α2‐Adrenergic Receptors in Neuronal and Glial Cultures: Characterization and Comparison

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Abstract
Membranes prepared from either neuronal or glial cultures contain .alpha.2-adrenergic receptors as determined by the characteristics of [3H]yohimbine ([3H]YOH) binding. The binding was rapid, reversible, saturable, dependent on the protein concentration used, and reached equilibrium by 5 min in membranes from both neuronal and glial cultures. Scatchard analyses of saturation isotherms revealed similar KD values of 13.7 .+-. 1.35 nM (n = 10) for neuronal cultures and 18.42 .+-. 2.34 nM (n = 10) for glial cultures. Glial cultures contained many more binding sites for [3H]YOH than neuronal cultures, having a Bmax of 1.6 .+-. 0.33 pmol/mg protein (n = 10) compared with 0.143 .+-. 0.018 pmol/mg protein (n = 10) in neurons. Drugs selective for .alpha.2-adrenergic receptors were the most effective displacers of [3H]YOH binding in both neuronal and glial cultures, i.e., the .alpha.2-adrenergic antagonists rauwolscine and yohimbine were better displacers than the other catecholamine antagonists prazosin, corynanthine, or propranolol. The agonists showed the same pattern with the .alpha.2-selective drugs clonidine and naphazoline being the most effective competitors for the [3H]YOH site. GTP and its nonhydrolyzable analog, 5''-guanylyl-imidodiphosphate, were able to lower the affinity of the .alpha.2-receptors for agonists but not antagonists in membranes from both neuronal and glial cultures, suggesting that the receptors are linked to a G protein in both cell types. The presence of .alpha.2-adrenergic receptors in neuronal cultures was also substantiated by light microscopic autoradiography of [3H]YOH binding. In summary, we have demonstrated that both neuronal and glial cultures contain .alpha.2-adrenoceptors.