Evidence for Pineal Gland Modulation of the Neuroendocrine-Thyroid Axis

Abstract

Experiments with rats and hamsters have provided evidence for an inhibitory action of the pineal gland on the neuroendocrine-thyroid axis. While maintenance of these animals in short photoperiod results in reduced levels of circulating thyroxin (T4), pinealectomy restores the levels to normal. Recent studies suggest that an active pineal gland produces a substance which inhibits thyrotrophin-releasing hormone release from the hypothalamus. Several investigators have concluded that endogenous melatonin, produced in the pineal gland, could account for the inhibitory action of the pineal gland on blood T4 levels. Although melatonin administration has been reported to inhibit blood T4 levels in both rats and hamsters, under certain experimental conditions melatonin administration can be demonstrated to have a counter-antithy-rotrophic effect resulting in increased blood levels of T4 and thyrotrophin. Assay of blood levels of melatonin of rats and hamsters under various experimental conditions will be necessary to distinguish physiological from pharmacological effects of melatonin. Lesion studies as well as studies with melatonin implants in the brain, suggest that the site of action is in the anterior hypothalamus. The effects of melatonin on the neuroendocrine-thyroid axis are similar to its effects on the neuroendocrine-gonadal axis, leading to the hypothesis of a common site of action for the thyroid and gonadal effects of melatonin. Although many pineal ‘factors’ have been postulated to account for the action of this gland, an action of melatonin on the serotonergic system of the brain stem could account for the data.

Effects of melatonin implants on plasma concentrations of testosterone, thyroxine and prolactin in the male silver fox (Vulpes vulpes)

“…There are also reports supporting the view that the pineal gland, via melatonin, has a modulatory effect on both gonadal and thyroid functions. Injections of melatonin for periods of 3-10 weeks reduced plasma thyroxine and TSH concentrations in hamsters, with the effects on gonadal and thyroid function being observed simultaneously (Vriend et al, 1979(Vriend et al, , 1982Vriend, 1983). Creighton & Rudeen (1988) observed that the effects of melatonin on the pituitary-gonadal and pituitary-thyroid axes were independently mediated via the central nervous system.…”

Influence of Melatonin on the Rate of Rana pipiens Tadpole Metamorphosis In Vivo and Regression of Thyroxine‐Treated Tail Tips In Vitro

“…The secretion of these hormones is on a circadian basis; their levels vary with the time of day and LD cycle as did the effect of melatonin in vivo. Furthermore, exogenous melatonin itself might influence the synthesis or release of these hormones (24).…”

“…Antagonism of thyroid hormone action by melatonin is suggested by the finding that T4 was more effective when given in the light (4,27) and that a 1 hr light pulse during the early dark of a 12L : 12D cycle partially counteracted the negative effect of administering T4 at that time (28). In rats, the pineal inhibited the neuroendocrine-thyroid axis (20,24). Since light stimulated thyroid gland activity in Rana catesbeiana (12), such inhibition might also exist in amphibians.…”

Pineal Dependence of the Syrian Hamster's Nocturnal Serum Melatonin Surge

“…The Syrian hamster is the species most often used to demonstrate the effk -ts of the pineal gland on the reproductive system and thyroid function [Reiter, 1980: M. Vriend, 1983). Previou, pineal melatonin content profiles [Panke ct al., 1979;Tamarkin et al, 19791 and the more recently studied serum concentration profiles [Brown et al 1981;Vaughan ct al., 1985a] both are characterized by a nocturnal melatonin surge in the late part of the dark phase in this species.…”