Vasopressin and oxytocin in the developing rat brain as shown by isoelectric focusing of radioimmunoassayable peptides.

Vasopressin and oxytocin content were measured by radioimmunoassay in tissue extracts from rat pituitary, hypothalamus and the extrahypothalamic brain from gestational day 14 until postnatal day 14. Assayable compounds were subsequently characterized by isoelectric focusing in gels. Fetal day 14 extracts contained only low amounts of both hormones (around 1-2 pg per fetus). Vasopressin content rapidly increased to levels of 41 ng in pituitary, 20 ng in hypothalamus, 0.3 ng in extrahypothalamic brain on postnatal day 14. Oxytocin content showed only measurable amounts around day 20 prenatally, reaching levels of, respectively, 10, 0.5 and 0.2 ng on postnatal day 14. The presence of radioimmunoassayable vasopressin in the extrahypothalamic brain on fetal day 14 (around 2 pg) confirms the early ontogeny of vasopressinergic innervation of the brain. Whenever measurable, immunoreactivity had an isoelectric point similar to that of synthetic vasopressin or oxytocin. The possibility that the detected amounts of vasopressin were due to vasotocin, which had been claimed to be present in fetal brain, was excluded by comparison of the regression coefficients of serial dilutions of the extracts in the radioimmunoassay with those of synthetic vasotocin or vasopressin.