Mitigating aberrant Cdk5 activation alleviates mitochondrial defects and motor neuron disease symptoms in spinal muscular atrophy.

Spinal muscular atrophy (SMA), the top genetic cause of infant mortality, is characterized by motor neuron degeneration. Mechanisms underlying SMA pathogenesis remain largely unknown. Here, we report that the activity of cyclin-dependent kinase 5 (Cdk5) and the conversion of its activating subunit p35 to the more potent activator p25 are significantly up-regulated in mouse models and human induced pluripotent stem cell (iPSC) models of SMA. The increase of Cdk5 activity occurs before the onset of SMA phenotypes, suggesting that it may be an initiator of the disease. Importantly, aberrant Cdk5 activation causes mitochondrial defects and motor neuron degeneration, as the genetic knockout of p35 in an SMA mouse model rescues mitochondrial transport and fragmentation defects, and alleviates SMA phenotypes including motor neuron hyperexcitability, loss of excitatory synapses, neuromuscular junction denervation, and motor neuron degeneration. Inhibition of the Cdk5 signaling pathway reduces the degeneration of motor neurons derived from SMA mice and human SMA iPSCs. Altogether, our studies reveal a critical role for the aberrant activation of Cdk5 in SMA pathogenesis and suggest a potential target for therapeutic intervention.

A dynamic calcium-force relationship model for sag behavior in fast skeletal muscle.

In vitro studies using isolated or skinned muscle fibers suggest that the sigmoidal relationship between the intracellular calcium concentration and force production may depend upon muscle type and activity. The goal of this study was to investigate whether and how the calcium-force relationship changes during force production under physiological conditions of muscle excitation and length in fast skeletal muscles. A computational framework was developed to identify the dynamic variation in the calcium-force relationship during force generation over a full physiological range of stimulation frequencies and muscle lengths in cat gastrocnemius muscles. In contrast to the situation in slow muscles such as the soleus, the calcium concentration for the half-maximal force needed to drift rightward to reproduce the progressive force decline, or sag behavior, observed during unfused isometric contractions at the intermediate length under low-frequency stimulation (i.e., 20 Hz). The slope at the calcium concentration for the half-maximal force was required to drift upward for force enhancement during unfused isometric contractions at the intermediate length under high-frequency stimulation (i.e., 40 Hz). The slope variation in the calcium-force relationship played a crucial role in shaping sag behavior across different muscle lengths. The muscle model with dynamic variations in the calcium-force relationship also accounted for the length-force and velocity-force properties measured under full excitation. These results imply that the calcium sensitivity and cooperativity of force-inducing crossbridge formation between actin and myosin filaments may be operationally altered in accordance with the mode of neural excitation and muscle movement in intact fast muscles.

Multimodal parameter spaces of a complex multi-channel neuron model.

One of the most common types of models that helps us to understand neuron behavior is based on the Hodgkin-Huxley ion channel formulation (HH model). A major challenge with inferring parameters in HH models is non-uniqueness: many different sets of ion channel parameter values produce similar outputs for the same input stimulus. Such phenomena result in an objective function that exhibits multiple modes (i.e., multiple local minima). This non-uniqueness of local optimality poses challenges for parameter estimation with many algorithmic optimization techniques. HH models additionally have severe non-linearities resulting in further challenges for inferring parameters in an algorithmic fashion. To address these challenges with a tractable method in high-dimensional parameter spaces, we propose using a particular Markov chain Monte Carlo (MCMC) algorithm, which has the advantage of inferring parameters in a Bayesian framework. The Bayesian approach is designed to be suitable for multimodal solutions to inverse problems. We introduce and demonstrate the method using a three-channel HH model. We then focus on the inference of nine parameters in an eight-channel HH model, which we analyze in detail. We explore how the MCMC algorithm can uncover complex relationships between inferred parameters using five injected current levels. The MCMC method provides as a result a nine-dimensional posterior distribution, which we analyze visually with solution maps or landscapes of the possible parameter sets. The visualized solution maps show new complex structures of the multimodal posteriors, and they allow for selection of locally and globally optimal value sets, and they visually expose parameter sensitivities and regions of higher model robustness. We envision these solution maps as enabling experimentalists to improve the design of future experiments, increase scientific productivity and improve on model structure and ideation when the MCMC algorithm is applied to experimental data.

Analyzing Modeled Torque Profiles to Understand Scale-Dependent Active Muscle Responses in the Hip Joint.

Animal locomotion is influenced by a combination of constituent joint torques (e.g., due to limb inertia and passive viscoelasticity), which determine the necessary muscular response to move the limb. Across animal size-scales, the relative contributions of these constituent joint torques affect the muscular response in different ways. We used a multi-muscle biomechanical model to analyze how passive torque components change due to an animal's size-scale during locomotion. By changing the size-scale of the model, we characterized emergent muscular responses at the hip as a result of the changing constituent torque profile. Specifically, we found that activation phases between extensor and flexor torques to be opposite between small and large sizes for the same kinematic motion. These results suggest general principles of how animal size affects neural control strategies. Our modeled torque profiles show a strong agreement with documented hindlimb torque during locomotion and can provide insights into the neural organization and muscle activation behavior of animals whose motion has not been extensively documented.

The Involvement of CaV1.3 Channels in Prolonged Root Reflexes and Its Potential as a Therapeutic Target in Spinal Cord Injury.

Spinal cord injury (SCI) results in not only the loss of voluntary muscle control, but also in the presence of involuntary movement or spasms. These spasms post-SCI involve hyperexcitability in the spinal motor system. Hyperactive motor commands post SCI result from enhanced excitatory postsynaptic potentials (EPSPs) and persistent inward currents in voltage-gated L-type calcium channels (LTCCs), which are reflected in evoked root reflexes with different timings. To further understand the contributions of these cellular mechanisms and to explore the involvement of LTCC subtypes in SCI-induced hyperexcitability, we measured root reflexes with ventral root recordings and motoneuron activities with intracellular recordings in an in vitro preparation using a mouse model of chronic SCI (cSCI). Specifically, we explored the effects of 1-(3-chlorophenethyl)-3-cyclopentylpyrimidine-2,4,6-(1H,3H,5H)-trione (CPT), a selective negative allosteric modulator of CaV1.3 LTCCs. Our results suggest a hyperexcitability in the spinal motor system in these SCI mice. Bath application of CPT displayed slow onset but dose-dependent inhibition of the root reflexes with the strongest effect on LLRs. However, the inhibitory effect of CPT is less potent in cSCI mice than in acute SCI (aSCI) mice, suggesting changes either in composition of CaV1.3 or other cellular mechanisms in cSCI mice. For intracellular recordings, the intrinsic plateau potentials, was observed in more motoneurons in cSCI mice than in aSCI mice. CPT inhibited the plateau potentials and reduced motoneuron firings evoked by intracellular current injection. These results suggest that the LLR is an important target and that CPT has potential in the therapy of SCI-induced muscle spasms.

Time Course of Alterations in Adult Spinal Motoneuron Properties in the SOD1(G93A) Mouse Model of ALS.

Although amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease, motoneuron electrical properties are already altered during embryonic development. Motoneurons must therefore exhibit a remarkable capacity for homeostatic regulation to maintain a normal motor output for most of the life of the patient. In the present article, we demonstrate how maintaining homeostasis could come at a very high cost. We studied the excitability of spinal motoneurons from young adult SOD1(G93A) mice to end-stage. Initially, homeostasis is highly successful in maintaining their overall excitability. This initial success, however, is achieved by pushing some cells far above the normal range of passive and active conductances. As the disease progresses, both passive and active conductances shrink below normal values in the surviving cells. This shrinkage may thus promote survival, implying the previously large values contribute to degeneration. These results support the hypothesis that motoneuronal homeostasis may be "hypervigilant" in ALS and a source of accumulating stress.

Differences in estimated persistent inward currents between ankle flexors and extensors in humans.

Persistent inward currents (PICs) are responsible for amplifying motoneuronal synaptic inputs and contribute to generating normal motoneuron activation. Delta-F (ΔF) is a well-established method that estimates PICs in humans indirectly from firing patterns of individual motor units. Traditionally, motor unit firing patterns are obtained by manually decomposing electromyography (EMG) signals recorded through intramuscular electrodes (iEMG). A previous iEMG study has shown that in humans the elbow extensors have higher ΔF than the elbow flexors. In this study, EMG signals were collected from the ankle extensors and flexors using high-density surface array electrodes during isometric sitting and standing at 10-30% maximum voluntary contraction. The signals were then decomposed into individual motor unit firings. We hypothesized that comparable to the upper limb, the lower limb extensor muscles (soleus) would have higher ΔF than the lower limb flexor muscles [tibialis anterior (TA)]. Contrary to our expectations, ΔF was higher in the TA than the soleus during sitting and standing despite the difference in cohort of participants and body positions. The TA also had significantly higher maximum discharge rate than the soleus while there was no difference in rate increase. When only the unit pairs with similar maximum discharge rates were compared, ∆F was still higher in the TA than the soleus. Future studies will focus on investigating the functional significance of the findings.NEW & NOTEWORTHY With the use of high-density surface array electrodes and convolutive blind source separation algorithm, thousands of motor units were decomposed from the soleus and tibialis anterior muscles. Persistent inward currents were estimated under seated and standing conditions via delta-F (∆F) calculation, and the results showed that unlike the upper limb, the flexor has higher ∆F than the extensor in the lower limb. Future studies will focus on functional significance of the findings.

Bursting interneurons in the deep dorsal horn develop increased excitability and sensitivity to serotonin after chronic spinal injury.

The loss of descending serotonin (5-HT) to the spinal cord contributes to muscle spasms in chronic spinal cord injury (SCI). Hyperexcitable motoneurons receive long-lasting excitatory postsynaptic potentials (EPSPs), which activate their persistent inward currents to drive muscle spasms. Deep dorsal horn (DDH) neurons with bursting behavior could be involved in triggering the EPSPs due to loss of inhibition in the chronically 5-HT-deprived spinal cord. Previously, in an acutely transected preparation, we found that bursting DDH neurons were affected by administration of the 5-HT1B/1D receptor agonist zolmitriptan, which suppressed their bursts, and by N-methyl-d-aspartate (NMDA), which enhanced their bursting behavior. Nonbursting DDH neurons were not influenced by these agents. In the present study, we investigate the firing characteristics of bursting DDH neurons following chronic spinal transection at T10 level in adult mice and examine the effects of replacing lost endogenous 5-HT with zolmitriptan. Terminal experiments using our in vitro preparation of the sacral cord were carried out ~10 wk postransection. Compared with the acute spinal stage of our previous study, DDH neurons in the chronic stage became more responsive to dorsal root stimulation, with burst duration doubling with chronic injury. The suppressive effects of zolmitriptan were stronger overall, but the facilitative effects of NMDA were weaker. In addition, the onset of DDH neuron activity preceded ventral root output and the firing rates of DDH interneurons correlated with the integrated long-lasting ventral root output. These results support a contribution of the bursting DDH neurons to muscle spasms following SCI and inhibition by 5-HT.NEW & NOTEWORTHY We investigate the firing characteristics of bursting deep dorsal horn (DDH) neurons following chronic spinal transection. DDH neurons in the chronic stage are different from those in the acute stage as noted by their increase in excitability overall and their differing responses serotonin (5-HT) and N-methyl-d-aspartate (NMDA) receptor agonists. Also, there is a strong relationship between DDH neuron activity and ventral root output. These results support a contribution of the bursting DDH neurons to muscle spasms following chronic spinal cord injury (SCI).

Exogenous neuromodulation of spinal neurons induces beta-band coherence during self-sustained discharge of hind limb motor unit populations.

The spontaneous or self-sustained discharge of spinal motoneurons can be observed in both animals and humans. Although the origins of this self-sustained discharge are not fully known, it can be generated by activation of persistent inward currents intrinsic to the motoneuron. If self-sustained discharge is generated exclusively through this intrinsic mechanism, the discharge of individual motor units will be relatively independent of one another. Alternatively, if increased activation of premotor circuits underlies this prolonged discharge of spinal motoneurons, we would expect correlated activity among motoneurons. Our aim is to assess potential synaptic drive by quantifying coherence during self-sustained discharge of spinal motoneurons. Electromyographic activity was collected from 20 decerebrate animals using a 64-channel electrode grid placed on the isolated soleus muscle before and following intrathecal administration of methoxamine, a selective α1-noradrenergic agonist. Sustained muscle activity was recorded and decomposed into the discharge times of ~10-30 concurrently active individual motor units. Consistent with previous reports, the self-sustained discharge of motor units occurred at low mean discharge rates with low-interspike variability. Before methoxamine administration, significant low-frequency coherence (<2 Hz) was observed, while minimal coherence was observed within higher frequency bands. Following intrathecal administration of methoxamine, increases in motor unit discharge rates and strong coherence in both the low-frequency and 15- to 30-Hz beta bands were observed. These data demonstrate beta-band coherence among motor units can be observed through noncortical mechanisms and that neuromodulation of spinal/brainstem neurons greatly influences coherent discharge within spinal motor pools.NEW & NOTEWORTHY The correlated discharge of spinal motoneurons is often used to describe the input to the motor pool. We demonstrate spinal/brainstem neurons devoid of cortical input can generate correlated motor unit discharge in the 15- to 30-Hz beta band, which is amplified through neuromodulation. Activity in the beta band is often ascribed to cortical drive in humans; however, these data demonstrate the capability of the mammalian segmental motor system to generate and modulate this coherent state of motor unit discharge.

Cross-Frequency Coupling in Descending Motor Pathways: Theory and Simulation.

Coupling of neural oscillations is essential for the transmission of cortical motor commands to motoneuron pools through direct and indirect descending motor pathways. Most studies focus on iso-frequency coupling between brain and muscle activities, i.e., cortico-muscular coherence, which is thought to reflect motor command transmission in the mono-synaptic corticospinal pathway. Compared to this direct pathway, indirect corticobulbospinal motor pathways involve multiple intermediate synaptic connections via spinal interneurons. Neuronal processing of synaptic inputs can lead to modulation of inter-spike intervals which produces cross-frequency coupling. This theoretical study aims to evaluate the effect of the number of synaptic layers in descending pathways on the expression of cross-frequency coupling between supraspinal input and the cumulative output of the motoneuron pool using a computer simulation. We simulated descending pathways as various layers of interneurons with a terminal motoneuron pool using Hogdkin-Huxley styled neuron models. Both cross- and iso-frequency coupling between the supraspinal input and the motorneuron pool output were computed using a novel generalized coherence measure, i.e., n:m coherence. We found that the iso-frequency coupling is only dominant in the mono-synaptic corticospinal tract, while the cross-frequency coupling is dominant in multi-synaptic indirect motor pathways. Furthermore, simulations incorporating both mono-synaptic direct and multi-synaptic indirect descending pathways showed that increased reliance on a multi-synaptic indirect pathway over a mono-synaptic direct pathway enhances the dominance of cross-frequency coupling between the supraspinal input and the motorneuron pool output. These results provide the theoretical basis for future human subject study quantitatively assessing motor command transmission in indirect vs. direct pathways and its changes after neurological disorders such as unilateral brain injury.

Botulinum Toxin Conditioning Enhances Motor Axon Regeneration in Mouse and Human Preclinical Models.

BACKGROUND: Peripheral axon regeneration is improved when the nerve lesion under consideration has recently been preceded by another nerve injury. This is known as the conditioning lesion effect (CLE). While the CLE is one of the most robust and well characterized means to enhance motor axon regeneration in experimental models, it is not considered a clinically feasible strategy. A pharmacological means to re-produce the CLE is highly desirable. OBJECTIVE: To test whether chemodenervation with a clinical grade formulation of botulinum toxin A (BoTX) would be sufficient to reproduce the CLE. METHODS: We examined the effects of a 1-week preconditioning administration of BoTX on motor axon regrowth in both a mouse tibial nerve injury and human embryonic stem cell (hESC)-based model. We assessed neuronal reinnervation in vivo (mice) with retrograde tracers and histological analysis of peripheral nerve tissue after injections into the triceps surae muscle group. We assessed motor neuron neurite outgrowth in vitro (hESC) after incubation in BoTX by immunohistochemistry and morphometric analysis. RESULTS: We found that BoTX conditioning treatment significantly enhanced outgrowth of both murine motor axons in vivo and human MN neurites in vitro. CONCLUSIONS: BoTX preconditioning represents a pharmacological candidate approach to enhance motor axon regeneration in specific clinical scenarios such as nerve transfer surgery. Further studies are needed to elucidate the molecular mechanism.

Neuromodulatory Inputs to Motoneurons Contribute to the Loss of Independent Joint Control in Chronic Moderate to Severe Hemiparetic Stroke.

In chronic hemiparetic stroke, increased shoulder abductor activity causes involuntary increases in elbow, wrist, and finger flexor activation, an abnormal muscle coactivation pattern known as the flexion synergy. Recent evidence suggests that flexion synergy expression may reflect recruitment of contralesional cortico-reticulospinal motor pathways following damage to the ipsilesional corticospinal tract. However, because reticulospinal motor pathways produce relatively weak post-synaptic potentials in motoneurons, it is unknown how preferential use of these pathways could lead to robust muscle activation. Here, we hypothesize that the descending neuromodulatory component of the ponto-medullary reticular formation, which uses the monoaminergic neurotransmitters norepinephrine and serotonin, serves as a gain control mechanism to facilitate motoneuron responses to reticulospinal motor commands. Thus, inhibition of the neuromodulatory component would reduce flexion synergy expression by disfacilitating spinal motoneurons. To test this hypothesis, we conducted a pre-clinical study utilizing two targeted neuropharmacological probes and inert placebo in a cohort of 16 individuals with chronic hemiparetic stroke. Test compounds included Tizanidine (TIZ), a noradrenergic α2 agonist and imidazoline ligand selected for its ability to reduce descending noradrenergic drive, and Isradipine, a dihyropyridine calcium-channel antagonist selected for its ability to post-synaptically mitigate a portion of the excitatory effects of monoamines on motoneurons. We used a previously validated robotic measure to quantify flexion synergy expression. We found that Tizanidine significantly reduced expression of the flexion synergy. A predominantly spinal action for this effect is unlikely because Tizanidine is an agonist acting on a baseline of spinal noradrenergic drive that is likely to be pathologically enhanced post-stroke due to increased reliance on cortico-reticulospinal motor pathways. Although spinal actions of TIZ cannot be excluded, particularly from Group II pathways, our finding is consistent with a supraspinal action of Tizanidine to reduce descending noradrenergic drive and disfacilitate motoneurons. The effects of Isradipine were not different from placebo, likely related to poor central bioavailability. These results support the hypothesis that the descending monoaminergic component of the ponto-medullary reticular formation plays a key role in flexion synergy expression in chronic hemiparetic stroke. These results may provide the basis for new therapeutic strategies to complement physical rehabilitation.

Robust and accurate decoding of motoneuron behaviour and prediction of the resulting force output.

KEY POINTS: The spinal alpha motoneuron is the only cell in the human CNS whose discharge can be routinely recorded in humans. We have reengineered motor unit collection and decomposition approaches, originally developed in humans, to measure the neural drive to muscle and estimate muscle force generation in the in vivo cat model. Experimental, computational, and predictive approaches are used to demonstrate the validity of this approach across a wide range of modes to activate the motor pool. The utility of this approach is shown through the ability to track individual motor units across trials, allowing for better predictions of muscle force than the electromyography signal, and providing insights in to the stereotypical discharge characteristics in response to synaptic activation of the motor pool. This approach now allows for a direct link between the intracellular data of single motoneurons, the discharge properties of motoneuron populations, and muscle force generation in the same preparation. ABSTRACT: The discharge of a spinal alpha motoneuron and the resulting contraction of its muscle fibres represents the functional quantum of the motor system. Recent advances in the recording and decomposition of the electromyographic signal allow for the identification of several tens of concurrently active motor units. These detailed population data provide the potential to achieve deep insights into the synaptic organization of motor commands. Yet most of our understanding of the synaptic input to motoneurons is derived from intracellular recordings in animal preparations. Thus, it is necessary to extend the new electrode and decomposition methods to recording of motor unit populations in these same preparations. To achieve this goal, we use high-density electrode arrays and decomposition techniques, analogous to those developed for humans, to record and decompose the activity of tens of concurrently active motor units in a hindlimb muscle in the in vivo cat. Our results showed that the decomposition method in this animal preparation was highly accurate, with conventional two-source validation providing rates of agreement equal to or superior to those found in humans. Multidimensional reconstruction of the motor unit action potential provides the ability to accurately track the same motor unit across multiple contractions. Additionally, correlational analyses demonstrate that the composite spike train provides better estimates of whole muscle force than conventional estimates obtained from the electromyographic signal. Lastly, stark differences are observed between the modes of activation, in particular tendon vibration produced quantal interspike intervals at integer multiples of the vibration period.

Altered Neuromodulatory Drive May Contribute to Exaggerated Tonic Vibration Reflexes in Chronic Hemiparetic Stroke.

Exaggerated stretch-sensitive reflexes are a common finding in elbow flexors of the contralesional arm in chronic hemiparetic stroke, particularly when muscles are not voluntarily activated prior to stretch. Previous investigations have suggested that this exaggeration could arise either from an abnormal tonic ionotropic drive to motoneuron pools innervating the paretic limbs, which could bring additional motor units near firing threshold, or from an increased influence of descending monoaminergic neuromodulatory pathways, which could depolarize motoneurons and amplify their responses to synaptic inputs. However, previous investigations have been unable to differentiate between these explanations, leaving the source(s) of this excitability increase unclear. Here, we used tonic vibration reflexes (TVRs) during voluntary muscle contractions of increasing magnitude to infer the sources of spinal motor excitability in individuals with chronic hemiparetic stroke. We show that when the paretic and non-paretic elbow flexors are preactivated to the same percentage of maximum prior to vibration, TVRs remain significantly elevated in the paretic arm. We also show that the rate of vibration-induced torque development increases as a function of increasing preactivation in the paretic limb, even though the amplitude of vibration-induced torque remains conspicuously unchanged as preactivation increases. It is highly unlikely that these findings could be explained by a source that is either purely ionotropic or purely neuromodulatory, because matching preactivation should control for the effects of a potential ionotropic drive (and lead to comparable tonic vibration reflex responses between limbs), while a purely monoaminergic mechanism would increase reflex magnitude as a function of preactivation. Thus, our results suggest that increased excitability of motor pools innervating the paretic limb post-stroke is likely to arise from both ionotropic and neuromodulatory mechanisms.

Acyloxyacyl hydrolase modulates pelvic pain severity.

Chronic pelvic pain causes significant patient morbidity and is a challenge to clinicians. Using a murine neurogenic cystitis model that recapitulates key aspects of interstitial cystitis/bladder pain syndrome (IC), we recently showed that pseudorabies virus (PRV) induces severe pelvic allodynia in BALB/c mice relative to C57BL/6 mice. Here, we report that a quantitative trait locus (QTL) analysis of PRV-induced allodynia in F2CxB progeny identified a polymorphism on chromosome 13, rs6314295 , significantly associated with allodynia (logarithm of odds = 3.11). The nearby gene encoding acyloxyacyl hydrolase ( Aoah) was induced in the sacral spinal cord of PRV-infected mice. AOAH-deficient mice exhibited increased vesicomotor reflex in response to bladder distension, consistent with spontaneous bladder hypersensitivity, and increased pelvic allodynia in neurogenic cystitis and postbacterial chronic pain models. AOAH deficiency resulted in greater bladder pathology and tumor necrosis factor production in PRV neurogenic cystitis, markers of increased bladder mast cell activation. AOAH immunoreactivity was detectable along the bladder-brain axis, including in brain sites previously correlated with human chronic pelvic pain. Finally, AOAH-deficient mice had significantly higher levels of bladder vascular endothelial growth factor, an emerging marker of chronic pelvic pain in humans. These findings indicate that AOAH modulates pelvic pain severity, suggesting that allelic variation in Aoah influences pelvic pain in IC.

Hyperexcitability in synaptic and firing activities of spinal motoneurons in an adult mouse model of amyotrophic lateral sclerosis.

Hyperexcitability is hypothesized to contribute to the degeneration of spinal motoneurons (MNs) in amyotrophic lateral sclerosis (ALS). Studies, thus far, have not linked hyperexcitability to the intrinsic properties of MNs in the adult ALS mouse model with the G93A-mutated SOD1 protein (mSOD1G93A). In this study, we obtained two types of measurements: ventral root recordings to assess motor output and intracellular recordings to assess synaptic properties of individual MNs. All studies were carried out in an in vitro preparation of the sacral spinal cords of mSOD1G93A mice and their non-transgenic (NT) littermates, both in the age range of 50-90days. Ventral root recordings revealed that maximum compound action potentials (coAPs) evoked by a short-train stimulation of corresponding dorsal roots were similar between the two types of mice. Although the progressive depression of coAPs was present during the train stimulation in all recordings, the coAP depression in mSOD1G93A mice was to a lesser extent, which suggests an increased firing tendency in mSOD1G93A MNs. Intracellular recordings showed no changes in fast excitatory postsynaptic potentials (EPSPs) in mSOD1G93A MNs. However, recording did show that oscillating EPSPs (oEPSPs) were induced by poly-EPSPs at a higher frequency and by less-intense electrical stimulation in mSOD1G93A MNs. These oEPSPs were dependent upon the activities of spinal network and N-methyl-d-aspartate receptors (NMDARs), and were subjected to riluzole modulation. Taken together, these findings revealed abnormal electrophysiology in mSOD1G93A MNs that could underlie ALS excitotoxicity.

Chronic electromyograms in treadmill running SOD1 mice reveal early changes in muscle activation.

KEY POINTS: The present study demonstrates that electromyograms (EMGs) obtained during locomotor activity in mice were effective for identification of early physiological markers of amyotrophic lateral sclerosis (ALS). These measures could be used to evaluate therapeutic intervention strategies in animal models of ALS. Several parameters of locomotor activity were shifted early in the disease time course in SOD1G93A mice, especially when the treadmill was inclined, including intermuscular phase, burst skew and amplitude of the locomotor bursts. The results of the present study indicate that early compensatory changes may be taking place within the neural network controlling locomotor activity, including spinal interneurons. Locomotor EMGs could have potential use as a clinical diagnostic tool. ABSTRACT: To improve our understanding of early disease mechanisms and to identify reliable biomarkers of amyotrophic lateral sclerosis (ALS), a progressive neurodegenerative disease, we measured electromyogram (EMG) activity in hind limb muscles of SOD1G93A mice. By contrast to clinical diagnostic measures using EMGs, which are performed on quiescent patients, we monitored activity during treadmill running aiming to detect presymptomatic changes in motor patterning. Chronic EMG electrodes were implanted into vastus lateralis, biceps femoris posterior, lateral gastrocnemius and tibialis anterior in mice from postnatal day 55 to 100 and the results obtained were assessed using linear mixed models. We evaluated differences in parameters related to EMG amplitude (peak and area) and timing (phase and skew, a measure of burst shape) when animals ran on level and inclined treadmills. There were significant changes in both the timing of activity and the amplitude of EMG bursts in SOD1G93A mice. Significant differences between wild-type and SOD1G93A mice were mainly observed when animals locomoted on inclined treadmills. All muscles had significant effects of mutation that were independent of age. These novel results indicate (i) locomotor EMG activity might be an early measure of disease onset; (ii) alterations in locomotor patterning may reflect changes in neuronal drive and compensation at the network level including altered activity of spinal interneurons; and (iii) the increased power output necessary on an inclined treadmill was important in revealing altered activity in SOD1G93A mice.

PICs in motoneurons do not scale with the size of the animal: a possible mechanism for faster speed of muscle contraction in smaller species.

The majority of studies on the electrical properties of neurons are carried out in rodents, and in particular in mice. However, the minute size of this animal compared with humans potentially limits the relevance of the resulting insights. To be able to extrapolate results obtained in a small animal such as a rodent, one needs to have proper knowledge of the rules governing how electrical properties of neurons scale with the size of the animal. Generally speaking, electrical resistances of neurons increase as cell size decreases, and thus maintenance of equal depolarization across cells of different sizes requires the underlying currents to decrease in proportion to the size decrease. Thus it would generally be expected that voltage-sensitive currents are smaller in smaller animals. In this study, we used in vivo preparations to record electrical properties of spinal motoneurons in deeply anesthetized adult mice and cats. We found that PICs do not scale with size, but instead are constant in their amplitudes across these species. This constancy, coupled with the threefold differences in electrical resistances, means that PICs contribute a threefold larger depolarization in the mouse than in the cat. As a consequence, motoneuronal firing rate sharply increases as animal size decreases. These differences in firing rates are likely essential in allowing different species to control muscles with widely different contraction speeds (smaller animals have faster muscle fibers). Thus from our results we have identified a possible new mechanism for how electrical properties are tuned to match mechanical properties within the motor output system.NEW & NOTEWORTHY The small size of the mouse warrants concern over whether the properties of their neurons are a scaled version of those in larger animals or instead have unique features. Comparison of spinal motoneurons in mice to cats showed unique features. Firing rates in the mouse were much higher, in large part due to relatively larger persistent inward currents. These differences likely reflect adaptations for controlling much faster muscle fibers in mouse than cat.

The potential for understanding the synaptic organization of human motor commands via the firing patterns of motoneurons.

Motoneurons are unique in being the only neurons in the CNS whose firing patterns can be easily recorded in human subjects. This is because of the one-to-one relationship between the motoneuron and muscle cell behavior. It has long been appreciated that the connection of motoneurons to their muscle fibers allows their action potentials to be amplified and recorded, but only recently has it become possible to simultaneously record the firing pattern of many motoneurons via array electrodes placed on the skin. These firing patterns contain detailed information about the synaptic organization of motor commands to the motoneurons. This review focuses on parameters in these firing patterns that are directly linked to specific features of this organization. It is now well established that motor commands consist of three components, excitation, inhibition, and neuromodulation; the importance of the third component has become increasingly evident. Firing parameters linked to each of the three components are discussed, along with consideration of potential limitations in their utility for understanding the underlying organization of motor commands. Future work based on realistic computer simulations of motoneurons may allow quantitative "reverse engineering" of human motoneuron firing patterns to provide good estimates of the relative amplitudes and temporal patterns of all three components of motor commands.