L-DOPA: a scapegoat for accelerated neurodegeneration in Parkinson's disease?

Hormonal mechanisms in the paraventricular nuclei associated with hyperalgesia in Parkinson's disease model rats

Pain is a major non-motor symptom of Parkinson's disease (PD). The relationship between hyperalgesia and neuropeptides originating from paraventricular nucleus (PVN) in 6-hydroxydopamine (6-OHDA) rats has already been investigated for oxytocin (OXT), but not yet for arginine vasopressin (AVP) and corticotropin-releasing hormone (CRH). The present study aimed to investigate the alterations in these neuropeptides following nociceptive stimulation in PD model rats and to examine the mechanisms of hyperalgesia. Dopaminergic nigrostriatal lesions were induced by injecting 6-OHDA into the medial forebrain bundle. Subcutaneous formalin injection into the vibrissa pad was performed in rats as a nociceptive stimulus in the orofacial region. Dopamine depletion's effect on nociception was assessed by counting the p-ERK-immunoreactive (-IR) cells in the trigeminal spinal subnucleus caudalis (Vc). The PD model rats induced by 6-OHDA injection (6-OHDA rats) showed a significantly higher number of p-ERK-IR cells in the Vc than the sham rats, confirming hyperalgesia in 6-OHDA rats. Then, we investigated the immunohistochemical responses to OXT, AVP, and CRH cells in the PVN and examined the changes in blood levels of these neuropeptides. As a result, formalin injection increased neuronal activity and blood levels of OXT and CRH in sham rats, but these were suppressed in the 6-OHDA rats. Contrarily, neuronal activity and blood level of AVP were unaffected by nociceptive stimuli and were significantly lower in 6-OHDA rats than in sham rats. Our findings suggest that OXT and CRH suppression is linked to hyperalgesia in PD, whereas AVP does not directly influence the observed hyperalgesia.

Advancements in ultrasound-mediated drug delivery for central nervous system disorders

INTRODUCTION

Central nervous system (CNS) disorders present major therapeutic challenges due to the presence of the blood - brain barrier (BBB) and disease heterogeneity. The BBB impedes most therapeutic agents, which restricts conventional treatments. Focused ultrasound (FUS) -assisted delivery offers a novel solution by temporarily disrupting the BBB and thereby enhancing drug delivery to the CNS.

AREAS COVERED

This review outlines the fundamental principles of FUS-assisted drug delivery technology, with an emphasis on its role in enhancing the spatial precision of therapeutic interventions and its molecular effects on the cellular composition of the BBB. Recent promising clinical studies are surveyed, and a comparative analysis of current US-assisted delivery system is provided. Additionally, the latest advancements and challenges of this technology are discussed.

EXPERT OPINION

FUS-mediated drug delivery shows promise, but the clinical translation of research findings is challenging. Key issues include safety, dosage optimization, and balancing efficacy with the risk of tissue damage. Continued research is crucial to address these challenges and bridge the gap between preclinical and clinical applications, and could transform treatments of CNS disorders.

Protopanaxadiols Eliminate Behavioral Impairments and Mitochondrial Dysfunction in Parkinson's Disease Mice Model

Currently, there are no effective therapies to cure Parkinson's disease (PD), which is the second most common neurodegenerative disease primarily characterized by motor dysfunction and degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). Protopanaxadiols (PPDs), including 20 (R)- protopanaxadiol (R-PPD) and 20 (S)- protopanaxadiol (S-PPD), are main metabolites of ginsenosides. The role of ginsenosides in neurodegenerative diseases has been thoroughly studied, however, it is unknown whether PPDs can attenuate behavioral deficits and dopaminergic neuron injury in PD model mice to date. Here, we administered PPDs to MPTP-induced PD model mice and monitored the effects on behavior and dopaminergic neurons to investigate the effects of R-PPD and S-PPD against PD. Our results showed that R-PPD and S-PPD (at a dose of 20 mg/kg, i.g.) treatment alleviated MPTP (30 mg/kg, i.p.) induced behavioral deficits. Besides, R-PPD and S-PPD protected MPP+-induced neuron injury and mitochondrial dysfunction, and reduced the abnormal expression of Cyt C, Bax, caspase-3 and Bcl-2. These findings demonstrate that R-PPD and S-PPD were potentially useful to ameliorate PD.

Selective dopaminergic neurotoxicity modulated by inherent cell-type specific neurobiology

Parkinson's disease (PD) is a debilitating neurodegenerative disease affecting millions of individuals worldwide. Hallmark features of PD pathology are the formation of Lewy bodies in neuromelanin-containing dopaminergic (DAergic) neurons of the substantia nigra pars compacta (SNpc), and the subsequent irreversible death of these neurons. Although genetic risk factors have been identified, around 90 % of PD cases are sporadic and likely caused by environmental exposures and gene-environment interaction. Mechanistic studies have identified a variety of chemical PD risk factors. PD neuropathology occurs throughout the brain and peripheral nervous system, but it is the loss of DAergic neurons in the SNpc that produce many of the cardinal motor symptoms. Toxicology studies have found specifically the DAergic neuron population of the SNpc exhibit heightened sensitivity to highly variable chemical insults (both in terms of chemical structure and mechanism of neurotoxic action). Thus, it has become clear that the inherent neurobiology of nigral DAergic neurons likely underlies much of this neurotoxic response to broad insults. This review focuses on inherent neurobiology of nigral DAergic neurons and how such neurobiology impacts the primary mechanism of neurotoxicity. While interactions with a variety of other cell types are important in disease pathogenesis, understanding how inherent DAergic biology contributes to selective sensitivity and primary mechanisms of neurotoxicity is critical to advancing the field. Specifically, key biological features of DAergic neurons that increase neurotoxicant susceptibility.

Toxic interactions between dopamine, α-synuclein, monoamine oxidase, and genes in mitochondria of Parkinson's disease

Parkinson's disease is characterized by its distinct pathological features; loss of dopamine neurons in the substantia nigra pars compacta and accumulation of Lewy bodies and Lewy neurites containing modified α-synuclein. Beneficial effects of L-DOPA and dopamine replacement therapy indicate dopamine deficit as one of the main pathogenic factors. Dopamine and its oxidation products are proposed to induce selective vulnerability in dopamine neurons. However, Parkinson's disease is now considered as a generalized disease with dysfunction of several neurotransmitter systems caused by multiple genetic and environmental factors. The pathogenic factors include oxidative stress, mitochondrial dysfunction, α-synuclein accumulation, programmed cell death, impaired proteolytic systems, neuroinflammation, and decline of neurotrophic factors. This paper presents interactions among dopamine, α-synuclein, monoamine oxidase, its inhibitors, and related genes in mitochondria. α-Synuclein inhibits dopamine synthesis and function. Vice versa, dopamine oxidation by monoamine oxidase produces toxic aldehydes, reactive oxygen species, and quinones, which modify α-synuclein, and promote its fibril production and accumulation in mitochondria. Excessive dopamine in experimental models modifies proteins in the mitochondrial electron transport chain and inhibits the function. α-Synuclein and familiar Parkinson's disease-related gene products modify the expression and activity of monoamine oxidase. Type A monoamine oxidase is associated with neuroprotection by an unspecific dose of inhibitors of type B monoamine oxidase, rasagiline and selegiline. Rasagiline and selegiline prevent α-synuclein fibrillization, modulate this toxic collaboration, and exert neuroprotection in experimental studies. Complex interactions between these pathogenic factors play a decisive role in neurodegeneration in PD and should be further defined to develop new therapies for Parkinson's disease.

Oxidation of dopamine and related catechols in dopaminergic brain regions in Parkinson's disease and during ageing in non-Parkinsonian subjects

The present study was performed to examine if catechol oxidation is higher in brains from patients with Parkinson's disease compared to age-matched controls, and if catechol oxidation increases with age. Brain tissue from Parkinson patients and age-matched controls was examined for oxidation of dopamine, 3,4-dihydroxyphenylacetic acid (DOPAC) and 3,4-dihydroxyphenylalanine (DOPA) to corresponding quinones, by measurement of 5-S-cysteinyl-dopamine, 5-S-cysteinyl-DOPAC and 5-S-cysteinyl-DOPA. The cysteinyl catechols are assumed to be biomarkers for DA, DOPAC and DOPA autoxidation and part of the biosynthetic pathway of neuromelanin. The concentrations of the 5-S-cysteinyl catechols were lower, whereas the 5-S-cysteinyl-DA/DA and 5-S-cysteinyl-DOPAC/DOPAC ratios tended to be higher in the Parkinson group compared to controls, which was interpreted as a higher degree of oxidation. High 5-S-cysteinyl-DA/DA ratios were found in the substantia nigra of a sub-population of the Parkinson group. Based on 5-S-cysteinyl-DA/DA ratios, dopamine oxidation was found to increase statistically significantly with age in the caudate nucleus, and non-significantly in the substantia nigra. In conclusion, the occurrence of 5-S-cysteinyl-DA, 5-S-cysteinyl-DOPAC and 5-S-cysteinyl-DOPA was demonstrated in dopaminergic brain areas of humans, a tendency for higher oxidation of DA in the Parkinson group compared to controls was observed as well as a statistically significant increase in DA oxidation with age. Possibly, autoxidation of DA and other catechols are involved in both normal and pathological ageing of the brain. This study confirms one earlier but small study, as well as complements one study on non-PD cases and one study on both PD cases and controls on NM bound or integrated markers or catechols.

α-Viniferin, a dietary phytochemical, inhibits Monoamine oxidase and alleviates Parkinson's disease associated behavioral deficits in a mice model

Parkinson's disease (PD) is one of the most prevalent age-related neurodegenerative disorders. Behavioral complexities worsen over time due to progressive dopaminergic (DArgic) neuronal loss at substantia nigra region of brain. Available treatments typically aim to increase dopamine (DA) levels at striatum. DA is degraded by Monoamine oxidase (MAO), thus dietary phytochemicals with MAO inhibitory properties can contribute to elevate DA levels and reduce the ailment. Characterization of naturally occurring dietary MAO inhibitors is inadequate. Based on available knowledge, we selected different classes of molecules and conducted a screening process to assess their potential as MAO inhibitors. The compounds mostly derived from food sources, broadly belonging to triterpenoids (ursane, oleanane and hopane), alkaloid, polyphenolics, monoterpenoids, alkylbenzene, phenylpropanoid and aromatic alcohol classes. Among all the molecules, highest level of MAO inhibition is offered by α-viniferin, a resveratrol trimer. Cell viability, mitochondrial morphology and reactive oxygen species (ROS) generation remained unaltered by 50 μM α-viniferin treatment in-vitro. Toxicity studies in Drosophila showed unchanged gross neuronal morphology, ROS level, motor activity or long-term survival. α-Viniferin inhibited MAO in mice brain and elevated striatal DA levels. PD-related akinesia and cataleptic behavior were attenuated by α-viniferin due to increase in striatal DA. Our study implies that α-viniferin can be used as an adjunct phytotherapeutic agent for mitigating PD-related behavioral deterioration.

Effects of curcumin to counteract levodopa-induced toxicity in zebrafish

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor dysfunction due to the death of dopaminergic neurons in the substantia nigra pars compacta. Currently, treatment of PD has focused on increasing dopamine levels, using a dopamine precursor, levodopa (L-DOPA) or stimulation of dopaminergic receptors. Prolonged use of L-DOPA is associated with the occurrence of motor complications and dyskinesia, attributed to neurotoxic effects of this drug. The aim of this study was to investigate the effects of curcumin (CUR), a lipophilic polyphenol, to counteract L-DOPA induced toxicity. Zebrafish larvae were pre-treated with CUR (0.05 µM) or vehicle dimethyl sulfoxide (DMSO) for 24 hr and subsequently exposed to L-DOPA (1 mM) or vehicle. Immediately and 24 hr after L-DOPA exposure, spontaneous swimming and dark/light behavioral tests were performed. In addition, levels of reactive oxygen species (ROS) and lipid peroxidation products were determined at the end of treatment. CUR significantly improved the motor impairment induced by 24 hr L-DOPA treatment, and reduced levels of ROS and lipoperoxidation products in zebrafish larvae. In conclusion, our results suggest that CUR acts as a neuroprotector against toxicity initiated by L-DOPA. Evidence suggests the observed effects of CUR are associated with its antioxidant properties.

Oxidative stress induced by sustained supraphysiological intrastriatal GDNF delivery is prevented by dose regulation

Despite its established neuroprotective effect on dopaminergic neurons and encouraging phase I results, intraputaminal GDNF administration failed to demonstrate significant clinical benefits in Parkinson's disease patients. Different human GDNF doses were delivered in the striatum of rats with a progressive 6-hydroxydopamine lesion using a sensitive doxycycline-regulated AAV vector. GDNF treatment was applied either continuously or intermittently (2 weeks on/2 weeks off) during 17 weeks. Stable reduction of motor impairments as well as increased number of dopaminergic neurons and striatal innervation were obtained with a GDNF dose equivalent to 3- and 10-fold the rat endogenous level. In contrast, a 20-fold increased GDNF level only temporarily provided motor benefits and neurons were not spared. Strikingly, oxidized DNA in the substantia nigra increased by 50% with 20-fold, but not 3-fold GDNF treatment. In addition, only low-dose GDNF allowed to preserve dopaminergic neuron cell size. Finally, aberrant dopaminergic fiber sprouting was observed with 20-fold GDNF but not at lower doses. Intermittent 20-fold GDNF treatment allowed to avoid toxicity and spare dopaminergic neurons but did not restore their cell size. Our data suggest that maintaining GDNF concentration under a threshold generating oxidative stress is a pre-requisite to obtain significant symptomatic relief and neuroprotection.

Bioactive strawberry fruit (Arbutus unedo L.) extract remedies paraquat-induced neurotoxicity in the offspring prenatally exposed rats

Background

Paraquat (1,1'-dimethyl-4-4'-bipyridinium dichloride) exposure is well-established as a neurotoxic agent capable of causing neurological deficits in offspring. This study aimed to investigate therapeutic effects of Arbutus unedo L. aqueous extract (AU) against paraquat (PQ) exposure.

Methods

For that the phytoconstituents of AU was determined by LC/MS, and then its antioxidant potential was assessed by DPPH and ABTS assays. The assessment included its impact on cell viability and mitochondrial metabolism using N27 dopaminergic cells. Additionally, we evaluated the effects of prenatal PQ exposure on motor coordination, dopamine levels, trace element levels, and total antioxidant capacity (TAC) in rat progeny.

Results

The phytochemical profile of AU extract revealed the presence of 35 compounds, primarily phenolic and organic acids, and flavonoids. This accounted for its strong in vitro antioxidant activities against DPPH and ABTS radicals, surpassing the activities of vitamin C. Our findings demonstrated that AU effectively inhibited PQ-induced loss of N27 rat dopaminergic neural cells and significantly enhanced their mitochondrial respiration. Furthermore, daily post-treatment with AU during the 21 days of the rat's pregnancy alleviated PQ-induced motor deficits and akinesia in rat progeny. These effects inhibited dopamine depletion and reduced iron levels in the striatal tissues. The observed outcomes appeared to be mediated by the robust antioxidant activity of AU, effectively counteracting the PQ-induced decrease in TAC in the blood plasma of rat progeny. These effects could be attributed to the bioactive compounds present in AU, including phenolic acids such as gallic acid and flavonoids such as quercetin, rutin, apigenin, glucuronide, and kaempferol, all known for their potent antioxidant capacity.

Discussion

In conclusion, this preclinical study provided the first evidence of the therapeutic potential of AU extract against PQ-induced neurotoxicity. These findings emphasize the need for further exploration of the clinical applicability of AU in mitigating neurotoxin-induced brain damage.

Antiparkinsonian medication masks motor signal progression in de novo patients

Patients not yet receiving medication provide insight to drug-naïve early physiology of Parkinson's Disease (PD). Wearable sensors can measure changes in motor features before and after introduction of antiparkinsonian medication. We aimed to identify features of upper limb bradykinesia, postural stability, and gait that measurably progress in de novo PD patients prior to the start of medication, and determine whether these features remain sensitive to progression in the period after commencement of antiparkinsonian medication. Upper limb motion was measured using an inertial sensor worn on a finger, while postural stability and gait were recorded using an array of six wearable sensors. Patients were tested over nine visits at three monthly intervals. The timepoint of start of medication was noted. Three upper limb bradykinetic features (finger tapping speed, pronation supination speed, and pronation supination amplitude) and three gait features (gait speed, arm range of motion, duration of stance phase) were found to progress in unmedicated early-stage PD patients. In all features, progression was masked after the start of medication. Commencing antiparkinsonian medication is known to lead to masking of progression signals in clinical measures in de novo PD patients. In this study, we show that this effect is also observed with digital measures of bradykinetic and gait motor features.

Lewy bodies, iron, inflammation and neuromelanin: pathological aspects underlying Parkinson's disease

Since the description of some peculiar symptoms by James Parkinson in 1817, attempts have been made to define its cause or at least to enlighten the pathology of "Parkinson's disease (PD)." The vast majority of PD subtypes and most cases of sporadic PD share Lewy bodies (LBs) as a characteristic pathological hallmark. However, the processes underlying LBs generation and its causal triggers are still unknown. ɑ-Synuclein (ɑ-syn, encoded by the SNCA gene) is a major component of LBs, and SNCA missense mutations or duplications/triplications are causal for rare hereditary forms of PD. Thus, it is imperative to study ɑ-syn protein and its pathology, including oligomerization, fibril formation, aggregation, and spreading mechanisms. Furthermore, there are synergistic effects in the underlying pathogenic mechanisms of PD, and multiple factors-contributing with different ratios-appear to be causal pathological triggers and progression factors. For example, oxidative stress, reduced antioxidative capacity, mitochondrial dysfunction, and proteasomal disturbances have each been suggested to be causal for ɑ-syn fibril formation and aggregation and to contribute to neuroinflammation and neural cell death. Aging is also a major risk factor for PD. Iron, as well as neuromelanin (NM), show age-dependent increases, and iron is significantly increased in the Parkinsonian substantia nigra (SN). Iron-induced pathological mechanisms include changes of the molecular structure of ɑ-syn. However, more recent PD research demonstrates that (i) LBs are detected not only in dopaminergic neurons and glia but in various neurotransmitter systems, (ii) sympathetic nerve fibres degenerate first, and (iii) at least in "brain-first" cases dopaminergic deficiency is evident before pathology induced by iron and NM. These recent findings support that the ɑ-syn/LBs pathology as well as iron- and NM-induced pathology in "brain-first" cases are important facts of PD pathology and via their interaction potentiate the disease process in the SN. As such, multifactorial toxic processes posted on a personal genetic risk are assumed to be causal for the neurodegenerative processes underlying PD. Differences in ratios of multiple factors and their spatiotemporal development, and the fact that common triggers of PD are hard to identify, imply the existence of several phenotypical subtypes, which is supported by arguments from both the "bottom-up/dual-hit" and "brain-first" models. Therapeutic strategies are necessary to avoid single initiation triggers leading to PD.

Oxidation Driven Reversal of PIP2-dependent Gating in GIRK2 Channels

Physiological activity of G protein gated inward rectifier K+ (GIRK, Kir3) channel, dynamically regulated by three key ligands, phosphoinositol-4,5-bisphosphate (PIP2), Gβγ, and Na+, underlies cellular electrical response to multiple hormones and neurotransmitters in myocytes and neurons. In a reducing environment, matching that inside cells, purified GIRK2 (Kir3.2) channels demonstrate low basal activity, and expected sensitivity to the above ligands. However, under oxidizing conditions, anomalous behavior emerges, including rapid loss of PIP2 and Na+-dependent activation and a high basal activity in the absence of any agonists, that is now paradoxically inhibited by PIP2. Mutagenesis identifies two cysteine residues (C65 and C190) as being responsible for the loss of PIP2 and Na+-dependent activity and the elevated basal activity, respectively. The results explain anomalous findings from earlier studies and illustrate the potential pathophysiologic consequences of oxidation on GIRK channel function, as well as providing insight to reversed ligand-dependence of Kir and KirBac channels.

Potential implications of polyphenolic compounds in neurodegenerative diseases

Neurodegenerative diseases are common chronic diseases related to progressive damage to the nervous system. Current neurodegenerative diseases present difficulties and despite extensive research efforts to develop new disease-modifying therapies, there is still no effective treatment for halting the neurodegenerative process. Polyphenols are biologically active organic compounds abundantly found in various plants. It has been reported that plant-derived dietary polyphenols may improve some disease states and promote health. Emerging pieces of evidence indicate that polyphenols are associated with neurodegenerative diseases. This review aims to overview the potential neuroprotective roles of polyphenols in most common neurodegenerative disorders, including Alzheimer's disease, Parkinson's disease, epilepsy, and ischemic stroke.

Simultaneous Measurement of Striatal Dopamine and Hydrogen Peroxide Transients Associated with L-DOPA Induced Rotation in Hemiparkinsonian Rats

Parkinson's disease (PD) is a neurodegenerative disorder commonly treated with levodopa (L-DOPA), which eventually induces abnormal involuntary movements (AIMs). The neurochemical contributors to these dyskinesias are unknown; however, several lines of evidence indicate an interplay of dopamine (DA) and oxidative stress. Here, DA and hydrogen peroxide (H2O2) were simultaneously monitored at discrete recording sites in the dorsal striata of hemiparkinsonian rats using fast-scan cyclic voltammetry. Mass spectrometry imaging validated the lesions. Hemiparkinsonian rats exhibited classic L-DOPA-induced AIMs and rotations as well as increased DA and H2O2 tone over saline controls after 1 week of treatment. By week 3, DA tone remained elevated beyond that of controls, but H2O2 tone was largely normalized. At this time point, rapid chemical transients were time-locked with spontaneous bouts of rotation. Striatal H2O2 rapidly increased with the initiation of contraversive rotational behaviors in lesioned L-DOPA animals, in both hemispheres. DA signals simultaneously decreased with rotation onset. The results support a role for these striatal neuromodulators in the adaptive changes that occur with L-DOPA treatment in PD and reveal a precise interplay between DA and H2O2 in the initiation of involuntary locomotion.

A Multi-Scale Computational Model of Levodopa-Induced Toxicity in Parkinson's Disease

Parkinson's disease (PD) is caused by the progressive loss of dopaminergic cells in substantia nigra pars compacta (SNc). The root cause of this cell loss in PD is still not decisively elucidated. A recent line of thinking has traced the cause of PD neurodegeneration to metabolic deficiency. Levodopa (L-DOPA), a precursor of dopamine, used as a symptom-relieving treatment for PD, leads to positive and negative outcomes. Several researchers inferred that L-DOPA might be harmful to SNc cells due to oxidative stress. The role of L-DOPA in the course of the PD pathogenesis is still debatable. We hypothesize that energy deficiency can lead to L-DOPA-induced toxicity in two ways: by promoting dopamine-induced oxidative stress and by exacerbating excitotoxicity in SNc. We present a systems-level computational model of SNc-striatum, which will help us understand the mechanism behind neurodegeneration postulated above and provide insights into developing disease-modifying therapeutics. It was observed that SNc terminals are more vulnerable to energy deficiency than SNc somas. During L-DOPA therapy, it was observed that higher L-DOPA dosage results in increased loss of terminals in SNc. It was also observed that co-administration of L-DOPA and glutathione (antioxidant) evades L-DOPA-induced toxicity in SNc neurons. Our proposed model of the SNc-striatum system is the first of its kind, where SNc neurons were modeled at a biophysical level, and striatal neurons were modeled at a spiking level. We show that our proposed model was able to capture L-DOPA-induced toxicity in SNc, caused by energy deficiency.

Pathophysiological Features of Nigral Dopaminergic Neurons in Animal Models of Parkinson's Disease

The degeneration of nigral dopaminergic neurons is considered the hallmark of Parkinson’s disease (PD), and it is triggered by different factors, including mitochondrial dysfunction, Lewy body accumulation, neuroinflammation, excitotoxicity and metal accumulation. Despite the extensive literature devoted to unravelling the signalling pathways involved in neuronal degeneration, little is known about the functional impairments occurring in these cells during illness progression. Of course, it is not possible to obtain direct information on the properties of the dopaminergic cells in patients. However, several data are available in the literature reporting changes in the function of these cells in PD animal models. In the present manuscript, we focus on dopaminergic neuron functional properties and summarize shared or peculiar features of neuronal dysfunction in different PD animal models at different stages of the disease in an attempt to design a picture of the functional modifications occurring in nigral dopaminergic neurons during disease progression preceding their eventual death.

Surface Plasmon Resonance Identifies High-Affinity Binding of l-DOPA to Siderocalin/Lipocalin-2 through Iron-Siderophore Action: Implications for Parkinson's Disease Treatment

l-3,4-Dihydroxyphenylalanine (l-DOPA), the dopamine precursor, remains the frontline treatment for Parkinson's disease (PD). With the treatment progress, l-DOPA efficacy decreases, necessitating higher and more frequent doses, with higher risks of dyskinesia. l-DOPA chelates iron through its catechol group, forming the l-DOPA:Fe complex; however, the fate of this complex is unknown. Catechol siderophore-like compounds are known to bind siderocalin (Scn)/lipocalin-2 to form stable siderophore:Fe:Scn complexes. Scn is upregulated in PD patients' substantia nigra and may play a role in PD pathophysiology. Therefore, in this study, we used the surface plasmon resonance (SPR) technique to examine the binding properties of l-DOPA to Scn. We found that l-DOPA formed a stable complex with Scn in the presence of Fe³⁺. Our analysis of the binding properties of l-DOPA precursors and metabolites indicates that the catechol group is necessary but not sufficient to form a stable complex with Scn. Finally, the affinity constant (K_d) of DOPA:Fe³⁺ binding with Scn (0.8 μM) was lower than l-DOPA plasma peak concentrations in l-DOPA preparations in the past six decades. Our results speculate a significant role for the l-DOPA-Scn complex in the decreased bioavailability of l-DOPA with the progress of PD.

A Multiscale, Systems-Level, Neuropharmacological Model of Cortico-Basal Ganglia System for Arm Reaching Under Normal, Parkinsonian, and Levodopa Medication Conditions

In order to understand the link between substantia nigra pars compacta (SNc) cell loss and Parkinson's disease (PD) symptoms, we developed a multiscale computational model that can replicate the symptoms at the behavioural level by incorporating the key cellular and molecular mechanisms underlying PD pathology. There is a modelling tradition that links dopamine to reward and uses reinforcement learning (RL) concepts to model the basal ganglia. In our model, we replace the abstract representations of reward with the realistic variable of extracellular DA released by a network of SNc cells and incorporate it in the RL-based behavioural model, which simulates the arm reaching task. Our results successfully replicated the impact of SNc cell loss and levodopa (L-DOPA) medication on reaching performance. It also shows the side effects of medication, such as wearing off and peak dosage dyskinesias. The model demonstrates how differential dopaminergic axonal degeneration in basal ganglia results in various cardinal symptoms of PD. It was able to predict the optimum L-DOPA medication dosage for varying degrees of cell loss. The proposed model has a potential clinical application where drug dosage can be optimised as per patient characteristics.

Long-Term Depression of Striatal DA Release Induced by mGluRs via Sustained Hyperactivity of Local Cholinergic Interneurons

The cellular mechanisms regulating dopamine (DA) release in the striatum have attracted much interest in recent years. By in vitro amperometric recordings in mouse striatal slices, we show that a brief (5 min) exposure to the metabotropic glutamate receptor agonist DHPG (50 μM) induces a profound depression of synaptic DA release, lasting over 1 h from DHPG washout. This long-term depression is sensitive to glycine, which preferentially inhibits local cholinergic interneurons, as well as to drugs acting on nicotinic acetylcholine receptors and to the pharmacological depletion of released acetylcholine. The same DHPG treatment induces a parallel long-lasting enhancement in the tonic firing of presumed striatal cholinergic interneurons, measured with multi-electrode array recordings. When DHPG is bilaterally infused in vivo in the mouse striatum, treated mice display an anxiety-like behavior. Our results demonstrate that metabotropic glutamate receptors stimulation gives rise to a prolonged depression of the striatal dopaminergic transmission, through a sustained enhancement of released acetylcholine, due to the parallel long-lasting potentiation of striatal cholinergic interneurons firing. This plastic interplay between dopamine, acetylcholine, and glutamate in the dorsal striatum may be involved in anxiety-like behavior typical of several neuropsychiatric disorders.

Cancer risk in Parkinson disease: An updated systematic review and meta-analysis

BACKGROUND AND PURPOSE

Increasing evidence suggests significant associations between Parkinson disease (PD) and cancer risks. We conducted an updated review of studies that examined the risks of various cancer among PD patients and how this differed when cancer preceded PD diagnosis or PD diagnosis preceded cancer.

METHODS

Four databases were searched for studies that examined the association between PD and incidence of cancer from database inception to 4 June 2021. Three independent reviewers screened the articles for eligibility and extracted study data. Pooled relative risk with 95% confidence intervals were calculated using a random effects model.

RESULTS

Forty studies involving 11 case-control studies, two nested case-control studies, 22 cohort studies, and five cross-sectional studies were included. Compared to controls, PD patients had lower risks of lung, genitourinary, gastrointestinal, and haematological cancers. Conversely, higher risks of melanoma and brain cancer were noted among PD patients. No association was found between PD and risk of female cancers. Subgroup analysis found negative associations between PD patients and risks of colon cancer, rectal cancer, and non-Hodgkin lymphoma.

CONCLUSIONS

Findings from our meta-analysis suggest PD patients had lower risks of lung, genitourinary, gastrointestinal, and haematological cancers and increased risks of melanoma and brain cancer. Future research to investigate the underlying mechanisms between PD and cancers is warranted.

Drosophila tubulin polymerization promoting protein mutants reveal pathological correlates relevant to human Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder with no known cure. PD is characterized by locomotion deficits, nigrostriatal dopaminergic neuronal loss, mitochondrial dysfunctions and formation of α-Synuclein aggregates. A well-conserved and less understood family of Tubulin Polymerization Promoting Proteins (TPPP) is also implicated in PD and related disorders, where TPPP exists in pathological aggregates in neurons in patient brains. However, there are no in vivo studies on mammalian TPPP to understand the genetics and neuropathology linking TPPP aggregation or neurotoxicity to PD. Recently, we discovered the only Drosophila homolog of human TPPP named Ringmaker (Ringer). Here, we report that adult ringer mutants display progressive locomotor disabilities, reduced lifespan and neurodegeneration. Importantly, our findings reveal that Ringer is associated with mitochondria and ringer mutants have mitochondrial structural damage and dysfunctions. Adult ringer mutants also display progressive loss of dopaminergic neurons. Together, these phenotypes of ringer mutants recapitulate some of the salient features of human PD patients, thus allowing us to utilize ringer mutants as a fly model relevant to PD, and further explore its genetic and molecular underpinnings to gain insights into the role of human TPPP in PD.

A Novel Method for Creating a Synthetic L-DOPA Proteome and In Vitro Evidence of Incorporation

Proteinopathies are protein misfolding diseases that have an underlying factor that affects the conformation of proteoforms. A factor hypothesised to play a role in these diseases is the incorporation of non-protein amino acids into proteins, with a key example being the therapeutic drug levodopa. The presence of levodopa as a protein constituent has been explored in several studies, but it has not been examined in a global proteomic manner. This paper provides a proof-of-concept method for enzymatically creating levodopa-containing proteins using the enzyme tyrosinase and provides spectral evidence of in vitro incorporation in addition to the induction of the unfolded protein response due to levodopa.

Influence of energy deficiency on the subcellular processes of Substantia Nigra Pars Compacta cell for understanding Parkinsonian neurodegeneration

Parkinson's disease (PD) is the second most prominent neurodegenerative disease around the world. Although it is known that PD is caused by the loss of dopaminergic cells in substantia nigra pars compacta (SNc), the decisive cause of this inexorable cell loss is not clearly elucidated. We hypothesize that "Energy deficiency at a sub-cellular/cellular/systems level can be a common underlying cause for SNc cell loss in PD." Here, we propose a comprehensive computational model of SNc cell, which helps us to understand the pathophysiology of neurodegeneration at the subcellular level in PD. The aim of the study is to see how deficits in the supply of energy substrates (glucose and oxygen) lead to a deficit in adenosine triphosphate (ATP). The study also aims to show that deficits in ATP are the common factor underlying the molecular-level pathological changes, including alpha-synuclein aggregation, reactive oxygen species formation, calcium elevation, and dopamine dysfunction. The model suggests that hypoglycemia plays a more crucial role in leading to ATP deficits than hypoxia. We believe that the proposed model provides an integrated modeling framework to understand the neurodegenerative processes underlying PD.

Apelin-13 Protects Dopaminergic Neurons against Rotenone-Induced Neurotoxicity through the AMPK/mTOR/ULK-1 Mediated Autophagy Activation

Parkinson’s disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. Several brain–gut peptides are able to exert neuroprotective effects on the nigrostriatal dopaminergic system. Apelin-13 is a neuropeptide, conveying potential neuroprotective activities. However, whether, and how, apelin-13 could antagonize rotenone-induced neurotoxicity has not yet been elucidated. In the present study, rotenone-treated SH-SY5Y cells and rats were used to clarify whether apelin-13 has protective effects on dopaminergic neurons, both in vivo and in vitro. The results showed that apelin-13 could protect SH-SY5Y cells from rotenone-induced injury and apoptosis. Apelin-13 was able to activate autophagy, and restore rotenone induced autophagy impairment in SH-SY5Y cells, which could be blocked by the autophagy inhibitor 3-Methyladenine. Apelin-13 activated AMPK/mTOR/ULK-1 signaling, AMPKα inhibitor compound C, as well as apelin receptor blockage via siRNA, which could block apelin-13-induced signaling activation, autophagy activation, and protective effects, in rotenone-treated SH-SY5Y cells. These results indicated that apelin-13 exerted neuroprotective properties against rotenone by stimulating AMPK/mTOR/ULK-1 signaling-mediated autophagy via the apelin receptor. We also observed that intracerebroventricular injection of apelin-13 could alleviate nigrostriatal dopaminergic neuron degeneration in rotenone-treated rats. Our findings provide new insights into the mechanism by which apelin-13 might attenuate neurotoxicity in PD.

Levodopa partially rescues microglial numerical, morphological, and phagolysosomal alterations in a monkey model of Parkinson's disease

Parkinson's disease (PD) is the most common neurodegenerative motor disorder. The mechanisms underlying the onset and progression of Levodopa (L-Dopa)-induced dyskinesia (LID) during PD treatment remain elusive. Emerging evidence implicates functional modification of microglia in the development of LID. Thus, understanding the link between microglia and the development of LID may provide the knowledge required to preserve or promote beneficial microglial functions, even during a prolonged L-Dopa treatment. To provide novel insights into microglial functional alterations in PD pathophysiology, we characterized their density, morphology, ultrastructure, and degradation activity in the sensorimotor functional territory of the putamen, using 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) cynomolgus monkeys. A subset of MPTP monkeys was treated orally with L-Dopa and developed LID similar to PD patients. Using a combination of light, confocal and transmission electron microscopy, our quantitative analyses revealed alterations of microglial density, morphology and phagolysosomal activity following MPTP intoxication that were partially normalized with L-Dopa treatment. In particular, microglial density, cell body and arborization areas were increased in the MPTP monkeys, whereas L-Dopa-treated MPTP animals presented a microglial phenotype similar to the control animals. At the ultrastructural level, microglia did not differ between groups in their markers of cellular stress or aging. Nevertheless, microglia from the MPTP monkeys displayed reduced numbers of endosomes, compared with control animals, that remained lower after L-Dopa treatment. Microglia from MPTP monkeys treated with L-Dopa also had increased numbers of primary lysosomes compared with non-treated MPTP animals, while secondary and tertiary lysosomes remained unchanged. Moreover, a decrease microglial immunoreactivity for CD68, considered a marker of phagocytosis and lysosomal activity, was measured in the MPTP monkeys treated with L-Dopa, compared with non-treated MPTP animals. Taken together, these findings revealed significant changes in microglia during PD pathophysiology that were partially rescued by L-Dopa treatment. Albeit, this L-Dopa treatment conferred phagolysosomal insufficiency on microglia in the dyskinetic Parkinsonian monkeys.

A Multi-Scale Computational Model of Excitotoxic Loss of Dopaminergic Cells in Parkinson's Disease

Parkinson's disease (PD) is a neurodegenerative disorder caused by loss of dopaminergic neurons in substantia nigra pars compacta (SNc). Although the exact cause of cell death is not clear, the hypothesis that metabolic deficiency is a key factor has been gaining attention in recent years. In the present study, we investigated this hypothesis using a multi-scale computational model of the subsystem of the basal ganglia comprising the subthalamic nucleus (STN), globus pallidus externa (GPe), and SNc. The proposed model is a multiscale model in that interaction among the three nuclei are simulated using more abstract Izhikevich neuron models, while the molecular pathways involved in cell death of SNc neurons are simulated in terms of detailed chemical kinetics. Simulation results obtained from the proposed model showed that energy deficiencies occurring at cellular and network levels could precipitate the excitotoxic loss of SNc neurons in PD. At the subcellular level, the models show how calcium elevation leads to apoptosis of SNc neurons. The therapeutic effects of several neuroprotective interventions are also simulated in the model. From neuroprotective studies, it was clear that glutamate inhibition and apoptotic signal blocker therapies were able to halt the progression of SNc cell loss when compared to other therapeutic interventions, which only slowed down the progression of SNc cell loss.

Anterior pituitary gland synthesises dopamine from l-3,4-dihydroxyphenylalanine (l-dopa)

Prolactin (PRL) is a hormone principally secreted by lactotrophs of the anterior pituitary gland. Although the synthesis and exocytosis of this hormone are mainly under the regulation of hypothalamic dopamine (DA), the possibility that the anterior pituitary synthesises this catecholamine remains unclear. The present study aimed to determine if the anterior pituitary produces DA from the precursor l-3,4-dihydroxyphenylalanine (l-dopa). Accordingly, we investigated the expression of aromatic l-amino acid decarboxylase (AADC) enzyme and the transporter vesicular monoamine transporter 2 (VMAT2) in the anterior pituitary, AtT20 and GH3 cells by immunofluorescence and western blotting. Moreover, we investigated the production of DA from l-dopa and its release in vitro. Then, we explored the effects of l-dopa with respect to the secretion of PRL from anterior pituitary fragments. We observed that the anterior pituitary, AtT20 and GH3 cells express both AADC and VMAT2. Next, we detected an increase in DA content after anterior pituitary fragments were incubated with l-dopa. Also, the presence of l-dopa increased DA levels in incubation media and reduced PRL secretion. Likewise, the content of cellular DA increased after AtT20 cells were incubated with l-dopa. In addition, l-dopa reduced corticotrophin-releasing hormone-stimulated adrenocorticotrophic hormone release from these cells after AADC activity was inhibited by NSD-1015. Moreover, DA formation from l-dopa increased apoptosis and decreased proliferation. However, in the presence of NSD-1015, l-dopa decreased apoptosis and increased proliferation rates. These results suggest that the anterior pituitary synthesises DA from l-dopa by AADC and this catecholamine can be released from this gland contributing to the control of PRL secretion. In addition, our results suggest that l-dopa exerts direct actions independently from its metabolisation to DA.

Phenotypic chemical and mutant screening of zebrafish larvae using an on-demand response to electric stimulation

Behavioral responses of zebrafish larvae to environmental cues are important functional readouts that should be evoked on-demand and studied phenotypically in behavioral, genetical and developmental investigations. Very recently, it was shown that zebrafish larvae execute a voluntary and oriented movement toward the positive electrode of an electric field along a microchannel. Phenotypic characterization of this response was not feasible due to larva's rapid movement along the channel. To overcome this challenge, a microfluidic device was introduced to partially immobilize the larva's head while leaving its mid-body and tail unrestrained in a chamber to image motor behaviors in response to electric stimulation, hence achieving quantitative phenotyping of the electrically evoked movement in zebrafish larvae. The effect of electric current on the tail-beat frequency and response duration of 5-7 days postfertilization zebrafish larvae was studied. Investigations were also performed on zebrafish exposed to neurotoxin 6-hydroxydopamine and larvae carrying a pannexin1a (panx1a) gene knockout, as a proof of principle applications to demonstrate on-demand movement behavior screening in chemical and mutant assays. We demonstrated for the first time that 6-hydroxydopamine leads to electric response impairment, levodopa treatment rescues the response and panx1a is involved in the electrically evoked movement of zebrafish larvae. We envision that our technique is broadly applicable as a screening tool to quantitatively examine zebrafish larvae's movements in response to physical and chemical stimulations in investigations of Parkinson's and other neurodegenerative diseases, and as a tool to combine recent advances in genome engineering of model organisms to uncover the biology of electric response.

Eugenol and its association with levodopa in 6-hydroxydopamine-induced hemiparkinsonian rats: Behavioural and neurochemical alterations

Parkinson's disease is a neurodegenerative disorder that affects the central nervous system and is mainly characterized by the loss of dopaminergic neurons and pro-oxidant mechanisms. Eugenol has been widely studied due to its anti-inflammatory and antioxidant activities, making it a promising neuroprotective agent. This study aimed to investigate the effects of eugenol and its combined action with levodopa in the 6-hydroxydopamine-induced Parkinson's disease model. Wistar rats were subjected to intrastriatal injection of 6-hydroxydopamine (21 μg) and then treated with eugenol (0.1, 1, or 10 mg/kg), levodopa (25 mg/kg) or their combination (eugenol 10 mg/kg + levodopa 12.5 mg/kg) orally for 14 days. On the 14th day, the animals were subjected to behavioural tests, and after euthanization and dissection of the brain areas, neurochemical analyses were performed. The results showed that eugenol reduced the oxidative stress and behavioural disturbances induced by 6-hydroxydopamine. The eugenol and levodopa combination was more effective in some behavioural parameters and body-weight gain in addition to promoting an increase in reduced glutathione levels compared to levodopa alone. Thus, the neuroprotective activity of eugenol was observed against motor and neurochemical disorders. Additionally, the eugenol and levodopa combination was promising when compared to conventional treatment.

Inhibitory potential of plant secondary metabolites on anti-Parkinsonian drug targets: Relevance to pathophysiology, and motor and non-motor behavioural abnormalities

Parkinson's disease (PD), a progressive neurodegenerative motor disorder, is caused due to the loss of dopaminergic neurons in the substantia nigra pars compacta region of mid-brain and the resultant depletion of the levels of the neurotransmitter dopamine. Although the pathophysiology of the disease is least understood, studies in animal models revealed oxidative stress, mitochondrial dysfunction and inflammation to be the major contributors. Dopamine replenishment therapy by oral administration of L-DOPA, the precursor of dopamine remains to be the therapeutic gold-standard for symptomatic treatment of PD. In addition, use of inhibitors of dopamine metabolizing enzymes (viz. monoamine oxidase-B: MAO-B; and catechol-O-methyltransferase: COMT) are the other strategies for amelioration of the motor abnormalities. Further, PD is associated with non-motor behavioural abnormalities as well, including cognitive impairment and mood disorders, which are caused due to cholinergic neurodegeneration, and thus inhibition of Acetylcholinesterase (AChE) is suggested. However, the currently used drugs against the three crucial enzymes (MAO-B, COMT and AChE) elicit several side effects, and thus the search for novel compounds continues, and plant-based compounds have promising potential in this regard. In the present study, we have used computational modeling to determine the efficiency of 40 plant-based natural products in inhibiting the three anti-Parkinsonian drug targets. Further, statistical analysis was performed to identify the properties of the compounds which are crucial for inhibition of the enzymes. While all the phytochemicals showed potential in inhibiting the enzymes, Rutin, Demethoxycurcumin and Acteoside were found to be most effective inhibitors of MAO-B, COMT and AChE respectively. Since most of the compounds are established anti-oxidant and anti-inflammatory molecules, they are surmised to confer neuroprotection in PD, and prevent progression of the disease.

Stress-induced phospho-ubiquitin formation causes parkin degradation

Mutations in the E3 ubiquitin ligase parkin are the most common known cause of autosomal recessive Parkinson’s disease (PD), and parkin depletion may play a role in sporadic PD. Here, we sought to elucidate the mechanisms by which stress decreases parkin protein levels using cultured neuronal cells and the PD-relevant stressor, L-DOPA. We find that L-DOPA causes parkin loss through both oxidative stress-independent and oxidative stress-dependent pathways. Characterization of the latter reveals that it requires both the kinase PINK1 and parkin’s interaction with phosphorylated ubiquitin (phospho-Ub) and is mediated by proteasomal degradation. Surprisingly, autoubiquitination and mitophagy do not appear to be required for such loss. In response to stress induced by hydrogen peroxide or CCCP, parkin degradation also requires its association with phospho-Ub, indicating that this mechanism is broadly generalizable. As oxidative stress, metabolic dysfunction and phospho-Ub levels are all elevated in PD, we suggest that these changes may contribute to a loss of parkin expression.

Age-dependent nigral dopaminergic neurodegeneration and α-synuclein accumulation in RGS6-deficient mice

Parkinson's is primarily a non-familial, age-related disorder caused by α-synuclein accumulation and the progressive loss of dopamine neurons in the substantia nigra pars compacta (SNc). G protein-coupled receptor (GPCR)-cAMP signaling has been linked to a reduction in human Parkinson's incidence and α-synuclein expression. Neuronal cAMP levels are controlled by GPCRs coupled to Gs or Gi/o, which increase or decrease cAMP, respectively. Regulator of G protein signaling 6 (RGS6) powerfully inhibits Gi/o signaling. Therefore, we hypothesized that RGS6 suppresses D2 autoreceptor- Gi/o signaling in SNc dopamine neurons promoting neuronal survival and reducing α-synuclein expression. Here we provide novel evidence that RGS6 critically suppresses late-age-onset SNc dopamine neuron loss and α-synuclein accumulation. RGS6 is restrictively expressed in human SNc dopamine neurons and, despite their loss in Parkinson's, all surviving neurons express RGS6. RGS6-/- mice exhibit hyperactive D2 autoreceptors with reduced cAMP signaling in SNc dopamine neurons. Importantly, RGS6-/- mice recapitulate key sporadic Parkinson's hallmarks, including: SNc dopamine neuron loss, reduced nigrostriatal dopamine, motor deficits, and α-synuclein accumulation. To our knowledge, Rgs6 is the only gene whose loss phenocopies these features of human Parkinson's. Therefore, RGS6 is a key regulator of D2R-Gi/o signaling in SNc dopamine neurons, protecting against Parkinson's neurodegeneration and α-synuclein accumulation.

An in silico investigation on the inhibitory potential of the constituents of Pomegranate juice on antioxidant defense mechanism: Relevance to neurodegenerative diseases

Elevation in the levels of reactive oxygen and nitrogen species (RONS), and downregulation of cellular antixoidants, have ubiquitously been reported from studies in animal models of neurodegenerative diseases, including Parkinson’s disease (PD) and Alzheimer’s disease (AD). Thus, plant-derived compounds are widely being investigated for their beneficial effects in these models. However, while studies have reported antioxidant potentials of several phytochemicals, a large number of studies have demonstrated different phytochemicals to be rather pro-oxidant and exaggerate oxidative stress (OS). One such study aimed to investigate possible ameliorative effect of Pomegranate juice (PJ) in rat model of toxin-induced parkinsonism revealed that PJ exacerbates OS, inflammation and promotes neurodegeneration. Thus, it remains to be investigated whether different constituents and metabolites of PJ are pro-oxidant or anti-oxidant. Using computational modeling, we investigated possible inhibitory potential of different constituents of PJ and their metabolites viz. delphinidin-3-glucoside, dimethylellagic acid-glucuronide, ellagic acid, ellagitannin, gallic acid, gallotannin 23, pelargonidin, punicalagin, urolithin A, urolithin A-glucuronide and urolithin B, on anti-oxidant defense system of the brain. The results indicate that the constituents of PJ have the potential to inhibit five key enzymes of the neuronal antioxidant defense system, viz. catalase, superoxide dismutase, glutathione peroxidase 4, glutathione reductase and glutathione-S-transferase. Thus, it is surmised that the constituents of PJ may contribute to OS and neurodegeneration by way of affecting antioxidant defense mechanism. This may particularly be more pronounced in neurodegenerative diseases, since neurons are known to be more vulnerable to OS. Thus, the present findings caution the use of PJ in patients prone to OS, especially those suffering from neurodegenerative diseases, and warrant further experimental studies to unveil the effects of individual components and metabolites of PJ on antioxidant defense system of brain.

L-DOPA-quinone Mediated Recovery from GIRK Channel Firing Inhibition in Dopaminergic Neurons

The oxidative degeneration of dopamine-releasing (DAergic) neurons in the substantia nigra pars compacta (SNc) has attracted much interest in preclinical research, due to its involvement in Parkinson's disease manifestations. Evidence exists on the participation of quinone derivatives in mitochondrial dysfunction, alpha synuclein protein aggregation, and protein degradation. With the aim to investigate the role of L-DOPA-quinone in DAergic neuron functions, we synthesized L-DOPA-quinone by use of 2-iodoxybenzoic acid and measured its activity in recovery from dopamine-mediated firing inhibition of SNc neurons. Noteworthy, L-DOPA-quinone counteracts firing inhibition in SNc DAergic neurons caused by GIRK opening. A possible mechanism to explain the effect of L-DOPA-quinone on GIRK channel has been proposed by computational models. Overall, the study showed the possibility that L-DOPA-quinone stabilizes GIRK in a preopen conformation through formation of a covalent adduct with cysteine-65 on the GIRK2 subunit of the protein.

1-O-Hexyl-2,3,5-Trimethylhydroquinone Ameliorates l-DOPA-Induced Cytotoxicity in PC12 Cells

1-O-Hexyl-2,3,5-trimethylhydroquinone (HTHQ) has previously been found to have effective anti-oxidant and anti-lipid-peroxidative activity. We aimed to elucidate whether HTHQ can prevent dopaminergic neuronal cell death by investigating the effect on l-DOPA-induced cytotoxicity in PC12 cells. HTHQ protected from both l-DOPA-induced cell death and superoxide dismutase activity reduction. When assessing the effect of HTHQ on oxidative stress-related signaling pathways, HTHQ inhibited l-DOPA-induced phosphorylation of sustained extracellular signal-regulated kinases (ERK1/2), p38 mitogen-activated protein kinase (MAPK), and c-Jun N-terminal kinase (JNK1/2). HTHQ also normalized l-DOPA-reduced Bcl-2-associated death protein (Bad) phosphorylation and Bcl-2-associated X protein (Bax) expression, promoting cell survival. Taken together, HTHQ exhibits protective effects against l-DOPA-induced cell death through modulation of the ERK1/2-p38MAPK-JNK1/2-Bad-Bax signaling pathway in PC12 cells. These results suggest that HTHQ may show ameliorative effects against oxidative stress-induced dopaminergic neuronal cell death, although further studies in animal models of Parkinson’s disease are required to confirm this.

Impact of Dopamine Oxidation on Dopaminergic Neurodegeneration

Parkinson's disease (PD) is the second most common neurodegenerative disease. The characteristic feature of PD is the progressive degeneration of the dopaminergic (DAergic) neurons in the substantia nigra (SN). DAergic neurons in the SN accumulate black and insoluble membrane structures known as neuromelanin during aging. The oxidation of dopamine (DA) to form neuromelanin generates many o-quinones, including DA o-quinones, aminochrome, and 5,6-indolequinone. The focus of this review is to discuss the role of DA oxidation in association with PD. The oxidation of DA produces oxidative products, inducing mitochondrial dysfunction, impaired protein degradation, α-synuclein aggregation into neurotoxic oligomers, and oxidative stress, in vitro. Recent studies have demonstrated that the DA content is critical for both DJ-1 knockout and A53T α-synuclein transgenic mice to develop PD pathological features, providing evidence for DA action in PD pathogenesis in vivo. The effects of L-DOPA, as the most effective anti-PD drug, are also briefly discussed.

Acute sensitivity of astrocytes in the Substantia Nigra to oxygen and glucose deprivation (OGD) compared with hippocampal astrocytes in brain slices

The Substantia Nigra is a brainstem nucleus critical for movement control. Although its dopamine-producing neurons degenerate in Parkinsons disease, little is known of the acute effects of ischemia in this region. We recently reported that oxygen and glucose deprivation (OGD) in brain slices, an in vitro ischemia model, evokes a profound depolarization and swelling of GABAergic neurons in the Substantia Nigra pars reticulata (SNr), but not dopaminergic neurons in the Substantia Nigra pars compacta (SNc). The current study characterised the effects of OGD on nigral astrocytes, and compared these with the established responses of astrocytes in the CA1 hippocampal region. Intracellular recordings were made from astrocytes at the border between SNc and SNr subregions, in midbrain slices from postnatal day 21-23 rats. Immunoreactivity for astrocyte-specific proteins was also assessed. OGD evoked a slow, then fast depolarization of nigral astrocytes. The fast phase developed during the anoxic depolarization (indicated by a fast negative shift of extracellular DC potential and increase in light transmittance) and rapid increase in extracellular K⁺ concentration in the SNr. This biphasic response resembled the OGD-evoked depolarization of hippocampal astrocytes. However, unlike the partial repolarization seen in hippocampal cells after reperfusion with O₂ and glucose, nigral astrocytes remained depolarized near 0 mV. In addition, immunoreactivity for glial fibrillary acidic protein-positive astrocytes markedly decreased in the Substantia Nigra after OGD, while in the hippocampus remained unchanged. These data indicate an acute post-ischemic withdrawal of astrocytic support in the Substantia Nigra, but not in the hippocampus.

Reversal of dopamine-mediated firing inhibition through activation of the dopamine transporter in substantia nigra pars compacta neurons

BACKGROUND AND PURPOSE

One of the hallmarks of ventral midbrain dopamine-releasing neurons is membrane hyperpolarization in response to stimulation of somato-dendritic D₂ receptors. At early postnatal age, under sustained dopamine, this inhibitory response is followed by a slow recovery, resulting in dopamine inhibition reversal (DIR). In the present investigation, we aimed to get a better insight into the cellular mechanisms underlying DIR.

EXPERIMENTAL APPROACH

We performed single-unit extracellular recordings with a multi-electrode array device and conventional patch-clamp recordings on midbrain mouse slices.

KEY RESULTS

While continuous dopamine (100 μM) perfusion gave rise to firing inhibition that recovered in 10 to 15 min, the same effect was not obtained with the D₂ receptor agonist quinpirole (100 nM). Moreover, firing inhibition caused by the GABA_B receptor agonist baclofen (300 nM) was reversed by dopamine (100 μM), albeit D₂ receptors had been blocked by sulpiride (10 μM). Conversely, the block of the dopamine transporter (DAT) with cocaine (30 μM) prevented firing recovery by dopamine under GABA_B receptor stimulation. Accordingly, in whole-cell recordings from single cells, the baclofen-induced outward current was counteracted by dopamine (100 μM) in the presence of sulpiride (10 μM), and this effect was prevented by the DAT antagonists cocaine (30 μM) and GBR12909 (2 μM).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the DAT plays a major role in DIR, mediating it under conditions of sustained dopamine exposure, and point to DAT as an important target for pharmacological therapies leading to prolonged enhancement of the dopaminergic signal.

Coating Dependent In Vitro Biocompatibility of New Fe-Si Nanoparticles

Magnetic nanoparticles offer multiple utilization possibilities in biomedicine. In this context, the interaction with cellular structures and their biological effects need to be understood and controlled for clinical safety. New magnetic nanoparticles containing metallic/carbidic iron and elemental silicon phases were synthesized by laser pyrolysis using Fe(CO)₅ vapors and SiH₄ gas as Fe and Si precursors, then passivated and coated with biocompatible agents, such as l-3,4-dihydroxyphenylalanine (l-DOPA) and sodium carboxymethyl cellulose (CMC-Na). The resulting magnetic nanoparticles were characterized by XRD, EDS, and TEM techniques. To evaluate their biocompatibility, doses ranging from 0⁻200 µg/mL hybrid Fe-Si nanoparticles were exposed to Caco2 cells for 24 and 72 h. Doses below 50 μg/mL of both l-DOPA and CMC-Na-coated Fe-Si nanoparticles induced no significant changes of cellular viability or membrane integrity. The cellular internalization of nanoparticles was dependent on their dispersion in culture medium and caused some changes of F-actin filaments organization after 72 h. However, reactive oxygen species were generated after exposure to 25 and 50 μg/mL of both Fe-Si nanoparticles types, inducing the increase of intracellular glutathione level and activation of transcription factor Nrf2. At nanoparticles doses below 50 μg/mL, Caco2 cells were able to counteract the oxidative stress by activating the cellular protection mechanisms. We concluded that in vitro biological responses to coated hybrid Fe-Si nanoparticles depended on particle synthesis conditions, surface coating, doses and incubation time.

On Cell Loss and Selective Vulnerability of Neuronal Populations in Parkinson's Disease

Significant advances have been made uncovering the factors that render neurons vulnerable in Parkinson's disease (PD). However, the critical pathogenic events leading to cell loss remain poorly understood, complicating the development of disease-modifying interventions. Given that the cardinal motor symptoms and pathology of PD involve the loss of dopamine (DA) neurons of the substantia nigra pars compacta (SNc), a majority of the work in the PD field has focused on this specific neuronal population. PD however, is not a disease of DA neurons exclusively: pathology, most notably in the form of Lewy bodies and neurites, has been reported in multiple regions of the central and peripheral nervous system, including for example the locus coeruleus, the dorsal raphe nucleus and the dorsal motor nucleus of the vagus. Cell and/or terminal loss of these additional nuclei is likely to contribute to some of the other symptoms of PD and, most notably to the non-motor features. However, exactly which regions show actual, well-documented, cell loss is presently unclear. In this review we will first examine the strength of the evidence describing the regions of cell loss in idiopathic PD, as well as the order in which this loss occurs. Secondly, we will discuss the neurochemical, morphological and physiological characteristics that render SNc DA neurons vulnerable, and will examine the evidence for these characteristics being shared across PD-affected neuronal populations. The insights raised by focusing on the underpinnings of the selective vulnerability of neurons in PD might be helpful to facilitate the development of new disease-modifying strategies and improve animal models of the disease.

Combination Treatment of Icariin and L-DOPA Against 6-OHDA-Lesioned Dopamine Neurotoxicity

Until now, the dopamine (DA) precursor, L-3,4-dihydroxyphenylalanine (L-DOPA), remains the gold standard effective drug therapy for Parkinson’s disease (PD) patients. Nevertheless, long-term chronic L-DOPA administration leads to the drug efficacy loss and severe adverse effects, such as L-DOPA-induced dyskinesia (LID). Icariin (ICA), a flavonoid that is extracted from Epimedium, has been proved to evoke neuroprotection against DA neuronal loss in PD animal models. Here, the present study detected the effects of ICA combined with L-DOPA on 6-hydroxydopamine (6-OHDA)-elicited DA neurotoxicity and L-DOPA-induced motor dysfunction as well. PC12 cells were applied to investigate the combination treatment of ICA and L-DOPA against 6-OHDA-lesioned neurotoxicity. In addition, rat substantia nigral stereotaxic injection of 6-OHDA-induced DA neuronal injury was performed to explore the neuroprotective effects mediated by ICA combined with L-DOPA. The pathological movement triggered by L-DOPA was determined by the abnormal involuntary movements (AIM) scores analysis. In PC12 cells, ICA combined with L-DOPA produced better neuroprotection from 6-OHDA-induced neurotoxicity than ICA or L-DOPA alone treatment. In parkinsonian 6-OHDA lesioned rats, ICA conferred DA neuroprotection as monotherapy and an enhancement benefit of L-DOPA treatment after daily administration of L-DOPA and ICA for 21 days. Moreover, ICA ameliorated the development of LID as evidenced by the lowered AIM scores without affecting L-DOPA-mediated efficacy. Furtherly, ICA attenuated neuroinflammation in 6-OHDA-induced DA neuronal loss and the development of LID in vivo. In conclusion, these findings suggest ICA might be a potential promising adjuvant to enhance L-DOPA efficacy and attenuate L-DOPA-produced adverse effects in PD.

Neuroinflammation in L-DOPA-induced dyskinesia: beyond the immune function

Neuroinflammation is a main component of Parkinson's disease (PD) neuropathology, where unremitting reactive microglia and microglia-secreted soluble molecules such as cytokines, contribute to the neurodegenerative process as part of an aberrant immune reaction. Besides, pro-inflammatory cytokines, predominantly TNF-α, play an important neuromodulatory role in the healthy and diseased brain, being involved in neurotransmitter metabolism, synaptic scaling and brain plasticity. Recent preclinical studies have evidenced an exacerbated neuroinflammatory reaction in the striatum of parkinsonian rats that developed dyskinetic responses following L-DOPA administration. These findings prompted investigation of non-neuronal mechanisms of L-DOPA-induced dyskinesia (LID) involving glial cells and glial-secreted soluble molecules. Hence, besides the classical mechanisms of LID that include abnormal corticostriatal neurotransmission and maladaptive changes in striatal medium spiny neurons (MSNs), here we review studies supporting a role of striatal neuroinflammation in the development of LID, with a focus on microglia and the pro-inflammatory cytokine TNF-α. Moreover, we discuss several mechanisms that have been involved in the development of LID, which are directly or indirectly under the control of TNF-α, and might be abnormally affected by its chronic overproduction and release by microglia in PD. It is proposed that TNF-α may contribute to the altered neuronal responses occurring in LID by targeting receptor trafficking and function in MSNs, but also dopamine synthesis in preserved dopaminergic terminals and serotonin metabolism in serotonergic neurons. Therapeutic approaches specifically targeting glial-secreted cytokines may represent a novel target for preventing or treating LID.

Differential spread of anoxic depolarization contributes to the pattern of neuronal injury after oxygen and glucose deprivation (OGD) in the Substantia Nigra in rat brain slices

Anoxic depolarization (AD) is an acute event evoked by brain ischemia, involving a profound loss of cell membrane potential and swelling that spreads over susceptible parts of the gray matter. Its occurrence is a strong predictor of the severity of neuronal injury. Little is known about this event in the Substantia Nigra, a midbrain nucleus critical for motor control. We tested the effects of oxygen and glucose deprivation (OGD), an in vitro model of brain ischemia, in rat midbrain slices. AD developed within 4min from OGD onset and spread in the Substantia Nigra pars reticulata (SNr), but not through the Substantia Nigra pars compacta (SNc). This differential effect involved a contrasting pattern of changes in membrane potential between dopamine-producing SNc and non-dopaminergic SNr neurons. A fast depolarization in SNr neurons was not followed by repolarization after the end of OGD, and was associated with swollen somata and beaded dendrites. In contrast, slowly developing depolarization of SNc neurons led to repolarization after OGD ended, and no changes in neuronal morphology were observed. The AD-resistance of the SNc involved smaller dysregulations of K⁺ and Ca²⁺ ions, and a slower loss of energy metabolites. Our results show that acute ischemia profoundly impairs the function and morphology of SNr neurons but not adjacent SNc neurons, and that the surprising higher tolerance of SNc neurons correlates with the resistance of the SNc region to AD. This differential response may affect the pattern of early neuronal injury that develops in the brainstem after acute ischemic insults.

The Relationship between Parkinson Disease and Brain Tumor: A Meta-Analysis

Objective

Epidemiological studies have investigated the association between Parkinson disease (PD) occurrence and the risk of brain tumors, while the results remain controversial. We performed a meta-analysis to clarify the exact relationship between PD and brain tumors.

Methods

A systematic literature search was conducted using PubMed, Embase, ScienceDirect and CBM (China Biology Medicine Disc) before February 2016. Eligible studies were those that reported risk estimates of brain tumors among patients with PD or vice versa. A random-effects model was used to calculate the pooled odds ratio (OR) of the outcomes. Subgroup analyses and sensitivity analysis were conducted to explore the potential sources of heterogeneity.

Results

In total, eight studies involving 329,276 participants met our inclusion criteria. The pooled OR was 1.51 (95%CI 1.21–1.89), indicating that PD carried a higher risk of brain tumor. Analyses by temporal relationship found that the occurrence of brain tumor was significantly higher after the diagnosis of PD (OR 1.55, 95% CI 1.18–2.05), but not statistically significant before PD diagnosis (OR 1.21, 95%CI 0.93–1.58). Subgroup analysis showed that gender differences, ethnicity differences and the characteristic of the tumor (benign or malignant) did not make much change in the association between brain tumor and PD.

Conclusions

Our meta-analysis collecting epidemiological studies suggested a positive association of PD with brain tumors, while the influence of anti-parkinson drugs and ascertainment bias could not be excluded. Further studies with larger sample size and more strict inclusion criteria should be conducted in the future.