The male hormone testosterone doesn't work in ways that had been assumed when it comes to masculinizing the brain during development and making males behave a certain way when they're adults, researchers found.

"It was known that testosterone and estrogen are essential for typical male behaviors in many vertebrate species," study senior author Dr. Nirao M. Shah, of the anatomy department at the University of California, San Francisco, said in a news release. "However, how these two hormones interact to control masculinization of the brain and behavior remained to be established."

Shah and colleagues genetically engineered mice to get rid of a pathway thought to play a role in how animals become masculinized. Yet, they found that the mutant mice still acted like males -- fighting and marking territory -- but with some differences. In particular, the extent and frequency of typical male behaviors varied between the mutant mice and the other mice.

The study, published in the April 29 issue of the journal Neuron, found that estrogen, which is virtually undetectable in the circulation of most male species, can be derived from circulating testosterone in males. In the brain, this testosterone-derived estrogen can control many behaviors that are typically linked to males.

"Our findings in conjunction with previous work suggest a model for the control of male pattern behaviors in which estrogen masculinizes the neural circuits for mating, fighting and territory marking, and testosterone and estrogen signaling generate the male typical levels of these behaviors," Shah said. "It will be interesting in future studies to identify the molecular and circuit level mechanisms that are controlled by these hormones."

More information

For more about testosterone, try the U.S. National Library of Medicine

SOURCE: Cell Press, news release, April 28, 2010

-- Randy Dotinga