Cholinergic control of excitability of spinal motoneurones in the salamander

Cholinergic Modulation of Locomotor Circuits in Vertebrates

Locomotion is a basic motor act essential for survival. Amongst other things, it allows animals to move in their environment to seek food, escape predators, or seek mates for reproduction. The neural mechanisms involved in the control of locomotion have been examined in many vertebrate species and a clearer picture is progressively emerging. The basic muscle synergies responsible for propulsion are generated by neural networks located in the spinal cord. In turn, descending supraspinal inputs are responsible for starting, maintaining, and stopping locomotion as well as for steering and controlling speed. Several neurotransmitter systems play a crucial role in modulating the neural activity during locomotion. For instance, cholinergic inputs act both at the spinal and supraspinal levels and the underlying mechanisms are the focus of the present review. Much information gained on supraspinal cholinergic modulation of locomotion was obtained from the lamprey model. Nicotinic cholinergic inputs increase the level of excitation of brainstem descending command neurons, the reticulospinal neurons (RSNs), whereas muscarinic inputs activate a select group of hindbrain neurons that project to the RSNs to boost their level of excitation. Muscarinic inputs also reduce the transmission of sensory inputs in the brainstem, a phenomenon that could help in sustaining goal directed locomotion. In the spinal cord, intrinsic cholinergic inputs strongly modulate the activity of interneurons and motoneurons to control the locomotor output. Altogether, the present review underlines the importance of the cholinergic inputs in the modulation of locomotor activity in vertebrates.

Differential pharmacological and sex-specific effects of antimuscarinic agents at the hypoglossal motor nucleus in vivo in rats

Successful cholinergic-noradrenergic pharmacotherapy for obstructive sleep apnea (OSA) is thought to be due to effects at the hypoglossal motor nucleus (HMN). Clinical efficacy varies with muscarinic-receptor (MR) subtype affinities. We hypothesized that oxybutynin (cholinergic agent in successful OSA pharmacotherapy) is an effective MR antagonist at the HMN and characterized its efficacy with other antagonists. We recorded tongue muscle activity of isoflurane anesthetized rats (121 males and 60 females, 7-13 per group across 13 protocols) in response to HMN microperfusion with MR antagonists with and without: (i) eserine-induced increased endogenous acetylcholine at the HMN and (ii) muscarine. Eserine-induced increased acetylcholine decreased tongue motor activity (p < 0.001) with lesser cholinergic suppression in females versus males (p = 0.017). Motor suppression was significantly attenuated by the MR antagonists atropine, oxybutynin, and omadacycline (MR2 antagonist), each p < 0.001, with similar residual activity between agents (p ≥ 0.089) suggesting similar efficacy at the HMN. Sex differences remained with atropine and oxybutynin (p < 0.001 to 0.05) but not omadacycline (p = 0.722). Muscarine at the HMN also decreased motor activity (p < 0.001) but this was not sex-specific (p = 0.849). These findings have translational relevance to antimuscarinic agents in OSA pharmacotherapy and understanding potential sex differences in HMN suppression with increased endogenous acetylcholine related to sparing nicotinic excitation.

Acetylcholine and spinal locomotor networks: The insider

This article aims to review studies that have investigated the role of neurons that use the transmitter acetylcholine (ACh) in controlling the operation of locomotor neural networks within the spinal cord. This cholinergic system has the particularity of being completely intraspinal. We describe the different effects exerted by spinal cholinergic neurons on locomotor circuitry by the pharmacological activation or blockade of this propriospinal system, as well as describing its different cellular and subcellular targets. Through the activation of one ionotropic receptor, the nicotinic receptor, and five metabotropic receptors, the M1 to M5 muscarinic receptors, the cholinergic system exerts a powerful control both on synaptic transmission and locomotor network neuron excitability. Although tremendous advances have been made in our understanding of the spinal cholinergic system's involvement in the physiology and pathophysiology of locomotor networks, gaps still remain, including the precise role of the different subtypes of cholinergic neurons as well as their pre- and postsynaptic partners. Improving our knowledge of the propriospinal cholinergic system is of major relevance to finding new cellular targets and therapeutics in countering the debilitating effects of neurodegenerative diseases and restoring motor functions after spinal cord injury.

Walking with Salamanders: From Molecules to Biorobotics

How do four-legged animals adapt their locomotion to the environment? How do central and peripheral mechanisms interact within the spinal cord to produce adaptive locomotion and how is locomotion recovered when spinal circuits are perturbed? Salamanders are the only tetrapods that regenerate voluntary locomotion after full spinal transection. Given their evolutionary position, they provide a unique opportunity to bridge discoveries made in fish and mammalian models. Genetic dissection of salamander neural circuits is becoming feasible with new methods for precise manipulation, elimination, and visualisation of cells. These approaches can be combined with classical tools in neuroscience and with modelling and a robotic environment. We propose that salamanders provide a blueprint of the function, evolution, and regeneration of tetrapod locomotor circuits.

The serotonin reuptake blocker citalopram destabilizes fictive locomotor activity in salamander axial circuits through 5-HT1A receptors

Serotoninergic (5-HT) neurons are powerful modulators of spinal locomotor circuits. Most studies on 5-HT modulation focused on the effect of exogenous 5-HT and these studies provided key information about the cellular mechanisms involved. Less is known about the effects of increased release of endogenous 5-HT with selective serotonin reuptake inhibitors. In mammals, such molecules were shown to destabilize the fictive locomotor output of spinal limb networks through 5-HT_1A receptors. However, in tetrapods little is known about the effects of increased 5-HT release on the locomotor output of axial networks, which are coordinated with limb circuits during locomotion from basal vertebrates to mammals. Here, we examined the effect of citalopram on fictive locomotion generated in axial segments of isolated spinal cords in salamanders, a tetrapod where raphe 5-HT reticulospinal neurons and intraspinal 5-HT neurons are present as in other vertebrates. Using electrophysiological recordings of ventral roots, we show that fictive locomotion generated by bath-applied glutamatergic agonists is destabilized by citalopram. Citalopram-induced destabilization was prevented by a 5-HT_1A receptor antagonist, whereas a 5-HT_1A receptor agonist destabilized fictive locomotion. Using immunofluorescence experiments, we found 5-HT-positive fibers and varicosities in proximity with motoneurons and glutamatergic interneurons that are likely involved in rhythmogenesis. Our results show that increasing 5-HT release has a deleterious effect on axial locomotor activity through 5-HT_1A receptors. This is consistent with studies in limb networks of turtle and mouse, suggesting that this part of the complex 5-HT modulation of spinal locomotor circuits is common to limb and axial networks in limbed vertebrates.NEW & NOTEWORTHY Little is known about the modulation exerted by endogenous serotonin on axial locomotor circuits in tetrapods. Using axial ventral root recordings in salamanders, we found that a serotonin reuptake blocker destabilized fictive locomotor activity through 5-HT_1A receptors. Our anatomical results suggest that serotonin is released on motoneurons and glutamatergic interneurons possibly involved in rhythmogenesis. Our study suggests that common serotoninergic mechanisms modulate axial motor circuits in amphibians and limb motor circuits in reptiles and mammals.

Balanced cholinergic modulation of spinal locomotor circuits via M2 and M3 muscarinic receptors

Neuromodulation ensures that neural circuits produce output that is flexible whilst remaining within an optimal operational range. The neuromodulator acetylcholine is released during locomotion to regulate spinal motor circuits. However, the range of receptors and downstream mechanisms by which acetylcholine acts have yet to be fully elucidated. We therefore investigated metabotropic acetylcholine receptor-mediated modulation by using isolated spinal cord preparations from neonatal mice in which locomotor-related output can be induced pharmacologically. We report that M2 receptor blockade decreases the frequency and amplitude of locomotor-related activity, whilst reducing its variability. In contrast, M3 receptor blockade destabilizes locomotor-related bursting. Motoneuron recordings from spinal cord slices revealed that activation of M2 receptors induces an outward current, decreases rheobase, reduces the medium afterhyperpolarization, shortens spike duration and decreases synaptic inputs. In contrast, M3 receptor activation elicits an inward current, increases rheobase, extends action potential duration and increases synaptic inputs. Analysis of miniature postsynaptic currents support that M2 and M3 receptors modulate synaptic transmission via different mechanisms. In summary, we demonstrate that M2 and M3 receptors have opposing modulatory actions on locomotor circuit output, likely reflecting contrasting cellular mechanisms of action. Thus, intraspinal cholinergic systems mediate balanced, multimodal control of spinal motor output.

Spinal cholinergic interneurons differentially control motoneuron excitability and alter the locomotor network operational range

While cholinergic neuromodulation is important for locomotor circuit operation, the specific neuronal mechanisms that acetylcholine employs to regulate and fine-tune the speed of locomotion are largely unknown. Here, we show that cholinergic interneurons are present in the zebrafish spinal cord and differentially control the excitability of distinct classes of motoneurons (slow, intermediate and fast) in a muscarinic dependent manner. Moreover, we reveal that m2-type muscarinic acetylcholine receptors (mAChRs) are present in fast and intermediate motoneurons, but not in the slow motoneurons, and that their activation decreases neuronal firing. We also reveal a strong correlation between the muscarinic receptor configuration on motoneurons and the ability of the animals to locomote at different speeds, which might serve as a plasticity mechanism to alter the operational range of the locomotor networks. These unexpected findings provide new insights into the functional flexibility of motoneurons and how they execute locomotion at different speeds.

Homeostatic regulation through GABA and acetylcholine muscarinic receptors of motor trigeminal neurons following sleep deprivation

Muscle tone is regulated across sleep-wake states, being maximal in waking, reduced in slow wave sleep (SWS) and absent in paradoxical or REM sleep (PS or REMS). Such changes in tone have been recorded in the masseter muscles and shown to correspond to changes in activity and polarization of the trigeminal motor 5 (Mo5) neurons. The muscle hypotonia and atonia during sleep depend in part on GABA acting upon both GABA_A and GABA_B receptors (Rs) and acetylcholine (ACh) acting upon muscarinic 2 (AChM2) Rs. Here, we examined whether Mo5 neurons undergo homeostatic regulation through changes in these inhibitory receptors following prolonged activity with enforced waking. By immunofluorescence, we assessed that the proportion of Mo5 neurons positively stained for GABA_ARs was significantly higher after sleep deprivation (SD, ~65%) than sleep control (SC, ~32%) and that the luminance of the GABA_AR fluorescence was significantly higher after SD than SC and sleep recovery (SR). Although, all Mo5 neurons were positively stained for GABA_BRs and AChM2Rs (100%) in all groups, the luminance of these receptors was significantly higher following SD as compared to SC and SR. We conclude that the density of GABA_A, GABA_B and AChM2 receptors increases on Mo5 neurons during SD. The increase in these receptors would be associated with increased inhibition in the presence of GABA and ACh and thus a homeostatic down-scaling in the excitability of the Mo5 neurons after prolonged waking and resulting increased susceptibility to muscle hypotonia or atonia along with sleep.

Impact of neural cell adhesion molecule deletion on regeneration after mouse spinal cord injury

The neural cell adhesion molecule (NCAM) plays important functional roles in development of the nervous system. We investigated the influence of a constitutive ablation of NCAM on the outcome of spinal cord injury. Transgenic mice lacking NCAM (NCAM-/-) were subjected to severe compression injury of the lower thoracic spinal cord using wild-type (NCAM+/+) littermates as controls. According to the single-frame motion analysis, the NCAM-/- mice showed reduced locomotor recovery in comparison to control mice at 3 and 6 weeks after injury, indicating an overall positive impact of NCAM on recovery after injury. Also the Basso Mouse Scale score was lower in NCAM-/- mice at 3 weeks after injury, whereas at 6 weeks after injury the difference between genotypes was not statistically significant. Worse locomotor function was associated with decreased monoaminergic and cholinergic innervation of the spinal cord caudal to the injury site and decreased axonal regrowth/sprouting at the site of injury. Astrocytic scar formation at the injury site, as assessed by immunohistology for glial fibrillary acidic protein at and around the lesion site was increased in NCAM-/- compared with NCAM+/+ mice. Migration of cultured monolayer astrocytes from NCAM-/- mice was reduced as assayed by scratch wounding. Numbers of Iba-1 immunopositive microglia were not different between genotypes. We conclude that constitutive NCAM deletion in young adult mice reduces recovery after spinal cord injury, validating the hypothesized beneficial role of this molecule in recovery after injury.

The polysialic acid mimetics 5-nonyloxytryptamine and vinorelbine facilitate nervous system repair

Polysialic acid (PSA) is a large negatively charged glycan mainly attached to the neural cell adhesion molecule (NCAM). Several studies have shown that it is important for correct formation of brain circuitries during development and for synaptic plasticity, learning and memory in the adult. PSA also plays a major role in nervous system regeneration following injury. As a next step for clinical translation of PSA based therapeutics, we have previously identified the small organic compounds 5-nonyloxytryptamine and vinorelbine as PSA mimetics. Activity of 5-nonyloxytryptamine and vinorelbine had been confirmed in assays with neural cells from the central and peripheral nervous system in vitro and shown to be independent of their function as serotonin receptor 5-HT1B/1D agonist or cytostatic drug, respectively. As we show here in an in vivo paradigm for spinal cord injury in mice, 5-nonyloxytryptamine and vinorelbine enhance regain of motor functions, axonal regrowth, motor neuron survival and remyelination. These data indicate that 5-nonyloxytryptamine and vinorelbine may be re-tasked from their current usage as a 5-HT1B/1D agonist or cytostatic drug to act as mimetics for PSA to stimulate regeneration after injury in the mammalian nervous system.

Synaptic modulation of excitatory synaptic transmission by nicotinic acetylcholine receptors in spinal ventral horn neurons

Nicotinic acetylcholine receptors (nAChRs) are distributed widely in the central nervous system and play important roles in higher brain functions, including learning, memory, and recognition. However, functions of the cholinergic system in spinal motoneurons remain poorly understood. In this study, we investigated the actions of presynaptic and postsynaptic nAChRs in spinal ventral horn neurons by performing whole-cell patch-clamp recordings on lumbar slices from male rats. The application of nicotine or acetylcholine generated slow inward currents and increased the frequency and amplitude of spontaneous excitatory postsynaptic currents (sEPSCs). Slow inward currents by acetylcholine or nicotine were not inhibited by tetrodotoxin (TTX) or glutamate receptor antagonists. In the presence of TTX, the frequency and amplitude of miniature excitatory postsynaptic currents (mEPSCs) were also increased by acetylcholine or nicotine. A selective α4β2 nicotinic receptor antagonist, dihydro-β-erythroidine hydrobromide (DhβE), significantly decreased nicotine-induced inward currents without affecting the enhancement of sEPSCs and mEPSCs. In addition, a selective α7 nicotinic receptor antagonist, methyllycaconitine, did not affect either nicotine-induced inward currents or the enhancement of sEPSCs and mEPSCs. These results suggest that α4β2 AChRs are localized at postsynaptic sites in the spinal ventral horn, non-α4β2 and non-α7 nAChRs are located presynaptically, and nAChRs enhance excitatory synaptic transmission in the spinal ventral horn.

The role of two-pore-domain background K⁺ (K₂p) channels in the thalamus

The thalamocortical system is characterized by two fundamentally different activity states, namely synchronized burst firing and tonic action potential generation, which mainly occur during the behavioral states of sleep and wakefulness, respectively. The switch between the two firing modes is crucially governed by the bidirectional modulation of members of the K2P channel family, namely tandem of P domains in a weakly inward rectifying K(+) (TWIK)-related acid-sensitive K(+) (TASK) and TWIK-related K(+) (TREK) channels, in thalamocortical relay (TC) neurons. Several physicochemical stimuli including neurotransmitters, protons, di- and multivalent cations as well as clinically used drugs have been shown to modulate K2P channels in these cells. With respect to modulation of these channels by G-protein-coupled receptors, PLCβ plays a unique role with both substrate breakdown and product synthesis exerting important functions. While the degradation of PIP2 leads to the closure of TREK channels, the production of DAG induces the inhibition of TASK channels. Therefore, TASK and TREK channels were found to be central elements in the control of thalamic activity modes. Since research has yet focused on identifying the muscarinic pathway underling the modulation of TASK and TREK channels in TC neurons, future studies should address other thalamic cell types and members of the K2P channel family.

Postsynaptic muscarinic m2 receptors at cholinergic and glutamatergic synapses of mouse brainstem motoneurons

In many brain areas, few cholinergic synapses are identified. Acetylcholine is released into the extracellular space and acts through diffuse transmission. Motoneurons, however, are contacted by numerous cholinergic terminals, indicating synaptic cholinergic transmission on them. The muscarinic m2 receptor is the major acetylcholine receptor subtype of motoneurons; therefore, we analyzed the localization of the m2 receptor in correlation with synapses by electron microscopic immunohistochemistry in the mouse trigeminal, facial, and hypoglossal motor nuclei. In all nuclei, m2 receptors were localized at the membrane of motoneuronal perikarya and dendrites. The m2 receptors were concentrated at cholinergic synapses located on the perikarya and most proximal dendrites. However, m2 receptors at cholinergic synapses represented only a minority (<10%) of surface m2 receptors. The m2 receptors were also enriched at glutamatergic synapses in both motoneuronal perikarya and dendrites. A relatively large proportion (20-30%) of plasma membrane-associated m2 receptors were located at glutamatergic synapses. In conclusion, the effect of acetylcholine on motoneuron populations might be mediated through a synaptic as well as diffuse type of transmission.

Cortical excitability in smoking and not smoking individuals with and without nicotine

RATIONAL

Activation of nicotinic acetylcholine receptors has a major neuromodulatory impact on central nervous system function. Beyond acute activation, chronic nicotine intake has long-lasting effects on cortical excitability in animal experiments, caused by receptor up- or down-regulation. Knowledge about the impact of nicotine on cortical excitability in humans, however, is limited.

OBJECTIVES

We therefore aimed to explore the effect of nicotine intake on cortical excitability in healthy human smokers and non-smokers.

METHODS

The primary motor cortex served as model, and cortical excitability was monitored via transcranial magnetic stimulation (TMS). Corticospinal excitability and intracortical excitability were recorded before and after application of nicotine patch in both groups. Corticospinal excitability was explored by motor threshold and input/output curve (I/O curve), and intracortical excitability was explored by means of paired-pulse TMS techniques (intracortical facilitation (ICF), short-latency intracortical inhibition (SICI), I-wave facilitation and short-latency afferent inhibition (SAI)).

RESULTS

The results show that smokers during nicotine withdrawal display increased corticospinal excitability with regard to the I/O curve (TMS intensity 150 % of resting motor threshold) compared to non-smokers and furthermore enhanced SAI and diminished ICF at the intracortical circuit level. After administration of nicotine, intracortical facilitation in smokers increased, while in non-smokers, inhibition (SICI, SAI) was enhanced.

CONCLUSION

Our results show that chronic nicotine consumption in smokers alters cortical excitability independent from acute nicotine consumption and that acute nicotine has different effects on motor cortical excitability in both groups.

From lamprey to salamander: an exploratory modeling study on the architecture of the spinal locomotor networks in the salamander

The evolutionary transition from water to land required new locomotor modes and corresponding adjustments of the spinal "central pattern generators" for locomotion. Salamanders resemble the first terrestrial tetrapods and represent a key animal for the study of these changes. Based on recent physiological data from salamanders, and previous work on the swimming, limbless lamprey, we present a model of the basic oscillatory network in the salamander spinal cord, the spinal segment. Model neurons are of the Hodgkin-Huxley type. Spinal hemisegments contain sparsely connected excitatory and inhibitory neuron populations, and are coupled to a contralateral hemisegment. The model yields a large range of experimental findings, especially the NMDA-induced oscillations observed in isolated axial hemisegments and segments of the salamander Pleurodeles waltlii. The model reproduces most of the effects of the blockade of AMPA synapses, glycinergic synapses, calcium-activated potassium current, persistent sodium current, and [Formula: see text]-current. Driving segments with a population of brainstem neurons yields fast oscillations in the in vivo swimming frequency range. A minimal modification to the conductances involved in burst-termination yields the slower stepping frequency range. Slow oscillators can impose their frequency on fast oscillators, as is likely the case during gait transitions from swimming to stepping. Our study shows that a lamprey-like network can potentially serve as a building block of axial and limb oscillators for swimming and stepping in salamanders.

Anatomical and electrophysiological plasticity of locomotor networks following spinal transection in the salamander

Recovery of locomotor behavior following spinal cord injury can occur spontaneously in some vertebrates, such as fish, urodele amphibians, and certain reptiles. This review provides an overview of the current status of our knowledge on the anatomical and electrophysiological changes occurring within the spinal cord that lead to, or are associated with the re-expression of locomotion in spinally-transected salamanders. A better understanding of these processes will help to devise strategies for restoring locomotor function in mammals, including humans.

K+ channel modulation causes genioglossus inhibition in REM sleep and is a strategy for reactivation

Rapid eye movement (REM) sleep is accompanied by periods of upper airway motor suppression that cause hypoventilation and obstructive apneas in susceptible individuals. A common idea has been that upper airway motor suppression in REM sleep is caused by the neurotransmitters glycine and γ-amino butyric acid (GABA) acting at pharyngeal motor pools to inhibit motoneuron activity. Data refute this as a workable explanation because blockade of this putative glycine/GABAergic mechanism releases pharyngeal motor activity in all states, and least of all in REM sleep. Here we summarize a novel motor-inhibitory mechanism that suppresses hypoglossal motor activity largely in REM sleep, this being a muscarinic receptor mechanism linked to G-protein-coupled inwardly rectifying potassium (GIRK) channels. We then outline how this discovery informs efforts to pursue therapeutic targets to reactivate hypoglossal motor activity throughout sleep via potassium channel modulation. One such target is the inwardly rectifying potassium channel Kir2.4 whose expression in the brain is almost exclusive to cranial motor nuclei.

The h-current in the substantia Nigra pars compacta neurons: a re-examination

The properties of the hyperpolarization-activated cation current (I_h) were investigated in rat substantia nigra - pars compacta (SNc) principal neurons using patch-clamp recordings in thin slices. A reliable identification of single dopaminergic neurons was made possible by the use of a transgenic line of mice expressing eGFP under the tyrosine hydroxylase promoter. The effects of temperature and different protocols on the I_h kinetics showed that, at 37°C and minimizing the disturbance of the intracellular milieu with perforated patch, this current actually activates at potentials more positive than what is generally indicated, with a half-activation potential of −77.05 mV and with a significant level of opening already at rest, thereby substantially contributing to the control of membrane potential, and ultimately playing a relevant function in the regulation of the cell excitability. The implications of the known influence of intracellular cAMP levels on I_h amplitude and kinetics were examined. The direct application of neurotransmitters (DA, 5-HT and noradrenaline) physiologically released onto SNc neurons and known to act on metabotropic receptors coupled to the cAMP pathway modify the I_h amplitude. Here, we show that direct activation of dopaminergic and of 5-HT receptors results in I_h inhibition of SNc DA cells, whereas noradrenaline has the opposite effect. Together, these data suggest that the modulation of I_h by endogenously released neurotransmitters acting on metabotropic receptors –mainly but not exclusively linked to the cAMP pathway- could contribute significantly to the control of SNc neuron excitability.

Identification of the mechanism mediating genioglossus muscle suppression in REM sleep

RATIONALE

Inhibition of pharyngeal motoneurons accompanies REM sleep and is a cause of hypoventilation and obstructive sleep apnea in humans. One explanation posits that the neurotransmitters glycine and γ-aminobutyric acid are responsible for REM sleep motor inhibition. However, blockade of that mechanism at cranial motor nuclei increases motor activity in all sleep-wake states, and least of all in REM sleep, arguing against it as a major mechanism of REM sleep pharyngeal motor inhibition.

OBJECTIVES

To identify the mechanism of REM sleep inhibition at the hypoglossal motor pool.

METHODS

Genioglossus and diaphragm activities were recorded in 34 rats across sleep-wake states. Microdialysis probes were implanted into the hypoglossal motor pool.

MEASUREMENTS AND MAIN RESULTS

Here we show that muscarinic receptor antagonism at the hypoglossal motor pool prevents the inhibition of genioglossus activity throughout REM sleep; likewise, with G-protein-coupled inwardly rectifying potassium (GIRK) channel blockade. Importantly, the genioglossus activating effects of these interventions were largest in REM sleep and minimal or often absent in other sleep-wake states. Finally, we showed that muscarinic inhibition of the genioglossus is functionally linked to GIRK channel activation.

CONCLUSIONS

We identify a powerful cholinergic-GIRK channel mechanism operating at the hypoglossal motor pool that has its largest inhibitory influence in REM sleep and minimal or no effects in other sleep-wake states. This mechanism is the major cause of REM sleep inhibition at a pharyngeal motor pool critical for effective breathing.

Muscarinic acetylcholine receptors enhance neonatal mouse hypoglossal motoneuron excitability in vitro

In brain stem slices from neonatal ( postnatal days 0–4) CD-1 mice, muscarinic ACh receptors (MAChRs) increased rhythmic inspiratory-related and tonic hypoglossal nerve discharge and depolarized single hypoglossal motoneurons (HMs) via an inward current without changing input resistance. These responses were blocked by the MAChR antagonist 1,1-dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP; 100 nM). MAChRs shifted voltage-dependent activation of the hyperpolarization-activated cation current to more positive levels. MAChRs increased the HM repetitive firing rate and decreased rheobase, with both effects being blocked by 4-DAMP. Muscarinic agonists reduced the afterhyperpolarization of single action potentials (APs), suggesting that small-conductance Ca2+-dependent K+ current inhibition increased the HM firing rate. Muscarinic agonists also reduced the AP amplitude and slowed its time course, suggesting that MAChRs inhibited voltage-gated Na+ channels. To compare muscarinic excitation of single HMs to muscarinic excitatory effects on motor output in thicker brain stem slices requiring higher extracellular K+ for rhythmic activity, we tested the effects of muscarinic agonists on single HM excitability in high-K+ artificial cerebrospinal fluid (aCSF). In high-K+ aCSF, muscarinic agonists still depolarized HMs and altered AP size and shape, as in standard aCSF, but did not increase the steady-state firing rate, decrease afterhyperpolarization, or alter threshold potential. These results indicate that the basic cellular response of HMs to muscarinic receptors is excitatory, via a number of distinct mechanisms, and that this excitatory response will be largely preserved in rhythmically active brain stem slices.

Muscarinic control of AMPA receptor responsiveness in mouse spinal cord motoneurons

A cholinergic synaptic input has been highlighted in recent years as an important modulator of motoneuron excitability. In the present study, it was examined whether, as in other regions, the cholinergic inputs on spinal motoneurons modulate the glutamatergic synaptic activity, evoked by electrical stimulation at the dorsolateral funiculus. The present results show that the pharmacological stimulation of muscarinic receptors produces a selective depression of AMPA-mediated glutamatergic activity in thoracic and lumbar spinal motoneurons. This effect does not involve a change in transmitter release and occurs both on synaptic currents and on AMPA responses evoked by photolysis of MNI-glutamate. Thus, it is concluded that muscarinic modulation occurs at the postsynaptic level. The results suggest that cholinergic inputs contacting motoneurons, which are known to increase motoneuron excitability, also directly interfere within coming synaptic inputs by changing their amplitudes.

Generation of the masticatory central pattern and its modulation by sensory feedback

The basic pattern of rhythmic jaw movements produced during mastication is generated by a neuronal network located in the brainstem and referred to as the masticatory central pattern generator (CPG). This network composed of neurons mostly associated to the trigeminal system is found between the rostral borders of the trigeminal motor nucleus and facial nucleus. This review summarizes current knowledge on the anatomical organization, the development, the connectivity and the cellular properties of these trigeminal circuits in relation to mastication. Emphasis is put on a population of rhythmogenic neurons in the dorsal part of the trigeminal sensory nucleus. These neurons have intrinsic bursting capabilities, supported by a persistent Na(+) current (I(NaP)), which are enhanced when the extracellular concentration of Ca(2+) diminishes. Presented evidence suggest that the Ca(2+) dependency of this current combined with its voltage-dependency could provide a mechanism for cortical and sensory afferent inputs to the nucleus to interact with the rhythmogenic properties of its neurons to adjust and adapt the rhythmic output. Astrocytes are postulated to contribute to this process by modulating the extracellular Ca(2+) concentration and a model is proposed to explain how functional microdomains defined by the boundaries of astrocytic syncitia may form under the influence of incoming inputs.

Neuromodulation of Vertebrate Locomotor Control Networks

Vertebrate locomotion must be adaptable in light of changing environmental, organismal, and developmental demands. Much of the underlying flexibility in the output of central pattern generating (CPG) networks of the spinal cord and brain stem is endowed by neuromodulation. This review provides a synthesis of current knowledge on the way that various neuromodulators modify the properties of and connections between CPG neurons to sculpt CPG network output during locomotion.

Transplanted L1 expressing radial glia and astrocytes enhance recovery after spinal cord injury

A major obstacle for the transplantation of neural stem cells (NSCs) into the lesioned spinal cord is their predominant astrocytic differentiation after transplantation. We took advantage of this predominant astrocytic differentiation of NSCs and expressed the paradigmatic beneficial neural cell adhesion molecule L1 in radial glial cells and reactive and nonreactive astrocytes as novel cellular vehicles to express L1 under the control of the promoter for the human glial fibrillary acidic protein (GFAP-L1 NSCs). Behavioral analysis and electrophysiological H-reflex recordings revealed that mice transplanted with GFAP-L1 NSCs showed enhanced locomotor recovery in comparison to mice injected with wild type (WT) NSCs or control mice injected with phosphate-buffered saline (PBS). This functional recovery was further accelerated in mice transplanted with L1-expressing radial glial cells that had been immunoisolated from GFAP-L1 NSCs (GFAP-L1-i cells). Morphological analysis revealed that mice grafted with GFAP-L1 NSCs exhibited increased neuronal differentiation and migration of transplanted cells, as well as increased soma size and cholinergic synaptic coverage of host motoneurons and increased numbers of endogenous catecholaminergic nerve fibers caudal to the lesion site. These findings show that L1-expressing astrocytes and radial glial cells isolated from GFAP-L1 NSC cultures represent a novel strategy for improving functional recovery after spinal cord injury, encouraging the use of the human GFAP promoter to target beneficial transgene expression in transplanted stem cells.

Changes in the Estimated Time Course of the Motoneuron Afterhyperpolarization Induced by Tendon Vibration

Group Ia afferents are activated vigorously with high-frequency tendon vibration and provide excitatory input to the agonist muscle and inhibitory input to the antagonist muscle group via inhibitory interneurons. The purpose of this experiment was to determine whether the afterhyperpolarization (AHP) time course in humans is altered in response to tendon vibration. The AHP time course is estimated using the interval death rate (IDR) analysis, a transform of the motor unit action potential train. Single motor units from tibialis anterior (TA) were recorded as subjects held low force dorsiflexor contractions for 600 s with and without vibration. The vibratory stimulus was superimposed on the low force contraction either to the tendon of the TA or the antagonist Achilles tendon. During TA tendon vibration, the time course of the AHP, as expressed by its time constant (τ), decreased from 35.5 ms in the previbration control condition to 31.3 ms during the vibration ( P = 0.003) and returned to 36.3 ms after the vibration was removed ( P = 0.002). The AHP τ during vibration of the antagonist Achilles tendon (38.6 ms) was greater than the previbration control condition (33.6 ms; P = 0.001). It is speculated that the reduction in AHP time constant with TA vibration may have resulted alone or in combination with a modulation of motoneuron gain, an alteration of persistent inward currents and/or the restructuring of synaptic noise. A decrease in firing probability, possibly reflecting Ia reciprocal inhibition, may have been responsible for the larger AHP time constant.

Postnatal development enhances the effects of cholinergic inputs on recruitment threshold and firing rate of rat oculomotor nucleus motoneurons

Changes in the electrophysiological and morphological characteristics of motoneurons (Mns) of the oculomotor nucleus during postnatal development have been reported, however synaptic modifications that take place concurrently with postnatal development in these Mns are yet to be elucidated. We investigated whether cholinergic inputs exert different effects on the recruitment threshold and firing rate of Mns during postnatal development. Rat oculomotor nucleus Mns were intracellularly recorded in brain slice preparations and separated in neonatal (4-7 postnatal days) and adult (20-30 postnatal days) age groups. Stimulation of the medial longitudinal fasciculus evoked a monosynaptic excitatory potential in Mns that was attenuated with atropine (1.5 μM, a muscarinic antagonist). Mns were silent at their resting membrane potential, and bath application of carbachol (10 μM, a cholinergic agonist) induced depolarization of the membrane potential and a sustained firing rate that were more pronounced in adult Mns. Pharmacological and immunohistochemical assays showed that these responses were attributable to muscarinic receptors located in the membrane of Mns. In addition, compared to control Mns, carbachol-exposed Mns exhibited a higher firing rate in response to the injection of the same amount of current, and a decrease in the current threshold required to achieve sustained firing. These latter effects were more pronounced in adult than in neonatal Mns. In conclusion, our findings suggest that cholinergic synaptic inputs are already present in neonatal Mns, and that the electrophysiological effects of such inputs on recruitment threshold and firing rate are enhanced with the postnatal development in oculomotor nucleus Mns. We propose that cholinergic input maturation could provide a greater dynamic range in adult Mns to encode the output necessary for graded muscle contraction.

Segmental oscillators in axial motor circuits of the salamander: distribution and bursting mechanisms

The rhythmic and coordinated activation of axial muscles that underlie trunk movements during locomotion are generated by specialized networks in the spinal cord. The operation of these networks has been extensively investigated in limbless swimming vertebrates. But little is known about the architecture and functioning of the axial locomotor networks in limbed vertebrates. We investigated the rhythm-generating capacity of the axial segmental networks in the salamander (Pleurodeles waltlii). We recorded ventral root activity from hemisegments and segments that were surgically isolated from the mid-trunk cord and chemically activated with bath-applied N-methyl-d-aspartate (NMDA). We provide evidence that the rhythmogenic capacity of the axial network is distributed along the mid-trunk spinal cord without an excitability gradient. We demonstrate that the burst generation in a hemisegment depends on glutamatergic excitatory interactions. Reciprocal glycinergic inhibition between opposite hemisegments ensures left-right alternation and lowers the rhythm frequency in segments. Our results further suggest that persistent sodium current contributes to the rhythmic regenerating process both in hemisegments and segments. Burst termination in hemisegments is not achieved through the activation of apamine-sensitive Ca(2+)-activated K(+) channels and burst termination in segments relies on crossed glycinergic inhibition. Together our results indicate that the basic design of the salamander axial network is similar to most of axial networks investigated in other vertebrates, albeit with some significant differences in the cellular mechanism that underlies segmental bursting. This finding supports the view of a phylogenetic conservation of basic building blocks of the axial locomotor network among the vertebrates.

Multiple Effects of Serotonin and Acetylcholine on Hyperpolarization-Activated Inward Current in Locomotor Activity-Related Neurons in Cfos-EGFP Mice

Hyperpolarization-activated inward current ( Ih) has been shown to be involved in production of bursting during various forms of rhythmic activity. However, details of Ih in spinal interneurons related to locomotion remain unknown. Using Cfos-EGFP transgenic mice (P6–P12) we are able to target the spinal interneurons activated by locomotion. Following a locomotor task, whole cell patch-clamp recordings were obtained from ventral EGFP+ neurons in spinal cord slices (T13–L4, 200–250 μm). Ih was found in 51% of EGFP+ neurons ( n = 149) with almost even distribution in lamina VII (51%), VIII (47%), and X (55%). Ih could be blocked by ZD7288 (10–20 μM) or cesium (1–1.5 mM) but was insensitive to barium (2–2.5 mM). Ih activated at −80.1 ± 9.2 mV with half-maximal activation −95.5 ± 13.3 mV, activation rate 10.0 ± 3.2 mV, time constant 745 ± 501 ms, maximal conductance 1.0 ± 0.7 nS, and reversal potential −34.3 ± 3.6 mV. 5-HT (15–20 μM) and ACh (20–30 μM) produced variable effects on Ih. 5-HT increased Ih in 43% of EGFP+ neurons ( n = 37), decreased Ih in 24%, and had no effect on Ih in 33% of the neurons. ACh decreased Ih in 67% of EGFP+ neurons ( n = 18) with unchanged Ih in 33% of the neurons. This study characterizes the Ih in locomotor-related interneurons and is the first to demonstrate the variable effects of 5-HT and ACh on Ih in rodent spinal interneurons. The finding of 5-HT and ACh-induced reduction of Ih in EGFP+ neurons suggests a novel mechanism that the motor system could use to limit the participation of certain neurons in locomotion.

Noradrenergic modulation of intrinsic and synaptic properties of lumbar motoneurons in the neonatal rat spinal cord

Although it is known that noradrenaline (NA) powerfully controls spinal motor networks, few data are available regarding the noradrenergic (NAergic) modulation of intrinsic and synaptic properties of neurons in motor networks. Our work explores the cellular basis of NAergic modulation in the rat motor spinal cord. We first show that lumbar motoneurons express the three classes of adrenergic receptors at birth. Using patch-clamp recordings in the newborn rat spinal cord preparation, we characterized the effects of NA and of specific agonists of the three classes of adrenoreceptors on motoneuron membrane properties. NA increases the motoneuron excitability partly via the inhibition of a K(IR) like current. Methoxamine (alpha(1)), clonidine (alpha(2)) and isoproterenol (beta) differentially modulate the motoneuron membrane potential but also increase motoneuron excitability, these effects being respectively inhibited by the antagonists prazosin (alpha(1)), yohimbine (alpha(2)) and propranolol (beta). We show that the glutamatergic synaptic drive arising from the T13-L2 network is enhanced in motoneurons by NA, methoxamine and isoproterenol. On the other hand, NA, isoproterenol and clonidine inhibit both the frequency and amplitude of miniature glutamatergic EPSCs while methoxamine increases their frequency. The T13-L2 synaptic drive is thereby differentially modulated from the other glutamatergic synapses converging onto motoneurons and enhanced by presynaptic alpha(1) and beta receptor activation. Our data thus show that the NAergic system exerts a powerful and complex neuromodulation of lumbar motor networks in the neonatal rat spinal cord.

Modulation of intrinsic spiking in spinal cord neurons

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

Muscarinic control of the excitability of hindlimb motoneurons in chronic spinal-transected salamanders

The excitability of spinal motoneurons (MNs) is regulated by acetylcholine via the activation of muscarinic receptors. The objective of the present study was to determine whether this cholinergic modulation of MN excitability is altered following a chronic spinal cord transection. Juvenile salamanders (Pleurodeles waltlii) were spinally transected at the mid-trunk level, and patch-clamp recordings from hindlimb MNs in spinal cord slices were performed 9-30 days after transection, with and without bath application of muscarine (20 mum). Our results showed that the input-output relationship was larger in MNs recorded 2 weeks after spinal transection than in MNs recorded 3-4 weeks after spinal transection. They further revealed that muscarine increased both the gain of MNs and the proportion of MNs that could exhibit plateau potentials and afterdischarges, whereas it decreased the amplitude of the medium afterhypolarizing potential. Moreover, muscarine had no effect on the hyperpolarization-activated cation current (I(h)), whereas it increased the inward rectifying K(+) current (I(Kir)) in MNs recorded > or = 2 weeks after spinal transection. We conclude that following chronic spinal cord injury, the muscarinic modulation of some intrinsic properties of MNs previously reported in acute spinal-transected animals [S. Chevallier et al. (2006)The Journal of Physiology, 570, 525-540] was preserved, whereas that of other intrinsic properties of MNs was suppressed, either transiently (I(Kir)) or definitively (I(h)). These alterations in muscarinic modulation of MN excitability may contribute to the spontaneous recovery of locomotion displayed in long-term chronic spinal-transected salamanders.

Muscarinic modulation of recruitment threshold and firing rate in rat oculomotor nucleus motoneurons

Above recruitment threshold, ocular motoneurons (Mns) show a firing rate linearly related with eye position. Current hypothesis suggests that synaptic inputs are determinant for establishing the recruitment threshold and firing rate gain in these Mns. We investigated this proposal by studying the cholinergic modulation in oculomotor nucleus Mns by intracellular recordings in rat brain slice preparation. All recorded Mns were silent at their resting membrane potential. Bath application of carbachol (10 microm) produced a depolarization and a sustained firing that was not silenced on returning membrane potential to the precarbachol value via DC injection. In response to similar membrane depolarization or equal-current steps, carbachol-exposed Mns produced a higher firing rate and a shorter spike afterhyperpolarization phase with lower amplitude. The relationship between injected current and firing rate (I-F) was linear in control and carbachol-exposed Mns. The slope of these relationships (I-F gain) decreased with carbachol exposure. Bath application of agonist and antagonist of nicotinic and muscarinic acetylcholine receptors in addition to immunohistochemical studies support the notion that muscarinic receptors are primarily involved in the preceding responses. We conclude that muscarinic inputs play an important role in determining the recruitment threshold and firing rate gain observed in oculomotor Mns in vivo.

Cellular and synaptic actions of acetylcholine in the lamprey spinal cord

This study investigated cellular and synaptic mechanisms of cholinergic neuromodulation in the in vitro lamprey spinal cord. Most spinal neurons tested responded to local application of acetylcholine (ACh) with depolarization and decreased input resistance. The depolarization persisted in the presence of either tetrodotoxin or muscarinic antagonist scopolamine and was abolished with nicotinic antagonist mecamylamine, indicating a direct depolarization through nicotinic ACh receptors. Local application of muscarinic ACh agonists modulated synaptic strength in the spinal cord by decreasing the amplitude of unitary excitatory and inhibitory postsynaptic potentials. The postsynaptic response to direct application of glutamate was unchanged by muscarinic agonists, suggesting a presynaptic mechanism. Cholinergic feedback from motoneurons was assessed using stimulation of a ventral root in the quiescent spinal cord while recording intracellularly from spinal motoneurons or interneurons. Mainly depolarizing potentials were observed, a portion of which was insensitive to removal of extracellular Ca2+, indicating electrotonic coupling. Hyperpolarizing potentials were also observed and were attenuated by the glycinergic antagonist strychnine, whereas depolarizing responses were potentiated by strychnine. Mecamylamine also reduced hyperpolarizing responses. The pharmacology of these responses suggests a Renshaw-like feedback pathway in lamprey. Immunohistochemistry for choline acetyltransferase, performed in combination with retrograde filling of motoneurons, demonstrated a population of nonmotoneuron cholinergic cells in the lamprey spinal cord. Thus endogenous cholinergic modulation of the lamprey spinal locomotor network is likely produced by both motoneurons and cholinergic interneurons acting via combined postsynaptic and presynaptic actions.

Afterhyperpolarization time-course and minimal discharge rate in low threshold motor units in humans

The alpha-motoneurone afterhyperpolarization (AHP) duration correlates with a number of its muscle unit properties in animal preparations. In humans, the interval death rate (IDR) analysis has been used to estimate the time course of human motoneurone AHP based on the pattern of motor unit firing. The purpose of this experiment was first, to examine the relationship between estimated AHP time course and the minimal firing rate of the motor unit and second, to examine the relationship between the AHP and motor unit contractile properties in the tibialis anterior (TA) muscle. Motor unit data were obtained from the TA muscle during low force isometric contractions lasting 600 s. Muscle unit twitch characteristics were determined using spike-triggered averaging (STA) and the motoneurone AHP time course was estimated using the IDR analysis. Minimal discharge rate and derecruitment threshold torque were determined for 2 s preceding motor unit derecruitment. The AHP time constant and minimal discharge rate were negatively correlated, whereas the derecruitment threshold torque was not associated with the AHP time constant. The estimated AHP duration, however, is considerably shorter than the mean ISI of the minimal discharge rate suggesting that synaptic noise and AHP duration are important factors in dictating the minimal discharge rate in low force voluntary contractions in humans. The AHP time constant did not vary significantly with motor unit twitch amplitude; however, significant positive relationships were found between the AHP time constant and the temporal properties of the motor unit twitch. The calculated AHP time course using the IDR analysis, therefore, is a reasonable estimate and coupled with motor unit properties attained with STA, it provides a powerful method to describe low-threshold motor units.

Resonant or not, two amplification modes of proprioceptive inputs by persistent inward currents in spinal motoneurons

Why do motoneurons possess two persistent inward currents (PICs), a fast sodium current and a slow calcium current? To answer this question, we replaced the natural PICs with dynamic clamp-imposed artificial PICs at the soma of spinal motoneurons of anesthetized cats. We investigated how PICs with different kinetics (1-100 ms) amplify proprioceptive inputs. We showed that their action depends on the presence or absence of a resonance created by the I(h) current. In resonant motoneurons, a fast PIC enhances the resonance and amplifies the dynamic component of Ia inputs elicited by ramp-and-hold muscle stretches. This facilitates the recruitment of these motoneurons, which likely innervate fast contracting motor units developing large forces, e.g., to restore balance or produce ballistic movements. In nonresonant motoneurons, in contrast, a fast PIC easily triggers plateau potentials, which leads to a dramatic amplification of the static component of Ia inputs. This likely facilitates the recruitment of these motoneurons, innervating mostly slowly contracting and fatigue-resistant motor units, during postural activities. Finally, a slow PIC may switch a resonant motoneuron to nonresonant by counterbalancing I(h), thus changing the action of the fast PIC. A modeling study shows that I(h) needs to be located on the dendrites to create the resonance, and it predicts that dendritic PICs amplify synaptic input in the same manner as somatic PICs.

Spinal cholinergic interneurons regulate the excitability of motoneurons during locomotion

To effect movement, motoneurons must respond appropriately to motor commands. Their responsiveness to these inputs, or excitability, is regulated by neuromodulators. Possible sources of modulation include the abundant cholinergic "C boutons" that surround motoneuron somata. In the present study, recordings from motoneurons in spinal cord slices demonstrated that cholinergic activation of m2-type muscarinic receptors increases excitability by reducing the action potential afterhyperpolarization. Analyses of isolated spinal cord preparations in which fictive locomotion was elicited demonstrated that endogenous cholinergic inputs increase motoneuron excitability during locomotion. Anatomical data indicate that C boutons originate from a discrete group of interneurons lateral to the central canal, the medial partition neurons. These results highlight a unique component of spinal motor networks that is critical in ensuring that sufficient output is generated by motoneurons to drive motor behavior.

The afterhyperpolarization conductance exerts the same control over the gain and variability of motoneurone firing in anaesthetized cats

Does the afterhyperpolarization current control the gain and discharge variability of motoneurones according to the same law? We investigated this issue in lumbar motoneurones of anaesthetized cats. Using dynamic clamp, we measured the conductance, time constant and driving force of the AHP current in a sample of motoneurones and studied how the gain was correlated to these quantities. To study the action of the AHP on the discharge variability and to compare it to its action on the gain, we injected an artificial AHP-like current in motoneurones. This increased the natural AHP. In three motoneurones, we abolished most of the natural AHP with the calcium chelator BAPTA to investigate the condition where the discharge was essentially controlled by the artificial AHP. Our results demonstrate that both the gain and the coefficient of variation of the firing rate are inversely proportional to the magnitude and to the time constant of the artificial AHP conductance. This indicates that the AHP exerts the same control over the gain and the variability. This mechanism ensures that the variability of the discharge is modulated with the gain. This guarantees a great regularity of the discharge when the motoneurone is in a low excitability state and hence good control of the force produced.

Tenascin-R restricts posttraumatic remodeling of motoneuron innervation and functional recovery after spinal cord injury in adult mice

Tenascin-R (TNR) is an extracellular glycoprotein in the CNS implicated in neural development and plasticity. Its repellent properties for growing axons in a choice situation with a conducive substrate in vitro have indicated that TNR may impede regeneration in the adult mammalian CNS. Here we tested whether constitutive lack of TNR has beneficial impacts on recovery from spinal cord injury in adult mice. Using the Basso, Beattie, Bresnahan (BBB) locomotor rating scale, we found that open-field locomotion in TNR-deficient (TNR-/-) mice recovered better that in wild-type (TNR+/+) littermates after compression of the thoracic spinal cord. We also designed, validated, and applied a motion analysis approach allowing numerical assessment of motor functions. We found, in agreement with the BBB score, that functions requiring low levels of supraspinal control such as plantar stepping improved more in TNR-/- mice. This was not the case for motor tasks demanding precision such as ladder climbing. Morphological analyses revealed no evidence that improved recovery of some functions in the mutant mice were attributable to enhanced tissue sparing or axonal regrowth. Estimates of perisomatic puncta revealed reduced innervation by cholinergic and GABAergic terminals around motoneurons in intact TNR-/- compared with TNR+/+ mice. Relative to nonlesioned animals, spinal cord repair was associated with increase in GABAergic and decrease of glutamatergic puncta in TNR-/- but not in TNR+/+ mice. Our results suggest that TNR restricts functional recovery by limiting posttraumatic remodeling of synapses around motoneuronal cell bodies where TNR is normally expressed in perineuronal nets.