Sex differences may not be set at birth

The news media love to write about research that shows differences between the brains of women and those of men, said Marc Breedlove, PhD. And most reports of these findings concentrate on the possibility that the differences are purely biological, developing in the fetus before birth.

But Breedlove emphasizes that some of those differences might occur in adulthood, depending on a person?s experience and the mixture of hormones circulating in each person?s blood. He presented evidence for this theory at a symposium on sex hormones at APA?s 1998 Annual Convention.

According to Breedlove, professor of psychology at the University of California at Berkeley, some sex differences in rats? brains are not hard-wired at all. Rather they change depending on the amount of male hormones, or androgens, circulating in the blood. One major difference between male and female rats is that the medial amygdala, the area of the brain that receives information about pheromones, is larger in male rats than in female rats. The medial amygdala is important for mating?without it, in fact, males fail to become sexually aroused.

The size of the medial amygdala appears to be controlled by the amount of androgens circulating in the rats, said Breedlove. He and his student Bradley Cooke find that if they castrate adult male rats?thus cutting off their supply of androgens?their medial amygdalas shrink to the size of a female medial amygdala, and they fail to become sexually aroused around females. If they castrate the adult males and then give them the androgen hormone testosterone, they can prevent shrinkage of the brain area. Furthermore, if the researchers give female rats testosterone, their medial amygdalas grow larger. These increases in volume result from an increase in the size of existing neurons.

Time of year and personal experience can also alter portions of the nervous system, said Breedlove. For example, one area of spinal chord neurons, called the spinal nucleus of the bulbocavernosus (SNB for short), grows and shrinks in male rodents depending on the season. This group of neurons stimulates the muscles attached to the base of the penis and helps control erection and ejaculation.

Normally, during the winter when it?s best not to have young, the SNB in rodents shrinks at the same time that the testes regress and hormone levels decline. But when Breedlove and his postdoctoral student Carol Hegstrom, PhD, put a group of male hamsters into winter-like conditions?fewer hours of daylight, cooler temperatures?and then put them into a cage with a receptive female hamster, hormone levels increased, their testes enlarged and their SNB returned to normal size.

'Sex differences [in the nervous system] can be explained by circulating androgens,' said Breedlove. And environment and experience can manipulate those androgens. This implies that differences in 'hormone levels in adults might cause sex differences found in the brains of humans.' In other words, the sex differences may not be set at birth.

?Beth Azar