Temporal oscillations of thyroid hormones in long term melatonin treated rats

Pharmacological doses of melatonin (0.5 mg/kg body wt. and 1.0 mg/kg body wt.) were chronically administered for 45 days to Wistar rats and 24 h rhythms of thyroid stimulating hormone (TSH), triiodothyronine (T₃) tetraiodothyronine (T₄) and melatonin were studied under semi-natural (LD12 : 12 h) conditions. Exogenous melatonin administration caused delays in the acrophases of T₃, T₄ and melatonin rhythm itself; whereas advances in the acrophases of TSH were observed, thus indicating chronic administration of melatonin could alter the characteristics of endocrine rhythms. Alterations in the amplitude and mesor values of these endocrine rhythms were also observed during melatonin administration. Exogenous administration of melatonin could influence the hormonal rhythms as a modulated internal zeitgeber and could simulate/mimic the conditions of altered photoperiod in the animals. Significant dose-dependent effects of melatonin were absent in the present study. It remains to be proven how exogenous administration of melatonin could influence these hormonal rhythms.