The biosynthetic activity of rat intermediate lobe melanotropes in vivo is inhibited by stimulation of dopamine D2 receptors. Individual melanotropes are innervated differentially by dopaminergic axons and vary in their levels of pro-opiomelanocortin (POMC) mRNA. We tested the hypothesis that placement of the lobe in primary culture, which removes the inhibitory innervation, would increase POMC mRNA levels and abolish the heterogeneity in POMC expression. POMC mRNA levels increased successively in untreated melanotropes when tested on culture Days 10, 16, and 20; however, some heterogeneity in POMC expression persisted. If treated with a D2 receptor agonist (1 microM bromocriptine) from culture Day 1, POMC mRNA levels were decreased significantly throughout the testing period when compared to untreated cells with the same time in culture. Although some melanotropes still expressed high POMC levels, preparations appeared more homogeneous by Day 20. Melanotrope responses were reversible, since POMC mRNA levels were down-regulated by application and up-regulated by withdrawal of a D2 receptor agonist. A short agonist treatment resulted in subpopulations that responded differently to the agonist, possibly representing a mechanism for fine-tuning peptide hormone release.