Histological evaluation of the dopaminergic regulation of proopiomelanocortin gene expression in the intermediate lobe of the rat pituitary, involving in situ hybridization and [3H]thymidine uptake measurement.

The melanotroph, a polyhedral secretory cell with an ovoid smooth nucleus, is the primary cell type of the intermediate lobe (IL) of the rat pituitary. The melanotrophs are not uniform, but differ in the tinctorial properties of their cytoplasm; some cells appear distinctly darker, others lighter, and cells staining in intermediate shades are also found. In addition, in situ hybridization using proopiomelanocortin (POMC) probes shows an uneven distribution of POMC mRNA among melanotrophs, indicating that different cells maintain different levels of biosynthetic activity. Dopaminergic drugs known to alter the secretion of POMC-related peptides from the IL produced parallel changes in histological staining properties and the amount of POMC mRNA per cell, as determined by in situ hybridization. Acute bromocriptine treatment (6 h) produced a dramatic reduction in grain counts over melanotroph cytoplasm (to 10% of the control levels). A similar reduction persisted after chronic treatment. Six hours after a single haloperidol injection, the grain counts were 180% of control levels. After chronic haloperidol treatment, they were further elevated to 300% of control levels. Chronic bromocriptine and haloperidol treatment also changed the thickness of the IL. Bromocriptine reduced and haloperidol treatment increased the number of cell layers in the IL by changing the rate of cell proliferation. Thus, haloperidol treatment significantly increased and bromocriptine treatment significantly decreased the number of melanotrophs labeled by [3H]thymidine. The mitotic index followed the same trend. These results suggest that dopamine regulation of the IL acts by two different mechanisms: POMC gene expression and cellular proliferation. The change in POMC gene expression is the cell's first rapid response. The influence on the cell cycle appears after subchronic and chronic treatment.