By David N. Leff
Many years ago - decades before political correctness - the chief of surgery at a prestigious American teaching hospital objected to women becoming surgeons. His reasoning: The brain of the female sex, being smaller than that of the male, doesn't contain enough DNA to do surgery.
This sounds quaint today, but current reports describe structural differences between the brains of men and women, as well as between heterosexual and homosexual men. From these cerebral anatomical dissimilarities they explain the behavioral differences between the sexes as due to brain development during gestation and childhood. In boys, testosterone secreted by the testes manages these changes.
Neuroendocrinologist Bruce McEwen, at The Rockefeller University in New York, made the point, "The emphasis on early developmental programming of brain structural sex differences was reinforced over several decades by the long-standing view that the brain is not capable of significant structural changes in adulthood."
He made this observation in a journal commentary titled, "Permanence of brain sex differences and structural plasticity of the adult brain." It accompanies a paper in the current Proceedings of the National Academy of Sciences, (PNAS), dated June 22, 1999, which demonstrates this cerebral malleability in male and female rats. Its title: "Brain sexual dimorphism controlled by adult circulating androgens." The article's senior author is neuroendocrinologist S. Marc Breedlove, at the University of California in Berkeley.
His paper points to one variable factor persisting throughout life, namely that the brain is constantly bathed and nourished by adult hormones that strongly influence the gender-related behavioral patterns of men and women.
Breedlove and his co-authors zeroed in on a region of the mammalian brain, the medial amygdala (MD), which is involved in reproductive behaviors, including response to olfactory pheromones and sexual arousal. In the brains of both sexes, this MD tissue is richly riddled with receptors for both male androgens - especially testosterone - and female estrogen. Normally, the amygdala is smaller in sexually mature women than in men. In rats, it's 65 percent larger in males than in females. The team set out to see if the male hormone testosterone could influence the size of this MD region.
After castrating sexually mature male rats, they implanted capsules of slow-release testosterone under the skin between the shoulder blades of both rodent sexes. Control males got empty capsules, and so did an aliquot of the gonadectomized males. After 30 days' exposure to the male hormone, the MD of females had grown to match that of male controls. In castrated males MD shrank to normal female volume. Those size shifts took place in the neurons' individual cell bodies.
"These experiments," the PNAS paper noted, "indicate that the observed sexual dimorphism in volume of the posterodorsal component of the medial amygdala is primarily a result of adult circulating androgen."
Other sexually dimorphic nuclei (neuronal bundles in the brain), the team pointed out - notably in the preoptic region of the hypothalamus - also are affected by adult hormone manipulations. Thus, rat castration considerably reduced this area.
The Rockefeller's McEwen offered another anatomic example: "Androgen actions in the adult spinal nucleus of the bulbocavernosus, which innervates the penis, regulate both size and connectivity of these motor neurons." (Contrapuntal nerve bundles innervate the vagina.)
This gender dichotomy is also for the birds. Breedlove and his co-authors recalled findings by others that administering testosterone to female Japanese quail swells their preoptic area to a par with male size, whereas gonadectomy of the male shrinks it. But they do not accept the concomitant suggestion that such plasticity helps the birds keep down body weight for flight. "The present findings in a mammalian species," they wrote, "speaks against this hypothesis." The team pointed out: "Adult testosterone treatment of female canaries enlarges the brain region higher vocal center, and induces the birds to produce male-like song."
But the birds of the air and the beasts of the field don't have this hormonal mind-bending gender influence all to themselves. Human behavior responds as well. The co-authors cite the effects of cross-sex hormones on transsexuals - individuals of one physical sex who feel psychologically closer to the opposite sex. They "display sex reversals in their cognitive abilities, emotional tendencies and libido," adding that "sex offenders are sometimes treated with antiandrogens to reduce their sex drive."
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McEwen speculated that hypothalamic nuclei, which differ in size between male and female rat brains, may also do so in humans. "There are some indications," he wrote, "that such nuclei may differ in size between homosexual and heterosexual males. ... Notable among these," he continued, "are hypothalamic nuclei for which sex differences are absent at birth, develop at around 4 years of age, and persist until around 50-60 years of age, where there is a decrease in volume at somewhat different rates in both sexes."
Breedlove surmised that these "sociosexual changes" reflect structural and physiological alterations "in steroid-sensitive areas within the brain." He concluded, "The volumetric sex reversal reported here substantiates the possibility that hormones in adulthood can dramatically affect a structure of the brain region concerned with sexual behavior."
However, McEwen opined that the Californian's suggestion that sexual dimorphisms of the human brain are due solely to circulating steroids in adulthood "is undoubtedly an overstatement of a valuable point." Rather, he proposed "morphological differences in the human brain are likely to reflect an interaction between developmental influences, experience, and hormone actions on the mature brain."