Sexual behavior can change the physical structure of the brain, animal research made public Wednesday suggests, so that the brains of sexually active males may be in some ways different from those who abstain.
In the first experimental evidence of its kind, a neuroscientist at UC Berkeley demonstrated in laboratory animals that differences in sexual behavior can alter the neurons that make up the nervous system and the brain.
The new research, published today in the British journal Nature, indicates that brain regions responsible for sexuality may not be dictated solely by genetics, as some researchers have suggested, but also may be strongly shaped by what an individual does.
Indeed, for some parts of the brain involved in sexual responses, experience can make all the difference, the study determined.
By itself, the finding is a remarkable observation in the neurobiology of behavior, brain experts said. But added to the volatile debate over the biological origins of homosexuality and sexual orientation, it takes on a charged social and political dimension as well.
“It adds fuel to the fire,” said UCLA neurobiologist Roger A. Gorski, who studies sexual differences in the human brain. The study “has specifically looked at sexual behavior and shown there is an effect” on the brain.
In an experiment with laboratory animals, Berkeley psychology professor Marc Breedlove discovered that the brain cells controlling movement in male rats could be changed by altering their sexual behavior.
He compared animals that were sexually active with those that were not. He focused on a bundle of nerve cells at the base of the spinal cord, called the SNB complex, that is active during copulation by controlling the penis.
To eliminate the effects of differing hormone levels on their behavior, the male rats were castrated and then were implanted with testosterone capsules to keep them interested in sex. One group was put in a cage with female rats given hormones to be continually receptive, while a control group was kept with unreceptive females.
Measured at the end of a four-week period, the nerve cells of the sexually active male rats were much smaller--and therefore perhaps more sensitive and responsive, Breedlove suggested--than the control group that did not engage in sex.
“These findings give us proof for what we theoretically know to be the case--that sexual experience can alter the structure of the brain, just as genes can alter it,” Breedlove said.
“It is possible that differences in sexual behavior cause, rather than are caused by, differences in brain structure.”
Marian Diamond, an authority at UC Berkeley on how learning affects the brain, and other neuroscientists said that Breedlove’s work reflects a growing scientific appreciation for how readily the adult brain can alter its cells and neural circuits in response to changes in the world around it.
“When we learn or when we acquire new abilities, those abilities are encoded in changes in neural structure,” said William T. Greenough, an authority at the University of Illinois on the neurobiology of learning. “It is well known that practice makes perfect in terms of sexual stamina in humans.”
“You can’t escape the conclusion that experience--in this case, sexual experience--has an effect on the size of the cells,” said Fred H. Gage, an expert in neural development at the Salk Institute for Biological Studies in La Jolla. “As with most provocative studies like this, it raises more questions than answers.”
In recent years, such studies of the human brain have triggered a fierce dispute over a range of gender and behavioral differences. Chief among them is whether sexual orientation is a matter of personal choice or an inborn, inherited imperative.
Research has shown that parts of the brain involved in sexual appetite and gratification are slightly smaller in women and homosexual men than in heterosexual men. A more recent study of transsexuals also detected differences in brain structure. So far, however, no one knows what causes them or what they may mean.
Diamond said she was surprised the neurons in the new study got smaller. Usually, neurons increase in size in response to stimulation.
At the same time, other work has shown that homosexuality might be a matter of heredity, but no one has yet found a gene that could control anything so complex as sexual behavior.
Dean Hamer, a molecular biologist at the National Institutes of Health who pioneered research into the genetics of homosexuality, said that Breedlove’s experiment was so extreme that he questioned whether it has any relevance to normal human sexual behavior.
“He exposed these rats to a very extreme difference in experience,” Hamer said. “He put some in the whorehouse and put others in the nunnery, so to speak. I doubt that is a significant factor in humans or even in most rats.”
In any case, he added, the experiment involved a part of the brain that has not been shown to be involved in sexual orientation.
“I don’t think it is experience [by itself] that caused the changes in brain structures in gay men and transsexuals,” Hamer said.
Janice Juraska, a psychologist at the University of Illinois in Urbana who studies sexuality and the brain, said that sexuality is the result of so many forces--psychological, social, environmental, cultural, historical and biological--that it is misleading to say it can be dictated by either heredity or behavior alone.
“Besides whatever biological predisposition there may be, there are differences in lifestyles--types and areas of sexual behavior--that may also be feeding into the brain,” she said. “Social experiences leave their mark on the nervous system too.
“Everybody wants it to be simple,” she said. “It is not simple.”