Messenger - Vol. 3, No. 3, Page 7 Spring 1994 On Research Endocrine studies spotlight reproductive behavior Candlelight and soft music may spell romance for human beings, but long, sunny days put golden hamsters and many other vertebrates in the mood. When exposed to long days in a laboratory, in fact, hamsters instinctively prepare to breed, while shorter days are a "turn off" for them. Yet, hamsters kept in darkness will eventually breed anyway, as they become "refractory," or resistant to photo-induced changes. Long days also trigger reproduction in birds. But they, too, eventually become immune to photostimulation-perhaps as a survival mechanism not yet understood by scientists. Such "photoperiodic" behavior is a function of the endocrine system, which controls long-term growth and reproduction through a system of glands that release hormones, explains Milton H. Stetson, director of the School of Life and Health Sciences. For 20 years, Stetson has studied endocrine responses in vertebrates to determine how biological mechanisms-such as the hormone melatonin-dictate reproductive behavior. Secreted by the pineal gland, an appendage of the brain found in most vertebrates, melatonin seems to work with light/dark cycles to regulate reproduction. Levels of melatonin within the body are highest at night, and lower in the daytime. "A hamster that is being stimulated by long days will shut down in response to melatonin treatment at one time of day, but not at another," Stetson reports. The pineal glands of certain fish species, if placed in culture and subjected to constant darkness, will continue to produce melatonin periodically in a nearly 24-hour circadian rhythm. But, the pineals of other fish species are clockless, responding to constant darkness with very high levels of melatonin output on a continuous basis. "We're trying to determine why one type of fish has a clock and the other doesn't," Stetson says. "We would also like to know what regulates melatonin production in the clockless fish pineals." Many animals exhibit photoperiodic behaviors. When exposed to short days characteristic of an Arctic winter, for example, lemmings turn white, making them 'invisible' to predators such as the fox. Before the advent of electric light, humans subjected to constant darkness near the Arctic Circle demonstrated a seasonal rhythm in menstrual cycles. That is, normal menstrual cycles would cease during the months of darkness and resume on a more regular basis when daily light/dark cycles reappeared. Thus, Stetson's basic physiological studies of hamsters, fish, birds and lemmings may ultimately lead to a better understanding of human reproductive cycles and survival of the species. -Ginger Pinholster