Recently, we demonstrated that methylene blue partially inhibited 
estradiol-benzoate-induced anterior pituitary hyperplasia in rats. Since central 
dopaminergic systems participate in the regulation of estrogen-induced anterior 
pituitary growth and tumor transformation, this study examined whether a 3-week 
treatment with methylene blue could affect anterior pituitary levels of dopamine 
(DA), dihydroxyphenylalanine (DOPA), and dihydroxyphenylacetic acid and dopamine 
(D-2) receptors in male rats. Compared to controls, methylene blue significantly 
decreased anterior pituitary weight, increased basal anterior pituitary DA 
levels, and inhibited estradiol benzoate-induced decreases in anterior pituitary 
DA concentrations. Furthermore, we found that methylene blue alone decreased 
anterior pituitary D-2 receptor number. Methylene blue given in combination with 
estradiol benzoate partially inhibited estradiol benzoate-induced anterior 
pituitary growth and estradiol benzoate-induced increases in D-2 receptor 
number. Estradiol benzoate-treated rats had significantly lower anterior 
pituitary DOPA accumulation after intraperitoneal administration of 
3,4-hydroxybenzyl-hydrazine dihydrochloride (NSD-1015), an irreversible 
inhibitor of L-aromatic amino acid decarboxylase whereas methylene blue did not 
affect anterior pituitary DOPA accumulation when compared to controls. Methylene 
blue decreased anterior pituitary prolactin levels and inhibited increases in 
anterior pituitary prolactin after estradiol benzoate administration. The 
present results suggest that anterior pituitary DA may play an important role in 
estrogen-induced anterior pituitary hyperplasia and tumor formation and that 
antioxidant drugs such as methylene blue may attenuate estrogen-induced 
pituitary growth. This may occur via increases in anterior pituitary DA levels 
associated with down-regulation of anterior pituitary D-2 receptors.