College of Arts and Sciences Department of Biology
David Freeman
David Freeman
Associate Professor
Ph.D. University of Connecticut
Phone: (901) 678-2959
E-Mail: dfreemn1@memphis.edu
Life Sciences 523
Laboratory Webpage:

Research Interests:

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 history.

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Last Updated: 2/4/13