[3H]SCH 23390 identifies D-1 binding sites in rat striatum and other brain areas

The specific in-vitro binding of [3H]SCH 23390 has been characterized and its use in the identification of D-1 sites in various brain regions examined. At a single ligand concentration (0.4 nM) the specific binding of [3H]SCH 23390 to striatal membranes was routinely 98% of total binding as defined using 10(-5) M cis-flupenthixol. Specific binding at 37 degrees C reached equilibrium at 15 min and was reversible with a t1/2 for dissociation of 14 min. Specific binding of [3H]SCH 23390 over a range of concentrations (0.01-3.5 nM) was saturable (Bmax 73 pmol g tissue-1) of high affinity (Kd 0.36 nM) and to a single population of binding sites. Specifically bound [3H]SCH 23390 (0.4 M) was stereo selectively displaced by the isomers of butaclamol and flupenthixol but not by the D-2 selective antagonist, sulpiride. 5-HT, noradrenaline and cinanserin caused little or no displacement. Specific binding of [3H]SCH 22390 (0.4 nM; as defined using 10(-5) M cis-flupenthixol) showed marked regional variation. Specific binding was highest in the striatum; high levels were also observed in the mesolimbic area and substantia nigra. Lower specific binding was found in the frontal cortex and superior colliculus with the lowest levels in cerebellar preparations. The inclusion of 3 X 10(-7) M cinanserin did not alter the extent of specific binding observed in any brain region. The properties of [3H]SCH 23390 suggest it to be an excellent ligand for identification of D-1 sites in a variety of brain regions.