Title: Circannual Variation in Thyroid Hormone Deiodinases in a Short‐Day Breeder
Abstract: At temperate latitudes, many mammals and birds show internally timed, long‐term changes in seasonal physiology, synchronised to the seasons by changing day length (photoperiod). Photoperiodic control of thyroid hormone levels in the hypothalamus dictates the timing. This is effected through reciprocal regulation of thyroid hormone deiodinase gene expression. The local synthesis of type 2 deiodinase ( D io2) promotes triiodothyronine ( T 3) production and summer biology, whereas type 3 deiodinase ( D io3) promotes T 3 degradation and winter biology. In the present study, we investigated the extent to which the hypothalamic expression of D io2 and D io3 is circannually regulated in the S oay sheep, a short‐day breeding mammal. M ale sheep were exposed to a long photoperiod ( LP ; 16 : 24 h light/dark cycle) or a short photoperiod ( SP ; 8 : 24 h light/dark cycle), for up to 28 weeks to establish four different endocrine states: (i) LP animals in a spring/summer‐like state of reproductive arrest; (ii) LP refractory ( LPR ) animals showing spontaneous reproductive reactivation; (iii) SP animals showing autumn/winter‐like reproductive activation; and (iv) SP refractory ( SPR ) animals showing spontaneous reproductive arrest. A complex pattern of hypothalamic Dio2 and D io3 expression was observed, revealing distinctive photoperiod‐driven and internally timed effects for both genes. The patterns of expression differed both spatially and temporally, with phases of peak D io2 expression in the median eminence and tuberoinfundibular sulcus, as well as in the paraventricular zone ( PVZ ) (maximal under LP ), whereas D io3 expression was always confined to the PVZ (maximal under SP ). These effects likely reflect the distinct roles of these enzymes in the localised control of hypothalamic T 3 levels. The spontaneous decline in D io2 and spontaneous increase in D io3 in LPR animals occurred with a corresponding decline in thyroid‐stimulating hormone β expression in the neighbouring pars tuberalis ( PT ), although this relationship did not hold for the corresponding D io2 increase/ D io3 decrease seen in SPR animals. We conclude that internally timed and spatially regulated changes in D io2 and D io3 expression may drive the cycling between breeding and nonbreeding states in long‐lived seasonal species, and may be either PT ‐dependent or PT ‐independent at different phases of the circannual cycle.
Publication Year: 2013
Publication Date: 2013-03-21
Language: en
Type: article
Indexed In: ['crossref', 'pubmed']
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Cited By Count: 72
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