A regulated secretory pathway in cultured hippocampal astrocytes

Glial cells have been reported to express molecules originally discovered i n neuronal and neuroendocrine cells, such as neuropeptides, neuropeptide pr ocessing enzymes, and ionic channels. To verify whether astrocytes may have regulated secretory vesicles, the primary cultures prepared from hippocamp i of embryonic and neonatal rats were used to investigate the subcellular l ocalization and secretory pathway followed by secretogranin II, a well know n marker for dense-core granules. By indirect immunofluorescence, SgII was detected in a large number of cultured hippocampal astrocytes, Immunoreacti vity for the granin was detected in the GoIgi complex and in a population o f dense-core vesicles stored in the cells. Subcellular fractionation experi ments revealed that SgII was stored in a vesicle population with a density identical to that of the dense-core secretory granules present in rat pheoc hromocytoma cells. In line with these data, biochemical results indicated t hat 40-50% of secretogranin II synthesized during 18-h labeling was retaine d intracellularly over a 4-h chase period and released after treatment with different secretagogues. The most effective stimulus appeared to be phorbo l ester in combination with ionomycin in the presence of extracellular Ca2, a treatment that was found to produce a large and sustained increase in i ntracellular calcium [Ca2+](i) transients, Our findings indicate that a reg ulated secretory pathway characterized by (i) the expression and stimulated exocytosis of a typical marker for regulated secretory granules, (ii) the presence of dense-core vesicles, and (iii) the ability to undergo [Ca2+](i) increase upon specific stimuli is present in cultured hippocampal astrocyt es.