Ultrastructure and immunocytochemistry of neurons in the supraoptic and paraventricular nuclei of the lizard Liolaemus cyanogaster. Evidence for the intracisternal location of the precursor of neurophysin.

The supraoptic (SON) and paraventricular (PVN) nuclei of the lizard Liolaemus cyanogaster c. were studied by use of histochemical, immunocytochemical and electron microscopic methods. The immunofluorescence staining for neurophysin was applied to methacrylate-embedded material before and after treatment of the sections with urea and trypsin. Pseudoisocyanine was applied to sections previously used for immunocytochemistry. The ultrastructural study showed that the SON and PVN neurons posses neurosecretory granules (nsg), distributed throughout the perikaryon, and large (2 to 12 micrometers) electron-dense droplets located within dilatations of the cisternae of the rough endoplasmic reticulum. Whereas the perikaryon (nsg) and the secretory droplets are stainable with pseudoisocyanine, only the former displays immunoreactivity for neurophysin. However, after treating the sections with urea and trypsin, the same secretory droplets become immunoreactive. It is suggested that the secretory droplets are sites of storage for the precursor of neurophysin, and that the tryptic digestion either triggers its conversion into neurophysin or exposes its immunoreactive sites. Based on the ultrastructure and the histochemical behavior of the secretory droplets, it is also postulated that they contain, in addition to peptides, a glycoprotein component.