Our brains are made up of nerve cells called neurons. These neurons are separated by tiny gaps called synapses. All functions of the brain and nervous system are based on communication among these neurons and across these synapses. The neurons relay information to each other by sending chemical messengers or neurotransmitters across the synapses throughout the neural network.
This is what occurs: Neurotransmitters are produced within a neuron. The neuron releases the neurotransmitter and it travels into the synapse space. The neurotransmitter may then be accepted by the next neuron attaching at a site called a receptor, thereby transmitting information from one nerve cell to another throughout the brain.
In order for these pathways to work effectively so that the message gets through, the neuron must produce and release enough of the neurotransmitter. In addition, the neurotransmitter must also stay in the synapse space long enough for it to bind with the receptor site.
After the neurotransmitter is released the excess or unused portion is then recaptured or reabsorbed by the original neuron that produced it. What sometimes seems to happen in individuals with ADHD is the neurotransmitter is prematurely reabsorbed back into neuron. When this occurs that portion of the neural network cannot relay messages in an adequate and timely way.
How Do Stimulants Come Into Play?
What stimulants seem to do is stimulate and increase the release of certain neurotransmitters, most notably dopamine and norepinephrine, and block or slow up how much of these chemicals are being reabsorbed back into the neuron from which they were released. As a result, more of the neurotransmitter is held in the synapse space between neurons long enough for it to properly bind to the receptor, thus messages within the brain are more effectively transmitted and received.
Developmental pediatrician, director of the National Center for Girls and Women with AD/HD, and internationally recognized ADHD expert, Dr. Patricia Quinn describes the joining of the neurotransmitter to the receptor as akin to a key fitting into a lock. “When the neurotransmitter (key) fits into the receptor (lock), it opens the door for messages to get though,” writes Dr. Quinn.
Research suggests that methylphenidate (a stimulant which includes the brand name medicines Ritalin, Metadate, and Concerta) primarily blocks the reuptake of dopamine and norepinephrine – that is, slows up how much of the neurotransmitter is being reabsorbed back into the neuron so that more is left in the synapse. And secondarily, methylphenidate seems to facilitate the direct release of neurotransmitter from within the neuron where it is produced and stored therefore sending more out into the synapse space.
Amphetamines (another type of stimulant that includes Dexedrine and Adderall) mostly increase the release of dopamine and norepinephrine from their storage sites into the synapse. A less significant mechanism of amphetamines is slowing the reuptake of the neurotransmitters. The differences in the way these stimulants work may explain why some people with ADHD respond to one type of stimulant medication better than another.
Dopamine and norepinephrine seem to play a key role in the areas of the brain responsible for regulating attention and executive function. The reason the stimulants are helpful in reducing symptoms of ADHD appears to be that they make these neurotransmitters more available, therefore improving activity and communication in those parts of the brain which operate on dopamine and norepinephrine and signal for specific tasks.
Brain imaging studies have demonstrated that when an individual is on stimulant medication there is evidence of increased metabolic activity in the prefrontal cortex, specific subcortical regions, and the cerebellum - all important centers for executive function. Thus, these areas of the brain appear more active and “turned on” to cognitive tasks when neurotransmitter levels are elevated.
The Effect on Motivation and Reward
Research has also found that dopamine helps regulate the accumbens and midbrain regions of the brain which are involved in a person’s ability to perceive reward and to be motivated by that reward to do actions. Thomas E. Brown, PhD is a leading expert in the assessment and treatment of ADHD. He is assistant clinical professor of psychiatry, Yale University School of Medicine, and associate director of the Yale Clinic for Attention and Related Disorders. Dr. Brown explains that “increased dopamine in the synapse can act almost as a kind of ‘Viagra’ to encourage the brain’s response to the task. Thus [stimulants] may counter the chronic problem with motivating oneself to do necessary, but not intrinsically interesting tasks.”
Stimulants do not cure ADHD. What they do is help to alleviate or reduce symptoms while the stimulant is active in a person’s system. Dr. Brown uses this analogy: “Taking stimulants is not like taking doses of an antibiotic to wipe out an infections; it is more like wearing eyeglasses that correct one’s vision while the glasses are being worn, but do nothing to fix one’s impaired eyes.”
Russell Barkley, PhD. Attention Deficit Hyperactivity Disorder: A Handbook for Diagnosis and Treatment. Guilford Press 2006.
Russell Barkley, PhD. Interview on PBS – Frontline: Medicating Kids
Thomas Brown, PhD. Attention Deficit Disorder: The Unfocused Mind in Children and Adults. Yale University Press 2005.
Nora Volkow, MD; Gene-Jack Wang, MD; Scott Kollins, PhD; Tim Wigal, PhD; Jeffrey Newcorn, MD; Frank Telang, MD; Joanna Fowler, PhD; Wei Zhu, PhD; Jean Logan, PhD; Yeming Ma, PhD; Kith Pradhan, MS; Christopher Wong, MS; James Swanson, PhD. Evaluating Dopamine Reward Pathway in ADHD. JAMA. 2009; 302(10):1084-1091.
Patricia Quinn, MD, and Judith Stern, MA. Putting on the Brakes: A Young People’s Guide to Understanding Attention Deficit Hyperactivity Disorder. Magination Press. American Psychological Association 2001.