Dopamine transporter loss in 6-OHDA Parkinson's model is unmet by parallel reduction in dopamine uptake

The chronic use of serotonin norepinephrine reuptake inhibitors facilitates dyskinesia priming in early Parkinson's disease

BACKGROUND

Parkinson's disease (PD) patients are frequently exposed to antidepressant medications (ADMs). Norepinephrine (NE) and serotonin (5HT) systems have a role in levodopa-induced dyskinesias (LID) pathophysiology.

METHODS

We performed a longitudinal analysis on the PPMI cohort including drug-naïve PD patients, who are progressively exposed to dopamine replacement therapies (DRTs) to test the effect of ADM exposure on LID development by the 4th year of follow-up.

RESULTS

LID prevalence (according to MDS UPDRS score 4.1 ≥ 1) was 16% (42/251); these patients were more likely women (p = 0.01), had higher motor (p < 0.001) and depression scores (p = 0.01) and lower putaminal DAT binding ratio (p = 0.01). LID were associated with the exposure time to L-DOPA (2.2 ± 1.07 vs 2.6 ± 0.9, p = 0.02) and to the exposure to ADMs, in particular to SNRI (4.8% vs 21.4%, p < 0.001). The latter persisted after correcting for significant covariates (e.g., disease duration, cognitive status, motor impairment, depression, dopaminergic denervation). A similar difference in LID prevalence in PD patients exposed vs non-exposed to SNRI was observed on matched data by the real-world TriNetX repository (22% vs 13%, p < 0.001).

DISCUSSION

This study supports the presence of an effect of SNRI on LID priming in patients with early PD. Independent prospective cohort studies are warranted to further verify such association.

Experimental Models to Study Immune Dysfunction in the Pathogenesis of Parkinson's Disease

Parkinson's disease (PD) is a chronic, age-related, progressive multisystem disease associated with neuroinflammation and immune dysfunction. This review discusses the methodological approaches used to study the changes in central and peripheral immunity in PD, the advantages and limitations of the techniques, and their applicability to humans. Although a single animal model cannot replicate all pathological features of the human disease, neuroinflammation is present in most animal models of PD and plays a critical role in understanding the involvement of the immune system (IS) in the pathogenesis of PD. The IS and its interactions with different cell types in the central nervous system (CNS) play an important role in the pathogenesis of PD. Even though culture models do not fully reflect the complexity of disease progression, they are limited in their ability to mimic long-term effects and need validation through in vivo studies. They are an indispensable tool for understanding the interplay between the IS and the pathogenesis of this disease. Understanding the immune-mediated mechanisms may lead to potential therapeutic targets for the treatment of PD. We believe that the development of methodological guidelines for experiments with animal models and PD patients is crucial to ensure the validity and consistency of the results.

Dopamine transmission in the tail striatum: Regional variation and contribution of dopamine clearance mechanisms

The striatum can be divided into four anatomically and functionally distinct domains: the dorsolateral, dorsomedial, ventral and the more recently identified caudolateral (tail) striatum. Dopamine transmission in these striatal domains underlies many important behaviours, yet little is known about this phenomenon in the tail striatum. Furthermore, the tail is divided anatomically into four divisions (dorsal, medial, intermediate and lateral) based on the profile of D1 and D2 dopamine receptor-expressing medium spiny neurons, something that is not seen elsewhere in the striatum. Considering this organisation, how dopamine transmission occurs in the tail striatum is of great interest. We recorded evoked dopamine release in the four tail divisions, with comparison to the dorsolateral striatum, using fast-scan cyclic voltammetry in rat brain slices. Contributions of clearance mechanisms were investigated using dopamine transporter knockout (DAT-KO) rats, pharmacological transporter inhibitors and dextran. Evoked dopamine release in all tail divisions was smaller in amplitude than in the dorsolateral striatum and, importantly, regional variation was observed: dorsolateral ≈ lateral > medial > dorsal ≈ intermediate. Release amplitudes in the lateral division were 300% of that in the intermediate division, which also exhibited uniquely slow peak dopamine clearance velocity. Dopamine clearance in the intermediate division was most dependent on DAT, and no alternative dopamine transporters investigated (organic cation transporter-3, norepinephrine transporter and serotonin transporter) contributed significantly to dopamine clearance in any tail division. Our findings confirm that the tail striatum is not only a distinct dopamine domain but also that each tail division has unique dopamine transmission characteristics. This supports that the divisions are not only anatomically but also functionally distinct. How this segregation relates to the overall function of the tail striatum, particularly the processing of multisensory information, is yet to be determined.

Dopamine Signaling in Substantia Nigra and Its Impact on Locomotor Function-Not a New Concept, but Neglected Reality

The mechanistic influences of dopamine (DA) signaling and impact on motor function are nearly always interpreted from changes in nigrostriatal neuron terminals in striatum. This is a standard practice in studies of human Parkinson's disease (PD) and aging and related animal models of PD and aging-related parkinsonism. However, despite dozens of studies indicating an ambiguous relationship between changes in striatal DA signaling and motor phenotype, this perseverating focus on striatum continues. Although DA release in substantia nigra (SN) was first reported almost 50 years ago, assessment of nigral DA signaling changes in relation to motor function is rarely considered. Whereas DA signaling has been well-characterized in striatum at all five steps of neurotransmission (biosynthesis and turnover, storage, release, reuptake, and post-synaptic binding) in the nigrostriatal pathway, the depth of such interrogations in the SN, outside of cell counts, is sparse. However, there is sufficient evidence that these steps in DA neurotransmission in the SN are operational and regulated autonomously from striatum and are present in human PD and aging and related animal models. To complete our understanding of how nigrostriatal DA signaling affects motor function, it is past time to include interrogation of nigral DA signaling. This brief review highlights evidence that changes in nigral DA signaling at each step in DA neurotransmission are autonomous from those in striatum and changes in the SN alone can influence locomotor function. Accordingly, for full characterization of how nigrostriatal DA signaling affects locomotor activity, interrogation of DA signaling in SN is essential.

Nigral-specific increase in ser31 phosphorylation compensates for tyrosine hydroxylase protein and nigrostriatal neuron loss: Implications for delaying parkinsonian signs

Compensatory mechanisms that augment dopamine (DA) signaling are thought to mitigate onset of hypokinesia prior to major loss of tyrosine hydroxylase (TH) in striatum that occurs in Parkinson's disease. However, the identity of such mechanisms remains elusive. In the present study, the rat nigrostriatal pathway was unilaterally-lesioned with 6-hydroxydopamine (6-OHDA) to determine whether differences in DA content, TH protein, TH phosphorylation, or D1 receptor expression in striatum or substantia nigra (SN) aligned with hypokinesia onset and severity at two time points. In striatum, DA and TH loss reached its maximum (>90%) 7 days after lesion induction. However, in SN, no DA loss occurred, despite ∼60% TH loss. Hypokinesia was established at 21 days post-lesion and maintained at 28 days. At this time, DA loss was ∼60% in the SN, but still of lesser magnitude than TH loss. At day 7 and 28, ser31 TH phosphorylation increased only in SN, corresponding to less DA versus TH protein loss. In contrast, ser40 TH phosphorylation was unaffected in either region. Despite DA loss in both regions at day 28, D1 receptor expression increased only in lesioned SN. These results support the concept that augmented components of DA signaling in the SN, through increased ser31 TH phosphorylation and D1 receptor expression, contribute as compensatory mechanisms against progressive nigrostriatal neuron and TH protein loss, and may mitigate hypokinesia severity.

Behavioral and neurochemical interactions of the tricyclic antidepressant drug desipramine with L-DOPA in 6-OHDA-lesioned rats. Implications for motor and psychiatric functions in Parkinson's disease

RATIONALE

The pharmacological effects of antidepressants in modulating noradrenergic transmission as compared to serotonergic transmission in a rat model of Parkinson's disease under chronic L-DOPA therapy are insufficiently explored.

OBJECTIVES

The aim of the present study was to investigate the effect of the tricyclic antidepressant desipramine administered chronically alone or jointly with L-DOPA, on motor behavior and monoamine metabolism in selected brain structures of rats with the unilateral 6-OHDA lesion.

METHODS

The antiparkinsonian activities of L-DOPA and desipramine were assessed behaviorally using a rotation test and biochemically based on changes in the tissue concentrations of noradrenaline, dopamine and serotonin and their metabolites, evaluated separately for the ipsi- and contralateral motor (striatum, substantia nigra) and limbic (prefrontal cortex, hippocampus) structures of rat brain by HPLC method.

RESULTS

Desipramine administered alone did not induce rotational behavior, but in combination with L-DOPA, it increased the number of contralateral rotations more strongly than L-DOPA alone. Both L-DOPA and desipramine + L-DOPA significantly increased DA levels in the ipsilateral striatum, substantia nigra, prefrontal cortex and the ipsi- and contralateral hippocampus. The combined treatment also significantly increased noradrenaline content in the ipsi- and contralateral striatum, while L-DOPA alone decreased serotonin level on both sides of the hippocampus.

CONCLUSIONS

The performed analysis of the level of monoamines and their metabolites in the selected brain structures suggests that co-modulation of noradrenergic and dopaminergic transmission in Parkinson's disease by the combined therapy with desipramine + L-DOPA may have some positive implications for motor and psychiatric functions but further research is needed to exclude potential negative effects.

Consequences of Acute or Chronic Methylphenidate Exposure Using Ex Vivo Neurochemistry and In Vivo Electrophysiology in the Prefrontal Cortex and Striatum of Rats

Methylphenidate (MPH) is among the main drugs prescribed to treat patients with attention-deficit and hyperactivity disease (ADHD). MPH blocks both the norepinephrine and dopamine reuptake transporters (NET and DAT, respectively). Our study was aimed at further understanding the mechanisms by which MPH could modulate neurotransmitter efflux, using ex vivo radiolabelled neurotransmitter assays isolated from rats. Here, we observed significant dopamine and norepinephrine efflux from the prefrontal cortex (PFC) after MPH (100 µM) exposure. Efflux was mediated by both dopamine and norepinephrine terminals. In the striatum, MPH (100 µM) triggered dopamine efflux through both sodium- and vesicular-dependent mechanisms. Chronic MPH exposure (4 mg/kg/day/animal, voluntary oral intake) for 15 days, followed by a 28-day washout period, increased the firing rate of PFC pyramidal neurons, assessed by in vivo extracellular single-cell electrophysiological recordings, without altering the responses to locally applied NMDA, via micro-iontophoresis. Furthermore, chronic MPH treatment resulted in decreased efficiency of extracellular dopamine to modulate NMDA-induced firing activities of medium spiny neurons in the striatum, together with lower MPH-induced (100 µM) dopamine outflow, suggesting desensitization to both dopamine and MPH in striatal regions. These results indicate that MPH can modulate neurotransmitter efflux in brain regions enriched with dopamine and/or norepinephrine terminals. Further, long-lasting alterations of striatal and prefrontal neurotransmission were observed, even after extensive washout periods. Further studies will be needed to understand the clinical implications of these findings.

Angiotensin-Converting Enzyme 2 Activation Mitigates Behavioral Deficits and Neuroinflammatory Burden in 6-OHDA Induced Experimental Models of Parkinson's Disease

Hypertension is reported to cause major brain disorders including Parkinson's disease (PD), apart from cardiovascular and chronic kidney disorders. Considering this, for the first time, we explored the effect of modulation of the ACE2/Ang (1-7)/MasR axis using diminazene aceturate (DIZE), an ACE2 activator, in 6-hydroxydopamine (6-OHDA) induced PD model. We found that DIZE treatment improved neuromuscular coordination and locomotor deficits in the 6-OHDA induced PD rat model. Further, the DIZE-mediated activation of ACE2 led to increased tyrosine hydroxylase (TH) and dopamine transporters (DAT) expression in the rat brain, indicating the protection of dopaminergic (DAergic) neurons from 6-OHDA induced neurotoxicity. Moreover, 6-OHDA induced activation of glial cells (astrocytes and microglia) and release of neuroinflammatory mediators were attenuated by DIZE treatment in both in vitro as well as in vivo models of PD. DIZE exerted its effect by activating ACE2 that produced Ang (1-7), a neuroprotective peptide. Ang (1-7) conferred its neuroprotective effect upon binding with the G-protein-coupled MAS receptor that led to the upregulation of cell survival proteins while downregulating apoptotic proteins. Importantly, these findings were further validated by using A-779, a MasR antagonist. The result showed that treatment with A-779 reversed the antioxidative and anti-inflammatory effects of DIZE by decreasing glial activation and neuroinflammatory markers. Although the role of ACE2 in PD pathology needs to be additionally confirmed using transgenic models in either ACE2 overexpressing or knockout mice, still, our study demonstrates that enhancing ACE2 activity could be a novel approach for ameliorating PD pathology.

Evaluation of 6-Hydroxydopamine and Rotenone In Vitro Neurotoxicity on Differentiated SH-SY5Y Cells Using Applied Computational Statistics

With the increase in life expectancy and consequent aging of the world’s population, the prevalence of many neurodegenerative diseases is increasing, without concomitant improvement in diagnostics and therapeutics. These diseases share neuropathological hallmarks, including mitochondrial dysfunction. In fact, as mitochondrial alterations appear prior to neuronal cell death at an early phase of a disease’s onset, the study and modulation of mitochondrial alterations have emerged as promising strategies to predict and prevent neurotoxicity and neuronal cell death before the onset of cell viability alterations. In this work, differentiated SH-SY5Y cells were treated with the mitochondrial-targeted neurotoxicants 6-hydroxydopamine and rotenone. These compounds were used at different concentrations and for different time points to understand the similarities and differences in their mechanisms of action. To accomplish this, data on mitochondrial parameters were acquired and analyzed using unsupervised (hierarchical clustering) and supervised (decision tree) machine learning methods. Both biochemical and computational analyses resulted in an evident distinction between the neurotoxic effects of 6-hydroxydopamine and rotenone, specifically for the highest concentrations of both compounds.

Anti-inflammatory and Neuroprotective Effects of Transcranial Ultrasound Stimulation on Parkinson's Disease

Low-intensity pulsed ultrasound (LIPUS) is a promising non-invasive neuromodulation tool for deep brain stimulation. Here, we investigated the impact of LIPUS, including neuroprotective effects, on the pathology of Parkinson's disease (PD) in an animal model. Sprague-Dawley rats were injected with 6-hydroxydopamine (6-OHDA) at two sites in the right striatum. LIPUS (1 MHz, 5% duty cycle, 1-Hz pulse repetition frequency, 15 min/d) stimulation was then applied to some of the rats (the 6-OHDA + LIPUS group) beginning 2 wk after the 6-OHDA administration, while the remaining rats (the 6-OHDA group) received no LIPUS stimulation. The 6-OHDA-induced inflammatory responses and expressions of neurotrophic factors were quantified with immunofluorescence activity. The safety of LIPUS was assessed using hematoxylin and eosin and Nissl staining. LIPUS treatment significantly inhibited 6-OHDA-induced glial activation and the phosphorylation of nuclear factor-κB p65 in the substantia nigra pars compacta. Further study revealed that LIPUS effectively preserved the levels of neurotrophic factors, dopamine transporter and tight junction proteins of the blood-brain barrier in the 6-OHDA + LIPUS group compared with the 6-OHDA group. These results indicate that LIPUS acts via multiple neuroprotective mechanisms in the PD rat model and suggest that LIPUS can be viewed as a potential treatment for PD.

Multifactoriality of Parkinson's Disease as Explored Through Human Neural Stem Cells and Their Transplantation in Middle-Aged Parkinsonian Mice

Parkinson's disease (PD) is an age-associated neurodegenerative disorder for which there is currently no cure. Cell replacement therapy is a potential treatment for PD; however, this therapy has more clinically beneficial outcomes in younger patients with less advanced PD. In this study, hVM1 clone 32 cells, a line of human neural stem cells, were characterized and subsequently transplanted in middle-aged Parkinsonian mice in order to examine cell replacement therapy as a treatment for PD. In vitro analyses revealed that these cells express standard dopamine-centered markers as well as others associated with mitochondrial and peroxisome function, as well as glucose and lipid metabolism. Four months after the transplantation of the hVM1 clone 32 cells, striatal expression of tyrosine hydroxylase was minimally reduced in all Parkinsonian mice but that of dopamine transporter was decreased to a greater extent in buffer compared to cell-treated mice. Behavioral tests showed marked differences between experimental groups, and cell transplant improved hyperactivity and gait alterations, while in the striatum, astroglial populations were increased in all groups due to age and a higher amount of microglia were found in Parkinsonian mice. In the motor cortex, nonphosphorylated neurofilament heavy was increased in all Parkinsonian mice. Overall, these findings demonstrate that hVM1 clone 32 cell transplant prevented motor and non-motor impairments and that PD is a complex disorder with many influencing factors, thus reinforcing the idea of novel targets for PD treatment that tend to be focused on dopamine and nigrostriatal damage.

The Association Between Parkinson's Disease and Attention-Deficit Hyperactivity Disorder

While Parkinson’s disease (PD) and attention-deficit hyperactivity disorder (ADHD) are two distinct conditions, it has been hypothesized that they share several overlapping anatomical and neurochemical changes. In order to investigate that hypothesis, this study used claims data from Taiwan’s Longitudinal Health Insurance Database 2000 to provide the significant nationwide population-based evidence of an increased risk of PD among ADHD patients, and the connection between the two conditions was not the result of other comorbidities. Moreover, this study showed that the patients with PD were 2.8 times more likely to have a prior ADHD diagnosis compared with those without a prior history of ADHD. Furthermore, an animal model of ADHD was generated by neonatally injecting rats with 6-hydroxydopamine (6-OHDA). These rats were subjected to behavior tests and the 99mTc-TRODAT-1 brain imaging at the juvenile stage. Compared to control group rats, the 6-OHDA rats showed a significantly reduced specific uptake ratio in the striatum, indicating an underlying PD-linked pathology in the brains of these ADHD phenotype-expressing rats. Overall, these results support that ADHD shares a number of anatomical and neurochemical changes with PD. As such, improved knowledge of the neurochemical mechanisms underlying ADHD could result in improved treatments for various debilitating neurological disorders, including PD.

Use of leptin-conjugated phosphatidic acid liposomes with resveratrol and epigallocatechin gallate to protect dopaminergic neurons against apoptosis for Parkinson's disease therapy

Complex liposomes were assembled with 1,2-distearoyl-sn-glycero-3-phosphocholine, dihexadecyl phosphate (DHDP), cholesterol and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (PA) to act as drug carriers for resveratrol (RES) and epigallocatechin gallate (EGCG). The liposomes were modified with leptin (Lep) on the surface to cross the blood-brain barrier (BBB) and to rescue degenerated dopaminergic neurons. The activity of RES and EGCG against neurotoxicity was investigated using an in vitro neurodegenerative model established by SH-SY5Y cells with an insult of 1-methyl-4-phenylpyridinium (MPP⁺). The results indicated that increasing the mole percentage of DHDP and PA increased the particle size and absolute zeta potential value, and improved the entrapment efficiency of RES and EGCG; however, this increase reduced the release rate of RES and EGCG and the grafting efficiency of Lep. The ability of Lep/RES-EGCG-PA-liposomes to cross the BBB was found to be higher than that of non-modified liposomes. Further, the addition of PA and Lep into liposomes enhanced cell viability and target efficiency. The immunofluorescence results demonstrated that the conjugation of Lep with liposomes enabled the docking of HBMECs and SH-SY5Y cells via Lep receptor, and enhanced their ability to permeate the BBB and cellular uptake. Immunofluorescence and western blot analysis also revealed that RES and EGCG encapsulated into liposomes could be a neural defensive strategy by reducing the apoptosis promotor protein Bcl-2 associated X protein and α-synuclein, and enhancement in the apoptosis inhibitor protein B cell lymphoma 2, tyrosine hydroxylase, and the dopamine transporter. Hence, Lep-PA-liposomes can be an excellent choice of potential delivery system for PD treatment.

Classic and evolving animal models in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disease with motor and non-motor symptoms. PD is characterized by the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) and deficiency of dopamine in the striatal region. The primary objective in PD research is to understand the pathogenesis, targets, and development of therapeutic interventions to control the progress of the disease. The anatomical and physiological resemblances between humans and animals gathered the researcher's attention towards the use of animals in PD research. Due to varying age of onset, symptoms, and progression rate, PD becomes heterogeneous which demands the variety of animal models to study diverse features of the disease. Parkinson is a multifactorial disorder, selection of models become important as not a single model shows all the biochemical features of the disease. Currently, conventional pharmacological, neurotoxin-induced, genetically modified and cellular models are available for PD research, but none of them recapitulate all the biochemical characteristics of the disease. In this review, we included the updated knowledge on the main features of currently available in vivo and in vitro models as well as their strengths and weaknesses.

Behavioral Tests in Neurotoxin-Induced Animal Models of Parkinson's Disease

Currently, neurodegenerative diseases are a major cause of disability around the world. Parkinson's disease (PD) is the second-leading cause of neurodegenerative disorder after Alzheimer's disease. In PD, continuous loss of dopaminergic neurons in the substantia nigra causes dopamine depletion in the striatum, promotes the primary motor symptoms of resting tremor, bradykinesia, muscle rigidity, and postural instability. The risk factors of PD comprise environmental toxins, drugs, pesticides, brain microtrauma, focal cerebrovascular injury, aging, and hereditary defects. The pathologic features of PD include impaired protein homeostasis, mitochondrial dysfunction, nitric oxide, and neuroinflammation, but the interaction of these factors contributing to PD is not fully understood. In neurotoxin-induced PD models, neurotoxins, for instance, 6-hydroxydopamine (6-OHDA), 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), 1-Methyl-4-phenylpyridinium (MPP+), paraquat, rotenone, and permethrin mainly impair the mitochondrial respiratory chain, activate microglia, and generate reactive oxygen species to induce autooxidation and dopaminergic neuronal apoptosis. Since no current treatment can cure PD, using a suitable PD animal model to evaluate PD motor symptoms' treatment efficacy and identify therapeutic targets and drugs are still needed. Hence, the present review focuses on the latest scientific developments in different neurotoxin-induced PD animal models with their mechanisms of pathogenesis and evaluation methods of PD motor symptoms.

Diabetes Causes Dysfunctional Dopamine Neurotransmission Favoring Nigrostriatal Degeneration in Mice

BACKGROUND

Numerous studies indicate an association between neurodegenerative and metabolic diseases. Although still a matter of debate, growing evidence from epidemiological and animal studies indicate that preexisting diabetes increases the risk to develop Parkinson's disease. However, the mechanisms of such an association are unknown.

OBJECTIVES

We investigated whether diabetes alters striatal dopamine neurotransmission and assessed the vulnerability of nigrostriatal neurons to neurodegeneration.

METHODS

We used streptozotocin-treated and genetically diabetic db/db mice. Expression of oxidative stress and nigrostriatal neuronal markers and levels of dopamine and its metabolites were monitored. Dopamine release and uptake were assessed using fast-scan cyclic voltammetry. 6-Hydroxydopamine was unilaterally injected into the striatum using stereotaxic surgery. Motor performance was scored using specific tests.

RESULTS

Diabetes resulted in oxidative stress and decreased levels of dopamine and its metabolites in the striatum. Levels of proteins regulating dopamine release and uptake, including the dopamine transporter, the Girk2 potassium channel, the vesicular monoamine transporter 2, and the presynaptic vesicle protein synaptobrevin-2, were decreased in diabetic mice. Electrically evoked levels of extracellular dopamine in the striatum were enhanced, and altered dopamine uptake was observed. Striatal microinjections of a subthreshold dose of the neurotoxin 6-hydroxydopamine in diabetic mice, insufficient to cause motor alterations in nondiabetic animals, resulted in motor impairment, higher loss of striatal dopaminergic axons, and decreased neuronal cell bodies in the substantia nigra.

CONCLUSIONS

Our results indicate that diabetes promotes striatal oxidative stress, alters dopamine neurotransmission, and increases vulnerability to neurodegenerative damage leading to motor impairment. © 2020 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

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.

GFR-α1 Expression in Substantia Nigra Increases Bilaterally Following Unilateral Striatal GDNF in Aged Rats and Attenuates Nigral Tyrosine Hydroxylase Loss Following 6-OHDA Nigrostriatal Lesion

Glial cell line-derived neurotrophic factor (GDNF) improved motor function in Parkinson's disease (PD) patients in Phase I clinical trials, and these effects persisted months after GDNF discontinuation. Conversely, phase II clinical trials reported no significant effects on motor improvement vs placebo. The disease duration and the quantity, infusion approach, and duration of GDNF delivery may affect GDNF efficacy in PD treatment. However, identifying mechanisms activated by GDNF that affect nigrostriatal function may reveal additional avenues to partially restore nigrostriatal function. In PD and aging models, GDNF affects tyrosine hydroxylase (TH) expression or phosphorylation in substantia nigra (SN), long after a single GDNF injection in striatum. In aged rats, the GDNF family receptor, GFR-α1, increases TH expression and phosphorylation in SN. To determine if GFR-α1 could be a mechanistic link in long-term GDNF impact, we conducted two studies; first to determine if a single unilateral striatal delivery of GDNF affected GFR-α1 and TH over time (1 day, 1 week, and 4 weeks) in the striatum or SN in aged rats, and second, to determine if soluble GFR-α1 could mitigate TH loss following 6-hydroxydopamine (6-OHDA) lesion. In aged rats, GDNF bilaterally increased ser31 TH phosphorylation and GFR-α1 expression in SN at 1 day and 4 weeks after GDNF, respectively. In striatum, GFR-α1 expression decreased 1 week after GDNF, only on the GDNF-injected side. In 6-OHDA-lesioned rats, recombinant soluble GFR-α1 mitigated nigral, but not striatal, TH protein loss following 6-OHDA. Together, these results show GDNF has immediate and long-term impact on dopamine regulation in the SN, which includes a gradual increase in GFR-α1 expression that may sustain TH expression and dopamine function therein.

Electrophysiological Characterization of Novel Effects of the Uptake-2 Blocker Decynium-22 (D-22) on Dopaminergic Neurons in the Substantia Nigra Pars Compacta

Extracellular levels of dopamine (DA) and other monoamines in the brain depend not only on the classic transporters encoded by SLC6A gene family such as DAT, NET and SERT, but also a more recently identified group of low-affinity/high-capacity 'Uptake-2' transporters, mainly OCT3 and PMAT. The most frequently used pharmacological tool in functional studies of Uptake-2 is decynium-22 (D-22) known to block these transporters. However, the effectiveness of this drug in enhancing extracellular DA remains uncertain. Our aim was to test the hypothesis that D-22 increases extracellular levels of DA released from the somatodendritic region of dopaminergic neurons in the substantia nigra pars compacta (SNc) by reducing the OCT3/PMAT-dependent component of DA uptake. Extracellular DA was assessed indirectly, by evoking D2-IPSCs in SNc neurons following stimulated release of this neurotransmitter in midbrain slices obtained from mice. Recordings were conducted after partial inhibition of DAT with nomifensine, and after application of L-DOPA which increased the releasable DA pool. Contrary to our expectations, D-22 reduced, rather than increased, the amplitude of D2-IPSCs. Other effects included inhibition of GABA_B-IPSCs and I_h current, and a reduction in firing frequency of nigral neurons. These results show that in addition to the previously known non-specific inhibitory action on α1 adrenoceptors, D-22 exerts additional off-target effects by inhibiting dopaminergic and GABAergic synaptic transmission in the SNc and the spontaneous (pacemaker) activity of nigral neurons. It remains to be established if these novel effects contribute to a reduction in spontaneous locomotor activity reported in previous studies after systemic drug administration.

Interactions of the tricyclic antidepressant drug amitriptyline with L-DOPA in the striatum and substantia nigra of unilaterally 6-OHDA-lesioned rats. Relevance to motor dysfunction in Parkinson's disease

Antidepressant drugs are recommended for the treatment of Parkinson's disease (PD)-associated depression but their role in the modulation of L-DOPA-induced behavioral and neurochemical markers is poorly explored. The aim of the present study was to examine the impact of the tricyclic antidepressant amitriptyline and L-DOPA, administered chronically alone or in combination, on rotational behavior, monoamine levels and binding of radioligands to their transporters in the dopaminergic brain structures of unilaterally 6-OHDA-lesioned rats. Binding of [³H]nisoxetine to noradrenaline transporter (NET), [³H]GBR 12,935 to dopamine transporter (DAT) and [³H]citalopram to serotonin transporter (SERT) were analyzed by autoradiography. Amitriptyline administered alone did not induce rotational behavior but in combination with L-DOPA increased the number of contralateral rotations much more strongly than L-DOPA alone. The combined treatment also significantly increased the tissue dopamine (DA) content in the ipsilateral striatum and substantia nigra (SN) vs. L-DOPA alone. 6-OHDA-mediated lesion of nigrostriatal DA neurons drastically reduced DAT and NET bindings in the ipsilateral striatum. In the ipsilateral SN, DAT binding decreased while NET binding rose. SERT binding increased significantly mainly in the SN. Amitriptyline administered alone or jointly with L-DOPA had no effect on DAT binding on the lesioned side, significantly decreased SERT binding in the striatum and SN while NET binding only in the SN. Since in the DA-denervated striatum, SERT is mainly responsible for reuptake of L-DOPA-derived DA while in the SN, SERT and NET are involved, the inhibition of these transporters by antidepressant drugs may improve dopaminergic transmission and consequently motor behavior.

Vitamin D protects dopaminergic neurons against neuroinflammation and oxidative stress in hemiparkinsonian rats

BACKGROUND

The deficiency in 1α, 25-dihydroxyvitamin D3 (VD3) seems to increase the risk for neurodegenerative pathologies, including Parkinson's disease (PD). The majority of its actions are mediated by the transcription factor, VD3 receptor (VD3R).

METHODS

The neuroprotective effects of VD3 were investigated on a PD model. Male Wistar rats were divided into the following groups: sham-operated (SO), 6-OHDA-lesioned (non-treated), and 6-OHDA-lesioned and treated with VD3 (7 days before the lesion, pre-treatment or for 14 days after the 6-OHDA striatal lesion, post-treatment). Afterwards, the animals were subjected to behavioral tests and euthanized for striatal neurochemical and immunohistochemical assays. The data were analyzed by ANOVA and the Tukey test and considered significant for p < 0.05.

RESULTS

We showed that pre- or post-treatments with VD3 reversed behavioral changes and improved the decreased DA contents of the 6-OHDA group. In addition, VD3 reduced the oxidative stress, increased (TH and DAT), and reduced (TNF-alpha) immunostainings in the lesioned striata. While significant decreases in VD3R immunoreactivity were observed after the 6-OHDA lesion, these changes were blocked after VD3 pre- or post-treatments. We showed that VD3 offers neuroprotection, decreasing behavioral changes, DA depletion, and oxidative stress. In addition, it reverses partially or completely TH, DAT, TNF-alpha, and VD3R decreases of immunoreactivities in the non-treated 6-OHDA group.

CONCLUSIONS

Taken together, VD3 effects could result from its anti-inflammatory and antioxidant actions and from its actions on VD3R. These findings should stimulate translational research towards the VD3 potential for prevention or treatment of neurodegenerative diseases, as PD.

2-Pentadecyl-2-Oxazoline Reduces Neuroinflammatory Environment in the MPTP Model of Parkinson Disease

Current pharmacological management of Parkinson disease (PD) does not provide for disease modification, but addresses only symptomatic features. Here, we explore a new approach to neuroprotection based on the use of 2-pentadecyl-2-oxazoline (PEA-OXA), the oxazoline derivative of the fatty acid amide signaling molecule palmitoylethanolamide (PEA), in an experimental model of PD. Daily oral treatment with PEA-OXA (10 mg/kg) significantly reduced behavioral impairments and neuronal cell degeneration of the dopaminergic tract induced by four intraperitoneal injections of the dopaminergic neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on 8-week-old male C57 mice. Moreover, PEA-OXA treatment prevented dopamine depletion, increased tyrosine hydroxylase and dopamine transporter activities, and decreased α-synuclein aggregation in neurons. PEA-OXA treatment also diminished nuclear factor-κB traslocation, cyclooxygenase-2, and inducible nitric oxide synthase expression and through upregulation of the nuclear factor E2-related factor 2 pathway, induced activation of Mn-superoxide dismutase and heme oxygenase-1. Further, PEA-OXA modulated microglia and astrocyte activation and preserved microtubule-associated protein-2 alterations. In conclusion, pharmacological activation of nuclear factor E2-related factor 2 pathways with PEA-OXA may be effective in the future therapy of PD.

Aspidosperma pyrifolium Mart: neuroprotective, antioxidant and anti-inflammatory effects in a Parkinson's disease model in rats

OBJECTIVES

Aspidosperma species are used for several diseases, especially for malaria in Brazil. Although the genus is object of pharmacological studies, almost none are found on Aspidosperma pyrifolium. We investigate neuroprotective, antioxidant and anti-inflammatory properties of the APSE-Aq fraction (benzoic acid glycosylated derivative) on Parkinson's disease model.

METHODS

Male Wistar rats were subjected to a 6-hydroxydopamine injection into the right striatum and treated or not with APSE-Aq (100 or 200 mg/kg, p.o.). The sham-operated group was injected with saline. Two weeks later, animals were subjected to behavioural, neurochemical and immunohistochemical evaluation. The data were analysed by ANOVA and Tukey test.

KEY FINDINGS

The APSE-Aq-treated group shows a partial recovery of behavioural changes as compared with the untreated-6-hydroxydopamine group. A partial recovery was also observed in nitrite contents and lipid peroxidation. APSE-Aq treatments significantly reversed decreases in striatal dopamine and metabolites in the untreated 6-hydroxydopamine group. Immunostainings for markers as tyrosine hydroxylase and dopamine transporter decreased in the untreated 6-hydroxydopamine group and values recovered after APSE-Aq treatments. Similar data were seen for TNF-alpha.

CONCLUSION

APSE-Aq presents neuroprotective, antioxidant and anti-inflammatory activities. Considering that APSE-Aq is chemically related to salicylic acid, it may act on similar targets.

Degree of dopaminergic degeneration measured by 99mTc-TRODAT-1 SPECT/CT imaging

To prevent and treat Parkinson’s disease in its early stages, it is essential to be able to detect the degree of early dopaminergic neuron degeneration. Dopamine transporters (DAT) in the striatum regulate synaptic dopamine levels, and striatal ^99mTc-TRODAT-1 single-photon emission computed tomography (-SPECT) imaging is a marker for presynaptic neuronal degeneration. However, the association between the degree of dopaminergic degeneration and in vivo^99mTc-TRODAT-1 SPECT imaging is unknown. Therefore, this study investigated the association between the degree of 6-hydroxydopamine (6-OHDA)-induced dopaminergic degeneration and DAT imaging using ^99mTc-TRODAT-1 SPECT in rats. Different degrees of nigrostriatal dopamine depletion were generated by injecting different doses of 6-OHDA (2, 4, and 8 μg) into the right medial forebrain bundle. The degree of nigrostriatal dopaminergic neuron degeneration was assessed by rotational behavior and immunohistochemical staining. The results showed that striatal ^99mTc-TRODAT-1 binding was significantly diminished both in the ipsilateral and the contralateral sides in the 4 and 8 μg 6-OHDA groups, and that DAT ^99mTc-TRODAT-1 binding in the ipsilateral striatum showed a high correlation to apomorphine-induced rotations at 8 weeks post-lesion (r = –0.887, P < 0.01). There were significant correlations between DAT ^99mTc-TRODAT-1 binding in the ipsilateral striatum and the amount of tyrosine hydroxylase immunoreactive neurons in the ipsilateral substantia nigra in the 2, 4, and 8 μg 6-OHDA groups at 8 weeks post-lesion (r = 0.899, P < 0.01). These findings indicate that striatal DAT imaging using ^99mTc-TRODAT-1 is a useful technique for evaluating the severity of dopaminergic degeneration.

Ceftriaxone reduces L-dopa-induced dyskinesia severity in 6-hydroxydopamine parkinson's disease model

BACKGROUND

Increased extracellular glutamate may contribute to l-dopa induced dyskinesia, a debilitating side effect faced by Parkinson's disease patients 5 to 10 years after l-dopa treatment. Therapeutic strategies targeting postsynaptic glutamate receptors to mitigate dyskinesia may have limited success because of significant side effects. Increasing glutamate uptake may be another approach to attenuate excess glutamatergic neurotransmission to mitigate dyskinesia severity or prolong the time prior to onset. Initiation of a ceftriaxone regimen at the time of nigrostriatal lesion can attenuate tyrosine hydroxylase loss in conjunction with increased glutamate uptake and glutamate transporter GLT-1 expression in a rat 6-hydroxydopamine model. In this article, we examined if a ceftriaxone regimen initiated 1 week after nigrostriatal lesion, but prior to l-dopa, could reduce l-dopa-induced dyskinesia in an established dyskinesia model.

METHODS

Ceftriaxone (200 mg/kg, intraperitoneal, once daily, 7 consecutive days) was initiated 7 days post-6-hydroxydopamine lesion (days 7-13) and continued every other week (days 21-27, 35-39) until the end of the study (day 39 postlesion, 20 days of l-dopa).

RESULTS

Ceftriaxone significantly reduced abnormal involuntary movements at 5 time points examined during chronic l-dopa treatment. Partial recovery of motor impairment from nigrostriatal lesion by l-dopa was unaffected by ceftriaxone. The ceftriaxone-treated l-dopa group had significantly increased striatal GLT-1 expression and glutamate uptake. Striatal tyrosine hydroxylase loss in this group was not significantly different when compared with the l-dopa alone group.

CONCLUSIONS

Initiation of ceftriaxone after nigrostriatal lesion, but prior to and during l-dopa, may reduce dyskinesia severity without affecting l-dopa efficacy or the reduction of striatal tyrosine hydroxylase loss. © 2017 International Parkinson and Movement Disorder Society.

What Mechanisms Are Responsible for the Reuptake of Levodopa-Derived Dopamine in Parkinsonian Striatum?

Levodopa is the most effective medication for motor symptoms in Parkinson's disease. However, various motor and non-motor complications are associated with levodopa treatment, resulting from altered levodopa-dopamine metabolism with disease progression and long-term use of the drug. The present review emphasizes the role of monoamine transporters other than the dopamine transporter in uptake of extracellular dopamine in the dopamine-denervated striatum. When dopaminergic neurons are lost and dopamine transporters decreased, serotonin and norepinephrine transporters compensate by increasing uptake of excessive extracellular dopamine in the striatum. Organic cation transporter-3 and plasma membrane monoamine transporter, low affinity, and high capacity transporters, also potentially uptake dopamine when high-affinity transporters do not work normally. Selective serotonin reuptake inhibitors and serotonin norepinephrine reuptake inhibitors are often administered to patients with Parkinson's disease presenting with depression, pain or other non-motor symptoms. Thus, it is important to address the potential of these drugs to modify dopamine metabolism and uptake through blockade of the compensatory function of these transporters, which could lead to changes in motor symptoms of Parkinson's disease.

Monoamine transporter contributions to l-DOPA effects in hemi-parkinsonian rats

l-DOPA is the standard treatment for Parkinson's disease (PD), but chronic treatment typically leads to abnormal involuntary movement or dyskinesia (LID) development. Although poorly understood, dyskinetic mechanisms involve a complex interaction between the remaining dopamine system and the semi-homologous serotonin and norepinephrine systems. Serotonin and norepinephrine transporters (SERT and NET, respectively) have affinity for dopamine uptake especially when dopamine transporters (DAT) are scant. Monoamine reuptake inhibitors have been reported to modulate l-DOPA's anti-parkinsonian effects, but DAT, SERT, and NET's contribution to dyskinesia has not been well delineated. The current investigation sought to uncover the differential expression and function of DAT, SERT, and NET in the l-DOPA-treated hemi-parkinsonian rat. Protein analysis of striatal monoamine transporters in unilateral sham or 6-hydroxydopamine-lesioned rats treated with l-DOPA (0 or 6 mg/kg) showed lesion-induced DAT loss and l-DOPA-induced gain in SERT:DAT and NET:DAT ratios in lesioned rats which positively correlated with dyskinesia expression, suggesting functional shifts among monoamine transporters in the dyskinetic state. SERT blockade with citalopram (3, 5 mg/kg) reduced LID while DAT and NET blockade with GBR-12909 (5, 10 mg/kg) and nisoxetine (5, 10 mg/kg), respectively, mildly exacerbated dyskinesia expression. Transporter inhibition did not significantly alter l-DOPA's ability to reverse motor deficit. Overall, DA and DAT loss with l-DOPA treatment appear to precipitate gain in SERT and NET function. Strong correlations with LID and direct behavioral comparisons of selective transporter blockade reveal novel implications for SERT, DAT, and NET as potential biomarkers and therapeutic targets in the hemi-parkinsonian model and dyskinetic PD patients.

Expanding the repertoire of L-DOPA's actions: A comprehensive review of its functional neurochemistry

Though a multi-facetted disorder, Parkinson's disease is prototypically characterized by neurodegeneration of nigrostriatal dopaminergic neurons of the substantia nigra pars compacta, leading to a severe disruption of motor function. Accordingly, L-DOPA, the metabolic precursor of dopamine (DA), is well-established as a treatment for the motor deficits of Parkinson's disease despite long-term complications such as dyskinesia and psychiatric side-effects. Paradoxically, however, despite the traditional assumption that L-DOPA is transformed in residual striatal dopaminergic neurons into DA, the mechanism of action of L-DOPA is neither simple nor entirely clear. Herein, focussing on its influence upon extracellular DA and other neuromodulators in intact animals and experimental models of Parkinson's disease, we highlight effects other than striatal generation of DA in the functional profile of L-DOPA. While not excluding a minor role for glial cells, L-DOPA is principally transformed into DA in neurons yet, interestingly, with a more important role for serotonergic than dopaminergic projections. Moreover, in addition to the striatum, L-DOPA evokes marked increases in extracellular DA in frontal cortex, nucleus accumbens, the subthalamic nucleus and additional extra-striatal regions. In considering its functional profile, it is also important to bear in mind the marked (probably indirect) influence of L-DOPA upon cholinergic, GABAergic and glutamatergic neurons in the basal ganglia and/or cortex, while anomalous serotonergic transmission is incriminated in the emergence of L-DOPA elicited dyskinesia and psychosis. Finally, L-DOPA may exert intrinsic receptor-mediated actions independently of DA neurotransmission and can be processed into bioactive metabolites. In conclusion, L-DOPA exerts a surprisingly complex pattern of neurochemical effects of much greater scope that mere striatal transformation into DA in spared dopaminergic neurons. Their further experimental and clinical clarification should help improve both L-DOPA-based and novel strategies for controlling the motor and other symptoms of Parkinson's disease.

Adaptive down-regulation of the serotonin transporter in the 6-hydroxydopamine-induced rat model of preclinical stages of Parkinson's disease and after chronic pramipexole treatment

Our recent study has indicated that a moderate lesion induced by bilateral 6-hydroxydopamine (6-OHDA) injections into the ventrolateral region of the caudate-putamen (CP) in rats, modeling preclinical stages of Parkinson's disease, induces a "depressive-like" behavior which is reversed by chronic treatment with pramipexole (PRA). The aim of the present study was to examine the influence of the above lesion and chronic PRA treatment on binding to the serotonin transporter (SERT) in different brain regions. As before, 6-OHDA (15 μg/2.5 μl) was administered bilaterally into the CP. PRA (1mg/kg) was injected subcutaneously twice a day for 2 weeks. Serotonergic and dopaminergic neurons of the dorsal raphe (DR) were immunostained for tryptophan hydroxylase and tyrosine hydroxylase, respectively, and were counted stereologically. Binding of [(3)H]GBR 12,935 to the dopamine transporter (DAT) and [(3)H]citalopram to SERT was analyzed autoradiographically. Intrastriatal 6-OHDA injections decreased the number of dopaminergic, but not serotonergic neurons in the DR. 6-OHDA reduced the DAT binding in the CP, and SERT binding in the nigrostriatal system (CP, substantia nigra (SN)), limbic system (ventral tegmental area (VTA), nucleus accumbens (NAC), amygdala, prefrontal cortex (PFCX), habenula, hippocampus) and DR. A significant positive correlation was found between DAT and SERT binding in the CP. Chronic PRA did not influence DAT binding but reduced SERT binding in the above structures, and deepened the lesion-induced losses in the core region of the NAC, SN, VTA and PFCX. The present study indicates that both the lesion of dopaminergic neurons and chronic PRA administration induce adaptive down-regulation of SERT binding. Moreover, although involvement of stimulation of dopaminergic transmission by chronic PRA in its "antidepressant" effect seems to be prevalent, additional contribution of SERT inhibition cannot be excluded.

Interpreting DTBZ binding data in rodent: Inherent variability and compensation

[11C]-dihydrotetrabenazine (DTBZ) Positron Emission Tomography was used to evaluate the vesicular monoamine transporter type 2 as an index of dopaminergic function in the striatum of adult Sprague-Dawley rats obtained from two different animal sources (Charles River Laboratories [CR] or UBC's Animal Care Centre [ACC]) and later submitted to two different unilateral lesions of the nigro-striatal pathway. The results showed a significant difference in the striatal binding potential (BP(ND)) at baseline (before lesioning) between the CR and ACC groups providing evidence that the origin of the animals, possibly due to differences in early environmental factors or breeding conditions associated with different animal vendors plays a role in the development of the adult dopaminergic system. Further, in both animal models, an increase in DTBZ BP(ND) was observed, after unilateral intervention, in the striatum contralateral to the lesion, likely reflecting compensatory effects. Based on these findings, we conclude that in unilateral models, the unlesioned side/hemisphere may not be an appropriate control and that care should be taken to control for the origin of the animals in any given study, especially in longitudinal and replication studies.

Pyrylium Salts as Reactive Matrices for MALDI-MS Imaging of Biologically Active Primary Amines

Many neuroactive substances, including endogenous biomolecules, environmental compounds, and pharmaceuticals possess primary amine functional groups. Among these are catecholamine neurotransmitters (e.g., dopamine), many substituted phenethylamines (e.g., amphetamine), as well as amino acids and neuropeptides. In most cases, mass spectrometric (ESI and MALDI) analyses of trace amounts of such compounds are challenging because of their poor ionization properties. We present a method for chemical derivatization of primary amines by reaction with pyrylium salts that facilitates their detection by MALDI-MS and enables the imaging of primary amines in brain tissue sections. A screen of pyrylium salts revealed that the 2,4-diphenyl-pyranylium ion efficiently derivatizes primary amines and can be used as a reactive MALDI-MS matrix that induces both derivatization and desorption. MALDI-MS imaging with such matrix was used to map the localization of dopamine and amphetamine in brain tissue sections and to quantitatively map the distribution of the neurotoxin β-N-methylamino-L-alanine.

Acrylamide increases dopamine levels by affecting dopamine transport and metabolism related genes in the striatal dopaminergic system

Dopaminergic system dysfunction is proved to be a possible mechanism in acrylamide (ACR) -induced neurotoxicity. The neurotransmitter dopamine (DA) has an increasingly important role in the dopaminergic system. Thus, the goal of this study is to evaluate effects of ACR on dopamine and its metabolite levels, dopamine transport and metabolic gene expression in dopaminergic neurons. Male Sprague-Dawley (SD) rats were dosed orally with ACR at 0 (saline), 20, 30, and 40 mg/kg/day for 20 days. Splayed hind limbs, reduced tail flick time and abnormal gait which preceded other neurologic parameters were observed in the above rats. ACR significantly increased dopamine levels, decreased 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA) contents in an area dependent manner in rat striatum. Immunohistochemical staining of the striatum revealed that the number of tyrosine hydroxylase (TH) positive cells significantly increased, while monoamine oxidase (MAO) positive cells were drastically reduced, which was consistent with changes in their mRNA and protein expressions. In addition, dopamine transporter (DAT) and vesicular monoamine transporter 2 (VMAT2) expression levels were both down-regulated in the striatum. These results suggest that dopamine levels increase significantly in response to ACR, presumably due to changes in the dopamine transport and metabolism related genes expression in the striatal dopaminergic neurons.

Intervention with exercise restores motor deficits but not nigrostriatal loss in a progressive MPTP mouse model of Parkinson's disease

Many studies have investigated exercise therapy in Parkinson's disease (PD) and have shown benefits in improving motor deficits. However, exercise does not slow down the progression of the disease or induce the revival of lost nigrostriatal neurons. To examine the dichotomy of behavioral improvement without the slowing or recovery of dopaminergic cell or terminal loss, we tested exercise therapy in an intervention paradigm where voluntary running wheels were installed half-way through our progressive PD mouse model. In our model, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is administered over 4 weeks with increased doses each week (8, 16, 24, 32-kg/mg). We found that after 4 weeks of MPTP treatment, mice that volunteered to exercise had behavioral recovery in several measures despite the loss of 73% and 53% tyrosine hydroxylase (TH) within the dorsolateral (DL) striatum and the substantia nigra (SN), respectively which was equivalent to the loss seen in the mice that did not exercise but were also administered MPTP for 4 weeks. Mice treated with 4 weeks of MPTP showed a 41% loss of vesicular monoamine transporter II (VMAT2), a 71% increase in the ratio of glycosylated/non-glycosylated dopamine transporter (DAT), and significant increases in glutamate transporters including VGLUT1, GLT-1, and excitatory amino acid carrier 1. MPTP mice that exercised showed recovery of all these biomarkers back to the levels seen in the vehicle group and showed less inflammation compared to the mice treated with MPTP for 4 weeks. Even though we did not measure tissue dopamine (DA) concentration, our data suggest that exercise does not alleviate motor deficits by sparing nigrostriatal neurons, but perhaps by stabilizing the extraneuronal neurotransmitters, as evident by a recovery of DA and glutamate transporters. However, suppressing inflammation could be another mechanism of this locomotor recovery. Although exercise will not be a successful treatment alone, it could supplement other pharmaceutical approaches to PD therapy.

[Functional properties of dopaminergic neurons obtained from fibroblasts of a patient with PARK2 form of Parkinson's disease]

OBJECTIVE

To carry out a functional analysis of dopamine transporter (DAT) in autosomal recessive Parkinson's disease caused by mutations in the PARK2 gene.

MATERIAL AND METHODS

Cultures of dopaminergic neurons were obtained from fibroblasts of a patient with PARK2 form of Parkinson's disease and a healthy donor with the use of the cell reprogramming technology. DAT expression in both cell cultures was assessed at the RNA and protein levels, and DAT activity was tested with the use of the fluorescent dopamine analogue ASP+.

RESULTS AND CONCLUSION

In the cells with PARK2 mutations, the level of DAT expression was significantly higher than in normal neurons, but the intensity of ASP+ capture by mutant dopaminergic neurons was 25% down from normal neurons. For the study of competitive inhibition of DAT, dopamine was added to the incubation medium containing ASP+: it was shown that dopamine binding by the normal cells was almost twice as much relative to PARK2 mutant neurons. Therefore, dopaminergic neurons carrying mutations in the PARK2 gene are characterized by functional failure of dopamine transport systems. One of cell mechanisms of compensation of this defect seems to be an early increase of expression of the DAT transporter protein.

Presynaptic Mechanisms of l-DOPA-Induced Dyskinesia: The Findings, the Debate, and the Therapeutic Implications

The dopamine (DA) precursor l-DOPA has been the most effective treatment for Parkinson’s disease (PD) for over 40 years. However, the response to this treatment changes with disease progression, and most patients develop dyskinesias (abnormal involuntary movements) and motor fluctuations within a few years of l-DOPA therapy. There is wide consensus that these motor complications depend on both pre- and post-synaptic disturbances of nigrostriatal DA transmission. Several presynaptic mechanisms converge to generate large DA swings in the brain concomitant with the peaks-and-troughs of plasma l-DOPA levels, while post-synaptic changes engender abnormal functional responses in dopaminoceptive neurons. While this general picture is well-accepted, the relative contribution of different factors remains a matter of debate. A particularly animated debate has been growing around putative players on the presynaptic side of the cascade. To what extent do presynaptic disturbances in DA transmission depend on deficiency/dysfunction of the DA transporter, aberrant release of DA from serotonin neurons, or gliovascular mechanisms? And does noradrenaline (which is synthetized from DA) play a role? This review article will summarize key findings, controversies, and pending questions regarding the presynaptic mechanisms of l-DOPA-induced dyskinesia. Intriguingly, the debate around these mechanisms has spurred research into previously unexplored facets of brain plasticity that have far-reaching implications to the treatment of neuropsychiatric disease.

Norepinephrine transporter inhibition with desipramine exacerbates L-DOPA-induced dyskinesia: role for synaptic dopamine regulation in denervated nigrostriatal terminals

Pharmacological dopamine (DA) replacement with Levodopa [L-dihydroxyphenylalanine (L-DOPA)] is the gold standard treatment of Parkinson's disease (PD). However, long-term L-DOPA treatment is complicated by eventual debilitating abnormal involuntary movements termed L-DOPA-induced dyskinesia (LID), a clinically significant obstacle for the majority of patients who rely on L-DOPA to alleviate PD-related motor symptoms. The manifestation of LID may in part be driven by excessive extracellular DA derived from L-DOPA, but potential involvement of DA reuptake in LID severity or expression is unknown. We recently reported that in 6-hydroxydopamine (6-OHDA)-lesioned striatum, norepinephrine transporter (NET) expression increases and may play a significant role in DA transport. Furthermore, L-DOPA preferentially inhibits DA uptake in lesioned striatum. Therefore, we hypothesized that desipramine (DMI), a NET antagonist, could affect the severity of LID in an established LID model. Whereas DMI alone elicited no dyskinetic effects in lesioned rats, DMI + L-DOPA-treated rats gradually expressed more severe dyskinesia compared with L-DOPA alone over time. At the conclusion of the study, we observed reduced NET expression and norepinephrine-mediated inhibition of DA uptake in the DMI + L-DOPA group compared with L-DOPA-alone group in lesioned striatum. LID severity positively correlated with striatal extracellular signal-regulated protein kinase phosphorylation among the three treatment groups, with increased ppERK1/2 in DMI + L-DOPA group compared with the L-DOPA- and DMI-alone groups. Taken together, these results indicate that the combination of chronic L-DOPA and NET-mediated DA reuptake in lesioned nigrostriatal terminals may have a role in LID severity in experimental Parkinsonism.

Ceftriaxone increases glutamate uptake and reduces striatal tyrosine hydroxylase loss in 6-OHDA Parkinson's model

Excess glutamatergic neurotransmission may contribute to excitotoxic loss of nigrostriatal neurons in Parkinson's disease (PD). Here, we determined if increasing glutamate uptake could reduce the extent of tyrosine hydroxylase (TH) loss in PD progression. The beta-lactam antibiotic, ceftriaxone, increases the expression of glutamate transporter 1 (GLT-1), a glutamate transporter that plays a major role in glutamate clearance in central nervous system and may attenuate adverse behavioral or neurobiological function in other neurodegenerative disease models. In association with >80% TH loss, we observed a significant decrease in glutamate uptake in the established 6-hydroxydopamine (6-OHDA) PD model. Ceftriaxone (200 mg/kg, i.p.) increased striatal glutamate uptake with >5 consecutive days of injection in nonlesioned rats and lasted out to 14 days postinjection, a time beyond that required for 6-OHDA to produce >70% TH loss (∼9 days). When ceftriaxone was given at the time of 6-OHDA, TH loss was ∼57% compared to ∼85% in temporally matched vehicle-injected controls and amphetamine-induced rotation was reduced about 2-fold. This attenuation of TH loss was associated with increased glutamate uptake, increased GLT-1 expression, and reduced Serine 19 TH phosphorylation, a calcium-dependent target specific for nigrostriatal neurons. These results reveal that glutamate uptake can be targeted in a PD model, decrease the rate of TH loss in a calcium-dependent manner, and attenuate locomotor behavior associated with 6-OHDA lesion. Given that detection of reliable PD markers will eventually be employed in susceptible populations, our results give credence to the possibility that increasing glutamate uptake may prolong the time period before locomotor impairment occurs.

ser31 Tyrosine hydroxylase phosphorylation parallels differences in dopamine recovery in nigrostriatal pathway following 6-OHDA lesion

Compensatory mechanisms in dopamine (DA) signaling have long been proposed to delay onset of locomotor symptoms during Parkinson's disease progression until ~ 80% loss of striatal DA occurs. Increased striatal dopamine turnover has been proposed to be a part of this compensatory response, but may occur after locomotor symptoms. Increased tyrosine hydroxylase (TH) activity has also been proposed as a mechanism, but the impact of TH protein loss upon site-specific TH phosphorylation in conjunction with the impact on DA tissue content is not known. The tissue content of DA was determined against TH protein loss in the striatum and substantia nigra (SN) following 6-hydroxydopamine lesion in the medial forebrain bundle in young Sprague-Dawley male rats. Although DA predictably decreased in both regions following 6-hydroxydopamine, there was a significant difference in DA loss between the striatum (75%) and SN (40%), despite similar TH protein loss. Paradoxically, there was a significant decrease in DA against remaining TH protein in striatum, but a significant increase in DA against remaining TH in SN. In the SN, increased DA per remaining TH protein was matched by increased ser31, but not ser40, TH phosphorylation. In striatum, both ser31 and ser40 phosphorylation decreased, reflecting decreased DA per TH. However, in control nigral and striatal tissue, only ser31 phosphorylation correlated with DA per TH protein. Combined, these results suggest that the phosphorylation of ser31 in the SN may be a mechanism to increase DA biosynthesis against TH protein loss in an in vivo model of Parkinson's disease. Properties of dopamine biosynthesis were evaluated in the 6-OHDA model of Parkinson's disease by studying the impact of tyrosine hydroxylase (TH) protein loss on its own phosphorylation and dopamine (DA) tissue content in rat nigrostriatal pathway. A dichotomous response was observed between striatum and substantia nigra in that dopamine per remaining TH decreased in striatum, but increased in substantia nigra. Phosphorylation at ser31 reflected these differences, indicating that ser31 phosphorylation may be critical to maintain dopamine with progressive TH protein loss. Drawings are from slides purchased from Motifolio (http://motifolio.com/).

Pre- and postsynaptic dopamine SPECT in idiopathic Parkinsonian diseases: a follow-up study

We prospectively evaluated the diagnostic contribution of (123)I-FP-Cit (DAT) and (123)I-IBZM (IBZM) SPECT in 29 patients with Parkinson's disease (PD) (74.4 ± 4.2 years) and 28 patients with atypical parkinsonian diseases (APD) (74.3 ± 9.2 years). Twelve had multiple system atrophy (MSA) and 16 progressive supranuclear palsy (PSP). Sixteen age-matched healthy controls (HC) were included. DAT and IBZM SPECTs were made at baseline and after 1 year in all PD patients and in 20 (DAT) and 18 (IBZM) of the APD patients, and after 3 years in 22 (DAT) and 17 (IBZM) of the PD patients and in 10 (DAT) and 10 (IBZM) of the APD patients. The relative DAT uptake decrease was faster in PD and PSP than in HC and MSA. In PSP the DAT uptake was lower than in MSA after 1 year but not after 3 years. Baseline IBZM uptake was not significantly different between patients and HC or between PD and APD. One year after initiated dopaminergic treatment the mean IBZM uptake in the MSA patients remained high compared to PSP and after 3 years compared to PD, PSP, and HC. Thus, the pattern of uptake of these ligands over time may be of value in discriminating between these diagnoses.

Multisite intracerebral microdialysis to study the mechanism of L-DOPA induced dopamine and serotonin release in the parkinsonian brain

L-DOPA is currently one of the best medications for Parkinson's disease. It was assumed for several years that its benefits and side effects were related to the enhancement of dopamine release in the dopamine-depleted striatum. The use of intracerebral microdialysis combined with a pharmacological approach has led to the discovery that serotonergic neurons are responsible for dopamine release induced by L-DOPA. The subsequent use of multisite microdialysis has further revealed that L-DOPA-stimulated dopamine release is widespread and related to the serotonergic innervation. The present Review emphasizes the functional impact of extrastriatal release of dopamine induced by L-DOPA in both the therapeutic and side effects of L-DOPA.