Docosahexaenoic Acid Promotes Recovery of Motor Function by Neuroprotection and Neuroplasticity Mechanisms

The omega-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA), has been shown to promote recovery of motor function after spinal cord injury. This is likely to be at least partly due to neuroprotective effects of DHA. However, recent studies have shown that DHA also supports neuroplasticity after injury, such as promoting sprouting of spared corticospinal tract (CST) axons. In this chapter, we review the published studies showing that DHA promotes recovery of motor function in rodent models of spinal cord injury (SCI), and consider the available data on the underlying mechanisms. This includes effects on inflammation and on neuronal and oligodendrocyte survival at the injury site, and effects on spared CST axons and serotonergic axons. Current data support the hypothesis that DHA promotes recovery of motor function by both neuroprotection and neuroplasticity mechanisms. The significance of this, and the implications of combining DHA with rehabilitation strategies, will be discussed

Computational Study of the Mechanisms Underlying Oscillation in Neuronal Locomotor Circuits

In this thesis we model two very different movement-related neuronal circuits, both of which produce oscillatory patterns of activity. In one case we study oscillatory activity in the basal ganglia under both normal and Parkinsonian conditions. First, we used a detailed Hodgkin-Huxley type spiking model to investigate the activity patterns that arise when oscillatory cortical input is transmitted to the globus pallidus via the subthalamic nucleus. Our model reproduced a result from rodent studies which shows that two anti-phase oscillatory groups of pallidal neurons appear under Parkinsonian conditions. Secondly, we used a population model of the basal ganglia to study whether oscillations could be locally generated. The basal ganglia are thought to be organised into multiple parallel channels. In our model, isolated channels could not generate oscillations, but if the lateral inhibition between channels is sufficiently strong then the network can act as a rhythm-generating ``pacemaker'' circuit. This was particularly true when we used a set of connection strength parameters that represent the basal ganglia under Parkinsonian conditions. Since many things are not known about the anatomy and electrophysiology of the basal ganglia, we also studied oscillatory activity in another, much simpler, movement-related neuronal system: the spinal cord of the Xenopus tadpole. We built a computational model of the spinal cord containing approximately 1,500 biologically realistic Hodgkin-Huxley neurons, with synaptic connectivity derived from a computational model of axon growth. The model produced physiological swimming behaviour and was used to investigate which aspects of axon growth and neuron dynamics are behaviourally important. We found that the oscillatory attractor associated with swimming was remarkably stable, which suggests that, surprisingly, many features of axonal growth and synapse formation are not necessary for swimming to emerge. We also studied how the same spinal cord network can generate a different oscillatory pattern in which neurons on both sides of the body fire synchronously. Our results here suggest that under normal conditions the synchronous state is unstable or weakly stable, but that even small increases in spike transmission delays act to stabilise it. Finally, we found that although the basal ganglia and the tadpole spinal cord are very different systems, the underlying mechanism by which they can produce oscillations may be remarkably similar. Insights from the tadpole model allow us to predict how the basal ganglia model may be capable of producing multiple patterns of oscillatory activity

Mechanisms of Left-Right Coordination in Mammalian Locomotor Pattern Generation Circuits: A Mathematical Modeling View

The locomotor gait in limbed animals is defined by the left-right leg coordination and locomotor speed. Coordination between left and right neural activities in the spinal cord controlling left and right legs is provided by commissural interneurons (CINs). Several CIN types have been genetically identified, including the excitatory V3 and excitatory and inhibitory V0 types. Recent studies demonstrated that genetic elimination of all V0 CINs caused switching from a normal left-right alternating activity to a left-right synchronized “hopping” pattern. Furthermore, ablation of only the inhibitory V0 CINs (V0D subtype) resulted in a lack of left-right alternation at low locomotor frequencies and retaining this alternation at high frequencies, whereas selective ablation of the excitatory V0 neurons (V0V subtype) maintained the left–right alternation at low frequencies and switched to a hopping pattern at high frequencies. To analyze these findings, we developed a simplified mathematical model of neural circuits consisting of four pacemaker neurons representing left and right, flexor and extensor rhythm-generating centers interacting via commissural pathways representing V3, V0D, and V0V CINs. The locomotor frequency was controlled by a parameter defining the excitation of neurons and commissural pathways mimicking the effects of N-methyl-D-aspartate on locomotor frequency in isolated rodent spinal cord preparations. The model demonstrated a typical left-right alternating pattern under control conditions, switching to a hopping activity at any frequency after removing both V0 connections, a synchronized pattern at low frequencies with alternation at high frequencies after removing only V0D connections, and an alternating pattern at low frequencies with hopping at high frequencies after removing only V0V connections. We used bifurcation theory and fast-slow decomposition methods to analyze network behavior in the above regimes and transitions between them. The model reproduced, and suggested explanation for, a series of experimental phenomena and generated predictions available for experimental testing

Multilevel Analysis of Locomotion in Immature Preparations Suggests Innovative Strategies to Reactivate Stepping after Spinal Cord Injury

Locomotion is one of the most complex motor behaviors. Locomotor patterns change during early life, reflecting development of numerous peripheral and hierarchically organized central structures. Among them, the spinal cord is of particular interest since it houses the central pattern generator (CPG) for locomotion. This main command center is capable of eliciting and coordinating complex series of rhythmic neural signals sent to motoneurons and to corresponding target-muscles for basic locomotor activity. For a long-time, the CPG has been considered a black box. In recent years, complementary insights from in vitro and in vivo animal models have contributed significantly to a better understanding of its constituents, properties and ways to recover locomotion after a spinal cord injury (SCI). This review discusses key findings made by comparing the results of in vitro isolated spinal cord preparations and spinal-transected in vivo models from neonatal animals. Pharmacological, electrical, and sensory stimulation approaches largely used to further understand CPG function may also soon become therapeutic tools for potent CPG reactivation and locomotor movement induction in persons with SCI or developmental neuromuscular disorder

Optogenetic control of spinal microcircuits : insights into locomotor rhythm and pattern generation

Mammalian locomotion is a complex task in which hundreds of muscles work together in a coordinated fashion. Neural networks in the spinal cord, known as central pattern generators (CPGs), carry all the components necessary to produce the cyclical pattern of muscle activity needed for locomotion. The fact that the locomotor CPG is innate and highly localized makes it outstanding as a subject to study how a complex, but concrete behavior, is produced by a neuronal network. Two fundamental aspects of the CPG are rhythm generation, and flexor-extensor coordination. These two properties have sometimes been linked together, such as in the half-center model, in which the alternating activity between flexors and extensors are the cause of the rhythm. Other models for rhythm generation have also been postulated, consequently no consensus exists regarding the overall structure of the CPG for locomotion. Pharmacological investigations have indicated that glutamatergic neurons as essential for locomotion. To further elucidate the function of these neurons, the work presented in this thesis has made use of a set of new tools to target glutamatergic neurons, and elucidate their specific contribution to locomotion. A mouse was produced that expressed the optically gated ion channel Channelrhodopsin-2, making it possible for the first time to selectively activate a genetically specific sub-population of neurons in the spinal cord. The experiments using this mouse show that glutamatergic neuron activation is sufficient to produce locomotor-like activity, both in the spinal cord, and in the hindbrain. With the use of another set of recently produced transgenic mice, it was possible to probe deeper into the structure of the CPG, and illuminate several key aspects of the organization of the network. Several proposed network models could be refuted and one in particular was promoted. The results show that the CPG network is build up from intrinsically rhythmic modules. Furthermore, a mouse without glutamatergic neurotransmission was examined. What was found was that the locomotion deficient mouse could produce locomotor-like activity under special conditions, and this activity depended solely on inhibitory interneurons, specifically, reciprocally connected Ia interneurons. Overall, glutamatergic neurons are shown to form intrinsically rhythmic modules that are indispensable for rhythm generation, and network function. The use of genetics and electrophysiology is a powerful combination that will continue to provide conclusions about how neural networks produce and control complex motor behavior

Biological Pattern Generation: The Cellular and Computational Logic of Networks in Motion

In 1900, Ramón y Cajal advanced the neuron doctrine, defining the neuron as the fundamental signaling unit of the nervous system. Over a century later, neurobiologists address the circuit doctrine: the logic of the core units of neuronal circuitry that control animal behavior. These are circuits that can be called into action for perceptual, conceptual, and motor tasks, and we now need to understand whether there are coherent and overriding principles that govern the design and function of these modules. The discovery of central motor programs has provided crucial insight into the logic of one prototypic set of neural circuits: those that generate motor patterns. In this review, I discuss the mode of operation of these pattern generator networks and consider the neural mechanisms through which they are selected and activated. In addition, I will outline the utility of computational models in analysis of the dynamic actions of these motor networks

Contributions of h- and Na+ /K+ pump currents to the generation of episodic and continuous rhythmic activities

Authors acknowledge studentships from the Natural Sciences and Engineering Research Council of Canada (NSERC-PGS-D: SS); Alberta Innovates (AIHS: SS and AL); Hotchkiss Brain Institute (SS and AL); and the Faculty of Veterinary Medicine (LY). This research was supported by grants from the Canadian Institute of Health Research (PW); an NSERC Discovery grant (PW); and National Institutes of Health, National Institute of Neurological Disorders and Stroke 1 R21 NS111355 (GC and Ronald L. Calabrese).Developing spinal motor networks produce a diverse array of outputs, including episodic and continuous patterns of rhythmic activity. Variation in excitability state and neuromodulatory tone can facilitate transitions between episodic and continuous rhythms; however, the intrinsic mechanisms that govern these rhythms and their transitions are poorly understood. Here, we tested the capacity of a single central pattern generator (CPG) circuit with tunable properties to generate multiple outputs. To address this, we deployed a computational model composed of an inhibitory half-center oscillator (HCO). Following predictions of our computational model, we tested the contributions of key properties to the generation of an episodic rhythm produced by isolated spinal cords of the newborn mouse. The model recapitulates the diverse state-dependent rhythms evoked by dopamine. In the model, episodic bursting depended predominantly on the endogenous oscillatory properties of neurons, with Na+/K+ ATPase pump (IPump) and hyperpolarization-activated currents (Ih) playing key roles. Modulation of either IPump or Ih produced transitions between episodic and continuous rhythms and silence. As maximal activity of IPump decreased, the interepisode interval and period increased along with a reduction in episode duration. Decreasing maximal conductance of Ih decreased episode duration and increased interepisode interval. Pharmacological manipulations of Ih with ivabradine, and IPump with ouabain or monensin in isolated spinal cords produced findings consistent with the model. Our modeling and experimental results highlight key roles of Ih and IPump in producing episodic rhythms and provide insight into mechanisms that permit a single CPG to produce multiple patterns of rhythmicity.Publisher PDFPeer reviewe

Flexor Dysfunction Following Unilateral Transient Ischemic Brain Injury Is Associated with Impaired Locomotor Rhythmicity

Functional motor deficits in hemiplegia after stroke are predominately associated with flexor muscle impairments in animal models of ischemic brain injury, as well as in clinical findings. Rehabilitative interventions often employ various means of retraining a maladapted central pattern generator for locomotion. Yet, holistic modeling of the central pattern generator, as well as applications of such studies, are currently scarce. Most modeling studies rely on cellular neural models of the intrinsic spinal connectivity governing ipsilateral flexor-extensor, as well as contralateral coupling inherent in the spinal cord. Models that attempt to capture the general behavior of motor neuronal populations, as well as the different modes of driving their oscillatory function in vivo is lacking in contemporary literature. This study aims at generating a holistic model of flexor and extensor function as a whole, and seeks to evaluate the parametric coupling of ipsilateral and contralateral half-center coupling through the means of generating an ordinary differential equation representative of asymmetric central pattern generator models of varying coupling architectures. The results of this study suggest that the mathematical predictions of the locomotor centers which drive the dorsiflexion phase of locomotion are correlated with the denervation-type atrophy response of hemiparetic dorsiflexor muscles, as well as their spatiotemporal activity dysfunction during in vivo locomotion on a novel precise foot placement task. Moreover, the hemiplegic solutions were found to lie in proximity to an alternative task-space solution, by which a hemiplegic strategy could be readapted in order to produce healthy output. The results revealed that there are multiple strategies of retraining hemiplegic solutions of the CPG. This solution may modify the hemiparetic locomotor pattern into a healthy output by manipulating inter-integrator couplings which are not damaged by damage to the descending drives. Ultimately, some modeling experiments will demonstrate that the increased reliance on intrinsic connectivity increases the stability of the output, rendering it resistant to perturbations originating from extrinsic inputs to the pattern generating center

A common role for astrocytes in rhythmic behaviours?

Authors acknowledge the Motor Neurone Disease (MND) Association UK (Miles/Apr18/863-791) and the Biotechnology and Biological Sciences Research Council (BBSRC; BB/M021793/1) for their funding and support.Astrocytes are a functionally diverse form of glial cell involved in various aspects of nervous system infrastructure, from the metabolic and structural support of neurons to direct neuromodulation of synaptic activity. Investigating how astrocytes behave in functionally related circuits may help us understand whether there is any conserved logic to the role of astrocytes within neuronal networks. Astrocytes are implicated as key neuromodulatory cells within neural circuits that control a number of rhythmic behaviours such as breathing, locomotion and circadian sleep-wake cycles. In this review, we examine the evidence that astrocytes are directly involved in the regulation of the neural circuits underlying six different rhythmic behaviours: locomotion, breathing, chewing, gastrointestinal motility, circadian sleep-wake cycles and oscillatory feeding behaviour. We discuss how astrocytes are integrated into the neuronal networks that regulate these behaviours, and identify the potential gliotransmission signalling mechanisms involved. From reviewing the evidence of astrocytic involvement in a range of rhythmic behaviours, we reveal a heterogenous array of gliotransmission mechanisms, which help to regulate neuronal networks. However, we also observe an intriguing thread of commonality, in the form of purinergic gliotransmission, which is frequently utilised to facilitate feedback inhibition within rhythmic networks to constrain a given behaviour within its operational range.PostprintPeer reviewe

Defining the impact of clinically modeled hindlimb stretching, exercise, & inactivity on functional recovery after spinal cord injury.

Spinal cord injury (SCI) is a devastating, life altering event that affects approximately 282,000 Americans. The most obvious side effect of SCI is paralysis due to damage to the spinal cord that disrupts ascending and descending pathways as well as central pattern generating circuitry. In addition to paralysis, patients suffer from other debilitating side effects including altered cardiovascular function, autonomic dysreflexia, neuropathic pain, spasticity, and contractures. In contrast to humans, rodents display spontaneous locomotor recovery following incomplete SCI due to in-cage activity/training. Previously, our laboratory has studied the effect of lack of in-cage training by utilizing custom designed rodent wheelchairs. The immobilized SCI animals had poor locomotor function and developed muscle contractures. Additional work by our lab was done to help alleviate the contractures by using clinically-modeled hindlimb stretching. It was found that clinically modeled stretching of rats with a thoracic SCI does not prevent contractures and surprisingly, causes a dramatic decrease in locomotor function that can persist even after stretching is stopped. Most recently, it has been discovered by our lab that stretching is dependent upon the presence of C-fibers (nociceptive afferents), as injured, stretched animals depleted of TRPV1+ C-fibers do not experience such dramatic detriments to their locomotor recovery. Increased sprouting of these nociceptive afferents occurs spontaneously after injury and has been associated with a myriad of other issues, such as autonomic dysreflexia and neuropathic pain. However, recent work has shown that nociceptive afferent sprouting can be prevented or reduced with increased activity and exercise. These findings are significant because stretch-based physical therapy is the most common approach for treating spasticity, contractures, and combating muscle atrophy after spinal cord injury in patients. The work presented in this dissertation aims to clarify the potential mechanisms for stretch-induced locomotor dysfunction in rodent models as well as provide rationale for future clinical and translational research that will be able to determine whether stretching has a negative impact in humans post-SCI. The following experiments revealed that the additional of applied exercise to the stretching protocol does not prevent locomotor dysfunction or the sprouting of nociceptive afferents. We also discovered that stretching animals with high thoracic contusion injuries similarly causes a drastic drop in locomotion, but with some key differences in ability to recover locomotor ability. Our studies suggest that stretching is likely maladaptive for functional locomotor recovery after SCI regardless of injury location or activity status