Ingrid Morales

Parkinson's disease as a result of aging

It is generally considered that Parkinsons disease is induced by specific agents that degenerate a clearly defined population of dopaminergic neurons. Data commented in this review suggest that this assumption is not as clear as is often thought and that aging may be critical for Parkinsons disease. Neurons degenerating in Parkinsons disease also degenerate in normal aging, and the different agents involved in the etiology of this illness are also involved in aging. Senescence is a wider phenomenon affecting cells all over the body, whereas Parkinsons disease seems to be restricted to certain brain centers and cell populations. However, reviewed data suggest that Parkinsons disease may be a local expression of aging on cell populations which, by their characteristics (high number of synaptic terminals and mitochondria, unmyelinated axons, etc.), are highly vulnerable to the agents promoting aging. The development of new knowledge about Parkinsons disease could be accelerated if the research on aging and Parkinsons disease were planned together, and the perspective provided by gerontology gains relevance in this field.

Ascorbate prevents cell death from prolonged exposure to glutamate in an in vitro model of human dopaminergic neurons.

Ascorbate (vitamin C) is a nonenzymatic antioxidant highly concentrated in the brain. In addition to mediating redox balance, ascorbate is linked to glutamate neurotransmission in the striatum, where it renders neuroprotection against excessive glutamate stimulation. Oxidative stress and glutamatergic overactivity are key biochemical features accompanying the loss of dopaminergic neurons in the substantia nigra that characterizes Parkinsons disease (PD). At present, it is not clear whether antiglutamate agents and ascorbate might be neuroprotective agents for PD. Thus, we tested whether ascorbate can prevent cell death from prolonged exposure to glutamate using dopaminergic neurons of human origin. To this purpose, dopamine‐like neurons were obtained by differentiation of SH‐SY5Y cells and then cultured for 4 days without antioxidant (antiaging) protection to evaluate glutamate toxicity and ascorbate protection as a model system of potential factors contributing to dopaminergic neuron death in PD. Glutamate dose dependently induced toxicity in dopaminergic cells largely by the stimulation of AMPA and metabotropic receptors and to a lesser extent by N‐methyl‐D‐aspartate and kainate receptors. At relatively physiological levels of extracellular concentration, ascorbate protected cells against glutamate excitotoxicity. This neuroprotection apparently relies on the inhibition of oxidative stress, because ascorbate prevented the pro‐oxidant action of the scavenging molecule quercetin, which occurred over the course of prolonged exposure, as is also seen with glutamate. Our findings show the relevance of ascorbate as a neuroprotective agent and emphasize an often underappreciated role of oxidative stress in glutamate excitotoxicity. Occurrence of a glutamate–ascorbate link in dopaminergic neurons may explain previous contradictions regarding their putative role in PD.

Striatal glutamate induces retrograde excitotoxicity and neuronal degeneration of intralaminar thalamic nuclei: their potential relevance for Parkinson's disease

An over‐stimulation of nigral glutamate (GLU) receptors has been proposed as a cause of the progression of the dopamine (DA) cell degeneration (excitotoxicity) which characterizes Parkinsons disease. The possible toxic action of striatal GLU (retrograde excitotoxicity) on these cells, and on other neurons which innervate the striatum and which also degenerate in Parkinsons disease (thalamostriatal cells of the intralaminar thalamic nuclei), is still practically unexplored. The retrograde excitotoxicity of striatal GLU on DAergic mesostriatal and GLUergic thalamostriatal cells was tested here by studying these cells 6 weeks after striatal perfusion of GLU by reverse microdialysis. GLU perfusion induced the striatal denervation of thalamic inputs (as revealed by vesicular glutamate transporter 2) and the remote degeneration of intralaminar neurons. In both centres, these effects were accompanied by microglial activation. Similar responses were not observed for nigrostriatal neurons, which showed no dopaminergic striatal denervation, no microglial activation in the substantia nigra and no changes in the number of dopaminergic cells in the substantia nigra. The inhibition of DAergic transmission increased the extrasynaptic GLU levels in the striatum (evaluated by microdialysis), an effect observed after the local administration of agonists and antagonists of DAergic transmission, and after the peripheral administration of levodopa (which increased the DA and decreased the GLU levels in the striatum of rats with an experimental DAergic denervation of this centre). The data presented show that striatal GLU induced a retrograde excitotoxicity which did not affect all striatal inputs in the same way and which could be involved in the cell degeneration of the intralaminar nuclei of the thalamus generally observed in Parkinsons disease.

Heterogeneous Dopamine Neurochemistry in the Striatum: The Fountain-Drain Matrix

In contrast to the relatively high attention paid to the structural heterogeneity of striatal dopamine (DA) innervation, little attention has been focused on the possible striatal heterogeneity for release and uptake of DA. By using amperometric methods, we found striatal regions showing a DA decrease during the medial forebrain bundle stimulation (drain areas) near to other zones that showed an increase in DA concentration (fountain areas). Both areas were intermixed to form a tridimensional matrix to regulate DA concentration throughout the striatum (fountain-drain matrix). The response to electrical stimuli of different amplitudes and durations and to different drugs (α-methyl-l-tyrosine, cocaine, γ-butyrolactone, and haloperidol) suggests that regional differences for both DA release/DA uptake and DA cell firing autoregulation are behind the striatal fountain-drain matrix. The high diversity of DA activity observed in the striatum is a new framework for analyzing experimental and clinical phenomena.

Mu-rhythm changes during the planning of motor and motor imagery actions.

Motor imagery is a mental representation of motor behavior which has been widely used to study the cognitive basis of movement. The assumption that real movements and motor imagery (virtual movements) use the same neurobiological basis has been questioned by functional magnetic resonance data. The functional similarity in the planning of real and virtual movements was studied here by analyzing event-related EEG recordings of the Mu-activity in the sensitive-motor cortex, pre-motor cortex and supplementary motor cortex. A visual stimulus (an arrow) which displayed the information needed for planning a motion (which can be executed or imaged later after the display of a second stimulus) induced a short-lasting phase-locked Mu-response (PLr) which was wider and more widespread when it was used for the motor planning of real or virtual movements than when it was passively watched. The phase-locked Mu-response was accompanied by a persistent decrease of the Mu-rhythms which were not phase-locked to stimuli (NPLr), a response which also was more marked and generalized when stimuli were used for motor planning than when they were passively observed. PLr and NPLr were similar during motor testing and imagery testing, suggesting that both tasks activated the Mu rhythms to a similar degree. This congruency between real and virtual movements was observed in the three cortical areas studied, where the amplitude, latency and duration of the phase-locked and non-phase-locked Mu response was similar in both cases. These noticeable similarities support the idea that the same cortical mechanisms are recruited during the planning of real and virtual movements, a fact that can be analyzed better when an event-related paradigm and a high time-resolution method are used.

The degeneration and replacement of dopamine cells in Parkinson's disease: the role of aging

Available data show marked similarities for the degeneration of dopamine cells in Parkinson’s disease (PD) and aging. The etio-pathogenic agents involved are very similar in both cases, and include free radicals, different mitochondrial disturbances, alterations of the mitophagy and the ubiquitin-proteasome system. Proteins involved in PD such as α-synuclein, UCH-L1, PINK1 or DJ-1, are also involved in aging. The anomalous behavior of astrocytes, microglia and stem cells of the subventricular zone (SVZ) also changes similarly in aging brains and PD. Present data suggest that PD could be the expression of aging on a cell population with high vulnerability to aging. The future knowledge of mechanisms involved in aging could be critical for both understanding the etiology of PD and developing etiologic treatments to prevent the onset of this neurodegenerative illness and to control its progression.

Self-induced accumulation of glutamate in striatal astrocytes and basal ganglia excitotoxicity

Excitotoxicity induced by high levels of extracellular glutamate (GLU) has been proposed as a cause of cell degeneration in basal ganglia disorders. This phenomenon is normally prevented by the astrocytic GLU‐uptake and the GLU‐catabolization to less dangerous molecules. However, high‐GLU can induce reactive gliosis which could change the neuroprotective role of astrocytes. The striatal astrocyte response to high GLU was studied here in an in vivo rat preparation. The transient striatal perfusion of GLU (1 h) by reverse microdialysis induced complex reactive gliosis which persisted for weeks and which was different for radial‐like glia, protoplasmic astrocytes and fibrous astrocytes. This gliosis was accompanied by a persistent cytosolic accumulation of GLU (immunofluorescence quantified by confocal microscope), which persisted for weeks (self‐induced glutamate accumulation), and which was associated to a selective decrease of glutamine synthetase activity. This massive and persistent self‐induced glutamate accumulation in striatal astrocytes could be an additional factor for the GLU‐induced excitotoxicity, which has been implicated in the progression of different basal ganglia disorders.

Striatal interaction among dopamine, glutamate and ascorbate

Despite evidence suggesting the interaction among glutamate (GLU), dopamine (DA) and ascorbic acid (AA) in the striatum, their actions are often studied separately. Microdialysis was used here to quantify the extracellular interaction among GLU-DA-AA in the striatum of rats, an interaction which was compared with those studied in the substantia nigra (SN). Perfusion of GLU by reverse microdialysis increased DA and decreased 3,4-dihydroxyphenylacetic acid (DOPAC) in the extracellular medium of the striatum, but increased both DA and DOPAC in the SN. The increase of extracellular DA-concentration induced by the local DA-perfusion decreased the extracellular level of GLU and glutamine, an effect that, as suggested by the GLU and glutamine increase observed after the haloperidol administration, probably involves the D2 dopamine receptor. Local administration of AA increased the extracellular DA, decreased DOPAC and had no effect on GLU and glutamine. Present data suggest that, in the striatum, GLU-release inhibits DA-uptake, DA-release inhibits GLU-release, and AA-release prevents DA-oxidation increasing its extracellular diffusion. These effects were different in the SN where GLU probably promoted the DA-release instead of inhibiting the DA-uptake as presumably occurred in the striatum. Present data denote a marked GLU-DA-AA interaction in the striatum, which might be relevant for the pharmacological control of basal ganglia disorders.

The astrocytic response to the dopaminergic denervation of the striatum

Increasing evidence suggests that the dopaminergic degeneration which characterizes Parkinsons disease starts in the striatal dopamine terminals and progresses retrogradely to the body of dopamine cells in the substantia nigra. The role of striatal astrocytes in the striatal initiation of the dopaminergic degeneration is little known. This work was aimed at studying the astrocytic response to the dopaminergic denervation of the striatum. The injection of 6‐hydroxydopamine (25 μg) in the lateral ventricle of adult Sprague–Dawley rats induced a fast (4 h) and selective (unaccompanied by unspecific lesions of striatal tissue or microgliosis) degeneration of the dopaminergic innervation of the striatum which was followed by a selective astrocytosis unaccompanied by microgliosis. This astrocytosis was severe and had a specific profile which included some (e.g. up‐regulation of glial fibrillary acidic protein, GS, S100β, NDRG2, vimentin) but not all (e.g. astrocytic proliferation or differentiation from NG2 cells, astrocytic scars, microgliosis) the characteristics observed after the non‐selective lesion of the striatum. This astrocytosis is similar to those observed in the parkinsonian striatum and, because it is was unaccompanied by changes in other striatal cells (e.g. by microgliosis), it may be suitable to study the role of striatal astrocytes during the dopaminergic denervation which characterizes the first stages of Parkinsons disease.

The degeneration of dopaminergic synapses in Parkinson's disease: A selective animal model.

Available evidence increasingly suggests that the degeneration of dopamine neurons in Parkinsons disease starts in the striatal axons and synaptic terminals. A selective procedure is described here to study the mechanisms involved in the striatal denervation of dopaminergic terminals. This procedure can also be used to analyze mechanisms involved in the dopaminergic re-innervation of the striatum, and the role of astrocytes and microglia in both processes. Adult Sprague-Dawley rats were injected in the lateral ventricles with increasing doses of 6-hydroxydopamine (12-50 μg), which generated a dose-dependent loss of dopaminergic synapses and axons in the striatum, followed by an axonal sprouting (weeks later) and by a progressive recovery of striatal dopaminergic synapses (months later). Both the degeneration and regeneration of the dopaminergic terminals were accompanied by astrogliosis. Because the experimental manipulations did not induce unspecific damage in the striatal tissue, this method could be particularly suitable to study the basic mechanisms involved in the distal degeneration and regeneration of dopaminergic nigrostriatal neurons, and the possible role of astrocytes and microglia in the dynamics of both processes.