( icates via a blood portal with the ante...)New brain hormone puts brakes on reproduction

These hormones are dumped into the bloodstream and make their way to the gonads, where in males they stimulate production of testosterone and the maturation of sperm. In females, the hormones stimulate production of estradiol, a sex steroid hormone and the body's main form of estrogen, and regulate ovulation, the production of fertile eggs.

Estradiol and testosterone, in turn, feed back on the pituitary to shut down production of pituitary hormones, establishing feedback that keeps the body's sex hormones on an even keel.

Estradiol also works higher in the brain, on the hypothalamus, to ramp down production of GnRH, but how this works has been a relative mystery. The new study provides an answer: Estradiol stimulates cells in the dorsomedial nucleus of the hypothalamus to produce GnIH, which appears to act directly on cells in the hypothalamus to turn off their production of GnRH.

"Here, we have a novel neural pathway mediating the regulatory actions of sex steroids," said Lance Kriegsfeld, UC Berkeley assistant professor of psychology.

"This is an example of the reproductive system being fine tuned," said George Bentley, UC Berkeley assistant professor of integrative biology. "We know a lot about the gross regulation of the reproductive system, but fine tuning hasn't been well understood at all."

GnIH was discovered five years ago in quail by Japanese researchers led by Kazuyoshi Tsutsui, a professor on the faculty of integrated arts and sciences at Hiroshima University. The discovery supplied one of the last remaining pieces of the bird's hormone system that controls reproduction. GnIH seemed to be the missing antagonist that switches off pituitary gonadotropins, and work by Tsutsui and Bentley in quail and white-crowned sparrows confirmed its role in turn
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Source:University of California - Berkeley