Chronic estrogen treatment promotes a functional uncoupling of the D2 dopamine receptor in rat anterior pituitary gland.
The effect of chronic estrogen treatment on the anterior pituitary D2 dopamine receptor was studied by treating rats with diethylstilbestrol (DES) over a 6-week period. DES treatment resulted in an increase in anterior pituitary weight and PRL content and serum PRL levels compared to those in sham-treated controls. The status of the anterior pituitary D2 dopamine receptor was evaluated using both radioligand binding and adenylate cyclase assays. [125I]N-(p-aminophenethyl)spiroperidol [( 125I]NAPS), a derivative of the D2-selective antagonist spiperone, was used to quantitate D2 receptors. Saturation analysis of [125I]NAPS binding indicated that DES treatment had no effect on the affinity or maximum binding capacity of the radioligand for the D2 receptor. Competition analysis with unlabeled D2 antagonists for [125I]NAPS binding also indicated that DES treatment did not affect antagonist interactions with the receptor. In contrast, the interactions of agonists with the D2 receptors from DES-treated rats were modified, as assessed through [125I]NAPS competition analysis. Using control tissue, agonist competition curves revealed both high and low affinity agonist binding states of the receptor. In the presence of guanine nucleotides, the high affinity agonist binding state is abolished, reflecting coupling of the receptor with a guanine nucleotide regulatory (G) protein. In DES-treated tissue, agonist competition curves indicated the presence of only low affinity agonist binding, with minimal effects of guanine nucleotides, suggesting uncoupling of receptor-G-protein interactions. The functionality of the D2 receptor was further assessed by examining dopaminergic inhibition of vasoactive intestinal peptide-stimulated adenylate cyclase activity. Although DES treatment resulted in a reduction of vasoactive intestinal peptide-stimulated enzyme activity itself, the ability of dopaminergic agonists to inhibit this activity was reduced by about 50%. These results suggest that estrogen is capable of attenuating the functional coupling of the D2 receptor with its biochemical effector system in the anterior pituitary gland.
Munemura
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Experimental Therapeutics Branch, National Institute of Neurological and Communicative Disorders and Stroke, Bethesda, Maryland 20892.
Agui
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Sibley
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D000262:Adenylyl Cyclases / Q000378:metabolism; D000818:Animals; D001058:Apomorphine / Q000378:metabolism; D001667:Binding, Competitive; D002462:Cell Membrane / Q000378:metabolism; D004054:Diethylstilbestrol / Q000494:pharmacology*; D019204:GTP-Binding Proteins / Q000378:metabolism; D006150:Guanine Nucleotides / Q000494:pharmacology; D006165:Guanylyl Imidodiphosphate / Q000494:pharmacology; D008297:Male; D009929:Organ Size / Q000187:drug effects; D010903:Pituitary Gland, Anterior / Q000033:anatomy & histology / Q000187:drug effects / Q000378:metabolism*; D011388:Prolactin / Q000097:blood / Q000378:metabolism; D051381:Rats; D011916:Rats, Inbred F344; D011954:Receptors, Dopamine / Q000187:drug effects / Q000378:metabolism*; D017448:Receptors, Dopamine D2; D013134:Spiperone / Q000031:analogs & derivatives / Q000378:metabolism; D014660:Vasoactive Intestinal Peptide / Q000494:pharmacology
D006150:Guanine Nucleotides; D011954:Receptors, Dopamine; D017448:Receptors, Dopamine D2; D006165:Guanylyl Imidodiphosphate; D014660:Vasoactive Intestinal Peptide; D013134:Spiperone; D004054:Diethylstilbestrol; D011388:Prolactin; C043186:N-(4-aminophenethyl)spiroperidol; D019204:GTP-Binding Proteins; D000262:Adenylyl Cyclases; D001058:Apomorphine