These proteins are synthesized in the cell bodies of the supraoptic and paraventricular regions of the hypothalamus. 
The disulfide-rich neurophysin protein is suggested to be congruent with the synthesis of insulin in which a precursor molecule of higher molecular weight is proteolytically cleaved and forms disulfide linkages.
Although not enough data has been obtained, it is hypothesized that there is a common precursor molecule between neurophysin and the two hormones it stabilizes.