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Inhibition of Reproductive Maturation and Somatic Growth of Fischer 344 Rats by Photoperiods Shorter than L14:D10 and by Gradually Decreasing Photoperiod¹

^a Department of Biology, College of William and Mary, Williamsburg, Virginia 23187-8795

ABSTRACT

Photoperiod is the major regulator of reproduction in temperate-zone mammals. Laboratory rats are generally considered to be nonphotoresponsive, but young male Fischer 344 (F344) rats have a uniquely robust response to short photoperiods of 8 h of light. Rats transferred at weaning from a photoperiod of 16 h to photoperiods of < 14 h of light slowed in both reproductive development and somatic growth rate. Those in photoperiods < 13 h of light underwent the strongest responses. The critical photoperiod of F344 rats can be defined as 13.5 h of light, but photoperiods of 12.5 h are required to fully suppress reproduction and somatic growth. This demonstrates that the 12-h photoperiod that is standard in some laboratory colonies would have significant effects on reproductive maturation and growth rate of this common rat strain. Young F344 rats in decreasing photoperiods that mimic natural change experienced delayed reproductive development and decreased growth rate to a greater extent and for a longer duration than those transferred at birth to a short photoperiod. The effects of gradual changes in photoperiod persisted for at least 12 wk after weaning. This indicates that young male F344 rats possess responses to photoperiod that would result in functional photoperiodism in a wild mammal.

FOOTNOTES

First decision: 10 April 2000.

¹ Supported by NIH Grant R15 DK51334 and Jeffress Research Grant J-356 to P.D.H. and by a Minor Research Grant to M.E.G. from a Howard Hughes Medical Institute Undergraduate Biological Sciences Education Program grant to the College of William and Mary.

² Correspondence: Paul D. Heideman, Department of Biology, College of William and Mary, P.O. Box 8795, Williamsburg, VA 23187-8795. FAX: 757 221 6483; pdheid{at}wm.edu

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