Thyroxine 5′-deiodinase activity in pineal gland and frontal cortex: Nighttime increase and the effect of either continuous light exposure or superior cervical ganglionectomy

Type II T₄ 5′-deiodinase (5′-D) activity was studied in pineal gland, frontal cortex, and anterior pituitary of male rats. Enzymatic activity was determined by the release of ¹²⁵I using [3′,5′-¹²⁵I]T₄ as substrate. Daytime levels of 5′-D activity were maximal for anterior pituitary (346.2 ± 142.5 fmol ¹²⁶I released/mg protein h), followed by pineal gland (61.5 ± 8.7 fmol ¹²⁵I released/mg protein-h) and frontal cortex (3.1 ± 0.5 fmol ¹²⁵I released/mg protein · h).Twenty-four-hour fluctuations of 5′-D activity were apparent in the pineal gland and cortex. For pineal, peak 5′-D activity (43.4 ± 11.4 fmol ¹²⁵I released/gland h) occurred at 0100 h, with values at this time being 8-fold greater than basal daytime levels. Superior cervical ganglionectomy (SCGX) or exposure to continuous light (LL) completely abolished the nocturnal rise of 5′-D activity. Treatment with iopanoic acid also inhibited 5′-D activity in pineal (32.1 ± 3.5 vs. 18.8 ± 1.7 fmol ¹²⁵I released/glandh) without influencing either the N-acetyltransferase activity or the melatonin content. In cortex, peak 5′-D activity occurred at 0500 h (5.3 ± 0.4 fmol ¹²⁶I released/mg protein-h), with values at this time being 1.5-fold greater than basal daytime levels. SCGX moderately depressed enzymatic activity, while LL exposure strikingly enhanced the nocturnal increase (11.7 ± 1.1 vs. 19.3 ± 2.4 fmol ¹²⁵I released/mg protein h at 0300 h). 5′-D activity exhibited no 24-h fluctuation in the anterior pituitary gland, and neither SCGX nor LL exposure affected enzyme levels in this tissue.Our data demonstrate the existence of a 24-h rhythm of type II T₄ 5′-D activity in pineal gland and cortex, with peak levels occurring at night; in pineal, the sympathetic neural input is indispensable for the rhythm, since SCGX prevented it. In cortex, the apparent paradoxical effect of LL and SCGX on 5′- D activity indicates that other mechanisms are involved in its regulation.