Evidence of a breakdown of corticostriatal connections in Parkinson's disease

Striatal spine plasticity in Parkinson's disease

Striatal dopamine (DA) denervation results in a significant loss of dendritic spines on medium spiny projection neurons in Parkinson's disease. In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated parkinsonian monkeys, spines contacted either by cortical or thalamic glutamatergic terminals are severely affected on both direct and indirect striatofugal neurons. In rodents, indirect pathway neurons appear to be more sensitive, at least in early stages of acute dopamine denervation. The remaining corticostriatal and thalamostriatal axo-spinous synapses undergo complex ultrastructural remodeling consistent with increased synaptic activity in the DA-denervated primate striatum, which may explain the pathophysiological overactivity of the corticostriatal system reported in various animal models of parkinsonism. The calcium-mediated regulation of the transcription factor myocyte enhancer factor 2 was recognized as a possible underlying mechanism for striatal spine plasticity. Future studies to determine how alterations in striatal spine plasticity contribute to the symptomatology of parkinsonism are warranted.

Extracerebellar role for Cerebellin1: modulation of dendritic spine density and synapses in striatal medium spiny neurons

Cerebellin1 (Cbln1) is a secreted glycoprotein that was originally isolated from the cerebellum and subsequently found to regulate synaptic development and stability. Cbln1 has a heterogeneous distribution in brain, but the only site in which it has been shown to have central effects is the cerebellar cortex, where loss of Cbln1 causes a reduction in granule cell-Purkinje cell synapses. Neurons of the thalamic parafascicular nucleus (PF), which provide glutamatergic projections to the striatum, also express high levels of Cbln1. We first examined Cbln1 in thalamostriatal neurons and then determined if cbln1 knockout mice exhibit structural deficits in striatal neurons. Virtually all PF neurons express Cbln1-immunoreactivity (-ir). In contrast, only rare Cbln1-ir neurons are present in the central medial complex, the other thalamic region that projects heavily to the dorsal striatum. In the striatum Cbln1-ir processes are apposed to medium spiny neuron (MSN) dendrites; ultrastructural studies revealed that Cbln1-ir axon terminals form axodendritic synapses with MSNs. Tract-tracing studies found that all PF cells retrogradely labeled from the striatum express Cbln1-ir. We then examined the dendritic structure of Golgi-impregnated MSNs in adult cbln1 knockout mice. MSN dendritic spine density was markedly increased in cbln1(-/-) mice relative to wildtype littermates, but total dendritic length was unchanged. Ultrastructural examination revealed an increase in the density of MSN axospinous synapses in cbln1(-/-) mice, with no change in postsynaptic density length. Thus, Cbln1 determines the dendritic structure of striatal MSNs, with effects distinct from those seen in the cerebellum.

Basal Ganglia circuits underlying the pathophysiology of levodopa-induced dyskinesia

Involuntary movements or dyskinesia, represent a debilitating complication of levodopa therapy for Parkinson's disease. Dyskinesia is, ultimately, experienced by the vast majority of the patients. Despite the importance of this problem, little was known about the cause of dyskinesia, a situation that has dramatically evolved in the last few years with a focus upon the molecular and signaling changes induced by chronic levodopa treatment. Departing from this, we here review the progress made in functional anatomy and neuroimaging that have had a tremendous impact on our understanding of the anatomo-functional organization of the basal ganglia in Parkinsonism and dyskinetic states, notably the demonstration that dyskinesia are linked to a pathological processing of limbic and cognitive information.

The synaptic pathology of alpha-synuclein aggregation in dementia with Lewy bodies, Parkinson's disease and Parkinson's disease dementia

Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are usually associated with loss of dopaminergic neurons. Loss of substantia nigra neurons and presence of Lewy body inclusions in some of the remaining neurons are the hallmark pathology seen in the final stages of the disease. Attempts to correlate Lewy body pathology to either cell death or severity of clinical symptoms, however, have not been successful. While the pathophysiology of the neurodegenerative process can hardly be explained by Lewy bodies, the clinical symptoms do indicate a degenerative process located at the presynapse resulting in a neurotransmitter deficiency. Recently it was shown that 90% or even more of alpha-synuclein aggregates in DLB cases were located at the presynapses in the form of very small deposits. In parallel, dendritic spines are retracted, whereas the presynapses are relatively preserved, suggesting a neurotransmitter deprivation. The same alpha-synuclein pathology can be demonstrated for PD. These findings give rise to the notion that not cell death but rather alpha-synuclein aggregate-related synaptic dysfunction causes the neurodegeneration. This opens new perspectives for understanding PD and DLB. If presynaptic alpha-synuclein aggregation, not neuronal loss, is the key issue of the neurodegenerative process, then PD and DLB may eventually be treatable in the future. The disease may progress via trans-synaptical spread, suggesting that stem cell transplants are of limited use. Future therapies may focus on the regeneration of synapses.

Striatal spine plasticity in Parkinson's disease: pathological or not?

Parkinson's disease (PD) is characterized by a dramatic loss of dopamine that underlies complex structural and functional changes in striatal projection neurons. A key alteration that has been reported in various rodent models and PD patients is a significant reduction in striatal dendritic spine density. Our recent findings indicate that striatal spine loss is also a prominent feature of parkinsonism in MPTP-treated monkeys. In these animals, striatal spine plasticity is tightly linked with the degree of striatal dopamine denervation. It affects predominantly the sensorimotor striatal territory (i.e. the post-commissural putamen) and targets both direct and indirect striatofugal neurons. However, electron microscopic 3D reconstruction studies demonstrate that the remaining spines in the dopamine-denervated striatum of parkinsonian monkeys undergo major morphological and ultrastructural changes characteristic of increased synaptic efficacy. Although both corticostriatal and thalamostriatal glutamatergic afferents display such plastic changes, the ultrastructural features of pre- and post-synaptic elements at these synapses are consistent with a higher strength of corticostriatal synapses over thalamic inputs in both normal and pathological conditions. Thus, striatal projection neurons and their glutamatergic afferents are endowed with a high degree of structural and functional plasticity. In parkinsonism, the striatal dopamine denervation induces major spine loss on medium spiny neurons and generates a significant remodeling of corticostriatal and thalamostriatal glutamatergic synapses, consistent with increased synaptic transmission. Future studies are needed to further characterize the mechanisms underlying striatal spine plasticity, and determine if it represents a pathological feature or compensatory process of PD.

Morphometric analysis in neurodegenerative disorders

The study of dendritic length and spine density has become a standard in the analysis of neuronal abnormalities since a considerable number of neurological diseases have their foundation in alterations in these structures. One of the best ways to study possible alterations in neuronal morphometry is the use of Golgi impregnation. Introduced more than a century ago, it is still the standard and state-of-the-art technique for visualization of neuronal architecture. We successfully applied the Golgi method to mouse, rat, monkey and human brain tissues for studying both the normal and abnormal morphology of neurons. We were able to discover subtle morphological alterations in neuronal dendrites and dendritic spines in different brain areas. Although Golgi preparations can be examined by electronic microscopy, we used light microscopy and Neurolucida reconstruction to quantitatively explore the relationship between total dendritic length and spine density in different types of neurons. This review summarizes the methodology used to quantify neuronal abnormalities and discusses the utility of these techniques in different models of neurodegeneration.

Cortical regulation of striatal medium spiny neuron dendritic remodeling in parkinsonism: modulation of glutamate release reverses dopamine depletion-induced dendritic spine loss

Striatal medium spiny neurons (MSNs) receive glutamatergic afferents from the cerebral cortex and dopaminergic inputs from the substantia nigra (SN). Striatal dopamine loss decreases the number of MSN dendritic spines. This loss of spines has been suggested to reflect the removal of tonic dopamine inhibitory control over corticostriatal glutamatergic drive, with increased glutamate release culminating in MSN spine loss. We tested this hypothesis in two ways. We first determined in vivo if decortication reverses or prevents dopamine depletion-induced spine loss by placing motor cortex lesions 4 weeks after, or at the time of, 6-hydroxydopamine lesions of the SN. Animals were sacrificed 4 weeks after cortical lesions. Motor cortex lesions significantly reversed the loss of MSN spines elicited by dopamine denervation; a similar effect was observed in the prevention experiment. We then determined if modulating glutamate release in organotypic cocultures prevented spine loss. Treatment of the cultures with the mGluR2/3 agonist LY379268 to suppress corticostriatal glutamate release completely blocked spine loss in dopamine-denervated cultures. These studies provide the first evidence to show that MSN spine loss associated with parkinsonism can be reversed and point to suppression of corticostriatal glutamate release as a means of slowing progression in Parkinson's disease.

Impact of dendritic spine preservation in medium spiny neurons on dopamine graft efficacy and the expression of dyskinesias in parkinsonian rats

Dopamine deficiency associated with Parkinson's disease (PD) results in numerous changes in striatal transmitter function and neuron morphology. Specifically, there is marked atrophy of dendrites and dendritic spines on striatal medium spiny neurons (MSN), primary targets of inputs from nigral dopamine and cortical glutamate neurons, in advanced PD and rodent models of severe dopamine depletion. Dendritic spine loss occurs via dysregulation of intraspine Cav1.3 L-type Ca(2+)channels and can be prevented, in animal models, by administration of the calcium channel antagonist, nimodipine. The impact of MSN dendritic spine loss in the parkinsonian striatum on dopamine neuron graft therapy remains unexamined. Using unilaterally parkinsonian Sprague-Dawley rats, we tested the hypothesis that MSN dendritic spine preservation through administration of nimodipine would result in improved therapeutic benefit and diminished graft-induced behavioral abnormalities in rats grafted with embryonic ventral midbrain cells. Analysis of rotational asymmetry and spontaneous forelimb use in the cylinder task found no significant effect of dendritic spine preservation in grafted rats. However, analyses of vibrissae-induced forelimb use, levodopa-induced dyskinesias and graft-induced dyskinesias showed significant improvement in rats with dopamine grafts associated with preserved striatal dendritic spine density. Nimodipine treatment in this model did not impact dopamine graft survival but allowed for increased graft reinnervation of striatum. Taken together, these results demonstrate that even with grafting suboptimal numbers of cells, maintaining normal spine density on target MSNs results in overall superior behavioral efficacy of dopamine grafts.

Slowly progressive dopamine cell loss--a model on which to test neuroprotective strategies for Parkinson's disease?

Making animal models of human disease is a very flawed process. Aspects of the disease can be imitated but models do not necessarily give reliable leads for treatment strategies. When Ungerstedt in Sweden first described the 6-hydroxydopamine (6-OHDA) treated rat model of Parkinson's disease /89/ we knew that the symptoms would not map readily to those of the human disease--rats have four legs after all. On the other hand, the neuropathology looked exactly like end-stage Parkinsonian pathology. That remained true even as we explored other types of neuropathology in the rats /24,43-46,80/. Many of today's treatments for Parkinsonism are developed from pharmacological studies on that model of rats with a chemically induced lesion. However, the 6-OHDA model does not address the important issue of a cure for the disease. The triggers and the time-course of dopamine (DA) cell death in rats are known for nearly every disease model - but for the human disease there is no equivalent knowledge. In the human, the neurons have been dying for a considerable time before the symptoms become obvious and they go on dying even with adequate symptomatic relief /94/, but after intracerebral administration of 6-OHDA to an animal the cells die quickly; all cells are destroyed in less than 5 days /42,88,89/. Thus, we were interested in developing an animal model of DA cell death with a slower time-course. After ibotenic acid injections into rat globus pallidus (GP), DA cells are lost from the ipsilateral substantia nigra over the slower time scale of about six weeks. This time scale has allowed us to test some interventions to prevent the cells from dying. Although some attempts have succeeded, cell death is prevented only for three weeks -beyond that treatments fail and DA cells die. At the moment, this model has at least opened a window into causes of neuronal death in a slower time scale /94/ than previous rodent models.

Effect of levodopa priming on dopamine neuron transplant efficacy and induction of abnormal involuntary movements in parkinsonian rats

Clinical trials of neural grafting for Parkinson's disease (PD) have produced variable, but overall disappointing, results. One particular disappointment has been the development of aberrant motor complications following dopamine (DA) neuron grafting. Despite a lack of consistent benefit, the utility of dopamine neuron replacement remains supported by clinical and basic data. In a continued effort to elucidate factors that might improve this therapy, we used a parkinsonian rat model to examine whether pregraft chronic levodopa affected graft efficacy and/or graft-induced dyskinesia (GID) induction. Indeed, all grafted PD patients to date have had a pregraft history of long-term levodopa. It is well established that long-term levodopa results in a plethora of long-lasting neurochemical alterations and genomic changes indicative of altered structural and synaptic plasticity. Thus, therapeutic dopamine terminal replacement in a striatal environment complicated by such changes could be expected to lead to abnormal or inappropriate connections between graft and host brain and to contribute to suboptimal efficacy and/or postgraft GID behaviors. To investigate the effect of pregraft levodopa, one group of parkinsonian rats received levodopa for 4 weeks prior to grafting. A second levodopa-naïve group was grafted, and the grafts were allowed to mature for 9 weeks prior to introducing chronic levodopa. We report here that, in parkinsonian rats, preexposure to chronic levodopa significantly reduces behavioral and neurochemical efficacy of embryonic dopamine grafts. Furthermore, dopamine terminal replacement prior to introduction of chronic levodopa is highly effective at preventing development of levodopa-induced dyskinesias, and GID-like behaviors occur regardless of pregraft levodopa status.

Striatal GPR88 expression is confined to the whole projection neuron population and is regulated by dopaminergic and glutamatergic afferents

GPR88, an orphan G protein-coupled receptor, was designated Strg/GPR88 for striatum-specific G protein-coupled receptor (K. Mizushima et al. (2000)Genomics, 69, 314-321). In this study, we focused on striatal GPR88 protein localization using a polyclonal antibody. We established that the distribution of immunoreactivity in rat brain matched that of GPR88 transcripts and provided evidence for its exclusive neuronal expression. GPR88 protein is abundant throughout the striatum of rat and primate, with expression limited to the two subsets of striatal projection medium spiny neurons (MSNs) expressing preprotachykinin-substance P or preproenkephalin mRNAs. Ultrastructural immunolabelling revealed the GPR88 concentration at post-synaptic sites along the somatodendritic compartments of MSNs, with pronounced preference for dendrites and dendritic spines. The GPR88-rich expression, in both striatal output pathways, designates this receptor as a potential therapeutic target for diseases involving dysfunction of the basal ganglia, such as Parkinson's disease. Hence, we investigated changes of GPR88 expression in a model of Parkinson's disease (unilateral 6-hydroxydopamine-lesioned rats) following repeated L-DOPA treatment. In dopamine-depleted striatum, GPR88 expression was differentially regulated, i.e. decreased in striatopallidal and increased in striatonigral MSNs. L-DOPA treatment led to a normalization of GPR88 levels through dopamine D1 and D2 receptor-mediated mechanisms in striatopallidal and striatonigral MSNs, respectively. Moreover, the removal of corticostriatal inputs, by ibotenate infusion, downregulated GPR88 in striatopallidal MSNs. These findings provide the first evidence that GPR88 is confined to striatal MSNs and indicate that L-DOPA-mediated behavioural effects in hemiparkinsonian rats may involve normalization of striatal GPR88 levels probably through dopamine receptor-mediated mechanisms and modulations of corticostriatal pathway activity.

Oxidative damage and neurodegeneration in manganese-induced neurotoxicity

Exposure to excessive manganese (Mn) levels results in neurotoxicity to the extrapyramidal system and the development of Parkinson's disease (PD)-like movement disorder, referred to as manganism. Although the mechanisms by which Mn induces neuronal damage are not well defined, its neurotoxicity appears to be regulated by a number of factors, including oxidative injury, mitochondrial dysfunction and neuroinflammation. To investigate the mechanisms underlying Mn neurotoxicity, we studied the effects of Mn on reactive oxygen species (ROS) formation, changes in high-energy phosphates (HEP), neuroinflammation mediators and associated neuronal dysfunctions both in vitro and in vivo. Primary cortical neuronal cultures showed concentration-dependent alterations in biomarkers of oxidative damage, F2-isoprostanes (F2-IsoPs) and mitochondrial dysfunction (ATP), as early as 2 h following Mn exposure. Treatment of neurons with 500 microM Mn also resulted in time-dependent increases in the levels of the inflammatory biomarker, prostaglandin E2 (PGE2). In vivo analyses corroborated these findings, establishing that either a single or three (100 mg/kg, s.c.) Mn injections (days 1, 4 and 7) induced significant increases in F2-IsoPs and PGE2 in adult mouse brain 24 h following the last injection. Quantitative morphometric analyses of Golgi-impregnated striatal sections from mice exposed to single or three Mn injections revealed progressive spine degeneration and dendritic damage of medium spiny neurons (MSNs). These findings suggest that oxidative stress, mitochondrial dysfunction and neuroinflammation are underlying mechanisms in Mn-induced neurodegeneration.

Focal nature of neurological disorders necessitates isotype-selective histone deacetylase (HDAC) inhibitors

Histone deacetylase (HDAC) inhibitors represent a promising new avenue of therapeutic options for a range of neurological disorders. Within any particular neurological disorder, neuronal damage or death is not widespread; rather, particular brain regions are preferentially affected. Different disorders exhibit distinct focal pathologies. Hence, understanding the region-specific effects of HDAC inhibitors is essential for targeting appropriate brain areas and reducing toxicity in unaffected areas. The outcome of HDAC inhibition depends on several factors, including the diversity in the central nervous system expression of HDAC enzymes, selectivity of a given HDAC inhibitor for different HDAC enzymes, and the presence or absence of cofactors necessary for enzyme function. This review will summarize brain regions associated with various neurological disorders and factors affecting the consequences of HDAC inhibition.

Molecular mechanisms of L-DOPA-induced dyskinesia

L-DOPA (L-3,4-dihydroxyphenylalanine) remains the most effective drug for the treatment of Parkinson's disease. However, chronic use causes dyskinesia, a complex motor phenomenon that consists of two components: the execution of involuntary movements in response to drug administration, and the 'priming' phenomenon that underlies these movements' establishment and persistence. A reinterpretation of recent data suggests that priming for dyskinesia results from nigral denervation and the loss of striatal dopamine input, which alters glutamatergic synaptic connectivity in the striatum. The subsequent response of the abnormal basal ganglia to dopaminergic drugs determines the manner and timing of dyskinesia expression. The combination of nigral denervation and drug treatment establishes inappropriate signalling between the motor cortex and the striatum, leading to persistent dyskinesia.

Striatal plasticity and medium spiny neuron dendritic remodeling in parkinsonism

Current approaches to Parkinson's Disease (PD) are largely based on our current understanding of the mechanisms that contribute to the death of nigrostriatal dopamine neurons. However, our understanding of the consequences of the loss of dopamine on the striatal target cells of nigrostriatal neurons is much less advanced. In particular, the compensatory changes that occur in striatal medium spiny neurons (MSNs) that have lost their normal dopamine input remains poorly understood. The compensatory changes may have either positive or negative effects. Among the alterations that occur in striatal cells of the dopamine-denervated striatum are dystrophic changes in the dendrites of MSNs, with a loss of dendritic length and dendritic spine number. Dendritic spines are the targets of convergent nigrostriatal dopamine and corticostriatal glutamate axons, and integrate these convergent signals to determine the nature of striatal output. The loss of these spines in the dopamine-denervated state may protect the MSN from overt excitotoxic death, but at the price of compromising MSN function. The loss of dendritic spines is thought be responsible for the gradual decrease in levodopa efficacy in late-stage PD, suggesting that therapeutic interventions need to be developed that target key downstream signaling complexes in medium spiny neurons.

Striatal and extrastriatal dopamine in the basal ganglia: an overview of its anatomical organization in normal and Parkinsonian brains

Degeneration of the nigrostriatal dopaminergic system is the characteristic neuropathological feature of Parkinson's disease and therapy is primarily based on a dopamine replacement strategy. Dopamine has long been recognized to be a key neuromodulator of basal ganglia function, essential for normal motor activity. The recent years have witnessed significant advances in our knowledge of dopamine function in the basal ganglia. Although the striatum remains the main functional target of dopamine, it is now appreciated that there is dopaminergic innervation of the pallidum, subthalamic nucleus, and substantia nigra. A new dopaminergic- thalamic system has also been uncovered, setting the stage for a direct dopamine action on thalamocortical activity. The differential distribution of D1 and D2 receptors on neurons in the direct and indirect striato-pallidal pathways has been re-emphasized, and cholinergic interneurons are recognized as an intermediary mediator of dopamine-mediated communication between the two pathways. The importance and specificity of dopamine in regulating morphological changes in striatal projection neurons provides further evidence for the complex and multifarious mechanisms through which dopamine mediates its functional effects in the basal ganglia. In this review, the role of basal ganglia dopamine and its functional relevance in normal and pathological conditions will be discussed.

Dopaminergic denervation and spine loss in the striatum of MPTP-treated monkeys

Striatal spine loss is a key pathological feature of human Parkinson's disease (PD) that can be induced after complete degeneration of the nigrostriatal dopaminergic system in rodent models of parkinsonism. In line with these observations, our findings reveal a significant (30-50%) reduction in spine density in both the caudate nucleus and putamen of severely DA-depleted striata of MPTP-treated monkeys; the sensorimotor post-commissural putamen being the most severely affected region for both dopamine depletion and spine loss. Using MPTP-treated monkeys with complete or partial striatal dopamine (DA) denervation, we also demonstrate that striatal spine loss is an early pathological feature of parkinsonism, which progresses along a positive rostrocaudal and mediolateral gradient in parallel with the extent of striatal dopamine denervation. Quantitative electron microscopy immunocytochemistry for D1 dopamine receptor (D1) in the striatum of control and severely DA-depleted animals revealed that both D1-immunoreactive and immunonegative spines are lost in the putamen of MPTP-treated monkeys. These data demonstrate that striatal spine loss in MPTP-treated monkeys is an early pathological event of parkinsonism, tightly correlated with the degree of nigrostriatal dopamine denervation that likely affects both direct and indirect striatofugal pathways.

The synaptic impact of the host immune response in a parkinsonian allograft rat model: Influence on graft-derived aberrant behaviors

Graft-induced dyskinesias (GIDs), side-effects found in clinical grafting trials for Parkinson's disease (PD), may be associated with the withdrawal of immunosuppression. The goal of this study was to determine the role of the immune response in GIDs. We examined levodopa-induced dyskinesias (LIDs), GID-like behaviors, and synaptic ultrastructure in levodopa-treated, grafted, parkinsonian rats with mild (sham), moderate (allografts) or high (allografts plus peripheral spleen cell injections) immune activation. Grafts attenuated amphetamine-induced rotations and LIDs, but two abnormal motor syndromes (tapping stereotypy, litter retrieval/chewing) emerged and increased with escalating immune activation. Immunohistochemical analyses confirmed immune activation and graft survival. Ultrastructural analyses showed increases in tyrosine hydroxylase-positive (TH+) axo-dendritic synapses, TH+ asymmetric specializations, and non-TH+ perforated synapses in grafted, compared to intact, striata. These features were exacerbated in rats with the highest immune activation and correlated statistically with GID-like behaviors, suggesting that immune-mediated aberrant synaptology may contribute to graft-induced aberrant behaviors.

The thalamostriatal systems: anatomical and functional organization in normal and parkinsonian states

Although we have gained significant knowledge in the anatomy and microcircuitry of the thalamostriatal system over the last decades, the exact function(s) of these complex networks remain(s) poorly understood. It is now clear that the thalamostriatal system is not a unique entity, but consists of multiple neural systems that originate from a wide variety of thalamic nuclei and terminate in functionally segregated striatal territories. The primary source of thalamostriatal projections is the caudal intralaminar nuclear group which, in primates, comprises the centromedian and parafascicular nuclei (CM/Pf). These two nuclei provide massive, functionally organized glutamatergic inputs to the whole striatal complex. There are several anatomical and physiological features that distinguish this system from other thalamostriatal projections. Although all glutamatergic thalamostriatal neurons express vGluT2 and release glutamate as neurotransmitter, CM/Pf neurons target preferentially the dendritic shafts of striatal projection neurons, whereas all other thalamic inputs are almost exclusively confined to the head of dendritic spines. This anatomic arrangement suggests that transmission of input from sources other than CM/Pf to the striatal neurons is likely regulated by dopaminergic afferents in the same manner as cortical inputs, while the CM/Pf axo-dendritic synapses do not display any particular relationships with dopaminergic terminals. A better understanding of the role of these systems in the functional circuitry of the basal ganglia relies on future research of the physiology and pathophysiology of these networks in normal and pathological basal ganglia conditions. Although much remains to be known about the role of these systems, recent electrophysiological studies from awake monkeys have provided convincing evidence that the CM/Pf-striatal system is the entrance for attention-related stimuli to the basal ganglia circuits. However, the processing and transmission of this information likely involves intrinsic GABAergic and cholinergic striatal networks, thereby setting the stage for complex physiological responses of striatal output neurons to CM/Pf activation. Finally, another exciting development that will surely generate significant interest towards the thalamostriatal systems in years to come is the possibility that CM/Pf may be a potential surgical target for movement disorders, most particularly Tourette syndrome and Parkinson's disease. Although the available clinical evidence is encouraging, these procedures remain empirical at this stage because of the limited understanding of the thalamostriatal systems.

Differential synaptic plasticity of the corticostriatal and thalamostriatal systems in an MPTP-treated monkey model of parkinsonism

Two cardinal features of Parkinson's disease (PD) pathophysiology are a loss of glutamatergic synapses paradoxically accompanied by an increased glutamatergic transmission to the striatum. The exact substrate of this increased glutamatergic drive remains unclear. The striatum receives glutamatergic inputs from the thalamus and the cerebral cortex. Using vesicular glutamate transporters (vGluTs) 1 and 2 as markers of the corticostriatal and thalamostriatal afferents, respectively, we examined changes in the synaptology and relative prevalence of striatal glutamatergic inputs in methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated monkeys using electron microscopic immunoperoxidase and confocal immunofluorescence methods. Our findings demonstrate that the prevalence of vGluT1-containing terminals is significantly increased in the striatum of MPTP-treated monkeys (51.9 +/- 3.5% to 66.5 +/- 3.4% total glutamatergic boutons), without any significant change in the pattern of synaptic connectivity; more than 95% of vGluT1-immunolabeled terminals formed axo-spinous synapses in both conditions. In contrast, the prevalence of vGluT2-immunoreactive terminals did not change after MPTP treatment (21.7 +/- 1.3% vs. 21.6 +/- 1.2% total glutamatergic boutons). However, a substantial increase in the ratio of axo-spinous to axo-dendritic synapses formed by vGluT2-immunoreactive terminals was found in the pre-caudate and post-putamen striatal regions of MPTP-treated monkeys, suggesting a certain degree of synaptic reorganization of the thalamostriatal system in parkinsonism. About 20% of putative glutamatergic terminals did not show immunoreactivity in striatal tissue immunostained for both vGluT1 and vGluT2, suggesting the expression of another vGluT in these boutons. These findings provide striking evidence that suggests a differential degree of plasticity of the corticostriatal and thalamostriatal system in PD.

High-resolution neuroanatomical tract-tracing for the analysis of striatal microcircuits

Although currently available retrograde tracers are useful tools for identifying striatal projection neurons, transported tracers often remained restricted within the neuronal somata and the thickest, main dendrites. Indeed, thin dendrites located far away from the cell soma as well as post-synaptic elements such as dendritic spines cannot be labeled unless performing intracellular injections. In this regard, the subsequent use of anterograde tracers for the labeling of striatal afferents often failed to unequivocally elucidate whether a given afferent makes true contacts with striatal projections neurons. Here we show that such a technical constraint can now be circumvented by retrograde tracing using rabies virus (RV). Immunofluorescence detection with a monoclonal antibody directed against the viral phosphoprotein resulted in a consistent Golgi-like labeling of striatal projection neurons, allowing clear visualization of small-size elements such as thin dendrites as well as dendritic spines. The combination of this retrograde tracing together with dual anterograde tracing of cortical and thalamic afferents has proven to be a useful tool for ascertaining striatal microcircuits. Indeed, by taking advantage of the trans-synaptic spread of RV, different subpopulations of local-circuit neurons modulating striatal efferent neurons can also be identified. At the striatal level, structures displaying labeling were visualized under the confocal laser-scanning microscope at high resolution. Once acquired, confocal stacks of images were firstly deconvoluted and then processed through 3D-volume rendering in order to unequivocally identify true contacts between pre-synaptic elements (axon terminals from cortical or thalamic sources) and post-synaptic elements (projection neurons and/or interneurons labeled with RV).

Dopamine depletion of the prefrontal cortex induces dendritic spine loss: reversal by atypical antipsychotic drug treatment

Dystrophic changes in dendrites of cortical neurons are present in several neuro-psychiatric disorders, including schizophrenia. The mechanisms that account for dendritic changes in the prefrontal cortex (PFC) in schizophrenia are unclear. Cognitive deficits in schizophrenia have been linked to compromised cortical dopamine function, and the density of the PFC dopamine innervation is decreased in schizophrenia. We determined if 6-hydroxydopamine lesions of the ventral tegmental area that disrupt the PFC dopamine innervation cause dystrophic changes in cortical neurons. Three weeks post-operatively we observed a marked decrease in basal dendritic length and spine density of layer V pyramidal cells in the prelimbic cortex; no change was seen in neurons of the motor cortex. We then examined rats in which the PFC dopamine innervation was lesioned and 3 weeks later were started on chronic treatment with an atypical (olanzapine) or typical (haloperidol) antipsychotic drug. Olanzapine but not haloperidol reversed lesion-induced changes in PFC pyramidal cell dendrites. These data suggest that dopamine regulates dendritic structure in PFC neurons. Moreover, the findings are consistent with a decrease in cortical dopaminergic tone contributing to the pathological changes in the cortex of schizophrenia, and suggest that the progressive cortical loss in schizophrenia may be slowed or reversed by treatment with atypical antipsychotic drugs.

Striatal proteomic analysis suggests that first L-dopa dose equates to chronic exposure

L-3,4-dihydroxypheylalanine (L-dopa)-induced dyskinesia represent a debilitating complication of therapy for Parkinson's disease (PD) that result from a progressive sensitization through repeated L-dopa exposures. The MPTP macaque model was used to study the proteome in dopamine-depleted striatum with and without subsequent acute and chronic L-dopa treatment using two-dimensional difference in-gel electrophoresis (2D-DIGE) and mass spectrometry. The present data suggest that the dopamine-depleted striatum is so sensitive to de novo L-dopa treatment that the first ever administration alone would be able (i) to induce rapid post-translational modification-based proteomic changes that are specific to this first exposure and (ii), possibly, lead to irreversible protein level changes that would be not further modified by chronic L-dopa treatment. The apparent equivalence between first and chronic L-dopa administration suggests that priming would be the direct consequence of dopamine loss, the first L-dopa administrations only exacerbating the sensitization process but not inducing it.

MPTP-induced deficits in striatal synaptic plasticity are prevented by glial cell line-derived neurotrophic factor expressed via an adeno-associated viral vector

This study determined the consequences of dopamine denervation of the striatum on synaptic plasticity and prevention of these changes with gene therapy using an adeno-associated viral vector (AAV) expressing glial cell line-derived neurotrophic factor (GDNF). C57BL6/J mice were injected with the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP); long-term depression (LTD) or potentiation (LTP) were measured in vitro. Fast-scan cyclic voltammetry measured electrically released dopamine from a functionally relevant pool in these same striatal slices. After MPTP, dopamine release and uptake were greatly diminished, and LTP and LTD were blocked in the striatal slices. The loss of plasticity resulted directly from the loss of dopamine since its application rescued synaptic plasticity. Striatal GDNF expression via AAV, before MPTP, significantly protected against the loss of dopamine and prevented the blockade of corticostriatal LTP. These data demonstrate that dopamine plays a role in supporting several forms of striatal plasticity and that GDNF expression via AAV prevents the loss of dopamine and striatal plasticity caused by MPTP. We propose that impairment of striatal plasticity after dopamine denervation plays a role in the symptomology of Parkinson's disease and that AAV expression of neurotrophic factors represents a tenable approach to protecting against or slowing these neurobiological deficits.

Cortical regulation of dopamine depletion-induced dendritic spine loss in striatal medium spiny neurons

The proximate cause of Parkinson's disease is striatal dopamine depletion. Although no overt toxicity to striatal neurons has been reported in Parkinson's disease, one of the consequences of striatal dopamine loss is a decrease in the number of dendritic spines on striatal medium spiny neurons (MSNs). Dendrites of these neurons receive cortical glutamatergic inputs onto the dendritic spine head and dopaminergic inputs from the substantia nigra onto the spine neck. This synaptic arrangement suggests that dopamine gates corticostriatal glutamatergic drive onto spines. Using triple organotypic slice cultures composed of ventral mesencephalon, striatum, and cortex of the neonatal rat, we examined the role of the cortex in dopamine depletion-induced dendritic spine loss in MSNs. The striatal dopamine innervation was lesioned by treatment of the cultures with the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) or by removing the mesencephalon. Both MPP+ and mesencephalic ablation decreased MSN dendritic spine density. Analysis of spine morphology revealed that thin spines were preferentially lost after dopamine depletion. Removal of the cortex completely prevented dopamine depletion-induced spine loss. These data indicate that the dendritic remodeling of MSNs seen in parkinsonism occurs secondary to increases in corticostriatal glutamatergic drive, and suggest that modulation of cortical activity may be a useful therapeutic strategy in Parkinson's disease.

Alterations in dendritic morphology of the prefrontal cortical and striatum neurons in the unilateral 6-OHDA-rat model of Parkinson's disease

We have studied the morphological changes of the dendrites of the pyramidal neurons of the prefrontal cortex (PFC) and the medium spiny neurons of the caudate-putamen (CPu) and nucleus accumbens (NAcc) induced by the injection of 6-hydroxydopamine (6-OHDA) into the substantia nigra pars compacta (SNc). The unilateral 6-OHDA-induced lesion of the SNc was made in Wistar rats to produce the Parkinson model lesion. Two weeks after the injection, the testing of rotational behavior caused by amphetamine injection was done to assess the animals with lesions. Four weeks after the 6-OHDA injection, the morphology of the pyramidal cells of Layer 5 of the PFC and the medium spiny neurons of the CPu and NAcc were quantified by modified Golgi-Cox staining. The results showed that the length of dendrites, the branching, and the density of dendritic spines on the medium spiny neurons of the same side of the caudate-putamen lesion were significantly decreased in rats with the unilateral 6-OHDA-induced lesion of the SNc. The pyramidal neurons of the PFC and medium spiny neurons of the NAcc showed a decrease in the density of dendritic spines without significant changes in dendritic length or arborization. Our data suggest that the SNc lesion with the 6-OHDA, Hemiparkinsonism animal model may lead to altered neuronal plasticity in the CPu, NAcc, and PFC that may have participated in the emergence of the behavioral changes observed in these animals.

Normalization and expression changes in predefined sets of proteins using 2D gel electrophoresis: a proteomic study of L-DOPA induced dyskinesia in an animal model of Parkinson's disease using DIGE

BACKGROUND

Two-Dimensional Difference In Gel Electrophoresis (2D-DIGE) is a powerful tool for measuring differences in protein expression between samples or conditions. However, to remove systematic variability within and between gels the data has to be normalized. In this study we examined the ability of four existing and four novel normalization methods to remove systematic bias in data produced with 2D-DIGE. We also propose a modification of an existing method where the statistical framework determines whether a set of proteins shows an association with the predefined phenotypes of interest. This method was applied to our data generated from a monkey model (Macaca fascicularis) of Parkinson's disease.

RESULTS

Using 2D-DIGE we analysed the protein content of the striatum from 6 control and 21 MPTP-treated monkeys, with or without de novo or long-term L-DOPA administration. There was an intensity and spatial bias in the data of all the gels examined in this study. Only two of the eight normalization methods evaluated ('2D loess+scale' and 'SC-2D+quantile') successfully removed both the intensity and spatial bias. In 'SC-2D+quantile' we extended the commonly used loess normalization method against dye bias in two-channel microarray systems to suit systems with three or more channels.Further, by using the proposed method, Differential Expression in Predefined Proteins Sets (DEPPS), several sets of proteins associated with the priming effects of L-DOPA in the striatum in parkinsonian animals were identified. Three of these sets are proteins involved in energy metabolism and one set involved proteins which are part of the microtubule cytoskeleton.

CONCLUSION

Comparison of the different methods leads to a series of methodological recommendations for the normalization and the analysis of data, depending on the experimental design. Due to the nature of 2D-DIGE data we recommend that the p-values obtained in significance tests should be used as rankings only. Individual proteins may be interesting as such, but by studying sets of proteins the interpretation of the results are probably more accurate and biologically informative.

Phenotype of striatofugal medium spiny neurons in parkinsonian and dyskinetic nonhuman primates: a call for a reappraisal of the functional organization of the basal ganglia

The classic view of anatomofunctional organization of the basal ganglia is that striatopallidal neurons of the "indirect" pathway express D2 dopamine receptors and corelease enkephalin with GABA, whereas striatopallidal neurons of the "direct" pathway bear D1 dopamine receptors and corelease dynorphin and substance P with GABA. Although many studies have investigated the pathophysiology of the basal ganglia after dopamine denervation and subsequent chronic levodopa (L-dopa) treatment, none has ever considered the possibility of plastic changes leading to profound reorganization and/or biochemical phenotype modifications of medium spiny neurons. Therefore, we studied the phenotype of striatal neurons in four groups of nonhuman primates, including the following: normal, parkinsonian, parkinsonian chronically treated with L-dopa without exhibiting dyskinesia, and parkinsonian chronically treated with L-dopa exhibiting overt dyskinesia. To identify striatal cells projecting to external (indirect) or internal (direct) segments of the globus pallidus, the retrograde tracer cholera toxin subunit B (CTb) was injected stereotaxically into the terminal areas. Using immunohistochemistry techniques, brain sections were double labeled for CTb and dopamine receptors, opioid peptides, or the substance P receptor (NK1). We also used HPLC-RIA to assess opioid levels throughout structures of the basal ganglia. Our results suggest that medium spiny neurons retain their phenotype because no variations were observed in any experimental condition. Therefore, it appears unlikely that dyskinesia is related to a phenotype modification of the striatal neurons. However, this study supports the concept of axonal collateralization of striatofugal cells that project to both globus pallidus pars externa and globus pallidus pars interna. Striatofugal pathways are not as segregated in the primate as previously considered.

Continuous dopamine-receptor treatment of Parkinson's disease: scientific rationale and clinical implications

Levodopa-induced motor complications are a common source of disability for patients with Parkinson's disease. Evidence suggests that motor complications are associated with non-physiological, pulsatile stimulation of dopamine receptors. In healthy brains, dopamine neurons fire continuously, striatal dopamine concentrations are relatively constant, and there is continuous activation of dopamine receptors. In the dopamine-depleted state, standard levodopa therapy does not normalise the basal ganglia. Rather, levodopa or other short-acting dopaminergic drugs induce molecular changes and altered neuronal firing patterns in basal ganglia neurons leading to motor complications. The concept of continuous dopaminergic stimulation proposes that continuous delivery of a dopaminergic drug will prevent pulsatile stimulation and avoid motor complications. In monkeys treated with MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and patients with Parkinson's disease, long-acting or continuous infusion of a dopaminergic drug reduces the risk of motor complications. The current challenge is to develop a long-acting oral formulation of levodopa that provides clinical benefits but avoids motor complications.

Neurone specific regulation of dendritic spines in vivo by post synaptic density 95 protein (PSD-95)

Post synaptic density protein 95 (PSD-95) is a postsynaptic adaptor protein coupling the NMDA receptor to downstream signalling pathways underlying plasticity. Mice carrying a targeted gene mutation of PSD-95 show altered behavioural plasticity including spatial learning, neuropathic pain, orientation preference in visual cortical cells, and cocaine sensitisation. These behavioural effects are accompanied by changes in long-term potentiation of synaptic transmission. In vitro studies of PSD-95 signalling indicate that it may play a role in regulating dendritic spine structure. Here, we show that PSD-95 mutant mice have alterations in dendritic spine density in the striatum (a 15% decrease along the dendritic length) and in the hippocampus (a localised 40% increase) without changes in dendritic branch patterns or gross neuronal architecture. These changes in spine density were accompanied by altered expression of proteins known to interact with PSD-95, including NR2B and SAP102, suggesting that PSD-95 plays a role in regulating the expression and activation of proteins found within the NMDA receptor complex. Thus, PSD-95 is an important regulator of neuronal structure as well as plasticity in vivo.

Selective elimination of glutamatergic synapses on striatopallidal neurons in Parkinson disease models

Parkinson disease is a common neurodegenerative disorder that leads to difficulty in effectively translating thought into action. Although it is known that dopaminergic neurons that innervate the striatum die in Parkinson disease, it is not clear how this loss leads to symptoms. Recent work has implicated striatopallidal medium spiny neurons (MSNs) in this process, but how and precisely why these neurons change is not clear. Using multiphoton imaging, we show that dopamine depletion leads to a rapid and profound loss of spines and glutamatergic synapses on striatopallidal MSNs but not on neighboring striatonigral MSNs. This loss of connectivity is triggered by a new mechanism-dysregulation of intraspine Cav1.3 L-type Ca(2+) channels. The disconnection of striatopallidal neurons from motor command structures is likely to be a key step in the emergence of pathological activity that is responsible for symptoms in Parkinson disease.

Striatal plasticity in parkinsonism: dystrophic changes in medium spiny neurons and progression in Parkinson's disease

Striatal dopamine loss in Parkinson's Disease (PD) sets into play a variety of compensatory responses to help counter dopamine depletion. Most of these changes involve surviving dopamine neurons, but there are also changes in striatal medium spiny neurons (MSNs), which are the major target of dopamine axons. Among these changes are decreases in MSN dendritic length and spine density, which may dampen excessive corticostriatal glutamatergic drive onto MSNs that occurs secondary to dopamine loss. An increasing knowledge of dendritic changes in PD suggests strategies for tracking progressive worsening of symptoms and is opening new ideas on novel therapeutic strategies for PD.