3H]Dihydroergocryptine binding to bovine striatal membranes defined by a low d-butaclamol concentration : Antagonism by substituted benzamides

Abstract

An apparent dopamine-D2 receptor binding system was characterized using [ 3H]dihydroergocryptine (DHE) as the ligand. Specific binding to bovine striatal membranes was defined as the difference between binding in the absence and in the presence of 4 nM d-butaclamol. These high affinity binding sites ( K d = 0.24 nM) were saturable ( B max = 75 fmoles/mg protein), apparently homogeneous, non-interacting, and stereospecific. Specific binding was greatest in areas rich in dopaminergic innervation. In competition experiments, apomorphine ( ic 50 = 100 nM) was fifty times more potent than dopamine which, in turn, was nine times more potent than l-norepinephrine and serotonin. Antipsychotic drugs including the substituted benzamides—metoclopramide, sulpiride and sultopride—were active in competing with [ 3H]DHE for these binding sites ( ic 50 = 100 nM ), and their affinities agreed well with their anti-dopaminergic potencies in vivo. Potencies of antipsychotic drugs in competing for DHE binding sites were increased compared to their potencies in competing for [ 3H]spiroperidol binding sites using 1 μM d-butaclamol as the masking ligand. This relative increase was about 10-fold greater for the substituted benzamides than for other antipsychotic drugs. The relative inactivity of substituted benzamides in competing for binding sites labeled by spiroperidol was accounted for by their greater selectivity for dopamine receptors. In binding studies using a non-selective radio-ligand, selectivity could be enhanced by defining specific binding sites with a low concentration of masking ligand.