My research spans organismal biology and physiology, with an emphasis on biological
rhythms, reproductive neuroendocrinology and seasonality. I currently investigate
the neural mechanisms that regulate seasonal cycles of reproduction in mammals, with
concentration on the pineal hormone melatonin. Synchronization of reproductive effort
with the external environment is crucial for successful reproduction in temperate
zone mammals; many species track changes in day length to time reproduction.
Exposure to long day lengths stimulate the reproductive system whereas short day lengths
in autumn causes involution of the reproductive system within 6 weeks; this inhibition
wanes after 20 weeks and the gonads spontaneously revert to the spring reproductive
phenotype in mid-winter, well in advance of the long day lengths of spring. The waning
of short-day inhibition of reproduction is thought to reflect development of refractoriness
of the neuroendocrine axis to short days and the pineal melatonin signals that transduce
effects of day length on the reproductive system.
The refractory state is reversed by exposure to many weeks of long, summer-like, day
lengths. With the exception of inhibition by short day lengths, little is known regarding
the neural melatonin target tissues that subserve the various phases of the seasonal
reproductive cycle. Melatonin acts at several neural sites to inhibit gonadotropin
secretion. Little is known about the relationships among the various structures in
the melatonin “pathway”.
My research established that refractoriness to melatonin occurs independently at multiple
brain sites in Siberian hamsters. This suggests that melatonin-sensitive neural circuits
may permit independent control of the several individual photoperiod traits such as
mating behavior, food intake, body weight regulation, immune function and prolactin
secretion. I plan to investigate this question using various neuroendocrine, behavioral
and physiological approaches.
A hamsters’ response to a particular day length depends on whether the current day
length is longer or shorter than those that preceded it; this has been termed the
“photoperiod history effect”. For example, an intermediate day length of 13 hours
of light per day elicits either gonadal growth or regression depending on whether
the animal was previously exposed to shorter or longer day lengths, respectively.
The sites and mechanisms by which melatonin forms photoperiod “memories” or where
these memories are stored remain largely uncharacterized. Recent results from my laboratory
identify the specific neural tissues that are involved in the formation of a photoperiod