Dopamine, Immunity, and Disease

1. Innate Immune Cells

The innate immune response is considered the first line of defense against invading pathogens and mediates rapid, nonspecific inflammatory responses. In innate cells, particularly myeloid cells, granulocytes, NK cells, and DCs, these responses are generally initiated by the exposure of extracellular and intracellular pattern recognition receptors (PRRs) to various stimuli associated with pathogens or other insults. The four main families of PRRs are toll-like receptors (TLRs), nucleotide-binding oligomerization domain-like receptors (NLRs), C-type lectin receptors, and RIG-1 like receptors (Janeway and Medzhitov, 2002; Takeuchi and Akira, 2010; Pinoli et al., 2017). TLRs are membrane proteins that are localized on endosomes and mediate extracellular recognition of pathogens, whereas NLRs are cytosolic proteins that recognize intracellular pathogens (Janeway and Medzhitov, 2002; Franchi et al., 2009; Kumar et al., 2011). There are currently 10 known functional TLRs in humans and 12 in mice, while there are 22 human NLRs (Kawai and Akira, 2008; Takeuchi and Akira, 2010). Formyl peptide receptors and scavenger receptors are also PRRs, as they bind N-formyl peptides produced by bacterial degradation and acetylated or oxidized low-density lipoproteins, respectively (Janeway and Medzhitov, 2002; Takeuchi and Akira, 2010; Pinoli et al., 2017).

PRRs act primarily by recognizing a variety of molecules known as pathogen-associated molecular patterns (PAMPs) or damage-associated molecular patterns (DAMPs), which signal the presence of danger. Examples of PAMPs and DAMPs include LPS, which is a component of Gram-negative bacterial cell walls; single- or double stranded RNA, which is associated with viral infection; β-glucans, which are components of fungal cell walls; and immunostimulants, such as polyinosinic:polycytidylic acid, which mimics activation caused by viral RNA. The expression and activation of PRRs on innate immune cells mediates coordinated effector responses upon contact with an invading pathogen (Janeway and Medzhitov, 2002; Li and Wu, 2021). Dopamine receptors and other dopamine-related proteins have been detected on most of these cells, suggesting the potential for broad effects of dopamine on many branches of the innate immune system (Pinoli et al., 2017).

b. Granulocytes

There has been relatively little research on the impact of dopamine on the four types of granulocytes: neutrophils, basophils, eosinophils, and mast cells. Dopamine has been shown to mediate functional changes in neutrophils, eosinophils, and mast cells (Fig. 4) with little current research on basophils. The majority of the research has been studied in neutrophils (Pinoli et al., 2017). These cells are relatively short-lived granulocytes derived from the bone marrow that quickly move into the blood. Neutrophils are the most abundant granulocyte in circulation (40%–70% of blood leukocytes) and are often the first line of host defense, inhibiting or eliminating invading pathogens through the generation of cytokines, ROS, neutrophil extracellular traps, and other factors. Recent studies have shown that neutrophils play a critical role in communication between the innate and adaptive immune response through direct interactions or cytokine signaling (Rosales, 2020). The expression of both D1-like and D2-like receptors on human neutrophils has been confirmed at both the mRNA and protein levels (Cosentino et al., 1999; Sookhai et al., 1999; McKenna et al., 2002; Pereira et al., 2003; Chen et al., 2014).

• Download figure
• Open in new tab
• Download powerpoint

Fig. 4

Dopamine receptor signaling in granulocytes. Current knowledge of the immunomodulatory effects of dopamine signaling in neutrophils, eosinophils, and mast cells is summarized. In neutrophils, dopamine acts through its receptors to increase formation of extracellular traps and to decrease NO production and transendothelial migration, while having reported bidirectional effects on neutrophil phagocytosis. Dopamine signaling in eosinophils has been shown to affect eosinophil counts with high doses of dopamine leading to eosinopenia and low doses of dopamine leading to eosinophilia. In mast cells, dopamine acting at D1-like receptors has been shown to increase degranulation while D2-like receptor stimulation decreases cytokine production. Created with BioRender.com.

Dopamine regulates neutrophil activity, as dopamine treatment (2.61 × 10⁻⁷ M) attenuated the expression of CD62l (L-selectin) and CD11b in neutrophils stimulated with N-formyl-methionyl-leucyl-phenylalanine, a potent neutrophil activator that mimics the actions of factors released by bacteria. This effect was not seen in resting neutrophils or in response to lower dopamine concentrations (2.61 × 10⁻¹⁰M) (Trabold et al., 2007). The downregulation of L-selectin and CD11b could substantially interfere with adherence to and rolling on activated endothelium, which is a primary function of these cells. Higher levels of dopamine (10⁻⁴M–10⁻⁵M) increased apoptosis in human neutrophils (Sookhai et al., 1999; Aslan et al., 2011), and the dopamine receptor antagonists chlorpromazine and pimozide blocked the increases in neutrophil counts associated with exposure to ovalbumin peptide in rats. In contrast, the dopamine receptor agonist apomorphine increased neutrophil numbers (Altenburg et al., 1995), suggesting that dopamine receptor activity could regulate neutrophil numbers and function and that higher levels of dopamine reduced neutrophil numbers, while lower levels may increase them.

Eosinophils are another granulocyte subset that are much less common than neutrophils (2%–3% of blood leukocytes) that have a key role in defense against helminth infection, injured tissue repair, and allergic diseases. These cells are released into the circulation like neutrophils but generally reside in tissues, mediating their effects through the release of cytoplasmic granules, cytokines, chemokines, and growth factors (Aoki et al., 2021). Very few studies have examined the dopaminergic system in these cells, but they have been shown to express all five subsets of dopamine receptors on their surface (McKenna et al., 2002). Eosinophils also express TH and can synthesize and release dopamine. Dopamine synthesis is slightly increased following activation with IL-5 and eotaxin, and inhibiting eosinophil TH activity with alpha-methyl-p-tyrosine decreased murine vascular relaxation in vitro. This suggests that the release of dopamine or other catecholamines is important for eosinophil function (Withers et al., 2017). In rats, administration of low-dose L-DOPA or apomorphine increased blood eosinophil count, while higher doses of these agents decreased blood eosinophil count, suggesting that dopamine synthesis and dopamine receptor activity affects eosinophil viability (Podolec et al., 1979). The usefulness of these findings is limited by our understanding of the dopaminergic system in eosinophils and is an area in need of further study.

Mast cells are myeloid lineage cells that originate in the bone marrow, but unlike other granulocytes, these cells reside in regions that are exposed to the external environment, such as mucosal and epithelial tissues. Mast cells are also common in connective tissues, and the many types of mast cells depend on the tissue environment in which they reside. These cells are important effectors of host defense against bacteria, venomous toxins, and triggers of allergic responses and anaphylaxis. During the immune host response, these cells release factors that rapidly recruit other innate and adaptive immune cells, which creates a balanced response to infection. Mast cells contain granules that store a variety of inflammatory mediators including cytokines and chemokines such as IL-4, C-C motif chemokine ligand 2 (CCL2), C-C motif chemokine ligand 5, and TNF-α; growth factors such as TGF-β and vascular endothelial growth factor; proteoglycans, proteases, and other enzymes; and biogenic amines such as histamine, serotonin, and dopamine (DeBruin et al., 2015). Mast cells express some dopamine receptors, and several studies indicate that dopamine plays an important role in mast cell function. Human synovial mast cells from patients with rheumatoid arthritis (RA) express D3, although these receptors were only seen on a subset of mast cells (Xue, Li et al., 2018). mRNA for D1 and D5 receptors was found in bone marrow and fetal skin derived mast cells (Mori et al., 2013). Bone marrow-derived murine mast cells also express TH and store dopamine in granules via a process that seems to be dependent on the presence of serglycin, an intracellular proteoglycan. Degranulation and the release of intracellular dopamine may be triggered by intracellular Ca²⁺ flux, which is mediated by IgE crosslinking (Ronnberg et al., 2012). Other studies have confirmed dopamine storage in murine (Freeman et al., 2001) and bovine mast cells (Edvinsson et al., 1977).

The release of dopamine is important for mast cell function, as dopamine-mediated (10⁻⁹M–10⁻⁷M) activation of D1-like receptors dose-dependently increased murine mast cell degranulation. Antagonizing D1-like receptors with SCH23390 also reduced ear swelling caused by passive cutaneous anaphylaxis, which is a mast cell–mediated process (Mori et al., 2013). Although not all D2-like receptors are present on mast cells, treating the rat cell line RBL-2H3 with bromocriptine, 7-OH-DPAT, haloperidol, and clozapine resulted in potent dose-dependent inhibition of degranulation (Seol et al., 2004). These ligands all bind to D2-like dopamine receptors, including D3, suggesting that D2-like receptors may also affect mast cell activity, although this cell type is actually derived from basophils, and it is not clear if it is entirely representative of mast cell biology (Passante and Frankish, 2009). Other studies suggest that D3 activity may be anti-inflammatory in mast cells, as there is a negative correlation between the numbers of D3-expressing mast cells in synovial fluid and disease severity in RA (Xue, Li et al., 2018). Furthermore, studies using bone marrow–derived mast cells from Drd3 knockout mice showed that D3 regulated the inhibitory effects of methamphetamine on LPS-induced cytokine production in mast cells. As methamphetamine does not bind to dopamine receptors, these data suggest that methamphetamine facilitates dopamine release in these cells by activating an autocrine signaling pathway by which the mast cells activate D3 to block inflammation (Xue et al., 2016). Taken together, these findings suggest that D1-like receptors may drive inflammatory activity in mast cells, while D2-like receptors may act in an anti-inflammatory manner.

Basophils are the largest and least common type of granulocyte, making up less than 1% of blood leukocytes. Basophils are very similar to mast cells but are found in the circulation rather than in tissues. After entering the peripheral blood, basophils transmigrate to various organs, such as the spleen, lymph nodes, and other inflammatory sites, where they can be activated. Like all granulocytes, these cells regulate innate inflammation and specialize in fighting parasites and regulating allergic responses. Basophils store inflammatory mediators such as IL-4 and TNF-α, as well as histamine, releasing them upon IgE crosslinking with surface receptors (Schwartz et al., 2016). The expression of dopamine receptors and other dopamine-related proteins on basophils is largely unclear, although human basophils do express VMAT2 (Anlauf et al., 2006), suggesting these cells can store monoamines in cytoplasmic vesicles. While there are no specific data on the functional effects of dopamine in this population, treatment with very low doses of epinephrine reduced histamine release (Mannaioni et al., 2010), and serotonin exposure significantly decreased IL-4 release (Schneider et al., 2011). These findings suggest that monoamines can regulate basophil function and that dopaminergic modulation may occur in regions with sufficient dopamine. For example, murine basophil activation leads to the release of serine proteases such as mMCP-8 and mMCP-11, which enhance microvascular permeability, allowing T-cells and other immune cells to transmigrate to sites of inflammation (Miyake and Karasuyama, 2017; Yamanishi et al., 2017). Dopamine has a known role in vascular regulation (Bhattacharya et al., 2008) and has been detected in the spleen and lymph nodes, which basophils frequent (Matt and Gaskill, 2020), suggesting that these cells are involved in this process. These interactions are currently unclear because of the lack of research on dopamine signaling in basophils, preventing a more comprehensive understanding of the role of dopamine in diseases characterized by basophilia, such as allergy or myeloproliferative disorders.

d. Dendritic Cells

Dendritic cells (DCs) are professional antigen-presenting cells derived from both myeloid and lymphoid progenitors. The primary function of these cells is to migrate to secondary lymphoid organs and interact with T-cells to promote the differentiation of various effector T-cell subsets (Patente et al., 2019). Both human and rodent DCs express all dopamine receptor subtypes and other dopamine-related proteins such as MAO-A and -B, TH, and VMAT2. As in macrophages, dopamine-β-hydroxylase is not expressed in DCs, and interestingly, DAT was also not found, suggesting that these cells may not be able to take up dopamine from the extracellular space (Chen, Tsai et al., 2012; Prado et al., 2012; Figueroa et al., 2017; Arce-Sillas et al., 2019). Human DCs can produce dopamine de novo, and intracellular dopamine levels increase in response to synthetic L-DOPA. Dopamine production seems to be regulated by D2-mediated cAMP signaling that activates TH, as D2 blockade and treatment with forskolin, an activator for adenylate cyclase, increased stored dopamine. By inhibiting D2, dopamine signals predominately through D1-like receptors, which canonically stimulate the formation of cAMP (Beaulieu and Gainetdinov, 2011), increase phosphorylation of TH, and increase dopamine production (Nakano et al., 2009). Once produced, dopamine is stored in vesicles near the plasma membrane, which release dopamine. It is possible that the release acts in an autocrine fashion, binding to DC dopamine receptors to regulate proper dopamine levels, as dopamine is important in their role in antigen presentation and T-cell activation. These and other effects of dopamine on DC activity are summarized in Fig. 5.

• Download figure
• Open in new tab
• Download powerpoint

Fig. 5

Dopamine receptor signaling in T-cells dendritic cells. Current knowledge of dopaminergic immunomodulation in dendritic cells and T-cells. Dopamine receptor balance on dendritic cells appear to impact T-cell differentiation. Increased expression of D1, D2, and D3 receptors on dendritic cells induces a T_h1 phenotype in T-cells. Increased D1 expression with low D2 expression drives T_h2 phenotype while increased D1, D3, and D5 along with low D4 expression has been linked to T_h17 phenotype. Double arrows before receptor expression indicate a stronger correlation of the corresponding receptor on dendritic cells that drive T-cell differentiation. On activated CD4⁺ T-cells, dopamine stimulates D1-like receptors to increase IL-5 and IL-17 production and stimulates D2-like receptors leading to reduced IL-2, IL-4, and IFN-y production. On CD8⁺ T-cells, dopamine stimulation of D1-like receptors leads to decrease cytotoxicity and decreased immune suppressive activity. By acting on D2-like receptors on resting CD8⁺ T-cells, dopamine increases IL-10 and TNF-α production. Created with BioRender.com.

When DCs interact with CD4⁺ T-cells, dopamine is released and activates D1-like receptors to increase cAMP signaling, which regulates T-helper (T_h)1–T_h2 polarization (Nakano et al., 2009). However, D2 may also be involved in this process, as blocking D2-like receptors in human monocyte-derived DCs with risperidone but not haloperidol inhibited DC production of T_h1 cytokines (IL-6, IL-8, TNF-α). Blocking D2-like receptors also increased T_h2 cytokine (IL-10) production and reduced T_h1 polarization in cocultured T-cells (Chen, Tsai et al., 2012). Interestingly, haloperidol did affect the activity of murine bone marrow–derived DC, reducing the expression of MHC class II, CD80, and CD86, and decreasing the production of IL-12p40, an important factor for DC maturation (Matsumoto et al., 2015). This finding suggests species-specific differences in receptor crosstalk on murine and human DC, as both haloperidol and risperidone have strong affinities for D2-like receptors, but risperidone has much higher affinity for serotonin receptors.

Several studies from the Pacheco group indicate that the specific D1-like receptor that mediates DC–T-cell interactions is D5, as D5-mediated increases in cAMP increase the activation of signal transducer and activator of transcription and drive inflammation (Prado et al., 2018). The LPS-induced maturation of murine DCs decreased D5 expression, which impaired the activation and proliferation of antigen-specific CD4⁺ T-cells in vitro. Transplanting these D5-deficient DCs into a mouse showed a significant reduction in the percentage of T_h17 cells infiltrating the CNS compared with that in wild-type animals (Prado et al., 2012). In the experimental autoimmune encephalomyelitis (EAE) mouse model of multiple sclerosis (MS), adoptive transfer of D5-deficient DCs reduced the severity of EAE and the frequency of inflammatory CD4⁺ T-cell subsets in the CNS. Reserpine-mediated dopamine depletion in wild-type DCs prior to transfer into EAE animals also reduced clinical severity, indicating that autocrine dopamine activation of D5 on DCs mediated the inflammatory effects of dopamine in this system (Prado et al., 2018). Because SCH23390 also blocks D5 activity, these data are supported by a separate study in which the inhibition of D1-like receptors with SCH-23390 blocked DC-mediated T_h17 differentiation in EAE mice, although treatment with the D4-selective antagonist L750667 induced T_h17 differentiation (Nakano et al., 2008). Adoptive transfer of D3-deficient DCs into mice had no effect on CD4⁺ T-cell responses but increased antigen cross presentation and CD8⁺ T-cell activity against tumors (Figueroa et al., 2017). This finding indicates that different dopamine receptors have discrete roles in antigen presentation and T-cell activation-associated signaling pathways in DCs. Because changes in antigen presentation impact the ability of the adaptive immune system to create memory, defining the overlapping and discrete dopamine pathways by which DCs regulate T-cell responses could be of substantial value in the treatment of autoimmunity.

2. Adaptive Immune Cells

The primary cells that mediate adaptive immunity are T- and B-lymphocytes, which interact with the innate immune system to generate specific responses to invading pathogens, killing infected cells and generating specific antibodies against pathogens. A critical feature of adaptive immunity is immune memory, which enables the body to rapidly respond to repeat insults using factors that specifically target those pathogens. Lymphocytes, particularly T-cells, can differentiate into numerous subpopulations that have a wide variety of functions depending on the maturation process and stimuli to which they are exposed. The major T-cell subsets are classified by surface expression of CD4 or CD8. The CD4⁺ “helper” T-cells coordinate the immune response, activating other adaptive immune cells, such as memory B-cells and effector T-cells, and innate immune cells, such as macrophages. These cells are activated by exposure to peptide antigens on the surface of antigen-presenting cells, such as DCs and macrophages and differentiate into T_h subtypes that rapidly proliferate and secrete various cytokines. The subsets into which CD4⁺ T-cells differentiate include T_h1, T_h2, T_h17, and others depending on the stimulus encountered and the type of immune function they need to execute. The CD4⁺ regulatory T-cell (T_reg) subset primarily prevents a response to self-antigens and suppresses detrimental immune responses. CD8⁺ cells, also called cytotoxic T-cells or cytotoxic T lymphocytes, are activated by CD4⁺ T-cells or antigen-presenting cells, producing cytotoxins such as perforin and granzyme, Fas ligand, and various cytokines that destroy target cells such as virus-infected cells or tumor cells.

The other major lymphocyte that mediates adaptive immunity is the B-cell, which primarily produces antibodies, although B-cells can also act as antigen-presenting cells and produce cytokines. Naïve or memory B-cells express B-cell receptors on the plasma membrane, and when this receptor is activated by a specific antigen, the B-cell proliferates and differentiates into a plasma cell, producing large quantities of specific antibodies to fight invading pathogens. Both T-cells and B-cells can develop into memory cells that can persist in a quiescent state for long periods of time until activated by the specific antigen to which they respond. Once the memory cell is activated, it generates a rapid adaptive immune response. Lymphocyte differentiation, the specific roles of lymphocytes subsets, and the formation of immune memory are central to the adaptive immune response, and this topic is an extraordinarily expansive and well-researched field, with numerous excellent reviews (Murphy et al., 2008; Chaplin, 2010; Kurosaki et al., 2015; Netea et al., 2020). The role of dopamine in adaptive immunity has also been well described, and many studies and reviews have discussed the different functions dopamine can modulate in T-lymphocytes (Pacheco et al., 2009; Gaskill et al., 2013; Levite, 2016; Levite et al., 2017), although there has been much less study of B-cells (Tsao et al., 1997; Meredith et al., 2006).

1. Inflammatory Regulation of Dopaminergic Machinery

As dopamine receptors, transporters, and metabolic machinery are expressed in nearly all immune cells, it is not surprising that there is significant crosstalk between the dopaminergic system and the immune function of these cells. While dopamine can modulate the responses of immune cells to inflammatory insults, inflammatory factors released from different immune cells can also impact dopaminergic machinery, altering the function and expression of dopamine receptors and impacting dopamine metabolism. In the CNS, cytokines and chemokines influence the development, maintenance, and functional properties of midbrain dopaminergic neurons (Zalcman et al., 1994; Ling et al., 1998; Dunn, 2006; Felger and Miller, 2012). Inflammatory cytokines such as IL-1β promote the differentiation of mesencephalic progenitor cells into dopaminergic neurons (Ling et al., 1998), and IL-6 modulates dopaminergic activity in various mesolimbic structures in a dose-dependent manner (Zalcman et al., 1994; Dunn, 2006).

Inflammation can also regulate dopamine synthesis and release. A number of studies show NO induced changes in dopamine regulation in different brain regions (Lorrain and Hull, 1993; Seilicovich et al., 1995; Hartung et al., 2011; Motahari et al., 2016). Acute exposure to IL-1β and IL-6 can stimulate TH expression and dopaminergic activity in vivo and in vitro (Abreu et al., 1994; Zalcman et al., 1994; Ling et al., 1998). In rice stem borer (Chilo suppressalis) hemocytes, exposure to LPS stimulates dopamine synthesis (Wu et al., 2015). Inflammatory cytokines also impair the conversion of phenylalanine to tyrosine (Felger et al., 2013), potentially limiting tyrosine availability for dopamine synthesis. In addition to altering the biosynthesis of dopamine, some cytokines can regulate dopamine storage. IL-1β and TNF-α can decrease the expression of VMAT2, thereby limiting the availability of presynaptic dopamine. Conversely, TGF-α increases VMAT2 expression, favoring the storage of presynaptic dopamine (Kazumori et al., 2004). In hMDMs, DAT is highly responsive to inflammatory stimuli, as LPS treatment reduces DAT-mediated uptake and increases DAT-mediated dopamine efflux by promoting an efflux-favoring conformation in these cells. These activities were shown to be part of a TLR4-dependent autocrine loop by which DAT dynamically regulates the amount of dopamine in the local microenvironment to modulate immune functions such as cytokine release and phagocytosis (Mackie et al., 2022).

These results indicate that inflammatory cytokines can exert complex regulatory effects on the amount of dopamine available in dopaminergic cells by modifying the biosynthesis rate and the storage of this neurotransmitter. Finally, it is possible that oxidative stress dysregulates dopamine receptors. In rat renal proximal tubes, oxidative stress caused hyperphosphorylation of DR1, leading to uncoupling of the receptor and G_αs via NF-κB (Banday et al., 2007; Fardoun et al., 2007). This effect was also induced by the TLR4 agonist LPS and may be one mechanism by which dysregulated dopamine receptor signaling can lead to hypertension (Wang, Luo et al., 2014; Yang, Villar et al., 2021). Together, these data support a bidirectional interaction between the immune system and dopamine metabolism, uptake, storage, and receptor signaling.

2. Dopamine-Mediated Impacts on the Inflammatory Response

Multiple studies have demonstrated that dopamine can impact the production of inflammatory mediators in various cell systems, although the precise receptors and signaling pathways involved in this process are complex and not well understood (Tarazona et al., 1995; Hasko et al., 1996, 2002; Capellino et al., 2010; Nakano et al., 2011; Gaskill et al., 2012; Zhang et al., 2015, Zhang, Jiang 2016; Arreola et al., 2016; Nolan and Gaskill, 2019; Yoshioka et al., 2020). These effects are broadly summarized in Table 1. As previously discussed, innate immune cells recognize PAMPs and DAMPs via PRRs. Dopamine often acts on pathways downstream of several of these PRRs, so a baseline understanding of their signaling processes is important for understanding dopamine signaling in the immune response. TLRs coordinate innate immune functions and initiate adaptive immune responses through activation of the NF-κB/activator protein 1 and/or IRF3/7 pathways. Similarly, NOD-like receptor and C-type lectin receptor (dectin-1) signaling activates NF-κB and inflammasomes during bacterial and antifungal infections, respectively. Antiviral immunity is most often mediated by the upregulation of type I interferons via cytoplasmic RIG-I/MDA5, which is a cytoplasmic caspase-recruiting domain helicase (Lee and Kim, 2007). Several of these signaling pathways converge on NF-κB and its downstream effectors. Production of many cytokines is regulated by NF-κB transcriptional activity (Liu et al., 2017), and NF-κB activation is the first step in inflammasome activation and the secretion of IL-1 cytokines in a variety of cell types (He et al., 2016; Herman and Pasinetti, 2018). Signaling pathways involve numerous secondary messengers, kinases, and other signaling effectors (Kawai and Akira, 2009; Takeuchi and Akira, 2010), many of which can be activated by distinct dopamine signaling pathways (Beaulieu and Gainetdinov, 2011; Beaulieu et al., 2015), demonstrating substantial crosstalk between PRRs and dopamine signaling.

The transcription factor NF-κB is central to the initiation and regulation of PRR-mediated inflammatory responses (Liu et al., 2017), and the impact of dopamine on NF-κB has been examined in a variety of immune cell subsets (Takeuchi and Fukunaga, 2003; Zhang et al., 2015; Nolan et al., 2019; Wang, Chen et al., 2019; Wu et al., 2020; Yoshioka et al., 2020; Yue et al., 2021). A number of these studies suggest that dopamine inhibits NF-κB, indicating anti-inflammatory activity. In the macrophage RAW264.7 cell line, very high levels of dopamine (10⁻³–10⁻⁵ M) reduced both LPS-induced expression of NO synthase and the expression of the NLRP3 inflammasome components NLRP3 and caspase-1 (Liu and Ding, 2019). Although this study did not directly demonstrate that dopamine impacted NF-κB, activation of this transcription factor is necessary for NLRP3 expression, and so the downregulation of NLRP3 indicates reductions in NF-κB activity (He et al., 2016; Herman and Pasinetti, 2018). Rotigotine (D2-like agonist) reduced TNF-α production and NF-κB activation in a mouse model of acute liver injury (Yue et al., 2021), and D2 signaling inhibited NF-κB activation by inhibiting Akt in a murine model of acute pancreatitis (Han et al., 2017). These effects are not unique to myeloid cells, as D2 activation in resting peripheral human T-cells was associated with IL-10 secretion, which regulates cytokine production via NF-κB (Besser et al., 2005). In CD3-stimulated human T-cells, D2 and D3 activation inhibited the dopamine-mediated release of IL-2, IFN-γ, and IL-4 (Ghosh et al., 2003). In a separate study, D4 blocked IL-2 production in CD3/CD28-stimulated human T-cells (Sarkar et al., 2006), indicating that under certain conditions, the activation of all three D2-like receptors can be anti-inflammatory. Dopamine treatment of astrocytes may also inhibit NF-κB and the NLRP3 inflammasome through D2-like receptor signaling (Shao et al., 2013; Zhu, Hu et al., 2018).

Dopamine signaling through D1-like receptors may also reduce NF-κB activity, potentially via PP2A activation (Wu et al., 2020), which is also linked to Akt inhibition (Manning and Toker, 2017). Dopamine (∼6–18 × 10⁻⁵M) also decreased inflammatory cytokine production and NLRP3 activity in murine lung tissue in ventilator-induced lung injury models (Yang et al., 2017). Treatment with A68,930 (D1-like agonist) inhibited NLRP3 activation in doxorubicin-treated murine cardiac myoblasts, as well as in mice with cardiac inflammation induced by doxorubicin, CNS inflammation induced by intracerebral hemorrhage, and acute kidney injury caused by renal ischemia/reperfusion (Wang, Nowrangi et al., 2018; Cao et al., 2020; Liu, Jin et al., 2021).

In murine BMDMs treated with the TLR2 ligand Pam3CSK4, D5 activation mediated the downregulation of NLRP3 (Wu et al., 2020), and signaling through D1 decreased NLRP3 activity by increasing NLRP3 ubiquitination in LPS-induced dopamine treated (1.5–2.5 × 10⁻⁴M) cells (Yan et al., 2015). These studies show that high levels of dopamine or specific D1-like receptor activation in rodents and rodent macrophages can downregulate NLRP3 activity. Several studies also suggest that this effect is mediated by cAMP signaling, although D1 signaling was not shown to act directly on cAMP production (Yan et al., 2015; Wang, Villar et al., 2018; Liu, Jin et al., 2021). The connection may therefore be indirect or circumstantial, as data in human macrophages suggest that D1-like receptors do not activate cAMP in this cell type (Nickoloff-Bybel et al., 2019).

Although these aforementioned studies showed an anti-inflammatory effect of D1-like receptor signaling, they examined the effects of high levels of dopamine or D1-specific agonists on existing inflammation. A number of other studies indicate an inflammatory role for dopamine, which can also be mediated by NF-κB and NLRP3 activity (Nakano et al., 2008, 2011; Gaskill et al., 2012; Parrado et al., 2012; Trudler et al., 2014; Wang, Villar et al., 2018; Nolan et al., 2019, 2020). In hMDMs, dopamine (10⁻⁸–10⁻⁶M) increased the production of the inflammatory mediators CCL2, IL-6, C-X-C motif chemokine ligand (CXCL) 8, IL-1β, CXCL9, and CXCL10 (Gaskill et al., 2012; Nolan et al., 2019). Similarly, dopamine (2 × 10⁻⁶M–10⁻⁵M) increased IL-6 and IL-1β production in activated primary murine microglia, NK cells, and BV-2 murine microglial cells (Trudler et al., 2014; Fan et al., 2018; Nolan et al., 2019), and one study showed increased ERK1/2 and p38MAPK activation in LPS-induced rodent primary microglia and BV2 cells relative to resting cells (Fan et al., 2018). IL-1β production was also increased in LPS-induced peritoneal macrophages (Kawano et al., 2018). In hMDMs, specifically blocking DAT activity enhanced LPS-mediated production of IL-6, TNF-α, and mitochondrial superoxide levels, showing that the regulation of dopamine uptake and release, as well as dopamine receptor activity, plays a role in dopaminergic immunomodulation in macrophages (Mackie et al., 2022).

These effects are not limited to myeloid cells. In naïve human CD4⁺ T-cells stimulated with anti-CD3/anti-CD28, dopamine (10⁻⁸–10⁻⁷M) increased the production of IL-5, IL-17, IL-1β, and IL-6 and the dopamine-mediated changes in IL-5 and IL-17 were blocked by the presence of SCH23390 (D1-like receptor antagonist) (Nakano et al., 2011). The secretion of TNF-α by resting human peripheral T-cells also increased in response to D1/D5 and D3 activation (Besser et al., 2005). Activation of D3 also drives inflammatory effects in several other human and rodent systems, biasing the generation of inflammatory T_h1 and T_h17 cells and suppressing the development of T_h2 cells (González et al., 2013; Ilani et al., 2004; Contreras et al., 2016). In a mouse model of intestinal inflammation, the transplantation of naïve Drd3^−/− T-cells resulted in milder weight loss and mucosal inflammation than the transplantation of wild-type cells (Contreras et al., 2016). In an MPTP model of PD, Drd3 knockout animals showed decreased microglial activation and reductions in TNF-α and IFN-γ production due to a lack of D3 signaling in T-cells (González et al., 2013). In addition to traditional immune cells, in human adipocytes, pharmacological stimulation of D2 induced the expression and release of IL-6 (Wang, Villar et al., 2018), and in human keratinocytes, D2 activation increased IL-6 and IL-8 levels (Parrado et al., 2012).

Inflammatory responses were also suppressed by SCH23390 in a SCID-mouse model of murine RA, while D2-like receptor suppression with haloperidol increased the accumulation of IL-6⁺ and IL-17⁺ T-cells (Nakano et al., 2011). In a murine model of EAE, SCH23390 (D1-like receptor antagonist) decreased IL-17 production (Nakano et al., 2008), and in a different murine model of EAE, dopamine was found to exacerbate the disease, as removal of dopamine reduced EAE severity. Much of this effect was found to be driven by D5 signaling in DCs, which increased the frequency of inflammatory CD4⁺ T-cells in the CNS and drove inflammation by blocking signal transducer and activator of transcription and inducing the activity of IL-12 and IL-23 in T-cells (Prado et al., 2018). Further, dopamine (10⁻⁶M–10⁻⁵M) increased the anti-inflammatory mediator IL-10 in LPS-induced hMDMs (Gaskill et al., 2012; Nolan et al., 2019), LPS-induced primary murine splenocytes, B-cells, and bone marrow–derived DCs (Kawano et al., 2018), although the effect of A77636 (D1-like receptor agonist) on splenocytes was inhibitory (Kawano et al., 2018).

Many studies of innate immune cells such as macrophages indicate that the regulation of NF-κB and the NLRP3 inflammasome is central to the inflammatory impact of dopamine. In hMDMs, dopamine increases the activation of NF-κB, leading to the expression of NLRP3 and IL-1β. Dopamine could also potentiate ATP-induced secretion of IL-1β but did not mediate the secretion of IL-1β on its own, which indicates that dopamine can prime but not activate the NLRP3 inflammasome by activating NF-κB (Nolan et al., 2020). Many previously discussed studies also show that dopamine or specific dopamine receptors increase and decrease NF-κB and NLRP3 inflammasome activity (Nakano et al., 2011; Shao et al., 2013; Trudler et al., 2014; Yan et al., 2015; Yamamoto et al., 2016; Wang, Nowrangi et al., 2018; Wu et al., 2020; Liu, Jin et al., 2021).

These data, along with other, similar research, indicate that NF-κB and the NLRP3 inflammasome may be central to dopaminergic modulation of inflammation, at least in macrophages and other types of innate immune cells. Notably, most studies only examined the NLRP3 inflammasome, and the effects of dopamine on other inflammasomes remain unclear. These data show the broad range of cell types on which dopamine can act and highlight the discrete roles of specific dopamine receptor subtypes, as well as how they could differ between cell types. While many of these data are conflicting, they demonstrate that dopamine acts differently on activated or stimulated cells than on unstimulated cells, mediating anti-inflammatory effects in the presence of inflammation and driving inflammation in non-inflammatory conditions.

It is likely that D1- and D2-like receptors mediate distinct effects, particularly across different cell types and activation states. The hypothesis that D1-like receptors are inflammatory and the D2-like receptors are anti-inflammatory is very appealing (Nolan and Gaskill, 2019) and is supported in many disease models, as well in a number of in vitro studies (Nakano et al., 2008; Nakano et al., 2011; Shao et al., 2013; Zhang et al., 2015; Osorio-Barrios et al., 2018; Prado et al., 2018; Zhu, Hu et al., 2018; Alam et al., 2021; Yue et al., 2021), many of which are discussed in the previous text. In hMDMs, there is a negative correlation between D1-like receptor expression and the anti-inflammatory cytokine IL-10, whereas dopamine increased IL-1β production in hMDMs lacking D2 (Nolan et al., 2019), and D1-like receptor signaling can increase TNF-α production in resting human CD4⁺ T-cells (Besser et al., 2005). Pharmacological inhibition of D1-like receptors using SCH23390, or knockout of Drd5 specifically on DCs reduced inflammation in an EAE model (Prado et al., 2018), antagonizing D1-like receptors also suppresses T_h17 and neutrophilic lung inflammation signaling (Nakagome et al., 2011), both suggesting that D1-like receptor signaling drives inflammation.

In further support of this hypothesis, D2 activation was anti-inflammatory in rodent models of pancreatitis (Han et al., 2017) and acute liver injury (Yue et al., 2021). D2 activation increased the secretion of the anti-inflammatory cytokine IL-10 in human T-cells (Besser et al., 2005), and silencing D2 receptors reduced renal inflammation, which was associated with increased Akt phosphorylation and decreased PP2A activity, suggesting that D2R signaling may act through the activation of PP2A and subsequent inhibition of Akt (Zhang, Jiang et al., 2016). In rodent models of brain injury, the D2 agonist quinpirole protected against glial cell–induced neuroinflammation by inhibiting NF-κB and the activation of αB-crystalline (Zhang et al., 2015; Alam et al., 2021). The precise cell type(s) affected by dopamine were not investigated in this study, but subsequent studies showed that the activation of astrocytic D2-like receptors induces αB-crystalline to alleviate neuroinflammation in response to MPTP-mediated injury and in stroke models (Shao et al., 2013; Qiu et al., 2016). Moreover, activation of astrocytic D2-like receptors inhibited NLRP3 inflammasome activation via a β-arrestin mediated mechanism (Zhu, Hu et al., 2018). While the PP2A/AKT axis has not been examined in astrocytes, β-arrestin-mediated recruitment of PP2A is a major mechanism by which D2-like receptors inhibit Akt (Beaulieu et al., 2005, 2015; Beaulieu and Gainetdinov, 2011). This finding suggests an important role of this signaling axis in mediating the anti-inflammatory effects of D2-like receptors.

While these studies and others support the inflammatory D1–anti-inflammatory D2 dichotomy, this concept is not consistent throughout the literature. For example, A68,930 (D1-like receptor agonist) inhibited NLRP3 activation in mouse lung (Jiang, Li et al., 2016), brain (Wang, Nowrangi et al., 2018), and kidney (Cao et al., 2020). Treatment with A77636 also inhibited inflammation in mouse splenocytes (Kawano et al., 2018), and while the effect was not specifically inflammatory, D5 signaling mediated an increase in T-cell activity (Franz et al., 2015). Extensive mechanistic studies show that both types of D1-like receptors individually inhibit NLRP3 activation in several types of activated murine macrophages (Yan et al., 2015; Liu and Ding, 2019; Wu et al., 2020; Liu, Wu et al., 2021). In the NG108-15 rodent glial cell line, D1-like signaling was also associated with the inhibition of NF-κB through PKA-mediated mechanisms, whereas D2-like activation was associated with the activation of NF-κB (Takeuchi and Fukunaga, 2003). D2 activation also increased the release of inflammatory cytokines by human adipocytes (Wang, Villar et al., 2018) and human keratinocytes (Parrado et al., 2012).

While it is possible that some of these differences may be associated with subtype-specific signaling (D5 is anti-inflammatory and D1 is proinflammatory), this seems unlikely, as both D1 (Yan et al., 2015) and D5 (Wu et al., 2020; Liu, Wu et al., 2021) have been shown to decrease inflammation in mouse model systems. Additionally, dopamine receptors of the same subtype can have distinct effects on the same cells, as D2 activation increases IL-10 in resting human T-cells, while D3 activation increases TNF-α in the same population (Besser et al., 2005). This bifurcation of dopamine-mediated effects on a dopamine receptor subtype seems to be particularly common in T-cells, in which D3 is more inflammatory and D2 is more anti-inflammatory, although this effect is clearly influenced by activation state. There may also be species-specific differences, such as dopamine being more inflammatory in human myeloid cells than rodent myeloid cells, but many additional studies of human immune cells are needed to determine this effect with any certainty. These data suggest that the effects of dopamine receptor signaling are heavily influenced by existing levels of inflammation and likely have distinct effects on different cell types.

3. Dopaminergic Regulation of Phagocytosis

Phagocytosis is a critical immune function in which cells engulf pathogens or debris and, in the case of pathogens, kill them through intracellular or extracellular mechanisms. Phagocytosis is primarily mediated by professional phagocytes, including monocytes, macrophages, DCs, and eosinophils, although most cell types have the capacity for some kind of phagocytosis. A wide range of studies across species indicate that dopamine can modulate phagocytic activity, but the precise mechanisms by which dopamine mediates these effects seem to differ across species. In Litopenaeus vannamei, a species of white prawn, dopamine (10⁻⁷M–10⁻⁶ M) significantly decreased gene expression of Drd4, G_i heterotrimeric G protein, and exocytosis-related proteins, while phagocytosis-related proteins and G_s heterotrimeric G protein were increased (Tong et al., 2020). In the rice stem borer (Chilo suppressalis), dopamine (10⁻⁵M) increased hemocyte phagocytosis via a D1-like receptor. This effect appeared to involve ERK1/2 activation (Wu et al., 2015), which can be induced by dopamine via cAMP/PKA signaling (Zhen et al., 1998; Gerits et al., 2008; Shao et al., 2020). While the implications of these findings to human subjects is unclear, these data suggest that the capacity of dopamine to stimulate the D1–cAMP axis may promote the phagocytic function of immune cells via the G_s heterotrimeric G protein pathway, which signals through the activation of adenylate cyclase and the production of cAMP (Marinissen and Gutkind, 2001).

Studies also suggest that dopamine modulates phagocytic activity in a number of vertebrate systems, although the mechanisms underlying this activity are not clear. In wall lizard splenic macrophages, dopamine induced bimodal changes in phagocytosis; higher concentrations of dopamine (10⁻⁷–10⁻⁵M) could decrease phagocytosis, while very low dopamine concentrations (10⁻¹¹–10⁻¹⁵M) could increase phagocytosis (Roy and Rai, 2004). Dopamine (10⁻⁸–10⁻⁷M) increased the phagocytic activity of murine BMDMs in the presence of IFN-γ, as well as in neutrophils (Sookhai et al., 1999), although another study showed that similar concentrations of dopamine (6.5 × 10⁻⁷M) reduced neutrophil phagocytosis (Wenisch et al., 1996). In mice, macrophage phagocytosis was decreased by spiperone (D2-like receptor antagonist) (Sternberg et al., 1987). Chicken macrophages treated with dopamine (0.65–1.6 × 10⁻⁶M) and guinea pig macrophages treated with bromocriptine (D2 receptor agonist) and pergolide (D2 receptor agonist) both showed an enhanced binding and phagocytosis of IgG-sensitive red blood cells (Ali et al., 1994; Gomez et al., 1999).

In each of these studies, changes in phagocytic activity were associated with dopamine receptor-induced changes in the expression of surface Fc-gamma receptor (Fcγ) receptors, which are critical regulators of the immune response that mediate a number of factors including antigen uptake (Sternberg et al., 1987). Dopamine-mediated changes in Fcγ receptors could not only disrupt phagocytosis and antigen presentation but also suggest a mechanism by which dopamine affects T-cell activation and promotes the development of autoimmunity (Junker et al., 2020). A D2-mediated effect on phagocytosis was seen in rodent peritoneal macrophages, but antagonizing D2-like receptors with domperidone increased phagocytosis in lactating rats (Carvalho-Freitas et al., 2008). Unlike studies showing effects through regulation of Fcγ expression, this effect was thought to be mediated by the regulation of serum prolactin levels, and phagocytosis was shown to be decreased at other time points in another study by this group (Carvalho-Freitas et al., 2008, 2011). Dopaminergic regulation of phagocytosis was also seen in hMDMs when DAT activity was blocked with nomifensine, downregulating LPS-induced phagocytic activity via autocrine signaling and suggesting that lower dopamine levels inhibit phagocytic activity in activated macrophages (Mackie et al., 2022). A decrease in phagocytosis, as well as ERK1/2 phosphorylation, was also seen in in response to dopamine (2 × 10⁻⁶M) treatment of LPS-activated BV2 microglia, but no effect on the phagocytic activity of resting cells was observed, suggesting autocrine control of phagocytosis by dopamine signaling in activated myeloid cells (Fan et al., 2018).

These differences could be due to the different mechanisms through which phagocytosis is regulated, such as Fcγ receptors, prolactin levels, or DAT/MAPK. Lower dopamine concentrations (≤10⁻⁶M) and D2 activation increased phagocytosis (Sternberg et al., 1987; Ali et al., 1994; Gomez et al., 1999) and were associated with changes in Fcγ receptors, while reducing D2 activity increased phagocytic activity and was associated with prolactin (Carvalho-Freitas et al., 2008, 2011). However, phagocytic activity was also reduced by the pan-dopamine agonist apomorphine, the D2/D3 agonist bromo-a-ergocryptine, and the β-adrenergic receptor antagonist propranolol in wall lizard macrophages. These data studies suggest that D1-like receptors can also drive phagocytosis, and many of these data suggest the involvement of the cAMP signaling pathway, as D2 activation inhibits cAMP production.

In support of this, in the wall lizard cells, treatment with phosphodiesterase IBMX, which increases cAMP levels, resulted in a greater reduction in phagocytosis when combined with dopamine. Signaling through MAPK is also downstream of cAMP in a number of pathways. However, the use of the cAMP analog db cAMP had a similar biphasic effect (Roy and Rai, 2004), making the directional effects of higher and lower cAMP levels unclear. Moreover, dopamine-mediated cAMP activity was notably absent from human macrophages (Nickoloff-Bybel et al., 2019), suggesting that this pathway might not function in human cells. Taken together, these data form a confusing picture of dopaminergic modulation of phagocytosis, suggesting that the differences in the effects of dopamine are due to the concentration of dopamine or ligand being used, the exposure time, the receptor being activated, and the species. Thus, while these data indicate that dopamine does regulate phagocytic function in multiple cell types, additional work with carefully chosen and controlled experimental conditions is needed to precisely define the mechanism by which this regulation occurs.

4. Dopaminergic Regulation of Chemotaxis

Chemotaxis is the mechanism by which cells move in response to extracellular chemical gradients, allowing immune cells to migrate to sites of inflammation or tissue damage when they detect chemokines and other factors secreted by cells at that site. Chemotaxis involves many chemokines, some of which are species- or cell-specific, and is initiated when soluble chemokines bind to their cognate receptors on immune cells (Rossi and Zlotnik, 2000; Hughes and Nibbs, 2018). A number of studies have shown that dopamine can impact the production of chemotactic factors, particularly CCL2, which is important in the pathogenesis of many chronic inflammatory conditions, as well as neurologic HIV (neuroHIV) (Eugenin et al., 2006; Covino et al., 2016; Fantuzzi et al., 2019; Das et al., 2021). Dopamine increased the mRNA expression of CXCL8 and CCL2 in activated PBMCs (Torres et al., 2005) and the injection of a D1-like agonist increased the mRNA expression levels of CCL2 and C-C motif chemokine ligand 7 in the rodent PFC (Saika et al., 2018). In hMDMs, dopamine (10⁻⁶M–10⁻⁸M) increased the production of CCL2, CXCL8, CXCL9, and CXCL10 (Gaskill et al., 2012; Nolan et al., 2019), while blocking DAT activity potentiated LPS-induced CCL2 production (Mackie et al., 2022). Both CCL2 and CXCL2 were reduced by D2 signaling in acute pancreatitis (Han et al., 2017; Saika et al., 2018), and in mouse renal proximal tubule cells, D2 knockdown was associated with an increase in CCL2 production. This effect was connected to the loss of PP2A activity, suggesting that the D2-like receptors may inhibit cytokine production via PP2A and Akt inhibition (Zhang, Jiang et al., 2016). In addition to CCL2, in primary murine splenocytes, peritoneal macrophages and NK cells, dopamine (10⁻⁵–10⁻⁶M) increased CXCL1 production (Kawano et al., 2018).

In addition to increasing the production of chemokines, dopamine itself can act as a chemoattractant and potentiate chemotaxis and chemokinesis. In neutrophils, high concentrations of dopamine (10⁻⁴ and 10⁻⁵ M) inhibited IL-8 mediated transendothelial migration (Sookhai et al., 2000). In mature human CD14⁺CD16⁺ monocytes, dopamine (10⁻⁶M–5 × 10⁻⁵M) and SKF38393 (D1-like agonist) increased chemokinesis and transmigration across a model of the blood–brain barrier (Coley et al., 2015; Calderon et al., 2017), suggesting a chemotactic role for D1-like receptors. A role for D1-like receptors, specifically D5, was also shown in murine CD4⁺ T-cells in the gut, as the formation of a heteromeric CCR9:D5 complex on these cells mediated inflammation-induced T-cell migration into the gut mucosa. These effects were mediated by ERK1/2 signaling and were specific to CCR9, as D5 did not form heteromers with other chemokine receptors (Osorio-Barrios et al., 2021).

D2-like receptor signaling has also been implicated in dopamine-mediated chemotaxis. In mice, D3 stimulation of T_regs reduced CCR9 expression and inhibited the migration of these cells into the lamina propria region of the gut upon intestinal inflammation (Ugalde et al., 2021). In naïve CD8⁺ T-cells, dopamine (10⁻⁷M) potentiated homing to CCL19, CCL21, and CXCL12 through D3 and G_αi-mediated Ca²⁺ mobilization (Watanabe et al., 2006), and treating peripheral T-cells from head and neck cancer patients with dopamine (10⁻⁸M) increased spontaneous migration and migration toward CXCL12 (Saussez et al., 2014). A role for D2-mediated G_αi signaling is supported by experiments using HEK293 cells transfected with D2-receptors. Treating these cells with quinpirole (D2-agonist) mediated a chemotactic response to IL-8 that required the activation of G_αi and G_βγ activity, while G_αs and G_αq receptors did not mediate these effects (Neptune and Bourne, 1997).

In addition to driving immune cell chemotaxis, dopamine can also influence inflammation and immune activity by modulating the movement of bacteria and pathogens. Antagonizing D1- and D2-like receptors blocked liver fluke (Clonorchis sinensis) larva chemotaxis into the bile duct, indicating that dopamine plays a role in the development of clonorchiasis (Dai et al., 2020). High levels of dopamine (5 × 10⁻⁵M) also increased the expression of flagellar motility-related genes and the swimming motility of the aquatic bacterium Vibrio harveyi, a Gram-negative bacterium that is a major pathogen of all aquatic organisms. This effect was blocked by chlorpromazine, suggesting that this effect was driven by D2-like receptors, although the effect could be influenced by the many effects of chlorpromazine on nondopaminergic receptors (Yang, Anh et al., 2014). Dopamine has biphasic effects on the chemotaxis of E. coli, repelling these bacteria at concentrations below 10⁻⁴M but attracting E. coli at higher concentrations. Although gut concentrations of dopamine are <10⁻⁴M at most times, higher concentrations are likely present in the microenvironment around the gut lumen and at the mucous layer where these compounds are secreted, and different foods could also elevate gut dopamine levels (Lopes and Sourjik, 2018; Matt and Gaskill, 2020).

Taken together, these data implicate several signaling pathways in the regulation of dopaminergic chemotaxis and suggest that many of the chemotactic effects of dopamine could be mediated through CCL2, CXCL9, and CXCL12. It is difficult to identify patterns relating to cell type, as D1-like activity promoted chemotaxis in monocytes, T-cells, and E. coli, while D2-like receptors drove chemotaxis in T-cells and other pathogens. This indicates that both D1- and D2-like receptor signaling can play a role in chemotactic activity. Pathways mediated by G_αi, G_βγ, and Akt primarily associated with D2-like receptors, while ERK1/2 and Ca²⁺ flux could be mediated by either receptor subtype. As with many functions, these effects are likely different in distinct cell types and activation states, but the current data make it difficult to further define this, as both types of receptors act on multiple cell types in both resting and activated states.

5. Dopaminergic Regulation of Oxidative Burst and Nitric Oxide Production

In addition to regulating the production of inflammatory mediators, phagocytic activity, and chemotaxis, dopamine also influences respiratory or oxidative bursts, as well as ROS and NO production. These bursts mediate the rapid release of ROS such as hydrogen peroxide (H₂O₂) and superoxide anion, which can be formed by the activity of superoxide dismutase. This process is common in phagocytes, such as myeloid cells and granulocytes, to degrade internalized bacteria and other particles as part of the immune response. These bursts can also affect cell signaling (Forman and Torres, 2002; Dahlgren and Karlsson, 1999). In neutrophils, dopamine may reduce respiratory bursts, as dopamine treatment (6.5 × 10⁻⁷M) of human neutrophils reduced ROS production (Wenisch et al., 1996), and high concentrations of dopamine (10⁻⁴ and 10⁻⁵ M) inhibited superoxide anion production in response to N-formylated N-formyl-methionyl-leucyl-phenylalanine stimulation (Yamazaki et al., 1989; Matsuoka, 1990). Similarly, high levels of dopamine (10⁻⁴M) and fenoldopam (D1-like agonist) reduced respiratory bursts in neutrophils isolated from the blood of patients with systemic inflammatory response syndrome (Sookhai et al., 1999). Lower concentrations of dopamine (2.61 × 10⁻⁷M or 2.61 × 10⁻¹⁰M) had no effect on the formation of H₂O₂-mediated oxidative bursts in human neutrophils (Trabold et al., 2007).

In myeloid cells such as rodent peritoneal macrophages, antagonizing D2-like receptors with domperidone increased spontaneous oxidative bursts of H₂O₂ (Carvalho-Freitas et al.,2008, 2011). Additionally, dopamine treatment (10⁻⁶–10⁻⁸M) of hemocytes from the white shrimp Litopenaeus vannamei significantly reduced respiratory bursts and superoxide dismutase activity (Cheng et al., 2005). These data suggest that dopamine generally inhibits the production of ROS and oxidative bursts in neutrophils, particularly at higher levels. This is supported by recent data showing exposure to dopamine (10⁻⁶–10⁻⁹M) broadly inhibited neutrophil activity in a D1-dependent manner (Marino et al., 2022). Further, data from hypertension studies showing that the activation of D1-like receptors, particularly D5, reduced the production of ROS in mitochondria through an autophagy-associated mechanism (Yang et al., 2006; Lee et al., 2021). In addition, studies showed that knocking out Drd2 in renal tubule cells increased renal ROS production (Yang, Cuevas et al., 2014) and that Drd2 knockout mice had increased levels of ROS due to aldosterone dysregulation (Armando et al., 2007).

Dopamine has also been shown to dysregulate NO production. NO is important in defense against infectious diseases, tumors, sterile inflammation, and other insults. NO is produced by several types of immune cells, mostly macrophages and granulocytes (Bogdan, 2001). In rodents, treatment with apomorphine (pan-dopamine receptor agonist) increased NO₂⁻ and NO₃⁻ in dialysate from the hypothalamus (Melis et al., 1996), suggesting that dopamine increases NO production. However, this conclusion is opposed by an array of in vitro studies. In LPS-induced primary rodent microglia and BV-2 cells, dopamine (2 × 10⁻⁶M) increased NO synthase (Fan et al., 2018). Inhibiting D1-like receptors with SCH23390 and D2-like receptors with sulpiride inhibited NO production by LPS-stimulated peritoneal macrophages (Hasko et al., 1996). Dopamine also enhanced LPS-mediated NO production in RAW264.7 cells, although only at higher concentrations (5 × 10⁻⁵M) and to a much lesser extent than other catecholamines (Chi et al., 2003). However, dopamine decreased the production of NO and NO synthase in LPS-stimulated primary rodent microglia as well as N9 and BV2 murine microglia through D1- and D2-like receptors (Chang and Liu, 2000; Farber et al., 2005; Yoshioka et al., 2016; Wang, Chen et al., 2019). These findings opposed other studies that suggested that dopamine increased NO production, but data from BV2 cells suggest that this could result from cytotoxicity due to the production of dopamine quinones (Beck et al., 2004) and may not be mediated by dopamine receptor signaling. These data suggest more research is needed in this area, but that dopamine receptor signaling may promote the production of NO in myeloid cells, although cytotoxic dopamine quinone formation may have the opposite effect.

6. Signaling Mechanisms Mediating Dopaminergic Immunomodulation

The discrete effects of specific dopamine receptors may also reflect D1- and D2-like receptor signaling through both convergent and divergent signaling cascades, depending on the cell system and dopamine receptors activated. For example, Akt inhibition via PP2A recruitment has been suggested to be central in dopamine-mediated anti-inflammatory effects in isolated cell systems and disease models (Tolstanova et al., 2015; Zhang, Jiang et al., 2016; Han et al., 2017; Wu et al., 2020; Yue et al., 2021). Signaling through Akt regulates a wide range of cellular processes, including cell growth, survival, metabolism, and inflammation (Manning and Toker, 2017). In the immune response, Akt modulates NF-κB activity by regulating IkappaB (IκB) kinase, which phosphorylates IκB to release and activate NF-κB (Dan et al., 2008; Cheng and Kane, 2013; Dorrington and Fraser, 2019). This suggests the dopamine-mediated inhibition of Akt could reduce inflammatory cytokine production. Akt inhibition is generally associated with D2-like receptor signaling (Beaulieu et al., 2004, 2007; Zhang, Jiang et al., 2016; Han et al., 2017; Zhu, Hu et al., 2018; Wu et al., 2020), but D1-like receptors can act on AKT (Beaulieu et al., 2005). The activity of this pathway, and the relative frequencies of D2- and D1-like receptors may explain the anti-inflammatory activity of these receptors in certain systems. Both D1- and D2-like receptors can also activate the phosphatidylinositol 3-kinase/Akt signaling cascade, although the immunologic impact of dopamine-mediated activation of this pathway is not well understood (Zhen et al., 2001; Brami-Cherrier et al., 2002; Nair et al., 2003; Nair and Sealfon, 2003; Iwakura et al., 2008; Mannoury la Cour et al., 2011; Chen, Ruan et al., 2012; Perreault et al., 2013; Radl et al., 2013; Mirones et al., 2014; Yan et al., 2020).

Some of the differences in the effects of dopamine on similar cell types may be due to variations in the capacity of D1-like receptors to activate the canonical D1–G_αs–cAMP signaling pathway. While studies have shown that cAMP signaling has both inflammatory and anti-inflammatory effects, in myeloid cells, cAMP signaling is largely associated with a decrease in inflammation (Serezani et al., 2008; Peters-Golden, 2009; Gerlo et al., 2011). Supporting this hypothesis, dopamine-mediated inhibition of NLRP3 in murine macrophages was linked to cAMP activation, and dopamine-mediated inhibition of NF-κB was associated with D1-mediated activation of PKA, a downstream effector of cAMP, in microglia and other cells lines (Takeuchi and Fukunaga, 2003; Yan et al., 2015; Wang, Chen et al., 2019). In contrast, in hMDM where dopamine activates NF-κB and primes the inflammasome (Nolan et al., 2020), D1-like receptors do not act through the G_αs-cAMP pathway (Nickoloff-Bybel et al., 2019). In T-cells, stimulation of D3 reduces cAMP, which increases naïve CD4⁺ T-cell activation (Franz et al., 2015), further suggesting that a lack of cAMP/PKA signaling supports dopamine-mediated inflammatory effects.

Differences in the capacity of distinct dopamine receptors to activate the MAPK cascade may also contribute to variations in the effects of dopamine. The MAPK signaling cascade modulates numerous cellular functions, including inflammatory cytokine release, in part via the activation of transcription factors such as activator protein 1 (Karin, 1995; Kaminska et al., 2009; Furler and Uittenbogaart, 2010; Cargnello and Roux, 2011; Kyriakis and Avruch, 2012). Dopamine can activate all members of the MAPK family through both D1- and D2-like receptors, although the precise mechanism is unclear and varies from system to system (Luo et al., 1998; Choi et al., 1999; Zhen et al., 2001; Beom et al., 2004; Wang et al., 2005; Lee et al., 2006; Huang et al., 2012; Franz et al., 2015). Many studies have connected the capacity of dopamine to regulate different members of this family with its effects on inflammation. In naïve CD4⁺ T-cells, D3 and D5 mediate ERK1/2 phosphorylation and promote the differentiation of inflammatory T_h1 cells (Franz et al., 2015). However, in DCs, D5 stimulation downregulated LPS-induced ERK1/2 phosphorylation but did not affect JNK or p38 MAPK phosphorylation. The D5-mediated reduction in ERK1/2 was associated with decreased IL-12 and IL-23 production in specific DC subsets, suggesting an inhibitory effect (Prado et al., 2012). Similarly, in primary microglia and BV-2 microglia, dopamine downregulated ERK phosphorylation, while in resting microglia dopamine increased p38 MAPK activity (Fan et al., 2018). Interestingly, in rat astrocytes, D1-like activation of ERK1/2 was associated with cell migration (Huang et al., 2012), supporting differences in the effects of dopamine-induced MAPK signaling between disparate cell types. Indeed, studies have shown both cell-type and receptor-specific differences in D2-like receptor-mediated ERK activation (Beom et al., 2004; Wang et al., 2005). As with cAMP and Akt signaling, this suggests discrete effects of dopamine receptor subtypes on MAPK activation, which may mediate the effects of dopamine on the inflammatory responses in these cells.

Overall, the data discussed in this section of the review indicate that dopamine influences a wide array of immune functions in both an inflammatory and anti-inflammatory manner. Data on most of these functions is relatively sparse, owing to the large variations in experimental design (species, dopamine/ligand concentration, disease model) and potential cell type– and species-specific differences in dopamine receptor expression and signaling bias. Thus, there is no clear consensus on the roles of distinct dopamine receptor subtypes in mediating these effects. The hypothesis that D1-like receptors are inflammatory while D2-like are anti-inflammatory is not broadly supported by the data, although there are certainly specific cell types or diseases, which will be discussed in depth in the following sections, in which this may be the case. Evidence suggests that dopamine may be more anti-inflammatory in stimulated cells and inflammatory in resting cells, and studies in human cells generally show an inflammatory effect of dopamine, although this may be due to the smaller numbers of studies done in humans relative to other systems. Data also suggest that the effects of specific dopamine receptors may differ between cell types and among the same cell type in different species. Signaling pathways activated by dopamine receptors are also likely to overlap, and the activation of multiple dopamine receptor subtypes could interact and result in opposing, additive, or novel outcomes.

Moving forward, careful evaluation of dopamine receptor expression in each model system, defining both raw receptor expression and the ratios of different dopamine receptors, is important in precisely defining the receptors and signaling mechanisms that mediate a particular effect. Many areas warrant further investigation, including the understanding of dopamine release in immune cells, examination of cell type–specific dopamine receptor signaling and specific immune activity, and delineation of dopamine-mediated inflammation from inflammation mediated by pharmacologic drugs that modify dopamine signaling. Broadly, these data show the importance of dopamine in the regulation of immune function and the need for further studies with carefully considered experimental designs to better understand and leverage dopaminergic immunomodulation for disease treatment. In the subsequent sections, we will discuss how the immunomodulatory effects discussed here impact different organs and diseases, highlighting the potential for dopaminergic immunomodulation in the treatment of a wide array of pathogenic conditions.

2. Parkinson’s Disease

PD is a progressive neurodegenerative condition that affects more than 6 million people globally (GBD 2016 Parkinson’s Disease Collaborators, 2018). It causes a variety of motor and nonmotor symptoms such as resting tremor, bradykinesia, rigidity, and gait and posture alterations, as well as cognitive impairment, anxiety, depression, sleep disturbances, and pain (Poewe et al., 2017; GBD 2016 Parkinson’s Disease Collaborators, 2018). Current data indicate that PD results in dysfunction and the loss of dopaminergic neurons in the SbN (Hirsch et al., 1988; Damier et al., 1999), which is likely related to the formation of α-synuclein protein aggregates known as Lewy bodies (Schulz-Schaeffer, 2010; Dagra et al., 2021). Elevated levels of α-synuclein are found in the CNS of PD patients (Tokuda et al., 2010; Irwin et al., 2012; Kouli et al., 2020) and can activate immune cells, such as microglia, macrophages, and T-cells, and astrocytes, increasing neuroinflammation, which is central to the progression of PD (Croisier et al., 2005; Thomas et al., 2007; Reynolds et al., 2008; Lee et al., 2010; Lindestam Arlehamn et al., 2020). Interestingly, recent data show that TH is upregulated in monocytes in the blood of PD patients and that TH expression in these cells is driven by TNF-α (Gopinath et al., 2021), suggesting that dopamine activity in peripheral myeloid cells may influence or be useful as a biomarker for PD progression.

In the CNS, microgliosis, astrogliosis and leukocyte infiltration are common histopathological findings in PD (Banati et al., 1998; Knott et al., 1999; Wilson et al., 2019; Kouli et al., 2020), and widespread microglial activation is present in PD patients (McGeer et al., 1988; Banati et al., 1998; Mirza et al., 2000; Imamura et al., 2003, 2005). In both humans and animal models, microglial activity correlates with neuronal death, starting prior to the death of dopaminergic neurons and exerting a neurotoxic effect on dopaminergic neurons that can result in neurodegeneration (Gao et al., 2002; Sugama et al., 2003, 2004;Ouchi et al., 2005; Zhang et al., 2005; Gerhard et al., 2006; Sawada et al., 2007; Kang et al., 2018). Neurotoxicity is mainly mediated by the production of ROS and the secretion of inflammatory cytokines such as TNF-α, IL-6 and IL-1β, which are elevated in the brain, cerebrospinal fluid (CSF), and serum in PD patients (Boka et al., 1994; Mogi et al., 1994; Müller et al., 1998; Imamura et al., 2005; Karpenko et al., 2018). The presence of ROS can exacerbate α-synuclein aggregation (Scudamore and Ciossek, 2018), and α-synuclein itself can activate microglia and astrocytes by binding to TLRs (Fellner et al., 2013; Kim et al., 2013; Daniele et al., 2015; Kouli et al., 2020; Sun et al., 2021) that are increased in PD brains (Shin et al., 2015; Dzamko et al., 2017; Maatouk et al., 2018; Ping et al., 2019). Increased TLR activation drives inflammation through NF-κB (Mogi et al., 2007; Reynolds et al., 2008; Pranski et al., 2013) and activates the inflammasome, which has been increasingly linked to PD pathology (Gordon et al., 2018; von Herrmann et al., 2018). Notably, inhibition of the NLRP3 inflammasome, which is primarily activated in macrophages and microglia (Gold and El Khoury, 2015; Chaurasia et al., 2018; Guermonprez and Helft, 2019; Voet et al., 2019), improves disease progression by ameliorating dopaminergic neurodegeneration, striatal dopamine depletion, the formation of α-synuclein aggregates, and the secretion of TNF-α, IL-6, and IL-1β (Mao et al., 2017; Gordon et al., 2018; Campolo et al., 2019; Lee et al., 2019; Ou et al., 2021).

Disruption of the dopaminergic system that triggers immune activation, particularly activation of the NLRP3 inflammasome, may create a feed-forward cycle and exacerbate the development of PD (Fig. 6). The loss of dopaminergic neurons means these cells are no longer available to regulate the concentration or spatial distribution of dopamine via DAT-mediated uptake, potentially exposing immune cells to aberrant dopamine concentrations that could drive inflammatory activity. These effects could be compounded by changes in the expression of dopamine receptors and other dopaminergic proteins on immune cells. While D1 expression is not altered in treated or untreated PD patients (Rinne et al., 1990; Shinotoh et al., 1993; Laihinen et al., 1994), D2 expression is initially upregulated and then decreases as the disease progresses (Rinne et al., 1990, 1995; Antonini et al., 1997; Kaasinen et al., 2000; Yang, Knight et al., 2021), correlating with PD severity (Antonini et al., 1995). Downregulation of D2 occurs mainly in the striatum of PD patients and not in the SbN, where the bulk of dopaminergic cell death occurs (Yang et al., 2021b), suggesting that dopaminergic neuronal death does not directly cause the decrease in D2. Despite the lack of direct effect on neuronal viability, the age-associated reduction in D2 (Antonini et al., 1993) may be one of the reasons that age is a risk factor for PD.