Sex Differences in the Brain

sex differences-April

When the Food and Drug Administration (FDA) ordered the makers of Ambien to cut their recommended dose in half for women, they were acknowledging that millions of women had been overdosing on the well-known sleep aid for 20 years. Soon after, CBS’s 60 Minutes highlighted this fact—and sex differences in general—in a segment that asked: Why did this happen, and are men and women treated equally in research and medicine?

April’s Cerebrum article, “Equal ≠ The Same: Sex Differences in the Human Brain,” addresses those questions, traces the evolution of the subject back to Darwin, looks at what he labels “the neurotrash” approach, and reports some important new research on the issue. The article is written by Larry Cahill, Ph.D., a professor in the Department of Neurobiology and Behavior at the University of California, Irvine. Cahill brings the perspective of someone who once worked in private industry (on memory-enhancing drugs at G.D. Searle & Company in Illinois). Later, at UC Irvine, his research led to discoveries about sex influences on emotional memory. Currently, he studies sex influences on brain function.

Cahill believes that the biomedical community has long operated on what is increasingly being viewed as a false assumption: that biological sex matters little, if at all, in most areas of medicine.” He answers the second question with a very direct, “No, today’s biomedical research establishment is not treating men and women equally.”

Since the 1970s, the hypothalamus, a region of the brain that regulates both sex hormones and sexual behaviors, was the focus for most neuroscientists studying sex differences in the brain. “They almost completely ignored possible sex influences on other areas of the brain, assuming that the sexes shared anything that was fundamental when it came to brain function,” he writes, adding: “To this day, neuroscientists overwhelmingly study only male animals.”

Recent studies cited by Cahill include one that found greater average inter-hemispheric connectivity in women compared to men—although what it means functionally is yet to be determined. Another study involved a comprehensive analysis of the patterns of expression in the brain of immune system–related genes in human aging and Alzheimer’s disease (AD) and found sex-specific patterns of gene expression in both conditions.

“In particular, they compared patterns of gene expression in two regions that are critical for higher cognitive function and known to develop AD-type pathology: the hippocampus and a region of the frontal cortex called the superior frontal gyrus,” writes Cahill. “The hippocampus was more prone to immune-type gene reactions in females than in males, while the superior frontal gyrus was more susceptible to immune-type gene reactions in males than in females. Studies such as this prove that the biological mechanisms of brain aging and disease cannot be assumed to be the same in men and women.”

Stay tuned to find out whether sex differences in the brain ultimately reveal the biological reason that Men Are from Mars, and Women Are from Venus, a book that Cahill would argue was published way before all the facts were in.

—Bill Glovin

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One Response to Sex Differences in the Brain

  1. Rosemary Shields says:

    Today, April 8th, is Gender Equality Day. Just a reminder that we are equal under the law, not necessarily the hormones that regulate our good health. Before the 1970s, most clinical trials were done on males. We’ve come a long way, baby, but more is needed to be done on research for females, as your article notes.

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