A nonrandom interneuronal pattern in the developing frog spinal cord

Light on a sensory interface linking the cerebrospinal fluid to motor circuits in vertebrates

The cerebrospinal fluid (CSF) is circulating around the entire central nervous system (CNS). The main function of the CSF has been thought to insure the global homeostasis of the CNS. Recent evidence indicates that the CSF also dynamically conveys signals modulating the development and the activity of the nervous system. The later observation implies that cues from the CSF could act on neurons in the brain and the spinal cord via bordering receptor cells. Candidate neurons to enable such modulation are the cerebrospinal fluid-contacting neurons (CSF-cNs) that are located precisely at the interface between the CSF and neuronal circuits. The atypical apical extension of CSF-cNs bears a cluster of microvilli bathing in the CSF indicating putative sensory or secretory roles in relation with the CSF. In the brainstem and spinal cord, CSF-cNs have been described in over two hundred species by Kolmer and Agduhr, suggesting an important function within the spinal cord. However, the lack of specific markers and the difficulty to access CSF-cNs hampered their physiological investigation. The transient receptor potential channel PKD2L1 is a specific marker of spinal CSF-cNs in vertebrate species. The transparency of zebrafish at early stages eases the functional characterization of pkd2l1⁺ CSF-cNs. Recent studies demonstrate that spinal CSF-cNs detect spinal curvature via the channel PKD2L1 and modulate locomotion and posture by projecting onto spinal interneurons and motor neurons in vivo. In vitro recordings demonstrated that spinal CSF-cNs are sensing pH variations mainly through ASIC channels, in combination with PKD2L1. Altogether, neurons contacting the CSF appear as a novel sensory modality enabling the detection of mechanical and chemical stimuli from the CSF and modulating the excitability of spinal circuits underlying locomotion and posture.

Six1 is a key regulator of the developmental and evolutionary architecture of sensory neurons in craniates

BACKGROUND

Various senses and sensory nerve architectures of animals have evolved during adaptation to exploit diverse environments. In craniates, the trunk sensory system has evolved from simple mechanosensory neurons inside the spinal cord (intramedullary), called Rohon-Beard (RB) cells, to multimodal sensory neurons of dorsal root ganglia (DRG) outside the spinal cord (extramedullary). The fish and amphibian trunk sensory systems switch from RB cells to DRG during development, while amniotes rely exclusively on the DRG system. The mechanisms underlying the ontogenic switching and its link to phylogenetic transition remain unknown.

RESULTS

In Xenopus, Six1 overexpression promoted precocious apoptosis of RB cells and emergence of extramedullary sensory neurons, whereas Six1 knockdown delayed the reduction in RB cell number. Genetic ablation of Six1 and Six4 in mice led to the appearance of intramedullary sensory neuron-like cells as a result of medial migration of neural crest cells into the spinal cord and production of immature DRG neurons and fused DRG. Restoration of SIX1 expression in the neural crest-linage partially rescued the phenotype, indicating the cell autonomous requirements of SIX1 for normal extramedullary sensory neurogenesis. Mouse Six1 enhancer that mediates the expression in DRG neurons activated transcription in Xenopus RB cells earlier than endogenous six1 expression, suggesting earlier onset of mouse SIX1 expression than Xenopus during sensory development.

CONCLUSIONS

The results indicated the critical role of Six1 in transition of RB cells to DRG neurons during Xenopus development and establishment of exclusive DRG system of mice. The study provided evidence that early appearance of SIX1 expression, which correlated with mouse Six1 enhancer, is essential for the formation of DRG-dominant system in mice, suggesting that heterochronic changes in Six1 enhancer sequence play an important role in alteration of trunk sensory architecture and contribute to the evolution of the trunk sensory system.

Noradrenergic innervation of the rat spinal cord caudal to a complete spinal cord transection: effects of olfactory ensheathing glia

Transplantation of olfactory bulb-derived olfactory ensheathing glia (OEG) combined with step training improves hindlimb locomotion in adult rats with a complete spinal cord transection. Spinal cord injury studies use the presence of noradrenergic (NA) axons caudal to the injury site as evidence of axonal regeneration and we previously found more NA axons just caudal to the transection in OEG- than media-injected spinal rats. We therefore hypothesized that OEG transplantation promotes descending coeruleospinal regeneration that contributes to the recovery of hindlimb locomotion. Now we report that NA axons are present throughout the caudal stump of both media- and OEG-injected spinal rats and they enter the spinal cord from the periphery via dorsal and ventral roots and along large penetrating blood vessels. These results indicate that the presence of NA fibers in the caudal spinal cord is not a reliable indicator of coeruleospinal regeneration. We then asked if NA axons appose cholinergic neurons associated with motor functions, i.e., central canal cluster and partition cells (active during fictive locomotion) and somatic motor neurons (SMNs). We found more NA varicosities adjacent to central canal cluster cells, partition cells, and SMNs in the lumbar enlargement of OEG- than media-injected rats. As non-synaptic release of NA is common in the spinal cord, more associations between NA varicosities and motor-associated cholinergic neurons in the lumbar spinal cord may contribute to the improved treadmill stepping observed in OEG-injected spinal rats. This effect could be mediated through direct association with SMNs and/or indirectly via cholinergic interneurons.

Activated notch disrupts the initial patterning of dopaminergic spinal cord neurons

Dopaminergic spinal cord neurons differentiate in the ventral spinal cord in a nonrandom dispersed pattern. To test whether Notch signaling was involved in generating the pattern of this neuron population as with others, we overexpressed a constitutively active form of Xenopus Notch (XotchDeltaE) in developing frog embryos. Overexpression was targeted to half the spinal cord by injecting activated Notch RNA into one blastomere at the two-cell stage. Injected animals showed morphological differences on the injected side including reduced numbers of dopaminergic spinal cord neurons. This is consistent with a role for Notch signaling in establishing the fate of this population in the developing spinal cord. At a later stage of development, dopaminergic neurons continued to differentiate on both sides of the spinal cord, but the difference between experimental and control columns remained constant. This is consistent with transient activation of Notch disrupting the fate of the earliest (primary) but not later (secondary) dopaminergic neurons. The precursors to secondary neurons appear to be refractory to Notch signaling at earlier stages of development.

Establishment of a ventral cell fate in the spinal cord

The neural plate is induced during gastrulation when the organizer affects the ectoderm around it. Recent experiments show that axial mesoderm can stimulate formation of specific ventral cell types in the spinal cord, including floor plate, motor neurons, and several types of interneurons. We have eliminated or disrupted axial mesoderm by using a variety of methods to show that ventral columns of intermittent dopaminergic neurons in the frog Xenopus also appear to be induced by axial mesoderm. Inversion of the dorsal-ventral neural axis by splitting the presumptive neural plate in vivo, produced two spinal cords with ectopic dopaminergic neurons. The location and number of neurons suggest that even a brief association with axial mesoderm can specify the identity of the first or primary dopaminergic neurons and that notochord retains the ability to induce cells to become secondary dopaminergic neurons.

Descending supraspinal pathways in amphibians: III. Development of descending projections to the spinal cord in Xenopus laevis with emphasis on the catecholaminergic inputs

In developmental stages of the clawed toad, Xenopus laevis, we describe the ontogeny of descending supraspinal connections, catecholaminergic projections in particular, by means of retrograde tracing techniques with dextran amines. Already at embryonic stages (stage 40), spinal projections from the reticular formation, raphe nuclei, Mauthner neurons, vestibular nuclei, the locus coeruleus, the interstitial nucleus of the medial longitudinal fasciculus, the posterior tubercle, and the periventricular nucleus of the zona incerta are well developed. At the beginning of the premetamorphic period (stage 46), spinal projections arise from the suprachiasmatic nucleus, the torus semicircularis, the pretectal region, and the ventral telencephalon. After stage 48, tectospinal and cerebellospinal projections develop, with spinal projections from the preoptic area following at stage 51. Rubrospinal projections are present at stage 50. During the prometamorphic period, spinal projections arise in the nucleus of the solitary tract, the lateral line nucleus, and the mesencephalic trigeminal nucleus. With in vitro double-labeling methods, based on retrograde tracing of dextran amines in combination with tyrosine hydroxylase (TH) immunohistochemistry, we show that at stage 40/41, catecholaminergic (CA) neurons in the posterior tubercle are the first to project to the spinal cord. Subsequently, at stage 43, new projections arise in the periventricular nucleus of the zona incerta and the locus coeruleus. The last CA projection to the spinal cord originates from neurons in the nucleus of the solitary tract at the beginning of prometamorphosis (stage 53). Our data show a temporal, rostrocaudal sequence in the development of the CA cell groups projecting to the spinal cord. Moreover, the early appearance of CA fibers, preterminals and terminal-like structures in dorsal, intermediate, and ventral zones of the embryonic spinal cord, suggests an important role for catecholamines during development in nociception, autonomic functions, and motor control at the spinal level.

Development of GABA-immunoreactive neuron patterning in the spinal cord

In the frog Xenopus laevis, gamma-aminobutyric acid (GABA)-immunoreactive spinal cord neurons (Kolmer-Agduhr cells) formed a dispersed pattern within two columns on either side of the midline. The cellular pattern became established during embryonic and larval development. The GABA-immunoreactive cells are cerebrospinal fluid (CSF)-contacting neurons that began to appear by 1.2 days (st 26) of development. This stage occurred shortly after neural tube closure (0.9 days, st 21) and followed the appearance of ultrastructural characteristics of CSF-contacting neurons. The pattern of GABA-immunoreactive cells emerged during embryogenesis, as their density increased. Each longitudinal column was heterogeneous, containing cells with and without GABA immunoreactivity. Spatial analysis at several embryonic and larval stages showed that the cells in each column formed a nonrandom, dispersed pattern even at early stages of differentiation. This one-dimensional pattern resembled that of dopamine-immunoreactive neurons, which are also located in the ventral spinal cord. The patterning of both cell types followed a different time course, but the ultimate spacing of the neurons remained comparable. These results suggested that the mechanism patterning the two cell types within the same region was similar but not identical and may involve related molecular mechanisms.

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.

Catecholamine systems in the brain of vertebrates: new perspectives through a comparative approach

A comparative analysis of catecholaminergic systems in the brain and spinal cord of vertebrates forces to reconsider several aspects of the organization of catecholamine systems. Evidence has been provided for the existence of extensive, putatively catecholaminergic cell groups in the spinal cord, the pretectum, the habenular region, and cortical and subcortical telencephalic areas. Moreover, putatively dopamine- and noradrenaline-accumulating cells have been demonstrated in the hypothalamic periventricular organ of almost every non-mammalian vertebrate studied. In contrast with the classical idea that the evolution of catecholamine systems is marked by an increase in complexity going from anamniotes to amniotes, it is now evident that the brains of anamniotes contain catecholaminergic cell groups, of which the counterparts in amniotes have lost the capacity to produce catecholamines. Moreover, a segmental approach in studying the organization of catecholaminergic systems is advocated. Such an approach has recently led to the conclusion that the chemoarchitecture and connections of the basal ganglia of anamniote and amniote tetrapods are largely comparable. This review has also brought together data about the distribution of receptors and catecholaminergic fibers as well as data about developmental aspects. From these data it has become clear that there is a good match between catecholaminergic fibers and receptors, but, at many places, volume transmission seems to play an important role. Finally, although the available data are still limited, striking differences are observed in the spatiotemporal sequence of appearance of catecholaminergic cell groups, in particular those in the retina and olfactory bulb.

Motoneurons of the axial swimming muscles in hatchling Xenopus tadpoles: features, distribution, and central synapses

Xenopus tadpole motoneurons make cholinergic synapses within the spinal cord. This excitation changes with longitudinal position and contributes to the excitation that controls motor activity and its longitudinal spread during swimming. To explore the anatomic constraints on this excitation, backfilling has been used to examine the anatomy and distribution of the whole population of spinal motoneurons, to define the extent of their central axons and to find where they make synapses. Motoneuron features show considerable variation but do not allow their separation into primary and secondary. Most motoneurons have descending central axons and it is likely that central synapses are made from these axons as longitudinal dendritic extent is very limited. Motoneuron density reaches a broad plateau over the mid-trunk region at 12-13 per 100 microm. Soma size does not change with longitudinal position, but the dorsoventral extent of the dendrites decreases caudally, whereas the central axon length increases. Motoneuron distribution data were used to estimate the longitudinal distribution of central motoneuron axons. This has a broad plateau at 12-14 per 100 microm over much of the trunk and only decreases significantly caudal to the anus. This distribution correlates with cholinergic excitation during swimming. Transmission electron microscopy of motoneurons backfilled with horseradish peroxidase was used to show that central motoneuron axons make en passant synapses with motoneuron dendrites and the dendrites of other unstained neurons. By using measures of synapse frequency and total dendrite length, trunk motoneurons are estimated to each receive 100-200 synapses.

Tyrosine hydroxylase-containing neurons in the spinal cord of the chicken. I. Development and analysis of catecholamine synthesis capabilities

1. The development of tyrosine hydroxylase-immunoreactive (TH-IR) neurons was examined in the spinal cord of the chick embryo and hatchling. 2. Two groups of TH-IR cells are described, both of which appear to reach their full complement in number relatively late in embryonic development. One group is comprised of numerous cells located ventral to the central canal which make direct contact with the lumen of the canal. The other group consists of large multipolar neurons that reside in the dorsal horn, more commonly along the outer margin of the gray matter within lamina I and II, and less frequently deeper in the dorsal horn within medial portions of laminae V, VI or VII. 3. TH-IR cells ventral to the central canal in the chick are comparable in location to dopamine (DA)-containing spinal cord cells in lower vertebrate species. In contrast, the dorsally-suited TH-IR cells in the chick are known only to occur in similar positions in higher vertebrates. Therefore, the chick is novel in that the presence of both groups of TH-IR cells appearing together in significant numbers within the spinal cord has not been shown in any other species studied to date. 4. The TH-containing cells in the chick cord do not appear to contain the catecholamine biosynthesis enzymes, DBH or PNMT. Moreover, using anti-DA immunocytochemistry, neither group of TH-IR cells demonstrated detectable levels of DA in control animals nor in animals pretreated with inhibitors of MAO (MAO-I). 5. However, a difference was noted though between the two TH-IR cell groups in terms of their responses to exogenously supplied L-DOPA, the immediate precursor to DA. With the administration of L-DOPA and a MAO-I to chick hatchlings, cells in the region ventral to the central canal stained intensely for DA. In contrast, the same treatment failed to produce DA-immunoreactive cells in the dorsal horn. 6. One reasonable hypothesis for these results is that the TH-IR cells ventral to the central canal contain an active form of AADC, the enzyme that converts L-DOPA to DA. With this interpretation, if these cells can produce DA from L-DOPA, yet do not appear to synthesize DA endogenously, it would appear that the TH enzyme contained in these cells occurs in an inactive form. Whether the TH enzyme in the dorsally located immunoreactive cells is also inactive is uncertain since it remains unclear whether they contain AADC.

Morphogenesis of catecholaminergic interneurons in the frog spinal cord

During embryonic and larval development of the frog Xenopus laevis, a bilateral population of cerebrospinal fluid-contacting neurons matures in the ventral spinal cord. These cells are catecholaminergic and form a dispersed or nonrandom pattern of spacing within each of their spinal cord columns. In order to test the mechanisms underlying pattern formation of these neurons, it is first necessary to understand their normal morphogenesis. Morphogenetic changes were examined by using immunocytochemistry for tyrosine hydroxylase as a cell marker. Immunoreactivity in spinal cord cells was detected as early as 1.4 days (stage 28) of embryonic development. Subsequently, these cells underwent changes in shape, and rapid, regressive changes in cell size. The population emerges gradually during development, but the major characteristic of nonrandom spacing, their dispersion from other catecholaminergic cells, is apparent at early stages of differentiation. Increases in cell density occur over an extended period of time and can be divided into an initial phase of large, rapid changes and a subsequent plateau phase of gradual changes. The two longitudinal columns of catecholaminergic cells that are characteristic of older animals become apparent just before hatching, when density increases until cells on both sides of the midline are present in the same region. Although the dispersed pattern exists within each column, cross-correlation analysis shows that there is a random relationship between cells in opposite columns. During larval development, the catecholaminergic cell domain expands in both a rostral and caudal direction. The morphogenetic changes of the catecholaminergic cell population begin to show how the cells become partitioned within the floor plate region of the spinal cord.