Seasonal rhythms of 6-sulphatoxymelatonin (aMT6s) excretion in female rats are abolished by growth of malignant tumors

The effect of development and growth of malignant tumors on pineal melatonin production was studied in two different hormone‐dependent tumor systems in female rats. Urinary excretion of 6‐sulphatoxymelatonin (aMT6s), the metabolic end product of melatonin, which parallels its production, was determined by radioimmunoassay at fortnightly or monthly intervals over the period of 1 year in female F344 Fischer rats bearing chemically‐induced mammary carcinomas and in BDII/Han rats with spontaneous endometrial carcinomas. Untreated Fischer rats and BDII/Han rats in which tumor growth was suppressed by treatment with a progestin served as controls. Based on the cosinor analysis, animals without tumors showed significant seasonal rhythms of aMT6s excretion, with peaks in August (Fischer rats) and in May (BDII/Han rats). In contrast, such rhythms were absent in animals with developing and manifest tumors. It is concluded that animals kept under constant environmental conditions still show seasonal rhythms of pineal activity. Tumor development and growth affect pineal activity leading to disturbance of these rhythms.