A new comprehensive review by researchers at the Montreal Neurological Institute (MNI), McGill University and the University of Cambridge, England provides vital insights into the neurological basis of addiction by investigating Parkinson's disease patients, who in some instances develop various addictions when undergoing medical treatment. The review, published in this week's (February 25) issue of the scientific journal Neuron, illustrates that persistently elevated levels of dopamine in the brain promote the development and maintenance of addictive behaviours.
Addiction is a complex health and societal problem that can destroy lives and damage communities. Brain imaging studies have shown that addiction severely alters brain areas critical to decision-making, learning and memory, and behavior control. In order to learn how to control or manage the disorder, it is necessary to understand the underlying biological mechanisms. Researchers have turned to Parkinson's disease to study addiction, successfully using one disease to learn about another. Although seemingly very different, dopamine plays a role in both disorders and some of the same systems in the brain are affected. Parksinson's disease is often thought as just affecting movement but, it also consists of cognitive, behavioural and mood symptoms, which are now being recognized as a major source of disability.
Dopamine is a neurotransmitter, or chemical messenger in the brain that is involved in brain processes that control movement, emotional response and the ability to experience pleasure, reward and pain. Parkinson's patients lack dopamine and are often treated with dopamine agonists, medication that mimics dopamine action.
"In some instances Parkinson's disease (PD) patients become addicted to their own medication, or develop behavioural addictions such as pathological gambling, compulsive shopping or hypersexuality," says Dr. Alain Dagher, neurologist at the MNI and co-author of the review.
"This is surprising because PD patients typically have a very low incidence of drug abuse and display a personality type that is the opposite of the typical addictive personality. These rare, addictive syndromes, which appear to result from excessive dopaminergic treatment, illustrate the link between dopamine, personality and addiction."
PD patients treated with dopamine agonists had an incidence of pathological gambling as high as 8% compared to less than 1% in the general population. In PD patients who develop addictive disorders, the problems started soon after starting dopaminergic therapy and stopped after treatment was discontinued. It was found that adjusting the dosage and combination of medication resolved the addictive symptoms, while maintaining the same motor benefit.
The phenomenon of addiction induced by dopamine medications can also tell us something about vulnerability to addiction in the general population. Not everyone is equally vulnerable, and it now appears that the propensity to become addicted is in part hereditary. Many of the genes implicated in addiction appear to affect brain levels of dopamine.
Studies show that that dopamine acts in an area of the brain known as the ventral striatum, which receives input from other areas such as the hippocampus and amygdala. It may be through this region that dopamine promotes addictive behaviours.
Understanding brain function that leads to drug addiction may help in the development of drugs to block drug-craving and drug-seeking behaviours in the general population as well as refine disease treatment for Parkinson's patients.
October 2009 marks the 75th anniversary of the MNI. The MNI is a McGill University research and teaching institute, dedicated to the study of the nervous system and neurological diseases. Founded in 1934 by the renowned Dr. Wilder Penfield, the MNI is one of the world's largest institutes of its kind. MNI researchers are world leaders in cellular and molecular neuroscience, brain imaging, cognitive neuroscience and the study and treatment of epilepsy, multiple sclerosis and neuromuscular disorders. The MNI, with its clinical partner, the Montreal Neurological Hospital (MNH), part of the McGill University Health Centre, continues to integrate research, patient care and training, and is recognized as one of the premier neuroscience centres in the world. At the MNI, we believe in investing in the faculty, staff and students who conduct outstanding research, provide advanced, compassionate care of patients and who pave the way for the next generation of medical advances. Highly talented, motivated people are the engine that drives research - the key to progress in medical care. A new building, the North Wing Expansion, is currently under construction and will house state-of-the-art brain imaging facilities. Once the construction is completed and the new building is fully equipped, the scientific community focused on brain imaging research at the MNI will be without equivalent anywhere in the world.
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