Food and neonatal androgen interact with photoperiod to inhibit reproductive maturation in Fischer 344 rats.

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Abstract
Laboratory rats generally do not respond reproductively to short days (SD) unless they are given treatments that unmask reproductive inhibition in SD. While young Fischer 344 (F344) rats are unusual among rat strains in that SD substantially inhibit their reproductive response, the inhibition is not as strong as in the classically photoresponsive species. Rats may have two components to photoresponsivenes: 1) an obligate inhibition by SD, and 2) a facultative inhibition in response to biologically relevant challenges. This study tested whether maturing male F344 rats, which clearly have an obligate inhibition of reproduction in SD, also have an additional, facultative inhibition of reproduction in SD in response to food restriction, a biologically reasonable challenge, or to neonatal androgen treatment, a pharmacological treatment that presumably alters organizational events in the development of the reproductive axis. Food restriction over a period of 13 wk strongly enhanced the inhibition of testicular growth by SD. Similarly, testosterone propionate (TP) treatment at 3 days of age strongly enhanced the inhibition of testicular growth by SD. Neonatal TP treatment along with SD inhibited testicular development almost as strongly as that observed in some commonly studied photoresponsive rodents, but for only half as many weeks. Thus, F344 rats possess an obligate inhibition of testicular development in SD that can be enhanced facultatively by food restriction and even more greatly enhanced by neonatal TP treatment. This combination of obligate and facultative responses to SD may have been important to wild rats ancestral to laboratory rats.