The scientists found that depressed female monkeys become socially withdrawn and have reduced body fat, low levels of activity, high heart rates and disruptions in hormone levels - all of which are known or suspected characteristics of major depression in women. Their research is based on female monkeys because women are 66 percent more likely than men to experience depression during their lifetimes.
"We believe these monkeys can be a useful model for learning more about depression in women," said Carol Shively, Ph.D., professor of pathology at Wake Forest Baptist. "Current ways to treat depression are only partially successful. This may be an important opportunity to develop and test new treatments."
She said that with current treatments, there's often a difference between men and women in effectiveness and side effects. The animal model will allow scientists to evaluate the effectiveness of treatment specifically in females, the population at greatest risk.
The study involved 36 adult female cynomolgus monkeys, who normally live in social groups in the wild. For most of the research, the animals lived in groups of four and their social interactions and behavior were observed. Depressive behavior included a slumped or collapsed body posture accompanied by a lack of response to events or objects in the environment in which other monkeys were interested.
The researchers found that the depressed monkeys had suppressed ovarian function, but continued to have menstrual periods. Irregular ovulation can lead to low estrogen levels, which have been associated in both women and monkeys with increased risk of disease in the arteries leading to the heart.
"This suggests the possibility that depressed women may have low ovarian function that goes unnoticed because they still have menstrual periods," said Shively. "If this turns out to be accurate, it could explain, in part, the observed relationship between coronary artery disease and depression."
The researchers said monkeys may be particularly useful for learning more about human depression for several reasons. In women, depression is often associated with changes in reproduction, including the beginning of menstruation (premenstrual syndrome or PMS), pregnancy, the post-partum period and perimenopause. Monkeys have menstrual cycles that are very similar to those of women.
"Further studies of monkeys may inform us about the nature of the relationship between reproductive function and mood in females," said Shively.
In addition, depression is higher among people with low levels of education and income. Some female monkeys face social stress that is similar to the stress that humans with low socioeconomic status experience. In some cases, these monkeys were also more prone to depression.
"What we've observed in these monkeys is the first animal model of social stress-related depression in females," said Shively.
In monkeys, the social stress comes from the social hierarchies they naturally form when they live in groups. Monkeys that are the low-status or "subordinate" animals in the group face more aggression, are more vigilant, spend less time being groomed - a friendly behavior by monkey standards - and spend more time alone. Previous research has shown that they have increased heart rates, more of the stress hormone cortisol and more cardiovascular disease than dominant monkeys.
Depressive disorders affect about 19 million Americans yearly. The lifetime prevalence rate of major depression in women is 21 percent, compared to 12.7 percent for men. Depression in women can have negative effects on children as well as marriage. In addition, women who develop depression before age 18 are less likely to obtain a college or graduate degree, reducing their future earning potential.
Other researchers involved in the project were Thomas Register, Ph.D., David Friedman, Ph.D., Timothy Morgan, Ph.D., Jalonda Thompson and Tasha Lanier, all from Wake Forest Baptist.
About Wake Forest University Baptist Medical Center: Wake Forest Baptist is an academic health system comprised of North Carolina Baptist Hospital and Wake Forest University Health Sciences, which operates the university's School of Medicine. The system comprises 1,282 acute care, psychiatric, rehabilitation and long-term care beds and is consistently ranked as one of "America's Best Hospitals" by U.S. News & World Report.