LOCALIZATION AND CHARACTERIZATION OF DOPAMINE-D-4 BINDING-SITES IN RAT AND HUMAN BRAIN BY USE OF THE NOVEL, D-4-RECEPTOR-SELECTIVE LIGAND [H-3] NGD-94-1 .2.

The dopamine D-4 selective ligand, [H-3]NGD 94-1, was used in these st udies to characterize binding sites in rat and human brain tissue by m embrane binding and autoradiography techniques. Autoradiographic analy sis of rat brain showed that specific [H-3]NGD 94-1 binding was greate st in entorhinal cortex, lateral septal nucleus, hippocampus and the m edial pre-optic area of the hypothalamus. This nonstriatal distributio n of [H-3]NGD 94-1 binding was distinct from the autoradiographic dist ribution of dopamine D-2 and D-3 receptor subtypes. In homogenate prep arations from rat brain regions, [H-3]NGD 94-1 binding sites were low in density (<30.0 fmol/mg protein). The low density of D-4 binding sit es was corroborated by autoradiographic comparisons in which binding d ensity for D-4 receptors as measured by [H-3]NGD 94-1 was only 1/7 of D-2 and 1/5 of D-3 receptor densities, despite corrections for differi ng radioligand binding characteristics. Pharmacological evaluation sho wed high affinity at rat [H-3]NGD 94-1 binding sites for compounds wit h known D-4 receptor affinity and little displacement by compounds wit h affinity for dopamine D-1/D-2/D-3 receptor subtypes. Specific, high- affinity [H-3]NGD 94-1 binding was also present in several human brain regions, including hippocampus, hypothalamus, dorsal medial thalamus, entorhinal cortex, prefrontal cortex and lateral septal nucleus. High -affinity [H-3]NGD 94-1 binding was not present in any human striatal region examined. The pharmacological profile of [H-3]NGD 94-1 binding sites in human brain was consistent with that previously demonstrated for cloned human D-4 receptors expressed in mammalian cells. These fin dings suggest that specific, high-affinity [H-3]NGD 94-1 binding exist s in rat and human brain and that these sites reflect populations of d opamine D-4 receptors with a distribution unique among dopamine recept or subtypes.