Scientists are also investigating the causes of ADHD at the neurotransmitter level. Neurotransmitters are often called chemical messengers. They are a central part of the communication system between the brain and the rest of the body. Neurotransmitters carry messages between nerve cells (or neurons) in the brain and the rest of the central nervous system. The central nervous system is the means by which the brain communicates with the rest of the body, via the spinal column. Since many messages originate at the neurotransmitter level, they are central to understanding brain disorders.
At one time, scientists thought that low levels of a neurotransmitter named dopamine caused ADHD. Dopamine is a neurotransmitter implicated in many psychiatric problems. Among other things, dopamine impacts movement, mood, motivation, and attention. More recent evidence suggests that the relationship between dopamine and ADHD is a bit more complicated. Individuals with ADHD seem to have an excessively efficient dopamine-removal system. They have a higher concentration of dopamine transporters called re-uptake inhibitors. When dopamine is removed too quickly, it doesn't have sufficient time to exert its effect. Researchers now believe that this overly-efficient transporter process may help to explain some ADHD symptoms.
Additional evidence connecting dopamine and ADHD comes from studies about the effect of certain medications on symptoms. For example, Ritalin® is a commonly used drug for ADHD. It is a stimulant medication that blocks dopamine transporters. Therefore, it slows the removal of dopamine after it is released. Researchers theorize that Ritalin helps reduce ADHD symptoms by giving dopamine more time to act on the cells. This in turn creates a better ability to manage mood, motivation, and attention. It should be noted that dopamine is implicated in many psychiatric conditions.
ADHD symptoms may also be caused by the reduction of two other neurotransmitters: norepinephrine and serotonin. These neurotransmitters influence brain function in the cerebellum. Norepinephrine is a catecholamine that is synthesized by dopamine. Catecholamines function several ways. Sometimes they function as a hormone. Other times they function as a neurotransmitter. Hormones are messengers that communicate with both the brain and the rest of the body, via the blood stream. In contrast, neurotransmitters communicate within the brain. Some chemicals operate as both a neurotransmitter and a hormone. When norepinephrine functions as a stress hormone, it affects attention. To complicate things further, some neurotransmitters affect the release of certain hormones. Therefore, the relationship between neurotransmitters and hormones is complex and fascinating; yet, it is beyond our needs. It is simply helpful to know that there are many valid explanations for the involvement of neurotransmitters and hormones in a disorder such as ADHD.
Serotonin is another neurotransmitter implicated in ADHD. It influences mood, social behavior, sleep, and memory. Low levels of serotonin may impair these important functions. As previously mentioned, some studies seem to indicate that there are different levels of neurotransmitters in the brain between the three subtypes of ADHD. Research studies on this topic and other suspected brain differences in ADHD are ongoing.