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.

Dual signal transduction pathways activated by TSH receptors in rat primary tanycyte cultures

“…Tanycytes sense biologically active hormones and metabolites from the ventricle and portal blood vessels, and they connect the ventricle and neighbouring pars tuberalis via networks of cisterna (Guerra et al 2010, Frayling et al 2011, Bolborea & Dale 2013, Balland et al 2014. They express receptors and/or transport proteins for a wide variety of known and unknown biologically active compounds (Graham et al 2003, Rodriguez et al 2005, Barrett et al 2007, Coppola et al 2007, Cottrell et al 2009, Nilaweera et al 2011, Shearer et al 2012, Dardente et al 2014. Notably, tanycytes express type 2 deiodinase (Dio2) at very high levels.…”

Waiting for the sun: The circannual program of reindeer is delayed by the recurrence of rhythmical melatonin secretion after the arctic night

“…In current working models for photoperiodic entrainment, as the duration of the nocturnal melatonin signal declines with increasing day length, it triggers a spring response by enhancing the amplitude of circadian rhythms of key transcriptional activators in the PT (Dardente et al, 2010;Matsumoto et al, 2010;Dardente et al, 2014). The present study challenges the assumption that circadian rhythmicity is important because both LL and DD block circadian rhythmicity in reindeer (Stokkan et al, 1994;Van Oort et al, 2005).…”

“…This timer converts changes in nightly duration of melatonin exposure into changes in the amplitude of expression of a transcription coactivator and developmental switch, Eya3. Eya3 promotes the expression of differentiated thyrotrophs in the PT, in turn governing hypothalamic function through thyroid hormonedependent mechanisms (Dardente et al, 2014). According to the current model (Dardente et al, 2010;Matsumoto et al, 2010;Dardente et al, 2014), shorter melatonin signals generate large amplitude daily oscillations of Eya3 RNA expression in the PT, while long-duration signals suppress the oscillation via a direct suppressive effect of melatonin in a critical time window occurring approximately 12 h after dark onset.…”

“…Eya3 promotes the expression of differentiated thyrotrophs in the PT, in turn governing hypothalamic function through thyroid hormonedependent mechanisms (Dardente et al, 2014). According to the current model (Dardente et al, 2010;Matsumoto et al, 2010;Dardente et al, 2014), shorter melatonin signals generate large amplitude daily oscillations of Eya3 RNA expression in the PT, while long-duration signals suppress the oscillation via a direct suppressive effect of melatonin in a critical time window occurring approximately 12 h after dark onset. Hence, induction of a physiological programme typical of spring, such as antler growth and moult in reindeer (Rangifer tarandus), is seen as a consequence of an increasing amplitude of a melatonin-dependent circadian rhythm of PT gene expression, induced by exposure to long days.…”

60 YEARS OF NEUROENDOCRINOLOGY: Regulation of mammalian neuroendocrine physiology and rhythms by melatonin

“…This therefore suggests that species-specific mechanisms within the hypothalamus regulate seasonal breeding status downstream of T 3 generation. For further detail, readers are referred to recent reviews of this topic (Yoshimura 2013, Dardente et al 2014, Wood & Loudon 2014.…”