Reduced MPTP toxicity in noradrenaline transporter knockout mice

Neurotoxicology of dopamine: Victim or assailant?

Since the identification of dopamine as a neurotransmitter in the mid-20th century, investigators have examined the regulation of dopamine homeostasis at a basic biological level and in human disorders. Genetic animal models that manipulate the expression of proteins involved in dopamine homeostasis have provided key insight into the consequences of dysregulated dopamine. As a result, we have come to understand the potential of dopamine to act as an endogenous neurotoxin through the generation of reactive oxygen species and reactive metabolites that can damage cellular macromolecules. Endogenous factors, such as genetic variation and subcellular processes, and exogenous factors, such as environmental exposures, have been identified as contributors to the dysregulation of dopamine homeostasis. Given the variety of dysregulating factors that impact dopamine homeostasis and the potential for dopamine itself to contribute to further cellular dysfunction, dopamine can be viewed as both the victim and an assailant of neurotoxicity. Parkinson's disease has emerged as the exemplar case study of dopamine dysregulation due to the genetic and environmental factors known to contribute to disease risk, and due to the evidence of dysregulated dopamine as a pathologic and pathogenic feature of the disease. This review, inspired by the talk, "Dopamine in Durham: location, location, location" presented by Dr. Miller for the Jacob Hooisma Memorial Lecture at the International Neurotoxicology Association meeting in 2023, offers a primer on dopamine toxicity covering endogenous and exogenous factors that disrupt dopamine homeostasis and the actions of dopamine as an endogenous neurotoxin.

Beta2-adrenoreceptor agonists and long-term risk of Parkinson's disease

INTRODUCTION

There is limited information on how the association between Parkinson's disease and the use of beta2-adrenoreceptor (β2AR) agonists varies among groups of short-, long-, and ultra-long-acting β2AR agonists (SABA, LABA and ultraLABA).

METHODS

In this prospective study of the Norwegian population, we estimated the incidence of Parkinson's disease according to exposure to β2AR agonists as a time-dependent variable by means of Cox regression. We adjusted for educational level, comorbidity and performed a sensitivity analysis excluding individuals with chronic obstructive pulmonary disease (COPD), all factors associated with smoking. Anticholinergics and corticosteroids as drugs with the same indication were analyzed for comparison.

RESULTS

In the follow-up period from 2005 to 2019, 15,807 incident Parkinson's cases were identified. After adjustments for sex, education and age as the timescale, SABA (Hazard ratio (HR) = 0.84; 95%CI: 0.79, 0.89; p < 0.001), LABA (HR = 0.85; 95%CI: 0.81, 0.90; p < 0.001) and ultraLABA (HR = 0.6; 95%CI: 0.49, 0.73; p < 0.001) were all associated with a lower risk of Parkinson's disease. After exclusion of COPD patients, corticosteroids and anticholinergics were no longer inversely associated, whereas β2AR agonists remained associated.

CONCLUSION

Of drugs with the same indication of use, only β2AR agonists remained inversely associated with PD risk after all adjustments, with ultraLABA displaying the overall strongest association. Although the precision of the estimate is limited by the modest number of exposed PD cases without COPD, the association is intriguing and suggest that longer-acting, more lipophilic, and thus likely more brain-penetrant β2AR agonists could be prioritized for further studies.

Levodopa responsive freezing of gait is associated with reduced norepinephrine transporter binding in Parkinson's disease

BACKGROUND

Freezing of gait (FOG) is a major cause of falling in Parkinson's disease (PD) and can be responsive or unresponsive to levodopa. Pathophysiology is poorly understood.

OBJECTIVE

To examine the link between noradrenergic systems, the development of FOG in PD and its responsiveness to levodopa.

METHODS

We examined norepinephrine transporter (NET) binding via brain positron emission tomography (PET) to evaluate changes in NET density associated with FOG using the high affinity selective NET antagonist radioligand [¹¹C]MeNER (2S,3S)(2-[α-(2-methoxyphenoxy)benzyl]morpholine) in 52 parkinsonian patients. We used a rigorous levodopa challenge paradigm to characterize PD patients as non-freezing (NO-FOG, N = 16), levodopa responsive freezing (OFF-FOG, N = 10), and levodopa-unresponsive freezing (ONOFF-FOG, N = 21), and also included a non-PD FOG group, primary progressive freezing of gait (PP-FOG, N = 5).

RESULTS

Linear mixed models identified significant reductions in whole brain NET binding in the OFF-FOG group compared to the NO-FOG group (-16.8%, P = 0.021) and regionally in the frontal lobe, left and right thalamus, temporal lobe, and locus coeruleus, with the strongest effect in right thalamus (P = 0.038). Additional regions examined in a post hoc secondary analysis including the left and right amygdalae confirmed the contrast between OFF-FOG and NO-FOG (P = 0.003). A linear regression analysis identified an association between reduced NET binding in the right thalamus and more severe New FOG Questionnaire (N-FOG-Q) score only in the OFF-FOG group (P = 0.022).

CONCLUSION

This is the first study to examine brain noradrenergic innervation using NET-PET in PD patients with and without FOG. Based on the normal regional distribution of noradrenergic innervation and pathological studies in the thalamus of PD patients, the implications of our findings suggest that noradrenergic limbic pathways may play a key role in OFF-FOG in PD. This finding could have implications for clinical subtyping of FOG as well as development of therapies.

The Neurotoxin DSP-4 Dysregulates the Locus Coeruleus-Norepinephrine System and Recapitulates Molecular and Behavioral Aspects of Prodromal Neurodegenerative Disease

The noradrenergic locus coeruleus (LC) is among the earliest sites of tau and α-synuclein pathology in Alzheimer's disease (AD) and Parkinson's disease (PD), respectively. The onset of these pathologies coincides with loss of noradrenergic fibers in LC target regions and the emergence of prodromal symptoms including sleep disturbances and anxiety. Paradoxically, these prodromal symptoms are indicative of a noradrenergic hyperactivity phenotype, rather than the predicted loss of norepinephrine (NE) transmission following LC damage, suggesting the engagement of complex compensatory mechanisms. Because current therapeutic efforts are targeting early disease, interest in the LC has grown, and it is critical to identify the links between pathology and dysfunction. We employed the LC-specific neurotoxin N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), which preferentially damages LC axons, to model early changes in the LC-NE system pertinent to AD and PD in male and female mice. DSP-4 (two doses of 50 mg/kg, one week apart) induced LC axon degeneration, triggered neuroinflammation and oxidative stress, and reduced tissue NE levels. There was no LC cell death or changes to LC firing, but transcriptomics revealed reduced expression of genes that define noradrenergic identity and other changes relevant to neurodegenerative disease. Despite the dramatic loss of LC fibers, NE turnover and signaling were elevated in terminal regions and were associated with anxiogenic phenotypes in multiple behavioral tests. These results represent a comprehensive analysis of how the LC-NE system responds to axon/terminal damage reminiscent of early AD and PD at the molecular, cellular, systems, and behavioral levels, and provides potential mechanisms underlying prodromal neuropsychiatric symptoms.

The Neurodegenerative Elderly Syndrome (NES) hypothesis: Alzheimer and Parkinson are two faces of the same disease

Increasing evidence suggests that Alzheimer's disease (AD) and Parkinson's disease (PD) share monoamine and alpha-synuclein (αSyn) dysfunctions, often beginning years before clinical manifestations onset. The triggers for these impairments and the causes leading these early neurodegenerative processes to become AD or PD remain unclear. We address these issues by proposing a radically new perspective to frame AD and PD: they are different manifestations of one only disease we call "Neurodegenerative Elderly Syndrome (NES)". NES goes through three phases. The seeding stage, which starts years before clinical signs, and where the part of the brain-body affected by the initial αSyn and monoamine dysfunctions, influences the future possible progression of NES towards PD or AD. The compensatory stage, where the clinical symptoms are still silent thanks to compensatory mechanisms keeping monoamine concentrations homeostasis. The bifurcation stage, where NES becomes AD or PD. We present recent literature supporting NES and discuss how this hypothesis could radically change the comprehension of AD and PD comorbidities and the design of novel system-level diagnostic and therapeutic actions.

Neuroprotective approaches to halt Parkinson's disease progression

One of the most significant threats in Parkinson's disease (PD) is neurodegeneration. Neurodegeneration at both nigral as well as non-nigral regions of the brain is considered responsible for disease progression in PD. The key factors that initiate neurodegeneration are oxidative stress, neuroinflammation, mitochondrial complex-1 inhibition, and abnormal α-synuclein (SNCA) protein aggregations. Nigral neurodegeneration results in motor symptoms (tremor, bradykinesia, rigidity, shuffling gait, and postural instability) whereas; non-nigral neurodegeneration is responsible for non-motor symptoms (depression, cognitive dysfunctions, sleep disorders, hallucination, and psychosis). The available therapies for PD aim at increasing dopamine levels. The medications such as Monoamine oxidase B (MAO-B) inhibitors, catechol o-methyltransferase (COMT) inhibitors, Dopamine precursor (Levodopa), dopamine agonists, and dopamine reuptake inhibitors drastically improve the motor symptoms and quality of life only in the early stages of the disease. However, dopa resistant motor symptoms (abnormality in posture, speech impediment, gait, and balance problems), dopa resistant non-motor signs (sleep problems, autonomic dysfunction, mood, and cognitive impairment, pain), and drug-related side effects (motor fluctuations, psychosis, and dyskinesias) are considered responsible for the failure of these therapies. Further, none of the treatments, alone or in combination, are capable of halting the disease progression in the long run. Therefore, there is a need to develop safe and efficient neuroprotective agents, which can slow or stop the disease progression for the better management of PD. In this review, an effort has been made to discuss the various mechanisms responsible for progressive neurodegeneration (disease progression) in PD and also multiple strategies available for halting disease progression.

Sustained chemogenetic activation of locus coeruleus norepinephrine neurons promotes dopaminergic neuron survival in synucleinopathy

Dopaminergic neuron degeneration in the midbrain plays a pivotal role in motor symptoms associated with Parkinson's disease. However, non-motor symptoms of Parkinson's disease and post-mortem histopathology confirm dysfunction in other brain areas, including the locus coeruleus and its associated neurotransmitter norepinephrine. Here, we investigate the role of central norepinephrine-producing neurons in Parkinson's disease by chronically stimulating catecholaminergic neurons in the locus coeruleus using chemogenetic manipulation. We show that norepinephrine neurons send complex axonal projections to the dopaminergic neurons in the substantia nigra, confirming physical communication between these regions. Furthermore, we demonstrate that increased activity of norepinephrine neurons is protective against dopaminergic neuronal depletion in human α-syn A53T missense mutation over-expressing mice and prevents motor dysfunction in these mice. Remarkably, elevated norepinephrine neurons action fails to alleviate α-synuclein aggregation and microgliosis in the substantia nigra suggesting the presence of an alternate neuroprotective mechanism. The beneficial effects of high norepinephrine neuron activity might be attributed to the action of norepinephrine on dopaminergic neurons, as recombinant norepinephrine treatment increased primary dopaminergic neuron cultures survival and neurite sprouting. Collectively, our results suggest a neuroprotective mechanism where noradrenergic neurons activity preserves the integrity of dopaminergic neurons, which prevents synucleinopathy-dependent loss of these cells.

Genetic lesions of the noradrenergic system trigger induction of oxidative stress and inflammation in the ventral midbrain

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor deficits caused by the loss of dopaminergic neurons in the substantia nigra (SN) and ventral tegmental area (VTA). However, clinical data revealed that not only the dopaminergic system is affected in PD. Postmortem studies showed degeneration of noradrenergic cells in the locus coeruleus (LC) to an even greater extent than that observed in the SN/VTA. Pharmacological models support the concept that modification of noradrenergic transmission can influence the PD-like phenotype induced by neurotoxins. Nevertheless, there are no existing data on animal models regarding the distant impact of noradrenergic degeneration on intact SN/VTA neurons. The aim of this study was to create a transgenic mouse model with endogenously evoked progressive degeneration restricted to noradrenergic neurons and investigate its long-term impact on the dopaminergic system. To this end, we selectively ablated the transcription initiation factor-IA (TIF-IA) in neurons expressing dopamine β-hydroxylase (DBH) by the Cre-loxP system. This mutation mimics a condition of nucleolar stress affecting neuronal survival. TIF-IA^DbhCre mice were characterized by selective, progressive degeneration of noradrenergic neurons, followed by phenotypic alterations associated with sympathetic system impairment. Our studies did not show any loss of tyrosine hydroxylase (TH)-positive cells in the SN/VTA of mutant mice; however, we observed increased indices of oxidative stress, enhanced markers of glial cell activation, inflammatory processes and isolated degenerating cells positive for FluoroJade C. These results were supported by gene expression profiling of VTA and SN from TIF-IA^DbhCre mice, revealing that 34 out of 246 significantly regulated genes in the SN/VTA were related to PD. Overall, our results shed new light on the possible negative influence of noradrenergic degeneration on dopaminergic neurons, reinforcing the neuroprotective role of noradrenaline.

Effects of Manipulation of Noradrenergic Activities on the Expression of Dopaminergic Phenotypes in Aged Rat Brains

This study investigated the effects of the pharmacological manipulation of noradrenergic activities on dopaminergic phenotypes in aged rats. Results showed that the administration of L-threo-3,4-dihydroxyphenylserine (L-DOPS) for 21 days significantly increased the expression of tyrosine hydroxylase (TH) and dopamine transporter (DAT) in the striatum and substantia nigra (SN) of 23-month-old rats. Furthermore, this treatment significantly increased norepinephrine/DA concentrations in the striatum and caused a deficit of sensorimotor gating as measured by prepulse inhibition (PPI). Next, old rats were injected with the α2-adrenoceptor antagonist 2-methoxy idazoxan or β2-adrenoceptor agonist salmeterol for 21 days. Both drugs produced similar changes of TH and DAT in the striatum and SN. Moreover, treatments with L-DOPS, 2-methoxy idazoxan, or salmeterol significantly increased the protein levels of phosphorylated Akt in rat striatum and SN. However, although a combination of 2-methoxy idazoxan and salmeterol resulted in a deficit of PPI in these rats, the administration of 2-methoxy idazoxan alone showed an opposite behavioral change. The in vitro experiments revealed that treatments with norepinephrine markedly increased mRNAs and proteins of ATF2 and CBP/p300 and reduced mRNA and proteins of HDAC2 and HDAC5 in MN9D cells. A ChIP assay showed that norepinephrine significantly increased CBP/p300 binding or reduced HDAC2 and HDAC5 binding on the TH promoter. The present results indicate that facilitating noradrenergic activity in the brain can improve the functions of dopaminergic neurons in aged animals. While this improvement may have biochemically therapeutic indication for the status involving the degeneration of dopaminergic neurons, it may not definitely include behavioral improvements, as indicated by using 2-methoxy idazoxan only.

What substance P might tell us about the prognosis and mechanism of Parkinson's disease?

The neuropeptide substance P (SP) plays an important role in neurodegenerative disorders, among which Parkinson's disease (PD). In the present work we have reviewed the involvement of SP and its preferred receptor (NK1-R) in motor and non-motor PD symptoms, in both PD animal models and patients. Despite PD is primarily a motor disorder, non-motor abnormalities, including olfactory deficits and gastrointestinal dysfunctions, can represent diagnostic PD predictors, according to the hypothesis that the olfactory and the enteric nervous system represent starting points of neurodegeneration, ascending to the brain via the sympathetic fibers and the vagus nerve. In PD patients, the α-synuclein aggregates in the olfactory bulb and the gastrointestinal tract, as well as in the dorsal motor nucleus of the vagus nerve often co-localize with SP, indicating SP-positive neurons as highly vulnerable sites of degeneration. Considering the involvement of the SP/NK1-R in both the periphery and specific brain areas, this system might represent a neuronal substrate for the symptom and disease progression, as well as a therapeutic target for PD.

ASL expression in ALDH1A1+ neurons in the substantia nigra metabolically contributes to neurodegenerative phenotype

Argininosuccinate lyase (ASL) is essential for the NO-dependent regulation of tyrosine hydroxylase (TH) and thus for catecholamine production. Using a conditional mouse model with loss of ASL in catecholamine neurons, we demonstrate that ASL is expressed in dopaminergic neurons in the substantia nigra pars compacta, including the ALDH1A1⁺ subpopulation that is pivotal for the pathogenesis of Parkinson disease (PD). Neuronal loss of ASL results in catecholamine deficiency, in accumulation and formation of tyrosine aggregates, in elevation of α-synuclein, and phenotypically in motor and cognitive deficits. NO supplementation rescues the formation of aggregates as well as the motor deficiencies. Our data point to a potential metabolic link between accumulations of tyrosine and seeding of pathological aggregates in neurons as initiators for the pathological processes involved in neurodegeneration. Hence, interventions in tyrosine metabolism via regulation of NO levels may be therapeutic beneficial for the treatment of catecholamine-related neurodegenerative disorders.

Andrographolide suppresses NLRP3 inflammasome activation in microglia through induction of parkin-mediated mitophagy in in-vitro and in-vivo models of Parkinson disease

Cellular communication linking microglia activation and dopaminergic neuronal loss play an imperative role in the progression of Parkinson's disease (PD); however, underlying molecular mechanisms are not precise and require further elucidation. NLR Family Pyrin Domain Containing 3 (NLRP3) inflammasome activation is extensively studied in context to microglial activation and progressive dopaminergic neuronal loss in PD. Several pathophysiological factors such as oxidative stress, mitochondrial dysfunction impaired mitophagy plays a crucial role in activating NLRP3 inflammasome complex. Hence, regulation of microglial activation through mitophagy could be a valuable strategy in controlling microglia mediated neurodegeneration. In this study we have developed a model of inflammasome activation by combining LPS with a mitochondrial complex-I inhibitor MPP⁺. The idea of using MPP⁺ after priming mouse microglia with LPS was to disrupt mitochondria and release reactive oxygen species, which act as Signal 2 in augmenting NLRP3 assembly, thereby releasing potent inflammatory mediators such as active interleukin-1 beta (IL-1β) and IL-18. LPS-MPP⁺ combination was seen to impaired the mitophagy by inhibiting the initial step of autophagosome formation as evidenced by protein expression and confocal imaging data. Treatment with Andrographolide promoted the parkin-dependent autophagic flux formation in microglia; resulting in the removal of defective mitochondria which in turn inhibit NLRP3 inflammasome activation. Additionally, the neuroprotective role of Andrographolide in inhibiting NLRP3 activation together with salvage ATP level via promoting parkin-dependent mitophagy was seen in the substantial nigra par compacta (SNpc) region of mice brain. Furthermore, Andrographolide rescued the dopaminergic neuron loss and improved the behavioural parameters in animal model. Collectively, our results reveal the role of mitophagy in the regulation of NLRP3 inflammasome by removing defective mitochondria. In addition, andrographolide was seen to abate NLRP3 inflammasome activation in microglia and rescue dopaminergic neuron loss.

Transcription Factors Phox2a/2b Upregulate Expression of Noradrenergic and Dopaminergic Phenotypes in Aged Rat Brains

The present study investigated the effects of forced overexpression of Phox2a/2b, two transcription factors, in the locus coeruleus (LC) of aged rats on noradrenergic and dopaminergic phenotypes in brains. Results showed that a significant increase in Phox2a/2b mRNA levels in the LC region was paralleled by marked enhancement in expression of DBH and TH per se. Furthermore, similar increases in TH protein levels were observed in the substantial nigra and striatum, as well as in the hippocampus and frontal cortex. Overexpression of Phox2 genes also significantly increased BrdU-positive cells in the hippocampal dentate gyrus and NE levels in the striatum. Moreover, this manipulation significantly improved the cognition behavior. The in vitro experiments revealed that norepinephrine treatments may increase the transcription of TH gene through the epigenetic action on the TH promoter. The results indicate that Phox2 genes may play an important role in improving the function of the noradrenergic and dopaminergic neurons in aged animals, and regulation of Phox2 gene expression may have therapeutic utility in aging or disorders involving degeneration of noradrenergic neurons.

Transgenic Mice Expressing Human α-Synuclein in Noradrenergic Neurons Develop Locus Ceruleus Pathology and Nonmotor Features of Parkinson's Disease

Degeneration of locus ceruleus (LC) neurons and dysregulation of noradrenergic signaling are ubiquitous features of Parkinson's disease (PD). The LC is among the first brain regions affected by α-synuclein (asyn) pathology, yet how asyn affects these neurons remains unclear. LC-derived norepinephrine (NE) can stimulate neuroprotective mechanisms and modulate immune cells, while dysregulation of NE neurotransmission may exacerbate disease progression, particularly nonmotor symptoms, and contribute to the chronic neuroinflammation associated with PD pathology. Although transgenic mice overexpressing asyn have previously been developed, transgene expression is usually driven by pan-neuronal promoters and thus has not been selectively targeted to LC neurons. Here we report a novel transgenic mouse expressing human wild-type asyn under control of the noradrenergic-specific dopamine β-hydroxylase promoter (DBH-hSNCA). These mice developed oligomeric and conformation-specific asyn in LC neurons, alterations in hippocampal and LC microglial abundance, upregulated GFAP expression, degeneration of LC fibers, decreased striatal DA metabolism, and age-dependent behaviors reminiscent of nonmotor symptoms of PD that were rescued by adrenergic receptor antagonists. These mice provide novel insights into how asyn pathology affects LC neurons and how central noradrenergic dysfunction may contribute to early PD pathophysiology.SIGNIFICANCE STATEMENT ɑ-Synuclein (asyn) pathology and loss of neurons in the locus ceruleus (LC) are two of the most ubiquitous neuropathologic features of Parkinson's disease (PD). Dysregulated norepinephrine (NE) neurotransmission is associated with the nonmotor symptoms of PD, including sleep disturbances, emotional changes such as anxiety and depression, and cognitive decline. Importantly, the loss of central NE may contribute to the chronic inflammation in, and progression of, PD. We have generated a novel transgenic mouse expressing human asyn in LC neurons to investigate how increased asyn expression affects the function of the central noradrenergic transmission and associated behaviors. We report cytotoxic effects of oligomeric and conformation-specific asyn, astrogliosis, LC fiber degeneration, disruptions in striatal dopamine metabolism, and age-dependent alterations in nonmotor behaviors without inclusions.

Locus coeruleus-norepinephrine: basic functions and insights into Parkinson's disease

The locus coeruleus is a pontine nucleus that produces much of the brain's norepinephrine. Despite its small size, the locus coeruleus is critical for a myriad of functions and is involved in many neurodegenerative and neuropsychiatric disorders. In this review, we discuss the physiology and anatomy of the locus coeruleus system and focus on norepinephrine's role in synaptic plasticity. We highlight Parkinson's disease as a disorder with motor and neuropsychiatric symptoms that may be understood as aberrations in the normal functions of locus coeruleus.

The Noradrenergic System in Parkinson's Disease

Nowadays it is well accepted that in Parkinson’s disease (PD), the neurodegenerative process occurs in stages and that damage to other areas precedes the neuronal loss in the substantia nigra pars compacta, which is considered a pathophysiological hallmark of PD. This heterogeneous and progressive neurodegeneration may explain the diverse symptomatology of the disease, including motor and non-motor alterations. In PD, one of the first areas undergoing degeneration is the locus coeruleus (LC). This noradrenergic nucleus provides extensive innervation throughout the brain and plays a fundamental neuromodulator role, participating in stress responses, emotional memory, and control of motor, sensory, and autonomic functions. Early in the disease, LC neurons suffer modifications that can condition the effectiveness of pharmacological treatments, and importantly, can lead to the appearance of common non-motor symptomatology. The noradrenergic system also exerts anti-inflammatory and neuroprotective effect on the dopaminergic degeneration and noradrenergic damage can consequently condition the progress of the disease. From the pharmacological point of view, it is also important to understand how the noradrenergic system performs in PD, since noradrenergic medication is often used in these patients, and drug interactions can take place when combining them with the gold standard drug therapy in PD, L-3,4-dihydroxyphenylalanine (L-DOPA). This review provides an overview about the functional status of the noradrenergic system in PD and its contribution to the efficacy of pharmacological-based treatments. Based on preclinical and clinical publications, a special attention will be dedicated to the most prevalent non-motor symptoms of the disease.

[The effect of the dopamine D2 receptor agonist cabergoline on the content of catecholamines and expression of BDNF mRNA in the rat midbrain and hypothalamus]

AIM

To study an effect of cabergoline on dopamine and noradrenaline concentration and BDNF mRNA level in the rat midbrain and hypothalamus.

MATERIAL AND METHODS

Twenty adult male Wistar rats were used in a single treatment paradigm: animals of the treatment group (n=10) received cabergoline (i.p., 0.5 mg/kg) and the control group (n=10) received an equivalent volume of the solvent. Quantitative analysis for the dopamine (DA) and noradrenaline (NA) was carried out using high-performance liquid chromatography (HPLC) coupled with electrochemical detection. BDNF mRNA levels were studied using quantitative RT-PCR.

RESULTS AND CONCLUSION

Cabergoline significantly increases NA concentration in the midbrain 24 hours after injection: 639.2±64.5 ng/g in the treatment group versus 398.0±66.0 ng/g in the control group (p<0.05), while mean content of DA is not significantly changed (211.4±16.3 ng/g vs 169.7±54.6 ng/g, respectively). Cabergoline does not affect hypothalamic DA and NA levels. The drug increases BDNF mRNA levels by 2-times in the midbrain, but not in the hypothalamus, 24 hours after injection.

The noradrenergic system is necessary for survival of vulnerable midbrain dopaminergic neurons: implications for development and Parkinson's disease

The cause of midbrain dopaminergic (mDA) neuron loss in sporadic Parkinson's disease (PD) is multifactorial, involving cell autonomous factors, cell-cell interactions, and the effects of environmental toxins. Early loss of neurons in the locus coeruleus (LC), the main source of ascending noradrenergic (NA) projections, is an important feature of PD and other neurodegenerative disorders. We hypothesized that NA afferents provide trophic support for vulnerable mDA neurons. We demonstrate that depriving mDA neurons of NA input increases postnatal apoptosis and decreases cell survival in young adult rodents, with relative sparing of calbindin-positive subpopulations known to be resistant to degeneration in PD. As a mechanism, we propose that the neurotrophin brain-derived neurotrophic factor (BDNF) modulates anterograde survival effects of LC inputs to mDA neurons. We demonstrate that the LC is rich in BDNF mRNA in postnatal and young adult brains. Early postnatal NA denervation reduces both BDNF protein and activation of TrkB receptors in the ventral midbrain. Furthermore, overexpression of BDNF in NA afferents in transgenic mice increases mDA neuronal survival. Finally, increasing NA activity in primary cultures of mDA neurons improves survival, an effect that is additive or synergistic in the presence of different concentrations of BDNF. Taken together, our results point to a novel mechanism whereby LC afferents couple BDNF effects and NA activity to provide anterograde trophic support for vulnerable mDA neurons. Early loss of NA activity and anterograde neurotrophin support may contribute to degeneration of vulnerable neurons in PD and other neurodegenerative disorders.

Norepinephrine upregulates the expression of tyrosine hydroxylase and protects dopaminegic neurons against 6-hydrodopamine toxicity

As a classic neurotransmitter in the brain, norepinephrine (NE) also is an important modulator to other neuronal systems. Using primary cultures from rat ventral mesencephalon (VM) and dopaminergic cell line MN9D, the present study examined the neuroprotective effects of NE and its effects on the expression of tyrosine hydroxylase (TH). The results showed that NE protected both VM cultures and MN9D cells against 6-hydroxydopamine-caused apoptosis, with possible involvement of adrenal receptors. In addition, treatment with NE upregulated TH protein levels in dose- and time-dependent manner. Further experiments to investigate the potential mechanisms underlying this NE-induced upregulation of TH demonstrated a marked increase in protein levels of the brain-derived neurotrophic factor (BDNF) and the phosphorylated extracellular signal-regulated protein kinase 1 and 2 (pERK1/2) in VM cultures treated with NE. In MN9D cells, a significantly increase of TH and pERK1/2 protein levels were observed after their transfection with BDNF cDNA or exposure to BDNF peptides. Treatment of VM cultures with K252a, an antagonist of the tropomyosin-related kinase B, blocked the upregulatory effects of NE on TH, BDNF and pERK1/2. Administration of MEK1 & MEK2 inhibitors also reversed NE-induced upregulation of TH and pERK1/2. Moreover, ChIP assay showed that treatment with NE or BDNF increased H4 acetylation in the TH promoter. These results suggest that the neuroprotection and modulation of NE on dopaminergic neurons are mediated via BDNF and MAPK/ERK pathways, as well as through epigenetic histone modification, which may have implications for the improvement of therapeutic strategies for Parkinson's disease.

Stimulation of noradrenergic transmission by reboxetine is beneficial for a mouse model of progressive parkinsonism

Parkinson's disease (PD) is the second most common neurodegenerative disorder and is characterized by motor deficits such as tremor, rigidity and bradykinesia. These symptoms are directly caused by the loss of dopaminergic neurons. However, a wealth of clinical evidence indicates that the dopaminergic system is not the only system affected in PD. Postmortem studies of brains from PD patients have revealed the degeneration of noradrenergic neurons in the locus coeruleus (LC) to the same or even greater extent than that observed in the dopaminergic neurons of substantia nigra (SN) and ventral tegmental area (VTA). Moreover, studies performed on rodent models suggest that enhancement of noradrenergic transmission may attenuate the PD-like phenotype induced by MPTP administration, a neurotoxin-based PD model. The aim of this study was to investigate whether chronic treatment with either of two compounds targeting the noradrenergic system (reboxetine or atipamezole) possess the ability to reduce the progression of a PD-like phenotype in a novel mouse model of progressive dopaminergic neurodegeneration induced by the genetic inhibition of rRNA synthesis in dopaminergic neurons, mimicking a PD-like phenotype. The results showed that reboxetine improved the parkinsonian phenotype associated with delayed progression of SN/VTA dopaminergic neurodegeneration and higher dopamine content in the striatum. Moreover, the alpha1-adrenergic agonist phenylephrine enhanced survival of TH+ neurons in primary cell cultures, supporting the putative neuroprotective effects of noradrenergic stimulation. Our results provide new insights regarding the possible influence of the noradrenergic system on dopaminergic neuron survival and strongly support the hypothesis regarding the neuroprotective role of noradrenaline.

Analysis of membrane transport mechanisms of endogenous substrates using chromatographic techniques

Membrane transporters are expressed in various bodily tissues and play essential roles in the homeostasis of endogenous substances and the absortion, distribution and/or excretion of xenobiotics. For transporter assays, radioisotope-labeled compounds have been mainly used. However, commercially available radioisotope-labeled compounds are limited in number and relatively expensive. Chromatographic analyses such as high-performance liquid chromatography with ultraviolet absorptiometry and liquid chromatography with tandem mass spectrometry have also been applied for transport assays. To elucidate the transport properties of endogenous substrates, although there is no difficulty in performing assays using radioisotope-labeled probes, the endogenous background and the metabolism of the compound after its translocation across cell membranes must be considered when the intact compound is assayed. In this review, the current state of knowledge about the transport of endogenous substrates via membrane transporters as determined by chromatographic techniques is summarized. Chromatographic techniques have contributed to our understanding of the transport of endogenous substances including amino acids, catecholamines, bile acids, prostanoids and uremic toxins via membrane transporters.

α-Synuclein and Noradrenergic Modulation of Immune Cells in Parkinson's Disease Pathogenesis

α-synuclein (α-syn) pathology and loss of noradrenergic neurons in the locus coeruleus (LC) are among the most ubiquitous features of Parkinson's disease (PD). While noradrenergic dysfunction is associated with non-motor symptoms of PD, preclinical research suggests that the loss of LC norepinephrine (NE), and subsequently its immune modulatory and neuroprotective actions, may exacerbate or even accelerate disease progression. In this review, we discuss the mechanisms by which α-syn pathology and loss of central NE may directly impact brain health by interrupting neurotrophic factor signaling, exacerbating neuroinflammation, and altering regulation of innate and adaptive immune cells.

Targeting the noradrenergic system for anti-inflammatory and neuroprotective effects: implications for Parkinson's disease

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-inflammatory 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 inflammatory 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-inflammatory 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.

Vagus nerve stimulation improves locomotion and neuronal populations in a model of Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is a progressive, neurodegenerative disorder with no disease-modifying therapies, and symptomatic treatments are often limited by debilitating side effects. In PD, locus coeruleus noradrenergic (LC-NE) neurons degenerate prior to substantia nigra dopaminergic (SN-DA) neurons. Vagus nerve stimulation (VNS) activates LC neurons, and decreases pro-inflammatory markers, allowing improvement of LC targets, making it a potential PD therapeutic.

OBJECTIVE

To assess therapeutic potential of VNS in a PD model.

METHODS

To mimic the progression of PD degeneration, rats received a systemic injection of noradrenergic neurotoxin DSP-4, followed one week later by bilateral intrastriatal injection of dopaminergic neurotoxin 6-hydroxydopamine. At this time, a subset of rats also had vagus cuffs implanted. After eleven days, rats received a precise VNS regimen twice a day for ten days, and locomotion was measured during each afternoon session. Immediately following final stimulation, rats were euthanized, and left dorsal striatum, bilateral SN and LC were sectioned for immunohistochemical detection of monoaminergic neurons (tyrosine hydroxylase, TH), α-synuclein, astrocytes (GFAP) and microglia (Iba-1).

RESULTS

VNS significantly increased locomotion of lesioned rats. VNS also resulted in increased expression of TH in striatum, SN, and LC; decreased SN α-synuclein expression; and decreased expression of glial markers in the SN and LC of lesioned rats. Additionally, saline-treated rats after VNS, had higher LC TH and lower SN Iba-1.

CONCLUSIONS

Our findings of increased locomotion, beneficial effects on LC-NE and SN-DA neurons, decreased α-synuclein density in SN TH-positive neurons, and neuroinflammation suggest VNS has potential as a novel PD therapeutic.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

Reduced noradrenergic innervation of ventral midbrain dopaminergic cell groups and the subthalamic nucleus in MPTP-treated parkinsonian monkeys

There is anatomical and functional evidence that ventral midbrain dopaminergic (DA) cell groups and the subthalamic nucleus (STN) receive noradrenergic innervation in rodents, but much less is known about these interactions in primates. Degeneration of NE neurons in the locus coeruleus (LC) and related brainstem NE cell groups is a well-established pathological feature of Parkinson's disease (PD), but the development of such pathology in animal models of PD has been inconsistent across species and laboratories. We recently demonstrated 30-40% neuronal loss in the LC, A5 and A6 NE cell groups of rhesus monkeys rendered parkinsonian by chronic administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In this study, we used dopamine-beta-hydroxylase (DβH) immunocytochemistry to assess the impact of this neuronal loss on the number of NE terminal-like varicosities in the substantia nigra pars compacta (SNC), ventral tegmental area (VTA), retrorubral field (RRF) and STN of MPTP-treated parkinsonian monkeys. Our findings reveal that the NE innervation of the ventral midbrain and STN of normal monkeys is heterogeneously distributed being far more extensive in the VTA, RRF and dorsal tier of the SNC than in the ventral SNC and STN. In parkinsonian monkeys, all regions underwent a significant (~50-70%) decrease in NE innervation. At the electron microscopic level, some DβH-positive terminals formed asymmetric axo-dendritic synapses in VTA and STN. These findings demonstrate that the VTA, RRF and SNCd are the main ventral midbrain targets of ascending NE inputs, and that these connections undergo a major break-down in chronically MPTP-treated parkinsonian monkeys. This severe degeneration of the ascending NE system may contribute to the pathophysiology of ventral midbrain and STN neurons in PD.

Brain regions and monoaminergic neurotransmitters that are involved in mouse ambulatory activity promoted by bupropion

Bupropion (BUP), a substituted phenyl-ethylamine, has been utilized for the treatment of depression and for smoking cessation, however, one concern is that BUP may increase a risk of psychosis similar to other substituted phenyl-ethylamine amphetamine (AMPH) and methamphetamine (MetAMPH). BUP promotes ambulation in mice and causes behavioral sensitization on the ambulation-promoting effect when repeatedly administered as well as AMPH and MetAMPH. The present study aimed to elucidate brain regions and monoaminergic neurotransmitters that are involved in the ambulation-promoting effect of BUP. c-Fos-like immunoreactivity (c-Fos-IR) mapping in brain in combination with measuring ambulatory activity was conducted to determine brain region(s) that is involved in the ambulatory effect of BUP. Three kinds of statistical analyses for c-Fos-IR in 24 brain regions consistently showed that c-Fos-IR in the Caudate putamen (CPu) is positively correlated with the ambulatory response to BUP. In addition, multiple regression analysis indicated that the ambulatory response is a function of c-Fos-IR not only in the CPu but also in the lateral septum nucleus (LS), median raphe nucleus (MnR), lateral globus pallidus (LGP), medial globus pallidus (MGP), locus coeruleus (LC) and ventral hypothalamic nucleus (VMH). Effects of BUP on monoaminergic neurotransmitters in the CPu were examined using in vivo microdialysis method, as the pharmacological experiments indicated that monoaminergic neurotransmitters, dopamine (DA) in particular, mediate the ambulatory response to BUP. Response of DA in the CPu to BUP was parallel to the ambulatory response, showing that DA in the CPu is involved in the ambulatory response to BUP. The present study also suggests that other brain regions such as the LC, the origin nucleus of norepinephrine (NE) neurons, and another neurotransmitter NE may also play some roles for the ambulatory response to BUP, however, further studies are needed to elucidate the roles.

Individual and Familial Susceptibility to MPTP in a Common Marmoset Model for Parkinson's Disease

INTRODUCTION

Insight into susceptibility mechanisms underlying Parkinson's disease (PD) would aid the understanding of disease etiology, enable target finding and benefit the development of more refined disease-modifying strategies.

METHODS

We used intermittent low-dose MPTP (0.5 mg/kg/week) injections in marmosets and measured multiple behavioral and neurochemical parameters. Genetically diverse monkeys from different breeding families were selected to investigate inter- and intrafamily differences in susceptibility to MPTP treatment.

RESULTS

We show that such differences exist in clinical signs, in particular nonmotor PD-related behaviors, and that they are accompanied by differences in neurotransmitter levels. In line with the contribution of a genetic component, different susceptibility phenotypes could be traced back through genealogy to individuals of the different families.

CONCLUSION

Our findings show that low-dose MPTP treatment in marmosets represents a clinically relevant PD model, with a window of opportunity to examine the onset of the disease, allowing the detection of individual variability in disease susceptibility, which may be of relevance for the diagnosis and treatment of PD in humans.

Progression and recovery of Parkinsonism in a chronic progressive MPTP-induction model in the marmoset without persistent molecular and cellular damage

Chronic exposure to low-dose 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in marmoset monkeys was used to model the prodromal stage of Parkinson's disease (PD), and to investigate mechanisms underlying disease progression and recovery. Marmosets were subcutaneously injected with MPTP for a period of 12weeks, 0.5mg/kg once per week, and clinical signs of Parkinsonism, motor- and non-motor behaviors were recorded before, during and after exposure. In addition, postmortem immunohistochemistry and proteomics analysis were performed. MPTP-induced parkinsonian clinical symptoms increased in severity during exposure, and recovered after MPTP administration was ended. Postmortem analyses, after the recovery period, revealed no alteration of the number and sizes of tyrosine hydroxylase (TH)-positive dopamine (DA) neurons in the substantia nigra. Also levels of TH in putamen and caudate nucleus were unaltered, no differences were observed in DA, serotonin or nor-adrenalin levels in the caudate nucleus, and proteomics analysis revealed no global changes in protein expression in these brain areas between treatment groups. Our findings indicate that parkinsonian symptoms can occur without detectable damage at the cellular or molecular level. Moreover, we show that parkinsonian symptoms may be reversible when diagnosed and treated early.

Compensatory mechanisms in genetic models of neurodegeneration: are the mice better than humans?

Neurodegenerative diseases are one of the main causes of mental and physical disabilities. Neurodegeneration has been estimated to begin many years before the first clinical symptoms manifest, and even a prompt diagnosis at this stage provides very little advantage for a more effective treatment as the currently available pharmacotherapies are based on disease symptomatology. The etiology of the majority of neurodegenerative diseases remains unknown, and even for those diseases caused by identified genetic mutations, the direct pathways from gene alteration to final cell death have not yet been fully elucidated. Advancements in genetic engineering have provided many transgenic mice that are used as an alternative to pharmacological models of neurodegenerative diseases. Surprisingly, even the models reiterating the same causative mutations do not fully recapitulate the inevitable neuronal loss, and some fail to even show phenotypic alterations, which suggests the possible existence of compensatory mechanisms. A better evaluation of these mechanisms may not only help us to explain why neurodegenerative diseases are mostly late-onset disorders in humans but may also provide new markers and targets for novel strategies designed to extend neuronal function and survival. The aim of this mini-review is to draw attention to this under-explored field in which investigations may reasonably contribute to unveiling hidden reserves in the organism.

Withdrawal from repeated administration of a low dose of reserpine induced opposing adaptive changes in the noradrenaline and serotonin system function: a behavioral and neurochemical ex vivo and in vivo studies in the rat

Reserpine is an inhibitor of the vesicular monoamine transporter 2 (VMAT2) and monoamine releaser, so it can be used as a pharmacological model of depression. In the present paper, we investigated the behavioral and neurochemical effects of withdrawal from acute and repeated administration of a low dose of reserpine (0.2 mg/kg) in Wistar Han rats. We demonstrated the behavioral and receptor oversensitivity (postsynaptic dopamine D1) during withdrawal from chronic reserpine. It was accompanied by a significant increase in motility in the locomotor activity test and climbing behavior in the forced swim test (FST). Neurochemical studies revealed that repeated but not acute administration the a low dose of reserpine triggered opposing adaptive changes in the noradrenergic and serotonin system function analyzed during reserpine withdrawal, i.e. 48 h after the last injection. The tissue concentration of noradrenaline was significantly decreased in the hypothalamus and nucleus accumbens only after repeated drug administration (by about 20% and 35% vs. control; p<0.05, respectively). On the other hand, the concentration of its extraneuronal metabolite, normetanephrine (NM) increased significantly in the VTA during withdrawal both from acute and chronic reserpine. The serotonin concentration was significantly reduced in the VTA after chronic reserpine (by about 40% vs. the control group, p<0.05) as well as its main metabolite, 5-HIAA (by about 30% vs. control; p<0.05) in the VTA and hypothalamus. Dopamine and its metabolites were not changed after acute or chronic reserpine administration. In vivo microdialysis studies clearly evidenced the lack of the effect of a single dose of reserpine, and its distinct effects after chronic treatment on the release of noradrenaline and serotonin in the rat striatum. In fact, the withdrawal from repeated administration of reserpine significantly increased an extraneuronal concentration of noradrenaline in the rat striatum but at the same time produced a distinct fall in the extraneuronal serotonin in this brain structure. On the basis of the presented behavioral and neurochemical experiments, we suggest that chronic administration of reserpine even in such low dose which not yet acted on the release of monoamines but produced an inhibition of VMAT2 caused a long-lasting disadvantageous effect of plasticity in the brain resembling depressive disorders.

Suppression of glia maturation factor expression prevents 1-methyl-4-phenylpyridinium (MPP⁺)-induced loss of mesencephalic dopaminergic neurons

Inflammation mediated by glial activation appears to play a critical role in the pathogenesis of Parkinson disease (PD). Glia maturation factor (GMF), a proinflammatory protein predominantly localized in the central nervous system was isolated, sequenced and cloned in our laboratory. We have previously demonstrated immunomodulatory and proinflammatory functions of GMF, but its involvement in 1-methyl-4-phenylpyridinium (MPP(+)), active metabolite of classical parkinsonian toxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), inducing loss of dopaminergic (DA) neurons has not been studied. Here we show that altered expression of GMF has direct consequences on the production of reactive oxygen species (ROS) and nuclear factor-kappa B (NF-κB)- mediated production of inflammatory mediators by MPP(+). We examined MPP(+)-induced DA neuronal loss in primary cultures of mouse mesencephalic neurons/glia obtained from GMF-deficient (GMF knockout (GMF-KO)) and GMF-containing wild-type (Wt) mice. We demonstrate that deficiency of GMF in GMF-KO neurons/glia led to decreased production of ROS and downregulation of NF-κB-mediated production of tumor necrosis factor-alpha (TNF-α) and interleukin-1beta (IL-1β) as compared to Wt neurons/glia. Additionally, overexpression of GMF induced DA neurodegeneration, whereas GMF downregulation by GMF-specific shRNA protected DA neurons from MPP-induced toxicity. Subsequently, GMF deficiency ameliorates antioxidant balance, as evidenced by the decreased level of lipid peroxidation, less ROS production along with increased level of glutathione; and attenuated the DA neuronal loss via the downregulation of NF-κB-mediated inflammatory responses. In conclusion, our overall data indicate that GMF modulates oxidative stress and release of deleterious agents by MPP(+) leading to loss of DA neurons. Our study provides new insights into the potential role of GMF and identifies targets for therapeutic interventions in neurodegenerative diseases.

Norepinephrine modulates the motility of resting and activated microglia via different adrenergic receptors

Microglia, the resident immune cells of the central nervous system (CNS), monitor the brain for disturbances of tissue homeostasis by constantly moving their fine processes. Microglia respond to tissue damage through activation of ATP/ADP receptors followed by directional process extension to the damaged area. A common feature of several neurodegenerative diseases is the loss of norepinephrine, which might contribute to the associated neuroinflammation. We carried out a high resolution analysis of the effects of norepinephrine (NE) on microglial process dynamics in acute brain slices from mice that exhibit microglia-specific enhanced green fluorescent protein expression. Bath application of NE to the slices resulted in significant process retraction in microglia. Analysis of adrenergic receptor expression with quantitative PCR indicated that resting microglia primarily express β2 receptors but switch expression to α2A receptors under proinflammatory conditions modeled by LPS treatment. Despite the differential receptor expression, NE caused process retraction in both resting and LPS-activated microglia cultured in the gelatinous substrate Matrigel in vitro. The use of subtype-selective receptor agonists and antagonists confirmed the involvement of β2 receptors in mediating microglial process dynamics in resting cells and α2A receptors in activated cells. Co-application of NE with ATP to resting microglia blocked the ATP-induced process extension and migration in isolated microglia, and β2 receptor antagonists prolonged ATP effects in brain slice tissues, suggesting the presence of cross-talk between adrenergic and purinergic signaling in microglia. These data show that the neurotransmitter NE can modulate microglial motility, which could affect microglial functions in pathogenic situations of either elevated or reduced NE levels.

Altered reward circuitry in the norepinephrine transporter knockout mouse

Synaptic levels of the monoamine neurotransmitters dopamine, serotonin, and norepinephrine are modulated by their respective plasma membrane transporters, albeit with a few exceptions. Monoamine transporters remove monoamines from the synaptic cleft and thus influence the degree and duration of signaling. Abnormal concentrations of these neuronal transmitters are implicated in a number of neurological and psychiatric disorders, including addiction, depression, and attention deficit/hyperactivity disorder. This work concentrates on the norepinephrine transporter (NET), using a battery of in vivo magnetic resonance imaging techniques and histological correlates to probe the effects of genetic deletion of the norepinephrine transporter on brain metabolism, anatomy and functional connectivity. MRS recorded in the striatum of NET knockout mice indicated a lower concentration of NAA that correlates with histological observations of subtle dysmorphisms in the striatum and internal capsule. As with DAT and SERT knockout mice, we detected minimal structural alterations in NET knockout mice by tensor-based morphometric analysis. In contrast, longitudinal imaging after stereotaxic prefrontal cortical injection of manganese, an established neuronal circuitry tracer, revealed that the reward circuit in the NET knockout mouse is biased toward anterior portions of the brain. This is similar to previous results observed for the dopamine transporter (DAT) knockout mouse, but dissimilar from work with serotonin transporter (SERT) knockout mice where Mn²⁺ tracings extended to more posterior structures than in wildtype animals. These observations correlate with behavioral studies indicating that SERT knockout mice display anxiety-like phenotypes, while NET knockouts and to a lesser extent DAT knockout mice display antidepressant-like phenotypic features. Thus, the mainly anterior activity detected with manganese-enhanced MRI in the DAT and NET knockout mice is likely indicative of more robust connectivity in the frontal portion of the reward circuit of the DAT and NET knockout mice compared to the SERT knockout mice.

Animal models of the non-motor features of Parkinson's disease

The non-motor symptoms (NMS) of Parkinson's disease (PD) occur in roughly 90% of patients, have a profound negative impact on their quality of life, and often go undiagnosed. NMS typically involve many functional systems, and include sleep disturbances, neuropsychiatric and cognitive deficits, and autonomic and sensory dysfunction. The development and use of animal models have provided valuable insight into the classical motor symptoms of PD over the past few decades. Toxin-induced models provide a suitable approach to study aspects of the disease that derive from the loss of nigrostriatal dopaminergic neurons, a cardinal feature of PD. This also includes some NMS, primarily cognitive dysfunction. However, several NMS poorly respond to dopaminergic treatments, suggesting that they may be due to other pathologies. Recently developed genetic models of PD are providing new ways to model these NMS and identify their mechanisms. This review summarizes the current available literature on the ability of both toxin-induced and genetically-based animal models to reproduce the NMS of PD.

Contributions of central and systemic inflammation to the pathophysiology of Parkinson's disease

Idiopathic Parkinson's disease (PD) represents a complex interaction between the inherent vulnerability of the nigrostriatal dopaminergic system, a possible genetic predisposition, and exposure to environmental toxins including inflammatory triggers. Evidence now suggests that chronic neuroinflammation is consistently associated with the pathophysiology of PD. Activation of microglia and increased levels of pro-inflammatory mediators such as TNF-α, IL-1β and IL-6, reactive oxygen species and eicosanoids has been reported after post-mortem analysis of the substantia nigra from PD patients and in animal models of PD. It is hypothesised that chronically activated microglia secrete high levels of pro-inflammatory mediators which damage neurons and further activate microglia, resulting in a feed forward cycle promoting further inflammation and neurodegeneration. Moreover, nigrostriatal dopaminergic neurons are more vulnerable to pro-inflammatory and oxidative mediators than other cell types because of their low intracellular glutathione concentration. Systemic inflammation has also been suggested to contribute to neurodegeneration in PD, as lymphocyte infiltration has been observed in brains of PD patients and in animal models of PD, substantiating the current theory of a fundamental role of inflammation in neurodegeneration. We will examine the current evidence in the literature which offers insight into the premise that both central and systemic inflammation may contribute to neurodegeneration in PD. We will discuss the emerging possibility of the use of diagnostic tools such as imaging technologies for PD patients. Finally, we will present the immunomodulatory therapeutic strategies that are now under investigation and in clinical trials as potential neuroprotective drugs for PD.

Noradrenaline and Parkinson's disease

Parkinson's disease (PD) is characterized by the degeneration of dopamine (DA) neurons in the substantia nigra pars compacta, and motor symptoms including bradykinesia, rigidity, and tremor at rest. These symptoms are exhibited when striatal dopamine concentration has decreased by around 70%. In addition to motor deficits, PD is also characterized by the non-motor symptoms. However, depletion of DA alone in animal models has failed to simultaneously elicit both the motor and non-motor deficits of PD, possibly because the disease is a multi-system disorder that features a profound loss in other neurotransmitter systems. There is growing evidence that additional loss of noradrenaline (NA) neurons of the locus coeruleus, the principal source of NA in the brain, could be involved in the clinical expression of motor as well as in non-motor deficits. In the present review, we analyze the latest evidence for the implication of NA in the pathophysiology of PD obtained from animal models of parkinsonism and from parkinsonian patients. Recent studies have shown that NA depletion alone, or combined with DA depletion, results in motor as well as in non-motor dysfunctions. In addition, by using selective agonists and antagonists of noradrenaline alpha receptors we, and others, have shown that α2 receptors are implicated in the control of motor activity and that α2 receptor antagonists can improve PD motor symptoms as well as l-Dopa-induced dyskinesia. In this review we argue that the loss of NA neurons in PD has an impact on all PD symptoms and that the addition of NAergic agents to dopaminergic medication could be beneficial in the treatment of the disease.

Monoamine transporters: vulnerable and vital doorkeepers

Transporters of dopamine, serotonin, and norepinephrine have been empirically used as medication targets for several mental illnesses in the last decades. These protein-targeted medications are effective only for subpopulations of patients with transporter-related brain disorders. Since the cDNA clonings in early 1990s, molecular studies of these transporters have revealed a wealth of information about the transporters' structure-activity relationship (SAR), neuropharmacology, cell biology, biochemistry, pharmacogenetics, and the diseases related to the human genes encoding these transporters among related regulators. Such new information creates a unique opportunity to develop transporter-specific medications based on SAR, mRNA, DNA, and perhaps transporter trafficking regulation for a number of highly relevant diseases including substance abuse, depression, schizophrenia, and Parkinson's disease.

Differential response of the central noradrenergic nervous system to the loss of locus coeruleus neurons in Parkinson's disease and Alzheimer's disease

In Parkinson's disease (PD), there is a significant loss of noradrenergic neurons in the locus coeruleus (LC) in addition to the loss of dopaminergic neurons in the substantia nigra (SN). The goal of this study was to determine if the surviving LC noradrenergic neurons in PD demonstrate compensatory changes in response to the neuronal loss, as observed in Alzheimer's disease (AD). Tyrosine hydroxylase (TH) and dopamine β-hydroxylase (DBH) mRNA expression in postmortem LC tissue of control and age-matched PD subjects demonstrated a significant reduction in the number of noradrenergic neurons in the LC of PD subjects. TH mRNA expression/neuron did not differ between control and PD subjects, but DBH mRNA expression/neuron was significantly elevated in PD subjects compared to control. This increase in DBH mRNA expression in PD subjects is not a response to neuronal loss because the amount of DBH mRNA expression/neuron in AD subjects was not significantly different from control. Norepinephrine transporter (NET) binding site concentration in the LC of PD subjects was significantly reduced over the cell body region as well as the peri-LC dendritic zone. In PD subjects, the loss of dendrites from surviving noradrenergic neurons was also apparent with TH-immunoreactivity (IR). This loss of LC dendritic innervation in PD subjects as measured by TH-IR was not due to LC neuronal loss because TH-IR in AD subjects was robust, despite a similar loss of LC neurons. These data suggest that there is a differential response of the noradrenergic nervous system in PD compared to AD in response to the loss of LC neurons.

Involvement of threonine 258 and serine 259 motif in amphetamine-induced norepinephrine transporter endocytosis

D-amphetamine (AMPH) down-regulates the norepinephrine transporter (NET), although the exact trafficking pathways altered and motifs involved are not known. Therefore, we examined the cellular and molecular mechanisms involved in AMPH-induced NET regulation in human placental trophoblast cells expressing the wild-type (WT)-hNET and the hNET double mutant (DM)-bearing protein kinase C (PKC)-resistant T258A + S259A motif. NET function and surface expression were significantly reduced in cells expressing WT-hNET but not in cells expressing hNET-DM following AMPH treatment. AMPH inhibited plasma membrane recycling of both WT-hNET and hNET-DM. In contrast, AMPH stimulated endocytosis of WT-hNET, and did not affect hNET-DM endocytosis. Although PKC or calcium/calmodulin- dependent kinase-II (CaMKII) inhibition or depletion of calcium failed to block AMPH-mediated down-regulation of WT-hNET, NET-specific blocker desipramine completely prevented AMPH-induced down-regulation. Furthermore, AMPH treatment had no effect on phospho-CaMKII immunoreactivity. The inhibitory potency of AMPH was highest on hNET-DM, intermediary on T258A and S259A single mutants and lowest on WT-hNET. Single mutants exhibited partial resistance to AMPH-mediated down-regulation. AMPH accumulation was similar in cells expressing WT-hNET or hNET-DM. The results demonstrate that reduced plasma membrane insertion and enhanced endocytosis account for AMPH-mediated NET down-regulation, and provide the first evidence that T258/S259 motif is involved only in AMPH-induced NET endocytosis that is desipramine-sensitive, but PKC and CaMKII independent.

MPTP-induced neuroblast apoptosis in the subventricular zone is not regulated by dopamine or other monoamine transporters

For 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to exert neurotoxicity on dopaminergic neurons, 1-methyl-4-phenylpyridinium (MPP+), a metabolite of MPTP, must be taken up into the dopaminergic neuron via the dopamine transporter (DAT). Previous reports have shown that MPTP also causes neuroblast apoptosis in the subventricular zone (SVZ) of adult mice. The aim of this study is to elucidate the role of DAT and other monoamine transporters including vesicular monoamine transporter 2 (VMAT2), the serotonin transporter (SERT), and the norepinephrine transporter (NET) on the neuroblast apoptosis induced by MPTP administration. There were no DAT-positive neuroblasts in the SVZ, whereas some neuroblasts were immunopositive for VMAT2 and SERT. To examine whether these transporters are involved in MPTP-induced neuroblast apoptosis in the SVZ, terminal deoxynucleotidyl transferase-mediated dUTP endlabeling (TUNEL)-positive cells were semiquantitatively analyzed after the injection of GBR12909 (GBR), a DAT inhibitor; tetrabenazine (TBZ), a VMAT2 inhibitor; fluoxetine (FLU), a SERT inhibitor, or desipramine (DES), a NET inhibitor, prior to MPTP injection. However, the injection of these transporter inhibitors had no influence on the MPTP-induced neuroblast apoptosis in the SVZ. It is likely that neither DAT nor other monoamine transporters are involved in MPTP-induced neuroblast apoptosis. The present findings suggest that the neurotoxicity of MPTP to neuroblasts in the SVZ does not require DAT or other monoamine transporters, and the apoptosis it induces may be executed through other unknown pathways.

Protective actions of the vesicular monoamine transporter 2 (VMAT2) in monoaminergic neurons

Vesicular monoamine transporters (VMATs) are responsible for the packaging of neurotransmitters such as dopamine, serotonin, norepinephrine, and epinephrine into synaptic vesicles. These proteins evolved from precursors in the major facilitator superfamily of transporters and are among the members of the toxin extruding antiporter family. While the primary function of VMATs is to sequester neurotransmitters within vesicles, they can also translocate toxicants away from cytosolic sites of action. In the case of dopamine, this dual role of VMAT2 is combined-dopamine is more readily oxidized in the cytosol where it can cause oxidative stress so packaging into vesicles serves two purposes: neurotransmission and neuroprotection. Furthermore, the deleterious effects of exogenous toxicants on dopamine neurons, such as MPTP, can be attenuated by VMAT2 activity. The active metabolite of MPTP can be kept within vesicles and prevented from disrupting mitochondrial function thereby sparing the dopamine neuron. The highly addictive drug methamphetamine is also neurotoxic to dopamine neurons by using dopamine itself to destroy the axon terminals. Methamphetamine interferes with vesicular sequestration and increases the production of dopamine, escalating the amount in the cytosol and leading to oxidative damage of terminal components. Vesicular transport seems to resist this process by sequestering much of the excess dopamine, which is illustrated by the enhanced methamphetamine neurotoxicity in VMAT2-deficient mice. It is increasingly evident that VMAT2 provides neuroprotection from both endogenous and exogenous toxicants and that while VMAT2 has been adapted by eukaryotes for synaptic transmission, it is derived from phylogenetically ancient proteins that originally evolved for the purpose of cellular protection.

Light and electron microscopic localization of alpha-1 adrenergic receptor immunoreactivity in the rat striatum and ventral midbrain

Electrophysiological and pharmacological studies have demonstrated that alpha-1 adrenergic receptor (alpha1AR) activation facilitates dopamine (DA) transmission in the striatum and ventral midbrain. However, because little is known about the localization of alpha1ARs in dopaminergic regions, the substrate(s) and mechanism(s) underlying this facilitation of DA signaling are poorly understood. To address this issue, we used light and electron microscopy immunoperoxidase labeling to examine the cellular and ultrastructural distribution of alpha1ARs in the caudate putamen, nucleus accumbens, ventral tegmental area, and substantia nigra in the rat. Analysis at the light microscopic level revealed alpha1AR immunoreactivity mainly in neuropil, with occasional staining in cell bodies. At the electron microscopic level, alpha1AR immunoreactivity was found primarily in presynaptic elements, with scarce postsynaptic labeling. Unmyelinated axons and about 30-50% terminals forming asymmetric synapses contained the majority of presynaptic labeling in the striatum and midbrain, while in the midbrain a subset of terminals forming symmetric synapses also displayed immunoreactivity. Postsynaptic labeling was scarce in both striatal and ventral midbrain regions. On the other hand, only 3-6% of spines displayed alpha1AR immunoreactivity in the caudate putamen and nucleus accumbens. These data suggest that the facilitation of dopaminergic transmission by alpha1ARs in the mesostriatal system is probably achieved primarily by pre-synaptic regulation of glutamate and GABA release.