Do We Think With Our Hormones?

WASHINGTON, 1999 June 21 [Reuters]

Ribald clichés about the part of the body men use for thinking may have some basis in truth, researchers said on Monday. They said tests on rats showed the “male” hormone testosterone, generated primarily in the testes, could change the size of part of the brain, not only in males but in females too.

“It is an important reminder that hormones aren’t just doing something to the body but that they are doing something to the brain,” said Marc Breedlove, a psychology professor at the University of California Berkeley who led the study. “As a psychologist I am happy to remind the public that the brain is always changing. Mostly we see it as a note of caution that any sex differences you see in the structure of the adult human brain could be due to sex differences in circulating steroids,” he said in a telephone interview.

Breedlove’s findings added to a growing body of evidence that the brain is not fixed at birth. “Most people conclude that sex differences are inborn, because most layfolk are used to thinking of the brain as something that doesn’t change, as something that is given to you at birth,” Breedlove said.

His team, led by graduate student Bradley Cooke, looked at a part of the rat brain involved in sexual arousal. The posterior dorsal component of the medial amygdala is 50 percent to 80 percent larger in male rats than in females. But after castration, which stops virtually all production of testosterone, this region shrank to about the size seen in females within 30 days, Breedlove’s team reported in the Proceedings of the National Academy of Sciences.

And giving testosterone to female rats for 30 days increased their medial amygdala to the size usually seen in males. In rats this region of the brain is involved in processing smells that cause sexual arousal. The castrated male rats showed little interest when they could smell a female in heat, while normal rats showed great interest.

Breedlove’s team did not test the hormone-charged female rats to see if their sexual behavior changed, but he said he doubted that it would, as sexual behavior is complex and not controlled by a single area of the brain.

But they said their findings had significance for sexual research. “Reports of structural differences between the brains of men and women, heterosexual and homosexual men, and male-to-female transsexuals and other men have been offered as evidence that the behavioral differences between these groups are likely caused by differences in the early development of the brain,” Breedlove’s group wrote.

They cited other studies that have also shown manipulation of hormones can affect the size of structures inside the brains of animals. “Human behavior is also subject to the activational effects of androgens (male hormones),” they wrote. “Transsexuals treated with cross-sex hormones display sex reversals in their cognitive abilities, emotional tendencies, and libido, and sex offenders are sometimes treated with anti-androgens to reduce their sex drive.”

They said the usual sexual differences in the structures of rat brains – at least in this part of the amygdala – are comparable to those seen in humans. “There are reports that there are also sex differences in the size of the overall amygdala in humans,” Breedlove said, but he said no one has looked to see if this particular region of the amygdala is larger in men as it is in rats.

Breedlove also suspects the same region is involved in smell, although he notes smell is much less important to human sexual behavior than it is to rat behavior.

He said his findings might also help explain why estrogen replacement therapy is being shown to help women think and remember better as they age.

It is possible that so-called sex hormones are very important to brain function, he said.