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The mammalian lumbar spinal cord experimentally isolated from supraspinal and afferent feedback input remains capable of expressing some basic locomotor function when appropriately stimulated. This ability has been attributed to spinal neural circuits referred to as central pattern generators (CPGs). In individuals with a severe spinal cord injury, rhythmic activity in paralyzed leg muscles can be generated by phasic proprioceptive feedback during therapist- or robotic-assisted stepping on a motorized treadmill. Here, we critically review to what extent the resulting motor output represents locomotor-like activity, and whether these motor patterns are the result of activation of CPGs, as commonly suggested in the literature. Attempts will be made to further delineate the pivotal roles played by mechanisms such as spinal proprioceptive reflexes and their alterations after spinal cord injury, the central excitability level, and by neurotransmitters critical for spinal locomotor activity. We will discuss the view that the muscle activity produced during assisted passive treadmill stepping is resulting from the entrainment of spinal reflex circuits by the cyclically generated proprioceptive feedback. We suggest that the activation of CPG circuits depends rather on the presence of a sustained tonic excitatory drive, as can be provided by electrical spinal cord stimulation, or by specific combinations of dopaminergic agonists, adrenergic/ dopaminergic precursors and/or 5-HT receptor agonists. Novel rehabilitation strategies using spinal cord stimulation and rhythmic-activity producing drugs during locomotor therapy will pave the way for clinically relevant advances in restoration of motor function in people with severe spinal cord injury.

Locomotion is one of the most complex motor behaviors. Locomotor patterns change during early life, reflecting development of numerous peripheral and hierarchically organized central structures. Among them, the spinal cord is of particular interest since it houses the central pattern generator (CPG) for locomotion. This main command center is capable of eliciting and coordinating complex series of rhythmic neural signals sent to motoneurons and to corresponding target-muscles for basic locomotor activity. For a long-time, the CPG has been considered a black box. In recent years, complementary insights from in vitro and in vivo animal models have contributed significantly to a better understanding of its constituents, properties and ways to recover locomotion after a spinal cord injury (SCI). This review discusses key findings made by comparing the results of in vitro isolated spinal cord preparations and spinal-transected in vivo models from neonatal animals. Pharmacological, electrical, and sensory stimulation approaches largely used to further understand CPG function may also soon become therapeutic tools for potent CPG reactivation and locomotor movement induction in persons with SCI or developmental neuromuscular disorder.

Background: No drug treatment capable of restoring locomotor capabilities in patients suffering a motor-complete spinal cord injury (SCI) has ever been developed. We assessed the safety and efficacy of an activator of spinal locomotor neurons in humans, which were shown in paraplegic animals to elicit temporary episodes of involuntary walking. Methods: Single administration of buspirone/levodopa/carbidopa (SpinalonTM), levodopa/carbidopa (ratio 4: 1), and buspirone or placebo was performed using a dose-escalation design in 45 subjects placed in supine position who had had an SCI classified as complete (AIS A) or motor-complete/sensory incomplete (AIS B) for at least 3 months. Blood samples before and at regular intervals (15, 30, 60, 120, 240 min) after treatment were collected for hematological and pharmacokinetic (PK) analyses. Electromyographic (EMG) activity of eight muscles (four per leg) was monitored prior to and at several time points after drug administration. Results: SpinalonTM (10-35 mg buspirone/100-350 mg levodopa/25-85 mg carbidopa) displayed no sign of safety concerns - only mild nausea was found in 3 cases. At higher doses, 50 mg/500 mg/125 mg SpinalonTM was considered to have reached maximum tolerated dose (MTD) since 3 out of 4 subjects experienced related adverse events including vomiting. PK analyses showed comparable data between groups suggesting no significant drugdrug interaction with SpinalonTM. Only the SpinalonTM–treated groups displayed significant EMG activity accompanied by locomotor-like characteristics - that is with rhythmic and bilaterally alternating bursts. Conclusion: Therefore, this study provides evidence of safety and preliminary efficacy following a single administration of SpinalonTM in subjects with SCI.

Homo sapiens constitute the only currently obligate bipedal mammals and, as it stands, upright bipedal locomotion is a defining characteristic of humans. Indeed, while the evolution to bipedalism has allowed for the upper limbs to be liberated from ground contact during ambulation, their role in locomotion is far from obsolete. Rather, there is reason to believe that arm swing offers important mechanical and neurological advantages to bipedal locomotion. In this short review, we present some compelling findings on the neural connections between the arms and legs during human locomotion. We begin with a description of the importance of arm swing during walking from a mechanical perspective. Then, we examine evidence for the existence of interappendicular connections that converge along with peripheral afferents, descending inputs, and propriospinal projections, onto the neural circuits innervating the muscles of the arms and legs. The varied effects of interappendicular coupling on the neural control of locomotion are also examined in cases of neurological injury. We use the insight gained from these collected works as well as those from our own studies on locomotor training to discuss strategies to use interappendicular connections to rehabilitate walking in individuals experiencing loss of function after debilitating spinal cord injury.

Most animal models of contused, compressed or transected spinal cord injury (SCI) require a laminectomy to be performed. However, despite advantages and disadvantages associated with each of these models, the laminectomy itself is generally associated with significant problems including longer surgery and anaesthesia (related post-operative complications), neuropathic pain, spinal instabilities, deformities, lordosis, and biomechanical problems, etc. This review provides an overview of findings obtained mainly from our laboratory that are associated with the development and characterization of a novel murine model of spinal cord transection that does not require a laminectomy. A number of studies successfully conducted with this model provided strong evidence that it constitutes a simple, reliable and reproducible transection model of complete paraplegia which is particularly useful for studies on large cohorts of wild-type or mutant animals - e.g., drug screening studies in vivo or studies aimed at characterizing neuronal and non-neuronal adaptive changes post-trauma. It is highly suitable also for studies aimed at identifying and developing new pharmacological treatments against aging associated comorbid problems and specific SCI-related dysfunctions (e.g., stereotyped motor behaviours such as locomotion, sexual response, defecation and micturition) largely related with 'command centers' located in lumbosacral areas of the spinal cord.

γ-aminobutyric acid (GABA) plays many of its key roles in embryonic development and functioning of the central nervous system (CNS) by acting on ligand gated chloride-permeable channels known as GABA A receptors (GABA A R). Classically, GABA A Rmediated synaptic communication is tailored to allow rapid and precise transmission of information to synchronize the activity of large populations of cells to generate and maintain neuronal networks oscillations. An alternative type of inhibition mediated by GABA A receptors, initially described about 25 years ago, is characterized by a tonic activation of receptors that react to ambient extracellular GABA. The receptors that mediate this action are wide-spread throughout the nerve cells but are located distant from the sites of GABA release, and therefore they have been called extrasynaptic GABA A receptors. The molecular nature of the extrasynaptic GABA A receptors and the tonic inhibitory current they generate have been characterized in many brain structures, and due to its relevance in controlling neuron excitability they have become attractive pharmacological targets for a variety of neurological disorders such as schizophrenia, epilepsy and Parkinson disease. In the spinal cord, early studies have implicated these receptors in anesthesia, chronic pain, motor control, and locomotion. This review highlights past and present developments in the field of extrasynaptic GABA A receptors and emphasizes their subunit containing distribution and physiological role in the spinal cord.

Background Individuals with spinal cord injury (SCI) have increased rates of glucose intolerance, insulin insensitivity, and type II diabetes caused mainly by the deconditioning of paralyzed muscle. The purpose of this systematic review was to determine the effectiveness of locomotor training in individuals with SCI on blood glucose control. Methods We searched studies on locomotor training for individuals with SCI with outcomes of glucose, insulin, or outcomes that could change glucose handling (i.e. increases in muscle mass, shifts in muscle fiber type composition, changes in transport proteins, or enzymes involved in glucose metabolism) in PubMed and EMBASE. Results Eleven studies (10 with incomplete SCI; 1 with complete SCI) were included in our review. Locomotor training included body weight supported treadmill training (BWSTT) with manual or robotic assistance, with and without functional electrical stimulation (FES), or involved FES-assisted over ground training. Six months of locomotor training in individuals with SCI resulted in significant decreases in glucose (15%) and insulin (33%) areas under the curve during oral glucose tolerance tests. Two to twelve months of locomotor training reversed some of the muscle atrophy - with muscle being the site of most glucose consumption, this is important for glucose control. Training also increased capacity for glucose storage, enzymes involved in glucose phosphorylation (hexokinase) and oxidation (citrate synthase), and glucose transport proteins (GLUT-4). Fiber type composition shifted to a slower fiber type, which favors glucose handling. There were no effects on fat mass. Conclusion Locomotor training in individuals with SCI (generally an incomplete injury) increases capacity to handle glucose and results in muscular changes that should reduce the risk of type II diabetes.

The aim of this review is to describe the rationale and main underlying reasons for undertaking, during clinical development, the study of drug candidates used separately and/or in combination with other technologies. To ease comprehension, reference will be made to the case of SpinalonTM, a new fixed-dose combination (FDC) product composed of levodopa/carbidopa/buspirone. This drug is capable of triggering, within minutes after a single administration orally, 45 minute- episodes of basic involuntary 'reflex' walking in paraplegic animals. Daily administration during one month was shown to lead to increased performance over time, with health benefits onto musculoskeletal and cardiovascular systems. A double-blind, dose-escalation, randomized phase I/IIa study with 45 spinal cord-injured subjects successfully provided the maximal tolerated dose (MTD) and preliminary evidence of efficacy. As an attempt to explore how efficacy may be optimized, a phase IIb study with 150 subjects was designed to compare the effects of repeated administration in different conditions (arms). Tests with a motorized treadmill, a harness for body weight support, a transdermal spinal cord stimulator and/or an exoskeleton were proposed because: 1) these devices are unlikely to alter safety but, 2) they are reasonably expected to increase spinal locomotor neuron activation, reflex walking induction, and musculoskeletal/cardiovascular benefits. This approach would normally allow the phase III study to demonstrate clearly, with fewer subjects and at lower costs, long-term benefits on health of SpinalonTM used in optimized conditions and settings. This innovative strategy in drug development may contribute to further describe the mechanisms of action as well as optimized conditions of use for patients. Adapted to the development of other products, such an approach may enable greater safety, efficacy, clinical utility and compliance to be sought for next-generation CNS drugs.

Locomotion is a complex motor behavior that may be expressed in different ways using a variety of strategies depending upon species and pathological or environmental conditions. Quadrupedal or bipedal walking, running, swimming, flying and gliding constitute some of the locomotor modes enabling the body, in all cases, to move from one place to another. Despite these apparent differences in modes of locomotion, both vertebrate and invertebrate species share, at least in part, comparable neural control mechanisms for locomotor rhythm and pattern generation and modulation. Significant advances have been made in recent years in studies of the genetic aspects of these control systems. Findings made specifically using Drosophila (fruit fly) models and preparations have contributed to further understanding of the key role of genes in locomotion. This review focuses on some of the main findings made in larval fruit flies while briefly summarizing the basic advantages of using this powerful animal model for studying the neural locomotor system.

Background: The basic motor patterns driving rhythmic limb movements during walking are generated by networks of neurons called central pattern generators (CPGs). Within motor control systems, neuromodulators are necessary for proper and efficient CPG function because they induce or regulate essential components of spinal network activity, including firing parameters of CPG neurons and network synaptic strength, allowing the network to change/adapt and sometimes to even become functional. Methods: The goal of this work is to focus on classical and recent findings addressing the role of neuromodulators such as glutamate, dopamine, acetylcholine and adenosine in eliciting, changing and sometimes terminating spinal CPG network function in rodents. Results: Neuromodulatory inputs onto CPG locomotor networks have been additionally related to inducing state changes such as locomotor timing, phasing and speed, and to the induction/maintenance of actual network function. These inputs originate from supraspinal centers such as the brainstem and from intraspinal neurotransmission. The isolated in vitro rodent spinal cord preparation is a powerful model for studies on locomotor network organization because of its physiological and anatomical accessibility, as well as the incorporation of various transgenic approaches to identify specific neuronal populations. Both roles are accomplished through the action of neuromodulators on ionotropic and metabotropic receptors mediating synaptic neurotransmission, which can be used by neurons that are intrinsic or extrinsic components of a CPG network itself. Conclusion: This article has hopefully provided a comprehensive overview of some of the main spinal mechanisms involved in the modulatory control of locomotor activity.

γ-aminobutyric acid (GABA) plays many of its key roles in embryonic development and functioning of the central nervous system (CNS) by acting on ligand gated chloride-permeable channels known as GABAA receptors (GABAAR). Classically, GABAARmediated synaptic communication is tailored to allow rapid and precise transmission of information to synchronize the activity of large populations of cells to generate and maintain neuronal networks oscillations. An alternative type of inhibition mediated by GABAA receptors, initially described about 25 years ago, is characterized by a tonic activation of receptors that react to ambient extracellular GABA. The receptors that mediate this action are wide-spread throughout the nerve cells but are located distant from the sites of GABA release, and therefore they have been called extrasynaptic GABAA receptors. The molecular nature of the extrasynaptic GABAA receptors and the tonic inhibitory current they generate have been characterized in many brain structures, and due to its relevance in controlling neuron excitability they have become attractive pharmacological targets for a variety of neurological disorders such as schizophrenia, epilepsy and Parkinson disease. In the spinal cord, early studies have implicated these receptors in anesthesia, chronic pain, motor control, and locomotion. This review highlights past and present developments in the field of extrasynaptic GABAA receptors and emphasizes their subunit containing distribution and physiological role in the spinal cord.

Much like locomotion or micturition, respiration is a rhythmic and stereotyped motor pattern controlled mainly by non-cortical structures including a complex circuit in the brainstem. Because tight regulation of lung ventilation is essential from the beginning of life, it has been presumed that the neural system regulating breathing is fixed, following a genetically predetermined developmental pattern. Here, we review evidence indicating that early life exposure to a non-systemic stress in the form of neonatal maternal separation (NMS) is sufficient to exert sex-specific consequences on the developmental trajectory of this vital homeostatic system that persist well into full maturity. At adulthood, male rats subjected to NMS are hypertensive and show an abnormally high hypoxic chemoreflex that correlates positively with respiratory instability during sleep. The effects are not observed in females. Investigation of the mechanisms this respiratory phenotype have highlighted the importance of 1) neuroendocrine influences on respiratory regulation and 2) stress-related imbalance between inhibitory (GABAergic) and excitatory (glutamatergic) modulation of the neural elements regulating breathing. These results provide new and valuable insight into the origins of respiratory disorders related to neural control dysfunction such as sleep disordered breathing.

Spinal cord injury (SCI) often results in permanent paralysis because there is little spontaneous repair. Neuronal injury in the central nervous system (CNS) causes breakage of axonal connections, release of myelin, inflammation and cell death at the lesion site. Many factors contribute to the failure of spontaneous repair after SCI, including the presence of growth inhibitory proteins in myelin, the inflammatory environment of the injured CNS, and the resulting signaling cascades that result in over-activation of Rho, a signaling switch in neurons and axons. In this review, we provide a general overview of growth inhibition in the CNS, and show evidence that most growth inhibitory proteins signal through a common intracellular pathway. Rho is a convergent signal for growth inhibition, and also for signaling some of the secondary consequences of inflammation after SCI. We review the preclinical evidence that targeting Rho is an effective way to stimulate axon regeneration and functional recovery in preclinical animal models. In the last part of the review, we describe the creation of Cethrin, a new investigational drug, and summarize the results of the Phase I/IIa clinical study to examine the safety, tolerability and efficacy of Cethrin in patients with acute SCI. We conclude with some insight for future clinical studies.

Although research on neural tissue repair has made enormous progress in recent years, spinal cord injury remains a devastating condition for which there is still no cure. In fact, recent estimates of prevalence in the United States reveal that spinal cord injury has undergone a five-fold increase in the last decades. Though, it has become the second most common neurological problem in North America after Alzheimer's disease. Despite modern trauma units and intensive care treatments, spinal cord injury remains associated with several comorbid conditions and unbearable health care costs. Regular administration of a plethora of symptomatic drug treatments aimed at controlling related-secondary complications and life-threatening problems in chronic spinal cord-injured patients has recently been reported. This article provides a thorough overview of the main drug classes and products currently used or in development for chronic spinal cord injury. Special attention is paid to a novel class of drug treatment designed to provide a holistic solution for several chronic complications and diseases related with spinal cord injury. There is clear evidence showing that new class can elicit 'on-demand' episodes of rhythmic and stereotyped walking activity in previously completely paraplegic animals and may consequently constitute a simple therapy against several physical inactivity-related comorbid problems. Understanding further pharmacological approaches to chronic spinal cord injury may improve both life expectancy and overall quality of life while reducing unsustainable cost increases associated with this debilitation condition.

Male sexual responses are reflexes mediated by the spinal cord and modulated by neural circuitries involving both the peripheral and central nervous system. While the brain interact with the reflexes to allow perception of sexual sensations and to exert excitatory or inhibitory influences, penile reflexes can occur despite complete transections of the spinal cord, as demonstrated by the reviewed animal studies on spinalization and human studies on spinal cord injury. Neurophysiological and neuropharmacological substrates of the male sexual responses will be discussed in this review, starting with the spinal mediation of erection and its underlying mechanism with nitric oxide (NO), followed by the description of the ejaculation process, its neural mediation and its coordination by the spinal generator of ejaculation (SGE), followed by the occurrence of climax as a multisegmental sympathetic reflex discharge. Brain modulation of these reflexes will be discussed through neurophysiological evidence involving structures such as the medial preoptic area of hypothalamus (MPOA), the paraventricular nucleus (PVN), the periaqueductal gray (PAG), and the nucleus para-gigantocellularis (nPGI), and through neuropharmacological evidence involving neurotransmitters such as serotonin (5-HT), dopamine and oxytocin. The pharmacological developments based on these mechanisms to treat male sexual dysfunctions will complete this review, including phosphodiesterase (PDE-5) inhibitors and intracavernous injections (ICI) for the treatment of erectile dysfunctions (ED), selective serotonin reuptake inhibitor (SSRI) for the treatment of premature ejaculation, and cholinesterase inhibitors as well as alpha adrenergic drugs for the treatment of anejaculation and retrograde ejaculation. Evidence from spinal cord injured studies will be highlighted upon each step.

Several motor behaviors such as locomotion, respiration, sexual function, and micturition are generated by rhythmic and stereotyped motor patterns of activity. In most cases, these functions are primarily controlled by signals and neuronal commands that originate from the brainstem and spinal cord. Defined as the storage and periodic elimination of urine, micturition requires a complex neural control system that coordinates the activities of a variety of effector organs including the smooth muscle of the urinary bladder and the smooth and striated muscle of the urethral sphincters. The lower urinary tract (LUT) reflex mechanisms, organized at the level of the lumbosacral spinal cord, are modulated predominantly by supraspinal controls. These LUT mechanisms include: (1) storage reflexes organized at the spinal level; (2) elimination reflexes organized at a supraspinal site in the pons; and (3) spinal storage reflexes modulated by inputs from the rostral pons. Precise coordination of the reciprocal functions of the urinary bladder and urethra and complex neural organization are required for normal function. Numerous neuropeptide/receptor systems are expressed in central and peripheral nervous system pathways that regulate the LUT and expression can also be found in both neural and non-neural (e.g., urothelium) components. Neuropeptides have tissue-specific distributions and functions in the LUT and exhibit neuroplastic changes in expression and function with LUT dysfunction with neural injury, inflammation, stress and disease. LUT dysfunction with abnormal voiding including urinary urgency, increased voiding frequency, nocturia, urinary incontinence, urinary retention, continence, detrusor dysynergia and/or pain may reflect a change in the balance of neuropeptides in central and peripheral bladder reflex pathways. LUT neuropeptide/receptor systems in LUT pathways may thus represent potential targets for therapeutic intervention.