Expression of the adrenergic phenotype in cultured fetal adrenal medullary cells: role of intrinsic and extrinsic factors

During embryogenesis of the rat the enzymes tryosine hydroxylase (TH) and dopamine-β-hydroxylase (DBH) are first detected by immunocytochemistry or biochemical assay on the 16th day of gestation (E 16). It is not until E 18 that the enzyme phenylethanolamine-N-methyltransferase (PNMT), which is required for biosynthesis of adrenaline, can be detected cytochemically or biochemically. In this study we sought to determine whether the delayed appearance of PNMT is consequent to invasion of the adrenal medulla by E 18 of cells destined to express PNMT, cues provided by the ingrowing splachnic nerves or the action of corticosterone (CS) secreted by the adrenal cortical anlage, a hormone which regulates PNMT in adult rats. When adrenal glands are removed on E 16 and placed in culture, PNMT cannot be detected cyto- or biochemically until 2 days later (E 16 + 2). While CS levels increase 100-fold in vivo between E 16 and E 18, the surge of CS is not necessary for expression of PNMT since (a) adrenals removed on E 16 and cultured in the absence of exogenous ACTH fail to increase CS yet still express PNMT and (b) addition of CS (10^?5M) to the cultures on E 16 does not alter the time of appearance of the enzyme. CS, on the other hand, increases the amount of PNMT protein and activity 3-fold with respect to control at all time points, without any effect on TH. We conclude that (a) it is the cells already present in the adrenal medulla at E 16 which differentiate to express PNMT; (b) the initial expression of PNMT is not controlled by nerves nor by corticosteroids; and (c) corticosteroids have a selective action on regulating the amount of PNMT, once it is expressed, but not TH enzyme protein. It remains to be determined whether the differentiation of PNMT is elicited by genetic or epigenetic signals.