Role of glucocorticoids in expression of the adrenergic phenotype in rat embryonic adrenal gland

Differentiation of the noradrenergic and adrenergic phenotypes was documented in rat embryonic adrenal chromaffin cells in vivo from 12.5 days of gestation (E12.5) to term. The initial appearance of three enzymes in the catecholaminergic pathway, tyrosine hydroxylase (T-OH), dopamine-β-hydroxylase (DBH), and phenylethanolamine-N-methyltransferase (PNMT) as well as endogenous catecholamines (CA), was followed by immunohistochemistry and histofluorescence. T-OH and DBH, were employed as indices of noradrenergic expression, whereas PNMT, the epinephrine-synthesizing enzyme, was used as an index of adrenergic expression. At E12.5, T-OH, DBH, and CA were present in cells of the sympathetic ganglia at the level of the adrenal anlage. By 13.5 days, cells containing T-OH, DBH, and CA, were observed between the sympathetic ganglia and developing adrenal, and within the adrenal itself. While T-OH, DBH, and CA were present in adrenal medullary cells from the earliest stages of adrenal development, PNMT, in contrast, was undetectable in ganglion primordia, migrating cells, or within the adrenal before 17 days. PNMT initially appeared at E17 in small clusters of cells scattered throughout the adrenal. The number of cells containing PNMT and the intensity of staining increased dramatically from E17 to term.A number of experimental manipulations were employed in vivo to investigate the role of glucocorticoids in differentiation of the adrenergic phenotype. Chronic or acute treatment of mothers and/or embryos with various glucocorticoids, adrenocorticotrophic hormone (ACTH), or S-adenosylmethionine (SAM) did not result in precocious appearance of PNMT. Moreover, the initial expression of PNMT was not prevented or delayed by embryonic hypophysectomy or by treatment with inhibitors of adrenocortical function. Consequently, the initial expression of PNMT on E17.0 is not dependent on normal glucocorticoid levels, cannot be induced prematurely by glucocorticoids, and is independent of the pituitary-adrenal axis. However, the ontogenetic increase in PNMT levels after initial expression has occurred does require intact pituitary-adrenal function. Our observations suggest that different mechanisms regulate initial expression and subsequent modulation of neurotransmitter phenotype.