Nigrostriatal denervation does not affect glutamate transporter mRNA expression but subsequent levodopa treatment selectively increases GLT1 mRNA and protein expression in the rat striatum

Serotonin 1A receptor agonist modulation of motor deficits and cortical oscillations by NMDA receptor interaction in parkinsonian rats

Although serotonin 1A (5-HT1A) receptor agonists are widely used as the additive compound to reduce l-dopa-induced dyskinesia in Parkinson's disease (PD), few studies focused on the effect and mechanism of 5-HT1A receptor agonist on the motor symptoms of PD. Unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats were used and implantation of electrodes was performed in the motor cortex of these rats. So the effect of 5-HT1A receptor agonist 8-OH-DPAT on motor behaviors and oscillatory activities were evaluated. In addition, 8-OH-DPAT combined with D2 receptor antagonist raclopride, NMDA receptor antagonist MK-801, or its agonist d-cycloserine (DCS) were co-administrated. 8-OH-DPAT administration significantly improved spontaneous locomotor activity and asymmetric forepaw function in 6-OHDA-lesioned rats. Meanwhile, 8-OH-DPAT identified selective modulation of the abnormal high beta oscillations (25-40 Hz) in the motor cortex of 6-OHDA-lesioned rats, without inducing pathological finely tuned gamma around 80 Hz. Different from 8-OH-DPAT, l-dopa treatment produced a prolonged improvement on motor performances and differential regulation of high beta and gamma oscillations. However, dopamine D2 receptor antagonist had no influence on the 8-OH-DPAT-mediated-motor behaviors and beta oscillations in 6-OHDA-lesioned rats. In contrast, subthreshold NMDA receptor antagonist MK-801 obviously elevated the 8-OH-DPAT-mediated-motor behaviors, while NMDA receptor agonist DCS partially impaired the 8-OH-DPAT-mediated symptoms in 6-OHDA-lesioned rats. This study suggests that 5-HT1A receptor agonist 8-OH-DPAT improves motor activity and modulates the oscillations in the motor cortex of parkinsonian rats. Different from l-dopa, 8-OH-DPAT administration ameliorates motor symptoms of PD through glutamatergic rather than the dopaminergic pathway.

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.

Glutamate Transporter GLT-1 as a Therapeutic Target for Substance Use Disorders

The development of new treatments for substance use disorders requires identification of targetable molecular mechanisms. Pathology in glutamatergic neurotransmission system in brain reward circuitry has been implicated in relapse to multiple classes of drugs. Glutamate transporter 1 (GLT-1) crucially regulates glutamatergic signaling by removing excess glutamate from the extrasynaptic space. The purpose of this review is to highlight the effects of addictive drug use on GLT-1 and glutamate uptake, and using GLT-1 as a target in addiction pharmacotherapy. Cocaine, opioids, ethanol, nicotine, amphetamines, and cannabinoids each affect GLT-1 expression and glutamate uptake, and restoring GLT-1 expression with N-acetylcysteine or ceftriaxone shows promise in correcting pre-clinical and clinical manifestations of drug addiction.

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.

Role of transcription factor yin yang 1 in manganese-induced reduction of astrocytic glutamate transporters: Putative mechanism for manganese-induced neurotoxicity

Astrocytes are the most abundant non-neuronal glial cells in the brain. Once relegated to a mere supportive role for neurons, contemporary dogmas ascribe multiple active roles for these cells in central nervous system (CNS) function, including maintenance of optimal glutamate levels in synapses. Regulation of glutamate levels in the synaptic cleft is crucial for preventing excitotoxic neuronal injury. Glutamate levels are regulated predominantly by two astrocytic glutamate transporters, glutamate transporter 1 (GLT-1) and glutamate aspartate transporter (GLAST). Indeed, the dysregulation of these transporters has been linked to several neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS), Alzheimer's disease (AD) and Parkinson's disease (PD), as well as manganism, which is caused by overexposure to the trace metal, manganese (Mn). Although Mn is an essential trace element, its excessive accumulation in the brain as a result of chronic occupational or environmental exposures induces a neurological disorder referred to as manganism, which shares common pathological features with Parkinsonism. Mn decreases the expression and function of both GLAST and GLT-1. Astrocytes are commonly targeted by Mn, and thus reduction in astrocytic glutamate transporter function represents a critical mechanism of Mn-induced neurotoxicity. In this review, we will discuss the role of astrocytic glutamate transporters in neurodegenerative diseases and Mn-induced neurotoxicity.

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

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

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.

High-frequency stimulation of the subthalamic nucleus modifies the expression of vesicular glutamate transporters in basal ganglia in a rat model of Parkinson's disease

Background

It has been suggested that glutamatergic system hyperactivity may be related to the pathogenesis of Parkinson’s disease (PD). Vesicular glutamate transporters (VGLUT1-3) import glutamate into synaptic vesicles and are key anatomical and functional markers of glutamatergic excitatory transmission. Both VGLUT1 and VGLUT2 have been identified as definitive markers of glutamatergic neurons, but VGLUT 3 is also expressed by non glutamatergic neurons. VGLUT1 and VGLUT2 are thought to be expressed in a complementary manner in the cortex and the thalamus (VL/VM), in glutamatergic neurons involved in different physiological functions. Chronic high-frequency stimulation (HFS) of the subthalamic nucleus (STN) is the neurosurgical therapy of choice for the management of motor deficits in patients with advanced PD. STN-HFS is highly effective, but its mechanisms of action remain unclear. This study examines the effect of STN-HFS on VGLUT1-3 expression in different brain nuclei involved in motor circuits, namely the basal ganglia (BG) network, in normal and 6-hydroxydopamine (6-OHDA) lesioned rats.

Results

Here we report that: 1) Dopamine(DA)-depletion did not affect VGLUT1 and VGLUT3 expression but significantly decreased that of VGLUT2 in almost all BG structures studied; 2) STN-HFS did not change VGLUT1-3 expression in the different brain areas of normal rats while, on the contrary, it systematically induced a significant increase of their expression in DA-depleted rats and 3) STN-HFS reversed the decrease in VGLUT2 expression induced by the DA-depletion.

Conclusions

These results show for the first time a comparative analysis of changes of expression for the three VGLUTs induced by STN-HFS in the BG network of normal and hemiparkinsonian rats. They provide evidence for the involvement of VGLUT2 in the modulation of BG cicuits and in particular that of thalamostriatal and thalamocortical pathways suggesting their key role in its therapeutic effects for alleviating PD motor symptoms.

Glutamate transporters: a key piece in the glutamate puzzle of major depressive disorder

Glutamatergic therapies are emerging as the new path for the treatment of Major Depression Disorder. Recent reports reviewing the use of glutamate activity modulators in the treatment of resistant depression advocate the importance of understanding the alterations of the diverse components of this complex system in mood disorders. In this postmortem study we used in situ hybridization and microarray analysis to evaluate the gene expression of the membrane transporters SLC1A2 and SLCA3 and the vesicular transporter SLCA17A7 in the hippocampus of Major Depressive Disorder (MDD) and Bipolar Disorder (BPD) subjects. Samples from 8 controls, 11 MDD and 6 BPD subjects were processed for in situ hybridization using cRNA probes for SLC1A2, SLC1A3 and SLC17A7. Laser capture microdissection was used to collect tissue from adjacent sections for microarray analysis. The results showed that the expression of the membrane transporters SLC1A2 and SLC1A3 was diminished in the MDD group compared to controls. The expression of the vesicular glutamate transporter SLC17A7 on the other hand was increased in MDD subjects. As for the BPD group, all three transporters showed trends similar to those observed in MDD, but the changes observed did not reach significance. We hypothesize that the decreased expression of the membrane glutamate transporters and the increased expression of the vesicular transporter in the hippocampus would affect the balance of the glutamatergic circuitry of the hippocampus, and that this effect may be a major contributor to depressive symptoms.

Riluzole neuroprotection in a Parkinson's disease model involves suppression of reactive astrocytosis but not GLT-1 regulation

Background

Riluzole is a neuroprotective drug used in the treatment of motor neurone disease. Recent evidence suggests that riluzole can up-regulate the expression and activity of the astrocyte glutamate transporter, GLT-1. Given that regulation of glutamate transport is predicted to be neuroprotective in Parkinson's disease, we tested the effect of riluzole in parkinsonian rats which had received a unilateral 6-hydroxydopamine injection into the median forebrain bundle.

Results

Rats were treated with intraperitoneal riluzole (4 mg/kg or 8 mg/kg), 1 hour before the lesion then once daily for seven days. Riluzole produced a modest but significant attenuation of dopamine neurone degeneration, assessed by suppression of amphetamine-induced rotations, preservation of tyrosine hydroxylase positive neuronal cell bodies in the substantia nigra pars compacta and attenuation of striatal tyrosine hydroxylase protein loss. Seven days after 6-hydroxydopamine lesion, reactive astrocytosis was observed in the striatum, as determined by increases in expression of glial fibrillary acidic protein, however the glutamate transporter, GLT-1, which is also expressed in astrocytes was not regulated by the lesion.

Conclusions

The results confirm that riluzole is a neuroprotective agent in a rodent model of parkinson's disease. Riluzole administration did not regulate GLT-1 levels but significantly reduced GFAP levels, in the lesioned striatum. Riluzole suppression of reactive astrocytosis is an intriguing finding which might contribute to the neuroprotective effects of this drug.

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.

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.

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.

Effects of coincident 5-HT1A receptor stimulation and NMDA receptor antagonism on L-DOPA-induced dyskinesia and rotational behaviors in the hemi-parkinsonian rat

RATIONALE

Serotonin 1A receptor (5-HT1AR) agonists reduce L-DOPA-induced dyskinesia and enhance motor function in experimental and clinical investigations of Parkinson's disease (PD). While the mechanism(s) by which these effects occur are unclear, recent research suggests that modulation of glutamate neurotransmission contributes.

OBJECTIVE

To further delineate the relationship between 5-HT1A receptors and glutamate, the current study examined the effects of the 5-HT1AR agonist, +/-8-OH-DPAT and the N-methyl-D-aspartic acid receptor (NMDAR) antagonist, MK-801, on L-DOPA-induced motor behavior.

MATERIALS AND METHODS

Unilateral 6-hydroxydopamine lesioned male Sprague-Dawley rats were rendered dyskinetic with 1 week of daily L-DOPA (12 mg/kg, i.p.) + benserazide (15 mg/kg, i.p.). On test days, one group of rats received pretreatments of: +/-8-OH-DPAT (0, 0.03, 0.1, 0.3 mg/kg, i.p.) or MK-801 (0, 0.03, 0.1, 0.3 mg/kg, i.p.). A second group was administered combined +/-8-OH-DPAT (0, 0.03 or 0.1 mg/kg, i.p.) + MK-801 (0, 0.1 mg/kg, i.p.). Pretreatments were followed by L-DOPA administration, after which, abnormal involuntary movements (AIMs) and rotations were monitored. To investigate effects on motor performance, subthreshold doses of +/-8-OH-DPAT (0.03 mg/kg, i.p.) + MK-801 (0.1 mg/kg, i.p.) were administered to L-DOPA-naïve hemiparkinsonian rats before the forepaw adjusting steps test.

RESULTS

Individually, both +/-8-OH-DPAT and MK-801 dose-dependently decreased L-DOPA-induced AIMs without affecting rotations. Combined subthreshold doses of +/-8-OH-DPAT+MK-801 reduced L-DOPA-induced AIMs and potently enhanced contralateral rotations without altering L-DOPA-induced motor improvements.

CONCLUSIONS

The current results indicate a functional interaction between 5-HT1AR and NMDAR that may improve pharmacological treatment of PD patients.

High-frequency stimulation of the subthalamic nucleus potentiates L-DOPA-induced neurochemical changes in the striatum in a rat model of Parkinson's disease

This study examined the cellular changes produced in the striatum by chronic L-DOPA treatment and prolonged subthalamic nucleus high-frequency stimulation (STN-HFS) applied separately, successively, or in association, in the 6-hydroxydopamine-lesioned rat model of Parkinson's disease (PD). Only animals showing severe L-DOPA-induced dyskinesias (LIDs) were included, and STN-HFS was applied for 5 d at an intensity efficient for alleviating akinesia without inducing dyskinesias. L-DOPA treatment alone induced FosB/deltaFosB immunoreactivity, exacerbated the postlesional increase in preproenkephalin, reversed the decrease in preprotachykinin, and markedly increased mRNA levels of preprodynorphin and of the glial glutamate transporter GLT1, which were respectively decreased and unaffected by the dopamine lesion. STN-HFS did not affect per se the postlesion changes in any of these markers. However, when applied in association with L-DOPA treatment, it potentiated the positive modulation exerted by L-DOPA on all of the markers examined and tended to exacerbate LIDs. After 5 d of L-DOPA withdrawal, the only persisting drug-induced responses were an elevation in preprodynorphin mRNA levels and in the number of FosB/deltaFosB-immunoreactive neurons. Selective additional increases in these two markers were measured when STN-HFS was applied subsequently to L-DOPA treatment. These data provide the first evidence that STN-HFS exacerbates the responsiveness of striatal cells to L-DOPA medication and suggest that STN-HFS acts specifically through an L-DOPA-modulated signal transduction pathway associated with LIDs in the striatum. They point to striatal cells as a primary site for the complex interactions between these two therapeutic approaches in PD and argue against a direct anti-dyskinetic action of STN-HFS.

Glutamate and GABA receptors and transporters in the basal ganglia: what does their subsynaptic localization reveal about their function?

GABA and glutamate, the main transmitters in the basal ganglia, exert their effects through ionotropic and metabotropic receptors. The dynamic activation of these receptors in response to released neurotransmitter depends, among other factors, on their precise localization in relation to corresponding synapses. The use of high resolution quantitative electron microscope immunocytochemical techniques has provided in-depth description of the subcellular and subsynaptic localization of these receptors in the CNS. In this article, we review recent findings on the ultrastructural localization of GABA and glutamate receptors and transporters in monkey and rat basal ganglia, at synaptic, extrasynaptic and presynaptic sites. The anatomical evidence supports numerous potential locations for receptor-neurotransmitter interactions, and raises important questions regarding mechanisms of activation and function of synaptic versus extrasynaptic receptors in the basal ganglia.

Unique anti-apoptotic activity of EAAC1 in injured motor neurons

Injured motor neurons of the adult rat can survive, whereas similar axotomy causes gradual motor neuron death in the adult mouse. We report that the decreased expression of the neuronal glutamate transporter excitatory amino-acid carrier 1 (EAAC1) following nerve injury is associated with motor neuron death in the mouse. Glutamate transporters play a crucial role in prevention of neuronal death by suppressing glutamate toxicity. However, the possible functional role of EAAC1 in preventing neuron death has not been resolved as compared with glial glutamate transporters such as GLT-1. Here, we have revealed a unique 'rescue' function of EAAC1, which is independent of removal of extracellular glutamate. During apoptotic stimuli, a mitochondrial protein, holocytochrome c synthetase (HCCS), translocates to outside the mitochondria, binds to and suppresses the X-linked inhibitor of apoptosis protein (XIAP), leading to activation of caspase-3. The N-terminus of EAAC1 can bind to HCCS, which interferes with the HCCS-XIAP association, and thereby maintain XIAP activity. This unique anti-apoptotic mechanism of EAAC1 functions in rescuing PC12 cells and motor neurons from NGF deprivation and nerve injury, respectively.

Effect of kynurenine 3-hydroxylase inhibition on the dyskinetic and antiparkinsonian responses to levodopa in Parkinsonian monkeys

Homeostatic interactions between dopamine and glutamate are central to the normal physiology of the basal ganglia. This relationship is altered in Parkinsonism and in levodopa-induced dyskinesias (LID), resulting in an upregulation of corticostriatal glutamatergic function. Kynurenic acid (KYNA), a tryptophan metabolite with antagonist activity at ionotropic glutamate receptors and the capability to inhibit glutamate release presynaptically, might therefore be of therapeutic value in LID. To evaluate this hypothesis, we used a pharmacological tool, the kynurenine 3-hydroxylase inhibitor Ro 61-8048, which raises KYNA levels acutely. Ro 61-8048 was tested in MPTP cynomolgus monkeys with a stable parkinsonian syndrome and reproducible dyskinesias after each dose of levodopa. Serum and CSF concentrations of KYNA and its precursor kynurenine increased dose-dependently after Ro 61-8048 administration, alone or in combination with levodopa. Coadministration of Ro 61-8048 with levodopa produced a moderate but significant reduction in the severity of dyskinesias while maintaining the motor benefit. These results suggest that elevation of KYNA levels through inhibition of kynurenine 3-hydroxylase constitutes a promising novel approach for managing LID in Parkinson's disease.

Denervation and repeated l-DOPA induce complex regulatory changes in neurochemical phenotypes of striatal neurons: Implication of a dopamine D1-dependent mechanism

Motor complications induced through repeated L-DOPA treatment in patients with Parkinson's disease are thought to be the consequence of molecular adaptations that occur in response to repeated dopamine receptors stimulation. Here, we studied the molecular changes taking place in the denervated striatum of unilaterally 6-OHDA-lesioned rats repeatedly treated with L-DOPA alone or combined to the D1 receptor antagonist SCH23390. We looked at the territorial patterns of expression of neurotensin (NT), dynorphin (DYN), enkephalin (ENK) and Nur77 (also known as NGFI-B) mRNA expression in the striatum and contrasted these with markers of glutamatergic transport and dopaminergic receptor functions. The denervation process induced NT and Nur77 mRNA expression in ENK-positive cells. Subsequent repeated L-DOPA treatment led to a sensitization of L-DOPA-induced rotational response and produced a second surge of NT induction, this time limited to DYN-positive cells and preferentially restricted to the lateral striatum. In this specific territory, the number of Nur77-positive cells was decreased, in response to L-DOPA, when compared to the medial part of the lesioned striatum. L-DOPA treatment increased dopamine D3 receptor and glutamate transporter 1 (GLT1) mRNA expression in the lesioned striatum and that, specifically in an area overlapping one of Nur77 decrease and of NT/DYN induction. The concomitant administration of SCH23390 with repeated L-DOPA treatment blocked the development of behavioral sensitization and the appearance of all L-DOPA-induced molecular reorganizations reported above. Our results showed that repeated L-DOPA treatment produces, in a denervated striatum, a complex pattern of genes regulation in both the direct and the indirect striatal output pathways. This phenomenon is located preferentially in a striatal area receiving converging inputs from the thalamus and sensorimotor cortex and is dependent upon D1 receptor stimulation.

Glutamatergic hypothesis of schizophrenia: involvement of Na+/K+-dependent glutamate transport

Hypothetical model based on deficient glutamatergic neurotransmission caused by hyperactive glutamate transport in astrocytes surrounding excitatory synapses in the prefrontal cortex is examined in relation to the aetiology of schizophrenia. The model is consistent with actions of neuroleptics, such as clozapine, in animal experiments and it is strongly supported by recent findings of increased expression of glutamate transporter GLT in prefrontal cortex of patients with schizophrenia. It is proposed that mechanisms regulating glutamate transport be investigated as potential targets for novel classes of neuroactive compounds with neuroleptic characteristics. Development of new efficient techniques designed specifically for the purpose of studying rapid activity-dependent translocation of glutamate transporters and associated molecules such as Na+, K+-ATPase is essential and should be encouraged.

l-dopa-induced reversal in striatal glutamate following partial depletion of nigrostriatal dopamine with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine

We have reported that 1 month following acute (20mg/kg x 4) or subchronic (30 mg/kg/day x 7d) administration of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, there is an increase or decrease, respectively, in the extracellular level of striatal glutamate as determined by in vivo microdialysis [Robinson S, Freeman P, Moore C, Touchon JC, Krentz L, Meshul CK (2003) Acute and subchronic MPTP administration differentially affects striatal glutamate synaptic function. Exp Neurol 180:73-86]. The goal of this study was to determine the effects of treatment with l-dopa (15 mg/kg) for 21 days on striatal glutamate starting on day 8 after the first dose of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine was administered to mice. Following acute administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, the increase in extracellular striatal glutamate due to lesion of the nigrostriatal pathway was completely reversed to a level below that found in the vehicle-treated group after l-dopa treatment. Subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment resulted in a decrease in striatal extracellular glutamate that was reversed to the level close to that observed in the vehicle-treated group. There was no change in the density of nerve terminal glutamate immunolabeling associated with the synaptic vesicle pool, suggesting that the alterations in extracellular glutamate most likely originated from the calcium-independent pool. There was a similar decrease in the relative density of tyrosine hydroxylase immunolabeling, a marker for dopamine terminals, within the dorsolateral striatum in both the acute and subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated groups that had been administered l-dopa. There was a decrease in the relative density of immunolabeling within the dorsolateral striatum for the glutamate transporter, GLT-1, following acute 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment in the groups administered either vehicle or l-dopa. There was no change in GLT-1 immunolabeling following subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. The results demonstrate that the reversal in the extracellular level of striatal glutamate following l-dopa treatment in both the acute and subchronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated groups is not due to changes in either striatal dopamine nerve terminals or in the density of the glutamate transporter, GLT-1.

Apomorphine-induced alterations in striatal and substantia nigra pars reticulata glutamate following unilateral loss of striatal dopamine

We have reported time-dependent changes in extracellular glutamate within the striatum at 1 and 3 months following a unilateral lesion of the nigrostriatal pathway using the neurotoxin, 6-hydroxydopamine (6-OHDA) (Meshul, C.K., Emre, N., Nakamura, C.M., Allen, C., Donohue, M.K., Buckman, J.F., 1999. Time-dependent changes in striatal glutamate synapses following a 6-hydroxydopamine lesion. Neurosci. 88, 1-16.). The aim of the present study was to determine the effects of such a lesion on glutamate within the substantia nigra pars reticulata (SN-PR) and the effect of subchronic administration of the dopamine D-1/D-2 agonist, apomorphine, on extracellular glutamate within both the striatum and the SN-PR using in vivo microdialysis. One month after the lesion, there is an increase in extracellular glutamate within the striatum and apomorphine treatment leads to a further increase. Within the SN-PR, a loss of striatal dopamine leads to a decrease in extracellular glutamate, while apomorphine treatment leads to a further decrease in nigral glutamate. Three months after a 6-OHDA lesion, there is a decrease in extracellular striatal glutamate, with apomorphine administration leading to essentially no further change in glutamate. The loss of striatal dopamine increased extracellular glutamate within the SN-PR while apomorphine administration resulted in a decrease in extracellular glutamate back to the value observed in the control group. The data suggests that the increase in striatal glutamate 1 month following a 6-OHDA lesion alone or following subchronic apomorphine is consistent with the hypothesis that a decrease in glutamate within the SN-PR leads to activation of the thalamo-cortico-striatal pathway. The decrease in striatal glutamate 3 months after a nigrostriatal lesion is also consistent with the observed increase in extracellular glutamate within the SN-PR, thus leading to a decrease in output of the thalamo-cortico-striatal pathway.

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.

Astroglial plasticity and glutamate function in a chronic mouse model of Parkinson's disease

Astrocytes play a major role in maintaining low levels of synaptically released glutamate, and in many neurodegenerative diseases, astrocytes become reactive and lose their ability to regulate glutamate levels, through a malfunction of the glial glutamate transporter-1. However, in Parkinson's disease, there are few data on these glial cells or their regulation of glutamate transport although glutamate cytotoxicity has been blamed for the morphological and functional decline of striatal neurons. In the present study, we use a chronic mouse model of Parkinson's disease to investigate astrocytes and their relationship to glutamate, its extracellular level, synaptic localization, and transport. C57/bl mice were treated chronically with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and probenecid (MPTP/p). From 4 to 8 weeks after treatment, these mice show a significant loss of dopaminergic terminals in the striatum and a significant increase in the size and number of GFAP-immunopositive astrocytes. However, no change in extracellular glutamate, its synaptic localization, or transport kinetics was detected. Nevertheless, the density of transporters per astrocyte is significantly reduced in the MPTP/p-treated mice when compared to controls. These results support reactive gliosis as a means of striatal compensation for dopamine loss. The reduction in transporter complement on individual cells, however, suggests that astrocytic function may be compromised. Although reactive astrocytes are important for maintaining homeostasis, changes in their ability to regulate glutamate and its associated synaptic functions could be important for the progressive nature of the pathophysiology associated with Parkinson's disease.

Glutamate transporters: animal models to neurologic disease

Glutamate is the primary excitatory amino acid neurotransmitter in the central nervous system and its activity is carefully modulated in the synaptic cleft by glutamate transporters. A number of glutamate transporters have been identified in the central nervous system and each has a unique physiologic property and distribution. Glutamate transporter dysfunction may either be an initiating event or part of a cascade leading to cellular dysfunction and ultimately cell death. Animal models of glutamate transporter dysfunction have revealed a significant role for these proteins in pathologic conditions such as neurodegenerative diseases, epilepsy, stroke, and central nervous system tumors. Recent work has focused on glutamate transporter biology in human diseases with an emphasis on how manipulation of these transporter proteins may lead to therapeutic interventions in neurologic disease.

Glutamate spillover in the striatum depresses dopaminergic transmission by activating group I metabotropic glutamate receptors

Cortical glutamate and substantia nigra dopamine (DA) afferents converge onto the dendritic spines of medium spiny neurons (MSNs) in the striatum where they act to modulate motor and cognitive functions. The released DA spills over from its synapse and is thought to regulate glutamatergic input by acting on distal DA receptors located on corticostriatal axon terminals. By monitoring evoked DA release directly using fast-scan cyclic voltammetry, we report a reciprocal modulation by glutamate spillover on evoked striatal DA release, induced by either glutamate uptake blockade or high-frequency stimulation of corticostriatal tracts. We demonstrate that this modulation is attributable to the activation of group I metabotropic glutamate receptors. Thus, under conditions in which glutamate escapes the confines of its synapse, it can elicit the presynaptic suppression of dopaminergic neurotransmission.

Group I metabotropic glutamate receptors in the monkey striatum: subsynaptic association with glutamatergic and dopaminergic afferents

Group I metabotropic glutamate receptors (mGluRs) are involved in long-term synaptic plasticity and neuroprotection in the striatum, but the specific role(s) of mGluR1 and mGluR5 remain poorly understood. In this study, we used electron-microscopic immunocytochemistry to compare the pattern of subsynaptic and subcellular distribution of mGluR1a and mGluR5 in relation to putative glutamatergic and dopaminergic inputs to the monkey striatum. At the light-microscopic level, both group I mGluRs are expressed in the striatal neuropil. In addition, numerous perikarya of striatal output neurons are immunostained for mGluR5, but much less frequently for mGluR1a. At the electron-microscopic level, immunoreactivity for both receptor subtypes is primarily expressed postsynaptically in dendrites and spines, although presynaptic mGluR1a labeling of glutamatergic thalamostriatal boutons and, less frequently, dopaminergic and corticostriatal terminals is also seen. In contrast to mGluR1a, mGluR5 immunoreactivity is rarely encountered presynaptically. In postsynaptic elements, 40-70% of immunoreactivity for both receptor subtypes is expressed intracellularly, whereas 30-60% is apposed to the plasma membrane. More than 80% of the labeling apposed to the plasma membrane is extrasynaptic. The remaining 20% is located at the edges of putative glutamatergic synapses or in the active zone of symmetric synapses. In mGluR5-, but not mGluR1a-immunostained sections, approximately 70% of dopaminergic symmetric synapses are labeled perisynaptically. These data emphasize the differential pattern of subsynaptic localization of the two group I mGluRs and provide various presynaptic and postsynaptic sites whereby mGluR1 and mGluR5 could mediate different, but complementary, effects on glutamatergic and dopaminergic transmission in the primate striatum.

CNS region-specific regulation of glial glutamate transporter expression

The neuronal cell death associated with certain neurodegenerative disorders as well as acute brain injuries is in part due to the reduced expression of glial glutamate transporters and the subsequent accumulation of toxic extracellular glutamate concentrations. Extracellular factors previously found to potently stimulate the expression of the glial glutamate transporters, GLT-1/EAAT2 and GLAST/EAAT1, in astroglial cultures of rat cerebral hemispheres are PACAP, TGF alpha, and EGF. In the present study, we sought to determine whether similar stimulatory influences apply for astroglia from other areas of the central nervous system (CNS). Immunoblot and real-time RT-PCR analysis of striatal astroglial cultures maintained for 72 h with PACAP, TGF alpha, or EGF revealed a prominent increase in GLT-1 and GLAST expression. In apparent contrast, all factors completely failed to affect GLT-1 and GLAST expression in astroglial cultures from the cerebellum, mesencephalon, and spinal cord between 36 h and 7 days. This failure was not due to the absence of functional recognition or transduction machineries for the extracellular factors as suggested by the additional observations that cerebellar, mesencephalic and spinal cord glia were capable of responding to stimulation with PACAP, TGF alpha, or EGF for 10 min with activation of CREB. Moreover, dibutyryl cyclic AMP (dbcAMP) potently promoted GLT-1 and/or GLAST expression in mesencephalic, cerebellar and spinal cord glia, further indicating that extracellular factors regulate glial glutamate transporter expression throughout the CNS. Together these findings identify PACAP, TGF alpha and EGF as potent regulators of glutamate transporter expression in striatal glia. In addition, these findings provide evidence for a CNS region-specific regulation of glial glutamate transport.