Testosterone's Manly Role

Researchers have uncovered some unexpected information about how sex hormones affect masculine physiology and behavior. Males are known to produce the androgen testosterone in the testes and then convert some of it into the estrogen 17β-estradiol in the brain. In addition, testosterone and 17β-estradiol are known to be responsible for male behavior in many vertebrates. But the respective contributions of these two structurally similar yet functionally different hormones was unclear until now. Nirao M. Shah of the University of California, San Francisco, and colleagues have shown that 17β-estradiol—but not testosterone—controls prenatal masculinization of the brain in mice (Neuron 2010, 66, 260). In fact, the brain of a prenatal mouse shows a surprising lack of androgen receptors for testosterone. After birth, 17β-estradiol signaling increases the number of androgen receptors. Their subsequent activation by testosterone controls the frequency and extent of typical male behaviors, including mating displays, fighting, and territorial marking. Even if a mouse is genetically engineered to lack androgen receptors within the brain, however, it still displays male behavior, although less often and to a lesser degree than normal mice, according to the researchers.