Counterantigonadotropic effect of melatonin administered via the drinking water

Plasma melatonin levels in Syrian hamsters were augmented by the addition of melatonin to the drinking water. Preliminary experiments in pinealectomized hamsters indicated that doses in the range of 2-5 micrograms/ml in the drinking water would produce variable levels of plasma melatonin, approximately in the physiological range (10-100 pg/ml). Two protocols were used, one to demonstrate the antigonadotropic effect of melatonin and the other to elicit the counter-antigonadotropic effect. In the antigonadotropic protocol, 100-g intact male hamsters were housed under a 14-h light, 10-h dark cycle for 10 weeks. Contrary to our hypothesis, there was no effect of melatonin in the drinking water on testis mass (dose range, 0.059-50 micrograms/ml). The counterantigonadotropic protocol involved the use of 100-g male hamsters that were blinded and housed under conditions similar to those of the first protocol. They received melatonin in concentrations ranging from 1.8-320 micrograms/ml. At 10 weeks, the blinded hamsters receiving no melatonin had the expected drop in testis mass to 0.56 g. In the range from 2.4-10 micrograms/ml melatonin, there was a highly variable response, but generally, higher doses resulted in higher testis mass. In the range of 20-320 micrograms/ml, the blind hamsters had mostly normal appearing large testes (mean, 3.3 g). The following conclusions were reached. 1) We produced the well known counterantigonadotropic effect of melatonin by administration via the drinking water ad libitum. 2) We could not produce the antigonadotropic effect by similar means.