Pascal Salin

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 Parkinsons 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.

Reversal of the adaptive response of neuropeptide Y neurons in the rat striatum to nigrostriatal dopamine deafferentation by theN-methyl-d-aspartate antagonist dizocilpine maleate

This study examined the effects of systemic treatments with dizocilpine maleate alone or in combination with unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopaminergic neurons on the number and staining intensity of neuropeptide Y-immunoreactive neurons in the rat striatum. In the combined condition, short-term and long-term treatments with dizocilpine maleate were started 19 days and 12 days after the lesion of the nigrostriatal dopaminergic pathway, respectively. As reported previously, the unilateral dopaminergic lesion elicited an increase in both the number and staining intensity of neuropeptide Y-immunoreactive neurons in the ipsilateral striatum. Short-term treatment with dizocilpine maleate at the dose of 0.2 mg/kg (four injections, 6 h apart, sacrifice 2 h after the final dose), which by itself did not modify neuropeptide Y immunostaining, totally suppressed the effect of the dopaminergic deafferentation on the number of neuropeptide Y-positive neurons but not that on the intraneuronal amount of labelling. When administered twice a day for eight days at the same dose of 0.2 mg/kg, dizocilpine maleate by itself elicited an increase in the number of neuropeptide Y-immunodetectable cells, paradoxically concomitant with a decrease in the levels of intraneuronal labelling. After combination of this treatment with unilateral lesion of the nigrostriatal dopaminergic pathway, the changes related to either the dizocilpine maleate treatment or the 6-hydroxydopamine-induced lesion totally disappeared, so that the number and staining intensity of neuropeptide Y-immunoreactive neurons in that condition did not differ from control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Progressive Parkinsonism by acute dysfunction of excitatory amino acid transporters in the rat substantia nigra

Parkinsons disease (PD) is characterized by the progressive degeneration of substantia nigra (SN) dopamine neurons, involving a multifactorial cascade of pathogenic events. Here we explored the hypothesis that dysfunction of excitatory amino acid transporters (EAATs) might be involved. Acutely-induced dysfunction of EAATs in the rat SN, by single unilateral injection of their substrate inhibitor l-trans-pyrrolidine-2,4-dicarboxylate (PDC), triggers a neurodegenerative process mimicking several PD features. Dopamine neurons are selectively affected, consistent with their sustained excitation by PDC measured by slice electrophysiology. The anti-oxidant N-acetylcysteine and the NMDA receptor antagonists ifenprodil and memantine provide neuroprotection. Besides oxidative stress and NMDA receptor-mediated excitotoxicity, glutathione depletion and neuroinflammation characterize the primary insult. Most interestingly, the degeneration progresses overtime with unilateral to bilateral and caudo-rostral evolution. Transient adaptive changes in dopamine function markers in SN and striatum accompany cell loss and axonal dystrophy, respectively. Motor deficits appear when neuron loss exceeds 50% in the most affected SN and striatal dopamine tone is dramatically reduced. These findings outline a functional link between EAAT dysfunction and several PD pathogenic mechanisms/pathological hallmarks, and provide a novel acutely-triggered model of progressive Parkinsonism.

Differential organization of cortical inputs to striatal projection neurons of the matrix compartment in rats

In prior studies, we described the differential organization of corticostriatal and thalamostriatal inputs to the spines of direct pathway (dSPNs) and indirect pathway striatal projection neurons (iSPNs) of the matrix compartment. In the present electron microscopic (EM) analysis, we have refined understanding of the relative amounts of cortical axospinous vs. axodendritic input to the two types of SPNs. Of note, we found that individual dSPNs receive about twice as many axospinous synaptic terminals from IT-type (intratelencephalically projecting) cortical neurons as they do from PT-type (pyramidal tract projecting) cortical neurons. We also found that PT-type axospinous synaptic terminals were about 1.5 times as common on individual iSPNs as IT-type axospinous synaptic terminals. Overall, a higher percentage of IT-type terminals contacted dSPN than iSPN spines, while a higher percentage of PT-type terminals contacted iSPN than dSPN spines. Notably, IT-type axospinous synaptic terminals were significantly larger on iSPN spines than on dSPN spines. By contrast to axospinous input, the axodendritic PT-type input to dSPNs was more substantial than that to iSPNs, and the axodendritic IT-type input appeared to be meager and comparable for both SPN types. The prominent axodendritic PT-type input to dSPNs may accentuate their PT-type responsiveness, and the large size of axospinous IT-type terminals on iSPNs may accentuate their IT-type responsiveness. Using transneuronal labeling with rabies virus to selectively label the cortical neurons with direct input to the dSPNs projecting to the substantia nigra pars reticulata, we found that the input predominantly arose from neurons in the upper layers of motor cortices, in which IT-type perikarya predominate. The differential cortical input to SPNs is likely to play key roles in motor control and motor learning.

Differential Effects of Corticostriatal and Thalamostriatal Deafferentation on Expression of the Glutamate Transporter GLT1 in the Rat Striatum

Abstract: This study compared the effects of the disruption of the two main presumably glutamatergic striatal inputs, the corticostriatal and thalamostriatal pathways, on GLT1 expression in the rat striatum, using in situ hybridization and immunohistochemistry. Unilateral ibotenate‐induced thalamic lesion produced no significant changes in striatal GLT1 mRNA labeling and immunostaining as assessed at 5 and 12 days postlesion. In contrast, significant increases in both parameters were measured after bilateral cortical lesion by superficial thermocoagulation. GLT1 mRNA levels increased predominantly in the dorsolateral part of the striatum; there, the increases were significant at 5 (+84%), 12 (+101%), and 21 (+45%) but not at 35 days postlesion. GLT1 immunostaining increased significantly and homogeneously by 17‐26% at 12 and 21 days postlesion. The increase in GLT1 expression at 12 days postlesion was further confirmed by western blot analysis; in contrast, a 36% decrease in glutamate uptake activity was measured at the same time point. These data indicate that striatal GLT1 expression depends on corticostriatal but not thalamostriatal innervation. Comparison of our results with previous data showing that cortical lesion by aspiration down‐regulates striatal GLT1 expression further suggests that differential changes in GLT1 expression, and thus presumably in glial cell function, may occur in the target striatum depending on the way the cortical neurons degenerate.

Antiparkinsonian action of a selective group III mGlu receptor agonist is associated with reversal of subthalamonigral overactivity

Activation of group III metabotropic glutamate (mGlu) receptors has been recently highlighted as a potential approach in the treatment of Parkinsons disease (PD). This study evaluates the antiparkinsonian action of systemic administration of the broad-spectrum agonist of group III mGlu receptors, 1-aminocyclopentane-1,3,4-tricarboxylic acid (ACPT-I), and investigates its site of action within the basal ganglia circuitry. Acute injection of ACPT-I reverses haloperidol-induced catalepsy, an index of akinesia in rodents. In a rat model of early PD based on partial bilateral nigrostriatal lesions, chronic (2weeks) administration of ACPT-I is required to efficiently alleviate the akinetic deficit evidenced in a reaction time task. This treatment counteracts the post-lesional increases in the gene expression of cytochrome oxidase subunit I, a metabolic marker of neuronal activity, in the overall subthalamic nucleus and in the lateral motor part of the substantia nigra pars reticulata (SNr) but has no effect in the globus pallidus. Paradoxically, ACPT-I administration in sham animals impairs performance and induces overexpression of cytochrome oxidase subunit I mRNA in the lateral SNr, and has no effect in the subthalamic nucleus or globus pallidus. Altogether, our results provide new evidence for the antiparkinsonian efficiency of group III mGlu receptor agonism, point to the regulation of the overactive subthalamo-nigral connection as a main site of action in an early stage of PD and underline the complex interplay between these receptors and the dopaminergic system to regulate basal ganglia function in control and PD conditions.

Striatal molecular signature of subchronic subthalamic nucleus high frequency stimulation in parkinsonian rat.

This study addresses the molecular mechanisms underlying the action of subthalamic nucleus high frequency stimulation (STN-HFS) in the treatment of Parkinsons disease and its interaction with levodopa (L-DOPA), focusing on the striatum. Striatal gene expression profile was assessed in rats with nigral dopamine neuron lesion, either treated or not, using agilent microarrays and qPCR verification. The treatments consisted in anti-akinetic STN-HFS (5 days), chronic L-DOPA treatment inducing dyskinesia (LIDs) or the combination of the two treatments that exacerbated LIDs. STN-HFS modulated 71 striatal genes. The main biological processes associated with the differentially expressed gene products include regulation of growth, of apoptosis and of synaptic transmission, and extracellular region is a major cellular component implicated. In particular, several of these genes have been shown to support survival or differentiation of striatal or of dopaminergic neurons. These results indicate that STN HFS may induce widespread anatomo-functional rearrangements in the striatum and create a molecular environment favorable for neuroprotection and neuroplasticity. STN-HFS and L-DOPA treatment share very few common gene regulation features indicating that the molecular substrates underlying their striatal action are mostly different; among the common effects is the down-regulation of Adrb1, which encodes the adrenergic beta-1- receptor, supporting a major role of this receptor in Parkinsons disease. In addition to genes already reported to be associated with LIDs (preprodynorphin, thyrotropin-releasing hormone, metabotropic glutamate receptor 4, cannabinoid receptor 1), the comparison between DOPA and DOPA/HFS identifies immunity-related genes as potential players in L-DOPA side effects.

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 Parkinsons 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.

Metabolic, synaptic and behavioral impact of 5‐week chronic deep brain stimulation in hemiparkinsonian rats

The long‐term effects and action mechanisms of subthalamic nucleus (STN) high‐frequency stimulation (HFS) for Parkinsons disease still remain poorly characterized, mainly due to the lack of experimental models relevant to clinical application. To address this issue, we performed a multilevel study in freely moving hemiparkinsonian rats undergoing 5‐week chronic STN HFS, using a portable constant‐current microstimulator. In vivo metabolic neuroimaging by 1H‐magnetic resonance spectroscopy (11.7 T) showed that STN HFS normalized the tissue levels of the neurotransmission‐related metabolites glutamate, glutamine and GABA in both the striatum and substantia nigra reticulata (SNr), which were significantly increased in hemiparkinsonian rats, but further decreased nigral GABA levels below control values; taurine levels, which were not affected in hemiparkinsonian rats, were significantly reduced. Slice electrophysiological recordings revealed that STN HFS was, uniquely among antiparkinsonian treatments, able to restore both forms of corticostriatal synaptic plasticity, i.e. long‐term depression and potentiation, which were impaired in hemiparkinsonian rats. Behavior analysis (staircase test) showed a progressive recovery of motor skill during the stimulation period. Altogether, these data show that chronic STN HFS efficiently counteracts metabolic and synaptic defects due to dopaminergic lesion in both the striatum and SNr. Comparison of chronic STN HFS with acute and subchronic treatment further suggests that the long‐term benefits of this treatment rely both on the maintenance of acute effects and on delayed actions on the basal ganglia network.

Neuropeptide Y Neurons in the Striatal Network. Functional Adaptive Responses to Impairment of Striatal Inputs

Neuropeptide Y (NPY), a 36 amino acid peptide enriched with tyrosine residues first isolated from porcine brain extracts (Tatemoto, 1982; Tatemoto et al, 1982), is thought to be the neuroactive member of the pancreatic polypeptide family (Di Maggio et al, 1985). It is one of the most abundant and widely distributed peptides isolated so far within the mammalian central nervous system and it shows a distinctive pattern of distribution as compared with other neuropeptides. The most salient feature of its distribution is the high concentration centred on the striatum, especially in the human brain (Adrian et al, 1983; Allen et al, 1983). Immunohistochemical studies have shown that NPY is present in numerous cell bodies and fibers in both the dorsal striatum or caudate putamen (CP) and the ventral striatum or nucleus accumbens (NAcc) (Adrian et al, 1983; Allen et al, 1983; De Quidt and Emson,1986; Smith et al, 1985). Within these structures and in related cortical areas NPY has been reported to coexist extensively with somatostatin and the enzyme nicotinamide adenine dinucleotide phosphatediaphorase (NADPH-D) (Chronwall et al, 1984; Gaspar et al, 1987; Kowall et al, 1987; Smith and Parent, 1986; Vincent et al, 1982; 1983). The presence of high levels of NPY in the basal ganglia nuclei has suggested that this peptide may play a fundamental role in the control of sensorimotor function and has led to detailed investigations on the morphological features and cellular relationships of the NPY neurons within these structures. The purpose of the present report is to briefly review data from biochemical and immunohistochemical studies in an attempt to clarify the anatomical and functional position of NPY elements in the striatal network (dorsal striatum).