Persistence of PAD and presynaptic inhibition of muscle spindle afferents after peripheral nerve crush

Direct evidence for decreased presynaptic inhibition evoked by PBSt group I muscle afferents after chronic SCI and recovery with step-training in rats

KEY POINTS

Presynaptic inhibition is modulated by supraspinal centres and primary afferents in order to filter sensory information, adjust spinal reflex excitability, and ensure smooth movement. After spinal cord injury (SCI), the supraspinal control of primary afferent depolarization (PAD) interneurons is disengaged, suggesting an increased role for sensory afferents. While increased H-reflex excitability in spastic individuals indicates a possible decrease in presynaptic inhibition, it remains unclear whether a decrease in sensory-evoked PAD contributes to this effect. We investigated whether the PAD evoked by hindlimb afferents contributes to the change in presynaptic inhibition of the H-reflex in a decerebrated rat preparation. We found that chronic SCI decreases presynaptic inhibition of the plantar H-reflex through a reduction in PAD evoked by posterior biceps-semitendinosus (PBSt) muscle group I afferents. We further found that step-training restored presynaptic inhibition of the plantar H-reflex evoked by PBSt, suggesting the presence of activity-dependent plasticity of PAD pathways activated by flexor muscle group I afferents.

ABSTRACT

Spinal cord injury (SCI) results in the disruption of supraspinal control of spinal networks and an increase in the relative influence of afferent feedback to sublesional neural networks, both of which contribute to enhancing spinal reflex excitability. Hyperreflexia occurs in ∼75% of individuals with a chronic SCI and critically hinders functional recovery and quality of life. It is suggested that it results from an increase in motoneuronal excitability and a decrease in presynaptic and postsynaptic inhibitory mechanisms. In contrast, locomotor training decreases hyperreflexia by restoring presynaptic inhibition. Primary afferent depolarization (PAD) is a powerful presynaptic inhibitory mechanism that selectively gates primary afferent transmission to spinal neurons to adjust reflex excitability and ensure smooth movement. However, the effect of chronic SCI and step-training on the reorganization of presynaptic inhibition evoked by hindlimb afferents, and the contribution of PAD has never been demonstrated. The objective of this study is to directly measure changes in presynaptic inhibition through dorsal root potentials (DRPs) and its association with plantar H-reflex inhibition. We provide direct evidence that H-reflex hyperexcitability is associated with a decrease in transmission of PAD pathways activated by posterior biceps-semitendinosus (PBSt) afferents after chronic SCI. More precisely, we illustrate that the pattern of inhibition evoked by PBSt group I muscle afferents onto both L4-DRPs and plantar H-reflexes evoked by the distal tibial nerve is impaired after chronic SCI. These changes are not observed in step-trained animals, suggesting a role for activity-dependent plasticity to regulate PAD pathways activated by flexor muscle group I afferents.

VGLUT1 synapses and P-boutons on regenerating motoneurons after nerve crush

Stretch-sensitive Ia afferent monosynaptic connections with motoneurons form the stretch reflex circuit. After nerve transection, Ia afferent synapses and stretch reflexes are permanently lost, even after regeneration and reinnervation of muscle by motor and sensory afferents is completed in the periphery. This loss greatly affects full recovery of motor function. However, after nerve crush, reflex muscle forces during stretch do recover after muscle reinnervation and reportedly exceed 140% baseline values. This difference might be explained by structural preservation after crush of Ia afferent synapses on regenerating motoneurons and decreased presynaptic inhibitory control. We tested these possibilities in rats after crushing the tibial nerve (TN), and using Vesicular GLUtamate Transporter 1 (VGLUT1) and the 65 kDa isoform of glutamic acid-decarboxylase (GAD65) as markers of, respectively, Ia afferent synapses and presynaptic inhibition (P-boutons) on retrogradely labeled motoneurons. We analyzed motoneurons during regeneration (21 days post crush) and after they reinnervate muscle (3 months). The results demonstrate a significant loss of VGLUT1 terminals on dendrites and cell bodies at both 21 days and 3 months post-crush. However, in both cellular compartments, the reductions were small compared to those observed after TN full transection. In addition, we found a significant decrease in the number of GAD65 P-boutons per VGLUT1 terminal and their coverage of VGLUT1 boutons. The results support the hypothesis that better preservation of Ia afferent synapses and a change in presynaptic inhibition could contribute to maintain or even increase the stretch reflex after nerve crush and by difference to nerve transection.

Electroacupuncture improves gait locomotion, H-reflex and ventral root potentials of spinal compression injured rats

This study explored the effect of electroacupuncture stimulation (EA) on alterations in the Hoffman reflex (H-reflex) response and gait locomotion provoked by spinal cord injury (SCI) in the rat. A compression lesion of the spinal cord was evoked by insufflating a Fogarty balloon located in the epidural space at the T8-9 spinal level of adult Wistar male rats (200-250 gr; n=60). In different groups of SCI rats, EA (frequencies: 2, 50 and 100Hz) was applied simultaneously to Huantiao (GB30), Yinmen (BL37), Jizhong (GV6) and Zhiyang (GV9) acupoints from the third post-injury day until the experimental session. At 1, 2, 3 and 4 post-injury weeks, the BBB scores of the SCI group of rats treated with EA at 50Hz showed a gradual but greater enhancement of locomotor activity than the other groups of rats. Unrestrained gait kinematic analysis of SCI rats treated with EA-50Hz stimulation showed a significant improvement in stride duration, length and speed (p<0.05), whereas a discrete recovery of gait locomotion was observed in the other groups of animals. After four post-injury weeks, the H-reflex amplitude and H-reflex/M wave amplitude ratio obtained in SCI rats had a noticeable enhancement (217%) compared to sham rats (n=10). Meanwhile, SCI rats treated with EA at 50Hz manifested a decreased facilitation of the H-reflex amplitude and H/M amplitude ratio (154%) and a reduced frequency-dependent amplitude depression of the H-reflex (66%). In addition, 50 Hz-EA treatment induced a recovery of the presynaptic depression of the Gs-VRP evoked by PBSt conditioning stimulation in the SCI rat (63.2±8.1%; n=9). In concordance with the latter, it could be suggested that 50 Hz-EA stimulation reduced the hyper-excitability of motoneurons and provokes a partial improvement of the locomotive performance and H reflex responses by a possible recovery of presynaptic mechanisms in the spinal cord of experimentally injured rats.

Measuring spinal presynaptic inhibition in mice by dorsal root potential recording in vivo

Presynaptic inhibition is one of the most powerful inhibitory mechanisms in the spinal cord. The underlying physiological mechanism is a depolarization of primary afferent fibers mediated by GABAergic axo-axonal synapses (primary afferent depolarization). The strength of primary afferent depolarization can be measured by recording of volume-conducted potentials at the dorsal root (dorsal root potentials, DRP). Pathological changes of presynaptic inhibition are crucial in the abnormal central processing of certain pain conditions and in some disorders of motor hyperexcitability. Here, we describe a method of recording DRP in vivo in mice. The preparation of spinal cord dorsal roots in the anesthetized animal and the recording procedure using suction electrodes are explained. This method allows measuring GABAergic DRP and thereby estimating spinal presynaptic inhibition in the living mouse. In combination with transgenic mouse models, DRP recording may serve as a powerful tool to investigate disease-associated spinal pathophysiology. In vivo recording has several advantages compared to ex vivo isolated spinal cord preparations, e.g. the possibility of simultaneous recording or manipulation of supraspinal networks and induction of DRP by stimulation of peripheral nerves.

Absence of effects of contralateral group I muscle afferents on presynaptic inhibition of Ia terminals in humans and cats

Crossed effects from group I afferents on reflex excitability and their mechanisms of action are not yet well understood. The current view is that the influence is weak and takes place indirectly via oligosynaptic pathways. We examined possible contralateral effects from group I afferents on presynaptic inhibition of Ia terminals in humans and cats. In resting and seated human subjects the soleus (SO) H-reflex was conditioned by an electrical stimulus to the ipsilateral common peroneal nerve (CPN) to assess the level of presynaptic inhibition (PSI_control). A brief conditioning vibratory stimulus was applied to the triceps surae tendon at the contralateral side (to activate preferentially Ia muscle afferents). The amplitude of the resulting H-reflex response (PSI_conditioned) was compared to the H-reflex under PSI_control, i.e., without the vibration. The interstimulus interval between the brief vibratory stimulus and the electrical shock to the CPN was -60 to 60 ms. The H-reflex conditioned by both stimuli did not differ from that conditioned exclusively by the ipsilateral CPN stimulation. In anesthetized cats, bilateral monosynaptic reflexes (MSRs) in the left and right L(7) ventral roots were recorded simultaneously. Conditioning stimulation applied to the contralateral group I posterior biceps and semitendinosus (PBSt) afferents at different time intervals (0-120 ms) did not have an effect on the ipsilateral gastrocnemius/soleus (GS) MSR. An additional experimental paradigm in the cat using contralateral tendon vibration, similar to that conducted in humans, was also performed. No significant differences between GS-MSRs conditioned by ipsilateral PBSt stimulus alone and those conditioned by both ipsilateral PBSt stimulus and contralateral tendon vibration were detected. The present results strongly suggest an absence of effects from contralateral group I fibers on the presynaptic mechanism of MSR modulation in relaxed humans and anesthetized cats.

Task-dependent modulation of primary afferent depolarization in cervical spinal cord of monkeys performing an instructed delay task

Task-dependent modulation of primary afferent depolarization (PAD) was studied in the cervical spinal cord of two monkeys performing a wrist flexion and extension task with an instructed delay period. We implanted two nerve cuff electrodes on proximal and distal parts of the superficial radial nerve (SR) and a recording chamber over a hemi-laminectomy in the lower cervical vertebrae. Antidromic volleys (ADVs) in the SR were evoked by intraspinal microstimuli (ISMS, 3-10 Hz, 3-30 microA) applied through a tungsten microelectrode, and the area of each ADV was measured. In total, 434 ADVs were evoked by ISMS in two monkeys, with onset latency consistently shorter in the proximal than distal cuffs. Estimated conduction velocity suggest that most ADVs were caused by action potentials in cutaneous fibers originating from low-threshold tactile receptors. Modulation of the size of ADVs as a function of the task was examined in 281 ADVs induced by ISMS applied at 78 different intraspinal sites. The ADVs were significantly facilitated during active movement in both flexion and extension (P<0.05), suggesting an epoch-dependent modulation of PAD. This facilitation started 400-900 ms before the onset of EMG activity. Such pre-EMG modulation is hard to explain by movement-induced reafference and probably is associated with descending motor commands.

Central suppression of regenerated proprioceptive afferents

Long after a cut peripheral nerve reinnervates muscle and restores force production in adult cats, the muscle does not respond reflexively to stretch. Motivated by the likelihood that stretch areflexia is related to problems with sensing and controlling limb position after peripheral neuropathies, we sought to determine the underlying mechanism. Electrophysiological and morphological measurements were made in anesthetized rats having one of the nerves to the triceps surae muscles either untreated or cut and immediately rejoined surgically many months earlier. First, it was established that reinnervated muscles failed to generate stretch reflexes, extending observations of areflexia to a second species. Next, multiple elements in the sensorimotor circuit of the stretch reflex were examined in both the PNS and CNS. Encoding of muscle stretch by regenerated proprioceptive afferents was remarkably similar to normal, although we observed some expected abnormalities, e.g., increased length threshold. However, the robust stretch-evoked sensory response that arrived concurrently at the CNS in multiple proprioceptive afferents produced synaptic responses that were either smaller than normal or undetectable. Muscle stretch failed to evoke detectable synaptic responses in 13 of 22 motoneurons, although electrical stimulation generated monosynaptic excitatory postsynaptic potentials that were indistinguishable from normal. The ineffectiveness of muscle stretch was not attributable therefore to dysfunction at synapses made between regenerated Ia afferents and motoneurons. Among multiple candidate mechanisms, we suggest that centrally controlled neural circuits may actively suppress the sensory information encoded by regenerated proprioceptive afferents to prevent recovery of the stretch reflex.