Targeting the noradrenergic system for anti-in flammatory and neuroprotective effects:implications for Parkinson’s disease

Eoin O’Neill, Andrew Harkin

Neuropsychopharmacology Research Group, School of Pharmacy and Pharmaceutical Sciences & Trinity College Institute of Neuroscience, Trinity College, Dublin, Ireland

Abstract Degeneration of the locus coeruleus noradrenergic system is thought to play a key role in the pathogenesis of Parkinson’s disease (PD), whereas pharmacological approaches to increase noradrenaline bioavailability may provide neuroprotection. Noradrenaline inhibits microglial activation and suppresses pro‐in flamma‐tory mediator production (e.g., tumor necrosis factor‐α, interleukin‐1β & inducible nitric oxide synthase activity), thus limiting the cytotoxicity of midbrain dopaminergic neurons in response to an in flamma‐tory stimulus. Neighbouring astrocyte populations promote a neurotrophic environment in response to β2‐adrenoceptor (β2‐AR) stimulation via the production of growth factors (e.g., brain derived neurotrophic factor, cerebral dopamine neurotrophic factor & glial cell derived neurotrophic factor which have shown promising neuroprotective and neuro‐restorative effects in the nigrostriatal dopaminergic system. More recent findings have demonstrated a role for the β2‐AR in down‐regulating expression levels of the human α‐synuclein gene SNCA and relative α‐synuclein protein abundance. Given that α‐synuclein is a major protein constituent of Lewy body pathology, a hallmark neuropathological feature in Parkinson’s disease,these findings could open up new avenues for pharmacological intervention strategies aimed at alleviating the burden of α‐synucleinopathies in the Parkinsonian brain. In essence, the literature reviewed herein supports our hypothesis of a tripartite neuroprotective role for noradrenaline in combating PD‐related neuropathology and motor dysfunction via (1) inhibiting nigral microglial activation & pro‐in flammatory mediator production, (2) promoting the synthesis of neurotrophic factors from midbrain astrocytes and(3) downregulating α‐synuclein gene expression and protein abundance in a β2‐AR‐dependent manner.Thus, taken together, either pharmacologically enhancing extra‐synaptic noradrenaline bioavailability or targeting glial β2‐ARs directly makes itself as a promising treatment option aimed at slowing/halting PD progression.

Key Words: noradrenaline; microglia; astrocytes; in flammation; Parkinson’s disease; neuroprotection; animal model; dopamie

Chronic microglial activation, pro‐inflammatory mediator production & oxidative stress can precipitate nigrostriatal neurodegeneration and ensuing motor dysfunction, as oc‐curs in Parkinson’s disease (PD) (Jenner, 2003). Toll‐like re‐ceptor 4 (TLR4) is expressed on brain‐resident microglia and TLR4‐mediated signalling can induce potent inflammatory responses and is thought to be implicated in the pathogenesis of PD (Panaro et al., 2008). Lipopolysaccharide (LPS) is an endotoxin and TLR4 agonist derived from Gram‐negative bacteria that has been previously used in rodents to study how inflammatory processes contribute to the pathogenesis of PD (Liu and Bing, 2011). Thus, promoting TLR4 signalling in nigral microglia with LPS is a useful method to recapitulate glial‐derived pro‐in flammatory mediator production and do‐paminergic neuropathology in experimental PD.

The locus coeruleus (LC), situated in the outermost layer of the pontine tegmentum is the main source of noradrener‐gic (NA) cell bodies in the central nervous system (CNS) and these cells are reduced at autopsy by approximately 60% in PD patients relative to age‐matched healthy controls (Marien et al., 2004). Consequently, NA inputs to the substantia nig‐ra (SN) and striatum are decreased and it has been suggested that this loss of NA tone is a signi ficant contributor to the progression of PD‐related neuropathology and symptom‐ology, including both motor and non‐motor aspects of the disease (Gesi et al., 2000; Vazey and Aston‐Jones, 2012). In‐deed, studies by (Remy et al., 2005) using [11C]RTI‐32 PET,anin vivomarker for dopamine & noradrenaline transporter binding on 8 and 12 PD patients with and without a history of depression respectively, have shown that the depressed cohort had lower [11C]RTI‐32 binding in the LC and several limbic areas such as the amygdala, thalamus, anterior cingu‐late cortex and in the ventral striatum, indicating NA loss in these brain regions. The severity of anxiety in depressed PD patients was further shown to be inversely correlated with[11C]RTI‐32 binding in most of these brain regions and the degree of apathy in these patients was inversely correlated with [11C]RTI‐32 binding in the ventral striatum, thus highlighting a link between degeneration of the LC‐NA sys‐tem and loss of NA innervation to limbic brain regions with a higher frequency of depression & anxiety in PD patients.

In an experimental setting, selective lesioning of the LC‐NA system and subsequent NA depletion is chiefly per‐formed through the use of N‐(2‐chloroethyl)‐N‐ethyl‐2‐bro‐mobenzylamine (DSP4) (Jonsson et al., 1981). Treatment with DSP4 (50 mg/kg; intraperitoneal (i.p.) injection every 2 weeks) commencing 6 months post systemic LPS challenge(5 mg/kg; i.p.) exacerbates the loss of dopaminergic neurons in the rodent substantia nigra pars compacta (SNpc)in vivowhereas pre‐treatment of primary mesencephalic neuron/glia cultures with submicromolar concentrations of nor‐adrenaline suppresses LPS‐induced microglial activation and proin flammatory mediator productionin vitro(Jiang et al., 2015). Interestingly, the same authors demonstrate that pre‐treatment of LPS‐challenged mesencephalic neuron/glia cultures derived from β2‐adrenergic receptor (AR)‐de ficient mice with NA does not totally abolish the neuroprotection observed, indicating that at a submicromolar level (10‐ to 100‐fold lower than its lower than its binding affinity to AR binding affinity to AR receptors) the neuroprotection afford‐ed by NA occurs at least partially, in a β2‐AR‐independent fashion. Indeed, data derived from the same studies show that noradrenaline, at submicromolar concentrations, exerts an anti‐in flammatory effect on activated microglia by atten‐uating LPS‐induced pro‐in flammatory mediator production(such as tumor necrosis factor‐α [TNFα] & nitric oxide)and inhibiting microglial nicotinamide adenine dinucle‐otide phosphate (NADPH) oxidase‐mediated superoxide production. Hence, degeneration of LC‐NA neurons (up to 80%), as occurs in PD even before the onset of dopami‐nergic neuronal loss (Zarow et al., 2003; Baloyannis et al.,2006), is likely to enhance neuroin flammation & PD‐related neuropathology whereas NA augmentation strategies could curtail these inflammatory processes & slow/halt disease progression.

Studies by Rommelfanger et al. (2007) have also shown that lesioning the LC‐NA system with N‐(2‐chloroeth‐yl)‐N‐ethyl‐2‐bromobenzylamine (DSP4), or dopamine β‐hydroxylase knockout (Dbh–/–) mice (which lack NA al‐together) display greater motor abnormalities than 1‐meth‐yl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP)‐injected mice with 80% dopaminergic (DA) nerve terminal loss. Data from the same study showed that acute pharmacological res‐toration of central noradrenaline levels with L‐threo‐DOPS however, improved motor function in Dbh–/–mice. More‐over, MPTP‐induced degeneration of the nigrostriatal DA system and ensuing dopamine loss is attenuated in noradren‐aline transporter knockout (NAT–/–) mice, a protective effect similarly conferred upon treatment with the specific NAT inhibitor nisoxetine (Rommelfanger et al., 2004). Thus, given that the LC‐NA system functionally modulates the survival rate of target DA neurons within the SN (Jiang et al., 2015),we hypothesized that the ensuing loss of LC noradrenergic in‐puts to the midbrain & striatum may be a major contributor to PD progression, whereas pharmacological enhancement of central noradrenergic tone may provide anti‐inflamma‐tory effects & provide neuroprotection against LPS‐induced PD‐related neuropathology and motor dysfunction.

Atomoxetine (marketed under brand name: Strattera by Eli Lilly and Company) is a selective noradrenaline reuptake inhibitor (NRI) approved by the US Food and Drug Admin‐istration for the treatment of attention de ficit hyperactivity disorder (ADHD). Atomoxetine blocks presynaptic nor‐adrenaline transporters (NAT) and thus, enhances extracel‐lular noradrenaline concentration which further promotes NA signalling. Idazoxan is a selective α2‐adrenoceptor an‐tagonist that blocks presynaptic α2‐AR’s (which normally act as auto‐receptors to regulate noradrenaline release) and thus potentiates extra‐synaptic noradrenaline bioavailabil‐ity. Previous data from our laboratory has demonstrated that treatment with atomoxetine (10 mg/kg; i.p.) attenuates LPS‐induced increases in TNFα, interleukin‐1β (IL‐1β)& inducible nitric oxide synthase (iNOS) gene expression levels and suppresses nuclear factor‐κB (NfκB) activation,findings which were accompanied by reductions in the mi‐croglial activation markers CD40 & CD11b in the rat cortex(O’Sullivan et al., 2009). Noradrenaline can affect a wide array of microglial functions through adrenergic signalling.Cultured rat microglia express mRNA encoding α1a‐, α2a‐,β1‐ and β2‐ARs (Mori et al., 2002). Nevertheless, it is likely that NA also influences other cell populations. Astrocytes in particular have been identi fied as a possible, primary ex‐tra‐synaptic target of noradrenaline and similar to microg‐lia, rat astrocytes express functional α1‐, α2‐, β1‐ and β2‐ARs(Hertz et al., 2004). We assessed the efficacy of treatment with atomoxetine alone or in combination with idazoxan in the inflammatory‐based intra‐nigral LPS rat model of PD.Treatment with atomoxetine (3 mg/kg; i.p.) alone or in com‐bination with idazoxan (1 mg/kg; i.p.) commenced 4 hours post unilateral lesioning of the substantia nigra with bacte‐rial LPS (10 μg/2 μL) and continued twice daily (b.i.d.) for 7 days. A previousin vivomicrodialysis study by (Swanson et al., 2006) investigating the effect of atomoxetine (3 mg/kg;i.p.) and idazoxan (1 mg/kg; i.p.) on extracellular noradren‐aline concentrations in freely moving rats has demonstrated a 7‐fold increase in cortical noradrenaline efflux greater than that of either compound alone, indicating a synergistic effect when these agents are administered in combination,and thus we deemed these treatment dosages appropriate for elevating extracellular noradrenaline concentrations in the rat brain. Overall, our data demonstrated that enhancing central extra‐synaptic noradrenergic bioavailability inhibits LPS‐induced microglial activation within the SN, abrogates DA neurodegeneration, attenuates the ensuing nigrostriatal dopamine loss and provides partial protection against the associated motor dysfunctionin vivo(Yssel et al., 2018).

Motor deficits are a salient feature of PD and become apparent in patients with ongoing DA neurodegeneration after an approximate 80% loss of striatal dopamine content(Cheng et al., 2010). Thus, we used a battery of motor func‐tion tests to assess the impact of pharmacologically enhanc‐ing NA tone on different aspects of PD‐related motor dys‐function relative to the human condition, namely forelimb akinesia (stepping test), asymmetric limb use (cylinder test& D‐amphetamine challenge) and skilled motor function(staircase test). Our data showed that in the stepping test of forelimb akinesia, intra‐nigral LPS injection reduced the number of contralateral adjusting steps made in the fore‐hand direction. Treatment with atomoxetine alone and in combination with idazoxan rescued these animals from LPS‐induced forelimb akinesia in the forehand direction as indexed by increases in the number of adjusting steps made at 7 and 13 days post‐lesioning relative to LPS‐injected rats treated with saline, thus indicating a protective role for NA‐augmentation against akinetic behaviour. Moreover,treatment with atomoxetine in combination with idazox‐an partially protected against LPS‐induced reductions in contralateral forelimb wall placements in the cylinder test and also reduced the number of amphetamine‐induced ipsilateral rotations in LPS‐lesioned rats, thus indicating a protective effect of enhancing NA tone against LPS‐induced asymmetric limb use. Intra‐nigral LPS reduced the contra‐lateral forelimb success rate in the staircase test at 7, but not 13 days post‐lesioning. In contrast to the stepping test and cylinder test, these deficits in skilled motor function were not restored by treatment with atomoxetine and/or idazox‐an. Hence, it seems likely that with respect to the varying levels of sensitivity across the behavioural tests, the detec‐tion of different aspects of motor dysfunction are therefore dependent on the extent of LPS‐induced degeneration of the nigrostriatal tract, and conversely the varying level of protection against motor dysfunction afforded by pharma‐cologically enhancing NA bioavailability likely depends on the degree of restored functionality of the neural circuitry responsible for performing these different motor tasks as discussed in the original research article.

Moreover, given that idazoxan only conferred a thera‐peutic benefit when co‐administered with atomoxetine,highlights that the therapeutic benefits in terms of motor improvements across the board are more attributable to blockade of NAT due to treatment with atomoxetine as opposed to antagonism of α2‐ARs post idazoxan admin‐istration. Although the influence of NA on subthalamic nuclei (STN) electrical activity remains unclear, it has been hypothesized that loss of NAergic input to this brain region increases the firing pattern of STN neurons resulting in hypokinesia. Conversely, improvements in motor function are observed following modulation of STN activity, through electrical LC stimulation or pharmacological manipula‐tions. Indeed, α2‐AR antagonism in the STN reduced motor abnormalities in 6‐hydroxydopamine (6‐OHDA) lesioned rats due to its heavy innervation by NAergic afferents ex‐pressing both α1‐ and α2‐AR (Belujon et al., 2007), lending support to the role of idazoxan and modulation of this brain region in mediating improvements in asymmetric limb use observed in the LPS PD model. Moreover, blockade of α2‐adrenoceptors exerts an ameliorative effect on levodopa(L‐DOPA)‐induced dyskinesia’s in MPTP parkinsonian mice and nonhuman primates (Grondin et al., 2000; Archer and Fredriksson, 2003). Similarly, the neuroprotective effect of 28‐day treatment with the α2‐adrenoceptor antagonist dexefaroxan (0.63 mg/kg; i.p.) on devascularisation‐induced degeneration of cholinergic neurons in the nucleus basalis was coincident with persistent NGF production in areas surrounding the cortical infarct (Debeir et al., 2004). It is possible that dexefaroxan‐induced α2‐AR blockade enhanc‐es NA tone, leading to the activation of astrocytic β2‐ARs culminating towards ampli fied NGF production (Lu et al.,1991; Semkova et al., 1996) and the protection of basal fore‐brain cholinergic neurons.

These behavioural improvements observed following ato‐moxetine treatment in the intra‐nigral LPS inflammatory model of PD in rats were forti fied by our post mortem anal‐ysis demonstrating a protective effect of atomoxetine treat‐ment against LPS‐mediated neurotoxicity of the nigrostri‐atal DA system. On average, intra‐nigral injection of LPS induced an approximate 66% loss of tyrosine hydroxylase(TH)+dopamine neurons within the SNpc and an associated 64% loss of TH+nerve terminals in the ipsilateral striatum.Treatment with atomoxetine alone or in combination with idazoxan abrogated the LPS‐induced DA neurodegenera‐tion within the SN and suppressed the affiliated DA nerve terminal loss in the striatum. Consistent with this immu‐nohistochemical data, intra‐nigral LPS induced an approxi‐mate 55% loss of dopamine content in the midbrain and an approximate 86% loss in the ipsilateral striatum. Treatment with atomoxetine alone almost completely attenuated the LPS‐induced loss of nigrostriatal dopamine content to levels comparable to that of vehicle‐injected control animals (Yssel et al., 2018).

Given that the midbrain contains the highest density of microglial cells in the entire rodent brain (Kim et al., 2000),the root, and indeed progression of this inflammatory‐de‐rived neuropathology and ensuing motor dysfunction is likely to be congruent on the activation state of nigral mi‐croglia. In essence, nigral microgliosis & subsequent pro‐in‐flammatory mediator production is a cellular prerequisite for LPS‐mediated neurotoxicity of dopamine neurons (Gayle et al., 2002). Treatment with atomoxetine alone or in com‐bination with idazoxan however, inhibited nigral microglial activation in response to lesioning with LPS. Intra‐nigral LPS injection lead to a robust increase in the number of Iba1+microglial cells within the SN with a morphological‐ly distinct phenotype (retracted processes & enlarged cell soma) from quiescent microglia (extended processes &smaller cell soma). Treatment with atomoxetine alone or in combination with idazoxan inhibited the LPS‐induced Iba1+nigral microgliosis and rendered the nigral microglia morphologically indicative of a quiescent phenotype. In support of the well documented anti‐in flammatory effect of noradrenaline on microglial activation and pro‐in flammato‐ry cytokine secretion (Dello Russo et al., 2004; McNamee et al., 2010a), in the present study, treatment with atomoxetine also attenuated the LPS‐induced increases in TNFα and IL‐1β pro‐in flammatory gene expression within the midbrain,a finding which is likely to have underpinned the attenu‐ated loss of nigrostriatal dopamine neurons and dopamine concentrations. Previous data has shown that stimulation of CNS β2‐adrenoceptors with clenbuterol (0.5 mg/kg; i.p.)suppresses NfκB activity and ameliorates expression of the NfκB‐inducible genes TNFα and intercellular cell adhe‐sion molecule‐1 (ICAM‐1) in response to central injection of bacterial LPS (1 μg/5μL; intracerebroventricular (icv)),whilst concurrently elevating the temporal expression of the NfκB‐inhibitory protein nuclear factor of kappa light poly‐peptide gene enhancer in B‐cells inhibitor α (IκBα) (Ryan et al., 2013). Moreover, noradrenaline negatively regulates the IL‐1 system in glial cellsviaupregulating IL‐1Ra and the IL‐1RII decoy receptorin vitro(McNamee et al., 2010b) andin vivo(McNamee et al., 2010c) and raises CNS expression levels of the broad spectrum anti‐in flammatory cytokine IL‐10 and its downstream signalling molecule suppressor of cy‐tokine signaling 3 (SOCS‐3) in a β‐adrenoceptor‐dependent manner (McNamee et al., 2010b). Similarly, enhancing NA tone ameliorates LPS (250 μg/kg; i.p.) induced increases in cortical IL‐1β, TNFα, iNOS, CD11b and CD40 gene expres‐sion (O’Sullivan et al., 2009), and decreases the elevated ex‐pression of the chemokines RANTES and interferon‐induc‐ible protein‐10 (IP‐10), as well as the cell adhesion molecules ICAM‐1 and vascular cell adhesion molecule‐1 (VCAM‐1)in the CNS following systemic in flammatory insult (O’Sul‐livan et al., 2010). Thus, both noradrenaline augmentation strategies and β2‐AR stimulation drive an anti‐in flammatory phenotype in the CNS and may be of great therapeutic value in conditions where in flammation contributes to neuropa‐thology (Figure 1).

Enhancing NA availability also induced a marked increase in growth factor production within the midbrain. Treatment with atomoxetine signi ficantly increased glial cell derived neu‐rotrophic factor (GDNF) and brain derived neurotrophic fac‐tor (BDNF) mRNA expression within the midbrain of LPS‐le‐sioned rats. Cerebral dopamine neurotrophic factor (CDNF)mRNA expression was also observably increased in these animals, albeit to a lesser extent to that of GDNF & BDNF.Here, we propose that treatment with atomoxetine is exerting a glial‐derived neurotrophic response to raised extra‐synaptic NA tone on foot of blockade of the NAT. The increases in GDNF are particularly promising from a neuro‐protective perspective, as delayed intra‐striatal delivery of this growth factor has previously been shown to prevent DA neurodegen‐eration within the SNpc, increase striatal dopamine concen‐tration and promote functional recovery in 6‐OHDA‐lesioned rats (Wang et al., 2002). Moreover, direct intra‐putamenal infusion of GDNF in 5 PD patients increases dopamine storage in the putamen, improves UPDRS motor scores and reduces medication‐induced dyskinesias (Gill et al., 2003).Thus, in conjunction with the anti‐inflammatory effect of atomoxetine treatment on activated microglia, we now show that atomoxetine also induces growth factor expression within the midbrain, most likely from astrocytes in an AR‐dependent manner. Thus, atomoxetine has a dual effect on midbrain glial cells, inducing a tonic inhibition on microglial activation &pro‐inflammatory gene expression, whilst concurrently pro‐moting the synthesis of neurotrophic factors from astrocytes.Both aspects are likely to contribute to the neuroprotection afforded by treatment with atomoxetine.

Noradrenaline is hypothesised to play a bi‐modal neuro‐protective role in the brain in a variety of neurodegenerative disease statesviaits interactions with glial cells, particularly by down‐regulating microglial pro‐inflammatory gene ex‐pression (Dello Russo et al., 2004; Jiang et al., 2015), and also by promoting a neurotrophic effect in the brainviaastrocyt‐ic growth factor production in a β‐AR dependent manner(Culmsee et al., 1999a, b). Pre‐treatment with NA protects primary rat hippocampal cells against Aβ1–42and Aβ25–35mediated toxicityviathe production of NGF and BDNF downstream of β‐AR signalling (Counts and Mufson, 2010).Moreover, in the multiple sclerosis field, CNS noradrenaline de ficiency exacerbates experimental autoimmune encephalitis(EAE), whereas dual treatment with the NRI atomoxetine (20 mg/kg; i.p.) and L‐threo‐DOPS (400 mg/kg; subcutaneous(s.c.)) administered three times weekly improves EAE clinical scores in NA‐depleted mice (Simonini et al., 2010). Similarly,the serotonin noradrenaline reuptake inhibitor venlafaxine suppresses CD3, CD8, IL‐12 p40, TNFα, IFN‐γ, CCL2 and RANTES gene transcripts in the CNS lesions of an experi‐mental adoptive myelin‐speci fic T‐cell model of EAE, whilst concomitantly upregulating BDNF expression in the in flamed spinal cord of these animals (Vollmar et al., 2009).

In addition to our findings, studies by Mittal et al. (2017)on the pharmaceutical history of over 4 million Norwe‐gians who were taking one of the β2‐AR agonists for other medical problems over an 11 year period has shown that usage of the β2‐AR agonist salbutamol (a brain‐penetrant asthma medication) was associated with a decreased risk of developing PD, and conversely, that blockade of the β2‐AR with the antagonist propranolol was associated with an increased risk of developing PD. Interestingly, out of 1126 Food and Drug Administration (FDA)‐approved drugs &compounds screened, 4 signi ficantly reduced alpha synucle‐in gene (SNCA) & protein expression (a major constituent of pathological Lewy Body inclusions in PD brains); three of them being the selective β2‐AR agonists metaproterenol,clenbuterol & salbutamol. Furthermore, treatment with clenbuterol (20 μM) reduced SNCA mRNA expression and α‐synuclein (α‐Syn) protein levels in SNCA‐triplication patient iPSC‐derived neuronal precursor cells and attenu‐ated the loss of TH+dopaminergic neurons in the SNpc of MPTP‐lesioned micein vivo. Moreover, the authors demon‐strate that the β2‐AR regulates the transcription of the hu‐man α‐synuclein gene SNCA through H3K27 acetylation(H3K27ac) of promoters and enhancers in the human SNCA locus and that treatment with clenbuterol is correlated with a decrease in H3K27ac levels and relative expression of SNCA mRNA levels. Data derived from the same study also shows that knockout of the β2‐AR gene (Adrb2) in murine primary neurons, RNA interference‐induced silencing of the β2‐AR in human SK‐N‐MC cells or chemical antagonism of the β2‐AR with the β‐blocker propranolol in SK‐N‐MC cells consistently increases SNCA mRNA expression and α‐Syn protein abundance. Conversely, transfection of SK‐N‐MC cells with ADRB2 constructs reduces α‐Syn SNCA mRNA levels and genetic silencing of the β2‐AR with siRNA’s or its blockade with propranolol abrogates the SNCA expres‐sion‐lowering effects induced by treatment with the β2‐AR agonist clenbuterol. Thus, these results are strong evidence indicating that the β2‐AR is linked to transcription of α‐syn and risk of developing PD, and that pharmacological stimu‐lation of glial β2‐ARs may provide neuroprotection.

Figure 1 Proposed anti-in flammatory mechanism of action of noradrenaline/β2-adrenoceptor (β2-AR) agonists on nigral microglia in the in flamed substantia nigra; a molecular signalling pathway towards neuroprotection.

In conclusion, we propose that noradrenaline is exerting a tripartite neuroprotective role in the pathologic CNS by(a) inhibiting microglial activation within the substantia nigra and attenuating the ensuing increases in pro‐in flam‐matory gene expression, (b) stimulating the production of neurotrophic factors from neighbouring astrocytes and (c)regulating SNCA gene expression and α‐syn protein levels.Our results demonstrate that pharmacological enhancement of extra‐synaptic NA bioavailabilityviablockade of the nor‐adrenaline transporter may be a pertinent candidate for con‐sideration as a future PD pharmacotherapy and in related illnesses where in flammation contributes to neuropathology.Future studies will aim to determine if pharmacologically targeting glial β2‐ARs directly will elicit anti‐in flammatory and neuroprotective effects in the intra‐nigral LPS and α‐syn transgenic rat models of PD.

Acknowledgments:The authors acknowledge the support of Trinity Foundation, Trinity College Dublin.

Author contributions:Both authors contributed to the writing of this manuscript.

Conflicts of interest:The authors have no con flicts of interest to declare.

Financial support:Eoin O’Neill was supported by a Trinity College postgraduate award.

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Open peer reviewer:Yi Pang, University of Mississippi Medical Center,USA.

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