Spinal Locomotor Research Articles

Volitional walking via upper limb muscle-controlled stimulation of the lumbar locomotor center in man.

Gait disturbance in individuals with spinal cord lesion is attributed to the interruption of descending pathways to the spinal locomotor center, whereas neural circuits below and above the lesion maintain their functional capability. An artificial neural connection (ANC), which bridges supraspinal centers and locomotor networks in the lumbar spinal cord beyond the lesion site, may restore the functional impairment. To achieve an ANC that sends descending voluntary commands to the lumbar locomotor center and bypasses the thoracic spinal cord, upper limb muscle activity was converted to magnetic stimuli delivered noninvasively over the lumbar vertebra. Healthy participants were able to initiate and terminate walking-like behavior and to control the step cycle through an ANC controlled by volitional upper limb muscle activity. The walking-like behavior stopped just after the ANC was disconnected from the participants even when the participant continued to swing arms. Furthermore, additional simultaneous peripheral electrical stimulation to the foot via the ANC enhanced this walking-like behavior. Kinematics of the induced behaviors were identical to those observed in voluntary walking. These results demonstrate that the ANC induces volitionally controlled, walking-like behavior of the legs. This paradigm may be able to compensate for the dysfunction of descending pathways by sending commands to the preserved locomotor center at the lumbar spinal cord and may enable individuals with paraplegia to regain volitionally controlled walking.

Sprouting of axonal collaterals after spinal cord injury is prevented by delayed axonal degeneration

After an incomplete spinal cord injury (SCI), partial recovery of locomotion is accomplished with time. Previous studies have established a functional link between extension of axon collaterals from spared spinal tracts and locomotor recovery after SCI, but the tissular signals triggering collateral sprouting have not been identified. Here, we investigated whether axonal degeneration after SCI contributes to the sprouting of collaterals from axons spared after injury. To this end, we evaluated collateral sprouting from BDA-labeled uninjured corticospinal axons after spinal cord hemisection (SCIH) in wild type (WT) mouse and WldS mouse strains, which shows a significant delay in Wallerian degeneration after injury. After SCIH, spared fibers of WT mice extend collateral sprouts to both intact and denervated sides of the spinal cord distant from the injury site. On the contrary, in the WldS mice collateral sprouting from spared fibers was greatly reduced after SCIH. Consistent with a role for collateral sprouting in functional recovery after SCI, locomotor recovery after SCIH was impaired in WldS mice compared to WT animals. In conclusion, our results identify axonal degeneration as one of the triggers for collateral sprouting from the contralesional uninjured fibers after an SCIH. These results open the path for identifying molecular signals associated with tissular changes after SCI that promotes collateral sprouting and functional recovery.

A hybrid electrical/chemical circuit in the spinal cord generates a transient embryonic motor behavior.

Spontaneous network activity is a highly stereotyped early feature of developing circuits throughout the nervous system, including in the spinal cord. Spinal locomotor circuits produce a series of behaviors during development before locomotion that reflect the continual integration of spinal neurons into a functional network, but how the circuitry is reconfigured is not understood. The first behavior of the zebrafish embryo (spontaneous coiling) is mediated by an electrical circuit that subsequently generates mature locomotion (swimming) as chemical neurotransmission develops. We describe here a new spontaneous behavior, double coiling, that consists of two alternating contractions of the tail in rapid succession. Double coiling was glutamate-dependent and required descending hindbrain excitation, similar to but preceding swimming, making it a discrete intermediary developmental behavior. At the cellular level, motoneurons had a distinctive glutamate-dependent activity pattern that correlated with double coiling. Two glutamatergic interneurons, CoPAs and CiDs, had different activity profiles during this novel behavior. CoPA neurons failed to show changes in activity patterns during the period in which double coiling appears, whereas CiD neurons developed a glutamate-dependent activity pattern that correlated with double coiling and they innervated motoneurons at that time. Additionally, double coils were modified after pharmacological reduction of glycinergic neurotransmission such that embryos produced three or more rapidly alternating coils. We propose that double coiling behavior represents an important transition of the motor network from an electrically coupled spinal cord circuit that produces simple periodic coils to a spinal network driven by descending chemical neurotransmission, which generates more complex behaviors.

Modulation of phase durations, phase variations, and temporal coordination of the four limbs during quadrupedal split-belt locomotion in intact adult cats.

Stepping along curvilinear paths produces speed differences between the inner and outer limb(s). This can be reproduced experimentally by independently controlling left and right speeds with split-belt locomotion. Here we provide additional details on the pattern of the four limbs during quadrupedal split-belt locomotion in intact cats. Six cats performed tied-belt locomotion (same speed bilaterally) and split-belt locomotion where one side (constant side) stepped at constant treadmill speed while the other side (varying side) stepped at several speeds. Cycle, stance, and swing durations changed in parallel in homolateral limbs with shorter and longer stance and swing durations on the fast side, respectively, compared with the slow side. Phase variations were quantified in all four limbs by measuring the slopes of the regressions between stance and cycle durations (rSTA) and between swing and cycle durations (rSW). For a given limb, rSTA and rSW were not significantly different from one another on the constant side whereas on the varying side rSTA increased relative to tied-belt locomotion while rSW became more negative. Phase variations were similar for homolateral limbs. Increasing left-right speed differences produced a large increase in homolateral double support on the slow side, while triple-support periods decreased. Increasing left-right speed differences altered homologous coupling, homolateral coupling on the fast side, and coupling between the fast hindlimb and slow forelimb. Results indicate that homolateral limbs share similar control strategies, only certain features of the interlimb pattern adjust, and spinal locomotor networks of the left and right sides are organized symmetrically.

Monoaminergic control of spinal locomotor networks in SOD1G93A newborn mice.

Mutations in the gene that encodes Cu/Zn-superoxide dismutase (SOD1) are the cause of approximately 20% of familial forms of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease characterized by the progressive loss of motor neurons. While ALS symptoms appear in adulthood, spinal motoneurons exhibit functional alterations as early as the embryonic and postnatal stages in the murine model of ALS, the SOD1 mice. Monoaminergic – i.e., dopaminergic (DA), serotoninergic (5-HT), and noradrenergic (NA) – pathways powerfully control spinal networks and contribute significantly to their embryonic and postnatal maturation. Alterations in monoaminergic neuromodulation during development could therefore lead to impairments in the motoneuronal physiology. In this study, we sought to determine whether the monoaminergic spinal systems are modified in the early stages of development in SOD1 mice. Using a post-mortem analysis by high performance liquid chromatography (HPLC), monoaminergic neuromodulators and their metabolites were quantified in the lumbar spinal cord of SOD1 and wild-type (WT) mice aged one postnatal day (P1) and P10. This analysis underscores an increased content of DA in the SOD1 lumbar spinal cord compared to that of WT mice but failed to reveal any modification of the other monoaminergic contents. In a next step, we compared the efficiency of the monoaminergic compounds in triggering and modulating fictive locomotion in WT and SOD1 mice. This study was performed in P1–P3 SOD1 mice and age-matched control littermates using extracellular recordings from the lumbar ventral roots in the in vitro isolated spinal cord preparation. This analysis revealed that the spinal networks of SOD1G93A mice could generate normal locomotor activity in the presence of NMA-5-HT. Interestingly, we also observed that SOD1 spinal networks have an increased sensitivity to NA compared to WT spinal circuits but exhibited similar DA responses.

Organization of left-right coordination of neuronal activity in the mammalian spinal cord locomotor CPG: insights from computational modeling

Different gaits of locomotion in mammals are based on the appropriate coordination of neuronal activity in the spinal cord controlling movements of left and right limbs. This left-right coordination is provided in the spinal cord by the commissural interneurons (CINs) whose axons cross the midline and affect neural circuits on the contralateral side of the cord. Several types of CINs have been genetically identified, including the excitatory V3 CINs and the inhibitory (V0D) and excitatory (V0V) V0 CINs. Talpalar et al. [1] recently demonstrated that (a) ablation of both V0 CIN types leads to a left-right synchronized, “hopping” activity at all locomotor frequencies, whereas (b) selective ablation of the excitatory V0V CINs maintains alternation at low frequencies but switches to synchronized activity at high frequencies while (c) ablation of only the inhibitory V0D CINs leads to a lack of left–right alternation at low frequencies, but maintains alternation at high frequencies. The genetically identified, ipsilaterally projecting excitatory V2a interneurons are recruited with an increase in locomotor speed [2] and contribute to left-right alternation at high locomotor frequencies [3,4]. Our objective was to construct and analyze a computational model of the bilaterally interacting central pattern generators (CPGs) that could reproduce and explain these findings. In our model, the CPG on each side of the cord consisted of flexor and extensor half-centers. Each neural population, including the half-centers and CIN populations, consisted of 50-200 neurons modeled in the Hodgkin-Huxley style. The intrinsic bursting of CPG neurons was based on a persistent sodium current in these neurons. During model construction, we assumed that left-right coordination of activity depends on the balance between the three CIN pathways providing interactions between the left and right CPGs: the V3-mediated pathway that supports left-right synchronization and the V0D- and V0V- mediated pathways that provide left-right alternation. The activity of each (left and right) inhibitory V0D population was driven by the ipsilateral flexor half-center. The recruitment of V0D neurons was progressively reduced with an increase in locomotor speed, because of the reduction of burst amplitude. The left and right V0V pathways could be organized in two ways: (1) the V0V activity on each side was driven by the ipsilateral flexor half-center and its action on contralateral circuits was mediated by an inhibitory population, or (2) the V0V activity was driven by the ipsilateral extensor half-center and it excited the contralateral circuits. In any case, the V0V activation was mediated by the ipsilateral V2a neurons progressively recruited with increasing locomotor speed. The model demonstrates: (1) a left-right alternating pattern under control conditions; (2) a synchronized hopping pattern at any frequency after removing both the V0 populations; (3) a synchronized pattern at low frequencies with alternation at high frequencies after removing the V0D populations; (4) an alternating pattern at low frequencies with synchronized hopping at high frequencies after removing either V0V or V2a populations. The model closely reproduces and suggests an explanation for the experimental data of Talpalar et al. [1], Zhong et al. [2], and Crone et al. [3,4], proposes the organization of commissural interactions in the spinal cord defining the left-right alternation at different locomotor speeds, and generates predictions for future experimental investigations.

Recovery of locomotion after partial spinal cord lesions in cats: assessment using behavioral, electrophysiological and imaging techniques.

This short review summarizes experimental findings made after spinal cord injury, mainly in cats. After a complete spinal injury, cats re-express hindlimb locomotion after 2-3 weeks because of a spinal locomotor circuitry named the central pattern generator or CPG. To investigate whether such circuits are also implicated in the recovery of locomotion after partial spinal lesions, we have used a dual spinal lesion paradigm. Essentially, after an initial unilateral hemisection, cats spontaneously recover quadrupedal locomotion. When a complete section is then performed 3 weeks after this hemisection, cats can walk with the hindlimbs within 24 hours compared to 2-3 weeks in cats with single complete spinal lesions demonstrating the importance of spinal mechanisms after partial lesions. Using kinematic and electromyographic methods to evaluate the changes throughout the dual lesion paradigm, we could show that the spinal cord reorganizes spontaneously without locomotor training or with training provided between the partial and complete spinal lesion. To assess spinal lesions we have used histology and magnetic resonance imaging (MRI). We will describe some advanced MRI techniques such as diffusion and magnetization transfer, which provide higher specificity to axon degeneration and demyelination. Examples of advanced MRI techniques in cats and humans are described, including the current limitations and perspectives.

The role of serotonin in the control of locomotor movements and strategies for restoring locomotion after spinal cord injury.

In this review we will discuss different ways for re-establishing serotonergic activity that can enhance recovery of coordinated plantar stepping after spinal cord injury in adult rats. It is well known that serotoninergic neurons located in the medulla are able to initiate locomotor activity. This effect is exerted by actions on motoneurons and on neurons of the locomotor CPG (Central Pattern Generator). Motoneuron and interneuron excitability is increased, and putative CPG interneurons display oscillatory behaviour in response to serotonin receptor activation. The medullary serotonergic nuclei play multiple roles in the control of locomotion, and they terminate on specific target neurons with different types of serotonergic receptors in the spinal cord. Activation of these serotonergic receptors can restore locomotor movements after spinal cord injury. Specifically, using defined serotonergic agonists the 5-HT2 receptors can be stimulated to control CPG activation as well as motoneuron output, while 5-HT7 receptors to control activity of the locomotor CPG. These results are consistent with the roles for these receptors during locomotion in intact rodents and in rodent brainstem-spinal cord in vitro preparations. The other possibility to encourage the remaining spinal cord circuitry below the total transection to control recovery of plantar hindlimb stepping is restoration of serotonergic innervation by intraspinal grafting of embryonic 5-HT neurons. Our data show that grafting of different populations of 5-HT neurons dissected from embryonic brainstem provides differential control over multiple components of the spinal locomotor circuitry through specific serotonin receptors. Moreover, we demonstrated that the best effect of motor recovery is obtained after grafting of neurons destined to form the B1, B2 and B3 descending 5-HT systems. Using only one of the subpopulations for intraspinal grafting, for example, B3 or the lateral group of 5-HT neurons, induces only partial recovery of plantar stepping with a clear lack of proper interlimb coordination. This confirms the hypothesis that transplantation of 5-HT neurons from specific embryonic sources is necessary to obtain optimal recovery of locomotor hindlimb movement.

Dopamine: a parallel pathway for the modulation of spinal locomotor networks.

The spinal cord contains networks of neurons that can produce locomotor patterns. To readily respond to environmental conditions, these networks must be flexible yet at the same time robust. Neuromodulators play a key role in contributing to network flexibility in a variety of invertebrate and vertebrate networks. For example, neuromodulators contribute to altering intrinsic properties and synaptic weights that, in extreme cases, can lead to neurons switching between networks. Here we focus on the role of dopamine in the control of stepping networks in the spinal cord. We first review the role of dopamine in modulating rhythmic activity in the stomatogastric ganglion (STG) and the leech, since work from these preparations provides a foundation to understand its role in vertebrate systems. We then move to a discussion of dopamine’s role in modulation of swimming in aquatic species such as the larval xenopus, lamprey and zebrafish. The control of terrestrial walking in vertebrates by dopamine is less studied and we review current evidence in mammals with a focus on rodent species. We discuss data suggesting that the source of dopamine within the spinal cord is mainly from the A11 area of the diencephalon, and then turn to a discussion of dopamine’s role in modulating walking patterns from both in vivo and in vitro preparations. Similar to the descending serotonergic system, the dopaminergic system may serve as a potential target to promote recovery of locomotor function following spinal cord injury (SCI); evidence suggests that dopaminergic agonists can promote recovery of function following SCI. We discuss pharmacogenetic and optogenetic approaches that could be deployed in SCI and their potential tractability. Throughout the review we draw parallels with both noradrenergic and serotonergic modulatory effects on spinal cord networks. In all likelihood, a complementary monoaminergic enhancement strategy should be deployed following SCI.

Enabling motor control in chronic spinal cord injury: found in translation.

Chronic spinal cord injury affects between 440 and 681 people in every million, depending on country (Bickenbach, 2013), and remains an unmet medical need. While there has been an exponential increase in our molecular and cellular understanding of spinal cord injury over the last three decades, with a considerable number of interventions producing functional neurological recovery in experimental models, not one has been translated into an FDA-approved treatment available in the clinic. Translating neurobiological concepts from basic neuroscience to clinical treatments is one of the toughest challenges and its success depends on a bidirectional dialogue (Curt, 2012). In the current issue of Brain , Susan Harkema and Victor Edgerton scale up the results of an earlier case report (Harkema et al. , 2011) to demonstrate that epidural stimulation, beginning >2 years after spinal cord injury, can restore voluntary movement in a select group of patients (Angeli et al. , 2014). Epidural stimulation (25 or 30 Hz, 1.5–2.5 V) was applied to the index patient from the initial case report and to three additional patients (neurological lesion level C7–T5), via an implanted device at spinal cord level L1-S1 as previously described (Harkema et al. , 2011). Two of the patients had lesions classified as grade A according to the American Spinal Injury Association Impairment Scale (AIS) (sensory and motor complete), while two had lesions classified as AIS B (sensory incomplete). The concept of lumbar stimulation to treat spinal injury is derived from research in the ‘spinal cat’ that revealed the presence of a locomotor centre in the lumbar spinal cord (Grillner et al. , 1969), a discovery that was further advanced in human patients by Milan Dimitrijevic. It was subsequently confirmed that both propriospinal and supraspinal afferents to spinal locomotor centres could facilitate motor output in patients with complete spinal …

Nanomolar Oxytocin Synergizes with Weak Electrical Afferent Stimulation to Activate the Locomotor CPG of the Rat Spinal Cord In Vitro

Synergizing the effect of afferent fibre stimulation with pharmacological interventions is a desirable goal to trigger spinal locomotor activity, especially after injury. Thus, to better understand the mechanisms to optimize this process, we studied the role of the neuropeptide oxytocin (previously shown to stimulate locomotor networks) on network and motoneuron properties using the isolated neonatal rat spinal cord. On motoneurons oxytocin (1 nM–1 μM) generated sporadic bursts with superimposed firing and dose-dependent depolarization. No desensitization was observed despite repeated applications. Tetrodotoxin completely blocked the effects of oxytocin, demonstrating the network origin of the responses. Recording motoneuron pool activity from lumbar ventral roots showed oxytocin mediated depolarization with synchronous bursts, and depression of reflex responses in a stimulus and peptide-concentration dependent fashion. Disinhibited bursting caused by strychnine and bicuculline was accelerated by oxytocin whose action was blocked by the oxytocin antagonist atosiban. Fictive locomotion appeared when subthreshold concentrations of NMDA plus 5HT were coapplied with oxytocin, an effect prevented after 24 h incubation with the inhibitor of 5HT synthesis, PCPA. When fictive locomotion was fully manifested, oxytocin did not change periodicity, although cycle amplitude became smaller. A novel protocol of electrical stimulation based on noisy waveforms and applied to one dorsal root evoked stereotypic fictive locomotion. Whenever the stimulus intensity was subthreshold, low doses of oxytocin triggered fictive locomotion although oxytocin per se did not affect primary afferent depolarization evoked by dorsal root pulses. Among the several functional targets for the action of oxytocin at lumbar spinal cord level, the present results highlight how small concentrations of this peptide could bring spinal networks to threshold for fictive locomotion in combination with other protocols, and delineate the use of oxytocin to strengthen the efficiency of electrical stimulation to activate locomotor circuits.

Spinal Glutamatergic Neurons Defined by EphA4 Signaling Are Essential Components of Normal Locomotor Circuits

EphA4 signaling is essential for the spatiotemporal organization of neuronal circuit formation. In mice, deletion of this signaling pathway causes aberrant midline crossing of axons from both brain and spinal neurons and the complete knock-outs (KOs) exhibit a pronounced change in motor behavior, where alternating gaits are replaced by a rabbit-like hopping gait. The neuronal mechanism that is responsible for the gait switch in these KO mice is not known. Here, using intersectional genetics, we demonstrate that a spinal cord-specific deletion of EphA4 signaling is sufficient to generate the overground hopping gait. In contrast, selective deletion of EphA4 signaling in forebrain neurons, including the corticospinal tract neurons, did not result in a change in locomotor pattern. The gait switch was attributed to the loss of EphA4 signaling in excitatory Vglut2+ neurons, which is accompanied by an increased midline crossing of Vglut2+ neurons in the ventral spinal cord. Our findings functionally define spinal EphA4 signaling in excitatory Vglut2+ neurons as required for proper organization of the spinal locomotor circuitry, and place these cells as essential components of the mammalian locomotor network.

Remote Control of Respiratory Neural Network by Spinal Locomotor Generators

During exercise and locomotion, breathing rate rapidly increases to meet the suddenly enhanced oxygen demand. The extent to which direct central interactions between the spinal networks controlling locomotion and the brainstem networks controlling breathing are involved in this rhythm modulation remains unknown. Here, we show that in isolated neonatal rat brainstem-spinal cord preparations, the increase in respiratory rate observed during fictive locomotion is associated with an increase in the excitability of pre-inspiratory neurons of the parafacial respiratory group (pFRG/Pre-I). In addition, this locomotion-induced respiratory rhythm modulation is prevented both by bilateral lesion of the pFRG region and by blockade of neurokinin 1 receptors in the brainstem. Thus, our results assign pFRG/Pre-I neurons a new role as elements of a previously undescribed pathway involved in the functional interaction between respiratory and locomotor networks, an interaction that also involves a substance P-dependent modulating mechanism requiring the activation of neurokinin 1 receptors. This neurogenic mechanism may take an active part in the increased respiratory rhythmicity produced at the onset and during episodes of locomotion in mammals.

Optogenetic Activation of Excitatory Premotor Interneurons Is Sufficient to Generate Coordinated Locomotor Activity in Larval Zebrafish

Neural networks in the spinal cord can generate locomotion in the absence of rhythmic input from higher brain structures or sensory feedback because they contain an intrinsic source of excitation. However, the molecular identity of the spinal interneurons underlying the excitatory drive within the locomotor circuit has remained unclear. Using optogenetics, we show that activation of a molecularly defined class of ipsilateral premotor interneurons elicits locomotion. These interneurons represent the excitatory module of the locomotor networks and are sufficient to produce a coordinated swimming pattern in zebrafish. They correspond to the V2a interneuron class and express the transcription factor Chx10. They produce sufficient excitatory drive within the spinal networks to generate coordinated locomotor activity. Therefore, our results define the V2a interneurons as the excitatory module within the spinal locomotor networks that is sufficient to initiate and maintain locomotor activity.

Nitric oxide-mediated modulation of the murine locomotor network

Spinal motor control networks are regulated by neuromodulatory systems to allow adaptability of movements. The present study aimed to elucidate the role of nitric oxide (NO) in the modulation of mammalian spinal locomotor networks. This was investigated with isolated spinal cord preparations from neonatal mice in which rhythmic locomotor-related activity was induced pharmacologically. Bath application of the NO donor diethylamine NONOate (DEA/NO) decreased the frequency and modulated the amplitude of locomotor-related activity recorded from ventral roots. Removal of endogenous NO with coapplication of a NO scavenger (PTIO) and a nitric oxide synthase (NOS) blocker [nitro-l-arginine methyl ester (l-NAME)] increased the frequency and decreased the amplitude of locomotor-related activity. This demonstrates that endogenously derived NO can modulate both the timing and intensity of locomotor-related activity. The effects of DEA/NO were mimicked by the cGMP analog 8-bromo-cGMP. In addition, the soluble guanylyl cyclase (sGC) inhibitor ODQ blocked the effects of DEA/NO on burst amplitude and frequency, although the frequency effect was only blocked at low concentrations of DEA/NO. This suggests that NO-mediated modulation involves cGMP-dependent pathways. Sources of NO were studied within the lumbar spinal cord during postnatal development (postnatal days 1–12) with NADPH-diaphorase staining. NOS-positive cells in the ventral horn exhibited a rostrocaudal gradient, with more cells in rostral segments. The number of NOS-positive cells was also found to increase during postnatal development. In summary, we have shown that NO, derived from sources within the mammalian spinal cord, modulates the output of spinal motor networks and is therefore likely to contribute to the fine-tuning of locomotor behavior.

Locomotor Rhythm Generation Linked to the Output of Spinal Shox2 Excitatory Interneurons

Locomotion is controlled by spinal networks that generate rhythm and coordinate left-right and flexor-extensor patterning. Defined populations of spinal interneurons have been linked to patterning circuits; however, neurons comprising the rhythm-generating kernel have remained elusive. Here, we identify an ipsilaterally projecting excitatory interneuron population, marked by the expression of Shox2 that overlaps partially with V2a interneurons. Optogenetic silencing or blocking synaptic output of Shox2 interneurons (INs) in transgenic mice perturbed rhythm without an effect on pattern generation, whereas ablation of the Shox2 IN subset coinciding with the V2a population was without effect. Most Shox2 INs are rhythmically active during locomotion and analysis of synaptic connectivity showed that Shox2 INs contact other Shox2 INs, commissural neurons, and motor neurons,with preference for flexor motor neurons. Our findings focus attention on a subset of Shox2 INs that appearto participate in the rhythm-generating kernel for spinal locomotion.

Modelling genetic reorganization in the mouse spinal cord affecting left–right coordination during locomotion

The spinal neural circuit contains inhibitory (CINi) and excitatory (CINe) commissural interneurons with axons crossing the mid-line. Direction of these axons to the other side of the cord is controlled by axon guidance molecules, such as Netrin-1 and DCC. The cord also contains glutamatergic interneurons, whose axon guidance involves the EphA4 receptor. In EphA4 knockout (KO) and Netrin-1 KO mice, the normal left-right alternating pattern is replaced with a synchronized hopping gait, and the cord of DCC KO mice exhibits uncoordinated left and right oscillations. To investigate the effects of these genetic transformations, we used a computational model of the spinal circuits containing left and right rhythm-generating neuron populations (RGs), each with a subpopulation of EphA4-positive neurons, and CINi and CINe populations mediating mutual inhibition and excitation between the left and right RGs. In the EphA4 KO circuits, half of the EphA4-positive axons crossed the mid-line and excited the contralateral RG neurons. In the Netrin-1 KO model, the number of contralateral CINi projections was significantly reduced, while in the DCC KO model, the numbers of both CINi and CINe connections were reduced. In our simulations, the EphA4 and Netrin-1 KO circuits switched from the left-right alternating pattern to a synchronized hopping pattern, and the DCC KO network exhibited uncoordinated left-right activity. The amplification of inhibitory interactions re-established an alternating pattern in the EphA4 and DCC KO circuits, but not in the Netrin-1 KO network. The model reproduces the genetic transformations and provides insights into the organization of the spinal locomotor network.

Removing sensory input disrupts spinal locomotor activity in the early postnatal period

Motor patterns driving rhythmic movements of our lower limbs during walking are generated by groups of neurons within the spinal cord, called central pattern generators (CPGs). After suffering a spinal cord injury (SCI), many descending fibers from our brain are severed or become nonfunctional, leaving the spinal CPG network without its initiating drive. Recent studies have focused on the importance of maintaining sensory stimulation to the limbs of SCI patients as a way to initiate and control the CPG locomotor network. We began assessing the role of sensory feedback to the locomotor CPG network using a neonatal mouse spinal cord preparation where the hindlimbs are still attached. Removing sensory feedback coming from the hindlimbs by way of a lower lumbar transection or by ventral root denervation revealed a positive correlation in the ability of sensory input deprivation to disrupt ongoing locomotor activity on older versus younger animals. The differences in the motor responses as a function of age could be correlated with the loss of excitatory activity from sensory afferents. Continued studies on this field could eventually provide key information that translates into the design of novel therapeutic strategies to treat patients who have suffered a SCI.

Local Injection of Lentivirus Encoding LINGO-1-shRNA Promotes Functional Recovery in Rats With Complete Spinal Cord Transection

We used a complete spinal cord transection model and locomotor function, histological, and immunohistochemical examinations to evaluate the effects of local injection of lentivirus/LINGO-1-short hairpin RNA (VL) on rats with spinal cord injury (SCI). To demonstrate the neuroregenerative and neuroprotective effects of LINGO-1 RNAi on complete transection SCI rats. LINGO-1 has been reported as a negative regulator of axonal sprouting and its antagonist was determined to improve functional outcomes in SCI rats. However, it has not been assessed whether blockade of LINGO-1 mediated by lentivirus vectors could stimulate neural recovery after SCI. Complete spinal cord transection was made at T10 level. Suspension of lentivirus vectors encoding LINGO-1-short hairpin RNA was injected into the lesion gap. Controls received control vectors in the same manner and the sham group was subjected to laminectomy only. The Basso-Beattie-Bresnahan scale and surface righting reflex test were used to evaluate functional outcomes. Finally, the spinal cords were harvested for histological and immunohistochemical analysis. The treatment with VL improved Basso-Beattie-Bresnahan scores and surface righting reflex after SCI. Tissue repair was facilitated and the cavity area was significantly decreased in VL-treated animals. More sprouting and myelinated nerve fibers were detected within the injured site in the VL group as compared with the control. In addition, the number of survival neurons and oligodendrocytes around the epicenter was notably higher under the VL condition. Local injection of lentivirus/LINGO-1-short hairpin RNA after complete transection of spinal cord resulted in meaningful histological and functional outcomes in rats. The mechanism of VL protection may be related to its promotion of axonal sprouting, remyelination, and cell survival.

Vestibular Lesion-Induced Developmental Plasticity in Spinal Locomotor Networks during Xenopus laevis Metamorphosis

During frog metamorphosis, the vestibular sensory system remains unchanged, while spinal motor networks undergo a massive restructuring associated with the transition from the larval to adult biomechanical system. We investigated in Xenopus laevis the impact of a pre- (tadpole stage) or post-metamorphosis (juvenile stage) unilateral labyrinthectomy (UL) on young adult swimming performance and underlying spinal locomotor circuitry. The acute disruptive effects on locomotion were similar in both tadpoles and juvenile frogs. However, animals that had metamorphosed with a preceding UL expressed restored swimming behavior at the juvenile stage, whereas animals lesioned after metamorphosis never recovered. Whilst kinematic and electrophysiological analyses of the propulsive system showed no significant differences in either juvenile group, a 3D biomechanical simulation suggested that an asymmetry in the dynamic control of posture during swimming could account for the behavioral restoration observed in animals that had been labyrinthectomized before metamorphosis. This hypothesis was subsequently supported by in vivo electromyography during free swimming and in vitro recordings from isolated brainstem/spinal cord preparations. Specifically, animals lesioned prior to metamorphosis at the larval stage exhibited an asymmetrical propulsion/posture coupling as a post-metamorphic young adult. This developmental alteration was accompanied by an ipsilesional decrease in propriospinal coordination that is normally established in strict left-right symmetry during metamorphosis in order to synchronize dorsal trunk muscle contractions with bilateral hindlimb extensions in the swimming adult. Our data thus suggest that a disequilibrium in descending vestibulospinal information during Xenopus metamorphosis leads to an altered assembly of adult spinal locomotor circuitry. This in turn enables an adaptive compensation for the dynamic postural asymmetry induced by the vestibular imbalance and the restoration of functionally-effective behavior.