Relative Efficacy of Melatonin and 5‐Methoxytryptamine in Terms of Their Antigonadotrophic and Counterantigonadotrophic Actions in Male Syrian Hamsters

Antigonadotrophic and counterantigonadotrophic activities of melatonin and 5‐methoxytryptamine (5‐MT) were quantitatively compared in male Syrian hamsters. In long day conditions, the daily afternoon administration of either 5, 15, 25, 50, 100 or 200 μg melatonin induced testicular regression within 10 wk; under the same circumstances, only the 200‐μg dosage of 5‐MT was able to suppress testicular weights. Thus, 5‐MT appears to have about one‐tenth the antigonadotrophic action of melatonin in the male Syrian hamster. In short days, the subcutaneous implantation of either 50 or 100 μg melatonin (every 2 wk in beeswax) prevented testicular regression whereas it required much larger doses of 5‐MT (1 mg every 2 wk in beeswax) to achieve the same counterantigonadotrophic action. In terms of both their antigonadotrophic and counterantigonadotrophic effects, hamsters seem to be more sensitive to melatonin than to 5‐MT.