In Situ Molecular Size of Agonist Dopamine D‐2 Binding Sites in Rat Striatum

Specific binding of [3H]N-propylnorapomorphine [( 3H]NPA) to 3,4-dihydroxyphenylethylamine (dopamine) D-2 receptors was investigated in rat striatum in vitro. For various dopamine receptor substances, the rank order of potency to inhibit [3H]NPA binding was spiroperidol greater than or equal to NPA greater than LY 171555 greater than SCH 23390 greater than SKF 38393. A single high-affinity binding site was found in membranes prepared in either Tris-citrate buffer or imidazole buffer; the affinity constants were 0.11 and 0.76 nM, respectively. The number of receptors (33 pmol/g wet weight) was independent of whether the membranes were prepared in Tris-citrate buffer or imidazole buffer and was similar to the number of receptors estimated by [3H]spiroperidol binding to dopamine receptors. Irradiation inactivation of frozen whole rat striata showed a monoexponential loss of [3H]NPA binding sites without a change in the binding affinity. The target size of the [3H]NPA binding site was 81,000 daltons, which shows that the functional molecular entity to bind the dopamine D-2 agonist was smaller than the molecular entity to bind the dopamine D-2 antagonist [3H]spiroperidol (target size, 137,000 daltons).