EVIDENCE FOR CIRCADIAN VARIATIONS OF THYROID-HORMONE CONCENTRATIONS AND TYPE-II 5'-IODOTHYRONINE DEIODINASE ACTIVITY IN THE RAT CENTRAL-NERVOUS-SYSTEM

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, respe ctively) activities were determined at 4-h intervals in different brai n 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 catal yzes 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 t o 40% (for T-4 and 5'D-II) of the daily mean value. 5'D-II activity sh owed 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 variat ion in any of the brain tissues investigated. Our results disclose cir cadian rhythms of 5'D-II activity and thyroid hormone concentrations i n 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 activa ted during the dark phase of the photoperiod. For all parameters under investigation, the patterns of variation observed were in part region ally specific, indicating that different regulatory mechanisms may be involved in generating the observed rhythms.