Prepubertal Sensitivity to Melatonin in Male Hamsters

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Abstract
The gonads of adult hamsters are regressed by 6-10 weeks exposure to short daylengths. This effect is mediated by the pineal gland and can be simulated by appropriately timed injections of melatonin. Exposure of newborn hamsters to short daylengths has no discernible effect on maturation of the hamster reproductive system. The hypothesis that prepubertal refractoriness to photoperiod reflects insensitivity of the neuroendocrine axis to melatonin was tested. Male hamsters exposed to 14 h of light/day (14L:10D) were injected once daily between 3 and 14 weeks of age with 25 µg melatonin or with an oil vehicle. Testicular condition was assessed at 7, 10 and 14 weeks of age. Animals treated with melatonin from 3 weeks of age had significantly smaller testes than did control oil treated hamsters at 7 and 10 weeks of age. The testes of other animals treated with melatonin beginning at 7 weeks of age were not significantly different from those of oil injected animals at 10 weeks of age and both of these groups had significantly larger testes than did animals treated with melatonin from 3 weeks of age. By week 14 the testes of all animals treated with melatonin showed substantial regression. Prepubertal hamsters are able to respond to the antigonadal actions of melatonin. It is suggested that failure of short day exposure to affect the rate of gonadal maturation is not due to target tissue insensitivity but rather reflects the intrinsic latency of the neuroendocrine system’s responsiveness to the antigonadal agents of the pineal gland.
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