Proenkephalin messenger RNA is expressed both in the rat anterior and posterior pituitary.

The presence of proenkephalin (PENK)-derived opioid peptides in the pituitary gland is well known. However, the cellular sources of their biosynthetic origin in all three pituitary lobes are less clear. In this study we identified the potential sites of synthesis by localizing the mRNA coding for PENK in the rat pituitary gland using in situ hybridization histochemistry. Numerous cells containing PENK mRNA were detected throughout the anterior lobe. Although suggested by previous reports, no mRNA signal could be detected in the intermediate lobe. Surprisingly, high levels of PENK mRNA were found in the posterior lobe. The cellular distribution in the neural lobe implies that pituicytes, a special class of glial cells, may express PENK mRNA.