Effect of constant light on DMBA mammary tumorigenesis in rats

A study of light, and mammary tumorigenesis was conducted in rats. One-hund red female Sprague-Dawley rats were divided by weight into two groups. One group was exposed to constant light (LL) from 26 days of age, and the secon d group was exposed to 8 h light and 16 h dark per day (LD). Both groups re ceived an 8 mg dose of a chemical carcinogen, dimethylbenzanthracene (DMBA) at 52 days of age. At 13 weeks post-DMBA, there were significantly fewer m ammary tumors in the LL group compared with the LD group. Constant light wa s clearly demonstrated to have a profound effect on mammary tissue developm ent. Although virgin, the majority of the LL rats (29/50) had gross evidenc e of lactation at 141 days of age. None of the LD rats (0/50) showed eviden ce of milk production. These results suggest that constant light not only s ubstantially accelerated mammary gland development, but pushed development of the tissue past the stage normally observed in virgin animals (to the la ctation stage). (C) 2000 Elsevier Science ireland Ltd. All rights reserved.