The spinal interneurons and properties of glutamatergic synapses in a primitive vertebrate cutaneous flexion reflex

A spinal organ of proprioception for integrated motor action feedback

Proprioception is essential for behavior and provides a sense of our body movements in physical space. Proprioceptor organs are thought to be only in the periphery. Whether the central nervous system can intrinsically sense its own movement remains unclear. Here we identify a segmental organ of proprioception in the adult zebrafish spinal cord, which is embedded by intraspinal mechanosensory neurons expressing Piezo2 channels. These cells are late-born, inhibitory, commissural neurons with unique molecular and physiological profiles reflecting a dual sensory and motor function. The central proprioceptive organ locally detects lateral body movements during locomotion and provides direct inhibitory feedback onto rhythm-generating interneurons responsible for the central motor program. This dynamically aligns central pattern generation with movement outcome for efficient locomotion. Our results demonstrate that a central proprioceptive organ monitors self-movement using hybrid neurons that merge sensory and motor entities into a unified network.

Subthreshold Electrical Noise Applied to the Plantar Foot Enhances Lower-Limb Cutaneous Reflex Generation

Reflex responses generated by cutaneous mechanoreceptors of the plantar foot are important for the maintenance of balance during postural tasks and gait. With aging, reflex generation, particularly from fast adapting type I receptors, is reduced, which likely contributes to impaired postural stability in this population. Therefore, improving reflex generation from these receptors may serve as a tool to improve balance performance. A mechanism to enhance reflexes may lie in the phenomenon of stochastic resonance, whereby the addition of certain intensities and frequencies of noise stimuli improves the performance of a system. This study was conducted to determine whether tactile noise stimuli could improve cutaneous reflex generation. In 12 healthy young adults, we evoked cutaneous reflex responses using a 0–50 Hz Gaussian noise vibration applied to the plantar heel. Concurrently, we applied one of six subthreshold intensities of electrical tactile noise to the plantar heel [0%, 20%, 40%, 60%, 80% or 100% (threshold)] and were able to analyze data from 0%, 20% and 40% trials. Across participants, it was found that the addition of a 20% perceptual threshold (PT) noise resulted in enhanced reflex responses when analyzed in both the time and frequency domains. These data provide evidence that cutaneous reflex generation can be enhanced via a stochastic resonance effect and that 20% PT is the optimal intensity of noise to do so. Therefore, the addition of noise stimuli may be a valuable clinical intervention to improve reflex responses associated with postural balance in populations with impairments.

The decision to move: response times, neuronal circuits and sensory memory in a simple vertebrate

All animals use sensory systems to monitor external events and have to decide whether to move. Response times are long and variable compared to reflexes, and fast escape movements. The complexity of adult vertebrate brains makes it difficult to trace the neuronal circuits underlying basic decisions to move. To simplify the problem, we investigate the nervous system and responses of hatchling frog tadpoles which swim when their skin is stimulated. Studying the neuron-by-neuron pathway from sensory to hindbrain neurons, where the decision to swim is made, has revealed two simple pathways generating excitation which sums to threshold in these neurons to initiate swimming. The direct pathway leads to short, and reliable delays like an escape response. The other includes a population of sensory processing neurons which extend firing to introduce noise and delay into responses. These neurons provide a brief, sensory memory of the stimulus, that allows tadpoles to integrate stimuli occurring within a second or so of each other. We relate these findings to other studies and conclude that sensory memory makes a fundamental contribution to simple decisions and is present in the brainstem of a basic vertebrate at a surprisingly early stage in development.

A simple decision to move in response to touch reveals basic sensory memory and mechanisms for variable response times

KEY POINTS

Short-term working memory and decision-making are usually studied in the cerebral cortex; in many models of simple decision making, sensory signals build slowly and noisily to threshold to initiate a motor response after long, variable delays. When touched, hatchling frog tadpoles decide whether to swim; we define the long and variable delays to swimming and use whole-cell recordings to uncover the neurons and processes responsible. Firing in sensory and sensory pathway neurons is short latency, and too brief and invariant to explain these delays, while recordings from hindbrain reticulospinal neurons controlling swimming reveal a prolonged and variable build-up of synaptic excitation which can reach firing threshold and initiate swimming. We propose this excitation provides a sensory memory of the stimulus and may be generated by small reverberatory hindbrain networks. Our results uncover fundamental network mechanisms that allow animals to remember brief sensory stimuli and delay simple motor decisions.

ABSTRACT

Many motor responses to sensory input, like locomotion or eye movements, are much slower than reflexes. Can simpler animals provide fundamental answers about the cellular mechanisms for motor decisions? Can we observe the 'accumulation' of excitation to threshold proposed to underlie decision making elsewhere? We explore how somatosensory touch stimulation leads to the decision to swim in hatchling Xenopus tadpoles. Delays measured to swimming in behaving and immobilised tadpoles are long and variable. Activity in their extensively studied sensory and sensory pathway neurons is too short-lived to explain these response delays. Instead, whole-cell recordings from the hindbrain reticulospinal neurons that drive swimming show that these receive prolonged, variable synaptic excitation lasting for nearly a second following a brief stimulus. They fire and initiate swimming when this excitation reaches threshold. Analysis of the summation of excitation requires us to propose extended firing in currently undefined presynaptic hindbrain neurons. Simple models show that a small excitatory recurrent-network inserted in the sensory pathway can mimic this process. We suggest that such a network may generate slow, variable summation of excitation to threshold. This excitation provides a simple memory of the sensory stimulus. It allows temporal and spatial integration of sensory inputs and explains the long, variable delays to swimming. The process resembles the 'accumulation' of excitation proposed for cortical circuits in mammals. We conclude that fundamental elements of sensory memory and decision making are present in the brainstem at a surprisingly early stage in development.

Principles Governing Locomotion in Vertebrates: Lessons From Zebrafish

Locomotor behaviors are critical for survival and enable animals to navigate their environment, find food and evade predators. The circuits in the brain and spinal cord that initiate and maintain such different modes of locomotion in vertebrates have been studied in numerous species for over a century. In recent decades, the zebrafish has emerged as one of the main model systems for the study of locomotion, owing to its experimental amenability, and work in zebrafish has revealed numerous new insights into locomotor circuit function. Here, we review the literature that has led to our current understanding of the neural circuits controlling swimming and escape in zebrafish. We highlight recent studies that have enriched our comprehension of key topics, such as the interactions between premotor excitatory interneurons (INs) and motoneurons (MNs), supraspinal and spinal circuits that coordinate escape maneuvers, and developmental changes in overall circuit composition. We also discuss roles for neuromodulators and sensory inputs in modifying the relative strengths of constituent circuit components to provide flexibility in zebrafish behavior, allowing the animal to accommodate changes in the environment. We aim to provide a coherent framework for understanding the circuitry in the brain and spinal cord of zebrafish that allows the animal to flexibly transition between different speeds, and modes, of locomotion.

Active mechanosensory feedback during locomotion in the zebrafish spinal cord

The investigation of mechanosensory feedback to locomotion has been hindered by the challenge of recording neurons in motion. Genetic accessibility and optical transparency of zebrafish larvae provide means to revisit this question. Glutamatergic Rohon-Beard (RB) and GABAergic CSF-contacting neurons (CSF-cNs) are spinal mechanosensory neurons. Recent studies combining bioluminescence, silencing and optogenetic activation show that mechanosensory neurons enhance speed and stabilize posture during locomotion. RB neurons can modulate speed by projecting onto glutamatergic premotor V2a interneurons during fast swimming, while CSF-cNs inhibit V0-v interneurons sustaining slow swimming. Sensory gating, either through inhibition of sensory interneurons (CoPA) or though the direct inhibition of primary motor neurons by CSF-cNs, mediates postural control. Advanced optical methods have shed light on the dynamics of sensorimotor integration during active locomotion unraveling implications for translational research.

Xenopus laevis as a Model Organism for the Study of Spinal Cord Formation, Development, Function and Regeneration

The spinal cord is the first central nervous system structure to develop during vertebrate embryogenesis, underscoring its importance to the organism. Because of its early formation, accessibility to the developing spinal cord in amniotes is challenging, often invasive and the experimental approaches amenable to model systems like mammals are limited. In contrast, amphibians, in general and the African-clawed frog Xenopus laevis, in particular, offer model systems in which the formation of the spinal cord, the differentiation of spinal neurons and glia and the establishment of spinal neuron and neuromuscular synapses can be easily investigated with minimal perturbations to the whole organism. The significant advances on gene editing and microscopy along with the recent completion of the Xenopus laevis genome sequencing have reinvigorated the use of this classic model species to elucidate the mechanisms of spinal cord formation, development, function and regeneration.

Mechanosensory neurons control the timing of spinal microcircuit selection during locomotion

Despite numerous physiological studies about reflexes in the spinal cord, the contribution of mechanosensory feedback to active locomotion and the nature of underlying spinal circuits remains elusive. Here we investigate how mechanosensory feedback shapes active locomotion in a genetic model organism exhibiting simple locomotion-the zebrafish larva. We show that mechanosensory feedback enhances the recruitment of motor pools during active locomotion. Furthermore, we demonstrate that inputs from mechanosensory neurons increase locomotor speed by prolonging fast swimming at the expense of slow swimming during stereotyped acoustic escape responses. This effect could be mediated by distinct mechanosensory neurons. In the spinal cord, we show that connections compatible with monosynaptic inputs from mechanosensory Rohon-Beard neurons onto ipsilateral V2a interneurons selectively recruited at high speed can contribute to the observed enhancement of speed. Altogether, our study reveals the basic principles and a circuit diagram enabling speed modulation by mechanosensory feedback in the vertebrate spinal cord.

Zebrafish transgenic constructs label specific neurons in Xenopus laevis spinal cord and identify frog V0v spinal neurons

A correctly functioning spinal cord is crucial for locomotion and communication between body and brain but there are fundamental gaps in our knowledge of how spinal neuronal circuitry is established and functions. To understand the genetic program that regulates specification and functions of this circuitry, we need to connect neuronal molecular phenotypes with physiological analyses. Studies using Xenopus laevis tadpoles have increased our understanding of spinal cord neuronal physiology and function, particularly in locomotor circuitry. However, the X. laevis tetraploid genome and long generation time make it difficult to investigate how neurons are specified. The opacity of X. laevis embryos also makes it hard to connect functional classes of neurons and the genes that they express. We demonstrate here that Tol2 transgenic constructs using zebrafish enhancers that drive expression in specific zebrafish spinal neurons label equivalent neurons in X. laevis and that the incorporation of a Gal4:UAS amplification cassette enables cells to be observed in live X. laevis tadpoles. This technique should enable the molecular phenotypes, morphologies and physiologies of distinct X. laevis spinal neurons to be examined together in vivo. We have used an islet1 enhancer to label Rohon-Beard sensory neurons and evx enhancers to identify V0v neurons, for the first time, in X. laevis spinal cord. Our work demonstrates the homology of spinal cord circuitry in zebrafish and X. laevis, suggesting that future work could combine their relative strengths to elucidate a more complete picture of how vertebrate spinal cord neurons are specified, and function to generate behavior. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 77: 1007-1020, 2017.

Decoding the organization of spinal circuits that control locomotion

Unravelling the functional operation of neuronal networks and linking cellular activity to specific behavioural outcomes are among the biggest challenges in neuroscience. In this broad field of research, substantial progress has been made in studies of the spinal networks that control locomotion. Through united efforts using electrophysiological and molecular genetic network approaches and behavioural studies in phylogenetically diverse experimental models, the organization of locomotor networks has begun to be decoded. The emergent themes from this research are that the locomotor networks have a modular organization with distinct transmitter and molecular codes and that their organization is reconfigured with changes to the speed of locomotion or changes in gait.

Sensory initiation of a co-ordinated motor response: synaptic excitation underlying simple decision-making

Key points

Deciding whether or how to initiate a motor response to a stimulus can be surprisingly slow and the underlying processes are not well understood.

The neuronal circuitry that allows frog tadpoles to swim in response to touch is well characterized and includes excitatory reticulospinal neurons that drive swim circuit neurons.

Build‐up of excitation to reticulospinal neurons is the key decision‐making step for swimming. Asymmetry in this build‐up between the two sides allows bilateral initiation at the same time as avoiding inappropriate co‐activation of motor antagonists.

Following stronger stimuli, reticulospinal neurons are excited through a trigeminal nucleus pathway and swimming starts first on the stimulated side. If this pathway fails or is lesioned, swimming starts later on the unstimulated side.

The mechanisms underlying initiation of a simple tadpole motor response may share similarities with more complex decisions in other animals, including humans.

Abstract

Animals take time to make co‐ordinated motor responses to a stimulus. How can sensory input initiate organized movements, activating all necessary elements at the same time as avoiding inappropriate co‐excitation of antagonistic muscles? In vertebrates, this process usually results in the activation of reticulospinal pathways. Young Xenopus tadpoles can respond to head‐skin touch by swimming, which may start on either side. We investigate how motor networks in the brain are organized, and whether asymmetries in touch sensory pathways avoid co‐activation of antagonists at the same time as producing co‐ordinated movements. We record from key reticulospinal neurons in the network controlling swimming. When the head skin is stimulated unilaterally, excitation builds up slowly and asymmetrically in these neurons such that those on both sides do not fire synchronously. This build‐up of excitation to threshold is the key decision‐making step and determines whether swimming will start, as well as on which side. In response to stronger stimuli, the stimulated side tends to ‘win’ because excitation from a shorter, trigeminal nucleus pathway becomes reliable and can initiate swimming earlier on the stimulated side. When this pathway fails or is lesioned, swimming starts later and on the unstimulated side. Stochasticity in the trigeminal nucleus pathway allows unpredictable turning behaviour to weaker stimuli, conferring potential survival benefits. We locate the longer, commissural sensory pathway carrying excitation to the unstimulated side and record from its neurons. These neurons fire to head‐skin stimuli but excite reticulospinal neurons indirectly. We propose that asymmetries in the sensory pathways exciting brainstem reticulospinal neurons ensure alternating and co‐ordinated swimming activity from the start.

Modelling the Effects of Electrical Coupling between Unmyelinated Axons of Brainstem Neurons Controlling Rhythmic Activity

Gap junctions between fine unmyelinated axons can electrically couple groups of brain neurons to synchronise firing and contribute to rhythmic activity. To explore the distribution and significance of electrical coupling, we modelled a well analysed, small population of brainstem neurons which drive swimming in young frog tadpoles. A passive network of 30 multicompartmental neurons with unmyelinated axons was used to infer that: axon-axon gap junctions close to the soma gave the best match to experimentally measured coupling coefficients; axon diameter had a strong influence on coupling; most neurons were coupled indirectly via the axons of other neurons. When active channels were added, gap junctions could make action potential propagation along the thin axons unreliable. Increased sodium and decreased potassium channel densities in the initial axon segment improved action potential propagation. Modelling suggested that the single spike firing to step current injection observed in whole-cell recordings is not a cellular property but a dynamic consequence of shunting resulting from electrical coupling. Without electrical coupling, firing of the population during depolarising current was unsynchronised; with coupling, the population showed synchronous recruitment and rhythmic firing. When activated instead by increasing levels of modelled sensory pathway input, the population without electrical coupling was recruited incrementally to unpatterned activity. However, when coupled, the population was recruited all-or-none at threshold into a rhythmic swimming pattern: the tadpole “decided” to swim. Modelling emphasises uncertainties about fine unmyelinated axon physiology but, when informed by biological data, makes general predictions about gap junctions: locations close to the soma; relatively small numbers; many indirect connections between neurons; cause of action potential propagation failure in fine axons; misleading alteration of intrinsic firing properties. Modelling also indicates that electrical coupling within a population can synchronize recruitment of neurons and their pacemaker firing during rhythmic activity.

Some groups of nerve cells in the brain are connected to each other electrically where their processes make contact and form specialized “gap” junctions. The simplest function of electrical connections is to make activity propagate faster by avoiding the delays resulting from chemical messengers at synaptic connections. In other cases, especially in higher brain regions where more spread out nerve cells may be connected by their axons, the function of electrical coupling is less clear. To understand this type of electrical connection better we have built computer models of a group of electrically coupled nerve cells in the brain which control swimming in very young frog tadpoles. We show that the coupling can be indirect, via other members of the group, and can profoundly influence the properties of the nerve cells which would be recorded during real experiments. The main role of the coupling is to synchronise the firing of the group so they are all recruited together when the tadpole is stimulated and then fire in a rhythm suitable to drive swimming movements. The results from this simple animal raise issues which will help to understand coupling in more complex brains.

Sensory gating of an embryonic zebrafish interneuron during spontaneous motor behaviors

In all but the simplest monosynaptic reflex arcs, sensory stimuli are encoded by sensory neurons that transmit a signal via sensory interneurons to downstream partners in order to elicit a response. In the embryonic zebrafish (Danio rerio), cutaneous Rohon-Beard (RB) sensory neurons fire in response to mechanical stimuli and excite downstream glutamatergic commissural primary ascending (CoPA) interneurons to produce a flexion response contralateral to the site of stimulus. In the absence of sensory stimuli, zebrafish spinal locomotor circuits are spontaneously active during development due to pacemaker activity resulting in repetitive coiling of the trunk. Self-generated movement must therefore be distinguishable from external stimuli in order to ensure the appropriate activation of touch reflexes. Here, we recorded from CoPAs during spontaneous and evoked fictive motor behaviors in order to examine how responses to self-movement are gated in sensory interneurons. During spontaneous coiling, CoPAs received glycinergic inputs coincident with contralateral flexions that shunted firing for the duration of the coiling event. Shunting inactivation of CoPAs was caused by a slowly deactivating chloride conductance that resulted in lowered membrane resistance and increased action potential threshold. During spontaneous burst swimming, which develops later, CoPAs received glycinergic inputs that arrived in phase with excitation to ipsilateral motoneurons and provided persistent shunting. During a touch stimulus, short latency glutamatergic inputs produced cationic currents through AMPA receptors that drove a single, large amplitude action potential in the CoPA before shunting inhibition began, providing a brief window for the activation of downstream neurons. We compared the properties of CoPAs to those of other spinal neurons and propose that glycinergic signaling onto CoPAs acts as a corollary discharge signal for reflex inhibition during movement.

Behavioral observation of Xenopus tadpole swimming for neuroscience labs

Neuroscience labs benefit from reliable, easily-monitored neural responses mediated by well-studied neural pathways. Xenopus laevis tadpoles have been used as a simple vertebrate model preparation in motor control studies. Most of the neuronal pathways underlying different aspects of tadpole swimming behavior have been revealed. These include the skin mechanosensory touch and pineal eye light-sensing pathways whose activation can initiate swimming, and the cement gland pressure-sensing pathway responsible for stopping swimming. A simple transection in the hindbrain can cut off the pineal eye and cement gland pathways from the swimming circuit in the spinal cord, resulting in losses of corresponding functions. Additionally, some pharmacological experiments targeting neurotransmission can be designed to affect swimming and, fluorescence-conjugated α-bungarotoxin can be used to label nicotinic receptors at neuromuscular junctions. These experiments can be readily adapted for undergraduate neuroscience teaching labs. Possible expansions of some experiments for more sophisticated pharmacological or neurophysiological labs are also discussed.

A developmental approach to predicting neuronal connectivity from small biological datasets: a gradient-based neuron growth model

Relating structure and function of neuronal circuits is a challenging problem. It requires demonstrating how dynamical patterns of spiking activity lead to functions like cognitive behaviour and identifying the neurons and connections that lead to appropriate activity of a circuit. We apply a “developmental approach” to define the connectome of a simple nervous system, where connections between neurons are not prescribed but appear as a result of neuron growth. A gradient based mathematical model of two-dimensional axon growth from rows of undifferentiated neurons is derived for the different types of neurons in the brainstem and spinal cord of young tadpoles of the frog Xenopus. Model parameters define a two-dimensional CNS growth environment with three gradient cues and the specific responsiveness of the axons of each neuron type to these cues. The model is described by a nonlinear system of three difference equations; it includes a random variable, and takes specific neuron characteristics into account. Anatomical measurements are first used to position cell bodies in rows and define axon origins. Then a generalization procedure allows information on the axons of individual neurons from small anatomical datasets to be used to generate larger artificial datasets. To specify parameters in the axon growth model we use a stochastic optimization procedure, derive a cost function and find the optimal parameters for each type of neuron. Our biologically realistic model of axon growth starts from axon outgrowth from the cell body and generates multiple axons for each different neuron type with statistical properties matching those of real axons. We illustrate how the axon growth model works for neurons with axons which grow to the same and the opposite side of the CNS. We then show how, by adding a simple specification for dendrite morphology, our model “developmental approach” allows us to generate biologically-realistic connectomes.

A functional scaffold of CNS neurons for the vertebrates: the developing Xenopus laevis spinal cord

In young and developing amphibians and fish the spinal cord is functional but remarkably simple compared with the adult. Is the pattern of neurons and their connections common across at least these lower vertebrates? Does this basic pattern extend into the brainstem? Could the development of simple functioning neuronal networks depend on very basic rules of connectivity and act as pioneer networks providing a substrate for the development of more complex and subtle networks. In this review of the functional neuron classes in the Xenopus laevis tadpole spinal cord up to hatching, we will consider progress and difficulties in using anatomy, transcription factor expression, physiology, and activity to define spinal neuron types. Even here it is not straightforward and is rarely possible to bring all the different strands of evidence together. But, we think we have a rather complete picture of the hatchling tadpole spinal neuron types and can define clear roles for most of them in behavior. Our present knowledge about the hatchling Xenopus spinal cord should set up many of the problems to be unraveled in the future by more developmentally oriented research.

The firing patterns of spinal neurons: in situ patch-clamp recordings reveal a key role for potassium currents

Neuron firing patterns underpin the detection and processing of stimuli, influence synaptic interactions, and contribute to the function of networks. To understand how intrinsic membrane properties determine firing patterns, we investigated the biophysical basis of single and repetitive firing in spinal neurons of hatchling Xenopus laevis tadpoles, a well-understood vertebrate model; experiments were conducted in situ. Primary sensory Rohon-Beard (RB) neurons fire singly in response to depolarising current, and dorsolateral (DL) interneurons fire repetitively. RB neurons exhibited a large tetrodotoxin-sensitive sodium current; in DL neurons, the sodium current density was significantly lower. High-voltage-activated calcium currents were similar in both neuron types. There was no evidence of persistent sodium currents, low-voltage-activated calcium currents, or hyperpolarisation-activated currents. In RB neurons, the potassium current was dominated by a tetraethylammonium-sensitive slow component (I(Ks) ); a fast component (I(Kf) ), sensitive to 4-aminopyridine, predominated in DL neurons. Sequential current-clamp and voltage-clamp recordings in individual neurons suggest that high densities of I(Ks) prevent repetitive firing; where I(Ks) is small, I(Kf) density determines the frequency of repetitive firing. Intermediate densities of I(Ks) and I(Kf) allow neurons to fire a few additional spikes on strong depolarisation; this property typifies a novel subset of RB neurons, and may activate escape responses. We discuss how this ensemble of currents and firing patterns underpins the operation of the Xenopus locomotor network, and suggest how simple mechanisms might underlie the similar firing patterns seen in the neurons of diverse species.

The role of a trigeminal sensory nucleus in the initiation of locomotion

While we understand how stimuli evoke sudden, ballistic escape responses, like fish fast-starts, a precise pathway from sensory stimulation to the initiation of rhythmic locomotion has not been defined for any vertebrate. We have now asked how head skin stimuli evoke swimming in hatchling frog tadpoles. Whole-cell recordings and dye filling revealed a nucleus of ∼20 trigeminal interneurons (tINs) in the hindbrain, at the level of the auditory nerve, with long, ipsilateral, descending axons. Stimulation of touch-sensitive trigeminal afferents with receptive fields anywhere on the head evoked large, monosynaptic EPSPs (∼5-20 mV) in tINs, at mixed AMPAR/NMDAR synapses. Following stimuli sufficient to elicit swimming, tINs fired up to six spikes, starting 4-8 ms after the stimulus. Paired whole-cell recordings showed that tINs produce small (∼2-6 mV), monosynaptic, glutamatergic EPSPs in the hindbrain reticulospinal neurons (descending interneurons, dINs) that drive swimming. Modelling suggested that summation of EPSPs from 18-24 tINs can make 20-50% of dINs fire. We conclude that: brief activity in a few sensory afferents is amplified by recruitment of many tINs; these relay summating excitation to hindbrain reticulospinal dINs; dIN firing then initiates activity for swimming on the stimulated side. During fictive swimming, tINs are depolarised and receive rhythmic inhibition but do not fire. Our recordings demonstrate a neuron-by-neuron pathway from head skin afferents to the reticulospinal neurons and motoneurons that drive locomotion in a vertebrate. This direct pathway, which has an important amplifier function, implies a simple origin for the complex routes to initiate locomotion in higher vertebrates.

Skin impulse excitation of spinal sensory neurons in developing Xenopus laevis (Daudin) tadpoles

Responses to gentle touch in young Xenopus tadpoles are mediated by spinal cord sensory Rohon-Beard neurons. Tadpoles also respond to noxious stimuli that elicit 'skin impulses', which propagate between epithelial cells over the whole body surface, somehow entering the CNS to generate a response. After hatching (~48 h post-fertilization), skin impulse signals enter the CNS only via cranial nerves, but previous evidence suggested the possibility of direct entry to the spinal cord before this (~24 h). We have used behavioural and electrophysiological methods to explore the developmental pattern of skin impulse entry into the spinal cord and the involvement of Rohon-Beard neurons. Lesioning confirmed that skin impulse signals can directly enter the spinal cord in young embryos, but access decreases over ~12 h and disappears soon after hatching. Electrical recordings from central Rohon-Beard axons in young embryos showed firing in response to skin impulses. However, unit recordings from Rohon-Beard somata showed that individuals that responded to touch within a characteristic, localised receptive field did not fire to skin impulses, whereas others from similar locations responded reliably. Developmental loss of skin impulse access to the spinal cord mirrored the known spread of sensitivity to gentle touch as the peripheral mechanosensory endings of Rohon-Beard neurons mature. Together, these results suggest that Rohon-Beard neurons respond to skin impulses only while immature, providing a transitory route for skin impulses to excite the CNS. In this way, Rohon-Beard neurons would mediate responses first to noxious and then to localised, gentle touch stimuli as the neurons developed.

Pharmacology of currents underlying the different firing patterns of spinal sensory neurons and interneurons identified in vivo using multivariate analysis

The operation of neuronal networks depends on the firing patterns of the network's neurons. When sustained current is injected, some neurons in the central nervous system fire a single action potential and others fire repetitively. For example, in Xenopus laevis tadpoles, primary-sensory Rohon-Beard (RB) neurons fired a single action potential in response to 300-ms rheobase current injections, whereas dorsolateral (DL) interneurons fired repetitively at 10-20 Hz. To investigate the basis for these differences in vivo, we examined drug-induced changes in the firing patterns of Xenopus spinal neurons using whole cell current-clamp recordings. Neuron types were initially separated through cluster analysis, and we compared results produced using different clustering algorithms. We used these results to develop a predictive function to classify subsequently recorded neurons. The potassium channel blocker tetraethylammonium (TEA) converted single-firing RB neurons to low-frequency repetitive firing but reduced the firing frequency of repetitive-firing DL interneurons. Firing frequency in DL interneurons was also reduced by the potassium channel blockers 4-aminopyridine (4-AP), catechol, and margatoxin; 4-AP had the greatest effect. The calcium channel blockers amiloride and nimodipine had few effects on firing in either neuron type but reduced action potential duration in DL interneurons. Muscarine, which blocks M-currents, did not affect RB neurons but reduced firing frequency in DL interneurons. These results suggest that potassium currents may control neuron firing patterns: a TEA-sensitive current prevents repetitive firing in RB neurons, whereas a 4-AP-sensitive current underlies repetitive firing in DL interneurons. The cluster and discriminant analysis described could help to classify neurons in other systems.

Genetic Analysis of the Touch Response in Zebrafish (Danio rerio)

Both mammals and zebrafish possess mechanosensory neurons that detect tactile sensation via free nerve endings. However, the basis for mechanotransduction and the unique cellular properties of these sensory neurons are poorly understood. We review the advantages of zebrafish for studies of the biological mechanisms involved in touch sensitivity. Importantly, Granato and colleagues (1996) demonstrated that a simple touch assay efficiently recovers mutations that affect sensory neurons.

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.

Development of motor rhythms in zebrafish embryos

The nervous system can generate rhythms of various frequencies; on the low-frequency side, we have the circuits regulating circadian rhythms with a 24-h period, while on the high-frequency side we have the motor circuits that underlie flight in a hummingbird. Given the ubiquitous nature of rhythms, it is surprising that we know very little of the cellular and molecular mechanisms that produce them in the embryos and of their potential role during the development of neuronal circuits. Recently, zebrafish has been developed as a vertebrate model to study the genetics of neural development. Zebrafish offer several advantages to the study of nervous system development including optical and electrophysiological analysis of neuronal activity even at the earliest embryonic stages. This unique combination of physiology and genetics in the same animal model has led to insights into the development of neuronal networks. This chapter reviews work on the development of zebrafish motor rhythms and speculates on birth and maturation of the circuits that produce them.

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.

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

Electrical coupling is important in rhythm generating systems. We examine its role in circuits controlling locomotion in a simple vertebrate model, the young Xenopus tadpole, where the hindbrain and spinal cord excitatory descending interneurons (dINs) that drive and maintain swimming have been characterised. Using simultaneous paired recordings, we show that most dINs are electrically coupled exclusively to other dINs (DC coupling coefficients ∼8.5%). The coupling shows typical low-pass filtering. We found no evidence that other swimming central pattern generator (CPG) interneurons are coupled to dINs or to each other. Electrical coupling potentials between dINs appear to contribute to their unusually reliable firing during swimming. To investigate the role of electrical coupling in swimming, we evaluated the specificity of gap junction blockers (18-β-GA, carbenoxolone, flufenamic acid and heptanol) in paired recordings. 18-β-GA at 40–60 μm produced substantial (84%) coupling block but few effects on cellular properties. Swimming episodes in 18-β-GA were significantly shortened (to ∼2% of control durations). At the same time, dIN firing reliability fell from nearly 100% to 62% of swimming cycles and spike synchronization weakened. Because dINs drive CPG neuron firing and are critical in maintaining swimming, the weakening of dIN activity could account for the effects of 18-β-GA on swimming. We conclude that electrical coupling among pre motor reticulospinal and spinal dINs, the excitatory interneurons that drive the swimming CPG in the hatchling Xenopus tadpole, may contribute to the maintenance of swimming as well as synchronization of activity.

The contribution of the NMDA receptor glycine site to rhythm generation during fictive swimming in Xenopus laevis tadpoles

The presence of the N-methyl-D-aspartate (NMDA) receptor glycine-binding site and its role in locomotor activity have been examined using fictive swimming in stage 42 Xenopus laevis frog tadpoles as a simple model system. The specific NMDA/glycine site blocker L-689560 (0.1-20 microm) impaired swimming rhythm generation and abolished NMDA-induced locomotor-like ventral root activity. D-serine (50 microm), an agonist at the NMDA/glycine site, increased the duration of skin stimulus-induced fictive swimming episodes, and produced slow modulations of burst frequency and amplitude. These effects of D-serine were reversed by L-689560. In some animals, D-serine also induced an alternative intense, non-locomotory form of rhythmic motor output termed struggling. Glycine (100 microm), another endogenous agonist at this site, triggered similar effects to D-serine, but only when applied in the presence of strychnine. Manipulations of endogenous glycine levels using sarcosine or ALX 5407 (inhibitors of the glycine re-uptake protein, GlyT1b), produced similar effects to glycine site agonists, including increased episode durations, and modulations in cycle period and burst amplitude. Sarcosine and ALX 5407 also induced struggling. In summary, these experiments support the hypothesis that NMDA receptors in the swimming network of Xenopus laevis tadpoles possess glycine-binding sites, not all of which are fully occupied under normal circumstances. Altering the strength of the NMDA receptor-mediated component of the synaptic drive for swimming by increasing or decreasing occupancy of this site potently influences the locomotor pattern.

Axon and dendrite geography predict the specificity of synaptic connections in a functioning spinal cord network

BACKGROUND

How specific are the synaptic connections formed as neuronal networks develop and can simple rules account for the formation of functioning circuits? These questions are assessed in the spinal circuits controlling swimming in hatchling frog tadpoles. This is possible because detailed information is now available on the identity and synaptic connections of the main types of neuron.

RESULTS

The probabilities of synapses between 7 types of identified spinal neuron were measured directly by making electrical recordings from 500 pairs of neurons. For the same neuron types, the dorso-ventral distributions of axons and dendrites were measured and then used to calculate the probabilities that axons would encounter particular dendrites and so potentially form synaptic connections. Surprisingly, synapses were found between all types of neuron but contact probabilities could be predicted simply by the anatomical overlap of their axons and dendrites. These results suggested that synapse formation may not require axons to recognise specific, correct dendrites. To test the plausibility of simpler hypotheses, we first made computational models that were able to generate longitudinal axon growth paths and reproduce the axon distribution patterns and synaptic contact probabilities found in the spinal cord. To test if probabilistic rules could produce functioning spinal networks, we then made realistic computational models of spinal cord neurons, giving them established cell-specific properties and connecting them into networks using the contact probabilities we had determined. A majority of these networks produced robust swimming activity.

CONCLUSION

Simple factors such as morphogen gradients controlling dorso-ventral soma, dendrite and axon positions may sufficiently constrain the synaptic connections made between different types of neuron as the spinal cord first develops and allow functional networks to form. Our analysis implies that detailed cellular recognition between spinal neuron types may not be necessary for the reliable formation of functional networks to generate early behaviour like swimming.

Role of type-specific neuron properties in a spinal cord motor network

Recent recordings from spinal neurons in hatchling frog tadpoles allow their type-specific properties to be defined. Seven main types of neuron involved in the control of swimming have been characterized. To investigate the significance of type-specific properties, we build models of each neuron type and assemble them into a network using known connectivity between: sensory neurons, sensory pathway interneurons, central pattern generator (CPG) interneurons and motoneurons. A single stimulus to a sensory neuron initiates swimming where modelled neuronal and network activity parallels physiological activity. Substitution of firing properties between neuron types shows that those of excitatory CPG interneurons are critical for stable swimming. We suggest that type-specific neuronal properties can reflect the requirements for involvement in one particular network response (like swimming), but may also reflect the need to participate in more than one response (like swimming and slower struggling).

The lower limb flexion reflex in humans

The flexion or flexor reflex (FR) recorded in the lower limbs in humans (LLFR) is a widely investigated neurophysiological tool. It is a polysynaptic and multisegmental spinal response that produces a withdrawal of the stimulated limb and resembles (having several features in common) the hind-paw FR in animals. The FR, in both animals and humans, is mediated by a complex circuitry modulated at spinal and supraspinal level. At rest, the LLFR (usually obtained by stimulating the sural/tibial nerve and by recording from the biceps femoris/tibial anterior muscle) appears as a double burst composed of an early, inconstantly present component, called the RII reflex, and a late, larger and stable component, called the RIII reflex. Numerous studies have shown that the afferents mediating the RII reflex are conveyed by large-diameter, low-threshold, non-nociceptive A-beta fibers, and those mediating the RIII reflex by small-diameter, high-threshold nociceptive A-delta fibers. However, several afferents, including nociceptive and non-nociceptive fibers from skin and muscles, have been found to contribute to LLFR activation. Since the threshold of the RIII reflex has been shown to correspond to the pain threshold and the size of the reflex to be related to the level of pain perception, it has been suggested that the RIII reflex might constitute a useful tool to investigate pain processing at spinal and supraspinal level, pharmacological modulation and pathological pain conditions. As stated in EFNS guidelines, the RIII reflex is the most widely used of all the nociceptive reflexes, and appears to be the most reliable in the assessment of treatment efficacy. However, the RIII reflex use in the clinical evaluation of neuropathic pain is still limited. In addition to its nocifensive function, the LLFR seems to be linked to posture and locomotion. This may be explained by the fact that its neuronal circuitry, made up of a complex pool of interneurons, is interposed in motor control and, during movements, receives both peripheral afferents (flexion reflex afferents, FRAs) and descending commands, forming a multisensorial feedback mechanism and projecting the output to motoneurons. LLFR excitability, mediated by this complex circuitry, is finely modulated in a state- and phase-dependent manner, rather as we observe in the FR in animal models. Several studies have demonstrated that LLFR excitability may be influenced by numerous physiological conditions (menstrual cycle, stress, attention, sleep and so on) and pathological states (spinal lesions, spasticity, Wallenberg's syndrome, fibromyalgia, headaches and so on). Finally, the LLFR is modulated by several drugs and neurotransmitters. In summary, study of the LLFR in humans has proved to be an interesting functional window onto the spinal and supraspinal mechanisms of pain processing and onto the spinal neural control mechanisms operating during posture and locomotion.

Glutamate and acetylcholine corelease at developing synapses

Most neurons release a single fast-acting low-molecular-weight transmitter at synapses to activate and open postsynaptic ion channels. We challenge this principle with evidence for corelease of the two major excitatory transmitters, glutamate and acetylcholine (ACh), from single identified neurons in the developing frog tadpole spinal cord. Whole-cell patch electrodes were used to record from single spinal neurons. When action potentials and inhibition were blocked, spontaneous miniature excitatory postsynaptic currents (mEPSCs) were recorded. These were fully blocked only by joint application of glutamate [alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) and N-methyl-D-aspartate receptor (NMDAR)] and nicotinic ACh receptor (nAChR) antagonists. Fast nAChR and slow NMDAR mEPSCs were isolated pharmacologically. We then show that some mEPSCs have both the fast nAChR rise and slow NMDAR decay and conclude that some individual synaptic vesicles corelease glutamate and ACh. Whole-cell recordings from pairs of neurons were then used to identify the spinal interneurons coreleasing the two excitatory transmitters. One anatomical class of interneuron with descending axons was found to excite other spinal neurons by activating nAChR, AMPAR, and NMDAR simultaneously at its synapses. Although Jonas, Bischofberger, and Sandkuhler [Jonas, P., Bischofberger, J. & Sandkuhler, J. (1998) Science 281, 419-424] showed that the inhibitory transmitters GABA and glycine can be coreleased at spinal synapses, the Xenopus tadpole provides a case where the two main CNS excitatory transmitters are released from single vesicles, and where the presynaptic neuron coreleasing two transmitters has been identified.

A Direct Comparison of Whole Cell Patch and Sharp Electrodes by Simultaneous Recording From Single Spinal Neurons in Frog Tadpoles

High-impedance, sharp intracellular electrodes were compared with whole cell patch electrodes by recording from single spinal neurons in immobilized frog tadpoles. A range of neuron properties were examined using sharp or patch test electrodes while making simultaneous recordings with a second control patch electrode. Overall, test patch electrodes did not significantly alter the activity recorded by the control electrode, and recordings from the two electrodes were essentially identical. In contrast, sharp electrode recordings differed from initial control patch recordings. In some cases, differences were due to real changes in neuron properties: the resting membrane potential became less negative and the neuron input resistance ( Ri) fell; this fall was larger for neurons with a higher Ri. In other cases, the control patch electrode revealed that differences were due to the recording properties of the sharp electrode: tip potentials were larger and more variable; resting potentials appeared to be more negative; and spike amplitude was attenuated. However, sharp electrode penetration did not, in most cases, significantly alter the pattern of neuron firing in response to injected current or the normal pattern of activity following sensory stimulation or during fictive swimming. We conclude that sharp electrodes introduce a significant leak to the membrane of tadpole spinal neurons compared with patch electrodes but that this does not change the fundamental firing characteristics or activity of the neurons.

shocked Gene is required for the function of a premotor network in the zebrafish CNS

The analysis of behavioral mutations in zebrafish can be a powerful strategy for identifying genes that regulate the function and development of neural circuits in the vertebrate CNS. A neurophysiological analysis of the shocked (sho) mutation that affects the initiation of swimming after mechanosensory stimulation was undertaken to identify the function of the sho gene product in the developing motor circuitry. The cutaneous Rohon-Beard (RB) mechanosensory neurons responded normally to stimulation, and muscle fibers were unaffected in sho embryos, suggesting that the output of the CNS is abnormal. Indeed whole cell patch recordings from mutant muscle cells showed normal spontaneous miniature endplate potentials, but abnormal touch-evoked endplate potentials. Furthermore, motor neuron recordings showed that bursts of rhythmic action potentials from synaptically dependent depolarizations are initiated in wild-type motor neurons after sensory stimulation or bath application of N-methyl-D-aspartate. These bursts presumably correspond to bouts of swimming. In sho motor neurons, the touch-evoked depolarizations were not sustained, resulting in an abbreviated burst of action potentials. The defective responses were not due to any obvious defect in sho motor neurons because their basic properties were normal. These results suggest that in sho embryos, there is aberrant motor processing within the CNS and that normal motor processing requires the sho gene product.

Dorsal spinal interneurons forming a primitive, cutaneous sensory pathway

In mammals, sensory projection pathways are provided by just three classes of spinal interneuron that develop from the roof-plate. We asked whether similar sensory projection interneurons are present primitively in a developing lower vertebrate where function can be more readily studied. Using an immobilized Xenopus tadpole spinal cord preparation, we define the properties and connections of spinal sensory projection interneurons using whole cell patch recordings from single neurons or pairs, identified by dye filling. Dorsolateral interneurons lie in the tadpole equivalent of the spinal dorsal horn, and have dorsally located dendrites and an ipsilateral ascending axon that projects into the midbrain. These neurons receive direct, mainly AMPA receptor (AMPAR)-mediated, excitation from skin touch sensory neurons. They in turn produce AMPAR and N-methyl-d-aspartate receptor (NMDAR)-mediated excitation of spinal locomotor pattern generator neurons rostrally on the same side of the cord. During swimming, they receive glycinergic modulatory inhibition from ascending interneurons in the spinal locomotor central pattern generator. We conclude that spinal dorsolateral interneurons are a primitive class of excitatory sensory projection neurons activated by ipsilateral cutaneous afferents and carrying excitation ipsilaterally and rostrally as far as the midbrain to initiate or accelerate swimming.

Distribution of prospective glutamatergic, glycinergic, and GABAergic neurons in embryonic and larval zebrafish

Zebrafish are an excellent model for studies of the functional organization of neuronal circuits, but little is known regarding the transmitter phenotypes of the neurons in their nervous system. We examined the distribution in spinal cord and hindbrain of neurons expressing markers of transmitter phenotype, including the vesicular glutamate transporter (VGLUT) genes for glutamatergic neurons, the neuronal glycine transporter (GLYT2) for glycinergic neurons, and glutamic acid decarboxylase (GAD65/67) for GABAergic neurons. All three markers were expressed in a large domain in the dorsal two-thirds of spinal cord, with additional, more ventral expression domains for VGLUT2 and GAD/GABA. In the large dorsal domain, dual in situ staining showed that GLYT2-positive cells were intermingled with VGLUT2 cells, with no dual-stained neurons. Many of the neurons in the dorsal expression domain that were positive for GABA markers at embryonic stages were also positive for GLYT2, suggesting that the cells might use both GABA and glycine, at least early in their development. The intermingling of neurons expressing inhibitory and excitatory markers in spinal cord contrasted markedly with the organization in hindbrain, where neurons expressing a particular marker were clustered together to form stripes that were visible running from rostral to caudal in horizontal sections and from dorsomedial to ventrolateral in cross sections. Dual labeling showed that the stripes of neurons labeled with one transmitter marker alternated with stripes of cells labeled for the other transmitter phenotypes. The differences in the distribution of excitatory and inhibitory neurons in spinal cord versus hindbrain may be tied to differences in their patterns of development and functional organization.

Neurotransmitter properties of spinal interneurons in embryonic and larval zebrafish

Many classes of spinal interneurons in zebrafish have been described based on morphology, but their transmitter phenotypes are largely unknown. Here we combine back-filling or genetic labeling of spinal interneurons with in situ staining for markers of neurotransmitter phenotypes, including the vesicular glutamate transporter (VGLUT) genes for glutamatergic neurons, the neuronal glycine transporter (GLYT2) for glycinergic neurons, and glutamic acid decarboxylase (GAD) for GABAergic neurons. Neurons positive for VGLUT include the commissural CoPA, MCoD, UCoD, and some of the CoSA neurons. The CiD interneurons, which have ipsilateral descending axons, were also VGLUT-positive, as were the ventrally located VeMe interneurons, whose descending axonal trajectory has not been clearly revealed. Cells positive for GLYT2 include the commissural CoLAs as well as some of the CoBL and CoSA neurons. The CiA cells were the only GLYT2-positive cells with an ipsilateral axon. Cells staining for GAD included, most notably, the dorsal longitudinal ascending (DoLA) and KA interneurons. Our approach allowed us to define the likely transmitter phenotypes of most of the known classes of spinal interneurons. These data provide a foundation for understanding the functional organization of the spinal networks in zebrafish.