This uncommon result has spawned research designed to unravel the mystery. Results now suggest a theory which accounts for the increase in mammary tumors. In this rat strain, atrazine diminishes the release of two hormones from the hypothalamus, luteinizing hormone and norepinephrine. Through a complex cascade of effects on other endocrine organs, this effect disrupts the estrous cycle and leads to a sustained increase in circulating prolactin.
Prolactin stimulates mitosis of cells in the breast.
Increasing the rate of mitosis increases the probability that normal background DNA damage will lead to formation of mutant cells and thus tumors. Two things may make EPA reluctant to take regulatory action based on this theory. First, it is novel. Although the basic endocrine physiology on which it is based is well-established, the effect of atrazine on the hypothalamus is something not frequently noted in toxicology. Second, not all the details of the interactions are understood as well as one might wish.