Hormonal Modulation of Cyclic Melatonin Production in the Pineal Gland of Rats and Syrian Hamsters: Effects of Thyroidectomy or Thyroxine Implant

Abstract

Night-time pineal levels of tryptophan, 5-hydroxytryptophan, serotonin, N-acetylserotonin, melatonin, 5-hydroxyindoleacetic acid and the activities of the two enzymes N-acetyltransferase and hydroxyindole-O-methyltransferase involved in the cyclic production of melatonin were determined in male albino rats and Syrian hamsters that were implanted with thyroxine or thyroidectomized two weeks earlier. Both treatments depressed nocturnal pineal melatonin content in rats and hamsters. The cause of this depression is not known, although minor alterations in the substrates and the enzymes involved in melatonin production were observed. The data suggest that alterations in thyroid hormone levels may increase the release of nocturnal melatonin from the pineal, thereby allowing less to accumulate in the gland.

TCDD Decreases Rapidly and Persistently Serum Melatonin Concentration Without Morphologically Affecting the Pineal Gland in TCDD‐Resistant Han/Wistar Rats

“…1988a &1990), it is quite conceivable that the metabolism of melatonin in the liver was enhanced; this possibility is further strengthened by a similar temporal occurrence of enzyme induction and melatonin depression. On the other hand, the temporal aspect tends to preclude an indirect mechanism through altered hormonal status, since although both thyroxine (Champney et a/. 1985) and corticosterone (Troiani et al 1988) may contribute to the regulation of melatonin levels, changes in their serum concentrations are not yet manifest on day 1 after TCDD exposure (Pohjanvirta et a/.…”

TCDD Reduces Serum Melatonin Levels in Long‐Evans Rats

“…Therefore, it is highly improbable that the impact should arise secondarily to TCDD hypophagia. On the other hand, both thyroxine administration and thyroidectomy depress nocturnal melatonin content in the pineal gland (Champney et al 1985), and a dose of 50 pg/kg TCDD decreases irreversibly serum thyroxine levels in L-E rats (Pohjanvirta et al 1989). Hence, TCDD might affect melatonin indirectly via altered thyroid hormone status.…”