Antiparkinsonian and antidyskinetic activity of drugs targeting central glutamatergic mechanisms

Effect of the Metabotropic Glutamate Receptor Type 5 Negative Allosteric Modulator Dipraglurant on Motor and Non-Motor Symptoms of Parkinson's Disease

Parkinson's disease (PD) patients suffer not only from the primary motor symptoms of the disease but also from a range of non-motor symptoms (NMS) that cause disability and low quality of life. Excessive glutamate activity in the basal ganglia resulting from degeneration of the nigrostriatal dopamine pathway has been implicated in the motor symptoms, NMS and dyskinesias in PD patients. In this study, we investigated the effects of a selective mGlu5 negative allosteric modulator (NAM), dipraglurant, in a rodent motor symptoms model of PD, but also in models of anxiety, depression and obsessive-compulsive disorder, all of which are among the most prevalent NMS symptoms. Dipraglurant is rapidly absorbed after oral administration, readily crosses the blood-brain barrier, and exhibits a high correlation between plasma concentration and efficacy in behavioral models. In vivo, dipraglurant dose-dependently reduced haloperidol-induced catalepsy, increased punished licks in the Vogel conflict-drinking model, decreased immobility time in the forced swim test, decreased the number of buried marbles in the marble-burying test, but had no effect on rotarod performance or locomotor activity. These findings suggest that dipraglurant may have benefits to address some of the highly problematic comorbid non-motor symptoms of PD, in addition to its antidyskinetic effect demonstrated in PD-LID patients.

Glycation modulates glutamatergic signaling and exacerbates Parkinson's disease-like phenotypes

Alpha-synuclein (aSyn) is a central player in the pathogenesis of synucleinopathies due to its accumulation in typical protein aggregates in the brain. However, it is still unclear how it contributes to neurodegeneration. Type-2 diabetes mellitus is a risk factor for Parkinson's disease (PD). Interestingly, a common molecular alteration among these disorders is the age-associated increase in protein glycation. We hypothesized that glycation-induced neuronal dysfunction is a contributing factor in synucleinopathies. Here, we dissected the impact of methylglyoxal (MGO, a glycating agent) in mice overexpressing aSyn in the brain. We found that MGO-glycation potentiates motor, cognitive, olfactory, and colonic dysfunction in aSyn transgenic (Thy1-aSyn) mice that received a single dose of MGO via intracerebroventricular injection. aSyn accumulates in the midbrain, striatum, and prefrontal cortex, and protein glycation is increased in the cerebellum and midbrain. SWATH mass spectrometry analysis, used to quantify changes in the brain proteome, revealed that MGO mainly increase glutamatergic-associated proteins in the midbrain (NMDA, AMPA, glutaminase, VGLUT and EAAT1), but not in the prefrontal cortex, where it mainly affects the electron transport chain. The glycated proteins in the midbrain of MGO-injected Thy1-aSyn mice strongly correlate with PD and dopaminergic pathways. Overall, we demonstrated that MGO-induced glycation accelerates PD-like sensorimotor and cognitive alterations and suggest that the increase of glutamatergic signaling may underly these events. Our study sheds new light into the enhanced vulnerability of the midbrain in PD-related synaptic dysfunction and suggests that glycation suppressors and anti-glutamatergic drugs may hold promise as disease-modifying therapies for synucleinopathies.

Receptor Ligands as Helping Hands to L-DOPA in the Treatment of Parkinson's Disease

Levodopa (LD) is the most effective drug in the treatment of Parkinson’s disease (PD). However, although it represents the “gold standard” of PD therapy, LD can cause side effects, including gastrointestinal and cardiovascular symptoms as well as transient elevated liver enzyme levels. Moreover, LD therapy leads to LD-induced dyskinesia (LID), a disabling motor complication that represents a major challenge for the clinical neurologist. Due to the many limitations associated with LD therapeutic use, other dopaminergic and non-dopaminergic drugs are being developed to optimize the treatment response. This review focuses on recent investigations about non-dopaminergic central nervous system (CNS) receptor ligands that have been identified to have therapeutic potential for the treatment of motor and non-motor symptoms of PD. In a different way, such agents may contribute to extending LD response and/or ameliorate LD-induced side effects.

Cholinergic activity and levodopa-induced dyskinesia: a multitracer molecular imaging study

Objective

To investigate the association between levodopa‐induced dyskinesias and striatal cholinergic activity in patients with Parkinson's disease.

Methods

This study included 13 Parkinson's disease patients with peak‐of‐dose levodopa‐induced dyskinesias, 12 nondyskinetic patients, and 12 healthy controls. Participants underwent 5‐[¹²³I]iodo‐3‐[2(S)‐2‐azetidinylmethoxy]pyridine single‐photon emission computed tomography, a marker of nicotinic acetylcholine receptors, [¹²³I]N‐ω‐fluoropropyl‐2β‐carbomethoxy‐3β‐(4‐iodophenyl)nortropane single‐photon emission computed tomography, to measure dopamine reuptake transporter density and 2‐[¹⁸F]fluoro‐2‐deoxyglucose positron emission tomography to assess regional cerebral metabolic activity. Striatal binding potentials, uptake values at basal ganglia structures, and correlations with clinical variables were analyzed.

Results

Density of nicotinic acetylcholine receptors in the caudate nucleus of dyskinetic subjects was similar to that of healthy controls and significantly higher to that of nondyskinetic patients, in particular, contralaterally to the clinically most affected side.

Interpretation

Our findings support the hypothesis that the expression of dyskinesia may be related to cholinergic neuronal excitability in a dopaminergic‐depleted striatum. Cholinergic signaling would play a role in maintaining striatal dopaminergic responsiveness, possibly defining disease phenotype and progression.

An updated meta-analysis of amantadine for treating dyskinesia in Parkinson's disease

In recent years, a few of randomized controlled trials (RCTs) about amantadine for treating dyskinesia in Parkinson's disease (PD) were completed. Here, we conducted a systematic literature review about the clinical research to provide the updated evidence for treating dyskinesia. Electronic search of Medline, PubMed, Cochrane Library, and other databases up to May 2016 for relevant studies was performed. We selected the Unified Parkinson's Disease Rating Scale IV (UPDRS IV) and Dyskinesia Rating Scales (DRS) as efficacy outcomes of amantadine on dyskinesia. Pooled data from included studies was then used to carry out meta-analysis. A total of eleven eligible RCTs that involved 356 PD patients with existing dyskinesia were included in the present study. The results of meta-analysis showed that amantadine significantly improved UPDRS IV (P < 0.0001) and DRS (P < 0.00001). Meanwhile, there was a mild reduction in Unified Parkinson's Disease Rating Scale III after amantadine treatment in advanced PD patients with dyskinesia (P = 0.01) compared with placebo. High dosage of amantadine obviously improved existing dyskinesia in PD, yet at the expense of the increased adverse events. It was necessary to detect the optimal therapeutic efficacy to balance the incidence of adverse events when we used amantadine to treat existing dyskinesia in PD patients.

Drug-Induced Dyskinesia, Part 1: Treatment of Levodopa-Induced Dyskinesia

Dyskinesias encompass a variety of different hyperkinetic phenomenologies, particularly chorea, dystonia, stereotypies, and akathisia. Levodopa-induced dyskinesia (LID) is one of the main types of drug-induced dyskinesia, occurring in patients with Parkinson's disease (PD) who have been treated with levodopa for long time, but this side effect may be encountered even within a few weeks or months after initiation of levodopa therapy. Based on the temporal pattern in relationship to levodopa dosing, LIDs are divided into "peak-dose dyskinesia," "diphasic dyskinesia," and "wearing off" or "off-period" dyskinesia, of which peak-dose dyskinesia is the most common, followed by off-period, and then diphasic dyskinesia. Treatment strategy includes identifying the kind of dyskinesia and tailoring treatment accordingly. Peak-dose dyskinesia is treated mainly by reducing individual doses of levodopa and adding amantadine and dopamine agonists, whereas off-period dystonia often responds to baclofen and botulinum toxin injections. Diphasic dyskinesias, occurring particularly in patients with young-onset PD, are the most difficult to treat. While fractionation of levodopa dosage is the most frequently utilized strategy, many patients require deep brain stimulation to control their troublesome motor fluctuations and LIDs. A variety of emerging (experimental) drugs currently in development promise to provide better control of LIDs and other levodopa-related complications in the near future.

Recent Advance in the Relationship between Excitatory Amino Acid Transporters and Parkinson's Disease

Parkinson's disease (PD) is the most common movement disorder disease in the elderly and is characterized by degeneration of dopamine neurons and formation of Lewy bodies. Glutamate is the major excitatory neurotransmitter in the central nervous system (CNS). If glutamate is not removed promptly in the synaptic cleft, it will excessively stimulate the glutamate receptors and induce excitotoxic effects on the CNS. With lack of extracellular enzyme to decompose glutamate, glutamate uptake in the synaptic cleft is mainly achieved by the excitatory amino acid transporters (EAATs, also known as high-affinity glutamate transporters). Current studies have confirmed that decreased expression and function of EAATs appear in PD animal models. Moreover, single unilateral administration of EAATs inhibitor in the substantia nigra mimics several PD features and this is a solid evidence supporting that decreased EAATs contribute to the process of PD. Drugs or treatments promoting the expression and function of EAATs are shown to attenuate dopamine neurons death in the substantia nigra and striatum, ameliorate the behavior disorder, and improve cognitive abilities in PD animal models. EAATs are potential effective drug targets in treatment of PD and thus study of relationship between EAATs and PD has predominant medical significance currently.

Glutamatergic pathways as a target for the treatment of dyskinesias in Parkinson's disease

PD (Parkinson's disease) is characterized by some typical motor features that are caused by striatal dopamine depletion and respond well to dopamine-replacement therapy with L-dopa. Unfortunately, the majority of PD patients treated with L-dopa develop abnormal involuntary movements (dyskinesias) within a few years. The mechanisms underlying the development of LIDs (L-dopa-induced dyskinesias) involve, on one hand, a presynaptic dysregulation of dopamine release and clearance and, on the other hand, an abnormal postsynaptic response to dopamine in the brain. There is a large amount of evidence that these dopamine-dependent mechanisms are modulated by glutamatergic pathways and glutamate receptors. The present article summarizes the pathophysiological role of glutamatergic pathways in LID and reviews pre-clinical and clinical results obtained using pharmacological modulators of different classes and subtypes of glutamate receptors to treat parkinsonian dyskinesias.

PSD-95 expression controls L-DOPA dyskinesia through dopamine D1 receptor trafficking

L-DOPA-induced dyskinesia (LID), a detrimental consequence of dopamine replacement therapy for Parkinson's disease, is associated with an alteration in dopamine D1 receptor (D1R) and glutamate receptor interactions. We hypothesized that the synaptic scaffolding protein PSD-95 plays a pivotal role in this process, as it interacts with D1R, regulates its trafficking and function, and is overexpressed in LID. Here, we demonstrate in rat and macaque models that disrupting the interaction between D1R and PSD-95 in the striatum reduces LID development and severity. Single quantum dot imaging revealed that this benefit was achieved primarily by destabilizing D1R localization, via increased lateral diffusion followed by increased internalization and diminished surface expression. These findings indicate that altering D1R trafficking via synapse-associated scaffolding proteins may be useful in the treatment of dyskinesia in Parkinson's patients.

Anti-dyskinetic mechanisms of amantadine and dextromethorphan in the 6-OHDA rat model of Parkinson's disease: role of NMDA vs. 5-HT1A receptors

Amantadine and dextromethorphan suppress levodopa (L-DOPA)-induced dyskinesia (LID) in patients with Parkinson's disease (PD) and abnormal involuntary movements (AIMs) in the unilateral 6-hydroxydopamine (6-OHDA) rat model. These effects have been attributed to N-methyl-d-aspartate (NMDA) antagonism. However, amantadine and dextromethorphan are also thought to block serotonin (5-HT) uptake and cause 5-HT overflow, leading to stimulation of 5-HT(1A) receptors, which has been shown to reduce LID. We undertook a study in 6-OHDA rats to determine whether the anti-dyskinetic effects of these two compounds are mediated by NMDA antagonism and/or 5-HT(1A) agonism. In addition, we assessed the sensorimotor effects of these drugs using the Vibrissae-Stimulated Forelimb Placement and Cylinder tests. Our data show that the AIM-suppressing effect of amantadine was not affected by the 5-HT(1A) antagonist WAY-100635, but was partially reversed by the NMDA agonist d-cycloserine. Conversely, the AIM-suppressing effect of dextromethorphan was prevented by WAY-100635 but not by d-cycloserine. Neither amantadine nor dextromethorphan affected the therapeutic effects of L-DOPA in sensorimotor tests. We conclude that the anti-dyskinetic effect of amantadine is partially dependent on NMDA antagonism, while dextromethorphan suppresses AIMs via indirect 5-HT(1A) agonism. Combined with previous work from our group, our results support the investigation of 5-HT(1A) agonists as pharmacotherapies for LID in PD patients.

Contribution of pre-synaptic mechanisms to L-DOPA-induced dyskinesia

Positron emission tomography (PET) imaging studies have shown that peak-dose dyskinesia is associated to abnormally high levels of synaptic dopamine (DA) in the caudate-putamen of dyskinetic L-DOPA-treated patients. High striatal extracellular DA levels have also been found in dyskinetic 6-OHDA-lesioned rats as compared to non-dyskinetic ones, suggesting that extracellular DA levels may play a key role in the induction of dyskinesia. In this article we review the evidences pointing to the serotonin system as the primary cause for the abnormally high levels of L-DOPA-derived extracellular DA in Parkinson's disease, and we discuss the feasibility of a therapeutic approach targeting this system.

Brain 5-HT(2A) receptors in MPTP monkeys and levodopa-induced dyskinesias

Levodopa-induced dyskinesias (LIDs) are abnormal involuntary movements induced by the chronic use of levodopa (l-Dopa) limiting the quality of life of Parkinson's disease (PD) patients. We evaluated changes of the serotonin 5-HT(2A) receptors in control monkeys, in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned monkeys and in l-Dopa-treated MPTP monkeys, without or with adjunct treatments to inhibit the expression of LID: CI-1041, a selective NR1A/2B subunit antagonist of glutamate N-methyl-d-aspartic acid (NMDA) receptor, or Cabergoline, a long-acting dopamine D(2) receptor agonist. All treatments were administered for 1 month and animals were killed 24 h after the last dose of l-Dopa. Striatal concentrations of serotonin were decreased in all MPTP monkeys investigated, as measured by high-performance liquid chromatography. [(3) H]Ketanserin-specific binding to 5-HT(2A) receptors was measured by autoradiography. l-Dopa treatment that induced dyskinesias increased 5-HT(2A) receptor-specific binding in the caudate nucleus and the anterior cingulate gyrus (AcgG) compared with control monkeys. Moreover, [(3) H]Ketanserin-specific binding was increased in the dorsomedial caudate nucleus in l-Dopa-treated MPTP monkeys compared with saline-treated MPTP monkeys. Nondyskinetic monkeys treated with CI-1041 or Cabergoline showed low 5-HT(2A) -specific binding in the posterior dorsomedial caudate nucleus and the anterior AcgG compared with dyskinetic monkeys. No significant difference in 5-HT(2A) receptor binding was observed in any brain regions examined in saline-treated MPTP monkeys compared with control monkeys. These results confirm the involvement of serotonergic pathways and the glutamate/serotonin interactions in LID. They also support targeting 5-HT(2A) receptors as a potential treatment for LID.

Altered AMPA receptor expression with treadmill exercise in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mouse model of basal ganglia injury

Dopamine depletion leads to impaired motor performance and increased glutamatergic-mediated hyperexcitability of medium spiny neurons in the basal ganglia. Intensive treadmill exercise improves motor performance in both saline treatment and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) mouse model of Parkinson's disease. In the present study, we investigated the effect of high-intensity treadmill exercise on changes in alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit expression, because these receptor channels confer the majority of fast excitatory neurotransmission in the brain, and their subunit composition provides a key mechanism for regulating synaptic strength and synaptic neuroplasticity and is important in modulating glutamatergic neurotransmission. Within the dorsolateral striatum of MPTP mice, treadmill exercise increased GluR2 subunit expression, with no significant effect on GluR1. Furthermore, neurophysiological studies demonstrated a reduction in the size of excitatory postsynaptic currents (EPSCs) in striatal medium spiny neurons (as determined by the input-output relationship), reduced amplitude of spontaneous EPSCs, and a loss of polyamine-sensitive inward rectification, all supportive of an increase in heteromeric AMPAR channels containing the GluR2 subunit. Phosphorylation of GluR2 at serine 880 in both saline-treated and MPTP mice suggests that exercise may also influence AMPAR trafficking and thus synaptic strength within the striatum. Finally, treadmill exercise also altered flip isoforms of GluR2 and GluR1 mRNA transcripts. These findings suggest a role for AMPARs in mediating the beneficial effects of exercise and support the idea that adaptive changes in GluR2 subunit expression may be important in modulating experience-dependent neuroplasticity of the injured basal ganglia.

Therapy for dyskinesias in Parkinson’s disease patients

Dyskinesia hampers the quality of life for most Parkinson’s disease patients following several years of therapy. However, the severity of L-Dopa-induced dyskinesia (LID) varies between patients, being quite tolerable in late-onset patients. Understanding the pathogenesis of LID has contributed to the development of a set of therapeutic strategies, including the choice, in early stages, of the least pulsatile regimen of dopamine-receptor activation. In cases where LIDs are already disabling, there is only a limited number of options: the optimization of ongoing DOPA-centered treatment, the utilization of glutamate antagonists and the exploration of the benefits of antipsychotic agents. More radical solutions are provided by deep brain stimulation in the subthalamic nucleus (or internal pallidus). This approach has proved efficacious in reducing LID, largely because it allows a reduction in dopaminergic daily doses. Stereotactic neurosurgery has fuelled several lines of investigation regarding the crosstalk between the basal ganglia and motor cortex. Here, we will present interesting evidence highlighting the potential for repetitive transcranial stimulation in reducing the occurrence of LID. The future may disclose important new avenues for the treatment of LIDs, given the current development of promising agents that might target different facets of dyskinesia, such as the impairment of striatal plasticity and non-Dopaminergic contributors such as adenosine, nitric oxide and the nucleotide cascade.

MK-801 inhibits L-DOPA-induced abnormal involuntary movements only at doses that worsen parkinsonism

Amantadine and dextromethorphan suppress levodopa (L-DOPA)-induced dyskinesia in Parkinson's disease patients and abnormal involuntary movements (AIMs) in the 6-hydroxydopamine (6-OHDA) rat model. These medications have been hypothesized to exert their therapeutic effects by a noncompetitive N-methyl-D-aspartate (NMDA) antagonist mechanism, but they also have known serotonin (5-HT) indirect agonist effects that could suppress AIMs. This raised the possibility that NMDA antagonists lacking 5-HTergic effects would not have the anti-dyskinetic action predicted by previous investigators. To test this hypothesis, we investigated MK-801, the most widely-studied NMDA antagonist. We found that chronic low-dose MK-801 (0.1 mg/kg) had no effect on development of AIMs or contraversive rotation. In addition, in L-DOPA-primed rats, low-dose MK-801 (0.1 mg/kg) had no effect on expression of AIMs, contraversive rotation, or sensorimotor function. Conversely, higher doses of MK-801 (0.2-0.3 mg/kg) suppressed expression of AIMs. However, as we show for the first time, anti-dyskinetic doses of MK-801 also suppressed L-DOPA-induced contralateral rotation and impaired sensorimotor function, likely due to non-specific interference of MK-801 with L-DOPA-induced behavior. We conclude that noncompetitive NMDA antagonists are unlikely to suppress dyskinesia clinically without worsening parkinsonism.

Assemblies of glutamate receptor subunits with post-synaptic density proteins and their alterations in Parkinson's disease

N-methyl-D-aspartate (NMDA) receptors have been implicated as a mediator of neuronal injury associated with many neurological disorders including ischemia, epilepsy, brain trauma, dementia and neurodegenerative disorders such as Parkinson's disease (PD) and Alzheimer's disease. To this, non-selective NMDA receptor antagonists have been tried and have been shown to be effective in many experimental animal models of disease, and some of these compounds have moved into clinical trials. However, the initial enthusiasm for this approach has waned, because the therapeutic index for most NMDA antagonists is quite poor, with significant adverse effects at clinically effective doses, thus limiting their utility. More recently, the concept that the exact pathways downstream NMDA receptor activation could represent a key variable element among neurological disorders has been put forward. In particular, variations in NMDA receptor subunit composition could be important in different disorders, both in the pathophysiological mechanisms of cell death and in the application of specific symptomatic therapies. As to PD, NMDA receptor complex has been shown to be altered in experimental models of parkinsonism and in PD in humans. Further, it has become increasingly evident that the NMDA receptor complex is intimately involved in the regulation of corticostriatal long-term potentiation, which is altered in experimental parkinsonism. The following sections will examine the modifications of specific NMDA receptor subunits as well as post-synaptic associated signalling complex including kinases and scaffolding proteins in experimental parkinsonism. These findings may allow the identification of specific molecular targets whose pharmacological or genetic manipulation might lead to innovative therapies for PD.

Evaluation of the D3 dopamine receptor selective antagonist PG01037 on L-dopa-dependent abnormal involuntary movements in rats

The D3 dopamine receptor selective antagonist PG01037 has been evaluated for the ability to attenuate L-dopa-associated abnormal involuntary movements (AIMs) in unilaterally lesioned male Sprague-Dawley rats, which is a model of L-dopa-dependent dyskinesia in patients with Parkinson's Disease. The intrinsic activity of PG01037 was determined using a) a forskolin-dependent adenylyl cyclase inhibition assay with transfected HEK 293 cells expressing either the human D2Long or D3 dopamine receptor subtype and b) an assay for agonist-associated mitogenesis. For the initial experiments, the 5-HT1A receptor selective partial agonist buspirone was used as a positive control to verify our ability to quantitate changes in total AIMs and AIMs minus locomotor scores. Subcutaneous (s.c.) administration of PG01037 was found to have minimal effect on AIMs score. However, it was observed that the in vivo efficacy of PG01037 increased when administered by intraperitoneal (i.p.) injection 15 min after L-dopa/benserazide administration, as compared to a 60 min, 30 min or 0 min pretreatment. It was also found that i.p. administration of PG01037 could inhibit involuntary movements after they had achieved maximum intensity. PG01037 was found to attenuate AIM scores in these animals in a dose dependent manner with IC(50) value equal to a) 7.4 mg/kg following L-dopa/benserazide administration (8 mg/kg each, i.p.) and b) 18.4 mg/kg following the administration of apomorphine (0.05 mg/kg, s.c.). However, PG01037 did not effectively inhibit SKF 81297-dependent abnormal involuntary movements. Rotarod studies indicate that PG01037 at a dose of 10 mg/kg did not adversely affect motor coordination of the unilaterally lesioned rats. Evaluation of lesioned rats using a cylinder test behavioral paradigm indicated that PG01037 did not dramatically attenuate the beneficial effects of L-dopa. These studies suggest that D3 dopamine receptor selective antagonists are potential pharmacotherapeutic candidates for the treatment of L-dopa-associated dyskinesia in patients with Parkinson's Disease.

Pharmacological modulation of glutamate transmission in a rat model of L-DOPA-induced dyskinesia: effects on motor behavior and striatal nuclear signaling

L-DOPA-induced dyskinesia (LID) in Parkinson's disease has been linked to altered dopamine and glutamate transmission within the basal ganglia. In the present study, we compared compounds targeting specific subtypes of glutamate receptors or calcium channels for their ability to attenuate LID and the associated activation of striatal nuclear signaling and gene expression in the rat. Rats with 6-hydroxydopamine lesions were treated acutely or chronically with L-DOPA in combination with the following selective compounds: antagonists of group I metabotropic glutamate receptors (mGluR), (2-methyl-1,3-thiazol-4-yl) ethynylpyridine (MTEP) for mGluR5 and (3-ethyl-2-methyl-quinolin-6-yl)-(4-methoxy-cyclohexyl)-methanone methane sulfonate (EMQMCM) for mGluR1; an agonist of group II mGluR, 1R,4R,5S,6R-2-oxa-4-aminobicyclo[3.1.0]hexane-4,6-dicarboxylate (LY379268); N-methyl-D-aspartate (NMDA)-R2B subunit (NR2B)-selective NMDA receptor antagonists 1-[2-(4-hydroxyphenoxy)ethyl]-4-[(4-methylphenyl)methyl]-4-piperidinol hydrochloride (Ro631908) and (+/-)-(R(*),S(*))-alpha-(4-hydroxyphenyl)-beta-methyl-4-(phenylmethyl)1-piperidine propanol (Ro256981); and an L-type calcium channel antagonist, 4-(4-benzofurazanyl)-1,-4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylic acid methyl 1-methylethyl ester (isradipine). Dyskinesia and rotarod performance were monitored during chronic drug treatment. The striatal expression of phospho-extracellular signal-regulated kinase (ERK) 1/2 and mitogen- and stress-activated kinase (MSK)-1, or prodynorphin mRNA was examined after acute or chronic treatment, respectively. In the acute treatment studies, only MTEP and EMQMCM significantly attenuated L-DOPA-induced phospho-ERK1/2 and/or phospho-MSK-1 expression, with MTEP being the most effective (70-80% reduction). In the chronic experiment, only MTEP significantly attenuated dyskinesia without adverse motor effects, whereas EMQMCM and LY379268 inhibited the L-DOPA-induced improvement in rotarod performance. The NR2B antagonist had positive antiakinetic effects but did not reduce dyskinesia. Only MTEP blocked the up-regulation of prodynorphin mRNA induced by L-DOPA. Among the pharmacological treatments examined, MTEP was most effective in inhibiting LID and the associated molecular alterations. Antagonism of mGluR5 seems to be a promising strategy to reduce dyskinesia in Parkinson's disease.

Levodopa-induced dyskinesias and their management

This paper reviews the epidemiology, pathophysiology, clinical features and rationale for managing dyskinesias associated with Parkinson's disease. These are a common clinical problem occurring in up to 90% of patients and more frequently affect those with early-onset. Dyskinesias have a negative impact on quality of life and are an important cause of disability. Their precise etiology is still poorly understood, although it is recognized that dopaminergic pre-synaptic and post-synaptic mechanisms are involved together with extra-dopaminergic factors. The phenomenology of dyskinesias encompasses a variable mixture of two prevalent features: dystonia and chorea. We have studied their time course following a single acute levodopa challenge and have found that dystonia occurs throughout the duration of the on period, whereas choreiform movements occur only at the peak of therapeutic dopaminergic motor responses. This allows a schematic relationship to be drawn between a short duration motor response and the occurrence of dystonia and chorea. There is currently no satisfactory treatment for dyskinesias. Managing the therapeutic window does not provide an adequate solution due to the appearance of a dyskinesia threshold dose that narrows the therapeutic margin. High frequency stimulation of the subthalamic nucleus probably has some specific anti-dyskinetic action, but is limited by the small number of patients who are candidates for this treatment. Research efforts are currently focused on the development of specific anti-dyskinetic medications. Their availability will certainly change the current clinical practice and will widen again the therapeutic window of dopaminergic medications that has now become too narrow.

Therapeutic use of dextromethorphan: Key learnings from treatment of pseudobulbar affect

A variety of neurological conditions and disease states are accompanied by pseudobulbar affect (PBA), an emotional disorder characterized by uncontrollable outbursts of laughing and crying. The causes of PBA are unclear but may involve lesions in neural circuits regulating the motor output of emotional expression. Several agents used in treating other psychiatric disorders have been applied in the treatment of PBA with some success but data are limited and these agents are associated with unpleasant side effects due to nonspecific activity in diffuse neural networks. Dextromethorphan (DM), a widely used cough suppressant, acts at receptors in the brainstem and cerebellum, brain regions implicated in the regulation of emotional output. The combination of DM and quinidine (Q), an enzyme inhibitor that blocks DM metabolism, has recently been tested in phase III clinical trials in patients with multiple sclerosis and amyotrophic lateral sclerosis and was both safe and effective in palliating PBA symptoms. In addition, clinical studies pertaining to the safety and efficacy of DM/Q in a variety of neurological disease states are ongoing.

Decreased expression of l-dopa-induced dyskinesia by switching to ropinirole in MPTP-treated common marmosets

Current concepts suggest that pulsatile stimulation of dopamine receptors following L-dopa administration leads to priming for dyskinesia in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP)-treated primates, while continuous dopaminergic stimulation with long-acting dopamine agonists does not. We investigated whether L-dopa-induced dyskinesia is reduced by switching to a dopamine agonist. MPTP-treated marmosets received chronic treatment with L-dopa or ropinirole in doses producing equivalent motor activity and reversal of motor deficits. Administration of L-dopa led to the rapid onset of moderate to severe dyskinesia, whereas ropinirole produced only mild dyskinesia. Animals initially treated with L-dopa were switched to an equivalent dose of ropinirole and those treated with ropinirole were switched to an equivalent dose of L-dopa for 56 days. L-dopa-primed animals that were switched to ropinirole showed a trend towards a reduction of dyskinesia intensity, whereas animals initially treated with ropinirole and switched to L-dopa showed a trend toward increased dyskinesia intensity. A subsequent, acute L-dopa challenge reversed motor deficits and induced intense dyskinesia in both groups. This suggests that L-dopa leads to the priming and expression of dyskinesia, but that expression is not maintained when switching to a long-acting dopamine agonist. In contrast, dopamine agonists may prime for dyskinesia, but do not lead to its full expression.

Tamoxifen effect on L-DOPA induced response complications in parkinsonian rats and primates

The contribution of striatal protein kinase C (PKC) isoform changes in levodopa (L-DOPA) induced motor response complications in parkinsonian rats was investigated and the ability of tamoxifen, an antiestrogen with a partial PKC antagonist property, to prevent these response alterations in 6-hydroxydopamine (6-OHDA) lesioned rats as well as in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) treated cynomologous monkeys was studied. Following treatment of adult male rats with L-DOPA twice daily for 3 weeks, protein levels of left (lesioned) and right (intact) striatal PKC isoforms were measured. Western blot analysis showed increased protein expression of both the novel PKC epsilon isoform and the atypical PKC lambda isoform ipsilateral to the lesion (174+/-17% for epsilon, 140+/-9% for lambda, of intact striatum in 6-OHDA lesioned plus chronic L-DOPA treated animals) in acute L-DOPA treated rats. No enhancement was observed in PKC immunoreactivity for other isoforms. Tamoxifen (5.0 mg/kg p.o.) significantly attenuated the L-DOPA induced augmentation of protein expression of PKC epsilon and PKC lambda, but had no effect on immunoreactivity for other PKC isoforms. In chronic L-DOPA treated parkinsonian rats, tamoxifen prevented (5.0 mg/kg p.o.) as well as ameliorated (5.0 mg/kg p.o.) the characteristic shortening in duration of motor response to L-DOPA challenge. In MPTP lesioned primates, similar to the ameliorative effect seen in rats, tamoxifen (1 and 3 mg/kg p.o) reduced the appearance of L-DOPA induced dyskinesia by 61% and 55% respectively (p<0.05). These results suggest that changes in specific striatal PKC isoforms contribute to the pathogenesis of L-DOPA induced motor complications and further that drugs able to selectively inhibit these signaling kinases might provide adjunctive benefit in the treatment of Parkinson's disease.

Competitive AMPA receptor antagonists

Glutamic acid (Glu) is the major excitatory neurotransmitter in the mammalian central nervous system (CNS) where it is involved in the physiological regulation of different processes. It has been well established that excessive endogenous Glu is associated with many acute and chronic neurodegenerative disorders such as cerebral ischaemia, epilepsy, amiotrophic lateral sclerosis, Parkinson's, and Alzheimer's disease. These data have consequently added great impetus to the research in this field. In fact, many Glu receptor antagonists acting at the N-methyl-D-aspartic acid (NMDA), 2-amino-3-(3-hydroxy-5-methylisoxazol-4-yl)propionic acid (AMPA), and/or kainic acid (KA) receptors have been developed as research tools and potential therapeutic agents. Ligands showing competitive antagonistic action at the AMPA type of Glu receptors were first reported in 1988, and the systemically active 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo[f]quinoxaline (NBQX) was first shown to have useful therapeutic effects in animal models of neurological disease in 1990. Since then, the quinoxaline template has represented the backbone of various competitive AMPA receptor antagonists belonging to different classes which had been developed in order to increase potency, selectivity and water solubility, but also to prolong the "in vivo" action. Compounds that present better pharmacokinetic properties and less serious adverse effects with respect to the others previously developed are undergoing clinical evaluation. In the near future, the most important clinical application for the AMPA receptor antagonists will probably be as neuroprotectant in neurodegenerative diseases, such as epilepsy, for the treatment of patients not responding to current therapies. The present review reports the history of competitive AMPA receptor antagonists from 1988 up to today, providing a systematic coverage of both the open and patent literature.

Effects of 2,3-benzodiazepine AMPA receptor antagonists on dopamine turnover in the striatum of rats with experimental parkinsonism

Although levodopa is the current "gold standard" for treatment of Parkinson's disease, there has been disputation on whether AMPA receptor antagonists can be used as adjuvant therapy to improve the effects of levodopa. Systemic administration of levodopa, the precursor of dopamine, increases brain dopamine turnover rate and this elevated turnover is believed to be essential for successful treatment of Parkinson's disease. However, long-term treatment of patients with levodopa often leads to development of dyskinesia. Therefore, drugs that feature potentiation of dopamine turnover rate and are able to reduce daily levodopa dosages might be used as adjuvant in the treatment of patients suffering from Parkinson's disease. To investigate such combined treatment, we have examined the effects of two non-competitive AMPA receptor antagonists, GYKI-52466 and GYKI-53405, alone or in combination with levodopa on dopamine turnover rate in 6-hydroxydopamine-lesioned striatum of the rat. We found here that repeated administration of levodopa, added with the peripheral DOPA decarboxylase inhibitor carbidopa, increased dopamine turnover rate after lesioning the striatum with 6-hydroxydopamine. Moreover, combination of levodopa with GYKI-52466 or GYKI-53405 further increased dopamine turnover enhanced by levodopa administration while the AMPA receptor antagonists by themselves failed to influence striatal dopamine turnover. We concluded from the present data that potentiation observed between levodopa and AMPA receptor antagonists may reflect levodopa-sparing effects in clinical treatment indicating the therapeutic potential of such combination in the management of Parkinson's disease.

Cellular and behavioral effects of 5-HT1A receptor agonist 8-OH-DPAT in a rat model of levodopa-induced motor complications

5-HT1A autoreceptor stimulation can act to attenuate supraphysiological swings in extracellular dopamine levels following long-term levodopa treatment and may be useful in the treatment and prevention of the motor complications. The purpose of this study was to investigate cellular and behavioral effects of 5-HT1A receptor agonist 8-OH-DPAT in a rat model of levodopa-induced motor complications. Two sets of experiments were performed. First, animals were treated with levodopa (50 mg/kg with benserazide 12.5 mg/kg, twice daily), intraperitoneally (i.p.) for 22 days. On day 23, animals received either 8-OH-DPAT (1 mg/kg, i.p.) or 8-OH-DPAT plus WAY-100635 (0.1 mg/kg, i.p) or vehicle with each levodopa dose. In the second set, animals were treated either with levodopa (50 mg/kg, i.p.) plus 8-OH-DPAT (1 mg/kg, i.p.) or levodopa (50 mg/kg, i.p.) plus vehicle, administered twice daily for 22 consecutive days. Our study showed that 8-OH-DPAT plus levodopa both prolonged the duration of the motor response and reduced peak turning. 8-OH-DPAT plus levodopa also decreased the frequency of failures to levodopa. Co-administration of WAY-100635, a 5-HT1A receptor antagonist, with 8-OH-DPAT eliminated the effect of 8-OH-DPAT on motor complications indicating that the observed 8-OH-DPAT responses were probably mediated at the 5-HT1A autoreceptor. Moreover, 8-OH-DPAT plus levodopa significantly reduced hyperphosphorylation of GluR1 at serine 845, which was closely associated with levodopa-induced motor complications.

Ovariectomy ameliorates dextromethorphan--induced memory impairment in young female rats

We have previously found that dextromethorphan (DM), over-the-counter cough suppressant, impairs memory retention in water maze task, when it is repeatedly administrated to adolescent female rats at high doses. In this study we examined first if ovariectomy ameliorates the DM-induced memory impairment in female rats, and then whether or not the DM effect is revived by estrogen replacement in ovariectomized female rats. Female rat pups received bilateral ovariectomy or sham operation on postnatal day (PND) 21, and then intraperitoneal DM (40 mg/kg) daily during PND 28-37. Rats were subjected to the Morris water maze task from PND 38, approximately 24 h after the last DM injection. In probe trial, goal quadrant dwell time was significantly reduced by DM in the sham operated group, however, the reduction by DM did not occur in the ovariectomy group. When 17beta-estradiol was supplied to ovariectomized females during DM treatment, the goal quadrant dwell time was significantly decreased, compared to the vehicle control group. Furthermore, a major effect of estrogen replacement was found in the escape latency during the last 3 days of initial learning trials. These results suggest that ovariectomy may ameliorate the adverse effect of DM treatment on memory retention in young female rats, and that estrogen replacement may revive it, i.e. estrogen may take a major role in DM-induced memory impairment in female rats.

Open-label pilot study of levetiracetam (Keppra) for the treatment of levodopa-induced dyskinesias in Parkinson's disease

We evaluated the tolerability and preliminary efficacy of levetiracetam (LEV; Keppra) in reducing levodopa-induced dyskinesias in Parkinson's disease (PD) in an open-label pilot study. Nine PD patients who were experiencing peak-dose dyskinesias for at least 25% of the awake day and were at least moderately disabling were treated with LEV in doses up to 3,000 mg for up to 60 days. The primary outcome measure was the percent of the awake day that patients spent on without dyskinesia or with nontroublesome dyskinesia (good on time). The mean dose of LEV at endpoint was 625+/-277 mg/day. LEV significantly improved percent of the awake day on without dyskinesia or with nontroublesome dyskinesia at endpoint compared to baseline (43% +/- 12% vs. 61% +/- 17%; P=0.02). Percent on time with troublesome dyskinesia decreased from 23% +/- 10% at baseline to 11% +/- 6% at endpoint, although not significantly. There was no significant increase in off time from baseline to endpoint. There was a 56% dropout rate, mostly due to somnolence. In PD patients who experienced peak-dose dyskinesia for at least 25% of the awake day, LEV significantly improved on time without dyskinesia or with nontroublesome dyskinesia.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

Pharmacokinetics of dextromethorphan after single or multiple dosing in combination with quinidine in extensive and poor metabolizers

Dextromethorphan (DM) pharmacological properties predict that the widely used cough suppressant could be used to treat several neuronal disorders, but it is rapidly metabolized after oral dosing. To find out whether quinidine (Q), a CYP2D6 inhibitor, could elevate and prolong DM plasma profiles, 2 multiple-dose studies identified the lowest oral dose of Q that could be used in a fixed combination with 3 doses of DM. A multiple-dose study in healthy subjects with an extensive or a poor enzyme metabolizer phenotype evaluated the safety and pharmacokinetic profile of a selected fixed-dose combination (AVP-923). Study 1 randomized 46 healthy subjects, who were extensive CYP2D6 metabolizers, to receive 0, 2.5, 10, 25, 50, or 75 mg Q twice daily in combination with 30 mg DM for 7 days. Plasma and urine samples were collected after the first and last doses for the assay of DM, dextrorphan (DX), and Q. Study 2 randomized 65 healthy extensive CYP2D6 metabolizers to 8 groups given twice-daily 45- or 60-mg DM doses combined with 0, 30, 45, or 60 mg Q for 7 days. The effects of increasing Q were not different with doses greater than 25 mg, whereas lower doses showed a dose-related increase in plasma DM concentrations. Urinary ratios of DM/DX showed a Q dose- and time-related increase in the number of subjects converted to the poor metabolizer phenotype that reached 100% on day 3 of dosing with 25 mg Q. Results from both studies indicated that 25 to 30 mg Q is adequate to maximally suppress O-demethylation of DM. Study 3 evaluated 7 extensive metabolizers and 2 poor metabolizers given an oral capsule every 12 hours containing 30 mg Q combined with 30 mg DM. DM plasma AUC values increased in both groups of subjects during the 8-day study. The mean urinary metabolic ratio (DM/DX) increased at least 27-fold in extensive metabolizers by day 8. There was no effect of Q on urinary metabolic ratios in poor metabolizers. Safety evaluations, including electrocardiograms, indicated that the combination was well tolerated, with no difference between extensive and poor metabolizer phenotypes.

Subthalamic nucleus lesion reverses motor abnormalities and striatal glutamatergic overactivity in experimental parkinsonism

Subthalamic nucleus (STN) is a target of choice for the neurosurgical treatment of Parkinson's disease (PD). The therapeutic effect of STN lesion in PD is classically ascribed to the rescue of physiological activity in the output structures of the basal ganglia, and little is known about the possible involvement of the striatum. In the present study, therefore, we electrophysiologically recorded in vitro single striatal neurons of DA-depleted rats unilaterally lesioned by 6-hydroxydopamine, treated or not with therapeutic doses of levodopa (l-DOPA), or with a consecutive ipsilateral STN lesion. We show that the beneficial motor effects produced in parkinsonian rats by STN lesion or l-DOPA therapy were paralleled by the normalization of overactive frequency and amplitude of striatal glutamate-mediated spontaneous excitatory postsynaptic currents (sEPSCs). Since neither l-DOPA treatment nor STN lesion affected sEPSCs kinetic properties, the reversal of these abnormalities in striatal excitatory synaptic transmission can be attributable to the normalization of glutamate release.

Effects of sarizotan on the corticostriatal glutamate pathways

The effects of sarizotan, a 5-HT(1A) agonist with additional affinity for D(3) and D(4) receptors, have been studied on the corticostriatal glutamate pathways using dual-probe microdialysis in the awake rat. Sarizotan given systemically (0.1-10 mg/kg s.c.) or perfused into the motor cortex (10 microM) produced 20-30% reduction of cortical and striatal glutamate levels. The inhibitory effects of the systemic sarizotan on cortical and striatal glutamate levels were counteracted by intracortical perfusion with the 5-HT(1A) antagonist WAY100135 (10 microM). These findings suggest that the anti-dyskinetic properties of sarizotan could be mediated via its 5-HT(1A) agonist actions in the motor cortex, leading to reduced activity in the corticostriatal glutamate pathways with reduced activation of the striatopallidal GABA pathway mediating motor inhibition.

The basal ganglia: anatomy, physiology, and pharmacology

The basal ganglia are perceived as important nodes in cortico-subcortical networks involved in the transfer, convergence, and processing of information in motor, cognitive, and limbic domains. How this integration might occur remains a matter of some debate, particularly given the consistent finding in anatomic and physiologic studies of functional segregation in cortico-subcortical loops. More recent theories, however, have raised the notion that modality-specific information might be integrated not spatially, but rather temporally, by coincident processing in discrete neuronal populations. Basal ganglia neurotransmitters, given their diverse roles in motor performance, learning, working memory, and reward-related activity are also likely to play an important role in the integration of cerebral activity. Further work will elucidate this to a greater extent, but for now, it is clear that the basal ganglia form an important nexus in the binding of cognitive, limbic, and motor information into thought and action.

Chronic L-DOPA treatment increases extracellular glutamate levels and GLT1 expression in the basal ganglia in a rat model of Parkinson's disease

There is growing experimental evidence for the implication of glutamate-mediated mechanisms both in the pathophysiology of Parkinson's disease and in the development of dyskinesias with long-term administration of L-3,4-dihydroxyphenylalanine (L-DOPA). However, the impact of this treatment on glutamate transmission in the basal ganglia has been poorly investigated. In this study, we examined the effects of 6-hydroxydopamine-induced lesion of nigral dopamine neurons with or without subsequent chronic L-DOPA treatment on several parameters of glutamate system function in the rat striatum and substantia nigra pars reticulata. All the lesioned animals treated with L-DOPA developed severe dyskinesias. Extracellular glutamate levels, measured by microdialysis in freely moving conditions, and gene expression of the glial glutamate transporter GLT1, assessed by in situ hybridization, were unaffected by dopamine lesion or L-DOPA treatment alone, but were both markedly increased on the lesion side of rats with subsequent L-DOPA treatment. No change in the expression of the vesicular glutamate transporters vGluT1 and vGluT2 was measured in striatum. These data show that chronic L-DOPA treatment leading to dyskinesias increases basal levels of glutamate function in basal ganglia. The L-DOPA-induced overexpression of GLT1 may represent a compensatory mechanism involving astrocytes to limit glutamate overactivity and subsequent toxic processes.

The NR2B-selective NMDA receptor antagonist CP-101,606 exacerbates L-DOPA-induced dyskinesia and provides mild potentiation of anti-parkinsonian effects of L-DOPA in the MPTP-lesioned marmoset model of Parkinson's disease

In Parkinson's disease (PD), degeneration of the dopaminergic nigrostriatal pathway leads to enhanced transmission at NMDA receptors containing NR2B subunits. Previous studies have shown that some, but not all, NR2B-containing NMDA receptor antagonists alleviate parkinsonian symptoms in animal models of PD. Furthermore, enhanced NMDA receptor-mediated transmission underlies the generation of L-DOPA-induced dyskinesia (LID). The subunit content of NMDA receptors responsible for LID is not clear. Here, we assess the actions of the NMDA antagonist CP-101,606 in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset model of Parkinson's disease. CP-101,606 is selective for NMDA receptors containing NR2B subunits, with higher affinity for NR1/NR2B complexes compared to ternary NR1/NR2A/NR2B complexes. CP-101,606 had no significant effect on parkinsonian symptoms when administered as monotherapy over a range of doses (0.1-10 mg/kg). CP-101,606 provided a modest potentiation of the anti-parkinsonian actions of L-DOPA (8 mg/kg), although, at doses of 1 and 3 mg/kg, CP-101,606 exacerbated LID. Results of this study provide further evidence of differences in the anti-parkinsonian activity and effects on LID of the NR2B subunit selective NMDA receptor antagonists. These distinctions may reflect disparities in action on NR1/NR2B as opposed to NR1/NR2A/NR2B receptors.

The N-Methyl-d-aspartate Receptor Inhibitory Potencies of Aromatic Inhaled Drugs of Abuse: Evidence for Modulation by Cation-π Interactions

Benzene and several close structural analogs are inhaled drugs of abuse with general anesthetic activity. By virtue of their pi electron clouds, they may engage in attractive electrostatic interactions with cationic atomic charges on protein targets. In this study, we tested the hypothesis that inhaled drugs of abuse inhibit human N-methyl-D-aspartate (NMDA) receptors with potencies that correlate with their abilities to engage in cation-pi interactions. Electrophysiological techniques were used to define the NR1/NR2B NMDA receptor inhibitory concentrations of volatile benzene analogs, and computer modeling was used to quantify their abilities to engage in cation-pi interactions and their molecular volumes. In addition, each compound's octanol/gas partition coefficient (a measure of hydrophobicity) was quantified. All 18 compounds inhibited human NR1/NR2B NMDA receptors reversibly and in a concentration-dependent manner. NMDA receptor inhibitory potency correlated strongly with the ability to engage in cation-pi interactions, weakly with hydrophobicity, and was independent of molecular volume. This is consistent with the hypothesis that cation-pi interactions enhance the binding of inhaled drugs of abuse to the NMDA receptor and suggests that the receptor binding site(s) for these drugs possesses significant cationic character.

Neurochemical Mechanisms Induced by High Frequency Stimulation of the Subthalamic Nucleus: Increase of Extracellular Striatal Glutamate and GABA in Normal and Hemiparkinsonian Rats

High frequency stimulation (HFS) (130 Hz) of the subthalamic nucleus (STN) provides beneficial effects in patients suffering from severe parkinsonism, but the mechanisms underlying these clinical results remain to be clarified. To date, very little is known concerning the effects of STN-HFS on neurochemical transmission in the different basal ganglia nuclei and in particular the striatum. This study examines the effects of STN-HFS in intact and hemiparkinsonian rats on extracellular striatal glutamate (Glu) and GABA levels by means of intracerebral microdialysis. Unilateral STN-HFS was found to induce a significant bilateral increase of striatal Glu and GABA both in intact and in dopamine-lesioned animals. In intact rats, these increases were reversed by local administration of the D1 antagonist SCH 23390, but were potentiated by the D2 antagonist sulpiride. Potentiation was also observed after local administration of both D1 and D2 antagonists whose amplitude was similar to that measured in hemiparkinsonian rats. These data furnish the first evidence that STN-HFS influences striatal amino-acid transmission and that this influence is modulated by dopamine. They provide evidence that the effects of STN-HFS are not only restricted to the direct STN targets, but also involve adaptive changes within other structures of the basal ganglia circuitry.

Noradrenergic Drugs for Levodopa-Induced Dyskinesia

Dyskinesia frequently mars the long-term therapeutic response to levodopa (LD) in Parkinson's disease (PD). New treatment strategies for levodopa-induced dyskinesia (LID) currently being investigated include some that target the nondopaminergic pathways. Indeed, LID in parkinsonism can be modulated by drugs acting on different neurotransmitters including glutamate, gamma-aminobutyric acid, noradrenaline, acetylcholine, serotonin, adenosine, and cholecystokinin. In many cases, the possibility of using specific compounds to counteract LID was raised by the previously shown efficacy of such compounds in the treatment of other types of dyskinesia. More data are now available on drugs that act on the noradrenergic system. Two studies have recently shown how the alpha-2 adrenoreceptor antagonist idazoxan can significantly reduce LID in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned primate model of parkinsonism and in patients with advanced PD. The experimental paper, which studied the antagonistic action of idazoxan on dyskinesia induced by both LD and apomorphine in marmosets with MPTP-induced parkinsonism, showed that the pharmacologic mechanisms underlying LID and apomorphine-induced dyskinesia in PD are probably distinct. LD, although not apomorphine-induced, dyskinesia was found to be influenced by adrenoreceptor antagonists. Indeed, the action of alpha-2 adrenoreceptor antagonists may involve the blockade of the action of noradrenaline synthesized from LD. The hypothesis is that because dopamine agonists are not metabolized to noradrenaline, alpha-2 adrenoreceptor antagonists do not reduce dyskinesia produced by such agents. This finding is particularly relevant in planning clinical studies in which LD or dopamine agonist challenges are used to assess the potential antidyskinetic properties of new drugs. The clinical study assessed the effects of idazoxan on LID in 18 patients with advanced PD: An improvement in LID, without the reappearance of parkinsonian symptoms, was observed. The practical outcome of this research is that, although the mechanisms underlying the manifestations and the priming process for dyskinesia have yet to be fully elucidated, a nondopaminergic approach to therapy may provide an effective way of preventing, or at least limiting, the expression of involuntary movements in PD.

Cyclic AMP responsive element binding protein phosphorylation and persistent expression of levodopa-induced response alterations in unilateral nigrostriatal 6-OHDA lesioned rats

Activation of cAMP responsive element binding protein (CREB) has been increasingly implicated in the formation and maintenance of long-term memory. To elucidate molecular mechanisms that underlie the persisting alterations in motor response occurring with levodopa (L-dopa) treatment of parkinsonian patients, we evaluated the time course of these changes in relation to the activation of striatal CREB in 6-hydroxydopamine (6-OHDA) lesioned animals. Three weeks of twice-daily L-dopa treatment reduced the duration of the rotational response to acute L-dopa challenge in hemiparkinsonian rats, which lasted about 5 weeks after withdrawal of chronic L-dopa therapy. This shortened response duration, resembling human wearing-off fluctuations, was associated with a marked increase in Ser-133 phosphorylated CREB (pCREB) immunoreactivity in medium spiny neurons in dorsolateral striatum in response to acute dopaminomimetic challenge. Intermittent treatment with the D1 receptor-preferring agonist SKF 38393, but not the D2 receptor-preferring agonist quinpirole, produced a similar rise in CREB phosphorylation. The time course of changes in CREB phosphorylation correlated with the time course of changes in motor behavior after cessation of chronic L-dopa therapy. Both the altered motor response duration and the degree of CREB phosphorylation were attenuated by the intrastriatal administration of CREB antisense or protein kinase A inhibitor Rp-cAMPS. The results suggest that region-specific Ser-133 CREB phosphorylation in D1 receptor containing spiny neurons contributes to the persistence of the motor response alterations produced by intermittent stimulation of striatal dopaminergic receptors.

Gene transfer of constitutively active protein kinase C into striatal neurons accelerates onset of levodopa-induced motor response alterations in parkinsonian rats

Alterations in motor response that complicate levodopa treatment of Parkinson's disease appear to involve sensitization of striatal ionotropic glutamate receptors. Since protein kinase C (PKC)-mediated phosphorylation regulates glutamatergic receptors of the alpha-amino-3-hydroxyl-5-methyl-4-isoxazole propionic acid (AMPA) subtype and has been linked to several forms of behavioral plasticity, activation of PKC signaling in striatal spiny neurons may also contribute to the motor plasticity changes associated with chronic levodopa therapy. To evaluate this possibility, we sought to augment PKC signaling by using Herpes Simplex Virus type 1 vectors (pHSVpkcDelta) to directly transfer the catalytic domain of the PKCbetaII gene into striatal neurons of parkinsonian rats. Microinjection of pHSVpkcDelta vectors lead to the persistent expression of PkcDelta (35% loss over 21 days) in medium spiny neurons together with an increase in serine 831 phosphorylation on AMPA receptor GluR1 subunits and hastened the appearance of the shortened response duration produced by chronic levodopa treatment (P<0.05). In pHSVpkcDelta-infected animals, intrastriatal injection of the PKC inhibitor NPC-15437 (1.0 microg) attenuated both the increased GluR1 phosphorylation (P<0.01) and the accelerated onset of the levodopa-induced response modifications (P<0.01). However, in rats that received levodopa treatment for 21 days without the gene transfer, intrastriatal NPC-15437 had no effect on the response shortening or on GluR1 S831 phosphorylation. The results suggest that an increase in PKC-mediated signaling, including, in part, phosphorylation of AMPA receptors, on striatal spiny neurons may be sufficient to promote the initial appearance, but not necessary the ultimate expression, of the levodopa-induced motor response changes occurring in a rodent model of the human motor complication syndrome.

Striatal glutamatergic mechanisms and extrapyramidal movement disorders

The nonphysiologic stimulation of striatal dopaminergic receptors, as a result of disease- or drug-related denervation or intermittent excitation, triggers adaptive responses in the basal ganglia which contribute to the appearance of parkinsonian symptoms and later to the dyskinesias and other alterations in motor response associated with dopaminergic therapy. Current evidence suggests that these altered responses involve activation of signal transduction cascades in striatal medium spiny neurons linking dopaminergic to coexpressed ionotropic glutamatergic receptors of the N-methyl-D-aspartate (NMDA) and Alpha-amino-3-hydroxy-5-methyl-4-isoxazole proprionic acid (AMPA) classes. These intraneuronal signaling pathways appear capable of modifying the phosphorylation state of NMDA and AMPA receptor subunits; resultant sensitization enhances cortical glutamatergic input which in turn modifies striatal output in ways that compromise motor behavior. The regulation of these spiny neuron glutamate receptors can also be affected by the activation state of coexpressed nondopaminergic receptors as well as by changes associated with Huntington's disease. These observations lend new insight into molecular mechanisms contributing to the integration of synaptic inputs to spiny neurons. They also suggest novel approaches to the pharmacotherapy of extrapyramidal motor dysfunction.