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Effects of long-term treatment with melatonin or melatonin plus ectopic pituitary transplants on testicular LH/hCG and prolactin receptors in juvenile Syrian hamsters (Mesocricetus auratus)

Summary. Juvenile hamsters were injected daily with melatonin and some were also given transplants of 2 pituitaries under the kidney capsule. Weights of the testes and the accessory reproductive glands were reduced after 8 and after 12 weeks of melatonin treatment, but remained unaltered in animals treated with ectopic pituitary transplants. Levels of testicular LH/hCG receptors were significantly reduced by daily melatonin injections for 8 and 12 weeks. The presence of pituitary transplants in melatonin-injected hamsters prevented these reductions, and increased LH/hCG receptors above control levels. These changes in testicular LH/hCG receptors were closely related to alterations in serum prolactin concentration induced by melatonin and pituitary transplants. After 8, but not after 12 weeks of treatment, testicular prolactin receptor levels were reduced by melatonin and maintained by the presence of pituitary transplants. We conclude that: (a) juvenile male hamsters become sensitive to the effects of daily melatonin injections when they reach maturity; (b) daily melatonin injections can reduce the levels of testicular LH/hCG and prolactin receptors; and (c) the effects of melatonin on LH/hCG and prolactin receptors are probably due to suppression of endogenous prolactin release.

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