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.
Highlights
Seasonality is a critical property of most organisms
To address a potential role for thyroid hormone (TH) turn-over beyond the photoperiodic response, we investigated the expression of Tshβ and Dio2/Dio3 within the medio-basal hypothalamus (MBH) of sheep under distinct physiological states: LP, LP refractory (LPR) obtained after prolonged LP exposure, short photoperiod (SP) and SP refractory (SPR) obtained after prolonged SP exposure [157]
TH metabolism within the MBH may intervene in the photoperiodic response but may be integral to longer-term timing processes such as circannual rhythms. At this stage several outstanding questions remain: first, since the same TSH/deiodinase/T3 pathway is triggered by LP in long-day breeders and in shortday breeders and non-photoperiodic species, how do we get opposite responses, or no response at all, of the hypothalamic–pituitary–gonadal axis? This is intriguing because the increased intra-hypothalamic availability of TH is uniformly linked to an anabolic state across seasonal species
Summary
Reviewed by: Valerie Simonneaux, Centre National de la Recherche Scientifique, France Sulay Tovar, University of Cologne, Germany. Living organisms show seasonality in a wide array of functions such as reproduction, fattening, hibernation, and migration. A common mechanism across all vertebrates is that these photoperiod-regulated systems alter hypothalamic thyroid hormone (TH) conversion. 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 focus on recent findings which indicate that, beyond the photoperiodic control of its conversion,TH might be involved in longerterm timing processes of seasonal programs. We examine the potential implication of kisspeptin and RFRP3, two RF-amide peptides expressed within the MBH, in seasonal rhythmicity
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