α 2 ‐Adrenergic Receptors in Neuronal and Glial Cultures: Characterization and Comparison

Membranes prepared from either neuronal or glial cultures contain α 2 ‐adrenergic receptors as determined by the characteristics of [ 3 H]yohimbine ([ 3 H]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 K D values of 13.7 ± 1.35 n M (n = 10) for neuronal cultures and 18.42 ± 2.34 n M (n = 10) for glial cultures. Glial cultures contained many more binding sites for [ 3 H]YOH than neuronal cultures, having a 5 max 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 α 2 ‐adrenergic receptors were the most effective displacers of [ 3 H]YOH binding in both neuronal and glial cultures, i.e., the α 2 ‐adrenergic antagonists rauwolscine and yohimbine were better displacers than the other catecholamine antagonists prazosin, corynan‐thine, or propranolol. The agonists showed the same pattern with the α 2 ‐selective drugs clonidine and naphazoline being the most effective competitors for the [ 3 H]YOH site. GTP and its nonhydrolyzable analog, 5′‐guanylyl‐imidodiphos‐phate, were able to lower the affinity of the α 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 α 2 ‐adrenergic receptors in neuronal cultures was also substantiated by light microscopic autoradiography of [ 3 H]YOH binding. In summary, we have demonstrated that both neuronal and glial cultures contain α‐adrenoceptors.