Female rats, like women, need each other to relieve stress
Stress can lead to anxiety and depression, which women are more likely to suffer from than men. However, most research is done on male lab animals - until now.
The female brain reacts differently to stressful situations, distress and depression than the male brain. Research in psycho-pharmacology - the study of drug-induced changes in mood and behaviour - however, is primarily done with male lab animals and therefore provides a distorted picture, Dutch researchers say.
Gert ter Horst, a professor of neurobiology at the University Medical Centre in Groningen , and his colleagues have examined the way male and female rats deal with stress. They published their findings in the scientific magazine Physiology & Behavior. "Gender differences have become a booming field of research only recently," they write.
"We have this running gag in the lab: 'If you know how a man responds to something, can you predict how a woman will? Exactly the opposite!,'" Ter Horst says. "But it is not a joke, really. When it comes to the brain, the male and female are completely different worlds."
Sex hormones
Chronical stress, Ter Horst explains, is often a precursor to anxiety disorders and depression. Therefore psychiatrists are very interested in the subject and much research is done on animals, like rats, whose brains have physiologic resemblances to the human brain. Most studies use male rats, because the cycle makes researching females more difficult. The female sex hormones have highs and lows and influence emotions, causing fluctuations in the test results.
"But the prevalence of anxiety disorders in women exceeds that in men two to three times," according to Ter Horst. "So it is very weird that the research is conducted only with male rats."
The rats tested in Groningen showed significant differences in the activities in certain areas of the brain that play an important role in emotions, such as the pre-frontal cortex. Under low stress circumstances, activities in this area of the brain are much lower amongst male rats than their female counterparts.
Exposure to stress increases brain activity in both sexes, but females have more difficulty sustaining that activity. At a certain point they overload, after which their brain sort of shuts down and psychiatric disorders can develop.
"There is a limit to brain activity", Ter Horst explains. "And because the natural level in the female brain activity is closer to that saturation point, they reach it sooner when suffering from stress."
The neurobiologist emphasises that the level of the basic brain activity is not caused by female sex hormones. The difference is rooted in the brain itself, research shows. Ter Horst also used female rats whose ovaries had been removed so that they no longer secreted sex hormones to the blood. Females without a cycle also had a higher level of brain activity than males - even more so than those with ovaries. "Apparently sex hormones mute the brain activities and mitigate the effect of stress."
Social support
Hormones do not make females more resilient to stress, however, because the hormone levels fluctuate and alter the brain activities. "Those fluctuations make women more vulnerable to stress. Depression and anxiety mostly strike during times when there are severe changes in hormone levels. Not so much during their monthly periods, but after giving birth, during puberty or during menopause."
Through the course of evolution, female animals seem to have developed a strategy to cope with this: social support. Women mammals lick each other or lie against each other in times of stress. Ter Horst and his team discovered that female rats recover from stress better when they are surrounded by those of the same sex. This form of social support did not seem to have much of an effect on the brain of male rats. A mixed group added to the stress level.
Through the course of evolution, female animals seem to have developed a strategy to cope with this: social support. Ter Horst and his team discovered that female rats recover from stress better when they are surrounded by those of the same sex. This form of social support did not seem to have much of an effect on the brain of male rats. A mixed group even added to the stress level.
The researchers worked with single sex groups only and the impact on females was striking. The social support stimulated the production of the neurotransmitter serotonin. That substance mutes the effects of stress the same way anti-depressants do. When the female rats are isolated, their serotonin system is not activated. "When you expose a rat to stress and then bring it back to its group, the others in the cage start taking care of the suffering animal. They will lick each other or lie against each other in times of stress, the males as well," says Ter Horst.
Ter Horst doesn't like translating his results to human situations, but here he makes an exception. "When women go through something stressful, they want to talk about it, again and again. That relieves their stress. Men don't have that desire so much."
