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

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.

Causes, consequences, and cures for neuroinflammation mediated via the locus coeruleus: noradrenergic signaling system

Aside from its roles in as a classical neurotransmitter involved in regulation of behavior, noradrenaline (NA) has other functions in the CNS. This includes restricting the development of neuroinflammatory activation, providing neurotrophic support to neurons, and providing neuroprotection against oxidative stress. In recent years, it has become evident that disruption of physiological NA levels or signaling is a contributing factor to a variety of neurological diseases and conditions including Alzheimer's disease (AD) and Multiple Sclerosis. The basis for dysregulation in these diseases is, in many cases, due to damage occurring to noradrenergic neurons present in the locus coeruleus (LC), the major source of NA in the CNS. LC damage is present in AD, multiple sclerosis, and a large number of other diseases and conditions. Studies using animal models have shown that experimentally induced lesion of LC neurons exacerbates neuropathology while treatments to compensate for NA depletion, or to reduce LC neuronal damage, provide benefit. In this review, we will summarize the anti-inflammatory and neuroprotective actions of NA, summarize examples of how LC damage worsens disease, and discuss several approaches taken to treat or prevent reductions in NA levels and LC neuronal damage. Further understanding of these events will be of value for the development of treatments for AD, multiple sclerosis, and other diseases and conditions having a neuroinflammatory component. The classical neurotransmitter noradrenaline (NA) has critical roles in modulating behaviors including those involved in sleep, anxiety, and depression. However, NA can also elicit anti-inflammatory responses in glial cells, can increase neuronal viability by inducing neurotrophic factor expression, and can reduce neuronal damage due to oxidative stress by scavenging free radicals. NA is primarily produced by tyrosine hydroxylase (TH) expressing neurons in the locus coeruleus (LC), a relatively small brainstem nucleus near the IVth ventricle which sends projections throughout the brain and spinal cord. It has been known for close to 50 years that LC neurons are lost during normal aging, and that loss is exacerbated in neurological diseases including Parkinson's disease and Alzheimer's disease. LC neuronal damage and glial activation has now been documented in a variety of other neurological conditions and diseases, however, the causes of LC damage and cell loss remain largely unknown. A number of approaches have been developed to address the loss of NA and increased inflammation associated with LC damage, and several methods are being explored to directly minimize the extent of LC neuronal cell loss or function. In this review, we will summarize some of the consequences of LC loss, consider several factors that likely contribute to that loss, and discuss various ways that have been used to increase NA or to reduce LC damage. This article is part of the 60th Anniversary special issue.

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.

Increasing CNS noradrenaline reduces EAE severity

The endogenous neurotransmitter noradrenaline (NA) is known to exert potent anti-inflammatory effects in glial cells, as well as provide neuroprotection against excitatory and inflammatory stimuli. These properties raise the possibility that increasing levels of NA in the central nervous system (CNS) could provide benefit in neurological diseases and conditions containing an inflammatory component. In the current study, we tested this possibility by examining the consequences of selectively modulating CNS NA levels on the development of clinical signs in experimental autoimmune encephalomyelitis (EAE). In mice immunized with myelin oligodendrocyte glycoprotein peptide to develop a chronic disease, pretreatment to selectively deplete CNS NA levels exacerbated clinical scores. Elevation of NA levels using the selective NA reuptake inhibitor atomoxetine did not affect clinical scores, while treatment of immunized mice with the synthetic NA precursor L-threo-3,4-dihydroxyphenylserine (L-DOPS) prevented further worsening. In contrast, treatment of mice with a combination of atomoxetine and L-DOPS led to significant improvement in clinical scores as compared to the control group. The combined treatment reduced astrocyte activation in the molecular layer of the cerebellum as assessed by staining for glial fibrillary protein but did not affect Th1 or Th17 type cytokine production from splenic T cells. These data suggest that selective elevation of CNS NA levels could provide benefit in EAE and multiple sclerosis without influencing peripheral immune responses.

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.

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.

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.

The effect of norepinephrine on endotoxin-mediated macrophage activation

The effect of norepinephrine (NE) on the production of tumor necrosis factor (TNF) by rat spleen macrophages was determined. Following activation with lipopolysaccharide, analysis of both secreted and cell-associated samples showed that TNF activity was significantly suppressed in the presence of 10 microM NE. With the addition of the beta-receptor antagonist propranolol a partial reversal of the suppressive effect of NE was noted whereas the addition of the mixed alpha-receptor antagonist phentolamine induced a more pronounced suppressive effect in the supernatant fraction. Similar results were obtained with epinephrine and isoproterenol. Control experiments confirmed that this effect of NE was mediated at the level of macrophage activation. Analysis of lymphocyte activating factors demonstrated a similar pattern of response. Since factors released by macrophages participate in many aspects of the immune response, these results support a functional role for sympathetic innervation of the spleen in immune regulation.