New research reveals that this type of dual diagnosis may stem from a common cause: developmental changes in the amygdala, a walnut-shaped part of the brain linked to fear, anxiety and other emotions.
A full report on why these “comorbid” disorders may develop appears in the December Behavioral Neuroscience, published by the American Psychological Association (APA).
Dual diagnosis is common yet difficult to treat. Addiction of all types – 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. Lead author Andrew Chambers, MD, cites clinical reports 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.
To find the scientific basis for this complex, seemingly intractable pairing, which has in the past been attributed to “self-medication,” Chambers' team at the Indiana University medical school compared the adult mood- and drug-related behavior 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.
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. And rats given repeated cocaine injections later showed even stronger expressions of the enduring changes in behavior – suggesting an overall hypersensitivity to the addictive process.
Given that the experimental and control rats were raised in the same tightly controlled conditions, the only difference being their brain status, researchers concluded that the integrity of the amygdala was the root cause of both impaired fear behavior and heightened drug response.
“Brain conditions may alter addiction vulnerability independently of drug history,” says Chambers. He and his colleagues concluded that someone's greater vulnerability to addiction, rather than a given drug's ability to alter the symptoms of mental illness for better or worse (usually worse), more fully explains the high rates of dual diagnosis.