U.S. researchers say a chemical marker that binds to plaque and tangle deposits indicates dementia may be linked to major depressive disorder in older people.

"This is the first study using [the marker] FDDNP to assess the abnormal protein levels in brains of older adults with severe depression," Dr. Gary Small, the Parlow-Solomon professor on aging at the University California, Los Angeles, said in a statement.

"The findings suggest that the higher protein load in critical brain regions may contribute to the development of severe depression in late life."

Using the biomarker viewed through a positron emission tomography brain scan, providing a "window into the brain," and pinpoint where in the brain these abnormal protein deposits are accumulating, said Small, the study's senior author.

Researchers compared the FDDNP brain scans of 20 older adults between ages 60-82 who had been diagnosed with major depressive disorder with the scans of 19 healthy controls of similar age, education and gender.

The study, published in the journal Archives of General Psychiatry, found that in patients with major depressive disorder, FDDNP binding was significantly higher throughout the brain and in critical brain regions involved in decision-making, complex reasoning, memory and emotions.

"The findings suggest that the higher protein load in critical brain regions may contribute to the development of severe depression in late life," Small said. (UPI)