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SLEEP AND TEMPERATURE REGULATION

Exogenous T₃ mimics long day lengths in Siberian hamsters

¹Department of Biology, University of Memphis, Memphis, Tennessee; and ²Department of Psychology and Institute for Mind and Biology, The University of Chicago, Chicago, Illinios

Submitted 6 October 2006 ; accepted in final form 25 January 2007

Siberian hamsters (Phodopus sungorus) exhibit seasonal cycles of reproduction driven by changes in day length. Day length is encoded endogenously by the duration of nocturnal melatonin (Mel) secretion from the pineal gland. Short-duration Mel signals stimulate reproduction and long-duration signals inhibit reproduction. The mechanism by which Mel signals are decoded at the level of neural target tissues remains uncharacterized. In Siberian hamsters, exposure to short day lengths or injections of Mel in long days results in a decrease in hypothalamic expression of type 2 iodothyronine deiodinase (Dio2) mRNA. Dio2 catalyzes the conversion of the thyroid hormone thyroxine to triiodothyronine (T₃). Thus exposure to short and long day lengths should decrease and increase hypothalamic T₃ concentrations, respectively. We tested the hypothesis that exogenous T₃ administered to short-day hamsters would mimic exposure to long day lengths with respect to gonadal stimulation. Hamsters gestated and raised in short day lengths that exhibited photoinhibition of the testes were given daily subutaneous injections of T₃ or saline vehicle for 4 wk beginning at week 12 of life. The results indicate that exogenous T₃ induced gonadal growth in short-day hamsters and delayed spontaneous gonadal development by an interval equal to the number of weeks during which T₃ was administered. T₃ injections delayed gonadal regression if given coincident with the transfer of hamsters from long to short day lengths. These results suggest that T₃ mimics long day exposure in Siberian hamsters and may serve as an intermediate step between the Mel rhythm and the reproductive response.

photoperiod; melatonin; thyroid hormone; reproduction

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