Maternal photoperiodic programming enlightens the internal regulation of thyroid-hormone deiodinases in tanycytes.

Seasonal rhythms in physiology are widespread among mammals living in temperate zones. These rhythms rely on the external photoperiodic signal to be entrained to the seasons, but persist under constant conditions, revealing their endogenous origin. Internal long-term timing (circannual cycles) can be revealed in the lab as photoperiodic-history dependent responses - i.e. the ability to respond differently to similar photoperiodic cues based on prior photoperiodic experience. In juveniles, history-dependence relies on the photoperiod transmitted by the mother to the fetus in utero, a phenomenon known as "maternal photoperiodic programming" (MPP). The response to photoperiod in mammals involves the nocturnal pineal hormone melatonin, which regulates a neuroendocrine network including thyrotropin in the pars tuberalis (PT) and deiodinases in tanycytes, and results in changes in thyroid hormone (TH) in the mediobasal hypothalamus. This review addresses MPP and discusses the latest findings on its impact on the thyrotropin/deiodinase network. Finally, commonalities between MPP and other instances of endogenous seasonal timing are considered, and a unifying scheme in which timing arises from a long-term communication between the PT and the hypothalamus and resultant spontaneous changes in local TH status, independently of the pineal melatonin signal is suggested. This article is protected by copyright. All rights reserved.