Nigral Dopamine Neurons Research Articles

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 Parkinson's 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 Parkinson's 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.

ErbB inhibitors ameliorate behavioral impairments of an animal model for schizophrenia: implication of their dopamine-modulatory actions

Ligands for ErbB receptors, including epidermal growth factor (EGF) and neuregulin-1, have a neurotrophic activity on midbrain dopaminergic neurons and are implicated in the pathophysiology of schizophrenia. Although ErbB kinase inhibitors ameliorate behavioral deficits of the schizophrenia model that was established by hippocampal lesioning of rat pups, the antipsychotic action of ErbB kinase inhibitors and its general applicability to other models are not fully characterized. Using a different animal model, here, we examined whether and how ErbB kinase inhibitors ameliorate the behavioral endophenotypes relevant to schizophrenia. The animal model for schizophrenia was prepared by exposing neonatal rats to the cytokine EGF. Intraventricular infusion of the ErbB1 inhibitors ZD1839 and PD153035 in these animals ameliorated the deficits in startle response and prepulse inhibition in a dose-dependent manner. The deficits of latent inhibition of fear learning were also alleviated by ZD1839 with its limited effects on body weight gain or locomotor activity. ZD1839 infusion also decreased the busting activity of nigral dopamine (DA) neurons and reduced pallidal DA metabolism, a result that mimics the anti-dopaminergic profile of risperidone and haloperidol in this brain region. ErbB inhibitors appear to have anti-dopaminergic actions to alleviate some of the behavioral deficits common to animal models for schizophrenia.

Bad News for Neuroprotective Therapies in PD?

While the etiopathogenesis of Parkinson’s disease (PD) continues to fuel intense debate [1], there is little doubt that the motor features of PD are caused by the degeneration of nigrostriatal dopamine (DA) neurons [2]. Clinical symptoms have been estimated to appear when at least 50% nigral DA neurons and 70% putaminal DA tissue contents are lost [3, 4]. Dopaminergic imaging suggests that at least 30% DA storage capacity (18F-dopa PET) and 50% of putamen DA transporters (DAT SPECT) have been lost by the onset of contralateral limb bradykinesia and rigidity [5–7]. Parkinsonian motor features are highly responsive to DA replacement therapy, which provides a stable symptomatic improvement for approximately 4–7 years. After this “honeymoon” period, the patients’ motor response to levodopa starts to fluctuate, and other complications, such as involuntary movements (dyskinesias), start to appear (reviewed in [8, 9]. The clinical history outlined above has assumed a continuing, gradual degeneration of nigrostriatal DA projections causing a progressive loss of DA terminals and DA storage capacity in the striatum (reviewed in [10]. The steady progression of this process is, however, challenged by a pathological investigation by Kordower and coworkers, published in the August issue of Brain [11]. In this study, putaminal and nigral sections from 28 subjects affected by idiopathic PD and 9 age-matched controls were immunohistochemically stained for the

Brain nigrostriatal system changes in rotenone-induced parkinsonism (quantitative immune-morphological study)

For studying one of the commonest diseases of the nervous system, parkinsonism, long-term course of injections of pesticide rotenone to Wistar rats was used, and thereafter changes of neurons and glial cells in the nigrostriatal regions of the brain were investigated by immunohistochemical methods. It was found that rats treated by rotenone were characterized by reduced motor activity and displayed characteristics of experimental parkinsonism. These changes were accompanied by a decreased tyrosine hydroxylase staining in the processes of the s. nigra dopamine neurons and aggregation of α-synuclein in their bodies, as well as by significant loss of dopamine cells in the rostral part of the s.nigra. Rotenone produced bilateral local destruction of brain tissue with surrounding activated astrocytes in the dorsal parts of the striatum bilaterally. One may conclude that a parkinsonian model induced by rotenone is characterized by degenerative changes of dopamine neurons in the s. nigra, with α-synuclein aggregation and local and symmetrical injury of the striatum (with the involvement of dopaminergic fibers, neurons, neuroglia and cerebral vessels), which presumably reflects rotenone-induced mitochondrial dysfunction.

Unaltered lactate and glucose transporter levels in the MPTP mouse model of Parkinson's disease

Metabolic impairment contributes to development of Parkinson's disease (PD). Mitochondrial dysfunction is involved in degeneration of nigral dopamine neurons. Also, in PD there are alterations in glucose metabolism in nigro-striatal pathways, and increased cerebral lactate levels have been found. We raise the question of whether changes in the amount transporters of energy substrates are involved in the pathogenesis of PD. We have used confocal immunofluorescence and immunogold postembedding electron microscopic techniques to study whether there are altered levels of the transporters for monocarboxylates (MCT1 and MCT2) and glucose (GLUT1) in the MPTP mouse model of PD. We found that MCT1 and GLUT1 were densely located in blood vessel endothelium, while MCT2 was present in perivascular astrocytic end feet processes in the substantia nigra and the striatum of control mice. We found that the localisation and densities of MCTs and GLUT1 were unaltered in the PD model. This is the first study reporting on the distribution of metabolic transporters in PD. Our results suggest that, although there are metabolic impairments in PD, the levels of MCT1, MCT2 and GLUT1 is not changed following dopaminergic neurodegeneration. This is in contrast to findings in other neurodegenerative disease, such as mesial temporal lobe epilepsy, where there are large alterations in MCT levels.

α-Synuclein–Induced Down-Regulation of Nurr1 Disrupts GDNF Signaling in Nigral Dopamine Neurons

Glial cell line-derived neurotrophic factor (GDNF) and its close relative neurturin are currently in clinical trials for neuroprotection in patients with Parkinson disease (PD). However, in animal models of PD, GDNF fails to protect nigral dopamine (DA) neurons against α-synuclein-induced neurodegeneration. Using viral vector delivery of human wild-type α-synuclein to nigral DA neurons in rats, we show that the intracellular response to GDNF is blocked in DA neurons that overexpress α-synuclein. This block is accompanied by reduced expression of the transcription factor Nurr1 and its downstream target, the GDNF receptor Ret. We found that Ret expression was also reduced in nigral DA neurons in PD patients. Conditional knockout of Nurr1 in mice resulted in reduced Ret expression and blockade of the response to GDNF, whereas overexpression of Nurr1 restored signaling, providing protection of nigral DA neurons against α-synuclein toxicity. These results suggest that Nurr1 is a regulator of neurotrophic factor signaling and a key player in the cellular defense against α-synuclein toxicity.

Expression of human E46K-mutated α-synuclein in BAC-transgenic rats replicates early-stage Parkinson's disease features and enhances vulnerability to mitochondrial impairment

Parkinson's disease (PD), the second most common neurodegenerative disorder, is etiologically heterogeneous, with most cases thought to arise from a combination of environmental factors and genetic predisposition; about 10% of cases are caused by single gene mutations. While neurotoxin models replicate many of the key behavioral and neurological features, they often have limited relevance to human exposures. Genetic models replicate known disease-causing mutations, but are mostly unsuccessful in reproducing major features of PD. In this study, we created a BAC (bacterial artificial chromosome) transgenic rat model of PD expressing the E46K mutation of α-synuclein, which is pathogenic in humans. The mutant protein was expressed at levels ~2–3-fold above endogenous α-synuclein levels. At 12months of age, there was no overt damage to the nigrostriatal dopamine system; however, (i) alterations in striatal neurotransmitter metabolism, (ii) accumulation and aggregation of α-synuclein in nigral dopamine neurons, and (iii) evidence of oxidative stress suggest this model replicates several preclinical features of PD. Further, when these animals were exposed to rotenone, a mitochondrial toxin linked to PD, they showed heightened sensitivity, indicating that α-synuclein expression modulates the vulnerability to mitochondrial impairment. We conclude that these animals are well-suited to examination of gene–environment interactions that are relevant to PD.

Pacemaker Rate and Depolarization Block in Nigral Dopamine Neurons: A Somatic Sodium Channel Balancing Act

Midbrain dopamine (DA) neurons are slow intrinsic pacemakers that undergo depolarization (DP) block upon moderate stimulation. Understanding DP block is important because it has been correlated with the clinical efficacy of chronic antipsychotic drug treatment. Here we describe how voltage-gated sodium (Na(V)) channels regulate DP block and pacemaker activity in DA neurons of the substantia nigra using rat brain slices. The distribution, density, and gating of Na(V) currents were manipulated by blocking native channels with tetrodotoxin and by creating virtual channels and anti-channels with dynamic clamp. Although action potentials initiate in the axon initial segment and Na(V) channels are distributed in multiple dendrites, selective reduction of Na(V) channel activity in the soma was sufficient to decrease pacemaker frequency and increase susceptibility to DP block. Conversely, increasing somatic Na(V) current density raised pacemaker frequency and lowered susceptibility to DP block. Finally, when Na(V) currents were restricted to the soma, pacemaker activity occurred at abnormally high rates due to excessive local subthreshold Na(V) current. Together with computational simulations, these data show that both the slow pacemaker rate and the sensitivity to DP block that characterizes DA neurons result from the low density of somatic Na(V) channels. More generally, we conclude that the somatodendritic distribution of Na(V) channels is a major determinant of repetitive spiking frequency.

Trophic factors differentiate dopamine neurons vulnerable to Parkinson's disease

Recent studies suggest a variety of factors characterize substantia nigra neurons vulnerable to Parkinson's disease, including the transcription factors pituitary homeobox 3 (Pitx3) and orthodenticle homeobox 2 (Otx2) and the trophic factor receptor deleted in colorectal cancer (DCC), but there is limited information on their expression and localization in adult humans. Pitx3, Otx2, and DCC were immunohistochemically localized in the upper brainstem of adult humans and mice and protein expression assessed using relative intensity measures and online microarray data. Pitx3 was present and highly expressed in most dopamine neurons. Surprisingly, in our elderly subjects no Otx2 immunoreactivity was detected in dopamine neurons, although Otx2 gene expression was found in younger cases. Enhanced DCC gene expression occurred in the substantia nigra, and higher amounts of DCC protein characterized vulnerable ventral nigral dopamine neurons. Our data show that, at the age when Parkinson's disease typically occurs, there are no significant differences in the expression of transcription factors in brainstem dopamine neurons, but those most vulnerable to Parkinson's disease rely more on the trophic factor receptor DCC than other brainstem dopamine neurons.

Bursting as a source of non‐linear determinism in the firing patterns of nigral dopamine neurons

Nigral dopamine (DA) neurons in vivo exhibit complex firing patterns consisting of tonic single-spikes and phasic bursts that encode information for certain types of reward-related learning and behavior. Non-linear dynamical analysis has previously demonstrated the presence of a non-linear deterministic structure in complex firing patterns of DA neurons, yet the origin of this non-linear determinism remains unknown. In this study, we hypothesized that bursting activity is the primary source of non-linear determinism in the firing patterns of DA neurons. To test this hypothesis, we investigated the dimension complexity of inter-spike interval data recorded in vivo from bursting and non-bursting DA neurons in the chloral hydrate-anesthetized rat substantia nigra. We found that bursting DA neurons exhibited non-linear determinism in their firing patterns, whereas non-bursting DA neurons showed truly stochastic firing patterns. Determinism was also detected in the isolated burst and inter-burst interval data extracted from firing patterns of bursting neurons. Moreover, less bursting DA neurons in halothane-anesthetized rats exhibited higher dimensional spiking dynamics than do more bursting DA neurons in chloral hydrate-anesthetized rats. These results strongly indicate that bursting activity is the main source of low-dimensional, non-linear determinism in the firing patterns of DA neurons. This finding furthermore suggests that bursts are the likely carriers of meaningful information in the firing activities of DA neurons.

Evidence that conditioned avoidance responses are reinforced by positive prediction errors signaled by tonic striatal dopamine

We conducted an experiment in which hedonia, salience and prediction error hypotheses predicted different patterns of dopamine (DA) release in the striatum during learning of conditioned avoidance responses (CARs). The data strongly favor the latter hypothesis. It predicts that during learning of the 2-way active avoidance CAR task, positive prediction errors generated when rats do not receive an anticipated footshock (which is better than expected) cause DA release that reinforces the instrumental avoidance action. In vivo microdialysis in the rat striatum showed that extracellular DA concentration increased during early CAR learning and decreased throughout training returning to baseline once the response was well learned. In addition, avoidance learning was proportional to the degree of DA release. Critically, exposure of rats to the same stimuli but in an unpredictable, unavoidable, and inescapable manner, did not produce alterations from baseline DA levels as predicted by the prediction error but not hedonic or salience hypotheses. In addition, rats with a partial lesion of substantia nigra DA neurons, which did not show increased DA levels during learning, failed to learn this task. These data represent clear and unambiguous evidence that it was the factor positive prediction error, and not hedonia or salience, which caused increase in the tonic level of striatal DA and which reinforced learning of the instrumental avoidance response.

Glucose Regulated Protein 78 Diminishes α-Synuclein Neurotoxicity in a Rat Model of Parkinson Disease

Accumulation of human wild-type (wt) α-synuclein (α-syn) induces neurodegeneration in humans and in experimental rodent models of Parkinson disease (PD). It also leads to endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR). We overexpressed glucose regulated protein 78, also known as BiP (GRP78/BiP), to test the hypothesis that this ER chaperone modulates the UPR, blocks apoptosis, and promotes the survival of nigral dopamine (DA) neurons in a rat model of PD induced by elevated level of human α-syn. We determined that α-syn activates ER stress mediators associated with pancreatic ER kinase-like ER kinase (PERK) and activating transcription factor-6 (ATF6) signaling pathways as well as proaoptotic CCAAT/-enhancer-binding protein homologous protein (CHOP) in nigral DA neurons. At the same time, overexpression of GRP78/BiP diminished α-syn neurotoxicity by down regulating ER stress mediators and the level of apoptosis, promoted survival of nigral tyrosine hydroxylase (TH) positive cells and resulted in higher levels of striatal DA, while eliminating amphetamine induced behavioral asymmetry. We also detected a complex between GRP78/BiP and α-syn that may contribute to prevention of the neurotoxicity caused by α-syn. Our data suggest that the molecular chaperone GRP78/BiP plays a neuroprotective role in α-syn-induced Parkinson-like neurodegeneration.

Dysregulated dopamine storage increases the vulnerability to α-synuclein in nigral neurons

Impairments in the capacity of dopaminergic neurons to handle cytoplasmic dopamine may be a critical factor underlying the selective vulnerability of midbrain dopamine neurons in Parkinson's disease. Furthermore, toxicity of α-synuclein in dopaminergic neurons has been suggested to be mediated by direct interaction between dopamine and α-synuclein through formation of abnormal α-synuclein species, although direct in vivo evidence to support this hypothesis is lacking. Here, we investigated the role of dopamine availability on α-synuclein mediated neurodegeneration in vivo. We found that overexpression of α-synuclein in nigral dopamine neurons in mice with deficient vesicular storage of dopamine led to a significant increase in dopaminergic neurodegeneration. Importantly, silencing the tyrosine hydroxylase enzyme – thereby reducing dopamine content in the nigral neurons – reversed the increased vulnerability back to the baseline level observed in wild-type littermates, but failed to eliminate it completely. Importantly, TH knockdown was not effective in altering the toxicity in the wild-type animals. Taken together, our data suggest that under normal circumstances, in healthy dopamine neurons, cytoplasmic dopamine is tightly controlled such that it does not contribute significantly to α-synuclein mediated toxicity. Dysregulation of the dopamine machinery in the substantia nigra, on the other hand, could act as a trigger for induction of increased toxicity in these neurons and could explain how these neurons become more vulnerable and die in the disease process.

Adeno-Associated Virus Type 2 Vector-Mediated Glial Cell Line-Derived Neurotrophic Factor Gene Transfer Induces Neuroprotection and Neuroregeneration in a Ubiquitin-Proteasome System Impairment Animal Model of Parkinson’s Disease

Background: The impairment of the ubiquitin-proteasome system (UPS) is a cellular mechanism underlying the neurodegenerative process in Parkinson’s disease (PD). A mouse model induced by the selective proteasome inhibitor lactacystin targeting on substantia nigra has been demonstrated to be valuable in investigating etiopathogenesis and neuroprotection for PD. Objective: In the present study, we used adeno-associated virus type 2 vector (AAV2) encoding glial cell line-derived neurotrophic factor (GDNF) injected into the striatum of this animal model to test the effectiveness and possible mechanisms of GDNF gene therapy. Results: Our results showed that AAV2-mediated GDNF gene therapy significantly attenuated lactacystin-induced loss of nigral dopamine (DA) neurons and striatal DA levels. Furthermore, we found that GDNF protein is mostly expressed in astrocytes in the subventricular zone (SVZ) and dentate gyrus (DG). AAV2-mediated GDNF therapy can induce neurogenesis in the SVZ and DG, and increase the number of nigral newborn DA neurons. Conclusion: These data indicate that AAV2-mediated GDNF gene therapy can protect the nigral DA neurons from the UPS impairment-induced degeneration, which may partly result from the nigral DA neuron regeneration in the brain, and such experimental results may have implications for the treatment of PD.

Viral-toxin interactions and Parkinson’s disease: poly(I:C) priming enhanced the neurodegenerative effects of paraquat

BackgroundParkinson’s disease (PD) has been linked with exposure to a variety of environmental and immunological insults (for example, infectious pathogens) in which inflammatory and oxidative processes seem to be involved. In particular, epidemiological studies have found that pesticide exposure and infections may be linked with the incidence of PD. The present study sought to determine whether exposure to a viral mimic prior to exposure to pesticides would exacerbate PD-like pathology.MethodsMice received a supra-nigral infusion of 5 μg of the double-stranded RNA viral analog, polyinosinic: polycytidylic acid (poly(I:C)), followed 2, 7 or 14 days later by administration of the pesticide, paraquat (nine 10 mg/kg injections over three weeks).ResultsAs hypothesized, poly(I:C) pre-treatment enhanced dopamine (DA) neuron loss in the substantia nigra pars compacta elicited by subsequent paraquat treatment. The augmented neuronal loss was accompanied by robust signs of microglial activation, and by increased expression of the catalytic subunit (gp91) of the NADPH oxidase oxidative stress enzyme. However, the paraquat and poly(I:C) treatments did not appreciably affect home-cage activity, striatal DA terminals, or subventricular neurogenesis.ConclusionsThese findings suggest that viral agents can sensitize microglial-dependent inflammatory responses, thereby rendering nigral DA neurons vulnerable to further environmental toxin exposure.

Impaired neurotransmission caused by overexpression of α-synuclein in nigral dopamine neurons.

We used in vivo amperometry to monitor changes in synaptic dopamine (DA) release in the striatum induced by overexpression of human wild-type α-synuclein in nigral DA neurons, induced by injection of an adeno-associated virus type 6 (AAV6)-α-synuclein vector unilaterally into the substantia nigra in adult rats. Impairments in DA release evolved in parallel with the development of degenerative changes in the nigrostriatal axons and terminals. The earliest change, seen 10 d after vector injection, was a marked, ≈50%, reduction in DA reuptake, consistent with an early dysfunction of the DA transporter that developed before any overt signs of axonal damage. At 3 wk, when the first signs of axonal damage were observed, the amount of DA released after a KCl pulse was reduced by 70-80%, and peak DA concentration was delayed, indicating an impaired release mechanism. At later time points, 8-16 wk, overall striatal innervation density was reduced by 60-80% and accompanied by abundant signs of axonal damage in the form of α-synuclein aggregates, axonal swellings, and dystrophic axonal profiles. At this stage DA release and reuptake were profoundly reduced, by 80-90%. The early changes in synaptic DA release induced by overexpression of human α-synuclein support the idea that early predegenerative changes in the handling of DA may initiate, and drive, a progressive degenerative process that hits the axons and terminals first. Synaptic dysfunction and axonopathy would thus be the hallmark of presymptomatic and early-stage Parkinson disease, followed by neuronal degeneration and cell loss, characteristic of more advanced stages of the disease.

Delayed caffeine treatment prevents nigral dopamine neuron loss in a progressive rat model of Parkinson's disease

Parkinson's disease (PD) is characterized by a prominent degeneration of nigrostriatal dopamine (DA) neurons with an accompanying neuroinflammation. Despite clinical and preclinical studies of neuroprotective strategies for PD, there is no effective treatment for preventing or slowing the progression of neurodegeneration. The inverse correlation between caffeine consumption and risk of PD suggests that caffeine may exert neuroprotection. Whether caffeine is neuroprotective in a chronic progressive model of PD has not been evaluated nor is it known if delayed caffeine treatment can stop DA neuronal loss. We show that a chronic unilateral intra-cerebroventricular infusion of 1-methyl-4-phenylpyridinium in the rat brain for 28days produces a progressive loss of DA and tyrosine hydroxylase in the ipsilateral striatum and a loss of DA cell bodies and microglial activation in the ipsilateral substantia nigra. Chronic caffeine consumption prevented the degeneration of DA cell bodies in the substantia nigra. Importantly, neuroprotection was still apparent when caffeine was introduced after the onset of the neurodegenerative process. These results add to the clinical relevance for adenosine receptors as a disease-modifying drug target for PD.

Apoptosis signal-regulating kinase 1 mediates MPTP toxicity and regulates glial activation.

Apoptosis signal-regulating kinase 1 (ASK1), a member of the mitogen-activated protein kinase 3 family, is activated by oxidative stress. The death-signaling pathway mediated by ASK1 is inhibited by DJ-1, which is linked to recessively inherited Parkinson's disease (PD). Considering that DJ-1 deficiency exacerbates the toxicity of the mitochondrial complex I inhibitor 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), we sought to investigate the direct role and mechanism of ASK1 in MPTP-induced dopamine neuron toxicity. In the present study, we found that MPTP administration to wild-type mice activates ASK1 in the midbrain. In ASK1 null mice, MPTP-induced motor impairment was less profound, and striatal dopamine content and nigral dopamine neuron counts were relatively preserved compared to wild-type littermates. Further, microglia and astrocyte activation seen in wild-type mice challenged with MPTP was markedly attenuated in ASK1−/− mice. These data suggest that ASK1 is a key player in MPTP-induced glial activation linking oxidative stress with neuroinflammation, two well recognized pathogenetic factors in PD. These findings demonstrate that ASK1 is an important effector of MPTP-induced toxicity and suggest that inhibiting this kinase is a plausible therapeutic strategy for protecting dopamine neurons in PD.

Progressive neurodegenerative and behavioural changes induced by AAV-mediated overexpression of α-synuclein in midbrain dopamine neurons

Parkinson's disease (PD) is characterised by the progressive loss of nigral dopamine neurons and the presence of synucleinopathy. Overexpression of α-synuclein in vivo using viral vectors has opened interesting possibilities to model PD-like pathology in rodents. However, the attempts made so far have failed to show a consistent behavioural phenotype and pronounced dopamine neurodegeneration. Using a more efficient adeno-associated viral (AAV) vector construct, which includes a WPRE enhancer element and uses the neuron-specific synapsin-1 promoter to drive the expression of human wild-type α-synuclein, we have now been able to achieve increased levels of α-synuclein in the transduced midbrain dopamine neurons sufficient to induce profound deficits in motor function, accompanied by reduced expression of proteins involved in dopamine neurotransmission and a time-dependent loss of nigral dopamine neurons, that develop progressively over 2–4months after vector injection. As in human PD, nigral cell loss was preceded by degenerative changes in striatal axons and terminals, and the appearance of α-synuclein positive inclusions in dystrophic axons and dendrites, supporting the idea that α-synuclein-induced pathology hits the axons and terminals first and later progresses to involve also the cell bodies. The time-course of changes seen in the AAV-α-synuclein treated animals defines distinct stages of disease progression that matches the pre-symptomatic, early symptomatic, and advanced stages seen in PD patients. This model provides new interesting possibilities for studies of stage-specific pathologic mechanisms and identification of targets for disease-modifying therapeutic interventions linked to early or late stages of the disease.

Intrinsic and integrative properties of substantia nigra pars reticulata neurons.

The GABA projection neurons of the substantia nigra pars reticulata (SNr) are output neurons for the basal ganglia and thus critical for movement control. Their most striking neurophysiological feature is sustained, spontaneous high frequency spike firing. A fundamental question is: what are the key ion channels supporting the remarkable firing capability in these neurons? Recent studies indicate that these neurons express tonically active type 3 transient receptor potential (TRPC3) channels that conduct a Na-dependent inward current even at hyperpolarized membrane potentials. When the membrane potential reaches -60 mV, a voltage-gated persistent sodium current (I(NaP)) starts to activate, further depolarizing the membrane potential. At or slightly below -50 mV, the large transient voltage-activated sodium current (I(NaT)) starts to activate and eventually triggers the rapid rising phase of action potentials. SNr GABA neurons have a higher density of I(NaT), contributing to the faster rise and larger amplitude of action potentials, compared with the slow-spiking dopamine neurons. I(NaT) also recovers from inactivation more quickly in SNr GABA neurons than in nigral dopamine neurons. In SNr GABA neurons, the rising phase of the action potential triggers the activation of high-threshold, inactivation-resistant Kv3-like channels that can rapidly repolarize the membrane. These intrinsic ion channels provide SNr GABA neurons with the ability to fire spontaneous and sustained high frequency spikes. Additionally, robust GABA inputs from direct pathway medium spiny neurons in the striatum and GABA neurons in the globus pallidus may inhibit and silence SNr GABA neurons, whereas glutamate synaptic input from the subthalamic nucleus may induce burst firing in SNr GABA neurons. Thus, afferent GABA and glutamate synaptic inputs sculpt the tonic high frequency firing of SNr GABA neurons and the consequent inhibition of their targets into an integrated motor control signal that is further fine-tuned by neuromodulators including dopamine, serotonin, endocannabinoids, and H₂O₂.