Quantitative analysis of dopaminergic loss in relation to functional territories in MPTP-treated monkeys

Dysfunction of motor cortices in Parkinson's disease

The cerebral cortex has long been thought to be involved in the pathophysiology of motor symptoms of Parkinson's disease. The impaired cortical function is believed to be a direct and immediate effect of pathologically patterned basal ganglia output, mediated to the cerebral cortex by way of the ventral motor thalamus. However, recent studies in humans with Parkinson's disease and in animal models of the disease have provided strong evidence suggesting that the involvement of the cerebral cortex is much broader than merely serving as a passive conduit for subcortical disturbances. In the present review, we discuss Parkinson's disease-related changes in frontal cortical motor regions, focusing on neuropathology, plasticity, changes in neurotransmission, and altered network interactions. We will also examine recent studies exploring the cortical circuits as potential targets for neuromodulation to treat Parkinson's disease.

Combining physical & cognitive training with iPSC-derived dopaminergic neuron transplantation promotes graft integration & better functional outcome in parkinsonian marmosets

Parkinson's disease (PD) is a relentlessly progressive and currently incurable neurodegenerative disease with significant unmet medical needs. Since PD stems from the degeneration of midbrain dopaminergic (DA) neurons in a defined brain location, PD patients are considered optimal candidates for cell replacement therapy. Clinical trials for cell transplantation in PD are beginning to re-emerge worldwide with a new focus on induced pluripotent stem cells (iPSCs) as a source of DA neurons since they can be derived from adult somatic cells and produced in large quantities under current good manufacturing practices. However, for this therapeutic strategy to be realized as a viable clinical option, fundamental translational challenges need to be addressed including the manufacturing process, purity and efficacy of the cells, the method of delivery, the extent of host reinnervation and the impact of patient-centered adjunctive interventions. In this study we report on the impact of physical and cognitive training (PCT) on functional recovery in the nonhuman primate (NHP) model of PD after cell transplantation. We observed that at 6 months post-transplant, the PCT group returned to normal baseline in their daily activity measured by actigraphy, significantly improved in their sensorimotor and cognitive tasks, and showed enhanced synapse formation between grafted cells and host cells. We also describe a robust, simple, efficient, scalable, and cost-effective manufacturing process of engraftable DA neurons derived from iPSCs. This study suggests that integrating PCT with cell transplantation therapy could promote optimal graft functional integration and better outcome for patients with PD.

Cortical Serotonergic and Catecholaminergic Denervation in MPTP-Treated Parkinsonian Monkeys

Decreased cortical serotonergic and catecholaminergic innervation of the frontal cortex has been reported at early stages of Parkinson's disease (PD). However, the limited availability of animal models that exhibit these pathological features has hampered our understanding of the functional significance of these changes during the course of the disease. In the present study, we assessed longitudinal changes in cortical serotonin and catecholamine innervation in motor-symptomatic and asymptomatic monkeys chronically treated with low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Densitometry and unbiased stereological techniques were used to quantify changes in serotonin and tyrosine hydroxylase (TH) immunoreactivity in frontal cortices of 3 control monkeys and 3 groups of MPTP-treated monkeys (motor-asymptomatic [N = 2], mild parkinsonian [N = 3], and moderate parkinsonian [N = 3]). Our findings revealed a significant decrease (P < 0.001) in serotonin innervation of motor (Areas 4 and 6), dorsolateral prefrontal (Areas 9 and 46), and limbic (Areas 24 and 25) cortical areas in motor-asymptomatic MPTP-treated monkeys. Both groups of symptomatic MPTP-treated animals displayed further serotonin denervation in these cortical regions (P < 0.0001). A significant loss of serotonin-positive dorsal raphe neurons was found in the moderate parkinsonian group. On the other hand, the intensity of cortical TH immunostaining was not significantly affected in motor asymptomatic MPTP-treated monkeys, but underwent a significant reduction in the moderate symptomatic group (P < 0.05). Our results indicate that chronic intoxication with MPTP induces early pathology in the corticopetal serotonergic system, which may contribute to early non-motor symptoms in PD.

Supratentorial and Infratentorial Lesions in Spinocerebellar Ataxia Type 3

Background: Spinocerebellar ataxia type 3 (SCA) is a cerebellum-dominant degenerative disorder that is characterized primarily by infratentorial damage, although less severe supratentorial involvement may contribute to the clinical manifestation. These impairments may result from the efferent loss of the cerebellar cortex and degeneration of the cerebral cortex. Method: We used the three-dimensional fractal dimension (3D-FD) method to quantify the morphological changes in the supratentorial regions and assessed atrophy in the relatively focal regions in patients with SCA3. A total of 48 patients with SCA3 and 50 sex- and age-matched healthy individuals, as the control group, participated in this study. The 3D-FD method was proposed to distinguish 97 automatic anatomical label regions of gray matter (left cerebrum: 45, right cerebrum: 45, cerebellum: 7) between healthy individuals and patients with SCA3. Results: Patients with SCA3 exhibited reduced brain complexity within both the traditional olivopontocerebellar atrophy (OPCA) pattern and specific supratentorial regions. The study results confirmed the extensive involvement of extracerebellar regions in SCA3. The atrophied regions of SCA3 in infratentorial and supratentorial cortex showed a wide range of overlapped areas as in two functional cortexes, namely cerebellum-related cortex and basal ganglia-related cortex. Conclusions: Our results found that the atrophy of the SCA3 are not only limited in the infratentorial regions. Both cerebellar related cortex and basal ganglia related cortex were affected in the disease process of SCA3. Our findings might correlate to the common symptoms of SCA3, such as ataxia, Parkinsonism, dysarthria, and dysmetria. SCA3 should no longer be considered a disease limited to the cerebellum and its connections; rather, it should be considered a pathology affecting the whole brain.

Pathophysiology of dyskinesia and behavioral disorders in non-human primates: the role of serotonergic fibers

The MPTP monkey model of Parkinson's disease (PD) has allowed huge advances regarding the understanding of the pathological mechanisms of PD and L-DOPA-induced adverse effects. Among the main findings were the imbalance between the efferent striatal pathways in opposite directions between the hypokinetic and hyperkinetic states of PD. In both normal and parkinsonian monkeys, the combination of behavioral and anatomical studies has allowed the deciphering of the cortico-basal ganglia circuits involved in both movement and behavioral disorders. A major breakthrough has then been made regarding the hypothesis of the involvement of serotonergic fibers in the conversion of L-DOPA to dopamine when dopaminergic neurons are dying and to release it, in an uncontrolled manner, as serotonergic neurons are deprived from the machinery required for buffering dopamine from the synaptic cleft. The crucial involvement of serotonergic fibers underlying L-DOPA-induced dyskinesia (LID) has been demonstrated in both rodent and monkey models of PD, in which dyskinesia induced by L-DOPA is abolished following lesion of the serotonergic system. Moreover, the role of serotonergic fibers goes well beyond dyskinesia, as lesioning of such serotonergic fibers by MDMA in the monkey also decreased other L-DOPA-induced adverse effects such as impulsive compulsive behaviors and visual hallucinations. The same pathological mechanism, i.e., an imbalance between serotonin and dopamine terminals may, therefore, favor L-DOPA-induced adverse effects according to the basal ganglia territory it inhabits. Further non-human primate studies will be needed to demonstrate the role of such a pathological mechanism in both movement and behavioral disorders driven by L-DOPA therapy but also to determine the causal link between serotonin lesions and the expression of non-motor symptoms like apathy, depression and anxiety, frequently observed in PD patients.

Chronic MPTP administration regimen in monkeys: a model of dopaminergic and non-dopaminergic cell loss in Parkinson's disease

Parkinson's disease (PD) is a progressive neurodegenerative disorder clinically characterized by cardinal motor deficits including bradykinesia, tremor, rigidity and postural instability. Over the past decades, it has become clear that PD symptoms extend far beyond motor signs to include cognitive, autonomic and psychiatric impairments, most likely resulting from cortical and subcortical lesions of non-dopaminergic systems. In addition to nigrostriatal dopaminergic degeneration, pathological examination of PD brains, indeed, reveals widespread distribution of intracytoplasmic inclusions (Lewy bodies) and death of non-dopaminergic neurons in the brainstem and thalamus. For that past three decades, the MPTP-treated monkey has been recognized as the gold standard PD model because it displays some of the key behavioral and pathophysiological changes seen in PD patients. However, a common criticism raised by some authors about this model, and other neurotoxin-based models of PD, is the lack of neuronal loss beyond the nigrostriatal dopaminergic system. In this review, we argue that this assumption is largely incorrect and solely based on data from monkeys intoxicated with acute administration of MPTP. Work achieved in our laboratory and others strongly suggest that long-term chronic administration of MPTP leads to brain pathology beyond the dopaminergic system that displays close similarities to that seen in PD patients. This review critically examines these data and suggests that the chronically MPTP-treated nonhuman primate model may be suitable to study the pathophysiology and therapeutics of some non-motor features of PD.

Neural Repair Strategies for Parkinson's Disease: Insights from Primate Models

Nonhuman primate models of Parkinson's disease (PD) have been invaluable to our understanding of the human disease and in the advancement of novel therapies for its treatment. In this review, we attempt to give a brief overview of the animal models of PD currently used, with a more comprehensive focus on the advantages and disadvantages presented by their use in the nonhuman primate. In particular, discussion addresses the 6-hydroxydopamine (6-OHDA), 1-methyl-1,2,3,6-tetrahydopyridine (MPTP), rotenone, paraquat, and maneb parkinsonian models. Additionally, the role of primate PD models in the development of novel therapies, such as trophic factor delivery, grafting, and deep brain stimulation, are described. Finally, the contribution of primate PD models to our understanding of the etiology and pathology of human PD is discussed.

Effect of Intrastriatal 6-OHDA Lesions on Extrastriatal Brain Structures in the Mouse

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive loss of midbrain dopaminergic neurons, resulting in motor and non-motor symptoms. The underlying pathology of non-motor symptoms is poorly understood. Discussed are pathological changes of extrastriatal brain structures. In this study, we characterized histopathological alterations of extrastriatal brain structures in the 6-hydroxydopamine (6-OHDA) PD animal model. Lesions were induced by unilateral stereotactic injections of 6-OHDA into the striatum or medial forebrain bundle of adult male mice. Loss of tyrosine hydroxylase positive (TH⁺) fibers as well as glia activation was quantified following stereological principles. Loss of dopaminergic innervation was further investigated by western-blotting. As expected, 6-OHDA injection into the nigrostriatal route induced retrograde degeneration of dopaminergic neurons within the substantia nigra pars compacta (SNpc), less so within the ventral tegmental area. Furthermore, we observed a region-specific drop of TH⁺ projection fiber density in distinct cortical regions. This pathology was most pronounced in the cingulate- and motor cortex, whereas the piriform cortex was just modestly affected. Loss of cortical TH⁺ fibers was not paralleled by microglia or astrocyte activation. Our results demonstrate that the loss of dopaminergic neurons within the substantia nigra pars compacta is paralleled by a cortical dopaminergic denervation in the 6-OHDA model. This model serves as a valuable tool to investigate mechanisms operant during cortical pathology in PD patients. Further studies are needed to understand why cortical dopaminergic innervation is lost in this model, and what functional consequence is associated with the observed denervation.

Roles of Multiple Globus Pallidus Territories of Monkeys and Humans in Motivation, Cognition and Action: An Anatomical, Physiological and Pathophysiological Review

The globus pallidus (GP) communicates with widespread cortical areas that support various functions, including motivation, cognition and action. Anatomical tract-tracing studies revealed that the anteroventral GP communicates with the medial prefrontal and orbitofrontal cortices, which are involved in motivational control; the anterodorsal GP communicates with the lateral prefrontal cortex, which is involved in cognitive control; and the posterior GP communicates with the frontal motor cortex, which is involved in action control. This organization suggests that distinct subdivisions within the GP play specific roles. Neurophysiological studies examining GP neurons in monkeys during behavior revealed that the types of information coding performed within these subdivisions differ greatly. The anteroventral GP is characterized by activities related to motivation, such as reward seeking and aversive avoidance; the anterodorsal GP is characterized by activity that reflects cognition, such as goal decision and action selection; and the posterior GP is characterized by activity associated with action preparation and execution. Pathophysiological studies have shown that GABA-related substances or GP lesions result in abnormal activity in the GP, which causes site-specific behavioral and motor symptoms. The present review article discusses the anatomical organization, physiology and pathophysiology of the three major GP territories in nonhuman primates and humans.

Primary motor cortex of the parkinsonian monkey: altered encoding of active movement

Abnormalities in the movement-related activation of the primary motor cortex (M1) are thought to be a major contributor to the motor signs of Parkinson's disease. The existing evidence, however, variably indicates that M1 is under-activated with movement, overactivated (due to a loss of functional specificity) or activated with abnormal timing. In addition, few models consider the possibility that distinct cortical neuron subtypes may be affected differently. Those gaps in knowledge were addressed by studying the extracellular activity of antidromically-identified lamina 5b pyramidal-tract type neurons (n = 153) and intratelencephalic-type corticostriatal neurons (n = 126) in the M1 of two monkeys as they performed a step-tracking arm movement task. We compared movement-related discharge before and after the induction of parkinsonism by administration of MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) and quantified the spike rate encoding of specific kinematic parameters of movement using a generalized linear model. The fraction of M1 neurons with movement-related activity declined following MPTP but only marginally. The strength of neuronal encoding of parameters of movement was reduced markedly (mean 29% reduction in the coefficients from the generalized linear model). This relative decoupling of M1 activity from kinematics was attributable to reductions in the coefficients that estimated the spike rate encoding of movement direction (-22%), speed (-40%), acceleration (-49%) and hand position (-33%). After controlling for MPTP-induced changes in motor performance, M1 activity related to movement itself was reduced markedly (mean 36% hypoactivation). This reduced activation was strong in pyramidal tract-type neurons (-50%) but essentially absent in corticostriatal neurons. The timing of M1 activation was also abnormal, with earlier onset times, prolonged response durations, and a 43% reduction in the prevalence of movement-related changes beginning in the 150-ms period that immediately preceded movement. Overall, the results are consistent with proposals that under-activation and abnormal timing of movement-related activity in M1 contribute to parkinsonian motor signs but are not consistent with the idea that a loss of functional specificity plays an important role. Given that pyramidal tract-type neurons form the primary efferent pathway that conveys motor commands to the spinal cord, the dysfunction of movement-related activity in pyramidal tract-type neurons is likely to be a central factor in the pathophysiology of parkinsonian motor signs.

Pathophysiology of L-dopa-induced motor and non-motor complications in Parkinson's disease

Involuntary movements, or dyskinesia, represent a debilitating complication of levodopa (L-dopa) therapy for Parkinson's disease (PD). L-dopa-induced dyskinesia (LID) are ultimately experienced by the vast majority of patients. In addition, psychiatric conditions often manifested as compulsive behaviours, are emerging as a serious problem in the management of L-dopa therapy. The present review attempts to provide an overview of our current understanding of dyskinesia and other L-dopa-induced dysfunctions, a field that dramatically evolved in the past twenty years. In view of the extensive literature on LID, there appeared a critical need to re-frame the concepts, to highlight the most suitable models, to review the central nervous system (CNS) circuitry that may be involved, and to propose a pathophysiological framework was timely and necessary. An updated review to clarify our understanding of LID and other L-dopa-related side effects was therefore timely and necessary. This review should help in the development of novel therapeutic strategies aimed at preventing the generation of dyskinetic symptoms.

Pallidal stimulation suppresses pathological dysrhythmia in the parkinsonian motor cortex

Although there is general consensus that deep brain stimulation (DBS) yields substantial clinical benefit in patients with Parkinson's disease (PD), the therapeutic mechanism of DBS remains a matter of debate. Recent studies demonstrate that DBS targeting the globus pallidus internus (GPi-DBS) suppresses pathological oscillations in firing rate and between-cell spike synchrony in the vicinity of the electrode but has negligible effects on population-level firing rate or the prevalence of burst firing. The present investigation examines the downstream consequences of GPi-DBS at the level of the primary motor cortex (M1). Multielectrode, single cell recordings were conducted in the M1 of two parkinsonian nonhuman primates (Macaca fasicularis). GPi-DBS that induced significant reductions in muscular rigidity also reduced the prevalence of both beta (12-30 Hz) oscillations in single unit firing rates and of coherent spiking between pairs of M1 neurons. In individual neurons, GPi-DBS-induced increases in mean firing rate were three times more common than decreases; however, averaged across the population of M1 neurons, GPi-DBS induced no net change in mean firing rate. The population-level prevalence of burst firing was also not affected by GPi-DBS. The results are consistent with the hypothesis that suppression of both pathological, beta oscillations and synchronous activity throughout the cortico-basal ganglia network is a major therapeutic mechanism of GPi-DBS.

Cortical glutamate levels decrease in a non-human primate model of dopamine deficiency

While Parkinson's disease is the result of dopaminergic dysfunction of the nigrostriatal system, the clinical manifestations of Parkinson's disease are brought about by alterations in multiple neural components, including cortical areas. We examined how 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration affected extracellular cortical glutamate levels by comparing glutamate levels in normal and MPTP-lesioned nonhuman primates (Macaca mulatta). Extracellular glutamate levels were measured using glutamate microelectrode biosensors. Unilateral MPTP-administration rendered the animals with hemiparkinsonian symptoms, including dopaminergic deficiencies in the substantia nigra and the premotor and motor cortices, and with statistically significant decreases in basal glutamate levels in the primary motor cortex on the side ipsilateral to the MPTP-lesion. These results suggest that the functional changes of the glutamatergic system, especially in the motor cortex, in models of Parkinson's disease could provide important insights into the mechanisms of this disease.

Decoding the dopamine signal in macaque prefrontal cortex: a simulation study using the Cx3Dp simulator

Dopamine transmission in the prefrontal cortex plays an important role in reward based learning, working memory and attention. Dopamine is thought to be released non-synaptically into the extracellular space and to reach distant receptors through diffusion. This simulation study examines how the dopamine signal might be decoded by the recipient neuron. The simulation was based on parameters from the literature and on our own quantified, structural data from macaque prefrontal area 10. The change in extracellular dopamine concentration was estimated at different distances from release sites and related to the affinity of the dopamine receptors. Due to the sparse and random distribution of release sites, a transient heterogeneous pattern of dopamine concentration emerges. Our simulation predicts, however, that at any point in the simulation volume there is sufficient dopamine to bind and activate high-affinity dopamine receptors. We propose that dopamine is broadcast to its distant receptors and any change from the local baseline concentration might be decoded by a transient change in the binding probability of dopamine receptors. Dopamine could thus provide a graduated ‘teaching’ signal to reinforce concurrently active synapses and cell assemblies. In conditions of highly reduced or highly elevated dopamine levels the simulations predict that relative changes in the dopamine signal can no longer be decoded, which might explain why cognitive deficits are observed in patients with Parkinson’s disease, or induced through drugs blocking dopamine reuptake.

Modeling Parkinson's disease in primates: The MPTP model

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) primate models of Parkinson's disease (PD) reproduce most, although not all, of the clinical and pathological hallmarks of PD. The present contribution presents the possibilities offered by the MPTP monkey models of PD to readers with minimal knowledge of PD, emphasizing the diversity of species, route and regimen of administration, symptoms and pathological features. Readers would eventually find out that there is not a single MPTP monkey model of PD but instead MPTP monkey models of PD, each addressing a specific experimental need.

Differential striatal spine pathology in Parkinson's disease and cocaine addiction: a key role of dopamine?

In the striatum, the dendritic tree of the two main populations of projection neurons, called "medium spiny neurons (MSNs)", are covered with spines that receive glutamatergic inputs from the cerebral cortex and thalamus. In Parkinson's disease (PD), striatal MSNs undergo an important loss of dendritic spines, whereas aberrant overgrowth of striatal spines occurs following chronic cocaine exposure. This review examines the possibility that opposite dopamine dysregulation is one of the key factors that underlies these structural changes. In PD, nigrostriatal dopamine degeneration results in a significant loss of dendritic spines in the dorsal striatum, while rodents chronically exposed to cocaine and other psychostimulants, display an increase in the density of "thin and immature" spines in the nucleus accumbens (NAc). In rodent models of PD, there is evidence that D2 dopamine receptor-containing MSNs are preferentially affected, while D1-positive cells are the main targets of increased spine density in models of addiction. However, such specificity remains to be established in primates. Although the link between the extent of striatal spine changes and the behavioral deficits associated with these disorders remains controversial, there is unequivocal evidence that glutamatergic synaptic transmission is significantly altered in both diseased conditions. Recent studies have suggested that opposite calcium-mediated regulation of the transcription factor myocyte enhancer factor 2 (MEF2) function induces these structural defects. In conclusion, there is strong evidence that dopamine is a major, but not the sole, regulator of striatal spine pathology in PD and addiction to psychostimulants. Further studies of the role of glutamate and other genes associated with spine plasticity in mediating these effects are warranted.

Physical activity is linked to ceruloplasmin in the striatum of intact but not MPTP-treated primates

Ceruloplasmin is a protective ferroxidase. Although some studies suggest that plasma ceruloplasmin levels are raised by exercise, the impact of exercise on brain ceruloplasmin is unknown. We have examined whether striatal ceruloplasmin is raised with treadmill exercise and/or is correlated with spontaneous physical activity in rhesus monkeys. Parkinson's disease is characterized by a loss in ceruloplasmin and, similarly, Parkinson's models lead to a loss in antioxidant defenses. Exercise might protect against Parkinson's disease and is known to prevent antioxidant loss in experimental models. We have therefore examined whether treadmill exercise prevents ceruloplasmin loss in monkeys treated unilaterally with the dopaminergic neurotoxin MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine). We found that exercise raised ceruloplasmin expression in the caudate and accumbens but not the putamen of intact monkeys. However, putamen ceruloplasmin was correlated with spontaneous activity in a home pen. MPTP alone did not cause unilateral loss of ceruloplasmin but blocked the impact of exercise on ceruloplasmin. Similarly, the correlation between putamen ceruloplasmin and activity was also lost with MPTP. MPTP elicited loss of tyrosine hydroxylase in the treated hemisphere; the remaining tyrosine hydroxylase was correlated with overall daily activity (spontaneous activity plus that induced by the treadmill). Thus, treadmill activity can raise ceruloplasmin but this impact and the link with spontaneous activity are both diminished in Parkinsonian primates. Furthermore, low overall physical activity predicts greater loss of dopaminergic phenotype in MPTP-treated primates. These data have implications for the maintenance of active lifestyles in both healthy and neurodegenerative conditions.

Animal models of Parkinson's disease: limits and relevance to neuroprotection studies

Over the last two decades, significant strides has been made toward acquiring a better knowledge of both the etiology and pathogenesis of Parkinson's disease (PD). Experimental models are of paramount importance to obtain greater insights into the pathogenesis of the disease. Thus far, neurotoxin-based animal models have been the most popular tools employed to produce selective neuronal death in both in vitro and in vivo systems. These models have been commonly referred to as the pathogenic models. The current trend in modeling PD revolves around what can be called the disease gene-based models or etiologic models. The value of utilizing multiple models with a different mechanism of insult rests on the premise that dopamine-producing neurons die by stereotyped cascades that can be activated by a range of insults, from neurotoxins to downregulation and overexpression of disease-related genes. In this position article, we present the relevance of both pathogenic and etiologic models as well as the concept of clinically relevant designs that, we argue, should be utilized in the preclinical development phase of new neuroprotective therapies before embarking into clinical trials.

Primary motor cortex of the parkinsonian monkey: differential effects on the spontaneous activity of pyramidal tract-type neurons

Dysfunction of primary motor cortex (M1) is thought to contribute to the pathophysiology of parkinsonism. What specific aspects of M1 function are abnormal remains uncertain, however. Moreover, few models consider the possibility that distinct cortical neuron subtypes may be affected differently. Those questions were addressed by studying the resting activity of intratelencephalic-type corticostriatal neurons (CSNs) and distant-projecting lamina 5b pyramidal-tract type neurons (PTNs) in the macaque M1 before and after the induction of parkinsonism by administration of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Contrary to previous reports, the general population of M1 neurons (i.e., PTNs, CSNs, and unidentified neurons) showed reduced baseline firing rates following MPTP, attributable largely to a marked decrease in PTN firing rates. CSN firing rates were unmodified. Although burstiness and firing patterns remained constant in M1 neurons as a whole and CSNs in particular, PTNs became more bursty post-MPTP and less likely to fire in a regular-spiking pattern. Rhythmic spiking (found in PTNs predominantly) occurred at beta frequencies (14-32 Hz) more frequently following MPTP. These results indicate that MPTP intoxication induced distinct modifications in the activity of different M1 neuronal subtypes. The particular susceptibility of PTNs suggests that PTN dysfunction may be an important contributor to the pathophysiology of parkinsonian motor signs.

Cholinergic mesencephalic neurons are involved in gait and postural disorders in Parkinson disease

Gait disorders and postural instability, which are commonly observed in elderly patients with Parkinson disease (PD), respond poorly to dopaminergic agents used to treat other parkinsonian symptoms. The brain structures underlying gait disorders and falls in PD and aging remain to be characterized. Using functional MRI in healthy human subjects, we have shown here that activity of the mesencephalic locomotor region (MLR), which is composed of the pedunculopontine nucleus (PPN) and the adjacent cuneiform nucleus, was modulated by the speed of imagined gait, with faster imagined gait activating a discrete cluster within the MLR. Furthermore, the presence of gait disorders in patients with PD and in aged monkeys rendered parkinsonian by MPTP intoxication correlated with loss of PPN cholinergic neurons. Bilateral lesioning of the cholinergic part of the PPN induced gait and postural deficits in nondopaminergic lesioned monkeys. Our data therefore reveal that the cholinergic neurons of the PPN play a central role in controlling gait and posture and represent a possible target for pharmacological treatment of gait disorders in PD.

Behavioral recovery in MPTP-treated monkeys: neurochemical mechanisms studied by intrastriatal microdialysis

Parkinson's disease (PD) patients express motor symptoms only after 60-80% striatal dopamine (DA) depletion. The presymptomatic phase of the disease may be sustained by biochemical modifications within the striatum. We used an appropriate specific 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) monkey model (Mounayar et al., 2007) to study the compensatory mechanisms operating in recovery from PD motor symptoms. We assessed the levels of DA and its metabolites (DOPAC, homovanillic acid), GABA, glutamate (Glu), serotonin (5-HT) and its metabolite (5HIAA) by repeated intracerebral microdialysis in awake animals before exposure to MPTP during full expression of the motor symptoms induced by MPTP and after recovery from these symptoms. Measurements were obtained from two functionally and anatomically different striatal areas: the associative-limbic territory and sensorimotor territory. Animals with motor symptoms displayed an extremely large decrease in levels of DA and its metabolites and an increase in Glu and GABA levels, as reported by other studies. However, we show here for the first time that serotonin levels increased in these animals. We found that increases in DA levels in the sensorimotor and/or associative-limbic territory and high levels of 5-HT and of its metabolite, 5HIAA, were associated with recovery from motor symptoms in this model. Determining whether similar changes in DA and 5-HT levels are involved in the compensatory mechanisms delaying the appearance of motor symptoms in the early stages of PD might make it possible to develop new treatment strategies for the disease.

Pharmacological characterization of psychosis-like behavior in the MPTP-lesioned nonhuman primate model of Parkinson's disease

Investigation of the pathophysiology of psychosis in Parkinson's disease (PD), as well as the assessment of potential novel therapeutics, has been limited by the lack of a well-validated animal model. MPTP-lesioned primates exhibit abnormal behaviors that are distinct from dyskinesia and parkinsonism and may represent behavioral correlates of neural processes related to psychosis in PD. Here we assess four types of behavior--agitation, hallucinatory-like responses to nonapparent stimuli, obsessive grooming, and stereotypies that are termed "psychosis-like"--and define their pharmacology using a psychosis-like behavior rating scale. By assessing the actions of drugs known to enhance or attenuate psychosis in PD patients, we find that the pharmacology of these behaviors recapitulates, in several respects, the pharmacology of psychosis in PD. Thus, levodopa and apomorphine elicited psychosis-like behaviors. Amantadine significantly decreased levodopa-induced dyskinesia but exacerbated psychosis-like behaviors. Haloperidol reduced psychosis-like behaviors but at the expense of increased parkinsonian disability while the atypical neuroleptics clozapine and quetiapine reduced psychosis-like behaviors without significant effect on parkinsonian disability. The response of different components of the psychotomimetic behavior suggested the involvement of both dopaminergic and nondopaminergic mechanisms in their expression.

Thalamic neuronal activity in dopamine-depleted primates: evidence for a loss of functional segregation within basal ganglia circuits

Different analyses of neuronal activity in primate models of Parkinson's disease (PD) have resulted in two different views on the effects of dopamine depletion. The first is based on the higher firing rate and bursty firing pattern, and assumes that dopamine depletion results in a hyperactivity of basal ganglia (BG) output structures. The second is based on the less-specific responses to passive joint manipulation and the excessive correlations between neuronal discharges, and assumes that dopamine depletion results in a loss of functional segregation in cortico-BG circuits. The aim of the present study was to test out the predictions of these two different views on thalamic neuronal activity. Three male vervet monkeys (Cercopithecus aethiops) were progressively intoxicated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Neuronal activities were characterized using standard analyses (firing rates and patterns, receptive fields, and cross-correlations) and compared between the normal, asymptomatic (before the stabilization of motor symptoms), and parkinsonian (with persistent akinesia and rigidity) stages of MPTP intoxication. The pallidonigral thalamus (receiving projections from the BG) was characterized in both the asymptomatic and parkinsonian states by (1) an unchanged firing rate and pattern and (2) a proliferation of nonspecific neurons and correlated pairs. In contrast, the cerebellar thalamus (receiving projections from the cerebellum), was characterized by no change (asymptomatic state) or minor changes (symptomatic state). Thus the major dysfunction after dopamine depletion appeared to be the loss of functional segregation within cortico-BG circuits, which could also be at the heart of parkinsonian pathophysiology.

Levodopa-induced dyskinesia in MPTP-treated macaques is not dependent on the extent and pattern of nigrostrial lesioning

The extent of nigrostriatal denervation is presumed to play a role in the genesis of levodopa-induced dyskinesia. Yet some parkinsonian patients who have been treated over a long period do not develop dyskinesia, raising the possibility that the pattern of denervation is as important as the extent of lesioning as a risk factor. Here we study the extent and pattern of nigrostriatal denervation in a homogeneous population of parkinsonian macaque monkeys chronically treated with levodopa. Based on the characteristics of the lesioning, non-dyskinetic animals could not be differentiated from those with dyskinesia. Indeed, the number of tyrosine-hydroxylase (TH)-immunopositive neurons in the substantia nigra pars compacta, striatal dopamine transporter (DAT) binding and TH immunostaining, as well as the overall TH striatal content measured by Western blotting were identical. Moreover, the patterns of lesioning assessed by a detailed analysis of the TH- and DAT-immunopositive striatal fibers were comparable in all functional quadrants and at all rostro-caudal levels considered. These data indicate that neither the extent nor the pattern of nigrostriatal lesioning are sufficient to explain the occurrence of levodopa-induced dyskinesia.

Effect of intrastriatal 6-OHDA lesion on dopaminergic innervation of the rat cortex and globus pallidus

The present study examined in the rat the effect of a partial lesion of the nigrostriatal dopaminergic pathway induced by intrastriatal injection of 6-hydroxydopamine (6-OHDA), on the dopaminergic innervation of the cortex and the globus pallidus as revealed using tyrosine hydroxylase (TH) immunoreactivity. Twenty-eight days after unilateral injection of 6-OHDA into the dorsal part of the striatum, TH-positive fiber density was reduced by 41% in the dorsal and central part of the structure, and was accompanied by a retrograde loss of 33% of TH-positive neurons in the substantia nigra (SN), while the ventral tegmental area was completely spared. In the SN, TH-positive cell loss was most severe in the ventral part of the structure (-55%). In the same animals, a substantial loss of TH-positive fibers was evident in the dorsal part of the globus pallidus, and involved both thick fibers of passage and thin varicose terminal axonal branches. In the cortex, a loss of TH-positive fibers was prominent in the cingulate area, moderate in the motor area and less affected in the insular area, while the noradrenergic innervation revealed using dopamine-beta-hydroxylase immunoreactivity was preserved in all of these cortical subregions. These results demonstrate that the intrastriatal 6-OHDA lesion model in rats produces a significant loss of dopaminergic axons in extrastriatal structures including the pallidum and cortex, which may contribute to functional sequelae in this animal model of Parkinson's disease.

Disruption of self-organized actions in monkeys with progressive MPTP-induced parkinsonism: II. Effects of reward preference

The motor and cognitive symptoms of Parkinson's disease (PD) are well documented, but little is known about the functionality of motivational processes mediated by the limbic circuits of basal ganglia. The aim of this study was to test the ability of motivational processes to direct and to urge behaviour, in four vervet monkeys (Cercopithecus aethiops) progressively intoxicated with systemic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) injections (0.3-0.4 mg/kg every 4-7 days). In the food preference task, the monkeys had to retrieve two types of directly visible food, simultaneously available in the wells of a reward board. At all stages of MPTP-induced parkinsonism, the monkeys continued to take their favourite food first. In the symbol discrimination task, the wells were covered with sliding plaques cued by symbols indicating the absence or presence of a reward, and the different types of food were blocked in separate sessions. Monkeys with mild or moderate parkinsonism made fewer attempts and took longer to retrieve non-preferred compared with preferred rewards. These results indicate that motivational processes are still able to direct (food preference task) and to urge (symbol discrimination task) behaviour in MPTP-lesioned monkeys. Such a functional preservation may be related to the relatively spared dopaminergic innervation of the limbic circuits that we found in our monkeys, in agreement with the literature on humans. Furthermore, the frequency of executive disorders (such as hesitations and freezing) appeared to be much lower with the preferred rewards. Thus, the preserved motivational processes may help to overcome executive dysfunction in the early stages of human PD.