Major depression is one of the most common mental disorders in the United States with 1 in 6 people experiencing a depressive episode during their lifetime. Depression varies in severity and by demographic indicators, and it is rapidly becoming a significant health concern worldwide. According to the World Health Organization, depression will be the second-leading cause of disability in the world by 2020.

Depression is thought to be caused by a combination of genetic, biological, environmental and psychological factors, but depression doesn't feel the same for everyone. However, researchers agree that psychiatric disorders are detrimental to the brain and may cause measurable changes to key areas of the brain.

One recent study provided strong insight to understanding the interaction among several brain areas in depression. The researchers identified two discrete circuits of parvalbumin-positive neurons in the ventral pallidum projecting to either the lateral habenula or ventral tegmental area, contributing to feelings of despair and helplessness.

This provides clear evidence that different brain circuitry is involved in different types of depressive behavior with specific symptoms. The researchers were able to alleviate or reverse such symptoms in mice, suggesting that multiple regions of the brain impact one another.

About twice as many women as men experience depression, and another recent study looked at why some women are more vulnerable to depression during hormonal fluctuations than others. For example, some women are more prone to depression after childbirth and during menopause — both times of significant fluctuations of estrogen but the reasons for why some women are overcome with depression during these periods and others remain unclear.

What is known, however, is that estradiol modulates the synthesis, availability and metabolism of serotonin, a key neurotransmitter in depression.

This study examined patterns of estrogen exposure during the reproductive years and risk of depression during the menopausal transition and early postmenopausal years. Data were collected at baseline and annually for 10 years, and included 1,306 regularly menstruating premenopausal women, aged 42 to 52 years.

The main outcome was incidence of high level of depressive symptoms, Center for Epidemiological Studies Depression Scale (CES-D) score at least 16, in the MT and initial postmenopausal years, independent of premenopausal depression symptoms. Risk factors examined were duration of estrogen exposure (menarche to MT), duration of hormonal birth control use, pregnancies, and lactation.

Notably, longer duration of estrogen exposure from the start of menstruation until the onset of menopause was significantly associated with a reduced risk of depression during the transition to menopause and for up to 10 years after menopause. Longer use of birth control was associated with a decreased risk of depression.

"Women are more vulnerable to depressive symptoms during and after the menopause transition because of fluctuating hormone changes," says Dr. JoAnn Pinkerton, executive director of the North American Menopause Society (NAMS). "This study additionally found a higher risk for depression in those with earlier menopause, fewer menstrual cycles over lifespan, or more frequent hot flashes."