Serotonin controls initiation of locomotion and afferent modulation of coordination via 5-HT7 receptors in adult rats

Covert actions of epidural stimulation on spinal locomotor circuits

Spinal circuitry produces the rhythm and patterning of locomotion. However, both descending and sensory inputs are required to initiate and adapt locomotion to the environment. Spinal cord injury (SCI) disrupts descending controls of the spinal cord, producing paralysis. Epidural stimulation (ES) is a promising clinical therapy for motor control recovery and is capable of reactivating the lumbar spinal locomotor networks, yet little is known about the effects of ES on locomotor neurons. Previously, we found that both sensory afferent pathways and serotonin exert mixed excitatory and inhibitory actions on lumbar interneurons involved in the generation of the locomotor rhythm, identified by the transcription factor Shox2. However, after chronic complete SCI, sensory afferent inputs to Shox2 interneurons become almost exclusively excitatory and Shox2 interneurons are supersensitive to serotonin. Here, we investigated the effects of ES on these SCI-induced changes. Inhibitory input from sensory pathways to Shox2 interneurons was maintained and serotonin supersensitivity was not observed in SCI mice that received daily sub-motor threshold ES. Interestingly, the effects of ES were maintained for at least three weeks after the ES was discontinued. In contrast, the effects of ES were not observed in Shox2 interneurons from mice that received ES after the establishment of the SCI-induced changes. Our results demonstrate mechanistic actions of ES at the level of identified spinal locomotor circuit neurons and the effectiveness of early treatment with ES on preservation of spinal locomotor circuitry after SCI, suggesting possible therapeutic benefits prior to the onset of motor rehabilitation.

Neuronal mechanisms regulating locomotion in adult Drosophila

The coordinated action of multiple leg joints and muscles is required even for the simplest movements. Understanding the neuronal circuits and mechanisms that generate precise movements is essential for comprehending the neuronal basis of the locomotion and to infer the neuronal mechanisms underlying several locomotor-related diseases. Drosophila melanogaster provides an excellent model system for investigating the neuronal circuits underlying motor behaviors due to its simple nervous system and genetic accessibility. This review discusses current genetic methods for studying locomotor circuits and their function in adult Drosophila. We highlight recently identified neuronal pathways that modulate distinct forward and backward locomotion and describe the underlying neuronal control of leg swing and stance phases in freely moving flies. We also report various automated leg tracking methods to measure leg motion parameters and define inter-leg coordination, gait and locomotor speed of freely moving adult flies. Finally, we emphasize the role of leg proprioceptive signals to central motor circuits in leg coordination. Together, this review highlights the utility of adult Drosophila as a model to uncover underlying motor circuitry and the functional organization of the leg motor system that governs correct movement.

Serotonergic dysfunction impairs locomotor coordination in spinal muscular atrophy

Neuromodulation by serotonin regulates the activity of neuronal networks responsible for a wide variety of essential behaviours. Serotonin (or 5-HT) typically activates metabotropic G protein-coupled receptors, which in turn initiate second messenger signalling cascades and induce short and long-lasting behavioural effects. Serotonin is intricately involved in the production of locomotor activity and gait control for different motor behaviours. Although dysfunction of serotonergic neurotransmission has been associated with mood disorders and spasticity after spinal cord injury, whether and to what extent such dysregulation is implicated in movement disorders has not been firmly established. Here, we investigated whether serotonergic neuromodulation is affected in spinal muscular atrophy (SMA), a neurodegenerative disease caused by ubiquitous deficiency of the SMN protein. The hallmarks of SMA are death of spinal motor neurons, muscle atrophy and impaired motor control, both in human patients and mouse models of disease. We used a severe mouse model of SMA, that closely recapitulates the severe symptoms exhibited by type I SMA patients, the most common and most severe form of the disease. Together, with mouse genetics, optogenetics, physiology, morphology and behavioural analysis, we report severe dysfunction of serotonergic neurotransmission in the spinal cord of SMA mice, both at early and late stages of the disease. This dysfunction is followed by reduction of 5-HT synapses on vulnerable motor neurons. We demonstrate that motor neurons innervating axial and trunk musculature are preferentially affected, suggesting a possible cause for the proximo-distal progression of disease, and raising the possibility that it may underlie scoliosis in SMA patients. We also demonstrate that the 5-HT dysfunction is caused by SMN deficiency in serotonergic neurons in the raphe nuclei of the brainstem. The behavioural significance of the dysfunction in serotonergic neuromodulation is underlined by inter-limb discoordination in SMA mice, which is ameliorated when selective restoration of SMN in 5-HT neurons is achieved by genetic means. Our study uncovers an unexpected dysfunction of serotonergic neuromodulation in SMA and indicates that, if normal function is to be restored under disease conditions, 5-HT neuromodulation should be a key target for therapeutic approaches.

Spinal motoneurons respond aberrantly to serotonin in a rabbit model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors that damage the developing central nervous system. Impaired motor control, including muscle stiffness and spasticity, is the hallmark of spastic CP. Rabbits that experience hypoxic-ischaemic (HI) injury in utero (at 70%-83% gestation) are born with muscle stiffness, hyperreflexia and, as recently discovered, increased 5-HT in the spinal cord. To determine whether serotonergic modulation of spinal motoneurons (MNs) contributes to motor deficits, we performed ex vivo whole cell patch clamp in neonatal rabbit spinal cord slices at postnatal day (P) 0-5. HI MNs responded to the application of α-methyl 5-HT (a 5-HT1 /5-HT2 receptor agonist) and citalopram (a selective 5-HT reuptake inhibitor) with increased amplitude and hyperpolarization of persistent inward currents and hyperpolarized threshold voltage for action potentials, whereas control MNs did not exhibit any of these responses. Although 5-HT similarly modulated MN properties of HI motor-unaffected and motor-affected kits, it affected sag/hyperpolarization-activated cation current (Ih ) and spike frequency adaptation only in HI motor-affected MNs. To further explore the differential sensitivity of MNs to 5-HT, we performed immunostaining for inhibitory 5-HT1A receptors in lumbar spinal MNs at P5. Fewer HI MNs expressed the 5-HT1A receptor compared to age-matched control MNs. This suggests that HI MNs may lack a normal mechanism of central fatigue, mediated by 5-HT1A receptors. Altered expression of other 5-HT receptors (including 5-HT2 ) likely also contributes to the robust increase in HI MN excitability. In summary, by directly exciting MNs, the increased concentration of spinal 5-HT in HI-affected rabbits can cause MN hyperexcitability, muscle stiffness and spasticity characteristic of CP. Therapeutic strategies that target serotonergic neuromodulation may be beneficial to individuals with CP. KEY POINTS: We used whole cell patch clamp electrophysiology to test the responsivity of spinal motoneurons (MNs) from neonatal control and hypoxia-ischaemia (HI) rabbits to 5-HT, which is elevated in the spinal cord after prenatal HI injury. HI rabbit MNs showed a more robust excitatory response to 5-HT than control rabbit MNs, including hyperpolarization of the persistent inward current and threshold voltage for action potentials. Although most MN properties of HI motor-unaffected and motor-affected kits responded similarly to 5-HT, 5-HT caused larger sag/hyperpolarization-activated cation current (Ih ) and altered repetitive firing patterns only in HI motor-affected MNs. Immunostaining revealed that fewer lumbar MNs expressed inhibitory 5-HT1A receptors in HI rabbits compared to controls, which could account for the more robust excitatory response of HI MNs to 5-HT. These results suggest that elevated 5-HT after prenatal HI injury could trigger a cascade of events that lead to muscle stiffness and altered motor unit development.

Contrasting walking styles map to discrete neural substrates in the mouse brainstem

Walking is a slow gait which is particularly adaptable to meet internal or external needs and is prone to maladaptive alterations that lead to gait disorders. Alterations can affect speed, but also style (the way one walks). While slowed speed may signify the presence of a problem, style represents the hallmark essential for clinical classification of gait disorders. However, it has been challenging to objectively capture key stylistic features while uncovering neural substrates driving these features. Here we revealed brainstem hotspots that drive strikingly different walking styles by employing an unbiased mapping assay that combines quantitative walking signatures with focal, cell type specific activation. We found that activation of inhibitory neurons that mapped to the ventromedial caudal pons induced slow motion-like style. Activation of excitatory neurons that mapped to the ventromedial upper medulla induced shuffle-like style. Contrasting shifts in walking signatures distinguished these styles. Activation of inhibitory and excitatory neurons outside these territories or of serotonergic neurons modulated walking speed, but without walking signature shifts. Consistent with their contrasting modulatory actions, hotspots for slow-motion and shuffle-like gaits preferentially innervated different substrates. These findings lay the basis for new avenues to study mechanisms underlying (mal)adaptive walking styles and gait disorders.

Graphical abstract

Spinal motoneurons respond aberrantly to serotonin in a rabbit model of cerebral palsy

Cerebral palsy (CP) is caused by a variety of factors that damage the developing central nervous system. Impaired motor control, including muscle stiffness and spasticity, is the hallmark of spastic CP. Rabbits that experience hypoxic-ischemic (HI) injury in utero (at 70-80% gestation) are born with muscle stiffness, hyperreflexia, and, as recently discovered, increased serotonin (5-HT) in the spinal cord. To determine whether serotonergic modulation of spinal motoneurons (MNs) contributes to motor deficits, we performed ex vivo whole cell patch clamp in neonatal rabbit spinal cord slices at postnatal day (P) 0-5. HI MNs responded to application of α-methyl 5-HT (a 5-HT 1 /5-HT 2 receptor agonist) and citalopram (a selective 5-HT reuptake inhibitor) with hyperpolarization of persistent inward currents and threshold voltage for action potentials, reduced maximum firing rate, and an altered pattern of spike frequency adaptation while control MNs did not exhibit any of these responses. To further explore the differential sensitivity of MNs to 5-HT, we performed immunohistochemistry for inhibitory 5-HT 1A receptors in lumbar spinal MNs at P5. Fewer HI MNs expressed the 5-HT 1A receptor compared to age-matched controls. This suggests many HI MNs lack a normal mechanism of central fatigue mediated by 5-HT 1A receptors. Other 5-HT receptors (including 5-HT 2 ) are likely responsible for the robust increase in HI MN excitability. In summary, by directly exciting MNs, the increased concentration of spinal 5-HT in HI rabbits can cause MN hyperexcitability, muscle stiffness, and spasticity characteristic of CP. Therapeutic strategies that target serotonergic neuromodulation may be beneficial to individuals with CP.

Key points

After prenatal hypoxia-ischemia (HI), neonatal rabbits that show hypertonia are known to have higher levels of spinal serotoninWe tested responsivity of spinal motoneurons (MNs) in neonatal control and HI rabbits to serotonin using whole cell patch clampMNs from HI rabbits showed a more robust excitatory response to serotonin than control MNs, including hyperpolarization of the persistent inward current and threshold for action potentials, larger post-inhibitory rebound, and less spike frequency adaptation Based on immunohistochemistry of lumbar MNs, fewer HI MNs express inhibitory 5HT 1A receptors than control MNs, which could account for the more robust excitatory response of HI MNs. These results suggest that after HI injury, the increased serotonin could trigger a cascade of events leading to muscle stiffness and altered motor unit development.

Role of Descending Serotonergic Fibers in the Development of Pathophysiology after Spinal Cord Injury (SCI): Contribution to Chronic Pain, Spasticity, and Autonomic Dysreflexia

As the nervous system develops, nerve fibers from the brain form descending tracts that regulate the execution of motor behavior within the spinal cord, incoming sensory signals, and capacity to change (plasticity). How these fibers affect function depends upon the transmitter released, the receptor system engaged, and the pattern of neural innervation. The current review focuses upon the neurotransmitter serotonin (5-HT) and its capacity to dampen (inhibit) neural excitation. A brief review of key anatomical details, receptor types, and pharmacology is provided. The paper then considers how damage to descending serotonergic fibers contributes to pathophysiology after spinal cord injury (SCI). The loss of serotonergic fibers removes an inhibitory brake that enables plasticity and neural excitation. In this state, noxious stimulation can induce a form of over-excitation that sensitizes pain (nociceptive) circuits, a modification that can contribute to the development of chronic pain. Over time, the loss of serotonergic fibers allows prolonged motor drive (spasticity) to develop and removes a regulatory brake on autonomic function, which enables bouts of unregulated sympathetic activity (autonomic dysreflexia). Recent research has shown that the loss of descending serotonergic activity is accompanied by a shift in how the neurotransmitter GABA affects neural activity, reducing its inhibitory effect. Treatments that target the loss of inhibition could have therapeutic benefit.

Shifts in Gait Signatures Mark the End of Lifespan in Mice, With Sex Differences in Timing

Reduced walking speed is a hallmark of functional decline in aging across species. An age-related change in walking style may represent an additional key marker signifying deterioration of the nervous system. Due to the speed dependence of gait metrics combined with slowing of gait during aging, it has been challenging to determine whether changes in gait metrics represent a change in style. In this longitudinal study we employed gait signatures to separate changes in walking style and speed in mice. We compared gait signatures at mature adult age with middle aged, old and geriatric time points and included female and male sub-cohorts to examine sex differences in nature or timing signature shifts. To determine whether gait signature shifts occurred independently from a decline in other mobility domains we measured balance and locomotor activity. We found that walking speed declined early, whereas gait signatures shifted very late during the aging process. Shifts represented longer swing time and stride length than expected for speed, as in slow motion, and were preceded by a decline in other mobility domains. The pattern of shifts was similar between female and male cohorts, but with sex differences in timing. We conclude that changes in walking style, speed and other mobility domains represent separate age-related phenomena. These findings call for careful, sex specific selection of type and timing of outcome measures in mechanistic or interventional studies. The pattern of age-related gait signature shifts is distinct from patterns seen in neurodegenerative conditions and may be a translatable marker for the end of the lifespan.

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

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

Unusual Quadrupedal Locomotion in Rat during Recovery from Lumbar Spinal Blockade of 5-HT7 Receptors

Coordination of four-limb movements during quadrupedal locomotion is controlled by supraspinal monoaminergic descending pathways, among which serotoninergic ones play a crucial role. Here we investigated the locomotor pattern during recovery from blockade of 5-HT₇ or 5-HT_2A receptors after intrathecal application of SB269970 or cyproheptadine in adult rats with chronic intrathecal cannula implanted in the lumbar spinal cord. The interlimb coordination was investigated based on electromyographic activity recorded from selected fore- and hindlimb muscles during rat locomotion on a treadmill. In the time of recovery after hindlimb transient paralysis, we noticed a presence of an unusual pattern of quadrupedal locomotion characterized by a doubling of forelimb stepping in relation to unaffected hindlimb stepping (2FL-1HL) after blockade of 5-HT₇ receptors but not after blockade of 5-HT_2A receptors. The 2FL-1HL pattern, although transient, was observed as a stable form of fore-hindlimb coupling during quadrupedal locomotion. We suggest that modulation of the 5-HT₇ receptors on interneurons located in lamina VII with ascending projections to the forelimb spinal network can be responsible for the 2FL-1HL locomotor pattern. In support, our immunohistochemical analysis of the lumbar spinal cord demonstrated the presence of the 5-HT₇ immunoreactive cells in the lamina VII, which were rarely 5-HT_2A immunoreactive.

Serotonergic Facilitation of Forelimb Functional Recovery in Rats with Cervical Spinal Cord Injury

Serotonergic agents can improve the recovery of motor ability after a spinal cord injury. Herein, we compare the effects of buspirone, a 5-HT1A receptor partial agonist, to fluoxetine, a selective serotonin reuptake inhibitor, on forelimb motor function recovery after a C4 bilateral dorsal funiculi crush in adult female rats. After injury, single pellet reaching performance and forelimb muscle activity decreased in all rats. From 1 to 6 weeks after injury, rats were tested on these tasks with and without buspirone (1-2 mg/kg) or fluoxetine (1-5 mg/kg). Reaching and grasping success rates of buspirone-treated rats improved rapidly within 2 weeks after injury and plateaued over the next 4 weeks of testing. Electromyography (EMG) from selected muscles in the dominant forelimb showed that buspirone-treated animals used new reaching strategies to achieve success after the injury. However, forelimb performance dramatically decreased within 2 weeks of buspirone withdrawal. In contrast, fluoxetine treatment resulted in a more progressive rate of improvement in forelimb performance over 8 weeks after injury. Neither buspirone nor fluoxetine significantly improved quadrupedal locomotion on the horizontal ladder test. The improved accuracy of reaching and grasping, patterns of muscle activity, and increased excitability of spinal motor-evoked potentials after buspirone administration reflect extensive reorganization of connectivity within and between supraspinal and spinal sensory-motor netxcopy works. Thus, both serotonergic drugs, buspirone and fluoxetine, neuromodulated these networks to physiological states that enabled markedly improved forelimb function after cervical spinal cord injury.

Three-Week Treadmill Exercise Enhances Persistent Inward Currents, Facilitates Dendritic Plasticity, and Upregulates the Excitability of Dorsal Raphe Serotonin Neurons in ePet-EYFP Mice

Exercise plays a key role in preventing or treating mental or motor disorders caused by dysfunction of the serotonergic system. However, the electrophysiological and ionic channel mechanisms underlying these effects remain unclear. In this study, we investigated the effects of 3-week treadmill exercise on the electrophysiological and channel properties of dorsal raphe nucleus (DRN). Serotonin (5-HT) neurons in ePet-EYFP mice, using whole-cell patch clamp recording. Treadmill exercise was induced in ePet-EYFP mice of P21–24 for 3 weeks, and whole-cell patch clamp recording was performed on EYFP-positive 5-HT neurons from DRN slices of P42–45 mice. Experiment data showed that 5-HT neurons in the DRN were a heterogeneous population with multiple firing patterns (single firing, phasic firing, and tonic firing). Persistent inward currents (PICs) with multiple patterns were expressed in 5-HT neurons and composed of Cav1.3 (Ca-PIC) and sodium (Na-PIC) components. Exercise hyperpolarized the voltage threshold for action potential (AP) by 3.1 ± 1.0 mV (control: n = 14, exercise: n = 18, p = 0.005) and increased the AP amplitude by 6.7 ± 3.0 mV (p = 0.031) and firing frequency by more than 22% especially within a range of current stimulation stronger than 70 pA. A 3-week treadmill exercise was sufficient to hyperpolarize PIC onset by 2.6 ± 1.3 mV (control: −53.4 ± 4.7 mV, n = 28; exercise: −56.0 ± 4.7 mV, n = 25, p = 0.050) and increase the PIC amplitude by 28% (control: 193.6 ± 81.8 pA; exercise: 248.5 ± 105.4 pA, p = 0.038). Furthermore, exercise hyperpolarized Na-PIC onset by 3.8 ± 1.8 mV (control: n = 8, exercise: n = 9, p = 0.049) and increased the Ca-PIC amplitude by 23% (p = 0.013). The exercise-induced enhancement of the PIC amplitude was mainly mediated by Ca-PIC and hyperpolarization of PIC onset by Na-PIC. Moreover, exercise facilitated dendritic plasticity, which was shown as the increased number of branch points by 1.5 ± 0.5 (p = 0.009) and dendritic branches by 2.1 ± 0.6 (n = 20, p = 0.001) and length by 732.0 ± 100.1 μm (p < 0.001) especially within the range of 50–200 μm from the soma. Functional analysis suggested that treadmill exercise enhanced Na-PIC for facilitation of spike initiation and Ca-PIC for regulation of repetitive firing. We concluded that PICs broadly existed in DRN 5-HT neurons and could influence serotonergic neurotransmission in juvenile mice and that 3-week treadmill exercise induced synaptic adaptations, enhanced PICs, and thus upregulated the excitability of the 5-HT neurons.

Control of Orienting Movements and Locomotion by Projection-Defined Subsets of Brainstem V2a Neurons

Spatial orientation requires the execution of lateralized movements and a change in the animal's heading in response to multiple sensory modalities. While much research has focused on the circuits for sensory integration, chiefly to the midbrain superior colliculus (SC), the downstream cells and circuits that engage adequate motor actions have remained elusive. Furthermore, the mechanisms supporting trajectory changes are still speculative. Here, using transneuronal viral tracings in mice, we show that brainstem V2a neurons, a genetically defined subtype of glutamatergic neurons of the reticular formation, receive putative synaptic inputs from the contralateral SC. This makes them a candidate relay of lateralized orienting commands. We next show that unilateral optogenetic activations of brainstem V2a neurons in vivo evoked ipsilateral orienting-like responses of the head and the nose tip on stationary mice. When animals are walking, similar stimulations impose a transient locomotor arrest followed by a change of trajectory. Third, we reveal that these distinct motor actions are controlled by dedicated V2a subsets each projecting to a specific spinal cord segment, with at least (1) a lumbar-projecting subset whose unilateral activation specifically controls locomotor speed but neither impacts trajectory nor evokes orienting movements, and (2) a cervical-projecting subset dedicated to head orientation, but not to locomotor speed. Activating the latter subset suffices to steer the animals' directional heading, placing the head orientation as the prime driver of locomotor trajectory. V2a neurons and their modular organization may therefore underlie the orchestration of multiple motor actions during multi-faceted orienting behaviors.

Noradrenergic Components of Locomotor Recovery Induced by Intraspinal Grafting of the Embryonic Brainstem in Adult Paraplegic Rats

Intraspinal grafting of serotonergic (5-HT) neurons was shown to restore plantar stepping in paraplegic rats. Here we asked whether neurons of other phenotypes contribute to the recovery. The experiments were performed on adult rats after spinal cord total transection. Grafts were injected into the sub-lesional spinal cord. Two months later, locomotor performance was tested with electromyographic recordings from hindlimb muscles. The role of noradrenergic (NA) innervation was investigated during locomotor performance of spinal grafted and non-grafted rats using intraperitoneal application of α₂ adrenergic receptor agonist (clonidine) or antagonist (yohimbine). Morphological analysis of the host spinal cords demonstrated the presence of tyrosine hydroxylase positive (NA) neurons in addition to 5-HT neurons. 5-HT fibers innervated caudal spinal cord areas in the dorsal and ventral horns, central canal, and intermediolateral zone, while the NA fiber distribution was limited to the central canal and intermediolateral zone. 5-HT and NA neurons were surrounded by each other’s axons. Locomotor abilities of the spinal grafted rats, but not in control spinal rats, were facilitated by yohimbine and suppressed by clonidine. Thus, noradrenergic innervation, in addition to 5-HT innervation, plays a potent role in hindlimb movement enhanced by intraspinal grafting of brainstem embryonic tissue in paraplegic rats.

Spike-induced ordering: Stochastic neural spikes provide immediate adaptability to the sensorimotor system

The functional advantages of using a stochastically spiking neural network (sSNN) instead of a nonspiking neural network (NS-NN) have remained largely unknown. We developed an architecture which enabled the parametric adjustment of the spikiness (i.e., impulsive dynamics and stochasticity) of the sSNN output and observed that stochastic spikes instantaneously induced the ordered motion of a dynamical system. We demonstrated the benefits of sSNNs using a musculoskeletal bipedal walker and, moreover, showed that the decrease in the spikiness of motor neuron output leads to a reduction in adaptability. Stochastic spikes may aid the adaptation of a biological system to sudden perturbations or environmental changes. Our architecture can easily be connected to the conventional NS-NN and may superimpose the on-site adaptability.

Most biological neurons exhibit stochastic and spiking action potentials. However, the benefits of stochastic spikes versus continuous signals other than noise tolerance and energy efficiency remain largely unknown. In this study, we provide an insight into the potential roles of stochastic spikes, which may be beneficial for producing on-site adaptability in biological sensorimotor agents. We developed a platform that enables parametric modulation of the stochastic and discontinuous output of a stochastically spiking neural network (sSNN) to the rate-coded smooth output. This platform was applied to a complex musculoskeletal–neural system of a bipedal walker, and we demonstrated how stochastic spikes may help improve on-site adaptability of a bipedal walker to slippery surfaces or perturbation of random external forces. We further applied our sSNN platform to more general and simple sensorimotor agents and demonstrated four basic functions provided by an sSNN: 1) synchronization to a natural frequency, 2) amplification of the resonant motion in a natural frequency, 3) basin enlargement of the behavioral goal state, and 4) rapid complexity reduction and regular motion pattern formation. We propose that the benefits of sSNNs are not limited to musculoskeletal dynamics. Indeed, a wide range of the stability and adaptability of biological systems may arise from stochastic spiking dynamics.

Contribution of 5-HT2 Receptors to the Control of the Spinal Locomotor System in Intact Rats

Applying serotonergic (5-HT) agonists or grafting of fetal serotonergic cells into the spinal cord improves locomotion after spinal cord injury. Little is known about the role of 5-HT receptors in the control of voluntary locomotion, so we administered inverse agonists of 5-HT₂ (Cyproheptadine; Cypr), 5-HT_2A neutral antagonist (Volinanserin; Volin), 5-HT_2C neutral antagonist (SB 242084), and 5-HT_2B/2C inverse agonist (SB 206553) receptors intrathecally in intact rats and monitored their effects on unrestrained locomotion. An intrathecal cannula was introduced at the low thoracic level and pushed caudally until the tip reached the L2/L3 or L5/L6 spinal segments. Locomotor performance was evaluated using EMG activity of hindlimb muscles during locomotion on a 2 m long runway. Motoneuron excitability was estimated using EMG recordings during dorsi- and plantar flexion at the ankle. Locomotion was dramatically impaired after the blockage of 5-HT_2A receptors. The effect of Cypr was more pronounced than that of Volin since in the L5/L6 rats Cypr (but not Volin) induced significant alteration of the strength of interlimb coordination followed by total paralysis. These agents significantly decreased locomotor EMG amplitude and abolished or substantially decreased stretch reflexes. Blocking 5-HT_2B/2C receptors had no effect either on locomotion or reflexes. We suggest that in intact rats serotonin controls timing and amplitude of muscle activity by acting on 5-HT_2A receptors on both CPG interneurons and motoneurons, while 5-HT_2B/2C receptors are not involved in control of the locomotor pattern in lumbar spinal cord.

Fictive Scratching Patterns in Brain Cortex-Ablated, Midcollicular Decerebrate, and Spinal Cats

Background: The spinal cord’s central pattern generators (CPGs) have been explained by the symmetrical half-center hypothesis, the bursts generator, computational models, and more recently by connectome circuits. Asymmetrical models, at odds with the half-center paradigm, are composed of extensor and flexor CPG modules. Other models include not only flexor and extensor motoneurons but also motoneuron pools controlling biarticular muscles. It is unknown whether a preferred model can explain some particularities that fictive scratching (FS) in the cat presents. The first aim of this study was to investigate FS patterns considering the aiming and the rhythmic periods, and second, to examine the effects of serotonin (5HT) on and segmental inputs to FS.

Methods: The experiments were carried out first in brain cortex-ablated cats (BCAC), then spinalized (SC), and for the midcollicular (MCC) preparation. Subjects were immobilized and the peripheral nerves were used to elicit the Monosynaptic reflex (MR), to modify the scratching patterns and for electroneurogram recordings.

Results: In BCAC, FS was produced by pinna stimulation and, in some cases, by serotonin. The scratching aiming phase (AP) initiates with the activation of either flexor or extensor motoneurons. Serotonin application during the AP produced simultaneous extensor and flexor bursts. Furthermore, WAY 100635 (5HT1A antagonist) produced a brief burst in the tibialis anterior (TA) nerve, followed by a reduction in its electroneurogram (ENG), while the soleus ENG remained silent. In SC, rhythmic phase (RP) activity was recorded in the soleus motoneurons. Serotonin or WAY produced FS bouts. The electrical stimulation of Ia afferent fibers produced heteronymous MRes waxing and waning during the scratch cycle. In MCC, FS began with flexor activity. Electrical stimulation of either deep peroneus (DP) or superficial peroneus (SP) nerves increased the duration of the TA electroneurogram. Medial gastrocnemius (MG) stretching or MG nerve electrical stimulation produced a reduction in the TA electroneurogram and an initial MG extensor burst. MRes waxed and waned during the scratch cycle.

Conclusion: Descending pathways and segmental afferent fibers, as well as 5-HT and WAY, can change the FS pattern. To our understanding, the half-center hypothesis is the most suitable for explaining the AP in MCC.

TIAM2S as a novel regulator for serotonin level enhances brain plasticity and locomotion behavior

TIAM2S, the short form of human T-cell lymphoma invasion and metastasis 2, can have oncogenic effects when aberrantly expressed in the liver or lungs. However, it is also abundant in healthy, non-neoplastic brain tissue, in which its primary function is still unknown. Here, we examined the neurobiological and behavioral significance of human TIAM2S using the human brain protein panels, a human NT2/D1-derived neuronal cell line model (NT2/N), and transgenic mice that overexpress human TIAM2S (TIAM2S-TG). Our data reveal that TIAM2S exists primarily in neurons of the restricted brain areas around the limbic system and in well-differentiated NT2/N cells. Functional studies revealed that TIAM2S has no guanine nucleotide exchange factor (GEF) activity and is mainly located in the nucleus. Furthermore, whole-transcriptome and enrichment analysis with total RNA sequencing revealed that TIAM2S-knockdown (TIAM2S-KD) was strongly associated with the cellular processes of the brain structural development and differentiation, serotonin-related signaling, and the diseases markers representing neurobehavioral developmental disorders. Moreover, TIAM2S-KD cells display decreased neurite outgrowth and reduced serotonin levels. Moreover, TIAM2S overexpressing TG mice show increased number and length of serotonergic fibers at early postnatal stage, results in higher serotonin levels at both the serum and brain regions, and higher neuroplasticity and hyperlocomotion in latter adulthood. Taken together, our results illustrate the non-oncogenic functions of human TIAM2S and demonstrate that TIAM2S is a novel regulator of serotonin level, brain neuroplasticity, and locomotion behavior.

Serotonergic modulation of sacral dorsal root stimulation-induced locomotor output in newborn rat

Descending neuromodulators from the brainstem play a major role in the development and regulation of spinal sensorimotor functions. Here, the contribution of serotonergic signaling in the lumbar spinal cord was investigated in the context of the generation of locomotor activity. Experiments were performed on in vitro spinal cord preparations from newborn rats (0-5 days). Rhythmic locomotor episodes (fictive locomotion) triggered by tonic electrical stimulations (2Hz, 30s) of a single sacral dorsal root were recorded from bilateral flexor-dominated (L2) and extensor-dominated (L5) ventral roots. We found that the activity pattern induced by sacral stimulation evolves over the 5 post-natal (P) day period. Although alternating rhythmic flexor-like motor bursts were expressed at all ages, the locomotor pattern of extensor-like bursting was progressively lost from P1 to P5. At later stages, serotonin (5-HT) and quipazine (5-HT2A receptor agonist) at concentrations sub-threshold for direct locomotor network activation promoted sacral stimulation-induced fictive locomotion. The 5-HT2A receptor antagonist ketanserin could reverse the agonist's action but was ineffective when fictive locomotion was already expressed in the absence of 5-HT (mainly before P2). Although inhibiting 5-HT7 receptors with SB266990 did not affect locomotor pattern organization, activating 5-HT1A receptors with 8-OH-DPAT specifically deteriorated extensor phase motor burst activity. We conclude that during the first 5 post-natal days in rat, serotonergic signaling in the lumbar cord becomes increasingly critical for the expression of fictive locomotion. Our findings therefore further underline the importance of both descending serotonergic and sensory afferent pathways in shaping locomotor activity during postnatal development. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Current Principles of Motor Control, with Special Reference to Vertebrate Locomotion

The vertebrate control of locomotion involves all levels of the nervous system from cortex to the spinal cord. Here, we aim to cover all main aspects of this complex behavior, from the operation of the microcircuits in the spinal cord to the systems and behavioral levels and extend from mammalian locomotion to the basic undulatory movements of lamprey and fish. The cellular basis of propulsion represents the core of the control system, and it involves the spinal central pattern generator networks (CPGs) controlling the timing of different muscles, the sensory compensation for perturbations, and the brain stem command systems controlling the level of activity of the CPGs and the speed of locomotion. The forebrain and in particular the basal ganglia are involved in determining which motor programs should be recruited at a given point of time and can both initiate and stop locomotor activity. The propulsive control system needs to be integrated with the postural control system to maintain body orientation. Moreover, the locomotor movements need to be steered so that the subject approaches the goal of the locomotor episode, or avoids colliding with elements in the environment or simply escapes at high speed. These different aspects will all be covered in the review.

Three-week treadmill training changes the electrophysiological properties of spinal interneurons in the mice

It was shown in previous studies that endurance training enhanced excitability of rat spinal motoneurons. However, the influence of the training on the spinal interneurons remains unclear. In this study, we investigated the training effects on spinal interneurons in dorsal and ventromedial area in mice (P42-P50). The electrophysiological properties of the interneurons were recorded from spinal cord slices (T13-L6) by whole-cell patch-clamp recording. The interneurons could be classified into three types based on their response to step currents: single spike (type 1), phasic firing (type 2), and tonic firing (type 3) in both control and trained mice. Interneurons collected from control mice possessed rheobase of 11.3 ± 6.0 pA and voltage threshold (V_th) of - 37.3 ± 4.7 mV. Treadmill training reduced the rheobase by 4.8 ± 1.5 pA and V_th by 3.1 ± 1.2 mV (P < 0.05). Furthermore, the training effects were dependent on the distribution and types of the interneurons. Treadmill training hyperpolarized V_th and decreased rheobase in ventromedial interneurons, while the significant change was observed only in the action potation height of the interneurons in dorsal horn. Treadmill training also hyperpolarized V_th and increased input resistance in type 3 interneurons, but none of these changes was shown in type 1 and 2 interneurons. Bath application of 5-HT (10-20 μM) increased the neuronal excitability in both control and trained mice. Serotonin had similar effect on membrane properties of the interneurons collected from both groups. This study suggested that treadmill training increased excitability of spinal interneurons of the mice and thus would make the spinal motor system easier to generate locomotion.

Central pattern generators in the brainstem and spinal cord: an overview of basic principles, similarities and differences

Central pattern generators (CPGs) are generally defined as networks of neurons capable of enabling the production of central commands, specifically controlling stereotyped, rhythmic motor behaviors. Several CPGs localized in brainstem and spinal cord areas have been shown to underlie the expression of complex behaviors such as deglutition, mastication, respiration, defecation, micturition, ejaculation, and locomotion. Their pivotal roles have clearly been demonstrated although their organization and cellular properties remain incompletely characterized. In recent years, insightful findings about CPGs have been made mainly because (1) several complementary animal models were developed; (2) these models enabled a wide variety of techniques to be used and, hence, a plethora of characteristics to be discovered; and (3) organizations, functions, and cell properties across all models and species studied thus far were generally found to be well-preserved phylogenetically. This article aims at providing an overview for non-experts of the most important findings made on CPGs in in vivo animal models, in vitro preparations from invertebrate and vertebrate species as well as in primates. Data about CPG functions, adaptation, organization, and cellular properties will be summarized with a special attention paid to the network for locomotion given its advanced level of characterization compared with some of the other CPGs. Similarities and differences between these networks will also be highlighted.

The Effect of Melatonin on Locomotor Behavior and Muscle Physiology in the Sea Cucumber Apostichopus japonicus

Melatonin is a highly conserved hormone in evolutionary history. It occurs in numerous organisms and plays a role in the endocrine and immune systems. Locomotor behavior is a basic behavior in animals and is an important indicator of circadian rhythms, which are coordinated by the nervous and endocrine systems. To date, the effect of melatonin on locomotor behavior has been studied in vertebrates, including syrian hamsters, sparrows, rats, zebrafish, goldfish, and flatworms. However, there have been few studies of the effects of melatonin on locomotor behavior in marine invertebrates. The goals of present study were to show the existence of melatonin in the sea cucumber Apostichopus japonicus and to evaluate its effect on locomotor activity. In addition, muscle tissues from control and melatonin-treated sea cucumbers were tested using ultra performance liquid chromatography and quadrupole time-of-flight mass spectrometry (UPLC-Q-TOF-MS) to determine the changes of metabolic activity in muscle. Melatonin was present in the coelomic fluid of A. japonicus at a concentration of ∼135.0 ng/L. The total distance traveled and number steps taken over 9 h after melatonin administration decreased with increasing concentration of the melatonin dose. Mean and maximum velocity of movement and stride length and stride frequency also decreased, but their differences were not statistically significant. Overall, these results suggest that melatonin administration had a sedative effect on A. japonicus. The levels of 22 different metabolites were altered in the muscle tissues of melatonin-treated sea cucumbers. Serotonin, 9-cis retinoic acid, all-trans retinoic acid, flavin mononucleotide in muscles were downregulated after melatonin administration. Moreover, a high free fatty acid (FFA) concentration and a decrease in the adenosine 5′-triphosphate (ATP) concentration in the muscle tissues of the melatonin-treated group were detected as well. These results suggest that the sedative effect of melatonin involves some other metabolic pathways, and the reduced locomotor modulator—serotonin, inhibited fatty acid oxidation and disturbed oxidative phosphorylation are potential physiological mechanisms that result in the inhibitory effect of melatonin on locomotion in sea cucumbers.

Wheel Running Improves Motor Function and Spinal Cord Plasticity in Mice With Genetic Absence of the Corticospinal Tract

Our previous studies showed that mutant mice with congenital absence of the corticospinal tract (CST) undergo spontaneous remodeling of motor networks to partially compensate for absent CST function. Here, we asked whether voluntary wheel running could further improve locomotor plasticity in CST-deficient mice. Adult mutant mice were randomly allocated to a “runners” group with free access to a wheel, or a “non-runners” group with no access to a wheel. In comparison with non-runners, there was a significant motor improvement including fine movement, grip strength, decreased footslip errors in runners after 8-week training, which was supported by the elevated amplitude of electromyography recording and increased neuromuscular junctions in the biceps. In runners, terminal ramifications of monoaminergic and rubrospinal descending axons were significantly increased in spinal segments after 12 weeks of exercise compared to non-runners. 5-ethynyl-2′-deoxyuridine (EDU) labeling showed that proliferating cells, 90% of which were Olig2-positive oligodendrocyte progenitors, were 4.8-fold more abundant in runners than in non-runners. In 8-week runners, RNAseq analysis of spinal samples identified 404 genes up-regulated and 398 genes down-regulated, and 69 differently expressed genes involved in signal transduction, among which the NF-κB, PI3K-Akt and cyclic AMP (cAMP) signaling were three top pathways. Twelve-week training induced a significant elevation of postsynaptic density protein 95 (PSD95), synaptophysin 38 and myelin basic protein (MBP), but not of brain derived neurotrophic factor (BDNF), glial cell line-derived neurotrophic factor (GDNF) and insulin like growth factor-1 (IGF-1). Thus, locomotor training activates multiple signaling pathways, contributes to neural plasticity and functional improvement, and might palliate locomotor deficits in patients.

Induced NB-3 Limits Regenerative Potential of Serotonergic Axons after Complete Spinal Transection

NB-3 (contactin-6) is a member of the contactin family and has a wide range of roles during central nervous system development and disease. Here, we found that NB-3 was simultaneously induced in the serotonergic raphespinal tract (sRST) axons and in the scar-forming cells after spinal cord injury (SCI). Regrowth of sRST axons was promoted in vivo by blocking NB-3 expression in either sRST axons or scar-forming cells when post-traumatic axons of the sRST tried to penetrate the glial scar. NB-3 deficiency promoted synapse reformation between sRST regenerative axons and motor neurons and enhanced the potential for electrical activity of muscle contraction and motor coordination. In vivo evidence also suggested that NB-3 induction in both sRST axons and scar-forming cells was required to mediate NB-3 signaling inhibition of sRST axon regeneration after SCI. Our findings suggest that NB-3 protein is a potential molecular target for future SCI treatments.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.

Molecular cloning and functional expression of the 5-HT7 receptor in Chinese mitten crab (Eriocheir sinensis)

Serotonin (5-HT) regulates numerous physiological functions and processes, such as light adaptation, food intake and ovarian maturation, and plays the role through 5-HT receptors. To our knowledge, this is the first study to isolate and characterize the serotonin receptor 7 (5-HT₇ receptor) cDNA encoded in Eriocheir sinensis, an economically important aquaculture species in China, by performing rapid-amplification of cDNA ends. The full-length of 5-HT₇ receptor gene cDNA is 2328 bp and encodes a polypeptide with 590 amino acids that are highly homologous with other crustaceans 5-HT₇ receptor genes. Analysis of the deduced amino acid sequence of the 5-HT₇, including 7 transmembrane domains and some common features of G protein-coupled receptors (GPCRs), indicated that 5-HT₇ receptor was a member of GPCRs family. A gene expression analysis of the 5-HT₇ receptor by RT-PCR revealed that the 5-HT₇ receptor transcripts were widely distributed in various tissues, in which high expression levels were observed in the cranial ganglia, thoracic ganglia and intestines. Further study about the effects of photoperiods on the 5-HT₇ expression in the tissues showed that a significantly increasing expression of the 5-HT₇ receptor was observed in the thoracic ganglia induced by constant light. In addition, in the eyestalks, the expression levels of 5-HT₇ mRNA in constant darkness and constant light were lower than control treatment. Then, the expression levels of the 5-HT₇ receptor in three feeding statuses displayed that there were significantly increasing expressions in the hepatopancreas and intestines after feeding, compared with before feeding and during the feeding period. Finally, the 5-HT₇ mRNA expression levels in stage III and stage IV were higher than the levels in stage I of ovarian development. Our experimental results showed that the 5-HT₇ receptor structurally belongs to GPCRs, and the thoracic ganglia and eyestalks are the important tissues of the 5-HT₇ receptor for light adaptation. The 5-HT₇ receptor may also be involved in the physiological regulation of the hepatopancreas and intestines after ingestion in E. sinensis. In addition, the 5-HT₇ receptor is involved in the process of ovarian maturation. The study provided a foundation for further research of light adaptation, digestive functions and ovarian maturation of the 5-HT₇ receptor in Decapoda.

Intraspinal Grafting of Serotonergic Neurons Modifies Expression of Genes Important for Functional Recovery in Paraplegic Rats

Serotonin (5-hydroxytryptamine; 5-HT) plays an important role in control of locomotion, partly through direct effects on motoneurons. Spinal cord complete transection (SCI) results in changes in 5-HT receptors on motoneurons that influence functional recovery. Activation of 5-HT_2A and 5-HT₇ receptors improves locomotor hindlimb movements in paraplegic rats. Here, we analyzed the mRNA of 5-HT_2A and 5-HT₇ receptors (encoded by Htr2a and Htr7 genes, resp.) in motoneurons innervating tibialis anterior (TA) and gastrocnemius lateralis (GM) hindlimb muscles and the tail extensor caudae medialis (ECM) muscle in intact as well as spinal rats. Moreover, the effect of intraspinal grafting of serotonergic neurons on Htr2a and Htr7 gene expression was examined to test the possibility that the graft origin 5-HT innervation in the spinal cord of paraplegic rats could reverse changes in gene expression induced by SCI. Our results indicate that SCI at the thoracic level leads to changes in Htr2a and Htr7 gene expression, whereas transplantation of embryonic serotonergic neurons modifies these changes in motoneurons innervating hindlimb muscles but not those innervating tail muscles. This suggests that the upregulation of genes critical for locomotor recovery, resulting in limb motoneuron plasticity, might account for the improved locomotion in grafted animals.

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.

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.

Rapid recovery and altered neurochemical dependence of locomotor central pattern generation following lumbar neonatal spinal cord injury

KEY POINTS

Spinal compression injury targeted to the neonatal upper lumbar spinal cord, the region of highest hindlimb locomotor rhythmogenicity, leads to an initial paralysis of the hindlimbs. Behavioural recovery is evident within a few days and approaches normal function within about 3 weeks. Fictive locomotion in the isolated injured spinal cord cannot be elicited by a neurochemical cocktail containing NMDA, dopamine and serotonin 1 day post-injury, but can 3 days post-injury as readily as in the uninjured spinal cord. Low frequency coordinated rhythmic activity can be elicited in the isolated uninjured spinal cord by NMDA + dopamine (without serotonin), but not in the isolated injured spinal cord. In both the injured and uninjured spinal cord, eliciting bona fide fictive locomotion requires the additional presence of serotonin.

ABSTRACT

Following incomplete compression injury in the thoracic spinal cord of neonatal mice 1 day after birth (P1), we previously reported that virtually normal hindlimb locomotor function is recovered within about 3 weeks despite substantial permanent thoracic tissue loss. Here, we asked whether similar recovery occurs following lumbar injury that impacts more directly on the locomotor central pattern generator (CPG). As in thoracic injuries, lumbar injuries caused about 90% neuronal loss at the injury site and increased serotonergic innervation below the injury. Motor recovery was slower after lumbar than thoracic injury, but virtually normal function was attained by P25 in both cases. Locomotor CPG status was tested by eliciting fictive locomotion in isolated spinal cords using a widely used neurochemical cocktail (NMDA, dopamine, serotonin). No fictive locomotion could be elicited 1 day post-injury, but could within 3 days post-injury as readily as in age-matched uninjured control spinal cords. Burst patterning and coordination were largely similar in injured and control spinal cords but there were differences. Notably, in both groups there were two main locomotor frequencies, but injured spinal cords exhibited a shift towards the higher frequency. Injury also altered the neurochemical dependence of locomotor CPG output, such that injured spinal cords, unlike control spinal cords, were incapable of generating low frequency rhythmic coordinated activity in the presence of NMDA and dopamine alone. Thus, the neonatal spinal cord also exhibits remarkable functional recovery after lumbar injuries, but the neurochemical sensitivity of locomotor circuitry is modified in the process.

LFP Oscillations in the Mesencephalic Locomotor Region during Voluntary Locomotion

Oscillatory rhythms in local field potentials (LFPs) are thought to coherently bind cooperating neuronal ensembles to produce behaviors, including locomotion. LFPs recorded from sites that trigger locomotion have been used as a basis for identification of appropriate targets for deep brain stimulation (DBS) to enhance locomotor recovery in patients with gait disorders. Theta band activity (6–12 Hz) is associated with locomotor activity in locomotion-inducing sites in the hypothalamus and in the hippocampus, but the LFPs that occur in the functionally defined mesencephalic locomotor region (MLR) during locomotion have not been determined. Here we record the oscillatory activity during treadmill locomotion in MLR sites effective for inducing locomotion with electrical stimulation in rats. The results show the presence of oscillatory theta rhythms in the LFPs recorded from the most effective MLR stimulus sites (at threshold ≤60 μA). Theta activity increased at the onset of locomotion, and its power was correlated with the speed of locomotion. In animals with higher thresholds (>60 μA), the correlation between locomotor speed and theta LFP oscillations was less robust. Changes in the gamma band (previously recorded in vitro in the pedunculopontine nucleus (PPN), thought to be a part of the MLR) were relatively small. Controlled locomotion was best achieved at 10–20 Hz frequencies of MLR stimulation. Our results indicate that theta and not delta or gamma band oscillation is a suitable biomarker for identifying the functional MLR sites.