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Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Dopamine D₂ receptor partial agonists represent a new generation of atypical antipsychotics. Cariprazine, which has received centralized market authorization from the European Medicines Agency in 2017 for the treatment of adult patients with schizophrenia (including those with predominant negative symptoms of schizophrenia) differs from the other two partial agonist antipsychotics aripiprazole and brexpiprazole due to its unique features. Cariprazine is a dopamine D₃ preferring D₃/D₂ partial agonist with very similar dopamine receptor subtype selectivity as dopamine. It has proven efficacy in the treatment of positive and negative symptoms of schizophrenia, as well as for relapse prevention. Further phase-3 clinical studies proved the efficacy of cariprazine in the acute treatment of manic or mixed episodes associated with bipolar I disorder, as well as in bipolar depression. For the adjunctive treatment of major depressive disorder, phase 3 studies are in progress.

A novel series of arylsulfonamides was prepared either by automated parallel or by traditional solution-phase synthesis. Several members of this compound library were identified as high-affinity dopamine D3 and D2 receptor ligands. The most interesting representative, compound 2, showed potent antipsychotic behaviour coupled with a beneficial cognitive and EPS profile.

The dopamine-D3 receptor is of special interest due to its postulated role in the pathophysiology and treatment of schizophrenia and Parkinson's Disease. Increasing evidences support the assumption that the D3 receptors are occupied to a high degree by dopamine at physiological conditions. Research on the functional role of the D3 receptors in brain has however been hampered by the lack of D3 selective ligands. In the present Positron Emission Tomography (PET) study the binding of the novel, putative dopamine-D3 receptor ligand, [11C]RGH-1756 was characterized in the cynomolgus monkey brain. [11C]RGH-1756 was rather homogenously distributed in brain and the regional binding potential (BP) values ranged between 0.17 and 0.48. Pretreatment with unlabelled RGH-1756 decreased radioligand binding to the level of the cerebellum in most brain areas. The regional BP values were lower after intravenous injection of a higher mass of RGH-1756, indicating saturable binding of [11C]RGH-1756. The D2/D3 antagonist raclopride partly inhibited the binding of [11C]RGH-1756 in several brain areas, including the striatum, mesencephalon and neocortex, whereas the 5HT(1A) antagonist WAY-100635 had no evident effect on [11C]RGH-1756 binding. Despite the promising binding characteristics of RGH-1756 in vitro the present PET-study indicates that [11C]RGH-1756 provides a low signal for specific binding to the D3 receptor in vivo. One explanation is that the favorable binding characteristics of RGH-1756 in vitro are not manifested in vivo. Alternatively, the results may support the hypothesis that the dopamine-D3 receptors are indeed occupied to a high extent by dopamine in vivo and thus not available for radioligand binding.

Remarkable advances during the past few years have sparked new interest in the molecular identification of receptor types and in the spatiotemporal regulation of their expression. Because dopamine (DA) receptors have been implicated in major neuropsychiatric illnesses, such as schizophrenia and affective disorders, both of which have significant heritable components, new research is now aimed at testing hypotheses which deal with the genetic regulation of DA receptor expression and with linkage of such expression to behavioral disorders. The purpose of this article, which is not intended to be an exhaustive review, is to examine the findings of two decades of genetic studies on DA receptor expression in terms of radioligand binding and inbred mouse and rat strains, as well as to discuss some of the pitfalls in past research and new strategies in current work.

Pharmacological treatment of schizophrenia has been traditionally dominated by dopamine D2 receptor antagonists, known to cause severe extrapyramidal side effects, which can be attributed to the blockade of D2 receptors in the striatum. Current antipsychotic research focuses on compounds with multireceptorial activity (different dopamine receptor subtypes, serotonine and adrenerg and muscarinic receptors has been sudied). Recent observations indicate, that control of both dopaminergic and serotoninergic systems is important for adequate antipsychotic therapy. It has been reported, that 5-HT1A receptor agonists reverse antipsychotic induced catalepsy.