Partial agonist actions at dopamine D2L receptors are modified by co-transfection of D3 receptors: Potential role of heterodimer formation

The First Negative Allosteric Modulator for Dopamine D2 and D3 Receptors, SB269652 May Lead to a New Generation of Antipsychotic Drugs

D₂ and D₃ dopamine receptors belong to the largest family of cell surface proteins in eukaryotes, the G protein-coupled receptors (GPCRs). Considering their crucial physiologic functions and their relatively accessible cellular locations, GPCRs represent one of the most important classes of therapeutic targets. Until recently, the only strategy to develop drugs regulating GPCR activity was through the identification of compounds that directly acted on the orthosteric sites for endogenous ligands. However, many efforts have recently been made to identify small molecules that are able to interact with allosteric sites. These sites are less well-conserved, therefore allosteric ligands have greater selectivity on the specific receptor. Strikingly, the use of allosteric modulators can provide specific advantages, such as an increased selectivity for GPCR subunits and the ability to introduce specific beneficial therapeutic effects without disrupting the integrity of complex physiologically regulated networks. In 2010, our group unexpectedly found that N-[(1r,4r)-4-[2-(7-cyano-1,2,3,4-tetrahydroisoquinolin-2-yl)ethyl]cyclohexyl]-1H-indole-2-carboxamide (SB269652), a compound supposed to interact with the orthosteric binding site of dopamine receptors, was actually a negative allosteric modulator of D₂- and D₃-receptor dimers, thus identifying the first allosteric small molecule acting on these important therapeutic targets. This review addresses the progress in understanding the molecular mechanisms of interaction between the negative modulator SB269652 and D₂ and D₃ dopamine receptor monomers and dimers, and surveys the prospects for developing new dopamine receptor allosteric drugs with SB269652 as the leading compound.

Trace amine-associated receptor 1 activation silences GSK3β signaling of TAAR1 and D2R heteromers

Trace amine-associated receptor 1 (TAAR1) activation by selective endogenous agonists modulates dopaminergic neurotransmission. This results in antipsychotic-like behavior in vivo which might be initiated by an interaction of TAAR1 and dopamine D2L receptor (D2R). Here we analyzed the functional link between TAAR1 and D2R using highly potent and selective TAAR1 agonists, and newly generated tools such as TAAR1 knock-out and TAAR1 overexpressing rats as well as specific anti-rat TAAR1 antibodies. We provide data from co-immunoprecipitation experiments supporting a functional interaction of the two receptors in heterologous cells and in brain tissue. Interaction of TAAR1 with D2R altered the subcellular localization of TAAR1 and increased D2R agonist binding affinity. Using specific β-arrestin 2 (βArr2) complementation assays we show that the interaction of TAAR1 with D2R reduced βArr2 recruitment to D2R. In addition, we report that besides Gαs-protein signaling TAAR1 also signals via βArr2. In the presence of D2R, cAMP signaling of TAAR1 was reduced while its βArr2 signaling was enhanced, resulting in reduced GSK3β activation. These results demonstrate that βArr2 signaling may be an important pathway for TAAR1 function and that the activation of the TAAR1-D2R complex negatively modulates GSK3β signaling. Given that patients with schizophrenia or bipolar disorder show increased GSK3β signaling, such a reduction of GSK3β signaling triggered by the interaction of D2R with activated TAAR1 further supports TAAR1 as a target for the treatment of psychiatric disorders.

Pharmacological modulation of abnormal involuntary DOI-induced head twitch response movements in male DBA/2J mice: II. Effects of D3 dopamine receptor selective compounds

We recently reported on the characterization of the hallucinogen 2,5-dimethoxy-4-methylamphetamine's (DOI) ability to elicit a head twitch response (HTR) in DBA/2J mice and the ability of D2 vs. D3 dopamine receptor selective compounds to modulate that response. For these studies, the ability of D3 vs. D2 dopamine receptor selective compounds to attenuate the DOI-dependent HTR was examined. WC 10, a D3 dopamine receptor weak partial agonist with 40-fold binding selectivity for D3 vs. D2 dopamine receptors, produced a dose-dependent decrease in the DOI-induced HTR (IC50 = 3.7 mg/kg). WC 44, a D3 receptor selective full agonist, also inhibited the DOI-induced HTR (IC50 = 5.1 mg/kg). The effect of two D3 receptor selective partial agonists, LAX-4-136 and WW-III-55, were also evaluated. These analogs exhibit 150-fold and 800-fold D3 vs. D2 binding selectivity, respectively. Both compounds inhibited the HTR with similar potency but with different maximum efficacies. At 10 mg/kg WW-III-55 inhibited the HTR by 95%, while LAX-4-136 administration resulted in a 50% reduction. In addition, DOI (5 mg/kg) was administered at various times after LAX-4-136 or WW-III-55 administration to compare the duration of action. The homopiperazine analog LAX-4-136 exhibited greater stability. An assessment of our test compounds on motor performance and coordination was performed using a rotarod test. None of the D3 dopamine receptor selective compounds significantly altered latency to fall, suggesting that these compounds a) did not attenuate the DOI-dependent HTR due to sedative or adverse motor effects and b) may have antipsychotic/antihallucinogenic activity.

Functional homomers and heteromers of dopamine D2L and D3 receptors co-exist at the cell surface

Human dopamine D(2long) and D(3) receptors were modified by N-terminal addition of SNAP or CLIP forms of O(6)-alkylguanine-DNA-alkyltransferase plus a peptide epitope tag. Cells able to express each of these four constructs only upon addition of an antibiotic were established and used to confirm regulated and inducible control of expression, the specificity of SNAP and CLIP tag covalent labeling reagents, and based on homogenous time-resolved fluorescence resonance energy transfer, the presence of cell surface D(2long) and D(3) receptor homomers. Following constitutive expression of reciprocal constructs, potentially capable of forming and reporting the presence of cell surface D(2long)-D(3) heteromers, individual clones were assessed for levels of expression of the constitutively expressed protomer. This was unaffected by induction of the partner protomer and the level of expression of the partner required to generate detectable cell surface D(2long)-D(3) heteromers was defined. Such homomers and heteromers were found to co-exist and using a reconstitution of function approach both homomers and heteromers of D(2long) and D(3) receptors were shown to be functional, potentially via trans-activation of associated G protein. These studies demonstrate the ability of dopamine D(2long) and D(3) receptors to form both homomers and heteromers, and show that in cells expressing each subtype a complex mixture of homomers and heteromers co-exists at steady state. These data are of potential importance both to disorders in which D(2long) and D(3) receptors are implicated, like schizophrenia and Parkinson disease, and also to drugs exerting their actions via these sites.

N-(4-(4-(2,3-dichloro- or 2-methoxyphenyl)piperazin-1-yl)butyl)heterobiarylcarboxamides with functionalized linking chains as high affinity and enantioselective D3 receptor antagonists

In the present report, the D3 receptor pharmacophore is modified in the 2,3-diCl- and 2-OCH(3)-phenylpiperazine class of compounds with the goal to improve D3 receptor affinity and selectivity. This extension of structure-activity relationships (SAR) has resulted in the identification of the first enantioselective D3 antagonists (R- and S-22) to be reported, wherein enantioselectivity is more pronounced at D3 than at D2, and that a binding region on the second extracellular loop (E2) may play a role in both enantioselectivity and D3 receptor selectivity. Moreover, we have discovered some of the most D3-selective compounds reported to date that show high affinity (K(i) = 1 nM) for D3 and approximately 400-fold selectivity over the D2 receptor subtype. Several of these analogues showed exquisite selectivity for D3 receptors over >60 other receptors, further underscoring their value as in vivo research tools. These lead compounds also have appropriate physical characteristics for in vivo exploration and therefore will be useful in determining how intrinsic activity at D3 receptors tested in vitro is related to behaviors in animal models of addiction and other neuropsychiatric disorders.