Chronic exercise increases excitability of lamina X neurons through enhancement of persistent inward currents and dendritic development in mice

KEY POINTS

Chronic exercise alters adaptability of spinal motor system in rodents. Multiple mechanisms are responsible for the adaptation, including regulation of neuronal excitability and change in dendritic morphology. Spinal interneurons in lamina X are a cluster of heterogeneous neurons playing multifunctional roles in the spinal cord, especially in regulating locomotor activity. Chronic exercise in juvenile mice increased excitability of these interneurons and facilitated dendritic development. Mechanisms underlying these changes remain unknown. Lamina X neurons expressed persistent inward currents (PICs) composed of calcium (Ca-PIC) and sodium (Na-PIC) components. The exercise-increased excitability of lamina X neurons was mediated by enhancing Ca-PIC and Na-PIC components and facilitating dendritic length. Na-PIC contributed more to lowering of PIC onset and Ca-PIC to increase of PIC amplitude. This study unveiled novel morphological and ionic mechanisms underlying adaptation of lamina X neurons in rodents during chronic exercise.

ABSTRACT

Chronic exercise has been shown to enhance excitability of spinal interneurons in rodents. However, the mechanisms underlying this enhancement remain unclear. In this study we investigated adaptability of lamina X neurons with three-week treadmill exercise in mice of P21-P24. Whole-cell path-clamp recording was performed on the interneurons from slices of T12-L4. The experimental results included: (1) Treadmill exercise reduced rheobase by 7.4±2.2 pA (control: 11.3±6.1 pA, n = 12; exercise: 3.8±4.6 pA, n = 13; P = 0.002) and hyperpolarized voltage threshold by 7.1±1.5 mV (control: -36.6±4.6 mV, exercise: -43.7±2.7 mV; P = 0.001). (2) Exercise enhanced persistent inward currents (PICs) with increase of amplitude (control: 140.6±56.3 pA, n = 25; exercise: 225.9±62.5 pA, n = 17; P = 0.001) and hyperpolarization of onset (control: -50.3±3.6 mV, exercise: -56.5±5.5 mV; P = 0.001). (3) PICs consisted of dihydropyridine-sensitive calcium (Ca-PIC) and tetrodotoxin-sensitive sodium (Na-PIC) components. Exercise increased amplitude of both components but hyperpolarized onset of Na-PIC only. (4) Exercise reduced derecruitment current of repetitive firing evoked by current bi-ramp and prolonged firing in falling phase of the bi-ramp. The derecruitment reduction was eliminated by bath application of 3 μM riluzole or 25 μM nimodipine, suggesting that both Na-PIC and Ca-PIC contributed to the exercise-prolonged hysteresis of firing. (5) Exercise facilitated dendritic development with significant increase in dendritic length by 285.1±113 μm (control: 457.8±171.8 μm, n = 12; exercise: 742.9±357 μm, n = 14; P = 0.019). We concluded that three-week treadmill exercise increased excitability of lamina X interneurons through enhancement of PICs and increase of dendritic length. This study provided insight into cellular and channel mechanisms underlying adaptation of the spinal motor system in exercise. Abstract figure legend A. B6 mice were randomly divided into control group and exercise group. Control group mice remained sedentary in the cage; exercise group mice completed 60 min treadmill runs each day (6 days/week) for a period of 3 weeks. B. Whole-cell patch clamp recordings were made from lumbar lamina X neurons after three-weeks exercise. C. Exercise facilitated development of dendrites of lamina X neurons. D. Exercise enhanced persistent inward currents. E. Exercise increased excitability of lamina X neurons by hyperpolarizing voltage threshold for action potential generation. This article is protected by copyright. All rights reserved.

V3 Interneurons Are Active and Recruit Spinal Motor Neurons during In Vivo Fictive Swimming in Larval Zebrafish

Survival for vertebrate animals is dependent on the ability to successfully find food, locate a mate, and avoid predation. Each of these behaviors requires motor control, which is set by a combination of kinematic properties. For example, the frequency and amplitude of motor output combine in a multiplicative manner to determine features of locomotion such as distance traveled, speed, force (thrust), and vigor. Although there is a good understanding of how different populations of excitatory spinal interneurons establish locomotor frequency, there is a less thorough mechanistic understanding for how locomotor amplitude is established. Recent evidence indicates that locomotor amplitude is regulated in part by a subset of functionally and morphologically distinct V2a excitatory spinal interneurons (Type II, nonbursting) in larval and adult zebrafish. Here, we provide direct evidence that most V3 interneurons (V3-INs), which are a developmentally and genetically defined population of ventromedial glutamatergic spinal neurons, are active during fictive swimming. We also show that elimination of the spinal V3-IN population reduces the proportion of active motor neurons (MNs) during fictive swimming but does not alter the range of locomotor frequencies produced. These data are consistent with V3-INs providing excitatory drive to spinal MNs during swimming in larval zebrafish and may contribute to the production of locomotor amplitude independently of locomotor frequency.

Zebrafish Paralogs brd2a and brd2b Are Needed for Proper Circulatory, Excretory and Central Nervous System Formation and Act as Genetic Antagonists during Development

Brd2 belongs to the BET family of epigenetic transcriptional co-regulators that act as adaptor-scaffolds for the assembly of chromatin-modifying complexes and other factors at target gene promoters. Brd2 is a protooncogene and candidate gene for juvenile myoclonic epilepsy in humans, a homeobox gene regulator in Drosophila, and a maternal-zygotic factor and cell death modulator that is necessary for normal development of the vertebrate central nervous system (CNS). As two copies of Brd2 exist in zebrafish, we use antisense morpholino knockdown to probe the role of paralog Brd2b, as a comparative study to Brd2a, the ortholog of human Brd2. A deficiency in either paralog results in excess cell death and dysmorphology of the CNS, whereas only Brd2b deficiency leads to loss of circulation and occlusion of the pronephric duct. Co-knockdown of both paralogs suppresses single morphant defects, while co-injection of morpholinos with paralogous RNA enhances them, suggesting novel genetic interaction with functional antagonism. Brd2 diversification includes paralog-specific RNA variants, a distinct localization of maternal factors, and shared and unique spatiotemporal expression, providing unique insight into the evolution and potential functions of this gene.

Corticospinal Circuits from the Sensory and Motor Cortices Differentially Regulate Skilled Movements through Distinct Spinal Interneurons

Little is known about the organizational and functional connectivity of the corticospinal (CS) circuits that are essential for voluntary movement. Here, we map the connectivity between CS neurons in the forelimb motor and sensory cortices and various spinal interneurons, demonstrating that distinct CS-interneuron circuits control specific aspects of skilled movements. CS fibers originating in the mouse motor cortex directly synapse onto premotor interneurons, including those expressing Chx10. Lesions of the motor cortex or silencing of spinal Chx10+ interneurons produces deficits in skilled reaching. In contrast, CS neurons in the sensory cortex do not synapse directly onto premotor interneurons, and they preferentially connect to Vglut3+ spinal interneurons. Lesions to the sensory cortex or inhibition of Vglut3+ interneurons cause deficits in food pellet release movements in goal-oriented tasks. These findings reveal that CS neurons in the motor and sensory cortices differentially control skilled movements through distinct CS-spinal interneuron circuits.

Cell-Type Dependent Effect of Surface-Patterned Microdot Arrays on Neuronal Growth

Surface micropatterns have been widely used as chemical cues to control the microenvironment of cultured neurons, particularly for neurobiological assays and neurochip designs. However, the cell-type dependency on the interactions between neurons and underlying micropatterns has been rarely investigated despite the inherent differences in the morphology of neuronal types. In this study, we used surface-printed microdot arrays to investigate the effect of the same micropatterns on the growth of mouse spinal interneuron, mouse hippocampal neurons, and rat hippocampal neurons. While mouse hippocampal neurons showed no significantly different growth on control and patterned substrates, we found the microdot arrays had different effects on early neuronal growth depending on the cell type; spinal interneurons tended to grow faster in length, whereas hippocampal neurons tended to form more axon collateral branches in response to the microdot arrays. Although there was a similar trend in the neurite length and branch number of both neurons changed across the microdot arrays with the expanded range of size and spacing, the dominant responses of each neuron, neurite elongation of mouse spinal interneurons and branching augmentation of rat hippocampal neurons were still preserved. Therefore, our results demonstrate that the same design of micropatterns could cause different neuronal growth results, raising an intriguing issue of considering cell types in neural interface designs.

Motoneuron-derived neurotrophin-3 is a survival factor for PAX2-expressing spinal interneurons

Rat spinal cord interneurons undergo programmed cell death shortly after birth. We investigated here whether cell death of interneurons could be regulated by trophic factors produced by motoneurons, one of their main targets. To test this hypothesis, we studied the effect of the selective destruction of motoneurons on the survival of interneurons in organotypic cultures of embryonic rat spinal cords. Motoneurons were eliminated by an anti-p75(NTR)-specific immunotoxin (192 IgG-saporin). We then observed a decrease of 28% in the number of ventral spinal interneurons immunoreactive (IR) for the homeoprotein PAX2. This was correlated with an increase in the number of apoptotic nuclei in the same area. Because neurotrophin-3 (NT-3) is specifically produced by motoneurons and because interneurons express the NT-3 high-affinity receptor trkC, we examined the role of NT-3 in the survival of PAX2-IR interneurons. Addition of NT-3 to 192 IgG-saporin-treated explants rescued ventral PAX2-IR interneurons. Depletion of secreted NT-3 by anti-NT-3 antibodies induced 66% loss of ventral PAX2-IR interneurons. We conclude that motoneuron-derived NT-3 is a trophic factor for ventral PAX2-IR interneurons.

Identification of an interneuronal population that mediates recurrent inhibition of motoneurons in the developing chick spinal cord

Studies on the development of synaptic specificity, embryonic activity, and neuronal specification in the spinal cord have all been limited by the absence of a functionally identified interneuron class (defined by its unique set of connections). Here, we identify an interneuron population in the embryonic chick spinal cord that appears to be the avian equivalent of the mammalian Renshaw cell (R-interneurons). These cells receive monosynaptic nicotinic, cholinergic input from motoneuron recurrent collaterals. They make predominately GABAergic connections back onto motoneurons and to other R-interneurons but project rarely to other spinal interneurons. The similarity between the connections of the developing R-interneuron, shortly after circuit formation, and the mature mammalian Renshaw cell raises the possibility that R-interneuronal connections are formed precisely from the onset. Using a newly developed optical approach, we identified the location of R-interneurons in a column, dorsomedial to the motor nucleus. Functional characterization of the R-interneuron population provides the basis for analyses that have so far only been possible for motoneurons.

Development of an identified spinal commissural interneuron population in an amniote: neurons of the avian Hofmann nuclei

The commissural interneurons of the Hofmann nuclei (HN) of the avian spinal cord (The axonal projections of the Hofmann nuclei in the spinal cord of the late stage chicken embryo, Anat Embryol (Berl), A.L. Eide, 1996, Vol 193, pp 543-557) provide a unique opportunity to describe the development of an identified spinal commissural axon projection and its terminal collaterals in an amniote vertebrate. Here, we use the lipophilic tracer Dil to label these and other commissural projections anterogradely and retrogradely from the time the HN neurons are born. [3H]thymidine birthdating shows that the final mitoses of HN neurons occur at stages 21-24 [developmental day (d) 4]. By direct comparison, this follows the generation of motoneurons and of large, dorsally located commissural interneurons. The first HN neurons reach the ventrolateral margin of the spinal cord by d6 by a radial migration through the ventral horn. Radial migration occurs after the extension of HN axons across the midline. Thus, HN neurons are determined to be commissural interneurons before attaining their definitive locations. The HN neurons subsequently aggregate into segmentally iterated clusters at the ventrolateral margin of the spinal cord by d8. Also by d8 their logitudinal axons attain mature extent in the ventral funiculus of the contralateral side and begin to sprout collaterals. The collaterals are directed predominantly toward the medial aspect of the ventral horn at all stages, forming by d12 a dense thicket of terminals that thins out over several segments to each side of the HN of origin. The initial direction of collateral outgrowth is largely appropriate for the mature termination pattern of the HN. Terminal arbors, however, are less focused at early developmental stages than at later stages.