University of California, Berkeley, researchers have discovered a new actor in the mammalian reproductive system, a hormone that fills a role long suspected, but until now undetected.

The hormone, called gonadotropin-inhibitory hormone (GnIH), puts the brakes on reproduction by directly inhibiting the action of the central hormone of the reproductive system - gonadotropin releasing hormone (GnRH). GnRH stimulates the pituitary gland to activate the reproductive system, whereas GnIH appears to reduce the effects of GnRH stimulation.

Researchers have long sought inhibitors of pituitary gonadotropins, but many had come to believe that such a direct inhibitor was unlikely in the complex cast of hormones and factors in the reproductive system. The inhibiting or braking hormone may complement the "gas pedal" role played by another recently discovered hormone, kisspeptin, that stimulates GnRH.

The discovery in rats, mice and hamsters of this new system for regulating reproduction strongly suggests that the hormone plays a similar role in the reproductive systems of humans.

The human reproductive system is regulated like a thermostat, with a number of hormones and factors to keep the body's hormones within the optimal range for fertility and successful mating. The main component is the brain's GnRH-producing hypothalamus, which communicates via a blood portal with the anterior pituitary and stimulates production of the hormones gonadotropin, luteinizing hormone and follicle stimulating hormone.

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.

Image credit: Nature.com