Dopamine Inhibition of Proopiomelanocortin Gene Expression in the Intermediate Lobe of the Pituitary

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Abstract
The effects of dopamine on proopiomelanocortin (POMC) gene expression were compared in primary cultures of the anterior and intermediate lobes of the rat pituitary. A single-stranded POMC complementary DNA was used to quantitate POMC messenger RNA levels. Treatment with dopamine (1 µM) for 48 h reduced POMC messenger RNA levels in the intermediate lobe by 77%, but had no effect on POMC gene expression in the anterior lobe. Dopamine D2 receptors were implicated in the response, as bromocriptine (100 nM) reproduced the dopamine inhibition. The responses to dopamine and bromocriptine were antagonized by haloperidol (10 µM). The decrease in POMC messenger RNA levels was dose dependent with ED50 values of about 50 and 0.1 nM for dopamine and bromocriptine, respectively. The accumulation of POMC-derived peptides, β-endorphin and α-melanocyte-stimulating hormone, over 2 days was measured by radioimmunnoassay and was shown to parallel the changes in POMC synthesis. The dopamine-induced inhibition of intermediate lobe POMC synthesis was unaffected by isoprenalin (5 µM) and corticotropin-releasing factor (10 nM), although these treatments had stimulatory effects when tested alone. Activating adenylate cyclase with forskolin (1 µM) or treatment with 8-bromocyclic adenosine monophosphate (1 mM) doubled POMC messenger RNA levels, and, when tested against these stimuli, bromocriptine still produced a 30% inhibition of POMC gene expression. These observations suggest that D2 receptor induced inhibition of POMC gene expression is not only mediated by a decrease in cyclic adenosine monophosphate levels. When cells were pretreated with pertussis toxin (100 ng/ml), the bromocriptine-induced inhibition was almost completely lost, suggesting that the dopaminergic inhibition is mediated by guanosine triphosphate binding proteins.

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