Neuroendocrine correlates of the critical day length response in the Soay sheep.

Abstract

In mammals, melatonin is the hormone responsible for synchronization of seasonal physiological cycles of physiology to the solar year. Melatonin is secreted by the pineal gland with a profile reflecting the duration of the night, and acts through melatonin-responsive cells in the pituitary pars tuberalis (PT), which in turn modulate hypothalamic thyroid hormone status. Recent models suggest that the actions of melatonin in the PT depend critically on day-length dependent changes in the expression of Eya3, which is a co-activator for thyrotropin beta-subunit (Tshβ) gene transcription. According to this model, short photoperiods suppress Eya3 and hence Tshβ expression, while long photoperiods produce the inverse effect. Studies underpinning this model have relied on step changes in photoperiod (from 8- to 16-h light / 24-h), and have not compared the sensitive ranges of photoperiods for changes in Eya3 and Tshβ expression with those for relevant downstream molecular and endocrine responses. We therefore performed a "critical day length" experiment in Soay sheep, in which animals acclimated to 8-h light / 24-h (SP) were exposed to a range of increased photoperiods spanning the range 11.75 to 16-h (LP), and responses at the level of the PT, hypothalamus and hormonal output were assessed. Although Eya3 and Tshβ both showed the predicted SP vs LP differences, they responded quite differently to intermediate photoperiods within this range, and, at the individual animal level, no clear Eya3 - Tshβ relationship could be seen. This result is inconsistent with a simple co-activator model for EYA3 action in the PT. Further downstream layers of non-linearity were also seen in terms of the Tshβ - dio2 and the dio2 - testosterone relationships. We conclude that the transduction of progressive changes in photoperiod into transitions in endocrine output is an emergent property of a multistep signaling cascade within the mammalian neuroendocrine system. This article is protected by copyright. All rights reserved.