People with diabetes are more prone to anxiety and depression than those with other chronic diseases that require similar levels of management. The reasons for this aren’t well understood, but Joslin Diabetes Center researchers have discovered one potential explanation.

Genetically modifying mice to make their brains resistant to insulin, the Joslin scientists first found that the animals exhibited behaviors that suggest anxiety and depression, and then pinpointed a mechanism that lowers levels of the key neurotransmitter dopamine in areas of the brain associated with those conditions.

“This is one of the first studies that directly shows that insulin resistance in the brain actually can produce a behavioral change,” says C. Ronald Kahn, MD, who is Joslin’s Chief Academic Officer and the Mary K. Iacocca Professor of Medicine at Harvard Medical School, and senior author on a paper reporting the work in the journal PNAS.

His research team assessed the genetically modified mice in multiple tests that place mice under stress and are commonly used to analyze drugs that treat anxiety and depression. Young mice behaved much like normal mice, but mice tested at 17 months of age (“which is starting late middle-age for mice,” notes Dr. Kahn) displayed significant behavioral disorders.

Examining the brains of these mice, the scientists found altered metabolism in mitochondria, which produce energy for cells. Among the changes, the mitochondria increased production of two enzymes that degrade dopamine, a major transmitter of brain behavior. “These mice release a normal amount of dopamine, but because of these changes in the mitochondria they metabolize that dopamine faster, and it’s not around as long,” explains Dr. Kahn. “We think that contributes to these behaviors, and in fact when we give the mice antidepressants that work by slowing dopamine degradation, we can correct some of the behavioral changes.”

Although the behavioral effects were not seen in the younger genetically modified mice, the scientists detected similar changes in their brain cells as well. It’s not clear why the changes in behavior might increase with age, Kahn says, but the effect is common among mouse models of neurological disorders, and is seen in the same human neurological diseases.