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A tissue culture model was established for the study of hypothalamic peptide synthesis and secretion. Microdissected explants of the paraventricular and supraoptic regions from Sprague-Dawley rats (neonates or young rats) were maintained in culture for up to 3 weeks. Studies were performed to evaluate vasopressin and oxytocin content (medium and tissue levels), immunocytochemical localization, and biosynthetic activity. Immunocytochemical staining for oxytocin, neurophysin, and neuron-specific enolase showed positive neurons in both the paraventricular and supraoptic cultures. In many cases, the neurons were large (30-40 microns) and bipolar, resembling the classic magnocellular neuron. Measurement of tissue and medium content showed the continued presence of vasopressin and oxytocin in the cultured explants. Even after 3 weeks, there were significant amounts of vasopressin present. Biosynthesis was evaluated by determining the incorporation of 35S-labeled cystine or cysteine into proteins and peptides. The medium and tissue extracts were separated by reverse-phase high-performance liquid chromatography. Results showed that most of the labeled peptides were released into the medium rather than stored. There were two labeled peaks in the medium, which chromatographically resembled native vasopressin and oxytocin. Treatment with a protein synthesis inhibitor, either puromycin or cycloheximide, resulted in a decrease in labeled peptides. A comparison of 35S-labeled cystine and cysteine showed that the latter was the label of choice, with significantly greater incorporation.

The potential role of central neuroendocrine changes in the development of spontaneous hypertension was evaluated. The developmental changes in blood pressure and hypothalamic and plasma levels of vasopressin (AVP) and oxytocin (OT) were determined in groups of SHR and WKY animals from 3 to 24 weeks of age. Hypothalamic OT content was significantly lower in 3-, 6-, and 12-week-old SHR rats compared to age-matched WKY animals. Hypothalamic AVP content was not different at 3 weeks of age, but was lower in the SHRs at 6 and 12 weeks. To localize strain differences in AVP and OT, specific hypothalamic nuclei were removed from 300 microns frozen brain sections, and hormone content measured. Paraventricular AVP and OT content was lower in the SHRs which had increased blood pressure (6, 12, and 24 weeks of age) but not in the prehypertensive groups (3 weeks of age). Neuropeptide content was unchanged in the supraoptic nucleus or median eminence. Plasma levels of AVP were increased in the SHR, while OT was unchanged. Thus, genetic hypertension is associated with specific and localized changes in hypothalamic AVP and OT. The fact that the peptide deficit occurred in the paraventricular nucleus, a region thought to be involved in the control of autonomic function, may have important implications in terms of the pathogenesis of hypertension.