**3.1 Mitochondrial dysfunction**

114 Etiology and Pathophysiology of Parkinson's Disease

a misfolded conformation in dopaminergic neurons. Indeed, dopamine promotes alphasynuclein protofibrils formation. Oxidative stress, which is known to be one of the main pathogenetic factors leading to the death of dopaminergic neurons in Parkinson's disease, has been identified as a major responsible for alpha-synuclein post-translational modifications, consisting in oxidation and nitration of specific aminoacid residues; these alterations in alpha-synuclein have been detected in nigral neurons of brain samples from patients with Parkinson's disease (Giasson et al 2000) and from animal models of disease

An increase in alpha-synuclein levels, correlated with the degree of nigrostriatal dopamine depletion, has been demonstrated in the substantia nigra of patients with sporadic Parkinson's disease (Chu et al 2007). It is conceivable to suppose that this accumulation of the toxic protein might derive from an increase in transcription and translation of the SNCA

The variability in codifying and non-codifying SNCA gene sequences has demonstrated to modify the risk of developing sporadic Parkinson's disease, probably through a mild modulatory effect on transcription activity and, as a consequence, on intraneuronal levels of alpha-synuclein. SNCA duplication was found in sporadic cases of Parkinson's disease. Increasing length in the dinucleotide repeat sequence (REP1) of the SNCA gene promoter has been demonstrated to be correlated with an increased risk of developing sporadic Parkinson's disease (Maraganore et al 2006; Brighina et al 2008). Furthermore, single nucleotide polymorphisms at the promoter region and at the 3' end of SNCA gene have

Anyway, aging remains the most evident risk factor for sporadic Parkinson's disease. An interesting hypothesis suggests that senescence of dopaminergic neurons could promote the accumulation of misfolded alpha-synuclein through two main mechanisms: slowing alpha-synuclein turnover through the impairment of clearance machinery; impairing the function of mitochondria and antioxidant systems, with consequent increase in oxidative

Great efforts have been done in order to identify genetic and environmental factors able to modulate the aging process. Caloric restriction represents the only environmental condition which has demonstrated to delay aging in a wide range of organisms. This evidence suggests that caloric restriction might guarantee protection from developing Parkinson's disease and could delay the disease progression. As it was supposed, caloric restriction has shown a protective effect in MPTP-induced animal models of Parkinson's disease, even if an epidemiologic correlation between diet and incidence of Parkinson's disease has not been demonstrated. The molecular basis of the protective role of caloric restriction in animal models has been elucidated in yeast, where a family of NAD+-dependent protein deacetylases, called sirtuins (SIRTs), have been identified as mediators of the changes induced by starvation in cells; many sirtuins analogs have been discovered in mammals. These proteins regulate cell homeostasis partially through an epigenetic mechanism: they modulate histones acetylation and chromatin condensation. SIRT1 is a major responsible for protective cell modifications during starvation, whereas other SIRTs seem to exert a specific deleterious effect on dopaminergic neurons. In particular, SIRT2, an alpha-tubulin deacetylase, promotes formation of alpha-tubulin oligomers, which destabilize microtubules and form toxic complexes with alpha-synuclein oligomers. The interaction of alphasynuclein oligomers with alpha-tubulin plays an important role in mediating alpha-

(Gao et al 2008).

stress.

gene or from a decrease in the protein degradation.

been found to modify the risk of disease.

Several epidemiologic and experimental data confirm the importance of mitochondrial derangement in the pathogenesis of Parkinson's disease. First of all, electron transport complex I represents the main molecular target of paraquat, rotenone and MPTP, the exogenous substances known to determine degeneration of human dopaminergic neurons. The selective toxicity of MPTP on dopaminergic neurons derives from its transformation to the metabolite 1-methyl-4-phenyl-pyridium (MPP+), which is concentrated within dopaminergic neurons by dopamine transporters. Even paraquat, which is structurally similar to MPP+, is carried into dopaminergic cells by the same transporter. In contrast to MPTP and paraquat, rotenone is not concentrated in nigral neurons, nevertheless it produces selective death of these cells. The development of parkinsonism after exposure to rotenone represents a further proof of the high susceptibility of dopaminergic neurons to mitochondrial impairment. Toxic derangement in complex I induces alpha-synuclein aggregation and formation of Lewy body-like inclusions (Betarbet et al 2000).

A reduced activity of the mitochondrial complexes I and IV has been observed in the substantia nigra of patients with Parkinson's disease and in animal models of disease (Palacino et al 2004). Decreased levels of coenzyme Q10 together with a higher ratio of oxidized vs. reduced Coenzyme Q10 have been observed in patients. An increase of cerebral lactate levels, suggesting an alteration in aerobic metabolism as a consequence of derangement of mitochondrial respiratory chain, has been detected by brain magnetic resonance spectroscopy in patients with Parkinson's disease (Henchcliffe et al 2008). Experiments performed on cytoplasmic hybrid cell lines from patients with Parkinson's disease suggest that the complex I deficit associated to the disease might be genetically determined by defects in mitochondrial DNA. A single nucleotide polymorphism within the gene encoding NADH dehydrogenase 3 of complex I has been associated to a decrease in the risk of sporadic Parkinson's disease, providing genetic evidence of the pathogenetic relevance of complex I activity (van der Walt et al 2003).

An endogenous substance able to impair mitochondrial function is salsolinol, which is synthesized from dopamine and acetaldehyde by salsolinol synthase. This molecule has been found in various regions of the brain, including striatum and substantia nigra. Salsolinol inhibits mitochondrial complex II and this effect may explain the fact that high levels of this molecule are able to induce degeneration of dopaminergic neurons (Storch et al 2000).

Alpha-Synuclein, Oxidative Stress and

and O2 by reduced glutathione.

dopamine oxidation.

al 1997).

in Parkinson's disease:

Autophagy Failure: Dangerous Liaisons in Dopaminergic Neurodegeneration 117

• Superoxide dismutase is a cytosolic, mithocondrial and extracellular metallo-protein

• Reduced glutathione is a tripeptide which exerts an important reducing activity of

• Glutathione peroxidase is an enzyme which catalyzes the reduction of H2O2 to water

A large amount of studies have demonstrated that oxidative damage plays a major pathogenetic role in Parkinson's disease and represents a key contributor to the loss of dopaminergic neurons (Jenner & Olanow 1998). The high concentration of dopamine is presumed to be essential to determine the high vulnerability of dopaminergic cells to oxidative stress. Dopamine itself does not seem to exert direct toxic effects at physiologic

• Auto-oxidation of dopamine leads to the production of semiquinones, which have an

• The enzymatic metabolism of dopamine, mediated by monoamine oxidase B, leads to the generation of H202; therefore, the inhibition of monoamine oxidase B by selective drugs may protect against production of some toxins and free radicals deriving from

The experimental stimulation of dopamine catabolism by monoamine oxidase B leads to an increase in oxidative stress levels, thus confirming the deleterious role of dopamine

Extensive studies performed on postmortem brain samples have provided evidence supporting the involvement of oxidative stress in the pathogenesis of Parkinson's disease. Elevated levels of reactive oxygen species have been detected by assessment of lipid peroxidation, protein oxidation and DNA damage in nigrostriatal regions of patients with

• Malondialdehyde, 4-hydroxy-2,3-nonenal and thiobarbituric acid reactive substances (TBARS) are markers of lipid peroxidation which are elevated in the substantia nigra

• Markers of oxidative damage to proteins, such as carbonyl modifications of aminoacid residues, are increased in substantia nigra of patients with Parkinson's disease (Alam et

• An increase in oxidative alterations of nucleic acids was detected in midbrain of

On the other hand, a dysregulation of multiple antioxidant systems has been demonstrated

• An increase in superoxide dismutase and catalase levels has been detected in striatum and midbrain of animal models (Keeney et al 2006). This feature probably represents a compensatory reaction of cell control systems, aimed at neutralizing the increasing

• A decrease of reduced glutathione levels was detected in the substantia nigra and corpus striatum of patients with Parkinson's disease and in animal models (Pearce et al 1997). Glutathione deficiency could be a consequence of the high levels of free radicals, which rapidly oxidize glutathione to glutathione disulfide, or, alternatively, it might represent an early pathogenetic feature which contributes to the high vulnerability of

which catalyzes the transformation of superoxide into O2 and H2O2.

concentrations, but toxic intermediates derive from its catabolism:

intrinsic toxic effect and generate reactive oxygen species.

derivatives on neuronal oxidoreductive equilibrium.

and striatum of diseased patients (Yoritaka et al 1996).

Parkinson's disease and in animal models:

diseased patients (Zhang et al 1999).

levels of reactive oxygen species.

dopaminergic neurons to oxidative stress.

reactive oxygen species through its conversion to glutathione disulfide.

Alpha-synuclein contains a mitochondrial targeting sequence. Over-expression of this protein favors its translocation to the mitochondria. The damage of these organelles seems to be one of the mechanisms mediating the toxic effects of alpha-synuclein oligomers.

An additional confirmation of the importance of mitochondrial function for the homeostasis of dopaminergic neurons is the finding that some proteins linked to hereditary forms of Parkinson's disease exert their function within mitochondria; furthermore, pathogenetic mutations of the genes encoding for some of these proteins are associated with the impairment of mitochondrial function, which might be the main neurotoxic action exerted by these mutations:


#### **3.2 Oxidative stress**

Free radicals include reactive oxygen species and reactive nitrogen species. During cell metabolism some of these substances are normally produced: superoxide anion, hydrogen peroxide, nitric oxide, peroxynitrite, nitroxyl and hydroxy radical. These products can be responsible for the damage of protein, DNA and lipid.

The brain contains a high amount of substances, such as phospholipids and free fatty acids, which are vulnerable to oxidative modifications and, as a consequence, are responsible for the high vulnerability of the central nervous system to the deleterious action of free radicals. Lipid peroxidation represents a key mechanism mediating the toxicity of free radicals on a wide range of cell organelles and functions. Free radicals initiate peroxidation of the membrane lipids making them lose a hydrogen atom from a methylene group, with formation of a diene; this product mediates the formation of a peroxy radical through reaction with oxygen; the peroxy radical abstracts a hydrogen atom from another lipid to form hydroperoxides, which mediates the propagation of lipid peroxidation.

In physiologic conditions free radicals are rapidly converted into non toxic molecules by antioxidants, which prevent oxidation of other molecules, thus protecting cell from oxidative stress. The cell antioxidant system include four main molecules:

• Catalase is a heme-protein localized in peroxisomes, which catalyzes the decomposition of hydrogen peroxide to water and oxygen.

Alpha-synuclein contains a mitochondrial targeting sequence. Over-expression of this protein favors its translocation to the mitochondria. The damage of these organelles seems to be one of the mechanisms mediating the toxic effects of alpha-synuclein oligomers. An additional confirmation of the importance of mitochondrial function for the homeostasis of dopaminergic neurons is the finding that some proteins linked to hereditary forms of Parkinson's disease exert their function within mitochondria; furthermore, pathogenetic mutations of the genes encoding for some of these proteins are associated with the impairment of mitochondrial function, which might be the main neurotoxic action exerted

• PTEN induced putative kinase 1 (PINK1/PARK6) is a kinase that localizes in the mitochondrial membrane. Its loss of function is associated with a decrease in complex I activity determining an increase in oxidative stress. Mutations of PINK1 gene are

• DJ-1 (PARK7) is a protein found to be mutated in recessive forms of Parkinson's disease. It plays a role in cell protection during oxidative stress, which induces the translocation of the protein to the mitochondrial matrix and the intermembrane space. The loss of DJ1 has been associated to depolarization and fragmentation of

• Leucine-rich repeat kinase 2 (LRRK2/PARK8) is a kinase linked to dominant forms of Parkinson's disease. It is mainly localized in the cytosol but a fraction can be found in the outer mitochondrial membrane. Pathogenetic mutations determine an increase in

• High temperature requirement proteinA2 (HTRA2/Omi/PARK13) is a mitochondrial protease which has been associated with recessive forms of Parkinson's disease. Its loss

Free radicals include reactive oxygen species and reactive nitrogen species. During cell metabolism some of these substances are normally produced: superoxide anion, hydrogen peroxide, nitric oxide, peroxynitrite, nitroxyl and hydroxy radical. These products can be

The brain contains a high amount of substances, such as phospholipids and free fatty acids, which are vulnerable to oxidative modifications and, as a consequence, are responsible for the high vulnerability of the central nervous system to the deleterious action of free radicals. Lipid peroxidation represents a key mechanism mediating the toxicity of free radicals on a wide range of cell organelles and functions. Free radicals initiate peroxidation of the membrane lipids making them lose a hydrogen atom from a methylene group, with formation of a diene; this product mediates the formation of a peroxy radical through reaction with oxygen; the peroxy radical abstracts a hydrogen atom from another lipid to form hydroperoxides, which mediates the propagation of lipid

In physiologic conditions free radicals are rapidly converted into non toxic molecules by antioxidants, which prevent oxidation of other molecules, thus protecting cell from

• Catalase is a heme-protein localized in peroxisomes, which catalyzes the decomposition

oxidative stress. The cell antioxidant system include four main molecules:

responsible for recessive forms of Parkinson's disease.

of function has revealed to produce mitochondrial damage.

responsible for the damage of protein, DNA and lipid.

of hydrogen peroxide to water and oxygen.

by these mutations:

mitochondria.

**3.2 Oxidative stress** 

peroxidation.

the kinase activity of the protein.


A large amount of studies have demonstrated that oxidative damage plays a major pathogenetic role in Parkinson's disease and represents a key contributor to the loss of dopaminergic neurons (Jenner & Olanow 1998). The high concentration of dopamine is presumed to be essential to determine the high vulnerability of dopaminergic cells to oxidative stress. Dopamine itself does not seem to exert direct toxic effects at physiologic concentrations, but toxic intermediates derive from its catabolism:


The experimental stimulation of dopamine catabolism by monoamine oxidase B leads to an increase in oxidative stress levels, thus confirming the deleterious role of dopamine derivatives on neuronal oxidoreductive equilibrium.

Extensive studies performed on postmortem brain samples have provided evidence supporting the involvement of oxidative stress in the pathogenesis of Parkinson's disease. Elevated levels of reactive oxygen species have been detected by assessment of lipid peroxidation, protein oxidation and DNA damage in nigrostriatal regions of patients with Parkinson's disease and in animal models:


On the other hand, a dysregulation of multiple antioxidant systems has been demonstrated in Parkinson's disease:


Alpha-Synuclein, Oxidative Stress and

postsynaptic receptors and mediate excitotoxicity.

Autophagy Failure: Dangerous Liaisons in Dopaminergic Neurodegeneration 119

Furthermore, reactive oxygen species have been identified as agents able to induce the increased release of glutamate during pathologic conditions. In fact, lipid peroxidation of presynaptic membrane seems to impair the function of transporters involved in the maintenance of calcium homeostasis, resulting in sustained elevation of this ion in the presynaptic terminal, which is depolarized and so releases glutamate into the synaptic cleft; moreover, oxidative stress directly impairs glutamate transporter function in astrocytes and neurons, leading to an increase in concentrations of extracellular glutamate, which can bind

Glutamate transport is an energy-dependent process, which explains, together with oxidative stress, the fact that mitochondrial dysfunction determine a decrease in the reuptake of

Based on this strong relationship among oxidative stress, mitochondrial dysfunction and excitotoxicity, it is easy to imagine an involvement of glutamate toxicity in the pathogenesis of Parkinson's disease, where free radicals and mitochondrial impairment are known to play a major role. In animal models of disease modifications in the abundance and phosphorylation status of the different NMDA receptor subunits have been detected (Dunah et al 2000). Anyway, the importance of excitotoxicity in the degeneration of nigral dopaminergic neurons might be increased by the complex set of changes that nigrostriatal dopamine depletion triggers in functional anatomy of basal ganglia circuitry. In particular, the reduction of the nigrostriatal dopaminergic transmission produces a strong increase in the glutammatergic activity of the subthalamic projection, normally inhibited by nigral neurons. This hyperactivity critically contributes to the onset of parkinsonian motor symptoms, as confirmed by the evidence that electric suppression of subthalamic nucleus through deep brain stimulation improves motor functions. Furthermore, these functional alterations in basal ganglia circuitry might have pathogenetic implications: overactivation of subthalamic glutammatergic transmission to substantia nigra could promote the excitotoxic damage of dopaminergic neurons, favoring neuronal death and accelerating disease progression; this concept has been confirmed by the beneficial effects obtained by blockade

glutamate and, as a consequence, an increase in neuron vulnerability to excitotoxicity.

of subthalamic activity in animal models of Parkinson's disease (Blandini et al 2001).

oxidative stress and is amplified by subthalamic overactivation.

**4.1 Impairment of the ubiquitin-proteasomal system** 

**4. Role of clearance mechanisms** 

clearence systems failure.

Therefore, excitotoxicity appears to be an important contributor to neuronal death in the substantia nigra of patients with Parkinson's disease; this mechanism correlates with

Emerging data support the view that dysregulation of alpha-synuclein clearance machinery might represent the key feature leading to the intraneuronal accumulation of this protein. These mechanisms include the ubiquitin-proteasome system and the autophagic-lysosomal pathways; the autophagic pathways involved in the degradation of alpha-synuclein are chaperone-mediated autophagy and macroautophagy (Webb et al 2000). Anyway, the accumulation of alpha-synuclein does not represent the only negative consequence of

Increasing evidences indicate that the ubiquitin–proteasome pathway is defective in Parkinson's disease. The finding that Lewy bodies are ubiquitin-positive aggregates has

The iron-catalyzed conversion of H202 into hydroxyl radicals is supposed to contribute to oxidative stress and neurodegeneration in Parkinson's disease; this remark is based on the finding of increased iron content in postmortem brain samples from diseased patients.

Various proteins linked to recessive hereditary forms of Parkinson's disease play an important role in cell response to oxidative stress. In particular, an increase in protein and lipid peroxidation has been demonstrated in parkin defective animal models, suggesting that a defect in the ubiquitin-proteasome pathway or in formation of Lewy bodies can favor the generation of free radicals.

The mechanisms mediating the neurotoxic effects of oxidative species in dopaminergic neurons are complex and only partially understood, but a central role seems to be played by the alterations that free radicals produce on alpha-synuclein conformational status and degradation. Indeed, it has been demonstrated that oxidative stress induces intraneuronal accumulation and aggregation of alpha-synuclein and formation of Lewy bodies-like inclusions. This effect might be produced through a double mechanism:


The deleterious effect of oxidative stress on the function of clearance systems might partially depend on direct alteration of proteins regulating and mediating these pathways; at the same time, oxidative stress produces ATP depletion through damage of mitochondrial membranes, thus interfering with all energy-dependent processes, such as the clearance of proteins and organelles.

#### **3.3 Excitotoxicity**

Several lines of experimental evidence indicate that a toxic effect exerted by high levels of glutamate in the synaptic cleft contributes to the neurodegeneration of dopaminergic neurons in Parkinson's disease. Excitotoxicity is a pathogenetic mechanism that has demonstrated a role in the neuronal death in different neurologic diseases, both acute and chronic (Ferrarese & Beal 2004).

The molecular processes that mediate glutamate effects on neuronal survival consist on a complex cascade of events that ultimately results in neuronal death. Overactivation of the N-methyl-D-aspartate (NMDA) receptors seem to mediate a great part of this neurotoxic effect through generation of a calcium overload. Indeed, NMDA receptors are permeable to calcium ions, so that their activation results in an increase in the cytoplasmatic concentration of this element; high intracellular levels of calcium are responsible for the activation of several pro-apoptotic pathways and for induction of oxidative stress through activation of oxygenases and perturbation of mitochondrial homeostasis.

Oxidative and nitrosative stress represent important processes both favoring and mediating excitotoxic damage. An increase in the production of free radicals has been demonstrated in neurons exposed to excitotoxic insults. Nitric oxide is produced in response to NMDA receptors activation thanks to the interaction of nitric oxide synthase with these receptors; nitric oxide contributes to the generation of free radicals and is involved in the activation of the pro-apoptotic cascade. These data suggest that oxidative stress gives an important contribution to the excitotoxic process downstream of glutamate receptor activation.

The iron-catalyzed conversion of H202 into hydroxyl radicals is supposed to contribute to oxidative stress and neurodegeneration in Parkinson's disease; this remark is based on the finding of increased iron content in postmortem brain samples from diseased patients. Various proteins linked to recessive hereditary forms of Parkinson's disease play an important role in cell response to oxidative stress. In particular, an increase in protein and lipid peroxidation has been demonstrated in parkin defective animal models, suggesting that a defect in the ubiquitin-proteasome pathway or in formation of Lewy bodies can favor

The mechanisms mediating the neurotoxic effects of oxidative species in dopaminergic neurons are complex and only partially understood, but a central role seems to be played by the alterations that free radicals produce on alpha-synuclein conformational status and degradation. Indeed, it has been demonstrated that oxidative stress induces intraneuronal accumulation and aggregation of alpha-synuclein and formation of Lewy bodies-like

• Reactive oxygen species directly generate post-translational modifications in alpha-

• High levels of free radicals determine a down-regulation of both the ubiquitinproteasome system and the autophagy-lysosomal pathway, thus impairing the entire

The deleterious effect of oxidative stress on the function of clearance systems might partially depend on direct alteration of proteins regulating and mediating these pathways; at the same time, oxidative stress produces ATP depletion through damage of mitochondrial membranes, thus interfering with all energy-dependent processes, such as the clearance of

Several lines of experimental evidence indicate that a toxic effect exerted by high levels of glutamate in the synaptic cleft contributes to the neurodegeneration of dopaminergic neurons in Parkinson's disease. Excitotoxicity is a pathogenetic mechanism that has demonstrated a role in the neuronal death in different neurologic diseases, both acute and

The molecular processes that mediate glutamate effects on neuronal survival consist on a complex cascade of events that ultimately results in neuronal death. Overactivation of the N-methyl-D-aspartate (NMDA) receptors seem to mediate a great part of this neurotoxic effect through generation of a calcium overload. Indeed, NMDA receptors are permeable to calcium ions, so that their activation results in an increase in the cytoplasmatic concentration of this element; high intracellular levels of calcium are responsible for the activation of several pro-apoptotic pathways and for induction of oxidative stress through activation of

Oxidative and nitrosative stress represent important processes both favoring and mediating excitotoxic damage. An increase in the production of free radicals has been demonstrated in neurons exposed to excitotoxic insults. Nitric oxide is produced in response to NMDA receptors activation thanks to the interaction of nitric oxide synthase with these receptors; nitric oxide contributes to the generation of free radicals and is involved in the activation of the pro-apoptotic cascade. These data suggest that oxidative stress gives an important contribution to the excitotoxic process downstream of glutamate receptor activation.

inclusions. This effect might be produced through a double mechanism:

degradative machinery of alpha-synuclein.

oxygenases and perturbation of mitochondrial homeostasis.

synuclein residues, enhancing the tendency of the protein to aggregate.

the generation of free radicals.

proteins and organelles.

chronic (Ferrarese & Beal 2004).

**3.3 Excitotoxicity** 

Furthermore, reactive oxygen species have been identified as agents able to induce the increased release of glutamate during pathologic conditions. In fact, lipid peroxidation of presynaptic membrane seems to impair the function of transporters involved in the maintenance of calcium homeostasis, resulting in sustained elevation of this ion in the presynaptic terminal, which is depolarized and so releases glutamate into the synaptic cleft; moreover, oxidative stress directly impairs glutamate transporter function in astrocytes and neurons, leading to an increase in concentrations of extracellular glutamate, which can bind postsynaptic receptors and mediate excitotoxicity.

Glutamate transport is an energy-dependent process, which explains, together with oxidative stress, the fact that mitochondrial dysfunction determine a decrease in the reuptake of glutamate and, as a consequence, an increase in neuron vulnerability to excitotoxicity.

Based on this strong relationship among oxidative stress, mitochondrial dysfunction and excitotoxicity, it is easy to imagine an involvement of glutamate toxicity in the pathogenesis of Parkinson's disease, where free radicals and mitochondrial impairment are known to play a major role. In animal models of disease modifications in the abundance and phosphorylation status of the different NMDA receptor subunits have been detected (Dunah et al 2000). Anyway, the importance of excitotoxicity in the degeneration of nigral dopaminergic neurons might be increased by the complex set of changes that nigrostriatal dopamine depletion triggers in functional anatomy of basal ganglia circuitry. In particular, the reduction of the nigrostriatal dopaminergic transmission produces a strong increase in the glutammatergic activity of the subthalamic projection, normally inhibited by nigral neurons. This hyperactivity critically contributes to the onset of parkinsonian motor symptoms, as confirmed by the evidence that electric suppression of subthalamic nucleus through deep brain stimulation improves motor functions. Furthermore, these functional alterations in basal ganglia circuitry might have pathogenetic implications: overactivation of subthalamic glutammatergic transmission to substantia nigra could promote the excitotoxic damage of dopaminergic neurons, favoring neuronal death and accelerating disease progression; this concept has been confirmed by the beneficial effects obtained by blockade of subthalamic activity in animal models of Parkinson's disease (Blandini et al 2001).

Therefore, excitotoxicity appears to be an important contributor to neuronal death in the substantia nigra of patients with Parkinson's disease; this mechanism correlates with oxidative stress and is amplified by subthalamic overactivation.
