Maturational changes in expression of enkephalin peptides in adrenal and extra-adrenal tissue of fetal and adult rabbits.

Met-enkephalin immunoreactivity (MET-ENKi), total enkephalin immunoreactivity (TOTAL MET-ENKi) and catecholamines were measured in adrenal and extra-adrenal tissue of fetal, newborn and adult rabbits. Met-enkephalin peptides were detected in adrenal and extra-adrenal tissue by 29 days of gestation. There were progressive increases in TOTAL MET-ENKi in both the adrenal and extra-adrenal tissue during development. In 29-day-old fetuses, MET-ENKi represented 43 and 50% of the peptide content in adrenal and extra-adrenal tissues respectively. By 3 days after birth, MET-ENKi represented only 15 and 7% of the peptide content in the same tissues. In the adult adrenals, 10% of enkephalin peptides were found as MET-ENKi. There were progressive increases in adrenal and extra-adrenal catecholamine content in the fetal and newborn rabbits throughout development. The changes in the ratio of MET-ENKi to TOTAL MET-ENKi peptides suggest differences in posttranslational processing of proenkephalin peptide during maturation. We speculate that enkephalin peptides derived from proenkephalin A are important during fetal and early newborn life and that extra-adrenal tissue may be an important source of these peptides during development.