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Evidence for Circadian Variations of Thyroid Hormone Concentrations and Type II 5′‐Iodothyronine Deiodinase Activity in the Rat Central Nervous System
Author(s) -
CamposBarros Angel,
Musa Alberto,
Flechner Anke,
Hessenius Carsten,
Gaio Ulla,
Meinhold Harald,
Baumgartner Andreas
Publication year - 1997
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1046/j.1471-4159.1997.68020795.x
Subject(s) - medicine , endocrinology , dio2 , circadian rhythm , deiodinase , thyroid , euthyroid , hormone , triiodothyronine , biology , melatonin , photoperiodism , chemistry
The 24‐h patterns of tissue thyroid hormone concentrations and type II 5′‐ and type III 5‐iodothyronine deiodinase (5′D‐II and 5D‐III, respectively) activities were determined at 4‐h intervals in different brain regions of male euthyroid rats entrained to a regular 12‐h light/12‐h dark cycle (lights on at 6:00 a.m.). Activity of 5′D‐II, which catalyzes the intracellular conversion of thyroxine (T 4 ) to 3,3′,5‐triiodo‐ l ‐thyronine (T 3 ) in the CNS, and the tissue concentrations of both T 4 and T 3 exhibited significant daily variations in all brain regions examined. Periodic regression analysis revealed significant circadian rhythms with amplitudes ranging from 9 to 23% (for T 3 ) and from 15 to 40% (for T 4 and 5′D‐II) of the daily mean value. 5′D‐II activity showed a marked nocturnal increase (1.3–2.1‐fold vs. daytime basal value), with a maximum at the end of the dark period and a minimum between noon and 4:00 p.m. 5D‐III did not exhibit circadian patterns of variation in any of the brain tissues investigated. Our results disclose circadian rhythms of 5′D‐II activity and thyroid hormone concentrations in discrete brain regions of rats entrained to a regular 12:12‐h light‐dark cycle and reveal that, in the rat CNS, T 3 biosynthesis is activated during the dark phase of the photoperiod. For all parameters under investigation, the patterns of variation observed were in part regionally specific, indicating that different regulatory mechanisms may be involved in generating the observed rhythms.