Perinodal astrocytic processes at nodes of Ranvier in developing normal and glial cell deficient rat spinal cord

The role of astrocytes from synaptic to non-synaptic plasticity

Information storage and transfer in the brain require a high computational power. Neuronal network display various local or global mechanisms to allow information storage and transfer in the brain. From synaptic to intrinsic plasticity, the rules of input-output function modulation have been well characterized in neurons. In the past years, astrocytes have been suggested to increase the computational power of the brain and we are only just starting to uncover their role in information processing. Astrocytes maintain a close bidirectional communication with neurons to modify neuronal network excitability, transmission, axonal conduction, and plasticity through various mechanisms including the release of gliotransmitters or local ion homeostasis. Astrocytes have been significantly studied in the context of long-term or short-term synaptic plasticity, but this is not the only mechanism involved in memory formation. Plasticity of intrinsic neuronal excitability also participates in memory storage through regulation of voltage-gated ion channels or axonal morphological changes. Yet, the contribution of astrocytes to these other forms of non-synaptic plasticity remains to be investigated. In this review, we summarized the recent advances on the role of astrocytes in different forms of plasticity and discuss new directions and ideas to be explored regarding astrocytes-neuronal communication and regulation of plasticity.

The morphology and spatial arrangement of astrocytes in the optic nerve head of the mouse

We evaluated the shapes, numbers, and spatial distribution of astrocytes within the glial lamina, an astrocyte-rich region at the junction of the retina and optic nerve. A primary aim was to determine how the population of astrocytes, collectively, partitions the axonal space in this region. Astrocyte processes labeled with glial fibrillary acidic protein (GFAP) compartmentalize ganglion cell axons into bundles, forming "glial tubes," and giving the glial architecture of the optic nerve head in transverse section a honeycomb appearance. The shapes of individual astrocytes were studied by using transgenic mice that express enhanced green fluorescent protein in isolated astrocytes (hGFAPpr-EGFP). Within the glial lamina the astrocytes were transverse in orientation, with thick, smooth primary processes emanating from a cytoplasmic expansion of the soma. Spaces between the processes of neighboring astrocytes were spatially aligned, to form the apertures through which the bundles of optic axons pass. The processes of individual astrocytes were far-reaching-they could span most of the width of the nerve-and overlapped the anatomical domains of other near and distant astrocytes. Thus, astrocytes in the glial lamina do not tile: each astrocyte participates in ensheathing approximately one-quarter of all of the axon bundles in the nerve, and each glial tube contains the processes of about nine astrocytes. This raises the mechanistic question of how, in glaucoma or other cases of nerve damage, the glial response can be confined to a circumscribed region where damage to axons has occurred.

Expression of fasciculation and elongation protein zeta-1 (FEZ1) in cultured rat neonatal astrocytes

Astrocytes play a more important role than simply providing physical support for neurons, however, the function(s) of type 1 and type 2 astrocytes (T1As, T2As), remains unclear. A DNA microarray was used to identify gene expression in cultured T1As and T2As isolated from postnatal day 1 rat cortex. Ninety-nine of the 138 differentially expressed genes were involved in a diverse number of processes. The fasciculation and elongation protein zeta-1 (FEZ1) gene was studied further because it has been suggested that it is not expressed by astrocytes. RT-PCR and Western blots confirmed the microarray data and showed that FEZ1 was present in T1 and T2As and is more highly expressed in T2As. Immunocytochemistry revealed that FEZ1 was located in the astrocytic cytoplasm and cell processes but not the nucleus. The results contribute to a clearer understanding of the two types of astrocytes.

Synaptophysin immunoreactivity in spinal white matter of young adult rats

Patterns of synaptophysin immunoreactivity were examined in the ventral and lateral funiculi of rat lumbosacral spinal cords. In normal young adults, dendrites from neurons in the spinal gray matter extended into the ventral and lateral white matter as finger-like projections, immunopositive for synaptophysin. These projections appeared to diminish in size as they extended peripherally and, in general, did not reach the surface of the spinal cord, so that the outer one-third to one-fourth of the funiculi contained little or no immunoreactivity. The spinal cords of some of the animals studied were X-irradiated on the third postnatal day. When examined 6 weeks to 5 months later, the pattern of synaptophysin immunoreactivity was found to be markedly altered in these animals. In general, the synaptophysin immunoreactivity in the white matter was less organized than in the non-irradiated rat. As a result, the finger-like projections, particularly into the lateral funiculi, were not as distinct, and the immunoreactivity appeared to be more diffusely distributed in the white matter. Further, the immunoreactivity was present throughout the thickness of the white matter in the irradiated animals and subpial concentrations were evident, especially along the lateral aspect of the spinal cord. Ultrastructural evaluation of the synaptic profiles revealed no differences between irradiated and non-irradiated animals. The synapses occurred on both the shafts of the dendrites and on the spines. In general, both dendrites and axon terminals were covered by astrocyte processes except at synaptic sites, and the synaptic complexes were surrounded by astrocyte processes. Although the mechanisms underlying the altered pattern of synaptophysin immunoreactivity are not yet understood, they may be related to radiation-induced effects on the glial populations previously reported by the investigators and/or to radiation-induced alterations in reorganization or maturation of dendritic trees.

Evidence that the influence of ganglion cell axons on astrocyte morphology is mediated by action spike activity during development

In many mammal retinas, the morphology of astrocytes is strongly influenced by nearby axons of ganglion cells. Astrocyte processes stretch along the axons, fine extensions of the processes contact node-like specialisation of the axon membrane and the morphology of the adult astrocyte is strongly determined by this relationship. The mechanism which attracts astrocyte processes to contact specific regions of the axon membrane is not known however. This study presents evidence that in the neonatal cat blocking the impulse activity of ganglion cells with the Na+-channel blocker tetrodotoxin (TTX) leads to a loss of the axon-related morphology of astrocytes. The morphological change induced in astrocytes by TTX was greater in younger animals and could not be detected in the adult. Conversely, if the TTX block was maintained for 4 postnatal weeks the changes induced in astrocytes persisted at least to 13 weeks. The TTX-induced loss of axon-related morphology in astrocytes suggests that the signal by which axons attract astrocyte processes to contact the axonal membrane in ways which modify astrocyte morphology is released by action spike activity during development.

Glial-glial and glial-neuronal interfaces in radiation-induced, glia-depleted spinal cord

This review summarises some of the major findings derived from studies using the model of a glia-depleted environment developed and characterised in this laboratory. Glial depletion is achieved by exposure of the immature rodent spinal cord to x-radiation which markedly reduces both astrocyte and oligodendrocyte populations and severely impairs myelination. This glia-depleted, hypomyelinated state presents a unique opportunity to examine aspects of spinal cord maturation in the absence of a normal glial population. An associated sequela within 2-3 wk following irradiation is the appearance of Schwann cells in the dorsal portion of the spinal cord. Characteristics of these intraspinal Schwann cells, their patterns of myelination or ensheathment, and their interrelations with the few remaining central glia have been examined. A later sequela is the development of Schwann cells in the ventral aspect of the spinal cord where they occur predominantly in the grey matter. Characteristics of these ventrally situated intraspinal Schwann cells are compared with those of Schwann cells located dorsally. Recently, injury responses have been defined in the glia-depleted spinal cord subsequent to the lesioning of dorsal spinal nerve roots. In otherwise normal animals, dorsal nerve root injury induces an astrocytic reaction within the spinal segments with which the root(s) is/are associated. Lesioning of the 4th lumbar dorsal root on the right side in irradiated or nonirradiated animals results in markedly different glial responses with little astrocytic scarring in the irradiated animals. Tracing studies reveal that these lesioned dorsal root axons regrow rather robustly into the spinal cord in irradiated but not in nonirradiated animals. To examine role(s) of glial cells in preventing this axonal regrowth, glial cells are now being added back to this glia-depleted environment through transplantation of cultured glia into the irradiated area. Transplanted astrocytes establish barrier-like arrangements within the irradiated cords and prevent axonal regrowth into the cord. Studies using other types of glial cultures (oligodendrocyte or mixed) are ongoing.

Astroglial control of oligodendrocyte survival mediated by PDGF and leukemia inhibitory factor-like protein

Programmed death and the identification of growth factors delaying this process in the oligodendrocyte lineage suggest that other cell types provide oligodendrogliotrophins. To determine their source, primary cultures of oligodendroblasts immunopurified from postnatal rat cerebrum were used to screen other cultured neural and non-neural cell types for the release of survival factors into a defined insulin-containing medium. In non-conditioned medium, oligodendroblasts died 1–2 days after undergoing terminal differentiation into oligodendrocytes, as defined by the onset of expression of galactocerebroside. In medium conditioned by astrocytes, unlike the other tested cell types, differentiated oligodendrocytes survived for weeks in a mature myelinogenic state. Survival was partially reduced by immunoabsorption of the medium with antibodies to platelet-derived growth factor and abolished by immunoabsorption with antibodies to leukemia inhibitory factor. By the same criterion, survival activity was not attributed to other astrocytic products, ciliary neurotrophic factor and basic fibroblast growth factor. Membrane ultrafiltration analysis indicated the activity corresponded to heat-labile protein smaller (M(r) = 10(−30) × 10(3)) than native rat leukemia inhibitory factor (M(r) = 43 × 10(3)). The astrocytic stimulus was > 4-fold more efficacious than other known oligodendrogliotrophic cytokines, including ciliary neurotrophic factor, neurotrophin-3 and leukemia inhibitory factor itself, tested singly or in combination, and promoted survival additively with these agents. These findings suggest that astrocytes function as paracrine regulators of oligodendroblast and oligodendrocyte survival and that their effect is mediated initially by platelet-derived growth factor and thereafter by a powerful cytokine related to leukemia inhibitory factor.

Distinct populations of identified glial cells in the developing rat spinal cord slice: ion channel properties and cell morphology

Four types of glial cells could be distinguished in the grey matter of rat spinal cord slices at postnatal days 1-19 (P1-P19), based on their pattern of membrane currents as revealed by the whole cell patch clamp technique, and by their morphological and immunocytochemical features. The recorded cells were labelled with Lucifer Yellow, which allowed the subsequent identification of cells using cell-type-specific markers. Astrocytes were identified by positive staining for glial fibrillary acidic protein (GFAP). These were morphologically characterized by multiple, very fine and short processes and electrophysiologically by symmetrical, non-decaying K+ selective currents. Oligodendrocytes were identified by a typical oligodendrocyte-like morphology, lack of GFAP staining and positive labelling with a combination of O1 and O4 antibodies (markers of the oligodendrocyte lineage), and their membrane was dominated by symmetrical, passive, decaying K+ currents. The third population of glial cells was also characterized by positive staining for O1/O4 or only for O4 antigens, lack of GFAP staining and, in some cells, oligodendrocyte-like morphology. However, these cells could be distinguished by the presence of inwardly rectifying (KIR), delayed outwardly rectifying (KDR) and A-type K+ currents (KA), representing the most likely glial precursor cells of the oligodendrocyte lineage. The fourth population of glial cells had small somata and a widespread network of long processes with no apparent orientation preference. In one case, processes were positively labelled with GFAP, while 30% were characterized by faint, diffuse staining. These cells expressed a complex pattern of voltage-gated channels, namely Na+, KDR, KA and KIR channels. In contrast to neurons, the amplitude of Na+ currents was at least one order of magnitude smaller than the K+ currents, and none of these cells showed the ability to generate action potentials in the current clamp mode. Since none of these cells could be labelled by oligodendrocyte markers we assume that they were either astrocytes or glial precursor cells of the astrocyte lineage. The four cell types were found in all regions of the grey matter. When randomly accessing the glial cells, the probability of recording from the oligodendrocyte precursor cells and the glial cells with Na+ currents decreased during development. At P1-P3, 50% of the cells revealed the Na+ current, while at P13-P15 only 18% did. Concomitantly, the number of glial cells with astrocyte- and oligodendrocyte-like membrane currents increased from 19 and 12% to 41 and 35.5% respectively.(ABSTRACT TRUNCATED AT 400 WORDS)

Astrocyte associations with nodes of Ranvier: ultrastructural analysis of HRP-filled astrocytes in the mouse optic nerve

Astrocytes are implicated in the function of nodes of Ranvier because their perinodal processes form contacts with the axonal membrane at nodes. We have filled astrocytes iontophoretically with horseradish peroxidase in the intact mouse optic nerve to resolve the precise relationship between perinodal processes and astrocyte three dimensional structure. We confirm that nodal contacts were formed either by single processes which almost completely enveloped nodes, or by delicate, finger-like projections from larger processes which made discrete nodal contacts. A single perinodal process can form multiple contacts with a node and nodes were contacted by processes from more than one astrocyte. Perinodal processes emanated from larger processes, which terminated as end-feet on blood vessels and at the pia, as well as collateral branches which subsequently ended at nodes; these latter may specifically subserve nodes. Perinodal contacts were also formed directly by the soma and cytoplasmic expansions of the cell body. Both primary processes and collateral branches formed multiple associations with nodes which often appeared in clusters. Thus, all astrocytes formed multiple contacts with nodes, blood vessels and the subpial glia limitans. We conclude that perinodal processes are not formed by a specialized astrocyte in the mouse optic nerve.

Three-dimensional morphology of astrocytes and oligodendrocytes in the intact mouse optic nerve

The three-dimensional morphology of astrocytes and oligodendrocytes was analysed in the isolated intact mature mouse optic nerve, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells, dye-filled with lysinated rhodamine dextran or horseradish peroxidase, respectively. These techniques enabled the entire process field of single dye-filled cells to be visualized in all planes and resolved the fine details of glial morphology. Morphometric analysis showed that the processes of all astrocytes had branches ending at the pial surface, on blood vessels, and freely in the nerve; branches ending in the nerve were described to end at nodes of Ranvier in the accompanying paper. Astrocytes were classified into a single morphological population in which each cell subserved multiple functions. The results of this study do not support the contention that astrocytes can be subdivided into two morphological and functional subtypes, namely type-1 and type-2, which have process ending either at the glia limitans or at nodes, respectively. Three-dimensional analysis of oligodendrocyte units, defined as the oligodendrocyte, its processes and the axons it ensheaths, showed the provision of single myelin segments for an average of 19 nearby axons (range 12-35) with a mean internodal length of 138 microns (range 50-350 microns). Mouse optic nerve oligodendrocytes were a homogeneous population and were markedly similar to those in the rat optic nerve. The results of our analysis of oligodendrocyte morphology are consistent with the view that the number and internodal length of myelin sheaths supported by a single oligodendrocyte are related to the diameter of the ensheathed axons.

Extracellular ionic and volume changes: the role in glia-neuron interaction

Activity-related changes in extracellular K+ concentration ([K+]e), pH (pHe) and extracellular volume were studied by means of ion-selective microelectrodes in the adult rat spinal cord in vivo and in neonatal rat spinal cords isolated from pups 3-14 days of age (P3-P14). Concomitantly with the ionic changes, the extracellular space (ECS) volume fraction (alpha), ECS tortuosity (lambda) and non-specific uptake (kappa'), three parameters affecting the diffusion of substances in nervous tissue, were studied in the rat spinal cord gray matter. In adult rats, repetitive electrical nerve stimulation (10-100 Hz) elicited increases in [K+]e of about 2.0-3.5 mM, followed by a post-stimulation K(+)-undershoot and triphasic alkaline-acid-alkaline changes in pHe with a dominating acid shift. The ECS volume in the adult rat occupies about 20% of the tissue, alpha = 0.20 +/- 0.003, lambda = 1.62 +/- 0.02 and kappa' = 4.6 +/- 0.4 x 10(-3) s-1 (n = 39). In contrast, in pups at P3-P6, the [K+]e increased by as much as 6.5 mM at a stimulation frequency of 10 Hz, i.e. K+ ceiling level was elevated, and there was a dominating alkaline shift. An increase in [K+]e as large as 1.3-2.5 mM accompanied by an alkaline shift was evoked by a single electrical stimulus. The K+ ceiling level and alkaline shifts decreased with age, while an acid shift, which was preceded by a small initial alkaline shift, appeared in the second postnatal week. In pups at P1-P2, the spinal cord was X-irradiated to block gliogenesis. The typical decrease in [K+]e ceiling level and the development of the acid shift in pHe at P10-P14 were blocked by X-irradiation. Concomitantly, continuous development of glial fibrillary acidic protein positive reaction was disrupted and densely stained astrocytes in gray matter at P10-P14 revealed astrogliosis. The alkaline, but not the acid, shift was blocked by Mg2+ and picrotoxin (10(-6) M). Acetazolamide enhanced the alkaline but blocked the acid shift. Furthermore, the acid shift was blocked, and the alkaline shift enhanced, by Ba2+, amiloride and SITS. Application of glutamate or gamma-aminobutyric acid evoked an alkaline shift in the pHe baseline at P3-P14 as well as after X-irradiation. The results suggest that the activity-related acid shifts in pHe are related to membrane transport processes in mature glia, while the alkaline shifts have a postsynaptic origin and are due to activation of ligand-gated ion channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Astroglial pattern in the spinal cord of the adult barbel (Barbus comiza)

The distribution and the structural, ultrastructural and immunohistochemical characteristics of the astroglial cells in the spinal cord of the adult barbel (Barbus comiza) have been studied by means of metallic impregnations (Golgi and gold-sublimate), immunohistochemical (GFAP and vimentin) and electron microscopic techniques. GFAP-positive cells were mainly distributed in the ependyma and in the periependymal region, but they have also been observed at subpial level in the anterior column. The ependymocytes were heterogeneous cells because they showed different immunohistochemical characteristics: GFAP-positive, vimentin-positive or non-immunoreactive cells. The radial astrocytes showed only GFAP immunoreactivity, and their processes ended at the subpial zone forming a continuous subpial glia limitans. Desmosomes and gap junctions between somata and processes of radial astrocytes were numerous, and a relationship between radial astroglial processes and the nodes of Ranvier was also described. The perivascular glia limitans was poorly developed and it was not complete in the blood vessels of the periependymal zone; in this case, the basal lamina was highly developed. An important characteristic in the barbel spinal cord was the existence of a zone with an abundant extracellular space near the ependyma. The presence of radial astroglial somata at subpial level, the existence of vimentin-positive ependymocytes and the abundant extracellular space in the periependymal zone is discussed in relation to the regeneration capacity and the continuous growth showed by fish. Moreover, the abundance of gliofilaments and desmosomes leads us to suggest that mechanical support might be an important function for the astroglial cells in the barbel spinal cord.

Molecular dissection of the myelinated axon

The membrane of the myelinated axon expresses a rich repertoire of physiologically active molecules: (1) Voltage-sensitive NA+ channels are clustered at high density (approximately 1,000/microns 2) in the nodal axon membrane and are present at lower density (< 25/microns 2) in the internodal axon membrane under the myelin. Na+ channels are also present within Schwann cell processes (in peripheral nerve) and perinodal astrocyte processes (in the central nervous system) which contact the Na+ channel-rich axon membrane at the node. In some demyelinated fibers, the bared (formerly internodal) axon membrane reorganizes and expresses a higher-than-normal Na+ channel density, providing a basis for restoration of conduction. The presence of glial cell processes, adjacent to foci of Na+ channels in immature and demyelinated axons, suggests that glial cells participate in the clustering of Na+ channels in the axon membrane. (2) "Fast" K+ channels, sensitive to 4-aminopyridine, are present in the paranodal or internodal axon membrane under the myelin; these channels may function to prevent reexcitation following action potentials, or participate in the generation of an internodal resting potential. (3) "Slow" K+ channels, sensitive to tetraethylammonium, are present in the nodal axon membrane and, in lower densities, in the internodal axon membrane; their activation produces a hyperpolarizing afterpotential which modulates repetitive firing. (4) The "inward rectifier" is activated by hyperpolarization. This channel is permeable to both Na+ and K+ ions and may modulate axonal excitability or participate in ionic reuptake following activity. (5) Na+/K(+)-ATPase and (6) Ca(2+)-ATPase are also present in the axon membrane and function to maintain transmembrane gradients of Na+, K+, and Ca2+. (7) A specialized antiporter molecule, the Na+/Ca2+ exchanger, is present in myelinated axons within central nervous system white matter. Following anoxia, the Na+/Ca2+ exchanger mediates an influx of Ca2+ which damages the axon. The molecular organization of the myelinated axon has important pathophysiological implications. Blockade of fast K+ channels and Na+/K(+)-ATPase improves action potential conduction in some demyelinated axons, and block of the Na+/Ca2+ exchanger protects white matter axons from anoxic injury. Modification of ion channels, pumps, and exchangers in myelinated fibers may thus provide an important therapeutic approach for a number of neurological disorders.

K+ and pH homeostasis in the developing rat spinal cord is impaired by early postnatal X-irradiation

Activity-related transient changes in extracellular K+ concentration ([K+]e) and pH (pHe) were studied by means of ion-selective microelectrodes in neonatal rat spinal cords isolated from pups 2-14 days of age. Pups 1 to 2 days old were X-irradiated to impair gliogenesis and spinal cords were isolated 2-13 days postirradiation (PI). In 2- to 14-day-old pups PI stimulation produced ionic changes that were the same as those in 3- to 6-day-old control (non-irradiated) pups; e.g. the [K+]e increased by 4.03 +/- 0.24 mM (mean +/- S.E.M., n = 30) at a stimulation frequency of 10 Hz and this was accompanied by an alkaline shift of 0.048 +/- 0.004 pH units (mean +/- S.E.M., n = 32) pH units. By contrast, stimulation in non-irradiated 10- to 14-day-old pups produced smaller [K+]e changes, of 1.95 +/- 0.12 mM (mean +/- S.E.M., n = 30), and an acid shift of 0.035 +/- 0.003 pH units which was usually preceded by a scarcely discernible initial alkaline shift, as is also the case in adult rats. Our results show that the decrease in [K+]e ceiling level and the development of the acid shift in pHe are blocked by X-irradiation. Concomitantly, typical continuous development of GFAP-positive reaction was disrupted and densely stained astrocytes in gray matter of 10- to 14-day-old pups PI revealed astrogliosis. In control 3- to 6-day-old pups and in pups PI the stimulation-evoked alkaline, but not the acid, shift was blocked by Mg2+ and picrotoxin (10(-6) M). The acid shift was blocked, and the alkaline shift enhanced, by acetazolamide, Ba2+, amiloride and SITS. Application of GABA evoked an alkaline shift in the pHe baseline which was blocked by picrotoxin and in HEPES-buffered solution. By contrast, the stimulus-evoked alkaline shifts were enhanced in HEPES-buffered solutions. The results suggest a dual mechanism of the stimulus-evoked alkaline shifts. Firstly, the activation of GABA-gated anion (Cl-) channels induces a passive net efflux of bicarbonate, which may lead to a fall in neuronal intracellular pH and to a rise in the pHe. Secondly, bicarbonate independent alkaline shifts may arise from synaptic activity resulting in a flux of acid equivalents.

Radial glia give rise to perinodal processes

Nodes of Ranvier in the central nervous system in mammals are characterized by the presence of perinodal astrocytic processes. This study examines the association between processes of radial glia and the axolemma at nodes of Ranvier in the spinal cord of the mature axolotl, an animal in which radial glia represent a large portion of the total glial population. The radial glial cells have their cell bodies located close to the central canal. Those situated dorsal to the canal send long processes to the dorsal surface of the spinal cord. Along this trajectory these processes coalesce into large fascicles in the midline and form the dorsal median septum. Slender branches rise from the processes in these fascicles and extend into the adjacent white matter to terminate in close apposition to the axolemma at nodes of Ranvier. This arrangement provides an intracellular pathway extending from the perinodal region to the surface of the spinal cord. Radial glia ventral to the central canal give rise to processes that project to the glia limitans adjacent to the ventral spinal artery. These ventrally projecting processes appear to be more irregular in their branching pattern than their dorsal counterparts. Multiple slender processes are seen in close apposition to the nodal axolemma of myelinated axons in the ventral white commissure, again providing an intracellular pathway that runs from the perinodal region to the cord surface. In one instance a radial glial process was observed to occupy a pocket formed by the invagination of the nodal axolemma. The axonal cytoplasm adjacent to the invagination contained a variety of organelles, e.g. multivesicular bodies, vesicles and endoplasmic reticulum, suggesting that this relationship between the radial glial process and the axon is more than a passive interaction. These observations are consistent with the view that processes of radial glial cells may regulate the extracellular environment adjacent to the nodal axolemma, and/or play an active role in the maintenance of the nodal membrane. The existence of perinodally-directed processes of radial glial cells in the salamander indicates that this axo-glial specialization reflects an important functional interaction preserved across a large segment of the phylogenetic scale.

Role of glia in K+ and pH homeostasis in the neonatal rat spinal cord

Stimulation-evoked transient changes in extracellular potassium ([K+]e) and pH (pHe) were studied in the neonatal rat spinal cords isolated from 3-13-day-old pups. In unstimulated pups the [K+]e baseline was elevated and pHe was more acid than that in Ringer's solution (3.5 mM K+, pH 7.3-7.35). The [K+]e and pHe in 3-6-day-old pups was 3.91 +/- 0.12 mM and pHe 7.19 +/- 0.01, respectively, while in 10-13-day-old pups it was 4.35 +/- 0.15 mM and 7.11 +/- 0.01, respectively. The [K+]e changes evoked in the dorsal horn by a single electrical stimulus were as large as 1.5-2.5 mM. Such changes in [K+]e are evoked in the adult rat spinal cord with stimulation at a frequency of 10-30 Hz. The maximal changes of 2.1-6.5 mM were found at a stimulation frequency of 10 Hz in 3-6-day-old animals. In older animals the [K+]e changes progressively decreased. The poststimulation K(+)-undershoot was found after a single stimulus as well as after repetitive stimulation. In 3-8-day-old pups, the stimulation evoked an alkaline shift, which was followed by a smaller poststimulation acid shift when the stimulation was discontinued. In pups 3-4-days-old the stimulation evoked the greatest alkaline shifts, i.e., by as much as 0.05 pH units after a single pulse and by about 0.1 pH units during stimulation at a frequency of 10 Hz. In 5-8-day-old pups, the alkaline shift became smaller and the poststimulation acid shift increased.(ABSTRACT TRUNCATED AT 250 WORDS)

Development of the rat corticospinal tract through an altered glial environment

The major corticospinal tract (CST) in the rat is located at the base of the dorsal funiculus. It is a late-developing tract, and the growth of its axons into the lumbosacral region of the spinal cord does not occur until postnatal days 5 and 6. This delay is taken advantage of in this study in order to evaluate the effects of a markedly reduced glial population on ingrowth of the CST axons into the lumbosacral spinal cord. A reduction of the glial population is achieved by exposure of this region of spinal cord to X-radiation at 3 days of age. Growth of CST axons into and through the lumbosacral spinal cord in rats in which this region has undergone a radiation-induced depletion of glial cells is compared with that in their non-irradiated littermate controls by axonal tracing techniques using horseradish peroxidase (HRP). The HRP was applied directly to the motor cortices of normal and irradiated rats, and at all ages studied, there was anterograde filling of CST axons and their growth cones. At 3 days postnatally, the age when the lumbosacral spinal cord was irradiated in the experimental animals, CST axons were present in the more rostral thoracic levels. CST axons were observed in the lumbar region of non-irradiated rats on day 5, and by day 7 they were present at sacral levels.(ABSTRACT TRUNCATED AT 250 WORDS)

Ion channel organization of the myelinated fiber

The myelinated axon provides a model in which it is possible to examine how various types of ion channels are incorporated into a membrane to form an excitable neuronal process. The available evidence now indicates that mammalian myelinated fibers contain a repertoire of physiologically active membrane molecules including at least four types of ion channels and an electrogenic Na+,K(+)-pump. Physiological properties of myelinated fibers reflect the distribution of these various types of channels and pumps, as well as interactions with myelinating Schwann cells in the PNS or oligodendrocytes in the CNS. A growing body of data also suggests a role for astrocytes and Schwann cells at nodes of Ranvier. This article reviews the current understanding of the ion channel organization of the mammalian myelinated fiber.

Macromolecular structure of axon membrane and action potential conduction in myelin deficient and myelin deficient heterozygote rat optic nerves

The macromolecular structure of the axon membrane in optic nerves from 25-day-old male littermate control and myelin deficient (md) rats and 16-month-old md heterozygotic rats was examined with quantitative freeze-fracture electron microscopy. The axon membrane of control optic nerves displayed an asymmetrical partitioning of intramembranous particles (IMPs); P-fracture faces of myelinated internodal axon membrane were more particulate than those of pre-myelinated axons (approximately 1600 v 1100 microns-2, respectively), while relatively few IMPs (approximately 150 microns-2) were present on external faces (E-faces) of internodal or pre-myelinated axon membrane. Amyelinated axons of md optic nerves also exhibited an asymmetrical partitioning of IMPs; protoplasmic membrane face (P-face) IMP densities, taken as a group, exhibited a wide range (approximately 600-2300 microns-2) and, in most regions, E-faces displayed a relatively low IMP density (approximately 175 microns-2). Axons of greater than 0.4 microns diameter exhibited significantly greater mean P-face IMP density than axons less than 0.4 microns diameter. Aggregations of E-face IMPs (approximately 350 microns-2) were occasionally observed along amyelinated axon membrane from md optic nerves. Optic nerves from md heterozygote rats exhibit myelin mosaicism, permitting examination of myelinated and amyelinated axon membrane along the same tract. The axon membrane exhibits different ultrastructure in these two domains. Myelinated internodal axon membrane from md heterozygote optic nerves exhibits similar P- and E-face IMP densities to those of control internodal axolemma (approximately 1800 and 140 microns-2, respectively). Amyelinated axons in the heterozygote exhibit a membrane structure similar to amyelinated axons in md optic nerve. P-face IMP density of large diameter (greater than 0.4 microns) amyelinated axons from md heterozygote optic nerves is significantly greater than that of small calibre (less than 0.4 microns) axons. In most regions, amyelinated axon membrane exhibits a relatively low E-face IMP density (approximately 200 microns-2); however, focal aggregations (approximately 400 microns-2) of E-face particles are present. Electrophysiological recordings demonstrate that amyelinated axons in md optic nerves support the conduction of action potentials. Compound action potentials in md optic nerves exhibit a monophasic configuration, even at 20-days postnatal, similar to that of pre-myelinated optic nerve of 7-day-old normal rats. Moreover, conduction velocities in the amyelinated 20-day-old md optic nerve are similar to those displayed by pre-myelinated axons from 7-day-old optic nerves.(ABSTRACT TRUNCATED AT 400 WORDS)

Subpial and perivascular astrocytes associated with nodes of Ranvier in the rat optic nerve

Recent evidence has suggested that there is a division of labour between two types of astrocytes in the rat optic nerve--one type extending processes to the pial surface and to blood vessels, the other extending processes to nodes of Ranvier (Miller et al., 1989b). Here we show that astrocytes and astrocyte processes located at the pial surface and around blood vessels in the rat optic nerve can also be associated with nodes of Ranvier, indicating that the division of labour between the two types of astrocytes in the nerve is not as strict as previously proposed.

The perinodal astrocyte

Several studies have demonstrated the presence of perinodal astrocyte processes at nodes of Ranvier in the central nervous system, suggesting that, in addition to the axon and oligodendrocyte, astrocytes participate in the formation of mature central nodes. The specific association between perinodal astrocyte processes and nodal membrane develops at the time of, or soon after, the appearance of relatively differentiated nodes of Ranvier. This interaction is likely to be mediated by cell adhesion molecules. J1 is a member of a family of glycoproteins that share a common carbohydrate epitope, designated L2/HNK-1, and that have been implicated in cell-cell interactions. This glycoprotein is concentrated at the interface between perinodal astrocyte processes and the nodal region of the axon. Moreover, N-CAM, which is a member of the same family as J1, and cytotactin, an extracellular matrix component produced by glia, are localized at the interface between the axon and perinodal astrocyte processes at nodes of Ranvier. The association of perinodal astrocyte processes with nodal membrane in the central nervous system is similar to that exhibited by perinodal Schwann cell processes at peripheral nodes, and similar functional properties have been suggested for these two glial cell processes, including production of nodal gap substance, buffering of perinodal extracellular ion concentration, and development and/or maintenance of nodal specializations in the axon membrane. Perinodal astrocyte and Schwann cell processes may also function as extraneuronal sites for the synthesis of voltage-sensitive sodium channels, to complement neuronal perikaryal synthesis and axonal transport. Ultrastructural studies on specialized patches of axon membrane within some unmyelinated, demyelinated, and dysmyelinated axons support the hypothesis of a specific role for perinodal astrocyte processes in the assembly, stabilization, and/or maintenance of axolemma with nodal characteristics. These observations suggest a multiplicity of functions for perinodal astrocyte processes at central nodes and implicate the astrocyte as an important component of the node of Ranvier.

Demyelination in canine distemper encephalomyelitis: an ultrastructural analysis

A morphological study of selected white matter lesions was carried out in three dogs with canine distemper encephalomyelitis. Two dogs had experimental infections while the third was a spontaneous case. Two stages were identified in the process of demyelination. The earliest evidence of myelin injury was a ballooning change in myelin sheaths involving single or multiple axons. This was followed by a progressive stripping of compact sheaths by the cytoplasmic fingers of phagocytic cells which infiltrated and removed myelin lamellae. Some axonal necrosis also accompanied these changes. Where demyelination occurred, canine distemper viral nucleocapsids were found in astrocytes, macrophages, ependymal cells and infiltrating lymphocytes. In contrast, oligodendrocytes were conspicuous by their apparent lack of infection. Thus it seems that myelin loss cannot be ascribed to oligodendrocyte infection. Perturbed astrocyte function following canine distemper viral infection may cause oedema of myelin sheaths, leading to ballooning and primary demyelination. Cells which phagocytosed myelin were mainly identified as microglial cells with lesser involvement by astrocytes. Rarely, oligodendrocytes also acted as macrophages. Myelin debris was engulfed in bulk or as small droplets into coated pits. Remyelination was present in established plaques although not in great abundance, perhaps due to the diminished oligodendrocyte numbers and a relative increase in immature forms of these cells. These observations are compared to similar changes observed in other demyelinating diseases of animals and man.

Fine structural relationships between astrocytes and the node of Ranvier in the amphibian and reptilian spinal cord

Associations between the axolemma of nodes of Ranvier and perinodal astrocytes in the amphibian and reptilian spinal cord were examined by electron microscopy. Our present results demonstrate the astrocytic processes at the nodes of Ranvier in the central nervous system of lower vertebrates. Such a presence in lower vertebrates is discussed in the framework of current views on neuron-glial interactions and their possible phylogenetic implications.

Effects of delayed myelination by oligodendrocytes and Schwann cells on the macromolecular structure of axonal membrane in rat spinal cord

The macromolecular structure of axonal membrane from dorsal funiculi of control and irradiated spinal cord of 45-day-old rats was examined with freeze-fracture electron microscopy. In control spinal cords, virtually all myelination is mediated by oligodendrocytes, and the internodal axonal membrane of these fibres displays highly asymmetrical partitioning of intramembranous particles (IMPs). The internodal P-face particle density is approximately 2350IMPs per micron 2, whereas the E-face IMP density is approximately 150 per micron 2. In control dorsal spinal roots, myelination is mediated by Schwann cells, and the ultrastructure of the internodal axolemma of the myelinated fibres is similar to that displayed by myelinated fibres of dorsal funiculi. On the internodal P-face of Schwann cell-myelinated fibres the IMP density is approximately 2350 per micron 2, whereas on the E-face the density is approximately 175 per micron 2. Irradiation of the lumbosacral spinal cord at 3 days of age results in a glial cell-deficient region within the spinal cord such that myelination in irradiated dorsal funiculi is delayed and subsequent myelination is mediated by both oligodendrocytes and Schwann cells. By 45 days of age, dorsal funiculi of irradiated spinal cords are well populated with fibres myelinated by oligodendrocytes and Schwann cells. However, fibres myelinated by oligodendrocytes display very thin myelin sheaths whereas Schwann cell-myelinated fibres exhibit myelin sheaths with normal thicknesses. Internodal membrane of fibres myelinated by Schwann cells and oligodendrocytes exhibit similar macromolecular structure, with approximately 2400 IMPs per micron 2 on P-faces and approximately 150 IMPs per micron 2 on E-faces. Occasional large (greater than 1.5 micron diameter) axons without glial-Schwann cell ensheathment are observed. These axons display a high density of P-face particles (approximately 2000 per micron 2) and a moderate density (approximately 350 per micron 2) of E-face IMPs on their fracture faces. These results demonstrate that CNS fibers exhibit similar axonal membrane ultrastructure irrespective of whether they are myelinated by Schwann cells or oligodendrocytes, or whether myelination is delayed. Moreover, when myelination does not occur, the axolemmal E-face IMP density, which may be related to the density of voltage-sensitive sodium channels, is not reduced.

The oligodendrocyte-type-2 astrocyte cell lineage is specialized for myelination

Astrocytes are one of the most numerous cell types in the vertebrate central nervous system (CNS) and yet their functions are largely unknown. In the rat optic nerve there are two distinct types of astrocyte: type-1 astrocytes develop from one type of precursor cell, and type-2 astrocytes develop from bipotential, oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells, that initially give rise to oligodendrocytes (which make myelin in the CNS), and then to type-2 astrocytes. Type-1 astrocytes form the glial limiting membrane at the periphery of the optic nerve and are probably responsible for glial scar formation following nerve transection. The functions of type-2 astrocytes, which, like oligodendrocytes, are found mainly in tracts of myelinated axons throughout the CNS, are unknown. In this report we provide evidence that processes from type-2 astrocytes contribute to the structure of nodes of Ranvier, suggesting that the O-2A cell lineage is specialized for constructing myelin sheaths and nodes in the mammalian CNS.

A quantitative study of developing axons and glia following altered gliogenesis in rat optic nerve

Axonal and glial cell development within rat optic nerve in which gliogenesis was altered by systemic injection of 5-azacytidine (5-AZ) was examined by quantitative electron microscopy. In neonatal (0-2 days) rat optic nerves, all fibers are premyelinated, and they exhibit a fairly uniform diameter (approximately 0.22 micron). These fibers occupy approximately 55% of the optic nerve volume. At this early age, glia within the optic nerve consist only of cells of astrocytic lineage and progenitor cells. These glia occupy approximately 28% of the optic nerve volume, and there are approximately 80 glial cells/optic nerve cross section. In 14-day-old normal optic nerves, myelinated and ensheathed fibers comprise approximately 17% and 9%, respectively, of the total number of axons. Mean axonal diameter of myelinated fibers is approximately 0.75 micron, while mean diameter for ensheathed axons is approximately 0.50 micron. By volume, these fibers occupy approximately 25% of the nerve, which is similar to the volume occupied by premyelinated axons in these nerves. At 14 days of age, there are approximately 300 glial cells/optic nerve transverse section, and these glia occupy approximately 37% of the volume in normal optic nerve. Oligodendroglia represent approximately 40% of total glial cells present, while astroglia and progenitor cell each comprise approximately 30% of the cells. In optic nerves from 14-day-old rats treated with 5-AZ, few myelinated fibers are present and the number of oligodendroglia is markedly reduced. Axonal diameter of premyelinated fibers is similar to that of age-matched controls. Myelinated and ensheathed fibers comprise approximately 2% of the total fibers present in 5-AZ-treated optic nerves, with the remaining fibers being premyelinated. The few myelinated and ensheathed fibers present in 5-AZ-treated optic nerves display similar axonal diameters to corresponding fibers from age-matched control tissue. Glial cells occupy approximately 40% of the nerve volume, and there are approximately 200 glia/nerve cross section in 5-AZ-treated rats. Astroglia comprise approximately 63% of the total glial cells, while approximately 12% of the cells are oligodendroglia. These results demonstrate that 5-AZ is a potent inhibitor of oligodendrogliogenesis, with a concomitant marked reduction in the number of myelinated fibers.

Molecular structure of the axolemma of developing axons following altered gliogenesis in rat optic nerve

Axonal and axolemmal development of fibers from rat optic nerves in which gliogenesis was severely delayed by systemic injection of 5-azacytidine (5-AZ) was examined by freeze-fracture electron microscopy. In neonatal (0-2 days) rat optic nerves, all fibers lack myelin, whereas in the adult, virtually all axons are myelinated. The axolemma of neonatal premyelinated fibers is relatively undifferentiated. The P-fracture face (P-face) displays a moderate (approximately 550/micron 2) density of intramembranous particles (IMPs), whereas the E-fracture face (E-face) has few IMPs (approximately 125/micron 2) present. By 14 days of age, approximately 25% of the axons within control optic nerves are ensheathed or myelinated, with the remaining axons premyelinated. The ensheathed and myelinated fibers display increased axonal diameter compared to premyelinated axons, and these larger caliber fibers exhibit marked axonal membrane differentiation. Notably, the P-face IMP density of ensheathed and myelinated fibers is substantially increased compared to premyelinated axolemma, and, at nodes of Ranvier, the density of E-face particles is moderately high (approximately 1300/micron 2), in comparison to internodal or premyelinated E-face axolemma. In optic nerves from 14-day-old 5-AZ-treated rats, few oligodendrocytes are present, and the percentage of myelinated fibers is markedly reduced. Despite delayed gliogenesis, some unensheathed axons within 5-AZ-treated optic nerves display an increased axonal diameter compared to premyelinated fibers. Most of these large caliber fibers also exhibit a substantial increase in P-face IMP density. Small (less than 0.4 micron) diameter unensheathed axons within treated optic nerves maintain a P-face IMP density similar to that of control premyelinated fibers. Regions of increased E-face particle density were not observed. The results demonstrate that some aspects of axolemma differentiation continue despite delayed gliogenesis and the absence of glial ensheathment, and suggest that axolemmal ultrastructure is, at least in part, independent of glial cell association.

Membrane structure of vesiculotubular complexes in developing axons in rat optic nerve: freeze-fracture evidence for sequential membrane assembly

Intra-axonal vesiculotubular complexes, located within developing axons in the optic nerve of eight-day-old rats, were examined by freeze-fracture electron microscopy. The clusters usually fill most of the cross section of the axon and extend for approximately 1 micron along the fibre axis. As seen in freeze-fracture, the E- and P-faces of the membranes comprising these clusters exhibit a paucity of intramembranous particles (i.m.ps). This i.m.p.-poor membrane structure is different from that of the axolemma per se, which contains i.m.p. densities of ca. 120 micron-2 on the E-face and ca. 400 micron-2 on the P-face. Since earlier studies indicate that the vesiculotubular complexes fuse with the axon membrane so as to contribute to membrane growth, it is suggested that axonal differentiation involves a sequential mode of membrane development, in which an initial growth of a relatively undifferentiated membrane bilayer is followed by in situ insertion of specialized proteins within specific membrane domains.

Organization of ion channels in the myelinated nerve fiber

The functional organization of the mammalian myelinated nerve fiber is complex and elegant. In contrast to nonmyelinated axons, whose membranes have a relatively uniform structure, the mammalian myelinated axon exhibits a high degree of regional specialization that extends to the location of voltage-dependent ion channels within the axon membrane. Sodium and potassium channels are segregated into complementary membrane domains, with a distribution reflecting that of the overlying Schwann or glial cells. This complexity of organization has important implications for physiology and pathophysiology, particularly with respect to the development of myelinated fibers.

Membrane ultrastructure of developing axons in glial cell deficient rat spinal cord

In order to investigate axolemmal development in a glial cell deficient environment, normal and irradiated dorsal funiculus in rat lumbosacral spinal cord was examined by freeze-fracture electron microscopy. At 3 days of age, normal fibres are all unmyelinated and of small (less than 0.5 micron) diameter. The unmyelinated axons have a moderate density (approximately 850 microns-2) of intramembranous particles (IMPs) on P-fracture faces and a low IMP density (approximately 300 microns-2) on E-faces. IMPs are homogeneously distributed along both fracture faces. By 19 days of age, the normal dorsal funiculus is well populated with myelinated axons and glial cells, as well as a sizable population of unmyelinated fibres. Nearly all of the myelinated fibres have a large (greater than 1.0 micron) diameter; whereas, most unmyelinated axons are of small (less than 0.5 micron) calibre. The axolemma of unmyelinated axons is relatively undifferentiated, with an asymmetrical distribution of IMPs (P-face: approximately 1100 microns-2; E-face: approximately 450 microns-2). Myelinated fibres show nodal and paranodal regions with P-face and E-face ultrastructure similar to previous descriptions. Internodal axolemma appears relatively homogeneous, with P-faces being highly particulate (approximately 2100 microns-2) and a low IMP density (approximately 200 microns-2) on E-faces. Following irradiation of the lumbosacral spinal cord at 3 days of age, there is a severe reduction in the number of glial cells and myelinated fibres in this region when the tissue is examined at 19 days of age. Despite the deficiency of glial cells in this tissue, axonal and axolemmal development continue. Numerous large (greater than 1.0 micron) diameter axons are present in this irradiated tissue. Large diameter axons show a high (approximately 2000 microns-2) density of IMPs on P-faces; E-face IMP density remains at approximately 440 micron-2. Small calibre axons also have an asymmetrical distribution of particles (P-face: approximately 1100 microns-2; E-face: 280 microns-2). The axolemmal E-faces of some glial cell deprived fibres exhibit regions with greater than normal (approximately 750 microns-2) density of IMPs. These results demonstrate that some aspects of axonal and axolemmal development continue in a glial cell deficient environment, and it is suggested that axolemmal ultrastructure is, at least in part, independent of glial cell association.

Glial proliferation in the irradiated rat spinal cord

The identity of mitotic cells in the ventral half of the irradiated spinal cord in 13-day-old rats was studied by light and electron microscopy. At this post-irradiation interval, astrocytes as well as oligodendrocytes are markedly reduced in both gray and white matter, and few myelin sheaths are present. Earlier studies showed incorporation of 3H-thymidine into cells identified light-microscopically as neuroglia. In the present study, a number of mitotic cells were identified in thick plastic sections. When adjacent thin sections were examined by electron microscopy, these mitotic cells were identified ultrastructurally as astroglia on the basis of the bundles of filaments in their cytoplasm and the irregular outline of the cell body and its processes. It is apparent from this study that astroglia proliferate prior to the delayed myelination that occurs later in the glial cell deprived ventral irradiated cord.