[3H]SCH 23390 binding to D1 dopamine receptors in the basal ganglia of the cat and primate: delineation of striosomal compartments and pallidal and nigral subdivisions.

The distribution of D1 dopamine receptors was studied autoradiographically in the basal ganglia of the cat, monkey and human. These receptor binding sites were labeled directly with the D1-selective antagonist [3H]SCH 23390, and ligand-binding assays were performed concurrently. Serial- or same-action analysis permitted comparisons among D1 binding distributions, acetylcholinesterase staining and tyrosine hydroxylase immunoreactivity. In all species studied, the dorsal striatum exhibited patches of particularly dense D1 binding in correspondence with acetylcholinesterase-poor striosomes. Highly patterned binding was present in the ventral striatum. Distinctions in binding density were observed among the subdivisions of the globus pallidus and of the substantia nigra. The external segment of the pallidum was extremely sparse in D1 binding, whereas the internal segment (or entopeduncular nucleus in the cat) was a site of high D1 binding density. The binding density was greatest in the core of the internal segment, and tyrosine hydroxylase-positive fibers surrounded and weakly dispersed themselves through this core. Weak binding was present in the ventral pallidum. In the substantia nigra, the pars reticulata demonstrated the densest binding, particularly medially. The pars compacta showed much sparser binding, though some of its tyrosine hydroxylase-positive neurons had dendrites extending ventrally into the zone of dense D1 binding in the pars reticulata. We conclude that [3H]SCH 23390-defined D1 binding is compartmentalized in the dorsal striatum and that, particularly in relation to the reported distributions of striatal D2 dopamine receptors, this is likely to be of functional significance in the dopaminergic modulation of intrastriatal neurotransmission as well as of afferent and efferent neurotransmission. The segregated localizations of D1 receptors in the substantia nigra suggest predominant activation of the pars reticulata, including ventral and medial regions adjacent to the densocellular zone. Specific pathways from compartments in the striatum to subdivisions of the pallidum may also be differentially modulated by dopamine acting via distinct receptor subtypes. At the level of the pallidum, such D1 modulation appears to be restricted to the internal segment, which projects to the thalamus, rather than to the external pallidum, which projects to the subthalamic nucleus.