Taking things to the extreme
How people with Parkinson’s shed light on the effects of too much dopamine
Many drugs affect the levels of dopamine in the brain, through the reward pathway, yet it is often difficult to study drug users directly. Instead, researchers look at other conditions that involve abnormal levels of dopamine, to get a better understanding of how it can affect the brain and body.
One group of patients given a therapeutic drug have become part of an unintended experiment in addiction. People with Parkinson’s disease develop muscle tremors and can also develop cognitive and behavioural symptoms, because of a lack of dopamine. These people are often given a dopamine precursor, L-dopa, or dopamine agonists (drugs that stimulate dopamine receptors in the brain) to help to control their physical symptoms.
A small percentage of these patients develop an addictive pattern of treatment use, taking more of these drugs than is needed. This overuse is then usually associated with dopamine dysregulation syndrome, which often results in impulse control disorders, including compulsive gambling, shopping, eating or seeking sex. This phenomenon is more commonly found in patients who were previously heavy drinkers or drug users.
The precise relationship between cause and effect is not yet known, but it is another link between the dopamine system and addiction. Some research suggests that as the brain becomes habituated to dopamine replacement therapy, more is needed to treat the physical symptoms of Parkinson’s, and this increase in dopamine causes an increase of reward-seeking behaviour, mediated by the amygdala and nucleus accumbens.
Interestingly, another side-effect of dopamine replacement therapy is psychosis, including hallucinations and confusion, which is relatively rare in untreated Parkinson’s but rises to 40 per cent prevalence in those treated with dopamine replacement drugs. Similar symptoms are found in people with schizophrenia, in which people have delusions and hallucinations. Recently, the use of some recreational drugs, like cocaine and cannabis, has been linked with an increased chance of developing psychoses with similar manifestations to schizophrenia. Studying this range of dopamine-related disorders gives us a further idea of the complex role of dopamine in addiction and psychosis, and how the two may overlap.Lead image:
Adapted from nottinghamvets/Flickr