Marcia Watson-Whitmyre

Reproductive Refractoriness in Hamsters: Environmental and Endocrine Etiologies

The golden hamster, Mesocricetus auratus, is typical of temperate-latitude rodents in its annual reproductive cycle. When housed under natural photoperiods, adult hamsters are reproductively active in the spring and summer months but inactive during the fall and winter (Vendrely et al. 1971a,b, 1972; see Figure 11-1 A). Laboratory experiments have demonstrated that the most important Zeitgeber for the annual reproductive cycle is the yearly variation in photoperiod: the entire breeding cycle can be reproduced under artificial daylengths when temperature and nutritional variables are held constant (Reiter 1968a). The critical daylength for the golden hamster is 12.5 hours of light per day: light cycles containing more than 12.5 hours of light are perceived as long days and are favorable to gonadal growth and maintenance, while those shorter than 12.5 hours of light result in gonadal regression (Gaston and Menaker 1967). The reproductive changes brought about by varying the daylength are mediated by changes in gonadotropin release (Berndtson and Desjardins 1974).

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

Abstract In this study we tested the hypothesis that exogenous melatonin exerts 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 hr) either at 1200 or 1900 hr (15 μg in 0.1 ml ethanol:saline 1:10) daily for 12 weeks. Testicular regression occurred in all animals of the 1900-hr injection group, while melatonin injected at noon was without effect. A third group of animals received small implants of melatonin subcutaneously at 0, 4, and 8 weeks. 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 release approximately 10-15 μg melatonin/day, and had no effect on testicular size, as these animals also remained on LD 14:10. After 12 weeks the animals of each group were sacrificed at 1- or 2-hr 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 hr after lights out (0200 hr) and levels remained elevated for approximately 4 hr. 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.