FRIDAY, Dec. 21 (HealthDay News) -- Developmental problems involving a walnut-shaped part of the brain called the amygdala -- linked to fear, anxiety and other emotions -- may explain why mental illness and addiction often appear together, researchers say.

Many kinds of addiction -- such as those for alcohol, drugs and nicotine -- occur in people with various kinds of mental illness, including depression, schizophrenia and anxiety disorders, according to background information in an American Psychological Association news release about the Indiana University study.

Two to five of every 10 anxious or depressed people, and four to eight of every 10 people with schizophrenia, biopolar disorder or antisocial personality, also have some form of addiction, according to epidemiological data.

In this study, published in the December issue of Behavioral Neuroscience, the researchers compared the behavior of adult rats whose amygdalas were surgically damaged in infancy and adult rats with intact amygdalas.

Rats with the damaged amygdalas showed less fear and caution than normal and were significantly more sensitive to cocaine after just one exposure to the drug. Rats with damaged amygdalas that received repeated cocaine injections developed "even stronger expressions of the enduring changes in behavior -- suggesting an overall hypersensitivity to the addictive process," the researchers noted.

Since both groups of rats were raised in the same controlled conditions, the damaged amygdala was the likely cause of the impaired fear behavior, as well as the heightened drug response, in the one group of rats, the study authors concluded.

In humans, disease and interactions between genetic and environmental factors can alter amygdala function or change the way it's 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," lead author Dr. Andrew Chambers explained in a prepared statement.

More information

The (U.S.) National Alliance on Mental Illness has more about dual diagnosis of substance abuse and mental illness.