Brain Sexual Dimorphism
by Heidi Anderson
Contrary to conventional thinking, adult brains can undergo significant structural changes. In their 1996 paper, “A brain sexual dimorphism controlled by adult circulating androgens,” scientists Bradley M. Cooke, Golnaz Tabibnia (now at UCLA), and S. Marc Breedlove from the Department of Psychology at UC Berkeley demonstrated that the size of a part of the rat brain linked to sexual behavior could be controlled by the application or withdrawal of sex hormones. The paper, along with a very interesting discussion by Bruce S. McEwen, is available online at the website of the Proceedings of the National Academy of Sciences (www.pnas.org).
Until recently, sex differences in the brains of mammals were thought to arise in much the same way as reproductive differences: by exposure to hormones during fetal development in the womb. For example, in a highly publicized Nature article, Breedlove presented evidence that the ring fingers of lesbian women and straight men tend to be significantly longer than their index fingers. He and others postulated that this happens because straight men and lesbians are exposed to more testosterone as fetuses than straight women, who tend to have index fingers longer than their ring fingers.
Theories of sex difference based in fetal development, which generally hold that the adult brain does not change and that sexual patterns set before birth persist into adulthood, have sparked mixed popular reactions. They have been embraced by people who feel strongly that sexual preference is determined solely by genetics, and denied by people who point to a vast cultural apparatus that creates and maintains gender roles.
Although the Berkeley team’s rat experiment touches upon a topic loaded with complex social implications, their experimental method was fairly straightforward. The posterodorsal nucleus of the medial amygdala (MePD) of the rat brain is implicated in sexual behaviors, including sexual arousal. As revealed by Nissl stains, the MePD of adult male rat brains is about 65% larger in volume than that of female rats. When the research team treated female rats with testosterone, however, they found that their MePDs would grow to the size of the male rats’ MePDs in about 30 days. Conversely, the MePD of a castrated male would shrink to the size of a female’s MePD in the same amount of time. Exposing castrated males to testosterone preserved the size of their MePDs indefinitely. Not only was the volume of the MePD altered by androgen treatment, but individual cell soma areas were enlarged as well. The group concluded that these physical sex characteristics in the brains of rats were entirely hormone-controlled, and could be altered in adult rats.
As further evidence that the sex-differentiated areas of adult brains could be changed by exposure to hormones, the group cited findings that adult female canaries treated with testosterone experience an enlarging of their brain’s vocal center and begin to produce male-like songs. In regard to humans, they remarked, “Transsexuals treated with cross-sex hormones display sex reversals in their cognitive abilities, emotional tendencies and libido, and sex offenders are sometimes treated with antiandrogens to reduce their sex drive. The sociosexual changes observed in these groups most likely reflect structural and physiological plasticity in steroid-sensitive areas within the brain.” And finally they stated “MePD sexual dimorphism in rats is quite comparable to reported sexual dimorphisms in the human brain and therefore supports the possibility that sexual dimorphisms of the human brain are caused solely by circulating steroids in adulthood.”
As McEwan also wrote, however, “this is undoubtedly an overstatement of a valuable point.” The morphological sexual differences among human brains must be the result of complex interactions among experiences, hormone actions, and developmental influences. However, the research of the Berkeley team implies that the physical structures of adult brains, and the behaviors that are controlled by them, are surprisingly flexible. Hormones may play an important role in many other examples of adult brain plasticity as well: currently the team is investigating the central nervous systems of Siberian hamsters, which undergo dramatic physiological changes from summer
The medial amygdala (MePD) area of the brain is implicated in sexual behavior and is substantially larger in male rat brains. MePD neuronal soma size in male control rats (SHAMS) is reduced to characteristically female size by castrating male rats (Castrates+B), but addition of testosterone (T) restores size in castrated male rats. MePD neuronal soma size in females can be increased to male size by treatment with testosterone.