title: Thyroid hormone and seasonal rhythmicity.
abstract: Living organisms show seasonality in a wide array of functions such as reproduction, fattening, hibernation, and migration. At temperate latitudes, changes in photoperiod maintain the alignment of annual rhythms with predictable changes in the environment. The appropriate physiological response to changing photoperiod in mammals requires retinal detection of light and pineal secretion of melatonin, but extraretinal detection of light occurs in birds. A common mechanism across all vertebrates is that these photoperiod-regulated systems alter hypothalamic thyroid hormone (TH) conversion. Here, we review the evidence that a circadian clock within the pars tuberalis of the adenohypophysis links photoperiod decoding to local changes of TH signaling within the medio-basal hypothalamus (MBH) through a conserved thyrotropin/deiodinase axis. We also focus on recent findings which indicate that, beyond the photoperiodic control of its conversion, TH might also be involved in longer-term timing processes of seasonal programs. Finally, we examine the potential implication of kisspeptin and RFRP3, two RF-amide peptides expressed within the MBH, in seasonal rhythmicity.
authors.affiliation: Physiologie de la Reproduction et des Comportements, INRA, UMR085 , Nouzilly , France ; CNRS, UMR7247 , Nouzilly , France ; Université François Rabelais de Tours , Tours , France ; Institut français du cheval et de l'équitation , Nouzilly , France.
authors.forename: Hugues
authors.initials: H
authors.lastname: Dardente
authors.affiliation: Department of Arctic and Marine Biology, University of Tromsø , Tromsø , Norway.
authors.forename: David G
authors.initials: DG
authors.lastname: Hazlerigg
authors.affiliation: School of Life Sciences, University of Nottingham , Nottingham , UK.
authors.forename: Francis J P
authors.initials: FJ
authors.lastname: Ebling
keywords: GnRH neurons; RF-amide; kisspeptins; melatonin rhythm; pars tuberalis; reproduction; seasonality