Pre-menopausal women have higher levels of a brain protein linked to depression than both younger and menopausal women, according to a recent study.

Researchers found elevated levels of the chemical monoamine oxidase-A (MAO-A) among women between the ages of 41 and 51. The new findings may explain the high rates of first-time depression seen among women in this transitional stage of life, known as perimenopause, according to a news release.

MAO-A is an enzyme that is a pro-oxidant and breaks down brain chemicals such as serotonin, norepinephrine and dopamine, which help to maintain normal mood.

"This is the first time that a biological change in the brain has been identified in perimenopause which is also associated with clinical depression," senior scientist Dr. Jeffrey Meyer of CAMH's Campbell Family Mental Health Research Institute said in a statement.

Dr. Meyer has previously linked high levels of MAO-A to major depressive disorder, depressed mood related to alcohol dependence and smoking cessation, and the period immediately after childbirth.

For the study, researchers conducted brain scans of three groups of women using a brain imaging technique called positron emission tomography (PET) in CAMH's Research Imaging Centre. Among the three groups of women, 19 were of reproductive age, 27 were in perimenopause, and 12 were in menopause.

Researchers found that, on average, levels of MAO-A were 34 percent higher in women with perimenopause than in the younger women, and 16 percent higher than those in menopause.

The women in perimenopause also reported a higher tendency to cry, based on a questionnaire called the Adult Crying Inventory, and this was associated with high MAO-A levels in the front part of the brain, the prefrontal cortex.

Researchers said their findings suggest new opportunities for prevention.

"Using PET imaging, we can test treatments to see if they can prevent this elevation of MAO-A, and potentially prevent clinical depression," Meyer said.

The findings were recently published in the journal JAMA Psychiatry.