Lack of depotentiation at basal ganglia output neurons in PD patients with levodopa-induced dyskinesia

Neuroplasticity in levodopa-induced dyskinesias: An overview on pathophysiology and therapeutic targets

Levodopa-induced dyskinesias (LIDs) are a common complication in patients with Parkinson's disease (PD). A complex cascade of electrophysiological and molecular events that induce aberrant plasticity in the cortico-basal ganglia system plays a key role in the pathophysiology of LIDs. In the striatum, multiple neurotransmitters regulate the different forms of physiological synaptic plasticity to provide it in a bidirectional and Hebbian manner. In PD, impairment of both long-term potentiation (LTP) and long-term depression (LTD) progresses with disease and dopaminergic denervation of striatum. The altered balance between LTP and LTD processes leads to unidirectional changes in plasticity that cause network dysregulation and the development of involuntary movements. These alterations have been documented, in both experimental models and PD patients, not only in deep brain structures but also at motor cortex. Invasive and non-invasive neuromodulation treatments, as deep brain stimulation, transcranial magnetic stimulation, or transcranial direct current stimulation, may provide strategies to modulate the aberrant plasticity in the cortico-basal ganglia network of patients affected by LIDs, thus restoring normal neurophysiological functioning and treating dyskinesias. In this review, we discuss the evidence for neuroplasticity impairment in experimental PD models and in patients affected by LIDs, and potential neuromodulation strategies that may modulate aberrant plasticity.

Levodopa-Induced Dyskinesias in Parkinson's Disease: An Overview on Pathophysiology, Clinical Manifestations, Therapy Management Strategies and Future Directions

Since its first introduction, levodopa has become the cornerstone for the treatment of Parkinson's disease and remains the leading therapeutic choice for motor control therapy so far. Unfortunately, the subsequent appearance of abnormal involuntary movements, known as dyskinesias, is a frequent drawback. Despite the deep knowledge of this complication, in terms of clinical phenomenology and the temporal relationship during a levodopa regimen, less is clear about the pathophysiological mechanisms underpinning it. As the disease progresses, specific oscillatory activities of both motor cortical and basal ganglia neurons and variation in levodopa metabolism, in terms of the dopamine receptor stimulation pattern and turnover rate, underlie dyskinesia onset. This review aims to provide a global overview on levodopa-induced dyskinesias, focusing on pathophysiology, clinical manifestations, therapy management strategies and future directions.

Dystonia, chorea, hemiballismus and other dyskinesias

Hyperkinesias are heterogeneous involuntary movements that significantly differ in terms of clinical and semeiological manifestations, including rhythm, regularity, speed, duration, and other factors that determine their appearance or suppression. Hyperkinesias are due to complex, variable, and largely undefined pathophysiological mechanisms that may involve different brain areas. In this chapter, we specifically focus on dystonia, chorea and hemiballismus, and other dyskinesias, specifically, levodopa-induced, tardive, and cranial dyskinesia. We address the role of neurophysiological studies aimed at explaining the pathophysiology of these conditions. We mainly refer to human studies using surface and invasive in-depth recordings, as well as spinal, brainstem, and transcortical reflexology and non-invasive brain stimulation techniques. We discuss the extent to which the neurophysiological abnormalities observed in hyperkinesias may be explained by pathophysiological models. We highlight the most relevant issues that deserve future research efforts. The potential role of neurophysiological assessment in the clinical context of hyperkinesia is also discussed.

Continuous Dopaminergic Stimulation Improves Cortical Maladaptive Changes in Advanced Parkinson's Disease

BACKGROUND

With the progression of Parkinson's disease (PD), pulsatile treatment with oral levodopa causes maladaptive changes within basal ganglia-thalamo-cortical circuits, which are clinically expressed as motor fluctuations and dyskinesias. At the level of the motor cortex, these changes may be detected using transcranial magnetic stimulation (TMS), as abnormal corticospinal and intracortical excitability and absent response to plasticity protocols.

OBJECTIVE

We investigated the effect of continuous dopaminergic stimulation on cortical maladaptive changes related to oral levodopa treatment.

METHODS

Twenty patients with advanced PD were tested using TMS within 1 week before and again 6 months after the introduction of levodopa-carbidopa intestinal gel. We measured resting and active motor thresholds, input/output curve, short interval intracortical inhibition curve, cortical silent period, and response to intermittent theta burst stimulation. Patients were clinically assessed with Part III and Part IV of the Movement Disorders Society Unified Parkinson's Disease Rating Scale.

RESULTS

Six months after the introduction of levodopa-carbidopa intestinal gel, motor fluctuations scores (P = 0.001) and dyskinesias scores (P < 0.001) were reduced. Resting and active motor threshold (P = 0.012 and P = 0.015) and x-intercept of input/output curve (P = 0.005) were also decreased, while short-interval intracortical inhibition and response to intermittent theta bust stimulation were improved (P = 0.026 and P = 0.031, respectively). Changes in these parameters correlated with clinical improvement.

CONCLUSIONS

In patients with advanced PD, switching from intermittent to continuous levodopa delivery increased corticospinal excitability and improved deficient intracortical inhibition and abnormal motor cortex plasticity, along with amelioration of motor fluctuations and dyskinesias. Continuous dopaminergic stimulation ameliorates maladaptive changes inflicted by chronic pulsatile dopaminergic stimulation. © 2022 International Parkinson and Movement Disorder Society.

Plasticity, genetics, and epigenetics in l-dopa-induced dyskinesias

l-Dopa-induced dyskinesias (LIDs) are a frequent complication in l-dopa-treated patients affected by Parkinson's disease (PD). In the last years, several progresses in the knowledge of LIDs mechanisms have led to the identification of several molecular and electrophysiologic events. A complex cascade of intracellular events underlies the pathophysiology of LIDs, and, among these, aberrant plasticity in the cortico-basal ganglia system, at striatal and cortical level, plays a key role. Furthermore, several recent studies have investigated genetic susceptibility and epigenetic modifications in LIDs pathophysiology that might have future relevance in clinical practice and pharmacologic research. These progresses might lead to the development of specific strategies not only to treat, but also to prevent or delay the development of LIDs in PD.

Parkinson's disease: Alterations of motor plasticity and motor learning

This chapter reviews the alterations in motor learning and motor cortical plasticity in Parkinson's disease (PD), the most common movement disorder. Impairments in motor learning, which is a hallmark of basal ganglia disorders, influence the performance of motor learning-related behavioral tasks and have clinical implications for the management of disturbance in gait and posture, and for rehabilitative management of PD. Although plasticity is classically induced and assessed in sliced preparation in animal models, in this review we have concentrated on the results from non-invasive brain stimulation techniques such as transcranial magnetic stimulation (TMS), transcranial alternating current stimulation (tACS) and transcranial direct current stimulation (tDCS) in patients with PD, in addition to a few animal electrophysiologic studies. The chapter summarizes the results from different cortical and subcortical plasticity investigations. Plasticity induction protocols reveal deficient plasticity in PD and these plasticity measures are modulated by medications and deep brain stimulation. There is considerable variability in these measures that are related to inter-individual variations, different disease characteristics and methodological considerations. Nevertheless, these pathophysiologic studies expand our knowledge of cortical excitability, plasticity and the effects of different treatments in PD. These tools of modulating plasticity and motor learning improve our understanding of PD pathophysiology and help to develop new treatments for this disabling condition.

Pathophysiological Mechanisms and Experimental Pharmacotherapy for L-Dopa-Induced Dyskinesia

L-dopa-induced dyskinesia (LID) is the most frequent motor complication associated with chronic L-dopa treatment in Parkinson’s disease (PD). Recent advances in the understanding of the pathophysiological mechanisms underlying LID suggest that abnormalities in multiple neurotransmitter systems, in addition to dopaminergic nigrostriatal denervation and altered dopamine release and reuptake dynamics at the synaptic level, are involved in LID development. Increased knowledge of neurobiological LID substrates has led to the development of several drug candidates to alleviate this motor complication. However, with the exception of amantadine, none of the pharmacological therapies tested in humans have demonstrated clinically relevant beneficial effects. Therefore, LID management is still one of the most challenging problems in the treatment of PD patients. In this review, we first describe the known pathophysiological mechanisms of LID. We then provide an updated report of experimental pharmacotherapies tested in clinical trials of PD patients and drugs currently under study to alleviate LID. Finally, we discuss available pharmacological LID treatment approaches and offer our opinion of possible issues to be clarified and future therapeutic strategies.

Insights into the Pathophysiology of Psychiatric Symptoms in Central Nervous System Disorders: Implications for Early and Differential Diagnosis

Different psychopathological manifestations, such as affective, psychotic, obsessive-compulsive symptoms, and impulse control disturbances, may occur in most central nervous system (CNS) disorders including neurodegenerative and neuroinflammatory diseases. Psychiatric symptoms often represent the clinical onset of such disorders, thus potentially leading to misdiagnosis, delay in treatment, and a worse outcome. In this review, psychiatric symptoms observed along the course of several neurological diseases, namely Alzheimer’s disease, fronto-temporal dementia, Parkinson’s disease, Huntington’s disease, and multiple sclerosis, are discussed, as well as the involved brain circuits and molecular/synaptic alterations. Special attention has been paid to the emerging role of fluid biomarkers in early detection of these neurodegenerative diseases. The frequent occurrence of psychiatric symptoms in neurological diseases, even as the first clinical manifestations, should prompt neurologists and psychiatrists to share a common clinico-biological background and a coordinated diagnostic approach.

Exploring Movement Impairments in Patients With Parkinson's Disease Using the Microsoft Kinect Sensor: A Feasibility Study

Background: Current assessments of motor symptoms in Parkinson's disease are often limited to clinical rating scales. Objectives: To develop a computer application using the Microsoft Kinect sensor to assess performance-related bradykinesia. Methods: The developed application (Motorgame) was tested in patients with Parkinson's disease and healthy controls. Participants were assessed with the Movement Disorder Society Unified Parkinson's disease Rating Scale (MDS-UPDRS) and standardized clinical side effect rating scales, i.e., UKU Side Effect Rating Scale and Simpson-Angus Scale. Additionally, tests of information processing (Symbol Coding Task) and motor speed (Token Motor Task), together with a questionnaire, were applied. Results: Thirty patients with Parkinson's disease and 33 healthy controls were assessed. In the patient group, there was a statistically significant (p < 0.05) association between prolonged time of motor performance in the Motorgame and upper body rigidity and bradykinesia (MDS-UPDRS) with the strongest effects in the right hand (p < 0.001). In the entire group, prolonged time of motor performance was significantly associated with higher Simson-Angus scale rigidity score and higher UKU hypokinesia scores (p < 0.05). A shortened time of motor performance was significantly associated with higher scores on information processing (p < 0.05). Time of motor performance was not significantly associated with Token Motor Task, duration of illness, or hours of daily physical activity. The Motorgame was well-accepted. Conclusions: In the present feasibility study the Motorgame was able to detect common motor symptoms in Parkinson's disease in a statistically significant and clinically meaningful way, making it applicable for further testing in larger samples.

Striatal Nurr1 Facilitates the Dyskinetic State and Exacerbates Levodopa-Induced Dyskinesia in a Rat Model of Parkinson's Disease

The transcription factor Nurr1 has been identified to be ectopically induced in the striatum of rodents expressing l-DOPA-induced dyskinesia (LID). In the present study, we sought to characterize Nurr1 as a causative factor in LID expression. We used rAAV2/5 to overexpress Nurr1 or GFP in the parkinsonian striatum of LID-resistant Lewis or LID-prone Fischer-344 (F344) male rats. In a second cohort, rats received the Nurr1 agonist amodiaquine (AQ) together with l-DOPA or ropinirole. All rats received a chronic DA agonist and were evaluated for LID severity. Finally, we performed single-unit recordings and dendritic spine analyses on striatal medium spiny neurons (MSNs) in drug-naïve rAAV-injected male parkinsonian rats. rAAV-GFP injected LID-resistant hemi-parkinsonian Lewis rats displayed mild LID and no induction of striatal Nurr1 despite receiving a high dose of l-DOPA. However, Lewis rats overexpressing Nurr1 developed severe LID. Nurr1 agonism with AQ exacerbated LID in F344 rats. We additionally determined that in l-DOPA-naïve rats striatal rAAV-Nurr1 overexpression (1) increased cortically-evoked firing in a subpopulation of identified striatonigral MSNs, and (2) altered spine density and thin-spine morphology on striatal MSNs; both phenomena mimicking changes seen in dyskinetic rats. Finally, we provide postmortem evidence of Nurr1 expression in striatal neurons of l-DOPA-treated PD patients. Our data demonstrate that ectopic induction of striatal Nurr1 is capable of inducing LID behavior and associated neuropathology, even in resistant subjects. These data support a direct role of Nurr1 in aberrant neuronal plasticity and LID induction, providing a potential novel target for therapeutic development.SIGNIFICANCE STATEMENT The transcription factor Nurr1 is ectopically induced in striatal neurons of rats exhibiting levodopa-induced dyskinesia [LID; a side-effect to dopamine replacement strategies in Parkinson's disease (PD)]. Here we asked whether Nurr1 is causing LID. Indeed, rAAV-mediated expression of Nurr1 in striatal neurons was sufficient to overcome LID-resistance, and Nurr1 agonism exacerbated LID severity in dyskinetic rats. Moreover, we found that expression of Nurr1 in l-DOPA naïve hemi-parkinsonian rats resulted in the formation of morphologic and electrophysiological signatures of maladaptive neuronal plasticity; a phenomenon associated with LID. Finally, we determined that ectopic Nurr1 expression can be found in the putamen of l-DOPA-treated PD patients. These data suggest that striatal Nurr1 is an important mediator of the formation of LID.

Effects of safinamide on the glutamatergic striatal network in experimental Parkinson's disease

OBJECTIVE

The aim of the study was to evaluate electrophysiological effects of safinamide on the intrinsic and synaptic properties of striatal spiny projection neurons (SPNs) and to characterize the possible therapeutic antiparkinsonian effect of this drug in dopamine (DA) denervated rats before and during levodopa (l-DOPA) treatment.

BACKGROUND

Current therapeutic options in Parkinson's disease (PD) are primarily DA replacement strategies that usually cause progressive motor fluctuations and l-DOPA-induced dyskinesia (LIDs) as a consequence of SPNs glutamate-induced hyperactivity. As a reversible and use-dependent inhibitor of voltage-gated sodium channels, safinamide reduces the release of glutamate and possibly optimize the effect of l-DOPA therapy in PD.

METHODS

Electrophysiological effects of safinamide (1-100 μM) were investigated by patch-clamp recordings in striatal slices of naïve, 6-hydroxydopamine (6-OHDA)-lesioned DA-denervated rats and DA-denervated animals chronically treated with l-DOPA. LIDs were assessed in vivo with and without chronic safinamide treatment and measured by scoring the l-DOPA-induced abnormal involuntary movements (AIMs). Motor deficit was evaluated with the stepping test.

RESULTS

Safinamide reduced the SPNs firing rate and glutamatergic synaptic transmission in all groups, showing a dose-dependent effect with half maximal inhibitory concentration (IC₅₀) values in the therapeutic range (3-5 μM). Chronic co-administration of safinamide plus l-DOPA in DA-denervated animals favored the recovery of corticostriatal long-term synaptic potentiation (LTP) and depotentiation of excitatory synaptic transmission also reducing motor deficits before the onset of LIDs.

CONCLUSIONS

Safinamide, at a clinically relevant dose, optimizes the effect of l-DOPA therapy in experimental PD reducing SPNs excitability and modulating synaptic transmission. Co-administration of safinamide and l-DOPA ameliorates motor deficits.

Synaptic GluN2A-Containing NMDA Receptors: From Physiology to Pathological Synaptic Plasticity

N-Methyl-d-Aspartate Receptors (NMDARs) are ionotropic glutamate-gated receptors. NMDARs are tetramers composed by several homologous subunits of GluN1-, GluN2-, or GluN3-type, leading to the existence in the central nervous system of a high variety of receptor subtypes with different pharmacological and signaling properties. NMDAR subunit composition is strictly regulated during development and by activity-dependent synaptic plasticity. Given the differences between GluN2 regulatory subunits of NMDAR in several functions, here we will focus on the synaptic pool of NMDARs containing the GluN2A subunit, addressing its role in both physiology and pathological synaptic plasticity as well as the contribution in these events of different types of GluN2A-interacting proteins.

Eltoprazine prevents levodopa-induced dyskinesias by reducing causal interactions for theta oscillations in the dorsolateral striatum and substantia nigra pars reticulate

Oscillatory activities within basal ganglia (BG) circuitry in L-DOPA induced dyskinesia (LID), a condition that occurs in patients with Parkinson disease (PD), are not well understood. The aims of this study were firstly to investigate oscillations in main BG input and output structures-the dorsolateral striatum (dStr) and substantia nigra pars reticulata (SNr), respectively- including the direction of oscillation information flow, and secondly to investigate the effects of 5-HT1A/B receptor agonism with eltoprazine on oscillatory activities and abnormal involuntary movements (AIMs) characteristic. To this end, we conducted local field potential (LFP) electrophysiology in the dStr and SNr of LID rats simultaneous with AIM scoring. The LFP data were submitted to power spectral density, coherence, and partial Granger causality analyses. AIM data were analyzed relative to simultaneous oscillatory activities, with and without eltoprazine. We obtained four major findings. 1) Theta band (5-8 Hz) oscillations were enhanced in the dStr and SNr of LID rats. 2) Theta power correlated with AIM scores in the 180-min period after the last LID-inducing L-DOPA injection, but not with daily summed AIM scores during LID development. 3) Oscillatory information flowed from the dStr to the SNr. 4) Chronic eltoprazine reduced BG theta activity in LID rats and normalized information flow directionality, relative to that in LID rats not given eltoprazine. These results indicate that dStr activity plays a determinative role in the causal interactions of theta oscillations and that serotonergic inhibition may suppress dyskinesia by reducing dStr-SNr theta activity and restoring theta network information flow.

Motor cortical circuits in Parkinson disease and dystonia

We review the motor cortical and basal ganglia involvement in two important movement disorders: Parkinson's disease (PD) and dystonia. Single and paired pulse transcranial magnetic stimulation studies showed altered excitability and cortical circuits in PD with decreased silent period, short interval intracortical inhibition, intracortical facilitation, long afferent inhibition, interhemispheric inhibition, and cerebellar inhibition, and increased long interval intracortical inhibition and short interval intracortical facilitation. In dystonia, there is decreased silent period, short interval intracortical inhibition, long afferent inhibition, interhemispheric inhibition, and increased intracortical facilitation. Plasticity induction protocols revealed deficient plasticity in PD and normal and exaggerated plasticity in dystonia. In the basal ganglia, there is increased β (14-30Hz) rhythm in PD and characteristic 5-18Hz band synchronization in dystonia. These motor cortical circuits, cortical plasticity, and oscillation profiles of the basal ganglia are altered with medications and deep brain stimulation treatment. There is considerable variability in these measures related to interindividual variations, different disease characteristics, and methodological considerations. Nevertheless, these pathophysiologic studies have expanded our knowledge of cortical excitability, plasticity, and oscillations in PD and dystonia, improved our understanding of disease pathophysiology, and helped to develop new treatments for these conditions.

Parkinson's disease-linked Parkin mutations impair glutamatergic signaling in hippocampal neurons

BACKGROUND

Parkinson's disease (PD)-associated E3 ubiquitin ligase Parkin is enriched at glutamatergic synapses, where it ubiquitinates multiple substrates, suggesting that its mutation/loss-of-function could contribute to the etiology of PD by disrupting excitatory neurotransmission. Here, we evaluate the impact of four common PD-associated Parkin point mutations (T240M, R275W, R334C, G430D) on glutamatergic synaptic function in hippocampal neurons.

RESULTS

We find that expression of these point mutants in cultured hippocampal neurons from Parkin-deficient and Parkin-null backgrounds alters NMDA and AMPA receptor-mediated currents and cell-surface levels and prevents the induction of long-term depression. Mechanistically, we demonstrate that Parkin regulates NMDA receptor trafficking through its ubiquitination of GluN1, and that all four mutants are impaired in this ubiquitinating activity. Furthermore, Parkin regulates synaptic AMPA receptor trafficking via its binding and retention of the postsynaptic scaffold Homer1, and all mutants are similarly impaired in this capacity.

CONCLUSION

Our findings demonstrate that pathogenic Parkin mutations disrupt glutamatergic synaptic transmission in hippocampal neurons by impeding NMDA and AMPA receptor trafficking. Such effects may contribute to the pathophysiology of PD in PARK2 patients.

Synaptic plasticity and levodopa-induced dyskinesia: electrophysiological and structural abnormalities

Parkinson's disease (PD) is a neurodegenerative disorder characterized by progressive degeneration of dopaminergic neurons located in the midbrain. The gold-standard therapy for PD is the restoration of dopamine (DA) levels through the chronic administration of the DA precursor levodopa (L-DOPA). Although levodopa therapy is the main therapeutic approach for PD, its use is limited by the development of very disabling dyskinetic movements, mainly due to the fluctuation of DA cerebral content. Experimental animal models of PD identified in DA D1/ERK-signaling pathway aberrant activation, occurring in striatal projection neurons, coupled with structural spines abnormalities, the molecular and neuronal basis of L-DOPA-induced dyskinesia (LIDs) occurrence. Different electrophysiological approaches allowed the identification of  the alteration of homeostatic structural and synaptic changes, the neuronal bases of LIDs either in vivo in parkinsonian patients or in vitro in experimental animals. Here, we report the most recent studies showing electrophysiological and morphological evidence of aberrant synaptic plasticity in parkinsonian patients during LIDs in different basal ganglia nuclei and also in cortical transmission, accounting for the complexity of the synaptic changes during dyskinesias. All together, these studies suggest that LIDs are associated with a loss of homeostatic synaptic mechanisms.

Motor complications in Parkinson's disease: Striatal molecular and electrophysiological mechanisms of dyskinesias

Long-term levodopa (l-dopa) treatment in patients with Parkinson´s disease (PD) is associated with the development of motor complications (ie, motor fluctuations and dyskinesias). The principal etiopathogenic factors are the degree of nigro-striatal dopaminergic loss and the duration and dose of l-dopa treatment. In this review article we concentrate on analysis of the mechanisms underlying l-dopa-induced dyskinesias, a phenomenon that causes disability in a proportion of patients and that has not benefited from major therapeutic advances. Thus, we discuss the main neurotransmitters, receptors, and pathways that have been thought to play a role in l-dopa-induced dyskinesias from the perspective of basic neuroscience studies. Some important advances in deciphering the molecular pathways involved in these abnormal movements have occurred in recent years to reveal potential targets that could be used for therapeutic purposes. However, it has not been an easy road because there have been a plethora of components involved in the generation of these undesired movements, even bypassing the traditional and well-accepted dopamine receptor activation, as recently revealed by optogenetics. Here, we attempt to unify the available data with the hope of guiding and fostering future research in the field of striatal activation and abnormal movement generation. © 2017 International Parkinson and Movement Disorder Society.

Field evoked potentials in the globus pallidus of non-human primates

Stimulation-induced field evoked potentials (fEPs) have been described in the basal ganglia output nuclei of patients with Parkinson's disease and dystonia. The aim of this study was to ascertain whether fEPs were inducible in the external (GPe) and internal (GPi) segments of the globus pallidus in normal non-human primates (NHPs). Microelectrode recording and stimulation was performed in the GPe and GPi of 2 healthy NHPs. Stimulus response curves of the fEP response to changing pulse width and amplitude examined strength-duration relationships and allowed for calculation of fEP chronaxie in the GPe and GPi. Traditional localization techniques were also used, including comparison of neuronal firing rates, optic tract activation, and internal capsule activation. Notable differences were seen in the fEPs found in GPe compared to the fEPs found in GPi. The GPe fEP had a smaller chronaxie time and larger positive deflection amplitude compared to GPi. In addition, an earlier negative deflection was identified in both nuclei and a late negative deflection was observed in the GPe in contrast to reported fEPs in patients with movement disorders. fEPs proved valuable as an ancillary method in localizing the GPe and GPi in NHPs and may be useful in the operating room during human GPi deep brain stimulation or pallidotomy procedures.

Subthalamic nucleus deep brain stimulation on motor-symptoms of Parkinson's disease: Focus on neurochemistry

Deep brain stimulation (DBS) has become a standard therapy for Parkinson's disease (PD) and it is also currently under investigation for other neurological and psychiatric disorders. Although many scientific, clinical and ethical issues are still unresolved, DBS delivered into the subthalamic nucleus (STN) has improved the quality of life of several thousands of patients. The mechanisms underlying STN-DBS have been debated extensively in several reviews; less investigated are the biochemical consequences, which are still under scrutiny. Crucial and only partially understood, for instance, are the complex interplays occurring between STN-DBS and levodopa (LD)-centred therapy in the post-surgery follow-up. The main goal of this review is to address the question of whether an improved motor control, based on STN-DBS therapy, is also achieved through the additional modulation of other neurotransmitters, such as noradrenaline (NA) and serotonin (5-HT). A critical issue is to understand not only acute DBS-mediated effects, but also chronic changes, such as those involving cyclic nucleotides, capable of modulating circuit plasticity. The present article will discuss the neurochemical changes promoted by STN-DBS and will document the main results obtained in microdialysis studies. Furthermore, we will also examine the preliminary achievements of voltammetry applied to humans, and discuss new hypothetical investigational routes, taking into account novel players such as glia, or subcortical regions such as the pedunculopontine (PPN) area. Our further understanding of specific changes in brain chemistry promoted by STN-DBS would further disseminate its utilisation, at any stage of disease, avoiding an irreversible lesioning approach.

Maladaptive Synaptic Plasticity in L-DOPA-Induced Dyskinesia

The emergence of L-DOPA-induced dyskinesia (LID) in patients with Parkinson disease (PD) could be due to maladaptive plasticity of corticostriatal synapses in response to L-DOPA treatment. A series of recent studies has revealed that LID is associated with marked morphological plasticity of striatal dendritic spines, particularly cell type-specific structural plasticity of medium spiny neurons (MSNs) in the striatum. In addition, evidence demonstrating the occurrence of plastic adaptations, including aberrant morphological and functional features, in multiple components of cortico-basal ganglionic circuitry, such as primary motor cortex (M1) and basal ganglia (BG) output nuclei. These adaptations have been implicated in the pathophysiology of LID. Here, we briefly review recent studies that have addressed maladaptive plastic changes within the cortico-BG loop in dyskinetic animal models of PD and patients with PD.

Paradoxical facilitation after depotentiation protocol can precede dyskinesia onset in early Parkinson's disease

Loss of dopamine, a key modulator of synaptic signalling, and subsequent pulsatile non-physiological levodopa replacement is believed to underlie altered neuroplasticity in Parkinson's disease (PD). Animal models suggest that maladaptive plasticity (e.g. deficient depotentiation at corticostriatal synapses) is key in the development of levodopa-induced dyskinesia (LID), a common complication following levodopa replacement in PD. Human studies using transcranial magnetic stimulation protocols have shown similar depotentiation deficit in patients with LID. We hypothesized that subtle depotentiation deficits should precede LID if these deficits are mechanistically linked to LID onset. Moreover, patients on pulsatile levodopa-based therapy may show these changes earlier than those treated with levodopa-sparing strategies. We recruited 22 early non-dyskinetic PD patients (<5 years since diagnosis) and 12 age-matched healthy controls. We grouped patients into those on Levodopa-Based (n = 11) and Levodopa-Sparing therapies (n = 11). Patients were selected to obtain groups matched for age and disease severity. We used a theta-burst stimulation protocol to investigate potentiation and depotentiation in a single session. We report significant depotentiation deficits in the Levodopa-Based group, compared to both Levodopa-Sparing and Healthy Control groups. Potentiation and Depotentiation responses were similar between Levodopa-Sparing and Healthy Control groups. Although differences persist after accounting for potential confounds (e.g. levodopa-equivalent dose), these results may yet be caused by differences in disease severity and cumulative levodopa-equivalent dose as discussed in the text. In conclusion, we show for the first time that paradoxical facilitation in response to depotentiation protocols can occur in PD even prior to LID onset.

Preliminary evidence for human globus pallidus pars interna neurons signaling reward and sensory stimuli

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Non-motor responses of human GPi neurons are described.

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Cells were identified that showed increased firing to reward-stimuli.

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Visual-sensory responses unrelated to reward also observed.

The globus pallidus pars interna (GPi) is a component of the basal ganglia, a network of subcortical nuclei that process motor, associative, and limbic information. While non-human primate studies have suggested a role for the GPi in non-motor functions, there have been no single-unit studies of non-motor electrophysiological behavior of human GPi neurons. We therefore sought to extend these findings by collecting single-unit recordings from awake patients during functional stereotactic neurosurgery targeting the GPi for deep brain stimulation. To assess cellular responses to non-motor information, patients performed a reward task where virtual money could be won, lost, or neither, depending on their performance while cellular activity was monitored. Changes in the firing rates of isolated GPi neurons after the presentation of reward-related stimuli were compared between different reward contingencies (win, loss, null). We observed neurons that modulated their firing rate significantly to the presentation of reward-related stimuli. We furthermore found neurons that responded to visual-stimuli more broadly. This is the first single-unit evidence of human GPi neurons carrying non-motor information. These results are broadly consistent with previous findings in the animal literature and suggest non-motor information may be represented in the single-unit activity of human GPi neurons.

Orexin A induces bidirectional modulation of synaptic plasticity: Inhibiting long-term potentiation and preventing depotentiation

The orexin system consists of two peptides, orexin A and B and two receptors, OX1R and OX2R. It is implicated in learning and memory regulation while controversy remains on its role in modulating hippocampal synaptic plasticity in vivo and in vitro. Here, we investigated effects of orexin A on two forms of synaptic plasticity, long-term potentiation (LTP) and depotentiation of field excitatory postsynaptic potentials (fEPSPs), at the Schaffer Collateral-CA1 synapse of mouse hippocampal slices. Orexin A (≧30 nM) attenuated LTP induced by theta burst stimulation (TBS) in a manner antagonized by an OX1R (SB-334867), but not OX2R (EMPA), antagonist. Conversely, at 1 pM, co-application of orexin A prevented the induction of depotentiation induced by low frequency stimulation (LFS), i.e. restoring LTP. This re-potentiation effect of sub-nanomolar orexin A occurred at LFS of 1 Hz, but not 2 Hz, and with LTP induced by either TBS or tetanic stimulation. It was significantly antagonized by SB-334867, EMPA and TCS-1102, selective OX1R, OX2R and dual OXR antagonists, respectively, and prevented by D609, SQ22536 and H89, inhibitors of phospholipase C (PLC), adenylyl cyclase (AC) and protein kinase A (PKA), respectively. LFS-induced depotentiation was antagonized by blockers of NMDA, A1-adenosine and type 1/5 metabotropic glutamate (mGlu1/5) receptors, respectively. However, orexin A (1 pM) did not affect chemical-induced depotentiation by agonists of these receptors. These results suggest that orexin A bidirectionally modulates hippocampal CA1 synaptic plasticity, inhibiting LTP via OX1Rs at moderate concentrations while inducing re-potentiation via OX1Rs and OX2Rs, possibly through PLC and AC-PKA signaling at sub-nanomolar concentrations.

Levodopa-induced plasticity: a double-edged sword in Parkinson's disease?

The long-term replacement therapy with the dopamine (DA) precursor 3,4-dihydroxy-l-phenylalanine (L-DOPA) is a milestone in the treatment of Parkinson's disease (PD). Although this drug precursor can be metabolized into the active neurotransmitter DA throughout the brain, its therapeutic benefit is due to restoring extracellular DA levels within the dorsal striatum, which lacks endogenous DA as a consequence of the neurodegenerative process induced by the disease. In the early phases of PD, L-DOPA treatment is able to restore both long-term depression (LTD) and long-term potentiation (LTP), two major forms of corticostriatal synaptic plasticity that are altered by dopaminergic denervation. However, unlike physiological DA transmission, this therapeutic approach in the advanced phase of the disease leads to abnormal peaks of DA, non-synaptically released, which are supposed to trigger behavioural sensitization, namely L-DOPA-induced dyskinesia. This condition is characterized by a loss of synaptic depotentiation, an inability to reverse previously induced LTP. In the advanced stages of PD, L-DOPA can also induce non-motor fluctuations with cognitive dysfunction and neuropsychiatric symptoms such as compulsive behaviours and impulse control disorders. Although the mechanisms underlying the role of L-DOPA in both motor and behavioural symptoms are still incompletely understood, recent data from electrophysiological and imaging studies have increased our understanding of the function of the brain areas involved and of the mechanisms implicated in both therapeutic and adverse actions of L-DOPA in PD patients.

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.