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Submitted on June 17, 2004
Accepted on November 11, 2004
Netherlands Institute for Brain Research (A.K, R.M.B., L.V.S.), Meibergdreef 33, 1105 AZ Amsterdam, The Netherlands; Department of Internal Medicine III (E.K., T.J.V.), Erasmus University Medical Center, 3015 GE Rotterdam, The Netherlands; Academic Medical Center (B.Z.D, E.F.), Department of Endocrinology and Metabolism, F5-171, Meibergdreef 9, 1105 AZ Amsterdam, The Netherlands
* To whom correspondence should be addressed. E-mail: A.Kalsbeek{at}nih.knaw.nl.
Type II deiodinase (D2) plays a key role in regulating thyroid-hormone dependent processes in, among others, the central nervous system (CNS), by accelerating the intracellular conversion of thyroxine (T4) into active thyroid hormone, T3. Just like the well-known daily rhythm of the hormones of the hypothalamo-pituitary-thyroid (HPT) axis, D2 activity also appears to show daily variations. However, the mechanisms involved in generating these daily variations, especially in the CNS, are not known. Therefore, we decided to investigate the role the master biological clock, located in the hypothalamus, plays with respect to D2 activity in the rat CNS, as well as the role of one of its main hormonal outputs, i.e. plasma corticosterone. D2 activity showed a significant daily rhythm in the pineal and pituitary gland, as well as in hypothalamic and cortical brain tissue, albeit with a different timing of its acrophase in the different tissues. Ablation of the biological clock abolished the daily variations of D2 activity in all 4 tissues studied. The main effect of the knock-out of the suprachiasmatic nuclei (SCN) was a reduction of nocturnal peak levels in D2 activity. Moreover, contrary to previous observations in SCN-intact animals, in SCN-lesioned animals the decreased levels of D2 activity are accompanied by decreased plasma levels of the thyroid hormones, suggesting that the SCN separately stimulates D2 activity as well as the HPT-axis.
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