Thyroid hormone and seasonal rhythmicity

Area of Science:

• * Endocrinology
• * Chronobiology
• * Animal Physiology

Background:

• * Organisms exhibit seasonal variations in functions like reproduction and migration.
• * Photoperiod changes synchronize annual rhythms with environmental shifts in temperate zones.
• * Vertebrates utilize distinct light detection methods (retinal vs. extraretinal) but share common photoperiod-regulated pathways.

Purpose of the Study:

• * To review evidence linking the pars tuberalis circadian clock to hypothalamic thyroid hormone (TH) conversion.
• * To explore the role of TH in longer-term seasonal timing.
• * To examine the involvement of kisspeptin and RFRP3 in seasonal rhythmicity.

Main Methods:

• * Review of existing scientific literature on photoperiodism, circadian rhythms, and thyroid hormone signaling.
• * Analysis of the thyrotropin/deiodinase axis in the context of seasonal regulation.
• * Examination of studies on RF-amide peptides (kisspeptin, RFRP3) in seasonal timing.

Main Results:

• * A conserved thyrotropin/deiodinase axis connects the pars tuberalis circadian clock to TH signaling in the medio-basal hypothalamus (MBH).
• * TH conversion is photoperiod-dependent, influencing seasonal programs.
• * Kisspeptin and RFRP3 peptides in the MBH may play a role in seasonal timing.

Conclusions:

• * The pars tuberalis acts as a critical interface for photoperiod decoding, influencing seasonal physiology via TH signaling.
• * Thyroid hormone's role extends beyond direct photoperiodic response to potentially mediate longer-term seasonal timing.
• * RF-amide peptides represent a novel area of investigation for understanding the neuroendocrine basis of seasonal rhythms.