Characterization of binding sites for [3H]spiroperidol.

Experiments were designed to investigate the biochemical properties of binding sites for [3H]spiroperidol ([3H]SPD) solubilized from canine caudate and to define the effect of detergent on the binding of the radioligand. Extraction of canine caudate with 0.75-1.0% digitonin was found to generate the maximum yield of binding sites for [3H]SPD while minimizing extraction of membrane proteins. Although binding sites were solubilized with 1.0% digitonin, a 10-fold reduction in detergent concentration was necessary to achieve maximal binding of [3H]SPD. The rank order of affinity for agonists and antagonists was consistent with the pharmacologic properties of the D2 subtype of the dopamine receptor. However, the binding of antagonists was found to be complex. Studies with some preparations of pooled canine caudate resulted in competition curves for the D2-selective antagonists domperidone and sulpiride that best fit a single-site model. Other preparations exhibited biphasic inhibition curves with these antagonists. The class of binding sites for [3H]SPD with low affinity for D2-selective antagonists constituted as much as 30-40% of the binding sites. Enrichment of solubilized binding sites for [3H]SPD was achieved by size exclusion HPLC followed by adsorption to DEAE-Sephadex and elution with buffer of increasing ionic strength. Enrichment of binding sites was accompanied by a decrease in the affinity of solubilized sites for [3H]SPD.