Distribution of dopamine D3 receptor expressing neurons in the human forebrain: comparison with D2 receptor expressing neurons.

The dopamine D2 and D3 receptors are members of the D2 subfamily that includes the D2, D3 and D4 receptor. In the rat, the D3 receptor exhibits a distribution restricted to mesolimbic regions with little overlap with the D2 receptor. Receptor binding and nonisotopic in situ hybridization were used to study the distribution of the D3 receptors and neurons positive for D3 mRNA in comparison to the D2 receptor/mRNA in subcortical regions of the human brain. D2 binding sites were detected in all brain areas studied, with the highest concentration found in the striatum followed by the nucleus accumbens, external segment of the globus pallidus, substantia nigra and ventral tegmental area, medial preoptic area and tuberomammillary nucleus of the hypothalamus. In most areas the presence of D2 receptor sites coincided with the presence of neurons positive for its mRNA. D3 binding sites and D3 mRNA positive neurons were most abundant in the limbic striatum and efferent structures, such as the nucleus accumbens, ventral striatum, substantia nigra, internal segment of the globus pallidus, anteroventral nucleus of the thalamus, and rostral pars reticulata of the substantia nigra. One important difference from the rat is that D3 receptors were virtually absent in the ventral tegmental area. D3 receptor and D3 mRNA positive neurons were observed in sensory, hormonal, and association regions such as the nucleus basalis, anteroventral, mediodorsal, and geniculate nuclei of the thalamus, mammillary nuclei, the basolateral, basomedial, and cortical nuclei of the amygdala. As revealed by simultaneous labeling for D3 and D2 mRNA, D3 mRNA was often expressed in D2 mRNA positive neurons. Neurons that solely expressed D2 mRNA were numerous and regionally widespread, whereas only occasional D3-positive-D2-negative cells were observed. The regions of relatively higher expression of the D3 receptor and its mRNA appeared linked through functional circuits, but co-expression of D2 and D3 mRNA suggests a functional convergence in many regions of the signals mediated by the two receptor subtypes.