Effect of continuous light on the incidence of 9,10-dimethyl-1,2-benzanthracene induced mammary tumors in female Holtzman rats.

The pineal has been recently implicated in mammary tumorigenesis. Effect of physiological pinealectomy brought about by subjecting female Holtzman rats to permanent lighting (24 h/day) from birth was studied on the incidence of 9,10-dimethyl-1,2-benzanthracene (DMBA) induced mammary tumors. The incidence of adenocarcinoma was 95% in animals maintained in continuous photoperiod in contrast to that (60%) observed in control animals maintained on a light/dark (10/14 h) schedule. Further, the latency period of tumor appearance (61.5 +/- 9 days) in the former group was found to be significantly shorter than that seen in the latter group (93.7 +/- 8.3 days). Explanations are offered for this difference in the observed incidence.