Axonal projections of Renshaw cells in the thoracic spinal cord

SPINAL V1 INHIBITORY INTERNEURON CLADES DIFFER IN BIRTHDATE, PROJECTIONS TO MOTONEURONS AND HETEROGENEITY

Spinal cord interneurons play a crucial role in shaping motor output, but their precise identity and circuit connectivity remain unclear. Focusing on the cardinal class of inhibitory V1 interneurons, we define the diversity of four major V1 subsets according to timing of neurogenesis, genetic lineage-tracing, synaptic output to motoneurons, and synaptic inputs from muscle afferents. Birthdating delineates two early-born (Renshaw and Pou6f2) and two late-born V1 clades (Foxp2 and Sp8) suggesting sequential neurogenesis gives rise to different V1 clades. Neurogenesis did not correlate with motoneuron targeting. Early-born Renshaw cells and late-born Foxp2-V1 interneurons both tightly coupled to motoneurons, while early-born Pou6f2-V1 and late-born Sp8-V1 interneurons did not. V1-clades also greatly differ in cell numbers and diversity. Lineage labeling of the Foxp2-V1 clade shows it contains over half of all V1 interneurons and provides the largest inhibitory input to motoneuron cell bodies. Foxp2-V1 subgroups differ in neurogenesis and proprioceptive input. Notably, one subgroup defined by Otp expression and located adjacent to the lateral motor column exhibits substantial input from proprioceptors, consistent with some Foxp2-V1 cells at this location forming part of reciprocal inhibitory pathways. This was confirmed with viral tracing methods for ankle flexors and extensors. The results validate the previous V1 clade classification as representing unique interneuron subtypes that differ in circuit placement with Foxp2-V1s forming the more complex subgroup. We discuss how V1 organizational diversity enables understanding of their roles in motor control, with implications for the ontogenetic and phylogenetic origins of their diversity.

SIGNIFICANCE STATEMENT

Spinal interneuron diversity and circuit organization represents a key challenge to understand the neural control of movement in normal adults and also during motor development and in disease. Inhibitory interneurons are a core element of these spinal circuits, acting on motoneurons either directly or via premotor networks. V1 interneurons comprise the largest group of inhibitory interneurons in the ventral horn and their organization remains unclear. Here we present a comprehensive examination of V1 subtypes according to neurogenesis, placement in spinal motor circuits and motoneuron synaptic targeting. V1 diversity increases during evolution from axial-swimming fishes to limb-based mammalian terrestrial locomotion and this is reflected in the size and heterogeneity of the Foxp2-V1 clade which is closely associated to limb motor pools. We show Foxp2-V1 interneurons establish the densest and more direct inhibitory synaptic input to motoneurons, especially on cell bodies. This is of further importance because deficits on motoneuron cell body inhibitory V1 synapses and on Foxp2-V1 interneurons themselves have recently been shown to be affected at early stages of pathology in motor neurodegenerative diseases like amyotrophic lateral sclerosis.

Modeling Electric Fields in Transcutaneous Spinal Direct Current Stimulation: A Clinical Perspective

Clinical findings suggest that transcutaneous spinal direct current stimulation (tsDCS) can modulate ascending sensitive, descending corticospinal, and segmental pathways in the spinal cord (SC). However, several aspects of the stimulation have not been completely understood, and realistic computational models based on MRI are the gold standard to predict the interaction between tsDCS-induced electric fields and anatomy. Here, we review the electric fields distribution in the SC during tsDCS as predicted by MRI-based realistic models, compare such knowledge with clinical findings, and define the role of computational knowledge in optimizing tsDCS protocols. tsDCS-induced electric fields are predicted to be safe and induce both transient and neuroplastic changes. This could support the possibility to explore new clinical applications, such as spinal cord injury. For the most applied protocol (2-3 mA for 20-30 min, active electrode over T10-T12 and the reference on the right shoulder), similar electric field intensities are generated in both ventral and dorsal horns of the SC at the same height. This was confirmed by human studies, in which both motor and sensitive effects were found. Lastly, electric fields are strongly dependent on anatomy and electrodes' placement. Regardless of the montage, inter-individual hotspots of higher values of electric fields were predicted, which could change when the subjects move from a position to another (e.g., from the supine to the lateral position). These characteristics underlines the need for individualized and patient-tailored MRI-based computational models to optimize the stimulation protocol. A detailed modeling approach of the electric field distribution might contribute to optimizing stimulation protocols, tailoring electrodes' configuration, intensities, and duration to the clinical outcome.

Spinal V1 neurons inhibit motor targets locally and sensory targets distally

Rostro-caudal coordination of spinal motor output is essential for locomotion. Most spinal interneurons project axons longitudinally to govern locomotor output, yet their connectivity along this axis remains unclear. In this study, we use larval zebrafish to map synaptic outputs of a major inhibitory population, V1 (Eng1⁺) neurons, which are implicated in dual sensory and motor functions. We find that V1 neurons exhibit long axons extending rostrally and exclusively ipsilaterally for an average of 6 spinal segments; however, they do not connect uniformly with their post-synaptic targets along the entire length of their axon. Locally, V1 neurons inhibit motor neurons (both fast and slow) and other premotor targets, including V2a, V2b, and commissural premotor neurons. In contrast, V1 neurons make robust long-range inhibitory contacts onto a dorsal horn sensory population, the commissural primary ascending neurons (CoPAs). In a computational model of the ipsilateral spinal network, we show that this pattern of short-range V1 inhibition to motor and premotor neurons underlies burst termination, which is critical for coordinated rostro-caudal propagation of the locomotor wave. We conclude that spinal network architecture in the longitudinal axis can vary dramatically, with differentially targeted local and distal connections, yielding important consequences for function.

Cell type and circuit modules in the spinal cord

The spinal cord contains an extraordinarily diverse population of interconnected neurons to process somatosensory information and execute movement. Studies of the embryonic spinal cord have elucidated basic principles underlying the specification of spinal cord neurons, while adult and postnatal studies have provided insight into cell type function and circuitry. However, the overarching principles that bridge molecularly defined subtypes with their connectivity, physiology, and function remain unclear. This review consolidates recent work in spinal neuron characterization, examining how molecular and spatial features of individual spinal neuron types relate to the reference points of connectivity and function. This review will focus on how spinal neuron subtypes are organized to control movement in the mouse.

Inhibitory Thoracic Interneurons are not Essential to Generate the Rostro-caudal Gradient of the Thoracic Inspiratory Motor Activity in Neonatal Rat

The inspiratory motor activities are greater in the intercostal muscles positioned at more rostral thoracic segments. This rostro-caudal gradient of the thoracic inspiratory motor activity is thought to be generated by the spinal interneurons. To clarify the involvement of the inhibitory thoracic interneurons in this rostro-caudal gradient, we examined the effects of 10 μM strychnine, an antagonist of glycine and GABA_A receptors, applied to the neonatal rat thoracic spinal cord. The respiratory-related interneuron activities were optically recorded from thoracic segments in the isolated neonatal rat brainstem-spinal cord preparations stained with voltage-sensitive dye, and the electrical inspiratory motor activities were obtained from the third and eleventh thoracic ventral roots (T3VR, T11VR). Although strychnine caused seizure-like activities in all of the ventral roots recorded, the inspiratory motor activities continued. The inspiratory optical signals in the rostral thoracic segments (T2-T5) were significantly larger than those in the caudal thoracic segments (T9-T11) regardless of the existence of strychnine. Similarly, the percent ratio of the amplitude of the inspiratory electrical activity in the T3VR under control and strychnine was significantly larger than that in the T11VR regardless of the existence of strychnine. Strychnine significantly increased the inspiratory activity in both the T3VR and T11VR. These results suggest that the glycinergic and GABAergic inhibitory interneurons are not essential to generate the rostro-caudal gradient in the neonatal rat thoracic inspiratory motor outputs, but these interneurons are likely to play a role in the inhibitory control of inspiratory motor output.

Expressions of VGLUT1/2 in the inspiratory interneurons and GAD65/67 in the inspiratory Renshaw cells in the neonatal rat upper thoracic spinal cord

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About half of the inspiratory interneurons in the ventromedial area of the third thoracic segment are glutamatergic.

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These glutamatergic interneurons may enhance the inspiratory intercostal motor activity.

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Inspiratory Renshaw cells exist in the ventromedial area of the third thoracic segments.

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Most of these Renshaw cells are GABAergic, and cause a single spike followed by ventral root stimulation at neonatal stage.

Although the inspiratory spinal interneurons are thought to provide a major fraction of the excitatory synaptic potentials to the inspiratory intercostal motoneurons, this has not been confirmed. To clarify whether some inspiratory spinal interneurons are glutamatergic, we obtained whole-cell recordings from the ventromedial area of the third thoracic segments in an isolated brainstem-spinal cord preparation from neonatal rat, and the recorded cells were filled with Lucifer Yellow for later visualization. We then examined the existence of mRNA of vesicular glutamate transporters 1 and/or 2 (VGLUT1/2) by performing in situ hybridization. To discriminate the interneurons from motoneurons, we electrically stimulated the third thoracic ventral root on the recorded side, and the results verified that the antidromic spike or excitatory postsynaptic potential was not evoked. In cases in which the ventral root stimulation evoked depolarizing postsynaptic potentials, we examined the existence of glutamic acid decarboxylase 65 and/or 67 (GAD65/67) mRNA using a mixed probe to verify whether the cell was truly a Renshaw cell. The long diameter of the recorded interneurons was 22 ± 8 μm; the short diameter was 13 ± 4 μm. The interneurons' input resistance was 598 ± 274 MΩ. The Renshaw cells had similar sizes and input resistance. Six of 11 interneurons expressed VGLUT1/2, and four of five Renshaw cells expressed GAD65/67. Our findings suggest that approximately one-half of the inspiratory interneurons in the ventromedial area of the neonatal rat thoracic spinal cord are glutamatergic, and these interneurons might enhance the inspiratory intercostal motor activity.

Escape from homeostasis: spinal microcircuits and progression of amyotrophic lateral sclerosis

In amyotrophic lateral sclerosis (ALS), loss of motoneuron function leads to weakness and, ultimately, respiratory failure and death. Regardless of the initial pathogenic factors, motoneuron loss follows a specific pattern: the largest α-motoneurons die before smaller α-motoneurons, and γ-motoneurons are spared. In this article, we examine how homeostatic responses to this orderly progression could lead to local microcircuit dysfunction that in turn propagates motoneuron dysfunction and death. We first review motoneuron diversity and the principle of α-γ coactivation and then discuss two specific spinal motoneuron microcircuits: those involving proprioceptive afferents and those involving Renshaw cells. Next, we propose that the overall homeostatic response of the nervous system is aimed at maintaining force output. Thus motoneuron degeneration would lead to an increase in inputs to motoneurons, and, because of the pattern of neuronal degeneration, would result in an imbalance in local microcircuit activity that would overwhelm initial homeostatic responses. We suggest that this activity would ultimately lead to excitotoxicity of motoneurons, which would hasten the progression of disease. Finally, we propose that should this be the case, new therapies targeted toward microcircuit dysfunction could slow the course of ALS.

Firing properties of Renshaw cells defined by Chrna2 are modulated by hyperpolarizing and small conductance ion currents Ih and ISK

Renshaw cells in the spinal cord ventral horn regulate motoneuron output through recurrent inhibition. Renshaw cells can be identified in vitro using anatomical and cellular criteria; however, their functional role in locomotion remains poorly defined because of the difficulty of functionally isolating Renshaw cells from surrounding motor circuits. Here we aimed to investigate whether the cholinergic nicotinic receptor alpha2 (Chrna2) can be used to identify Renshaw cells (RCs(α2)) in the mouse spinal cord. Immunohistochemistry and electrophysiological characterization of passive and active RCs(α2) properties confirmed that neurons genetically marked by the Chrna2-Cre mouse line together with a fluorescent reporter mouse line are Renshaw cells. Whole-cell patch-clamp recordings revealed that RCs(α2) constitute an electrophysiologically stereotyped population with a resting membrane potential of -50.5 ± 0.4 mV and an input resistance of 233.1 ± 11 MΩ. We identified a ZD7288-sensitive hyperpolarization-activated cation current (Ih) in all RCs(α2), contributing to membrane repolarization but not to the resting membrane potential in neonatal mice. Additionally, we found RCs(α2) to express small calcium-activated potassium currents (I(SK)) that, when blocked by apamin, resulted in a complete attenuation of the afterhyperpolarisation potential, increasing cellular firing frequency. We conclude that RCs(α2) can be genetically targeted through their selective Chrna2 expression and that they display currents known to modulate rebound excitation and firing frequency. The genetic identification of Renshaw cells and their electrophysiological profile is required for genetic and pharmacological manipulation as well as computational simulations with the aim to understand their functional role.