Developmental changes in levels of proopiomelanocortin intron A-containing heterogeneous nuclear RNA and mature messenger RNA in the anterior and neurointermediate lobes of the rat pituitary.

The POMC cells of the rat pituitary undergo dynamic phenotypic changes during differentiation. Here we have determined that alterations in the relative levels of a POMC precursor RNA species and POMC mRNA occur during development and may represent another level at which the POMC phenotype is developmentally regulated. We performed solution hybridization/nuclease protection assays using a POMC exon 1/intron A splice junction probe to quantitate levels of both intron A-containing POMC heterogeneous nuclear (hnRNA) and fully processed POMC mRNA in separated anterior and neurointermediate lobes of the fetal, neonatal, and adult pituitary. The levels of POMC hnRNA per anterior lobe increased 7-fold from embryonic day 15 to adulthood (0.022 to 0.159 fmol/lobe), while POMC mRNA levels increased 121-fold (0.15 to 18.2 fmol/lobe). POMC hnRNA levels per neurointermediate lobe increased 23-fold from embryonic day 18 to adulthood (0.024 to 0.54 fmol/lobe), while POMC mRNA levels increased 69-fold (0.65 to 44.6 fmol/lobe). Thus, both anterior and neurointermediate lobes contain higher relative abundances of POMC hnRNA compared to mRNA during early development. These subsequently decrease (from 1:7 to 1: approximately 110 in the anterior lobe and from 1:27 to 1:83 in the intermediate lobe over the ages examined) as the levels of POMC mRNA in both anterior and neurointermediate lobe increase at a greater rate than POMC hnRNA as development progresses. These results provide the first measurements of POMC mRNA and hnRNA levels during ontogeny and suggest that there may be a developmental change in the regulation of POMC primary transcript processing.