Substance abuse frequently occurs alongside depression, anxiety disorders, schizophrenia, and other psychiatric problems, and individuals affected by both mental illness and substance abuse are less likely to respond to treatment. A study conducted by researchers from the Indiana University School of Medicine indicates that damage to the amygdala, a region of the brain associated with fear, anxiety, and other emotions, may affect an individual's likelihood of developing a mental illness in addition to a substance abuse problem. The researchers found that rats with amygdala lesions displayed behavioral abnormalities and were also more sensitive to cocaine than healthy rats.
The study examined the effects of neonatal amygdala lesions (NAML) and cocaine use on the behavior of rats. In humans, damage to the amygdala can result from temporal lobe epilepsy, tumors, early brain injury, or even a combination of genetic and environmental factors, including early trauma, that can change how the amygdala functions. The researchers induced NAML in 25 rats and compared their behavior to the behavior of 24 healthy rats, studying the rats' responses to a new environment, their social behavior, and their anxiety-related behavior. They then injected the rats with either cocaine or saline over a period of five days. After an additional two weeks, they injected all of the rats with cocaine to test how a history of cocaine use, as opposed to a single injection, affected their behavior. Rats with NAML did not respond appropriately to situations that should have seemed ambiguous or threatening to them. They were more active in a new environment, were less afraid while inside a maze that was lifted off the ground, and continued to socialize when a predator's scent was present in their environment. These rats were also more sensitive to the effects of cocaine. Rats receiving cocaine injections had higher activity levels in general than rats injected with saline, but the rats with NAML had the highest activity levels. Two weeks later when all rats received cocaine injections, rats with NAML who had previously received cocaine injections still had the highest activity levels.
The link between mental illness and drug use is hard to explain, and a number of factors could be involved. Individuals might use drugs to alleviate the symptoms of a mental illness, and substance abuse could also trigger mental illness or worsen a preexisting but undiagnosed disorder. The results from this study, however, indicate that neurobiological abnormalities could result in both psychiatric symptoms and an increased vulnerability to the effects of cocaine, explaining why mental illness and substance abuse commonly occur together.
About half of individuals with a mental illness also have a substance abuse problem, and these individuals are more likely to be violent, not comply with treatment, and suffer relapses. Treating only one problem will not always fix the other, and an integrated approach, which addresses both problems at the same time, is crucial. Future research should investigate the connection between mental illness and substance abuse. NAML did have an effect on the rats' behavior as well as their sensitivity to cocaine, and further research on the subject might provide further insight into the connection between mental illness and substance abuse.
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