Developmental changes in the membrane current pattern, K+ buffer capacity, and morphology of glial cells in the corpus callosum slice

Recent studies indicated that glial cells in tissue culture can express a variety of different voltage-gated channels, while little is known about the presence of such channels in glial cells in vivo. We used a mouse corpus callosum slice preparation, in which after postnatal day 5 (P5) more than 99% of all perikarya belong to glial cells (Sturrock, 1976), to study the current patterns of glial cells during their development in situ. We combined the patch-clamp technique with intracellular labeling using Lucifer yellow (LY) and subsequent ultrastructural characterization. In slices of mice from P6 to P8, we predominantly found cells expressing delayed-rectifier K+ currents. They were similar to those described for cultured glial precursor cells (Sontheimer et al., 1989). A-type K+ currents or Na+ currents were not or only rarely observed, in contrast to cultured glial precursors. LY labeling revealed that numerous thin processes extended radially from the perikaryon of these cells, and ultrastructural observations suggested that they resemble immature glial cells. In slices of older mice (P10-13), when myelination of the corpus callosum has already commenced, many cells were characterized by an almost linear current-voltage relationship. This current pattern was similar to cultured oligodendrocytes (Sontheimer et al., 1989). Most processes of LY-filled cells with such a current profile extended parallel to each other. Electron microscopy showed that these processes surround thick, unmyelinated axons. We suggest that cells with oligodendrocyte-type electrophysiology are promyelinating oligodendrocytes. In contrast to cultured oligodendrocytes, membrane currents of promyelinating oligodendrocytes in the slice decayed during the voltage command. This decay was due not to inactivation, but to a marked change in the potassium equilibrium potential within the voltage jump. This implies that, in the more mature corpus callosum, small membrane polarizations in a physiological range can lead to extensive changes in the K+ gradient across the glial membrane within a few milliseconds

Expression of the axonal cell adhesion molecules axonin 1 and ng cam during the development of the chick retinotectal system

Cell surface glycoproteins expressed on growth cones and axons during brain development have been postulated to be involved in the cell-cell interactions that guide axons into their target area. Nevertheless, an unequivocal description of the mechanism by which such molecules exert control over the pathway of a growing axon has not been done. As a crucial requirement in support of a relevant involvement of an axonal surface molecule in growth cone guidance, this molecule should be expressed in the growth cone. The developing retinotectal system provides an excellent opportunity to test whether a particular neuronal surface molecule fulfills the requirement of the spatiotemporal coincidence between its appearance and the emergence of growth cones because its setup follows the rule of chronotopy, i.e., the position of axons in a certain site is determined by the time of their arrival. We have analyzed axonin-1 and the neuron-glia cell adhesion molecule (Ng-CAM), two axonal surface molecules that promote neurite growth in vitro, for their expression in the retina and in the retinotectal system of the chick throughout its development. At stage 18, both axonin-like (A-LI) and Ng-CAM-like immunoreactivity (Ng-CAM-LI) are clearly present in the area where first retinal ganglion cells (RGCs) are generated. The immunoreactivity spreads synchronously with the formation of RGCs over the developing retina. From stage 32 on, the inner plexiform layer is also stained according to its temporospatial gradient of maturation. In later stages, the outer plexiform layer and the inner segments of photoreceptors also show immunoreactivity. The development of A-LI and Ng-CAM-LI along the optic nerve, chiasm, optic tract, and in the superficial layers of the optic tectum follows the chronotopic pattern of axons, as was found by earlier morphological investigations. Older axons loose their A-LI. This allows to localize the position of newly formed axons. The fact that A-LI and Ng-CAM-LI parallel the formation and maturation of axons suggests that axonin-1 and Ng-CAM may play an important role in the organization of the retinotectal system

GABA- and glutamate-activated currents in glial cells of the mouse corpus callosum slice

Whole-cell transmitter-activated currents were recorded with the patch-clamp technique from glial cells in thin frontal brain slices of the corpus callosum. In slices from 6- to 8-day-old mice, glioblasts were predominantly found, while oligodendrocytes were predominant in slices from 10- to 13-day-old mice. These developmental stages could be readily distinguished by their K+ channel pattern and their morphology and ultrastructural features. Both cell types expressed GABA and glutamate receptors in this in situ preparation. GABA responses showed similarities to those described for GABAA receptors, i.e., they were mimicked by muscimol, blocked by bicuculline, and enhanced by pentobarbital. Glutamate responses showed similarities to those of the kainate/quisqualate receptor subtype. The amplitude of GABA-activated currents recorded in oligodendrocytes was significantly smaller than that from glioblasts, while glutamate responses did not show marked differences in either cell type

Calcium entry through kainate receptors and resulting potassium- channel blockade in Bergmann glial cells

Glutamate receptors, the most abundant excitatory transmitter receptors in the brain, are not restricted to neurons; they have also been detected on glial cells. Bergmann glial cells in mouse cerebellar slices revealed a kainate-type glutamate receptor with a sigmoid current-to-voltage relation, as demonstrated with the patch-clamp technique. Calcium was imaged with fura-2, and a kainate-induced increase in intracellular calcium concentration was observed, which was blocked by the non-N-methyl-D-aspartate (NMDA) glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and by low concentrations of external calcium, indicating that there was an influx of calcium through the kainate receptor itself. The entry of calcium led to a marked reduction in the resting (passive) potassium conductance of the cell. Purkinje cells, which have glutamatergic synapses, are closely associated with Bergmann glial cells and therefore may provide a functionally important stimulus