3H]Spiroperidol Binding in Rat Striatum: Two High‐Affinity Sites of Differing Selectivities

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Abstract
[3H]Spiroperidol binding to homogenates of rat striatum is saturable and shows monophasic or biphasic saturation isotherms under specified conditions. In poorly washed membrane fragment preparations, saturation isotherms of [3H]spiroperidol binding are monophasic, revealing an apparently homogeneous set of sites with Kd 0.6 .+-. 0.3 nM and density 440 .+-. 80 f[femto]mol/mg protein. Equilibrium displacement studies of [3H]spiroperidol binding at this site indicate an .alpha.-adrenergic component in addition to the previously described dopaminergic component. In thoroughly washed membrane fragment preparations, saturation isotherms are clearly biphasic, showing an additional high-affinity site with an approximate Kd of 24 .+-. 10 pM and an approximate density of 110 .+-. 20 fmol/mg protein at a protein concentration of 2.0 mg/ml. Selectivity at this site appears classically dopaminergic; apparently the lower affinity site is the primary source of the .alpha.-adrenergic component of spiroperidol binding.