By incorporating more female-specific models in biomedical research, scientists are gaining deeper insight into how male and female brains differ at the molecular level, with important implications for drug development and disease treatment.
For decades, biomedical research focused almost exclusively on male subjects. Researchers justified this approach by claiming that including females would introduce excessive variability due to hormonal cycles. This assumption, however, was eventually proven wrong. Studies conducted in the 1990s demonstrated that female animals were no more variable than their male counterparts, exposing a significant oversight in the scientific community.
This realization prompted widespread calls for greater representation of both sexes in research. The National Institutes of Health responded in 2016 with the Sex as a Biological Variable policy, which required scientists to incorporate sex as a key factor in research design, analysis, and reporting across human and animal studies. While this marked a significant step forward, questions remain about how effectively it has addressed sex bias in the field.
Catherine Woolley, a neuroendocrinologist at Northwestern University, has been at the forefront of investigating sex-based differences in brain function at the molecular level. Her team discovered that estradiol, a form of estrogen, influences synaptic transmission in the hippocampus of both male and female brains. Importantly, they also found that estrogen receptors behave differently in each sex, underscoring the need for inclusive research to develop more precisely targeted therapies.
The consequences of male-focused research became particularly evident in studies of conditions like post-traumatic stress disorder and depression, both of which disproportionately affect women. Rebecca Shansky, a neuroscientist at Northeastern University, recognized this gap and began studying both male and female animals. Her research revealed that stress and fear alter brain structure and function differently in each sex, demonstrating the wealth of information that had been overlooked by studying only one sex.
The distribution of microglia (green) across the different cortical layers (orange) in the mouse visual cortex differs in female and male brains when recovering from ketamine anesthesia.
© Siegert group / ISTA
Research into ketamine, a common general anesthetic, has further highlighted sex-based differences in brain function. Sandra Siegert, a neuroscientist at the Institute of Science and Technology Austria, and her colleagues observed that ketamine interacts with microglia, the brain’s immune cells, in ways that differ between the sexes. By tracking microglial activity in animals recovering from ketamine, the team found that the drug triggered significantly more microglia-neuron connections in female brains than in male ones. Further investigation revealed that ketamine elevated stress hormone levels only in female mice, driving the heightened microglial response.
Clinical observations have long shown sex-based patterns in neurological and psychiatric conditions. Women are more likely to experience mood disorders, while men are more frequently diagnosed with attention-deficit/hyperactivity disorder. To explore whether genetic differences contribute to these patterns, Alex DeCasien, a neurogeneticist at the National Institute on Aging, and her team analyzed gene expression in nearly 170 brain tissue samples from 15 men and 15 women. They identified over 3,000 genes whose expression varied by sex, findings that may help explain differences in disease susceptibility. While environmental and social factors also play a role, the research underscores the value of studying both male and female brains.
In a complementary study, Emory University neurologist Thomas Wingo and his colleagues examined the proteomes of more than 1,200 post-mortem brain samples. Their analysis revealed that over 30 proteins exhibited sex-biased expression, potentially shedding light on why certain neurological conditions are more prevalent in one sex than the other. While Wingo noted that environmental influences cannot be ruled out, these findings could ultimately guide the development of more effective, sex-specific treatments.
Differences between sexes also extend to pain perception. Women consistently report more persistent and severe pain than men, a pattern supported by animal studies. Geoffroy Laumet, a neuroimmunologist at Michigan State University, and his team examined inflammatory cytokines in mice and discovered that males exhibited stronger anti-inflammatory signaling than females, likely influenced by sex hormones. As a result, inflammation and pain resolved more quickly in male mice. These findings could eventually lead to more effective pain management strategies tailored specifically for women.
