Inhibitory neurones of a motor pattern generator in Xenopus revealed by antibodies to glycine

Mechanisms Underlying the Recruitment of Inhibitory Interneurons in Fictive Swimming in Developing Xenopus laevis Tadpoles

Developing spinal circuits generate patterned motor outputs while many neurons with high membrane resistances are still maturing. In the spinal cord of hatchling frog tadpoles of unknown sex, we found that the firing reliability in swimming of inhibitory interneurons with commissural and ipsilateral ascending axons was negatively correlated with their cellular membrane resistance. Further analyses showed that neurons with higher resistances had outward rectifying properties, low firing thresholds, and little delay in firing evoked by current injections. Input synaptic currents these neurons received during swimming, either compound, unitary current amplitudes, or unitary synaptic current numbers, were scaled with their membrane resistances, but their own synaptic outputs were correlated with membrane resistances of their postsynaptic partners. Analyses of neuronal dendritic and axonal lengths and their activities in swimming and cellular input resistances did not reveal a clear correlation pattern. Incorporating these electrical and synaptic properties into a computer swimming model produced robust swimming rhythms, whereas randomizing input synaptic strengths led to the breakdown of swimming rhythms, coupled with less synchronized spiking in the inhibitory interneurons. We conclude that the recruitment of these developing interneurons in swimming can be predicted by cellular input resistances, but the order is opposite to the motor-strength-based recruitment scheme depicted by Henneman's size principle. This form of recruitment/integration order in development before the emergence of refined motor control is progressive potentially with neuronal acquisition of mature electrical and synaptic properties, among which the scaling of input synaptic strengths with cellular input resistance plays a critical role.SIGNIFICANCE STATEMENT The mechanisms on how interneurons are recruited to participate in circuit function in developing neuronal systems are rarely investigated. In 2-d-old frog tadpole spinal cord, we found the recruitment of inhibitory interneurons in swimming is inversely correlated with cellular input resistances, opposite to the motor-strength-based recruitment order depicted by Henneman's size principle. Further analyses showed the amplitude of synaptic inputs that neurons received during swimming was inversely correlated with cellular input resistances. Randomizing/reversing the relation between input synaptic strengths and membrane resistances in modeling broke down swimming rhythms. Therefore, the recruitment or integration of these interneurons is conditional on the acquisition of several electrical and synaptic properties including the scaling of input synaptic strengths with cellular input resistances.

Orderly compartmental mapping of premotor inhibition in the developing zebrafish spinal cord

In vertebrates, faster movements involve the orderly recruitment of different types of spinal motor neurons. However, it is not known how premotor inhibitory circuits are organized to ensure alternating motor output at different movement speeds. We found that different types of commissural inhibitory interneurons in zebrafish form compartmental microcircuits during development that align inhibitory strength and recruitment order. Axonal microcircuits develop first and provide the most potent premotor inhibition during the fastest movements, followed by perisomatic microcircuits, and then dendritic microcircuits that provide the weakest inhibition during the slowest movements. The conversion of a temporal sequence of neuronal development into a spatial pattern of inhibitory connections provides an "ontogenotopic" solution to the problem of shaping spinal motor output at different speeds of movement.

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

KEY POINTS

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

ABSTRACT

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

The Pharmacology of Vertebrate Spinal Central Pattern Generators

Central pattern generators are networks of neurons capable of generating an output pattern of spike activity in a relatively stereotyped, rhythmic pattern that has been found to underlie vital functions like respiration and locomotion. The central pattern generator for locomotion in vertebrates seems to share some basic building blocks. Activation and excitation of activity is driven by descending, sensory, and intraspinal glutamatergic neurons. NMDA receptor activation may also lead to the activation of oscillatory properties in individual neurons that depend on an array of ion channels situated in those neurons. Coordination across joints or the midline of the animal is driven primarily by glycinergic inhibition. In addition to these processes, numerous modulatory mechanisms alter the function of the central pattern generator. These include metabotropic amino acid receptors activated by rhythmic release of glutamate and GABA as well as monoamines, ACh, and peptides. Function and stability of the central pattern generator is also critically dependent on the array of ion channels found in neurons that compose these oscillators, including Ca2+and voltage-gated K+channels and Ca2+channels.

Immunogold cytochemistry in neuroscience

The complexity of the central nervous system calls for immunocytochemical procedures that allow target proteins to be localized with high precision and with opportunities for quantitation. Immunogold procedures stand out as particularly powerful in this regard. Although these procedures have found wide application in the neuroscience community, they present limitations and pitfalls that must be taken into account. At the same time, these procedures offer potentials that remain to be fully realized.

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

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

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

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

Transmitter-phenotypes of commissural interneurons in the lumbar spinal cord of newborn mice

Commissural interneurons (CINs) are a necessary component of central pattern generators (CPGs) for locomotion because they mediate the coordination of left and right muscle activity. The projection patterns and relative locations of different classes of CINs in the ventromedial part of the rodent lumbar cord have been described (Eide et al. [1999] J Comp Neurol 403:332-345; Stokke et al. [2002] J Comp Neurol 446:349-359; Nissen et al. [2005] J Comp Neurol 483:30-47). However, the distribution and relative prevalence of different CIN neurotransmitter phenotypes in the ventral region of the mammalian spinal cord where the locomotor CPG is localized is unknown. In this study we describe the relative proportions and anatomical locations of putative inhibitory and excitatory CINs in the lumbar spinal cord of newborn mice. To directly visualize potential neurotransmitter phenotypes we combined retrograde labeling of CINs with in situ hybridization against the glycine transporter, GlyT2, or the vesicular glutamate transporter, vGluT2, in wildtype mice and in transgenic mice expressing eGFP driven by the promoters of glutamic acid decarboxylase (GAD) 65, GAD67, or GlyT2. Our study shows that putative glycinergic, GABAergic, and glutamatergic CINs are expressed in almost equal numbers, with a small proportion of CINs coexpressing GlyT2 and GAD67::eGFP, indicating a putative combined glycinergic/GABAergic phenotype. These different CIN phenotypes were intermingled in laminas VII and VIII. Our results suggest that glycinergic, GABAergic, and glutamatergic CINs are the principal CIN phenotypes in the CPG region of the lumbar spinal cord in the newborn mouse. We compare these results to descriptions of CIN neurotransmitter phenotypes in other vertebrate species.

Transmitter phenotypes of commissural interneurons in the lamprey spinal cord

The fundamental network for locomotion in all vertebrates contains a central pattern generator or CPG that produces the required motor output in the spinal cord. In the lamprey spinal cord different classes of interneuron's forming the core CPG circuitry have been characterized based on their morphological and electrophysiological features. The commissural interneuron's (C-INs) represent one essential component of CPG that have been implicated in controlling left-right alternation of the motor activity during swimming. However, it is still unclear if the C-INs displays a homogenous neurotransmitter phenotype and how they are distributed. In this paper we investigated the segmental distribution of glycine, glutamate and GABA-immunoreactive (ir) C-INs by combining retrograde Neurobiotin tracing with specific antibodies for these transmitters. The C-INs were more abundant in caudal and rostral segments adjacent to the injection site and their number gradually decreased in more distal segments, suggesting that these interneurons project over a short distance. The glycine-ir neurons represented around 50% of the total C-INs, while glutamate-ir neurons represented only 29%. Both types of C-INs were homogenously distributed over different segments along the spinal cord. Finally, no Neurobiotin labeled C-INs displayed GABA-ir, although many interneurons were ir to GABA, suggesting that GABAergic interneurons are not directly responsible for controlling left-right alternation of activity during locomotion in lamprey. Overall, these results show that the C-INs display a gradual rostrocaudal distribution and consist of both glycine- and glutamate-ir neurons. The difference in the proportion of inhibitory and excitatory C-INs represents an anatomical substrate that can ensure the predominance of alternating activity during locomotion.

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

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

Group I mGluRs increase locomotor network excitability in Xenopus tadpoles via presynaptic inhibition of glycinergic neurotransmission

The group I metabotropic glutamate receptor agonist (S)-3,5-dihyroxyphenylglycine (DHPG) increases the frequency of rhythmic swimming activity in Xenopus tadpoles. This study explores the possibility that group I receptor modulation occurs in part via depression of inhibitory synaptic transmission. Applications of the glycine receptor antagonist strychnine occluded the effects of DHPG, providing preliminary evidence that group I receptors affect motor network output by reducing glycinergic transmission. This evidence was supported further by intracellular and whole-cell patch-clamp recordings from presumed motorneurons. DHPG applications produced two prominent effects: (i) during swimming activity, glycinergic mid-cycle IPSPs were reduced in amplitude; and (ii) during quiescent periods, the frequency of spontaneous miniature IPSPs was also reduced. No change in membrane potential or input resistance following group I receptor activation was detected. The reduction in fast synaptic inhibition provides a plausible explanation for the increased excitability of the locomotor network, although other contributory mechanisms activated in parallel by group I receptors cannot be discounted. Aspects of this work have been published previously in abstract form [R. J. Chapman & K. T. Sillar (2003) SFN Abstracts 277.8].

Glycine-immunoreactive neurons in the developing spinal cord of the sea lamprey: comparison with the gamma-aminobutyric acidergic system

The development and cellular distribution of the inhibitory neurotransmitter glycine in the spinal cord of the sea lamprey were studied by immunocytochemistry and double immunofluorescence and compared with the distribution of gamma-aminobutyric acid (GABA). Results in lamprey embryos and prolarvae reveal that the appearance of glycine-immunoreactive (-ir) spinal neurons precedes that of GABA-ir neurons. Throughout development, glycine-ir cells in the lateral and dorsomedial gray matter of the spinal cord are more numerous than the GABA-ir cells. Only a subset of these neurons shows colocalization of GABA and glycine, suggesting that they are primarily disparate neuronal populations. In contrast, most cerebrospinal fluid (CSF)-contacting neurons of the central canal walls are strongly GABA-ir, and only a portion of them are faintly glycine-ir. Some edge cells (lamprey intraspinal mechanoreceptors) were glycine-ir in larvae and adults. The glycine-ir and GABA-ir neuronal populations observed in the adult spinal cord were similar to those found in larvae. Comparison of glycine-ir and GABA-ir fibers coursing longitudinally in the spinal cord of adult lamprey revealed large differences in diameter between these two types of fiber. Commissural glycine-ir fibers appear in prolarvae and become numerous at larval stages, whereas crossed GABA-ir are scarce. Taken together, results in this primitive vertebrate indicate that the spinal glycinergic cells do not arise by biochemical shift of preexisting GABAergic cells but instead suggest that glycine is present in the earliest circuitry of the developing lamprey spinal cord, where it might act transiently as an excitatory transmitter.

Immunogold quantification of amino acids and proteins in complex subcellular compartments

An increasing number of imaging techniques are in use to study the localization of molecules involved in cell-to-cell signaling. Here we describe the use of immunogold procedures to detect and quantify molecules on electron micrographs. To measure the areas of the subcellular compartments under investigation, the protocol uses an overlay screen with an array of regularly spaced points. On the basis of this, the densities of the gold-labeled molecules can be calculated. Despite the limited lateral resolution of the immunogold method as used by many investigators ( approximately 30 nm), it is possible to measure the content of molecules associated with tiny tissue compartments, e.g., synaptic vesicles and different types of membrane, such as plasma membranes and vesicle membranes. The quantification protocol can be carried out without using computer programs. The entire protocol can be completed in approximately 15 d.

Modulation of a spinal locomotor network by metabotropic glutamate receptors

We have explored the potential involvement of the three main classes of metabotropic glutamate receptor in the modulation of a spinal locomotor network using tadpoles of the anuran amphibian Xenopus laevis. Selective activation of group I receptors in Xenopus embryos and young larvae using the general group I agonist DHPG [(S)-3,5-dihyroxyphenylglycine] significantly increased the frequency of swimming and the number of spontaneously occurring swimming episodes, as monitored by extracellular recordings from ventral roots. Group I receptor activation was without significant effect on the duration or amplitude of motor bursts, the duration of swimming episodes, or the head-to-tail delay in the propagation of swimming activity. Activation of either group II or group III receptors, however, following bath applications of the specific agonists APDC [(2R,4R)-aminopyrrolidine-2,4-dicarboxylic acid] and L-AP4 (L-2-amino-4-phosphonobutanoate), respectively, produced a net inhibitory effect on many of the parameters of fictive swimming at both developmental stages, including a reduction in swimming frequency and episode duration, along with a significant reduction in motor burst amplitude and duration in larval animals only. Applications of selective antagonists provide evidence for activation of all three groups during swimming. The group II and III antagonists EGLU (1-ethyl-2-benzimidazolinone) and MAP4 [(S)-2-amino-2-methyl-4-phosphonobutanoate], respectively, increased, while group I antagonists, CPCCOEt and MPEP, decreased swim frequency. Our findings thus provide evidence for the presence and endogenous activation of three classes of metabotropic glutamate receptor which may function as an intrinsic modulatory control system during fictive swimming in Xenopus tadpoles.

Increased expression of phosphate-activated glutaminase in hippocampal neurons in human mesial temporal lobe epilepsy

Patients with mesial temporal lobe epilepsy (MTLE) have increased basal concentrations of extracellular glutamate in the epileptogenic versus the non-epileptogenic hippocampus. Such elevated glutamate levels have been proposed to underlie the initiation and maintenance of recurrent seizures, and a key question is what causes the elevation of glutamate in MTLE. Here, we explore the possibility that neurons in the hippocampal formation contain higher levels of the glutamate synthesizing enzyme phosphate-activated glutaminase (PAG) in patients with MTLE versus patients with other forms of temporal lobe epilepsy (non-MTLE). Increased PAG immunoreactivity was recorded in subpopulations of surviving neurons in the MTLE hippocampal formation, particularly in CA1 and CA3 and in the polymorphic layer of the dentate gyrus. Immunogold analysis revealed that PAG was concentrated in mitochondria. Double-labeling experiments indicated a positive correlation between the mitochondrial contents of PAG protein and glutamate, as well as between PAG enzyme activity and PAG protein as determined by Western blots. These data suggest that the antibodies recognize an enzymatically active pool of PAG. Western blots and enzyme activity assays of hippocampal homogenates revealed no change in PAG between MTLE and non-MTLE, despite a greatly (>50%) reduced number of neurons in the MTLE hippocampal formation compared to non-MTLE. Thus, the MTLE hippocampal formation contains an increased concentration and activity of PAG per neuron compared to non-MTLE. This increase suggests an enhanced capacity for glutamate synthesis-a finding that might contribute to the disrupted glutamate homeostasis in MTLE.

Zebrafish bandoneon mutants display behavioral defects due to a mutation in the glycine receptor beta-subunit

Bilateral alternation of muscle contractions requires reciprocal inhibition between the two sides of the hindbrain and spinal cord, and disruption of this inhibition should lead to simultaneous activation of bilateral muscles. At 1 day after fertilization, wild-type zebrafish respond to mechanosensory stimulation with multiple fast alternating trunk contractions, whereas bandoneon (beo) mutants contract trunk muscles on both sides simultaneously. Similar simultaneous contractions are observed in wild-type embryos treated with strychnine, a blocker of the inhibitory glycine receptor (GlyR). This result suggests that glycinergic synaptic transmission is defective in beo mutants. Muscle voltage recordings confirmed that muscles on both sides of the trunk in beo are likely to receive simultaneous synaptic input from the CNS. Recordings from motor neurons revealed that glycinergic synaptic transmission was missing in beo mutants. Furthermore, immunostaining with an antibody against GlyR showed clusters in wild-type neurons but not in beo neurons. These data suggest that the failure of GlyRs to aggregate at synaptic sites causes impairment of glycinergic transmission and abnormal behavior in beo mutants. Indeed, mutations in the GlyR beta-subunit, which are thought to be required for proper localization of GlyRs, were identified as the basis for the beo mutation. These data demonstrate that GlyRbeta is essential for physiologically relevant clustering of GlyRs in vivo. Because GlyR mutations in humans lead to hyperekplexia, a motor disorder characterized by startle responses, the zebrafish beo mutant should be a useful animal model for this condition.

Metamodulation of a spinal locomotor network by nitric oxide

Flexibility in the output of spinal networks can be accomplished by the actions of neuromodulators; however, little is known about how the process of neuromodulation itself may be modulated. Here we investigate the potential "meta"-modulatory hierarchy between nitric oxide (NO) and noradrenaline (NA) in Xenopus laevis tadpoles. NO and NA have similar effects on fictive swimming; both potentiate glycinergic inhibition to slow swimming frequency and GABAergic inhibition to reduce episode durations. In addition, both modulators have direct effects on the membrane properties of motor neurons. Here we report that antagonism of noradrenergic pathways with phentolamine dramatically influences the effect of the NO donor S-nitroso-N-acetylpenicillamine (SNAP) on swimming frequency, but not its effect on episode durations. In contrast, scavenging extracellular NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (PTIO) does not influence any of the effects of NA on fictive swimming. These data place NO above NA in the metamodulatory hierarchy, strongly suggesting that NO works via a noradrenergic pathway to control glycine release but directly promotes GABA release. We confirmed this possibility using intracellular recordings from motor neurons. In support of a natural role for NO in the Xenopus locomotor network, PTIO not only antagonized all of the effects of SNAP on swimming but also, when applied on its own, modulated both swimming frequency and episode durations in addition to the underlying glycinergic and GABAergic pathways. Collectively, our results illustrate that NO and NA have parallel effects on motor neuron membrane properties and GABAergic inhibition, but that NO serially metamodulates glycinergic inhibition via NA.

Commissural propriospinal connections between the lateral aspects of laminae III-IV in the lumbar spinal cord of rats

It has been established that there is a strong functional link between sensory neural circuits on the two sides of the spinal cord. In one of our recent studies we provided a morphological confirmation of this functional phenomenon, presenting evidence for the presence of a direct commissural connection between the lateral aspects of the dorsal horn on the two sides of the lumbar spinal cord. By using a combination of neural tracing and immunocytochemical detection of neural markers like vesicular glutamate transporters, glutamic acid decarboxylase, glycine transporter, and met-enkephalin (which are characteristic of various subsets of excitatory and inhibitory neurons), we investigated here the distribution, synaptic relations, and neurochemical characteristics of the commissural axon terminals. We found that the cells of origin of commissural fibers in the lateral aspect of the dorsal horn were confined to laminae III-IV and projected to the corresponding area of the contralateral gray matter. Most of the commissural axon terminals established synaptic contacts with dendrites. Axospinous or axosomatic synaptic contacts were found in limited numbers. We demonstrated that interactions among commissural neurons also exist. More than three-fourths of the labeled axon terminals were immunostained for glutamic acid decarboxylase and/or glycine transporter, but none of them showed positive immunoreaction for met-enkephalin and vesicular glutamate transporters. The results indicate that there is a substantial reciprocal commissural synaptic interaction between the lateral aspects of laminae III-IV on the two sides of the lumbar spinal cord and that this pathway may transmit both inhibitory and excitatory signals to their postsynaptic targets.

Glycine, GABA and their transporters in pancreatic islets of Langerhans: evidence for a paracrine transmitter interplay

To elucidate the possible roles of the CNS neurotransmitters glycine and GABA in neuroendocrine paracrine signalling, we investigated their localizations, and those of their transport proteins, by confocal immunofluorescence and quantitative post-embedding immuno-electron microscopy in the pancreatic islets of Langerhans. We show that A-cells contain glycine in synaptic-like microvesicles as well as in secretory granules. A-cells express the macromolecules necessary to: (1) concentrate glycine within both organelle types before release (the vesicular GABA/glycine transporter VGAT=VIAAT); and to (2) take up the transmitter from the extracellular space (the plasma membrane glycine transporter GLYT2). Also B-cells have glycine in their microvesicles and granules, but the microvesicle/cytosol ratio is lower than in A-cells, consistent with the presence of GABA (which competes with glycine for vesicular uptake) in the cytosol at a much higher concentration in B-cells than in A-cells. Both A- and B-cells contain GABA in their microvesicles and secretory granules, and the membranes of the two organelle types contain VGAT in both cell types. A-cells as well as B-cells express a plasma membrane transporter GAT3 that mediates uptake of GABA. The localization of VGAT in the cores of A-cell secretory granules, and in the secretory granule membranes in both cell types, indicates novel aspects of the mechanisms for release of glycine and GABA. The discovery that both A- and B-cells possess the molecular machinery for the evoked release of both glycine and GABA from synaptic-like microvesicles suggests that both of the principal inhibitory transmitters in the brain participate in paracrine signalling in the pancreas.

An immunogold investigation of the distribution of GABA and glycine in nerve terminals on the somata of spherical bushy cells in the anteroventral cochlear nucleus of guinea pig

Spherical bushy neurons in the anteroventral cochlear nucleus receive glutamatergic primary terminals from the cochlear nerve and terminals of noncochlear (i.e. nonprimary) origin, many of which colocalize gamma-aminobutyric acid (GABA) and glycine. Here the relationship between GABA and glycine in these terminals has been investigated using postembedding immunogold labelling. A significant negative correlation was found between the density of terminal labelling for GABA and for glycine in four guinea pigs. Terminals could be divided into three categories, GABA-only, glycine-only, or colocalizing depending on whether they had a significantly higher labelling density for either amino acid than the primary terminals. The overall labelling density in all four animals was significantly greater for GABA in GABA-only terminals than colocalizing ones but similar for glycine in both. Within the terminals, the labelling density over synaptic vesicles, nonvesicular regions of cytoplasm and mitochondria was also investigated. No significant difference was detected in the labelling density of vesicles compared with nonvesicular regions for either amino acid. However, a significant difference was found between the overall labelling density over mitochondria and nonvesicular regions for both. There was also significantly more mitochondrial GABA labelling in GABA-only terminals compared to colocalizing terminals but mitochondrial glycine labelling was similar in glycine-only and colocalizing terminals. Thus the level of GABA is higher in single than in colocalizing terminals, particularly in the mitochondria, but similar for glycine in both. It is possible therefore that the presence of glycine affects the level of GABA in the nonprimary terminals but that the presence of GABA does not affect the level of glycine.

Neurotransmitter properties of spinal interneurons in embryonic and larval zebrafish

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

Ontogeny of gamma-aminobutyric acid-immunoreactive neurons in the rhombencephalon and spinal cord of the sea lamprey

The development of neurons expressing gamma-aminobutyric acid (GABA) in the rhombencephalon and spinal cord of the sea lamprey (Petromyzon marinus) was studied for the first time with an anti-GABA antibody. The earliest GABA-immunoreactive (GABAir) neurons appear in late embryos in the basal plate of the isthmus, caudal rhombencephalon, and rostral spinal cord. In prolarvae, the GABAir neurons of the rhombencephalon appear to be distributed in spatially restricted cellular domains that, at the end of the prolarval period, form four longitudinal GABAir bands (alar dorsal, alar ventral, dorsal basal, and ventral basal). In the spinal cord, we observed only three GABAir longitudinal bands (dorsal, intermediate, and ventral). The larval pattern of GABAir neuronal populations was established by the 30-mm stage, and the same populations were observed in premetamorphic and adult lampreys. The ontogeny of GABAergic populations in the lamprey rhombencephalon and spinal cord is, in general, similar to that previously described in mouse and Xenopus.

Xenopus embryonic spinal neurons recorded in situ with patch-clamp electrodes--conditional oscillators after all?

The central pattern generator for swimming Xenopus embryo is organized as two half-centres linked by reciprocal inhibition. Microelectrode recordings suggest that Xenopus neurons are poorly excitable, necessitating a key role for postinhibitory rebound in the operation of the central pattern generator. However the Xenopus central pattern generator seems unusual in that the component neurons apparently have no intrinsic or conditional rhythmogenic properties. We have re-examined the firing properties of Xenopus embryo spinal neurons by making patch-clamp recordings in situ from intact spinal cord. Recordings made from 99 neurons were divided into three groups. Central pattern generator neurons overwhelmingly (44/51) fired trains of action potentials in response to current injection. Just over half of the sensory interneurons (13/22) also fired trains of action potentials. Neurons that received no synaptic inputs during swimming mostly fired just one or two action potentials (22/26). Thirty-four neurons were identified morphologically. Commissural (8/12) and descending (6/6) interneurons, key components of the spinal central pattern generator, fired repetitive trains of action potentials during current injection. Neurons that were not part of the central pattern generator did not demonstrate this preponderance for repetitive firing. Analysis of the interspike intervals during current injection revealed that the majority of central pattern generators, descending and commissural interneurons, could readily fire at frequencies up to twice that of swimming. We suggest that Xenopus neurons can be considered as conditional oscillators: in the presence of unpatterned excitation they exhibit an ability to fire rhythmically. This property makes the Xenopus embryonic central pattern generator more similar to other model central pattern generators than has hitherto been appreciated.

The neuronal targets for GABAergic reticulospinal inhibition that stops swimming in hatchling frog tadpoles

In most animals locomotion can be started and stopped by specific sensory cues. We are using a simple vertebrate, the hatchling Xenopus tadpole, to study a neuronal pathway that turns off locomotion. In the tadpole, swimming stops when the head contacts solid objects or the water's surface meniscus. The primary sensory neurons are in the trigeminal ganglion and directly excite inhibitory reticulospinal neurons in the hindbrain. These project axons into the spinal cord and release GABA to inhibit spinal neurons and stop swimming. We ask whether there is specificity in the types of spinal neuron inhibited. We used single-neuron recording to determine which classes of spinal neurons receive inhibition when the head skin is pressed. Ventral motoneurons and premotor interneurons involved in generating the swimming rhythm receive reliable GABAergic inhibition. More dorsal inhibitory premotor interneurons are inhibited less reliably and some are excited. Dorsal sensory pathway interneurons that start swimming following a touch to the trunk skin do not appear to receive such inhibition. There is therefore specificity in the formation of descending inhibitory connections so that more ventral neurons producing swimming are most strongly inhibited.

Coordinated motor activity in simulated spinal networks emerges from simple biologically plausible rules of connectivity

The spinal motor circuits of the Xenopus embryo have been simulated in a 400-neuron network. To explore the consequences of differing patterns of synaptic connectivity within the network for the generation of the motor rhythm, a system of biologically plausible rules was devised to control synapse formation by three parameters. Each neuron had an intrinsic probability of synapse formation (P(soma), specified by a space constant lambda) that was a monotonically decreasing function of its soma location in the rostro-caudal axis of the simulated network. The neurons had rostral and caudal going axons of specified length (L(axon)) associated with a probability of synapse formation (P(axon)). The final probability of synapse formation was the product of P(soma) and P(axon). Realistic coordinated activity only occurred when L(axon) and the probabilities of interconnection were sufficiently high. Increasing the values of the three network parameters reduced the burst duration, cycle period, and rostro-caudal delay and increased the reliability with which the network functioned as measured by the coefficient of variance of these parameters. Whereas both L(axon) and P(axon) had powerful and consistent effects on network output, the effects of lambda on burst duration and rostro-caudal delay were more variable and depended on the values of the other two parameters. This network model can reproduce the rostro-caudal coordination of swimming without using coupled oscillator theory. The changes in network connectivity and resulting changes in activity explored by the model mimic the development of the motor pattern for swimming in the real embryo.

Fast inhibitory synapses: targets for neuromodulation and development of vertebrate motor behaviour

Locomotor networks must possess the inherent flexibility to adapt their output. In this review we discuss evidence from a simple vertebrate locomotor network that suggests fast inhibitory synapses are important targets for the forms of neuromodulation that afford this flexibility. Two important inhibitory transmitters, glycine and GABA, are present in the CNS of Xenopus tadpoles, where they each play distinct roles in the control of swimming. Glycine, but not GABA, contributes to the inhibitory mid-cycle component of each swim-cycle, the strength of which determines the frequency of swimming. Meanwhile, GABA release onto the swim network prematurely terminates swimming episodes. Hence, glycine controls how fast, whilst GABA controls how far the tadpole swims. Our work has focused on how the amines serotonin (5-HT) and noradrenaline (NA), and more recently the gas nitric oxide (NO), selectively target glycine and GABA release in the spinal cord to modulate swimming. In particular, we have identified three brainstem populations of nitrergic neurons, which suggests that nitric oxide may co-localise with 5-HT, NA and GABA. Here we review this work and suggest a hierarchy of brainstem modulatory systems, with NO acting as a metamodulator.

Immunoreactivity for Taurine Characterizes Subsets of Glia, GABAergic and non-GABAergic Neurons in the Neo- and Archicortex of the Rat, Cat and Rhesus Monkey: Comparison with Immunoreactivity for Homocysteic Acid

The cerebral cortex is an area rich in taurine (2-aminoethanesulphonic acid), but only limited information exists regarding its cellular distribution. We therefore examined taurine-like immunoreactivity in the cerebral cortex of the rat, cat and macaque monkey using antiserum directed against glutaraldehyde-conjugated taurine. Immunostaining was assessed at the light and electron microscopic level, and patterns obtained in light microscopic studies were compared to those produced with antiserum to gamma-aminobutyric acid (GABA) and homocysteic acid (HCA). In all three species, strong taurine-like immunoreactive perivascular endothelial cells, pericytes and oligodendrocytes were found. These cells were located throughout the neuropil, which itself showed a low level of immunoreactivity. In rats and cats, a small number of weakly taurine-enriched neurons were observed, particularly in superficial layers. In all cortical areas of the macaque, however, glial staining was matched by strong, selective staining of subpopulations of cortical neurons which were distributed in a bilaminar pattern involving layers II/III and VI. In addition, in primary visual cortex, area 17, immunopositive neurons were also present in sublayer IVCbeta, while in the hippocampus strongly taurine-positive neurons were most conspicuous in the granule cell layer of the dentate gyrus. In all regions, strongly taurine-positive neurons constituted only a subpopulation of the neurons occupying a given layer. Examination of adjacent sections for GABA immunoreactivity showed that the most strongly taurine-positive neurons in layers II/III were immunoreactive for GABA. The cells located in layers IVCbeta and VI, and the granule cells of the dentate gyrus, however, were GABA-negative. The morphological features of these latter groups suggested that the antiserum to taurine identifies subsets of spiny stellate, small pyramidal and dentate granule cells. None of these neurons showed immunoreactivity with antiserum to HCA in the primate; HCA-positive glia were found along the pial and white matter boundaries of the cortex, and showed no overlap with strongly taurine-positive glial elements. Although a transmitter role for taurine may be unlikely, particularly in view of its enrichment in subpopulations of both inhibitory and excitatory cells, the capacity of taurine to influence membrane-associated functions in excitable tissues, and its selective distribution demonstrated here, provides the potential for a contribution to communication between cortical cells.

Characterization and Function of Spinal Excitatory Interneurons with Commissural Projections in Xenopus laevis embryos

The multipolar somata of dorsolateral commissural (dlc) interneurons (Roberts and Clarke, 1982) lie in a superficial dorsolateral position in the spinal cord of Xenopus laevis embryos. By applying horseradish peroxidase to one-half of the 100 microm diameter spinal cord, these neurons have been backfilled. Their dendritic branching pattern, commissural axonal projection and distribution near the time of hatching is described. Using Lucifer yellow-filled microelectrodes a population of sensory interneurons with dlc morphology has been identified. They have multipolar somata in a dorsolateral superficial position, obliquely projecting dendrites and a ventral commissural axon. They receive presumed monosynaptic excitation in response to electrical stimulation of sensory neurites in the skin on the same side as the soma. During fictive swimming activity in curarized embryos the dlc interneurons are rhythmically inhibited in time with ventral root discharge on the same side. Dlc interneurons can fire multiple impulses and can turn on fictive swimming when stimulated by intracellular current injection. Skin stimulation is followed by excitatory postsynaptic potentials (EPSPs) in contralateral ventral rhythmic neurons. These EPSPs are reduced by the application of NMDA receptor antagonist. We conclude that dlc interneurons are excited by primary skin afferent Rohon - Beard neurons, carry sensory information across the spinal cord to excite neurons on the opposite side by release of an excitatory amino acid transmitter and participate in reflexes and in the initiation of swimming.

Mutual Re-excitation with Post-Inhibitory Rebound: A Simulation Study on the Mechanisms for Locomotor Rhythm Generation in the Spinal Cord of Xenopus Embryos

We have used computer simulations as one way to test the hypothesis that locomotor rhythm production for swimming in frog embryo spinal cord depends on rebound from inhibition and is sustained by mutual re-excitation among spinal excitatory interneurons. All simulations were based on physiological and anatomical data on the neurons and circuitry of Xenopus embryo spinal cord. Model 'neurons' had resistively coupled axon, soma, and dendrite compartments. Membrane properties were based on Hodgkin - Huxley equations with resting potential at - 75 mV and where soma and dendrite had reduced K+ and Na+ conductance and slowed K+ conductance. These 'neurons' fired a single non-overshooting spike both to depolarizing current and after hyperpolarizing current given during imposed depolarization. Synapses were made on to the dendrite. Inhibitory and excitatory synaptic channels had Nernst potentials of - 80 and 0 mV, time constants for opening of 1 ms, and closing of 6 and 75 ms. When the short inhibitory postsynaptic potential occurred on the long (N-methyl-D-aspartate-type) excitatory postsynaptic potential, it led to rebound firing. A four 'neuron' symmetrical network was built with reciprocal inhibition and where excitatory 'neurons' re-excited themselves and the inhibitory 'neuron' on their own side. The rhythmic alternating activity with one spike per cycle produced reliably by this network was self-sustaining, initiated by a brief synaptic input, and closely resembled the spinal cord motor pattern during swimming. The robustness of this activity pattern was investigated by varying cellular and synaptic parameters, initiating inputs, and network connectivity. We conclude that cellular, synaptic, and network properties are all important and that mutual re-excitation, a form of positive feedback, could sustain motor rhythm production in the Xenopus embryo spinal cord.

The Isolation and Identification of Spinal Neurons That Control Movement in the Xenopus Embryo

A procedure for isolating spinal neurons of the Xenopus embryo has been devised. Using a variety of methods four of the eight previously described categories of neuron can be distinguished in vitro. Firstly, since many anatomical features of the neurons survived dissociation, sensory neurons and glycinergic inhibitory interneurons could be identified after isolation using anatomical criteria. Secondly, by selective labelling of the neurons in the intact spinal cord with fluorescein-conjugated dextran amines prior to dissociation, three classes of isolated neuron could be identified: motoneurons, putative excitatory interneurons and, once again, inhibitory interneurons. The identity of the inhibitory interneurons has been confirmed using glycine immunocytochemistry. The physiological properties of the isolated neurons were similar to those of their in vivo counterparts. The dissociated neurons exhibited strong membrane accommodation and outward rectification, both of which could be blocked by the injection of Cs+ ions. The neurons also retained their receptors for the agonists N-methyl-d-aspartate, kainate, quisqualate, glycine and GABA. Both the membrane properties and pharmacological sensitivity of the isolated neurons therefore appeared to be unaltered by the dissociation procedure. Thus the four classes of neuron from the Xenopus embryonic nervous system that can be identified after isolation constitute the basis for a valid model for detailed study of how the properties of individual neurons contribute to the functioning of the circuitry that underlies locomotor pattern generation in vertebrates.

Nitric oxide selectively tunes inhibitory synapses to modulate vertebrate locomotion

We have explored the possible modulation by nitric oxide (NO) of inhibitory synaptic transmission mediated by either glycine or GABA during episodes of rhythmic fictive swimming in postembryonic Xenopus laevis tadpoles. Extracellular ventral-root recordings suggest a stage-dependent increase in the reliability and extent of the NO donor S-nitroso-n-acetylpenicillamine (SNAP; 0.1-1 mm) to inhibit swimming by reducing the frequency and shortening the duration of swim episodes. These effects of SNAP on the swimming rhythm at both developmental stages are corroborated by intracellular recordings from presumed motor neurons with sharp microelectrodes, which also suggest that NO inhibits swimming by facilitating both glycinergic and GABAergic inhibition. However, we found no evidence for NO modulation of the excitatory drive for swimming. In addition to presynaptic effects on inhibitory transmitter release, a pronounced postsynaptic membrane depolarization ( approximately 5-10 mV) and conductance decrease ( approximately 10-20%) are associated with bath application of SNAP. Hence, NO exerts inhibitory effects on swimming through multiple but selective actions on both the electrical properties of spinal neurons and on particular synaptic interconnections. The presynaptic and postsynaptic effects of NO act in concert to tune inhibitory synapses.

Sensory activation and role of inhibitory reticulospinal neurons that stop swimming in hatchling frog tadpoles

Activity in neuronal networks underlying locomotion and other rhythmic actions can start and stop in response to specific sensory stimuli. In vertebrate locomotion, some reticulospinal neurons such as Mauthner neurons can initiate activity, but the neurons controlling stopping are not defined. We have studied swimming in tadpoles of the frog, Xenopus, which is started by touching the skin and stops when the head contacts a solid surface. Using an immobilized tadpole preparation, the same stimuli control fictive swimming. When head contact is imitated by pressure to the head skin sensory neurons in the trigeminal ganglion are active, spinal neurons receive GABAergic inhibition, and swimming stops. Here we record intracellularly from neurons in the hindbrain that are excited by pressure or electrical stimulation to the head skin. By intracellular filling with neurobiotin, we identify these anatomically as mid-hindbrain reticulospinal neurons (MHRs). These have contralateral descending projections to the spinal cord and GABA-like immunoreactivity. They are rhythmically inhibited during fictive swimming. Individual MHRs reliably stopped ongoing swimming when brief firing was induced by intracellular current injection. The ability of individual MHRs to stop swimming was blocked by the GABA(A) antagonist bicuculline. Our evidence indicates that MHRs receive direct excitation from trigeminal sensory neurons and in turn release GABA to directly inhibit spinal neurons and turn off the swimming central pattern generator.

Cell-specific expression of the glutamine transporter SN1 suggests differences in dependence on the glutamine cycle

Glutamine is involved in a variety of metabolic processes, including recycling of the neurotransmitters glutamate and gamma-aminobutyric acid (GABA). The system N transporter SN1 mediates efflux as well as influx of glutamine in glial cells [Chaudhry et al. (1999), Cell, 99, 769-780]. We here report qualitative and quantitative data on SN1 protein expression in rat. The total tissue concentrations of SN1 in brain and in kidney are half and one-quarter, respectively, of that in liver, but the average concentration of SN1 could be higher in astrocytes than in hepatocytes. Light and electron microscopic immunocytochemistry shows that glutamatergic, GABAergic and, surprisingly, purely glycinergic boutons are ensheathed by astrocytic SN1 laden processes, indicating a role of glutamine in the production of all three rapid transmitters. A dedication of SN1 to neurotransmitter recycling is further supported by the lack of SN1 immunoreactivity in oligodendrocytes (cells rich in glutamine but without perisynaptic processes). All neuronal structures appear unlabelled implying that a different protein mediates glutamine uptake into nerve endings. In several regions, SN1 immunoreactivity is higher in association with GABAergic than glutamatergic synapses, in agreement with observations that exogenous glutamine increases output of transmitter glutamate but not GABA. Nerve terminals with low transmitter reuptake or high prevailing firing frequency are associated with high SN1 immunoreactivity in adjacent glia. Bergmann glia and certain other astroglia contain very low levels of SN1 immunoreactivity compared to most astroglia, including retinal Müller cells, indicating the possible existence of SN isoforms and alternative mechanisms for transmitter recycling.

Spike-independent release of ATP from Xenopus spinal neurons evoked by activation of glutamate receptors

As the release of ATP from neurons has only been directly studied in a few cases, we have used patch sniffing to examine ATP release from Xenopus spinal neurons. ATP release was detected following intracellular current injection to evoke spikes. However, spiking was not essential as both glutamate and NMDA could evoke release of ATP in the presence of TTX. Neither acetylcholine nor high K(+) was effective at inducing ATP release in the presence of TTX. Although Cd(2+) blocked glutamate-evoked release of ATP suggesting a dependence on Ca(2+) entry, neither omega-conotoxin-GVIA nor nifedipine prevented ATP release. N-type and L-type channels are thus not essential for glutamate-evoked ATP release. That glutamate receptors can elicit release in the absence of spiking suggests a close physical relationship between these receptors, the Ca(2+) channels and release sites. As the dependence of ATP release on the influx of Ca(2+) through Ca(2+) channel subtypes differs from that of synaptic transmitter release, ATP may be released from sites that are distinct from those of the principal transmitter. In addition to its role as a fast transmitter, ATP may thus be released as a consequence of the activation of excitatory glutamatergic synapses and act to signal information about activity patterns in the nervous system.

Defining classes of spinal interneuron and their axonal projections in hatchling Xenopus laevis tadpoles

Neurobiotin was injected into individual spinal interneurons in the Xenopus tadpole to discern their anatomical features and complete axonal projection patterns. Four classes of interneuron are described, with names defining their primary axon projection: Dorsolateral ascending and commissural interneurons are predominantly multipolar cells with somata and dendrites exclusively in the dorsal half of the spinal cord. Ascending interneurons have unipolar somata located in the dorsal half, but their main dendrites are located in the ventral half of the spinal cord. Descending interneurons show bigger variance in their anatomy, but the majority are unipolar, and they all have a descending primary axon. Dorsolateral commissural interneurons are clearly defined using established criteria, but the others are not, so cluster analysis was used. Clear discriminations can be made, and criteria are established to characterize the three classes of interneuron with ipsilateral axonal projections. With identifying criteria established, the distribution and axonal projection patterns of the four classes of interneuron are described. By using data from gamma-aminobutyric acid immunocytochemistry, the distribution of the population of ascending interneurons is defined. Together with the results from the axonal projection data, this allows the ascending interneuron axon distribution along the spinal cord to be estimated. By making simple assumptions and using existing information about the soma distributions of the other interneurons, estimates of their axon distributions are made. The possible functional roles of the four interneuron classes are discussed.

Spatiotemporal pattern of nicotinamide adenine dinucleotide phosphate-diaphorase reactivity in the developing central nervous system of premetamorphic Xenopus laevis tadpoles

We have catalogued the progressive appearance of putative nitric oxide synthase (NOS)-containing neurons in the developing central nervous system (CNS) of Xenopus laevis. Xenopus embryos and larvae were processed in wholemount and in cross section using nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry as a marker for NOS within the CNS. The temporal sequence of NADPH-d reactivity identified discrete groups and subgroups of neurons in the forebrain, midbrain, and hindbrain on the basis of their morphology, location, and order of appearance during development. A proportion of these groups of neurons appeared to be important in sensory processing and motor control. Staining also appeared at specific stages in the spinal cord, the retina, and the skin. After the appearance of labelling, NADPH-d reactivity continued in each of the cell groups throughout the stages examined. We found no evidence for staining that subsequently disappeared at later stages in any cell group, indicating a persistent rather than transient role for NO in the Xenopus tadpole CNS. These results are discussed in light of recent findings on possible roles for NADPH-d-positive cell groups within the developing motor circuitry.

Early functional organization of spinal neurons in developing lower vertebrates

The spinal neurons in the embryos and young larvae of two amphibians (Xenopus and Triturus) and two fish (Oryzias and Brachydanio) are described and compared. They can be placed into a limited number of common neuron classes: Rohon-Beard sensory, dorsolateral and dorsolateral commissural sensory interneurons, inhibitory ascending interneurons, two classes of inhibitory commissural interneuron, excitatory descending interneurons, motoneurons and possible sensory Kolmer-Agdhur neurons. In Triturus and other urodeles, there are also giant dorsolateral commissural sensory interneurons. The functions of the spinal neurons in simple flexion responses and swimming are considered in relation to evidence mainly from the Xenopus tadpole.

The development of neuromodulatory systems and the maturation of motor patterns in amphibian tadpoles

The relative simplicity of the amphibian tadpole nervous system has been utilised as a model for the mechanisms underlying the generation and development of vertebrate locomotion. In this paper, we review evidence on the role of descending brainstem projections in the maturation and intrinsic modulation of tadpole spinal motor networks. Three transmitter systems that have been investigated utilise the biogenic amines serotonin (5HT) and noradrenaline (NA) and the inhibitory amino acid gamma-aminobutyric acid (GABA). The distribution, development and spinal targets of these systems will be reviewed. More recent data on the role of nitric oxide (NO) will also be discussed. This ubiquitous gaseous signalling molecule is known to play a crucial role in the developing nervous system, but until recently, had not been directly implicated in the brain regions involved in motor control. NO appears to be produced by three homologous brainstem clusters in the developing motor networks of two closely related amphibian species, Xenopus laevis and Rana temporaria but, surprisingly, it plays contrasting roles in these species. Given the presumed co-localisation and interaction of nitric oxide with conventional neurotransmitters, we discuss the potential relationship of nitrergic neurons with 5HT, NA and GABA in these amphibian models.

Effects of high-potassium-induced depolarization on amino acid chemistry of the dorsal cochlear nucleus in rat brain slices

High K+ was used to depolarize glia and neurons in order to study the effects on amino acid release from and concentrations within the dorsal cochlear nucleus (DCN) of brain slices. The release of glutamate, gamma-aminobutyrate (GABA) and glycine increased significantly during exposure to 50 mM K+, while glutamine and serine release decreased significantly during and/or after exposure, respectively. After 10 min of exposure to 50 mM K+, glutamine concentrations increased in all three layers of DCN slices, to more than 5 times the values in unexposed slices. In the presence of a glutamate uptake blocker, L-trans-pyrrolidine-2,4-dicarboxylic acid (PDC), glutamine concentrations in all layers did not increase as much during 50 mM K+. Similar but smaller changes occurred for serine. Mean ATP concentrations were lower in 50 mM K(+)-exposed slices compared to control. The results suggest that depolarization, such as during increased neural activity, can greatly affect amino acid metabolism in the cochlear nucleus.

Protein phosphatase-1 regulation in the induction of long-term potentiation: heterogeneous molecular mechanisms

Protein phosphatase inhibitor-1 (I-1) has been proposed as a regulatory element in the signal transduction cascade that couples postsynaptic calcium influx to long-term changes in synaptic strength. We have evaluated this model using mice lacking I-1. Recordings made in slices prepared from mutant animals and also in anesthetized mutant animals indicated that long-term potentiation (LTP) is deficient at perforant path-dentate granule cell synapses. In vitro, this deficit was restricted to synapses of the lateral perforant path. LTP at Schaffer collateral-CA1 pyramidal cell synapses remained normal. Thus, protein phosphatase-1-mediated regulation of NMDA receptor-dependent synaptic plasticity involves heterogeneous molecular mechanisms, in both different dendritic subregions and different neuronal subtypes. Examination of the performance of I-1 mutants in spatial learning tests indicated that intact LTP at lateral perforant path-granule cell synapses is either redundant or is not involved in this form of learning.

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.

The distribution of NADPH-diaphorase-labelled interneurons and the role of nitric oxide in the swimming system of Xenopus laevis larvae

The possible involvement of the free radical gas nitric oxide (NO) in the modulation of spinal rhythm-generating networks has been studied using Xenopus laevis larvae. Using NADPH-diaphorase histochemistry, three putative populations of nitric oxide synthase (NOS)-containing cells were identified in the brainstem. The position and morphology of the largest and most caudal population suggested that a proportion of these neurons is reticulospinal. The possible contribution of nitrergic neurons to the control of swimming activity was examined by manipulating exogenous and endogenous NO concentrations in vivo with an NO donor (SNAP, 100–500 micromol l(−)(1)) and NOS inhibitors (l-NAME and l-NNA, 0.5-5 mmol l(−)(1)), respectively. In the presence of SNAP, swim episode duration decreased and cycle period increased, whereas the NOS inhibitors had the opposite effects. We conclude from these data that the endogenous release of NO from brainstem neurons extrinsic to the spinal cord of Xenopus laevis larvae exerts a continuous modulatory influence on swimming activity, functioning like a ‘brake’. Although the exact level at which NO impinges upon the swimming rhythm generator has yet to be determined, the predominantly inhibitory effect of NO suggests that the underlying mechanisms of NO action could involve modulation of synaptic transmission and/or direct effects on neuronal membrane properties.

Development, neurochemical properties, and axonal projections of a population of last-order premotor interneurons in the white matter of the chick lumbosacral spinal cord

There is general agreement that last-order premotor interneurons-a set of neurons that integrate activities generated by the spinal motor apparatus, sensory information and volleys arising from higher motor centres, and transmit the integrated signals to motoneurons through monosynaptic contacts-play crucial roles in the initiation and maintenance of spinal motor activities. Here, we demonstrate the development, neurochemical properties, and axonal projections of a unique group of last-order premotor interneurons within the ventrolateral aspect of the lateral funiculus of the chick lumbosacral spinal cord. Neurons expressing immunoreactivity for neuron-specific enolase were first detected in the ventrolateral white matter at embryonic day 9 (E9). The numbers of immunoreactive neurons were significantly increased at E10-E12, while most of them were gradually concentrated in small segmentally arranged nuclei (referred to as major nuclei of Hofmann) protruding from the white matter in a necklace like fashion dorsal to the ventral roots. The major nuclei of Hofmann became more prominent at E12-E16, but substantial numbers of cells were still located within the ventrolateral white matter (referred to as minor nucleus of Hofmann). The distribution of immunoreactive neurons achieved by E16 was maintained during later developmental stages and was also characteristic of adult animals. After injection of Phaseolus vulgaris-leucoagglutinin unilaterally into the minor nucleus of Hofmann, labeled fibres were detected in the ventrolateral white matter ipsilateral to the injection site. Ascending and descending fibres were revealed throughout the entire rostro-caudal length of the lumbosacral spinal cord. Axon terminals were predominantly found within the lateral motor column and the ventral regions of lamina VII ipsilateral to the injection site. Several axon varicosities made close appositions with somata and dendrites of motoneurons, which were identified as synaptic contacts in a consecutive electron microscopic study. With the postembedding immunogold method, 21 of 97 labeled terminals investigated were immunoreactive for glycine and 2 of them showed immunoreactivity for gamma-aminobutyric acid (GABA). The axon trajectories of neurons within the minor nucleus of Hofmann suggest that some of these cells might represent a population of last-order premotor interneurons. J. Exp. Zool. 286:157-172, 2000.