EphA4 defines a class of excitatory locomotor-related interneurons

Investigation of central pattern generators in the spinal cord of chicken embryos

For most quadrupeds, locomotion involves alternating movements of the fore- and hindlimbs. In birds, however, while walking generally involves alternating movements of the legs, to generate lift and thrust, the wings are moved synchronously with each other. Neural circuits in the spinal cord, referred to as central pattern generators (CPGs), are the source of the basic locomotor rhythms and patterns. Given the differences in the patterns of movement of the wings and legs, it is likely that the neuronal components and connectivity of the CPG that coordinates wing movements differ from those that coordinate leg movements. In this study, we used in vitro preparations of embryonic chicken spinal cords (E11-E14) to compare the neural responses of spinal CPGs that control and coordinate wing flapping with those that control alternating leg movements. We found that in response to N-methyl-D-aspartate (NMDA) or a combination of NMDA and serotonin (5-HT), the intact chicken spinal cord produced rhythmic outputs that were synchronous both bilaterally and between the wing and leg segments. Despite this, we found that this rhythmic output was disrupted by an antagonist of glycine receptors in the lumbosacral (legs), but not the brachial (wing) segments. Thus, our results provide evidence of differences between CPGs that control the wings and legs in the spinal cord of birds.

Mechanistic and therapeutic implications of EphA-4 receptor tyrosine kinase in the pathogenesis of Alzheimer's disease

Erythropoietin-producing hepatoma (Eph) receptors belong to a family of tyrosine kinase receptors that plays a pivotal role in the development of the brain. Eph can be divided broadly into two groups, namely, EphA and EphB, comprising nine and five members, respectively. In recent years, the role of EphA-4 has become increasingly apparent in the onset of Alzheimer's disease (AD). Emerging evidence suggests that EphA-4 results in synaptic dysfunction, which in turn promotes the progression of AD. Moreover, pharmacological or genetic ablation of EphA-4 in the murine model of AD can alleviate the symptoms. The current review summarizes different pathways by which EphA-4 can influence pathogenesis. Since, majority of the studies had reported the protective effect of EphA-4 inhibition during AD, designing therapeutics based on decreasing its enzymatic activity might be necessary for introducing the novel interventions. Therefore, the review described peptide and nanobodies inhibitors of EphA-4 that exhibit the potential to modulate EphA-4 and could be used as lead molecules for the targeted therapy of AD.

EphA4 Is Required for Neural Circuits Controlling Skilled Reaching

Skilled forelimb movements are initiated by feedforward motor commands conveyed by supraspinal motor pathways. The accuracy of reaching and grasping relies on internal feedback pathways that update ongoing motor commands. In mice lacking the axon guidance molecule EphA4, axonal misrouting of the corticospinal tract and spinal interneurons is manifested, leading to a hopping gait in hindlimbs. Moreover, mice with a conditional forebrain deletion of EphA4, display forelimb hopping in adaptive locomotion and exploratory reaching movements. However, it remains unclear how loss of EphA4 signaling disrupts function of forelimb motor circuit and skilled reaching and grasping movements. Here we investigated how neural circuits controlling skilled reaching were affected by the loss of EphA4. Both male and female C57BL/6 wild-type, heterozygous EphA4^+/-, and homozygous EphA4^-/- mice were used in behavioral and in vivo electrophysiological investigations. We found that EphA4 knock-out (-/-) mice displayed impaired goal-directed reaching movements. In vivo intracellular recordings from forelimb motor neurons demonstrated increased corticoreticulospinal excitation, decreased direct reticulospinal excitation, and reduced direct propriospinal excitation in EphA4 knock-out mice. Cerebellar surface recordings showed a functional perturbation of the lateral reticular nucleus-cerebellum internal feedback pathway in EphA4 knock-out mice. Together, our findings provide in vivo evidence at the circuit level that loss of EphA4 disrupts the function of both feedforward and feedback motor pathways, resulting in deficits in skilled reaching.SIGNIFICANCE STATEMENT The central advances of this study are the demonstration that null mutation in the axon guidance molecule EphA4 gene impairs the ability of mice to perform skilled reaching, and identification of how these behavioral deficits correlates with discrete neurophysiological changes in central motor pathways involved in the control of reaching. Our findings provide in vivo evidence at the circuit level that loss of EphA4 disrupts both feedforward and feedback motor pathways, resulting in deficits in skilled reaching. This analysis of motor circuit function may help to understand the pathophysiological mechanisms underlying movement disorders in humans.

Roles of axon guidance molecules in neuronal wiring in the developing spinal cord

The spinal cord receives, relays and processes sensory information from the periphery and integrates this information with descending inputs from supraspinal centres to elicit precise and appropriate behavioural responses and orchestrate body movements. Understanding how the spinal cord circuits that achieve this integration are wired during development is the focus of much research interest. Several families of proteins have well-established roles in guiding developing spinal cord axons, and recent findings have identified new axon guidance molecules. Nevertheless, an integrated view of spinal cord network development is lacking, and many current models have neglected the cellular and functional diversity of spinal cord circuits. Recent advances challenge the existing spinal cord axon guidance dogmas and have provided a more complex, but more faithful, picture of the ontogenesis of vertebrate spinal cord circuits.

Diversity of molecularly defined spinal interneurons engaged in mammalian locomotor pattern generation

Mapping the expression of transcription factors in the mouse spinal cord has identified ten progenitor domains, four of which are cardinal classes of molecularly defined, ventrally located interneurons that are integrated in the locomotor circuitry. This review focuses on the properties of these interneuronal populations and their contribution to hindlimb locomotor central pattern generation. Interneuronal populations are categorized based on their excitatory or inhibitory functions and their axonal projections as predictors of their role in locomotor rhythm generation and coordination. The synaptic connectivity and functions of these interneurons in the locomotor central pattern generators (CPGs) have been assessed by correlating their activity patterns with motor output responses to rhythmogenic neurochemicals and sensory and descending fibers stimulations as well as analyzing kinematic gait patterns in adult mice. The observed complex organization of interneurons in the locomotor CPG circuitry, some with seemingly similar physiological functions, reflects the intricate repertoire associated with mammalian motor control and is consistent with high transcriptional heterogeneity arising from cardinal interneuronal classes. This review discusses insights derived from recent studies to describe innovative approaches and limitations in experimental model systems and to identify missing links in current investigational enterprise.

Organization of the Mammalian Locomotor CPG: Review of Computational Model and Circuit Architectures Based on Genetically Identified Spinal Interneurons(1,2,3)

The organization of neural circuits that form the locomotor central pattern generator (CPG) and provide flexor–extensor and left–right coordination of neuronal activity remains largely unknown. However, significant progress has been made in the molecular/genetic identification of several types of spinal interneurons, including V0 (V0_D and V0_V subtypes), V1, V2a, V2b, V3, and Shox2, among others. The possible functional roles of these interneurons can be suggested from changes in the locomotor pattern generated in mutant mice lacking particular neuron types. Computational modeling of spinal circuits may complement these studies by bringing together data from different experimental studies and proposing the possible connectivity of these interneurons that may define rhythm generation, flexor–extensor interactions on each side of the cord, and commissural interactions between left and right circuits. This review focuses on the analysis of potential architectures of spinal circuits that can reproduce recent results and suggest common explanations for a series of experimental data on genetically identified spinal interneurons, including the consequences of their genetic ablation, and provides important insights into the organization of the spinal CPG and neural control of locomotion.

Assembly and function of spinal circuits for motor control

Control of movement is a fundamental and complex task of the vertebrate nervous system, which relies on communication between circuits distributed throughout the brain and spinal cord. Many of the networks essential for the execution of basic locomotor behaviors are composed of discrete neuronal populations residing within the spinal cord. The organization and connectivity of these circuits is established through programs that generate functionally diverse neuronal subtypes, each contributing to a specific facet of motor output. Significant progress has been made in deciphering how neuronal subtypes are specified and in delineating the guidance and synaptic specificity determinants at the core of motor circuit assembly. Recent studies have shed light on the basic principles linking locomotor circuit connectivity with function, and they are beginning to reveal how more sophisticated motor behaviors are encoded. In this review, we discuss the impact of developmental programs in specifying motor behaviors governed by spinal circuits.

EphA4-mediated ipsilateral corticospinal tract misprojections are necessary for bilateral voluntary movements but not bilateral stereotypic locomotion

In this study, we took advantage of the reported role of EphA4 in determining the contralateral spinal projection of the corticospinal tract (CST) to investigate the effects of ipsilateral misprojections on voluntary movements and stereotypic locomotion. Null EphA4 mutations produce robust ipsilateral CST misprojections, resulting in bilateral corticospinal tracts. We hypothesize that a unilateral voluntary limb movement, not a stereotypic locomotor movement, will become a bilateral movement in EphA4 knock-out mice with a bilateral CST. However, in EphA4 full knock-outs, spinal interneurons also develop bilateral misprojections. Aberrant bilateral spinal circuits could thus transform unilateral corticospinal control signals into bilateral movements. We therefore studied mice with conditional forebrain deletion of the EphA4 gene under control by Emx1, a gene expressed in the forebrain that affects the developing CST but spares brainstem motor pathways and spinal motor circuits. We examined two conditional knock-outs targeting forebrain EphA4 during performance of stereotypic locomotion and voluntary movement: adaptive locomotion over obstacles and exploratory reaching. We found that the conditional knock-outs used alternate stepping, not hopping, during overground locomotion, suggesting normal central pattern generator function and supporting our hypothesis of minimal CST involvement in the moment-to-moment control of stereotypic locomotion. In contrast, the conditional knock-outs showed bilateral voluntary movements under conditions when single limb movements are normally produced and, as a basis for this aberrant control, developed a bilateral motor map in motor cortex that is driven by the aberrant ipsilateral CST misprojections. Therefore, a specific change in CST connectivity is associated with and explains a change in voluntary movement.

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.

EphrinB3/EphA4-mediated guidance of ascending and descending spinal tracts

The spinal cord contains many descending and ascending longitudinal tracts whose development appears to be controlled by distinct guidance systems. We identified a population of dorsal spinal neurons marked by coexpression of the transcription factor Zic2 and the guidance receptor EphA4. Zic2+;EphA4+ neurons are surrounded by mechanosensory terminals, suggesting innervation by mechanoreceptor afferents. Their axons form an ipsilateral ascending pathway that develops during embryogenesis and projects within the ventral aspect of the dorsal funiculus, the same location as the descending corticospinal tract (CST), which develops postnatally. Interestingly, the same guidance mechanism, namely, ephrinB3-induced EphA4 forward signaling, is required for the guidance of both ascending and descending axon tracts. Our analysis of conditional EphA4 mutant mice also revealed that the development of the dorsal funiculus occurs independently of EphA4 expression in descending CST axons and is linked to the distribution of Zic2+;EphA4+ spinal neurons and the formation of the ascending pathway.

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.

Neuronal activity in the isolated mouse spinal cord during spontaneous deletions in fictive locomotion: insights into locomotor central pattern generator organization

We explored the organization of the spinal central pattern generator (CPG) for locomotion by analysing the activity of spinal interneurons and motoneurons during spontaneous deletions occurring during fictive locomotion in the isolated neonatal mouse spinal cord, following earlier work on locomotor deletions in the cat. In the isolated mouse spinal cord, most spontaneous deletions were non-resetting, with rhythmic activity resuming after an integer number of cycles. Flexor and extensor deletions showed marked asymmetry: flexor deletions were accompanied by sustained ipsilateral extensor activity, whereas rhythmic flexor bursting was not perturbed during extensor deletions. Rhythmic activity on one side of the cord was not perturbed during non-resetting spontaneous deletions on the other side, and these deletions could occur with no input from the other side of the cord. These results suggest that the locomotor CPG has a two-level organization with rhythm-generating (RG) and pattern-forming (PF) networks, in which only the flexor RG network is intrinsically rhythmic. To further explore the neuronal organization of the CPG, we monitored activity of motoneurons and selected identified interneurons during spontaneous non-resetting deletions. Motoneurons lost rhythmic synaptic drive during ipsilateral deletions. Flexor-related commissural interneurons continued to fire rhythmically during non-resetting ipsilateral flexor deletions. Deletion analysis revealed two classes of rhythmic V2a interneurons. Type I V2a interneurons retained rhythmic synaptic drive and firing during ipsilateral motor deletions, while type IIV2a interneurons lost rhythmic synaptic input and fell silent during deletions. This suggests that the type I neurons are components of the RG, whereas the type II neurons are components of the PF network.We propose a computational model of the spinal locomotor CPG that reproduces our experimental results. The results may provide novel insights into the organization of spinal locomotor networks.

Myelin-derived ephrinB3 restricts axonal regeneration and recovery after adult CNS injury

Recovery of neurological function after traumatic injury of the adult mammalian central nervous system is limited by lack of axonal growth. Myelin-derived inhibitors contribute to axonal growth restriction, with ephrinB3 being a developmentally important axonal guidance cue whose expression in mature oligodendrocytes suggests a role in regeneration. Here we explored the in vivo regeneration role of ephrinB3 using mice lacking a functional ephrinB3 gene. We confirm that ephrinB3 accounts for a substantial portion of detergent-resistant myelin-derived inhibition in vitro. To assess in vivo regeneration, we crushed the optic nerve and examined retinal ganglion fibers extending past the crush site. Significantly increased axonal regeneration is detected in ephrinB3(-/-) mice. Studies of spinal cord injury in ephrinB3(-/-) mice must take into account altered spinal cord development and an abnormal hopping gait before injury. In a near-total thoracic transection model, ephrinB3(-/-) mice show greater spasticity than wild-type mice for 2 mo, with slightly greater hindlimb function at later time points, but no evidence for axonal regeneration. After a dorsal hemisection injury, increased corticospinal and raphespinal growth in the caudal spinal cord are detected by 6 wk. This increased axonal growth is accompanied by improved locomotor performance measured in the open field and by kinematic analysis. Thus, ephrinB3 contributes to myelin-derived axonal growth inhibition and limits recovery from adult CNS trauma.

Change in the balance of excitatory and inhibitory midline fiber crossing as an explanation for the hopping phenotype in EphA4 knockout mice

Neuronal networks in the spinal cord termed central pattern generators (CPGs) are responsible for the generation of rhythmic movements, such as walking. The axon guidance molecule EphA4 has been suggested to play a role in the configuration of spinal CPG networks in mammals. In EphA4 knockout (EphA4-KO) mice, the normal alternating walking pattern is replaced by a rabbit-like hopping gait, which can be reproduced when locomotor-like activity is induced in the isolated spinal cord. This hopping phenotype has been explained by an abnormal midline crossing of ipsilateral axons. Here, we investigated the nature of this overcrossing in heterozygous EphA4 (EphA4(lacZ/+) ) mice that showed normal alternating gait and homozygous EphA4 (EphA4(lacZ/lacZ) ) mice with hopping gait. Localized lesions showed that the hopping phenotype is maintained by fibers crossing in the ventral commissure. Using transgenic mouse lines in which glutamatergic, GABAergic and glycinergic neurons are intrinsically labeled, we showed a significant increase in the number of crossing excitatory β-galactosidase-positive neurons and a decrease in the number of inhibitory neurons crossing the midline in EphA4(lacZ/lacZ) mice compared with EphA4(lacZ/+) mice. These results show that the hopping phenotype is the result of a change in the balance between excitatory and inhibitory signals across the midline and that EphA4-positive neurons play an essential role in the mammalian CPG.

Do pacemakers drive the central pattern generator for locomotion in mammals?

Locomotor disorders profoundly impact quality of life of patients with spinal cord injury. Understanding the neuronal networks responsible for locomotion remains a major challenge for neuroscientists and a fundamental prerequisite to overcome motor deficits. Although neuronal circuitry governing swimming activities in lower vertebrates has been studied in great details, determinants of walking activities in mammals remain elusive. The manuscript reviews some of the principles relevant to the functional organization of the mammalian locomotor network and mainly focuses on mechanisms involved in rhythmogenesis. Based on recent publications supplemented with new experimental data, the authors will specifically discuss a new working hypothesis in which pacemakers, cells characterized by inherent oscillatory properties, might be functionally integrated in the locomotor network in mammals.

Spatiotemporal dynamics of rhythmic spinal interneurons measured with two-photon calcium imaging and coherence analysis

In rhythmic neural circuits, a neuron often fires action potentials with a constant phase to the rhythm, a timing relationship that can be functionally significant. To characterize these phase preferences in a large-scale, cell type-specific manner, we adapted multitaper coherence analysis for two-photon calcium imaging. Analysis of simulated data showed that coherence is a simple and robust measure of rhythmicity for calcium imaging data. When applied to the neonatal mouse hindlimb spinal locomotor network, the phase relationships between peak activity of >1,000 ventral spinal interneurons and motor output were characterized. Most interneurons showed rhythmic activity that was coherent and in phase with the ipsilateral motor output during fictive locomotion. The phase distributions of two genetically identified classes of interneurons were distinct from the ensemble population and from each other. There was no obvious spatial clustering of interneurons with similar phase preferences. Together, these results suggest that cell type, not neighboring neuron activity, is a better indicator of an interneuron's response during fictive locomotion. The ability to measure the phase preferences of many neurons with cell type and spatial information should be widely applicable for studying other rhythmic neural circuits.

Compensation for variable intrinsic neuronal excitability by circuit-synaptic interactions

Recent theoretical and experimental work indicates that neurons tune themselves to maintain target levels of excitation by modulating ion channel expression and synaptic strengths. As a result, functionally equivalent circuits can produce similar activity despite disparate underlying network and cellular properties. To experimentally test the extent to which synaptic and intrinsic conductances can produce target activity in the presence of variability in neuronal intrinsic properties, we used the dynamic clamp to create hybrid two-cell circuits built from four types of stomatogastric neurons coupled to the same model Morris-Lecar neuron by reciprocal inhibition. We measured six intrinsic properties (input resistance, minimum membrane potential, firing rate in response to +1 nA of injected current, slope of the frequency-current curve, spike height, and spike voltage threshold) of dorsal gastric, gastric mill, lateral pyloric, and pyloric dilator neurons from male crabs of the species Cancer borealis. The intrinsic properties varied twofold to sevenfold in each cell type. We coupled each biological neuron to the Morris-Lecar model with seven different values of inhibitory synaptic conductance and also used the dynamic clamp to add seven different values of an artificial h-conductance, thus creating 49 different circuits for each biological neuron. Despite the variability in intrinsic excitability, networks formed from each neuron produced similar circuit performance at some values of synaptic and h-conductances. This work experimentally confirms results from previous modeling studies; tuning synaptic and intrinsic conductances can yield similar circuit outputs from neurons with variable intrinsic excitability.

Synaptic patterning of left-right alternation in a computational model of the rodent hindlimb central pattern generator

Establishing, maintaining, and modifying the phase relationships between extensor and flexor muscle groups is essential for central pattern generators in the spinal cord to coordinate the hindlimbs well enough to produce the basic walking rhythm. This paper investigates a simplified computational model for the spinal hindlimb central pattern generator (CPG) that is abstracted from experimental data from the rodent spinal cord. This model produces locomotor-like activity with appropriate phase relationships in which right and left muscle groups alternate while extensor and flexor muscle groups alternate. Convergence to this locomotor pattern is slow, however, and the range of parameter values for which the model produces appropriate output is relatively narrow. We examine these aspects of the model's coordination of left-right activity through investigation of successively more complicated subnetworks, focusing on the role of the synaptic architecture in shaping motoneuron phasing. We find unexpected sensitivity in the phase response properties of individual neurons in response to stimulation and a need for high levels of both inhibition and excitation to achieve the walking rhythm. In the absence of cross-cord excitation, equal levels of ipsilateral and contralateral inhibition result in a strong preference for hopping over walking. Inhibition alone can produce the walking rhythm, but contralateral inhibition must be much stronger than ipsilateral inhibition. Cross-cord excitatory connections significantly enhance convergence to the walking rhythm, which is achieved most rapidly with strong crossed excitation and greater contralateral than ipsilateral inhibition. We discuss the implications of these results for CPG architectures based on unit burst generators.

Functional organization of V2a-related locomotor circuits in the rodent spinal cord

Studies of mammalian locomotion have been greatly facilitated by the use of the isolated rodent spinal cord preparation that retains the locomotor circuits needed to execute the movement. Physiological and molecular genetic experiments in this preparation have started to unravel the basic circuit organization responsible for walking in mammals. Here, we review these experiments with a focus on the functional role of excitatory V2a interneurons in the mammalian locomotor network. With regard to these neurons and other network structures we also discuss similarities and differences between the mammalian walking central pattern generator (CPG) and the fish swimming CPG.

Synaptic integration of rhythmogenic neurons in the locomotor circuitry: the case of Hb9 interneurons

Innovative molecular and genetic techniques have recently led to the identification of genetically defined populations of ipsilaterally projecting excitatory interneurons with probable functions in the rhythm-generating kernel of the central pattern generators (CPGs). The role of interneuronal populations in specific motor function is determined by their synaptic inputs, intrinsic properties, and target neurons. In this review we examine whether Hb9-expressing interneurons (Hb9 INs) fulfill a set of criteria that are the hallmarks of rhythm generators in the locomotor circuitry. Induced locomotor-like activity in this distinct population of ventral interneurons is in phase with bursts of motor activity, raising the possibility that they are part of the locomotor generator. To increase our understanding of the integrative function of Hb9 INs in the locomotor CPG, we investigated the cellular mechanisms underlying their rhythmic activity and examined the properties of synaptic inputs from low-threshold afferents and possible synaptic contacts with segmental motoneurons. Our findings suggest that the rhythmogenic Hb9 INs are integral components of the sensorimotor circuitry that regulate locomotor-like activity in the spinal cord.

Probing spinal circuits controlling walking in mammals

Locomotion in mammals is a complex motor act that involves the activation of a large number of muscles in a well-coordinated pattern. Understanding the network organization of the intrinsic spinal networks that control the locomotion, the central pattern generators, has been a challenge to neuroscientists. However, experiments using the isolated rodent spinal cord and combining electrophysiology and molecular genetics to dissect the locomotor network have started to shed new light on the network structure. In the present review, we will discuss findings that have revealed the role of designated populations of neurons for the key network functions including coordinating muscle activity and generating rhythmic activity. These findings are summarized in proposed organizational principles for the mammalian segmental CPG.

A cluster of cholinergic premotor interneurons modulates mouse locomotor activity

Mammalian motor programs are controlled by networks of spinal interneurons that set the rhythm and intensity of motor neuron firing. Motor neurons have long been known to receive prominent "C bouton" cholinergic inputs from spinal interneurons, but the source and function of these synaptic inputs have remained obscure. We show here that the transcription factor Pitx2 marks a small cluster of spinal cholinergic interneurons, V0(C) neurons, that represents the sole source of C bouton inputs to motor neurons. The activity of these cholinergic interneurons is tightly phase locked with motor neuron bursting during fictive locomotor activity, suggesting a role in the modulation of motor neuron firing frequency. Genetic inactivation of the output of these neurons impairs a locomotor task-dependent increase in motor neuron firing and muscle activation. Thus, V0(C) interneurons represent a defined class of spinal cholinergic interneurons with an intrinsic neuromodulatory role in the control of locomotor behavior.

Electrophysiological and pharmacological properties of locomotor activity-related neurons in cfos-EGFP mice

Although locomotion is known to be generated by networks of spinal neurons, knowledge of the properties of these neurons is limited. Using neonatal transgenic mice that express enhanced green fluorescent protein (EGFP) driven by the c-fos promoter, we visualized EGFP-positive neurons in spinal cord slices from animals that were subjected to a locomotor task or drug cocktail [N-methyl-D-aspartate, serotonin (5-HT), dopamine, and acetylcholine (ACh)]. The activity-dependent expression of EGFP was also induced in dorsal root ganglion neurons with electrical stimulation of the neurons. Following 60-90 min of swimming, whole cell patch-clamp recordings were made from EGFP+ neurons in laminae VII, VIII, and X from slices of segments T(12) to L(4). The EGFP+ neurons (n = 55) could be classified into three types based on their responses to depolarizing step currents: single spike, phasic firing, and tonic firing. Membrane properties observed in these neurons include hyperpolarization-activated inward currents (29/55), postinhibitory rebound (11/55), and persistent-inward currents (31/55). Bath application of 10-40 microM 5-HT and/or ACh increased neuronal excitability or output with hyperpolarization of voltage threshold and changes in membrane potential. 5-HT also increased input resistance, reduced the afterhyperpolarization (AHP), and induced membrane oscillations, whereas ACh reduced the input resistance and increased the AHP. In this study, we demonstrate a new way of identifying neurons active in locomotion. Our results suggest that the EGFP+ neurons are a heterogeneous population of interneurons. The actions of 5-HT and ACh on these neurons provide insights into the neuronal properties modulated by these transmitters for generation of locomotion.

Defining the excitatory neurons that drive the locomotor rhythm in a simple vertebrate: insights into the origin of reticulospinal control

Important questions remain about the origin of the excitation that drives locomotion in vertebrates and the roles played by reticulospinal neurons. In young Xenopus tadpoles, paired whole-cell recordings reveal reticulospinal neurons that directly excite swimming circuit neurons in the brainstem and spinal cord. They form part of a column of neurons (dINs) with ipsilateral descending projections which fire reliably and rhythmically in time with swimming. We ask if, at this early stage of development, these reticulospinal neurons are themselves the primary source of rhythmic drive to spinal cord neurons on each cycle of swimming. Loose-patch recordings in the hindbrain and spinal cord from neurons active during fictive swimming distinguished dINs from other neurons by spike shape. These recordings showed that reticulospinal dINs in the caudal hindbrain (rhombomeres 7–8) fire significantly earlier on each swimming cycle than other, ipsilateral, swimming circuit neurons. Whole-cell recordings showed that fast EPSCs typically precede, and probably drive, spikes in most swimming circuit neurons. However, the earliest-firing reticulospinal dINs spike too soon to be driven by underlying fast EPSCs. We propose that rebound following reciprocal inhibition can contribute to early reticulospinal dIN firing during swimming and show rebound firing in dINs following evoked, reciprocal inhibitory PSPs. Our results define reticulospinal neurons that are the source of the primary, descending, rhythmic excitation that drives spinal cord neurons to fire during swimming. These neurons are an integral part of the rhythm generating circuitry. We discuss the origin of these reticulospinal neurons as specialised members of a longitudinally distributed population of excitatory interneurons extending from the brainstem into the spinal cord.

Modulation of intrinsic spiking in spinal cord neurons

The vertebrate spinal cord is equipped with a number of neuronal networks that underlie repetitive patterns of behavior as locomotion. Activity in such networks is mediated not only by intrinsic cellular properties but also by synaptic coupling. In this study, we focused on the modulation of the intrinsic activity by 5-hydroxytryptamine (5-HT, serotonin) and the cholinergic agonist muscarine in spinal cord cultures (embryonic age 14 rats). We investigated theses cultures (slices and dissociated cells) at the network level using multielectrode arrays (MEAs) and at the cellular level using whole cell patch clamp. All cultures showed bursting network activity and intrinsic activity when gamma-aminobutyric acid, glycine, and glutamate transmission was blocked. Using MEAs, we observed an increase of the intrinsic activity in the ventral part of the slices with 5-HT and muscarine. In single-cell recordings we found that 43 and 35% of the cells that were silent in the absence of fast synaptic activity were transformed into intrinsically spiking cells by 5-HT and muscarine, respectively. We tested the hypothesis that these neuromodulators act via modulation of the persistent sodium currents (I(NaP)) in these neurons. We found that 5-HT increased threefold the amplitude of I(NaP), specifically in the nonintrinsically spiking cells, and thus switched these cells into intrinsically spiking cells via activation of 5-HT(2) receptor and the phospholipase C pathway. In contrast, the effect of muscarine on nonintrinsically spiking neurons seems to be independent of I(NaP). We conclude from these findings that serotoninergic and cholinergic modulation can turn silent into spontaneously spiking neurons and thus initiate new sources of activity for rhythm generation in spinal networks.

Circuits controlling vertebrate locomotion: moving in a new direction

Neurobiologists have long sought to understand how circuits in the nervous system are organized to generate the precise neural outputs that underlie particular behaviours. The motor circuits in the spinal cord that control locomotion, commonly referred to as central pattern generator networks, provide an experimentally tractable model system for investigating how moderately complex ensembles of neurons generate select motor behaviours. The advent of novel molecular and genetic techniques coupled with recent advances in our knowledge of spinal cord development means that a comprehensive understanding of how the motor circuitry is organized and operates may be within our grasp.

V3 spinal neurons establish a robust and balanced locomotor rhythm during walking

A robust and well-organized rhythm is a key feature of many neuronal networks, including those that regulate essential behaviors such as circadian rhythmogenesis, breathing, and locomotion. Here we show that excitatory V3-derived neurons are necessary for a robust and organized locomotor rhythm during walking. When V3-mediated neurotransmission is selectively blocked by the expression of the tetanus toxin light chain subunit (TeNT), the regularity and robustness of the locomotor rhythm is severely perturbed. A similar degeneration in the locomotor rhythm occurs when the excitability of V3-derived neurons is reduced acutely by ligand-induced activation of the allatostatin receptor. The V3-derived neurons additionally function to balance the locomotor output between both halves of the spinal cord, thereby ensuring a symmetrical pattern of locomotor activity during walking. We propose that the V3 neurons establish a regular and balanced motor rhythm by distributing excitatory drive between both halves of the spinal cord.

Genetic ablation of V2a ipsilateral interneurons disrupts left-right locomotor coordination in mammalian spinal cord

The initiation and coordination of activity in limb muscles are the main functions of neural circuits that control locomotion. Commissural neurons connect locomotor circuits on the two sides of the spinal cord, and represent the known neural substrate for left-right coordination. Here we demonstrate that a group of ipsilateral interneurons, V2a interneurons, plays an essential role in the control of left-right alternation. In the absence of V2a interneurons, the spinal cord fails to exhibit consistent left-right alternation. Locomotor burst activity shows increased variability, but flexor-extensor coordination is unaffected. Anatomical tracing studies reveal a direct excitatory input of V2a interneurons onto commissural interneurons, including a set of molecularly defined V0 neurons that drive left-right alternation. Our findings imply that the neural substrate for left-right coordination consists of at least two components; commissural neurons and a class of ipsilateral interneurons that activate commissural pathways.

Asymmetric operation of the locomotor central pattern generator in the neonatal mouse spinal cord

The rhythmic voltage oscillations in motor neurons (MNs) during locomotor movements reflect the operation of the pre-MN central pattern generator (CPG) network. Recordings from MNs can thus be used as a method to deduct the organization of CPGs. Here, we use continuous conductance measurements and decomposition methods to quantitatively assess the weighting and phase tuning of synaptic inputs to different flexor and extensor MNs during locomotor-like activity in the isolated neonatal mice lumbar spinal cord preparation. Whole cell recordings were obtained from 22 flexor and 18 extensor MNs in rostral and caudal lumbar segments. In all flexor and the large majority of extensor MNs the extracted excitatory and inhibitory synaptic conductances alternate but with a predominance of inhibitory conductances, most pronounced in extensors. These conductance changes are consistent with a "push-pull" operation of locomotor CPG. The extracted excitatory and inhibitory synaptic conductances varied between 2 and 56% of the mean total conductance. Analysis of the phase tuning of the extracted synaptic conductances in flexor and extensor MNs in the rostral lumbar cord showed that the flexor-phase-related synaptic conductance changes have sharper locomotor-phase tuning than the extensor-phase-related conductances, suggesting a modular organization of premotor CPG networks consisting of reciprocally coupled, but differently composed, flexor and extensor CPG networks. There was a clear difference between phase tuning in rostral and caudal MNs, suggesting a distinct operation of CPG networks in different lumbar segments. The highly asymmetric features were preserved throughout all ranges of locomotor frequencies investigated and with different combinations of locomotor-inducing drugs. The asymmetric nature of CPG operation and phase tuning of the conductance profiles provide important clues to the organization of the rodent locomotor CPG and are compatible with a multilayered and distributed structure of the network.

Persistent sodium current contributes to induced voltage oscillations in locomotor-related hb9 interneurons in the mouse spinal cord

Neurochemically induced membrane voltage oscillations and firing episodes in spinal excitatory interneurons expressing the HB9 protein (Hb9 INs) are synchronous with locomotor-like rhythmic motor outputs, suggesting that they contribute to the excitatory drive of motoneurons during locomotion. Similar to central pattern generator neurons in other systems, Hb9 INs are interconnected via electrical coupling, and their rhythmic activity does not depend on fast glutamatergic synaptic transmission. The primary objective of this study was to determine the contribution of fast excitatory and inhibitory synaptic transmission and subthreshold voltage-dependent currents to the induced membrane oscillations in Hb9 INs in the postnatal mouse spinal cord. The non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) reduced the amplitude of voltage oscillations but did not alter their frequency. CNQX suppressed rhythmic motor activity. Blocking glycine and GABAA receptor-mediated inhibitory synapses as well as cholinergic transmission did not change the properties of CNQX-resistant membrane oscillations. However, disinhibition triggered new episodes of slow motor bursting that were not correlated with induced locomotor-like rhythms in Hb9 INs. Our observations indicated that fast excitatory and inhibitory synaptic inputs did not control the frequency of induced rhythmic activity in Hb9 INs. We next examined the contribution of persistent sodium current (INaP) to subthreshold membrane oscillations in the absence of primary glutamatergic, GABAergic and glycinergic synaptic drive to Hb9 INs. Low concentrations of riluzole that blocked the slow-inactivating component of sodium current gradually suppressed the amplitude and reduced the frequency of voltage oscillations. Our finding that INaP regulates locomotor-related rhythmic activity in Hb9 INs independently of primary synaptic transmission supports the concept that these neurons constitute an integral component of the rhythmogenic locomotor network in the mouse spinal cord.

Heterogeneity of V2-derived interneurons in the adult mouse spinal cord

Spinal neurons and networks that generate rhythmic locomotor activity remain incompletely defined, prompting the use of molecular biological strategies to label populations of neurons in the postnatal mouse. During spinal cord development, expression of Lhx3 in the absence of Isl1 specifies a V2 interneuronal fate. In this study, postnatal V2-derived interneurons were identified by yellow fluorescent protein (YFP) expression in the double-transgenic offspring of Lhx3Cre/+ x thy1-loxP-stop-loxP-YFP mice. While some motoneurons were labelled, several populations of interneurons predominantly located in lamina VII could also be distinguished. Small interneurons were located throughout the spinal cord whereas larger interneurons were concentrated in the lumbar enlargement. Some V2-derived interneurons were propriospinal, with axons that bifurcated in the lateral funiculus. V2-derived interneurons gave rise to populations of both excitatory and inhibitory interneurons in approximately equal proportions, as demonstrated by in situ hybridization with VGLUT2 mRNA. Immunohistochemical studies revealed YFP+ boutons throughout the spinal cord. Both glutamatergic and glycinergic YFP+ boutons were observed in lamina IX where many apposed motoneuron somata. GABAergic YFP+ boutons were also observed in lamina IX, and they did not form P-boutons. At P0, more than half of the YFP+ interneurons expressed Chx10 and thus were derived from the V2a subclass. In adult mice, there was an increase in Fos expression in V2-derived interneurons following locomotion, indicating that these neurons are active during this behaviour. The heterogeneity of V2-derived interneurons in adult mice indicates that physiologically distinct subpopulations, including last-order interneurons, arise from these embryonically defined neurons.

Phenotype of V2-derived interneurons and their relationship to the axon guidance molecule EphA4 in the developing mouse spinal cord

The ventral spinal cord consists of interneuron groups arising from distinct, genetically defined, progenitor domains along the dorsoventral axis. Many of these interneuron groups settle in the ventral spinal cord which, in mammals, contains the central pattern generator for locomotion. In order to better understand the locomotor networks, we have used different transgenic mice for anatomical characterization of one of these interneuron groups, called V2 interneurons. Neurons in this group are either V2a interneurons marked by the postmitotic expression of the transcription factor Chx10, or V2b interneurons which express the transcription factors Gata2 and Gata3. We found that all V2a and most V2b interneurons were ipsilaterally projecting in embryos as well as in newborns. V2a interneurons were for the most part glutamatergic while V2b interneurons were mainly GABAergic or glycinergic. Furthermore, we demonstrated that a large proportion of V2 interneurons expressed the axon guidance molecule EphA4, a molecule previously shown to be important for correct organization of locomotor networks. We also showed that V2 interneurons and motor neurons alone did not account for all EphA4-expressing neurons in the spinal cord. Together, these findings enable a better interpretation of neural networks underlying locomotion, and open up the search for as yet unknown components of the mammalian central pattern generator.

Nck adaptor proteins control the organization of neuronal circuits important for walking

The intracellular signaling targets used by mammalian axon guidance receptors to organize the nervous system in vivo are unclear. The Nck1 and Nck2 SH2/SH3 adaptors (collectively Nck) can couple phosphotyrosine (pTyr) signals to reorganization of the actin cytoskeleton and are therefore candidates for linking guidance cues to the regulatory machinery of the cytoskeleton. We find that selective inactivation of Nck in the murine nervous system causes a hopping gait and a defect in the spinal central pattern generator, which is characterized by synchronous firing of bilateral ventral motor neurons. Nck-deficient mice also show abnormal projections of corticospinal tract axons and defective development of the posterior tract of the anterior commissure. These phenotypes are consistent with a role for Nck in signaling initiated by different classes of guidance receptors, including the EphA4 receptor tyrosine kinase. Our data indicate that Nck adaptors couple pTyr guidance signals to cytoskeletal events required for the ipsilateral projections of spinal cord neurons and thus for normal limb movement.

Excitatory components of the mammalian locomotor CPG

Locomotion in mammals is to a large degree controlled directly by intrinsic spinal networks, called central pattern generators (CPGs). The overall function of these networks is governed by interaction between inhibitory and excitatory neurons. In the present review, we will discuss recent findings addressing the role of excitatory synaptic transmission for network function including the role of specific excitatory neuronal populations in coordinating muscle activity and in generating rhythmic activity.

EphA4-Dependent Axon Guidance Is Mediated by the RacGAP α2-Chimaerin

Neuronal network formation in the developing nervous system is dependent on the accurate navigation of nerve cell axons and dendrites, which is controlled by attractive and repulsive guidance cues. Ephrins and their cognate Eph receptors mediate many repulsive axonal guidance decisions by intercellular interactions resulting in growth cone collapse and axon retraction of the Eph-presenting neuron. We show that the Rac-specific GTPase-activating protein alpha2-chimaerin binds activated EphA4 and mediates EphA4-triggered axonal growth cone collapse. alpha-Chimaerin mutant mice display a phenotype similar to that of EphA4 mutant mice, including aberrant midline axon guidance and defective spinal cord central pattern generator activity. Our results reveal an alpha-chimaerin-dependent signaling pathway downstream of EphA4, which is essential for axon guidance decisions and neuronal circuit formation in vivo.

Strategies for delineating spinal locomotor rhythm-generating networks and the possible role of Hb9 interneurones in rhythmogenesis

Despite significant advances in our understanding of pattern generation in invertebrates and lower vertebrates, there have been barriers to the application of the principles learned to the definition of networks underlying mammalian locomotion. Major difficulties have arisen in identifying spinal interneurones in preparations which allow study of neuronal intrinsic properties and the role of identified interneurones in locomotor networks. Recent genetic technologies in which selective expression of fluorescent proteins in specific populations of mouse spinal neurones have provided new avenues of investigation. In this review, we focus on the generation of locomotor rhythm and outline criteria that rhythm-generating neurones might be expected to fulfill. We then examine the extent to which a recently identified population of spinal interneurones, Hb9 interneurones, fulfill these criteria. Finally, we suggest that Hb9 interneurones could be involved in an asymmetric model of locomotor rhythmogenesis through projections of electrotonically coupled rhythm-generating modules to flexor pattern formation half-centres. The principles learned from studying this population of interneurones have led to strategies to systematically evaluate neurones that may be involved in locomotor rhythmogenesis.

Segmental, synaptic actions of commissural interneurons in the mouse spinal cord

Left-right alternation depends on activity in commissural interneurons (CINs) that have axons crossing in the midline. In this study, we investigate the CIN connectivity to local motor neurons using a newly developed preparation of the in vitro neonatal mouse spinal cord that allows us to identify all classes of CINs. Nineteen of 29 short-range CINs with axonal projections <1.5 segments (sCINs) directly excited, directly inhibited, or indirectly inhibited contralateral motor neurons in the quiescent spinal cord. Excitation was glutamatergic and inhibition was mixed glycinergic and/or GABAergic. Long-range CINs were also found to have input to local, contralateral motor neurons. Thirteen of 29 descending CINs had similar synaptic connectivity to contralateral motor neurons as the sCINs, including direct excitation and direct and indirect inhibition. Some (9 of 23) rostrally projecting ascending CINs, and a few (2 of 10) CINs with bifurcating axons that both ascend and descend, indirectly inhibited local, contralateral motor neurons. Rhythmic firing during locomotor-like activity was observed in a number of CINs with segmental synaptic effects on contralateral motor neurons. This study outlines the basic connectivity pattern of CINs in the mouse spinal cord on a segmental level. Our study suggests that, based on observed synaptic connectivity, both short- and long-range CINs are likely involved in segmental left-right coordination and that the CIN system is organized into a dual-inhibitory and single-excitatory system. These systems are organized in a way that they could provide appropriate coordination during locomotion.

Role of glutamate in locomotor rhythm generating neuronal circuitry

Central pattern generators (CPGs) are defined as neuronal circuits capable of producing a rhythmic and coordinated output without the influence of sensory input. The locomotor and respiratory neuronal circuits are two of the better-characterized CPGs, although much work remains to fully understand how these networks operate. Glutamatergic neurons are involved in most neuronal circuits of the nervous system and considerable efforts have been made to study glutamate receptors in nervous system signaling using a variety of approaches. Because of the complexity of glutamate-mediated signaling and the variety of receptors triggered by glutamate, it has been difficult to pinpoint the role of glutamatergic neurons in neuronal circuits. In addition, glutamate is an amino acid used by every cell, which has hampered identification of glutamatergic neurons. Glutamatergic excitatory neurotransmission is dependent on the release from glutamate-filled presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6-8, which function as vesicular glutamate transporters (VGLUTs). Recent data describe that Vglut2 (Slc17a6) null mutant mice die immediately after birth due to a complete loss of the stable autonomous respiratory rhythm generated by the pre-Bötzinger complex. Surprisingly, we found that basal rhythmic locomotor activity is not affected in Vglut2 null mutant embryos. With this perspective, we discuss data regarding presence of VGLUT1, VGLUT2 and VGLUT3 positive neuronal populations in the spinal cord.

Persistent sodium currents participate in fictive locomotion generation in neonatal mouse spinal cord

The persistent sodium current (I(Na(P))) has been implicated in the regulation of synaptic integration, intrinsic membrane properties, and rhythm generation in many types of neurons. We characterized I(Na(P)) in commissural interneurons (CINs) in the neonatal (postnatal days 0-3) mouse spinal cord; it is activated at subthreshold potentials, inactivates slowly, and can be blocked by low concentrations of riluzole. The role of I(Na(P)) in locomotor pattern generation was examined by applying riluzole during fictive locomotion induced by NMDA, serotonin, and dopamine or by stimulation of the cauda equina. Blockade of I(Na(P)) has marginal effects on the locomotion frequency but progressively weakens the rhythmic firing and locomotor-related membrane oscillation of CINs and motoneurons (MNs) and the locomotor-like bursts in ventral roots, until the motor pattern ceases. Riluzole directly affects the intrinsic firing properties of CINs and MNs, reducing their ability to fire repetitively during tonic depolarizations and raising their spike threshold. At the same time, riluzole has little effects on the strength of spike-evoked synaptic transmission onto CINs and MNs. Our results suggest that I(Na(P)) is essential for the generation of the locomotor pattern and acts in part by regulating the frequency of interneuron firing in the central pattern generator for locomotion.

Biological pattern generation: the cellular and computational logic of networks in motion

In 1900, Ramón y Cajal advanced the neuron doctrine, defining the neuron as the fundamental signaling unit of the nervous system. Over a century later, neurobiologists address the circuit doctrine: the logic of the core units of neuronal circuitry that control animal behavior. These are circuits that can be called into action for perceptual, conceptual, and motor tasks, and we now need to understand whether there are coherent and overriding principles that govern the design and function of these modules. The discovery of central motor programs has provided crucial insight into the logic of one prototypic set of neural circuits: those that generate motor patterns. In this review, I discuss the mode of operation of these pattern generator networks and consider the neural mechanisms through which they are selected and activated. In addition, I will outline the utility of computational models in analysis of the dynamic actions of these motor networks.

Vesicular glutamate transporter 2 is required for central respiratory rhythm generation but not for locomotor central pattern generation

Glutamatergic excitatory neurotransmission is dependent on glutamate release from presynaptic vesicles loaded by three members of the solute carrier family, Slc17a6-8, which function as vesicular glutamate transporters (VGLUTs). Here, we show that VGLUT2 (Slc17a6) is required for life ex utero. Vglut2 null mutant mice die immediately after birth because of the absence of respiratory behavior. Investigations at embryonic stages revealed that neural circuits in the location of the pre-Bötzinger (PBC) inspiratory rhythm generator failed to become active. However, neurons with bursting pacemaker properties and anatomical integrity of the PBC area were preserved. Vesicles at asymmetric synapses were fewer and malformed in the Vglut2 null mutant hindbrain, probably causing the complete disruption of AMPA/kainate receptor-mediated synaptic activity in mutant PBC cells. The functional deficit results from an inability of PBC neurons to achieve synchronous activation. In contrast to respiratory rhythm generation, the locomotor central pattern generator of Vglut2 null mutant mice displayed normal rhythmic and coordinated activity, suggesting differences in their operating principles. Hence, the present study identifies VGLUT2-mediated signaling as an obligatory component of the developing respiratory rhythm generator.

Long ascending propriospinal projections from lumbosacral to upper cervical spinal cord in the rat

The retrograde tracer cholera toxin beta-subunit (CTB) was used to trace long ascending propriospinal projections from neurons in the lumbosacral spinal cord to the upper cervical (C3) gray matter in adult male Sprague-Dawley rats. Following large 0.5 microl CTB injections restricted mainly to the upper cervical ventral horn (n=5), there were many lumbosacral CTB-positive neurons (14-17/section) in the intermediate gray and ventral horn (dorsal lamina VIII, medial VII extending into X) contralaterally, with fewer at corresponding ipsilateral locations. Labeled cells (4-8/section) were also observed in contralateral laminae IV-VI and the lateral spinal nucleus, with fewer ipsilaterally. Few labeled cells (<2/section) were observed in superficial laminae I-II. Smaller (0.15 microl) microinjections of CTB restricted to the upper cervical ventral gray matter labeled cells in contralateral laminae VII-VIII (approximately 6-9/section) with fewer ipsilaterally. There were relatively fewer (<2/section) in the intermediate dorsal horn and very few (<1/section) in lamina I. Larger (0.5 microl) CTB injections encompassing the C3 dorsal and ventral gray matter on one side labeled significantly more CTB-positive neurons (>6/section) in contralateral lamina I compared to ventral horn injections. These results suggest direct projections from ventromedially located neurons of lumbar and sacral segments to the contralateral ventral gray matter of upper cervical segments, as well as from neurons in the intermediate but not superficial dorsal horn. They further suggest that some lumbosacral superficial dorsal horn neurons project to the upper cervical dorsal horn. These propriospinal projections may be involved in coordinating head and neck movements during locomotion or stimulus-evoked motor responses.

Inter-enlargement pathways in the ventrolateral funiculus of the adult rat spinal cord

The ventrolateral funiculus (VLF) in the spinal cord contains important ascending and descending pathways related to locomotion and interlimb coordination. The primary purpose of this descriptive study was to investigate the distribution of inter-enlargement pathways in the adult rat spinal cord with an emphasis on the VLF. We made discrete unilateral injections of Fluoro-Gold (FG) into the right VLF at thoracic segment (T) 9, and either unilateral or bilateral injections of Fluoro-Ruby (FR) into the intermediate gray matter at the cervical (C) 5-6, C7-8, or lumbar (L) 2 segmental levels. Inter-enlargement neurons with ascending axons in the right VLF were found bilaterally in laminae VII and VIII throughout the rostral lumbar spinal cord (L1-L3) and predominantly contralaterally in the caudal lumbosacral (L4-S1) spinal cord. Following left unilateral FR injections at C5-6 or C7-8 and right unilateral VLF injections of FG at T9, very few double-labeled neurons could be found anywhere in the lumbar spinal cord. Similar injections of FR at L2 revealed an almost symmetrical bilateral distribution of double-labeled neurons throughout the cervical spinal cord (C1-8). These results describe ascending and descending pathways within the spinal cord that interconnect the two enlargements and involve both commissural and ipsilateral interneurons. The majority of inter-enlargement neurons had axons within the VLF at T9. These observations support the hypothesis that the VLF contains long ascending and descending axons with propriospinal inter-enlargement, commissural and ipsilateral connections that are anatomically well-suited to mediate interlimb coordination.

LOCOMOTOR CIRCUITS IN THE MAMMALIAN SPINAL CORD

Intrinsic spinal networks, known as central pattern generators (CPGs), control the timing and pattern of the muscle activity underlying locomotion in mammals. This review discusses new advances in understanding the mammalian CPGs with a focus on experiments that address the overall network structure as well as the identification of CPG neurons. I address the identification of excitatory CPG neurons and their role in rhythm generation, the organization of flexor-extensor networks, and the diverse role of commissural interneurons in coordinating left-right movements. Molecular and genetic approaches that have the potential to elucidate the function of populations of CPG interneurons are also discussed.

alx, a zebrafish homolog of Chx10, marks ipsilateral descending excitatory interneurons that participate in the regulation of spinal locomotor circuits

Recent molecular genetic studies suggest that the expression of transcription factors in the developing spinal cord helps determine the morphological and physiological properties of neurons. Using the zebrafish preparation, we have examined the properties of neurons marked by alx, a zebrafish homolog of mammalian Chx10. We performed morphological and physiological studies using transgenic zebrafish expressing fluorescent reporter constructs in cells that had at any time point expressed alx (alx neurons). Our data reveal that zebrafish alx neurons are all ipsilateral descending neurons that are positive for vesicular glutamate transporter 2, suggesting that they are glutamatergic excitatory interneurons. Patch recordings show that earlier-born neurons are active during stronger movements such as escapes and fast swimming (strong movement class), whereas later-born ones are involved in sustained weak swimming (weak movement class). Paired recordings between alx neurons and motoneurons show that neurons of the strong movement class make frequent monosynaptic excitatory connections onto motoneurons. Thus, neurons of this class are likely premotor interneurons that regulate motoneuron activity during escapes and fast swimming. We also show the existence of a monosynaptic connection between an alx neuron of the weak movement class and a motoneuron. Collectively, our data demonstrate that alx marks ipsilateral descending neurons that are involved in the regulation of motoneuron activity during forms of locomotion, such as escape and swimming.

Deciphering the organization and modulation of spinal locomotor central pattern generators

Networks within our spinal cord generate the basic pattern underlying walking. Over the past decade, much progress has been made in our understanding of their function in a variety of vertebrate species. A significant hurdle has been the identification of candidate populations of neurons that are involved in pattern generation in the spinal cord. Recently,systems neuroscientists in collaboration with molecular biologists have begun to dissect the circuitry underlying spinal locomotor networks. These advances have combined genetic and electrophysiological techniques using in vitro preparations of the mouse spinal cord. This review will discuss new advances in the field of spinal locomotor networks with emphasis on the mouse.

Many of the behaviors fundamental to animal life, such as breathing,chewing and locomotion, are rhythmic activities controlled by neuronal networks. Discerning which neurons are members of these networks, their synaptic connectivity and their individual electrophysiological properties is essential to our understanding of how rhythmic motor behaviors are produced. It is well known that the spinal cord contains the basic circuitry to produce locomotion. However, identifying neurons and connections within spinal networks is challenging because cells that comprise the locomotor network form part of a heterogeneous mix of interneurons within the ventral spinal cord. Recently, the merging of electrophysiological and genetic approaches has provided new tools to identify classes of interneurons within the spinal cord that contribute to network function. These new findings will be discussed in this review.

Activity of Renshaw cells during locomotor-like rhythmic activity in the isolated spinal cord of neonatal mice

In the present study, we examine the activity patterns of and synaptic inputs to Renshaw cells (RCs) during fictive locomotion in the newborn mouse using visually guided recordings from GABAergic cells expressing glutamic acid decarboxylase 67-green fluorescent protein (GFP). Among the GFP-positive neurons in the lumbar ventral horn, RCs were uniquely identified as receiving ventral root-evoked short-latency EPSPs that were markedly reduced in amplitude by nicotinic receptor blockers mecamylamine or tubocurarine. During locomotor-like rhythmic activity evoked by bath application of 5-HT and NMDA, 50% of the recorded RCs fired in-phase with the ipsilateral L2 flexor-related rhythm, whereas the rest fired in the extensor phase. Each population of RCs fired throughout the corresponding locomotor phase. All RCs received both excitatory and inhibitory synaptic inputs during the locomotor-like rhythmic activity. Blocking nicotinic receptors with mecamylamine markedly reduced the rhythmic excitatory drive, indicating that these rhythmic inputs originate mainly from motor neurons (MNs). Inhibitory synaptic inputs persisted in the presence of the nicotinic blocker. Part of this inhibitory drive and remaining excitatory drive could be from commissural interneurons because the present study also shows that RCs receive direct crossed inhibitory and excitatory synaptic inputs. However, rhythmic synaptic inputs in RCs were also observed in hemicord preparations in the presence of mecamylamine. These results show that, during locomotor activity, RC firing properties are modulated not only by MNs but also by the ipsilateral and contralateral locomotor networks.

Persistent responses to brief stimuli: feedback excitation among brainstem neurons

The ability of brief stimuli to trigger prolonged neuronal activity is a fundamental requirement in nervous systems, common to motor responses and short-term memory. Bistable membrane properties and network feedback excitation have both been proposed as suitable mechanisms to sustain such persistent responses. There is now good experimental evidence for membrane bistability. In contrast, the long-standing hypotheses based on positive feedback excitation have yet to be supported by direct evidence for mutual excitatory connections between appropriate neurons. In young frog tadpoles (Xenopus), we show that a small region of caudal hindbrain and rostral spinal cord is sufficient to generate prolonged swimming in response to a brief stimulus. We used paired whole-cell patch recordings to identify hindbrain neurons in this region that actively excite spinal neurons to drive sustained swimming. We show directly that some of these hindbrain neurons make reciprocal excitatory connections with each other. We use a population model of the hindbrain network to illustrate how feedback excitation can provide a robust mechanism to generate persistent responses. Our recordings provide direct evidence for feedback excitation among neurons within a network that drives a prolonged response. Its presence in a lower brain region early in development suggests that it is a basic feature of neuronal network design.