The Dopamine D3 Receptor as an Emerging Therapeutic Target in Parkinson’s Disease: Structural Advances, Signaling Bias and Neuroprotective Perspectives

Abstract

The dopamine D₃ receptor (D₃R) has long been considered a secondary target in the treatment of Parkinson’s disease (PD), with therapeutic strategies primarily focused on D₂ receptor–mediated motor control. However, accumulating evidence now supports D₃R as a functionally distinct dopaminergic receptor subtype with specific relevance to non-motor symptom domains and dopaminergic signaling under hypodopaminergic conditions. Recent advances in high-resolution structural biology have elucidated the molecular basis of D₃R/D₂R discrimination, revealing how subtle residue-level and microstructural differences within a conserved G protein–coupled receptor framework shape ligand recognition and receptor activation. In parallel, the emergence of ligand-dependent biased signaling has refined current understanding of D₃R pharmacology. Selected ligands can preferentially engage Gα_i/o-mediated pathways while limiting β-arrestin recruitment and associated regulatory processes, providing a mechanistic rationale for more stable modulation of mesolimbic dopaminergic circuits involved in affective and motivational regulation. Beyond symptomatic modulation, preclinical studies suggest that D₃R signaling may influence neuronal resilience, synaptic plasticity, and adaptive responses to dopaminergic injury; however, such effects remain experimental and have not been demonstrated in clinical PD. This review integrates recent structural, signaling, and functional insights into D₃R biology, with particular emphasis on biased agonism and emerging therapeutic concepts. Although D₃R-targeted strategies do not currently represent disease-modifying interventions, they offer a rational framework for the development of next-generation dopaminergic therapies aimed at improving precision, tolerability, and long-term signaling stability in Parkinson’s disease.

1. Introduction

Dopamine D₂ and D₃ receptors belong to the D₂-like subfamily of G protein-coupled receptors (GPCRs), which primarily couple to Gα_i/o proteins and regulate a broad range of motor, motivational, and affective functions [1]. Structurally, these receptors share a high degree of homology, with approximately 78% amino acid identity across their transmembrane domains [2,3]. Despite this close structural similarity, the dopamine D₂ receptor (D₂R) and D₃ receptor (D₃R) exhibit distinct pharmacological properties and functional roles, differences that are particularly relevant to the pathophysiology of Parkinson’s disease (PD) [4,5].

One of the most prominent distinctions between D₂R and D₃R lies in their affinity for dopamine. The D₃R displays a substantially higher affinity than the D₂R, with in vitro studies reporting differences ranging from 10- to 100-fold, depending on the experimental system and ligand context [1]. In recombinant cellular models, such as HEK-293 cells, dopamine affinity at D₃R can be up to two orders of magnitude greater than that observed at D₂R [3,6]. Complementary in vivo approaches, including microdialysis and receptor-binding studies, further indicate that D₃R can be activated by very low dopamine concentrations within the basal extracellular range, estimated at approximately 3–5 nM [7]. Consistent with this observation, the reported K_i of dopamine for D₃R is around 30 nM, placing it well within physiological synaptic concentrations [8]. In contrast, D₂R exhibits a significantly lower affinity for dopamine, particularly when present in its G protein–uncoupled state [9].

These affinity differences acquire relevance in the context of PD. As dopaminergic tone progressively declines during disease progression, the high-affinity D₃R is expected to remain functionally engaged even under conditions of pronounced dopamine depletion, while D₂R-mediated signaling becomes impaired at earlier stages [10,11,12]. This divergence provides a mechanistic framework to explain not only the early deterioration of D₂R-dependent motor functions but also the selective preservation or dysregulation of processes mediated by D₃R-enriched circuits [5].

Beyond pharmacological considerations, the functional dissociation between D₂R and D₃R is further shaped by their distinct neuroanatomical distribution. While D₂R are broadly expressed throughout the dorsal striatum and are closely associated with motor control [13], D₃R expression is preferentially enriched in mesolimbic regions, including the nucleus accumbens, amygdala, olfactory tubercle, and islands of Calleja, areas critically involved in motivation, emotional regulation, and affective behavior [14]. Consequently, the relative preservation or selective dysfunction of D₃R-mediated signaling may contribute to the emergence of non-motor symptoms in PD, such as depression, anxiety, anhedonia, and motivational deficits, which frequently precede or accompany the classical motor manifestations of the disease [15,16,17].

Against this background, interest in the dopamine D₃R as a therapeutic target has re-emerged, driven not only by its involvement in non-motor symptomatology but also by recent advances that have refined its pharmacological and mechanistic characterization. The availability of high-resolution structural data, including X-ray crystallography and cryo-electron microscopy studies of D₃R in inactive and active Gα_i-coupled conformations, has provided a molecular framework to understand subtype selectivity and receptor activation beyond global architectural similarities [3,18]. In parallel, increasing recognition of ligand-dependent biased signaling has reshaped the conceptual landscape of D₃R pharmacology, demonstrating that selective ligands can stabilize receptor conformations that preferentially engage Gα_i/o-mediated pathways while limiting alternative signaling routes. This property offers a potential strategy to dissociate therapeutic efficacy from adverse effects associated with non-selective dopaminergic stimulation [19,20].

Moreover, accumulating preclinical evidence suggests that D₃R-mediated signaling may influence processes related to neuronal resilience, neuroinflammatory modulation, and synaptic plasticity. Although these effects remain experimental, they extend the relevance of D₃R beyond symptomatic modulation and support continued investigation of D₃R-centered strategies within a mechanistic and translational framework [10,21,22].

Collectively, these developments reposition the D₃R as a strategically relevant target in PD, situated at the intersection of structural biology, receptor signaling, and circuit-level dysfunction. Here, we propose that the D₃R should be viewed as a precision modulatory node for non-motor domains in PD, whose therapeutic value lies in the quality and controllability of dopaminergic signaling rather than in maximal dopaminergic drive. In this review, we integrate recent structural, pharmacological, and functional advances related to the dopamine D₃R, with particular emphasis on biased signaling and neuroprotective perspectives, in order to critically evaluate its therapeutic potential within the comprehensive management of Parkinson’s disease.

2. D3 Receptor Structure and Selective Ligand Recognition

As a class A G protein–coupled receptor (GPCR), the D₃R adopts the canonical seven-transmembrane (7TM) helical architecture and exhibits high sequence homology with the dopamine D₂ receptor (D₂R), particularly across the transmembrane segments that form the orthosteric ligand-binding pocket [2]. This extensive structural conservation has long posed a significant challenge for the development of truly subtype-selective ligands, as small-molecule recognition is largely governed by subtle microenvironmental differences within an otherwise highly conserved binding cavity [3,23,24]. This limitation is particularly relevant in Parkinson’s disease, where pharmacological agents targeting D₂/D₃ receptors frequently produce overlapping therapeutic effects and adverse outcomes due to insufficient receptor subtype discrimination [25,26].

For many years, the absence of high-resolution structural information limited mechanistic insight into the molecular determinants governing D₂/D₃ selectivity and receptor activation [27,28]. This landscape began to shift in 2010, when the first X-ray crystal structure of the human D₃R was solved in an inactive-state conformation stabilized by the antagonist eticlopride [3]. This landmark structure provided a detailed map of the D₃R orthosteric pocket and revealed a principle that has since guided rational design: despite broad overall similarity to D₂R, discrete residue-level differences and local packing variations within the 7TM bundle can yield disproportionate effects on ligand affinity and subtype preference [3]. Subsequent inactive-state structures of D₂R enabled direct receptor-to-receptor comparisons and reinforced the idea that D₂/D₃ selectivity is not driven by global architectural divergence but rather by microstructural and dynamic differences within and above the orthosteric site that shape ligand entry, pose, and stabilization [6]. These structural principles are illustrated in Figure 1, which compares inactive and active conformations of D₂R and D₃R and highlights localized differences within the orthosteric binding pocket and intracellular domains.

Although highly informative, inactive-state structures provide only partial insight into PD-relevant pharmacology, particularly when the goal is to understand how chemically distinct agonists can elicit divergent functional outcomes [29,30]. In this regard, the advent of cryo-electron microscopy (cryo-EM) has been transformative by enabling visualization of active-state conformations of D₂R and D₃R in complex with Gi proteins [6,18,31,32]. These active-state assemblies have clarified conserved hallmarks of GPCR activation, most notably the outward rearrangement of transmembrane helix 6, coordinated movements of TM5 and TM7, and the formation of an intracellular cavity permissive for G-protein engagement [29]. At the same time, they have delineated receptor-specific structural features that may contribute to the high apparent sensitivity of D₃R to low dopaminergic tone, providing a structural framework to interpret its functional behavior under hypodopaminergic conditions characteristic of Parkinson’s disease [1,18].

Recent cryo-EM studies have further expanded this structural framework. Structural genomics analyses have resolved active-state conformations of all five human dopamine receptor subtypes bound to the clinically used agonist rotigotine, revealing conserved activation mechanisms together with receptor-specific determinants that shape ligand recognition and G-protein coupling selectivity across the dopaminergic receptor family [31]. In parallel, high-resolution cryo-EM structures of the human D₃ receptor in complex with selective bitopic agonists have uncovered an additional selectivity interface involving the TM2–ECL1–TM1 region, which expands the classical secondary binding pocket and provides new opportunities for the rational design of subtype-selective dopaminergic ligands [32]. Together, these advances highlight how subtle extracellular and vestibular structural features can influence ligand binding modes and functional selectivity at D₃R.

A central interpretive challenge remains the disentanglement of conformational changes that are intrinsic to receptor activation from those imposed by ligand chemistry, a limitation that has become increasingly evident with the expansion of active-state GPCR structures [29,30]. In practice, the structural differences observed between inactive and active complexes likely reflect a composite of (i) a conserved activation trajectory (for example, the canonical outward movement of TM6 and reorganization of intracellular motifs enabling Gα_i coupling) and (ii) ligand-dependent stabilization of distinct microstates within the binding pocket and extracellular vestibule [33,34]. This distinction is particularly relevant for D₂R and D₃R because active-state structures have been captured with agonists that are chemically heterogeneous (e.g., pramipexole-like and other scaffolds), whereas inactive-state structures often involve antagonists or inverse agonists with fundamentally different pharmacophores [3,6,18]. Thus, apparent “state differences” may conflate activation-linked rearrangements with ligand-specific remodeling of local interaction networks, an overlap that likely underpins the functional diversity observed across D₃-directed chemotypes [19,30].

From a therapeutic perspective in PD, these structural advances are more than descriptive. The availability of both inactive and active conformations of the dopamine D₃ receptor provides a molecular framework to rationalize how subtype-selective ligands can exploit subtle yet functionally consequential differences between D₃R and D₂R, despite their high overall structural similarity [3,18]. Notably, these data also support structure-guided strategies aimed at functional selectivity, including the design of ligands that preferentially stabilize signaling-competent conformations associated with favorable intracellular coupling profiles [30,34]. Moreover, they provide a foundation for the development of bitopic ligands that engage the orthosteric site while extending into secondary recognition regions, such as the extracellular vestibule, thereby improving subtype discrimination and enabling finer control over receptor microstates [32,35]. Collectively, advances in structural biology have repositioned D₃R from a difficult-to-separate receptor subtype into a tractable target for precision pharmacology, with direct implications for the rational design of therapies that maximize benefit, particularly within non-motor domains, while mitigating liabilities associated with broader D2R engagement [36,37].

3. D3 Receptor Signaling: Gi/o Pathways, β-Arrestin Modulation and Biased Agonism

The D₃R predominantly couples to Gα_i/o heterotrimeric G proteins, engaging a canonical signaling cascade characterized by inhibition of adenylyl cyclase (AC) and a consequent reduction in intracellular cAMP levels. This pathway underlies the classical modulatory role of D₃R in neuronal excitability and dopaminergic tone [1,38]. However, recent studies indicate that D₃R exhibits a more restricted coupling profile within the Gi/o/z family than previously appreciated [39,40,41]. In contrast to the D₂ receptor (D₂R), which shows robust coupling to multiple Gi/o subtypes, D₃R displays limited functional interaction with the Gα_i1, Gα_i2, and Gα_i3 proteins and preferentially engages the Gα_o and Gα_z isoforms. Structural and mutational analyses suggest that this selectivity arises from specific molecular determinants located at the interface between the receptor intracellular loop 2 (ICL2) and the α5 helix of the G protein [39]. In particular, steric constraints and electrostatic interactions involving residues such as Asp350 in Gαi and Thr142 in the D₃R ICL2 region may hinder productive coupling with certain Gi proteins, thereby shaping receptor–G protein selectivity [40,41].

These findings highlight that D₃R signaling cannot be fully explained by a uniform Gi/o-mediated mechanism. Instead, D₃R exhibits a context- and ligand-dependent engagement of intracellular pathways, a feature that contributes to its functional divergence from the closely related D₂ receptor and provides a mechanistic basis for differential dopaminergic signaling outcomes [19,30]. This selective coupling profile may also contribute to the signaling bias observed for several D₃R ligands, further supporting the concept of functionally selective dopaminergic pharmacology.

Compared with D₂R, activation of D₃R is associated with a more selective and nuanced modulation of intracellular signaling cascades, including extracellular signal-regulated kinase (ERK) and protein kinase B (Akt) pathways. Importantly, activation of these effectors appears to depend less on robust, receptor-wide signaling and more on cellular context, receptor microenvironment, and ligand-specific conformational states [20,34]. These characteristics suggest that D₃R operates as a finely tunable signaling node rather than a binary on-off switch, a property that may be particularly relevant in neurodegenerative conditions such as PD [37].

A distinctive functional hallmark of D₃R is its relatively low propensity for desensitization and internalization following prolonged stimulation, especially when compared with D₂R [19,42]. This behavior implies a reduced reliance on β-arrestin-dependent regulatory mechanisms and results in more sustained signaling under chronic activation conditions [19]. Such signaling stability is especially relevant in the context of long-term dopaminergic therapies, where receptor desensitization, tolerance, and loss of efficacy contribute substantially to adverse clinical outcomes [43,44].

Within this framework, the concept of biased signaling has emerged as a central paradigm for understanding D₃R pharmacology. Biased agonism refers to the ability of a ligand to preferentially activate specific intracellular signaling pathways through a single receptor, rather than uniformly engaging all available effectors [30,38]. In the case of D₃R, several ligands have been shown to stabilize receptor conformations that favor Gα_i/o-mediated signaling while minimally recruiting β-arrestins. This property provides a mechanistic basis to dissociate therapeutically desirable effects from signaling processes associated with receptor desensitization, tolerance, and adverse outcomes [19,20].

From a mechanistic perspective, Gα_i/o-biased D₃R signaling involves the stabilization of active receptor conformations that promote efficient G-protein coupling while constraining β-arrestin engagement [19,45]. Activation of Gα_i/o leads to suppression of adenylyl cyclase (AC) activity, reduced intracellular cAMP accumulation, and downstream attenuation of protein kinase A (PKA) signaling. This signaling axis intersects with the differential engagement of MAPK/ERK and PI3K–Akt pathways, which have been implicated in neuronal survival, cellular stress resilience, and the maintenance of synaptic plasticity [9,14,46]. In parallel, D₃R-mediated Gα_i/o signaling modulates neuronal excitability through inhibition of voltage-gated calcium channels (N- and P/Q-type) and activation of G protein-gated inwardly rectifying potassium (GIRK) channels, thereby exerting fine control over neuronal firing patterns and mesolimbic dopaminergic tone [9,14].

Conversely, limited recruitment of β-arrestins attenuates receptor internalization and downstream regulatory processes, dampens pro-inflammatory signaling, and reduces maladaptive compensatory responses, collectively favoring a more stable and sustained functional output during chronic stimulation. In this context, functionally biased agonists [45], including bitopic ligands and experimental compounds such as SK608, have demonstrated pharmacological profiles consistent with preferential Gα_i/o signaling, reduced tolerance liability, and engagement of pathways associated with neuroprotection and synaptic maintenance [19]. These ligand-dependent differences in signaling quality are schematically illustrated in Figure 2.

In Parkinson’s disease, these signaling properties acquire direct translational relevance. Biased activation of D₃R may contribute to the modulation of non-motor symptoms, including affective and motivational disturbances, while potentially enabling a more refined influence on motor circuits with a reduced risk of desensitization and off-target effects associated with broad dopaminergic stimulation [18,32]. In aggregate, biased D₃R signaling emerges as a promising pharmacological framework to enhance therapeutic efficacy while improving tolerability, supporting its consideration as a component of next-generation dopaminergic strategies in the comprehensive management of PD.

4. Functional Role of D3 Receptor in Parkinson’s Disease

PD is classically defined by nigrostriatal dopaminergic degeneration and the ensuing disruption of basal ganglia circuitry. Yet, the clinical phenotype extends well beyond motor impairment, encompassing prominent non-motor symptoms, including depression, anxiety, apathy, anhedonia and motivational deficits, that frequently precede, accompany, or outlast fluctuations in motor control [47,48,49]. Within this broader clinical framework, the D₃R occupies a distinctive functional niche. Its preferential enrichment in mesolimbic and associative circuits, together with its high affinity for dopamine, positions D₃R as a key modulator of affective–motivational processing under conditions of reduced dopaminergic tone [10,14,50].

In contrast to the D₂R, which is widely expressed across the dorsal striatum and tightly coupled to motor output, D₃R expression is biased toward ventral striatal and limbic nodes, including the nucleus accumbens, ventral pallidum, amygdala and related mesolimbic structures [14,27,50]. These regions are critically involved in reward valuation, reinforcement learning, goal-directed behavior, and emotional regulation, functional domains that are disproportionately affected in PD, particularly within its non-motor symptom spectrum [47,48]. From a mechanistic standpoint, progressive dopaminergic depletion is expected to differentially reshape signaling dynamics within these circuits. Because D₃R displays high agonist affinity and remains responsive at low extracellular dopamine concentrations, it may function as a sensitive gain controller of mesolimbic dopaminergic tone, thereby influencing behavioral outputs even under hypodopaminergic conditions [1,51]. This framework provides a biologically plausible substrate for the selective vulnerability and, in some cases, the early emergence of non-motor neuropsychiatric features in PD.

Consistent with this circuit logic, experimental evidence across genetic, pharmacological, and lesion-based models supports an involvement of D₃R in affective and motivational phenotypes. Reduced D₃R function, achieved through receptor inhibition or loss-of-function paradigms, has been associated with anxiety-like and depressive-like behaviors, whereas preferential D₃R activation can produce antidepressant- and anxiolytic-like effects in selected experimental settings [52,53]. Although such findings cannot be directly equated to clinical efficacy, they reinforce the concept that D₃R participates in the regulatory architecture of mood and motivation under hypodopaminergic conditions.

A defining aspect of PD management is the requirement for long-term dopaminergic replacement and/or dopaminomimetic therapy, which is frequently complicated over time by wearing-off phenomena, motor fluctuations, and variability in treatment response [54,55]. Within this therapeutic landscape, D₃R is functionally relevant in two complementary ways. First, D₃R shows a comparatively lower propensity for agonist-driven internalization and desensitization than D₂R, suggesting that D₃R-mediated signaling may remain more stable during chronic stimulation, an attractive property given the time-dependent waning of therapeutic response observed with long-term dopaminergic treatment [56,57]. Second, D₃R is closely linked to mesolimbic reward circuitry, and dopaminergic agonists with high D₂/D₃ affinity, particularly those displaying relative D₃ preference, are consistently implicated in reinforcement-related behavioral plasticity and impulse control disorders (ICDs) in PD. This association supports the view that pharmacological engagement of D₃R-enriched circuits may influence not only symptom control but also reinforcement learning and impulse regulation [58,59,60].

This dual role becomes particularly evident when considering ICDs and related behavioral syndromes, which represent well-recognized complications of dopamine agonist therapy in PD and are thought to reflect, at least in part, altered signaling within mesolimbic reward circuits [60,61,62]. Given the preferential expression of the dopamine D₃ receptor (D₃R) in ventral striatal and limbic regions, its pharmacological engagement has been proposed as a mechanistic link between the intended therapeutic modulation of motivational deficits and the unintended amplification of maladaptive reward-driven behaviors [14,58]. Accordingly, the functional role of D₃R in Parkinson’s disease is best interpreted within a precision pharmacology framework, in which the same receptor node that may support improvements in affective-motivational domains can also contribute to vulnerability for behavioral adverse effects, depending on ligand pharmacological properties, dosing regimens, and patient-specific circuit states [59,62].

The emergence of biased signaling further refines how D₃R function can be conceptualized in PD. Rather than treating D₃R activation as a single pharmacological output, ligand-dependent stabilization of distinct receptor conformations implies that D₃R engagement may be tuned toward Gα_i/o-dominant signaling with limited β-arrestin recruitment. Such bias may reduce receptor regulatory processes that contribute to tolerance and variability under chronic stimulation [30]. Functionally, this signaling profile could support a more predictable modulation of limbic dopaminergic tone while limiting pathways associated with desensitization and broader network adaptation [19,63]. Importantly, this framework does not imply guaranteed clinical benefit; instead, it provides a mechanistic rationale for why next-generation D₃R ligands, particularly those engineered for functional selectivity, may better align therapeutic intent with tolerability [14,37].

5. Therapeutic Potential: From Agonists to Biased Ligands and Neuroprotective Concepts

Advances in the structural and functional characterization of the D₃R have enabled a more precise appraisal of pharmacological strategies that are both plausible and mechanistically justified for therapeutic exploration in PD. This progress has been driven in part by the integration of structural biology with modern signaling frameworks, which increasingly support structure-guided and pathway-aware ligand design [10,37]. Rather than positioning D₃R as a primary target for motor symptom control, converging evidence indicates that its principal therapeutic value may lie in the selective modulation of dopaminergic circuits involved in non-motor symptomatology, including affective and motivational domains linked to mesolimbic function and dopamine agonist exposure [14,64]. In parallel, contemporary D₃R-focused syntheses and PD-specific reviews discuss emerging preclinical evidence that D₃R signaling may engage cellular programs relevant to neuronal resilience and plasticity under hypodopaminergic conditions, framing these concepts as mechanistic opportunities rather than established disease-modifying effects [10,43,65].

D₃-preferring dopamine agonists represent the most established pharmacological approach to engage D₃R in PD. Compounds such as pramipexole, ropinirole, and rotigotine, while not strictly subtype-selective, display a higher relative affinity for D₃R compared with D₂R and are widely used in clinical practice [66,67]. Beyond their effects on motor symptoms, these agents have demonstrated efficacy in alleviating non-motor manifestations, including depression, apathy, and motivational disturbances, which are increasingly recognized as core components of the PD phenotype [47,68]. In particular, pramipexole has been consistently shown to exert antidepressant and anxiolytic effects in PD patients, effects that cannot be fully explained by improvements in motor function alone and are also recapitulated in preclinical models of depression and dopaminergic dysfunction [69,70]. These observations are mechanistically congruent with the preferential mesolimbic distribution of D₃R and with its high agonist affinity, which enables sustained receptor engagement under conditions of diminished dopaminergic tone, a defining feature of PD progression [1,50].

Beyond clinically established D₂/D₃-preferring agonists, several experimental D₃R-preferring or partial agonists have also been evaluated in preclinical models of Parkinson’s disease. Among these, the D₃R-preferring agonist PD-128907 and the more recently developed D₃R partial agonist PD13R have provided important proof-of-concept evidence for D₃R-directed pharmacology. In 6-hydroxydopamine (6-OHDA) lesions, PD-128907 was reported to reverse motivational deficits and improve affective-like behavioral alterations, suggesting a role for D₃R activation in domains related to motivation and reward processing [16]. However, studies in levodopa-primed MPTP nonhuman primates indicated that although PD-128907 produced antiparkinsonian motor effects comparable to apomorphine, it did not significantly attenuate levodopa-induced dyskinesia. These observations highlight that D₃R-preferring agonism alone does not necessarily translate into improved control of motor complications [71]. More recently, the D₃R partial agonist PD13R has expanded this experimental framework. In a nonhuman primate model of Parkinson’s disease, PD13R suppressed the expression of levodopa-induced dyskinesia while maintaining antiparkinsonian efficacy, suggesting that more selective or functionally differentiated D₃R ligands may offer advantages over earlier D₃R-preferring compounds [72]. Collectively, these experimental studies broaden the therapeutic landscape of D₃R-targeted pharmacology and illustrate that the translational potential of these ligands depends not only on receptor preference but also on ligand efficacy profile, model context, and the symptom domain being evaluated.

Despite these benefits, the long-term clinical use of D₂/D₃ agonists is accompanied by important limitations. Chronic dopaminergic stimulation may lead to fluctuating therapeutic responses and behavioral complications, including impulse control disorders and reward-related behavioral dysregulation. These effects are thought to arise, at least in part, from excessive stimulation of mesolimbic dopaminergic circuits enriched in D₃ receptors. Consequently, while currently available D₂/D₃ agonists demonstrate that dopaminergic modulation of D₃R-containing circuits can provide meaningful symptomatic benefit, their clinical profile also highlights the need for more selective pharmacological strategies capable of preserving therapeutic effects while minimizing adverse behavioral outcomes.

These considerations raise an important translational question: whether highly selective D₃R-targeted ligands or broader D₂/D₃ agonists are better suited to address the combined motor and non-motor symptom burden of Parkinson’s disease. Selective modulation of D₃R, particularly through biased agonists, may offer greater precision for affective and motivational domains, consistent with the preferential enrichment of D₃R within mesolimbic circuits involved in reward processing and motivational behavior [14,50]. However, pharmacological agents with combined D₂/D₃ activity may retain a practical advantage when both motor and non-motor manifestations need to be controlled simultaneously, because they engage limbic circuits enriched in D₃R while also maintaining D₂-mediated signaling within striatal pathways that are more directly linked to motor regulation. This broader receptor engagement may help explain the clinical profile of widely used dopamine agonists such as pramipexole, ropinirole, and rotigotine, which improve motor symptoms while also alleviating selected non-motor manifestations of Parkinson’s disease [66,67].

Within this framework, strategies centered on D₃R selectivity are better viewed not as universal replacements for conventional D₂/D₃ agonists, but rather as candidates for more targeted therapeutic applications, particularly when modulation of limbic dopaminergic signaling and stabilization of non-motor symptoms are prioritized. For comparison, pharmacological approaches targeting D₁-like receptors have also regained interest in Parkinson’s disease. Agonists and partial agonists acting on D₁/D₅ receptors can provide robust motor benefit through facilitation of the striatonigral pathway and restoration of direct pathway signaling [64]. However, current clinical development of D₁-directed agents has focused primarily on motor outcomes, and their potential relevance for affective or motivational symptoms remains less clearly defined than that proposed for D₃R- or D₂/D₃-directed strategies.

In this context, the recognition that D₃R signaling can be functionally biased has catalyzed interest in ligands engineered to preferentially engage Gα_i/o-mediated pathways while minimizing β-arrestin recruitment. From a mechanistic standpoint, this strategy aims to preserve signaling axes associated with therapeutic benefit, such as modulation of affective and motivational circuits and stabilization of dopaminergic tone, while attenuating receptor desensitization, tolerance development, and signaling pathways linked to adverse outcomes [14,63].

Although biased D₃R agonists remain largely in preclinical or early experimental stages, compounds such as SK608, as well as emerging bitopic ligands, have provided proof-of-concept that functional selectivity at D₃R is achievable. Experimental pharmacological studies demonstrate that these ligands can stabilize receptor conformations preferentially coupled to Gα_i/o signaling, with reduced engagement of regulatory pathways associated with receptor desensitization. This profile offers a rational framework to improve the predictability and durability of dopaminergic signaling [5,14,19]. Importantly, biased agonism does not imply enhanced efficacy per se, but rather greater control over signaling quality, an attribute that may be particularly valuable in the context of long-term PD management [46].

Beyond symptomatic modulation, a body of preclinical evidence suggests that D₃R activation may influence neuronal survival, plasticity, and repair-related processes. Seminal studies led by Joyce and colleagues demonstrated that D₃R-preferring agonists attenuate dopaminergic neuron loss, promote axonal sprouting, and improve functional recovery in toxin-based models of PD, including 6-OHDA paradigms [65,73]. Subsequent experimental work has implicated D₃R signaling in the regulation of oxidative stress responses, neurotrophic factor pathways, and synaptic remodeling, further supporting a role for this receptor in adaptive responses to dopaminergic injury [10,43,74,75].

Crucially, these findings should be interpreted with appropriate caution. At present, neuroprotective effects associated with D₃R activation remain mechanistic and experimental, and there is insufficient evidence to support disease-modifying claims in clinical PD. Nevertheless, the consistency of these observations across model systems supports the view that D₃R may participate in adaptive responses to dopaminergic injury, warranting continued investigation within carefully designed translational frameworks.

6. Future Directions and Outstanding Questions

Despite substantial progress in elucidating the structural, signaling, and functional properties of the D₃R, several key questions remain that will determine its translational relevance in PD. Addressing these gaps will require coordinated advances across structural biology, pharmacology, and clinical neuroscience.

A central challenge is the development of ligands with improved subtype selectivity and functional precision. Although structural data have clarified determinants of D₃/D₂ discrimination, translating these insights into compounds with robust receptor selectivity, favorable pharmacokinetic properties, and adequate central nervous system penetration remains a non-trivial task. In parallel, the field would benefit from standardized experimental frameworks to quantify biased signaling at D₃R, enabling meaningful comparisons across ligands, cellular contexts, and experimental systems.

Another unresolved issue concerns the circuit-specific roles of D₃R in PD. While its preferential enrichment within mesolimbic regions supports involvement in non-motor symptom domains, the extent to which D₃R signaling contributes to interactions between limbic and motor networks, particularly under conditions of chronic dopaminergic therapy, remains incompletely understood. Integrating receptor-level insights with circuit-resolved approaches, including advanced neuroimaging, electrophysiology, and human-derived cellular models, will be essential to clarify these dynamics.

From a translational perspective, there is a critical need for biomarkers capable of reporting D₃R engagement in vivo. Further refinement of D₃-selective positron emission tomography (PET) ligands and complementary pharmacodynamic readouts would facilitate target validation, dose optimization, and patient stratification in early-stage clinical studies. Such tools are especially important given the heterogeneity of PD phenotypes and the variable expression and progression of non-motor symptoms.

Finally, the putative neuroprotective and neurorestorative roles of D₃R warrant cautious but systematic evaluation. Although preclinical findings are intriguing, future work must rigorously define the conditions under which D₃R signaling influences neuronal resilience, distinguish direct receptor-mediated effects from secondary network-level adaptations, and establish whether these mechanisms can be meaningfully harnessed in human disease.

7. Conclusions

Overall, recent advances in structural biology and receptor pharmacology have refined the understanding of the D₃R as a functionally distinct component of the dopaminergic system in PD. Rather than acting as a primary mediator of motor control, D₃R emerges as a receptor strategically positioned to modulate mesolimbic circuits, non-motor symptom domains, and signaling dynamics under conditions of dopaminergic depletion. High-resolution structural insights, together with growing recognition of ligand-dependent signaling bias, have provided a mechanistic basis to reconsider how D₃R may be pharmacologically engaged with greater precision.

While currently available clinical agents only partially exploit these properties, concepts such as functional selectivity and biased agonism offer a rational framework to refine dopaminergic interventions and improve long-term signaling stability. Importantly, evidence supporting neuroprotective or disease-modifying roles of D₃R remains preliminary and largely confined to preclinical settings. At present, the most compelling value of D3R lies in its potential to support selective and modulable dopaminergic signaling, particularly in relation to non-motor features of PD.