Presynaptic inhibition of soleus Ia afferent terminals in Parkinson's disease

Effect of Motor State on Postactivation Depression of the Soleus H-Reflex in Parkinson's Disease During Deep Brain Stimulation and Dopaminergic Medication Treatment: A Pilot Study

PURPOSE

Postactivation depression of the Hoffmann reflex is reduced in Parkinson's disease (PD), but how the recovery is influenced by the state of the muscle is unknown. The present pilot study examined postactivation depression in PD at rest and during a voluntary contraction while patients were off treatment and while receiving medications and/or deep brain stimulation.

METHODS

The authors recruited nine patients with PD treated with implanted deep brain stimulation and examined postactivation depression under four treatment conditions. Paired pulses were delivered 25 to 300 ms apart, and soleus Hoffmann reflex recovery was tested at rest and during voluntary plantar flexion. Trials were matched for background muscle activity and compared with 10 age-matched controls.

RESULTS

Patients with Parkinson disease who were OFF medications (OFF meds) and OFF stimulation (OFF stim) at rest showed less postactivation depression at the 300 ms interpulse interval (86.1% ± 21.0%) relative to control subjects (36.4% ± 6.1%; P < 0.05). Postactivation depression was restored when dopaminergic medication and/or deep brain stimulation was applied. Comparisons between resting and active motor states revealed that the recovery curves were similar OFF meds/OFF stim owing to faster recovery in PD seen at rest. In contrast, the effect of the motor state was different ON meds/OFF stim and ON meds/ON stim (both P < 0.05), with a nonsignificant trend OFF meds/ON stim (P > 0.08). During a contraction, recovery curves were similar between all treatment conditions in PD and control.

CONCLUSIONS

Disrupted Hoffmann reflex recovery is restored to control levels in PD patients at rest when receiving medications and/or deep brain stimulation or when engaged in voluntary contraction.

Loss of presynaptic inhibition for step initiation in parkinsonian individuals with freezing of gait

KEY POINTS

Individuals with freezing of gait (FoG) due to Parkinson's disease (PD) have small and long anticipatory postural adjustments (APAs) associated with delayed step initiation. Individuals with FoG ('freezers') may require functional reorganization of spinal mechanisms to perform APAs due to supraspinal dysfunction. As presynaptic inhibition (PSI) is centrally modulated to allow execution of supraspinal motor commands, it may be deficient in freezers during APAs. We show that freezers presented PSI in quiet stance (control task), but they presented loss of PSI (i.e. higher ratio of the conditioned H-reflex relative to the test H-reflex) during APAs before step initiation (functional task), whereas non-freezers and healthy control individuals presented PSI in both the tasks. The loss of PSI in freezers was associated with both small APA amplitudes and FoG severity. We hypothesize that loss of PSI during APAs for step initiation in freezers may be due to FoG.

ABSTRACT

Freezing of gait (FoG) in Parkinson's disease involves deficient anticipatory postural adjustments (APAs), resulting in a cessation of step initiation due to supraspinal dysfunction. Individuals with FoG ('freezers') may require functional reorganization of spinal mechanisms to perform APAs. As presynaptic inhibition (PSI) is centrally modulated to allow execution of supraspinal motor commands, here we hypothesized a loss of PSI in freezers during APA for step initiation, which would be associated with FoG severity. Seventy individuals [27 freezers, 22 non-freezers, and 21 age-matched healthy controls (HC)] performed a 'GO'-commanded step initiation task on a force platform under three conditions: (1) without electrical stimulation, (2) test Hoffman reflex (H-reflex) and (3) conditioned H-reflex. They also performed a control task (quiet stance). In the step initiation task, the H-reflexes were evoked on the soleus muscle when the amplitude of the APA exceeded 10-20% of the mean baseline mediolateral force. PSI was quantified by the ratio of the conditioned H-reflex relative to the test H-reflex in both the tasks. Objective assessment of FoG severity (FoG-ratio) was performed. Freezers presented lower PSI levels during quiet stance than non-freezers and HC (P < 0.05). During step initiation, freezers presented loss of PSI and lower APA amplitudes than non-freezers and HC (P < 0.05). Significant correlations were only found for freezers between loss of PSI and FoG-ratio (r = 0.59, P = 0.0005) and loss of PSI and APA amplitude (r = -0.35, P < 0.036). Our findings suggest that loss of PSI for step initiation in freezers may be due to FoG.

A Role for Dystonia-Associated Genes in Spinal GABAergic Interneuron Circuitry

Spinal interneurons are critical modulators of motor circuit function. In the dorsal spinal cord, a set of interneurons called GABApre presynaptically inhibits proprioceptive sensory afferent terminals, thus negatively regulating sensory-motor signaling. Although deficits in presynaptic inhibition have been inferred in human motor diseases, including dystonia, it remains unclear whether GABApre circuit components are altered in these conditions. Here, we use developmental timing to show that GABApre neurons are a late Ptf1a-expressing subclass and localize to the intermediate spinal cord. Using a microarray screen to identify genes expressed in this intermediate population, we find the kelch-like family member Klhl14, implicated in dystonia through its direct binding with torsion-dystonia-related protein Tor1a. Furthermore, in Tor1a mutant mice in which Klhl14 and Tor1a binding is disrupted, formation of GABApre sensory afferent synapses is impaired. Our findings suggest a potential contribution of GABApre neurons to the deficits in presynaptic inhibition observed in dystonia.

Resistance training with instability is more effective than resistance training in improving spinal inhibitory mechanisms in Parkinson's disease

This study assessed 1) the effects of 12 wk of resistance training (RT) and resistance training with instability (RTI) on presynaptic inhibition (PSI) and disynaptic reciprocal inhibition (DRI) of patients with Parkinson's disease (PD); 2) the effectiveness of RT and RTI in moving PSI and DRI values of patients toward values of age-matched healthy controls (HC; Z-score analysis); and 3) associations between PSI and DRI changes and clinical outcomes changes previously published. Thirteen patients in RT group, 13 in RTI group, and 11 in a nonexercising control group completed the trial. While RT and RTI groups performed resistance exercises twice a week for 12 wk, only the RTI group used unstable devices. The soleus H reflex was used to evaluate resting PSI and DRI before and after the experimental protocol. The HC (n = 31) was assessed at pretest only. There were significant group × time interactions for PSI (P < 0.0001) and DRI (P < 0.0001). RTI was more effective than RT in increasing the levels of PSI (P = 0.0154) and DRI (P < 0.0001) at posttraining and in moving PSI [confidence interval (CI) 0.1-0.5] and DRI (CI 0.6-1.1) levels to those observed in HC. There was association between DRI and quality of life changes (r = -0.69, P = 0.008) and a strong trend toward association between PSI and postural instability changes (r = 0.60, P = 0.051) after RTI. RTI increased PSI and DRI levels more than RT, reaching the average values of the HC. Thus RTI may cause plastic changes in PSI and DRI pathways that are associated with some PD clinical outcomes.

NEW & NOTEWORTHY

Patients with Parkinson's disease (PD) have motor dysfunction. Spinal inhibitory mechanisms are important for modulating both supraspinal motor commands and sensory feedback at the spinal level. Resistance training with instability was more effective than resistance training in increasing the levels of presynaptic inhibition and disynaptic reciprocal inhibition of lower limb at rest of the patients with PD, reaching the average values of the healthy controls.

Evidence for cutaneous and corticospinal modulation of presynaptic inhibition of Ia afferents from the human lower limb

1. Presynaptic inhibition of soleus muscle Ia afferent fibres, produced by stimulation of group I afferents in the common peroneal nerve, was assessed from changes in the H reflex at long conditioning intervals, in six normal subjects. 2. Stimulation of the ipsilateral sural nerve at the malleolus, just before stimulation of the common peroneal nerve at the head of the fibula, decreased the presynaptic inhibition. This effect was strongest during voluntary plantar flexion and weaker during dorsiflexion or at rest. 3. Stimulation of other cutaneous nerve branches serving the dorsum of the ipsilateral foot, and also the contralateral sural nerve, decreased presynaptic inhibition. Adequate stimulation of low threshold cutaneous mechanoreceptors by light brushing of both distal dorsal and plantar surfaces of the ipsilateral foot decreased presynaptic inhibition. 4. Stimulation of the ipsilateral plantar nerves increased presynaptic inhibition, but this action is attributed to activation of group I afferents from the intrinsic muscles of the foot. 5. Transcranial magnetic stimulation of the lower limb area of the contralateral motor cortex decreased presynaptic inhibition. This effect was strongest during voluntary plantar flexion and weaker during dorsiflexion or at rest. 6. The actions of cutaneous and corticospinal pathways completely occluded each other. However, when both effects were adjusted to be liminal, a spatial facilitation between them was observed. 7. It is concluded that in man, as in the cat, cutaneous and corticospinal axons converge on interneurones that inhibit the machinery of presynaptic inhibition of group Ia afferents. These actions may be responsible for the modulation of presynaptic inhibition which has been observed to precede and accompany a wide range of human movements.

Presynaptic inhibition in humans

Presynaptic inhibition plays an important role in controlling sensory processing of information in humans, as in other animals. However, because of experimental constraints the methods for measuring presynaptic inhibition are necessarily more indirect in humans. The most common method uses the modulation of the H-reflex by vibratory or electrical inputs. However, these stimuli can produce postsynaptic as well as presynaptic changes so it is important to use very short periods of stimulation and measure changes at a latency where presynaptic changes predominate. In addition, the stimuli should be superimposed upon a steady background of EMG activity, preferably in a single motor unit, to maintain the postsynaptic state at a constant level. Recent studies indicate that presynaptic inhibition is used as part of the program for voluntary movement and that it can be rapidly and dramatically adapted to the task being carried out. This task-dependent modulation is produced by pattern generators within the central nervous system as well as sensory feedback from the periphery, but the relative importance of the two remains uncertain. Clinical disorders, such as spasticity, affect the ability of humans to modulate presynaptic inhibition, and contribute to the deficits observed. Improved methods for treating the symptoms pharmacologically and electrically can improve function in these patients.