by Pat McBroom
The first evidence that sexual experience can create differences in neurons or cell bodies of the nervous system has been produced by a Berkeley psychology professor.
The structural changes were found at the base of the spinal cord in neurons that control erection and ejaculation in male rats.
"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," said Marc Breedlove.
Breedlove's experiment was published in the Oct. 23 issue of the British scientific journal Nature.
The findings also throw new light on beliefs that sexual orientation, particularly homosexuality, is inherited or under the exclusive control of genes.
"It doesn't nullify the notion of a hereditary component in homosexuality," said Breedlove. "To my mind the scientific literature is clear: there is a genetic factor."
But, he added, this work suggests that sexual orientation may also be affected by experience.
"You can't assume that because you find a structural difference in the brain, that it was caused by genes. You don't know how the difference got there," said Breedlove.
A part of the hypothalamus, a brain structure associated with sexual appetite, is smaller in women and homosexual men than in heterosexual men. The difference might be inborn, but it could also be the result of experience, said Breedlove.
Breedlove's experiment with male rats compared sexually active animals with ones that were not. Castrated and then injected with androgens, all rats were equally prepared for copulation, but only half, caged with receptive females, had sexual activity. The others, caged with unreceptive females, had no sexual activity.
Four weeks later, examination of a part of the spinal cord that controls muscular activity of the penis showed clear differences between the two groups of male rats. Motorneurons of the sexually active rats were considerably smaller than in the other group.
"Somehow the extensive sexual experience affected the morphology of these neurons," said Breedlove.
One explanation is that because smaller motorneurons are more active and fire more frequently, they were being primed for further action, said Breedlove. But it is possible, he added, the neurons shrank because they were overworked.