Cytodifferentiation of the supraoptic nucleus correlated with vasopressin synthesis in the rat.

This study demonstrates that neurons in the supraoptic nucleus attain many of the prerequisites for functional activity prior to birth. Immunoassayable vasopressin (VP) was detected in the hypothalamo-neurohypophyseal system (HNS) of the rat as early as 17 days post-coitus (dpc). Vasopressin concentrations increased 3--6-fold daily from an average of 21 pg/animal on 17 dpc to 5984 pg/animal at 21 dpc. The daily increases were highly significant (P less than 0.001). Between 21 dpc and the morning of the day of birth on 22 dpc, a further significant increase (P less than 0.05) occurred to a mean level of 9672 pg VP/animal. Birth usually occurred on the afternoon of the 22nd day. Parturition did not seem to deplete VP stores in the HNS. Differentiation of the magnocellular neurons in the supraoptic nucleus closely paralleled the appearance and increases in VP. It was first possible to dintinguish a supraopic nucleus in the 17 dpc rat and to identify dense core granules in the developing neurons of the nucleus. Cytodifferentiation of the magnocellular neurons was essentially complete by 21 dpc. Synaptic contacts could not be found on the soma and dendrites of the supraoptic neurons until 21 dpc and were extremely rare throughout the period examined.