Effect of dopamine on immune cell proliferation in mice

Dopaminergic regulation of inflammation and immunity in Parkinson's disease: friend or foe?

Parkinson's disease (PD) is a neurodegenerative disease affecting 7-10 million people worldwide. Currently, there is no treatment available to prevent or delay PD progression, partially due to the limited understanding of the pathological events which lead to the death of dopaminergic neurons in the substantia nigra in the brain, which is known to be the cause of PD symptoms. The current available treatments aim at compensating dopamine (DA) deficiency in the brain using its precursor levodopa, dopaminergic agonists and some indirect dopaminergic agents. The immune system is emerging as a critical player in PD. Therefore, immune-based approaches have recently been proposed to be used as potential antiparkinsonian agents. It has been well-known that dopaminergic pathways play a significant role in regulating immune responses in the brain. Although dopaminergic agents are the primary antiparkinsonian treatments, their immune regulatory effect has yet to be fully understood. The present review summarises the current available evidence of the immune regulatory effects of DA and its mimics and discusses dopaminergic agents as antiparkinsonian drugs. Based on the current understanding of their involvement in the regulation of neuroinflammation in PD, we propose that targeting immune pathways involved in PD pathology could offer a better treatment outcome for PD patients.

Dopamine, Immunity, and Disease

The neurotransmitter dopamine is a key factor in central nervous system (CNS) function, regulating many processes including reward, movement, and cognition. Dopamine also regulates critical functions in peripheral organs, such as blood pressure, renal activity, and intestinal motility. Beyond these functions, a growing body of evidence indicates that dopamine is an important immunoregulatory factor. Most types of immune cells express dopamine receptors and other dopaminergic proteins, and many immune cells take up, produce, store, and/or release dopamine, suggesting that dopaminergic immunomodulation is important for immune function. Targeting these pathways could be a promising avenue for the treatment of inflammation and disease, but despite increasing research in this area, data on the specific effects of dopamine on many immune cells and disease processes remain inconsistent and poorly understood. Therefore, this review integrates the current knowledge of the role of dopamine in immune cell function and inflammatory signaling across systems. We also discuss the current understanding of dopaminergic regulation of immune signaling in the CNS and peripheral tissues, highlighting the role of dopaminergic immunomodulation in diseases such as Parkinson's disease, several neuropsychiatric conditions, neurologic human immunodeficiency virus, inflammatory bowel disease, rheumatoid arthritis, and others. Careful consideration is given to the influence of experimental design on results, and we note a number of areas in need of further research. Overall, this review integrates our knowledge of dopaminergic immunology at the cellular, tissue, and disease level and prompts the development of therapeutics and strategies targeted toward ameliorating disease through dopaminergic regulation of immunity. SIGNIFICANCE STATEMENT: Canonically, dopamine is recognized as a neurotransmitter involved in the regulation of movement, cognition, and reward. However, dopamine also acts as an immune modulator in the central nervous system and periphery. This review comprehensively assesses the current knowledge of dopaminergic immunomodulation and the role of dopamine in disease pathogenesis at the cellular and tissue level. This will provide broad access to this information across fields, identify areas in need of further investigation, and drive the development of dopaminergic therapeutic strategies.

Dopamine attenuates lipopolysaccharide-induced expression of proinflammatory cytokines by inhibiting the nuclear translocation of NF-κB p65 through the formation of dopamine quinone in microglia

Many reports have indicated that dopamine has immunomodulatory effects on peripheral immune cells. The purpose of this study was to reveal the immunomodulatory effect of dopamine on the expression of proinflammatory cytokines in microglial cells, which are the immune cells of the central nervous system. In murine microglial cell line BV-2 cells, pretreatment with dopamine for 24 h attenuated the lipopolysaccharide (LPS)-induced expression of proinflammatory cytokines such as tumor-necrosis factor-α, interleukin-1β, and interleukin-6. Neither (5R)-8-chloro-3-methyl-5-phenyl-1,2,4,5-tetrahydro-3-benzazepin-7-ol; hydrochloride (SCH-23390) nor sulpiride, which are dopamine D₁-like and D₂-like receptor antagonists, respectively, affected the attenuation of LPS-induced expression of cytokines by dopamine. In addition, pretreatment with neither (-)-(6aR,12bR)-4,6,6a,7,8,12b-Hexahydro-7-methylindolo[4,3-a]phenanthridin (CY208-243) nor bromocriptine, dopamine D₁-like and D₂-like receptor agonists, respectively, was effective in doing so. However, N-acetylcysteine (NAC), which inhibits dopamine oxidation to dopamine quinone, did inhibit this attenuated expression. Dopamine increased the level of quinoproteins, and this increase was inhibited by NAC. Western blot and immunocytochemical analyses revealed that dopamine inhibited LPS-induced nuclear translocation of nuclear factor-kappa B (NF-κB) p65. Dopamine also attenuated the expression of cytokines and the nuclear translocation of NF-κB p65 induced by LPS in mouse microglial cells in primary culture. These results suggest that dopamine attenuated LPS-induced expression of cytokines by inhibiting the nuclear translocation of NF-κB p65 through the formation of dopamine quinone in microglial cells.

Where Is Dopamine and how do Immune Cells See it?: Dopamine-Mediated Immune Cell Function in Health and Disease

Dopamine is well recognized as a neurotransmitter in the brain, and regulates critical functions in a variety of peripheral systems. Growing research has also shown that dopamine acts as an important regulator of immune function. Many immune cells express dopamine receptors and other dopamine related proteins, enabling them to actively respond to dopamine and suggesting that dopaminergic immunoregulation is an important part of proper immune function. A detailed understanding of the physiological concentrations of dopamine in specific regions of the human body, particularly in peripheral systems, is critical to the development of hypotheses and experiments examining the effects of physiologically relevant dopamine concentrations on immune cells. Unfortunately, the dopamine concentrations to which these immune cells would be exposed in different anatomical regions are not clear. To address this issue, this comprehensive review details the current information regarding concentrations of dopamine found in both the central nervous system and in many regions of the periphery. In addition, we discuss the immune cells present in each region, and how these could interact with dopamine in each compartment described. Finally, the review briefly addresses how changes in these dopamine concentrations could influence immune cell dysfunction in several disease states including Parkinson's disease, multiple sclerosis, rheumatoid arthritis, inflammatory bowel disease, as well as the collection of pathologies, cognitive and motor symptoms associated with HIV infection in the central nervous system, known as NeuroHIV. These data will improve our understanding of the interactions between the dopaminergic and immune systems during both homeostatic function and in disease, clarify the effects of existing dopaminergic drugs and promote the creation of new therapeutic strategies based on manipulating immune function through dopaminergic signaling. Graphical Abstract.

The effects of pessimism on cell-mediated immunity in rats

We used a recently developed ambiguous-cue interpretation (ACI) paradigm to investigate whether 'optimism' and 'pessimism' as behavioural traits may be interrelated with immune functions in rodents. To this aim, in a series of ACI tests (cognitive bias screening, CBS), we identified rats that displayed 'pessimistic' and 'optimistic' traits. We found significant differences in immune biomarkers between 'optimistic' and 'pessimistic' animals. Moreover 'pessimism' was associated with significantly lower relative weight of the spleen and thymus, significantly decreased proliferative activity of splenocytes. Pessimism was associated with an increased production of interleukin-(IL)1β and IL-4, activin A, l-selectin, interferon (IFN)-γ and some chemokines and receptors for advanced glycation endproducts. The findings indicate an inflammatory profile in "pessimistic" animals.

Immunomodulatory Effects Mediated by Dopamine

Dopamine (DA), a neurotransmitter in the central nervous system (CNS), has modulatory functions at the systemic level. The peripheral and central nervous systems have independent dopaminergic system (DAS) that share mechanisms and molecular machinery. In the past century, experimental evidence has accumulated on the proteins knowledge that is involved in the synthesis, reuptake, and transportation of DA in leukocytes and the differential expression of the D1-like (D1R and D5R) and D2-like receptors (D2R, D3R, and D4R). The expression of these components depends on the state of cellular activation and the concentration and time of exposure to DA. Receptors that are expressed in leukocytes are linked to signaling pathways that are mediated by changes in cAMP concentration, which in turn triggers changes in phenotype and cellular function. According to the leukocyte lineage, the effects of DA are associated with such processes as respiratory burst, cytokine and antibody secretion, chemotaxis, apoptosis, and cytotoxicity. In clinical conditions such as schizophrenia, Parkinson disease, Tourette syndrome, and multiple sclerosis (MS), there are evident alterations during immune responses in leukocytes, in which changes in DA receptor density have been observed. Several groups have proposed that these findings are useful in establishing clinical status and clinical markers.

Dopamine and T cells: dopamine receptors and potent effects on T cells, dopamine production in T cells, and abnormalities in the dopaminergic system in T cells in autoimmune, neurological and psychiatric diseases

Dopamine, a principal neurotransmitter, deserves upgrading to 'NeuroImmunotransmitter' thanks to its multiple, direct and powerful effects on most/all immune cells. Dopamine by itself is a potent activator of resting effector T cells (Teffs), via two independent ways: direct Teffs activation, and indirect Teffs activation by suppression of regulatory T cells (Tregs). The review covers the following findings: (i) T cells express functional dopamine receptors (DRs) D1R-D5R, but their level and function are dynamic and context-sensitive, (ii) DR membranal protein levels do not necessarily correlate with DR mRNA levels, (iii) different T cell types/subtypes have different DR levels and composition and different responses to dopamine, (iv) autoimmune and pro-inflammatory T cells and T cell leukaemia/lymphoma also express functional DRs, (v) dopamine (~10(-8) M) activates resting/naive Teffs (CD8(+) >>>CD4(+) ), (vi) dopamine affects Th1/Th2/Th17 differentiation, (vii) dopamine inhibits already activated Teffs (i.e. T cells that have been already activated by either antigen, mitogen, anti-CD3 antibodies cytokines or other molecules), (viii) dopamine inhibits activated Tregs in an autocrine/paracrine manner. Thus, dopamine 'suppresses the suppressors' and releases the inhibition they exert on Teffs, (ix) dopamine affects intracellular signalling molecules and cascades in T cells (e.g. ERK, Lck, Fyn, NF-κB, KLF2), (x) T cells produce dopamine (Tregs>>>Teffs), can release dopamine, mainly after activation (by antigen, mitogen, anti-CD3 antibodies, PKC activators or other), uptake extracellular dopamine, and most probably need dopamine, (xi) dopamine is important for antigen-specific interactions between T cells and dendritic cells, (xii) in few autoimmune diseases (e.g. multiple sclerosis/SLE/rheumatoid arthritis), and neurological/psychiatric diseases (e.g. Parkinson disease, Alzheimer's disease, Schizophrenia and Tourette), patient's T cells seem to have abnormal DRs expression and/or responses to dopamine or production of dopamine, (xiii) drugs that affect the dopaminergic system have potent effects on T cells (e.g. dopamine=Intropin, L-dopa, bromocriptine, haloperidol, quinpirole, reserpine, pergolide, ecopipam, pimozide, amantadine, tetrabenazine, nomifensine, butaclamol). Dopamine-induced activation of resting Teffs and suppression of Tregs seem beneficial for health and may also be used for immunotherapy of cancer and infectious diseases. Independently, suppression of DRs in autoimmune and pro-inflammatory T cells, and also in cancerous T cells, may be advantageous. The review is relevant to Immunologists, Neurologists, Neuroimmunologists, Hematologists, Psychiatrists, Psychologists and Pharmacologists.

Vaccination strategies for Parkinson disease: induction of a swift attack or raising tolerance?

Parkinson disease is the second most common neurodegenerative disease in the world, but there is currently no available cure for it. Current treatments only alleviate some of the symptoms for a few years, but they become ineffective in the long run and do not stop the disease. Therefore it is of outmost importance to develop therapeutic strategies that can prevent, stop, or cure Parkinson disease. A very promising target for these therapies is the peripheral immune system due to its probable involvement in the disease and its potential as a tool to modulate neuroinflammation. But for such strategies to be successful, we need to understand the particular state of the peripheral immune system during Parkinson disease in order to avoid its weaknesses. In this review we examine the available data regarding how dopamine regulates the peripheral immune system and how this regulation is affected in Parkinson disease; the specific cytokine profiles observed during disease progression and the alterations documented to date in patients' peripheral blood mononuclear cells. We also review the different strategies used in Parkinson disease animal models to modulate the adaptive immune response to salvage dopaminergic neurons from cell death. After analyzing the evidence, we hypothesize the need to prime the immune system to restore natural tolerance against α-synuclein in Parkinson disease, including at the same time B and T cells, so that T cells can reprogram microglia activation to a beneficial pattern and B cell/IgG can help neurons cope with the pathological forms of α-synuclein.

Neuro-immune modulation of the thymus microenvironment (review)

The thymus is the primary site for T-cell lympho-poiesis. Its function includes the maturation and selection of antigen specific T cells and selective release of these cells to the periphery. These highly complex processes require precise parenchymal organization and compartmentation where a plethora of signalling pathways occur, performing strict control on the maturation and selection processes of T lymphocytes. In this review, the main morphological characteristics of the thymus microenvironment, with particular emphasis on nerve fibers and neuropeptides were assessed, as both are responsible for neuro-immune‑modulation functions. Among several neurotransmitters that affect thymus function, we highlight the dopaminergic system as only recently has its importance on thymus function and lymphocyte physiology come to light.

Association of leukocytosis with amphetamine and cocaine use

OBJECTIVE

Determining the etiology of unexplained leukocytosis in asymptomatic patients may incur unnecessary testing, cost, and prolonged emergency department stay. The objective was to delineate if use of amphetamines and/or cocaine is a factor.

METHODS

For two years we reviewed all psychiatric patients presenting for medical clearance with exclusions for infection, epilepsy, trauma, or other nonpsychiatric medical conditions.

RESULTS

With a total of 1,206 patients, 877 (72.7%) amphetamines/cocaine-negative drug screen controls had mean WBC 8.4 ± 2.6 × 10³/µL. The 240 (19.9%) amphetamines-positive, cocaine-negative, patients had WBC 9.4 ± 3.3 × 10³/µL (P < 0.0001). The 72 (6.0%) amphetamines-negative, cocaine-positive, patients had WBC 7.1 ± 1.8 × 10³/µL (P < 0.0001). The remaining 17 (1.4%) amphetamines/cocaine-positive patients had WBC 10.0 ± 4.2 × 10³/µL (P = 0.01). Amphetamines-positive patients had a supranormal WBC ratio significantly higher than controls (23.8% versus 14.8%, P = 0.001), whereas only one cocaine-positive patient had a supranormal WBC count, with significantly lower ratio (1.4%, P = 0.0003).

CONCLUSION

Use of amphetamines, not cocaine, may be associated with idiopathic leukocytosis. This may be explained by unique pharmacologic, neuroendocrine, and immunomodulatory differences.

Dopamine receptor D3 expressed on CD4+ T cells favors neurodegeneration of dopaminergic neurons during Parkinson's disease

Emerging evidence has demonstrated that CD4(+) T cells infiltrate into the substantia nigra (SN) in Parkinson's disease (PD) patients and in animal models of PD. SN-infiltrated CD4(+) T cells bearing inflammatory phenotypes promote microglial activation and strongly contribute to neurodegeneration of dopaminergic neurons. Importantly, altered expression of dopamine receptor D3 (D3R) in PBLs from PD patients has been correlated with disease severity. Moreover, pharmacological evidence has suggested that D3R is involved in IFN-γ production by human CD4(+) T cells. In this study, we examined the role of D3R expressed on CD4(+) T cells in neurodegeneration of dopaminergic neurons in the SN using a mouse model of PD. Our results show that D3R-deficient mice are strongly protected against loss of dopaminergic neurons and microglial activation during 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD. Notably, D3R-deficient mice become susceptible to MPTP-induced neurodegeneration and microglial activation upon transfer of wild-type (WT) CD4(+) T cells. Furthermore, RAG1 knockout mice, which are devoid of T cells and are resistant to MPTP-induced neurodegeneration, become susceptible to MPTP-induced loss of dopaminergic neurons when reconstituted with WT CD4(+) T cells but not when transferred with D3R-deficient CD4(+) T cells. In agreement, experiments analyzing activation and differentiation of CD4(+) T cells revealed that D3R favors both T cell activation and acquisition of the Th1 inflammatory phenotype. These findings indicate that D3R expressed on CD4(+) T cells plays a fundamental role in the physiopathology of MPTP-induced PD in a mouse model.

Distorted expression of dopamine receptor genes in systemic lupus erythematosus

Several observations suggest that alterations in the neurotransmitter dopamine and/or its receptors could be associated with the pathophysiology of lupus. We therefore assessed expression of the five dopamine receptor genes in a cohort of patients. We found that all receptors are expressed in lupus peripheral blood cells. We also discovered that dopamine receptor 2 gene (DR2) was underexpressed, and that DR4 was overexpressed in lupus patients, as compared to controls. Cell sorting of peripheral T- and B-lymphocytes disclosed that the altered DR2 and DR4 expressions were borne by T-cells. These distorted expressions of DR2 and DR4 could influence immune functions in lupus through several mechanisms. Since DR2 can be effective in regulating the activation and differentiation of naive CD4⁺ cells by promoting polarization toward regulatory T-cells, the underexpression of DR2 we have observed may account, at least in part, for the reduction of regulatory T-cell function and/or numbers in lupus. In addition to providing novel insight into disease pathogenesis, our findings may have therapeutic implications. Because DR4 can be effective in triggering T-cell quiescence, its overexpression on lupus T cells suggests that inducing quiescence using DR4-specific agonists may represent a useful strategy in the treatment of lupus.

Dopamine D1-like receptor antagonist attenuates Th17-mediated immune response and ovalbumin antigen-induced neutrophilic airway inflammation

Allergic airway inflammation is generally considered a Th2-type immune response. Recent studies, however, demonstrated that Th17-type immune responses also play important roles in this process, especially in the pathogenesis of neutrophilic airway inflammation, a hallmark of severe asthma. We previously reported that dendritic cells release dopamine to naive CD4(+) T cells in Ag-specific cell-cell interaction, in turn inducing Th17 differentiation through dopamine D1-like receptor (D1-like-R). D1-like-R antagonist attenuates Th17-mediated diseases such as experimental autoimmune encephalomyelitis and autoimmune diabetes. However, the effect of antagonizing D1-like-R on Th17-mediated airway inflammation has yet to be studied. In this study, we examined whether D1-like-R antagonist suppresses OVA-induced neutrophilic airway inflammation in OVA TCR-transgenic DO11.10 mice and then elucidated the mechanism of action. DO11.10 mice were nebulized with OVA or PBS, and some mice received D1-like-R antagonist orally before OVA nebulization. D1-like-R antagonist significantly suppressed OVA-induced neutrophilic airway inflammation in DO11.10 mice. It also inhibited the production of IL-17 and infiltration of Th17 cells in the lung. Further, D1-like-R antagonist suppressed the production of IL-23 by lung CD11c(+) APCs. In contrast, D1-like-R antagonist did not increase Foxp3(+) regulatory T cells in the lung. D1-like-R antagonist neither suppressed nonspecific LPS-induced neutrophilic airway inflammation nor OVA-induced eosinophilic airway inflammation. These results indicate that D1-like-R antagonist could suppress Th17-mediated neutrophilic airway inflammation, raising the possibility that antagonizing D1-like-R serves as a promising new strategy for treating neutrophil-dominant severe asthma.

Dopamine, vesicular transporters and dopamine receptor expression and localization in rat thymus and spleen

The localization of dopamine stores and the expression and localization of vesicular monoamine transporter (VMAT) type-1 and 2 and of dopamine D1-like and D2-like receptor subtypes were investigated in rat thymus and spleen by immunohistochemical, immunochemical techniques and by RT-PCR. In the thymus dopamine immunoreactivity was developed in the cortico-medullary junction and in the medulla, but not in the thymic cortex. In the spleen, dopamine stores were found in reticular structures in the white pulp border and in the white pulp, but not in the red one. Both thymus and spleen expressed VMAT-1 and VMAT-2 immunoreactivity as well as dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity. Immunohistochemistry revealed VMAT-1, VMAT-2 and dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity primarily in the thymic cortical-medulla transitional zone and to a lesser extent in the medulla but not in the cortex. In the spleen, VMAT-1, VMAT-2 and dopamine D1, D2, D3, D4 and D5 receptor immunoreactivity was located primarily in the white pulp border and to a lesser extent in the white pulp. These findings indicate that both thymus and spleen express a dopaminergic system characterized by the presence of dopamine, vesicular monoamine transporters and the five subtypes of dopamine receptors. The presence of these dopaminergic markers suggests that dopamine likely originating from immune cells and/or from sympathetic neuroeffector plexus is released in the lymphoid microenvironment. Based on the microanatomical localization of dopaminergic markers investigated, a role of dopamine in maturation and selection of lymphocytes and activation of immune responses is suggested.

Proclivity to self-injurious behavior in MRL-lpr mice: implications for autoimmunity-induced damage in the dopaminergic system

Systemic lupus erythematosus is frequently accompanied by psychiatric manifestations of unknown origin. Although damage of central neurons had been documented, little is known about neurotransmitter systems affected by the autoimmune/inflammatory process. Recent studies on lupus-prone MRL-lpr mice point to imbalanced dopamine function and neurodegeneration in dopamine-rich brain regions. We follow up on anecdotal observations of singly housed mice developing chest wounds. Compulsive grooming and/or skin biting accounted for open lesions, lending itself to the operational term 'self-injurious behavior' (SIB). Low incidence of spontaneous SIB increased significantly after repeated injections of dopamine-2/3 receptor (D2/D3R) agonist quinpirole (QNP). To further probe the dopaminergic circuitry and examine whether SIB is associated with development of lupus-like disease, we compared behavioral responses among cohorts that differed in the immune status. Two-week treatment with QNP (intraperitoneal, 0.5 mg kg(-1) body weight per day) induced SIB in 60% of diseased MRL-lpr mice, and exacerbated their splenomegaly. Although increased grooming and stereotypy were observed in less symptomatic MRL+/+ controls, only one mouse (10%) developed SIB. Similarly, SIB was not seen in young, asymptomatic groups despite dissimilar ambulatory responses to QNP. In situ hybridization revealed treatment-independent upregulation of D2R mRNA in substantia nigra of diseased MRL-lpr mice. The above results suggest that development of systemic autoimmunity alters sensitivity of the dopaminergic system and renders MRL-lpr mice prone to SIB. Although pathogenic factors were not examined, we hypothesize that immune and endocrine mechanisms jointly contribute to early neuronal damage, which underlies behavioral deficiency in the adulthood.

Catecholamines-crafty weapons in the inflammatory arsenal of immune/inflammatory cells or opening pandora's box?

It is well established that catecholamines (CAs), which regulate immune and inflammatory responses, derive from the adrenal medulla and from presynaptic neurons. Recent studies reveal that T cells also can synthesize and release catecholamines which then can regulate T cell function. We have shown recently that macrophages and neutrophils, when stimulated, can generate and release catecholamines de novo which, then, in an autocrine/paracrine manner, regulate mediator release from these phagocytes via engagement of adrenergic receptors. Moreover, regulation of catecholamine-generating enzymes as well as degrading enzymes clearly alter the inflammatory response of phagocytes, such as the release of proinflammatory mediators. Accordingly, it appears that phagocytic cells and lymphocytes may represent a major, newly recognized source of catecholamines that regulate inflammatory responses.

Circulating interleukin-15 and RANTES chemokine in Parkinson's disease

Interleukin-15 promotes T-cell proliferation, induction of cytolytic effector cells including natural killer (NK) and cytotoxic cells and stimulates B-cell to proliferate and secrete immunoglobulins. RANTES is a C-C beta chemokine with strong chemoattractant activity for T lymphocytes and monocytes.

OBJECTIVES

The objective of our study was to find out whether IL-15 and RANTES are involved in the possible inflammatory reactions of PD.

PATIENTS AND METHODS

We measured by immunoassay serum IL-15 and RANTES levels in 41 patients with PD in comparison with serum levels in 19 healthy subjects age and sex-matched. IL-15 and RANTES levels were correlated with sex, age, disease duration. H-Y stage and the UPDRS III score in all the studied groups and were also correlated with treatment status in PD patients.

RESULTS

The PD group presented with significantly increased RANTES levels as compared to the control group (P = 0.0009). No difference was observed as regards IL-15 levels. A strong and significant correlation between RANTES levels and UPDRS III score was observed in PD patients (R(s) = 0.42, P = 0.007). Untreated patients had significantly higher RANTES levels as compared to the controls.

CONCLUSIONS

Our findings may suggest a recruitment of activated monocytes, macrophages and T lymphocytes to sites of inflammation in the central nervous system of PD patients.

Dopamine plasma membrane transporter (DAT) in rat thymus and spleen: an immunochemical and immunohistochemical study

The expression of the dopamine plasma membrane transporter (DAT) was investigated in rat thymus and spleen by immunochemical and immunohistochemical techniques. Antibodies raised against a peptide mapping near the amino terminus of DAT were bound to a single band of approximately 76 kDa in thymus and spleen membranes as well as in striatal and kidney membranes which were used as dopaminergic reference tissues. Reverse transcription-polymerase chain reaction analysis revealed that both thymus and spleen expressed DAT mRNA. Immunohistochemistry revealed in rat thymus a DAT immune reaction in the wall of arteries located in septa of connective tissue as well as in the medulla, with a reticular localization and an apparent negative reaction of thymocytes. In the spleen, DAT immunoreactivity was located primarily in the red-white pulp marginal zone, within small cells, likely corresponding to lymphocytes and in the wall of white pulp arteries. The presence of a dopamine transporter suggests that dopamine released in the lymphoid microenvironment may contribute to neuroimmune modulation. It cannot be excluded a different activity of dopamine in primary and secondary immune organs, such as maturation and selection of lymphocytes and activation of immune responses in the spleen.

Effekte von Dopamin auf die zelluläre und humorale Immunantwort von Patienten mit Sepsis

In vitro and in vivo studies have demonstrated that apart from its hemodynamic action dopamine can modulate immune responses. Dopamine reduces the synthesis of proinflammatory and induces the synthesis of anti-inflammatory mediators. Dopamine inhibits neurohormone synthesis, lymphocyte proliferation and platelet aggregation. It reduces the phagocytic activity of neutrophils and induces apoptosis. Particularly with regard to sepsis, where high serum dopamine levels are reached by enhanced endogenous production, exogenous application and impaired clearance, this immunomodulation may have a clinical impact. This review summarizes dopamine-mediated immunomodulating effects to advance the knowledge regarding dopamine as an immune regulator under septic conditions.

Close encounters of the monoamine kind: immune cells betray their nervous disposition

Here we review the evidence for immune cells expressing multiple components of the serotonergic and dopaminergic systems that are more commonly associated with the central nervous system (CNS). We discuss where and how peripheral encounters with these biogenic monoamines occur and posit reasons as to why the immune system would wish to deploy these pathways. A full taxonomy of serotonergic and dopaminergic constituents and their workings in component cells of the immune system should facilitate the formulation of novel therapeutic approaches in diseases characterized by immune dysfunction and potentially provide a range of surrogate peripheral markers for registering and monitoring disturbances within the CNS.

Changes in innate and acquired immune responses in mice with targeted deletion of the dopamine transporter gene

The dopamine transporter (DAT) is responsible for the re-uptake of dopamine into presynaptic nerve terminals and thereby controls dopaminergic neurotransmission. Deletion of DAT results in a hyperdopaminergic phenotype and DAT(-/-) mice are characterized by pituitary hypoplasia, impaired maternal behavior, and increased locomotion. From earlier studies, we have evidence that the activity of the central dopaminergic system may play a role in determining immune reactivity and disease susceptibility. To further explore the functional relation between the dopaminergic system and the immune system, we investigated the activity of the immune system in DAT(-/-) mice. We show that in vitro, splenocytes from DAT(-/-) mice displayed reduced natural killer cell activity and reduced mitogen-induced cytokine responses. In contrast, LPS-induced cytokine production by macrophages was enhanced. In vivo, the cellular response to immunization with ovalbumine (OVA-induced delayed type hypersensitivity response) was significantly reduced. Interestingly, the OVA-induced humoral response (anti-OVA IgG) was increased in DAT(-/-) mice compared to wild-type animals. Plasma levels of catecholamines and corticosterone did not differ significantly between DAT(-/-) and wild-type animals. In conclusion, we show in the present study that interfering with the dopaminergic system has major consequences for both the acquired and the innate immune response.

Dopamine exerts no acute effects on Kv1.3 in activated encephalitogenic T cells

Apart from a central function in the extrapyramidal motor system, dopamine has been suggested to play a role in neuroimmune interactions. Particularly in diseases of the central nervous system, such as multiple sclerosis, alterations in dopamine homeostasis might have immunological consequences. We investigated potential effects of dopamine stabilized by ascorbic acid on specifically activated encephalitogenic T cells at the peak of activation. Those cells exhibited an upregulation of voltage-sensitive K+ channels which play a role in many neurotransmitter responses of lymphocytes and fulfilled a prerequisite to respond to dopamine, i.e. stable expression of mRNA for dopamine receptors DRD1, DRD2 and DRD3. However, whole-cell and perforated whole-cell recordings revealed no change in voltage-sensitive K+ currents. Moreover, T cell proliferation was not changed in the presence of dopamine. Previously reported dopamine effects on T cells may be explained by a comparatively lower activation of the cells under investigation, suggesting an activation dependence of dopamine effects that may not be mediated by K+ channels. Alternatively, the occurrence of dopamine degradation products under unprotected conditions may account for the changes reported. Nevertheless, care should be taken when using the dopamine-protecting anti-oxidant ascorbic acid, since we found that it markedly inhibited both K+ currents and lymphocyte proliferation at higher concentrations.

Clinical review: immunomodulatory effects of dopamine in general inflammation

Large quantitaties of inflammatory mediators are released during the course of endotoxaemia. These mediators in turn can stimulate the sympathetic nervous system (SNS) to release catecholamines, which ultimately regulate inflammation-associated impairment in tissue perfusion, myocardial impairment and vasodilatation. Treatment of sepsis is based on surgical and/or antibiotic therapy, appropriate fluid management and application of vasoactive catecholamines. With respect to the latter, discussions on the vasopressor of choice are still ongoing. Over the past decade dopamine has been considered the 'first line' vasopressor and is frequently used to improve organ perfusion and blood pressure. However, there is a growing body of evidence that dopamine has deleterious side effects; therefore, its clinical relevance seems to be more and more questionable. Nevertheless, it has not been convincingly demonstrated that other catecholamines are superior to dopamine in this respect. Apart from its haemodynamic action, dopamine can modulate immune responses by influencing the cytokine network. This leads to inhibition of expression of adhesion molecules, inhibition of cytokine and chemokine production, inhibition of neutrophil chemotaxis and disturbed T-cell proliferation. In the present review we summarize our knowledge of the immunomodulatory effects of dopamine, with an emphasis on the mechanisms by which these effects are mediated.

The enhancement of T cell proliferation by l-dopa is mediated peripherally and does not involve interleukin-2

Although several recent studies suggest that dopamine may have a significant role in the regulation of immune cell function, the mechanisms involved remain unclear. We undertook this study to clarify the actions of dopamine in vivo on lymphocyte proliferation and to determine whether such effects are mediated by alterations in interleukin-2 (IL-2) production and/or IL-2 receptor (IL-2R) expression. Administration of the dopamine precursor, L-dopa, for 5 days to BALB/c mice significantly increased Concanavalin A (ConA)-induced proliferation of lymphocytes. This effect was blocked by treatment with dopamine receptor antagonists as well as a peripheral dopamine synthesis inhibitor, suggesting a peripheral action of dopamine. L-dopa treatment had no effect on IL-2 production or IL-2R expression, indicating that the effects of dopamine on lymphocyte proliferation are not indirectly mediated by alterations in the IL-2 system.

Central catecholamine depletion inhibits peripheral lymphocyte responsiveness in spleen and blood

Experimental and clinical evidence has demonstrated extensive communication between the CNS and the immune system. To analyse the role of central catecholamines in modulating peripheral immune functions, we injected the neurotoxin 6-hydroxydopamine (6-OHDA) i.c.v. in rats. This treatment significantly reduced brain catecholamine content 2, 4 and 7 days after injection, and in the periphery splenic catecholamine levels were reduced 4 days after treatment. Central catecholamine depletion induced an inhibition of splenic and blood lymphocyte proliferation and splenic cytokine production and expression (interleukin-2 and interferon-gamma) 7 days after injection. In addition, central treatment with 6-OHDA reduced the percentage of spleen and peripheral blood natural killer (CD161 +) cells, and T-cytotoxic (CD8 +) cells in peripheral blood. The reduction in splenocyte proliferation was not associated with a glucocorticoid alteration but was completely abolished by prior peripheral sympathectomy. These data demonstrate a crucial role of central and peripheral catecholamines in modulating immune function.

Catecholamines in a macrophage cell line

This study provides the first evidence for catecholamine synthesis and release in the RAW264.7 cell line, an important macrophage model. Although catecholamines were low in unstimulated cells, activation with lipopolysaccharide (LPS) induced tyrosine hydroxylase (TH) mRNA and increased extracellular norepinephrine and intracellular dopamine within 48 h. The catecholamine synthesis inhibitor alpha-methyl-para-tyrosine (alpha-mpt) decreased extracellular norepinephrine levels, suggesting release and rapid turnover of newly synthesized norepinephrine. High concentrations of dopamine or norepinephrine (>/=100 microM) decreased proliferation and increased apoptosis of macrophages. These anti-proliferative effects were prevented by simultaneous treatment with the anti-oxidant ascorbic acid. Pre-incubation with a glutathione synthesis inhibitor (L-buthionine-[S,R]-sulfoximine [L-BSO]) increased sensitivity to catecholamine-stimulated apoptosis, suggesting that glutathione protects macrophages from both endogenous and exogenous catecholamines.

Catecholamines in murine bone marrow derived mast cells

Cultured murine bone marrow derived mast cells (BMMC) were found to store high levels of dopamine (3753+/-844 pg/10(7) cells) and occasionally produce norepinephrine and epinephrine. The catecholamine synthesis inhibitor, alpha-methyl-para-tyrosine, decreased intracellular catecholamine concentrations, and activation with ionomycin stimulated dopamine release. Neither dopaminergic receptor antagonists nor exogenous dopamine < or =10 microM affected IL-3-induced cell proliferation. High exogenous dopamine (20-100 microM) decreased proliferation and increased apoptosis, and the anti-oxidant ascorbic acid prevented these effects. Increased expression of the anti-apoptotic factor Bcl-2 or loss of pro-apoptotic Bax expression attenuated dopamine-induced apoptosis, suggesting the apoptosis proceeds through a mitochondrial pathway.

Circulating dopamine level, in lung carcinoma patients, inhibits proliferation and cytotoxicity of CD4+ and CD8+ T cells by D1 dopamine receptors: an in vitro analysis

Besides cardiovascular and renal functions, the role of dopamine in periphery as an endogenous regulator of immune functions is in the limelight. In human malignancy, depression of T cell functions is known. Interestingly, recent evidences indicate significant elevation of plasma dopamine in malignancy due to stress of the disease process. Therefore, this study evaluates whether this increased plasma dopamine exerts any influence on the proliferation and cytotoxicity of CD4+ and CD8+ T cells. Patients with lung carcinoma were selected for this study due to the high prevalence rate of this kind of cancer in developing countries and also due to strong positive biochemical and psychological criteria of stress in most of the patients. Results showed significant elevation of plasma dopamine (48.6 +/- 5.1 pg/ml) in lung cancer patients than normal controls (10.2 +/- 0.9 pg/ml). In vitro dopamine concentration, simulating the plasma concentration of the patients, significantly inhibited the proliferation and cytotoxicity of T cells of these patients and also of the normal volunteers, in presence of their respective serum. The mechanism has been attributed to be D1 class of dopamine receptor mediated elevation of intracellular cAMP in these cell populations. The results may be of significance in understanding the role of peripheral dopamine as an immunomodulator in human health and diseases.

Dopamine, a neurotransmitter, influences the immune system

Dopamine (DA) is a monoamine neurotransmitter of both central and peripheral nervous system. Its role in the neural-immune communication has been discussed in the present review. Results reveal that in vivo damage or stimulation of specific central dopaminergic system suppresses or enhances functional activities of the immune effector cells. The possible influences of other immunomodulators of the brain by altering brain DA may be the underlying mechanism. Direct effects of DA on the immune effector cells are also contradictory, it is suppressive in vitro, while in pharmacological doses, it is mostly stimulatory in vivo. The possible mechanisms have been discussed. Lastly, future areas of relevance on DA and immunity have been highlighted to advance our knowledge regarding DA as an immune regulator.