REGIONAL, DEVELOPMENTAL, AND CELL CYCLE-DEPENDENT DIFFERENCES IN MU-OPIOID, DELTA-OPIOID, AND KAPPA-OPIOID RECEPTOR EXPRESSION AMONG CULTURED MOUSE ASTROCYTES

The diversity of opioid receptor expression was examined in astrocytes in low-density and non-dividing (confluent) cultures from the cerebra l cortex, hippocampus, cerebellum, and striatum of 1-day-old mice. mu, delta, and kappa opioid receptor expression was assessed in individua l cells immunocytochemically, by using flow cytometry, and functionall y by examining agonist-induced changes in intracellular calcium ([Ca2](i)). Significant spatial and temporal differences were evident in th e pattern of expression of mu, delta, and kappa receptors among astroc ytes. In low-density cultures, greater proportions of astrocytes expre ssed mu-opioid receptor immunoreactivity in the cerebral cortex and hi ppocampus (26-34%) than in the cerebellum or striatum (7-12%). At conf luence, a greater percentage of astrocytes in cerebellar (26%) and str iatal (30%) cultures expressed mu-immunoreactivity. Fewer astrocytes p ossessed delta-immunoreactivity in low-density striatal cultures (8%) compared to other regions (16-22%). The proportion of delta receptor-e xpressing astrocytes declined in the cerebellum but increased in the h ippocampus. kappa-opioid receptors were uniformly expressed by 27-34% of astrocytes from all regions, except in cortical cultures, where the proportion of kappa expressing cells was 38% at low-density and decre ased to 22% at confluence. Selective mu (PLO 17; H-Tyr-Pro-Phe (N-Me) -D-Pro-NH2, delta ([D-Pen(2), D-Pen(5)] enkephalin), or kappa (U50,488 H; l-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeneacetamide methanesulfonate ) opioid receptor agonists increased [Ca2+](i) in subpopulations of as trocytes indicating the presence of functional receptors. Lastly, opio id receptor immunofluorescence varied during the cell division cycle. A greater proportion of astrocytes in the G(2)/M phase of the cell cyc le were mu or delta receptor immunofluorescent than at G(0)/G(1). When astrocytes were reversibly arrested in G(1), significantly fewer cell s expressed delta receptor immunofluorescence; however, upon reentry i nto the cell cycle immunofluorescent cells reappeared. in conclusion, opioid phenotype varies considerably among individual cultured astrocy tes, and this diversity was determined by regional and developmental(a ge and cell cycle dependent) differences in the brain. These in vitro findings suggest astroglia contribute to regional and developmental id iosyncrasies in opioid function within the brain. (C) 1998 Wiley-Liss, Inc.