SEASONAL INFLUENCE ON 7,12-DIMETHYLBENZ[A]ANTHRACENE-INDUCED MAMMARY CARCINOGENESIS IN SPRAGUE-DAWLEY RATS UNDER CONTROLLED LABORATORY CONDITIONS

There is good evidence in some species, including rats, that circannua l rhythms are innate and can occur even under constant environmental c onditions. Such circannual rhythms, e.g. in hormone levels and immune system function, may influence tumourigenesis. This prompted us to stu dy 7,12-dimethylbenz[a]anthracene(DMBA)-induced mammary carcinogenesis at different seasons of the year in female Sprague-Dawley rats under constant environmental conditions (photoperiod, temperature, air humid ity, food). DMBA was administered orally ar a dose of 5 mg per rat at the first day of the experiment and then at weekly intervals up to a t otal dose of 20 mg per rat. Rats were palpated once weekly for the pre sence of mammary tumours. After 13 weeks, they were necropsied for exa mination of the number and size of mammary rumours. Age-matched groups of 36-99 rats were used per experiment. When the experiment was perfo rmed twice within 2 years during the same season (spring/summer), tumo ur incidence (56 and 61%) and tumour burden were almost equal, indicat ing that data obtained in this way were reproducible. However, the sam e experiment performed in autumn yielded a significantly lower tumour incidence (34%) and tumour burden. When the experiment was started dur ing winter, tumour incidence was similar to the spring/summer groups, but tumour burden was lower. The data indicate a seasonal variation in the development and growth of DMBA-induced breast cancer in Sprague-D awley rats. One possible explanation for this phenomenon may be the se asonal variation in pineal melatonin production and immune function pr eviously reported in rodents under constant environmental conditions.