is affected by it. Scientists have discovered a connection between the levels of chemicals in the brain called ‘monoamines’, specifically dopamine and norepinephrine, and the symptoms of ADHD. These are two chemicals that can act as ‘neurotransmitters’, or chemical messengers, that deliver messages throughout the brain and body signalling when to move or react.
Dopamine (DA) is best known for its role in signalling reward-motivated behaviour. In other words, it tells the body when to move or do something because of the positive effects the behaviour could have. For example, in people with non-ADHD brains, if they know they have to do the laundry or their homework their dopamine neurotransmitters react to the idea that ‘it’ll be worth having clean clothes or a finished paper in the end’. People with ADHD, however, have less dopamine in their brains, and therefore find it difficult to do boring or uninteresting tasks because their brain doesn’t make the connection that it’ll be ‘worth it’ (rewarding) in the end.
Norepinephrine (NE) is similar to dopamine but reacts to stress rather than reward. Fight-or-flight is a common example for the role of norepinephrine, as it signals the body to move when danger is near. It tells the body to move its muscles and increase its heart rate to get away from the danger. When talking about ADHD, the ‘danger’ isn’t necessarily life-or-death but it could be another type of stress, such as an upcoming deadline or the possibility of getting in trouble for not sitting still. When brains not affected by ADHD recognise these stresses, norepinephrine tells the body to get moving and focus to avoid the unwanted result. Lower levels of this chemical in the brains of people with ADHD make procrastination and ‘acting out’ more common results of these stresses, as the body doesn’t get the signal to get moving or concentrate on the task at hand.
This study done on ADHD in rats (actually, something called ‘spontaneous hyperactivity’ that has the same symptoms as ADHD) was done to try to find out what causes these lowered amounts of dopamine and norepinephrine in the brains of individuals with ADHD. The researchers believed that a hormone called glucocorticoid (GR) might have something to do with how much DA and NE were developed in the brian, so they decided to focus on levels of this hormone. They divided hyperactive rats into three groups: one group was given a drug called Dexamethasone (DEX), which is a GR agonist (causes more action of GR); another group was given a drug called RU486, a GR inhibitor (causes less action of GR); the last group wasn’t given any drugs and was tested along with the other two groups as a control. All of the rats had to run through a maze, and were monitored for symptoms of ADHD: hyperactiveness, impulsiveness, and distractedness.
What the researchers found was that the rats given DAX had reduced symptoms of ADHD and were able to perform better on the maze than the other two groups. The rats given RU486 performed worse, and the control rats showed typical ADHD behaviour. The researchers concluded that these observations showed positive effects from GA agonists (increased GA activity), meaning it was possible that the increased GA made producing DA and NE in the brain easier. They noted higher levels of DA and NE in the brains of these rats than in those of the control group, supporting this conclusion.
Although this finding doesn’t explain 100% what causes ADHD symptoms, it does open up a new area for researchers to look into further. Almost all medications currently given to ADHD patients are related to increasing DA and NE levels, and this new evidence could help regulate medications that help the brain develop and regulate more of these neurotransmitters on its own. Maybe after more research is done on rats scientists will be able to test for the same effects on humans.
Chen, Y., Zheng, X., Xie, L., Huang, L., Ke, Z., Zheng, J., … & Hu, J. (2017). Glucocorticoids/glucocorticoid receptors effect on dopaminergic neurotransmitters in ADHD rats. Brain research bulletin, 131, 214-220.
