A volume decrease in specific parts of the brain's hippocampus -- long identified as a hub of mood and memory processing -- was linked to bipolar disorder in a study led by researchers at The University of Texas Health Science Center at Houston (UTHealth). The research was published today in Molecular Psychiatry, part of the Nature Publishing Group.

"Our study is one of the first to locate possible damage of bipolar disorder in specific subfields within the hippocampus," said Bo Cao, Ph.D., first and corresponding author and a postdoctoral fellow in the Department of Psychiatry and Behavioral Sciences at McGovern Medical School at UTHealth. "This is something that researchers have been trying to answer. The theory was that different subfields of the hippocampus may have different functions and may be affected differently in different mood disorders, such as bipolar disorder and major depression disorder."

Cao hopes the study, which was funded in part by the National Institute of Mental Health (NIMH), will pioneer future research on details within the hippocampus as a marker for precise diagnosis and positive treatment response of bipolar disorder.

Approximately 6 million Americans suffer from bipolar disorder. Bipolar I disorder is characterized by mood changes that can swing from a high-energy, manic state to a low-energy, depressive state. The disorder can affect sleep, energy level and the ability to think clearly, according to the National Institutes of Health. It can interfere with a person's ability to work and perform daily living activities, and could lead to suicide attempts. Patients with bipolar II disorder do not experience the full-blown manic episodes, but may have a less severe high-energy state.

The research team used a combination of magnetic resonance imaging (MRI) and a state-of-the-art segmentation approach to discover differences in the volumes of subfields of the hippocampus, a seahorse-shaped region in the brain. Subjects with bipolar disorder were compared to healthy subjects and subjects with major depressive disorder.

Researchers found that subjects with bipolar disorder had reduced volumes in subfield 4 of the cornu ammonis (CA), two cellular layers and the tail portion of hippocampus. The reduction was more severe in patients with bipolar I disorder than other mood disorders investigated.

Further, in patients with bipolar I disorder, the volumes of certain areas such as the right CA 1 decreased as the illness duration increased. Volumes of other CA areas and hippocampal tail were more reduced in subjects who had more manic episodes.