Fluoxetine facilitates use-dependent excitability of human primary motor cortex

Serotonergic and noradrenergic contributions to motor cortical and spinal motoneuronal excitability in humans

Animal models indicate that motor behaviour is shaped by monoamine neuromodulators released diffusely throughout the brain and spinal cord. As an alternative to conducting a single study to explore the effects of neuromodulators on the human motor system, we have identified and collated human experiments investigating motor effects of well-characterised drugs that act on serotonergic and noradrenergic networks. In doing so, we present strong neuropharmacology evidence that human motor pathways are affected by neuromodulators across both healthy and clinical populations, insight that cannot be determined from a single reductionist experiment. We have focused our review on the effects that monoaminergic drugs have on muscle responses to non-invasive stimulation of the motor cortex and peripheral nerves, and other closely related tests of motoneuron excitability, and discuss how these measurement techniques elucidate the effects of neuromodulators at motor cortical and spinal motoneuronal levels. Although there is some heterogeneity in study methods, we find drugs acting to enhance extracellular concentrations of serotonin tend to reduce the excitability of the human motor cortex, and enhanced extracellular concentrations of noradrenaline increases motor cortical excitability by enhancing intracortical facilitation and reducing inhibition. Both monoamines tend to enhance the excitability of spinal motoneurons. Overall, this review details the importance of neuromodulators for the output of human motor pathways and suggests that commonly prescribed monoaminergic drugs target the motor system in addition to their typical psychiatric/neurological indications.

Voluntary activation of muscle in humans: does serotonergic neuromodulation matter?

Ionotropic inputs to motoneurones have the capacity to depolarise and hyperpolarise the motoneurone, whereas neuromodulatory inputs control the state of excitability of the motoneurone. Intracellular recordings of motoneurones from in vitro and in situ animal preparations have provided extraordinary insight into the mechanisms that underpin how neuromodulators regulate neuronal excitability. However, far fewer studies have attempted to translate the findings from cellular and molecular studies into a human model. In this review, we focus on the role that serotonin (5-HT) plays in muscle activation in humans. 5-HT is a potent regulator of neuronal firing rates, which can influence the force that can be generated by muscles during voluntary contractions. We firstly outline structural and functional characteristics of the serotonergic system, and then describe how motoneurone discharge can be facilitated and suppressed depending on the 5-HT receptor subtype that is activated. We then provide a narrative on how 5-HT effects can influence voluntary activation during muscle contractions in humans, and detail how 5-HT may be a mediator of exercise-induced fatigue that arises from the central nervous system.

Serotonergic Facilitation of Forelimb Functional Recovery in Rats with Cervical Spinal Cord Injury

Serotonergic agents can improve the recovery of motor ability after a spinal cord injury. Herein, we compare the effects of buspirone, a 5-HT1A receptor partial agonist, to fluoxetine, a selective serotonin reuptake inhibitor, on forelimb motor function recovery after a C4 bilateral dorsal funiculi crush in adult female rats. After injury, single pellet reaching performance and forelimb muscle activity decreased in all rats. From 1 to 6 weeks after injury, rats were tested on these tasks with and without buspirone (1-2 mg/kg) or fluoxetine (1-5 mg/kg). Reaching and grasping success rates of buspirone-treated rats improved rapidly within 2 weeks after injury and plateaued over the next 4 weeks of testing. Electromyography (EMG) from selected muscles in the dominant forelimb showed that buspirone-treated animals used new reaching strategies to achieve success after the injury. However, forelimb performance dramatically decreased within 2 weeks of buspirone withdrawal. In contrast, fluoxetine treatment resulted in a more progressive rate of improvement in forelimb performance over 8 weeks after injury. Neither buspirone nor fluoxetine significantly improved quadrupedal locomotion on the horizontal ladder test. The improved accuracy of reaching and grasping, patterns of muscle activity, and increased excitability of spinal motor-evoked potentials after buspirone administration reflect extensive reorganization of connectivity within and between supraspinal and spinal sensory-motor netxcopy works. Thus, both serotonergic drugs, buspirone and fluoxetine, neuromodulated these networks to physiological states that enabled markedly improved forelimb function after cervical spinal cord injury.

Selective serotonin reuptake inhibitors for functional independence and depression prevention in early stage of post-stroke: A meta-analysis

BACKGROUND

The efficacy and safety of selective serotonin reuptake inhibitors (SSRIs) for functional independence and depression prevention in early stage of post-stroke (within 1 month after stroke onset) are still unclear.

METHODS

Relevant randomized controlled trials (RCTs) comparing early SSRIs therapy with placebo were sought from PubMed, Cochrane Library, Medline, and Embase. Primary outcomes were functional independence and depression occurrence. Secondary outcomes contained the improvement of Fugl-Meyer motor scale (FMMS) score and adverse events. We used fixed or random effects model to pooled effect estimates. And we chose risk ratio (RR) or mean differences (MDs) with the 95% confidence intervals (CIs) for data analysis.

RESULTS

We included 10 RCTs with total 5370 patients. The outcome of functional independence showed no significant difference between SSRIs and placebo group (RR, 1.28; 95% CI, 0.96-1.72; P = .10; I = 92%). However, depression occurrence differed significantly between these 2 groups, which favored SSRIs group (RR, 0.78; 95% CI, 0.67-0.90; P = .001; I = 23%). In addition, we observed that the side effects of SSRIs were seizure and nausea. Except psychiatric disorders/insanity rate was less in SSRIs group than placebo group (RR, 0.66; 95% CI, 0.48-0.90; P = .009) (I = 0%), other adverse events were revealed non-significant in our meta-analysis.

CONCLUSIONS

Our meta-analysis revealed that early SSRIs therapy were effective to prevent post-stroke depression. However, SSRIs did not improve patient's post-stroke functional independence. In addition to increase the occurrence of seizure and nausea, SSRIs were relatively safe.

Mapping the physiological and molecular markers of stress and SSRI antidepressant treatment in S100a10 corticostriatal neurons

In mood disorders, psychomotor and sensory abnormalities are prevalent, disabling, and intertwined with emotional and cognitive symptoms. Corticostriatal neurons in motor and somatosensory cortex are implicated in these symptoms, yet mechanisms of their vulnerability are unknown. Here, we demonstrate that S100a10 corticostriatal neurons exhibit distinct serotonin responses and have increased excitability, compared with S100a10-negative neurons. We reveal that prolonged social isolation disrupts the specific serotonin response which gets restored by chronic antidepressant treatment. We identify cell-type-specific transcriptional signatures in S100a10 neurons that contribute to serotonin responses and strongly associate with psychomotor and somatosensory function. Our studies provide a strong framework to understand the pathogenesis and create new avenues for the treatment of mood disorders.

The effect of psychotropic drugs on cortical excitability and plasticity measured with transcranial magnetic stimulation: Implications for psychiatric treatment

OBJECTIVE

Repetitive transcranial magnetic stimulation (rTMS) is an emerging treatment for neuropsychiatric disorders. Patients in rTMS treatment typically receive concomitant psychotropic medications, which affect neuronal excitability and plasticity and may interact to affect rTMS treatment outcomes. A greater understanding of these drug effects may have considerable implications for optimizing multi-modal treatment of psychiatric patients, and elucidating the mechanism(s) of action (MOA) of rTMS.

METHOD

We summarized the empirical literature that tests how psychotropic drugs affect cortical excitability and plasticity, using varied experimental TMS paradigms.

RESULTS

Glutamate antagonists robustly attenuate plasticity, largely without changes in excitability per se; antiepileptic drugs show the opposite pattern of effects, while calcium channel blockers attenuate plasticity. Benzodiazepines have moderate and variable effects on plasticity, and negligible effects on excitability. Antidepressants with potent 5HT transporter inhibition reduce both excitability and alter plasticity, while antidepressants with other MOAs generally lack either effect. Catecholaminergic drugs, cholinergic agents and lithium have minimal effects on excitability but exhibit robust and complex, non-linear effects in TMS plasticity paradigms.

LIMITATIONS

These effects remain largely untested in sustained treatment protocols, nor in clinical populations. In addition, how these medications impact clinical response to rTMS remains largely unknown.

CONCLUSIONS

Psychotropic medications exert robust and varied effects on cortical excitability and plasticity. We encourage the field to more directly and fully investigate clinical pharmaco-TMS studies to improve outcomes.

The effects of a single dose of fluoxetine on practice-dependent plasticity

OBJECTIVE

To determine whether a single dose of fluoxetine increases corticomotoneuronal excitability, motor performance and practice-dependent plasticity.

METHODS

Twelve healthy adults completed this placebo-controlled, pseudo-randomized, double-blind crossover study. Transcranial magnetic stimulation (TMS) was used to assess corticomotoneuronal excitability, and two uni-axial accelerometers measured kinetics of fastest possible ballistic voluntary thumb movements and TMS-evoked thumb movements. Six hours after administration of either 20 mg of the serotonin reuptake inhibitor fluoxetine or placebo, participants practiced ballistic thumb movements in the direction opposite to the TMS-evoked thumb movements. The primary outcome of this study was the proportion of thumb movements that fell within the target-training zone (TTZ) during and for 30 min after the practice.

RESULTS

All participants demonstrated practice-dependent plasticity evidenced by an increase of TMS-evoked thumb movements falling into the TTZ (P = 0.045), with no difference between drugs. There was a significant increase in peak acceleration of the practiced voluntary thumb movements (P = 0.002), but no DRUG by TIME interaction. Motor-evoked potential amplitudes were not changed by drug intake or practice.

CONCLUSIONS

A single dose of 20 mg of fluoxetine did not enhance corticomotoneuronal excitability, performance of a ballistic thumb movement task, or practice-dependent plasticity in healthy adults.

SIGNIFICANCE

Longer administration fluoxetine may be necessary to enhance motor performance and plasticity.

Early Selective Serotonin Reuptake Inhibitors for Recovery after Stroke: A Meta-Analysis and Trial Sequential Analysis

BACKGROUND

Potential benefits and risks of early (≤30 days from stroke onset) selective serotonin reuptake inhibitors (SSRIs) treatment for neurologic functional recovery after stroke are not fully understood.

METHODS

We searched PubMed, Embase, and the Cochrane Library to identify randomized controlled trials that assessed SSRI medications during the initial ictus after stroke versus placebo. Primary outcome was decrease in National Institutes of Health Stroke Scale (NIHSS) score. Secondary outcomes included the improvement of Barthel index, functional independence (modified Rankin Scale score 0-2 at the end of follow-up), the incidence of depression, and adverse events including diarrhea, insomnia, hepatic enzyme disorders, seizure, and intracranial hemorrhage. We used fixed effects models or random effects models to estimate weighted mean differences (WMDs) and relative risks (RRs) with 95% confidence intervals (CIs) according to heterogeneity.

RESULTS

Eight trials were included, with 1549 patients. Compared with placebo, decrease in NIHSS was greater in SSRI-treated patients (WMD, 0.82; 95% CI, 0.31-1.33; P = .002). Trial sequential analysis showed that the cumulative z curve crossed the trial sequential monitoring boundary for benefit, establishing sufficient and conclusive evidence. Early SSRI treatment also promoted Barthel index (WMD, 5.32; 95% CI, 1.65-8.99; P = .005) and functional independence (RR, 2.54; 95% CI, 1.82-3.55; P < .0001). There was no difference in the incidence of depression and adverse events between groups. No evidence of publication bias was detected.

CONCLUSIONS

The early SSRIs treatment reduces the defective neurologic function in patients undergoing rehabilitation after stroke.

Enhancement of motor coordination by applying high frequency repetitive TMS on the sensory cortex

The sensory function plays an important role for successful motor performance. We investigated the modulating effects of high frequency repetitive transcranial magnetic stimulation (rTMS) on sensory discrimination and motor coordination. Twenty healthy participants were assigned into two random groups; the real- and sham-rTMS group. Total of 900 rTMS pulses at a frequency of 10Hz (stimulus intensity of 90% RMT) were given over deltoid representational areas of the somatosensory cortex. Sensory discrimination ability was evaluated using two-point discrimination test. Motor coordination was measured by the latency difference between the synchronized contraction of deltoid and abductor pollicis brevis muscles before and after rTMS. The sensory discrimination was significantly increased only in the deltoid area and the difference in the latency of synchronized contraction of two muscles was significantly shortened after real-rTMS compared sham condition, which had tendency of negative correlation following real-rTMS condition. The results of this study demonstrated rTMS-induced enhancement of sensorimotor integration, which may contribute to develop effective therapeutic strategies for rehabilitation of various sensorimotor disorders in the clinical setting.

Vagus nerve stimulation delivered during motor rehabilitation improves recovery in a rat model of stroke

Neural plasticity is widely believed to support functional recovery following brain damage. Vagus nerve stimulation paired with different forelimb movements causes long-lasting map plasticity in rat primary motor cortex that is specific to the paired movement. We tested the hypothesis that repeatedly pairing vagus nerve stimulation with upper forelimb movements would improve recovery of motor function in a rat model of stroke. Rats were separated into 3 groups: vagus nerve stimulation during rehabilitation (rehab), vagus nerve stimulation after rehab, and rehab alone. Animals underwent 4 training stages: shaping (motor skill learning), prelesion training, postlesion training, and therapeutic training. Rats were given a unilateral ischemic lesion within motor cortex and implanted with a left vagus nerve cuff. Animals were allowed 1 week of recovery before postlesion baseline training. During the therapeutic training stage, rats received vagus nerve stimulation paired with each successful trial. All 17 trained rats demonstrated significant contralateral forelimb impairment when performing a bradykinesia assessment task. Forelimb function was recovered completely to prelesion levels when vagus nerve stimulation was delivered during rehab training. Alternatively, intensive rehab training alone (without stimulation) failed to restore function to prelesion levels. Delivering the same amount of stimulation after rehab training did not yield improvements compared with rehab alone. These results demonstrate that vagus nerve stimulation repeatedly paired with successful forelimb movements can improve recovery after motor cortex ischemia and may be a viable option for stroke rehabilitation.

Drug-induced changes in cortical inhibition in medication overuse headache

BACKGROUND

We investigated whether chronic headache related to medication overuse (MOH) is associated with changes in brain mechanisms regulating inhibitory cortical responses compared with healthy volunteers and episodic migraineurs recorded between attacks, and whether these changes differ according to the drug overused.

SUBJECTS AND METHODS

We studied 40 MOH patients whose symptoms were related to triptans alone, non-steroidal anti-inflammatory drugs (NSAIDs) or both medications combined, 12 migraineurs and 13 healthy volunteers. We used high-intensity transcranial magnetic stimulation over the primary motor cortex to assess the silent period from contracted perioral muscles.

RESULTS

In MOH patients the cortical silent period differed according to the type of headache medication overused: in patients overusing triptans alone it was shorter than in healthy volunteers (44.7 ± 14.2 vs. 108.1 ± 30.1 ms), but similar to that reported in migraineurs (59.9 ± 30.4 ms), whereas in patients overusing NSAIDs alone or triptans and NSAIDs combined duration of silent period was within normal limits (80.6 ± 46.4 and 103.8 ± 47.2 ms).

CONCLUSIONS

Compared with episodic migraineurs, MOH patients overusing triptans have no significant change in cortical inhibition, whereas those overusing NSAIDs have an increase in cortical inhibitory mechanisms. We attribute these changes to medication-induced neural adaptation promoted by changes in central serotonin neurotransmission.

Assessment and modulation of neural plasticity in rehabilitation with transcranial magnetic stimulation

Despite intensive efforts to improve outcomes after acquired brain injury, functional recovery is often limited. One reason for this limitation is the challenge in assessing and guiding plasticity after brain injury. In this context, transcranial magnetic stimulation (TMS), a noninvasive tool of brain stimulation, could play a major role. TMS has been shown to be a reliable tool for measuring plastic changes in the motor cortex associated with interventions in the motor system, such as motor training and motor cortex stimulation. In addition, as illustrated by the experience in promoting recovery from stroke, TMS is a promising therapeutic tool to minimize motor, speech, cognitive, and mood deficits. In this review, we will focus on stroke to discuss how TMS can provide insights into the mechanisms of neurologic recovery and how it can be used for measurement and modulation of plasticity after an acquired brain insult.

Dopamine in motor cortex is necessary for skill learning and synaptic plasticity

Preliminary evidence indicates that dopamine given by mouth facilitates the learning of motor skills and improves the recovery of movement after stroke. The mechanism of these phenomena is unknown. Here, we describe a mechanism by demonstrating in rat that dopaminergic terminals and receptors in primary motor cortex (M1) enable motor skill learning and enhance M1 synaptic plasticity. Elimination of dopaminergic terminals in M1 specifically impaired motor skill acquisition, which was restored upon DA substitution. Execution of a previously acquired skill was unaffected. Reversible blockade of M1 D1 and D2 receptors temporarily impaired skill acquisition but not execution, and reduced long-term potentiation (LTP) within M1, a form of synaptic plasticity critically involved in skill learning. These findings identify a behavioral and functional role of dopaminergic signaling in M1. DA in M1 optimizes the learning of a novel motor skill.

Chronic dose effects of reboxetine on motor skill acquisition and cortical excitability

BACKGROUND

Enhancement of cortical excitability is thought to be beneficial for synaptic plasticity associated with motor skill acquisition. Single dose application of the selective norepinephrine reuptake inhibitor reboxetine (RBX) increases motor cortex excitability. In this study, we tested if a chronic dose application of RBX improved motor skill acquisition and modulated cortical excitability.

METHODS

The study was randomised, double blind and placebo-controlled. Twelve healthy subjects received four milligram RBX twice a day for four days preceded by two milligram RBX twice a day for two days. Each subject served as his own control. The time interval between the verum and the placebo session was 16 days or more. Measurement of cortical excitability by means of paired pulse transcranial magnetic stimulation (ppTMS) was conducted before and after the motor skill acquisition task in each session. The task was to lift two fingers of the right hand at once while the hand was positioned sprawled out on the table. The movements were self-paced and subjects had to perform as many moves as possible in 60 sec. Between seven blocks of self-paced movements six blocks with 60 single trials at a fixed interstimulus intervall were presented. Two equally difficult versions of the task using different finger combinations were established in order to avoid carry over effects in performance. The finger movements were recorded with a three-dimensional ultrasound movement analysis system (Zebris).

RESULTS

All subjects had substantial gain in performance across the selfpaced blocks. Average increase in number of correct moves was 87% (from 27.8 to 51.9). There was no significant difference neither between the versions of the task nor between placebo vs. verum. Also, there was no significant difference between first and second session, indicating that there was no carry over effect in performance. ppTMS revealed no significant differences in cortical excitability between groups.

CONCLUSION

The newly developed skill acquisition task yields robust single subject gain of performance. As the two versions of the task do not interact, it is suitable to be used in cross-over designs. In contrast to studies using single doses of RBX, motor cortex excitability seems to be unaffected in a steady-state induced by repeated drug applications. This could explain why RBX did not modulate motor behavior.

Pharmacological modulation of plasticity in the human motor cortex

Ischemic cerebral stroke is the leading cause of long-term disability among adults in industrialized countries. One fundamental but still not sufficiently solved question is how to improve disability after stroke. Here, evidence will be reviewed on how pharmacological treatment modulates plasticity and learning in the intact human motor cortex. It will be argued that these data may be useful for advancing the concepts of pharmacotherapy for recovery after stroke.

Rehabilitation of the Older Adult with Stroke

Stroke is an increasing public health concern throughout the world as the leading cause of long-term disability. It is well known that there exist differences related to epidemiology, pathophysiology, comorbidity, and functional outcome of stroke patients with advanced age compared with the young. Factors that have been suggested to influence this disparity include age-related complications, availability of resources, lack of aggressive management, and possible diminished capacity for neuroplasticity. This article reviews the current medical and rehabilitative aspects of stroke and the possible disparities related to advanced age.

Atomoxetine Enhances a Short-Term Model of Plasticity in Humans

OBJECTIVE

To evaluate the role of 2 noradrenergic drugs in modulating use-dependent plasticity in humans.

DESIGN

Double-blind, randomized, and placebo-controlled crossover design.

SETTING

A laboratory in a hospital.

PARTICIPANTS

A convenience sample of 10 healthy subjects.

INTERVENTION

An established paradigm that measures motor memory as a short-term model of use-dependent plasticity. Subjects attended 3 sessions, separated by at least 1 week to allow drug washout. Subjects received atomoxetine (Strattera), venlafaxine (Effexor), or placebo.

MAIN OUTCOME MEASURE

Increase in the proportion of movements into the training target zone (TTZ), an indicator of enhanced plasticity.

RESULTS

Atomoxetine, but not venlafaxine, significantly increased movements into the TTZ.

CONCLUSIONS

These results support a role for norepinephrine in enhancing cortical plasticity and suggest potential benefits in using these drugs for improving motor recovery after stroke.

Chronic administration of selective serotonin reuptake inhibitor (SSRI) paroxetine modulates human motor cortex excitability in healthy subjects

The aim of the study was to investigate the effect of chronic administration of paroxetine (selective serotonin reuptake inhibitor: SSRI) on motor cortex excitability in healthy subjects by means of transcranial magnetic stimulation (TMS), functional magnetic resonance imaging (fMRI) and behavioral motor tests. In a randomized, double-blind, crossover study, twenty-one right-handed subjects received 20 mg daily of either paroxetine or a placebo over a period of 30 days separated by a period of 3 months wash-out. The TMS study is presented here correlated with some results of the motor behavior study (finger tapping test) and the fMRI study (primary sensorimotor cortex (S1M1) volume of activation). TMS was used to test motor threshold (MT), motor evoked potential recruitment curve (RC), cortical silent period (CSP) and paired-pulse intracortical inhibition and facilitation (ICI, ICF). Chronic administration of paroxetine did not modulate ICI or CSP but induced a significant enhancement of mean ICF (ANOVA P=0.04), which significantly correlated with increase of speed in a finger tapping test (P=0.02). This suggests a modulation of cortical interneuronal excitatory pathways without changes in the excitability of cortical inhibitory GABAergic interneurons. A decrease of RC (ANOVA P=0.05) was also observed after 30 days intake of paroxetine in comparison with placebo and was associated with changes of fMRI activation intensity (left S1M1 hypoactivation, ), without changes of S1M1 activation volume. Finally, the different modulation of RC and ICF after chronic administration of paroxetine compared to single dose (opposite effects) emphasizes the different pharmacological action of the drug at cortical level depending on its acute or long-term administration.

The NMDA antagonist memantine affects training induced motor cortex plasticity--a study using transcranial magnetic stimulation

BACKGROUND

Training of a repetitive synchronised movement of two limb muscles leads to short-term plastic changes in the primary motor cortex, which can be assessed by transcranial magnetic stimulation (TMS) mapping. We used this paradigm to study the effect of memantine, a NDMA antagonist, on short-term motor cortex plasticity in 20 healthy human subjects, and we were especially interested in possible differential effects of different treatment regimens. In a randomised double-blinded cross over study design we therefore administered placebo or memantine either as a single dosage or as an ascending dosage over 8 days. Before and after one hour of motor training, which consisted of a repetitive co-contraction of the abductor pollicis brevis (APB) and the deltoid muscle, we assessed the motor output map of the APB muscle by TMS under the different conditions.

RESULTS

We found a significant medial shift of the APB motor output map after training in the placebo condition, indicating training-induced short-term plastic changes in the motor cortex. A single dosage of memantine had no significant effect on this training-induced plasticity, whereas memantine administered in an ascending dosage over 8 days was able to block the cortical effect of the motor training. The memantine serum levels after 8 days were markedly higher than the serum levels after a single dosage of memantine, but there was no individual correlation between the shift of the motor output map and the memantine serum level. Besides, repeated administration of a low memantine dosage also led to an effective blockade of training-induced cortical plasticity in spite of serum levels comparable to those reached after single dose administration, suggesting that the repeated administration was more important for the blocking effect than the memantine serum levels.

CONCLUSION

We conclude that the NMDA-antagonist memantine is able to block training-induced motor cortex plasticity when administered over 8 days, but not after administration of a single dose. This differential effect might be mainly due to the prolonged action of memantine at the NMDA receptor. These findings must be considered if clinical studies are designed, which aim at evaluating the potency of memantine to prevent "maladaptive" plasticity, e.g. after limb amputation.