Astrocytosis in parkinsonism: considering tripartite striatal synapses in physiopathology?

The current concept of basal ganglia organization and function in physiological and pathophysiological conditions excludes the most numerous cells in the brain, i.e., the astrocytes, present with a ratio of 10:1 neuron. Their role in neurodegenerative condition such as Parkinson's disease (PD) remains to be elucidated. Before embarking into physiological investigations of the yet-to-be-identified "tripartite" synapses in the basal ganglia in general and the striatum in particular, we therefore characterized anatomically the PD-related modifications in astrocytic morphology, the changes in astrocytic network connections and the consequences on the spatial relationship between astrocytic processes and asymmetric synapses in normal and PD-like conditions in experimental and human PD. Our results unravel a dramatic regulation of striatal astrocytosis supporting the hypothesis of a key role in (dys) regulating corticostriatal transmission. Astrocytes and their various properties might thus represent a therapeutic target in PD.

Immediate-early gene expression in structures outside the basal ganglia is associated to l-DOPA-induced dyskinesia.

Long-term l-3,4-dihydroxyphenylalanine (l-DOPA) treatment in Parkinson's disease (PD) leads to l-DOPA-induced dyskinesia (LID), a condition thought to primarily involve the dopamine D1 receptor-expressing striatal medium spiny neurons. Activation of the D1 receptor results in increased expression of several molecular markers, in particular the members of the immediate-early gene (IEG) family, a class of genes rapidly transcribed in response to an external stimulus. However, several dopaminoceptive structures in the brain that are likely to be affected by the exogenously produced DA have received little attention although they might play a key role in mediating those l-DOPA-induced abnormal behaviours. ΔFosB, ARC, FRA2 and Zif268 IEGs expression patterns were thus characterised, using unbiased stereological methods, in the whole brain of dyskinetic and non-dyskinetic rats to identify brain nuclei displaying a transcriptional response specifically related to LID. Within the basal ganglia, the striatum and the substantia nigra pars reticulata showed an increased expression of all four IEGs in dyskinetic compared to non-dyskinetic rats. Outside the basal ganglia, there was a striking increased expression of the four IEGs in the motor cortex, the bed nucleus of the stria terminalis, the dorsal hippocampus, the pontine nuclei, the cuneiform nucleus and the pedunculopontine nuclei. Moreover, the zona incerta and the lateral habenula displayed an overexpression of ΔFosB, ARC and Zif268. Among these structures, the IEG expression in the striatum, the bed nucleus of the stria terminalis, the lateral habenula, the pontine nuclei and the cuneiform nucleus correlate with LID severity. These results illustrate a global transcriptional response to a dyskinetic state in the whole brain suggesting the possible involvement of these structures in LID.

Endogenous morphine-like compound immunoreactivity increases in parkinsonism.

Morphine is endogenously synthesized in the central nervous system and endogenous dopamine is thought to be necessary for endogenous morphine formation. As Parkinson's disease results from the loss of dopamine and is associated with central pain, we considered how endogenous morphine is regulated in the untreated and l-DOPA-treated parkinsonian brain. However, as the cellular origin and overall distribution of endogenous morphine remains obscure in the pathological adult brain, we first characterized the distribution of endogenous morphine-like compound immunoreactive cells in the rat striatum. We then studied changes in the endogenous morphine-like compound immunoreactivity of medium spiny neurons in normal, Parkinson's disease-like and l-DOPA-treated Parkinson's disease-like conditions in experimental (rat and monkey) and human Parkinson's disease. Our results reveal an unexpected dramatic upregulation of neuronal endogenous morphine-like compound immunoreactivity and levels in experimental and human Parkinson's disease, only partially normalized by l-DOPA treatment. Our data suggest that endogenous morphine formation is more complex than originally proposed and that the parkinsonian brain experiences a dramatic upregulation of endogenous morphine immunoreactivity. The functional consequences of such endogenous morphine upregulation are as yet unknown, but based upon the current knowledge of morphine signalling, we hypothesize that it is involved in fatigue, depression and pain symptoms experienced by patients with Parkinson's disease.

High frequency stimulation of the entopeduncular nucleus sets the cortico-basal ganglia network to a new functional state in the dystonic hamster.

High frequency stimulation (HFS) of the internal pallidum is effective for the treatment of dystonia. Only few studies have investigated the effects of stimulation on the activity of the cortex-basal ganglia network. We here assess within this network the effect of entopeduncular nucleus (EP) HFS on the expression of c-Fos and cytochrome oxidase subunit I (COI) in the dt(sz)-hamster, a well-characterized model of paroxysmal dystonia. In dt(sz)-hamsters, we identified abnormal activity in motor cortex, basal ganglia and thalamus. These structures have already been linked to the pathophysiology of human dystonia. EP-HFS (i) increased striatal c-Fos expression in controls and dystonic hamsters and (ii) reduced thalamic c-Fos expression in dt(sz)-hamsters. EP-HFS had no effect on COI expression. The present results suggest that EP-HFS induces a new network activity state which may improve information processing and finally reduces the severity of dystonic attacks in dt(sz)-hamsters.

Receptor recycling mediates plasma membrane recovery of dopamine D1 receptors in dendrites and axons after agonist-induced endocytosis in primary cultures of striatal neurons.

The pharmacological stimulation of G-protein-coupled receptor induces receptor internalization. Receptor's fate after the step of internalization remains poorly characterized despite its incidence on the neuronal responsiveness. In this context, we studied the dopamine (DA) D1 receptor (D1R) trafficking in a model of striatal neuronal culture that endogenously express the D1R. We first characterized by immunohistochemistry the spatial distribution of the compartments involved in the endocytic pathways and then the D1R trafficking in dendrites and axons. In dendrites, immunohistochemical analysis showed that acute stimulation by the D1R agonist SKF 82958 (1 microM) induces an internalization of D1R in early endosomes labeled with Alexa-488-conjugated transferrin. We show that, 20 min after removal of the agonist, the D1R immunolabeling pattern returns to the basal state in dendrites and in axons. Recovery was unaffected by cycloheximide (70 microM) but was prevented by monensin (100 microM) that inhibits endosomal acidification and receptor recycling. These data suggest that dendritic and axonal D1Rs are internalized after agonist stimulation and targeted to the recycling pathway demonstrating that the machinery involved in GPCR endocytosis and recycling is functional both in dendrites and in axons. Temporal characteristics observed for the recovery of D1R density to the basal state and those observed for the resensitization process strongly suggest that D1R recycling supports the receptor resensitization.

Spatial and temporal localization during embryonic and fetal human development of the transcription factor SIM2 in brain regions altered in Down syndrome.

Human SIM2 is the ortholog of Drosophila single-minded (sim), a master regulator of neurogenesis and transcriptional factor controlling midline cell fate determination. We previously localized SIM2 in a chromosome 21 critical region for Down syndrome (DS). Here, we studied SIM2 gene using a new approach to provide insights in understanding of its potential role in human development. For the first time, we showed SIM2 spatial and temporal expression pattern during human central nervous system (CNS) development, from embryonic to fetal stages. Additional investigations were performed using a new optic microscopy technology to compare signal intensity and cell density [M. Rachidi, C. Lopes, S. Gassanova, P.M. Sinet, M. Vekemans, T. Attie, A.L. Delezoide, J.M. Delabar, Regional and cellular specificity of the expression of TPRD, the tetratricopeptide Down syndrome gene, during human embryonic development, Mech. Dev. 93 (2000) 189--193]. In embryonic stages, SIM2 was identified predominantly in restricted regions of CNS, in ventral part of D1/D2 diencephalic neuroepithelium, along the neural tube and in a few cell subsets of dorsal root ganglia. In fetal stages, SIM2 showed differential expression in pyramidal and granular cell layers of hippocampal formation, in cortical cells and in cerebellar external granular and Purkinje cell layers. SIM2 expression in embryonic and fetal brain could suggest a potential role in human CNS development, in agreement with Drosophila and mouse Sim mutant phenotypes and with the conservation of the Sim function in CNS development from Drosophila to Human. SIM2 expression in human fetal brain regions, which correspond to key structures for cognitive processes, correlates well with the behavioral phenotypes of Drosophila Sim mutants and transgenic mice overexpressing Sim2. In addition, SIM2-expressing brain regions correspond to the altered structures in DS patients. All together, these findings suggest a potential role of SIM2 in CNS development and indicate that SIM2 overexpression could participate to the pathogenesis of mental retardation in Down syndrome patients.