Male and female hamsters were exposed to natural photoperiodic and temperature conditions beginning on November 13 and groups of hamsters were necropsied either on January 15 or on April 10. Throughout the study, half of the hamsters (control hamsters) received subcutaneous implants of beeswax (25 mg) every other week while the other half received beeswax pellets containing 1 mg melatonin. Control beeswax treated males necropsied in January had atrophic testes and accessory sex organs while the uteri of the control females were infantile in appearance. Melatonin treatment prevented the atrophic responses of the testes and accessory sex organs in males and the uteri in females. Melatonin treated hamsters killed in January also had higher plasma levels of immuno-reactive LH while in females both pituitary and plasma levels of LH were elevated after melatonin treatment. By April 10, the testes and accessory organs of control males and the uteri of control females had regenerated to the adult condition and these organs were comparable in size to those in melatonin treated hamsters. The pituitary glands of control males contained slightly more LH and much more FSH than the pituitaries of melatonin treated males. Melatonin treated females necropsied in April had pituitary and plasma levels of LH and FSH similar to those in beeswax treated females killed at the same time. The data show that chronic melatonin treatment prevents atrophy of the reproductive organs of male and female hamsters which is associated with the shortened days of the winter.
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