TSH-β as a signal of long photoperiod: Insights from field and natural photoperiod studies

The neuroendocrine pathways regulating seasonal activation of the hypothalamic pituitary gonad axis were first characterized a decade ago. Initially, identified in Japanese quail, the photoreduction pathway involves the long-day induction of thyroid stimulating hormone-β (TSH β), in the pars tuberalis of the anterior pituitary gland. Targeted back to the medial basal hypothalamus, via specialized glycosylation, TSH-β has been proposed to induce the reciprocal switching of deiodinase expression in the medial basal hypothalamus. Following its characterization in quail, the components of this pathway has been extended to a number of other seasonal breeders. Yet the majority of these studies have relied on artificial photoperiod manipulations in laboratory environments. However, emerging data from both avian and mammalian species using naturalistic photoperiods, or field studies, is challenging the canonically established photoinducible neuroendocrine pathways controlling seasonal reproduction. Here we present evidence from recent studies in both a seasonally breeding songbird and Japanese quail measuring changes in gene expression for the photoinducible components of the neuroendocrine system regulating seasonal reproduction. Combined with data from studies in sheep we found evidence that challenges the role of TSH β in particular with respect to seasonal reproduction. These findings highlight the importance of including consideration of natural conditions in future studies of seasonal reproduction.