Chronic embryonic MK-801 exposure disrupts the somatotopic organization of cutaneous nerve projections in the chick spinal cord

Action-based sensory encoding in spinal sensorimotor circuits

The concept of a modular organisation of the spinal withdrawal reflex circuits has proven to be fundamental for the understanding of how the spinal cord is organised and how the sensorimotor circuits translate sensory information into adequate movement corrections. Recent studies indicate that a task-related body representation is engraved at the network level through learning-dependent mechanisms involving an active probing procedure termed 'somatosensory imprinting' during development. It was found that somatosensory imprinting depends on the tactile input that is associated with spontaneous movements that occur during sleep and results in elimination of erroneous connections and establishment of correct connections. In parallel studies it was found that the strength of the first order tactile synapses in rostrocaudally elongated zones in the adult dorsal horn in the lower lumbar cord is related to the modular organisation of the withdrawal reflexes. Hence, the topographical organisation of the tactile input to this spinal area seems to be action-based rather than a simple body map as previously thought. Far from being innate and adult like at birth, the adult organisation seems to emerge from an initial 'floating' and diffuse body representation with many inappropriate connections through profound activity-dependent rearrangements of afferent synaptic connections. It is suggested that somatosensory imprinting plays a key role in the self-organisation of the spinal cord during development.

The development of vestibulo-ocular circuitry in the chicken embryo

This article reviews studies of the organization and development of the vestibulo-ocular reflex arc in the chicken embryo. It summarizes some of the principal features that characterize the development of this circuit, including the gradual clustering of motoneurons in the oculomotor nucleus into functionally identifiable motoneuron pools, the patterning of vestibular projection neurons into coherent clusters with specific axonal trajectories and terminations onto the oculomotor motoneuron pools, the reverse order of synapse formation during development (motoneuron to muscle, then vestibular projection neuron to motoneuron), and the selectivity of initial synaptic termination at both the ultimate and penultimate relays within the reflex arc. Reference to studies in other vertebrate species is made to provide a comparative context, and potential mechanisms are discussed that may contribute to the underlying synaptic specificity in this circuit.

Posthatching locomotor experience alters locomotor development in chicks

We have previously demonstrated that, even though chicks are very precocial and can locomote within hours of hatching, they require a period of time to develop a mature stable walk. As an example, 1- to 2-day-old animals move with disproportionately small stride lengths compared with 10- to 14-day-old animals. The purpose of this study was to determine whether the maturation of walking, including the development of a mature stride length, depends on locomotor experience. We also investigated the development and experience-dependence nature of head bobbing, an optokinetic behavior that occurs during walking in birds. Chicks were randomly assigned to one of three groups receiving either increased locomotor experience (i.e., treadmill exercise), decreased locomotor experience (i.e., decreased housing space), or no alteration in locomotor experience. To assess the dependence of locomotor maturation on N-methyl-D-aspartate (NMDA)-type glutamate receptors, animals in each group were either given an NMDA antagonist (MK-801, 1 mg/kg intramuscularly daily) or saline control. Locomotor characteristics (stride length, leg support durations, horizontal head excursions) were quantified from videotaped recordings of chicks walking overground unrestrained on posthatching days 1, 2, 4, 6, 8, and 10. Animals subject to exercise restriction for at least 6 days moved with shortened stride lengths compared with age-matched treadmill-exercised or control animals, a change that was maintained for the duration of the study. NMDA antagonism also resulted in shortened stride lengths. Head bobbing behavior matured during the same posthatching time period. The rate of this maturation was also decreased by exercise restriction. Thus locomotor experience is required for normal development of locomotor behavior, even in very precocial animals. These results are discussed in terms of the possible neuroanatomical and neurophysiological mechanisms underlying experience- and activity-dependent changes during motor development.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Mechanisms that initiate spontaneous network activity in the developing chick spinal cord

Many developing networks exhibit a transient period of spontaneous activity that is believed to be important developmentally. Here we investigate the initiation of spontaneous episodes of rhythmic activity in the embryonic chick spinal cord. These episodes recur regularly and are separated by quiescent intervals of many minutes. We examined the role of motoneurons and their intraspinal synaptic targets (R-interneurons) in the initiation of these episodes. During the latter part of the inter-episode interval, we recorded spontaneous, transient ventral root depolarizations that were accompanied by small, spatially diffuse fluorescent signals from interneurons retrogradely labeled with a calcium-sensitive dye. A transient often could be resolved at episode onset and was accompanied by an intense pre-episode (approximately 500 ms) motoneuronal discharge (particularly in adductor and sartorius) but not by interneuronal discharge monitored from the ventrolateral funiculus (VLF). An important role for this pre-episode motoneuron discharge was suggested by the finding that electrical stimulation of motor axons, sufficient to activate R-interneurons, could trigger episodes prematurely. This effect was mediated through activation of R-interneurons because it was prevented by pharmacological blockade of either the cholinergic motoneuronal inputs to R-interneurons or the GABAergic outputs from R-interneurons to other interneurons. Whole-cell recording from R-interneurons and imaging of calcium dye-labeled interneurons established that R-interneuron cell bodies were located dorsomedial to the lateral motor column (R-interneuron region). This region became active before other labeled interneurons when an episode was triggered by motor axon stimulation. At the beginning of a spontaneous episode, whole-cell recordings revealed that R-interneurons fired a high-frequency burst of spikes and optical recordings demonstrated that the R-interneuron region became active before other labeled interneurons. In the presence of cholinergic blockade, however, episode initiation slowed and the inter-episode interval lengthened. In addition, optical activity recorded from the R-interneuron region no longer led that of other labeled interneurons. Instead the initial activity occurred bilaterally in the region medial to the motor column and encompassing the central canal. These findings are consistent with the hypothesis that transient depolarizations and firing in motoneurons, originating from random fluctuations of interneuronal synaptic activity, activate R-interneurons, which then trigger the recruitment of the rest of the spinal interneuronal network. This unusual function for R-interneurons is likely to arise because the output of these interneurons is functionally excitatory during development.

Development of specific connectivity between premotor neurons and motoneurons in the brain stem and spinal cord

Astounding progress has been made during the past decade in understanding the general principles governing the development of the nervous system. An area of prime physiological interest that is being elucidated is how the neural circuitry that governs movement is established. The concerted application of molecular biological, anatomical, and electrophysiological techniques to this problem is yielding gratifying insight into how motoneuron, interneuron, and sensory neuron identities are determined, how these different neuron types establish specific axonal projections, and how they recognize and synapse upon each other in patterns that enable the nervous system to exercise precise control over skeletal musculature. This review is an attempt to convey to the physiologist some of the exciting discoveries that have been made, within a context that is intended to link molecular mechanism to behavioral realization. The focus is restricted to the development of monosynaptic connections onto skeletal motoneurons. Principal topics include the inductive mechanisms that pattern the placement and differentiation of motoneurons, Ia sensory afferents, and premotor interneurons; the molecular guidance mechanisms that pattern the projection of premotor axons in the brain stem and spinal cord; and the precision with which initial synaptic connections onto motoneurons are established, with emphasis on the relative roles played by cellular recognition versus electrical activity. It is hoped that this review will provide a guide to understanding both the existing literature and the advances that await this rapidly developing topic.

Locomotor plasticity after spinal injury in the chick

Functional recovery after spinal cord injury likely depends, in part, on the reorganization of undamaged spinal circuitry. Segmental afferent input from the limbs remains largely intact after spinal injury and may provide an important source of activation and regulation of the spinal circuits that have lost descending input as a result of the injury. This purpose of this study was to investigate the contribution of cutaneous afferent inputs to the recovery of motor function after spinal injury in the chick. After lateral thoracic spinal hemisection, the motion of the ipsilateral limb was impaired during both walking and swimming. By 2 weeks postoperatively, limb motion recovered to preoperative values for walking but not for swimming. It was hypothesized that phasic afferent inputs experienced during walking, but not swimming, contributed to recovery of limb motion during walking. When a source of phasic cutaneous input was provided during swim training sessions, limb motion gradually improved to preoperative values. After 2 weeks of training, this improved motion was retained even after the source of cutaneous stimulation was removed. The proposed mechanism is an experience-dependent strengthening of the circuits activated during the improved limb motion, leading to a permanent change in limb action during swimming. Thus, the afferent inputs experienced during movement repetition are important during the acquisition of learned movements after spinal injury. These results are discussed in terms of behavioral, physiological, and anatomical evidence for spinal plasticity in other species. It is concluded that the spinal cord has significant plastic capabilities, and efforts should be directed toward maximizing the contribution of this plasticity to functional recovery after spinal cord injury.

Highly reproducible spatiotemporal patterns of mammalian embryonic movements at the developmental stage of the earliest spontaneous motility

The principles underlying the variations in patterns of mammalian embryonic movements have not been established. In an attempt to clarify the mechanism that is responsible for the variations in motor patterns, we carried out a precise quantitative spatiotemporal analysis of movements in mouse embryos, using a transplacental perfusion method for the in vitro maintenance of live mammalian embryos. Episodes of spontaneous movements at the inception of motility, at embryonic day 12.5, occurred once every few minutes, lasted for several seconds and consisted of successive movements of body regions, the spatiotemporal patterns of which varied from episode to episode. By analysing and categorizing the patterns of these movements, we found that embryonic movements follow relatively few restricted patterns with respect to the order of the movements of body regions. A further analysis of episodes at high spatiotemporal resolution revealed that most of the episodes in a major category could be classified into two distinct subtypes. Each of these subtypes had its own highly reproducible spatiotemporal patterns of movement. Overall, these results show that early embryonic movements follow relatively few rather stereotyped patterns, and random local fluctuations have little effect on such movement patterns. The appearance of one pattern out of several rather stereotyped patterns may be the main cause of apparent variations in patterns of early embryonic movements. The stereotyped patterns may represent important orderly characteristics of spontaneous embryonic activities that may be involved in the development of orderly structures and functions in higher animals.

Transformations in embryonic motility in chick: kinematic correlates of type I and II motility at E9 and E12

Soon after hatching, chicks exhibit an array of adaptive, coordinated behaviors. Chick embryos also acquire nearly 18 days of movement experience, referred to as embryonic motility, before hatching. The chick expresses three forms of motility, types I, II, and III, and each emerges at a different stage of embryonic development. Although much is known about the mechanisms associated with motility at early embryonic stages and at the onset of hatching, the transformations in behavior and underlying mechanisms are not fully understood. Thus the purpose of this study was to determine how motility is modified during the first expected transformation, from type I to type II. It was hypothesized that kinematic features for motility at embryonic day 12 (E12) would differ significantly from features at E9 because type II motility emerges during E11. Embryos were video taped for extended intervals in ovo at E9 or E12 and entire sequences of motility were computer digitized for kinematic analyses. Results reported here indicate that several of the kinematic features characteristic of motility at E9 are also reliable features at E12. On the basis of these findings, a kinematic definition of type I motility is posed for use in subsequent behavioral studies. Several parameters distinguished motility at E12 from E9. The most notable difference between ages was the less regular timing of repetitive limb movements at E12, a finding consistent with recent reports suggesting early motility is an emergent product of a transient neural network rather than a specialized pattern generator. As predicted from established definitions for type II motility, startle-like movements were common at E12; however, they also were present in many kinematic plots at E9, suggesting the discreet age-dependent boundaries in the established definition for type II motility may require modification. Some age-related differences, such as increased intralimb coordination and excursion velocity, may be prerequisites for adaptive behavior after hatching.

Brain waves and brain wiring: the role of endogenous and sensory-driven neural activity in development

Neural activity is critical for sculpting the intricate circuits of the nervous system from initially imprecise neuronal connections. Disrupting the formation of these precise circuits may underlie many common neurodevelopmental disorders, ranging from subtle learning disorders to pervasive developmental delay. The necessity for sensory-driven activity has been widely recognized as crucial for infant brain development. Recent experiments in neurobiology now point to a similar requirement for endogenous neural activity generated by the nervous system itself before sensory input is available. Here we use the formation of precise neural circuits in the visual system to illustrate the principles of activity-dependent development. Competition between the projections from lateral geniculate nucleus neurons that receive sensory input from the two eyes shapes eye-specific connections from an initially diffuse projection into ocular dominance columns. When the competition is altered during a critical period for these changes, by depriving one eye of vision, the normal ocular dominance column pattern is disrupted. Before ocular dominance column formation, the highly ordered projection from retina to lateral geniculate nucleus develops. These connections form before the retina can respond to light, but at a time when retinal ganglion cells spontaneously generate highly correlated bursts of action potentials. Blockade of this endogenous activity, or biasing the competition in favor of one eye, results in a severe disruption of the pattern of retinogeniculate connections. Similar spontaneous, correlated activity has been identified in many locations in the developing central nervous system and is likely to be used during the formation of precise connections in many other neural systems. Understanding the processes of activity-dependent development could revolutionize our ability to identify, prevent, and treat developmental disorders resulting from disruptions of neural activity that interfere with the formation of precise neural circuits.

Mechanisms of spontaneous activity in developing spinal networks

Developing networks of the chick spinal cord become spontaneously active early in development and remain so until hatching. Experiments using an isolated preparation of the spinal cord have begun to reveal the mechanisms responsible for this activity. Whole-cell and optical recordings have shown that spinal neurons receive a rhythmic, depolarizing synaptic drive and experience rhythmic elevations of intracellular calcium during spontaneous episodes. Activity is expressed throughout the neuraxis and can be produced by different parts of the cord and by the isolated brain stem, suggesting that it does not depend upon the details of network architecture. Two factors appear to be particularly important for the production of endogenous activity. The first is the predominantly excitatory nature of developing synaptic connections, and the second is the presence of prolonged activity-dependent depression of network excitability. The interaction between high excitability and depression results in an equilibrium in which episodes are expressed periodically by the network. The mechanism of the rhythmic bursting within an episode is not understood, but it may be due to a "fast" form of network depression. Spontaneous embryonic activity has been shown to play a role in neuron and muscle development, but is probably not involved in the initial formation of connections between spinal neurons. It may be important in refining the initial connections, but this possibility remains to be explored.

Blockade and recovery of spontaneous rhythmic activity after application of neurotransmitter antagonists to spinal networks of the chick embryo

We studied the regulation of spontaneous activity in the embryonic (day 10-11) chick spinal cord. After bath application of either an excitatory amino acid (AP-5 or CNQX) and a nicotinic cholinergic (DHbetaE or mecamylamine) antagonist, or glycine and GABA receptor (bicuculline, 2-hydroxysaclofen, and strychnine) antagonists, spontaneous activity was blocked for a period (30-90 min) but then reappeared in the presence of the drugs. The efficacy of the antagonists was assessed by their continued ability to block spinal reflex pathways during the reappearance of spontaneous activity. Spontaneous activity ceased over the 4-5 hour monitoring period when both sets of antagonists were applied together. After application of glycine and GABA receptor antagonists, the frequency of occurrence of spontaneous episodes slowed and became highly variable. By contrast, during glutamatergic and nicotinic cholinergic blockade, the frequency of occurrence of spontaneous episodes initially slowed and then recovered to stabilize near the predrug level of activity. Whole-cell recordings made from ventral spinal neurons revealed that this recovery was accompanied by an increase in the amplitude of spontaneously occurring synaptic events. We also measured changes in the apparent equilibrium potential of the rhythmic, synaptic drive of ventral spinal neurons using voltage or discontinuous current clamp. After excitatory blockade, the apparent equilibrium potential of the rhythmic synaptic drive shifted approximately 10 mV more negative to approximately -30 mV. In the presence of bicuculline, the apparent equilibrium potential of the synaptic drive shifted toward the glutamate equilibrium potential. Considered with other evidence, these findings suggest that spontaneous rhythmic output is a general property of developing spinal networks, and that GABA and glycinergic networks alter their function to compensate for the blockade of excitatory transmission.

Receptive properties of embryonic chick sensory neurons innervating skin

Receptive properties of embryonic chick sensory neurons innervating skin. J. Neurophysiol. 78: 2560-2568, 1997. We describe a new in vitro skin-nerve preparation from chick embryos that allows detailed study of the functional properties of developing sensory neurons innervating skin. Functionally single sensory afferents were isolated by recording from their axons in microdissected filaments of the cutaneous femoralis medialis nerve, which innervates skin of the thigh. A total of 157 single neurons were characterized from embryos [embryonic days 17-21 (E17-E21), n = 115] and hatchlings up to 3 wk old (n = 42). Neurons were initially classified on the basis of their conduction velocity; those conducting below 1.0 m/s were being classified as C fibers and faster conducting fibers as A fibers. The proportions of A and C fibers encountered in embryonic and hatchling preparations were not very different, indicating that myelination and axon growth proceeds quite slowly over the period studied. Afferent fibers that could subserve nociceptive and nonnociceptive functions were identified in the time period studied. Subpopulations of low-threshold myelinated afferent units exhibited rapidly or slowly adapting discharges to constant force stimuli and could have tactile functions. Many afferent fibers responded to noxious heat and were excited and sensitized by exposure to inflammatory mediators, suggesting that they are nociceptors. The behavior of these units changed in several respects over the period studied. The discharge of C fibers to noxious heat increased with age as did their mechanical thresholds. A substantial population of heat-responsive neurons (34% of the A fibers) present in embryos were not encountered in hatchling chicks. This indicates that substantial changes in the physiological response properties of sensory afferents occur after hatching. We conclude that this new preparation can be used for quantitative assessment of the receptive properties of developing sensory neurons and has considerable potential for the investigation of factors, such as neurotrophins, that specify and influence the functional phenotype of sensory neurons during embryonic development in vivo.

Evidence that intersegmental competition influences the segmental distribution of the central terminals of muscle sensory afferents in the chicken embryo

The possibility that muscle sensory afferents that enter the spinal cord at different segmental levels compete for terminal space in the lateral motor column was tested by unilaterally ablating neural crest cells destined to generate dorsal root ganglia (DRGs). Newly migrating neural crest was surgically removed from several lumbosacral segments on one side of the spinal cord on day 3 of embryogenesis (d3). The resultant experimental preparations developed without DRGs on the operated side at the segmental levels where the ablation was performed. At d18 the central projection from the intact DRG immediately caudal to the operated segments was anatomically mapped with a lipophilic axonal tracer. The central projection from the same DRG on the opposite, unoperated side was mapped as a control. The density of terminals along the longitudinal axis of the lateral motor column was quantified in serial sections with an image analysis program. On both the operated and control sides, the majority of terminals were found within the segment of entry and the rostral and caudal neighboring spinal segments. However, the rostral neighboring segment received a larger proportion of terminals on the operated side compared to the control side. Thus, the absence of primary afferents entering the spinal cord at specific segments leads to a shift of the afferent projection to the lateral motor column from the neighboring segments towards the deafferented segments.

Ethanol exposure alters the development of serotonergic neurons in chick spinal cord

Exposure to ethanol is known to alter the development of the serotonergic system. However, previous studies have examined large populations of cells and have not determined the effects of ethanol on individual serotonergic neurons. In the present study, the effects of various concentrations of ethanol on the development of single serotonergic neurons in the chick embryo spinal cord were determined using immunohistochemical techniques. Between embryonic day 7 (E7) and E14, ethanol administrations produced in ovo alcohol concentrations of: a) low dose, 30-60 mg/dl, b) medium dose, 150-200 mg/dl or c) high dose, 240-300 mg/dl. In animals exposed to the medium and high ethanol doses, the normal developmental increase in cross-sectional area of the somata was not observed. At all stages examined, the numbers of primary and nonprimary processes were significantly lower in ethanol-treated groups compared to controls. These data indicate that ethanol exposure induces dose-dependent alterations in the development of identified spinal cord neurons. The ethanol-induced changes may be involved in the motor dysfunction observed after embryonic ethanol exposure.

Overexpression of nerve growth factor in epidermis of transgenic mice preserves excess sensory neurons but does not alter the somatotopic organization of cutaneous nerve projections

To determine how target-derived nerve growth factor (NGF) affects sensory neuronal survival and the development of topographic nerve projections in the spinal cord, anatomical studies were performed on transgenic mice that overexpress NGF in skin and other keratinized epithelial structures. Transgenic animals showed a 100% increase in the number of sensory neurons in specific dorsal root ganglia and exhibited significantly more fibers immunoreactive for calcitonin gene-related peptide in the dorsal horn compared to control animals. This confirms earlier studies which suggested that naturally occurring sensory neuronal death is decreased, or eliminated, in the transgenic mice. Nerve labeling studies showed that the somatotopic organization of cutaneous nerve projections was not altered in the transgenic animals. These data suggest that neuronal death does not act to remove sensory neurons that project to inappropriate regions of the spinal cord.