Heterodimerization between D1 and D2 receptors is a recently discovered, novel mechanism by which D1 and D2 receptors activate Gq-mediated signaling in the brain. Although first met with skepticism, evidence for the existence of functional D1/D2 heterodimers under physiological conditions has become more and more convincing.

Heterodimerization between D1 and D2 receptors is linked to their coexpression in the same cell. The localization of D1 and D2 receptors has been extensively studied in the striatum. After D1 and D2 receptors were cloned 20 years ago, in situ hybridization studies suggested that there are two main populations of neurons in the striatum: one population that predominantly expresses D1 receptors and projects mono-synaptically to the substantia nigra (called the striato-nigral or direct pathway), and the other population that expresses D2 receptors and projects over several synapses to the substantia nigra (called the striato-pallidal or indirect pathway).

When these studies were followed up using single cell PCR and immunohistochemistry (IHC) using antibodies against D1 and D2 receptors, the percentage of neurons coexpressing both receptors increased to 15-30 percent, or even 100 percent for some IHC studies. One argument for the inconsistency between the early in situ hybridization studies and the newer studies had been that in situ hybridization may not be as sensitive as single cell PCR or IHC. However, the main problem with IHC is that different antibodies were used in different studies, and it is known that not all available antibodies against D2 receptors are really specific.

Recently, mice that express green fluorescent protein under the control of either the D1 or the D2 promoter have been developed that allow for selective labeling of D1- and D2-positive MSNs, respectively. The findings with D1- and D2-GFP mice are more in line with the original in situ hybridization studies showing relatively low overlap of expression in the dorsal striatum (5-7 percent of MSNs) and higher overlap in the ventral striatum (around 20 percent).

The problem of antibody specificity has also been a problem for studying heterodimers in the striatum. Therefore, in a recent study, Susan George's laboratory at the University of Toronto took great effort in testing the specificity of their antibodies (Perreault et al., 2010). One important control they included was knockout mice in which the D1 or the D2 receptor gene was inactivated. Immunohistochemistry for D1 and D2 did not show any signal in these mice, indicating specificity of the employed antibodies. Moreover, colocalization studies with these antibodies showed a degree of overlap that was comparable to what had been observed in the classical in situ hybridization studies and the recent studies using D1- and D2-GFP mice. Last, the authors used FRET technology and demonstrated coexpression at a spatial resolution that supports a direct interaction between both receptors in vivo.

The laboratory of Fang Liu, also at the University of Toronto, has now found that heterodimerization may be increased under pathological conditions. Using immunoprecipitation experiments, Pei et al. found increased coupling between D1Rs and D2Rs in the striatum and the cortex of patients with major depression (Pei et al., 2010). Perreault et al. found increased affinity for SKF83959, a heterodimer specific dopamine receptor agonist in the globus pallidus of patients with schizophrenia (Perreault et al., 2010). Since the globus pallidus is the main output structure of the indirect pathway of the striatum, it raises the question whether the degree of D1 and D2 receptor coexpression may be increased under pathological conditions

Obviously, both postmortem findings will need replication using higher subject numbers. Due to the confounding effects of postmortem tissue analysis and medication, PET imaging studies could greatly benefit this analysis. Imaging could be done earlier in the disease process and under drug-naïve conditions. The challenge here may be the development of appropriate tracers that are both suitable for PET imaging and specific for detecting heterodimers.

That SKF83959 selectively activates heterodimers raises the possibility for the development of selective antagonists. If increased heterodimers are indeed involved in the pathophysiology of depression and schizophrenia, they may be good targets for treating negative symptoms such as anhedonia and avolition that are associated with both disorders. They may also help against psychosis, though we would then expect that D1 receptor antagonists, which block heterodimer-mediated signaling, would be effective antipsychotics.

References:

Perreault ML, Hasbi A, Alijaniaram M, Fan T, Varghese G, Fletcher PJ, Seeman P, O'Dowd BF, George SR The dopamine D1-D2 receptor heteromer localizes in dynorphin/enkephalin neurons: increased high affinity state following amphetamine and in schizophrenia. J Biol Chem 285:36625-36634. Abstract

Pei L, Li S, Wang M, Diwan M, Anisman H, Fletcher PJ, Nobrega JN, Liu F Uncoupling the dopamine D1-D2 receptor complex exerts antidepressant-like effects. Nat Med 16:1393-1395. Abstract

View all comments by Christoph Kellendonk

Christoph nicely summarized key aspects of the paper in the context of the relevant literature. In addition, I feel the paper makes an important contribution because it draws attention to a signaling mechanism that may help explain some of the more contentious effects of dopamine.

View all comments by Jeremy Seamans