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 anti bodies), as well as functionally by examining the effect of K-receptor activation on intracellular calcium ([Ca2+](i)) homeostasis and DNA s ynthesis. On days 6-7, as many as 50% of the astrocytes displayed kapp a-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 astr ocytes 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 astrocyt es were kappa-opioid receptor (KAs) immunoreactive or displayed functi onal increases in [Ca2+](i). Furthermore, U69,593 (100 nM) treatment f ailed to inhibit BrdU incorporation at 9 days in vitro. Experimental m anipulations showed that kappa-receptor activation increases astroglia l [Ca2+](i) both through influx via L-type channels and through mobili zation of intracellular stores (which is an important Ca2+ signaling p athway 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 [Ca 2+](i) and inhibits cell proliferation. Moreover, the proportion of as trocytes expressing kappa-receptors was greatest during a period of ra pid cell growth suggesting that they are preferentially expressed by p roliferating astrocytes.