Natural and synthetic analogues of melatonin and related compounds. II. Effects on plasma thyroid hormones and cholesterol levels in male Syrian hamsters.

Synthetic or natural analogues of the pineal indole, melatonin, were injected separately every evening (1700 hours) for 7 (Exp. 1) or 10 (Exp. 2) weeks into adult male Syrian hamsters maintained in 14 hours of light and 10 hours of darkness each day. Plasma thyroxine (T4) levels were significantly depressed by 25 micrograms/day of melatonin (aMT) in both experiments. Injecting 25 micrograms/day either of acetyl methoxytryptophol or of synthetic analogues (hexanoyl methoxytryptamine, propionyl methoxytryptophol, or 6-chloro-melatonin) in Exp. 1 or of a natural analogue (N-acetylserotonin, 6-hydroxy-melatonin, hydroxytryptophol, or methoxytryptophol) in Exp. 2 had no effect on the circulating T4 levels. Plasma levels of triiodothyronine (T3) and thyrotropin (TSH) were unaffected in either experiment. Since none of the tested melatonin analogues is capable of suppressing circulating T4 concentration when given in a dose at which melatonin is reproducibly effective, the pineal-induced suppression of T4 is most likely mediated by melatonin. Plasma cholesterol levels were elevated only in hamsters receiving 6-chloromelatonin injections. However, plasma triglyceride levels were significantly higher than the diluent treated controls in Exp. 1 after injections of melatonin, acetyl methoxytryptophol, propionyl methoxytryptophol and 6-chloromelatonin. Interscapular brown adipose tissue was significantly heavier in melatonin treated animals.