Abstract

The dopamine D₄ selective ligand, [³H]NGD 94–1, was used in these studies to characterize binding sites in rat and human brain tissue by membrane binding and autoradiography techniques. Autoradiographic analysis of rat brain showed that specific [³H]NGD 94–1 binding was greatest in entorhinal cortex, lateral septal nucleus, hippocampus and the medial preoptic area of the hypothalamus. This nonstriatal distribution of [³H]NGD 94–1 binding was distinct from the autoradiographic distribution of dopamine D₂ and D₃ receptor subtypes. In homogenate preparations from rat brain regions, [³H]NGD 94–1 binding sites were low in density (<30.0 fmol/mg protein). The low density of D₄binding sites was corroborated by autoradiographic comparisons in which binding density for D₄ receptors as measured by [³H]NGD 94–1 was only 1/7 of D₂ and 1/5 of D₃ receptor densities, despite corrections for differing radioligand binding characteristics. Pharmacological evaluation showed high affinity at rat [³H]NGD 94–1 binding sites for compounds with known D₄ receptor affinity and little displacement by compounds with affinity for dopamine D₁/D₂/D₃receptor subtypes. Specific, high-affinity [³H]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 [³H]NGD 94–1 binding was not present in any human striatal region examined. The pharmacological profile of [³H]NGD 94–1 binding sites in human brain was consistent with that previously demonstrated for cloned human D₄ receptors expressed in mammalian cells. These findings suggest that specific, high-affinity [³H]NGD 94–1 binding exists in rat and human brain and that these sites reflect populations of dopamine D₄ receptors with a distribution unique among dopamine receptor subtypes.