Belgium
— Female hormones circulating in the brain determine
masculine behavior, at least in mice. Estrogen--the
quintessential female hormone responsible for regulating
the reproductive cycle--turns lady mice into wannabe male
mice when it is allowed to penetrate the brain during
development, according to new research.
Neuroscientist Julie
Bakker of the University of Liege <http://www.ulg.ac.be/>
in Belgium and her colleagues proved this in the course
of solving one of the longstanding riddles of brain
development. Although it had long been known that a
certain protein--alpha-fetoprotein (AFP)--plays a key
role in mouse brain development by binding to estrogen,
it was unclear whether AFP facilitates the development of
female brains by carrying the hormone or simply by
blocking it from entering the brain.
The scientists used
female mice incapable of producing AFP and set them loose
in a Plexiglass aquarium with a sexually active male. To
give them extra incentive to mate, they also received
injections and capsules of female hormones. But, unlike
their wild cousins, the AFP-deficient mice showed little
interest in the male's advances and their brains had
fewer cells devoted to producing certain chemicals
critical to reproduction, just like males. Furthermore,
when placed in a cage with a sexually receptive female,
the mice without AFP tried to mimic their male
counterparts by mounting the female and engaging in
pelvic thrusting.
These observations still
didn't constitute definitive proof that AFP helped female
mice stay ladylike by blocking estrogen, however. So the
researchers prevented AFP-deficient mice from being
exposed to estrogen while still in the womb. Without AFP
and without estrogen during brain development, these mice
proved just as behaviorally female as their wild cousins,
the researchers report in the February issue of Nature
Neuroscience <http://www.nature.com/neuro/index.html>.
In addition to revealing that it is the lack of estrogen
that makes the brains of female mice feminine, the
findings indicate that it is the presence of the hormone
that makes male mice behave as males.
In primates, including
humans, androgen--not estrogen--plays the key role in
making men's brains masculine and AFP does not bind to
estrogens. But the appropriately named sex hormone
binding globulin (SHBG) may play a similar role to that
of AFP, Bakker notes, keeping women feminine and allowing
baby boys develop masculine behavior.
Further Reading
— Sex
Differences in the Brain
