IMMUNOCYTOCHEMICAL LOCALIZATION OF CHROMOGRANIN-A IN THE NORMAL AND STIMULATED HYPOTHALAMONEUROHYPOPHYSEAL SYSTEM OF THE RAT

Chromogranin A (CGA) is a calcium-binding glycoprotein thought to be t he precursor of several peptides with defined biological activity. Chr omogranin A has been localized in most endocrine cells and many neuron s in the CNS. Here we studied its expression in neurons of the hypotha lamo-neurohypophysial system, which secrete the neurohormones oxytocin and vasopressin. Light and electron microscopic immunocytochemistry a nd immunoblot analysis with antibodies specific for CGA revealed high levels of chromogranin A immunoreactivity throughout the hypothalamo-n eurohypophysial system. In the supraoptic and paraventricular nuclei, it was characterized by intracytoplasmic labelling of magnocellular so mata and processes and of certain astrocytes. Extensive labelling of f ibres and dilatations characterized the internal layer of the median e minence and the neurohypophysis, transit and terminal site of the neur osecretory axons, respectively. Tanycyte-like cells in the median emin ence also displayed reaction. Simultaneous immunofluorescence showed t hat oxytocinergic and vasopressinergic neurons contain chromogranin A. Electron microscopy revealed that chromogranin A immunoreactivity (vi sualized by pre- embedding immunoperoxidase or silver-enhanced colloid al gold techniques) was associated with neurosecretory granules in hyp othalamo-neurohypophysial system neurons. In astrocytes and pituicytes , it was seen over the cytoplasm and glial filaments. In tissue from c olchicine-treated or immobilization-stressed rats, it was clear that c hromogranin A immunoreactivity in the hypothalamus was confined to the hypothalamo-neurohypophysial system. In rats in which neurohypophysia l secretion was strongly stimulated by dehydration, immunocytochemistr y showed that hypothalamoneurohypophysial system immunoreactivity sign ificantly increased in the magnocellular nuclei but decreased in the n eurohypophysis. On the other hand, chromogranin A distribution was not markedly affected by stress or lactation. These observations demonstr ate that chromogranin A is present in neurons and, to a lesser degree, glial cells of the hypothalamoneurohypophysial system and that its ex pression is closely related to that of the neurohypophysial peptides.