Quantitative changes in neurohypophyseal peptides in the developing brain

Abstract

Quantitative changes in the content of neurohypophyseal peptides: neurophysins, oxytocin and vasopressin, were determined in the developing fetus and followed through day 30 post-gestation. In the fetus, no neurohypophyseal peptides were found until day 13. Neurophysins then increased dramatically on day 14. The hormones, vasopressin and oxytocin, were not detected at day 13 and were measured at low levels on day 14. A 350 fold molar excess of neurophysin to hormone existed at day 14. From day 14 to day 19 the total content of neurophysin decreased while the content of vasopressin and oxytocin slowly increased. At day 19, there was a near molar equivalency between the content of neurophysin and that of the neurohypophyseal hormones. From day 18 to day 22 there was a sharp increase in the content of vasopressin while the content of neurophysin and oxytocin increased less dramatically. At term there was a molar excess of vasopressin and the molar ratio of neurophysin to hormone at the time of delivery was 0.12. Large molecular forms of VP and OT were identified in saline extracts of the 16–17 day fetus. Small (natural) vasopressin was obtained by acidification or incubation with 6 M urea. The amount of “big vasopressin” decreased from greater than 90% at day 16 to less than 40% at day 19. The content of vasopressin in the pituitary increased rapidly in the first days after birth while the content of oxytocin and neurophysin remained low until 8 days and then increased from day 8 to day 21 of life. Finally the ratio of vasopressin to oxytocin reached unity at 30 days and the ratio of neurophysin to hormone reached unity at the same time. Assays to detect vasotocin gave negative results throughout the time studied. We postulate that precursor forms of neurophysin and vasopressin may be present during development in concentrations which are not found in the adult. These precursor forms may explain the discrepancies between the moles of neurophysin and hormone present at any given time.