Rostrocaudal progression in the development of periodic spontaneous activity in fetal rat spinal motor circuits in vitro

Characterisation of the consequences of maternal immune activation on distinct cell populations in the developing rat spinal cord

Maternal immune activation (MIA) during gestation has been implicated in the development of neurological disorders such as schizophrenia and autism. Epidemiological studies have suggested that the effect of MIA may depend on the gestational timing of the immune challenge and the region of the central nervous system (CNS) in question. This study investigated the effects of MIA with 100 μg/kg lipopolysaccharide at either Embryonic days (E)12 or E16 on the oligodendrocytes, microglia and astrocytes of the offspring spinal cord. At E16, MIA decreased the number of olig2⁺ and Iba‐1⁺ cells in multiple grey and white matter regions of the developing spinal cord 5 h after injection. These decreases were not observed at postnatal day 14. In contrast, MIA at E12 did not alter Olig2⁺ or Iba‐1⁺ cell number in the developing spinal cord 5 h after injection, however, Olig2⁺ cell number was decreased in the ventral grey matter of the P14 spinal cord. No changes were observed in glial fibrillary acidic protein (GFAP) expression at P14 following MIA at either E12 or E16. These data suggest that E16 may be a window of immediate vulnerability to MIA during spinal cord development, however, the findings also suggest that the developmental process may be capable of compensation over time. Potential changes in P14 animals following the challenge at E12 are indicative of the complexity of the effects of MIA during the developmental process.

Prenatal exposure to nicotine disrupts synaptic network formation by inhibiting spontaneous correlated wave activity

Correlated spontaneous activity propagating over a wide region of the central nervous system is expressed during a specific period of embryonic development. We previously demonstrated using an optical imaging technique with a voltage-sensitive dye that this wave-like activity, which we referred to as the depolarization wave, is fundamentally involved in the early process of synaptic network formation. We found that the in ovo application of bicuculline/strychnine or d-tubocurarine, which blocked the neurotransmitters mediating the wave, significantly reduced functional synaptic expression in the brainstem sensory nucleus. This result, particularly for d-tubocurarine, an antagonist of nicotinic acetylcholine receptors, suggested that prenatal nicotine exposure associated with maternal smoking affects the development of neural circuit formation by interfering with the correlated wave. In the present study, we tested this hypothesis by examining the effects of nicotine on the correlated activity and assessing the chronic action of nicotine in ovo on functional synaptic expression along the vagal sensory pathway. In ovo observations of chick embryo behavior and electrical recording using in vitro preparations showed that the application of nicotine transiently increased embryonic movements and electrical bursts associated with the wave, but subsequently inhibited these activities, suggesting that the dominant action of the drug was to inhibit the wave. Optical imaging with the voltage-sensitive dye showed that the chronic exposure to nicotine in ovo markedly reduced functional synaptic expression in the higher-order sensory nucleus of the vagus nerve, the parabrachial nucleus. The results suggest that prenatal nicotine exposure disrupts the initial formation of the neural circuitry by inhibiting correlated spontaneous wave activity.

Spinal motoneuron firing properties mature from rostral to caudal during postnatal development of the mouse

Key points

Many mammals are born with immature motor systems that develop through a critical period of postnatal development.

In rodents, postnatal maturation of movement occurs from rostral to caudal, correlating with maturation of descending supraspinal and local spinal circuits.

We asked whether development of fundamental electrophysiological properties of spinal motoneurons follows the same rostro‐caudal sequence.

We show that in both regions, repetitive firing parameters increase and excitability decreases with development; however, these characteristics mature earlier in cervical motoneurons.

We suggest that in addition to autonomous mechanisms, motoneuron development depends on activity resulting from their circuit milieu.

Abstract

Altricial mammals are born with immature nervous systems comprised of circuits that do not yet have the neuronal properties and connectivity required to produce future behaviours. During the critical period of postnatal development, neuronal properties are tuned to participate in functional circuits. In rodents, cervical motoneurons are born prior to lumbar motoneurons, and spinal cord development follows a sequential rostro‐caudal pattern. Here we asked whether birth order is reflected in the postnatal development of electrophysiological properties. We show that motoneurons of both regions have similar properties at birth and follow the same developmental profile, with maximal firing increasing and excitability decreasing into the third postnatal week. However, these maturative processes occur in cervical motoneurons prior to lumbar motoneurons, correlating with the maturation of premotor descending and local spinal systems. These results suggest that motoneuron properties do not mature by cell autonomous mechanisms alone, but also depend on developing premotor circuits.

Many mammals are born with immature motor systems that develop through a critical period of postnatal development.

In rodents, postnatal maturation of movement occurs from rostral to caudal, correlating with maturation of descending supraspinal and local spinal circuits.

We asked whether development of fundamental electrophysiological properties of spinal motoneurons follows the same rostro‐caudal sequence.

We show that in both regions, repetitive firing parameters increase and excitability decreases with development; however, these characteristics mature earlier in cervical motoneurons.

We suggest that in addition to autonomous mechanisms, motoneuron development depends on activity resulting from their circuit milieu.

Development of Spontaneous Activity in the Avian Hindbrain

Spontaneous activity in the developing central nervous system occurs before the brain responds to external sensory inputs, and appears in the hindbrain and spinal cord as rhythmic electrical discharges of cranial and spinal nerves. This spontaneous activity recruits a large population of neurons and propagates like a wave over a wide region of the central nervous system. Here, we review spontaneous activity in the chick hindbrain by focusing on this large-scale synchronized activity. Asynchronous activity that is expressed earlier than the above mentioned synchronized activity and activity originating in midline serotonergic neurons are also briefly mentioned.

Connectivity of pacemaker neurons in the neonatal rat superficial dorsal horn

Pacemaker neurons with an intrinsic ability to generate rhythmic burst-firing have been characterized in lamina I of the neonatal spinal cord, where they are innervated by high-threshold sensory afferents. However, little is known about the output of these pacemakers, as the neuronal populations that are targeted by pacemaker axons have yet to be identified. The present study combines patch-clamp recordings in the intact neonatal rat spinal cord with tract-tracing to demonstrate that lamina I pacemaker neurons contact multiple spinal motor pathways during early life. Retrograde labeling of premotor interneurons with the trans-synaptic pseudorabies virus PRV-152 revealed the presence of burst-firing in PRV-infected lamina I neurons, thereby confirming that pacemakers are synaptically coupled to motor networks in the spinal ventral horn. Notably, two classes of pacemakers could be distinguished in lamina I based on cell size and the pattern of their axonal projections. Whereas small pacemaker neurons possessed ramified axons that contacted ipsilateral motor circuits, large pacemaker neurons had unbranched axons that crossed the midline and ascended rostrally in the contralateral white matter. Recordings from identified spino-parabrachial and spino-periaqueductal gray neurons indicated the presence of pacemaker activity within neonatal lamina I projection neurons. Overall, these results show that lamina I pacemakers are positioned to regulate both the level of activity in developing motor circuits and the ascending flow of nociceptive information to the brain, thus highlighting a potential role for pacemaker activity in the maturation of pain and sensorimotor networks in the central nervous system.

Electrical stimulation of transplanted motoneurons improves motor unit formation

Motoneurons die following spinal cord trauma and with neurological disease. Intact axons reinnervate nearby muscle fibers to compensate for the death of motoneurons, but when an entire motoneuron pool dies, there is complete denervation. To reduce denervation atrophy, we have reinnervated muscles in Fisher rats from local transplants of embryonic motoneurons in peripheral nerve. Since growth of axons from embryonic neurons is activity dependent, our aim was to test whether brief electrical stimulation of the neurons immediately after transplantation altered motor unit numbers and muscle properties 10 wk later. All surgical procedures and recordings were done in anesthetized animals. The muscle consequences of motoneuron death were mimicked by unilateral sciatic nerve section. One week later, 200,000 embryonic day 14 and 15 ventral spinal cord cells, purified for motoneurons, were injected into the tibial nerve 10-15 mm from the gastrocnemii muscles as the only neuron source for muscle reinnervation. The cells were stimulated immediately after transplantation for up to 1 h using protocols designed to examine differential effects due to pulse number, stimulation frequency, pattern, and duration. Electrical stimulation that included short rests and lasted for 1 h resulted in higher motor unit counts. Muscles with higher motor unit counts had more reinnervated fibers and were stronger. Denervated muscles had to be stimulated directly to evoke contractions. These results show that brief electrical stimulation of embryonic neurons, in vivo, has long-term effects on motor unit formation and muscle force. This muscle reinnervation provides the opportunity to use patterned electrical stimulation to produce functional movements.

Electrical stimulation of embryonic neurons for 1 hour improves axon regeneration and the number of reinnervated muscles that function

Motoneuron death after spinal cord injury or disease results in muscle denervation, atrophy, and paralysis. We have previously transplanted embryonic ventral spinal cord cells into the peripheral nerve to reinnervate denervated muscles and to reduce muscle atrophy, but reinnervation was incomplete. Here, our aim was to determine whether brief electrical stimulation of embryonic neurons in the peripheralnerve changes motoneuron survival, axon regeneration, and muscle reinnervation and function because neural depolarization is crucial for embryonic neuron survival and may promote activity-dependent axon growth. At 1 week after denervation by sciatic nerve section, embryonic day 14 to 15 cells were purified for motoneurons, injected into the tibial nerve of adult Fischer rats, and stimulated immediatelyfor up to 1 hour. More myelinated axons were present in tibial nerves 10 weeks after transplantation when transplants had been stimulated acutely at 1 Hz for 1 hour. More muscles were reinnervated if the stimulation treatment lasted for 1 hour. Reinnervation reduced muscle atrophy, with or without the stimulation treatment. These data suggest that brief stimulation of embryonic neurons promotes axon growth, which has a long-term impact on muscle reinnervation and function. Muscle reinnervation is important because it may enable the use of functional electrical stimulation to restore limb movements.

Large-scale synchronized activity in the embryonic brainstem and spinal cord

In the developing central nervous system, spontaneous activity appears well before the brain responds to external sensory inputs. One of the earliest activities is observed in the hindbrain and spinal cord, which is detected as rhythmic electrical discharges of cranial and spinal motoneurons or oscillations of Ca(2+)- and voltage-related optical signals. Shortly after the initial expression, the spontaneous activity appearing in the hindbrain and spinal cord exhibits a large-scale correlated wave that propagates over a wide region of the central nervous system, maximally extending to the lumbosacral cord and to the forebrain. In this review, we describe several aspects of this synchronized activity by focusing on the basic properties, development, origin, propagation pattern, pharmacological characteristics, and possible mechanisms underlying the generation of the activity. These profiles differ from those of the respiratory and locomotion pattern generators observed in the mature brainstem and spinal cord, suggesting that the wave is primordial activity that appears during a specific period of embryonic development and plays some important roles in the development of the central nervous system.

Timing of developmental sequences in different brain structures: physiological and pathological implications

The developing brain is not a small adult brain. Voltage- and transmitter-gated currents, like network-driven patterns, follow a developmental sequence. Studies initially performed in cortical structures and subsequently in subcortical structures have unravelled a developmental sequence of events in which intrinsic voltage-gated calcium currents are followed by nonsynaptic calcium plateaux and synapse-driven giant depolarising potentials, orchestrated by depolarizing actions of GABA and long-lasting NMDA receptor-mediated currents. The function of these early patterns is to enable heterogeneous neurons to fire and wire together rather than to code specific modalities. However, at some stage, behaviourally relevant activities must replace these immature patterns, implying the presence of programmed stop signals. Here, we show that the developing striatum follows a developmental sequence in which immature patterns are silenced precisely when the pup starts locomotion. This is mediated by a loss of the long-lasting NMDA-NR2C/D receptor-mediated current and the expression of a voltage-gated K(+) current. At the same time, the descending inputs to the spinal cord become fully functional, accompanying a GABA/glycine polarity shift and ending the expression of developmental patterns. Therefore, although the timetable of development differs in different brain structures, the g sequence is quite similar, relying first on nonsynaptic events and then on synaptic oscillations that entrain large neuronal populations. In keeping with the 'neuroarcheology' theory, genetic mutations or environmental insults that perturb these developmental sequences constitute early signatures of developmental disorders. Birth dating developmental disorders thus provides important indicators of the event that triggers the pathological cascade leading ultimately to disease.

Pharmacological mechanisms underlying switching from the large-scale depolarization wave to segregated activity in the mouse central nervous system

During the early development of the nervous system, synchronized activity is observed in a variety of structures, and is considered to play a fundamental role in neural development. One of the most striking examples of such activity is the depolarization wave reported in chick and rat embryos. In the accompanying paper (Momose-Sato et al., 2012), we have demonstrated that a depolarization wave is also present in the mouse embryo by showing large-scale optical waves, which spread remarkably over the central nervous system, including the spinal cord, hindbrain, cerebellum, midbrain, and forebrain. In the present study, we examined the pharmacological nature of the mouse depolarization wave and its developmental changes. We show here that two types of switching in pharmacological characteristics occur during development. One is that the depolarization wave is strongly dependent on nicotinic acetylcholine receptors during the early developmental stage [embryonic day (E)11-12], but is dominated by glutamate at the later stage (E13 onwards). The second is that γ-aminobutyric acid (GABA), which acts as an excitatory mediator of the depolarization wave during the early phase, becomes an inhibitory modulator by E14. These changes seemed to occur earlier in the hindbrain than in the spinal cord. Furthermore, we show that the second switch causes the loss of synchronization over the network, resulting in the disappearance of the depolarization wave and segregation of the activity into discrete regions of the medulla and spinal cord. We suggest that pharmacological switching is a possible mechanism underlying replacement of the primordial correlated network by a mature neuronal circuit.

Spontaneous depolarization wave in the mouse embryo: origin and large-scale propagation over the CNS identified with voltage-sensitive dye imaging

Spontaneous embryonic movements, called embryonic motility, are produced by correlated spontaneous activity in the cranial and spinal nerves, which is driven by brainstem and spinal networks. Using optical imaging with a voltage-sensitive dye, we have revealed previously that this correlated activity is a widely propagating wave of neural depolarization, which we termed the depolarization wave. We have observed in the chick and rat embryos that the activity spread over an extensive region of the CNS, including the spinal cord, hindbrain, cerebellum, midbrain and forebrain. One important consideration is whether a depolarization wave with similar characteristics occurs in other species, especially in different mammals. Here, we provide evidence for the existence of the depolarization wave in the mouse embryo by showing that the widely propagating wave appeared independently of the localized spontaneous activity detected previously with Ca(2+) imaging. Furthermore, we mapped the origin of the depolarization wave and revealed that the wave generator moved from the rostral spinal cord to the caudal cord as development proceeded, and was later replaced with mature rhythmogenerators. The present study, together with an accompanying paper that describes pharmacological properties of the mouse depolarization wave, shows that a synchronized wave with common characteristics is expressed in different species, suggesting fundamental roles in neural development.

Primary afferent terminals acting as excitatory interneurons contribute to spontaneous motor activities in the immature spinal cord

Patterned, spontaneous activity plays a critical role in the development of neuronal networks. A robust spontaneous activity is observed in vitro in spinal cord preparations isolated from immature rats. The rhythmic ventral root discharges rely mainly on the depolarizing/excitatory action of GABA and glycine early during development, whereas at later stages glutamate drive is primarily responsible for the rhythmic activity and GABA/glycine are thought to play an inhibitory role. However, rhythmic discharges mediated by the activation of GABA(A) receptors are recorded from dorsal roots (DRs). In the present study, we used the in vitro spinal cord preparation of neonatal rats to identify the relationship between discharges that are conducted antidromically along DRs and the spontaneous activity recorded from lumbar motoneurons. We show that discharges in DRs precede those in ventral roots and that primary afferent depolarizations (PADs) start earlier than EPSPs in motoneurons. EPSP-triggered averaging revealed that the action potentials propagate not only antidromically in the DR but also centrally and trigger EPSPs in motoneurons. Potentiating GABAergic antidromic discharges by diazepam increased the EPSPs recorded from motoneurons; conversely, blocking DR bursts markedly reduced these EPSPs. High intracellular concentrations of chloride are maintained in primary afferent terminals by the sodium-potassium-chloride cotransporter NKCC1. Blocking these cotransporters by bumetanide decreased both dorsal and ventral root discharges. We conclude that primary afferent fibers act as excitatory interneurons and that GABA, through PADs reaching firing threshold, is still playing a key role in promoting spontaneous activity in neonates.

Contribution of the potassium-chloride co-transporter KCC2 to the modulation of lumbar spinal networks in mice

Spontaneous activity is observed in most developing neuronal circuits, such as the retina, hippocampus, brainstem and spinal cord. In the spinal cord, spontaneous activity is important for generating embryonic movements critical for the proper development of motor axons, muscles and synaptic connections. A spontaneous bursting activity can be recorded in vitro from ventral roots during perinatal development. The depolarizing action of the inhibitory amino acids γ-aminobutyric acid and glycine is widely proposed to contribute to spontaneous activity in several immature systems. During development, the intracellular chloride concentration decreases, leading to a shift of equilibrium potential for Cl(-) ions towards more negative values, and thereby to a change in glycine- and γ-aminobutyric acid-evoked potentials from depolarization/excitation to hyperpolarization/inhibition. The up-regulation of the outward-directed Cl(-) pump, the neuron-specific potassium-chloride co-transporter type 2 KCC2, has been shown to underlie this shift. Here, we investigated whether spontaneous and locomotor-like activities are altered in genetically modified mice that express only 8-20% of KCC2, compared with wild-type animals. We show that a reduced amount of KCC2 leads to a depolarized equilibrium potential for Cl(-) ions in lumbar motoneurons, an increased spontaneous activity and a faster locomotor-like activity. However, the left-right and flexor-extensor alternating pattern observed during fictive locomotion was not affected. We conclude that neuronal networks within the spinal cord are more excitable in KCC2 mutant mice, which suggests that KCC2 strongly modulates the excitability of spinal cord networks.

The role of inhibitory neurotransmission in locomotor circuits of the developing mammalian spinal cord

Neuronal circuits generating the basic coordinated limb movements during walking of terrestrial mammals are localized in the spinal cord. In these neuronal circuits, called central pattern generators (CPGs), inhibitory synaptic transmission plays a crucial part. Inhibitory synaptic transmission mediated by glycine and GABA is thought to be essential in coordinated activation of muscles during locomotion, in particular, controlling temporal and spatial activation patterns of muscles of each joint of each limb on the left and right side of the body. Inhibition is involved in other aspects of locomotion such as control of speed and stability of the rhythm. However, the precise roles of neurotransmitters and their receptors mediating inhibitory synaptic transmission in mammalian spinal CPGs remain unclear. Moreover, many of the inhibitory interneurones essential for output pattern of the CPG are yet to be identified. In this review, recent advances on these issues, mainly from studies in the developing rodent spinal cord utilizing electrophysiology, molecular and genetic approaches are discussed.

GABAergic and glycinergic interneuron expression during spinal cord development: dynamic interplay between inhibition and excitation in the control of ventral network outputs

A key objective of neuroscience research is to understand the processes leading to mature neural circuitries in the central nervous system (CNS) that enable the control of different behaviours. During development, network-constitutive neurons undergo dramatic rearrangements, involving their intrinsic properties, such as the blend of ion channels governing their firing activity, and their synaptic interactions. The spinal cord is no exception to this rule; in fact, in the ventral horn the maturation of motor networks into functional circuits is a complex process where several mechanisms cooperate to achieve the development of motor control. Elucidating such a process is crucial in identifying neurons more vulnerable to degenerative or traumatic diseases or in developing new strategies aimed at rebuilding damaged tissue. The focus of this review is on recent advances in understanding the spatio-temporal expression of the glycinergic/GABAergic system and on the contribution of this system to early network function and to motor pattern transformation along with spinal maturation. During antenatal development, the operation of mammalian spinal networks strongly depends on the activity of glycinergic/GABAergic neurons, whose action is often excitatory until shortly before birth when locomotor networks acquire the ability to generate alternating motor commands between flexor and extensor motor neurons. At this late stage of prenatal development, GABA-mediated excitation is replaced by synaptic inhibition mediated by glycine and/or GABA. At this stage of spinal maturation, the large majority of GABAergic neurons are located in the dorsal horn. We propose that elucidating the role of inhibitory systems in development will improve our knowledge on the processes regulating spinal cord maturation.

Switching of the transmitters that mediate hindbrain correlated activity in the chick embryo

Widely propagating correlated neuronal activity is a hallmark of the developing nervous system. The activity is usually mediated by multiple transmitters, and the contribution of gap junctions has also been suggested in several systems. In some structures, such as the retina and spinal cord, it has been shown that the dominant transmitter mediating the correlated wave switches from acetylcholine to glutamate during development, although the functional significance of this phenomenon has not been clarified. An important question is whether such a transmitter switch occurs in other systems, especially in the brain. In the present study, we demonstrate that the major transmitter mediating correlated wave activity in the embryonic chick hindbrain changes from acetylcholine/gamma-aminobutyric acid (GABA)/glycine to glutamate/GABA as development proceeds. The results show for the first time that the dominant transmitter switches from acetylcholine to glutamate in a region other than the retina and spinal cord. This finding sheds more light on the role of nicotinic acetylcholine receptors in the generation of correlated wave activity, which is considered to regulate the development of the nervous system.

Origin of the earliest correlated neuronal activity in the chick embryo revealed by optical imaging with voltage-sensitive dyes

Spontaneous correlated neuronal activity during early development spreads like a wave by recruiting a large number of neurons, and is considered to play a fundamental role in neural development. One important and as yet unresolved question is where the activity originates, especially at the earliest stage of wave expression. In other words, which part of the brain differentiates first as a source of the correlated activity, and how does it change as development proceeds? We assessed this issue by examining the spatiotemporal patterns of the depolarization wave, the optically identified primordial correlated activity, using the optical imaging technique with voltage-sensitive dyes. We surveyed the region responsible for the induction of the evoked and spontaneous depolarization waves in chick embryos, and traced its developmental changes. The results showed that the wave initially originated in a restricted area near the obex and was generated by multiple regions at later stages. We suggest that the upper cervical cord/lower medulla near the obex is the kernel that differentiates first as the source of the correlated activity, and that regional and temporal differences in neuronal excitability might underlie the developmental profile of wave generation in early chick embryos.

Central respiratory rhythmogenesis is abnormal in lbx1- deficient mice

Lbx1 is a transcription factor that determines neuronal cell fate and identity in the developing medulla and spinal cord. Newborn Lbx1 mutant mice die of respiratory distress during the early postnatal period. Using in vitro brainstem-spinal cord preparations we tested the hypothesis that Lbx1 is necessary for the inception, development and modulation of central respiratory rhythmogenesis. The inception of respiratory rhythmogenesis at embryonic day 15 (E15) was not perturbed in Lbx1 mutant mice. However, the typical age-dependent increase in respiratory frequency observed in wild-type from E15 to P0 was not observed in Lbx1 mutant mice. The slow respiratory rhythms in E18.5 Lbx1 mutant preparations were increased to wild-type frequencies by application of substance P, thyrotropin releasing hormone, serotonin, noradrenaline, or the ampakine drug 1-(1,4-benzodioxan-6-yl-carbonyl) piperidine. Those data suggest that respiratory rhythm generation within the pre-Bötzinger complex (preBötC) is presumably functional in Lbx1 mutant mice with additional neurochemical drive. This was supported by anatomical data showing that the gross structure of the preBötC was normal, although there were major defects in neuronal populations that provide important modulatory drive to the preBötC including the retrotrapezoid nucleus, catecholaminergic brainstem nuclei, nucleus of the solitary tract, and populations of inhibitory neurons in the ventrolateral and dorsomedial medullary nuclei. Finally, we determined that those defects were caused by abnormalities of neuronal specification early in development or subsequent neuronal migration.

Dual personality of GABA/glycine-mediated depolarizations in immature spinal cord

The inhibitory action of glycine and GABA in adult neurons consists of both shunting incoming excitations and moving the membrane potential away from the action potential (AP) threshold. By contrast, in immature neurons, inhibitory postsynaptic potentials (IPSPs) are depolarizing; it is generally accepted that, despite their depolarizing action, these IPSPs are inhibitory because of the shunting action of the Cl(-) conductance increase. Here we investigated the integration of depolarizing IPSPs (dIPSPs) with excitatory inputs in the neonatal rodent spinal cord by means of both intracellular recordings from lumbar motoneurons and a simulation using the compartment model program "Neuron." We show that the ability of IPSPs to suppress suprathreshold excitatory events depends on E(Cl) and the location of inhibitory synapses. The depolarization outlasts the conductance changes and spreads electrotonically in the somatodendritic tree, whereas the shunting effect is restricted and local. As a consequence, dIPSPs facilitated AP generation by subthreshold excitatory events in the late phase of the response. The window of facilitation became wider as E(Cl) was more depolarized and started earlier as inhibitory synapses were moved away from the excitatory input. GAD65/67 immunohistochemistry demonstrated the existence of distal inhibitory synapses on motoneurons in the neonatal rodent spinal cord. This study demonstrates that small dIPSPs can either inhibit or facilitate excitatory inputs depending on timing and location. Our results raise the possibility that inhibitory synapses exert a facilitatory action on distant excitatory inputs and slight changes of E(Cl) may have important consequences for network processing.

Spontaneous depolarization waves of multiple origins in the embryonic rat CNS

During development, correlated neuronal activity plays an important role in the establishment of the central nervous system (CNS). We have previously reported that a widely propagating correlated neuronal activity, termed the depolarization wave, is evoked by various sensory inputs. A remarkable feature of the depolarization wave is that it spreads broadly through the brain and spinal cord. In the present study, we examined whether the depolarization wave occurs spontaneously in the embryonic rat CNS and, if so, where it originates. In E15-16 rat embryos, spontaneous optically-revealed signals appeared in association with the rhythmic discharges of cranial motoneurons and propagated widely with similar characteristics to the evoked depolarization wave. At E15, the spontaneous wave mostly originated in the cervical to upper lumbar cords. At E16, the wave was predominantly generated in the lumbosacral cord although a wave associated with the second oscillatory burst was initiated in the rostral cord. At E16, a few waves also originated in the rostral ventrolateral medulla and the dorsomedial pons. When the influence of the caudal cord was removed by transecting the spinal cord, the contribution of the medulla and pons became more significant. These results show that the depolarization wave can be triggered by the spontaneous activity of multiple neuronal populations which are distributed widely from the pons to the lumbosacral cord, although the spinal cord usually plays a predominant role. This network possibly works as a self-distributing system that maintains the incidence and complicated patterns of the correlated activity in the developing CNS.

Spatial restriction of spontaneous activity towards the rostral primary initiating zone during development of the embryonic mouse hindbrain

In the developing embryonic mouse hindbrain, we have previously shown that synchronized spontaneous activity is driven by midline serotonergic neurons at E11.5. This is mediated, at least in part, by the 5-HT2A receptor, which is expressed laterally in the hindbrain. Activity at E11.5 is widespread within the hindbrain tissue, propagating from the midline to more lateral regions. Using rapid acquisition of [Ca2+]i events along the midline, we now show that the rostral midline, primarily in the region of former rhombomere r2, is the primary initiating zone for this activity. We propose that at E11.5, the combined events along the rostral-caudal axis in combination with events propagating along the medial-lateral axis could assign positional information to developing neurons within the hindbrain. With further development, to E13.5, both the lateral and caudal dimensions of spontaneous activity retract to the rostral midline, occupying an area only 14% of that exhibited at E11.5. We also show that increased levels of [K+]o (to 8 mM) at E13.5 are able to increase the spread of spontaneous activity laterally and rostro-caudally. This suggests that spontaneous activity in the hindbrain depends in a dynamic way on the dominant initiating zone of the rostral midline, and that this relationship changes over development.

Glycine receptors regulate interneuron differentiation during spinal network development

Glycinergic and GABAergic excitatory chloride-mediated signaling is often the first form of activity to emerge in the nascent nervous system and has been proposed to be essential for several aspects of nervous system development. However, few studies have examined the effects of disrupting glycinergic transmission. Here we perturbed glycinergic transmission in vivo from the onset of development in zebrafish and examined its impact on the formation of the locomotor circuitry. Targeted knockdown of the embryonic glycine receptor alpha2-subunit disrupted rhythm-generating networks and reduced the frequency of spontaneous glycinergic and glutamatergic events. Immunohistochemistry revealed a reduction in the number of spinal interneurons without affecting sensory and motor neurons. This effect was accompanied by a concomitant increase in the number of mitotic cells, suggesting that glycine receptors regulate interneuron differentiation during early development. Despite the loss of many interneurons, a subthreshold rhythm-generating circuit was still capable of forming. These data provide evidence that glycine receptors, in addition to their role in neurotransmission, regulate interneuron differentiation during development of this central neural network.

Serotoninergic modulation of chloride homeostasis during maturation of the locomotor network in zebrafish

During development, neural networks progress through important functional changes such as the generation of spontaneous activity, the expression of a depolarizing chloride gradient, and the appearance of neuromodulation. Little is known about how these processes are integrated to yield mature behaviors. We showed previously that, during the maturation of the locomotor network of the zebrafish, endogenous serotonin (5HT) increased motor activity by reducing intervals of inactivity, without affecting the active swim periods that are the target of 5HT in other and more mature preparations. Because membrane properties were constant during the rest intervals, we examined here whether 5HT modulates chloride homeostasis. We compared the effects of blocking (inward) chloride cotransport with bumetanide to the effects of 5HT and its antagonists, both behaviorally by video imaging and cellularly by whole-cell and gramicidin-perforated patch recordings. Bumetanide mimicked the effects of 5HT antagonists, by prolonging rest intervals without affecting the properties of swim episodes (duration; frequency; extent of depolarization) either behaviorally or during fictive swimming. Furthermore, bumetanide and 5HT antagonists suppressed the amplitude of depolarizing responses evoked by ionophoresis of glycine onto spinal neurons in the presence of tetrodotoxin and transiently suppressed the amplitude of responses to glycine measured after fictive swimming. The effects of bumetanide contrasted with and occluded the effects of 5HT. We suggest that, during development, endogenous 5HT modulates chloride homeostasis during the quiescent intervals and thereby offsets the long periods of quiescence commonly observed in developing networks to allow expression of sustained and behaviorally relevant activity.

Primary role of the serotonergic midline system in synchronized spontaneous activity during development of the embryonic mouse hindbrain

In the developing embryonic mouse hindbrain, we have shown that previously widespread synchronized spontaneous activity at E11.5 retracts to the initiating zone of the rostral hindbrain by E13.5, and ceases completely by E14.5. We now confirm that at E11.5 and E13.5, the primary driver of spontaneous activity is serotonergic input, while other transmitters (GABA, glutamate, NE, and ATP) have only modulatory roles. Using immunocytochemistry, we also show that at E13.5, 5-HT-positive neurons in the midline extend over a larger rostro-caudal distance than at E11.5, and that in the presumptive initiating zone, cell bodies occupy a band that extends 200 microm laterally on each side of the midline, with extensive axonal processes. The 5-HT2A receptor retains expression in lateral tissue over this developmental time. We find that in addition to being sensitive to 5-HT receptor antagonists, spontaneous activity is also abolished by blockers of gap junctions, and is increased in frequency and lateral spread by application of ammonium, presumably via increased intracellular pH augmenting gap junction conductance. Thus, 5-HT neurons of the midline remain the primary drivers of spontaneous activity at several stages of development in the hindbrain, relying in part on gap junctional communication during initiation of activity.

Rhythmic neuronal discharge in the medulla and spinal cord of fetal rats in the absence of synaptic transmission

Spontaneous rhythmic neuronal activity is generated in the developing vertebrate nervous system. The patterned activity spreads diffusely throughout the fetal neuraxis. Here we demonstrate the ability of the fetal rat spinal cord and medulla to generate and transmit robust rhythmic patterns in the absence of synaptic activity. Regular rhythmic discharges were produced by fetal tissue bathed in low or zero [Ca(2+)](o) solution. The activity persisted in the presence of antagonists to neurotransmitter receptors that are known to mediate synaptic-mediated events associated with fetal rhythms. A combination of ventral root recordings and optical imaging using voltage-sensitive dyes demonstrated the extensive spread of rhythmic discharge in spinal cord and medullary neuronal populations of in vitro preparations. Whole cell recordings from medullary slices were performed to examine the ionic conductances and revealed the importance of persistent sodium conductances for generation of rhythmic activity in hypoglossal (XII) motoneurons. Rhythmic bursting in XII motoneurons persisted in the presence of gap junction blockers, although the amplitude of synchronized motor discharge recorded from nerve roots was diminished. We propose that nonsynaptically mediated conductances, potentially by extracellular ionic flux and/or ephaptic and electrotonic interactions mechanisms, act in concert with neurochemical transmission and gap junctions to promote the diffuse spread of rhythmic motor patterns in the developing nervous system.

Optical Imaging of Large-Scale Correlated Wave Activity in the Developing Rat CNS

Correlated neuronal activity plays a fundamental role in the development of the nervous system. Using a multiple-site optical recording technique with a fast voltage-sensitive dye, we previously reported a novel form of correlated activity in the chick embryo, which showed wide propagation throughout the CNS. In this study, we report that similar wave activity is generated in the embryonic rat CNS. Electrical stimulation applied to the cervical cord evoked wave activity that traveled over a wide region of the CNS including the medulla, pons, midbrain, diencephalon, and spinal cord. Small signals were also detected from the cerebellum and part of the cerebrum. Stimulation applied to the cranial nerves such as the trigeminal and vagus nerves evoked waves with similar patterns, indicating that the wave is triggered by external sensory inputs. This wave activity was inhibited by glutamate-, acetylcholine-, GABA- and glycine-receptor antagonists in addition to gap junction blockers such as octanol and 18β-glycyrrhetinic acid. In the immunohistochemical study, significant immunoreactivity of connexin26 and connexin32 was also observed. Wave activity detected with a voltage-sensitive dye was accompanied by a Ca2+-wave, indicating that it not only provides electrical synchrony but also biochemical signals associated with [Ca2+]i elevation. These characteristics of the wave activity are similar to those of the depolarization wave reported in the chick embryo, suggesting that the large-scale depolarization wave is globally generated across different species.

Ion Channel Development, Spontaneous Activity, and Activity-Dependent Development in Nerve and Muscle Cells

At specific stages of development, nerve and muscle cells generate spontaneous electrical activity that is required for normal maturation of intrinsic excitability and synaptic connectivity. The patterns of this spontaneous activity are not simply immature versions of the mature activity, but rather are highly specialized to initiate and control many aspects of neuronal development. The configuration of voltage- and ligand-gated ion channels that are expressed early in development regulate the timing and waveform of this activity. They also regulate Ca2+influx during spontaneous activity, which is the first step in triggering activity-dependent developmental programs. For these reasons, the properties of voltage- and ligand-gated ion channels expressed by developing neurons and muscle cells often differ markedly from those of adult cells. When viewed from this perspective, the reasons for complex patterns of ion channel emergence and regression during development become much clearer.

Changes on the properties of glycine receptors during neuronal development

Glycine receptors (GlyRs) play a major role in the excitability of spinal cord and brain stem neurons. During development, several properties of these receptors undergo significant changes resulting in major modifications of their physiological functions. For example, the receptor structure switches from a monomeric alpha or heteromeric alpha 2 beta in immature neurons to an alpha 1 beta receptor type in mature neurons. Together with these changes in receptor subunits, the postsynaptic cluster size increases with development. Parallel to these modifications, the apparent receptor affinity to glycine and strychnine, as well as that of Zn(2+) and ethanol increases with time. The mature receptor is characterized by a slow desensitizing current and high sensitivity to modulation by protein kinase C. Also, the high level of glycinergic transmission in immature spinal neurons modulates neuronal excitability causing membrane depolarization and changes in intracellular calcium. Due to these properties, chronic inhibition of glycinergic transmission affects neurite outgrowth and produces changes in the level of synaptic transmission induced by GABA(A) and AMPA receptors. Finally, the high level of plasticity found in immature GlyRs is likely associated to changes in cytoskeleton dynamics.

Interneurone bursts are spontaneously associated with muscle contractions only during early phases of mouse spinal network development: a study in organotypic cultures

For a short time during development immature circuits in the spinal cord and other parts of the central nervous system spontaneously generate synchronous patterns of rhythmic activity. In the case of the spinal cord, it is still unclear how strongly synchronized bursts generated by interneurones are associated with motoneurone firing and whether the progressive decline in spontaneous bursting during circuit maturation proceeds in parallel for motoneurone and interneurone networks. We used organotypic cocultures of spinal cord and skeletal muscle in order to investigate the ontogenic evolution of endogenous spinal network activity associated with the generation of coordinate muscle fibre contractions. A combination of multiunit electrophysiological recordings, videomicroscopy and optical flow computation allowed us to measure the correlation between interneurone firing and motoneurone outputs after 1, 2 and 3 weeks of in vitro development. We found that, in spinal organotypic slices, there is a developmental switch of spontaneous activity from stable bursting to random patterns after the first week in culture. Conversely, bursting recorded in the presence of strychnine and bicuculline became increasingly regular with time in vitro. The time course of spontaneous activity maturation in organotypic slices is similar to that previously reported for the spinal cord developing in utero. We also demonstrated that spontaneous bursts of interneurone action potentials strongly correlate with muscular contractions only during the first week in vitro and that this is due to the activation of motoneurones via AMPA-type glutamate receptors. These results indicate the occurrence in vitro of motor network development regulating bursting inputs from interneurones to motoneurones.

Multiple Spontaneous Rhythmic Activity Patterns Generated by the Embryonic Mouse Spinal Cord Occur Within a Specific Developmental Time Window

Spontaneous rhythmic activity is a ubiquitous phenomenon in developing neural networks and is assumed to play an important role in the elaboration of mature circuitry. Here we describe the day-by-day evolution of spontaneous activity in the embryonic mouse spinal cord and show that, at a specific developmental stage, 2 distinct rhythms coexist. On embryonic days E12.5 and E13.5, we observed a single type of regularly recurring short spike-episodes synchronized across cervical, thoracic, and lumbar levels. By E14.5, in addition to this motor rhythm, another type of spontaneous synchronous activity appeared, characterized by much longer lasting episodes separated by longer time intervals. On E15.5, these long episodes disappeared. Short episodes were less numerous and more irregular except at the cervical level where a rhythm was occasionally observed. By E16.5, this cervical rhythm became more robust, whereas the lumbar level fell almost silent. Surprisingly, at E17.5, spontaneous activity resumed at caudal levels, now characterized by numerous erratic short episodes. A striking ontogenetic feature of spontaneous activity was the occurrence of long episodes only at E14.5. Although concomitant at all levels of the spinal cord, long episodes displayed different patterns along the spinal cord, with tonic firing at the thoracic level and rhythmic discharge with occasional sequences of left/right alternation at the lumbar level. Thus at E14.5, the originally synchronized network has started to segregate into more specialized subnetworks. In conclusion, this work suggests that ongoing spontaneous rhythms do not follow a smooth evolution during maturation, but rather undergo profound changes at very specific stages.

Serotonin patterns locomotor network activity in the developing zebrafish by modulating quiescent periods

Developing neural networks follow common trends such as expression of spontaneous, recurring activity patterns, and appearance of neuromodulation. How these processes integrate to yield mature, behaviorally relevant activity patterns is largely unknown. We examined the integration of serotonergic neuromodulation and its role in the functional organization of the accessible locomotor network in developing zebrafish at behavioral and cellular levels. Locally restricted populations of serotonergic neurons and their projections appeared in the hindbrain and spinal cord of larvae after hatching (approximately day 2). However, 5-HT affected the swimming pattern only from day 4 on, when sustained spontaneous swimming appeared. 5-HT and its agonist quipazine increased motor output by reducing intervals of inactivity, observed behaviorally (by high-speed video) and in recordings from spinal neurons during fictive swimming (by whole-cell current clamp). 5-HT and quipazine had little effect on the properties of the activity periods, such as the duration of swim episodes and swim frequency. Further, neuronal input resistance, rheobasic current, and resting potential were not affected significantly. The 5-HT antagonists methysergide and ketanserin decreased motor output by prolonging the periods of inactivity with little effect on the active swim episode or neuronal properties. Our results suggest that 5-HT neuromodulation is integrated early in development of the locomotor network to increase its output by reducing periods of inactivity with little effect on the activity periods, which in contrast are the main targets of 5-HT neuromodulation in neonatal and adult preparations.

The ontogeny of mammalian sleep: a reappraisal of alternative hypotheses

Newborn mammals spend as much as 90% or more of their time in a sleep state characterized by frequent twitches, rapid eye movements (REMs), and irregular respiratory cycles. These motor and respiratory patterns resemble the phasic motor/respiratory components of adult REM sleep, and as a consequence, this sleep state is traditionally viewed as an immature form of REM sleep. An alternative view is that a significant portion of what has been called REM sleep in these species is a form of spontaneous activity typical of the immature nervous system. In this review, we compare and contrast these two opposing views about the ontogenetic origins of REM sleep, and review the evidence most often cited to support the idea that REM sleep is present in newborn altricial mammals. Critical review of this evidence indicates that REM sleep may not be present at birth in these species; rather, it appears that all mammals early in development exhibit spontaneous, dissociated activity that progressively becomes organized into the distinct states of REM and non-rapid eye movement sleep.

Ontogeny of rhythmic motor patterns generated in the embryonic rat spinal cord

Patterned spontaneous activity is generated in developing neuronal circuits throughout the CNS including the spinal cord. This activity is thought to be important for activity-dependent neuronal growth, synapse formation, and the establishment of neuronal networks. In this study, we examine the spatiotemporal distribution of motor patterns generated by rat spinal cord and medullary circuits from the time of initial axon outgrowth through to the inception of organized respiratory and locomotor rhythmogenesis during late gestation. This includes an analysis of the neuropharmacological control of spontaneous rhythms generated within the spinal cord at different developmental stages. In vitro spinal cord and medullary-spinal cord preparations isolated from rats at embryonic ages (E)13.5-E21.5 were studied. We found age-dependent changes in the spatiotemporal pattern, neurotransmitter control, and propensity for the generation of spontaneous rhythmic motor discharge during the prenatal period. The developmental profile of the neuropharmacological control of rhythmic bursting can be divided into three periods. At E13.5-E15.5, the spinal networks comprising cholinergic and glycinergic synaptic interconnections are capable of generating rhythmic activity, while GABAergic synapses play a role in supporting the spontaneous activity. At late stages (E18.5-E21.5), glutamate drive acting via non- N-methyl-d-aspartate (non-NMDA) receptors is primarily responsible for the rhythmic activity. During the middle stage (E16.5-E17.5), the spontaneous activity results from the combination of synaptic drive acting via non-NMDA glutamatergic, nicotinic acetylcholine, glycine, and GABA(A) receptors. The modulatory actions of chloride-mediated conductances shifts from predominantly excitatory to inhibitory late in gestation.

Development of the locomotor network in zebrafish

The zebrafish is a leading model for studies of vertebrate development and genetics. Its embryonic motor behaviors are easy to assess (e.g. for mutagenic screens), the embryos develop rapidly (hatching as larvae at 2 days) and are transparent, permitting calcium imaging and patch clamp recording in vivo. We review primarily the recent advances in understanding the cellular basis for the development of motor activities in the developing zebrafish. The motor activities are generated largely in the spinal cord and hindbrain. In the embryo these segmented structures possess a relatively small number of repeating sets of identifiable neurons. Many types of neurons as well as the two types of muscle cells have been classified based on their morphologies. Some of the molecular signals for cellular differentiation have been identified recently and mutations affecting cell development have been isolated. Embryonic motor behaviors appear in sequence and consist of an early period of transient spontaneous coiling contractions, followed by the emergence of twitching responses to touch, and later by the ability to swim. Coiling contractions are generated by an electrically coupled network of a subset of spinal neurons whereas a chemical (glutamatergic and glycinergic) synaptic drive underlies touch responses and swimming. Swimming becomes sustained in larvae once the neuromodulatory serotonergic system develops. These results indicate many similarities between developing zebrafish and other vertebrates in the properties of the synaptic drive underlying locomotion. Therefore, the zebrafish is a useful preparation for gaining new insights into the development of the neural control of vertebrate locomotion. As the types of neurons, transmitters, receptors and channels used in the locomotor network are being defined, this opens the possibility of combining cellular neurophysiology with forward and reverse molecular genetics to understand the principles of locomotor network assembly and function.

Development of posture and locomotion: an interplay of endogenously generated activities and neurotrophic actions by descending pathways

The adult pattern of locomotion is observed at the end of the second postnatal week in the rat. The in vitro spinal cord isolated from immature rats has served as a valuable preparation to study the mechanisms underlying the development of locomotion. Although the rat is unable to walk at birth, because of an immature posture, its spinal cord networks can generate at least two kinds of motor patterns in vitro. One activity is called 'fictive locomotion' because it shares several common features with locomotion observed in vivo. This fictive locomotor pattern is rarely observed spontaneously and its release requires either pharmacological or electrical stimulation of the spinal cord. A second endogenously generated activity observed in this preparation occurs spontaneously and exhibits phase relationships between motor outputs that are quite different from the fictive locomotor pattern. Here we review some of the developmental functions this spontaneous activity may subserve. It is likely a major trigger for the maturation of lumbar networks in the fetus, at a stage when inputs from both the periphery and supraspinal structures are weak. Pathways descending from the brainstem arrive in the lumbar enlargement during the last week in utero and the first two postnatal weeks. These pathways, through the neurotransmitters they contain, especially monoamines, are essential for the expression of some neuronal properties and may regulate several ongoing developmental processes.

Postural modifications and neuronal excitability changes induced by a short-term serotonin depletion during neonatal development in the rat

Serotonin (5-HT) plays an important role both in the development and in the recovery of locomotion after spinalization in vertebrates. We investigated the contribution of the serotonergic system to the maturation of the lumbar motoneurons and networks in the neonatal rat. A 5-HT synthesis inhibitor, p-chlorophenylalanine (PCPA), was administered daily from the first postnatal day (P0) onward. This protocol depleted serotonin in the spinal cord within 3-4 d, as demonstrated by immunohistochemistry. PCPA-treated rats exhibited postural changes characterized by lesser flexion at the knee and ankle levels and lesser extension of the hip. Posture was asymmetric, suggesting possible deficits in the interlimb coordination. Intracellular recordings were made at P3-5 from motoneurons innervating different hindlimb muscles, using the in vitro brainstem-spinal cord-nerve-attached preparation. In PCPA-treated rats, the conduction velocity of motoneurons was increased, and their excitability was decreased (because of higher rehobase and input conductance) compared with sham animals. In accordance with postural observations, changes were more pronounced in hip extensor/knee flexor than in ankle extensor motoneurons. The maturation of repetitive firing properties was stopped by PCPA treatment, although PCPA, applied in vitro, had no effect on membrane properties. The spontaneous endogenously generated activity, which is a characteristic of immature networks, was increased in PCPA-treated rats, suggesting that developing lumbar networks are sensitive to 5-HT levels. Serotonin may play a critical role during development in regulating the balance between the excitability of motoneurons and that of interneurons. Interneuronal excitability is crucial for the activity-dependent development of spinal cord networks.

Descending 5-hydroxytryptamine raphe inputs repress the expression of serotonergic neurons and slow the maturation of inhibitory systems in mouse embryonic spinal cord

Spontaneous synchronous rhythmic activities are a common feature of immature neuronal networks. Although the mechanisms underlying such activities have been studied extensively, whether they might be controlled by modulatory information remains questionable. Here, we investigated the role of descending serotonergic (5-HT) inputs from the medulla to the spinal cord in the maturation of rhythmic activity. We found that in spinal cords maintained, as a whole, in organotypic culture without the medulla, the maturation of spontaneous activity is similar to that found in spinal cords developed in utero. Interestingly, in organotypic cultures without the medulla (i.e., devoid of descending inputs), numerous intraspinal neurons expressed 5-HT, unlike in spinal cords cultivated in the presence of the medulla or matured in utero. We demonstrated that this 5-HT expression was specifically dependent on the absence of 5-HT fibers and was repressed by 5-HT itself via activation of 5-HT(1A) receptors. Finally, to verify whether the expression of 5-HT intraspinal neurons could compensate for the lack of descending 5-HT fibers and play a role in the development of spontaneous activity, we blocked the 5-HT synthesis using p-chlorophenylalanine methyl ester in cultures devoid of the medulla. Surprisingly, we found that this pharmacological treatment did not prevent the development of spontaneous activity but accelerated the maturation of intraspinal inhibition at the studied stages. Together, our data indicate that descending 5-HT raphe inputs (1) repress the expression of spinal serotonergic neurons and (2) slow the maturation of inhibitory systems in mouse spinal cord.

Early expression of glycine and GABA(A) receptors in developing spinal cord neurons. Effects on neurite outgrowth

Using fluorometric and immunocytochemical techniques, we found that high glycine concentrations or blockade of glycine receptors increases neurite outgrowth in developing mouse spinal cord neurons. Glycine- and GABA(A)-activated currents were demonstrated during applications of glycine and GABA (50-100 microM) in 5 days in vitro (DIV) neurons. Long application (> or =10 min) of 100 microM glycine desensitized the membrane response by more than 95%. Application of glutamate in the absence of external Mg(2+), at several membrane potentials, did not produce any detectable membrane response in these cells. Immunocytochemical studies with NR1 and GluR1 antibodies showed a delayed appearance of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptors respectively. Spontaneous synaptic activity was readily observed in 5 DIV neurons. The use of various receptor antagonists (strychnine, bicuculline, DL-2-amino-5-phosphonovalerate [APV], 6-cyano-7-nitroquinoxaline-2,3-dione [CNQX]) revealed that this activity was predominantly glycinergic, and to a smaller extent, GABAergic. In the presence of bicuculline, APV and CNQX, we detected abundant spontaneous depolarizing potentials which often reached the action potential threshold. Further evidence for functional synaptic activity was provided by the detection of co-localization of gephyrin and synaptophysin at 5 DIV using confocal microscopy. Fluorometric studies with Fluo-3, a Ca(2+) indicator, in 5 DIV cultures showed the presence of spontaneous fluctuations associated with tetrodotoxin-sensitive synaptic events. The number of neurons displaying these fluctuations was significantly increased (>100%) when the cells were bathed in a strychnine-containing solution. On the other hand, these synaptically mediated Ca(2+) events were blocked by the co-application of strychnine and bicuculline. This suggests that glycine and GABA(A) receptors provide a fundamental regulation of both neuronal excitability and intracellular Ca(2+) at this early time of development.The neurotrophic effects of agonists and antagonists for glycine, GABA(A) and glutamate receptors were examined in neurons cultured for 2 or 5 DIV. From all the agonists used, only high concentrations of glycine increased neurite outgrowth in 5 DIV neurons. We found that strychnine also increased neurite outgrowth, whereas tetrodotoxin (1 microM), nimodipine (4 microM) and bicuculline (20 microM) completely blocked it. On the other hand, APV (50 microM) and CNQX (20 microM) were unable to affect neurite outgrowth. These data suggest that spinal glycine receptors depress neurite outgrowth by shunting neuronal excitability. Outgrowth induction possibly results from the enhanced activity found after the inhibition of glycinergic activity. We postulate that this resets the intracellular calcium at a concentration that favors neurite outgrowth.

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.

A critical role of the strychnine-sensitive glycinergic system in spontaneous retinal waves of the developing rabbit

In the developing vertebrate retina, spontaneous electric activity occurs rhythmically in the form of propagating waves and is believed to play a critical role in activity-dependent visual system development, including the establishment of precise retinal and geniculate circuitry. To elucidate how spontaneous retinal waves encode specific developmental cues at various developmental stages, it is necessary to understand how the waves are generated and regulated. Using Ca(2+) imaging and patch clamp in a flat-mount perinatal rabbit retinal preparation, this study demonstrates that, in addition to the cholinergic system, a strychnine-sensitive system in the inner retina plays an obligatory and developmentally regulated role in the initiation and propagation of spontaneous retinal waves. This system, which is believed to be the glycinergic network, provided an excitatory drive during early retinal development. It then became inhibitory after postnatal day 1 (P1) to P2, an age when a number of coordinated transitions in neurotransmitter systems occurred concomitantly, and finally contributed to the complete inhibition and disappearance of spontaneous waves after P7-P9. This glycinergic contribution was notably distinct from that of the ionotropic GABAergic system, which was found to exert an inhibitory but nonessential influence on the early wave formation. Blocking glycine- and GABA-gated anion currents had opposing effects on spontaneous retinal waves between embryonic day 29 and P0, suggesting that Cl(-) transporters, particularly R(+)-butylindazone-sensitive K-Cl cotransporters, may have a synapse- and/or cell type-specific distribution pattern, in addition to an age-dependent expression pattern in the inner retina. Overall, the results revealed an important reliance of spontaneous retinal waves on dynamic and coordinated interactions among multiple, nonredundant neurotransmitter systems.

Developing networks play a similar melody

During development, when synapses start to be established, a primitive form of network-driven activity provides most of the synaptic activity. This pattern enables a high degree of synchrony in immature neurons in spite of the small number of functional synapses and could participate in activity-dependent growth and synapse formation. Relying on the giant depolarizing potentials that provide most of the synaptic activity in the developing hippocampus, this article reviews the common properties and generating mechanisms of these patterns, and particularly the role of the early depolarizing action of GABA(A) and glycine receptors and the sequential expression of GABA and glutamate synapses. Patterns similar to giant depolarizing potentials have been observed in a wide range of structures and species suggesting that there is a temporal template throughout evolution that constitutes an essential step in the formation of functional networks.

Perinatal development of lumbar motoneurons and their inputs in the rat

The rat is quite immature at birth and a rapid maturation of motor behavior takes place during the first 2 postnatal weeks. Lumbar motoneurons undergo a rapid development during this period. The last week before birth represents the initial stages of motoneuron differentiation, including regulation of the number of cells and the arrival of segmental and first supraspinal afferents. At birth, motoneurons are electrically coupled and receive both appropriate and inappropriate connections from the periphery; the control from supraspinal structures is weak and exerted mainly through polysynaptic connections. During the 1st postnatal week, inappropriate sensori-motor contacts and electrical coupling disappear, the supraspinal control increases gradually and myelin formation is responsible for an increased conduction velocity in both descending and motor axons. Both N-methyl-D-aspartate (NMDA) and non-NMDA receptors are transiently overexpressed in the neonatal spinal cord. The contribution of non-NMDA receptors to excitatory amino acid transmission increases with age. Activation of gamma-aminobutyric acid(A) and glycine receptors leads to membrane depolarization in embryonic motoneurons but to hyperpolarization in older motoneurons. The firing properties of motoneurons change with development: they are capable of more repetitive firing at the end of the 1st postnatal week than before birth. However, maturation does not proceed simultaneously in the motor pools innervating antagonistic muscles; for instance, the development of repetitive firing of ankle extensor motoneurons lags behind that of flexor motoneurons. The spontaneous embryonic and neonatal network-driven activity, detected at the levels of motoneurons and primary afferent terminals, may play a role in neuronal maturation and in the formation and refinement of sensorimotor connections.

Formation of the central pattern generator for locomotion in the rat and mouse

It is well known that in the neonatal rat spinal cord preparation, alternating rhythmic bursts in the left and right ventral roots in a given lumbar segment can be induced by bath-application of N-methyl-D-aspartate or 5-hydroxytryptamine. Alternation between L2 and L5 ventral roots on the same side, representing the activity of flexor and extensor muscles, respectively, can be observed as well. In the prenatal period in the rat, alternation between the left and right ventral roots is established between embryonic day (E) 16.5 and E18.5. The alternation between the L2 and L5 ventral roots emerges at E20.5. Recent findings show that locomotor-like rhythmic activity with similar characteristics can be induced in the neonatal mouse preparation. In the lumbar spinal cord in the neonatal mouse, it is likely that the rhythm-generating network is distributed throughout the lumbar region with a rostro-caudal gradient, a situation similar to that in the neonatal and fetal rat spinal cord. With this review we hope to highlight the dramatic changes that neuronal networks generating locomotor-like activity undergo during the prenatal development of the rat. Moreover, the distribution of the neuronal network generating the locomotor rhythm in the neonatal rat and mouse spinal cord is compared.

Spontaneous and locomotor-related GABAergic input onto primary afferents in the neonatal rat

The in vitro brain stem-spinal cord preparation of neonatal rats (0-5 days old) was used to examine the contribution of GABAA (gamma-aminobutyric acid) receptors to the spontaneous and locomotor-related antidromic firing in the dorsal roots of neonatal rats. Spontaneous bursts of antidromic discharges were generated by the underlying afferent terminal depolarizations reaching spiking threshold. The number of antidromic action potentials increased significantly in saline solution with Cl- concentration reduced to 50% of control. Bath application of the GABAA receptor antagonist bicuculline, at low concentrations (1-2 microM), or picrotoxin blocked the antidromic discharges in the dorsal roots almost completely. The increase in Cl- conductance was therefore mediated by an activation of GABAA receptors. Increasing the concentration of bicuculline to 10-20 microM never blocked these discharges further. On the contrary, in half of the preparations, the number of antidromic action potentials was higher in the presence of high concentrations of bicuculline (10-20 microM) than in the presence of picrotoxin or low concentrations of bicuculline. This suggests that bicuculline, at high concentrations, may have other effects, in addition to blocking GABAA receptors. Dorsal root firing was observed during fictive locomotion induced by bath application of excitatory amino acids and serotonin. A rhythmical pattern was often demonstrated. Bicuculline at low concentrations caused a decrease of the antidromic discharge whereas, at high concentrations, bursts of discharges appeared. A double-bath with a barrier built at the L3 level was then used to separate the mechanisms which generate locomotion from those mediating primary afferent depolarizations. Excitatory amino acids and serotonin were perfused in the rostral pool only. Decreasing the concentration of chloride in the caudal bath caused a sharp increase in the number of antidromic action potentials recorded from the L5 dorsal root. These discharges, which were modulated in phase with the locomotor rhythm, were blocked by bicuculline. These data demonstrate the existence of a locomotor-related GABAergic input onto primary afferent terminals in the neonatal rat.

Development of motor behaviour

The development of motor behaviour depends on the differentiation of underlying circuitry. Recent work with the zebrafish brings the simple swimming behaviour of lower vertebrates and their embryos into focus as a suitable model to study the development of motor circuitry and its genetic control. Changes in connectivity and excitability contribute to the development of swimming in this simple system. In the chick embryo, limb motor circuitry is spontaneously active before motor axons reach their muscle targets, and it has properties in common with the spontaneously active networks in the retina. The early rhythmic activity responsible for embryonic movement is probably a generalised property of developing spinal networks that precedes, and may be required for, the completion of functional locomotor circuitry.

Bridging the gap - from ion channels to networks and behaviour

A major challenge for current research in neuroscience is to understand the intrinsic operation of the functional modules of the central nervous system, such as those formed by cortical columns and the neuronal networks controlling motor behaviour. Most vertebrate experimental models used in network analyses involve developing nervous systems, which are in rapid transition with regard to their cellular properties and the expression of different ion channels. Recent advances in our understanding of the cellular and circuit properties of motor networks are making it possible to decipher the mechanisms involved in vertebrate motor pattern generation.