Sodium channel expression detected with antibody 7493 in A2B5+ and A2B5- astrocytes from rat optic nerve in vitro

Functions of fibroblast growth factor (FGF)-2 and FGF-5 in astroglial differentiation and blood-brain barrier permeability: evidence from mouse mutants

Multiple evidence suggests that fibroblast growth factors (FGFs), most prominently FGF-2, affect astroglial proliferation, maturation, and transition to a reactive phenotype in vitro, and after exogenous administration, in vivo. Whether this reflects a physiological role of endogenous FGF is unknown. Using FGF-2 and FGF-5 single- and double mutant mice we show now a region-specific reduction of glial fibrillary acidic protein (GFAP), but not of S100 in gray matter astrocytes. FGF-2 is apparently the major regulator of GFAP, because in mice deficient for FGF-2, GFAP is distinctly reduced in cortex and striatum, whereas in FGF-5-/- animals only a reduction in the midbrain tegmentum can be observed. In FGF-2-/-/FGF-5-/- double mutant animals, GFAP-immunoreactivity is reduced in all three brain regions. Cortical astrocytes cultured from FGF-2-/-/FGF-5-/- double mutant mice revealed reduced levels of GFAP, but not S100 as compared with wild-type littermates. This phenotype could be rescued by exogenous FGF-2 but not FGF-5 (10 ng/ml). Electron microscopy revealed reduced levels of intermediate filaments in perivascular astroglial endfeet. This defect was accompanied by enhanced permeability of the blood-brain barrier (BBB), as detected by albumin extravasation. Levels of the tight junction proteins Occludin and ZO-1 were reduced in blood vessels of FGF-2-/-/FGF-5-/- double mutant mice as compared with wild-type littermates. Our data support the notion that endogenous FGF-2 and FGF-5 regulate GFAP expression in a region-specific manner. The observed defect in astroglial differentiation is accompanied by a defect in BBB function arguing for an indirect or direct role of FGFs in the regulation of BBB permeability in vivo.

Expression of voltage-activated chloride currents in acute slices of human gliomas

Using whole-cell patch-clamp recordings, we identified a novel voltage-activated chloride current that was selectively expressed in glioma cells from 23 patient biopsies. Chloride currents were identified in 64% of glioma cells studied in acute slices of nine patient biopsies. These derived from gliomas of various pathological grades. In addition, 98% of cells acutely isolated or in short-term culture from 23 patients diagnosed with gliomas showed chloride current expression. These currents, which we termed glioma chloride currents activated at potentials >45 mV, showed pronounced outward rectification, and were sensitive to bath application of the presumed Cl- channel specific peptide chlorotoxin (approximately 600 nM) derived from Leiurus scorpion venom. Interestingly, low grade tumours (e.g., pilocytic astrocytomas), containing more differentiated, astrocyte-like cells showed expression of glioma chloride currents in concert with voltage-activated sodium and potassium currents also seen in normal astrocytes. By contrast, high grade tumours (e.g., glioblastoma multiforme) expressed almost exclusively chloride currents, suggesting a gradual loss of Na+ currents and gain of Cl- currents with increasing pathological tumour grade. To expand on the observation that these chloride currents are glioma-specific, we introduced experimental tumours in scid mice by intracranial injection of D54MG glioma cells and subsequently recorded from tumour cells and adjacent normal glial cells in acute slices. We consistently observed expression of chlorotoxin-sensitive chloride channels in implanted glioma cells, but without evidence for expression of chloride channels in surrounding "normal" host glial cells, suggesting that these chloride channels are probably a glioma-specific feature. Finding of this novel glioma specific Cl- channel in gliomas in situ and it's selective binding of chlorotoxin may provide a way to identify or target glioma cells in the future.

Glial ontogeny and glial neoplasia: the search for closure

The last ten years have seen rapid progress in both our understanding of the normal progression and control of gliogenesis and in the laboratory techniques necessary to sustain and study most glial cell types, including progenitor cells of both type-1 astrocyte (T1A) and oligodendrocyte-type-2 astrocyte (T2A) lineage. These studies have direct relevance for the lineage analysis of human gliomas, optimizing in vitro glioma models, and suggesting potentially fertile new grounds for glioma biology research. We do not yet known whether malignant transformation occurs only in mature glia that then 'de-differentiate' into cells with glial precursor phenotypes and behavior characteristics, whether neoplastic transformation occurs in O-2A progenitor cells, or whether both mechanisms may occur in different patients. However, preliminary results suggest that astrocytomas can arise from two different glial lineages, that oligodendrogliomas and mixed oligo-astrocytomas arise exclusively from the O-2A lineage, and that medulloblastomas may also have a connection with the O-2A lineage. An ontogeny-based glioma classification system may lead to better prognostic patient data and better predict patient response to treatment than currently available classification systems. Available data from the study of developmental glial biology raises serious doubts about the fidelity and relevance of in vitro glioma models that rely on culture media supplemented with animal serum and suggest that relying on chemically-defined media conditioned by astrocytes may be the better research strategy. Findings from the study of normal gliogenesis also suggest that growth factors are likely to act as much more than simple mitogens in glioma biology. Potentially fertile areas of research for glioma biology include studying the cooperative effect of multiple growth factors, potential growth factor effects as survival factors, inhibitors of differentiation, and differentiation inducers, and studying potential positive humoral feedback loops between glioma cells and normal glial cells, as well as normal non-glial cells, within and surrounding each glioma.

Isoform-specific expression of sodium channels in astrocytes in vitro: immunocytochemical observations

The expression of sodium channel alpha-subunit isoforms in astrocytes cultured from P-0 rat spinal cord and P-7 rat optic nerve was examined utilizing immunocytochemical methods with antibodies generated against conserved and isoform-specific amino acid sequences of the rat brain sodium channel. In spinal cord cultures at 5 days in vitro (DIV), both stellate and flat astrocytes were immunostained with antibody SP20, which recognizes a conserved sequence common to sodium channel types I, II/IIA, and III. Antibody SP11-I, which is directed against a subtype-specific sequence in sodium channel I, did not yield detectable staining in spinal cord astrocytes. Antibody SP11-II, which is directed against a subtype-specific sequence in sodium channel II, immunostained both stellate and flat spinal cord astrocytes, although with less intensity than SP20. Antibody SP32-III, which is directed against a subtype-sequence in sodium channel III, immunostained stellate but not flat spinal cord astrocytes. SP20, SP11-II, and SP32-III staining persisted in stellate spinal cord astrocytes through 14-21 DIV, while SP20 and SP11-II immunostaining in flat spinal cord astrocytes was attenuated with time in culture. In optic nerve cultures at 5 DIV, SP20 staining was present in both stellate and flat astrocytes, but at reduced levels compared to spinal cord astrocytes. With increased time in culture SP20 staining was maintained in stellate optic nerve astrocytes but was gradually lost in flat optic nerve astrocytes. Stellate optic nerve astrocytes exhibited low levels of staining with SP11-I, SP11-II, and SP32-III. Flat optic nerve astrocytes lacked or displayed very low SP11-II staining, and SP11-I and SP32-III staining was not detectable. These observations demonstrate that cultures astrocytes are immunoreactive to antibodies generated against conserved and isotype-specific peptide sequences of rat brain sodium channels, and further suggest that there are different patterns of sodium channel expression between flat vs. stellate astrocytes and in astrocytes derived from different regions of the CNS.

Voltage-dependent sodium and potassium currents in cultured trout astrocytes

Voltage-gated ionic currents were recorded from cultured trout astrocytes with the whole-cell variation of the patch-clamp technique. In a subpopulation of astrocytes depolarizations above -40 mV activated a fast transient inward current that was identified as a sodium current by ion substitution experiments, its current reversal potential, and its TTX-sensitivity. Regarding threshold of activation, peak current voltage, and amplitude this current closely resembled those previously described for mammalian astrocytes. Voltage-dependence of inactivation and kinetics, however, markedly differed from the "glial-like" sodium current occurring in mammalian hippocampal or optic nerve astrocytes, since the sodium current of trout astrocytes exhibited a faster time course of activation and decay and a more depolarized steady-state inactivation curve with midpoints close to -60 mV. During a period of 2 weeks in culture the biophysical properties of the sodium current did not change significantly, albeit a continuous decrease in current density was observed. At depolarizing voltage steps positive to -40 mV, additionally voltage-gated potassium outward currents were evoked, which could be separated into a steady-state current with delayed rectifier properties and an inactivating component resembling the A-type current. Moreover, in a subpopulation of astrocytes an inward potassium current was elicited at hyperpolarizing potentials, which exhibited biophysical features consistent with the potassium inward rectifier of mammalian astrocytes.

Voltage-dependent ion channels in glial cells

Glial cells, although non-excitable, express a wealth of voltage-activated ion channels that are typically characteristic of excitable cells. Since these channels are also observed in acutely isolated cells and in brain slices, they have to be considered functional in the intact brain. Numerous studies over the past 10 years have yielded detailed characterizations of glial channels permitting comparison of their properties to those of their neuronal counterparts. While for the most part such comparisons have demonstrated a high degree of similarity, they also provide evidence for the expression of some uniquely glial ion channels. An increasing number of studies indicate that the expression of "glial" channels is influenced by the cells' microenvironment. For example, the presence of neurons can induce or inhibit (depending on the preparation and type of channel studied) the expression of glial ion channels. Like ion channels in excitable cells, glial channels can be functionally regulated by activation of second-messenger pathways, allowing for short-term modulation of their membrane properties. Although the extent to which most of the characterized ion channels are involved in glial function is presently unclear, a growing body of data suggests that certain channels play an active role in glial function. Thus inwardly rectifying K+ channels in concert with delayed rectifying K+ channels are thought to be involved in the removal and redistribution of excess K+ in the brain, a process referred to as "spatial buffering". Glial K+ channels may also be crucial in modulating glial proliferation. Cl- channels and stretch-activated cation channels are believed to be involved in volume regulation. Na+ channels appear to be important in fueling the glial Na+/K(+)-pump, and Ca2+ channels are likely involved in numerous cellular events in which intracellular Ca2+ is a critical second messenger.

The expression of rat brain voltage-sensitive Na+ channel mRNAs in astrocytes

Astrocytes from various regions of CNS have been shown to express voltage-activated Na+ currents. To date, three distinct subtypes (I, II and III) of Na+ channels have been cloned from rat brain. We have applied a combined technique of reverse transcription and polymerase chain reaction (RT-PCR) to examine the expression of rat brain Na+ channels in rat astrocytes in vivo and in vitro. Five PCR primer sets were used to amplify coding or 3' non-coding regions of subtype I, II, and III Na+ channels. We were able to amplify all three of these rat brain Na+ channel subtypes from rat optic nerve, which does not have neuronal cell bodies but does contain astrocytes known to express voltage-sensitive Na+ channels. In studies on cultured spinal cord astrocytes, we were also able to amplify all three subtypes of rat brain Na+ channel mRNAs. In control experiments, RT-PCR was performed on RNAs prepared from several rat tissues, including brain, skeletal muscle, and liver. Rat brain was shown to express the three Na+ channel subtypes as expected. In rat skeletal muscle, subtype I and III Na+ channel mRNAs, but not subtype II, were amplified. In rat liver, Na+ channel messages were not detectable. The present study provides the first direct evidence that astrocytes in vivo and in vitro express rat brain voltage-sensitive Na+ channel mRNAs, which have been considered as mainly neuronal-type Na+ channel messages.

Voltage-dependent sodium channel alpha subunit immunoreactivity is expressed by distinct cell types of the cat and monkey retina

Polyclonal (7493 and 7317) and monoclonal (mAb3) antibodies, generated to the alpha subunit of the voltage-gated sodium channel (alpha NaCh), were employed to assess the cell types containing alpha NaCh-like immunoreactivity in the mature cat and monkey retina. Immunoblot analyses of retinal proteins in the cat revealed that the polyclonal and monoclonal antibodies we employed labeled a band in the 260-kDa region which corresponds to the molecular mass of the alpha subunit of the NaCh. In both the cat and monkey, these antibodies immunolabeled several distinct types of retinal cells. With the polyclonal antibodies immunoreactivity was observed in ganglion cells and their intraretinal axons, in horizontal cells, and unexpectedly, in cones. In addition, in both species, a limited number of heavily labeled profiles, presumed to be bipolar cells, were seen in the inner nuclear layer. In cat and monkey the monoclonal antibody labeled axons in the fiber layer, ganglion cell somata, and a continuous band of immunoreactive cell bodies (presumed bipolar cells) situated in the outer half of the inner nuclear layer. By immunolabeling isolated cells dissociated from the cat retina, it was possible to demonstrate unequivocally that a population of bipolar cells was labeled by the monoclonal and the polyclonal antibodies we employed. The differences in the labeling observed with the monoclonal antibody as compared to the polyclonal antibodies were interpreted as reflecting the presence of different alpha-subunit subtypes in the mammalian retina. Collectively, our findings suggest that alpha NaCh-like proteins are expressed by a more diverse population of retinal cells than expected on the basis of previous physiological and immunohistochemical studies.

Spinal cord astrocytes in vitro: phenotypic diversity and sodium channel immunoreactivity

The expression of sodium channels in morphologically and antigenically distinct astrocytes derived from neonatal rat spinal cords was examined at various times in culture. During the course of this study [2-40 days in vitro (DIV)], nine morphologies of glial fibrillary acidic protein (GFAP)+ cells were distinguished: 1a) flat, fibroblast-like; 1b) elongated, with generally few, short processes; 1c) triangular soma with three short, stubby processes; 1d) bipolar with long, slender processes; 1e) bipolar with broad, flared processes; 1f) stellate with radially oriented slender processes extending from a small to moderate-sized soma; 1g) multiple short, stubby processes extending from a moderate-sized soma; 1h) flat, roundish shape with either a smooth edge ("pancake"-like) or numerous very short processes; and 1i) broad, elongated cell body with orthogonally oriented short, spike-like processes. Not all cell types were present at all times in culture. Each type of astrocyte displayed sodium channel immunoreactivity at some time in culture; however, different types of astrocytes exhibited different patterns, over time, of sodium channel staining. Sodium channel immunoreactivity in all astrocyte types was reduced to low levels by 14 DIV, and was not detectable at 40 DIV. Except for types 1b and 1e, A2B5 staining was present on all astrocyte morphologies at some time in culture, and was generally attenuated with longer times in vitro; in contrast to cultures derived from neonatal rat optic nerve, A2B5 staining does not distinguish unequivocally between the various classes of morphologically different astrocytes derived from spinal cord. O4 immunoreactivity was consistently observed only on bipolar, elongated, and process-bearing astrocytes, though not all process-bearing astrocytes were O4+. These results demonstrate that astrocytes derived from neonatal spinal cord are morphologically and antigenically heterogeneous. Moreover, while spinal cord astrocytes express sodium channels, these astrocytes exhibit a time-course of channel expression that is different from astrocytes derived from several other CNS regions where sodium channel staining is maintained even for extended times in culture, suggesting a regional modulation of astrocyte function.

Different Na+ currents in P0- and P7-derived hippocampal astrocytes in vitro: evidence for a switch in Na+ channel expression in vivo

Hippocampal astrocytes, derived from postnatal day zero (P0) rats, appear to be pluripotential with respect to sodium current expression in vitro, and display Na+ currents with h infinity midpoints close to -65 up to 5 days in vitro (DIV), and Na+ currents with midpoints close to -85 mV at 6 DIV and thereafter. These astrocytes also exhibit a biphasic pattern of Na+ current density, which is expressed at moderate levels at early times in vitro and decreases throughout the first 5 DIV, prior to expressing a second peak for the duration of time in culture. These observations have been interpreted as suggesting that astrocytes in these cultures display a 'switch' in Na+ channel biosynthesis, so that they express different types of Na+ current (with different h infinity curves) at early and later times in culture. To test the hypothesis that a similar switch in Na+ channel expression occurs in vivo, we have used patch-clamp methods to study Na+ current expression in astrocytes derived from rat hippocampus at various stages of postnatal development, P0, P4, P5 and P7. We observed a biphasic distribution of Na+ current density, which was highest in P0- and P7-derived astrocytes (18 pA/pF and 10.3 pA/pF, respectively); astrocytes derived at P4 and P5 did not express sodium currents. While P0-derived astrocytes show depolarized h infinity curves (midpoints close to -65 mV) at early times in culture, P7-derived astrocytes, studied at comparable times in vitro, display hyperpolarized h infinity curves (midpoints close to -85 mV).(ABSTRACT TRUNCATED AT 250 WORDS)

Three types of sodium channels in adult rat dorsal root ganglion neurons

Several types of Na+ currents have previously been demonstrated in dorsal root ganglion (DRG) neurons isolated from neonatal rats, but their expression in adult neurons has not been studied. Na+ current properties in adult dorsal root ganglion (DRG) neurons of defined size class were investigated in isolated neurons maintained in primary culture using a combination of microelectrode current clamp, patch voltage clamp and immunocytochemical techniques. Intracellular current clamp recordings identified differing relative contributions of TTX-sensitive and -resistant inward currents to action potential waveforms in DRG neuronal populations of defined size. Patch voltage clamp recordings identified three distinct kinetic types of Na+ current differentially distributed among these size classes of DRG neurons. 'Small' DRG neurons co-express two types of Na+ current: (i) a rapidly-inactivating, TTX-sensitive 'fast' current and (ii) a slowly-activating and -inactivating, TTX-resistant 'slow' current. The TTX-sensitive Na+ current in these cells was almost completely inactivated at typical resting potentials. 'Large' cells expressed a single TTX-sensitive Na+ current identified as 'intermediate' by its inactivation rate constants. 'Medium'-sized neurons either co-expressed 'fast' and 'slow' current or expressed only 'intermediate' current. Na+ channel expression in these size classes was also measured by immunocytochemical techniques. An antibody against brain-type Na+ channels (Ab7493)10 labeled small and large neurons with similar intensity. These results demonstrate that three types of Na+ currents can be detected which correlate with electrogenic properties of physiologically and anatomically distinct populations of adult rat DRG neurons.

Identification of mouse type-2-like astrocytes: Demonstration of glutamate and GABA transmitter activated responses

We have identified mouse type-2-like astrocytes and examined some of their electrophysiological properties. Cultures were prepared from P4 mouse neopallia. We demonstrate that mouse type-2-like astrocytes can be identified using the following criteria: presence of glial fibrillary acidic protein (GFAP), presence of chondroitin sulfate polysaccharide, and presence of gamma-aminobutyric acid (GABA). A2B5-binding is not a sufficient criterion to identify O2A lineage cells in mouse neopallial glial cultures since the monoclonal antibody A2B5 binds not only to O2A lineage cells but also to a subpopulation of large, flat type-1-like astrocytes. Mouse type-2-like astrocytes have resting membrane potentials of -76.2 +/- 2.1 mV-i.e., similar to that of mouse type-1-like astrocytes. The input resistance of 44.2 +/- 0.5 M omega is an order of magnitude greater than that of type-1-like astrocytes suggesting the type-2-like astrocytes are not extensively electrically coupled either to each other or to type-1-like astrocytes. Glutamate application caused an 8.8 +/- 1.7 mV depolarization of type-2-like astrocytes. Application of glutamate to barium treated astrocytes caused a fast depolarization with a peak amplitude of 21.4 +/- 1.8 mV; the cells repolarized from this peak by about 10 mV and upon removal of glutamate returned to its pre-glutamate value. Application of GABA caused a transient depolarization of 14.0 +/- 1.7 mV. The presence of barium resulted in a steady-state GABA-induced depolarization of 10.3 +/- 2.0 mV. Neither SITS nor beta-alanine interfered with the amplitude of the glutamate and GABA responses.

Sodium channel expression in optic nerve astrocytes chronically deprived of axonal contact

Immunocytochemical and electrophysiological methods were used to examine the effect of retinal ablation on the expression of sodium channels within optic nerve astrocytes in situ and in vitro. Enucleation was performed at postnatal day 3 (P3), and electron microscopy of the enucleated optic nerves at P28-P40 revealed complete degeneration of retinal ganglion axons, resulting in optic nerves composed predominantly of astrocytes. In contrast to control (non-enucleated) optic nerve astrocytes, which exhibited distinct sodium channel immunoreactivity following immunostaining with antibody 7493, the astrocytes in enucleated optic nerves did not display sodium channel immunoreactivity in situ. Cultures obtained from enucleated optic nerves consisted principally (greater than 90%) of glial fibrillary acidic protein (GFAP)+/A2B5- ("type-1") astrocytes, as determined by indirect immunofluorescence; GFAP+/A2B5+ ("type-2") astrocytes were not present, nor were GFAP-/A2B5+ (O-2A) progenitor cells. Sodium channel immunoreactivity was not present in GFAP+/A2B5- astrocytes obtained from enucleated optic nerves; in contrast, GFAP+/A2B5- astrocytes from control optic nerves exhibited 7493 immunostaining for the first 4-6 days in culture. Sodium current expression, studied using whole-cell patch-clamp recording, was attenuated in cultured astrocytes derived from enucleated optic nerves. Whereas 39 of 50 type-1 astrocytes cultured from intact optic nerves showed measurable sodium currents at 1-7 days in vitro, sodium currents were present in only 6 of 38 astrocytes cultured from enucleated optic nerves. Mean sodium current densities in astrocytes from the enucleated optic nerves (0.66 +/- 0.3 pA/pF) were significantly smaller than in astrocytes from control optic nerves (7.15 +/- 1.1 pA/pF). The h infinity-curves of sodium currents were similar in A2B5- astrocytes from enucleated and control rat optic nerves. These results suggest that there is neuronal modulation of sodium channel expression in type-1 optic nerve astrocytes, and that, following chronic loss of axonal association in vivo, sodium channel expression is down-regulated in this population of optic nerve astrocytes.

Glial ion channels

It now appears that most of the ion channels discovered in glia are similar or identical to their neuronal equivalents. Recent studies show that glial cells can sense and respond to neuronal signals and that neurons may influence both the development and maintenance of ion channel expression of certain glial cells. Although they lack excitability, glia are probably active participants in brain function.

Two types of Na(+)-currents in cultured rat optic nerve astrocytes: changes with time in culture and with age of culture derivation

Na+ channel expression was studied in cultures of rat optic nerve astrocytes using whole-cell voltage-clamp recordings. Astrocytes from postnatal day 7 rat optic nerve (RON) expressed two distinct types of Na+ currents, which had significantly different h infinity curves. Stellate, GFAP+/A2B5+ astrocytes showed currents with h infinity curve midpoints close to -65 mV, similar to Na+ currents in most neurons. In contrast, flat fibroblast-like GFAP+/A2B5- astrocytes showed Na+ currents with h infinity midpoints around -85 mV, almost 20 mV more hyperpolarized than in neurons or A2B5+ astrocytes. Interestingly, Na+ current expression was maintained in A2B5+ astrocytes but began to decrease in A2B5- astrocytes after 6 days in vitro (DIV) and fell to or below the level of detection (i.e., 1 pA/pF) at 12 DIV. Astrocytes cultured from neonatal rats (P0) are almost exclusively GFAP+/A2B5-. These cells did not display measurable Na+ currents when studied at 2 DIV; however, Na+ current was observed after 5 DIV in A2B5- astrocytes from these neonatal (P0) cultures. These findings were substantiated by immunocytochemical experiments using 7493, an antibody raised against purified rat brain Na+ channels; in P0-derived astrocyte cultures 7493 antibody staining was initially lacking (up to 3 DIV), but it was prominent in cultures after 5 DIV, suggesting that Na+ current expression in RON astrocytes occurs postnatally.

Functions and distribution of voltage-gated sodium and potassium channels in mammalian Schwann cells

Recent patch-clamp studies on freshly isolated mammalian Schwann cells suggest that voltage-gated sodium and potassium channels, first demonstrated in cells under culture conditions, are present in vivo. The expression of these channels, at least at the cell body region, appears to be dependent on the myelinogenic and proliferative states of the Schwann cell. Specifically, myelin elaboration is accompanied by a down regulation of functional potassium channel density at the cell body. One possibility to account for this is a progressive regionalization of ion channels on a Schwann cell during myelin formation. In adult myelinating Schwann cells, voltage-gated potassium channels appear to be localized at the paranodal region. Theoretical calculations have been made of activity-dependent potassium accumulations in various compartments of a mature myelinated nerve fibre; the largest potassium accumulation occurs not at the nodal gap but rather at the adjacent 2-4 microns length of periaxonal space at the paranodal junction. Schwann cell potassium channels at the paranode may contribute to ionic regulation during nerve activities.

Specificity of cell-cell coupling in rat optic nerve astrocytes in vitro

Intercellular coupling was studied in cultured rat optic nerve astrocytes individually characterized by A2B5 antibody staining. The presence of cell coupling was assessed by injecting single cells with the low molecular weight fluorescent dye Lucifer yellow and noting dye passage into adjacent cells; cell coupling was also studied by analyzing the decay phase of current transients recorded in response to small voltage steps using whole-cell patch-clamp recording. Cell coupling was restricted to A2B5- astrocytes, the majority of which had a flat fibroblast-like appearance and was never observed in A2B5+ stellate-shaped astrocytes. Furthermore, A2B5- astrocytes showed coupling only to A2B5- and never to A2B5+ astrocytes. Analysis of current transients provided an additional indicator for cell coupling. Astrocytes that showed dye coupling to at least one neighboring cell required the sum of two exponential functions to fit current transients, whereas a single exponential function sufficed to fit transients in cells that were not dye coupled. The specificity of cell coupling in cultured rat optic nerve astrocytes suggests that predominantly A2B5- astrocytes comprise a coupled glial syncytium; this physiological feature of these cells may be a specialized adaptation for "spatial buffering," the transport of K+ away from areas of focal extracellular accumulation. On the other hand, A2B5+ astrocytes form an uncoupled subpopulation of rat optic nerve glial cells that may serve different functions.