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

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 greater than 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 greater than 7-fold from 68.75 +/- 11.0 fmol to 491 +/- 97.8 fmol during this time period. ACTH/microgram of soluble protein decreased during postnatal development from 0.006 +/- 0.01 to 0.005 +/- 0.001 fmol/microgram of protein and alpha-MSH increased from 0.06 +/- 0.01 fmol/microgram of protein to 0.11 +/- 0.009 fmol/microgram of protein between P1 and P7, decreased to 0.015 +/- 0.003 fmol/microgram 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.