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1.  Differing effects of intracortical circuits on plasticity 
Practice of a motor task leads to an increase in amplitude of motor-evoked potentials (MEP) in the exercised muscle. This is termed practice-dependent plasticity, and is abolished by the NMDA antagonist dextromethorphan and the GABAA agonist lorazepam. Here, we sought to determine whether specific subtypes of GABAA circuits are responsible for this effect on MEPs by comparing the action of the non-selective agonist, lorazepam with that of the selective GABAA-alpha1 receptor agonist, zolpidem. In 7 healthy subjects, transcranial magnetic stimulation (TMS) was used to quantify changes in amplitude of motor-evoked potentials (MEP) after practice of a ballistic motor task. In addition we measured how the same drugs affected the excitability of a number of MEP amplitudes and cortical inhibitory circuits (short-interval intracortical inhibition (SICI), short-interval afferent inhibition (SAI) and long-interval intracortical inhibition (LICI)). This allowed us to explore correlations between drug effects on measures of cortical excitability and practice dependent plasticity of MEPs.
As previously reported, lorazepam increased SICI and decreased SAI, while zolpidem only decreased SAI. The new findings were that practice-dependent plasticity of MEPs was impaired by lorazepam but not zolpidem, and that this was negatively correlated with lorazepam-induced changes in SICI but not SAI. This suggests that the intracortical circuits involved in SICI (and not neurons expressing GABAA-alpha1 receptor subunits that are implicated in SAI) may be involved in controlling the amount of practice-dependent MEP plasticity.
PMCID: PMC3019102  PMID: 19048237
plasticity; motor cortex; transcranial magnetic stimulation; GABA; SICI
2.  Abnormal motor cortex excitability in preclinical and very early Huntington’s disease 
Biological psychiatry  2009;65(11):959-965.
In Huntington’s disease (HD), the cerebral cortex is involved early in the disease process. The study of cortical excitability can therefore contribute to understanding HD pathophysiology.
Using transcranial magnetic stimulation (TMS) we examined motor cortex excitability in 8 premanifest HD gene carriers, 8 very early symptomatic HD patients and 22 healthy controls. Electrophysiological measures were correlated with the clinical stage of HD to identify motor cortical dysfunction prior to overt clinical disease onset.
Premanifest and early manifest HD patients had higher resting and active motor cortex thresholds than controls (p=0.024). At rest, recruitment of motor evoked potentials was more gradual in both patient groups than in controls (p=0.001). When active, recruitment and the duration of the cortical silent period were similar in all groups. There was a tendency for short latency intra-cortical inhibition (SICI) to have a higher threshold in all patients taken together but not in each group separately. Short latency afferent inhibition (SAI) was reduced in early manifest patients compared with controls and premanifest patients (p<0.001) and in contrast to all other measures was inversely associated with predicted years to onset of HD signs (p=0.013, adjusted R2=0.32) and the UHDRS motor score (p=0.001, adjusted R2=0.5). A combination of age, CAG repeat length, and SAI strongly predicted the UHDRS motor score (p=0.001, adjusted R2=0.68).
Since reduced excitatory and inhibitory corticospinal thresholds and MEP recruitment in patients at rest do not correlate with symptom severity, they may be a consequence of carrying the HD mutation. In contrast, SAI correlated with the severity of motor signs and may reflect the disease state.
PMCID: PMC2998173  PMID: 19200948
transcranial magnetic stimulation; UHDRS; Huntington’s disease; corticospinal system; short latency afferent inhibition
3.  Abnormal motor cortex plasticity in premanifest and very early manifest Huntington's disease 
Cognition is affected early in Huntington's disease, and in HD animal models there is evidence that this reflects abnormal synaptic plasticity. We investigated whether there is evidence for abnormal synaptic plasticity using the human motor cortex-rTMS model, and if so, if there is any difference between premanifest HD gene carriers and very early manifest HD patients or any relationship with ratings of the severity of motor signs.
Fifteen HD gene carriers (7 premanifest, 8 very early manifest) and 14 control participants were given a continuous train of 100 bursts of theta burst stimulation (cTBS: three pulses at 50 Hz and 80% AMT repeated every 200ms). The size of the motor evoked potential was measured at regular intervals until 21 minutes after cTBS.
HD gene carriers and controls responded differently to theta burst stimulation (F4.9,131.9=1.37, p=0.048) with controls having more inhibition than HD gene carriers (F1,27=13.3, p=0.001). Across all time points mean inhibition differed between the groups (F2,26=6.32, p=0.006); controls had more inhibition than either HD gene carrier subgroup (p=0.006 for premanifest and p=0.009 for early symptomatic) whereas there was no difference between premanifest and early symptomatic HD gene carriers. The measure of cortical plasticity was not associated with any clinical ratings (UHDRS motor score, estimate of age at onset).
Motor cortex plasticity is abnormal in HD gene carriers but is not closely linked to the development of motor signs of HD.
PMCID: PMC2997479  PMID: 19828482
repetitive transcranial magnetic stimulation; UHDRS; Huntington's disease; synaptic plasticity; theta burst stimulation
Brain : a journal of neurology  2009;132(Pt 10):2871-2877.
Dystonia is characterised by two main pathophysiological abnormalities: reduced excitability of inhibitory systems at many levels of the sensorimotor system, and increased plasticity of neural connections in sensorimotor circuits at a brainstem and spinal level. A surprising finding in two recent papers has been the fact that abnormalities of inhibition similar to those in organic dystonia are also seen in patients who have psychogenic dystonia. To try to determine the critical feature that might separate organic and psychogenic conditions, we investigated cortical plasticity in a group of 10 patients with psychogenic dystonia and compared the results with those obtained in a matched group of 10 patients with organic dystonia and 10 healthy individuals. We confirmed the presence of abnormal motor cortical inhibition (short interval intracortical inhibition, SICI) in both organic and psychogenic groups. However, we found that plasticity (paired associative stimulation, PAS) was abnormally high only in the organic group, while there was no difference between the plasticity measured in psychogenic patients and healthy controls. We conclude that abnormal plasticity is a hallmark of organic dystonia; furthermore it is not a consequence of reduced inhibition since the latter is seen in psychogenic patients who have normal plasticity.
PMCID: PMC2997979  PMID: 19690095
associative plasticity; organic dystonia; psychogenic dystonia; paired associative stimulation; transcranial magnetic stimulation
5.  Modulatory effects of 5Hz rTMS over the primary somatosensory cortex in focal dystonia – an fMRI-TMS study 
Dystonia is associated with impaired somatosensory ability. The electrophysiological method of repetitive transcranial magnetic stimulation (rTMS) can be used for non-invasive stimulation of the human cortex and can alter cortical excitability and associated behaviour. Among others, rTMS can alter/improve somatosensory discrimation abilities, as shown in healthy controls.
We applied 5Hz-rTMS over the left primary somatosensory cortex (S1) in 5 patients with right-sided writer's dystonia and 5 controls. We studied rTMS effects on tactile discrimination accuracy and concomitant rTMS-induced changes in hemodynamic activity measured by functional magnetic resonance imaging (fMRI).
Prior to rTMS, patients performed worse on the discrimination task than controls even though fMRI showed greater task-related activation bilaterally in the basal ganglia (BG). In controls, rTMS led to improved discrimination; fMRI revealed this was associated with increased activity of the stimulated S1, bilateral premotor cortex and BG. In dystonia patients, rTMS had no effect on discrimination; fMRI showed similar cortical effects to controls except for no effects in BG.
Improved discrimination after rTMS in controls is linked to enhanced activation of S1 and BG. Failure of rTMS to increase BG activation in dystonia may be associated with the lack of effect on sensory discrimination in this group and may reflect impaired processing in BG-S1 connections. Alternatively, the increased BG activation seen in the baseline state without rTMS may reflect a compensatory strategy that saturates a BG contribution to this task.
PMCID: PMC2929458  PMID: 20058321
writer's cramp; primary dystonia; basal ganglia; sensory discrimination; sensorimotor cortex; premotor cortex; fMRI; TMS; repetitive TMS
6.  The behaviour of the long-latency stretch reflex in patients with Parkinson's disease 
The size of the long-latency stretch reflex was measured in a proximal (triceps) and distal (flexor pollicis longus) muscle in 47 patients with Parkinson's disease, and was compared with that seen in a group of 12 age-matched normal control subjects. The patients were classified clinically into four groups according to the degree of rigidity at the elbow or tremor. Stretch reflexes were evaluated while the subject was exerting a small force against a constant preload supplied by a torque motor, and the size of the reflex response was measured as fractional increase over basal levels of activity. When stretches were given at random intervals by increasing the force exerted by the motor by a factor of 2 or 3, there was a clear trend for the more severely affected patients to have larger long latency responses in the triceps muscle, although there was no change in the size of the short-latency, spinal component of the response. In contrast, there was no change in the size of the long-latency response of the flexor pollicis longus in any group of patients with Parkinson's disease. Despite any differences in reflex size, the inherent muscle stiffness of both muscles appeared to be normal in all groups of patients with Parkinson's disease, since the displacement trajectory of the limb following the force increase was the same as control values in the short (25 ms) period before reflex compensation could intervene. In 20 of the patients and in seven of the control subjects, servo-controlled, ramp positional disturbances were given to the thumb. Up to a velocity of 300°/s, the size of the long-latency stretch reflex was proportional to the log velocity of stretch. This technique revealed, in both moderately and severely rigid patients, increases in the reflex sensitivity of the flexor pollicis longus, which had not been clear using step torque stretches alone. However, whether using ramp or step displacements, long latency stretch reflex gain was not closely related to rigidity; reflex size was within the normal range in many patients with severe rigidity. Enhanced long latency stretch reflexes thus contribute to, but may not be solely responsible for, rigidity in Parkinson's disease.
PMCID: PMC1027261  PMID: 6842198

Results 1-6 (6)