Genetically defined inhibitory neurons in the mouse spinal cord dorsal horn: a possible source of rhythmic inhibition of motoneurons during fictive locomotion

Multimodal sensory control of motor performance by glycinergic interneurons of the mouse spinal cord deep dorsal horn

Sensory feedback is integral for contextually appropriate motor output, yet the neural circuits responsible remain elusive. Here, we pinpoint the medial deep dorsal horn of the mouse spinal cord as a convergence point for proprioceptive and cutaneous input. Within this region, we identify a population of tonically active glycinergic inhibitory neurons expressing parvalbumin. Using anatomy and electrophysiology, we demonstrate that deep dorsal horn parvalbumin-expressing interneuron (dPV) activity is shaped by convergent proprioceptive, cutaneous, and descending input. Selectively targeting spinal dPVs, we reveal their widespread ipsilateral inhibition onto pre-motor and motor networks and demonstrate their role in gating sensory-evoked muscle activity using electromyography (EMG) recordings. dPV ablation altered limb kinematics and step-cycle timing during treadmill locomotion and reduced the transitions between sub-movements during spontaneous behavior. These findings reveal a circuit basis by which sensory convergence onto dorsal horn inhibitory neurons modulates motor output to facilitate smooth movement and context-appropriate transitions.

Controlled delivery of a neurotransmitter-agonist conjugate for functional recovery after severe spinal cord injury

Despite considerable unmet medical needs, effective pharmacological treatments that promote functional recovery after spinal cord injury remain limited. Although multiple pathological events are implicated in spinal cord injuries, the development of a microinvasive pharmacological approach that simultaneously targets the different mechanisms involved in spinal cord injury remains a formidable challenge. Here we report the development of a microinvasive nanodrug delivery system that consists of amphiphilic copolymers responsive to reactive oxygen species and an encapsulated neurotransmitter-conjugated KCC2 agonist. Upon intravenous administration, the nanodrugs enter the injured spinal cord due to a disruption in the blood-spinal cord barrier and disassembly due to damage-triggered reactive oxygen species. The nanodrugs exhibit dual functions in the injured spinal cord: scavenging accumulated reactive oxygen species in the lesion, thereby protecting spared tissues, and facilitating the integration of spared circuits into the host spinal cord through targeted modulation of inhibitory neurons. This microinvasive treatment leads to notable functional recovery in rats with contusive spinal cord injury.

Characterization of Dmrt3-Derived Neurons Suggest a Role within Locomotor Circuits

Neuronal networks within the spinal cord, collectively known as the central pattern generator (CPG), coordinate rhythmic movements underlying locomotion. The transcription factor doublesex and mab-3-related transcription factor 3 (DMRT3) is involved in the differentiation of the dorsal interneuron 6 class of spinal cord interneurons. In horses, a non-sense mutation in the Dmrt3 gene has major effects on gaiting ability, whereas mice lacking the Dmrt3 gene display impaired locomotor activity. Although the Dmrt3 gene is necessary for normal spinal network formation and function in mice, a direct role for Dmrt3-derived neurons in locomotor-related activities has not been demonstrated. Here we present the characteristics of the Dmrt3-derived spinal cord interneurons. Using transgenic mice of both sexes, we characterized interneurons labeled by their expression of Cre driven by the endogenous Dmrt3 promoter. We used molecular, retrograde tracing and electrophysiological techniques to examine the anatomical, morphological, and electrical properties of the Dmrt3-Cre neurons. We demonstrate that inhibitory Dmrt3-Cre neurons receive extensive synaptic inputs, innervate surrounding CPG neurons, intrinsically regulate CPG neuron's electrical activity, and are rhythmically active during fictive locomotion, bursting at frequencies independent to the ventral root output. The present study provides novel insights on the character of spinal Dmrt3-derived neurons, data demonstrating that these neurons participate in locomotor coordination.SIGNIFICANCE STATEMENT In this work, we provide evidence for a role of the Dmrt3 interneurons in spinal cord locomotor circuits as well as molecular and functional insights on the cellular and microcircuit level of the Dmrt3-expressing neurons in the spinal cord. Dmrt3 neurons provide the first example of an interneuron population displaying different oscillation frequencies. This study presents novel findings on an under-reported population of spinal cord neurons, which will aid in deciphering the locomotor network and will facilitate the design and development of therapeutics for spinal cord injury and motor disorders.

And yet it moves: Recovery of volitional control after spinal cord injury

Preclinical and clinical neurophysiological and neurorehabilitation research has generated rather surprising levels of recovery of volitional sensory-motor function in persons with chronic motor paralysis following a spinal cord injury. The key factor in this recovery is largely activity-dependent plasticity of spinal and supraspinal networks. This key factor can be triggered by neuromodulation of these networks with electrical and pharmacological interventions. This review addresses some of the systems-level physiological mechanisms that might explain the effects of electrical modulation and how repetitive training facilitates the recovery of volitional motor control. In particular, we substantiate the hypotheses that: (1) in the majority of spinal lesions, a critical number and type of neurons in the region of the injury survive, but cannot conduct action potentials, and thus are electrically non-responsive; (2) these neuronal networks within the lesioned area can be neuromodulated to a transformed state of electrical competency; (3) these two factors enable the potential for extensive activity-dependent reorganization of neuronal networks in the spinal cord and brain, and (4) propriospinal networks play a critical role in driving this activity-dependent reorganization after injury. Real-time proprioceptive input to spinal networks provides the template for reorganization of spinal networks that play a leading role in the level of coordination of motor pools required to perform a given functional task. Repetitive exposure of multi-segmental sensory-motor networks to the dynamics of task-specific sensory input as occurs with repetitive training can functionally reshape spinal and supraspinal connectivity thus re-enabling one to perform complex motor tasks, even years post injury.

Functional Characterization of Lamina X Neurons in ex-Vivo Spinal Cord Preparation

Functional properties of lamina X neurons in the spinal cord remain unknown despite the established role of this area for somatosensory integration, visceral nociception, autonomic regulation and motoneuron output modulation. Investigations of neuronal functioning in the lamina X have been hampered by technical challenges. Here we introduce an ex-vivo spinal cord preparation with both dorsal and ventral roots still attached for functional studies of the lamina X neurons and their connectivity using an oblique LED illumination for resolved visualization of lamina X neurons in a thick tissue. With the elaborated approach, we demonstrate electrophysiological characteristics of lamina X neurons by their membrane properties, firing pattern discharge and fiber innervation (either afferent or efferent). The tissue preparation has been also probed using Ca²⁺ imaging with fluorescent Ca²⁺ dyes (membrane-impermeable or -permeable) to demonstrate the depolarization-induced changes in intracellular calcium concentration in lamina X neurons. Finally, we performed visualization of subpopulations of lamina X neurons stained by retrograde labeling with aminostilbamidine dye to identify sympathetic preganglionic and projection neurons in the lamina X. Thus, the elaborated approach provides a reliable tool for investigation of functional properties and connectivity in specific neuronal subpopulations, boosting research of lamina X of the spinal cord.

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.

Rebound from Inhibition: Self-Correction against Neurodegeneration?

Neural networks play a critical role in establishing constraints on excitability in the central nervous system. Several recent studies have suggested that network dysfunction in the brain and spinal cord are compromised following insult by a neurodegenerative trigger and might precede eventual neuronal loss and neurological impairment. Early intervention of network excitability and plasticity might therefore be critical in resetting hyperexcitability and preventing later neuronal damage. Here, the behavior of neurons that generate burst firing upon recovery from inhibitory input or intrinsic membrane hyperpolarization (rebound neurons) is examined in the context of neural networks that underlie rhythmic activity observed in areas of the brain and spinal cord that are vulnerable to neurodegeneration. In a non-inflammatory rodent model of spongiform neurodegenerative disease triggered by retrovirus infection of glia, rebound neurons are particularly vulnerable to neurodegeneration, likely due to an inherently low calcium buffering capacity. The dysfunction of rebound neurons translates into a dysfunction of rhythmic neural circuits, compromising normal neurological function and leading to eventual morbidity. Understanding how virus infection of glia can mediate dysfunction of rebound neurons, induce hyperexcitability and loss of rhythmic function, pathologic features observed in neurodegenerative disorders ranging from epilepsy to motor neuron disease, might therefore suggest a common pathway for early therapeutic intervention.

Functional Recovery from Neural Stem/Progenitor Cell Transplantation Combined with Treadmill Training in Mice with Chronic Spinal Cord Injury

Most studies targeting chronic spinal cord injury (SCI) have concluded that neural stem/progenitor cell (NS/PC) transplantation exerts only a subclinical recovery; this in contrast to its remarkable effect on acute and subacute SCI. To determine whether the addition of rehabilitative intervention enhances the effect of NS/PC transplantation for chronic SCI, we used thoracic SCI mouse models to compare manifestations secondary to both transplantation and treadmill training, and the two therapies combined, with a control group. Significant locomotor recovery in comparison with the control group was only achieved in the combined therapy group. Further investigation revealed that NS/PC transplantation improved spinal conductivity and central pattern generator activity, and that treadmill training promoted the appropriate inhibitory motor control. The combined therapy enhanced these independent effects of each single therapy, and facilitated neuronal differentiation of transplanted cells and maturation of central pattern generator activity synergistically. Our data suggest that rehabilitative treatment represents a therapeutic option for locomotor recovery after NS/PC transplantation, even in chronic SCI.

Unique Spatiotemporal Neuromodulation of the Lumbosacral Circuitry Shapes Locomotor Success after Spinal Cord Injury

Spinal cord epidural stimulation has resulted in the initiation of voluntary leg movements and improvement in postural, bladder, and sexual function. However, one of the limitations in reaching the full potential of epidural stimulation for therapeutic purposes in humans has been the identification of optimal stimulation configurations that can neuromodulate the spinal cord for stepping. In the present work, we investigated the mechanisms underlying the specificity of interaction between the rostral and caudal spinal cord circuitries in enabling locomotion in spinal rats (n = 10) by epidural spinal cord stimulation. By using unique spatiotemporal epidural stimulation parameters of the lumbar and sacral spinal cords, a robust stepping pattern in spinal rats was observed with only six training sessions and as early as 3 weeks post-injury. Electrophysiological evidence reveals that in addition to frequency of stimulation pulses at the stimulation sites, the relative timing between stimulation pulses applied at the lumbar (L2) and sacral (S1) segments of the spinal cord heavily impacted stepping performance. Best stepping was established at a higher stimulation frequency (40 Hz vs. 5, 10, 15, and 20Hz) and at specific relative time-intervals between the stimulation pulses (L2 pulse applied at 18-25 msec after the onset of the S1 pulse; S1 pulse applied 0-7 msec after the L2 pulse). Our data suggest that controlling pulse-to-pulse timing at multiple stimulation sources provides a novel strategy to optimize spinal stepping by fine-tuning the physiological state of the locomotor networks. These findings hold direct relevance to the clinician who will incorporate electrical stimulation strategies for optimizing control of locomotion after complete paralysis.

Decoding the organization of spinal circuits that control locomotion

Unravelling the functional operation of neuronal networks and linking cellular activity to specific behavioural outcomes are among the biggest challenges in neuroscience. In this broad field of research, substantial progress has been made in studies of the spinal networks that control locomotion. Through united efforts using electrophysiological and molecular genetic network approaches and behavioural studies in phylogenetically diverse experimental models, the organization of locomotor networks has begun to be decoded. The emergent themes from this research are that the locomotor networks have a modular organization with distinct transmitter and molecular codes and that their organization is reconfigured with changes to the speed of locomotion or changes in gait.

BDNF Induced by Treadmill Training Contributes to the Suppression of Spasticity and Allodynia After Spinal Cord Injury via Upregulation of KCC2

BACKGROUND

Spasticity and allodynia are major sequelae that affect the quality of life and daily activities of spinal cord injury (SCI) patients. Although rehabilitation ameliorates spasticity and allodynia, the molecular mechanisms involved in these processes remain elusive.

OBJECTIVE

To investigate the molecular mechanisms by which rehabilitation ameliorates spasticity and allodynia after SCI in rats.

METHODS

The expression levels of brain-derived neurotrophic factor (BDNF) and potassium-chloride cotransporter-2 (KCC2), as well as the localization of KCC2, were examined in the lumbar enlargements of untrained and treadmill-trained thoracic SCI model rats. Spasticity and allodynia were determined via behavioral and electrophysiological analyses. The effects of BDNF on spasticity, allodynia, and KCC2 activation were determined by inhibition of BDNF signaling via intrathecal administration of TrkB-IgG. The effects of SCI and training on the expression levels of functional phospholipase C-γ in the lumbar enlargement were also examined.

RESULTS

Treadmill training after SCI upregulated endogenous BDNF expression and posttranslational modification of KCC2 in the lumbar enlargement significantly. There were also significant correlations between increased KCC2 expression and ameliorated spasticity and allodynia. Administration of TrkB-IgG abrogated the training-induced upregulation of KCC2 and beneficial effects on spasticity and allodynia. The expression level of functional phospholipase C-γ was reduced significantly after SCI, which may have contributed to the change in the function of BDNF, whereby it did not trigger short-term downregulation or induce long-term upregulation of KCC2 expression secondary to training.

CONCLUSIONS

BDNF-mediated restoration of KCC2 expression underlies the suppression of spasticity and allodynia caused by rehabilitation.

Plasticity of motor network and function in the absence of corticospinal projection

Despite the obvious clinical interest, our understanding of how developmental mechanisms are redeployed during degeneration and regeneration after brain and spinal cord injuries remains quite rudimentary. In animal models of spinal cord injury, although spontaneous regeneration of descending axons is limited, compensation by intact corticospinal axons, descending tracts from the brainstem, and local intrinsic spinal networks all contribute to the recovery of motor function. Here, we investigated spontaneous motor compensation and plasticity that occur in the absence of corticospinal tract, using Celsr3|Emx1 mice in which the corticospinal tract is completely and specifically absent as a consequence of Celsr3 inactivation in the cortex. Mutant mice had no paresis, but displayed hyperactivity in open-field, and a reduction in skilled movements in food pellet manipulation tests. The number of spinal motoneurons was reduced and their terminal arbors at neuromuscular junctions were atrophic, which was reflected in electromyography deficits. Rubrospinal projections, calretinin-positive propriospinal projections, afferent innervation of motoneurons by calretinin-positive segmental interneurons, and terminal ramifications of monoaminergic projections were significantly increased. Contrary to control animals, mutants also developed a severe and persistent disability of forelimb use following the section of the rubrospinal tract at the C4 spinal level. These observations demonstrate for the first time that the congenital absence of the corticospinal tract induces spontaneous plasticity, both at the level of the motor spinal cord and in descending monoaminergic and rubrospinal projections. Such compensatory mechanisms could be recruited in case of brain or spinal cord lesion or degeneration.

V1 and v2b interneurons secure the alternating flexor-extensor motor activity mice require for limbed locomotion

Reciprocal activation of flexor and extensor muscles constitutes the fundamental mechanism that tetrapod vertebrates use for locomotion and limb-driven reflex behaviors. This aspect of motor coordination is controlled by inhibitory neurons in the spinal cord; however, the identity of the spinal interneurons that serve this function is not known. Here, we show that the production of an alternating flexor-extensor motor rhythm depends on the composite activities of two classes of ventrally located inhibitory neurons, V1 and V2b interneurons (INs). Abrogating V1 and V2b IN-derived neurotransmission in the isolated spinal cord results in a synchronous pattern of L2 flexor-related and L5 extensor-related locomotor activity. Mice lacking V1 and V2b inhibition are unable to articulate their limb joints and display marked deficits in limb-driven reflex movements. Taken together, these findings identify V1- and V2b-derived neurons as the core interneuronal components of the limb central pattern generator (CPG) that coordinate flexor-extensor motor activity.

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.

Participation of a persistent sodium current and calcium-activated nonspecific cationic current to burst generation in trigeminal principal sensory neurons

The properties of neurons participating in masticatory rhythmogenesis are not clearly understood. Neurons within the dorsal trigeminal principal sensory nucleus (dPrV) are potential candidates as components of the masticatory central pattern generator (CPG). The present study examines in detail the ionic mechanisms controlling burst generation in dPrV neurons in rat (postnatal day 8-12) brain stem slices using whole cell and perforated patch-clamp methods. Nominal extracellular Ca(2+) concentration transformed tonic discharge in response to a maintained step pulse of current into rhythmical bursting in 38% of nonbursting neurons. This change in discharge mode was suppressed by riluzole, a persistent Na(+) current (INaP) antagonist. Veratridine, which suppresses the Na(+) channel inactivation mechanism, induced rhythmical bursting in nonbursting neurons in normal artificial cerebrospinal fluid, suggesting that INaP contributes to burst generation. Nominal extracellular Ca(2+) exposed a prominent afterdepolarizing potential (ADP) following a single spike induced by a 3-ms current pulse, which was suppressed, but not completely blocked, by riluzole. Application of BAPTA, a Ca(2+) chelator, intracellularly, or flufenamic acid, a Ca(2+)-activated nonspecific cationic channel (ICAN) antagonist, extracellularly to the bath, suppressed rhythmical bursting and the postspike ADP. Application of drugs to alter Ca(2+) release from endoplasmic reticulum also suppressed bursting. Finally, voltage-clamp methods demonstrated that nominal Ca(2+) facilitated INaP and induced ICAN. These data demonstrate for the first time that the previously observed induction in dPrV neurons of rhythmical bursting in nominal Ca(2+) is mediated by enhancement of INaP and onset of ICAN, which are dependent on intracellular Ca(2+).

Neuronal correlates of the dominant role of GABAergic transmission in the developing mouse locomotor circuitry

GABA and glycine are the primary fast inhibitory neurotransmitters in the mammalian spinal cord, but they differ in their regulatory functions, balancing neuronal excitation in the locomotor circuitry in the mammalian spinal cord. This review focuses on the unique role of GABAergic transmission during the assembly of the locomotor circuitry, from early embryonic stages when GABA(A) receptor-activated membrane depolarizations increase network excitation, to the period of early postnatal development, when GABAergic inhibition plays a primary role in coordinating the patterns of locomotor-like motor activity. To gain insight into the mechanisms that underlie the dominant contribution of GABAergic transmission to network activity during that period, we examined the morphological and electrophysiological properties of a subpopulation of GABAergic commissural interneurons that fit well with their putative function as integrated components of the rhythm-coordinating networks in the mouse spinal cord.

Imaging spinal neuron ensembles active during locomotion with genetically encoded calcium indicators

Advances in molecular-genetic tools for labeling neuronal subtypes, and the emerging development of robust genetic probes for neural activity, are likely to revolutionize our understanding of the functional organization of neural circuits. In principle, these tools should be able to detect activity at cellular resolution for large ensembles of identified neuron types as they participate in specific behaviors. This report describes the use of genetically encoded calcium indicators (GECIs), combined with two-photon microscopy, to characterize V1 interneurons, known to be critical for setting the duration of the step cycle. All V1 interneurons arise from a common precursor population and express engrailed-1 (En1). Our data show that although neighboring interneurons that arise from the same developmental lineage and share many features, such as projection patterns and neurotransmitter profiles, they are not irrevocably committed to having the same pattern of activity.

Early spread of hyperexcitability to caudal dorsal horn networks after a chemically-induced lesion of the rat spinal cord in vitro

Hyperexcitability of dorsal horn neurons has been shown to play a key role in neuropathic pain following chronic experimental spinal cord injury. With a neonatal in vitro spinal cord injury model, we show that a chemically-induced lesion leads to rapid gain-of-function of sublesional dorsal horn networks biased to hyperexcitation. The expression of the GABA synthetic enzyme GAD65 was significantly reduced at the same level of the spinal cord, suggesting a compromised inhibitory system. We propose that our model could be useful to test early approaches to contrast spinal cord injury-induced central sensitization of dorsal horn circuits.

TTX-resistant NMDA receptor-mediated membrane potential oscillations in neonatal mouse Hb9 interneurons

Conditional neuronal membrane potential oscillations have been identified as a potential mechanism to help support or generate rhythmogenesis in neural circuits. A genetically identified population of ventromedial interneurons, called Hb9, in the mouse spinal cord has been shown to generate TTX-resistant membrane potential oscillations in the presence of NMDA, serotonin and dopamine, but these oscillatory properties are not well characterized. Hb9 interneurons are rhythmically active during fictive locomotor-like behavior. In this study, we report that exogenous N-Methyl-D-Aspartic acid (NMDA) application is sufficient to produce membrane potential oscillations in Hb9 interneurons. In contrast, exogenous serotonin and dopamine application, alone or in combination, are not sufficient. The properties of NMDA-induced oscillations vary among the Hb9 interneuron population; their frequency and amplitude increase with increasing NMDA concentration. NMDA does not modulate the T-type calcium current (I_Ca(T)), which is thought to be important in generating locomotor-like activity, in Hb9 neurons. These results suggest that NMDA receptor activation is sufficient for the generation of TTX-resistant NMDA-induced membrane potential oscillations in Hb9 interneurons.

Distributions of active spinal cord neurons during swimming and scratching motor patterns

The spinal cord can generate motor patterns underlying several kinds of limb movements. Many spinal interneurons are multifunctional, contributing to multiple limb movements, but others are specialized. It is unclear whether anatomical distributions of activated neurons differ for different limb movements. We examined distributions of activated neurons for locomotion and scratching using an activity-dependent dye. Adult turtles were stimulated to generate repeatedly forward swimming, rostral scratching, pocket scratching, or caudal scratching motor patterns, while sulforhodamine 101 was applied to the spinal cord. Sulforhodamine-labeled neurons were widely distributed rostrocaudally, dorsoventrally, and mediolaterally after each motor pattern, concentrated bilaterally in the deep dorsal horn, the lateral intermediate zone, and the dorsal to middle ventral horn. Labeled neurons were common in all hindlimb enlargement segments and the pre-enlargement segment following swimming and scratching, but a significantly higher percentage were in the rostral segments following swimming than rostral scratching. These findings suggest that largely the same spinal regions are activated during swimming and scratching, but there are some differences that may indicate locations of behaviorally specialized neurons. Finally, the substantial inter-animal variability following a single kind of motor pattern may indicate that essentially the same motor output is generated by anatomically variable networks.

Requirement of neuronal connexin36 in pathways mediating presynaptic inhibition of primary afferents in functionally mature mouse spinal cord

Electrical synapses formed by gap junctions containing connexin36 (Cx36) promote synchronous activity of interneurones in many regions of mammalian brain; however, there is limited information on the role of electrical synapses in spinal neuronal networks. Here we show that Cx36 is widely distributed in the spinal cord and is involved in mechanisms that govern presynaptic inhibition of primary afferent terminals. Electrophysiological recordings were made in spinal cord preparations from 8- to 11-day-old wild-type and Cx36 knockout mice. Several features associated with presynaptic inhibition evoked by conditioning stimulation of low threshold hindlimb afferents were substantially compromised in Cx36 knockout mice. Dorsal root potentials (DRPs) evoked by low intensity stimulation of sensory afferents were reduced in amplitude by 79% and in duration by 67% in Cx36 knockouts. DRPs were similarly affected in wild-types by bath application of gap junction blockers. Consistent with presynaptic inhibition of group Ia muscle spindle afferent terminals on motoneurones described in adult cats, conditioning stimulation of an adjacent dorsal root evoked a long duration inhibition of monosynaptic reflexes recorded from the ventral root in wild-type mice, and this inhibition was antagonized by bicuculline. The same conditioning stimulation failed to inhibit monosynaptic reflexes in Cx36 knockout mice. Immunofluorescence labelling for Cx36 was found throughout the dorsal and ventral horns of the spinal cord of juvenile mice and persisted in mature animals. In deep dorsal horn laminae, where interneurones involved in presynaptic inhibition of large diameter muscle afferents are located, cells were extensively dye-coupled following intracellular neurobiotin injection. Coupled cells displayed Cx36-positive puncta along their processes. Our results indicate that gap junctions formed by Cx36 in spinal cord are required for maintenance of presynaptic inhibition, including the regulation of transmission from Ia muscle spindle afferents. In addition to a role in presynaptic inhibition in juvenile animals, the persistence of Cx36 expression among spinal neuronal populations in the adult mouse suggests that the contribution of electrical synapses to integrative processes in fully mature spinal cord may be as diverse as that found in other areas of the CNS.

Characterization of last-order premotor interneurons by transneuronal tracing with rabies virus in the neonatal mouse spinal cord

We characterized the interneurons involved in the control of ankle extensor (triceps surae [TS] muscles) motoneurons (MNs) in the lumbar enlargement of mouse neonates by retrograde transneuronal tracing using rabies virus (RV). Examination of the kinetics of retrograde transneuronal transfer at sequential intervals post inoculation enabled us to determine the time window during which only the first-order interneurons, i.e., interneurons likely monosynaptically connected to MNs (last-order interneurons [loINs]) were RV-infected. The infection of the network resulted exclusively from a retrograde transport of RV along the motor pathway. About 80% of the loINs were observed ipsilaterally to the injection. They were distributed all along the lumbar enlargement, but the majority was observed in L4 and L5 segments where TS MNs were localized. Most loINs were distributed in laminae V-VII, whereas the most superficial laminae were devoid of RV infection. Contralaterally, commissural loINs were found essentially in lamina VIII of all lumbar segments. Groups of loINs were characterized by their chemical phenotypes using dual immunolabeling. Glycinergic neurons connected to TS MNs represented 50% of loINs ipsilaterally and 10% contralaterally. As expected, the ipsilateral glycinergic loINs included Renshaw cells, the most ventral neurons expressing calbindin. We also demonstrated a direct connection between a group of cholinergic interneurons observed ipsilaterally in L3 and the rostral part of L4, and TS MNs. To conclude, transneuronal tracing with RV, combined with an immunohistochemical detection of neuronal determinants, allows a very specific mapping of motor networks involved in the control of single muscles.

Fictive locomotion in the adult decerebrate and spinal mouse in vivo

Recently, transgenic mice have been created with mutations affecting the components of the mammalian spinal central pattern generator (CPG) for locomotion; however, it has currently only been possible to evoke fictive locomotion in mice, using neonatal in vitro preparations. Here, we demonstrate that it is possible to evoke fictive locomotion in the adult decerebrate mouse in vivo using l-3,4-dihydroxyphenylalanine methyl ester hydrochloride (l-DOPA) and 5-hydroxytryptophan (5HTP) following injection of the monoaminoxiadase inhibitor Nialamide. We investigate the effects of afferent stimulation and spinalization as well as demonstrate the possibility of simultaneous intracellular recording of rhythmically active motoneurones. Our results demonstrate that several features of the mouse locomotor CPG are similar to those that have been observed in rat, cat, rabbit and monkey suggesting a fairly conserved organisation and allowing for future results in transgenic mice to be extrapolated to existing knowledge of CPG components and circuitry obtained in larger species.

Low-threshold calcium currents contribute to locomotor-like activity in neonatal mice

In this study, we examined the contribution of a low-threshold calcium current [I(Ca(T))] to locomotor-related activity in the neonatal mouse. Specifically, the role of I(Ca(T)) was studied during chemically induced, locomotor-like activity in the isolated whole cord and in a genetically distinct population of ventromedial spinal interneurons marked by the homeobox gene Hb9. In isolated whole spinal cords, cycle frequency was decreased in the presence of low-threshold calcium channel blockers, which suggests a role for I(Ca(T)) in the network that produces rhythmic, locomotor-like activity. Additionally, we used Hb9 interneurons as a model to study the cellular responses to application of low-threshold calcium channel blockers. In transverse slice preparations from transgenic Hb9::enhanced green fluorescent protein neonatal mice, N-methyl-d-aspartate-induced membrane potential oscillations in identified Hb9 interneurons also slowed in frequency with application of nickel when fast, spike-mediated, synaptic transmission was blocked with TTX. Voltage-clamp and immunolabeling experiments confirmed expression of I(Ca(T)) and channels, respectively, in Hb9 interneurons located in the ventromedial spinal cord. Taken together, these results provide support that T-type calcium currents play an important role in network-wide rhythm generation during chemically evoked, fictive locomotor activity.

Long-duration perforated patch recordings from spinal interneurons of adult mice

It has been very difficult to record from interneurons in acute slices of the lumbar spinal cord from mice >3 wk of age. The low success rate and short recording times limit in vitro experimentation on mouse spinal networks to neonatal and early postnatal periods when locomotor networks are still developmentally immature. To overcome this limitation and enable investigation of mature locomotor network neurons, we have established a reliable procedure to record from spinal cord neurons in slices from adult, behaviorally mature mice of any age. Two key changes to the established neonate procedure were implemented. First, we remove the cord by a dorsal laminectomy from a deeply anesthetized animal. This enables respiration and other vital functions to continue up to the moment the maximally oxygenated lumbar spinal cord is removed, improving the health of the slices. Second, since adult spinal cord interneurons appear more sensitive to the intracellular dialysis that occurs during whole cell recordings, we introduced perforated patch recordings to the procedure. Stable recordings up to 12 h in duration were obtained with our new method. This will allow investigation of changes in mature neuronal properties in disease states or after spinal cord injury and allow prolonged recordings of responses to drug application that were previously impossible.

Acute inhibition of signalling phenotype of spinal GABAergic neurons by tumour necrosis factor-alpha

Spinal application of TNFα induces both allodynia and hyperalgesia, and at least part of the pronociceptive effects of TNFα have been suggested as due to the impaired function of spinal inhibitory neurons (disinhibition). The present study explores the effects of TNFα on the signalling phenotype of spinal GABAergic neurons identified in transgenic mice expressing green fluorescent protein at the glutamic acid decarboxylase 67 (GAD67) promoter. Acute application of TNFα directly inhibits the excitability of a subset of GAD67(+) spinal neurons. TNFα-induced inhibition was dependent on the activation of p38 mitogen-activated protein kinase (MAPK) within these GAD67(+) neurons. TNFα receptor 1 (TNFR1) but not receptor 2 (TNFR2) was identified on spinal GAD67(+) neurons, suggesting that TNFα signals through TNFR1. Voltage-clamp recordings of GAD67(+) neurons indicated that the inhibitory effect of TNFα was through suppression of the hyperpolarization-activated cation current (I(h)). This study defines a novel mechanism of spinal disinhibition mediated by a TNFα-TNFR1-p38 pathway within GABAergic inhibitory interneurons.

Locomotor-related activity of GABAergic interneurons localized in the ventrolateral region in the isolated spinal cord of neonatal mice

Inhibitory neurons are an essential element of the locomotor network in the mammalian spinal cord. However, little is known about the firing pattern and synaptic modulation during locomotion in the majority of them. In this study, we performed whole cell recording in visually identified ventrolaterally located GABAergic neurons (VL-GNs) in the rostral (L2 segment) and caudal (L5 segment) lumbar cord using isolated spinal cord preparations taken from glutamate decarboxylase 67-green fluorescent protein (GAD67-GFP) knock-in mouse neonates. These neurons did not respond to electrical stimulation of the ventral root, indicating that they were not Renshaw cells. Ninety-five percent of VL-GNs in the L2 segment and fifty percent of those in the L5 segment showed significant rhythmic firing during locomotor-like rhythmic activity induced by bath application of 5-HT and NMDA. Seventy percent of these neurons fired mainly during the extensor phase, and twenty-five percent fired mainly during the flexor phase. Voltage-clamp recordings revealed that most of these neurons received rhythmic inhibition during the nonfiring phase and excitatory synaptic inputs during the firing phase. Morphological examination of recorded neurons filled with neurobiotin showed that their soma was located lateral to the motoneuron pool and that they extended their processes into the local ipsilateral ventromedial region and dorsal regions. The present study indicates that these GABAergic interneurons located in the ventrolateral region adjacent to the motoneuron pool are rhythmically active during locomotion and involved in the inhibitory modulation of local locomotor network in the lumbar spinal cord.

Development and functional organization of spinal locomotor circuits

The coordination and timing of muscle activities during rhythmic movements, like walking and swimming, are generated by intrinsic spinal motor circuits. Such locomotor networks are operational early in development and are found in all vertebrates. This review outlines and compares recent advances that have revealed the developmental and functional organization of these fundamental spinal motor networks in limbed and non-limbed animals. The comparison will highlight common principles and divergence in the organization of the spinal locomotor network structure in these different species as well as point to unresolved issues regarding the assembly and functioning of these networks.

Whither motoneurons?

In the preceding series of articles, the history of vertebrate motoneuron and motor unit neurobiological studies has been discussed. In this article, we select a few examples of recent advances in neuroscience and discuss their application or potential application to the study of motoneurons and the control of movement. We conclude, like Sherrington, that in order to understand normal, traumatized, and diseased human behavior, it is critical to continue to study motoneuron biology using all available and emerging tools. This article is part of a Special Issue entitled Historical Review.

Trans-spinal direct current stimulation modulates motor cortex-induced muscle contraction in mice

The present study investigated the effect of trans-spinal direct current (tsDC) on the firing rate, pattern, and amplitude of spontaneous activity of the tibial nerve and on the magnitude of cortically elicited triceps surae (TS) muscle contractions. The effect of combined tsDC and repetitive cortical electrical stimulation (rCES) on the amplitude of cortically elicited TS twitches was also investigated. Stimulation was applied by two disk electrodes (0.79 cm(2)): one was located subcutaneously over the vertebral column (T(10)-L(1)) and was used to deliver anodal DC (a-tsDC) or cathodal DC (c-tsDC) (density range: ± 0.64 to ± 38.2 A/m(2)), whereas the other was located subcutaneously on the lateral aspect of the abdomen and served as a reference. While the application of a-tsDC significantly increased the spike frequency and amplitude of spontaneous discharges compared with c-tsDC, c-tsDC made the spontaneous discharges more rhythmic. Cortically elicited TS twitches were depressed during a-tsDC and potentiated after termination. Conversely, cortically elicited TS twitches were enhanced during c-tsDC and depressed after termination. While combined a-tsDC and rCES produced similar effects as a-tsDC alone, combined c-tsDC and rCES showed the greatest increase in cortically elicited TS twitches. tsDC appears to be a powerful neurostimulation tool that can differentially modulate spinal cord excitability and corticospinal transmission.

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.

Functional subdivision of feline spinal interneurons in reflex pathways from group Ib and II muscle afferents; an update

A first step towards understanding the operation of a neural network is identification of the populations of neurons that contribute to it. Our aim here is to reassess the basis for subdivision of adult mammalian spinal interneurons that mediate reflex actions from tendon organs (group Ib afferents) and muscle spindle secondary endings (group II afferents) into separate populations. Re-examining the existing experimental data, we find no compelling reasons to consider intermediate zone interneurons with input from group Ib afferents to be distinct from those co-excited by group II afferents. Similar patterns of distributed input have been found in subpopulations that project ipsilaterally, contralaterally or bilaterally, and in both excitatory and inhibitory interneurons; differences in input from group I and II afferents to individual interneurons showed intra- rather than inter-population variation. Patterns of reflex actions evoked from group Ib and II afferents and task-dependent changes in these actions, e.g. during locomotion, may likewise be compatible with mediation by premotor interneurons integrating information from both group I and II afferents. Pathological changes after injuries of the central nervous system in humans and the lineage of different subclasses of embryonic interneurons may therefore be analyzed without need to consider subdivision of adult intermediate zone interneurons into subpopulations with group Ib or group II input. We propose renaming these neurons 'group I/II interneurons'.

The use of PRV-Bartha to define premotor inputs to lumbar motoneurons in the neonatal spinal cord of the mouse

Background

The neonatal mouse has become a model system for studying the locomotor function of the lumbar spinal cord. However, information about the synaptic connectivity within the governing neural network remains scarce. A neurotropic pseudorabies virus (PRV) Bartha has been used to map neuronal connectivity in other parts of the nervous system, due to its ability to travel trans-neuronally. Its use in spinal circuits regulating locomotion has been limited and no study has defined the time course of labelling for neurons known to project monosynaptically to motoneurons.

Methodology/Principal Findings

Here we investigated the ability of PRV Bartha, expressing green and/or red fluorescence, to label spinal neurons projecting monosynaptically to motoneurons of two principal hindlimb muscles, the tibialis anterior (TA) and gastrocnemius (GC). As revealed by combined immunocytochemistry and confocal microscopy, 24–32 h after the viral muscle injection the label was restricted to the motoneuron pool while at 32–40 h the fluorescence was seen in interneurons throughout the medial and lateral ventral grey matter. Two classes of ipsilateral interneurons known to project monosynaptically to motoneurons (Renshaw cells and cells of origin of C-terminals) were consistently labeled at 40 h post-injection but also a group in the ventral grey matter contralaterally. Our results suggest that the labeling of last order interneurons occurred 8–12 h after motoneuron labeling and we presume this is the time taken by the virus to cross one synapse, to travel retrogradely and to replicate in the labeled cells.

Conclusions/Significance

The study establishes the time window for virally - labelling monosynaptic projections to lumbar motoneurons following viral injection into hindlimb muscles. Moreover, it provides a good foundation for intracellular targeting of the labeled neurons in future physiological studies and better understanding the functional organization of the lumbar neural networks.

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.

Spinal interneurons providing input to the final common path during locomotion

As the nexus between the nervous system and the skeletomuscular system, motoneurons effect all behavior. As such, motoneuron activity must be well regulated so as to generate appropriately timed and graded muscular contractions. Accordingly, motoneurons receive a large number of both excitatory and inhibitory synaptic inputs from various peripheral and central sources. Many of these synaptic contacts arise from spinal interneurons, some of which belong to spinal networks responsible for the generation of locomotor activity. Although the complete definition of these networks remains elusive, it is known that the neural machinery necessary to generate the basic rhythm and pattern of locomotion is contained within the spinal cord. One approach to gaining insights into spinal locomotor networks is to describe those spinal interneurons that directly control the activity of motoneurons, so-called last-order interneurons. In this chapter, we briefly survey the different populations of last-order interneurons that have been identified using anatomical, physiological, and genetic methodologies. We discuss the possible roles of these identified last-order interneurons in generating locomotor activity, and in the process, identify particular criteria that may be useful in identifying putative last-order interneurons belonging to spinal locomotor networks.

Synaptically activated burst-generating conductances may underlie a group-pacemaker mechanism for respiratory rhythm generation in mammals

Breathing, chewing, and walking are critical life-sustaining behaviors in mammals that consist essentially of simple rhythmic movements. Breathing movements in particular involve the diaphragm, thorax, and airways but emanate from a network in the lower brain stem. This network can be studied in reduced preparations in vitro and using simplified mathematical models that make testable predictions. An iterative approach that employs both in vitro and in silico models argues against canonical mechanisms for respiratory rhythm in neonatal rodents that involve reciprocal inhibition and pacemaker properties. We present an alternative model in which emergent network properties play a rhythmogenic role. Specifically, we show evidence that synaptically activated burst-generating conductances-which are only available in the context of network activity-engender robust periodic bursts in respiratory neurons. Because the cellular burst-generating mechanism is linked to network synaptic drive we dub this type of system a group pacemaker.