kappa-Opioid receptor expression defines a phenotypically distinct subpopulation of astroglia: Relationship to Ca2+ mobilization, development, and the antiproliferative effect of opioids

To assess the role of kappa-opioid receptors in astrocyte development, the effect of kappa-agonists on the growth of astroglia derived from 1-2-day-old mouse cerebra was examined in vitro. kappa-Opioid receptor expression was assessed immunocytochemically (using KA8 and KOR1 antibodies), as well as functionally by examining the effect of K-receptor activation on intracellular calcium ([Ca2+](i)) homeostasis and DNA synthesis. On days 6-7, as many as 50% of the astrocytes displayed kappa-receptor (KA8) immunoreactivity or exhibited increases in [Ca2+](i) in response to kappa-agonist treatment (U69,593 or U50,488H). Exposure to U69,593 (100 nM) for 72 h caused a significant reduction in number and proportion of glial fibrillary acidic protein-immunoreactive astrocytes incorporating bromodeoxyuridine (BrdU) that could be prevented by co-administering the kappa-antagonist, nor-binaltorphimine (300 nM). In contrast, on day 14, only 5 or 14%, respectively, of the astrocytes were kappa-opioid receptor (KAs) immunoreactive or displayed functional increases in [Ca2+](i). Furthermore, U69,593 (100 nM) treatment failed to inhibit BrdU incorporation at 9 days in vitro. Experimental manipulations showed that kappa-receptor activation increases astroglial [Ca2+](i) both through influx via L-type channels and through mobilization of intracellular stores (which is an important Ca2+ signaling pathway in cell division). Collectively, these results indicate that a subpopulation of developing astrocytes express kappa-opioid receptors in vitro, and suggest that the activation of K-receptors mobilizes [Ca2+](i) and inhibits cell proliferation. Moreover, the proportion of astrocytes expressing kappa-receptors was greatest during a period of rapid cell growth suggesting that they are preferentially expressed by proliferating astrocytes.