NEWS RELEASE, 10/20/97

FOR IMMEDIATE RELEASE

BERKELEY--The first evidence that sexual experience can create differences in neurons or cell bodies of the nervous system has been produced by a professor of psychology at the University of California, Berkeley.

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, professor of psychology at UC Berkeley

Breedlove's experiment will be published in a brief communication to 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, as scientists have always believed.

"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.

Prior research has shown that a part of the hypothalamus, a brain structure associated with sexual appetite and gratification, is smaller in women and homosexual men than in heterosexual men. It has been inferred that the difference might be inborn, but it could also be the result of experience, said Breedlove.

Breedlove's experiment with male rats compared animals that were sexually active with ones that were not. Castrated and then injected with androgens, all of the rats were equally prepared for copulation, but only half the group had sexual activity. Those animals were placed in cages with females given hormones to keep them continuously receptive. The others were in cages with unreceptive females and had no sexual activity.

Four weeks into the experiment, 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.

The 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, that the neurons shrank because they were overworked.