Distribution of D1 and D2 dopamine receptors in the basal ganglia of the cat determined by quantitative autoradiography

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Abstract
The patterns of dopamine D1 and D2 receptors were examined in the corpus striatum and related structures in the cat brain by quantitative autoradiography after in vitro radioligand binding with [3H]SCH23390 (D1 antagonist) and [3H]spiperone (D2 antagonist). Highly specific binding for both radioligands occurs in striatal structures known to receive dopaminergic input: the caudate nucleus, putamen, nucleus accumbens, and olfactory tubercle. However, the density of binding varies from one structure to another, and the density distribution within striatal nuclei is heterogeneous. In all but one portion of the striatum, the concentration of bound D1 radioligand ranges from 46 to 230% more than that of the D2 radioligand. The exception to this difference occurs at caudal putamenal levels where the two radioligands bind in equal concentrations. (≈︁ 220 fmol/mg tissue wet-weight). The highest density of both D1 and D2 radioligand binding occurs in irregular zones in the head and body of the caudate nucleus. Such high-density zones of D2 radioligand binding appear mainly in the dorsolateral part of the caudate's head. For the D1 radioligand, the high-density zones are more widespread throughout the caudate nucleus, nucleus accumbens, and putamen. The D2 radioligand binding (but not the D1) also exhibits low-density zones at more caudal levels of the caudate nucleus, and these are often in register with the high-density zones of D1 radioligand binding. In the putamen, inverted concentration gradients, exist for the two radioligands: the [3H]SCH23390 gradient runs from higher levels rostrally to lower levels caudally. The lowest levels of bound [3H]spiperone in the striatum occur in the nucleus accumbens-olfactory tubercle area, whereas the lowest binding of [3H]SCH23390 occurs in the caudal putamen. Pallidal and nigral structures show marked disparities in binding of the two different radioligands. The D2 radioligand binding in the globus pallidus (80 ± 8 fmol/mg tissue wet-weight) is about twice that in the entopeduncular nucleus and pars reticulata of the substantia nigra, the latter two having equal levels (35 ± 3 fmol/mg). No specific binding of the D2 radioligand occurs in the ventral pallidum. In contrast, D1 radioligand binding is highest in the entopeduncular nucleus (217 ± 6 fmol/mg) and in the pars reticulata of the substantia nigra (198 ± 2 fmol/mg) and moderate in the ventral pallidum (135 ± 15 fmol/mg). In the globus pallidus, no detectable D1 radioligand binding occurs. The binding density of [3H]SCH23390 is comparable in both subdivisions of the substantia nigra, but [3H]spiperone binding is four times greater in the pars compacta (154 ± 12 fmol/mg) than in the pars reticulata. The heterogeneity of dopamine receptors is discussed with respect to other evidence for compartmentation in the striatum and the neurochemical organization of the striatopallidal projection system.