The influence of pineal ablation and administration of melatonin on growth and spread of hamster melanoma

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Abstract
Removal of the pineal organ resulted in increase in the growth of transplanted melanoma in hamsters. Administration of melatonin, which is the principle indole found in pineal tissue, to pinealectomized animals abolished this effect. Therefore it was concluded that the effect of pinealectomy on tumor growth is due to a lack of endogenous melatonin. Administration of large doses of melatonin (4 mg/ day) to intact animals, however, did not influence the rate of tumor growth, indicating that the drug has no direct effect on tumor growth and that the changes produced by melatonin deficiency is perhaps due to a complex reaction involving centers in the hypothalamus and brain stem. Certain morphologic changes were noted in the pineal organs of animals treated with exogenous melatonin. Although the biologic significance of these changes is not clear, study of the electron micrographs indicates that the pinealocytes of the melatonin‐treated animals are in a state of increased activity.