NEWS RELEASE, 06/21/99
UC Berkeley researchers find first evidence in mammals that sex hormones can change the size of a brain structure
By Patricia McBroom, Public Affairs
BERKELEY-- The first evidence in mammals that circulating male sex hormones can dramatically change the size of a brain region has been found by researchers at the University of California, Berkeley.
The structure under study is associated with sexual arousal in male rats. Its size seems to be completely controlled in adult animals by levels of androgens or male hormones that circulate through the bloodstream, according to research carried out by Bradley M. Cooke, a graduate student under the direction of S. Marc Breedlove, UC Berkeley professor of psychology.
Circulating androgens have long been known to fluctuate with environmental experiences. The new evidence means that for the brain as well, regions associated with sexual behavior and sex differences are thoroughly open to environmental influence, said Breedlove.
The research, sponsored by the National Institute of Neurological Diseases and Stroke, is published today (June 21) in the Proceedings of the National Academy of Sciences. A third co-author is former UC Berkeley student Golnaz Tabibnia.
"Most people assume that sex differences in the brain arise before birth during fetal development," said Breedlove. "But here we have dramatic changes in adulthood."
Breedlove said the researchers could completely reverse male-female differences in this brain region strictly through altering blood hormonal levels in the animals by castrating the male rats and giving androgens to the females.
He said the evidence should act as a warning to many people taking steroidal hormones.
"Hormones are powerful. If they can change this brain region so dramatically, surely they can change other areas of the brain as well."
The only other evidence that sex hormones can alter the size of brain structures was found in birds and was considered a special adaptation of birds.
The region studied by Breedlove and his students was the medial amygdala, correlated in male rats with sexual arousal in response to the sense of smell. It is normally 65 percent larger in male rats than in females and is rich in receptors for both androgens and estrogens.
This region is not as important for sexual arousal in humans as in rats, but Breedlove believes that other regions of the brain which show sex differences in mammals, including humans, may also be dramatically affected by circulating hormones.
"Now we can say that if you find a sex difference in the adult human brain, you have to consider the possibility that circulating hormones are responsible," said Breedlove.
According to considerable research on human beings, circulating androgens are responsible for libido or sexual arousal in both sexes. Women's ovaries produce tiny amounts of testosterone. There is little evidence that estrogen plays any role in sexual arousal in women.
Breedlove said it has also been shown that all kinds of experiences can change the level of the primary circulating androgen, testosterone, in men. Before any competition, for instance, whether a track meet or a chess match, hormone levels rise. After the match, testosterone levels remain high in the winner and fall in the loser.
Besides sexual arousal, these hormones are associated with a wide range of other male behaviors, including energy levels and aggression. That research is complicated, however, by the fact that fluctuations in hormone levels do not seem to affect either arousal or aggression in men.
Within the normal range, high testosterone levels do not make men more aggressive or more aroused, nor do low levels make them less so.
"Within normal ranges, every man has all the testosterone he needs," said Breedlove, noting that even a tiny amount of testosterone, as small as a tenth of normal levels, is enough to be fully effective.
"It's clear that if you take it all away, there is a dramatic effect," said Breedlove, "but otherwise, we know of no correlations between fluctuations in the levels of circulating androgens and libido in men."
Breedlove said the relevance of his research in female rats was difficult to assess. This region of the brain is probably not central to arousal in female rats. Also the female rats could not display female sexual behavior because their ovaries had been removed, depriving them of all sex steroids.
But, he added, "if androgens can influence brain regions this powerfully, there is no doubt that estrogen can as well."
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