Proenkephalin (Penk) gene expression is high in the adult hamster adrenal medulla and it is comparable to that found in both the hamster and rat striatum. In addition, Penk gene expression in the hamster adrenal medulla is more typical of adult mammalian adrenals than the rat. Since the nature of Penk gene expression in the developing hamster adrenal is not known, it was examined and compared to that found in the striatum were adult levels in the adrenal and striatum are similar. The results show that Penk gene expression progressively increases in the developing hamster adrenal to peak on postnatal day 4. There is then a small decline to adult levels by postnatal day 12 when the morphology of the developing adrenal resembles the adult. Functional splanchnic nerve activity, as assessed by the ability of reserpine to induce increases in adrenal tyrosine hydroxylase mRNA, is not present until after postnatal day 4. Therefore, early increases in Penk gene expression are independent of splanchnic nerve activity. Adrenal EC peptides resulting from the developmental increases in Penk gene expression appear to be unprocessed and proenkephalin-like. This is based on the very low levels of free enkephalin (met-enkephalin) detected in the adrenals from both newborn and adult hamsters (1-5% of total EC peptide levels). In the developing hamster striatum, Penk gene expression remains low and unchanged until postnatal day 4 and increases six-fold by adulthood. Free enkephalin (met-enkephalin) levels remain high (between 36 and 88% of total EC peptide levels) in the developing and adult hamster striatum. Therefore the results show early increases in adrenal Penk gene expression in the developing hamster that are independent of splanchnic nerve activity and adult Penk gene expression which is high and dependent on splanchnic nerve activity. This differs from what is observed in the frequently studied rat. However, developmental changes in the hamster striatum are similar to those in the rat.
