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
                                                    -Ginger Pinholster