Spike-timing-related plasticity is preserved in Parkinson's disease and is enhanced by dopamine: Evidence from transcranial magnetic stimulation

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.

On the Use of TMS to Investigate the Pathophysiology of Neurodegenerative Diseases

Neurodegenerative diseases are a collection of disorders that result in the progressive degeneration and death of neurons. They are clinically heterogenous and can present as deficits in movement, cognition, executive function, memory, visuospatial awareness and language. Transcranial magnetic stimulation (TMS) is a non-invasive brain stimulation tool that allows for the assessment of cortical function in vivo. We review how TMS has been used for the investigation of three neurodegenerative diseases that differ in their neuroanatomical axes: (1) Motor cortex-corticospinal tract (motor neuron diseases), (2) Non-motor cortical areas (dementias), and (3) Subcortical structures (parkinsonisms). We also make four recommendations that we hope will benefit the use of TMS in neurodegenerative diseases. Firstly, TMS has traditionally been limited by the lack of an objective output and so has been confined to stimulation of the motor cortex; this limitation can be overcome by the use of concurrent neuroimaging methods such as EEG. Given that neurodegenerative diseases progress over time, TMS measures should aim to track longitudinal changes, especially when the aim of the study is to look at disease progression and symptomatology. The lack of gold-standard diagnostic confirmation undermines the validity of findings in clinical populations. Consequently, diagnostic certainty should be maximized through a variety of methods including multiple, independent clinical assessments, imaging and fluids biomarkers, and post-mortem pathological confirmation where possible. There is great interest in understanding the mechanisms by which symptoms arise in neurodegenerative disorders. However, TMS assessments in patients are usually carried out during resting conditions, when the brain network engaged during these symptoms is not expressed. Rather, a context-appropriate form of TMS would be more suitable in probing the physiology driving clinical symptoms. In all, we hope that the recommendations made here will help to further understand the pathophysiology of neurodegenerative diseases.

Comparing kinematic changes between a finger-tapping task and unconstrained finger flexion-extension task in patients with Parkinson's disease

Repetitive finger tapping is a well-established clinical test for the evaluation of parkinsonian bradykinesia, but few studies have investigated other finger movement modalities. We compared the kinematic changes (movement rate and amplitude) and response to levodopa during a conventional index finger-thumb-tapping task and an unconstrained index finger flexion-extension task performed at maximal voluntary rate (MVR) for 20 s in 11 individuals with levodopa-responsive Parkinson's disease (OFF and ON) and 10 healthy age-matched controls. Between-task comparisons showed that for all conditions, the initial movement rate was greater for the unconstrained flexion-extension task than the tapping task. Movement rate in the OFF state was slower than in controls for both tasks and normalized in the ON state. The movement amplitude was also reduced for both tasks in OFF and increased in the ON state but did not reach control levels. The rate and amplitude of movement declined significantly for both tasks under all conditions (OFF/ON and controls). The time course of rate decline was comparable for both tasks and was similar in OFF/ON and controls, whereas the tapping task was associated with a greater decline in MA, both in controls and ON, but not OFF. The findings indicate that both finger movement tasks show similar kinematic changes during a 20-s sustained MVR, but that movement amplitude is less well sustained during the tapping task than the unconstrained finger movement task. Both movement rate and amplitude improved with levodopa; however, movement rate was more levodopa responsive than amplitude.

Safety of transcranial magnetic stimulation in Parkinson's disease: a review of the literature

BACKGROUND

Transcranial magnetic stimulation (TMS) has been used in both physiological studies and, more recently, the therapy of Parkinson's disease (PD). Prior TMS studies in healthy subjects and other patient populations demonstrate a slight risk of seizures and other adverse events. Our goal was to estimate these risks and document other safety concerns specific to PD patients.

METHODS

We performed an English-Language literature search through PudMed to review all TMS studies involving PD patients. We documented any seizures or other adverse events associated with these studies. Crude risks were calculated per subject and per session of TMS.

RESULTS

We identified 84 single pulse (spTMS) and/or paired-pulse (ppTMS) TMS studies involving 1091 patients and 77 repetitive TMS (rTMS) studies involving 1137 patients. Risk of adverse events was low in all protocols. spTMS and ppTMS risk per patient for any adverse event was 0.0018 (95% CI: 0.0002-0.0066) per patient and no seizures were encountered. Risk of an adverse event from rTMS was 0.040 (95% CI: 0.029-0.053) per patient and no seizures were reported. Other adverse events included transient headaches, scalp pain, tinnitus, nausea, increase in pre-existing pain, and muscle jerks. Transient worsening of Parkinsonian symptoms was noted in one study involving rTMS of the supplementary motor area (SMA).

CONCLUSION

We conclude that current TMS and rTMS protocols do not pose significant risks to PD patients. We would recommend that TMS users in this population follow the most recent safety guidelines but do not warrant additional precautions.

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.

Evidence for high-fidelity timing-dependent synaptic plasticity of human motor cortex

A single transcranial magnetic stimulation (TMS) pulse typically evokes a short series of spikes in corticospinal neurons [known as indirect (I)-waves] which are thought to arise from transynaptic input. Delivering a second pulse at inter-pulse intervals (IPIs) corresponding to the timing of these I-waves leads to a facilitation of the response, and if stimulus pairs are delivered repeatedly, a persistent LTP-like increase in excitability can occur. This has been demonstrated at an IPI of 1.5 ms, which corresponds to the first I-wave interval, in an intervention referred to as ITMS (I-wave TMS), and it has been argued that this may have similarities with timing-dependent plasticity models. Consequently, we hypothesized that if the second stimulus is delivered so as not to coincide with I-wave timing, it should lead to LTD. We performed a crossover study in 10 subjects in which TMS doublets were timed to coincide (1.5-ms IPI, ITMS1.5) or not coincide (2-ms IPI, ITMS2) with I-wave firing. Single pulse motor-evoked potential (MEP) amplitude, resting motor threshold (RMT), and short-interval cortical inhibition (SICI) were measured from the first dorsal interosseous (FDI) muscle. After ITMS1.5 corticomotor excitability was increased by ∼60% for 15 min ( P < 0.05) and returned to baseline by 20 min. Increasing the IPI by just 500 μs to 2 ms reversed the aftereffect, and MEP amplitude was significantly reduced (∼35%, P < 0.05) for 15 min before returning to baseline. This reduction was not associated with an increase in SICI, suggesting a reduction in excitatory transmission rather than an increase in inhibitory efficacy. RMT also remained unchanged, suggesting that these changes were not due to changes in membrane excitability. Amplitude-matching ITMS2 did not modulate excitability. The results are consistent with timing-dependent synaptic LTP/D-like effects and suggest that there are plasticity mechanisms operating in the human motor cortex with a temporal resolution of the order of a few hundreds of microseconds.

Momentary reward induce changes in excitability of primary motor cortex

OBJECTIVE

To investigate the human primary motor cortex (M1) excitability changes induced by momentary reward.

METHODS

To test the changes in excitatory and inhibitory functions of M1, motor-evoked potentials (MEPs), short-interval intracortical inhibition (SICI) and short-latency afferent inhibition (SAI) were tested in the abductor pollicis brevis (APB) muscle of non-dominant hand in 14 healthy volunteers by transcranial magnetic stimulation (TMS) during a behavioral task in which subjects were pseudorandomly received either reward target or non-target stimuli in response to a cue. To control sensorimotor and attention effects, a sensorimotor control task was done replacing the reward target with non-reward target.

RESULTS

The SICI was increased, and the SAI was decreased significantly during the presentation of the reward target stimuli. Those changes were not evident during non-reward target stimuli in the sensorimotor control task, indicating that this change is specific to momentary reward.

CONCLUSIONS

Momentary rewarding is associated with change in intracortical inhibitory circuits of M1.

SIGNIFICANCE

TMS may be a useful probe to study the reward system in health and in many diseases in which its dysfunction is suspected.

Dopamine influences primary motor cortex plasticity and dorsal premotor-to-motor connectivity in Parkinson's disease

We investigated abnormal premotor to motor (PMd-to-M1) connectivity in Parkinson's disease (PD) with repetitive transcranial magnetic stimulation (rTMS). We studied 28 patients off and on dopaminergic therapy and 28 healthy subjects. We delivered 5 Hz rTMS over M1 before and after conditioning PMd with 5 Hz rTMS. In healthy subjects, motor-evoked potentials (MEPs) elicited by M1-rTMS were facilitated and PMd-rTMS left MEPs unchanged. In patients, before PMd-rTMS, M1-rTMS induced no MEP facilitation, whereas after PMd-rTMS, it significantly facilitated MEPs only when patients were on therapy. In the second experiment, we delivered M1-rTMS under 3 different attention-demanding tasks: eyes closed, attention directed to the stimulated hand, and attention directed to the nonstimulated hand. In healthy subjects, a more pronounced MEP facilitation was present when subjects directed attention to the stimulated hand. In patients, the MEP facilitation was present when attention was directed to the stimulated hand only when patients were on therapy. Finally, we delivered M1-rTMS in patients on therapy while they were looking at the stimulated hand, before and after 1 Hz PMd-rTMS. PMd-rTMS reduced the attention-induced MEP facilitation. We conclude that in addition to abnormal M1 plasticity, the reduced MEP facilitation in PD also reflects altered PMd-to-M1 connectivity.

Maladaptive striatal plasticity in L-DOPA-induced dyskinesia

Dopamine (DA) replacement therapy with l-DOPA remains the most effective treatment for Parkinson's disease, but causes dyskinesia (abnormal involuntary movements) in the vast majority of the patients. The basic mechanisms of l-DOPA-induced dyskinesia (LID) have become the object of intense research focusing on neurochemical and molecular adaptations in the striatum. Here we review this vast literature and highlight trends that converge into a unifying pathophysiological interpretation. We propose that the core molecular alteration of striatal neurons in LID consists in an inability to turn down supersensitive signaling responses downstream of DA D1 receptors (where supersensitivity is primarily caused by DA denervation). The sustained activation of intracellular signaling pathways induced by each dose of l-DOPA leads to abnormal cellular plasticity and high bioenergetic expenditure. The over-exploitation of signaling pathways and energy reserves during treatment impairs the ability of striatal neurons to dynamically gate cortically driven motor commands. LID thus exemplifies a disorder where 'too much' molecular plasticity leads to plasticity failure in the striatum.