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Effect of daily injections and constant release implants of melatonin on the endogenous pineal melatonin rhythm in golden hamsters

Effect of daily injections and constant release implants of melatonin on the endogenous pineal melatonin rhythm in golden hamsters

Proceedings of the Society for Experimental Biology and Medicine 174(1): 119-122

Exogenous melatonin may exert its effects on the reproductive system of hamsters by directly or indirectly altering the endogenous rhythm of melatonin production and release. Melatonin was injected in male hamsters housed on LD 14:10 (lights 0600-2000 h) either at 1200 or 1900 h (15 .mu.g in 0.1 ml ethanol:saline 1:10) daily for 12 wk. Testicular regression occurred in all animals of the 1900 h injection group, while melatonin injected at noon was without effect. A 3rd group of animals received small implants of melatonin s.c. at 0, 4, and 8 wk. Implants were 4 mm in length and contained a melatonin:beeswax mixture (1:25) drawn up into polyethylene tubing (2.2 mm i.d.). These implants released .apprx. 10-15 .mu.g melatonin/day, and had no effect on testicular size as these animals also remained on LD 14:10. After 12 wk the animals of each group were sacrificed at 1 or 2 h intervals around the clock. Pineals were saved and assayed for melatonin content. In each group the nocturnal rhythm of pineal melatonin was similar; peak melatonin levels were achieved 6 h after lights out (0200 h) and levels remained elevated for .apprx. 4 h. These results exclude a mode of action of exogenous melatonin on the pineal melatonin rhythm as a basis for the testicular response to melatonin in hamsters. They also pose some interesting questions of feedback regulation by melatonin on its own production and release.

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