The distribution and function of neurons coexpressing the dopamine D1 and D2 receptors in the basal ganglia and mesolimbic system are unknown. We found a subset of medium spiny neurons coexpressing D1 and D2 receptors in varying densities throughout the basal ganglia, with the highest incidence in nucleus accumbens and globus pallidus and the lowest incidence in caudate putamen. These receptors formed D1-D2 receptor heteromers that were localized to cell bodies and presynaptic terminals. In rats, selective activation of D1-D2 heteromers increased grooming behavior and attenuated AMPA receptor GluR1 phosphorylation by calcium/calmodulin kinase II␣ in nucleus accumbens, implying a role in reward pathways. D1-D2 heteromer sensitivity and functional activity was up-regulated in rat striatum by chronic amphetamine treatment and in globus pallidus from schizophrenia patients, indicating that the dopamine D1-D2 heteromer may contribute to psychopathologies of drug abuse, schizophrenia, or other disorders involving elevated dopamine transmission. Dopaminergic signaling within basal ganglia occurs within at least two populations of GABAergic medium spiny neurons (MSNs), 2 containing the neuropeptides dynorphin (DYN) and substance P, or containing enkephalin (ENK) (1, 2). There is controversy as to the colocalization of dopamine D1 and D2 receptors (D1R, D2R) within these two neuronal subtypes. It is generally agreed that whereas D1R is largely segregated to the DYN/substance P-expressing direct striatonigral pathway, D2R is predominantly localized to the ENK-expressing indirect striatopallidal pathway, which is supported by recent studies using fluorophore-tagged promoter elements of D1R and D2R in bacterial artificial chromosome transgenic mice to quantify the cells expressing the receptors within these pathways (3, 4). Although strict segregation of D1R and D2R has been suggested (5, 6) it was noted in these studies that although ϳ60% of MSNs in nucleus accumbens (NAc) expressed D1R, ϳ50% expressed D2R. This indicated a certain fraction of NAc MSNs expressed both receptors, a finding consistent with numerous studies showing colocalization of D1R and D2R in a proportion of striatal neurons (7-11).In keeping with these findings, we have shown that D1R and D2R interacted in striatum to form a D1-D2 heteromeric complex that could be immunoprecipitated (9, 12). The D1-D2 heteromer was distinct from its constituent receptors in that it coupled to Gq/11 to activate phospholipase C and generate intracellular calcium release, representing a novel signaling pathway directly linking dopamine action to calcium (9, 13). However, despite growing evidence that neurons coexpressing D1R and D2R embody a significant fraction of MSNs in NAc, their regional distribution, phenotypic characterization, and functional role within basal ganglia are entirely unknown. Given the apparent relationship between dopamine receptor expression and DYN or ENK content, we hypothesized that neurons expressing the D1-D2 heteromer would contain both DYN and ...

We provided evidence for the formation of a novel phospholipase C-mediated calcium signal arising from coactivation of D1 and D2 dopamine receptors. In the present study, robust fluorescence resonance energy transfer showed that these receptors exist in close proximity indicative of D1-D2 receptor heterooligomerization. The close proximity of these receptors within the heterooligomer allowed for cross-phosphorylation of the D2 receptor by selective activation of the D1 receptor. D1-D2 receptor heterooligomers were internalized when the receptors were coactivated by dopamine or either receptor was singly activated by the D1-selective agonist (Ϯ)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide (SKF 81297) or the D2-selective agonist quinpirole. The D2 receptor expressed alone did not internalize after activation by quinpirole except when coexpressed with the D1 receptor. D1-D2 receptor heterooligomerization resulted in an altered level of steady-state cell surface expression compared with D1 and D2 homooligomers, with increased D2 and decreased D1 receptor cell surface density. Together, these results demonstrated that D1 and D2 receptors formed heterooligomeric units with unique cell surface localization, internalization, and transactivation properties that are distinct from that of D1 and D2 receptor homooligomers.The dopamine receptor family is subdivided into two distinct subclasses, based on structural similarities, pharmacological profiles, and signal transduction mechanisms, into D1-and D2-like receptors. Although D1 and D2 receptor subclasses are biochemically distinct in that D1 receptors couple positively and the D2 receptors couple negatively to adenylyl cyclase, many physiological functions are known to be mediated by the coactivation of both receptors. For example, the augmentative effect of cocaine on locomotion and intracranial self-stimulation is mediated by the activation of both D1 and D2 receptors (Kita et al., 1999). Dopamine receptor synergism could occur either at the level of neuronal networks through D1-and D2-like receptors expressed in separate neuronal populations or, on the other hand, within the same neurons through convergent postreceptor mechanisms. The latter mechanism of D1-D2 receptor synergism is possible because dopamine receptor subclasses are colocalized in rat brain, with colocalization of D1-and D2-like receptors in virtually every neuron in the neonatal striatum (Aizman et al., 2000). Furthermore, our own studies have demonstrated robust colocalization of D1 and D2 receptors in a subset of neurons in human caudate nucleus, rat striatum, and cortex . These data suggest, therefore, that functional synergism could occur within individual neurons. In fact, the coactivation of both D1 and D2 receptors has been shown to result in a significant increase in action potential frequency in neurons of the substantia nigra pars reticulata (Waszczak et al., 2002) and a potentiation of arachidonic acid release in Chinese hamster ovary cells coexpre...

In basal ganglia a significant subset of GABAergic medium spiny neurons (MSNs) coexpress D1 and D2 receptors (D1R and D2R) along with the neuropeptides dynorphin (DYN) and enkephalin (ENK). These coexpressing neurons have been recently shown to have a region-specific distribution throughout the mesolimbic and basal ganglia circuits. While the functional relevance of these MSNs remains relatively unexplored, they have been shown to exhibit the unique property of expressing the dopamine D1–D2 receptor heteromer, a novel receptor complex with distinct pharmacology and cell signaling properties. Here we showed that MSNs coexpressing the D1R and D2R also exhibited a dual GABA/glutamate phenotype. Activation of the D1R–D2R heteromer in these neurons resulted in the simultaneous, but differential regulation of proteins involved in GABA and glutamate production or vesicular uptake in the nucleus accumbens (NAc), ventral tegmental area (VTA), caudate putamen and substantia nigra (SN). Additionally, activation of the D1R–D2R heteromer in NAc shell, but not NAc core, differentially altered protein expression in VTA and SN, regions rich in dopamine cell bodies. The identification of a MSN with dual inhibitory and excitatory intrinsic functions provides new insights into the neuroanatomy of the basal ganglia and demonstrates a novel source of glutamate in this circuit. Furthermore, the demonstration of a dopamine receptor complex with the potential to differentially regulate the expression of proteins directly involved in GABAergic inhibitory or glutamatergic excitatory activation in VTA and SN may potentially provide new insights into the regulation of dopamine neuron activity. This could have broad implications in understanding how dysregulation of neurotransmission within basal ganglia contributes to dopamine neuronal dysfunction.