Postnatal development of ACTH and [alpha]-MSH in the medulla oblongata of rat: [alpha]-MSH is the predominant peptide

Abstract

ACTH and [alpha]-MSH levels were measured by radioimmunoassays in extracts of the caudal medulla oblongata of developing rats on postnatal (P) days 1-42 at 7 day intervals, and in adult rats. From P1 to adulthood, ACTH increased >11-fold from 7.2+/-1.9 fmol to 82.4+/-12.6 fmol per medulla section (mean+/-S.E.M.). In comparison, [alpha]-MSH increased >7-fold from 68.75+/-11.0 fmol to 491+/-97.8 fmol during this time period. ACTH/[mu]g of soluble protein decreased during postnatal development from 0.006+/-0.01 to 0.005+/-0.001 fmol/[mu]g of protein and [alpha]-MSH increased from 0.06+/-0.01 fmol/[mu]g of protein to 0.11+/-0.009 fmol/[mu]g of protein between P1 and P7, decreased to 0.015+/-0.003 fmol/[mu]g of protein by P42 and increased to 0.03+/-0.006 fmol/protein per unit protein by adulthood. These data indicate a significant shift in the levels of [alpha]-MSH detected during development with a decrease in the concentration of material occurring from early postnatal development (P1-P7) to adulthood, which does not appear to be solely related to a regional increase in protein. These studies, as well as radioimmunoassays for ACTH and [alpha]-MSH in combination with sizing chromatography of pooled extracts at P1, P7 and the adult, demonstrated the predominance of [alpha]-MSH at all ages.