Glial Cells

Voltage-gated Na+ channels in glia: properties and possible functions

Glial cells are nervous-system cells that have classically been considered to be inexcitable. Despite their lack of electrical excitability, they can express voltage-activated Na+ channels with properties similar to the Na+ channels used by excitable cells to generate action potentials. The functional role that these voltage-activated Na+ channels play in glia is unclear. Three functions have been proposed: (1) glial cells might synthesize Na+ channels and donate them to adjacent neurons, thereby reducing the biosynthetic load of neurons; (2) Na+ channels might endow glial cells with the ability to sense electric activity of neighboring neurons, and might thus play a role in neuro-glial communication; and (3) Na+ influx through voltage-gated Na+ channels could be important to fuel the glial (Na+,K+)-ATPase, thereby facilitating and possibly modulating K+ uptake from the extracellular space.

Heterogeneous morphology and tracer coupling patterns of retinal oligodendrocytes

The present study characterizes the morphology and tracer coupling patterns of oligodendrocytes in the myelinated band of the rabbit retina, as revealed by intracellular injection of biocytin or Lucifer yellow in an isolated superfused preparation. Based on the observed heterogeneity in morphology, we have grouped the presumptive oligodendrocytes into three categories termed 'parallel', 'stratified' and 'radial'. Most parallel oligodendrocytes were tracer coupled to nearby oligodendrocytes and astrocytes, whereas the stratified and radial oligodendrocytes rarely showed coupling. We conclude that the different categories of oligodendrocytes may be stages in a developmental series, with radial oligodendrocytes being premyelinating cells, parallel oligodendrocytes being mature myelinating cells and the stratified cells representing a transition between these categories.

Histamine inactivation in the brain: aspects of N-methylation

This report deals with molecular and anatomical site of histamine N-methylation assumed to be the exclusive route of HA inactivation. The methyl transfer from the -S-CH3 of S-adenosyl-L-methionine to the ring (tele)-nitrogen of histamine, appears as much more complex than a one-step transformation. It seems that -S-CH3 is transformed before being transferred to the nitrogen of the acceptor probably via methanol (formaldehyde) formation. For localizations of transmethylation of neuronal histamine we assume at least a two-compartment model in which glia participate to a significant extent. The uptake of neuronal HA into glial cells might be the first step of histamine inactivation.

Ion channels in rabbit cultured fibroblasts

Large outward currents are recorded with the whole-cell patch-clamp technique on depolarization of rabbit cultured fibroblasts. Our findings suggest that these outward currents consist of two voltage-dependent components, one of which also depends on cytoplasmic calcium concentration. Total replacement of external Cl- by the large anion ascorbate does not affect the amplitude of the currents, indicating that both components must be carried by K+. Consistent with these findings with whole-cell currents, in single channel recordings from fibroblasts we found that most patches contain high-conductance potassium-selective channels whose activation depends on both membrane potential and the calcium concentration at the cytoplasmic surface of the membrane. In a smaller number of patches, a second population of high-conductance calcium-independent potassium channels is observed having different voltage-dependence. The calcium- and voltage-dependence suggest that these two channels correspond with the two components of outward current seen in the whole-cell recordings. The single channel conductance of both channels in symmetrical KCl (150 mM) is 260-270 pS. Both channels are highly selective for K+ over both Na+ and Cl-. The conductance of the channels when outward current is carried by Rb+ is considerably smaller than when it is carried by K+. Some evidence is adduced to support the hypothesis that these potassium channel populations may be involved in the control of cell proliferation.

The presence of voltage-gated sodium, potassium and chloride channels in rat cultured astrocytes

Patch-clamp recording from the plasmalemma of rat cultured astrocytes reveals the presence of both voltage-dependent sodium and voltage-dependent potassium conductances. These conductances are similar but not identical to the corresponding conductances in the axolemma. Whereas the h infinity relation of the sodium channels has the same voltage dependence as in the nodal axolemma, the peak current-voltage relation is shifted by about 30 mV along the voltage axis in the depolarizing direction. It is speculated that the glial cells synthesize sodium and potassium channels for later insertion into the axolemma of neighbouring axons. The astrocytes also express a plasmalemmal voltage-dependent anion conductance that is turned on at about -40 mV (that is, near the resting potential of the cultured astrocytes). The channels involved are large enough to be just permeable to glutamate but not to ascorbate. It is suggested that the conductance of this channel for chloride plays a physiological role in the spatial buffering of potassium by glial cells.

A qualitative and quantitative ultrastructural study of glial cells in the developing visual cortex of the rat

(i) This paper provides new information on the time course and fine structural features of glial cell differentiation, on the relative frequencies of glioblasts, astroblasts, astrocytes, oligodendrocytes and microglial cells, and on neuron: glia ratios in visual cortex of the rat between birth and maturity. The analyses were done on montages of electron micrographs of 75 pm wide strips extending the full depth of the cortex from animals 12 h and 4, 6, 8, 10, 12, 14, 20, 24, 90 and 180 days old (six montages from two or three animals at each age). (ii) At birth, and up to 4 days, most non-neuronal cells are poorly differentiated, irregularly shaped cells with dark nuclei (glioblasts). A few at this stage and progressively larger numbers over the next few days, can be recognized asastroblastsby the presence of a distinctive form of granular reticulum (distended cisterns with a moderately electron dense content), and some also by their position in contact with the subpial or perivascular basal laminae. Astroblasts enlarge, develop processes and transform into immature astrocytes: their nuclei become paler, the granular reticulum is no longer distended, and glial filaments begin to accumulate.Mature astrocyteswith pale nuclei, filaments and a low concentration of perikaryal organelles in a pale cytoplasmic matrix predominate at 24 days, and at 3-6 months 51 % of all glial cells are astrocytes. (iii) Concentrations of glioblasts (at 0 and 4 days) and subsequently of cells of the astrocytic lineage are apparent in the most superficial and in the deepest cortical layers, and an additional small peak is seen at the level of layer IV in the adult animals. The superficial concentration is probably associated with the subpial glia limitans and the layer IV concentration with the high density of synapses in this region; several probable explanations are considered for the concentration in layer VI. (iv) Processes ofradial glial cellsare apparent from birth to day 8 but not thereafter. No evidence was found for transformation of radial glia into astrocytes. A peak in phagocytic activity by immature microglial cells at days 6-8 suggests the possibility of loss of radial glial processes by degeneration rather than transformation. (v)Oligodendroblasts, intermediate in morphology between glioblasts and light oligodendrocytes, appear suddenly in the deep cortex and subcortical white matter at day 6 and are rapidly replaced bylight oligodendrocytes. These are large, organelle-rich cells with characteristically distended Golgi saccules, and are the only oligodendrocytes present during early myelination, which begins at day 10. Early in the 3rd postnatal week some light oligodendrocytes are replaced bymedium oligodendrocytes, which are smaller and darker, with abundant orderly stacks of granular reticulum.Dark oligodendrocytesare first apparent at the end of the 3rd week, account for about one-third of all oligodendrocytes at day 24, predominate at day 40 and constitute 90 % of all oligodendrocytes at 3 and 6 months, at which time oligodendrocytes comprise 39% of all cortical glial cells. We suggest that the progression from light to medium oligodendrocytes does not simply represent a diminution in the overall level of synthetic activity but that different components of the myelin sheath are being synthesized at the two stages. (vi)Microgliaare present from birth but are seen in significant numbers at days 6—10 and thereafter. Some are relatively mature in appearance, even in the youngest animals, and almost all are similar to the resting microglia of adult brain by day 16. At 3-6 months, 8 % of all cortical glial cells are identified as microglia and these cells are fairly evenly distributed throughout the cortical depth but are surprisingly and consistently poorly represented in layer VI. From day 6 to the end of the 2nd postnatal week, cells with poorly differentiated cytoplasm (many free polyribosomes), but containing phagocytosed products of cell degeneration, are identified asimmature microglia. However, it is possible that such cells do not mature into classical resting microglia but that they represent a different cell type. (vii) Theneuron: glia ratiois 4.54 at birth, rises to 5.09 at 4 days, and falls to approximately 2.5 at days 12-24. At 3-6 months the ratio is 2.13.