title: Photorefractoriness in mammals: dissociating a seasonal timer from the circadian-based photoperiod response.
abstract: In seasonal animals, prolonged exposure to constant photoperiod induces photorefractoriness, causing spontaneous reversion in physiology to that of the previous photoperiodic state. This study tested the hypothesis that the onset of photorefractoriness is correlated with a change in circadian expression of clock genes in the suprachiasmatic nucleus (circadian pacemaker) and the pars tuberalis (PT, a melatonin target tissue). Soay sheep were exposed to summer photoperiod (16-h light) for either 6 or 30 wk to produce a photostimulated and photorefractory physiology, and seasonal changes were tracked by measuring the long-term prolactin cycles. Animals were killed at 4-h intervals throughout 24 h. Contrary to the hypothesis, the 24-h rhythmic expression of clock genes (Rev-erbalpha, Per1, Per2, Bmal1, Cry1) in the suprachiasmatic nucleus and PT reflected the ambient photoperiod/melatonin signal and not the changing physiology. Contrastingly, the PT expression of alpha-glycoprotein hormone subunit (alphaGSU) and betaTSH declined in photorefractory animals toward a short day-like endocrinology. We conclude that the generation of long-term endocrine cycles depends on the interaction between a circadian-based, melatonin-dependent timer that drives the initial photoperiodic response and a non-circadian-based timer that drives circannual rhythmicity in long-lived species. Under constant photoperiod the two timers can dissociate, leading to the apparent refractory state.
authors.affiliation: The Queen's Medical Research Instiute, 47 Little France Crescent, Edinburgh EH16 4TJ, Scotland, United Kingdom. g.lincoln@hrsu.mrc.ac.uk
authors.forename: Gerald A
authors.initials: GA
authors.lastname: Lincoln
authors.forename: Jonathan D
authors.initials: JD
authors.lastname: Johnston
authors.forename: Hakan
authors.initials: H
authors.lastname: Andersson
authors.forename: Gabriela
authors.initials: G
authors.lastname: Wagner
authors.forename: David G
authors.initials: DG
authors.lastname: Hazlerigg
mesh_terms: D056930:ARNTL Transcription Factors; D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins; D001683:Biological Clocks / Q000235:genetics / Q000502:physiology*; D018797:Cell Cycle Proteins; D002940:Circadian Rhythm / Q000502:physiology*; D056931:Cryptochromes; D004268:DNA-Binding Proteins / Q000235:genetics; D005260:Female; D005420:Flavoproteins / Q000235:genetics; D015870:Gene Expression / Q000502:physiology; D015292:Glycoprotein Hormones, alpha Subunit / Q000235:genetics; D006801:Humans; D008297:Male; D009687:Nuclear Proteins / Q000235:genetics; D056953:Nuclear Receptor Subfamily 1, Group D, Member 1; D056950:Period Circadian Proteins; D017440:Photoperiod; D010903:Pituitary Gland, Anterior / Q000378:metabolism / Q000502:physiology; D011388:Prolactin / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000235:genetics; D012621:Seasons; D012756:Sheep; D013493:Suprachiasmatic Nucleus / Q000502:physiology; D037322:Thyrotropin, beta Subunit / Q000235:genetics; D014157:Transcription Factors / Q000235:genetics; D042461:Vascular Endothelial Growth Factor A / Q000235:genetics
chemical_list: D056930:ARNTL Transcription Factors; D051792:Basic Helix-Loop-Helix Proteins; D018797:Cell Cycle Proteins; D056931:Cryptochromes; D004268:DNA-Binding Proteins; D005420:Flavoproteins; D015292:Glycoprotein Hormones, alpha Subunit; D009687:Nuclear Proteins; D056953:Nuclear Receptor Subfamily 1, Group D, Member 1; D056950:Period Circadian Proteins; D011388:Prolactin; D018160:Receptors, Cytoplasmic and Nuclear; D037322:Thyrotropin, beta Subunit; D014157:Transcription Factors; D042461:Vascular Endothelial Growth Factor A; C538960:BMAL1 protein, human; C538961:Bmal1 protein, mouse; C538963:CRY1 protein, human; C538964:Cry1 protein, mouse; C064353:NR1D1 protein, human; C539022:PER1 protein, human; C421902:PER2 protein, human; C539023:Per1 protein, mouse; C513312:Per2 protein, mouse