Glutamate receptors and Parkinson's disease: opportunities for intervention

Glutamatergic dysfunction in neurodegenerative diseases focusing on Parkinson's disease: Role of glutamate modulators

Parkinson's disease (PD) is a prevalent neurodegenerative disorder resulting from the degeneration of dopamenergic neurons in the substantia nigra pars compacta (SNpc). Research has predominantly centered on understanding the dysfunction of dopaminergic neurotransmission in PD. Recently, more studies discussed the potential role of other neurotransmitters in PD neuropathology. One of the most important non-dopaminergic neurotransmitters involved in the pathogenesis of PD is glutamate, which is widely involved in glutamatergic neurotransmission in different brain regions, including SNpc. The development and progression of PD neuropathology and levodopa-induced dyskinesias (LID) are associated with glutamate neurotoxicity. Therefore, this review seeks to explore the possible involvement of glutamatergic signaling in PD development and assess the therapeutic potential of glutamate receptor antagonists in treating the disorder.

Automated radiosynthesis of mGluR5 PET tracer [18F]FPEB from aryl-chloro precursor and validation for clinical application

The radioligand [18F]FPEB, used for PET imaging of the brain's metabotropic glutamate receptor subtype 5 (mGluR5), undergoes a thorough validation process to ensure its safety, efficacy, and quality for clinical use. The process starts by optimizing the synthesis of [18F]FPEB to achieve high radiochemical yield and purity. This study focuses on optimizing the radiolabeling process using an aryl-chloro precursor and validating the GMP production for clinical applications. Fully automated radiolabeling was achieved via one-step nucleophilic substitution reaction. [18F]FPEB was produced and isolated in high radioactivity and radiochemical purity. Throughout the validation process, thorough quality control measures are implemented. Radiopharmaceutical batch release criteria are established, including testing for physical appearance, filter integrity, pH, radiochemical purity, molar activity, radiochemical identity, chemical impurity, structural identity, stability, residual solvent, sterility, and endotoxin levels. In conclusion, the validation of [18F]FPEB involved a comprehensive process of synthesis optimization, quality control, which ensure the safety, efficacy, and quality of [18F]FPEB, enabling its reliable use in clinical PET. Here, we successfully radiolabeled and validated [18F]FPEB using aryl-chloro precursor according to GMP production for clinical application.

Translational molecular imaging and drug development in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder that primarily affects elderly people and constitutes a major source of disability worldwide. Notably, the neuropathological hallmarks of PD include nigrostriatal loss and the formation of intracellular inclusion bodies containing misfolded α-synuclein protein aggregates. Cardinal motor symptoms, which include tremor, rigidity and bradykinesia, can effectively be managed with dopaminergic therapy for years following symptom onset. Nonetheless, patients ultimately develop symptoms that no longer fully respond to dopaminergic treatment. Attempts to discover disease-modifying agents have increasingly been supported by translational molecular imaging concepts, targeting the most prominent pathological hallmark of PD, α-synuclein accumulation, as well as other molecular pathways that contribute to the pathophysiology of PD. Indeed, molecular imaging modalities such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) can be leveraged to study parkinsonism not only in animal models but also in living patients. For instance, mitochondrial dysfunction can be assessed with probes that target the mitochondrial complex I (MC-I), while nigrostriatal degeneration is typically evaluated with probes designed to non-invasively quantify dopaminergic nerve loss. In addition to dopaminergic imaging, serotonin transporter and N-methyl-D-aspartate (NMDA) receptor probes are increasingly used as research tools to better understand the complexity of neurotransmitter dysregulation in PD. Non-invasive quantification of neuroinflammatory processes is mainly conducted by targeting the translocator protein 18 kDa (TSPO) on activated microglia using established imaging agents. Despite the overwhelming involvement of the brain and brainstem, the pathophysiology of PD is not restricted to the central nervous system (CNS). In fact, PD also affects various peripheral organs such as the heart and gastrointestinal tract - primarily via autonomic dysfunction. As such, research into peripheral biomarkers has taken advantage of cardiac autonomic denervation in PD, allowing the differential diagnosis between PD and multiple system atrophy with probes that visualize sympathetic nerve terminals in the myocardium. Further, α-synuclein has recently gained attention as a potential peripheral biomarker in PD. This review discusses breakthrough discoveries that have led to the contemporary molecular concepts of PD pathophysiology and how they can be harnessed to develop effective imaging probes and therapeutic agents. Further, we will shed light on potential future trends, thereby focusing on potential novel diagnostic tracers and disease-modifying therapeutic interventions.

Angiotensin II increases the firing activity of pallidal neurons and participates in motor control in rats

The globus pallidus has emerged as a crucial node in the basal ganglia motor control circuit under both healthy and parkinsonian states. Previous studies have shown that angiotensin II (Ang II) and angiotensin subtype 1 receptor (AT1R) are closely related to Parkinson's disease (PD). Recent morphological study revealed the expression of AT1R in the globus pallidus of mice. To investigate the functions of Ang II/AT1R on the globus pallidus neurons of both normal and parkinsonian rats, electrophysiological recordings and behavioral tests were performed in the present study. Electrophysiological recordings showed that exogenous and endogenous Ang II mainly excited the globus pallidus neurons through AT1R. Behavioral tests further demonstrated that unilateral microinjection of Ang II into the globus pallidus induced significantly contralateral-biased swing in elevated body swing test (EBST), and bilateral microinjection of Ang II into the globus pallidus alleviated catalepsy and akinesia caused by haloperidol. AT1R was involved in Ang II-induced behavioral effects. Immunostaining showed that AT1R was expressed in the globus pallidus of rats. On the basis of the present findings, we concluded that pallidal Ang II/AT1R alleviated parkinsonian motor deficits through activating globus pallidus neurons, which will provide a rationale for further investigations into the potential of Ang II in the treatment of motor disorders originating from the basal ganglia.

Galvanic vestibular stimulation down-regulated NMDA receptors in vestibular nucleus of PD model

Parkinsonian symptoms relief by electrical stimulation is constructed by modulating neural network activity, and Galvanic vestibular stimulation (GVS) is known to affect the neural activity for motor control by activating the vestibular afferents. However, its underlying mechanism is still elusive. Due to the tight link from the peripheral vestibular organ to vestibular nucleus (VN), the effect by GVS was investigated to understand the neural mechanism. Using Sprague Dawley (SD) rats, behavioral response, extracellular neural recording, and immunohistochemistry in VN were conducted before and after the construction of Parkinson's disease (PD) model. Animals' locomotion was tested using rota-rod, and single extracellular neuronal activity was recorded in VN. The immunohistochemistry detected AMPA and NMDA receptors in VN to assess the effects by different amounts of electrical charge (0.018, 0.09, and 0.18 coulombs) as well as normal and PD with no GVS. All PD models showed the motor impairment, and the loss of TH⁺ neurons in medial forebrain bundle (mfb) and striatum was observed. Sixty-five neuronal extracellular activities (32 canal & 33 otolith) were recorded, but no significant difference in the resting firing rates and the kinetic responding gain were found in the PD models. On the other hand, the numbers of AMPA and NMDA receptors increased after the construction of PD model, and the effect by GVS was significantly evident in the change of NMDA receptors (p < 0.018). In conclusion, the increased glutamate receptors in PD models were down-regulated by GVS, and the plastic modulation mainly occurred through NMDA receptor in VN.

SNPeffect: identifying functional roles of SNPs using metabolic networks

Genetic sources of phenotypic variation have been a focus of plant studies aimed at improving agricultural yield and understanding adaptive processes. Genome-wide association studies identify the genetic background behind a trait by examining associations between phenotypes and single-nucleotide polymorphisms (SNPs). Although such studies are common, biological interpretation of the results remains a challenge; especially due to the confounding nature of population structure and the systematic biases thus introduced. Here, we propose a complementary analysis (SNPeffect) that offers putative genotype-to-phenotype mechanistic interpretations by integrating biochemical knowledge encoded in metabolic models. SNPeffect is used to explain differential growth rate and metabolite accumulation in A. thaliana and P. trichocarpa accessions as the outcome of SNPs in enzyme-coding genes. To this end, we also constructed a genome-scale metabolic model for Populus trichocarpa, the first for a perennial woody tree. As expected, our results indicate that growth is a complex polygenic trait governed by carbon and energy partitioning. The predicted set of functional SNPs in both species are associated with experimentally characterized growth-determining genes and also suggest putative ones. Functional SNPs were found in pathways such as amino acid metabolism, nucleotide biosynthesis, and cellulose and lignin biosynthesis, in line with breeding strategies that target pathways governing carbon and energy partition.

Molecular docking studies of N-substituted 4-methoxy-6-oxo-1-aryl-pyridazine-3-carboxamide derivatives as potential modulators of glutamate receptors

Introduction: The virtual target-oriented screening is a necessary stage of modern drug-design. In the present study, the affinity of pyridazine derivatives for the most promising antiparkinsonian biotargets – I–III groups of metabotropic and ionotropic NMDA-glutamate receptors – was evaluated.

Materials and methods: Docking of the studied ligands to the active sites of biotargets – mGluR5, mGluR3, mGluR8, NMDA GluN2B – was performed using AutoDockVina. Base of the preparation of ligands and proteins – AutoDockTools-1.5.6. A Discovery Studio Visualizer 2017/R2 was used to visualize the interpretation of the results.

Results and discussion: A high degree of the affinity is predicted for group III of the metabotropic mGlu8 receptors – binding energy from -5.0 to -8.7 kcal/mol, compared to -6.1 kcal/mol of that of the reference drug (L-AP4), as well as for the ionotropic NMDA GluN2B receptors –binding energy from -8.7 to -11.6 kcal/mol, compared to -11.3 kcal/mol of that of ifenprodil.

Conclusion: The prospects of the searching for glutamate receptor modulators in a number of n-substituted 4-methoxy-6-oxo-1-aryl-pyridazine-3-carboxamide derivatives are proved. Some aspects of the structure-affinity relationship are discussed.

Revisiting the Radiosynthesis of [18F]FPEB and Preliminary PET Imaging in a Mouse Model of Alzheimer's Disease

[¹⁸F]FPEB is a positron emission tomography (PET) radiopharmaceutical used for imaging the abundance and distribution of mGluR5 in the central nervous system (CNS). Efficient radiolabeling of the aromatic ring of [¹⁸F]FPEB has been an ongoing challenge. Herein, five metal-free precursors for the radiofluorination of [¹⁸F]FPEB were compared, namely, a chloro-, nitro-, sulfonium salt, and two spirocyclic iodonium ylide (SCIDY) precursors bearing a cyclopentyl (SPI5) and a new adamantyl (SPIAd) auxiliary. The chloro- and nitro-precursors resulted in a low radiochemical yield (<10% RCY), whereas both SCIDY precursors and the sulfonium salt precursor produced [¹⁸F]FPEB in the highest RCYs of 25% and 36%, respectively. Preliminary PET/CT imaging studies with [¹⁸F]FPEB were conducted in a transgenic model of Alzheimer’s Disease (AD) using B6C3-Tg(APPswe,PSEN1dE9)85Dbo/J (APP/PS1) mice, and data were compared with age-matched wild-type (WT) B6C3F1/J control mice. In APP/PS1 mice, whole brain distribution at 5 min post-injection showed a slightly higher uptake (SUV = 4.8 ± 0.4) than in age-matched controls (SUV = 4.0 ± 0.2). Further studies to explore mGluR5 as an early biomarker for AD are underway.

The levels of the NMDA receptor co-agonist D-serine are reduced in the substantia nigra of MPTP-lesioned macaques and in the cerebrospinal fluid of Parkinson's disease patients

Dysfunction of NMDA receptor (NMDAR)-mediated transmission is supposed to contribute to the motor and non-motor symptoms of Parkinson’s Disease (PD), and to L-DOPA-induced dyskinesia. Besides the main agonist L-glutamate, two other amino acids in the atypical D-configuration, D-serine and D-aspartate, activate NMDARs. In the present work, we investigated the effect of dopamine depletion on D-amino acids metabolism in the brain of MPTP-lesioned Macaca mulatta, and in the serum and cerebrospinal fluid of PD patients. We found that MPTP treatment increases D-aspartate and D-serine in the monkey putamen while L-DOPA rescues both D-amino acids levels. Conversely, dopaminergic denervation is associated with selective D-serine reduction in the substantia nigra. Such decrease suggests that the beneficial effect of D-serine adjuvant therapy previously reported in PD patients may derive from the normalization of endogenous D-serine levels and consequent improvement of nigrostriatal hypoglutamatergic transmission at glycine binding site. We also found reduced D-serine concentration in the cerebrospinal fluid of L-DOPA-free PD patients. These results further confirm the existence of deep interaction between dopaminergic and glutamatergic neurotransmission in PD and disclose a possible direct influence of D-amino acids variations in the changes of NMDAR transmission occurring under dopamine denervation and L-DOPA therapy.

Electrical stimulation or MK-801 in the inferior colliculus improve motor deficits in MPTP-treated mice

The inferior colliculus (IC) is an important midbrain relay station for the integration of descending and ascending auditory information. Additionally, the IC has been implicated in processing sensorimotor responses. Glutamatergic and GABAergic manipulations in the IC can improve motor deficits as demonstrated by the animal model of haloperidol-induced catalepsy. However, how the IC influences motor function remains unclear. We investigated the effects of either intracollicular deep brain stimulation (DBS) or microinjection of the glutamatergic antagonist MK-801 or the agonist NMDA in C57BL/6J mice chronically treated with saline or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). After DBS or microinjections, the mice were submitted to rotarod and open field tests, respectively. DBS in the IC was effective to increase the time spent on the rotarod in MPTP-treated mice. After unilateral microinjection of MK-801, but not NMDA, MPTP-treated mice increased the distance travelled in the open field (p < 0.05). In conclusion, intracollicular DBS or MK-801 microinjection can improve motor performance in parkinsonian mice suggesting the IC as a new and non-conventional therapeutic target in motor impairment.

Automated cGMP-compliant radiosynthesis of [18 F]-(E)-PSS232 for brain PET imaging of metabotropic glutamate receptor subtype 5

(E)-3-(Pyridin-2-yl ethynyl)cyclohex-2-enone O-(3-(2-[¹⁸ F]-fluoroethoxy)propyl) oxime ([¹⁸ F]-(E)-PSS232, [¹⁸ F]2a) is a recently developed radiotracer that can be used to visualize metabotropic glutamate receptor subtype 5 (mGlu₅ ) in vivo. The mGlu₅ has become an attractive therapeutic and diagnostic target owing to its role in many neuropsychiatric disorders. Several carbon-11-labeled and fluorine-18-labeled radiotracers have been developed to measure mGlu₅ receptor occupancy in the human brain. The radiotracer [¹⁸ F]2a, which is used as an analogue for [¹¹ C]ABP688 ([¹¹ C]1) and has a longer physical half-life, is a selective radiotracer that exhibits high binding affinity for mGlu₅ . Herein, we report the fully automated radiosynthesis of [¹⁸ F]2a using a commercial GE TRACERlab™ FX-_FN synthesizer for routine production and distribution to nearby satellite clinics. Nucleophilic substitution of the corresponding mesylate precursor with cyclotron-produced [¹⁸ F]fluoride ion at 100°C in dimethyl sulfoxide (DMSO), followed by high-performance liquid chromatography (HPLC) purification and formulation, readily provided [¹⁸ F]2a with a radiochemical yield of 40 ± 2% (decay corrected, n = 5) at the end of synthesis. Radiochemical purity for the [¹⁸ F]-(E)-conformer was greater than 95%. Molar activity was determined to be 63.6 ± 9.6 GBq/μmol (n = 5), and the overall synthesis time was 70 minutes.

Ceftriaxone reverses deficits of behavior and neurogenesis in an MPTP-induced rat model of Parkinson's disease dementia

Hyperactivity of the glutamatergic system is involved in excitotoxicity and neurodegeneration in Parkinson's disease (PD) so that glutamatergic modulation maybe a potential therapeutic target for PD. Ceftriaxone (CEF) has been reported to increase glutamate uptake by increasing glutamate transporter expression and has been demonstrated neuroprotective effects in animal study. The aim of this study was to determine the effects of CEF on behavior and neurogenesis in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD rat model. MPTP was stereotaxically injected into the substantia nigra pars compacta (SNc) of male Wistar rats. Starting on the same day after MPTP lesioning (day 0), the rats were injected daily with either CEF or saline for 14days and underwent a T-maze test on days 8-10 and an object recognition test on days 12-14, then the brain was taken for histological evaluation on day 15. The results showed that MPTP lesioning resulted in decreased motor function, working memory, and object recognition and reduced neurogenesis in the substantial nigra and dentate gyrus of the hippocampus. These behavioral and neuronal changes were prevented by CEF treatment. To our knowledge, this is the first study showing that CEF prevents loss of neurogenesis in the brain of PD rats. CEF may therefore have clinical potential in the treatment of PD.

Negative Allosteric Modulators of Metabotropic Glutamate Receptors Subtype 5 in Addiction: a Therapeutic Window

BACKGROUND

Abundant evidence at the anatomical, electrophysiological, and molecular levels implicates metabotropic glutamate receptor subtype 5 (mGluR5) in addiction. Consistently, the effects of a wide range of doses of different mGluR5 negative allosteric modulators (NAMs) have been tested in various animal models of addiction. Here, these studies were subjected to a systematic review to find out if mGluR5 NAMs have a therapeutic potential that can be translated to the clinic.

METHODS

Literature on consumption/self-administration and reinstatement of drug seeking as outcomes of interest published up to April 2015 was retrieved via PubMed. The review focused on the effects of systemic (i.p., i.v., s.c.) administration of the mGluR5 NAMs 3-((2-Methyl-4-thiazolyl)ethynyl)pyridine (MTEP) and 2-Methyl-6-(phenylethynyl)pyridine (MPEP) on paradigms with cocaine, ethanol, nicotine, and food in rats.

RESULTS

MTEP and MPEP were found to reduce self-administration of cocaine, ethanol, and nicotine at doses ≥1mg/kg and 2.5mg/kg, respectively. Dose-response relationship resembled a sigmoidal curve, with low doses not reaching statistical significance and high doses reliably inhibiting self-administration of drugs of abuse. Importantly, self-administration of cocaine, ethanol, and nicotine, but not food, was reduced by MTEP and MPEP in the dose range of 1 to 2mg/kg and 2.5 to 3.2mg/kg, respectively. This dose range corresponds to approximately 50% to 80% mGluR5 occupancy. Interestingly, the limited data found in mice and monkeys showed a similar therapeutic window.

CONCLUSION

Altogether, this review suggests a therapeutic window for mGluR5 NAMs that can be translated to the treatment of substance-related and addictive disorders.

Synthesis and evaluation of N-(methylthiophenyl)picolinamide derivatives as PET radioligands for metabotropic glutamate receptor subtype 4

In recent years, mGlu4 has received great research attention because of the potential benefits of mGlu4 activation in treating numerous brain disorders, such as Parkinson's disease (PD). A specific mGlu4 PET radioligand could be an important tool in understanding the role of mGlu4 in both healthy and disease conditions, and also for the development of new drugs. In this study, we synthesized four new N-(methylthiophenyl)picolinamide derivatives 11-14. Of these ligands, 11 and 14 showed high in vitro binding affinity for mGlu4 with IC50 values of 3.4nM and 3.1nM, respectively, and suitable physicochemical parameters. Compound 11 also showed enhanced metabolic stability and good selectivity to other mGluRs. [(11)C]11 and [(11)C]14 were radiolabeled using the [(11)C]methylation of the thiophenol precursors 20a and 20c with [(11)C]CH3I in 19.0% and 34.8% radiochemical yields (RCY), and their specific activities at the end of synthesis (EOS) were 496±138GBq/μmol (n=6) and 463±263GBq/μmol (n=4), respectively. The PET studies showed that [(11)C]11 accumulated fast into the brain and had higher uptake, slower washout and 25% better contrast than [(11)C]2, indicating improved imaging characteristics as PET radiotracer for mGlu4 compared to [(11)C]2. Therefore, [(11)C]11 will be a useful radioligand to investigate mGlu4 in different biological applications.

Structural covariance networks of striatum subdivision in patients with Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder associated with the striatum. Previous studies indicated that subdivisions of the striatum with distinct functional connectivity profiles contribute to different pathogeneses in PD. Segregated structural covariance (SC) pattern between the striatum and neocortex observed in healthy subjects, however, remain unknown in PD. The purpose of this study is to map and compare the subregional striatal SC network organization between 30 healthy controls and 48 PD patients and to investigate their association with the disease severity. The striatal SC network was statistically inferred by correlating the mean gray matter (GM) volume of six striatal subdivisions (including the bilateral dorsal caudate, superior ventral striatum, inferior ventral striatum, dorsal caudal putamen, dorsal rostral putamen, and ventral rostral putamen) with the entire neocortical GM volume in voxel-wise manner. The PD patients revealed marked atrophy in the striatum, cerebellum, and extra-striatum neocortices. As predicted, segregated striatal SC network patterns were observed in both groups. This suggests that in PD, pathological processes occurring in the striatum affect the same striato-cortical networks that covary with the striatum in healthy brains. The PD patients further demonstrated atypical striatal SC patterns between the caudate, parahippocampus temporal cortices, and cerebellum, which corresponded to dopaminergic associated network. The areas with significant group differences in SC were further associated with disease severity. Our findings support previous studies indicating that PD is associated with altered striato-cortical networks, and suggest that structural changes in the striatum may result in a cascade of alterations in multiple neocortices.

Unprecedented therapeutic potential with a combination of A2A/NR2B receptor antagonists as observed in the 6-OHDA lesioned rat model of Parkinson's disease

In Parkinson's disease, the long-term use of dopamine replacing agents is associated with the development of motor complications; therefore, there is a need for non-dopaminergic drugs. This study evaluated the potential therapeutic impact of six different NR2B and A2A receptor antagonists given either alone or in combination in unilateral 6-OHDA-lesioned rats without (monotherapy) or with (add-on therapy) the co-administration of L-Dopa: Sch-58261+ Merck 22; Sch-58261+Co-101244; Preladenant + Merck 22; Preladenant + Radiprodil; Tozadenant + Radiprodil; Istradefylline + Co-101244. Animals given monotherapy were assessed on distance traveled and rearing, whereas those given add-on therapy were assessed on contralateral rotations. Three-way mixed ANOVA were conducted to assess the main effect of each drug separately and to determine whether any interaction between two drugs was additive or synergistic. Additional post hoc analyses were conducted to compare the effect of the combination with the effect of the drugs alone. Motor activity improved significantly and was sustained for longer when the drugs were given in combination than when administered separately at the same dose. Similarly, when tested as add-on treatment to L-Dopa, the combinations resulted in higher levels of contralateral rotation in comparison to the single drugs. Of special interest, the activity observed with some combinations could not be described by a simplistic additive effect and involved more subtle synergistic pharmacological interactions. The combined administration of A2A/NR2B-receptor antagonists improved motor behaviour in 6-OHDA rats. Given the proven translatability of this model such a combination may be expected to be effective in improving motor symptoms in patients.

Ceftriaxone prevents and reverses behavioral and neuronal deficits in an MPTP-induced animal model of Parkinson's disease dementia

Glutamatergic hyperactivity plays an important role in the pathophysiology of Parkinson's disease (PD). Ceftriaxone increases expression of glutamate transporter 1 (GLT-1) and affords neuroprotection. This study was aimed at clarifying whether ceftriaxone prevented, or reversed, behavioral and neuronal deficits in an 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD rat model. Male Wistar rats were injected daily with either ceftriaxone starting 5 days before or 3 days after MPTP lesioning (day 0) or saline and underwent a bar-test on days 1-7, a T-maze test on days 9-11, and an object recognition test on days 12-14, then the brains were taken for histological evaluation on day 15. Dopaminergic degeneration in the substantia nigra pars compacta and striatum was observed on days 3 and 15. Motor dysfunction in the bar test was observed on day 1, but disappeared by day 7. In addition, lesioning resulted in deficits in working memory in the T-maze test and in object recognition in the object recognition task, but these were not observed in rats treated pre- or post-lesioning with ceftriaxone. Lesioning also caused neurodegeneration in the hippocampal CA1 area and induced glutamatergic hyperactivity in the subthalamic nucleus, and both changes were suppressed by ceftriaxone. Increased GLT-1 expression and its co-localization with astrocytes were observed in the striatum and hippocampus in the ceftriaxone-treated animals. To our knowledge, this is the first study showing a relationship between ceftriaxone-induced GLT-1 expression, neuroprotection, and improved cognition in a PD rat model. Ceftriaxone may have clinical potential for the prevention and treatment of dementia associated with PD.

Small-molecule inhibitors at the PSD-95/nNOS interface attenuate MPP+-induced neuronal injury through Sirt3 mediated inhibition of mitochondrial dysfunction

Post-synaptic density protein 95 (PSD-95) links neuronal nitric oxide synthase (nNOS) with the N-methyl-D-aspartic acid (NMDA) receptor in the central nervous system, and this molecular complex has been implicated in regulating neuronal excitability in several neurological disorders. Here, small-molecule inhibitors of the PSD-95/nNOS interaction, IC87201 and ZL006 were tested for neuroprotective effects in an in vitro Parkinson's disease (PD) model. We now report that IC87201 and ZL006 reduced MPP(+)-induced neuronal injury and apoptotic cell death in a dose-dependent manner in cultured cortical neurons. These protective effects were associated with suppressed mitochondrial dysfunction, as evidenced by decreased reactive oxygen species (ROS) generation, cytochrome c release, mitochondrial membrane potential (MMP) collapse, and the preserved mitochondrial complex I activity and ATP synthesis. IC87201 and ZL006 also preserved intracellular homeostasis through mitigating mitochondrial Ca(2+) uptake and promoting mitochondrial Ca(2+) buffering capacity. Moreover, treatment with IC87201 and ZL006 significantly increased the expression of Sirt3 after MPP(+) exposure, and knockdown of Sirt3 using specific targeted small interfere RNA (siRNA) partially nullified the protective effects induced by these two inhibitors. These data strongly support the hypothesis that targeting the PSD-95/nNOS interaction produces neuroprotective effects and may represent a novel class of therapeutics for PD as well as other neurological diseases where detrimental NMDA receptor signaling plays a major role.

Therapeutic potential of metabotropic glutamate receptor 4-positive allosteric modulator TAS-4 in rodent models of movement disorders

Parkinson's disease (PD) is a very serious neurological disorder, and current methods of treatment fail to achieve long-term control. Previous studies suggest that stimulation of the metabotropic glutamate receptor 4 (mGluR4) represents a promising new approach to the symptomatic treatment of Parkinson's disease (PD). Preclinical models using both agonists and positive allosteric modulators of mGluR4 have demonstrated the potential for this receptor for the treatment of PD. In the present study, we describe the pharmacological characterization of an mGluR4 PAM, N-(2, 4-dichlorophenyl) pyridine-2-carboxamide (TAS-4), in several rodent PD models. TAS-4 is a potent and selective mGluR4 PAM of the human mGluR4 receptor (EC50- 287.8nM). TAS-4 showed efficacy alone or when administered in combination with l-DOPA. When administered alone, TAS-4 exhibited efficacy in reversing haloperidol-induced catalepsy. In addition, acute TAS-4 dose-dependently potentiated contralateral turning behavior induced by a threshold dose of l-3,4-dihydroxyphenylalanine (l-DOPA, 4mg/kg i.p.), a classical test for antiparkinson drug screening. Subchronic (28days, twice a day) TAS-4 (10mg/kg i.p.)+l-DOPA (4mg/kg i.p.) did not induce sensitization to turning behavior or abnormal involuntary movements during the course of treatment. Moreover, subchronic administration of a fully effective dose of l-DOPA (8mg/kg i.p.) significantly induces sensitization to turning behavior or abnormal involuntary movements. Results showed that TAS-4, in association with a low dose of l-DOPA, displayed antiparkinsonian activity similar to that produced by a full dose of l-DOPA without exacerbating abnormal motor side effects.

Hippocampal neuronal cells that accumulate α-synuclein fragments are more vulnerable to Aβ oligomer toxicity via mGluR5--implications for dementia with Lewy bodies

BACKGROUND

In dementia with Lewy bodies (DLB) abnormal interactions between α-synuclein (α-syn) and beta amyloid (Aβ) result in selective degeneration of neurons in the neocortex, limbic system and striatum. However, factors rendering these neurons selectively vulnerable have not been fully investigated. The metabotropic glutamate receptor 5 (mGluR5) has been shown to be up regulated in DLB and might play a role as a mediator of the neurotoxic effects of Aβ and α-syn in vulnerable neuronal populations. In this context, the main objective of the present study was to investigate the role of mGluR5 as a mediator of the neurotoxic effects of α-syn and Aβ in the hippocampus.

RESULTS

We generated double transgenic mice over-expressing amyloid precursor protein (APP) and α-syn under the mThy1 cassette and investigated the relationship between α-syn cleavage, Aβ, mGluR5 and neurodegeneration in the hippocampus. We found that compared to the single tg mice, the α-syn/APP tg mice displayed greater accumulation of α-syn and mGluR5 in the CA3 region of the hippocampus compared to the CA1 and other regions. This was accompanied by loss of CA3 (but not CA1) neurons in the single and α-syn/APP tg mice and greater loss of MAP 2 and synaptophysin in the CA3 in the α-syn/APP tg. mGluR5 gene transfer using a lentiviral vector into the hippocampus CA1 region resulted in greater α-syn accumulation and neurodegeneration in the single and α-syn/APP tg mice. In contrast, silencing mGluR5 with a lenti-shRNA protected neurons in the CA3 region of tg mice. In vitro, greater toxicity was observed in primary hippocampal neuronal cultures treated with Aβ oligomers and over-expressing α-syn; this effect was attenuated by down-regulating mGluR5 with an shRNA lentiviral vector. In α-syn-expressing neuronal cells lines, Aβ oligomers promoted increased intracellular calcium levels, calpain activation and α-syn cleavage resulting in caspase-3-dependent cell death. Treatment with pharmacological mGluR5 inhibitors such as 2-Methyl-6-(phenylethynyl)pyridine (MPEP) and 3-((2-Methyl-4-thiazolyl)ethynyl)pyridine (MTEP) attenuated the toxic effects of Aβ in α-syn-expressing neuronal cells.

CONCLUSIONS

Together, these results support the possibility that vulnerability of hippocampal neurons to α-syn and Aβ might be mediated via mGluR5. Moreover, therapeutical interventions targeting mGluR5 might have a role in DLB.

Effects of ceftriaxone on the behavioral and neuronal changes in an MPTP-induced Parkinson's disease rat model

Hyperactivity of the glutamatergic system is involved in excitotoxicity and neurodegeneration in Parkinson's disease (PD) and treatment with drugs modulating glutamatergic activity may have beneficial effects. Ceftriaxone has been reported to increase glutamate uptake by increasing glutamate transporter expression. The aim of this study was to determine the effects of ceftriaxone on working memory, object recognition, and neurodegeneration in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD rat model. MPTP was stereotaxically injected into the substantia nigra pars compacta (SNc) of male Wistar rats. Then, starting the next day (day 1), the rats were injected daily with either ceftriaxone (200 mg/kg/day, i.p.) or saline for 14 days and underwent a T-maze test on days 8-10 and an object recognition test on days 12-14. MPTP-lesioned rats showed impairments of working memory in the T-maze test and of recognition function in the object recognition test. The treatment of ceftriaxone decreased the above MPTP-induced cognitive deficits. Furthermore, this study provides evidence that ceftriaxone inhibits MPTP lesion-induced dopaminergic degeneration in the nigrostriatal system, microglial activation in the SNc, and cell loss in the hippocampal CA1 area. In conclusion, these data support the idea that hyperactivity of the glutamatergic system is involved in the pathophysiology of PD and suggest that ceftriaxone may be a promising pharmacological tool for the development of new treatments for the dementia associated with PD.

Radiosynthesis of N-(4-chloro-3-[(11)C]methoxyphenyl)-2-picolinamide ([(11)C]ML128) as a PET radiotracer for metabotropic glutamate receptor subtype 4 (mGlu4)

N-(Chloro-3-methoxyphenyl)-2-picolinamide (3, ML128, VU0361737) is an mGlu4 positive allosteric modulator (PAM), which is potent and centrally penetrating. 3 is also the first mGlu4 PAM to show efficacy in a preclinical Parkinson disease model upon systemic dosing. As a noninvasive medical imaging technique and a powerful tool in neurological research, positron emission tomography (PET) offers a possibility to investigate mGlu4 expression in vivo under physiologic and pathological conditions. We synthesized a carbon-11 labeled ML128 ([(11)C]3) as a PET radiotracer for mGlu4, and characterized its biological properties in Sprague Dawley rats. [(11)C]3 was synthesized from N-(4-chloro-3-hydroxyphenyl)-2-picolinamide (2) using [(11)C]CH3I. Total synthesis time was 38±2.2min (n=7) from the end of bombardment to the formulation. The radioligand [(11)C]3 was obtained in 27.7±5.3% (n=5) decay corrected radiochemical yield based on the radioactivity of [(11)C]CO2. The radiochemical purity of [(11)C]3 was >99%. Specific activity was 188.7±88.8GBq/mol (n=4) at the end of synthesis (EOS). PET images were conducted in 20 normal male Sprague Dawley rats including 11 control studies, 6 studies blocking with an mGlu4 modulator (4) to investigate specificity and 3 studies blocking with an mGlu5 modulator (MTEP) to investigate selectivity. These studies showed fast accumulation of [(11)C]3 (peak activity between 1-3min) in several brain areas including striatum, thalamus, hippocampus, cerebellum, and olfactory bulb following with fast washout. Blocking studies with the mGlu4 modulator 4 showed 22-28% decrease of [(11)C]3 accumulation while studies of selectivity showed only minor decrease supporting good selectivity over mGlu5. Biodistribution studies and blood analyses support fast metabolism. Altogether this is the first PET imaging ligand for mGlu4, in which the labeled ML128 was used for imaging its in vivo distribution and pharmacokinetics in brain.

Glycine site agonists of the N-methyl-D-aspartate receptor and Parkinson's disease: a hypothesis

Limitations of current pharmacological approaches to Parkinson's disease (PD) highlight the need for the development of nondopaminergic therapeutic strategies. The potential role of glutamatergic neurotransmission modulators, including those active at the N-methyl-D-aspartate receptor (NMDAR), is presently under investigation. Most literature proposes the use of NMDAR antagonists based on neurodegenerative theories of NMDAR function. Nevertheless, NMDAR antagonism has proven disappointing in clinical trials and may be associated with serious adverse events. More recent theories indicate that NMDAR target selectivity may be a cardinal prerequisite for efficacy, with present efforts being devoted primarily to development of NMDAR-NR2B subunit antagonists. We propose a novel hypothesis according to which NMDAR stimulation, accomplished through allosteric modulation via the glycine modulatory site, may be beneficial in late-phase PD. This hypothesis stems from: (1) meta-analysis of randomized controlled trials performed in schizophrenia, indicating that glycine site agonists (eg, glycine, D-serine) alleviate antipsychotic-induced parkinsonian symptoms; (2) clinical observations indicating that NMDAR hypofunction is associated with motor disturbances; (3) results of a preliminary D-serine trial in PD; (4) data indicating glycine efficacy in a rat tardive dyskinesia model; and (5) no evidence of excitotoxic damage following chronic high-dose glycine nutritional supplementation. This hypothesis is discussed in the context of glycine site agonist effects on intrasynaptic NMDAR subunits and striatal synaptic plasticity.

Brain energy metabolism in glutamate-receptor activation and excitotoxicity: role for APC/C-Cdh1 in the balance glycolysis/pentose phosphate pathway

Recent advances in the field of brain energy metabolism strongly suggest that glutamate receptor-mediated neurotransmission is coupled with molecular signals that switch-on glucose utilization pathways to meet the high energetic requirements of neurons. Failure to adequately coordinate energy supply for neurotransmission ultimately results in a positive amplifying loop of receptor over-activation leading to neuronal death, a process known as excitotoxicity. In this review, we revisited current concepts in excitotoxic mechanisms, their involvement in energy substrate utilization, and the signaling pathways that coordinate both processes. In particular, we have focused on the novel role played by the E3 ubiquitin ligase, anaphase-promoting complex/cyclosome (APC/C)-Cdh1, in cell metabolism. Our laboratory identified 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase-3 (PFKFB3) -a key glycolytic-promoting enzyme- as an APC/C-Cdh1 substrate. Interestingly, APC/C-Cdh1 activity is inhibited by over-activation of glutamate receptors through a Ca(2+)-mediated mechanism. Furthermore, by inhibiting APC/C-Cdh1 activity, glutamate-receptors activation promotes PFKFB3 stabilization, leading to increased glycolysis and decreased pentose-phosphate pathway activity. This causes a loss in neuronal ability to regenerate glutathione, triggering oxidative stress and delayed excitotoxicity. Further investigation is critical to identify novel molecules responsible for the coupling of energy metabolism with glutamatergic neurotransmission and excitotoxicity, as well as to help developing new therapeutic strategies against neurodegeneration.

G protein-coupled receptor heterocomplexes in neuropsychiatric disorders

G protein-coupled receptors (or GPCRs) represent the largest family of membrane proteins in the human genome and are the target of approximately half of all therapeutic drugs. GPCRs contain a conserved structure of seven transmembrane domains. Their amino terminus is located extracellularly, whereas the carboxy terminus extends into the cytoplasm. Accumulating evidence suggests that GPCRs exist and function as monomeric entities. Nevertheless, more recent findings indicate that GPCRs can also form dimers or even higher order oligomers. The differential pharmacological and signaling properties of GPCR heteromeric complexes hint that their physiological effects may be different as compared to those obtained in tissue cultures that express a particular GPCR. In this chapter, we review current data on the role of GPCR heteromerization in receptor signaling, as well as its potential implication in neuropsychiatric disorders such as schizophrenia, depression, and Parkinson's disease.

Allosteric modulation of the group III mGlu4 receptor provides functional neuroprotection in the 6-hydroxydopamine rat model of Parkinson's disease

BACKGROUND AND PURPOSE

We recently reported that broad spectrum agonist-induced activation of presynaptic group III metabotropic glutamate (mGlu) receptors within the substantia nigra pars compacta using L-2-amino-4-phosphonobutyrate provided functional neuroprotection in the 6-hydroxydopamine lesion rat model of Parkinson's disease. The aim of this study was to establish whether selective activation of the mGlu(4) receptor alone could afford similar functional neuroprotection.

EXPERIMENTAL APPROACH

The neuroprotective effects of 8 days of supranigral treatment with a positive allosteric modulator of mGlu(4) receptors, (+/-)-cis-2-(3,5-dichlorphenylcarbamoyl)cyclohexanecarboxylic acid (VU0155041), were investigated in rats with unilateral 6-hydroxydopamine lesions. The effects of VU0155041 treatment on motor function were assessed using both habitual (cylinder test) and forced (adjusted stepping, amphetamine-induced rotations) behavioural tests. Nigrostriatal tract integrity was examined by analysis of tyrosine hydroxylase, dopa decarboxylase or dopamine levels in the striatum and tyrosine hydroxylase-positive cell counts in the substantia nigra pars compacta.

KEY RESULTS

VU0155041 provided around 40% histological protection against a unilateral 6-hydroxydopamine lesion as well as significant preservation of motor function. These effects were inhibited by pre-treatment with (RS)-α-cyclopropyl-4-phosphonophenylglycine, confirming a receptor-mediated response. Reduced levels of inflammatory markers were also evident in the brains of VU0155041-treated animals.

CONCLUSIONS AND IMPLICATIONS

Allosteric potentiation of mGlu(4) receptors in the substantia nigra pars compacta provided neuroprotective effects in the 6-hydroxydopamine rat model A reduced inflammatory response may contribute, in part, to this action. In addition to the reported symptomatic effects, activation of mGlu(4) receptors may also offer a novel approach for slowing the progressive degeneration observed in Parkinson's disease.

Animal foraging and the evolution of goal-directed cognition

Foraging- and feeding-related behaviors across eumetazoans share similar molecular mechanisms, suggesting the early evolution of an optimal foraging behavior called area-restricted search (ARS), involving mechanisms of dopamine and glutamate in the modulation of behavioral focus. Similar mechanisms in the vertebrate basal ganglia control motor behavior and cognition and reveal an evolutionary progression toward increasing internal connections between prefrontal cortex and striatum in moving from amphibian to primate. The basal ganglia in higher vertebrates show the ability to transfer dopaminergic activity from unconditioned stimuli to conditioned stimuli. The evolutionary role of dopamine in the modulation of goal-directed behavior and cognition is further supported by pathologies of human goal-directed cognition, which have motor and cognitive dysfunction and organize themselves, with respect to dopaminergic activity, along the gradient described by ARS, from perseverative to unfocused. The evidence strongly supports the evolution of goal-directed cognition out of mechanisms initially in control of spatial foraging but, through increasing cortical connections, eventually used to forage for information.

Iterative experimental and virtual high-throughput screening identifies metabotropic glutamate receptor subtype 4 positive allosteric modulators

Activation of metabotropic glutamate receptor subtype 4 has been shown to be efficacious in rodent models of Parkinson's disease. Artificial neural networks were trained based on a recently reported high throughput screen which identified 434 positive allosteric modulators of metabotropic glutamate receptor subtype 4 out of a set of approximately 155,000 compounds. A jury system containing three artificial neural networks achieved a theoretical enrichment of 15.4 when selecting the top 2 % compounds of an independent test dataset. The model was used to screen an external commercial database of approximately 450,000 drug-like compounds. 1,100 predicted active small molecules were tested experimentally using two distinct assays of mGlu(4) activity. This experiment yielded 67 positive allosteric modulators of metabotropic glutamate receptor subtype 4 that confirmed in both experimental systems. Compared to the 0.3 % active compounds in the primary screen, this constituted an enrichment of 22 fold.

Glutamate-based depression GBD

We describe a new term: glutamate-based depression (GBD). GBD is defined as a chronic depressive illness associated with environmental stress and diseases associated with altered glutamate neurotransmission. We hypothesize that glutamate-induced over-activation of extrasynaptic NMDA receptors in the subgenual cingulate area called Brodmann's 25 plays an important role in the etiology of depression and may be responsible for the high incidence of co-morbid depression associated in diseases with glutamate etiology. While depression is a syndrome with multiple possible etiologies, we propose that a disruption in glutamatergic neurotransmission may underline a substantial proportion of clinically observed depression. The high rates of depressive symptoms associated with various disorders in which altered glutamatergic functions have been identified, may suggest a common pathophysiological mechanism is underlying the diverse clinical presentations.

Evaluation of permselective membranes for optimization of intracerebral amperometric glutamate biosensors

Monitoring of extracellular brain glutamate concentrations by intracerebral biosensors is a promising approach to further investigate the role of this important neurotransmitter. However, amperometric biosensors are typically hampered by Faradaic interference caused by the presence of other electroactive species in the brain, such as ascorbic acid, dopamine, and uric acid. Various permselective membranes are often used on biosensors to prevent this. In this study we evaluated the most commonly used membranes, i.e. nafion, polyphenylenediamine, polypyrrole, polyaniline, and polynaphthol using a novel silica-based platinum electrode. First we selected the membranes with the highest sensitivity for hydrogen peroxide in vitro and an optimal selectivity against electrochemical interferents. Then we evaluated the performances of these membranes in a short lasting (3-4h) in vivo experiment. We found that best in vitro performance was accomplished with biosensors that were protected by a poly(m-phenylenediamine) membrane deposited onto the platinum electrode by cyclic voltammetry. However, post-implantation evaluation of these membranes showed poor selectivity against dopamine. Combination with a previously applied nafion layer did not protect the sensors against acute biofouling; indeed it was even counter effective. Finally, we investigated the ability of our biosensors to monitor the effect of glutamate transport blocker DL-TBOA on modulating glutamate concentrations in the prefrontal cortex of anaesthetized rats. The optimized biosensors recorded a rapid 35-fold increase in extracellular glutamate, and are considered suitable for further exploration in vivo.

A(2A)/D(2) receptor heteromerization in a model of Parkinson's disease. Focus on striatal aminoacidergic signaling

The present manuscript mainly summarizes the basic concepts and the molecular mechanisms underlying adenosine A(2A)-dopamine D(2) receptor-receptor interactions in the basal ganglia. Special emphasis is placed on neurochemical, behavioral and electrophysiological findings supporting the functional role that A(2A)/D(2) heteromeric receptor complexes located on striato-pallidal GABA neurons and corticostriatal glutamate terminals play in the regulation of the so called "basal ganglia indirect pathway". Furthermore, the role of A(2A)/mGluR(5) synergistic interactions in striatal neuron function and dysfunction is discussed. The functional consequences of the interactions between striatal adenosine A(2A), mGluR(5) and dopamine D(2) receptors on striatopallidal GABA release and motor behavior dysfunctions suggest the possibility of simultaneously targeting these receptors in Parkinson's disease treatment. This article is part of a Special Issue entitled Brain Integration. This article is part of a Special Issue entitled: Brain Integration.

Anxiety in Parkinson's disease: a critical review of experimental and clinical studies

Parkinson's disease (PD) is the second most common neurodegenerative disorder affecting about 1% of the population older than 60 years. Classically, PD is considered as a movement disorder, and its diagnosis is based on the presence of a set of cardinal motor signs that are the consequence of a pronounced death of dopaminergic neurons in the substantia nigra pars compacta. There is now considerable evidence showing that the neurodegenerative processes leading to sporadic PD begin many years before the appearance of the characteristic motor symptoms, and that additional neuronal fields and neurotransmitter systems are also involved in PD, including olfactory structures, amygdala, caudal raphe nuclei, locus coeruleus, and hippocampus. Accordingly, adrenergic and serotonergic neurons are also lost, which seems to contribute to the anxiety in PD. Non-motor features of PD usually do not respond to dopaminergic medication and probably form the major current challenge in the clinical management of PD. Additionally, most studies performed with animal models of PD have investigated their ability to induce motor alterations associated with advanced phases of PD, and some studies begin to assess non-motor behavioral features of the disease. The present review attempts to examine results obtained from clinical and experimental studies to provide a comprehensive picture of the neurobiology and current and potential treatments for anxiety in PD. The data reviewed here indicate that, despite their high prevalence and impact on the quality of life, anxiety disorders are often under-diagnosed and under-treated in PD patients. Moreover, there are currently few clinical and pre-clinical studies underway to investigate new pharmacological agents for relieving these symptoms, and we hope that this article may inspire clinicians and researchers devote to the studies on anxiety in PD to change this scenario. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Effects of pallidal neurotensin on haloperidol-induced parkinsonian catalepsy: behavioral and electrophysiological studies

OBJECTIVE

The globus pallidus plays a critical role in movement regulation. Previous studies have indicated that the globus pallidus receives neurotensinergic innervation from the striatum, and systemic administration of a neurotensin analog could produce antiparkinsonian effects. The present study aimed to investigate the effects of pallidal neurotensin on haloperidol-induced parkinsonian symptoms.

METHODS

Behavioral experiments and electrophysiological recordings were performed in the present study.

RESULTS

Bilateral infusions of neurotensin into the globus pallidus reversed haloperidol-induced parkinsonian catalepsy in rats. Electrophysiological recordings showed that microinjection of neurotensin induced excitation of pallidal neurons in the presence of systemic haloperidol administration. The neurotensin type-1 receptor antagonist SR48692 blocked both the behavioral and the electrophysiological effects induced by neurotensin.

CONCLUSION

Activation of pallidal neurotensin receptors may be involved in neurotensin-induced antiparkinsonian effects.

Discovery of 3-substituted aminocyclopentanes as potent and orally bioavailable NR2B subtype-selective NMDA antagonists

A series of 3-substituted aminocyclopentanes has been identified as highly potent and selective NR2B receptor antagonists. Incorporation of a 1,2,4-oxadiazole linker and substitution of the pendant phenyl ring led to the discovery of orally bioavailable analogues that showed efficient NR2B receptor occupancy in rats. Unlike nonselective NMDA antagonists, the NR2B-selective antagonist 22 showed no adverse affects on motor coordination in the rotarod assay at high dose. Compound 22 was efficacious following oral administration in a spinal nerve ligation model of neuropathic pain and in an acute model of Parkinson's disease in a dose dependent manner.

Metabotropic glutamate receptor 5 antagonist protects dopaminergic and noradrenergic neurons from degeneration in MPTP-treated monkeys

Degeneration of the dopaminergic nigrostriatal system and of noradrenergic neurons in the locus coeruleus are important pathological features of Parkinson's disease. There is an urgent need to develop therapies that slow down the progression of neurodegeneration in Parkinson's disease. In the present study, we tested whether the highly specific metabotropic glutamate receptor 5 antagonist, 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine, reduces dopaminergic and noradrenergic neuronal loss in monkeys rendered parkinsonian by chronic treatment with low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Weekly intramuscular 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injections (0.2-0.5 mg/kg body weight), in combination with daily administration of 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine or vehicle, were performed until the development of parkinsonian motor symptoms in either of the two experimental groups (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine versus 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle). After 21 weeks of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment, all 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated animals displayed parkinsonian symptoms, whereas none of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated monkeys were significantly affected. These behavioural observations were consistent with in vivo positron emission tomography dopamine transporter imaging data, and with post-mortem stereological counts of midbrain dopaminergic neurons, as well as striatal intensity measurements of dopamine transporter and tyrosine hydroxylase immunoreactivity, which were all significantly higher in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated animals than in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated monkeys. The 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine treatment also had a significant effect on the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced loss of norepinephrine neurons in the locus coeruleus and adjoining A5 and A7 noradrenaline cell groups. In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated animals, almost 40% loss of tyrosine hydroxylase-positive norepinephrine neurons was found in locus coeruleus/A5/A7 noradrenaline cell groups, whereas the extent of neuronal loss was lower than 15% of control values in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated monkeys. Our data demonstrate that chronic treatment with the metabotropic glutamate receptor 5 antagonist, 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine, significantly reduces 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity towards dopaminergic and noradrenergic cell groups in non-human primates. This suggests that the use of metabotropic glutamate receptor 5 antagonists may be a useful strategy to reduce degeneration of catecholaminergic neurons in Parkinson's disease.

Heteromerization of G protein-coupled receptors: relevance to neurological disorders and neurotherapeutics

Because G protein-coupled receptors (GPCRs) are numerous, widely expressed and involved in major physiological responses, they represent a relevant therapeutic target for drug discovery, particularly regarding pharmacological treatments of neurological disorders. Among the biological phenomena regulating receptor function, GPCR heteromerization is an important emerging area of interest and investigation. There is increasing evidence showing that heteromerization contributes to the pharmacological heterogeneity of GPCRs by modulating receptor ontogeny, activation and recycling. Although in many cases the physiological relevance of receptor heteromerization has not been fully established, the unique pharmacological and functional properties of heteromers are likely to lead to new strategies in clinical medicine. This review describes the main GPCR heteromers and their implications for major neurological disorders such as Parkinson's disease, schizophrenia and addiction. A better understanding of molecular mechanisms underlying drug interactions related to the targeting of receptor heteromers could provide more specific and efficient therapeutic agents for the treatment of brain diseases.

Alterations in mGluR5 expression and signaling in Lewy body disease and in transgenic models of alpha-synucleinopathy--implications for excitotoxicity

Dementia with Lewy bodies (DLB) and Parkinson's Disease (PD) are neurodegenerative disorders of the aging population characterized by the abnormal accumulation of alpha-synuclein (alpha-syn). Previous studies have suggested that excitotoxicity may contribute to neurodegeneration in these disorders, however the underlying mechanisms and their relationship to alpha-syn remain unclear. For this study we proposed that accumulation of alpha-syn might result in alterations in metabotropic glutamate receptors (mGluR), particularly mGluR5 which has been linked to deficits in murine models of PD. In this context, levels of mGluR5 were analyzed in the brains of PD and DLB human cases and alpha-syn transgenic (tg) mice and compared to age-matched, unimpaired controls, we report a 40% increase in the levels of mGluR5 and beta-arrestin immunoreactivity in the frontal cortex, hippocampus and putamen in DLB cases and in the putamen in PD cases. In the hippocampus, mGluR5 was more abundant in the CA3 region and co-localized with alpha-syn aggregates. Similarly, in the hippocampus and basal ganglia of alpha-syn tg mice, levels of mGluR5 were increased and mGluR5 and alpha-syn were co-localized and co-immunoprecipitated, suggesting that alpha-syn interferes with mGluR5 trafficking. The increased levels of mGluR5 were accompanied by a concomitant increase in the activation of downstream signaling components including ERK, Elk-1 and CREB. Consistent with the increased accumulation of alpha-syn and alterations in mGluR5 in cognitive- and motor-associated brain regions, these mice displayed impaired performance in the water maze and pole test, these behavioral alterations were reversed with the mGluR5 antagonist, MPEP. Taken together the results from study suggest that mGluR5 may directly interact with alpha-syn resulting in its over activation and that this over activation may contribute to excitotoxic cell death in select neuronal regions. These results highlight the therapeutic importance of mGluR5 antagonists in alpha-synucleinopathies.

Glutamate receptors as therapeutic targets for Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder characterized by motor symptoms including tremor and bradykinesia. The primary pathophysiology underlying PD is the degeneration of dopaminergic neurons of the substantia nigra pars compacta. Loss of these neurons causes pathological changes in neurotransmission in the basal ganglia motor circuit. The ability of ionotropic and metabotropic glutamate receptors to modulate neurotransmission throughout the basal ganglia suggests that these receptors may be targets for reversing the effects of altered neurotransmission in PD. Studies in animal models suggest that modulating the activity of these receptors may alleviate the primary motor symptoms of PD as well as side effects induced by dopamine replacement therapy. Moreover, glutamate receptor ligands may slow disease progression by delaying progressive dopamine neuron degeneration. Antagonists of NMDA receptors have shown promise in reversing motor symptoms, levodopa-induced dyskinesias, and neurodegeneration in preclinical PD models. The effects of drugs targeting AMPA receptors are more complex; while antagonists of these receptors exhibit utility in the treatment of levodopa-induced dyskinesias, AMPA receptor potentiators show promise for neuroprotection. Pharmacological modulation of metabotropic glutamate receptors (mGluRs) may hold even more promise for PD treatment due to the ability of mGluRs to fine-tune neurotransmission. Antagonists of mGluR5, as well as activators of group II mGluRs and mGluR4, have shown promise in several animal models of PD. These drugs reverse motor deficits in addition to providing protection against neurodegeneration. Glutamate receptors therefore represent exciting targets for the development of novel pharmacological therapies for PD.

beta-N-methylamino-l-alanine induces oxidative stress and glutamate release through action on system Xc(-)

beta-N-methylamino-l-alanine (BMAA) is a non-protein amino acid implicated in the neurodegenerative disease amyotrophic lateral sclerosis/Parkinson-dementia complex (ALS/PDC) on Guam. BMAA has recently been discovered in the brains of Alzheimer's patients in Canada and is produced by various species of cyanobacteria around the world. These findings suggest the possibility that BMAA may be of concern not only for specific groups of Pacific Islanders, but for a much larger population. Previous studies have indicated that BMAA can act as an excitotoxin by acting on the NMDA receptor. We have shown that the mechanism of neurotoxicity is actually three-fold; it involves not only direct action on the NMDA receptor, but also activation of metabotropic glutamate receptor 5 (mGluR5) and induction of oxidative stress. We now explore the mechanism by which BMAA activates the mGluR5 receptor and induces oxidative stress. We found that BMAA inhibits the cystine/glutamate antiporter (system Xc(-)) mediated cystine uptake, which in turn leads to glutathione depletion and increased oxidative stress. BMAA also appears to drive glutamate release via system Xc(-) and this glutamate induces toxicity through activation of the mGluR5 receptor. Therefore, the oxidative stress and mGluR5 activation induced by BMAA are both mediated through action at system Xc(-). The multiple mechanisms of BMAA toxicity, particularly the depletion of glutathione and enhanced oxidative stress, may account for its ability to induce complex neurodegenerative diseases.

Adenosine receptor ligands and PET imaging of the CNS

Advances in radiotracer chemistry have resulted in the development of novel molecular imaging probes for adenosine receptors (ARs). With the availability of these molecules, the function of ARs in human pathophysiology as well as the safety and efficacy of approaches to the different AR targets can now be determined. Molecular imaging is a rapidly growing field of research that allows the identification of molecular targets and functional processes in vivo. It is therefore gaining increasing interest as a tool in drug development because it permits the process of evaluating promising therapeutic targets to be stratified. Further, molecular imaging has the potential to evolve into a useful diagnostic tool, particularly for neurological and psychiatric disorders. This chapter focuses on currently available AR ligands that are suitable for molecular neuroimaging and describes first applications in healthy subjects and patients using positron emission tomography (PET).

Pathway-based approaches for analysis of genomewide association studies

Published genomewide association (GWA) studies typically analyze and report single-nucleotide polymorphisms (SNPs) and their neighboring genes with the strongest evidence of association (the "most-significant SNPs/genes" approach), while paying little attention to the rest. Borrowing ideas from microarray data analysis, we demonstrate that pathway-based approaches, which jointly consider multiple contributing factors in the same pathway, might complement the most-significant SNPs/genes approach and provide additional insights into interpretation of GWA data on complex diseases.