A new study at Indiana University medical school has revealed that mental illness and drug addiction may co-occur due to disturbance in part of the brain.
The researchers found that this type of dual diagnosis may stem from a common cause due to developmental changes in the amygdala, a walnut-shaped part of the brain linked to fear, anxiety and other emotions.
Addiction to nicotine, alcohol and drugs is often found in people with a wide variety of mental illnesses, including anxiety disorders, unipolar and bipolar depression, schizophrenia, and borderline and other personality disorders.
The research led by Andrew Chambers, MD, reported that at least half the people who seek help with addiction or mental-health treatment have co-occurring disorders.
Epidemiological data says that from two to five of every 10 anxious or depressed people, and from four to eight of every 10 people with schizophrenia, bipolar disorder, or antisocial personality, also have some type of addiction.
The researchers compared the adult mood- and drug-related behaviour of two groups of adult rats: those whose amygdalas were surgically damaged in infancy and those whose amygdalas were left intact but who underwent a sham surgery, to equalize their treatment.
They discovered that rats with damaged (lesioned) amygdalas grew up abnormally under-responsive to ambiguous or potentially threatening stimuli.
Not showing the normal caution, they moved significantly more in response to novelty, showed significantly less fear in an elevated maze, and kept socializing even when exposed to the scent of a predator.
Crucially, these same rats also were significantly more sensitive to cocaine after just one exposure.
The rats, who were given repeated cocaine injections later showed even stronger expressions of the enduring changes in behaviour - suggesting an overall hypersensitivity to the addictive process.
"Brain conditions may alter addiction vulnerability independently of drug history," Chambers said.
Dr. Chambers cites the relatively rare cases of temporal lobe epilepsy, tumours or early brain injury.
The complex interactions among subtle genetic and environmental factors changes the way the amygdala functions or is connected to the rest of the brain during childhood and adolescence.
"Early emotional trauma, paired with a certain genetic background, may alter the early development of neural networks intrinsic to the amygdala, resulting in a cascade of brain effects and functional changes that present in adulthood as a dual-diagnosis disorder," he added.
A full report appears in the December Behavioral Neuroscience, published by the American Psychological Association.