Effects of injections and/or chronic implants of melatonin and 5-methoxytryptamine on plasma thyroid hormones in male and female Syrian hamsters

Plasma thyroid hormone concentrations in male and female Syrian hamsters were evaluated in five experiments after two pineal indoles, melatonin (MEL) and 5-methoxytryptamine (5-MT), were administered as chronic s.c. implants and/or daily afternoon injections. Circulating concentrations of thyroxine (T₄), oridinarily maintained by the long photoperiod (LD 14:10), were inhibited by daily afternoon injections of 25 μg MEL but not 5-MT in male (experiment 1) and female (experiment 2) Syrian hamsters. The suppressive effect of MEL injections on T₄ concentration in the long photoperiod was prevented in both sexes by a s.c. beeswax pellet containing either 1 mg MEL or 5-MT. In the third experiment, various doses of MEL or 5-MT were injected each afternoon for 12 weeks to compare the effectiveness of these two indoles in reducing T₄ concentrations in hamsters maintained in the long photoperiod. Only the highest dose (200 μg) of 5-MT effectively suppressed T₄ concentrations whereas all doses of MEL (≥ 5 μg) significantly reduced plasma T₄. MEL or 5-MT (1-1,000 μg) were implanted in beeswax pellets in male hamsters exposed to short photoperiod (LD 10:14) to determine if either indole could prevent the short photoperiod-induced suppression of T₄ (experiments 4 and 5). Regression and covariance analyses showed a significant log dose-related elevation of T₄ in these hamsters, indicating equal potency of MEL and 5-MT in preventing the short photoperiod-induced suppression of T₄. 5-MT given by either injection or implantation raised plasma triiodothyronine (T₃) in female (experiment 2) but not in male hamsters. Otherwise, photoperiod, MEL and 5-MT had no consistent effect on plasma T₃ concentrations. In conclusion, it appears that MEL is considerably more effective than 5-MT in inhibiting T₄ levels when administered as a daily afternoon injection. On the other hand, when implanted s.c. as part of a beeswax pellet, 5-MT appears to be as effective as MEL in preventing suppression of T₄ that otherwise occurs as a result of either repeated afternoon injections of MEL (experiment 1) or exposure to short photoperiod (experiment 5).