STRUCTURAL BIOLOGY
Female hormone found to play key role in male birds' ability to sing

Jim Barlow, Life Sciences Editor
(217) 333-5802; b-james3@uiuc.edu

3/1/2001

CHAMPAIGN, Ill. -- Why do male but not female zebra finches sing?

Scientists for 20 years have known that males develop the correct brain pathway but researchers didn’t know why. Mystery solved: Male brains produce enough of the so-called female hormone estrogen at exactly the right time.

Estrogen's role has been suspected but never proven. A new study in which University of Illinois scientists isolated and monitored living brain slices in culture gave a clear picture. The work -- funded by the National Institutes of Health -- appeared in the February issue of Nature Neuroscience.

"It is estrogen that makes the male songbird capable of singing," said David F. Clayton, a professor of cell and structural biology and researcher at the UI Beckman Institute for Advanced Science and Technology. "Estrogen is the signal that says, `OK, let's build the circuitry.’ There may be other things going on, too, but, in a front-line sense, estrogen determines if a bird's brain is going to make it or not. At age 20 days, when the male and female brains are the same, the decision has to be made."

A surprising discovery, he said, was that male brain slices synthesized estrogen autonomously. The slices were kept by themselves in a culture dish, yet, in the absence of gonads, they continued to make estrogen on their own. Researchers had known that brain development was sensitive to estrogens and other similar steroid hormones, but they had assumed these hormones had to come from the gonads.

"The idea of neuronal steroids has been bounced around in the literature, but this is the first clear demonstration that the brain is actually making estrogen," Clayton said.

By the age of 30 days, a male finch's brain will have developed a synaptic projection -- called the HVC-RA pathway and visible in the brain slices to the naked eye -- between two nuclei of the brain. The resulting circuitry links the high vocal center (HVC) in the neostriatum with the robust nucleus of the archistriatum (RA).

Because females do not have this circuitry, they cannot sing.
Clayton's doctoral student Carl Clayton Holloway, who has since graduated, designed the in-vitro experiments. Initially, they observed that, when kept separate, male slices developed the projection but the females did not. When male and female slices were in the same culture dish, however, projections grew in the female slices, suggesting that the males produce a diffusible agent.

The researchers next added tamoxifen and fadrozole into samples containing slices from both sexes to determine if the agent was estrogen. Both estrogen-inhibiting drugs, administered separately, blocked normal male development and female slices developed normally.

"These results strongly indicate that male slices produce estrogen, which is necessary for their own masculine development and sufficient to induce masculine development in females," Clayton and Holloway wrote.

Both male and female slices produce estrogen, but only males have enough to trigger the growth of axons between the two nuclei, the researchers reported.