Differential origin of reticulospinal drive to motoneurons innervating trunk and hindlimb muscles in the mouse revealed by optical recording

Lumbar V3 interneurons provide direct excitatory synaptic input onto thoracic sympathetic preganglionic neurons, linking locomotor, and autonomic spinal systems

Although sympathetic autonomic systems are activated in parallel with locomotion, the neural mechanisms mediating this coordination are incompletely understood. Sympathetic preganglionic neurons (SPNs), primarily located in the intermediate laminae of thoracic and upper lumbar segments (T1-L2), increase activation of tissues and organs that provide homeostatic and metabolic support during movement and exercise. Recent evidence suggests integration between locomotor and autonomic nuclei occurs within the brainstem, initiating both descending locomotor and sympathetic activation commands. However, both locomotor and sympathetic autonomic spinal systems can be activated independent of supraspinal input, in part due to a distributed network involving propriospinal neurons. Whether an intraspinal mechanism exists to coordinate activation of these systems is unknown. We hypothesized that ascending spinal neurons located in the lumbar region provide synaptic input to thoracic SPNs. Here, we demonstrate that synaptic contacts from locomotor-related V3 interneurons (INs) are present in all thoracic laminae. Injection of an anterograde tracer into lumbar segments demonstrated that 8-20% of glutamatergic input onto SPNs originated from lumbar V3 INs and displayed a somatotopographical organization of synaptic input. Whole cell patch clamp recording in SPNs demonstrated prolonged depolarizations or action potentials in response to optical activation of either lumbar V3 INs in spinal cord preparations or in response to optical activation of V3 terminals in thoracic slice preparations. This work demonstrates a direct intraspinal connection between lumbar locomotor and thoracic sympathetic networks and suggests communication between motor and autonomic systems may be a general function of the spinal cord.

Excitatory and Inhibitory Descending Commissural Interneurons Differentially Integrate Supraspinal and Segmental Sensory Signals

The limited information about how descending inputs from the brain and sensory inputs from the periphery use spinal cord interneurons (INs) is a major barrier to understanding how these inputs may contribute to motor functions under normal and pathologic conditions. Commissural interneurons (CINs) are a heterogeneous population of spinal INs that has been implicated in crossed motor responses and bilateral motor coordination (ability to use the right and left side of the body in a coordinated manner) and, therefore, are likely involved in many types of movement (e.g., dynamic posture stabilization, jumping, kicking, walking). In this study, we incorporate mouse genetics, anatomy, electrophysiology, and single-cell calcium imaging to investigate how a subset of CINs, those with descending axons called dCINs, are recruited by descending reticulospinal and segmental sensory signals independently and in combination. We focus on two groups of dCINs set apart by their principal neurotransmitter (glutamate and GABA) and identified as VGluT2+ dCINs and GAD2+ dCINs. We show that VGluT2+ and GAD2+ dCINs are both extensively recruited by reticulospinal and sensory input alone but that VGluT2+ and GAD2+ dCINs integrate these inputs differently. Critically, we find that when recruitment depends on the combined action of reticulospinal and sensory inputs (subthreshold inputs), VGluT2+ dCINs, but not GAD2+ dCINs, are recruited. This difference in the integrative capacity of VGluT2+ and GAD2+ dCINs represents a circuit mechanism that the reticulospinal and segmental sensory systems may avail themselves of to regulate motor behaviors both normally and after injury.SIGNIFICANCE STATEMENT The way supraspinal and peripheral sensory inputs use spinal cord interneurons is fundamental to defining how motor functions are supported both in health and disease. This study, which focuses on dCINs, a heterogeneous population of spinal interneurons critical for crossed motor responses and bilateral motor coordination, shows that both glutamatergic (excitatory) and GABAergic (inhibitory) dCINs can be recruited by supraspinal (reticulospinal) or peripheral sensory inputs. Additionally, the study demonstrates that in conditions where the recruitment of dCINs depends on the combined action of reticulospinal and sensory inputs, only excitatory dCINs are recruited. The study uncovers a circuit mechanism that the reticulospinal and segmental sensory systems may avail themselves of to regulate motor behaviors both normally and after injury.

Deconstructing the modular organization and real-time dynamics of mammalian spinal locomotor networks

Locomotion empowers animals to move. Locomotor-initiating signals from the brain are funneled through descending neurons in the brainstem that act directly on spinal locomotor circuits. Little is known in mammals about which spinal circuits are targeted by the command and how this command is transformed into rhythmicity in the cord. Here we address these questions leveraging a mouse brainstem-spinal cord preparation from either sex that allows locating the locomotor command neurons with simultaneous Ca²⁺ imaging of spinal neurons. We show that a restricted brainstem area - encompassing the lateral paragigantocellular nucleus (LPGi) and caudal ventrolateral reticular nucleus (CVL) - contains glutamatergic neurons which directly initiate locomotion. Ca²⁺ imaging captures the direct LPGi/CVL locomotor initiating command in the spinal cord and visualizes spinal glutamatergic modules that execute the descending command and its transformation into rhythmic locomotor activity. Inhibitory spinal networks are recruited in a distinctly different pattern. Our study uncovers the principal logic of how spinal circuits implement the locomotor command using a distinct modular organization.

Zebrafish Erc1b mediates motor innervation and organization of craniofacial muscles in control of jaw movement

BACKGROUND

Movement of the lower jaw, a common behavior observed among vertebrates, is required for eating and processing food. This movement is controlled by signals sent from the trigeminal motor nerve through neuromuscular junctions (NMJs) to the masticatory muscles. Dysfunctional jaw movements contribute to craniomandibular disorders, yet the pathophysiology of these disorders is not well understood, as limited studies have been conducted on the molecular mechanisms of jaw movement.

RESULTS

Using erc1b/kimm533 genetic loss of function mutant, we evaluated lower jaw muscle organization and innervation by the cranial motor nerves in developing zebrafish. Using time-lapse confocal imaging of the erc1b mutant in a transgenic fluorescent reporter line, we found delayed trigeminal nerve growth and disrupted nerve branching architecture during muscle innervation. By automated 3D image analysis of NMJ distribution, we identified an increased number of small, disorganized NMJ clusters in erc1b mutant larvae compared to WT siblings. Using genetic replacement experiments, we determined the Rab GTPase binding domain of Erc1b is required for cranial motor nerve branching, but not NMJ organization or muscle attachment.

CONCLUSIONS

We identified Erc1b/ERC1 as a novel component of a genetic pathway contributing to muscle organization, trigeminal nerve outgrowth, and NMJ spatial distribution during development that is required for jaw movement.

Crossed activation of thoracic trunk motoneurons by medullary reticulospinal neurons

Activation of contralateral muscles by supraspinal neurons, or crossed activation, is critical for bilateral coordination. Studies in mammals have focused on the neural circuits that mediate cross activation of limb muscles, but the neural circuits involved in crossed activation of trunk muscles are still poorly understood. In this study, we characterized functional connections between reticulospinal (RS) neurons in the medial and lateral regions of the medullary reticular formation (medMRF and latMRF) and contralateral trunk motoneurons (MNs) in the thoracic cord (T7 and T10 segments). To do this, we combined electrical microstimulation of the medMRF and latMRF and calcium imaging from single cells in an ex vivo brain stem-spinal cord preparation of neonatal mice. Our findings substantiate two spatially distinct RS pathways to contralateral trunk MNs. Both pathways originate in the latMRF and are midline crossing, one at the level of the spinal cord via excitatory descending commissural interneurons (reticulo-commissural pathway) and the other at the level of the brain stem (crossed RS pathway). Activation of these RS pathways may enable different patterns of bilateral trunk coordination. Possible implications for recovery of trunk function after stroke or spinal cord injury are discussed.NEW & NOTEWORTHY We identify two spatially distinct reticulospinal pathways for crossed activation of trunk motoneurons. Both pathways cross the midline, one at the level of the brain stem and the other at the level of the spinal cord via excitatory commissural interneurons. Jointly, these pathways provide new opportunities for repair interventions aimed at recovering trunk functions after stroke or spinal cord injury.

Glutamatergic neurons of the gigantocellular reticular nucleus shape locomotor pattern and rhythm in the freely behaving mouse

Because of their intermediate position between supraspinal locomotor centers and spinal circuits, gigantocellular reticular nucleus (GRN) neurons play a key role in motor command. However, the functional contribution of glutamatergic GRN neurons in initiating, maintaining, and stopping locomotion is still unclear. Combining electromyographic recordings with optogenetic manipulations in freely behaving mice, we investigate the functional contribution of glutamatergic brainstem neurons of the GRN to motor and locomotor activity. Short-pulse photostimulation of one side of the glutamatergic GRN did not elicit locomotion but evoked distinct motor responses in flexor and extensor muscles at rest and during locomotion. Glutamatergic GRN outputs to the spinal cord appear to be gated according to the spinal locomotor network state. Increasing the duration of photostimulation increased motor and postural tone at rest and reset locomotor rhythm during ongoing locomotion. In contrast, photoinhibition impaired locomotor pattern and rhythm. We conclude that unilateral activation of glutamatergic GRN neurons triggered motor activity and modified ongoing locomotor pattern and rhythm.

Brainstem Steering of Locomotor Activity in the Newborn Rat

Control of locomotion relies on motor loops conveying modulatory signals between brainstem and spinal motor circuits. We investigated the steering control of the brainstem reticular formation over the spinal locomotor networks using isolated brainstem-spinal cord preparations of male and female neonatal rats. First, we performed patch-clamp recordings of identified reticulospinal cells during episodes of fictive locomotion. This revealed that a spinal ascending phasic modulation of reticulospinal cell activity is already present at birth. Half of the cells exhibited tonic firing during locomotion, while the other half emitted phasic discharges of action potentials phase locked to ongoing activity. We next showed that mimicking the phasic activity of reticulospinal neurons by applying patterned electrical stimulation bilaterally at the ventral caudal medulla level triggered fictive locomotion efficiently. Moreover, the brainstem stimuli-induced locomotor rhythm was entrained in a one-to-one coupling over a range of cycle periods (2-6 s). Additionally, we induced turning like motor outputs by either increasing or decreasing the relative duration of the stimulation trains on one side of the brainstem compared to the other. The ability of the patterned descending command to control the locomotor output depended on the functional integrity of ventral reticulospinal pathways and the involvement of local spinal central pattern generator circuitry. Altogether, this study provides a mechanism by which brainstem reticulospinal neurons relay steering and speed commands to the spinal locomotor networks.SIGNIFICANCE STATEMENT Locomotor function allows the survival of most animal species while sustaining the expression of fundamental behaviors. Locomotor activities adapt from moment to moment to behavioral and environmental changes. We show that the brainstem can control the spinal locomotor network outputs through phasic descending commands that alternate bilaterally. Manipulating the periodicity and/or the relative durations of the left and right descending commands at the brainstem level is efficient to set the locomotor speed and sustain directional changes.

Reticulospinal Systems for Tuning Motor Commands

The pontomedullary reticular formation (RF) is a key site responsible for integrating descending instructions to execute particular movements. The indiscrete nature of this region has led not only to some inconsistencies in nomenclature, but also to difficulties in understanding its role in the control of movement. In this review article, we first discuss nomenclature of the RF, and then examine the reticulospinal motor command system through evolution. These command neurons have direct monosynaptic connections with spinal interneurons and motoneurons. We next review their roles in postural adjustments, walking and sleep atonia, discussing their roles in movement activation or inhibition. We propose that knowledge of the internal organization of the RF is necessary to understand how the nervous system tunes motor commands, and that this knowledge will underlie strategies for motor functional recovery following neurological injuries or diseases.

Human Postural Control

From ancient Greece to nowadays, research on posture control was guided and shaped by many concepts. Equilibrium control is often considered part of postural control. However, two different levels have become increasingly apparent in the postural control system, one level sets a distribution of tonic muscle activity (“posture”) and the other is assigned to compensate for internal or external perturbations (“equilibrium”). While the two levels are inherently interrelated, both neurophysiological and functional considerations point toward distinct neuromuscular underpinnings. Disturbances of muscle tone may in turn affect movement performance. The unique structure, specialization and properties of skeletal muscles should also be taken into account for understanding important peripheral contributors to postural regulation. Here, we will consider the neuromechanical basis of habitual posture and various concepts that were rather influential in many experimental studies and mathematical models of human posture control.

Influence of Brain Stem on Axial and Hindlimb Spinal Locomotor Rhythm Generating Circuits of the Neonatal Mouse

The trunk plays a pivotal role in limbed locomotion. Yet, little is known about how the brain stem controls trunk activity during walking. In this study, we assessed the spatiotemporal activity patterns of axial and hindlimb motoneurons (MNs) during drug-induced fictive locomotor-like activity (LLA) in an isolated brain stem-spinal cord preparation of the neonatal mouse. We also evaluated the extent to which these activity patterns are affected by removal of brain stem. Recordings were made in the segments T7, L2, and L5 using calcium imaging from individual axial MNs in the medial motor column (MMC) and hindlimb MNs in lateral motor column (LMC). The MN activities were analyzed during both the rhythmic and the tonic components of LLA, the tonic component being used as a readout of generalized increase in excitability in spinal locomotor networks. The most salient effect of brain stem removal was an increase in locomotor rhythm frequency and a concomitant reduction in burst durations in both MMC and LMC MNs. The lack of effect on the tonic component of LLA indicated specificity of action during the rhythmic component. Cooling-induced silencing of the brain stem reproduced the increase in rhythm frequency and accompanying decrease in burst durations in L2 MMC and LMC, suggesting a dependency on brain stem neuron activity. The work supports the idea that the brain stem locomotor circuits are operational already at birth and further suggests an important role in modulating trunk activity. The brain stem may influence the axial and hindlimb spinal locomotor rhythm generating circuits by extending their range of operation. This may represent a critical step of locomotor development when learning how to walk in different conditions and environments is a major endeavor.

Cellular reactions and compensatory tissue re-organization during spontaneous recovery after spinal cord injury in neonatal mice

Following incomplete spinal cord injuries, neonatal mammals display a remarkable degree of behavioral recovery. Previously, we have demonstrated in neonatal mice a wholesale re-establishment and reorganization of synaptic connections from some descending axon tracts (Boulland et al.: PLoS One 8 (2013)). To assess the potential cellular mechanisms contributing to this recovery, we have here characterized a variety of cellular sequelae following thoracic compression injuries, focusing particularly on cell loss and proliferation, inflammation and reactive gliosis, and the dynamics of specific types of synaptic terminals. Early during the period of recovery, regressive events dominated. Tissue loss near the injury was severe, with about 80% loss of neurons and a similar loss of axons that later make up the white matter. There was no sign of neurogenesis, no substantial astroglial or microglial proliferation, no change in the ratio of M1 and M2 microglia and no appreciable generation of the terminal complement peptide C5a. One day after injury the number of synaptic terminals on lumbar motoneurons had dropped by a factor of 2, but normalized by 6 days. The ratio of VGLUT1/2+ to VGAT+ terminals remained similar in injured and uninjured spinal cords during this period. By 24 days after injury, when functional recovery is nearly complete, the density of 5-HT+ fibers below the injury site had increased by a factor of 2.5. Altogether this study shows that cellular reactions are diverse and dynamic. Pronounced recovery of both excitatory and inhibitory terminals and an increase in serotonergic innervation below the injury, coupled with a general lack of inflammation and reactive gliosis, are likely to contribute to the recovery. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 928-946, 2017.

Elevated intracellular Na+ concentrations in developing spinal neurons

Over 25 years ago it was first reported that intracellular chloride levels (Cl^-_in ) were higher in developing neurons than in maturity. This finding has had significant implications for understanding the excitability of developing networks and recognizing the underlying causes of hyperexcitability associated with disease and neural injury. While there is some evidence that intracellular sodium levels (Na⁺_in ) change during the development of non-neural cells, it has largely been assumed that Na⁺_in is the same in developing and mature neurons. Here, using the sodium indicator SBFI, we test this idea and find that Na⁺_in is significantly higher in embryonic spinal motoneurons and interneurons than in maturity. We find that Na⁺_in reaches ~ 60 mM in mid-embryonic development and is then reduced to ~ 30 mM in late embryonic development. By retrogradely labeling motoneurons with SBFI we can reliably follow Na⁺_in levels in vitro for hours. Bursts of spiking activity, and blocking voltage-gated sodium channels did not influence observed motoneuron sodium levels. On the other hand, Na⁺_in was reduced by blocking the Na⁺ -K⁺ -2Cl^- cotransporter NKCC1, and was highly sensitive to changes in external Na⁺ and a blocker of the Na⁺ /K⁺ ATPase. Our findings suggest that the Na⁺ gradient is weaker in embryonic neuronal development and strengthens in maturity in a manner similar to that of Cl^- .

A Neonatal Mouse Spinal Cord Compression Injury Model

Spinal cord injury (SCI) typically causes devastating neurological deficits, particularly through damage to fibers descending from the brain to the spinal cord. A major current area of research is focused on the mechanisms of adaptive plasticity that underlie spontaneous or induced functional recovery following SCI. Spontaneous functional recovery is reported to be greater early in life, raising interesting questions about how adaptive plasticity changes as the spinal cord develops. To facilitate investigation of this dynamic, we have developed a SCI model in the neonatal mouse. The model has relevance for pediatric SCI, which is too little studied. Because neural plasticity in the adult involves some of the same mechanisms as neural plasticity in early life¹, this model may potentially have some relevance also for adult SCI. Here we describe the entire procedure for generating a reproducible spinal cord compression (SCC) injury in the neonatal mouse as early as postnatal (P) day 1. SCC is achieved by performing a laminectomy at a given spinal level (here described at thoracic levels 9-11) and then using a modified Yasargil aneurysm mini-clip to rapidly compress and decompress the spinal cord. As previously described, the injured neonatal mice can be tested for behavioral deficits or sacrificed for ex vivo physiological analysis of synaptic connectivity using electrophysiological and high-throughput optical recording techniques¹. Earlier and ongoing studies using behavioral and physiological assessment have demonstrated a dramatic, acute impairment of hindlimb motility followed by a complete functional recovery within 2 weeks, and the first evidence of changes in functional circuitry at the level of identified descending synaptic connections¹.

Organization of pontine reticulospinal inputs to motoneurons controlling axial and limb muscles in the neonatal mouse

Using optical recording of synaptically mediated calcium transients and selective spinal lesions, we investigated the pattern of activation of spinal motoneurons (MNs) by the pontine reticulospinal projection in isolated brain stem-spinal cord preparations from the neonatal mouse. Stimulation sites throughout the region where the pontine reticulospinal neurons reside reliably activated MNs at cervical, thoracic, and lumbar levels. Activation was similar in MNs ipsi- and contralateral to the stimulation site, similar in medial and lateral motor columns that contain trunk and limb MNs, respectively, and similar in the L2 and L5 segments that predominantly contain flexor and extensor MNs, respectively. In nonlesioned preparations, responses in both ipsi- and contralateral MNs followed individual stimuli in stimulus trains nearly one-to-one (with few failures). After unilateral hemisection at C1 on the same side as the stimulation, responses had substantially smaller magnitudes and longer latencies and no longer followed individual stimuli. After unilateral hemisection at C1 on the side opposite to the stimulation, the responses were also smaller, but their latencies were not affected. Thus we distinguish two pontine reticulospinal pathways to spinal MNs, one uncrossed and the other crossed, of which the uncrossed pathway transmits more faithfully and appears to be more direct.

Lhx3-Chx10 reticulospinal neurons in locomotor circuits

Motor behaviors result from the interplay between the brain and the spinal cord. Reticulospinal neurons, situated between the supraspinal structures that initiate motor movements and the spinal cord that executes them, play key integrative roles in these behaviors. However, the molecular identities of mammalian reticular formation neurons that mediate motor behaviors have not yet been determined, thus limiting their study in health and disease. In the medullary reticular formation of the mouse, we identified neurons that express the transcription factors Lhx3 and/or Chx10, and demonstrate that these neurons form a significant component of glutamatergic reticulospinal pathways. Lhx3-positive medullary reticular formation neurons express Fos following a locomotor task in the adult, indicating that they are active during walking. Furthermore, they receive functional inputs from the mesencephalic locomotor region and have electrophysiological properties to support tonic repetitive firing, both of which are necessary for neurons that mediate the descending command for locomotion. Together, these results suggest that Lhx3/Chx10 medullary reticular formation neurons are involved in locomotion.

A neonatal mouse spinal cord injury model for assessing post-injury adaptive plasticity and human stem cell integration

Despite limited regeneration capacity, partial injuries to the adult mammalian spinal cord can elicit variable degrees of functional recovery, mediated at least in part by reorganization of neuronal circuitry. Underlying mechanisms are believed to include synaptic plasticity and collateral sprouting of spared axons. Because plasticity is higher in young animals, we developed a spinal cord compression (SCC) injury model in the neonatal mouse to gain insight into the potential for reorganization during early life. The model provides a platform for high-throughput assessment of functional synaptic connectivity that is also suitable for testing the functional integration of human stem and progenitor cell-derived neurons being considered for clinical cell replacement strategies. SCC was generated at T9–T11 and functional recovery was assessed using an integrated approach including video kinematics, histology, tract tracing, electrophysiology, and high-throughput optical recording of descending inputs to identified spinal neurons. Dramatic degeneration of axons and synaptic contacts was evident within 24 hours of SCC, and loss of neurons in the injured segment was evident for at least a month thereafter. Initial hindlimb paralysis was paralleled by a loss of descending inputs to lumbar motoneurons. Within 4 days of SCC and progressively thereafter, hindlimb motility began to be restored and descending inputs reappeared, but with examples of atypical synaptic connections indicating a reorganization of circuitry. One to two weeks after SCC, hindlimb motility approached sham control levels, and weight-bearing locomotion was virtually indistinguishable in SCC and sham control mice. Genetically labeled human fetal neural progenitor cells injected into the injured spinal cord survived for at least a month, integrated into the host tissue and began to differentiate morphologically. This integrative neonatal mouse model provides opportunities to explore early adaptive plasticity mechanisms underlying functional recovery as well as the capacity for human stem cell-derived neurons to integrate functionally into spinal circuits.

Glutamatergic reticulospinal neurons in the mouse: developmental origins, axon projections, and functional connectivity

Subcortical descending glutamatergic neurons, such as reticulospinal (RS) neurons, play decisive roles in the initiation and control of many motor behaviors in mammals. However, little is known about the mechanisms used by RS neurons to control spinal motor networks because most of the neuronal elements involved have not been identified and characterized. In this review, we compare, in the embryonic mouse, the timing of developmental events that lead to the formation of synaptic connections between RS and spinal cord neurons. We then summarize our recent research in the postnatal mouse on the organization of synaptic connections between RS neurons and lumbar axial motoneurons (MNs), hindlimb MNs, and commissural interneurons. Finally, we give a brief account of some of the most recent studies on the intrinsic capabilities for plasticity of the mammalian RS system. The present review should give an updated insight into how functional specificity in RS motor networks emerges.

Templated agarose scaffolds for the support of motor axon regeneration into sites of complete spinal cord transection

Bioengineered scaffolds have the potential to support and guide injured axons after spinal cord injury, contributing to neural repair. In previous studies we have reported that templated agarose scaffolds can be fabricated into precise linear arrays and implanted into the partially injured spinal cord, organizing growth and enhancing the distance over which local spinal cord axons and ascending sensory axons extend into a lesion site. However, most human injuries are severe, sparing only thin rims of spinal cord tissue in the margins of a lesion site. Accordingly, in the present study we examined whether template agarose scaffolds seeded with bone marrow stromal cells secreting Brain-Derived Neurotrophic Factor (BDNF) would support regeneration into severe, complete spinal cord transection sites. Moreover, we tested responses of motor axon populations originating from the brainstem. We find that templated agarose scaffolds support motor axon regeneration into a severe spinal cord injury model and organize axons into fascicles of highly linear configuration. BDNF significantly enhances axonal growth. Collectively, these findings support the feasibility of scaffold implantation for enhancing central regeneration after even severe central nervous system injury.

Vestibular-mediated synaptic inputs and pathways to sympathetic preganglionic neurons in the neonatal mouse

To assess when vestibulosympathetic projections become functional postnatally, and to establish a preparation in which vestibulosympathetic circuitry can be characterized more precisely, we used an optical approach to record VIIIth nerve-evoked synaptic inputs to thoracic sympathetic preganglionic neurons (SPNs) in newborn mice. Stimulation of the VIIIth nerve was performed in an isolated brainstem-spinal cord preparation after retrogradely labelling with the fluorescent calcium indicator Calcium Green 1-conjugated dextran amine, the SPNs and the somatic motoneurons (MNs) in the thoracic (T) segments T2, 4, 6, 8, 10 and 12. Synaptically mediated calcium responses could be visualized and recorded in individual SPNs and MNs, and analysed with respect to latency, temporal pattern, magnitude and synaptic pharmacology. VIIIth nerve stimulation evoked responses in all SPNs and MNs investigated. The SPN responses had onset latencies from 90 to 200 ms, compared with much shorter latencies in MNs, and were completely abolished by mephenesin, a drug that preferentially reduces polysynaptic over monosynaptic transmission. Bicuculline and picrotoxin, but not strychnine, increased the magnitudes of the SPN responses without changing the onset latencies, suggesting a convergence of concomitant excitatory and inhibitory synaptic inputs. Lesions strategically placed to test the involvement of direct vestibulospinal pathways versus indirect pathways within the brainstem showed that vestibulosympathetic inputs in the neonate are mediated predominantly, if not exclusively, by the latter. Thus, already at birth, synaptic connections in the vestibulosympathetic reflex are functional and require the involvement of the ventrolateral medulla as in adult mammals.

Organization of functional synaptic connections between medullary reticulospinal neurons and lumbar descending commissural interneurons in the neonatal mouse

The medullary reticular formation (MRF) of the neonatal mouse is organized so that the medial and lateral MRF activate hindlimb and trunk motoneurons (MNs) with differential predominance. The goal of the present study was to investigate whether this activation is polysynaptic and mediated by commissural interneurons with descending axons (dCINs) in the lumbar spinal cord. To this end, we tested the polysynapticity of inputs from the MRF to MNs and tested for the presence of selective inputs from medial and lateral MRF to 574 individual dCINs in the L2 segment of the neonatal mouse. Reticulospinal-mediated postsynaptic Ca(2+) responses in MNs were reduced in the presence of mephenesin and after a midline lesion, suggesting the involvement of dCINs in mediating the responses. Consistent with this, stimulation of reticulospinal neurons in the medial or lateral MRF activated 51% and 57% of ipsilateral dCINs examined (255 and 352 dCINs, respectively) and 52% and 46% of contralateral dCINs examined (166 and 133 dCINs, respectively). The proportion of dCINs that responded specifically to stimulation of medial or lateral MRF was similar to the proportions of dCINs that responded to both MRF regions or to neither. The three responsive dCIN populations had largely overlapping spatial distributions. We demonstrate the existence of dCIN subpopulations sufficient to mediate responses in lumbar motoneurons from reticulospinal pathways originating from the medial and lateral MRF. Differential control of trunk and hindlimb muscles by the medullary reticulospinal system may therefore be mediated in part by identifiable dCIN populations.

Segmental patterns of vestibular-mediated synaptic inputs to axial and limb motoneurons in the neonatal mouse assessed by optical recording

Proper control of movement and posture occurs partly via descending projections from the vestibular nuclei to spinal motor circuits. Days before birth in rodents, vestibulospinal neurons develop axonal projections that extend to the spinal cord. How functional these projections are just after birth is unknown. Our goal was to assess the overall functional organization of vestibulospinal inputs to spinal motoneurons in a brainstem-spinal cord preparation of the neonatal mouse (postnatal day (P) 0-5). Using calcium imaging, we recorded responses evoked by electrical stimulation of the VIIIth nerve, in many motoneurons simultaneously throughout the spinal cord (C2, C6, T7, L2 and L5 segments), in the medial and lateral motor columns. Selective lesions in the brainstem and/or spinal cord distinguished which tracts contributed to the responses: those in the cervical cord originated primarily from the medial vestibulospinal tracts but with a substantial contribution from the lateral vestibulospinal tract; those in the thoracolumbar cord originated exclusively from the lateral vestibulospinal tract. In the thoracolumbar but not the cervical cord, excitatory commissural connections mediated vestibular responses in contralateral motoneurons. Pharmacological blockade of GABA(A) receptors showed that responses involved a convergence of excitatory and inhibitory inputs which in combination produced temporal response patterns specific for different segmental levels. Our results show that by birth vestibulospinal projections in rodents have already established functional synapses and are organized to differentially regulate activity in neck and limb motoneurons in a tract- and segment-specific pattern similar to that in adult mammals. Thus, this particular set of descending projections develops several key features of connectivity appropriately at prenatal stages. We also present novel information about vestibulospinal inputs to axial motoneurons in mammals, providing a more comprehensive platform for future studies into the overall organization of vestibulospinal inputs and their role in regulating postural stability.

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.

Imaging synaptically mediated responses produced by brainstem inputs onto identified spinal neurons in the neonatal mouse

Descending inputs to spinal cord neurons in mammals have previously been characterized functionally using microelectrode recording of single neurons, a technique with high spatial and temporal resolution but low yield. Consequently our knowledge about the functional connections between the brain and the spinal cord has been accumulating at a very low pace. Here we describe a high throughput optical recording approach in an ex vivo brainstem-spinal cord preparation of the neonatal mouse that permits screening many spinal neurons simultaneously for synaptic inputs from descending axons. The fluorescent calcium indicator calcium green dextran amine was loaded retrogradely into specific spinal neuron populations, including motoneurons (MNs) of the medial and lateral motor columns and two populations of interneurons with descending axons (dINs) in the ventral funiculus. Focal electrical stimulation of brainstem neuron populations with descending axons generated synaptic responses revealed by transient increases in intracellular calcium concentration in all four populations of spinal neurons. The resultant fluorescence signals could be readily visualized in individual MNs directly through the ventral white matter. In the more deeply located dINs, responses could be readily visualized in individual neurons from the surface of an oblique cut through the spinal cord. The rapid optical investigation of functional connections between brainstem descending neurons and various populations of spinal neurons in the living mammalian preparation should help uncover some of the key features of supraspinal sensory and motor control and provide a valuable tool for examining the re-innervation of spinal neurons by descending axons after spinal cord regeneration.