Effect of varying photoperiods on mammary morphology, DNA synthesis, and hormone profile in female rats.

A clear positive correlation between circulating levels of prolactin (Prl) and morphologic development as well as DNA synthetic index in the mammary gland was established in young virgin Holtzman rats exposed to constant light from birth. The observed elevated level of circulating Prl by virtue of its morphogenic and mitogenic properties induced changes in mammary epithelium [numerous actively differentiating terminal end buds into alveolar buds (AB)] highly susceptible for the action of 7,12-dimethylbenz[a]anthracene [(DMBA) CAS: 57-97-6]. Conversely, substitution treatment with melatonin in such a model caused a significant decrease in both Prl and 17 beta-estradiol (E2) levels as well as in the morphologic and DNA synthetic pattern of the mammary gland. Administration of 2-bromo-alpha- ergocryptin in these animals caused a significant decrease in the plasma level of Prl (without affecting the level of E2) and a decrease in the density of AB and in DNA synthesis. These changes impaired the mammary gland responsiveness to DMBA as seen from the significant decrease in the incidence of mammary carcinoma.