Spinal Locomotor Research Articles

Ferrocenyl-containing silicone nanocomposites as materials for neuronal interfaces

A synthetic method involving hydrosilylation reactions was developed to produce nanocomposites of elastic ferrocenyl-containing silicone rubber (EFSR) and multi-walled carbon nanotubes (MWCNT). The EFSR-MWCNT nanocomposites have a satisfactory elongation at break ∼80%, tensile strength (2.4 MPa), as well as electrical conductivity comparable to that of semiconductors (7∙10 −5 S·cm −1 ), all of which are necessary for application as neuronal implants. A novel prototype of a spinal cord neuronal interface based on EFSR-MWCNT was created as a prosthetic for impaired neuronal functions and to access spinal sensorimotor networks. Ferrocenyl groups in nanocomposites increase the charge injection that declines the risks of negative effects of electrical stimulation including nerve tissue damage. • Ferrocenyl-containing silicone nanocomposites with carbon nanotubes were obtained. • The nanocomposites are semiconductors with suitable tensile properties. • A novel prototype of a spinal cord neural interface was created. • The nanocomposite electrodes promote spinal cord stimulation and locomotor circuitry.

Effects of Electrical Spinal Cord Stimulation on the Activity of the Hypothalamic‐Pituitary‐Adrenocortical System and the Sensitivity of the Gastric Mucosa to Ulcerogenic Stimuli

Our previous results of two areas of unrelated researches are the basis of the present study. We demonstrated previously that the hypothalamic‐pituitary‐adrenocortical (HPA) axis is gastroprotective component of the brain‐gut axis and stress‐induced glucocorticoids are gastroprotective hormones but not ulcerogenic ones (Filaretova et al., 2016). At the same time recently electrical spinal cord stimulation began to be used for both experimental studies of motor functions regulation and rehabilitation of motor functions in patients with spinal cord injury (Gerasimenko et al., 2021; Moshonkina et al., 2016). The spinal cord stimulation directed to the activation of spinal locomotor related networks affected visceral systems as well. The aim of the present experimental work was to verify whether electrical spinal cord stimulation may activate the HPA axis and affect a sensitivity of the gastric mucosa to ulcerogenic stimuli in rats. Two ulcerogenic stimuli were used in anesthetized rats: 1) prolonged gastric ischemia/reperfusion (I/R, 30 min occlusion of celiac artery followed by 3 h of reperfusion); 2) indomethacin administration (IM, 35 mg/kg, sc). For spinal stimulation, stainless steel wire electrodes (AS632; Cooner Wire, California, USA) were fixed in fasciae between T11‐12 and L1–L2 vertebrae of anesthetized animals. Electrodes for recording motor responses to stimulation placed to the muscles of the hind limbs. A‐M System, Model 2100 stimulator was used for spinal cord stimulation. At the beginning motor thresholds determined by stimulation with monopolar rectangular pulses (1 ms). Then the spinal cord was stimulated with a frequency 30 Hz and a subthreshold current (350–700 μA) in order to exclude the influence of movements for 20 min. Sham‐stimulated (control) rats were subjected to the same manipulations except for the stimulation itself. The concentration of corticosterone in blood plasma was determined using commercial ELISA kits. The spinal cord stimulation resulted in an increase of blood glucocorticoid level 1 h later suggesting an activation of the HPA axis. The spinal stimulation, applied 1 h before the onset of ulcerogenic stimuli, also significantly attenuated gastric erosion formation induced by I/R and IM as well. The data suggest that the electrical spinal cord stimulation may activate the HPA axis and affect a sensitivity of the gastric mucosa to ulcerogenic stimuli in rats producing gastroprotective effect. We consider the results as a basis for further research to verify a possibility of an involvement of glucocorticoids in the mechanisms that may provide the beneficial effects of spinal neuromodulation.

The effects of temperature changes on the mammalian spinal locomotor CPG network and the role of TRP channels in functional compensation

Preliminary data suggests that spinal cord neural networks that contain intact sensory pathways, which include dorsal root ganglia (DRG) and thermal transient receptor potential (TRP) channels are more resilient to gradual temperature changes than isolated preparations. Moreover, how acute temperature changes affect mammalian CPG‐driven motor activity remains uncertain. We studied how acute and gradual temperature changes impact the stability of motor networks with or without sensory pathways. More specifically, we aim to investigate: 1. the difference between how acute temperature changes versus gradual temperature changes can have an impact on the stability and robustness of motor networks; 2. how sensory inputs and TRP channels affect the adaptation of spinal central pattern generator (CPG) networks to changes in temperature. We compared changes in motor neuron bursting activity between ventral L2 and L5 roots of neonatal mouse spinal cord preparations under acute (from 20℃ directly to 29℃) and gradual (gradually from 20℃, to 23℃, to 26℃, and to 29℃) temperature change conditions. Spinal cord preparations both with and without DRGs were compared. We found that the locomotor activities were increased as temperature increase under both chronic and acute changes; however, spinal cord preparations without intact DRG showed activities with less stable phasing and more overall motor bursting variability compared to those preparations with intact DRGs (on L3, L4, and L5 spinal levels). Our results suggest that the lumbar spinal locomotor CPG network activity shows more susceptibility and less adaptation to temperature changes in the absence of sensory feedback provided by DRG sensory afferents. Further studies will be conducted in the presence of TRP channel blockers to preparations with DRGs to confirm the potential compensatory role that sensory feedback afferents convey to spinal CPG networks.

Feasibility of transcutaneous spinal direct current stimulation combined with locomotor training after spinal cord injury.

Feasibility study, consisting of random-order, cross-over study of a single intervention session, followed by a parallel-arm study of 16 sessions. To investigate the feasibility of a novel combinatorial approach with simultaneous delivery of transcutaneous spinal direct current stimulation (tsDCS) and locomotor training (tsDCS + LT) after spinal cord injury, compared to sham stimulation and locomotor training (sham + LT), and examine preliminary effects on walking function. Clinical research center in the southeastern United States. Eight individuals with chronic incomplete spinal cord injury (ISCI) completed the two-part protocol. Feasibility was assessed based on safety (adverse responses), tolerability (pain, spasticity, skin integrity), and protocol achievement (session duration, intensity). Walking function was assessed with the 10 m and 6 min walk tests. There were no major adverse responses. Minimal reports of skin irritation and musculoskeletal pain were consistent between groups. Average training peak heart rate as percent of maximum (mean(SD); tsDCS + LT: 66 (4)%, sham + LT: 69 (10)%) and Borg ratings of perceived exertion (tsDCS + LT: 17.5 (1.2), sham + LT: 14.4 (1.8)) indicate both groups trained at high intensities. Walking speed gains exceeded the minimal clinically important difference (MCID) in three of four who received tsDCS + LT (0.18 (0.29) m/s) and one of four in sham + LT (-0.05 (0.23) m/s). Gains in walking endurance exceeded the MCID in one of four in each group (tsDCS + LT: 36.4 (69.0) m, sham + LT: 4.9 (56.9) m). Combinatorial tsDCS and locomotor training is safe and feasible for individuals with chronic ISCI, even those with considerable walking impairment. Study outcomes support the need to investigate the efficacy of this approach.

A new transgenic reporter line reveals expression of protocadherin 9 at a cellular level within the zebrafish central nervous system

The wiring of neuronal networks is far from understood. One outstanding question is how neurons of different types link up to form subnetworks within the greater context. Cadherins have been suggested to create an inclusion code where interconnected neurons express the same subtypes. Here, we have used a CRISPR/Cas9 knock-in approach to generate a transgenic zebrafish reporter line for protocadherin 9 (pcdh9), which is predominantly expressed within the central nervous system. Expression of eGFP was detected in subsets of neurons in the cerebellum, retina and spinal cord, in both larvae and juveniles. A closer characterization of the spinal locomotor network revealed that a portion of distinct classes of both excitatory and inhibitory interneurons, as well as motor neurons, expressed pcdh9. This transgenic line could thus be used to test the cadherin network hypothesis, through electrophysiological characterization of eGFP positive cells, to show if these are synaptically connected and form a discrete network within the spinal cord.

Activity of Spinal Interneurons during Forward and Backward Locomotion.

Higher vertebrates are capable not only of forward but also backward and sideways locomotion. Also, single steps in different directions are generated for postural corrections. While the networks responsible for the control of forward walking (FW) have been studied in considerable detail, the networks controlling steps in other directions are mostly unknown. Here, to characterize the operation of the spinal locomotor network during FW and backward walking (BW), we recorded the activity of individual spinal interneurons from L4 to L6 during both FW and BW evoked by epidural stimulation (ES) of the spinal cord at L5-L6 in decerebrate cats of either sex. Three groups of neurons were revealed. Group 1 (45%) had a similar phase of modulation during both FW and BW. Group 2 (27%) changed the phase of modulation in the locomotor cycle depending on the direction of locomotion. Group 3 neurons were modulated during FW only (Group 3a, 21%) or during BW only (Group 3b, 7%). We suggest that Group 1 neurons belong to the network generating the vertical component of steps (the limb elevation and lowering) because it should operate similarly during locomotion in any direction, while Groups 2 and 3 neurons belong to the networks controlling the direction of stepping. Results of this study provide new insights into the organization of the spinal locomotor circuits, advance our understanding of ES therapeutic effects, and can potentially be used for the development of novel strategies for recuperation of impaired balance control, which requires the generation of corrective steps in different directions.SIGNIFICANCE STATEMENT Animals and humans can perform locomotion in different directions in relation to the body axis (forward, backward, sideways). While the networks that control forward walking have been studied in considerable detail, the networks controlling steps in other directions are unknown. Here, by recording the activity of the same spinal neurons during forward and backward walking, we revealed three groups of neurons forming, respectively, the network operating similarly during stepping in different directions, the network changing its operation with a change in the direction of stepping, and the network operating only during locomotion in a specific direction. These networks presumably control different aspects of the step. The obtained results provide new insights into the organization of the spinal locomotor networks.

Long descending commissural V0v neurons ensure coordinated swimming movements along the body axis in larval zebrafish

Developmental maturation occurs in slow swimming behavior in larval zebrafish; older larvae acquire the ability to perform slow swimming while keeping their head stable in the yaw dimension. A class of long-distance descending commissural excitatory V0v neurons, called MCoD neurons, are known to develop in a later phase of neurogenesis, and participate in slow swimming in older larvae. We hypothesized that these MCoD neurons play a role in coordinating the activities of trunk muscles in the diagonal dimension (e.g., the rostral left and the caudal right) to produce the S-shaped swimming form that contributes to the stability of the head. Here, we show that MCoD neurons do indeed play this role. In larvae in which MCoD neurons were laser-ablated, the swimming body form often adopted a one-sided (C-shaped) bend with reduced appearance of the normal S-shaped bend. With this change in swimming form, the MCoD-ablated larvae exhibited a greater degree of head yaw displacement during slow swimming. In mice, the long-distance descending commissural V0v neurons have been implicated in diagonal interlimb coordination during walking. Together with this, our study suggests that the long-distance descending commissural V0v neurons form an evolutionarily conserved pathway in the spinal locomotor circuits that coordinates the movements of the diagonal body/limb muscles.

Decoding the Mouse Spinal Cord Locomotor Neural Network Using Tissue Clearing, Tissue Expansion and Tiling Light Sheet Microscopy Techniques

Decoding the Mouse Spinal Cord Locomotor Neural Network Using Tissue Clearing, Tissue Expansion and Tiling Light Sheet Microscopy Techniques

Single Cell Transcriptomic Analysis of Spinal Dmrt3 Neurons in Zebrafish and Mouse Identifies Distinct Subtypes and Reveal Novel Subpopulations Within the dI6 Domain.

The spinal locomotor network is frequently used for studies into how neuronal circuits are formed and how cellular activity shape behavioral patterns. A population of dI6 interneurons, marked by the Doublesex and mab-3 related transcription factor 3 (Dmrt3), has been shown to participate in the coordination of locomotion and gaits in horses, mice and zebrafish. Analyses of Dmrt3 neurons based on morphology, functionality and the expression of transcription factors have identified different subtypes. Here we analyzed the transcriptomes of individual cells belonging to the Dmrt3 lineage from zebrafish and mice to unravel the molecular code that underlies their subfunctionalization. Indeed, clustering of Dmrt3 neurons based on their gene expression verified known subtypes and revealed novel populations expressing unique markers. Differences in birth order, differential expression of axon guidance genes, neurotransmitters, and their receptors, as well as genes affecting electrophysiological properties, were identified as factors likely underlying diversity. In addition, the comparison between fish and mice populations offers insights into the evolutionary driven subspecialization concomitant with the emergence of limbed locomotion.

Trpm5 channels encode bistability of spinal motoneurons and ensure motor control of hindlimbs in mice

Bistable motoneurons of the spinal cord exhibit warmth-activated plateau potential driven by Na+ and triggered by a brief excitation. The thermoregulating molecular mechanisms of bistability and their role in motor functions remain unknown. Here, we identify thermosensitive Na+-permeable Trpm5 channels as the main molecular players for bistability in mouse motoneurons. Pharmacological, genetic or computational inhibition of Trpm5 occlude bistable-related properties (slow afterdepolarization, windup, plateau potentials) and reduce spinal locomotor outputs while central pattern generators for locomotion operate normally. At cellular level, Trpm5 is activated by a ryanodine-mediated Ca2+ release and turned off by Ca2+ reuptake through the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump. Mice in which Trpm5 is genetically silenced in most lumbar motoneurons develop hindlimb paresis and show difficulties in executing high-demanding locomotor tasks. Overall, by encoding bistability in motoneurons, Trpm5 appears indispensable for producing a postural tone in hindlimbs and amplifying the locomotor output.

Epidural spinal cord stimulation as an intervention for motor recovery after motor complete spinal cord injury.

Spinal cord injury (SCI) commonly results in permanent loss of motor, sensory, and autonomic function. Recent clinical studies have shown that epidural spinal cord stimulation may provide a beneficial adjunct for restoring lower extremity and other neurological functions. Herein, we review the recent clinical advances of lumbosacral epidural stimulation for restoration of sensorimotor function in individuals with motor complete SCI and we discuss the putative neural pathways involved in this promising neurorehabilitative approach. We focus on three main sections: review recent clinical results for locomotor restoration in complete SCI; discuss the contemporary understanding of electrical neuromodulation and signal transduction pathways involved in spinal locomotor networks; and review current challenges of motor system modulation and future directions toward integrative neurorestoration. The current understanding is that initial depolarization occurs at the level of large diameter dorsal root proprioceptive afferents that when integrated with interneuronal and latent residual supraspinal translesional connections can recruit locomotor centers and augment downstream motor units. Spinal epidural stimulation can initiate excitability changes in spinal networks and supraspinal networks. Different stimulation parameters can facilitate standing or stepping, and it may also have potential for augmenting myriad other sensorimotor and autonomic functions. More comprehensive investigation of the mechanisms that mediate the transformation of dysfunctional spinal networks to higher functional states with a greater focus on integrated systems-based control system may reveal the key mechanisms underlying neurological augmentation and motor restoration after severe paralysis.

L‐DOPA and 5‐HTP modulation of air‐stepping in newborn rats

In newborn rats, L-DOPA increases the occurrence of air-stepping activity without affecting movement characteristics. L-DOPA administration increases the spinal content of dopamine in a dose-dependent manner. Injection of 5-HTP increases the spinal serotonin content but does not trigger air-stepping. 5-HTP counteracts the pro-locomotor action of L-DOPA. Less dopamine and serotonin are synthesized when L-DOPA and 5-HTP are administered as a cocktail. The catecholamine precursor, L-3,4-dihydroxyphenylalanine (L-DOPA), is a well-established pharmacological agent for promoting locomotor action in vertebrates, including triggering air-stepping activities in the neonatal rat. Serotonin is also a well-known neuromodulator of the rodent spinal locomotor networks. Here, using kinematic analysis, we compared locomotor-related activities expressed by newborn rats in response to varying doses of L-DOPA and the serotonin precursor 5-hydroxytryptophan (5-HTP) administered separately or in combination. L-DOPA alone triggered episodes of air-stepping in a dose-dependent manner (25-100mg/kg), notably determining the duration of locomotor episodes, but without affecting step cycle frequency or amplitude. In contrast, 5-HTP (25-150mg/kg) was ineffective in instigating air-stepping, but altered episode durations of L-DOPA-induced air-stepping, and decreased locomotor cycle frequency. High performance liquid chromatography revealed that L-DOPA, which was undetectable in control conditions, accumulated in a dose-dependent manner in the lumbar spinal cord 30min after its administration. This was paralleled by an increase in dopamine levels, whereas the spinal content of noradrenaline and serotonin remained unaffected. In the same way, the spinal levels of serotonin increased in parallel with the dose of 5-HTP without affecting the levels of dopamine and noradrenaline. When both precursors are administrated, they counteract each other for the production of serotonin and dopamine. Our data thus indicate for the first time that both L-DOPA and 5-HTP exert opposing neuromodulatory actions on air-stepping behaviour in the developing rat, and we speculate that competition for the production of dopamine and serotonin occurs when they are administered as a cocktail.

Modeling spinal locomotor circuits for movements in developing zebrafish.

Many spinal circuits dedicated to locomotor control have been identified in the developing zebrafish. How these circuits operate together to generate the various swimming movements during development remains to be clarified. In this study, we iteratively built models of developing zebrafish spinal circuits coupled to simplified musculoskeletal models that reproduce coiling and swimming movements. The neurons of the models were based upon morphologically or genetically identified populations in the developing zebrafish spinal cord. We simulated intact spinal circuits as well as circuits with silenced neurons or altered synaptic transmission to better understand the role of specific spinal neurons. Analysis of firing patterns and phase relationships helped to identify possible mechanisms underlying the locomotor movements of developing zebrafish. Notably, our simulations demonstrated how the site and the operation of rhythm generation could transition between coiling and swimming. The simulations also underlined the importance of contralateral excitation to multiple tail beats. They allowed us to estimate the sensitivity of spinal locomotor networks to motor command amplitude, synaptic weights, length of ascending and descending axons, and firing behavior. These models will serve as valuable tools to test and further understand the operation of spinal circuits for locomotion.

Optogenetic Activation of V1 Interneurons Reveals the Multimodality of Spinal Locomotor Networks in the Neonatal Mouse.

In the spinal cord, classes of interneurons have been studied in vitro to determine their role in producing or regulating locomotion. It is unclear whether all locomotor behaviors are produced by the same circuitry or engage different subsets of neurons. Here, in neonatal mice of either sex, we test this idea by comparing the actions of a class of spinal, inhibitory interneuron (V1) expressing channelrhodopsin driven by the engrailed-1 transcription factor on the rhythms elicited by different methods. We find that, although the overall locomotor activities in vitro are similar, V1 interneuron depolarization produces opposite effects depending of the mode of activation of the locomotor circuitry. The differential behavior of V1 neurons suggests that their function depends on how the locomotor rhythm is activated and is consistent with the idea that the functional organization of the corresponding locomotor networks also differs.SIGNIFICANCE STATEMENT The neural networks dictating the execution of fictive locomotion are located in the spinal cord. It is generally assumed that the mode of activation of these spinal networks should not change the recruitment or function of neurons. Here, we manipulated the activity of a class of interneuron (V1), which targets these networks and found that their activation induces opposite effects depending on the mode of activation. This suggests that the mode of activation of the spinal networks differentially recruits either V1 interneurons or other interneurons, or both.

Locomotion dependent neuron-glia interactions control neurogenesis and regeneration in the adult zebrafish spinal cord

Physical exercise stimulates adult neurogenesis, yet the underlying mechanisms remain poorly understood. A fundamental component of the innate neuroregenerative capacity of zebrafish is the proliferative and neurogenic ability of the neural stem/progenitor cells. Here, we show that in the intact spinal cord, this plasticity response can be activated by physical exercise by demonstrating that the cholinergic neurotransmission from spinal locomotor neurons activates spinal neural stem/progenitor cells, leading to neurogenesis in the adult zebrafish. We also show that GABA acts in a non-synaptic fashion to maintain neural stem/progenitor cell quiescence in the spinal cord and that training-induced activation of neurogenesis requires a reduction of GABAA receptors. Furthermore, both pharmacological stimulation of cholinergic receptors, as well as interference with GABAergic signaling, promote functional recovery after spinal cord injury. Our findings provide a model for locomotor networks’ activity-dependent neurogenesis during homeostasis and regeneration in the adult zebrafish spinal cord.

Role of DSCAM in the Development of Neural Control of Movement and Locomotion

Locomotion results in an alternance of flexor and extensor muscles between left and right limbs generated by motoneurons that are controlled by the spinal interneuronal circuit. This spinal locomotor circuit is modulated by sensory afferents, which relay proprioceptive and cutaneous inputs that inform the spatial position of limbs in space and potential contacts with our environment respectively, but also by supraspinal descending commands of the brain that allow us to navigate in complex environments, avoid obstacles, chase prey, or flee predators. Although signaling pathways are important in the establishment and maintenance of motor circuits, the role of DSCAM, a cell adherence molecule associated with Down syndrome, has only recently been investigated in the context of motor control and locomotion in the rodent. DSCAM is known to be involved in lamination and delamination, synaptic targeting, axonal guidance, dendritic and cell tiling, axonal fasciculation and branching, programmed cell death, and synaptogenesis, all of which can impact the establishment of motor circuits during development, but also their maintenance through adulthood. We discuss herein how DSCAM is important for proper motor coordination, especially for breathing and locomotion.

Inhibition and Facilitation of the Spinal Locomotor Central Pattern Generator and Reflex Circuits by Somatosensory Feedback From the Lumbar and Perineal Regions After Spinal Cord Injury.

Somatosensory feedback from peripheral receptors dynamically interacts with networks located in the spinal cord and brain to control mammalian locomotion. Although somatosensory feedback from the limbs plays a major role in regulating locomotor output, those from other regions, such as lumbar and perineal areas also shape locomotor activity. In mammals with a complete spinal cord injury, inputs from the lumbar region powerfully inhibit hindlimb locomotion, while those from the perineal region facilitate it. Our recent work in cats with a complete spinal cord injury shows that they also have opposite effects on cutaneous reflexes from the foot. Lumbar inputs increase the gain of reflexes while those from the perineal region decrease it. The purpose of this review is to discuss how somatosensory feedback from the lumbar and perineal regions modulate the spinal locomotor central pattern generator and reflex circuits after spinal cord injury and the possible mechanisms involved. We also discuss how spinal cord injury can lead to a loss of functional specificity through the abnormal activation of functions by somatosensory feedback, such as the concurrent activation of locomotion and micturition. Lastly, we discuss the potential functions of somatosensory feedback from the lumbar and perineal regions and their potential for promoting motor recovery after spinal cord injury.

Spinal Locomotion in Cats Following Spinal Cord Injury: A Prospective Study.

Simple SummaryFunctional neurorehabilitation promotes neural reorganization by stimulating subjects without deep pain perception, leading to a faster recovery when compared to spontaneous recovery, and achieving fewer compensatory errors, or even deviations to neuropathic or adaptive pain pathways, such as spasticity. The present study demonstrates the importance of intensive and repetition-based functional neurorehabilitation, which is essential for subjects classified as grade 0 according to the modified Frankel scale. This article aimed to evaluate the safety and efficacy of intensive neurorehabilitation in paraplegic cats, with no deep pain perception (grade 0 on the modified Frankel scale), with more than three months of injury. Nine cats, admitted to the Arrábida Veterinary Hospital/Arrábida Animal Rehabilitation Center (CRAA), were subjected to a 12-week intensive functional neurorehabilitation protocol, based on ground and underwater treadmill locomotor training, electrostimulation, and kinesiotherapy exercises, aiming to obtain a faster recovery to ambulation and a modulated locomotor pattern of flexion/extension. Of the nine cats that were admitted in this study, 56% (n = 5) recovered from ambulation, 44% of which (4/9) did so through functional spinal locomotion by reflexes, while one achieved this through the recovery of deep pain perception. These results suggest that intensive neurorehabilitation can play an important role in ambulation recovery, allowing for a better quality of life and well-being, which may lead to a reduction in the number of euthanasia procedures performed on paraplegic animals.

Computational Modeling of Spinal Locomotor Circuitry in the Age of Molecular Genetics.

Neuronal circuits in the spinal cord are essential for the control of locomotion. They integrate supraspinal commands and afferent feedback signals to produce coordinated rhythmic muscle activations necessary for stable locomotion. For several decades, computational modeling has complemented experimental studies by providing a mechanistic rationale for experimental observations and by deriving experimentally testable predictions. This symbiotic relationship between experimental and computational approaches has resulted in numerous fundamental insights. With recent advances in molecular and genetic methods, it has become possible to manipulate specific constituent elements of the spinal circuitry and relate them to locomotor behavior. This has led to computational modeling studies investigating mechanisms at the level of genetically defined neuronal populations and their interactions. We review literature on the spinal locomotor circuitry from a computational perspective. By reviewing examples leading up to and in the age of molecular genetics, we demonstrate the importance of computational modeling and its interactions with experiments. Moving forward, neuromechanical models with neuronal circuitry modeled at the level of genetically defined neuronal populations will be required to further unravel the mechanisms by which neuronal interactions lead to locomotor behavior.

Spinal Cord Injury Alters Spinal Shox2 Interneurons by Enhancing Excitatory Synaptic Input and Serotonergic Modulation While Maintaining Intrinsic Properties in Mouse.

Neural circuitry generating locomotor rhythm and pattern is located in the spinal cord. Most spinal cord injuries (SCIs) occur above the level of spinal locomotor neurons; therefore, these circuits are a target for improving motor function after SCI. Despite being relatively intact below the injury, locomotor circuitry undergoes substantial plasticity with the loss of descending control. Information regarding cell type-specific plasticity within locomotor circuits is limited. Shox2 interneurons (INs) have been linked to locomotor rhythm generation and patterning, making them a potential therapeutic target for the restoration of locomotion after SCI. The goal of the present study was to identify SCI-induced plasticity at the level of Shox2 INs in a complete thoracic transection model in adult male and female mice. Whole-cell patch-clamp recordings of Shox2 INs revealed minimal changes in intrinsic excitability properties after SCI. However, afferent stimulation resulted in mixed excitatory and inhibitory input to Shox2 INs in uninjured mice which became predominantly excitatory after SCI. Shox2 INs were differentially modulated by serotonin (5-HT) in a concentration-dependent manner in uninjured conditions but following SCI, 5-HT predominantly depolarized Shox2 INs. 5-HT7 receptors mediated excitatory effects on Shox2 INs from both uninjured and SCI mice, but activation of 5-HT2B/2C receptors enhanced excitability of Shox2 INs only after SCI. Overall, SCI alters sensory afferent input pathways to Shox2 INs and 5-HT modulation of Shox2 INs to enhance excitatory responses. Our findings provide relevant information regarding the locomotor circuitry response to SCI that could benefit strategies to improve locomotion after SCI.SIGNIFICANCE STATEMENT Current therapies to gain locomotor control after spinal cord injury (SCI) target spinal locomotor circuitry. Improvements in therapeutic strategies will require a better understanding of the SCI-induced plasticity within specific locomotor elements and their controllers, including sensory afferents and serotonergic modulation. Here, we demonstrate that excitability and intrinsic properties of Shox2 interneurons, which contribute to the generation of the locomotor rhythm and pattering, remain intact after SCI. However, SCI induces plasticity in both sensory afferent pathways and serotonergic modulation, enhancing the activation and excitation of Shox2 interneurons. Our findings will impact future strategies looking to harness these changes with the ultimate goal of restoring functional locomotion after SCI.