Photoperiodic Regulation on Dio2/3 expression in Middle Aged Siberian Hamsters

Abstract

Photoperiodism is the biological ability to measure day length, which provides a reliable cue for time of year. Changes in photoperiod induce seasonal reproductive, metabolic and immunological adaptations to allow animals to anticipate the upcoming season. Siberian hamsters decrease body mass, halt reproductive behaviors, and increase anxiety- and depressive-like responses in short days. Melatonin plays an important role in conveying both circadian and photoperiodic information. Melatonin signaling also regulates hypothalamic levels of thyroid hormones, which has been implicated in the rapid photoperiod mediated changes in physiology. As rodents age, they become unresponsive to short days and continue to maintain reproductive function. The goal of this study is to determine whether changes in gene expression of Dio2/Dio3 correlate with photoperiodic changes in body mass and behavior of late adult hamsters. Although studies have been conducted in juvenile animals, Dio gene expression in their aged counterparts remains unspecified. We predict that middle-aged hamsters (10-12 months) housed in short days will express decreased responsiveness to photoperiod relative to their long day housed counterparts. Middle-aged male Siberian hamsters were individually housed in either short (SD, 8h: 16h) or long (LD, 16h: 8h) day conditions for 4-8 weeks, then tested for anxiety and depressive-like behaviors using the open field, elevated plus, and forced swim tests. Daily food intake and body mass were assessed once per week during the study. At the end of the experiment, hippocampal and hypothalamic tissues were collected for qPCR analysis of Dio3/Dio2 expression. There was no change in Dio3 expression between long day and short day housed hamsters, but Dio2 expression was blunted. This study may provide a novel mechanism by which photoperiodic signals are transduced to regulate body mass and behavioral changes.