Locomotor rhythm maintenance: electrical coupling among premotor excitatory interneurons in the brainstem and spinal cord of young Xenopus tadpoles

Specific brainstem neurons switch each other into pacemaker mode to drive movement by activating NMDA receptors

Rhythmic activity is central to brain function. In the vertebrate CNS, the neuronal circuits for breathing and locomotion involve inhibition and also neurons acting as pacemakers, but identifying the neurons responsible has proven difficult. By studying simple hatchling Xenopus laevis tadpoles, we have already identified a population of electrically coupled hindbrain neurons (dINs) that drive swimming. During rhythm generation, dINs release glutamate to excite each other and activate NMDA receptors (NMDARs). The resulting depolarization enables a network mechanism for swimming rhythm generation that depends on reciprocal inhibition between antagonistic right and left sides. Surprisingly, a surgically isolated hemi-CNS without inhibition can still generate swimming-like rhythms. We have now discovered that activation of NMDARs transforms dINs, which normally fire singly to current injection, into pacemakers firing within the normal swimming frequency range (10-25 Hz). When dIN firing is blocked pharmacologically, this NMDAR activation produces 10 Hz membrane potential oscillations that persist when electrical coupling is blocked but not when the voltage-dependent gating of NMDARs by Mg²+ is removed. The NMDA-induced oscillations and pacemaker firing at swimming frequency are unique to the dIN population and do not occur in other spinal neurons. We conclude that NMDAR-mediated self-resetting switches critical neurons that drive swimming into pacemaker mode only during locomotion where it provides an additional, parallel mechanism for rhythm generation. This allows rhythm generation in a half-CNS and raises the possibility that such concealed pacemaker properties may be present underlying rhythm generation in other vertebrate brain networks.

Chapter 11--novel mechanism for hyperreflexia and spasticity

We established that hyperreflexia is delayed after spinal transection in the adult rat and that passive exercise could normalize low frequency-dependent depression of the H-reflex. We were also able to show that such passive exercise will normalize hyperreflexia in patients with spinal cord injury (SCI). Recent results demonstrate that spinal transection results in changes in the neuronal gap junction protein connexin 36 below the level of the lesion. Moreover, a drug known to increase electrical coupling was found to normalize hyperreflexia in the absence of passive exercise, suggesting that changes in electrical coupling may be involved in hyperreflexia. We also present results showing that a measure of spasticity, the stretch reflex, is rendered abnormal by transection and normalized by the same drug. These data suggest that electrical coupling may be dysregulated in SCI, leading to some of the symptoms observed. A novel therapy for hyperreflexia and spasticity may require modulation of electrical coupling.

Segmental oscillators in axial motor circuits of the salamander: distribution and bursting mechanisms

The rhythmic and coordinated activation of axial muscles that underlie trunk movements during locomotion are generated by specialized networks in the spinal cord. The operation of these networks has been extensively investigated in limbless swimming vertebrates. But little is known about the architecture and functioning of the axial locomotor networks in limbed vertebrates. We investigated the rhythm-generating capacity of the axial segmental networks in the salamander (Pleurodeles waltlii). We recorded ventral root activity from hemisegments and segments that were surgically isolated from the mid-trunk cord and chemically activated with bath-applied N-methyl-d-aspartate (NMDA). We provide evidence that the rhythmogenic capacity of the axial network is distributed along the mid-trunk spinal cord without an excitability gradient. We demonstrate that the burst generation in a hemisegment depends on glutamatergic excitatory interactions. Reciprocal glycinergic inhibition between opposite hemisegments ensures left-right alternation and lowers the rhythm frequency in segments. Our results further suggest that persistent sodium current contributes to the rhythmic regenerating process both in hemisegments and segments. Burst termination in hemisegments is not achieved through the activation of apamine-sensitive Ca(2+)-activated K(+) channels and burst termination in segments relies on crossed glycinergic inhibition. Together our results indicate that the basic design of the salamander axial network is similar to most of axial networks investigated in other vertebrates, albeit with some significant differences in the cellular mechanism that underlies segmental bursting. This finding supports the view of a phylogenetic conservation of basic building blocks of the axial locomotor network among the vertebrates.

Functional organization of V2a-related locomotor circuits in the rodent spinal cord

Studies of mammalian locomotion have been greatly facilitated by the use of the isolated rodent spinal cord preparation that retains the locomotor circuits needed to execute the movement. Physiological and molecular genetic experiments in this preparation have started to unravel the basic circuit organization responsible for walking in mammals. Here, we review these experiments with a focus on the functional role of excitatory V2a interneurons in the mammalian locomotor network. With regard to these neurons and other network structures we also discuss similarities and differences between the mammalian walking central pattern generator (CPG) and the fish swimming CPG.

Roles for multifunctional and specialized spinal interneurons during motor pattern generation in tadpoles, zebrafish larvae, and turtles

The hindbrain and spinal cord can produce multiple forms of locomotion, escape, and withdrawal behaviors and (in limbed vertebrates) site-specific scratching. Until recently, the prevailing view was that the same classes of central nervous system neurons generate multiple kinds of movements, either through reconfiguration of a single, shared network or through an increase in the number of neurons recruited within each class. The mechanisms involved in selecting and generating different motor patterns have recently been explored in detail in some non-mammalian, vertebrate model systems. Work on the hatchling Xenopus tadpole, the larval zebrafish, and the adult turtle has now revealed that distinct kinds of motor patterns are actually selected and generated by combinations of multifunctional and specialized spinal interneurons. Multifunctional interneurons may form a core, multipurpose circuit that generates elements of coordinated motor output utilized in multiple behaviors, such as left-right alternation. But, in addition, specialized spinal interneurons including separate glutamatergic and glycinergic classes are selectively activated during specific patterns: escape-withdrawal, swimming and struggling in tadpoles and zebrafish, and limb withdrawal and scratching in turtles. These specialized neurons can contribute by changing the way central pattern generator (CPG) activity is initiated and by altering CPG composition and operation. The combined use of multifunctional and specialized neurons is now established as a principle of organization across a range of vertebrates. Future research may reveal common patterns of multifunctionality and specialization among interneurons controlling diverse movements and whether similar mechanisms exist in higher-order brain circuits that select among a wider array of complex movements.

How neurons generate behavior in a hatchling amphibian tadpole: an outline

Adult nervous systems are so complex that understanding how they produce behavior remains a real challenge. We chose to study hatchling Xenopus tadpoles where behavior is controlled by a few thousand neurons but there is a very limited number of types of neuron. Young tadpoles can flex, swim away, adjust their trajectory, speed-up and slow-down, stop when they contact support and struggle when grasped. They are sensitive to touch, pressure, noxious stimuli, light intensity and water currents. Using whole-cell recording has led to rapid progress in understanding central networks controlling behavior. Our methods are illustrated by an analysis of the flexion reflex to skin touch. We then define the seven types of neuron that allow the tadpole to swim when the skin is touched and use paired recordings to investigate neuron properties, synaptic connections and activity patterns. Proposals on how the swim network operates are evaluated by experiment and network modeling. We then examine GABAergic inhibitory pathways that control swimming but also produce tonic inhibition to reduce responsiveness when the tadpole is at rest. Finally, we analyze the strong alternating struggling movements the tadpole makes when grasped. We show that the mechanisms for rhythm generation here are very different to those during swimming. Although much remains to be explained, study of this simple vertebrate has uncovered basic principles about the function and organization of vertebrate nervous systems.

From circuits to behaviour: motor networks in vertebrates

Neural networks in the hindbrain and spinal cord generate the simple patterns of motor activity that are necessary for breathing and locomotion. These networks function autonomously, producing simple yet flexible rhythmic motor behaviours that are highly responsive to sensory inputs and central control. This review outlines recent advances in our understanding of the genetic programmes controlling the assembly and functioning of circuits in the hindbrain and spinal cord that are responsible for respiration and locomotion. In addition, we highlight the influence that target-derived retrograde signaling and experience-dependent mechanisms have on establishing connectivity, particularly with respect to sensory afferent innervation of the spinal cord.

Firing and cellular properties of V2a interneurons in the rodent spinal cord

Previous studies have shown that a group of ventrally located neurons, designated V2a interneurons, play a key role in maintaining locomotor rhythmicity and in ensuring appropriate left-right alternation during locomotion (Crone et al., 2008, 2009). These V2a interneurons express the transcription factor Chx10. The aim of the present study was to characterize the locomotor-related activity of individual V2a interneurons, their cellular properties, and their detailed anatomical attributes in Chx10-GFP mice. A dorsal horn-removed preparation was developed to allow for visual whole-cell patch recordings from V2a interneurons along the entire lumbar spinal cord while at the same time leaving enough of the spinal cord intact to generate fictive locomotion. During drug-evoked locomotor-like activity, a large proportion of Chx10 cells showed rhythmic firing or membrane potential fluctuations related to either flexor or extensor activity in every lumbar segment. Chx10 cells received predominantly rhythmic excitatory input. Chx10 neurons displayed a wide variety of firing and potential rhythmogenic properties. However, none of these properties was obviously related to the observed rhythmicity during locomotor-like activity. In dual recordings, we found no evidence of Chx10 neuron interconnectivity. Intracellular fills revealed diverse projection patterns with most Chx10 interneurons being local with projections to the central pattern generator and motor neuron regions of the spinal cord and others with long ascending and/or descending branches. These data are compatible with V2a neurons having a role in regulating segmental left-right alternation and ipsilateral motor neuron firing with little effect on rhythm generation.

Rhythmogenesis in axial locomotor networks: an interspecies comparison

During locomotion, specialized neural networks referred to as "central pattern generators" ensure precise temporal relations between the axial segments, both in limbed and limbless vertebrates. These neural networks are intrinsically capable of generating coordinated patterns of rhythmic activity in the absence of sensory feedback or descending command from higher brain centers. Rhythmogenesis in these neural circuits lies on several mechanisms, both at the cellular and the network levels. In this chapter, we compare the anatomical organization of the axial networks, the role of identified spinal neurons, and their interactions in rhythmogenesis in four species: lamprey, zebrafish, Xenopus tadpole, and salamander. The comparison suggests that several principles in axial network design are phylogenetically conserved among vertebrates.

Glutamate drives the touch response through a rostral loop in the spinal cord of zebrafish embryos

Characterizing connectivity in the spinal cord of zebrafish embryos is not only prerequisite to understanding the development of locomotion, but is also necessary for maximizing the potential of genetic studies of circuit formation in this model system. During their first day of development, zebrafish embryos show two simple motor behaviors. First, they coil their trunks spontaneously, and a few hours later they start responding to touch with contralateral coils. These behaviors are contemporaneous until spontaneous coils become infrequent by 30 h. Glutamatergic neurons are distributed throughout the embryonic spinal cord, but their contribution to these early motor behaviors in immature zebrafish is still unclear. We demonstrate that the kinetics of spontaneous coiling and touch-evoked responses show distinct developmental time courses and that the touch response is dependent on AMPA-type glutamate receptor activation. Transection experiments suggest that the circuits required for touch-evoked responses are confined to the spinal cord and that only the most rostral part of the spinal cord is sufficient for triggering the full response. This rostral sensory connection is presumably established via CoPA interneurons, as they project to the rostral spinal cord. Electrophysiological analysis demonstrates that these neurons receive short latency AMPA-type glutamatergic inputs in response to ipsilateral tactile stimuli. We conclude that touch responses in early embryonic zebrafish arise only after glutamatergic synapses connect sensory neurons and interneurons to the contralateral motor network via a rostral loop. This helps define an elementary circuit that is modified by the addition of sensory inputs, resulting in behavioral transformation.

Measured motion: searching for simplicity in spinal locomotor networks

Spinal interneurons are organized into networks that control the activity and output of the motor system. This review outlines recent progress in defining the rules that govern the assembly and function of spinal motor networks, focusing on three main areas. We first examine how subtle variations in the wiring diagrams and organization of locomotor networks in different vertebrates permits animals to adapt their motor programs to the demands of their physical environment. We discuss how the membrane properties of spinal interneurons, and their synaptic interactions, underlie the modulation of motor circuits and encoded motor behaviors. We also describe recent molecular genetic approaches to map and manipulate the connectivity and interactions of spinal interneurons and to assess the impact of such perturbations on network function and motor behavior.

Defining the excitatory neurons that drive the locomotor rhythm in a simple vertebrate: insights into the origin of reticulospinal control

Important questions remain about the origin of the excitation that drives locomotion in vertebrates and the roles played by reticulospinal neurons. In young Xenopus tadpoles, paired whole-cell recordings reveal reticulospinal neurons that directly excite swimming circuit neurons in the brainstem and spinal cord. They form part of a column of neurons (dINs) with ipsilateral descending projections which fire reliably and rhythmically in time with swimming. We ask if, at this early stage of development, these reticulospinal neurons are themselves the primary source of rhythmic drive to spinal cord neurons on each cycle of swimming. Loose-patch recordings in the hindbrain and spinal cord from neurons active during fictive swimming distinguished dINs from other neurons by spike shape. These recordings showed that reticulospinal dINs in the caudal hindbrain (rhombomeres 7–8) fire significantly earlier on each swimming cycle than other, ipsilateral, swimming circuit neurons. Whole-cell recordings showed that fast EPSCs typically precede, and probably drive, spikes in most swimming circuit neurons. However, the earliest-firing reticulospinal dINs spike too soon to be driven by underlying fast EPSCs. We propose that rebound following reciprocal inhibition can contribute to early reticulospinal dIN firing during swimming and show rebound firing in dINs following evoked, reciprocal inhibitory PSPs. Our results define reticulospinal neurons that are the source of the primary, descending, rhythmic excitation that drives spinal cord neurons to fire during swimming. These neurons are an integral part of the rhythm generating circuitry. We discuss the origin of these reticulospinal neurons as specialised members of a longitudinally distributed population of excitatory interneurons extending from the brainstem into the spinal cord.

Electrical coupling synchronises spinal motoneuron activity during swimming in hatchling Xenopus tadpoles

The role of electrical coupling between neurons in the swimming rhythm generator of Xenopus embryos has been studied using pharmacological blockade of gap junctions. A conspicuous effect of 18beta-glycyrrhetinic acid (18beta-GA) and carbenoxolone, which have been shown to block electrical coupling in this preparation, was to increase the duration of ventral root bursts throughout the spinal cord during swimming. The left-right coordination, the swimming frequency and the duration of swimming episodes were not affected by concentrations of 18beta-GA which significantly increased burst durations. However, the longitudinal coupling was affected such that 18beta-GA led to a significant correlation between rostrocaudal delays and cycle periods, which is usually only present in older larval animals. Patch clamp recordings from spinal motoneurons tested whether gap junction blockers affect the spike timing and/or firing pattern of motoneurons during fictive swimming. In the presence of 18beta-GA motoneurons continued to fire a single, but broader action potential in each cycle of swimming, and the timing of their spikes relative to the ventral root burst became more variable. 18beta-GA had no detectable effect on the resting membrane potential of motoneurons, but led to a significant increase in input resistance, consistent with the block of gap junctions. This effect did not result in increased firing during swimming, despite the fact that multiple spikes can occur in response to current injection. Applications of 18beta-GA at larval stage 42 had no discernible effect on locomotion. The results, which suggest that electrical coupling primarily functions to synchronize activity in synergistic motoneurons during embryo swimming, are discussed in the context of motor system development.