DOPAMINERGIC REGULATION OF SECRETORY GRANULE-ASSOCIATED PROTEINS IN RAT INTERMEDIATE PITUITARY

The biosynthesis of peptides requires the synthesis of the prohormone, several biosynthetic processing enzymes, and other granule constituen ts. We have investigated the regulated expression of proopiomelanocort in (POMC) and five enzymes essential for the processing of POMC to sma ller, bioactive peptides in intermediate pituitary melanotropes. Rats were treated with a dopaminergic agonist (bromocriptine) or antagonist (haloperidol) for periods ranging from 1 h to 5 days, followed by ana lyses of mRNA levels and protein biosynthetic rates. Multiplex RNase p rotection assays showed that bromocriptine treatment caused a striking decrease in POMC mRNA levels, and significant decreases in mRNA level s for prohormone convertase 2 (PC2), carboxypeptidase H (CPH), and pep tidylglycine alpha-amidating monooxygenase (PAM). Smaller increases in mRNA levels were seen after haloperidol stimulation. Protein biosynth etic rates changed more profoundly than mRNA levels at short drug trea tment times, indicating a role for translational effects after treatme nt with bromocriptine and with haloperidol. The homogeneous population of melanotropes in the intermediate lobe of the pituitary allows a qu antitative analysis of transcript levels and biosynthetic rates. POMC mRNA levels are 200-1,000-fold higher than levels of any of the proces sing enzyme mRNAs, and POMC biosynthetic rates exceed those of PC2, PC 1, and PAM by 1,000-10,000-fold.