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

The effect of development and growth of malignant tumors on pineal melatoni n production was studied in two different hormone-dependent tumor systems i n female rats. Urinary excretion of 6-sulphatoxymelatonin (aMT6s), the meta bolic end product of melatonin, which parallels its production, was determi ned by radioimmunoassay at fortnightly or monthly intervals over the period of 1 year in female F344 Fischer rats bearing chemically-induced mammary c arcinomas and in BDII/Han rats with spontaneous endometrial carcinomas. Unt reated Fischer rats and BDII/Han rats in which tumor growth was suppressed by treatment with a progestin served as controls, Based on the cosinor anal ysis, animals without tumors showed significant seasonal rhythms of aMT6s e xcretion, with peaks in August (Fischer rats) and in May (BDII/Han rats). I n contrast, such rhythms were absent in animals with developing and manifes t tumors. It is concluded that animals kept under constant environmental co nditions still show seasonal rhythms of pineal activity. Tumor development and growth affect pineal activity leading to disturbance of these rhythms.