Low temperature-induced circulating triiodothyronine accelerates seasonal testicular regression

In temperate zones, animals restrict breeding to specific seasons to maximize the survival of their offspring. Birds have evolved highly sophisticated mechanisms of seasonal regulation, and their testicular mass can change 100-fold within a few weeks. Recent studies on Japanese quail revealed that seasonal gonadal development is regulated by central thyroid hormone activation within the hypothalamus, depending on the photoperiodic changes. By contrast, the mechanisms underlying seasonal testicular regression remain unclear. Here we show the effects of short day and low temperature on testicular regression in quail. Low temperature stimulus accelerated short dayinduced testicular regression by shutting down the hypothalamus-pituitary-gonadal axis and inducing meiotic arrestandgermcell apoptosis. Induction of T₃ coincided with the climax of testicular regression. Temporal gene expression analysis over the course of apoptosis revealed the suppression of LH response genes and activation of T₃ response genes involved in amphibian metamorphosis within the testis. Daily ip administration of T₃ mimicked the effects of low temperature stimulus on germ cell apoptosis and testicular mass. Although type 2 deiodinase, a thyroid hormone- activating enzyme, in the brown adipose tissue generates circulating T₃ under low-temperature conditions in mammals, there is no distinct brown adipose tissue in birds. In birds, type 2 deiodinase is induced by low temperature exclusively in the liver, which appears to be caused by increased food consumption. We conclude that birds use low temperature-induced circulating T₃ not only for adaptive thermoregulation but also to trigger apoptosis to accelerate seasonal testicular regression.