Abstract
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), ordinarily 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).
