Mental illness and addiction 'link'
Updated on 03 December 2007
Mental illness and drug addiction often occur together because they stem from problems in the same part of the brain, new research suggests.
Scientists traced their origins to developmental changes in the amygdala, a walnut-shaped fragment of the brain linked to fear, anxiety and other emotions.
Addiction to tobacco, alcohol or drugs is frequently found in people with a wide range of mental illnesses including anxiety disorders, normal and "bipolar" depression and schizophrenia.
Drug dependency is also associated with certain dysfunctional personalities. Research has shown that between four to eight out of 10 people with schizophrenia, bipolar disorder, or an "antisocial" personality, also have some type of addiction.
The reason for this coupling of problems has been a mystery up to now. Some experts have attributed it to self medication - a desire on the part of patients to fight their demons with drugs.
But new animal research in the US suggests a more fundamental biological explanation.
Scientists at Indiana University School of Medicine in Indianapolis compared mood and drug-related behaviour in two groups of adult rats.
One group had their amygdalas surgically damaged in infancy, while the brains of the other rats were left intact.
Animals with amygdala lesions grew up abnormally under-responsive to potentially threatening or risky situations. They lacked the caution of normal rats, were more curious when faced with novel stimuli, and showed significantly less fear in an elevated maze. When exposed to the scent of a predator, they seemed to ignore the danger and went on socialising.
The same rats were significantly more sensitive to cocaine after just one exposure. Rats given repeated cocaine injections later showed even stronger behaviour changes.
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