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 circannual rhythms are innate and can occur even under constant environmental conditions. Such circannual rhythms, e.g. in hormone levels and immune system function, may influence tumourigenesis. This prompted us to study 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 humidity, food). DMBA was administered orally at a dose of 5 mg per rat at the first day of the experiment and then at weekly intervals up to a total dose of 20 mg per rat. Rats were palpated once weekly for the presence of mammary tumours. After 13 weeks, they were necropsied for examination of the number and size of mammary tumours. Age-matched groups of 36-99 rats were used per experiment. When the experiment was performed twice within 2 years during the same season (spring/summer), tumour incidence (56 and 61%) and tumour burden were almost equal, indicating that data obtained in this way were reproducible. However, the same experiment performed in autumn yielded a significantly lower tumour incidence (34%) and tumour burden. When the experiment was started during 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-Dawley rats. One possible explanation for this phenomenon may be the seasonal variation in pineal melatonin production and immune function previously reported in rodents under constant environmental conditions.