Motor cortex plasticity can indicate vulnerability to motor fluctuation and high L-DOPA need in drug-naïve Parkinson's disease

Short-term plasticity of the motor cortex compensates for bradykinesia in Parkinson's disease

Patients with Parkinson's disease (PD) show impaired short-term potentiation (STP) mechanisms in the primary motor cortex (M1). However, the role played by this neurophysiological abnormality in bradykinesia pathophysiology is unknown. In this study, we used a multimodal neuromodulation approach to test whether defective STP contributes to bradykinesia. We evaluated STP by measuring motor-evoked potential facilitation during 5 Hz-repetitive transcranial magnetic stimulation (rTMS) and assessed repetitive finger tapping movements through kinematic techniques. Also, we used transcranial alternating current stimulation (tACS) to drive M1 oscillations and experimentally modulate bradykinesia. STP was assessed during tACS delivered at beta (β) and gamma (γ) frequency, and during sham-tACS. Data were compared to those recorded in a group of healthy subjects. In PD, we found that STP was impaired during sham- and γ-tACS, while it was restored during β-tACS. Importantly, the degree of STP impairment was associated with the severity of movement slowness and amplitude reduction. Moreover, β-tACS-related improvements in STP were linked to changes in movement slowness and intracortical GABA-A-ergic inhibition during stimulation, as assessed by short-interval intracortical inhibition (SICI). Patients with prominent STP amelioration had greater SICI reduction (cortical disinhibition) and less slowness worsening during β-tACS. Dopaminergic medications did not modify β-tACS effects. These data demonstrate that abnormal STP processes are involved in bradykinesia pathophysiology and return to normal levels when β oscillations increase. STP changes are likely mediated by modifications in GABA-A-ergic intracortical circuits and may represent a compensatory mechanism against β-induced bradykinesia in PD.

Additive Effect of Dopaminergic Medication on Gait Under Single and Dual-Tasking Is Greater Than of Deep Brain Stimulation in Advanced Parkinson Disease With Long-Duration Deep Brain Stimulation

INTRODUCTION

Patients with advanced Parkinson disease (PD) often experience problems with mobility, including walking under single- (ST) and dual-tasking (DT) conditions. The effects of deep brain stimulation in the subthalamic nucleus (DBS) versus dopaminergic medication (Med) on these conditions are not well investigated.

MATERIALS AND METHODS

We used two ST and two DT-gait paradigms to evaluate the effect of DBS and dopaminergic medication on gait parameters in 14 PD patients (mean age 66 ± 8 years) under DBS_OFF/Med_ON, DBS_ON/Med_OFF, and DBS_ON/Med_ON conditions. They performed standardized 20-meter walks with convenient and fast speed. To test DT capabilities, they performed a checking-boxes and a subtraction task during fast-paced walking. Quantitative gait analysis was performed using a tri-axial accelerometer (Dynaport, McRoberts, The Netherlands). Dual-task costs (DTC) of gait parameters and secondary task performance were compared intraindividually between DBS_OFF/Med_ON vs DBS_ON/Med_ON, and DBS_ON/Med_OFF vs DBS_ON/Med_ON to estimate responsiveness.

RESULTS

Dopaminergic medication increased gait speed and cadence at convenient speed. It increased cadence and decreased number of steps at fast speed, and improved DTC of cadence during the checking boxes and DTC of cadence and number of steps during the subtraction tasks. DBS only improved DTC of cadence during the checking boxes and DTC of gait speed during the subtraction task.

CONCLUSION

Dopaminergic medication showed larger additional effects on temporal gait parameters under ST and DT conditions in advanced PD than DBS. These results, after confirmation in independent studies, should be considered in the medical management of advanced PD patients with gait and DT deficits.

Motor cortical plasticity and its correlation with motor symptoms in Parkinson's disease

Background

The relationship between abnormal cortical plasticity and parkinsonian symptoms remains unclear in Parkinson's disease (PD).

Objective

We studied the relationship between their symptoms and degree of Long-term potentiation (LTP)-like effects induced by quadripulse magnetic stimulation (QPS) over the primary motor cortex, which has a small inter-individual variability in humans.

Methods

Participants were 16 PD patients (drug-naïve or treated with L-DOPA monotherapy) and 13 healthy controls (HC). LTP-like effects by QPS were compared between three conditions (HC､PD with or without L-DOPA). In PD, correlation analyses were performed between clinical scores (MDS-UPDRS, MMSE and MoCA-J) and the degree of LTP-like effects induced by QPS.

Results

In PD, QPS-induced LTP-like effect was reduced and restored by L-DOPA. The degree of the LTP was negatively correlated with MDS-UPDRS Part I and III scores, but not with MMSE and MoCA-J. In the sub-scores, upper limb bradykinesia and rigidity showed a negative correlation with the LTP-like effect whereas the tremor had no correlation.

Conclusions

Our results suggest that motor cortical plasticity relate with mechanisms underlying bradykinesia and rigidity in the upper limb muscles. LTP induced by QPS may be used as an objective marker of parkinsonian symptoms.

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Quadripulse magnetic stimulation (QPS) was applied to early PD patients.

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L-DOPA restored QPS-induced LTP of the primary motor cortex in early PD patients.

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The degree of LTP was negatively correlated with the severity of motor symptoms.

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Upper limb bradykinesia and rigidity had a strong negative correlation with LTP.

Clinical neurophysiology of Parkinson's disease and parkinsonism

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This review is part of the series on the clinical neurophysiology of movement disorders and focuses on Parkinson’s disease and parkinsonism.

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The pathophysiology of cardinal parkinsonian motor symptoms and myoclonus are reviewed.

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The recordings from microelectrode and deep brain stimulation electrodes are reported in detail.

This review is part of the series on the clinical neurophysiology of movement disorders. It focuses on Parkinson’s disease and parkinsonism. The topics covered include the pathophysiology of tremor, rigidity and bradykinesia, balance and gait disturbance and myoclonus in Parkinson’s disease. The use of electroencephalography, electromyography, long latency reflexes, cutaneous silent period, studies of cortical excitability with single and paired transcranial magnetic stimulation, studies of plasticity, intraoperative microelectrode recordings and recording of local field potentials from deep brain stimulation, and electrocorticography are also reviewed. In addition to advancing knowledge of pathophysiology, neurophysiological studies can be useful in refining the diagnosis, localization of surgical targets, and help to develop novel therapies for Parkinson’s disease.

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.

Depotentiation of associative plasticity is intact in Parkinson's disease with mild dyskinesia

OBJECTIVE

Depotentiation of homosynaptic plasticity of the primary motor cortex (M1) is impaired in patients with Parkinson's disease (PD) who have developed dyskinesias. In this exploratory study, we tested whether this holds true for heterosynaptic plasticity induced by paired associative stimulation (PAS).

METHODS

Dyskinetic (n=11) and Non-dyskinetic (n=11), levodopa-treated PD patients were tested in M1 with PAS_25ms alone, PAS_25ms preceded by continuous theta-burst stimulation of the cerebellum (cTBS_CB-PAS) as a method to evoke a larger plastic response in M1, and each of these two interventions followed by a depotentiation protocol (cTBS_150pulses) to M1.

RESULTS

PAS_25ms and cTBS_CB-PAS_25ms induced long-term potentiation (LTP)-like responses in both groups of PD patients, with cTBS_CB significantly boosting the plastic response. Both these LTP-like responses could be depotentiated by cTBS₁₅₀, in both groups of patients.

CONCLUSIONS

Cerebellar stimulation enhances heterosynaptic plasticity in PD irrespective of dyskinesias. Depotentiation mechanisms of heterosynaptic plasticity are preserved in PD patients, including those with dyskinesias. The lack of depotentiation of LTP-like plasticity as a hallmark of dyskinesia in PD patients is not absolute. The ability to depotentiate LTP-like plasticity may potentially depend on the type of plasticity induced (homosynaptic or heterosynaptic), the circuits involved in these responses and the adequacy of dopaminergic stimulation.

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.

Evolving concepts on bradykinesia

Bradykinesia is one of the cardinal motor symptoms of Parkinson's disease and other parkinsonisms. The various clinical aspects related to bradykinesia and the pathophysiological mechanisms underlying bradykinesia are, however, still unclear. In this article, we review clinical and experimental studies on bradykinesia performed in patients with Parkinson's disease and atypical parkinsonism. We also review studies on animal experiments dealing with pathophysiological aspects of the parkinsonian state. In Parkinson's disease, bradykinesia is characterized by slowness, the reduced amplitude of movement, and sequence effect. These features are also present in atypical parkinsonisms, but the sequence effect is not common. Levodopa therapy improves bradykinesia, but treatment variably affects the bradykinesia features and does not significantly modify the sequence effect. Findings from animal and patients demonstrate the role of the basal ganglia and other interconnected structures, such as the primary motor cortex and cerebellum, as well as the contribution of abnormal sensorimotor processing. Bradykinesia should be interpreted as arising from network dysfunction. A better understanding of bradykinesia pathophysiology will serve as the new starting point for clinical and experimental purposes.

Abnormal cerebellar processing of the neck proprioceptive information drives dysfunctions in cervical dystonia

The cerebellum can influence the responsiveness of the primary motor cortex (M1) to undergo spike timing-dependent plastic changes through a complex mechanism involving multiple relays in the cerebello-thalamo-cortical pathway. Previous TMS studies showed that cerebellar cortex excitation can block the increase in M1 excitability induced by a paired-associative stimulation (PAS), while cerebellar cortex inhibition would enhance it. Since cerebellum is known to be affected in many types of dystonia, this bidirectional modulation was assessed in 22 patients with cervical dystonia and 23 healthy controls. Exactly opposite effects were found in patients: cerebellar inhibition suppressed the effects of PAS, while cerebellar excitation enhanced them. Another experiment comparing healthy subjects maintaining the head straight with subjects maintaining the head turned as the patients found that turning the head is enough to invert the cerebellar modulation of M1 plasticity. A third control experiment in healthy subjects showed that proprioceptive perturbation of the sterno-cleido-mastoid muscle had the same effects as turning the head. We discuss these finding in the light of the recent model of a mesencephalic head integrator. We also suggest that abnormal cerebellar processing of the neck proprioceptive information drives dysfunctions of the integrator in cervical dystonia.