Mental and physical disorders caused by chronic stress are constantly increasing, putting a significant burden on society. They affect both men and women but not necessarily in the same way. Although much scientific evidence suggests that males and females deal differently with stress, the causes of these differences are not yet fully understood, and in any event, personalized treatments for both genders have not yet been achieved.
Scientific excellence requires diversity – research conducted by both men and women and people from different backgrounds and with varied worldviews. The need for diversity extends to scientific experiments themselves, but even today the vast majority of studies in the life sciences are conducted on only on male mice so they can skew the results and the ability to extrapolate from them to humans.
Until the 1980s, clinical trials of new drugs were conducted on men alone. The accepted view was that including women was unnecessary, and that it would only complicate the research, bringing into play new variables such as menstruation and hormonal changes. For the same reasons, preclinical studies avoided using female animals until very recently. But it’s now known that animals on a molecular and behavioral level, the variability among males is usually greater than among females, so there is no reason to suppose that females would complicate the experiments any more than males.
A new study by researchers from the Weizmann Institute of Science in Rehovot addresses this challenge, revealing in unprecedented detail how the brains of male and female mice respond differently to stress.
How do male, female brains respond differently to stress?
In the study, published in Cell Reports under the title “Sex shapes cell-type-specific transcriptional signatures of stress exposure in the mouse hypothalamus,” researchers from Prof. Alon Chen’s joint laboratory at the Weizmann Institute and the Max Planck Institute of Psychiatry in Munich discovered that a subcategory of brain cells responds to stress in a totally different manner in males and females. The findings could lead to a better understanding of health conditions affected by chronic stress including anxiety, depression, and even obesity and diabetes.
Researchers in Chen’s lab, which specializes in studying the response to stress, suggested that innovative research methods could help to change the picture. Previous studies in other labs had uncovered certain sex differences in the response to stress, but those findings were obtained using research methods that could mask significant differences in the responses of specific cells or even entirely erase the roles played by relatively rare cells. In contrast, his lab uses advanced methods that allow scientists to analyze brain activity at an unprecedented resolution – on the level of the individual cell – and could therefore shed new light on the differences between the sexes. The team said they could pave the way toward personalized therapies for these disorders.
“We turned the most sensitive research lens possible onto the area of the brain that acts as a central hub of the stress response in mammals, the paraventricular nucleus (PVN) of the hypothalamus,” said Dr. Elena Brivio, who led the study. “By sequencing the RNA molecules in that part of the brain on the level of the individual cell, we were able to map the stress response in male and female mice along three main axes: how each cell type in that part of the brain responds to stress, how each cell type previously exposed to chronic stress responds to a new stress experience, and how these responses differ between males and females.”
The researchers mapped out gene expression in more than 35,000 individual cells, generating a huge amount of data that provides a picture of stress response that’s unprecedented in its scope and in highlighting the differences between how males and females perceive and process stress. As part of the study and in keeping with the principles of open-access science, the researchers decided to make the entire detailed mapping publicly available on a dedicated interactive website that went live at the same time the study was published, providing other researchers with convenient, user-friendly access to the data.
“The website will, for example, allow researchers who are focusing on a specific gene to see how that gene’s expression changes in a certain cell type in response to stress, in males as well as females,” Brivio said.
The comprehensive mapping has already allowed the researchers to identify a long list of differences in gene expression – between males and females and between chronic and acute stress. The data showed that certain brain cells respond differently to stress in males and females: Some cells are more susceptible to stress in females and some to stress in males.
The most significant difference was found in a type of brain cell called the oligodendrocyte – a subtype of glial cell that provides support to nerve cells and plays an important role in regulating brain activity. In males, exposure to stress conditions, especially chronic stress, changed not only the gene expression in these cells and their interactions with surrounding nerve cells but also their very structure. In females, however, no significant change was observed in these cells, and they were not susceptible to stress exposure.
“Neurons attract most of the scientific attention, but they make up only about a third of all brain cells. Our method enables us to see a much richer and fuller picture, including all the cell types and their interactions in the part of the brain under study,” added Dr. Juan Pablo Lopez, a former postdoctoral fellow in Chen’s group and now the head of a research group at the neuroscience department of the famed Karolinska Institute in Sweden.
In basic research, it’s still common to conduct experiments only on males. “Our findings show that, when it comes to stress-related health conditions, from depression to diabetes, it’s very important to take the sex variable into account, since it has a significant impact on how different brain cells respond to stress,” Chen explained.
“Even if a study does not specifically focus on the differences between males and females, it’s essential to include female animals in the research, especially in neuroscience and behavioral science, just as it is important to implement the most sensitive research methods to obtain as complete a picture of brain activity as possible,” Brivio concluded.
