We have shown that pituitary vasoactive intestinal peptide (VIP) mediates the effects of estrogen on lactotrope hyperplasia, angiogenesis and hyperprolactinemia, and reduces the pituitary content of transforming growth factor beta beta1 (TGF-beta1, an inhibitor of lactotrope proliferation). Dopamine agonists reverse lactotrope hyperplasia and hyperprolactinemia and also reduce the pituitary VIP content in hyperestrogenized rats. To elucidate the interaction of bromocriptine (BC) and pituitary VIP, a VIP receptor antagonist (VA), BC, or both drugs were administered for 5 days to F344 rats treated with diethylstilbestrol (DES). Both BC and VA similarly blocked the effects of DES on pituitary weight and pituitary content of prolactin (PRL), proliferating cell nuclear antigen, and vascular endothelial growth factor, without evidence of synergism. The estrogen effect on pituitary TGF-beta1 was completely inhibited by VA, but only partially by BC. On the contrary, serum PRL was close to the normal levels in the BC group 2 h after the first dose, while VA only reduced serum PRL after 5 days. DES increased VIP and VIP mRNA levels specifically at the pituitary, this effect being partially blocked by BC. These data suggest that the dopamine agonists inhibit lactotrope proliferation and angiogenesis by blocking the autocrine/paracrine action of VIP. On the other hand, the dopamine agonists inhibit the estrogen-induced hyperprolactinemia by acting through different pathways than those implicated in the proliferative process.
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