Therapeutic efficacy of the BKCa channel opener chlorzoxazone in a mouse model of Fragile X syndrome.

Fragile X syndrome (FXS) is an X-linked neurodevelopmental disorder characterized by several behavioral abnormalities, including hyperactivity, anxiety, sensory hyper-responsiveness, and autistic-like symptoms such as social deficits. Despite considerable efforts, effective pharmacological treatments are still lacking, prompting the need for exploring the therapeutic value of existing drugs beyond their original approved use. One such repurposed drug is chlorzoxazone which is classified as a large-conductance calcium-dependent potassium (BKCa) channel opener. Reduced BKCa channel functionality has been reported in FXS patients, suggesting that molecules activating these channels could serve as promising treatments for this syndrome. Here, we sought to characterize the therapeutic potential of chlorzoxazone using the Fmr1-KO mouse model of FXS which recapitulates the main phenotypes of FXS, including BKCa channel alterations. Chlorzoxazone, administered either acutely or chronically, rescued hyperactivity and acoustic hyper-responsiveness as well as impaired social interactions exhibited by Fmr1-KO mice. Chlorzoxazone was more efficacious in alleviating these phenotypes than gaboxadol and metformin, two repurposed treatments for FXS that do not target BKCa channels. Systemic administration of chlorzoxazone modulated the neuronal activity-dependent gene c-fos in selected brain areas of Fmr1-KO mice, corrected aberrant hippocampal dendritic spines, and was able to rescue impaired BKCa currents recorded from hippocampal and cortical neurons of these mutants. Collectively, these findings provide further preclinical support for BKCa channels as a valuable therapeutic target for treating FXS and encourage the repurposing of chlorzoxazone for clinical applications in FXS and other related neurodevelopmental diseases.

CSF1R inhibition at chronic stage after spinal cord injury modulates microglia proliferation.

Traumatic spinal cord injury (SCI) induces irreversible autonomic and sensory-motor impairments. A large number of patients exhibit chronic SCI and no curative treatment is currently available. Microglia are predominant immune players after SCI, they undergo highly dynamic processes, including proliferation and morphological modification. In a translational aim, we investigated whether microglia proliferation persists at chronic stage after spinal cord hemisection and whether a brief pharmacological treatment could modulate microglial responses. We first carried out a time course analysis of SCI-induced microglia proliferation associated with morphological analysis up to 84 days post-injury (dpi). Second, we analyzed outcomes on microglia of an oral administration of GW2580, a colony stimulating factor-1 receptor tyrosine kinase inhibitor reducing selectively microglia proliferation. After SCI, microglia proliferation remains elevated at 84 dpi. The percentage of proliferative microglia relative to proliferative cells increases over time reaching almost 50% at 84 dpi. Morphological modifications of microglia processes are observed up to 84 dpi and microglia cell body area is transiently increased up to 42 dpi. A transient post-injury GW2580-delivery at two chronic stages after SCI (42 and 84 dpi) reduces microglia proliferation and modifies microglial morphology evoking an overall limitation of secondary inflammation. Finally, transient GW2580-delivery at chronic stage after SCI modulates myelination processes. Together our study shows that there is a persistent microglia proliferation induced by SCI and that a pharmacological treatment at chronic stage after SCI modulates microglial responses. Thus, a transient oral GW2580-delivery at chronic stage after injury may provide a promising therapeutic strategy for chronic SCI patients.

Muscarinic cholinergic modulation of cardiovascular variables in spinal cord injured rats.

Spinal cord injury (SCI) leads not only to major impairments in sensorimotor control but also to dramatic dysregulation of autonomic functions including major cardiovascular disturbances. Consequently, individuals with SCI endure daily episodic hypo/hypertension and are at increased risk for cardiovascular disease. Several studies have suggested that an intrinsic spinal coupling mechanism between motor and sympathetic neuronal networks exist and that propriospinal cholinergic neurons may be responsible for a synchronized activation of both somatic and sympathetic outputs. We therefore investigated in the present study, the effect of cholinergic muscarinic agonists on cardiovascular parameters in freely moving adult rats after SCI. Female Sprague-Dawley rats were implanted with radiotelemetry sensors for long-term in vivo monitoring of blood pressure (BP). From BP signal, we calculated heart rate (HR) and respiratory frequency. We first characterized the physiological changes occurring after a SCI performed at the T3-T4 level in our experimental model system. We then investigated the effects on BP, HR and respiration, of the muscarinic agonist oxotremorine using one variant that crossed the blood brain barrier (Oxo-S) and one that does not (Oxo-M) in both Pre- and Post-SCI animals. After SCI, both HR and respiratory frequency increased. BP values exhibited an immediate profound drop before progressively increasing over the three-week post-lesion period but remained below control values. A spectral analysis of BP signal revealed the disappearance of the low frequency component of BP (0.3-0.6 Hz) referred to as Mayer waves after SCI. In Post-SCI animals, central effects mediated by Oxo-S led to an increase in HR and MAP, a slowdown in respiratory frequency and to an increased power in the 0.3-0.6 Hz frequency band. This study unravels some of the mechanisms by which muscarinic activation of spinal neurons could contribute to partial restoration of BP after SCI.

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.

Increased surface P2X4 receptors by mutant SOD1 proteins contribute to ALS pathogenesis in SOD1-G93A mice.

Amyotrophic lateral sclerosis (ALS) is a fatal motoneuron (MN) disease characterized by protein misfolding and aggregation leading to cellular degeneration. So far neither biomarker, nor effective treatment has been found. ATP signaling and P2X4 receptors (P2X4) are upregulated in various neurodegenerative diseases. Here we show that several ALS-related misfolded proteins including mutants of SOD1 or TDP-43 lead to a significant increase in surface P2X4 receptor density and function in vitro. In addition, we demonstrate in the spinal the cord of SOD1-G93A (SOD1) mice that misfolded SOD1-G93A proteins directly interact with endocytic adaptor protein-2 (AP2); thus, acting as negative competitors for the interaction between AP2 and P2X4, impairing constitutive P2X4 endocytosis. The higher P2X4 surface density was particularly observed in peripheral macrophages of SOD1 mice before the onset and during the progression of ALS symptoms positioning P2X4 as a potential early biomarker for ALS. P2X4 expression was also upregulated in spinal microglia of SOD1 mice during ALS and affect microglial inflammatory responses. Importantly, we report using double transgenic SOD1 mice expressing internalization-defective P2X4mCherryIN knock-in gene or invalidated for the P2X4 gene that P2X4 is instrumental for motor symptoms, ALS progression and survival. This study highlights the role of P2X4 in the pathophysiology of ALS and thus its potential for the development of biomarkers and treatments. We also decipher the molecular mechanism by which misfolded proteins related to ALS impact P2X4 trafficking at early pathological stage in cells expressing-P2X4.

Impact of acute and chronic nicotine administration on midbrain dopaminergic neuron activity and related behaviours in TRPV1 knock-out juvenile mice.

The addictive properties of nicotine, the main alkaloid in tobacco and tobacco-derived products, largely depend on its action on the activity of midbrain dopamine (DA) neurons. The transient receptor potential vanilloid 1 (TRPV1) channel has also been examined as an emerging contributor to addiction-related symptoms due to its ability to modulate midbrain neurons. Thus, the objective of our study was to explore the role of TRPV1 receptors (TRPV1Rs) on nicotine-induced behaviours and associated response of DA neuron activity. Both wild type juvenile mice and juvenile mice with invalidation of the TRPV1R gene were exposed to acute or chronic nicotine 0.3 mg/kg administration. We analysed locomotor activity in response to the drug. In addition, we performed cell-attached and whole-cell recordings from ventral tegmental area (VTA) neurons after nicotine exposure. Our results showed that the genetic deletion of TRPV1Rs reduced nicotine-induced locomotor sensitization. In addition, it provided evidence in support of TRPV1Rs being regulators of inhibitory synaptic transmission in the VTA. However, TRPV1Rs did not seem to modulate either nicotine-induced conditioning place preference or nicotine-evoked electrical activity of DA neurons. In conclusion, TRPV1Rs modulate nicotine-induced psychomotor sensitization in mice independently of a control on VTA DA neuron activity. Thus, TRPV1R control may depend on another key player of the mesolimbic circuit.

Developmentally Regulated Modulation of Lumbar Motoneurons by Metabotropic Glutamate Receptors: A Cellular and Behavioral Analysis in Newborn Mice.

The present study explores the impact of metabotropic glutamate receptor (mGluR) activation on activity-dependent synaptic plasticity (ADSP) and the intrinsic membrane properties of lumbar motoneurons (MNs) using a combination of biochemical, pharmacological, electrophysiological and behavioral techniques. Using spinal cord slices from C57BL/6JRJ mice at two developmental stages, 1-3 and 8-12 postnatal days (P1-P3; P8-P12, respectively), we found that ADSP expressed at glutamatergic synapses between axons conveyed in the ventrolateral funiculus (VLF) and MNs, involved mGluR activation. Using specific agonists of the three groups of mGluRs, we observed that mGluR stimulation causes subtype-specific and developmentally regulated modulation of the ADSP and synaptic transmission at VLF-MN synapses as well as the intrinsic membrane properties of MNs. RT-qPCR analysis revealed a downregulation of mGluR gene expression with age in the ventral part of the lumbar spinal cord. Interestingly, the selective harvest by laser microdissection of MNs innervating the Gastrocnemius and Tibialis anterior muscles unraveled that the level of Grm2 expression is higher in Tibialis MNs compared to Gastrocnemius MNs suggesting a specific mGluR gene expression profile in these two MN pools. Finally, we assessed the functional impact of mGluR modulation on electrically induced bouts of fictive locomotion in the isolated spinal cord preparation of P1-P3 mice, and in vivo during spontaneous episodes of swimming activity in both P1-P3 and P8-P12 mouse pups. We observed that the mGluR agonists induced distinct and specific effects on the motor burst amplitudes and period of the locomotor rhythms tested and that their actions are function of the developmental stage of the animals. Altogether our data show that the metabotropic glutamatergic system exerts a complex neuromodulation in the developing spinal lumbar motor networks and provide new insights into the expression and modulation of ADSP in MNs.

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.

Increased surface P2X4 receptor regulates anxiety and memory in P2X4 internalization-defective knock-in mice.

ATP signaling and surface P2X4 receptors are upregulated selectively in neurons and/or glia in various CNS disorders including anxiety, chronic pain, epilepsy, ischemia, and neurodegenerative diseases. However, the cell-specific functions of P2X4 in pathological contexts remain elusive. To elucidate P2X4 functions, we created a conditional transgenic knock-in P2X4 mouse line (Floxed P2X4mCherryIN) allowing the Cre activity-dependent genetic swapping of the internalization motif of P2X4 by the fluorescent mCherry protein to prevent constitutive endocytosis of P2X4. By combining molecular, cellular, electrophysiological, and behavioral approaches, we characterized two distinct knock-in mouse lines expressing noninternalized P2X4mCherryIN either exclusively in excitatory forebrain neurons or in all cells natively expressing P2X4. The genetic substitution of wild-type P2X4 by noninternalized P2X4mCherryIN in both knock-in mouse models did not alter the sparse distribution and subcellular localization of P2X4 but increased the number of P2X4 receptors at the surface of the targeted cells mimicking the pathological increased surface P2X4 state. Increased surface P2X4 density in the hippocampus of knock-in mice altered LTP and LTD plasticity phenomena at CA1 synapses without affecting basal excitatory transmission. Moreover, these cellular events translated into anxiolytic effects and deficits in spatial memory. Our results show that increased surface density of neuronal P2X4 contributes to synaptic deficits and alterations in anxiety and memory functions consistent with the implication of P2X4 in neuropsychiatric and neurodegenerative disorders. Furthermore, these conditional P2X4mCherryIN knock-in mice will allow exploring the cell-specific roles of P2X4 in various physiological and pathological contexts.

Locomotion and dynamic posture: neuro-evolutionary basis of bipedal gait.

Body displacement during locomotion is a major challenge for motor control, requiring complex synergistic postural regulation and the integrated functioning of all body musculature, including that of the four limbs, trunk and neck. Despite the obvious pivotal role played by the trunk during locomotion, most studies devoted to understanding the neural basis of locomotor control have only addressed the operation of the neural circuits driving leg movements, and relatively little is known of the networks that control trunk muscles in limbed vertebrates. This review addresses this issue, both in animals and humans. We first review studies addressing the central role played by central pattern generator (CPG) circuit interactions within the spinal cord in coordinating trunk and hind limb muscle activities in a variety of vertebrates, and present evidence that vestibulo-spinal reflexes are differentially involved in trunk and hind limb control. We finally highlight the role of the various components that participate in maintaining dynamic equilibrium during stepping, including connective tissues. We propose that many aspects of the organization of the motor systems involved in trunk-hind limb movement control in vertebrates have been highly conserved throughout evolution.

Specific features and survival of French bulldogs with congenital pulmonic stenosis: a prospective cohort study of 66 cases.

INTRODUCTION: The objectives of this study were to characterize the epidemiological, clinical, and echocardiographic features of French bulldogs (FBs) with congenital pulmonic stenosis and document their survival times and risk factors for cardiac death (CD). ANIMALS: This study included 66 FBs with congenital pulmonic stenosis. METHODS: Prospective cohort study including a survival analysis to assess time to CD. RESULTS: In most cases (53/66, 80%), at least two obstructive lesions were observed, most commonly valvular and supravalvular (42/66, 64%), with pulmonary trunk hypoplasia in 40/66 (61%) of cases. The median Doppler-derived peak trans-stenotic pressure gradient (ΔP) was very high: 170 mmHg (range = 34-291 mmHg), with ΔP ≥ 200 mmHg in 33% of FBs. Among the 51 FBs with an available follow-up and that did not undergo surgical valvuloplasty, 21/51 (41%) died, 67% (14/21) of deaths being CD. The median survival time from diagnosis to CD was 2.8 years (interquartile range = 0.8-4.6 years). Univariate Cox proportional hazard analyses revealed that age (hazard ratio [HR] = 2.3 per 1 year increase; p = 0.02), clinical signs at presentation (HR = 3.7; p = 0.03), ΔP (HR = 1.2 per 10 mmHg increase; p = 0.01), right ventricular dilation (HR = 5.0; p = 0.04), severe tricuspid regurgitation (HR = 7.6; p = 0.001), and right-sided congestive heart failure (HR = 4.8; p = 0.05) were associated with time to CD. After adjustment for age and ΔP, tricuspid regurgitation remained significantly associated with time to CD (HR = 5.1; p = 0.02). CONCLUSIONS: Pulmonic stenosis in FBs is commonly severe and complex, with at least 2 obstructive lesions in most cases, a high incidence of pulmonary trunk hypoplasia and CD, and strong association between prognosis and tricuspid regurgitation severity.

Cholinergic-mediated coordination of rhythmic sympathetic and motor activities in the newborn rat spinal cord.

Here, we investigated intrinsic spinal cord mechanisms underlying the physiological requirement for autonomic and somatic motor system coupling. Using an in vitro spinal cord preparation from newborn rat, we demonstrate that the specific activation of muscarinic cholinergic receptors (mAchRs) (with oxotremorine) triggers a slow burst rhythm in thoracic spinal segments, thereby revealing a rhythmogenic capability in this cord region. Whereas axial motoneurons (MNs) were rhythmically activated during both locomotor activity and oxotremorine-induced bursting, intermediolateral sympathetic preganglionic neurons (IML SPNs) exhibited rhythmicity solely in the presence of oxotremorine. This somato-sympathetic synaptic drive shared by MNs and IML SPNs could both merge with and modulate the locomotor synaptic drive produced by the lumbar motor networks. This study thus sheds new light on the coupling between somatic and sympathetic systems and suggests that an intraspinal network that may be conditionally activated under propriospinal cholinergic control constitutes at least part of the synchronizing mechanism.

3-D motion capture for long-term tracking of spontaneous locomotor behaviors and circadian sleep/wake rhythms in mouse.

BACKGROUND: Locomotor activity provides an index of an animal's behavioral state. Here, we report a reliable and cost-effective method that allows long-term (days to months) simultaneous tracking of locomotion in mouse cohorts (here consisting of 24 animals). NEW METHOD: The technique is based on a motion capture system used mainly for human movement study. A reflective marker was placed on the head of each mouse using a surgical procedure and labeled animals were returned to their individual home cages. Camera-recorded data of marker displacement resulting from locomotor movements were then analyzed with custom built software. To avoid any data loss, data files were saved every hour and automatically concatenated. Long-term recordings (up to 3 months) with high spatial (<1mm) and temporal (up to 100Hz) resolution of animal movements were obtained. RESULTS: The system was validated by analyzing the spontaneous activity of mice from post-natal day 30-90. Daily motor activity increased up to 70days in correspondence with maturational changes in locomotor performance. The recorded actigrams also permitted analysis of circadian and ultradian rhythms in cohort sleep/wake behavior. COMPARISON WITH EXISTING METHOD(S): In contrast to traditional session-based experimental approaches, our technique allows locomotor activity to be recorded with minimal experimenter manipulation, thereby minimizing animal stress. CONCLUSIONS: Our method enables the continuous long-term (up to several months) monitoring of tens of animals, generating manageable amounts of data at minimal costs without requiring individual dedicated devices. The actigraphic data collected allows circadian and ultradian analysis of sleep/wake behaviors to be performed.

Distinct and developmentally regulated activity-dependent plasticity at descending glutamatergic synapses on flexor and extensor motoneurons.

Activity-dependent synaptic plasticity (ADSP) is paramount to synaptic processing and maturation. However, identifying the ADSP capabilities of the numerous synapses converging onto spinal motoneurons (MNs) remain elusive. Using spinal cord slices from mice at two developmental stages, 1-4 and 8-12 postnatal days (P1-P4; P8-P12), we found that high-frequency stimulation of presumed reticulospinal neuron axons in the ventrolateral funiculus (VLF) induced either an NMDA receptor-dependent-long-term depression (LTD), a short-term depression (STD) or no synaptic modulation in limb MNs. Our study shows that P1-P4 cervical MNs expressed the same plasticity profiles as P8-P12 lumbar MNs rather than P1-P4 lumbar MNs indicating that ADSP expression at VLF-MN synapses is linked to the rostrocaudal development of spinal motor circuitry. Interestingly, we observed that the ADSP expressed at VLF-MN was related to the functional flexor or extensor MN subtype. Moreover, heterosynaptic plasticity was triggered in MNs by VLF axon tetanisation at neighbouring synapses not directly involved in the plasticity induction. ADSP at VLF-MN synapses specify differential integrative synaptic processing by flexor and extensor MNs and could contribute to the maturation of spinal motor circuits and developmental acquisition of weight-bearing locomotion.

Age-Related Changes in Pre- and Postsynaptic Partners of the Cholinergic C-Boutons in Wild-Type and SOD1G93A Lumbar Motoneurons.

Large cholinergic synaptic terminals known as C-boutons densely innervate the soma and proximal dendrites of motoneurons that are prone to neurodegeneration in amyotrophic lateral sclerosis (ALS). Studies using the Cu/Zn-superoxide dismutase (SOD1) mouse model of ALS have generated conflicting data regarding C-bouton alterations exhibited during ALS pathogenesis. In the present work, a longitudinal study combining immunohistochemistry, biochemical approaches and extra- and intra-cellular electrophysiological recordings revealed that the whole spinal cholinergic system is modified in the SOD1 mouse model of ALS compared to wild type (WT) mice as early as the second postnatal week. In WT motoneurons, both C-bouton terminals and associated M2 postsynaptic receptors presented a complex age-related dynamic that appeared completely disrupted in SOD1 motoneurons. Indeed, parallel to C-bouton morphological alterations, analysis of confocal images revealed a clustering process of M2 receptors during WT motoneuron development and maturation that was absent in SOD1 motoneurons. Our data demonstrated for the first time that the lamina X cholinergic interneurons, the neuronal source of C-boutons, are over-abundant in high lumbar segments in SOD1 mice and are subject to neurodegeneration in the SOD1 animal model. Finally, we showed that early C-bouton system alterations have no physiological impact on the cholinergic neuromodulation of newborn motoneurons. Altogether, these data suggest a complete reconfiguration of the spinal cholinergic system in SOD1 spinal networks that could be part of the compensatory mechanisms established during spinal development.

Origin of thoracic spinal network activity during locomotor-like activity in the neonatal rat.

Effective quadrupedal locomotor behaviors require the coordination of many muscles in the limbs, back, neck, and tail. Because of the spinal motoneuronal somatotopic organization, motor coordination implies interactions among distant spinal networks. Here, we investigated some of the interactions between the lumbar locomotor networks that control limb movements and the thoracic networks that control the axial muscles involved in trunk movement. For this purpose, we used an in vitro isolated newborn rat spinal cord (from T2 to sacrococcygeal) preparation. Using extracellular ventral root recordings, we showed that, while the thoracic cord possesses an intrinsic rhythmogenic capacity, the lumbar circuits, if they are rhythmically active, will entrain the rhythmicity of the thoracic circuitry. However, if the lumbar circuits are rhythmically active, these latter circuits will entrain the rhythmicity of the thoracic circuitry. Blocking the synaptic transmission in some thoracic areas revealed that the lumbar locomotor network could trigger locomotor bursting in distant thoracic segments through short and long propriospinal pathways. Patch-clamp recordings revealed that 72% of the thoracic motoneurons (locomotor-driven motoneurons) expressed membrane potential oscillations and spiking activity coordinated with the locomotor activity expressed by the lumbar cord. A biphasic excitatory (glutamatergic)/inhibitory (glycinergic) synaptic drive was recorded in thoracic locomotor-driven motoneurons. Finally, we found evidence that part of this locomotor drive involved a monosynaptic component coming directly from the lumbar locomotor network. We conclude that the lumbar locomotor network plays a central role in the generation of locomotor outputs in the thoracic cord by acting at both the premotoneuronal and motoneuronal levels.

Multiple monoaminergic modulation of posturo-locomotor network activity in the newborn rat spinal cord.

Studies devoted to understanding locomotor control have mainly addressed the functioning of the neural circuits controlling leg movements and relatively little is known of the operation of networks that activate trunk muscles in coordination with limb movements. The aim of the present work was (1) to identify the exogenous neurotransmitter cocktail that most strongly activates postural thoracic circuitry; (2) to investigate how the biogenic amines serotonin (5-HT), dopamine (DA), and noradrenaline (NA) modulate the coordination between limb and axial motor networks. Experiments were carried out on in vitro isolated spinal cord preparations from newborn rats. We recorded from ventral roots to monitor hindlimb locomotor and axial postural network activity. Each combination of the three amines with excitatory amino acids (EAAs) elicited coordinated rhythmic motor activity at all segmental levels with specific characteristics. The variability in cycle period was similar with 5-HT and DA while it was significantly higher with NA. DA elicited motor bursts of smaller amplitude in thoracic segments compared to 5-HT and NA, while both DA and NA elicited motor bursts of higher amplitude than 5-HT in the lumbar and sacral segments. The amines modulated the phase relationships of bursts in various segments with respect to the reference lumbar segment. At the thoracic level there was a phase lag between all recorded segments in the presence of 5-HT, while DA and NA elicited synchronous bursting. At the sacral level, 5-HT and DA induced an intersegmental phase shift while relationships became phase-locked with NA. Various combinations of EAAs with two or even all three amines elicited rhythmic motor output that was more variable than with one amine alone. Our results provide new data on the coordinating processes between spinal cord networks, demonstrating that each amine has a characteristic "signature" regarding its specific effect on intersegmental phase relationships.

Monoaminergic control of spinal locomotor networks in SOD1G93A newborn mice.

Mutations in the gene that encodes Cu/Zn-superoxide dismutase (SOD1) are the cause of approximately 20% of familial forms of amyotrophic lateral sclerosis (ALS), a fatal neurodegenerative disease characterized by the progressive loss of motor neurons. While ALS symptoms appear in adulthood, spinal motoneurons exhibit functional alterations as early as the embryonic and postnatal stages in the murine model of ALS, the SOD1 mice. Monoaminergic - i.e., dopaminergic (DA), serotoninergic (5-HT), and noradrenergic (NA) - pathways powerfully control spinal networks and contribute significantly to their embryonic and postnatal maturation. Alterations in monoaminergic neuromodulation during development could therefore lead to impairments in the motoneuronal physiology. In this study, we sought to determine whether the monoaminergic spinal systems are modified in the early stages of development in SOD1 mice. Using a post-mortem analysis by high performance liquid chromatography (HPLC), monoaminergic neuromodulators and their metabolites were quantified in the lumbar spinal cord of SOD1 and wild-type (WT) mice aged one postnatal day (P1) and P10. This analysis underscores an increased content of DA in the SOD1 lumbar spinal cord compared to that of WT mice but failed to reveal any modification of the other monoaminergic contents. In a next step, we compared the efficiency of the monoaminergic compounds in triggering and modulating fictive locomotion in WT and SOD1 mice. This study was performed in P1-P3 SOD1 mice and age-matched control littermates using extracellular recordings from the lumbar ventral roots in the in vitro isolated spinal cord preparation. This analysis revealed that the spinal networks of SOD1(G93A) mice could generate normal locomotor activity in the presence of NMA-5-HT. Interestingly, we also observed that SOD1 spinal networks have an increased sensitivity to NA compared to WT spinal circuits but exhibited similar DA responses.

Activity-dependent synaptic plasticity and metaplasticity in spinal motor networks.

Throughout life, neuronal network properties are modulated according to both external and internal stimuli. These adaptive capabilities of the central nervous system (CNS) have been generically termed "plasticity". One prominent form of CNS plasticity is the capability of synapses to change their strength. Synaptic strength is not a constant value but depends at each moment on the synapse's past activity. These changes in transmission efficacy are called activity-dependent synaptic plasticity (ADSP) and result in an increase (potentiation) or a decrease (depression) in synaptic strength. The ability of synapses to express one type of ADSP can change as a function of previous plasticity and previous activation of synapses. This plasticity of synaptic plasticity has been termed metaplasticity. ADSP and metaplasticity are now regarded as essential mechanisms for normal information processing in neuronal networks. Rhythmic activities such as locomotion are generated by rhythmically active central neuronal networks called central pattern generators (CPGs) that possess the intrinsic ability to generate rhythmic and organized activity in the absence of sensory inputs. The CPG activity arises from a complex dynamic interaction between synaptic transmission, intrinsic membrane properties and neuromodulatory inputs. A growing body of evidence suggests that the spinal cord matches the plastic and metaplastic properties found in other parts of the CNS, both under normal conditions and after spinal cord injury. Here, findings describing ADSP and its neuromodulation in vertebrate sensorimotor networks are reviewed, followed by a discussion of the potential role of ADSP and neuromodulation in the physiology and pathophysiology of motor circuit assembly.

Cholinergic partition cells and lamina x neurons induce a muscarinic-dependent short-term potentiation of commissural glutamatergic inputs in lumbar motoneurons.

Acetylcholine and the activation of muscarinic receptors influence the activity of neural networks generating locomotor behavior in the mammalian spinal cord. Using electrical stimulations of the ventral commissure, we show that commissural muscarinic (CM) depolarizations could be induced in lumbar motoneurons. We provide a detailed electrophysiological characterization of the muscarinic receptors and the membrane conductance involved in these responses. Activation of the CM terminals, originating from lamina X neurons and partition cells, induced a pathway-specific short-term potentiation (STP) of commissural glutamatergic inputs in motoneurons. This STP is occluded in the presence of the muscarinic antagonist atropine. During fictive locomotion, the activation of the commissural pathways transiently enhanced the motor output in a muscarinic-dependent manner. This study describes for the first time a novel regulatory mechanism of synaptic strength in spinal locomotor networks. Such cellular mechanisms would endow the locomotor central pattern generators with adaptive processes needed to generate appropriate synaptic inputs to motoneurons during different motor tasks.