Melatonin causes reproductive organs to shrink in lab animals; scientists suspect the supplement causes similar problems in humans

• Shift workers and travelers who pop melatonin pills to stave off drowsiness or jetlag have another reason to be cautious about taking the supplement, say Japanese and University of California, Berkeley, researchers.
• A new study shows that melatonin, a hormone available without prescription, has broader effects in the brain than once thought.
• In experiments on the Japanese quail, the researchers found that melatonin switches on a recently discovered hormone called gonadotropin inhibitory hormone (GnIH), which has been found to have the opposite effect to the key hormone priming the body for sex -- gonadotropin releasing hormone (GnRH).
• In birds, switching off GnRH causes the gonads -- testes and ovary -- to shrink as part of the birds' yearly cycle.
• Though the role of melatonin is likely to be more complex in humans, the fact that the hormone has such a big effect on birds suggests it could have significant though unnoticed effects in humans, said George E. Bentley, an assistant professor of integrative biology at UC Berkeley.
• Bentley, along with graduate student Takayoshi Ubuka, senior endocrinologist Kazuyoshi Tsutsui, a professor on the faculty of integrated arts and sciences, and their colleagues at Hiroshima University, report their findings this week in the Online Early Edition of the Proceedings of the National Academy of Sciences.
• GnIH's discovery got a lot of attention at the time because it was one of the last remaining pieces of the brain's hormone system that controls reproduction.
• GnIH seemed to be that missing antagonist, and work by Tsutsui and Bentley confirmed its role in turning down production of GnRH and thus switching off the gonads.
• Tsutsui, Ubuka and Ukena also are affiliated with the Core Research for Evolutional Science and Technology at Tokyo's Japan Science and Technology Corporation.