Picture the person with OCD (obsessive compulsive disorder)…he spends hours washing, and can literally be stuck in the shower or sink. Now picture the person with ADHD (attention deficit hyperactivity disorder)…he frequently flips from one activity to the next, just a constant stream of motion, and unable to do the same movement for very long. So one spends all their time in one place in rituals, while the other impulsively flips to various activity, unable to sustain attention. These two disorders appear to be different, but they are related with regards to the lack of control of their movements (and thoughts).
In addition, there are other neuropsychiatric disorders which have problems with the lack of control of movements (and thoughts), including Parkinson’s, Tourette’s, Huntington’s, and psychosis. What is interesting is that all of these disorders are linked to dysfunctions of the basal ganglia, which mediates movement (and thoughts related to movement) in the brain.
A new study has revealed that OCD may be triggered by the activation of the direct pathway in the basal ganglia, while ADHD is triggered by the disruption of the indirect pathway of the basal ganglia. These two pathways were previously studied in isolation, but they appear to work simultaneously to help us control our actions.
If you look at OCD, ADHD, Parkinson’s disease, and psychosis, they all involve problems with the control of movement (and thoughts):
- In OCD, you have repetitive thoughts and actions- you are engrossed in rituals
- In ADHD, you have impulsive movements- you are shifting constantly
- In Parkinson’s, you have shuffling gait and slowed thoughts- you are slow, have difficulty initiating movement, and meandering
- In psychosis, you have erratic thoughts and behaviors- you are disorganized
When you look at what is happening in the brain, these actions and thoughts are due to the various disruptions in the direct and/or indirect pathways of the basal ganglia. They appear to work in concert to balance the control of movement.
In the study, mice were tested and were trained to press levers for rewards. When the direct pathway was manipulated and disrupted, the mice “froze” and stopped pressing the lever. When the indirect pathway was activated, suddenly, the mice were unable to stay in one place, and chose to do something else, like explore other parts of the area.
From this experiment, it appears that the role of the direct pathway is to sustain action, while the indirect pathway allows (or prevents) switching to a different action and can stop the action as well.
So in Parkinson’s, a drug used to treat it, l-dopa (Sinemet), activates the direct pathway and inhibits the indirect pathway. If you give too much l-dopa, then they develop uncontrollable, repetitive movements, as seen in OCD.
In psychosis, a drug used to treat it, haloperidol (Haldol), activates the indirect pathway. If you give too much haloperidol, then they can develop slowed movements and thoughts, as seen in Parkinson’s.
So you see that these seemingly different and unrelated disorders are in fact related, in that they have the problem with lack of control of movement (and thoughts). These new insights can lead to better treatments for all the disorders described in this article.