Levels of Neurohypophyseal Peptides in the Rat during the First Month of Life. I. Basal Levels in Plasma, Pituitary, and Hypothalamus*

Vasopressin, oxytocin, and neurophysin were measured by RIA in the pituitary, hypothalamus, and plasma (except oxytocin) of the rat during the first month of life. In plasma, vasopressin was less than 1.7 microU/ml in most animals. Neurophysin was elevated above adult levles on day 2 and decreased with age. The three peptides were present in the pituitary at birth, but in amounts less than 1% of the adult level. The vasopressin content of the pituitary increased rapidly in the first days after birth, while the levels of oxytocin and neurophysin remained low until 8 days and then increased between 8-21 days. The ratio of vasopressin to oxytocin in the pituitary was 4.4 at birth and reached unity (the ratio in the adult) at 30 days. At birth, the moles of neurophysin in the pituitary relative to the moles of hormone (oxytocin plus vasopressin) was low (0.15), largely due to a molar excess of vasopressin. The ratio of neurophysin to hormone reached unity at 21-30 days. Assays to detect vasotocin gave negative results. It is postulated that a precursor neurophysin which was related to vasopressin was present in the fetal rat but was not measured in our study.