Central catecholamine neurotoxin administration. 1. Immunological changes associated with the suppression of experimental autoimmune encephalomyelitis

Chemogenetic activation of locus coeruleus neurons ameliorates the severity of multiple sclerosis

BACKGROUND

Most current disease-modifying therapies approved for multiple sclerosis (MS) are immunomodulatory drugs that counteract the aberrant activity of the immune system. Hence, new pharmacological interventions that drive anti-inflammatory activity and neuroprotection would represent interesting alternative therapeutic approaches or complementary strategies to treat progressive forms of MS. There is evidence of reduced noradrenaline levels and alterations to locus coeruleus (LC) noradrenergic neurons in MS patients, as well as in animal models of this disease, potentially factors contributing to the pathophysiology. Drugs that enhance noradrenaline appear to have some beneficial effects in MS, suggesting their potential to dampen the underlying pathology and disease progression.

METHODS

Therefore, we explored the consequences of chronic LC noradrenergic neurons activation by chemogenetics in experimental autoimmune encephalomyelitis (EAE) mice, the most widely used experimental model of MS. LC activation from the onset or the peak of motor symptoms was explored as two different therapeutic approaches, assessing the motor and non-motor behavioral changes as EAE progresses, and studying demyelination, inflammation and glial activation in the spinal cord and cerebral cortex during the chronic phase of EAE.

RESULTS

LC activation from the onset of motor symptoms markedly alleviated the motor deficits in EAE mice, as well as their anxiety-like behavior and sickness, in conjunction with reduced demyelination and perivascular infiltration in the spinal cord and glial activation in the spinal cord and prefrontal cortex (PFC). When animals exhibited severe paralysis, LC activation produced a modest alleviation of EAE motor symptoms and it enhanced animal well-being, in association with an improvement of the EAE pathology at the spinal cord and PFC level. Interestingly, the reduced dopamine beta-hydroxylase expression associated with EAE in the spinal cord and PFC was reversed through chemogenetic LC activation.

CONCLUSION

Therefore, clear anti-inflammatory and neuroprotective effects were produced by the selective activation of LC noradrenergic neurons in EAE mice, having greater benefits when LC activation commenced earlier. Overall, these data suggest noradrenergic LC neurons may be targets to potentially alleviate some of the motor and non-motor symptoms in MS.

Hormones in experimental autoimmune encephalomyelitis (EAE) animal models

Multiple sclerosis (MS) is an inflammatory demyelinating disease of the central nervous system (CNS) in which activated immune cells attack the CNS and cause inflammation and demyelination. While the etiology of MS is still largely unknown, the interaction between hormones and the immune system plays a role in disease progression, but the mechanisms by which this occurs are incompletely understood. Several in vitro and in vivo experimental, but also clinical studies, have addressed the possible role of the endocrine system in susceptibility and severity of autoimmune diseases. Although there are several demyelinating models, experimental autoimmune encephalomyelitis (EAE) is the oldest and most commonly used model for MS in laboratory animals which enables researchers to translate their findings from EAE into human. Evidences imply that there is great heterogeneity in the susceptibility to the induction, the method of induction, and the response to various immunological or pharmacological interventions, which led to conflicting results on the role of specific hormones in the EAE model. In this review, we address the role of endocrine system in EAE model to provide a comprehensive view and a better understanding of the interactions between the endocrine and the immune systems in various models of EAE, to open up a ground for further detailed studies in this field by considering and comparing the results and models used in previous studies.

Role of the End-Point Mediators of Sympathoadrenal and Sympathoneural Stress Axes in the Pathogenesis of Experimental Autoimmune Encephalomyelitis and Multiple Sclerosis

The role of stress effector systems in the initiation and progression of multiple sclerosis (MS) and experimental autoimmune encephalomyelitis (EAE), the most commonly used experimental model of MS, has strongly been suggested. To corroborate this notion, alterations in activity of the sympathoadrenal and sympathoneural axes of sympathoadrenal system (a major communication pathway between the central nervous system and the immune system), mirrored in altered release of their end-point mediators (adrenaline and noradrenaline, respectively), are shown to precede (in MS) and/or occur during development of MS and EAE in response to immune cell activation (in early phase of disease) and disease-related damage of sympathoadrenal system neurons and their projections (in late phase of disease). To add to the complexity, innate immunity cells and T-lymphocytes synthesize noradrenaline that may be implicated in a local autocrine/paracrine self-amplifying feed-forward loop to enhance myeloid-cell synthesis of proinflammatory cytokines and inflammatory injury. Furthermore, experimental manipulations targeting noradrenaline/adrenaline action are shown to influence clinical outcome of EAE, in a disease phase-specific manner. This is partly related to the fact that virtually all types of cells involved in the instigation and progression of autoimmune inflammation and target tissue damage in EAE/MS express functional adrenoceptors. Although catecholamines exert majority of immunomodulatory effects through β₂-adrenoceptor, a role for α-adrenoceptors in EAE pathogenesis has also been indicated. In this review, we summarize all aforementioned aspects of immunopathogenetic action of catecholamines in EAE/MS as possibly important for designing new strategies targeting their action to prevent/mitigate autoimmune neuroinflammation and tissue damage.

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.

Adrenergic and dopaminergic modulation of immunity in multiple sclerosis: teaching old drugs new tricks?

Multiple sclerosis (MS) is an autoimmune disorder of the CNS characterized by inflammation, demyelination and axonal loss. Classical evidence in experimental allergic encephalomyelitis, the animal model of MS, support the relevance of sympatoadrenergic as well as of dopaminergic mechanisms. In MS patients, dysregulation of adrenergic and dopaminergic pathways contribute to the disease in immune system cells as well as in glial cells. Available evidence is summarized and discussed also in the light of the novel role of dopamine, noradrenaline and adrenaline as transmitters in immune cells, providing a conceptual frame to exploit the potential of several dopaminergic and adrenergic agents, already in clinical use for non-immune indications and with a usually favourable risk-benefit profile, as add-on drugs to conventional immunomodulating therapies in MS.

The sympathetic nervous system modulates CD4(+)FoxP3(+) regulatory T cells via a TGF-beta-dependent mechanism

CD4(+)FoxP3(+) Tregs are essential mediators of the peripheral immune response to self-antigens. Accordingly, the homeostatic regulation of Treg activity and number would impact on the immune response to both self- and non-self antigens. Because the sympathetic nervous system (SNS) interacts chemically and physically with the central and peripheral immune system and exerts a direct influence on antigen-presenting cells and effector lymphocytes, we have investigated the effect of chemical ablation of the SNS on the number and function of peripheral Treg. Removal of murine peripheral sympathetic innervation by 6-hydroxydopamine induced an increase in splenic and lymph node CD4(+)FoxP3(+) Tregs by a TGF-beta-dependent mechanism. Further, this increase in Tregs coincides with an inhibition of the induction of experimental autoimmune encephalomyelitis. Our results demonstrate that the SNS is an important contributor to the maintenance of peripheral Treg and TGF-beta acts as a bridge between the immune system and the nervous system. Neurological events mediated by the SNS, such as a stress response, may affect the number of T cells that regulate an immune response. Additionally, targeting Tregs via the SNS may be a novel approach to the prevention or treatment of autoimmune diseases.

Neuroimmune regulation in immunocompetence, acute illness, and healing

Adaptive immunocompetence is maintained by growth hormone (GH), prolactin (PRL), and vasopressin (VP). Innate or natural immunocompetence depends on cytokines, hormones (especially of the hypothalamus-pituitary-adrenal axis), and catecholamines. The acute phase response (APR, or acute febrile illness) is an emergency defense reaction whereby the adaptive, T cell-dependent, immune reactions are suppressed and the innate immune function is dramatically amplified. Infection and various forms of injury induce APR. Cytokines [interleukin (IL)-1beta, tumor necrosis factor-alpha, and IL-6] stimulate corticotropin-releasing hormone (CRH) and VP secretion and cause a "sympathetic outflow." Colony-stimulating factors activate leukocytes. CRH is a powerful activator of the pituitary adrenocortical axis and elevates glucocorticoid (GC) levels. Cytokines, GCs, and catecholamines play fundamental roles in the amplification of natural immune defense mechanisms. VP supports the APR at this stage. However, VP remains active and is elevated for a longer period than is CRH. VP, but not CRH, is elevated during chronic inflammatory diseases. VP controls adaptive immune function and stimulates adrenocorticotropic hormone (ACTH) and PRL secretion. PRL maintains the function of the thymus and of the T cell-dependent adaptive immune system. The ACTH-adrenal axis stimulates natural immunity and of suppressor/regulatory T cells, which suppress the adaptive immune system. VP also has a direct effect on lymphoid cells, the significance of which remains to be elucidated. It is suggested that VP regulates the process of recovery from acute illness.

Noradrenergic regulation of inflammatory gene expression in brain

It is now well accepted that inflammatory events contribute to the pathogenesis of numerous neurological disorders, including multiple sclerosis (MS), Alzheimer's disease (AD), Parkinson's disease, and AID's dementia. Whereas inflammation in the periphery is subject to rapid down regulation by increases in anti-inflammatory molecules and the presence of scavenging soluble cytokine receptors, the presence of an intact blood-brain barrier may limit a similar autoregulation from occurring in brain. Mechanisms intrinsic to the brain may provide additional immunomodulatory functions, and whose dysregulation could contribute to increased inflammation in disease. The findings that noradrenaline (NA) reduces cytokine expression in microglial, astroglial, and brain endothelial cells in vitro, and that modification of the noradrenergic signaling system occurs in some brain diseases having an inflammatory component, suggests that NA could act as an endogenous immunomodulator in brain. Furthermore, accumulating studies indicate that modification of the noradrenergic signaling system occurs in some neurodiseases. In this article, we will briefly review the evidence that NA can modulate inflammatory gene expression in vitro, summarize data supporting a similar immunomodulatory role in brain, and present recent data implicating a role for NA in attenuating the cortical inflammatory response to beta amyloid protein.

Locus ceruleus and immunity. III. Compromised immune function (antibody production, hypersensitivity skin reactions and experimental allergic encephalomyelitis) in rats with lesioned locus ceruleus is restored by magnetic fields applied to the brain

This study deals with the relationship between the immunosuppression induced by electrolytic lesions placed into the nucleus locus cerules and the immunopotentiation produced by micromagnets implanted to the parietal area of the skull. The following groups of rats were set up: LC, rats with lesioned locus ceruleus; ShL, sham-lesioned animals bearing non-magnetic beads in the brain parietal region; M, rats with micromagnets of 60 mT influx density in the parietal part of the skull; LCM, animals with impaired locus ceruleus and magnetic beads placed in the parietal area of the skull; and IC, intact control rats. Animals of all groups were tested for plaque-forming cell response, circulating antibodies to sheep red blood cells and bovine serum albumin, Arthus and delayed hypersensitivity skin reactions to bovine serum albumin and old tuberculin, and experimental allergic encephalomyelitis. In LC-rats, humoral and cell-mediated immune reactions were compromised. On the other hand, immune responses in M-rats were significantly potentiated. In LCM-rats, however, the immunosuppression induced by destruction of the locus ceruleus was abrogated by prolonged exposure of the brain parietal region to the magnetic fields, i.e. immune reactivity of LCM-rats was quite similar to that of control IC- and ShL-animals. Several mechanisms may account for the immunomodulating effects produced by lesioning of the locus ceruleus and exposure of the brain to magnetic fields. Noradrenergic, serotoninergic, dopaminergic and peptidergic neurotransmitters, as well as growth hormones and immunopeptides, produced within the central nervous system or elsewhere, may be implicated as necessary for the interactions among the brain, immune apparatus and magnetic fields.

Locus ceruleus and immunity. II. Suppression of experimental allergic encephalomyelitis and hypersensitivity skin reactions in rats with lesioned locus ceruleus

Rats with lesions restricted to the locus ceruleus were tested for immune inflammatory reactions. In these rats, Arthus and delayed skin hypersensitivity reactions to bovine serum albumin and old tuberculin were suppressed. The ablation of locus ceruleus completely inhibited the development of clinical signs of experimental allergic encephalomyelitis, markedly diminished the occurrence and intensity of lesions in the central nervous system, and significantly reduced the production of antibody against the rat brain myelin basic protein. These results establish a link between the locus ceruleus and immune inflammatory reactions in the rat.

Enkephalins, brain and immunity: modulation of immune responses by methionine-enkephalin injected into the cerebral cavity

There is a large number of interactions at molecular and cellular levels between the nervous system and the immune system. It has been demonstrated that the opioid neuropentapeptide methionine-enkephalin (Met-Enk) is involved in humoral and cell-mediated immune reactions. Met-Enk injected peripherally produces a dual and dose-dependent immunomodulatory effect: high doses suppress, whereas low doses potentiate the immune reactivity. The present mini-review concerns the immunological activity of Met-Enk after its administration into the lateral ventricles of the rat brain, and describes the extraordinary capacity of centrally applied Met-Enk to regulate/modulate the immune function. This survey is composed of sections dealing with (a) the role of opioid peptides in the central nervous system (CNS); (b) the activity of opioid peptides in the immune system; (c) the application of Met-Enk into the cerebral cavity; (d) the influence of centrally administered Met-Enk on nonspecific local inflammatory reaction; (e) the effect of Met-Enk injected intracerebroventricularly (i.c.v.) on specific delayed hypersensitivity skin reaction, experimental allergic encephalomyelitis, anaphylactic shock, plaque-forming cell response, and hemagglutinin production; (f) the central antagonizing action of quaternary naltrexone, an opioid antagonist that does not cross the brain-blood barrier, on Met-Enk-induced immunomodulation; (g) the alteration of immune responsiveness by i.c.v. injection of enkephalinase-degrading enzymes; (h) the participation of the brain-blood/blood-brain barrier in the CNS-immune system interaction; and (i) the role of opioid receptors in immunological activity of Met-Enk. A hypothesis has been advanced for the reaction of Met-Enk and opioid receptor sitting on the cell membrane. This concept suggests that the constellation of chemical residues of enkephalin and receptor in the microenvironment determines the binding between the opioid partners. The plurality of conformational structures of enkephalins and receptors makes possible their involvement in a variety of processes which occur in different physiological systems, including the nervous system and the immune system, and intercommunications between the two systems.

Prazosin suppresses development of axonal damage in rats inoculated for experimental allergic encephalomyelitis

The effectiveness of the alpha 1-adrenergic antagonist prazosin for preventing monoaminergic axonal damage in the spinal cords of rats that were inoculated for experimental allergic encephalomyelitis (EAE) was assessed using immunohistochemistry. Prazosin injections (2 mg, i.p.) given twice daily from day 7 to day 15 postinoculation significantly reduced paralysis, spinal cord inflammation and monoaminergic axonal damage compared to saline injections. A close positive correlation between severity of inflammation and severity of axonal damage was found for both prazosin- and saline-treated rats that were inoculated for EAE. These findings confirmed previous observations of suppression of the development of clinical signs of EAE by prazosin treatment and supported the hypothesis that some factor associated with spinal cord inflammation may be responsible for the bulbospinal monoaminergic axonal damage that occurs during EAE.

Hypothalamic noradrenergic pathways exert an influence on neuroendocrine and clinical status in experimental autoimmune encephalomyelitis

The immunomodulatory action of corticosteroids and the ability of central noradrenergic systems to activate the hypothalamic-pituitary-adrenal (HPA) axis led us to investigate the relationship between neuroendocrine status and the clinical course of encephalomyelitis (EAE) following adrenalectomy and depletion of noradrenaline (NA) centrally or peripherally. A significant inverse correlation was found between hypothalamic NA and serum corticosterone (CS) at peak clinical signs of EAE in all the sham groups or when NA was depleted only in the peripheral nervous system. A positive correlation was found between serum CS and disease severity, and in all experimental groups with intact peripheral and/or central noradrenergic pathways a uniformly increased splenic NA content was also observed at peak disease. Administration of 6-OHDA i.p. to neonatal or adult Lewis rats produced a significant depletion of splenic NA alone which resulted in increased disease severity, despite the fact that circulating CS was elevated. Thus the rise in the NA content of lymphoid tissue at peak clinical signs contributes to recovery. A single i.c.v. injection of 6-OHDA into the hypothalamic region resulted in an 80% reduction in NA content, which subsequently modified the clinical severity of EAE. Serum CS levels rose preclinically in the treated group and remained high despite milder clinical disease than that seen in the sham group. The overriding immunoregulatory influence of glucocorticoids is demonstrated by the rapid onset of clinical EAE and morbidity in adrenalectomized animals. However, the strong inverse correlation found between hypothalamic NA and circulating CS indicates that regulation of the HPA axis may ultimately be controlled by central sympathetic pathways.

Suppression of clinical weakness in experimental autoimmune encephalomyelitis associated with weight changes, and post-decapitation convulsions after intracisternal-ventricular administration of 6-hydroxydopamine

Selective depletion of central nervous system norepinephrine (NE) by the neurotoxin 6-hydroxydopamine (6-OHDA) in rats subsequently inoculated with myelin basic protein (MBP) and complete Freund's adjuvant (CFA) produced experimental autoimmune encephalomyelitis (EAE) without the usual expected degree of weakness. The preservation of strength occurred in spite of continued weight loss. Post-decapitation myoclonic convulsive kick latency and kick number, which are known to depend on spinal cord NE, agreed well with the degree of weakness through the clinical disease course. The only difference between EAE groups was that the stronger 6-OHDA pretreated EAE animals did not have an elevated pons-medulla NE compared to saline intracisternal-ventricular (i.c.v.) pretreated controls. We conclude that 6-OHDA can influence the clinical course of weakness by interfering with central noradrenergic activity independent of other features associated with disease in EAE. This effect of 6-OHDA may be exerted through alteration of the blood-spinal cord barrier function and/or central nervous system blood flow.

Monoamine-containing fiber plexuses in the spinal cord of guinea pigs during paralysis, recovery and relapse stages of chronic relapsing experimental allergic encephalomyelitis

Immunohistochemical techniques were used to examine the morphology and distribution of monoamine- and substance P-containing fibers in the spinal cords of guinea pigs in acute paralytic, remission and relapse stages of chronic relapsing experimental allergic encephalomyelitis. During the initial paralytic attack, focal regions of axonal distortion appeared in the white matter of the cervical and thoracic cord; and axon terminal depletion in the gray matter of the caudal spinal cord was pronounced. This neuropathology persisted throughout remission and was exacerbated during relapse of paralysis. These results suggest that axonal damage is an important component of the pathophysiology of this autoimmune disease.