Dopamine D-2 receptors in rat caudate-putamen: the lateral to medial gradient does not correspond to dopaminergic innervation.

The topography of dopamine D-2 receptors within the rat caudate-putamen (CPU) was characterized by measuring [3H]spiroperidol binding to membranes from dissected regions of the CPU and to thin sections of rat forebrain using quantitative autoradiography. Specific binding, defined using 1 microM (+)-butaclamol, was considerably higher in the lateral CPU than the medial CPU, without any obvious gradient in the dorsoventral axis. The difference in Bmax between the lateral and medial CPU was similar whether determined by digital subtraction autoradiography of coronal forebrain sections or by [3H]spiroperidol binding to membrane homogenates derived from CPU subregions. No regional differences were observed in the affinity of [3H]spiroperidol for D-2 sites. There were no lateral-to-medial differences in the dopaminergic innervation of the CPU, as determined by the measurement of the levels of dopamine, homovanillic acid and dihydroxyphenylacetic acid in subregions of the CPU using high pressure liquid chromatography. Thus, regional variations in the density of D-2 receptors within the CPU do not correspond to the degree of dopaminergic innervation. The functional importance of the organization of this dopamine receptor system is discussed in relation to the lateral-to-medial organization of the CPU.