Acute dopamine boost has a negative effect on plasticity of the primary motor cortex in advanced Parkinson's disease

Treatment and physiology in Parkinson's disease and dystonia: using transcranial magnetic stimulation to uncover the mechanisms of action

Transcranial magnetic stimulation (TMS) has served as an important technological breakthrough in the field of the physiology of movement disorders over the last three decades. TMS has grown popular owing to the ease of application as well as its painless and noninvasive character. The technique has provide important insights into understanding the pathophysiology of movement disorders, particularly Parkinson's disease and dystonia. The basic applications have included the study of motor cortex excitability, functioning of excitatory and inhibitory circuits, study of interactions between sensory and motor systems, and the plasticity response of the brain. TMS has also made important contributions to understanding the response to treatments such as dopaminergic medications, botulinum toxin injections, and deep brain stimulation surgery. This review summarizes the knowledge gained to date with TMS in Parkinson's disease and dystonia, and highlights the current challenges in the use of TMS technology.

Cerebellar influence on motor cortex plasticity: behavioral implications for Parkinson's disease

Normal motor behavior involves the creation of appropriate activity patterns across motor networks, enabling firing synchrony, synaptic integration, and normal functioning of these networks. Strong topography-specific connections among the basal ganglia, cerebellum, and their projections to overlapping areas in the motor cortices suggest that these networks could influence each other's plastic responses and functions. The defective striatal signaling in Parkinson's disease (PD) could therefore lead to abnormal oscillatory activity and aberrant plasticity at multiple levels within the interlinked motor networks. Normal striatal dopaminergic signaling and cerebellar sensory processing functions influence the scaling and topographic specificity of M1 plasticity. Both these functions are abnormal in PD and appear to contribute to the abnormal M1 plasticity. Defective motor map plasticity and topographic specificity within M1 could lead to incorrect muscle synergies, which could manifest as abnormal or undesired movements, and as abnormal motor learning in PD. We propose that the loss of M1 plasticity in PD reflects a loss of co-ordination among the basal ganglia, cerebellar, and cortical inputs which translates to an abnormal plasticity of motor maps within M1 and eventually to some of the motor signs of PD. The initial benefits of dopamine replacement therapy on M1 plasticity and motor signs are lost during the progressive course of disease. Levodopa-induced dyskinesias in patients with advanced PD is linked to a loss of M1 sensorimotor plasticity and the attenuation of dyskinesias by cerebellar inhibitory stimulation is associated with restoration of M1 plasticity. Complimentary interventions should target reestablishing physiological communication between the striatal and cerebellar circuits, and within striato-cerebellar loop. This may facilitate correct motor synergies and reduce abnormal movements in PD.

Maladaptive plasticity in levodopa-induced dyskinesias and tardive dyskinesias: old and new insights on the effects of dopamine receptor pharmacology

Maladaptive plasticity can be defined as behavioral loss or even development of disease symptoms resulting from aberrant plasticity changes in the human brain. Hyperkinetic movement disorders, in the neurological or psychiatric realms, have been associated with maladaptive neural plasticity that can be expressed by functional changes such as an increase in transmitter release, receptor regulation, and synaptic plasticity or anatomical modifications such as axonal regeneration, sprouting, synaptogenesis, and neurogenesis. Recent evidence from human and animal models provided support to the hypothesis that these phenomena likely depend on altered dopamine turnover induced by long-term drug treatment. However, it is still unclear how and where these altered mechanisms of cortical plasticity may be localized. This study provides an up-to-date overview of these issues together with some reflections on future studies in the field, particularly focusing on two specific disorders (levodopa-induced dyskinesias in Parkinson’s disease patients and tardive dyskinesias in schizophrenic patients) where the modern neuroimaging approaches have recently provided new fundamental insights.

Morphological and electrophysiological changes in intratelencephalic-type pyramidal neurons in the motor cortex of a rat model of levodopa-induced dyskinesia

Levodopa-induced dyskinesia (LID) is a major complication of long-term dopamine replacement therapy for Parkinson's disease, and becomes increasingly problematic in the advanced stage of the disease. Although the cause of LID still remains unclear, there is accumulating evidence from animal experiments that it results from maladaptive plasticity, resulting in supersensitive excitatory transmission at corticostriatal synapses. Recent work using transcranial magnetic stimulation suggests that the motor cortex displays the same supersensitivity in Parkinson's disease patients with LID. To date, the cellular mechanisms underlying the abnormal cortical plasticity have not been examined. The morphology of the dendritic spines has a strong relationship to synaptic plasticity. Therefore, we explored the spine morphology of pyramidal neurons in the motor cortex in a rat model of LID. We used control rats, 6-hydroxydopamine-lesioned rats (a model of Parkinson's disease), 6-hydroxydopamine-lesioned rats chronically treated with levodopa (a model of LID), and control rats chronically treated with levodopa. Because the direct pathway of the basal ganglia plays a central role in the development of LID, we quantified the density and size of dendritic spines in intratelencephalic (IT)-type pyramidal neurons in M1 cortex that project to the striatal medium spiny neurons in the direct pathway. The spine density was not different among the four groups. In contrast, spine size became enlarged in the Parkinson's disease and LID rat models. The enlargement was significantly greater in the LID model than in the Parkinson's disease model. This enlargement of the spines suggests that IT-type pyramidal neurons acquire supersensitivity to excitatory stimuli. To confirm this possibility, we monitored miniature excitatory postsynaptic currents (mEPSCs) in the IT-type pyramidal neurons in M1 cortex using whole-cell patch clamp. The amplitude of the mEPSCs was significantly increased in the LID model compared with the control. This indicates that the IT-type pyramidal neurons become hyperexcited in the LID model, paralleling the enlargement of spines. Thus, spine enlargement and the resultant hyperexcitability of IT-type pyramidal neurons in M1 cortex might contribute to the abnormal cortical neuronal plasticity in LID.

Dopaminergic meso-cortical projections to m1: role in motor learning and motor cortex plasticity

Although the architecture of a dopaminergic (DA) system within the primary motor cortex (M1) was well characterized anatomically, its functional significance remained obscure for a long time. Recent studies in rats revealed that the integrity of DA fibers in M1 is a prerequisite for successful acquisition of motor skills. This essential contribution of DA for motor learning is plausible as it modulates M1 circuitry at multiple levels thereby promoting plastic changes that are required for information storage: at the network level, DA increases cortical excitability and enhances the stability of motor maps. At the cellular level, DA induces the expression of learning-related genes via the transcription factor c-Fos. At the level of synapses, DA is required for the formation of long-term potentiation, a mechanism that likely is a fingerprint of a motor memory trace within M1. DA fibers innervating M1 originate within the midbrain, precisely the ventral tegmental area (VTA) and the medial portion of substantia nigra (SN). Thus, they could be part of the meso-cortico-limbic pathway – a network that provides information about saliency and motivational value of an external stimulus and is commonly referred as “reward system.” However, the behavioral triggers of the release of dopamine in M1 are not yet identified. As alterations in DA transmission within M1 occur under various pathological conditions such as Parkinson disease or ischemic and traumatic brain injury, a deeper understanding of the interaction of VTA/SN and M1 may reveal a deeper insight into a large spectrum of neurological disorders.

The Enemy within: Propagation of Aberrant Corticostriatal Learning to Cortical Function in Parkinson's Disease

Motor dysfunction in Parkinson’s disease is believed to arise primarily from pathophysiology in the dorsal striatum and its related corticostriatal and thalamostriatal circuits during progressive dopamine denervation. One function of these circuits is to provide a filter that selectively facilitates or inhibits cortical activity to optimize cortical processing, making motor responses rapid and efficient. Corticostriatal synaptic plasticity mediates the learning that underlies this performance-optimizing filter. Under dopamine denervation, corticostriatal plasticity is altered, resulting in aberrant learning that induces inappropriate basal ganglia filtering that impedes rather than optimizes cortical processing. Human imaging suggests that increased cortical activity may compensate for striatal dysfunction in PD patients. In this Perspective article, we consider how aberrant learning at corticostriatal synapses may impair cortical processing and learning and undermine potential cortical compensatory mechanisms. Blocking or remediating aberrant corticostriatal plasticity may protect cortical function and support cortical compensatory mechanisms mitigating the functional decline associated with progressive dopamine denervation.

Motor cortical plasticity in Parkinson's disease

In Parkinson's disease (PD), there are alterations of the basal ganglia (BG) thalamocortical networks, primarily due to degeneration of nigrostriatal dopaminergic neurons. These changes in subcortical networks lead to plastic changes in primary motor cortex (M1), which mediates cortical motor output and is a potential target for treatment of PD. Studies investigating the motor cortical plasticity using non-invasive transcranial magnetic stimulation (TMS) have found altered plasticity in PD, but there are inconsistencies among these studies. This is likely because plasticity depends on many factors such as the extent of dopaminergic loss and disease severity, response to dopaminergic replacement therapies, development of l-DOPA-induced dyskinesias (LID), the plasticity protocol used, medication, and stimulation status in patients treated with deep brain stimulation (DBS). The influences of LID and DBS on BG and M1 plasticity have been explored in animal models and in PD patients. In addition, many other factors such age, genetic factors (e.g., brain derived neurotropic factor and other neurotransmitters or receptors polymorphism), emotional state, time of the day, physical fitness have been documented to play role in the extent of plasticity induced by TMS in human studies. In this review, we summarize the studies that investigated M1 plasticity in PD and demonstrate how these afore-mentioned factors affect motor cortical plasticity in PD. We conclude that it is important to consider the clinical, demographic, and technical factors that influence various plasticity protocols while developing these protocols as diagnostic or prognostic tools in PD. We also discuss how the modulation of cortical excitability and the plasticity with these non-invasive brain stimulation techniques facilitate the understanding of the pathophysiology of PD and help design potential therapeutic possibilities in this disorder.

Imaging of Dopamine in PD and Implications for Motor and Neuropsychiatric Manifestations of PD

Parkinson’s disease (PD) is characterized by dopamine depletion in the putamen, which leads to motor dysfunction. As the disease progresses, a substantial degree of dopamine depletion also occurs in caudate and nucleus accumbens. This may explain a number of neuropsychiatric manifestations, including depression, apathy, and cognitive decline. Dopamine replacement therapy partially restores motor function but long-term treatment is often associated with motor complications (motor fluctuations and dyskinesias). Positron emission tomography (PET) studies suggest that the dopamine release rate is substantially higher in PD subjects with motor complications compared to stable responders. Notably, this differential pattern of dopamine release is already present in the early stages of the disease, before motor complications become clinically apparent. Converging evidence suggests that striatal dopamine depletion in PD leads to reduced plasticity in the primary motor cortex and, presumably, in non-motor cortical areas as well. Although dopamine replacement therapy tends to restore physiological plasticity, treatment-induced motor, and neuropsychiatric complications could be related to abnormalities in corticostriatal synaptic plasticity.

Alterations of mean diffusivity in brain white matter and deep gray matter in Parkinson's disease

Although Parkinson's disease is a neurodegenerative disease primarily involving basal ganglia and midbrain, the deficit of white matter is also involved during the disease progression. As the diffusion tensor imaging method is sensitive to the microstructural changes, we investigated the microstructural alterations in white matter and deep gray matter in patients with Parkinson's disease. Brain images of 64 patients and sex- and age-matched 64 healthy controls were obtained from a 3T MRI scanner. Tract-based spatial statistics were used to compare the mean diffusivity of the white matter tract between the groups. Voxel-based analysis was used to compare the mean diffusivity of the subcortical gray matter between the groups. There were white matter deficits in the corticofugal tract, cingulum, uncinate fasciculus, crus of fornix or stria terminalis, corpus callosum, external capsule, superior longitudinal fasciculus, posterior thalamic radiation including optic radiation, and the tracts adjacent to the precuneus and supramarginal gyrus, as indicated by higher mean diffusivity in Parkinson's disease patients than in controls. There were also deficits in the left putamen, pallidum, thalamus, and caudate as indicated by higher mean diffusivity in Parkinson's disease patients than in controls. Using diffusion tensor imaging and multi-methods of image analysis, we successfully characterized and visualized brain white matter and deep gray matter areas with microstructural deficits in Parkinson's disease patients.

Study of cerebello-thalamocortical pathway by transcranial magnetic stimulation in Parkinson's disease

BACKGROUND

Although functional changes in the activation of the cerebellum in Parkinson's disease (PD) patients have been consistently described, it is still debated whether such altered cerebellar activation is a natural consequence of PD pathophysiology or rather it involves compensatory mechanisms.

OBJECTIVE/HYPOTHESIS

We used different forms of cerebellar transcranial magnetic stimulation to evaluate the hypothesis that altered cerebello-cortical interactions can be observed in PD patients and to evaluate the role of dopaminergic treatment.

METHODS

We studied the effects of a single cerebellar magnetic pulse over the excitability of the contralateral primary motor cortex tested with motor-evoked potentials (MEPs) (cerebellar-brain inhibition-CBI) in a group of 16 PD patients with (ON) and without dopaminergic treatment (OFF), and in 16 age-matched healthy controls. Moreover, we also tested the effects of cerebellar continuous theta-burst stimulation (cTBS) on MEP amplitude, short intracortical inhibition (SICI) and short intracortical facilitation (SICF) tested in the contralateral M1 in 13 PD patients in ON and OFF and in 16 age-matched healthy controls.

RESULTS

CBI was evident in controls but not in PD patients, even when tested in both ON and OFF conditions. Similarly, cerebellar cTBS reduced MEP amplitude and SICI in controls but not in PD patients under any condition.

CONCLUSION(S)

These results demonstrate that PD patients have deficient short-latency and long-lasting cerebellar-thalamocortical inhibitory interactions that cannot be promptly restored by standard dopaminergic medication.

Parkinson's disease

There have been a large number of basic research studies of noninvasive brain stimulation in Parkinson's disease. Initial work focused on measuring: (1) the excitability of corticospinal output with threshold and input-output measures, and (2) the effectiveness of intracortical γ-aminobutyric acid (GABA)ergic inhibitory systems using short-interval intracortical inhibition (SICI), long-interval intracortical inhibition (LICI), and silent period measures. Early suggestions of increased excitability and reduced inhibition have been progressively modified. There are conflicting reports on changes in excitability, silent period, and LICI, and the more consistent reduction in SICI is now viewed as a superimposed excitation rather than a primary deficit in a GABAergic mechanism. A small number of studies have suggested that premovement increases in corticospinal excitability may be prolonged in Parkinson's disease, consistent with the suggestion of slower buildup of the motor command to move; there are also modifications of interhemispheric connections in patients with mirror movements. Transcranial magnetic stimulation (TMS) has also been used to explore the involvement of motor cortex and cerebellum in resting and postural tremors by examining how readily they can be reset by single TMS pulses over each area. It can also probe the effects of deep brain stimulation of motor cortex excitability. Finally, new TMS techniques that examine synaptic plasticity in motor cortex have shown reduced excitability in patients off therapy which is restored when on therapy. Data are also emerging about the possible role of cortical plasticity in compensating for gradual loss of dopaminergic function prior to onset of clinical symptoms.

Non-homogeneous effect of levodopa on inhibitory circuits in Parkinson's disease and dyskinesia

INTRODUCTION

Levodopa-induced dyskinesia in patients with Parkinson's disease (PD) has been shown to be associated with an abnormal plasticity in the motor cortex. We investigated whether changes in the excitability of inhibitory and excitatory motor circuits could underlie maladaptive mechanisms associated with dyskinesia.

METHODS

Using single and paired transcranial magnetic stimulation (TMS), we studied motor threshold, silent period (SP) duration, intracortical facilitation (ICF), short intracortical inhibition (SICI) and low- and high-intensity long intracortical inhibition (LICI) in 10 dyskinetic and 10 non-dyskinetic patients, matched for disease and treatment duration, before (OFF state) and after (ON state) levodopa, and in 10 healthy controls.

RESULTS

In the OFF state, the two groups of patients showed similar motor cortex excitability with a reduced SICI compared to controls. LICI was weaker and increasing stimulation intensity had a lower effect on SP duration in dyskinetic patients than in controls. In dyskinetic patients, in contrast to non-dyskinetic patients, levodopa failed to increase SICI and SP duration, and potentiated to a lesser extent the effect of increasing the stimulation intensity on LICI. Although levodopa improved motor symptoms to a similar extent in both dyskinetic and non-dyskinetic patients, it failed to activate effectively the excitability of the inhibitory systems in dyskinetic patients.

DISCUSSION

These findings suggest that dyskinesia is associated with an abnormal effect of levodopa on cortical motor inhibitory circuits.