Inhibition of Pineal Type‐II 5′‐Deiodinase Does Not Affect the Nocturnal Increase of N‐Acetyltransferase Activity and Melatonin Content in Either Euthyroid or Thyroidectomized Rats

The presence of type‐II thyroxine 5′‐deiodinase (5′‐D) activity in rat pineal gland has been previously described. In the present paper, 5′‐D activity, N‐acetyltransferase (NAT) activity, and melatonin content were measured in the same rat pineal. Each of these constituents exhibits a nocturnal increase with peak values at 0100 h for melatonin (1.20 ± 0.12 ng/gland) and at 0300 h for both 5′‐D (39.5 ± 11.9 fmol/gland/h) and NAT (8.38 ± 1.04 nmol/gland/h) activities. In vivo treatment with iopanoic acid (IOP) completely prevented the nocturnal increase in 5′‐D activity (14.1 ± 2.6 fml/gland/h at 0300 h) with no modification in either the NAT activity or melatonin content. Thyroidectomy greatly enhanced the 5′‐D activity during the dark period (102.9 ± 10.2 vs. 31.6 ± 4.2 fmol/gland/h), reaching a peak at 0200 h; thyroidectomy, however, did not affect daytime pineal 5′‐D activity (3.11 ± 0.78 vs. 2.5 + 0.92 fmol/gland/h). Treatment of rats with IOP acid completely inhibited the pineal 5′‐D activity in both control (7.86 ± 0.88 fmol/gland/h) and thyroidectomized animals (2.24 ± 1.10 fmol/gland/h) with no change in the melatonin content of the gland (1.21 ± 0.32 vs. 0.99 ± 0.18 ng/gland).