MK801 suppresses the L-DOPA-induced increase of glutamate in striatum of hemi-Parkinson rats

4'-fluorocannabidiol associated with capsazepine restrains L-DOPA-induced dyskinesia in hemiparkinsonian mice: Contribution of anti-inflammatory and anti-glutamatergic mechanisms

We tested the efficacy of 4'-fluorocannabidiol (4'-F-CBD), a semisynthetic cannabidiol derivative, and HU-910, a cannabinoid receptor 2 (CB2) agonist in resolving l-DOPA-induced dyskinesia (LID). Specifically, we were interested in studying whether these compounds could restrain striatal inflammatory responses and rescue glutamatergic disturbances characteristic of the dyskinetic state. C57BL/6 mice were rendered hemiparkinsonian by unilateral striatal lesioning with 6-OHDA. Abnormal involuntary movements were then induced by repeated i.p. injections of l-DOPA + benserazide. After LID was installed, the effects of a 3-day treatment with 4'-F-CBD or HU-910 in combination or not with the TRPV1 antagonist capsazepine (CPZ) or CB2 agonists HU-308 and JWH015 were assessed. Immunostaining was conducted to investigate the impacts of 4'-F-CBD and HU-910 (with CPZ) on inflammation and glutamatergic synapses. Our results showed that the combination of 4'-F-CBD + CPZ, but not when administered alone, decreased LID. Neither HU-910 alone nor HU-910+CPZ were effective. The CB2 agonists HU-308 and JWH015 were also ineffective in decreasing LID. Both combination treatments efficiently reduced microglial and astrocyte activation in the dorsal striatum of dyskinetic mice. However, only 4'-F-CBD + CPZ normalized the density of glutamate vesicular transporter-1 (vGluT1) puncta colocalized with the postsynaptic density marker PSD95. These findings suggest that 4'-F-CBD + CPZ normalizes dysregulated cortico-striatal glutamatergic inputs, which could be involved in their anti-dyskinetic effects. Although it is not possible to rule out the involvement of anti-inflammatory mechanisms, the decrease in striatal neuroinflammation markers by 4'-F-CBD and HU-910 without an associated reduction in LID indicates that they are insufficient per se to prevent LID manifestations.

Synergistic effect of serotonin 1A and serotonin 1B/D receptor agonists in the treatment of L-DOPA-induced dyskinesia in 6-hydroxydopamine-lesioned rats

BACKGROUND

The gold standard for symptomatic relief of Parkinson's disease (PD) is L-DOPA. However, long-term treatment often leads to motor complications such as L-DOPA-induced dyskinesia (LID). While amantadine (Gocovri™) is the only approved therapy for dyskinesia in PD patients on the American market, it is associated with neurological side effects and limited efficacy. Thus, there remains a high unmet need for addressing LID in PD patients worldwide.

OBJECTIVE

The objective of this study was to evaluate the efficacy, safety and performance compared to approved treatments of the serotonin receptor 1A (5-HT1A) and 5-HT1B/D agonists buspirone and zolmitriptan in the 6-hydroxydopamine unilaterally lesioned rat model for PD.

METHODS

The hemiparkinsonian 6-OHDA-lesioned rats underwent chronic treatment with L-DOPA to induce dyskinesia and were subsequently used for efficacy testing of buspirone, zolmitriptan and comparison with amantadine, measured as abnormal involuntary movement (AIM) scores after L-DOPA challenge. Safety testing was performed in model and naïve animals using forelimb adjusting, rotarod and open field tests.

RESULTS

5-HT1A and 5-HT1B/D agonism effectively reduced AIM scores in a synergistic manner. The drug combination of buspirone and zolmitriptan was safe and did not lead to tolerance development following sub-chronic administration. Head-to-head comparison with amantadine showed superior performance of buspirone and zolmitriptan in the model.

CONCLUSIONS

The strong anti-dyskinetic effect found with combined 5-HT1A and 5-HT1B/D agonism renders buspirone and zolmitriptan together a meaningful treatment for LID in PD.

Contributive Role of TNF-α to L-DOPA-Induced Dyskinesia in a Unilateral 6-OHDA Lesion Model of Parkinson's Disease

Our present objective was to better characterize the mechanisms that regulate striatal neuroinflammation in mice developing L-DOPA-induced dyskinesia (LID). For that, we used 6-hydroxydopamine (6-OHDA)-lesioned mice rendered dyskinetic by repeated intraperitoneal injections of 3,4-dihydroxyphenyl-L-alanine (L-DOPA) and quantified ensuing neuroinflammatory changes in the dopamine-denervated dorsal striatum. LID development was associated with a prominent astrocytic response, and a more moderate microglial cell reaction restricted to this striatal area. The glial response was associated with elevations in two pro-inflammatory cytokines, tumor necrosis factor-α (TNF-α) and interleukin-1β. Treatment with the phytocannabinoid cannabidiol and the transient receptor potential vanilloid-1 (TRPV-1) channel antagonist capsazepine diminished LID intensity and decreased TNF-α levels without impacting other inflammation markers. To possibly reproduce the neuroinflammatory component of LID, we exposed astrocyte and microglial cells in culture to candidate molecules that might operate as inflammatory cues during LID development, i.e., L-DOPA, dopamine, or glutamate. Neither L-DOPA nor dopamine produced an inflammatory response in glial cell cultures. However, glutamate enhanced TNF-α secretion and GFAP expression in astrocyte cultures and promoted Iba-1 expression in microglial cultures. Of interest, the antidyskinetic treatment with cannabidiol + capsazepine reduced TNF-α release in glutamate-activated astrocytes. TNF-α, on its own, promoted the synaptic release of glutamate in cortical neuronal cultures, whereas cannabidiol + capsazepine prevented this effect. Therefore, we may assume that the release of TNF-α by glutamate-activated astrocytes may contribute to LID by exacerbating corticostriatal glutamatergic inputs excitability and maintaining astrocytes in an activated state through a self-reinforcing mechanism.

Glutamate-induced excitotoxicity in Parkinson's disease: The role of glial cells

Glutamate is the major excitatory neurotransmitter in the central nervous system. Glutamate transmission efficiency depends on the correct functionality and expression of a plethora of receptors and transporters, located both on neurons and glial cells. Of note, glutamate reuptake by dedicated transporters prevents its accumulation at the synapse as well as non-physiological spillover. Indeed, extracellular glutamate increase causes aberrant synaptic signaling leading to neuronal excitotoxicity and death. Moreover, extrasynaptic glutamate diffusion is strongly associated with glia reaction and neuroinflammation. Glutamate-induced excitotoxicity is mainly linked to an impaired ability of glial cells to reuptake and respond to glutamate, then this is considered a common hallmark in many neurodegenerative diseases, including Parkinson's disease (PD). In this review, we discuss the function of astrocytes and microglia in glutamate homeostasis, focusing on how glial dysfunction causes glutamate-induced excitotoxicity leading to neurodegeneration in PD.

Change of Extracellular Glutamate Level in Striatum during Deep Brain Stimulation of the Entopeduncular Nucleus in Rats

Objective

Globus pallidus interna (GPi) is acknowledged as an essential treatment for advanced Parkinson’s disease (PD). Nonetheless, the neurotransmitter study about its results is undiscovered. The goal of this research was to examine influences of entopeduncular nucleus (EPN) stimulation, identical to human GPi, in no-lesioned (NL) rat and 6-hydroxydopamine (6-HD)-lesioned rat on glutamate change in the striatum.

Methods

Extracellular glutamate level changes in striatum of NL category, NL with deep brain stimulation (DBS) category, 6-HD category, and 6-HD with DBS category were examined using microdialysis and high-pressure liquid chromatography. Tyrosine hydroxylase (TH) immunoreactivities in substantia nigra and striatum of the four categories were also analyzed.

Results

Extracellular glutamate levels in the striatum of NL with DBS category and 6-HD with DBS category were significantly increased by EPN stimulation compared to those in the NL category and 6-HD category. EPN stimulation had no significant effect on the expression of TH in NL or 6-HD category.

Conclusion

Clinical results of GPi DBS are not only limited to direct inhibitory outflow to thalamus. They also include extensive alteration within basal ganglia.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

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

Loss of the preferential control over the striato-nigral direct pathway by striatal NMDA receptors in a rat model of Parkinson's disease

By using multi-probe microdialysis we previously demonstrated that endogenous glutamate differentially regulates the activity of the striatal output pathways in vivo, through N-methyl-d-aspartate (NMDA) receptors containing the GluN2A or GluN2B subunits. Using the same approach, we presently investigate whether reverse dialysis of NMDA in the striatum differentially affects GABA release in the striatum and in striatal target areas, i.e. globus pallidus (GP) and substantia nigra reticulata (SNr). Moreover, we ask whether this control is altered under parkinsonian conditions. Intrastriatal NMDA perfusion (10 min) evoked GABA release more potently in SNr (1-100 μM) than in other regions (10-100 μM), suggesting preferential control over striato-nigral projection neurons. Intrastriatal NMDA more potently stimulated glutamate levels in the striatum (1-100 μM) and SNr (1-10 μM) than in GP (10 μM). Striatal dopamine denervation with 6-hydroxydopamine caused a leftward shift in the NMDA concentration-response curve. Intrastriatal NMDA elevated GABA levels at 0.1 μM (all regions) and 1 μM (striatum and GP only), but not at higher concentrations, indicating that, compared to naïve animals, the GABA response in SNr was attenuated. Attenuation of the glutamate response was also observed in SNr (NMDA effective only at 0.1 μM). Conversely, the glutamate response in GP was widened (NMDA effective in the 0.1-1 μM range). We conclude that NMDA preferentially stimulates the activity of the striato-nigral direct pathway under physiological conditions. In Parkinson's disease, dopamine loss compromises the NMDA ability to stimulate striato-nigral neurons, thus shifting the NMDA control towards the striato-pallidal ones.

The effect of striatal dopamine depletion on striatal and cortical glutamate: A mini-review

Understanding the interplay between the neurotransmitters dopamine and glutamate in the striatum has become the highlight of several theories of neuropsychiatric illnesses, such as schizophrenia. Using in vivo brain imaging in humans, alterations in dopamine and glutamate concentrations have been observed in several neuropsychiatric disorders. However, it is unclear a priori how alterations in striatal dopamine should modulate glutamate concentrations in the basal ganglia. In this selective mini-review, we examine the consequence of reducing striatal dopamine functioning on glutamate concentrations in the striatum and cortex; regions of interest heavily examined in the human brain imaging studies. We examine the predictions of the classical model of the basal ganglia, and contrast it with findings in humans and animals. The review concludes that chronic dopamine depletion (>4months) produces decreases in striatal glutamate levels which are consistent with the classical model of the basal ganglia. However, acute alterations in striatal dopamine functioning, specifically at the D2 receptors, may produce opposite affects. This has important implications for models of the basal ganglia and theorizing about neurochemical alterations in neuropsychiatric diseases. Moreover, these findings may help guide a priori hypotheses for (1)H-MRS studies measuring glutamate changes given alterations in dopaminergic functioning in humans.

The preferential nNOS inhibitor 7-nitroindazole and the non-selective one N(G)-nitro-L-arginine methyl ester administered alone or jointly with L-DOPA differentially affect motor behavior and monoamine metabolism in sham-operated and 6-OHDA-lesioned rats

Reciprocal interactions between nitrergic and dopaminergic systems play a key role in the control of motor behavior. In the present study, we performed a comparative analysis of motor behavior (locomotor activity, catalepsy, rotational behavior) and monoamine metabolism in the striatum and substantia nigra of unilaterally sham-operated and 6-OHDA-lesioned rats treated with the preferential neuronal nitric oxide synthase (nNOS) inhibitor 7-nitroindazole (7-NI) or the non-selective one N(G)-nitro-L-arginine methyl ester (L-NAME), alone or in combination with L-DOPA. Each NOS inhibitor given alone (50mg/kg) induced a distinct catalepsy 30 min after injection but only 7-NI impaired spontaneous locomotion after 10 min. In 6-OHDA-lesioned rats, chronic L-DOPA (25mg/kg) induced 2.5-h long contralateral rotations. 7-NI (30 and 50mg/kg) markedly reduced the intensity of L-DOPA-induced contralateral rotations while extending their duration until 4.5h whereas L-NAME (50 and 100mg/kg) only tended to attenuate their intensity without affecting the duration. 7-NI but not L-NAME significantly increased endogenous tissue DA levels in the nigrostriatal system of both sham-operated and 6-OHDA-lesioned rats. In L-DOPA-treated group, 7-NI significantly enhanced the L-DOPA-derived tissue DA content in this system and decreased the level of the intracellular DA metabolite DOPAC produced by monoamine oxidase (MAO). In contrast to 7-NI, L-NAME decreased markedly DA content and did not affect DOPAC level in the ipsilateral striatum. It means that the differences in 7-NI and L-NAME-mediated modulation of L-DOPA-induced behavioral and biochemical effects resulted not only from the inhibition of NOS activity but also from differences in their ability to inhibit MAO.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Alterations in the motor cortical and striatal glutamatergic system and D-serine levels in the bilateral 6-hydroxydopamine rat model for Parkinson's disease

Parkinson's disease (PD) is hallmarked by progressive degeneration of the substantia nigra pars compacta (SNc) neurons and is associated with aberrant glutamatergic activity. However, studies on the glutamatergic system in the motor cortex and striatum, two motor loop-related areas, are lacking in the clinically relevant bilateral SNc 6-hydroxydopamine (6-OHDA) rat model, and therefore led to the rationale behind the present investigations. Using Western blotting, the expression levels of the glial glutamate transporters, GLT-1 and GLAST, as well as xCT, the specific subunit of system xc(-), and the vesicular glutamate transporters, VGLUT1 and 2 were investigated at two different time points (1 week and 2 weeks) post-lesion. In addition, the total content of glutamate was measured. Moreover, the total D-serine levels were, to the best of our knowledge, studied for the first time in these two PD-related areas in the bilateral 6-OHDA rat model. In the motor cortex, no significant changes were observed in the different glutamate transporter expression levels in the bilaterally-lesioned rats. In the striatum, GLAST expression was significantly decreased at both time points whereas VGLUT1 and 2 expressions were significantly decreased 2 weeks after bilateral 6-OHDA lesion. Interestingly, bilateral 6-OHDA SNc lesion resulted in an enhancement of the total d-serine content in both motor cortex and striatum at 1 week post-lesion suggesting its possible involvement in the pathophysiology of PD. In conclusion, this study demonstrates disturbed glutamate and D-serine regulation in the bilateral SNc-lesioned brain which could contribute to the behavioral impairments in PD.

Progressive brain metabolic changes under deep brain stimulation of subthalamic nucleus in parkinsonian rats

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is an efficient neurosurgical treatment for advanced Parkinson's disease. Non-invasive metabolic neuroimaging during the course of DBS in animal models may contribute to our understanding of its action mechanisms. Here, DBS was adapted to in vivo proton magnetic resonance spectroscopy at 11.7 T in the rat to follow metabolic changes in main basal ganglia structures, the striatum, and the substantia nigra pars reticulata (SNr). Measurements were repeated OFF and ON acute and subchronic (7 days) STN-DBS in control and parkinsonian (6-hydroxydopamine lesion) conditions. Acute DBS reversed the increases in glutamate, glutamine, and GABA levels induced by the dopamine lesion in the striatum but not in the SNr. Subchronic DBS normalized GABA in both the striatum and SNr, and glutamate in the striatum. Taurine levels were markedly decreased under subchronic DBS in the striatum and SNr in both lesioned and unlesioned rats. Microdialysis in the striatum further showed that extracellular taurine was increased. These data reveal that STN-DBS has duration-dependent metabolic effects in the basal ganglia, consistent with development of adaptive mechanisms. In addition to counteracting defects induced by the dopamine lesion, prolonged DBS has proper effects independent of the pathological condition. Non-invasive metabolic neuroimaging might be useful to understand the physiological mechanisms of deep brain stimulation (DBS). Here, we demonstrate the feasibility of repeated high-field proton magnetic resonance spectroscopy of basal ganglia structures under subthalamic nucleus DBS in control and parkinsonian rats. Results show that DBS has both rapid and delayed effects either dependent or independent of disease state.

NMDA receptor antagonism potentiates the L-DOPA-induced extracellular dopamine release in the subthalamic nucleus of hemi-parkinson rats

Long term treatment with L-3,4-dihydroxyphenylalanine (L-DOPA) is associated with several motor complications. Clinical improvement of this treatment is therefore needed. Lesions or high frequency stimulation of the hyperactive subthalamic nucleus (STN) in Parkinson's disease (PD), alleviate the motor symptoms and reduce dyskinesia, either directly and/or by allowing the reduction of the L-DOPA dose. N-methyl-D-aspartate (NMDA) receptor antagonists might have similar actions. However it remains elusive how the neurochemistry changes in the STN after a separate or combined administration of L-DOPA and a NMDA receptor antagonist. By means of in vivo microdialysis, the effect of L-DOPA and/or MK 801, on the extracellular dopamine (DA) and glutamate (GLU) levels was investigated for the first time in the STN of sham and 6-hydroxydopamine-lesioned rats. The L-DOPA-induced DA increase in the STN was significantly higher in DA-depleted rats compared to shams. MK 801 did not influence the L-DOPA-induced DA release in shams. However, MK 801 enhanced the L-DOPA-induced DA release in hemi-parkinson rats. Interestingly, the extracellular STN GLU levels remained unchanged after nigral degeneration. Furthermore, administration of MK 801 alone or combined with L-DOPA did not alter the STN GLU levels in both sham and DA-depleted rats. The present study does not support the hypothesis that DA-ergic degeneration influences the STN GLU levels neither that MK 801 alters the GLU levels in lesioned and non-lesioned rats. However, NMDA receptor antagonists could be used as a beneficial adjuvant treatment for PD by enhancing the therapeutic efficacy of l-DOPA at least in part in the STN.

Reverse microdialysis of a 5-HT2A receptor antagonist alters extracellular glutamate levels in the striatum of the MPTP mouse model of Parkinson's disease

Clinical observations have suggested that antagonism of 5-HT2A receptors may benefit patients with parkinsonian symptomatology. The mechanism of the antiparkinsonian effects of 5-HT2A receptor antagonists has not been fully elucidated. We have shown that the selective 5-HT2A receptor antagonist M100907 [R-(+)-alpha-(2,3-dimethoxyphenyl)-1-[2-(4-fluorophenethyl)]-4-piperidinemethanol] improved motor impairments in mice treated with the parkinsonian neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). In Parkinson's disease (PD) patients and animal models of parkinsonism dopamine denervation is associated with increased cortico-striatal glutamatergic transmission. We hypothesized that 5-HT2A receptor antagonists may exert their antiparkinsonian effects by decreasing striatal glutamate. Here, using in vivo microdialysis, we have shown an increased basal level of extracellular striatal glutamate when measured 3weeks after MPTP administration. The local administration of M100907 to the striatum significantly decreased striatal extracellular glutamate levels in MPTP-treated and saline treated mice. Basal extracellular serotonin (5-HT) levels were also elevated, whereas dopamine (DA) levels were significantly reduced in the striatum of MPTP-treated mice. Infusion of M100907 into the striatum produced no effect on dopamine or 5-HT levels. Local application of tetrodotoxin suppressed glutamate, 5-HT and DA concentrations in striatal dialysates in the presence or absence of M100907. The striatal expression of the glutamate transporter GLT1 was unchanged. However, there was an upregulation of the expression of 5-HT2A receptors in the striatum of MPTP-treated animals. Our data provide further evidence of enhanced glutamatergic neurotransmission in parkinsonism and demonstrate that blocking 5-HT2A receptors in the striatum will normalize glutamatergic neurotransmission.

Differential effects of the NMDA receptor antagonist MK-801 on dopamine receptor D1- and D2-induced abnormal involuntary movements in a preclinical model

Dopamine-replacement therapy with l-DOPA is still the gold standard treatment for Parkinson's disease (PD). One drawback is the common development of l-DOPA-induced dyskinesia (LID) in patients, which can be as disabling as the disease itself. There is no satisfactory adjunct therapy available. Glutamatergic transmission in the basal ganglia circuitry has been shown to be an important player in the development of LID. The N-methyl-d-aspartate (NMDA) receptor antagonist MK-801 has previously been shown to reduce l-DOPA-induced abnormal involuntary movements (AIMs) in a rat preclinical model but only at concentrations that worsen parkinsonism. We investigated the contribution of the direct and indirect striatofugal pathways to these effects. In the direct pathway, dopamine D1 receptors (D1R) are expressed, whereas in the indirect pathway, dopamine D2 receptors (D2R) are expressed. We used the 6-hydroxydopamine-lesioned hemi-parkinsonian rat model initially primed with l-DOPA to induce dyskinesia. When the rats were then primed and probed with the D1R agonist SKF81297, co-injection of MK-801 worsened the D1R-induced limb, axial, and orolingual (LAO) AIMs by 18% (predominantly dystonic axial AIMs) but did not aggravate parkinsonian hypokinesia as reflected by a surrogate measure of ipsiversive rotations in this model. In contrast, when the rats were then primed and probed with the D2R agonist quinpirole, co-injection of MK-801 reduced D2R-induced LAO AIMs by 89% while inducing ipsiversive rotations. The data show that only inhibition of the indirect striatopallidal pathway is sufficient for the full anti-dyskinetic/pro-parkinsonian effects of the NMDA receptor antagonist MK-801, and that MK-801 modestly worsens dyskinesias that are due to activation of the direct striatonigral pathway alone. This differential activation of the glutamatergic systems in D1R- and D2R-mediated responses is relevant to current therapy for PD which generally includes a mixture of dopamine agonists and l-DOPA.

Molsidomine, a nitric oxide donor, modulates rotational behavior and monoamine metabolism in 6-OHDA lesioned rats treated chronically with L-DOPA

Some biochemical and histological studies of Parkinson's disease patients' brains and 6-OHDA-lesioned rats suggest that dopaminergic dennervation of the striatum leads to the nitrergic system hypofunction in this structure. Hence, recently the modulation of nitric oxide (NO)- soluble guanylyl cyclase-cyclic GMP signaling is considered to be a new target for the treatment of Parkinson's disease. The aim of our study was to examine the impact of chronic combined treatment with low doses of the NO donor molsidomine (2 and 4mg/kg) and L-DOPA (12.5 and 25mg/kg) on rotational behavior and monoamine metabolism in the striatum (STR) and substantia nigra (SN) of unilaterally 6-OHDA-lesioned rats. Chronic administration of molsidomine at a dose of 2mg/kg jointly with 25mg/kg of L-DOPA significantly decreased the number of contralateral rotations when compared to L-DOPA alone. Other combinations of the examined drug doses were less effective. The tissue DA levels in the ipsilateral STR and SN after the last chronic doses of molsidomine (2mg/kg) and L-DOPA (12.5 or 25mg/kg), were significantly higher than after L-DOPA alone. Chronic L-DOPA treatment alone or jointly with a lower dose of molsidomine decreased 5-HT levels and accelerated its catabolism in the examined structures. However, combination of a higher dose of molsidomine with L-DOPA (25mg/kg) did not reduce 5-HT content while its catabolism was less intensive. The obtained results show that low doses of molsidomine can modulate rotational behavior and tissue DA and 5-HT concentrations in the STR and SN of 6-OHDA-lesioned rats treated chronically with L-DOPA.

Does MPTP intoxication in mice induce metabolite changes in the nucleus accumbens? A ¹H nuclear MRS study

Using in vivo ¹H NMR spectroscopy in a mouse model of Parkinson's disease, we previously showed that glutamate concentrations in the dorsal striatum were highest after dopamine denervation associated with an increase in gamma-aminobutyric acid (GABA) and (Gln) glutamine levels. The aim of this study was to determine whether the changes previously observed in the motor part of the striatum were reproduced in a ventral part of the striatum, the nucleus accumbens (NAc). This study was carried out on controls and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-intoxicated mice. In vivo spectra were acquired for a voxel (8 μL) in the dorsal striatum, and in the NAc (1.56 μL). NMR acquisitions were first performed 10 days after the last MPTP injection in a basal condition [after saline intraperitoneal (i.p.) injection] and then in the same animal the week after basal NMR acquisitions, after acute levodopa administration (200 mg kg⁻¹, i.p.). Immunohistochemistry was used to determine the levels of (Glu) glutamate, glutamine synthetase (GS) and glutamic acid decarboxylase (GAD) isoform 67 in these two structures. The Glu, Gln and GABA concentrations obtained in the basal state were higher in the NAc of MPTP-intoxicated mice which have the higher dopamine denervation in the ventral tegmental area (VTA) and in the dorsal striatum. Levodopa decreased the levels of these metabolites in MPTP-intoxicated mice to levels similar to those in controls. In parallel, immunohistochemical staining showed that glutamate, GS and GAD67 immunoreactivity increased in the dorsal striatum of MPTP-intoxicated mice and in the NAc for animals with a severe dopamine denervation in VTA. These findings strongly supported a hyperactivity of the glutamatergic cortico-striatal pathway and changes in glial activity when the dopaminergic denervation in the VTA and substantia nigra pars compacta (SNc) was severe.

Striatal glutamate release in L-DOPA-induced dyskinetic animals

L-DOPA-induced dyskinesia is a common side effect developed after chronic treatment with 3,4-dihydroxyphenyl-l-alanine (l-DOPA) in Parkinson's disease. The biological mechanisms behind this side effect are not fully comprehended although involvement of dopaminergic, serotonergic, and glutamatergic systems has been suggested. The present study utilizes in vivo amperometry to investigate the impact from unilateral 6-hydroxydopamine lesions and l-DOPA (4 mg/kg, including benserazide 15 mg/kg) -induced dyskinetic behavior on striatal basal extracellular glutamate concentration and potassium-evoked glutamate release in urethane-anesthetized rats. Recordings were performed before and after local L-DOPA application in the striatum. In addition, effects from the 5-HT(1A) receptor agonist (2R)-(+)-8-hydroxy-2-(di-n-propylamino)tetralin hydrobromide (8-OHDPAT; 1 mg/kg) was assessed on glutamate release and on dyskinetic behavior. The results revealed a bilateral ≈ 30% reduction of basal extracellular glutamate concentration and attenuated potassium-evoked glutamate release after a unilateral dopamine-depletion in L-DOPA naïve animals. In dyskinetic subjects, basal glutamate concentration was comparable to normal controls, although potassium-evoked glutamate release was reduced to similar levels as in drug naïve dopamine-lesioned animals. Furthermore, acute striatal L-DOPA administration attenuated glutamate release in all groups, except in the dopamine-lesioned striatum of dyskinetic animals. Co-administration of 8-OHDPAT and L-DOPA decreased dyskinesia in dopamine-lesioned animals, but did not affect potassium-evoked glutamate release, which was seen in normal animals. These findings indicate altered glutamate transmission upon dopamine-depletion and dyskinesia.

The effect of adenosine A(2A) receptor antagonists on hydroxyl radical, dopamine, and glutamate in the striatum of rats with altered function of VMAT2

It has been shown that a decreased vesicular monoamine transporter (VMAT2) function and the disruption of dopamine (DA) storage is an early contributor to oxidative damage of dopamine neurons in Parkinson's disease (PD). In our previous study, we demonstrated that adenosine A(2A) receptor antagonists suppressed oxidative stress in 6-hydroxydopamine-treated rats suggesting that this effect may account for neuroprotective properties of drugs. In the present study, rats were injected with reserpine (10 mg/kg sc) and 18 h later the effect of the adenosine A(2A) receptor antagonists 8-(3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) on extracellular DA, glutamate and hydroxyl radical formation was studied in the rat striatum using in vivo microdialysis. By disrupting VMAT2 function, reserpine depleted DA stores, and increased glutamate and hydroxyl radical levels in the rat striatum. CSC (1 mg/kg) but not ZM 241385 (3 mg/kg) increased extracellular DA level and production of hydroxyl radical in reserpinised rats. Both antagonists decreased the reserpine-induced increase in extracellular glutamate. L-3,4-Dihydroxyphenylalanine (L-DOPA) (25 mg/kg) significantly enhanced extracellular DA, had no effect on reserpine-induced hydroxyl radical production and decreased extracellular glutamate concentration. CSC but not ZM 241385 given jointly with L-DOPA increased the effect of L-DOPA on extracellular DA and augmented the reserpine-induced hydroxyl radical production. CSC and ZM 241385 did not influence extracellular glutamate level, which was decreased by L-DOPA. It seems that by decreasing the MAO-dependent DA metabolism rate, CSC raised cytosolic DA and by DA autoxidation, it induced hydroxyl radical overproduction. Thus, the methylxanthine A(2A) receptor antagonists bearing properties of MAO-B inhibitor, like CSC, may cause a risk of oxidative stress resulting from dysfunctional DA storage mechanism in early PD.

Effects of subthalamic nucleus lesions and stimulation upon corticostriatal afferents in the 6-hydroxydopamine-lesioned rat

Abnormalities of striatal glutamate neurotransmission may play a role in the pathophysiology of Parkinson's disease and may respond to neurosurgical interventions, specifically stimulation or lesioning of the subthalamic nucleus (STN). The major glutamatergic afferent pathways to the striatum are from the cortex and thalamus, and are thus likely to be sources of striatal neuronally-released glutamate. Corticostriatal terminals can be distinguished within the striatum at the electron microscopic level as their synaptic vesicles contain the vesicular glutamate transporter, VGLUT1. The majority of terminals which are immunolabeled for glutamate but are not VGLUT1 positive are likely to be thalamostriatal afferents. We compared the effects of short term, high frequency, STN stimulation and lesioning in 6-hydroxydopamine (6OHDA)-lesioned rats upon striatal terminals immunolabeled for both presynaptic glutamate and VGLUT1. 6OHDA lesions resulted in a small but significant increase in the proportions of VGLUT1-labeled terminals making synapses on dendritic shafts rather than spines. STN stimulation for one hour, but not STN lesions, increased the proportion of synapses upon spines. The density of presynaptic glutamate immuno-gold labeling was unchanged in both VGLUT1-labeled and -unlabeled terminals in 6OHDA-lesioned rats compared to controls. Rats with 6OHDA lesions+STN stimulation showed a decrease in nerve terminal glutamate immuno-gold labeling in both VGLUT1-labeled and -unlabeled terminals. STN lesions resulted in a significant decrease in the density of presynaptic immuno-gold-labeled glutamate only in VGLUT1-labeled terminals. STN interventions may achieve at least part of their therapeutic effect in PD by normalizing the location of corticostriatal glutamatergic terminals and by altering striatal glutamatergic neurotransmission.

Striatal NTS1 , dopamine D2 and NMDA receptor regulation of pallidal GABA and glutamate release--a dual-probe microdialysis study in the intranigral 6-hydroxydopamine unilaterally lesioned rat

The current microdialysis study elucidates a functional interaction between the striatal neurotensin NTS(1) receptor and the striatal dopamine D(2) and N-methyl-d-aspartic acid (NMDA) receptors in the regulation of striatopallidal gamma-aminobutyric acid (GABA) and glutamate levels after an ipsilateral intranigral 6-hydroxydopamine-induced lesion of the ascending dopamine pathways to the striatum. Lateral globus pallidus GABA levels were higher in the lesioned group while no change was observed in striatal GABA and glutamate levels. The 6-hydroxydopamine-induced lesion did not alter the ability of intrastriatal NT (10 nm) to counteract the decrease in pallidal GABA and glutamate levels induced by the dopamine D(2) -like receptor agonist quinpirole (10 μm). A more pronounced increase in the intrastriatal NMDA- (10 μm) induced increase in pallidal GABA levels was observed in the lesioned group while it attenuated the increase in striatal glutamate levels and amplified the increase in pallidal glutamate levels compared with that observed in the controls. NT enhanced the NMDA-induced increase in pallidal GABA and glutamate and striatal glutamate levels; these effects were counteracted by the NTS(1) antagonist SR48692 (100 nm) in both groups. These findings demonstrate an inhibitory striatal dopamine D(2) and an excitatory striatal NMDA receptor regulation of striatopallidal GABA transmission in both groups. These actions are modulated by NT via antagonistic NTS(1) /D(2) and facilitatory NTS(1) /NMDA receptor-receptor interactions, leading to enhanced glutamate drive of the striatopallidal GABA neurons associated with motor inhibition, effects which all are counteracted by SR48692. Thus, NTS(1) antagonists in combination with conventional treatments may provide a novel therapeutic strategy in Parkinson's disease.

Neuroglial plasticity at striatal glutamatergic synapses in Parkinson's disease

Striatal dopamine denervation is the pathological hallmark of Parkinson's disease (PD). Another major pathological change described in animal models and PD patients is a significant reduction in the density of dendritic spines on medium spiny striatal projection neurons. Simultaneously, the ultrastructural features of the neuronal synaptic elements at the remaining corticostriatal and thalamostriatal glutamatergic axo-spinous synapses undergo complex ultrastructural remodeling consistent with increased synaptic activity (Villalba and Smith, 2011). The concept of tripartite synapses (TS) was introduced a decade ago, according to which astrocytes process and exchange information with neuronal synaptic elements at glutamatergic synapses (Araque et al., 1999a). Although there has been compelling evidence that astrocytes are integral functional elements of tripartite glutamatergic synaptic complexes in the cerebral cortex and hippocampus, their exact functional role, degree of plasticity and preponderance in other CNS regions remain poorly understood. In this review, we discuss our recent findings showing that neuronal elements at cortical and thalamic glutamatergic synapses undergo significant plastic changes in the striatum of MPTP-treated parkinsonian monkeys. We also present new ultrastructural data that demonstrate a significant expansion of the astrocytic coverage of striatal TS synapses in the parkinsonian state, providing further evidence for ultrastructural compensatory changes that affect both neuronal and glial elements at TS. Together with our limited understanding of the mechanisms by which astrocytes respond to changes in neuronal activity and extracellular transmitter homeostasis, the role of both neuronal and glial components of excitatory synapses must be considered, if one hopes to take advantage of glia–neuronal communication knowledge to better understand the pathophysiology of striatal processing in parkinsonism, and develop new PD therapeutics.

Effect of adenosine A(2A) receptor antagonists and L-DOPA on hydroxyl radical, glutamate and dopamine in the striatum of 6-OHDA-treated rats

A_2A adenosine receptor antagonists have been proposed as a new therapy of PD. Since oxidative stress plays an important role in the pathogenesis of PD, we studied the effect of the selective A_2A adenosine receptor antagonists 8-(-3-chlorostyryl)caffeine (CSC) and 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385) on hydroxyl radical generation, and glutamate (GLU) and dopamine (DA) extracellular level using a microdialysis in the striatum of 6-OHDA-treated rats. CSC (1 mg/kg) and ZM 241385 (3 mg/kg) given repeatedly for 14 days decreased the production of hydroxyl radical and extracellular GLU level, both enhanced by prior 6-OHDA treatment in dialysates from the rat striatum. CSC and ZM 241385 did not affect DA and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanilic acid (HVA) extracellular levels in the striatum of 6-OHDA-treated rats. l-DOPA (6 mg/kg) given twice daily for two weeks in the presence of benserazide (3 mg/kg) decreased striatal hydroxyl radical and glutamate extracellular level in 6-OHDA-treated rats. At the same time, l-DOPA slightly but significantly increased the extracellular levels of DOPAC and HVA. A combined repeated administration of l-DOPA and CSC or ZM 241385 did not change the effect of l-DOPA on hydroxyl radical production and glutamate extracellular level in spite of an enhancement of extracellular DA level by CSC and elevation of extracellular level of DOPAC and HVA by ZM 241385. The data suggest that the 6-OHDA-induced damage of nigrostriatal DA-terminals is related to oxidative stress and excessive release of glutamate. Administration of l-DOPA in combination with CSC or ZM 241385, by restoring striatal DA-glutamate balance, suppressed 6-OHDA-induced overproduction of hydroxyl radical.

Amantadine attenuates levodopa-induced dyskinesia in mice and rats preventing the accompanying rise in nigral GABA levels

Amantadine is the only drug marketed for treating levodopa-induced dyskinesia. However, its impact on basal ganglia circuitry in the dyskinetic brain, particularly on the activity of striatofugal pathways, has not been evaluated. We therefore used dual probe microdialysis to investigate the effect of amantadine on behavioral and neurochemical changes in the globus pallidus and substantia nigra reticulata of 6-hydroxydopamine hemi-lesioned dyskinetic mice and rats. Levodopa evoked abnormal involuntary movements (AIMs) in dyskinetic mice, and simultaneously elevated GABA release in substantia nigra reticulata (∼3-fold) but not globus pallidus. Glutamate levels were unaffected in both areas. Amantadine (40 mg/kg, i.p.), ineffective alone, attenuated (∼50%) AIMs expression and prevented the GABA rise. Moreover, it unraveled a facilitatory effect of levodopa on pallidal glutamate levels. Levodopa also evoked AIMs expression and a GABA surge (∼2-fold) selectively in the substantia nigra of dyskinetic rats. However, different from mice, glutamate levels rose simultaneously. Amantadine, ineffective alone, attenuated (∼50%) AIMs expression preventing amino acid increase and leaving unaffected pallidal glutamate. Overall, the data provide neurochemical evidence that levodopa-induced dyskinesia is accompanied by activation of the striato-nigral pathway in both mice and rats, and that the anti-dyskinetic effect of amantadine partly relies on the modulation of this pathway.

Role of the primary motor cortex in L-Dopa-induced dyskinesia and its modulation by 5-HT1A receptor stimulation

While serotonin 5-HT1A receptor (5-HT1AR) agonists reduce L-DOPA-induced dyskinesias (LID) by normalizing activity in the basal ganglia neurocircuitry, recent evidence suggests putative 5-HT1AR within the primary motor cortex (M1) may also contribute. To better characterize this possible mechanism, c-fos immunohistochemistry was first used to determine the effects of systemic administration of the full 5-HT1AR agonist ±8-OH-DPAT on L-Dopa-induced immediate early gene expression within M1 and the prefrontal cortex (PFC) of rats with unilateral medial forebrain bundle (MFB) dopamine (DA) lesions. Next, in order to determine if direct stimulation of 5-HT1AR within M1 attenuates the onset of LID, rats with MFB lesions were tested for L-Dopa-induced abnormal involuntary movements (AIMs) and rotations following M1 microinfusions of ±8-OH-DPAT with or without coadministration of the 5-HT1AR antagonist WAY100635. Finally, ±8-OH-DPAT was infused into M1 at peak dyskinesia to determine if 5-HT1AR stimulation attenuates established L-Dopa-induced AIMs and rotations. While no treatment effects were seen within the PFC, systemic ±8-OH-DPAT suppressed L-Dopa-induced c-fos within M1. Intra-M1 5-HT1AR stimulation diminished the onset of AIMs and this effect was reversed by WAY100635 indicating receptor specific effects. Finally, continuous infusion of ±8-OH-DPAT into M1 at peak dyskinesia alleviated L-Dopa-induced AIMs. Collectively, these findings support an integral role for M1 in LID and its modulation by local 5-HT1AR.

Local modulation of striatal glutamate efflux by serotonin 1A receptor stimulation in dyskinetic, hemiparkinsonian rats

Serotonin 1A receptor (5-HT(1A)R) agonists reduce both L-DOPA- and D1 receptor (D1R) agonist-mediated dyskinesia, but their anti-dyskinetic mechanism of action is not fully understood. Given that 5-HT(1A)R stimulation reduces glutamatergic neurotransmission in the dopamine-depleted striatum, 5-HT(1A)R agonists may diminish dyskinesia in part through modulation of pro-dyskinetic striatal glutamate levels. To test this, rats with unilateral medial forebrain bundle dopamine or sham lesions were primed with L-DOPA (12 mg/kg+benserazide, 15 mg/kg, sc) or the D1R agonist SKF81297 (0.8 mg/kg, sc) until abnormal involuntary movements (AIMs) stabilized. On subsequent test days, rats were treated with vehicle or the 5-HT(1A)R agonist ±8-OH-DPAT (1.0 mg/kg, sc), followed by L-DOPA or SKF81297, or intrastriatal ±8-OH-DPAT (7.5 or 15 mM), followed by L-DOPA. In some cases, the 5-HT(1A)R antagonist WAY100635 was employed to determine receptor-specific effects. In vivo microdialysis was used to collect striatal samples for analysis of extracellular glutamate levels during AIMs assessment. Systemic and striatal ±8-OH-DPAT attenuated L-DOPA-induced dyskinesia and striatal glutamate efflux while WAY100635 reversed ±8-OH-DPAT's effects. Interestingly, systemic ±8-OH-DPAT diminished D1R-mediated AIMs without affecting glutamate. These findings indicate a novel anti-dyskinetic mechanism of action for 5-HT(1A)R agonists with implications for the improved treatment of Parkinson's disease.

In vivo brain microdialysis: advances in neuropsychopharmacology and drug discovery

INTRODUCTION: Microdialysis is an important in vivo sampling technique, useful in the assay of extracellular tissue fluid. The technique has both pre-clinical and clinical applications but is most widely used in neuroscience. The in vivo microdialysis technique allows measurement of neurotransmitters such as acetycholine (ACh), the biogenic amines including dopamine (DA), norepinephrine (NE) and serotonin (5-HT), amino acids such as glutamate (Glu) and gamma aminobutyric acid (GABA), as well as the metabolites of the aforementioned neurotransmitters, and neuropeptides in neuronal extracellular fluid in discrete brain regions of laboratory animals such as rodents and non-human primates. AREAS COVERED: In this review we present a brief overview of the principles and procedures related to in vivo microdialysis and detail the use of this technique in the pre-clinical measurement of drugs designed to be used in the treatment of chemical addiction, neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD) and as well as psychiatric disorders such as attention-deficit/hyperactivity disorder (ADHD) and schizophrenia. This review offers insight into the tremendous utility and versatility of this technique in pursuing neuropharmacological investigations as well its significant potential in rational drug discovery. EXPERT OPINION: In vivo microdialysis is an extremely versatile technique, routinely used in the neuropharmacological investigation of drugs used for the treatment of neurological disorders. This technique has been a boon in the elucidation of the neurochemical profile and mechanism of action of several classes of drugs especially their effects on neurotransmitter systems. The exploitation and development of this technique for drug discovery in the near future will enable investigational new drug candidates to be rapidly moved into the clinical trial stages and to market thus providing new successful therapies for neurological diseases that are currently in demand.

Time-dependent changes in GLT-1 functioning in striatum of hemi-Parkinson rats

Striatal dopamine loss in Parkinson's disease is accompanied by a dysregulation of corticostriatal glutamatergic neurotransmission. Within this study, we investigated striatal expression and activity of the glial high-affinity Na(+)/K(+)-dependent glutamate transporters, GLT-1 and GLAST, in the 6-hydroxydopamine hemi-Parkinson rat model at different time points after unilateral 6-hydroxydopamine injection into the medial forebrain bundle. Using semi-quantitative Western blotting and an ex vivo D-[(3)H]-aspartate uptake assay, we showed a time-dependent bilateral effect of unilateral 6-hydroxydopamine lesioning on the expression as well as activity of GLT-1. At 3 and 12 weeks post-lesion, striatal GLT-1 function was bilaterally upregulated whereas at 5 weeks there was no change. Even though our data do not allow a straightforward conclusion as for the role of glutamate transporters in the pathogenesis of the disease, they do clearly demonstrate a link between disturbed glutamatergic neurotransmission and glutamate transporter functioning in the striatum of a rat model for Parkinson's disease.

Metabolic changes detected in vivo by 1H MRS in the MPTP-intoxicated mouse

We used in vivo proton ((1)H) Magnetic Resonance Spectroscopy (MRS) to measure the levels of the main excitatory amino acid, glutamate (Glu) and also glutamine (Gln) and GABA in the striatum and cerebral cortex in the MPTP-intoxicated mouse, a model of dopaminergic denervation, before and after dopamine (DA) replacement. The study was performed at 9.4T on control mice (n = 8) and MPTP-intoxicated mice (n = 8). In vivo spectra were acquired in a voxel (8 microL) centered in the striatum, and in the cortex (4.6 microL). Three days after basal MRS acquisitions new spectra were acquired in the striatum and cortex, after levodopa (200 mg.kg(-1)). Glu, Gln and GABA concentrations obtained in the basal state were significantly increased in the striatum of MPTP-lesioned mice (Glu: 20.2 +/- 0.8 vs 11.4 +/- 0.9 mM, p < 0.001; Gln: 5.4 +/- 1.6 vs 2.0 +/- 0.6 mM, p < 0.05; GABA: 3.6 +/- 0.8 vs 1.6 +/- 0.2 mM, p < 0.05). Levodopa lowered metabolites concentrations in the striatum of MPTP-lesioned mice (Glu: 20.2 +/- 0.8 vs 11.2 +/- 0.4 mM (+ Ldopa), p < 0.001; Gln: 5.4 +/- 1.6 vs 1.6 +/- 0.4 mM (+ Ldopa), p < 0.05; GABA: 3.6 +/- 0.8 vs 1.7 +/- 0.4 mM (+ Ldopa), p < 0.01). Metabolite levels in the striatum of MPTP-intoxicated mice + levodopa were not significantly different from those in the striatum of controls. No change was found in the cortex after DA denervation and after DA replacement between the two animals groups. These results strongly support a predominant change in striatal Glu synaptic activity in the cortico-striatal pathway. Acute levodopa administration reverses the increase of metabolites in the striatum.

Potential future neuroprotective therapies for neurodegenerative disorders and stroke

The cellular mechanisms underlying neuronal loss and neurodegeneration have been an area of interest in the last decade. Although neurodegenerative diseases such as Alzheimer disease, Parkinson disease, and Huntington disease each have distinct clinical symptoms and pathologies, they all share common mechanisms such as protein aggregation, oxidative injury, inflammation, apoptosis, and mitochondrial injury that contribute to neuronal loss. Although cerebrovascular disease has different causes from the neurodegenerative disorders, many of the same common disease mechanisms come into play following a stroke. Novel therapies that target each of these mechanisms may be effective in decreasing the risk of disease, abating symptoms, or slowing down their progression. Although most of these therapies are experimental, and require further investigation, a few seem to offer promise.

Site-specific action of L-3,4-dihydroxyphenylalanine in the striatum but not globus pallidus and substantia nigra pars reticulata evokes dyskinetic movements in chronic L-3,4-dihydroxyphenylalanine-treated 6-hydroxydopamine-lesioned rats

Dyskinesia eventually develops in the majority of Parkinson's disease patients treated with l-3,4-dihydroxyphenylalanine (l-DOPA). We have investigated the effect of an acute and local administration of L-DOPA, GABA and glutamate to provoke dyskinetic movements in three basal ganglia structures (striatum, globus pallidus (GP) and substantia nigra pars reticulata (SNr)) of chronically L-DOPA-treated, unilaterally 6-hydroxydopamine-lesioned rats. We demonstrated that L-DOPA administration into the lesioned striatum using the technique of reverse in vivo microdialysis was an effective trigger to switch on dyskinesia. Notably, local L-DOPA perfusion at the same concentration in the ipsilateral GP and SNr did not provoke significant dyskinetic behaviour. Neither GABA nor glutamate triggered dyskinetic movements in the striatum, GP or SNr. We postulate a site-specific action of L-DOPA for the evocation of already established dyskinesia since L-DOPA in the striatum but not in the GP or SNr switched on dyskinetic behaviour.

Striatal GPR88 expression is confined to the whole projection neuron population and is regulated by dopaminergic and glutamatergic afferents

GPR88, an orphan G protein-coupled receptor, was designated Strg/GPR88 for striatum-specific G protein-coupled receptor (K. Mizushima et al. (2000)Genomics, 69, 314-321). In this study, we focused on striatal GPR88 protein localization using a polyclonal antibody. We established that the distribution of immunoreactivity in rat brain matched that of GPR88 transcripts and provided evidence for its exclusive neuronal expression. GPR88 protein is abundant throughout the striatum of rat and primate, with expression limited to the two subsets of striatal projection medium spiny neurons (MSNs) expressing preprotachykinin-substance P or preproenkephalin mRNAs. Ultrastructural immunolabelling revealed the GPR88 concentration at post-synaptic sites along the somatodendritic compartments of MSNs, with pronounced preference for dendrites and dendritic spines. The GPR88-rich expression, in both striatal output pathways, designates this receptor as a potential therapeutic target for diseases involving dysfunction of the basal ganglia, such as Parkinson's disease. Hence, we investigated changes of GPR88 expression in a model of Parkinson's disease (unilateral 6-hydroxydopamine-lesioned rats) following repeated L-DOPA treatment. In dopamine-depleted striatum, GPR88 expression was differentially regulated, i.e. decreased in striatopallidal and increased in striatonigral MSNs. L-DOPA treatment led to a normalization of GPR88 levels through dopamine D1 and D2 receptor-mediated mechanisms in striatopallidal and striatonigral MSNs, respectively. Moreover, the removal of corticostriatal inputs, by ibotenate infusion, downregulated GPR88 in striatopallidal MSNs. These findings provide the first evidence that GPR88 is confined to striatal MSNs and indicate that L-DOPA-mediated behavioural effects in hemiparkinsonian rats may involve normalization of striatal GPR88 levels probably through dopamine receptor-mediated mechanisms and modulations of corticostriatal pathway activity.

Assessment of metabolic changes in the striatum of a rat model of parkinsonism: an in vivo (1)H MRS study

Degeneration of the dopaminergic neurons of the substantia nigra pars compacta in Parkinson's disease induces an abnormal activation of the glutamatergic neurotransmission system within the basal ganglia network and related structures. The aim of this study was to use proton MRS to show metabolic changes in the striatum of 6-hydroxydopamine-lesioned rats, a rodent animal model of Parkinson's disease. Animals were examined before and after extensive lesioning of the nigral dopaminergic neurons and after acute administration of L-3,4-dihydroxyphenylalanine. No significant alterations in glutamate concentrations, assessed by the MR signal dominated by glutamate with minor contributions from glutamine and gamma-aminobutyric acid, could be measured. The total choline/total creatine ratio was found to be reduced in the striatum of the ipsilateral hemisphere.

In vivo microdialysis in Parkinson's research

Parkinson's disease (PD) is a progressive neurodegenerative disorder that is primarily characterized by the degeneration of dopamine (DA) neurons in the nigrostriatal system, which in turn produces profound neurochemical changes within the basal ganglia, representing the neural substrate for parkinsonian motor symptoms. The pathogenesis of the disease is still not completely understood, but environmental and genetic factors are thought to play important roles. Research into the pathogenesis and the development of new therapeutic intervention strategies that will slow or stop the progression of the disease in human has rapidly advanced by the use of neurotoxins that specifically target DA neurons. Over the years, a broad variety of experimental models of the disease has been developed and applied in diverse animal species. The two most common toxin models used employ 6-hydroxydopamine (6-OHDA) and the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/1-methyl-4-phenilpyridinium ion (MPTP/MPP+), either given systemically or locally applied into the nigrostriatal pathway, to resemble PD features in animals. Both neurotoxins selectively and rapidly destroy catecolaminergic neurons, although with different mechanisms. Since in vivo microdialysis coupled to high-performance liquid chromatography is an established technique for studying physiological, pharmacological, and pathological changes of a wide range of low molecular weight substances in the brain extracellular fluid, here we review the most prominent animal and human data obtained by the use of this technique in PD research.

Evaluation of behavioural effects of a selective NMDA NR1A/2B receptor antagonist in the unilateral 6-OHDA lesion rat model

The degeneration of the dopaminergic nigrostriatal pathway in Parkinson's disease (PD) is associated with altered transmission at striatal NMDA receptors containing NR2B subunits. We investigated a potential novel therapeutic compound, 4-trifluoromethoxy-N-(2-trifluoromethyl-benzyl)-benzamidine (BZAD-01), a selective NMDA NR1A/2B receptor antagonist for PD and compared it with levodopa, the standard treatment for PD. This study also evaluated whether combining levodopa and BZAD-01 gave better improvements of parkinsonian symptoms. Parkinsonism was induced by microinjection of the toxin, 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle (MFB) of 40 Sprague-Dawley rats. Parkinsonism and the efficacy of drugs were assessed using a battery of behavioural tests including balance beam, apomorphine-induced rotation, body axis bias or "curling", head position bias and disengage sensorimotor latency test. Immunohistochemistry was performed on post-mortem tissue to estimate the loss of dopaminergic neurons. The main effects were that BZAD-01 co-administration prevented chronic levodopa-induced potentiation of apomorphine rotation. However levodopa-treated rats were slower than either controls or BZAD-01-treated rats in the locomotor test. The improvement in the apomorphine rotation test suggests that BZAD-01 may be a useful adjunct to levodopa monotherapy.

Association of protein phosphatase 1 gamma 1 with spinophilin suppresses phosphatase activity in a Parkinson disease model

Sustained nigrostriatal dopamine depletion increases the serine/threonine phosphorylation of multiple striatal proteins that play a role in corticostriatal synaptic plasticity, including Thr(286) phosphorylation of calcium/calmodulin-dependent protein kinase IIalpha (CaMKIIalpha). Mechanisms underlying these changes are unclear, but protein phosphatases play a critical role in the acute modulation of striatal protein phosphorylation. Here we show that dopamine depletion for periods ranging from 3 weeks to 10 months significantly reduces the total activity of protein phosphatase (PP) 1, but not of PP2A, in whole lysates of rat striatum, as measured using multiple substrates, including Thr(286)-autophosphorylated CaMKIIalpha. Striatal PP1 activity is partially inhibited by a fragment of the PP1-binding protein neurabin-I, Nb-(146-493), because of the selective inhibition of the PP1gamma(1) isoform. The fraction of PP1 activity that is insensitive to Nb-(146-493) was unaffected by dopamine depletion, demonstrating that dopamine depletion specifically reduces the activity of PP1 isoforms that are sensitive to Nb-(146-493) (i.e. PP1gamma(1)). However, total striatal levels of PP1gamma(1) or any other PP1 isoform were unaffected by dopamine depletion, and our previous studies showed that total levels of the PP1 regulatory/targeting proteins DARPP-32, spinophilin, and neurabin were also unchanged. Rather, co-immunoprecipitation experiments demonstrated that dopamine depletion increases the association of PP1gamma(1) with spinophilin in striatal extracts. In combination, these data demonstrate that striatal dopamine depletion inhibits a specific synaptic phosphatase by increasing PP1gamma(1) interaction with spinophilin, perhaps contributing to hyperphosphorylation of synaptic proteins and disruptions of synaptic plasticity and/or dendritic morphology.

Metabolic interactions between glutamatergic and dopaminergic neurotransmitter systems are mediated through D(1) dopamine receptors

Interactions between the dopaminergic and glutamatergic neurotransmission systems were investigated in the adult brain of wild-type (WT) and transgenic mice lacking the dopamine D(1) or D(2) receptor subtypes. Activity of the glutamine cycle was evaluated by using (13)C NMR spectroscopy, and striatal activity was assessed by c-Fos expression and motor coordination. Brain extracts from (1,2-(13)C(2)) acetate-infused mice were prepared and analyzed by (13)C NMR to determine the incorporation of the label into the C4 and C5 carbons of glutamate and glutamine. D(1)R(-/-) mice showed a significantly higher concentration of cerebral (4,5-(13)C(2)) glutamine, consistent with an increased activity of the glutamate-glutamine cycle and of glutamatergic neurotransmission. Conversely, D(2)R(-/-) mice did not show any significant changes in (4,5-(13)C(2)) glutamate or (4,5-(13)C(2)) glutamine, suggesting that alterations in glutamine metabolism are mediated through D(1) receptors. This was confirmed with D(1)R(-/-) and WT mice treated with reserpine, a dopamine-depleting drug, or with reserpine followed by L-DOPA, a dopamine precursor. Exposure to reserpine increased (4,5-(13)C(2)) glutamine in WT to levels similar to those found in untreated D(1)R(-/-) mice. These values were the same as those reached in the reserpine-treated D(1)R(-/-) mice. Treatment of WT animals with L-DOPA returned (4,5-(13)C(2)) glutamine levels to normal, but this was not verified in D(1)R(-/-) animals. Reserpine impaired motor coordination and decreased c-Fos expression, whereas L-DOPA restored both variables to normal values in WT but not in D(1)R(-/-). Together, our results reveal novel neurometabolic interactions between glutamatergic and dopaminergic systems that are mediated through the D(1), but not the D(2), dopamine receptor subtype.

Metabolic changes detected by proton magnetic resonance spectroscopy in vivo and in vitro in a murin model of Parkinson's disease, the MPTP-intoxicated mouse

Parkinson's disease is a neurodegenerative disorder characterized by the progressive loss of the dopaminergic neurons in the substantia nigra pars compacta, which project to the striatum. The aim of this study was to analyze in vivo and in vitro consequences of dopamine depletion on amount of metabolites in a mouse model of Parkinson's disease using proton (1)H magnetic resonance spectroscopy (MRS). The study was performed on control mice (n = 7) and MPTP-intoxicated mice (n = 7). All the experiments were performed at 9.4 T. For in vivo MRS acquisitions, mice were anesthetized and carefully placed on an animal handling system with the head centered in birdcage coil used for both excitation and signal reception. Spectra were acquired in a voxel (8 microL) centered in the striatum, applying a point-resolved spectroscopy sequence (TR = 4000 ms, TE = 8.8 ms). After in vivo MRS acquisitions, mice were killed; successful lesion verified by tyrosine hydroxylase immunolabeling on the substantia nigra pars compacta and in vitro MRS acquisitions performed on perchloric extracts of anterior part of mice brains. In vitro spectra were acquired using a standard one-pulse experiment. The absolute concentrations of metabolites were determined using jmrui (Lyon, France) from (1)H spectra obtained in vivo on striatum and in vitro on perchloric extracts. Glutamate (Glu), glutamine (Gln), and GABA concentrations obtained in vivo were significantly increased in striatum of MPTP-lesioned mice (Glu: 15.5 +/- 2.5 vs. 12.9 +/- 1.0 mmol/L, p < 0.05; Gln: 2.3 +/- 0.9 vs. 1.8 +/- 0.6 mmol/L, p < 0.05; GABA: 2.3 +/- 0.9 vs. 1.3 +/- 0.6 mmol/L, p < 0.05). The in vitro results confirmed these results, Glu (10.9 +/- 2.5 vs. 7.9 +/- 1.7 micromol/g, p < 0.05), Gln (6.8 +/- 2.9 vs. 4.3 +/- 1.0 micromol/g, p < 0.05), and GABA (2.9 +/- 0.9 vs. 1.5 +/- 0.4 micromol/g, p < 0.01). The present study strongly supports a hyperactivity of the glutamatergic cortico-striatal pathway hypothesis after dopaminergic denervation in association with an increase of striatal GABA levels. It further shows an increased of striatal Gln concentrations, perhaps as a strategy to protect neurons from Glu excitotoxic injury after striatal dopamine depletion.

Glutamate measurement in Parkinson's disease using MRS at 3 T field strength

Loss of nigral dopamine neurons in Parkinson's disease induces abnormal activation of glutamate systems in the basal ganglia. The purpose of this study was to assess these changes in the lentiform nucleus using MRS with optimized glutamate sensitivity (TE-averaged method). Ten patients with Parkinson's disease and 10 healthy controls were examined. Compared with healthy controls, no significant differences in glutamate were measured in patients, but a trend to lower total creatine was observed.