The fact that prolactin regulates the synthesis, release, and turnover of dopamine in hypothalamic neurons explains the initial increase in libido and erectile function observed in a study of male rats with acute hyperprolactinemia; however, a subsequent decrease in erectile function was observed as prolactin levels remained elevated, suggesting a down regulation of dopamine receptors secondary to chronically elevated levels of prolactin (Figure 2). Here, PRL is linked to hyperprolactinemia.