Expression of potassium channels during postnatal differentiation of rabbit Müller glial cells.

The postnatal maturation of Müller glial cells from immature radial glial cells is accompanied by specific changes in the activity of distinct types of K+ channels, as shown by whole-cell and cell-attached records on freshly isolated cells from retinae of young (postnatal days 1-30, P1-P30) and adult rabbits. (i) The density of inwardly rectifying currents, providing the main K+ conductance in adult Müller cells, was very low (0.8 pA/pF) from P1 to P6 but increased rapidly thereafter until a relatively stable level of 11.0 pA/pF was established at P17. (ii) Transient (A-type) K+ currents were expressed in all immature cells at a high density (9.6 pA/pF). After P12, both the percentage of cells with A-type currents and the peak amplitudes of the currents (2.8 pA/pF) declined. (iii) Delayed rectifying K+ currents developed slowly until after P30. (iv) The postnatal maturation of radial glial cells was accompanied by a strong decrease in the activity of large-conductance, Ca2+-activated K+ channels, the open probability of which (measured at the resting membrane potential) decreased from 0.69 at P2-4 to 0.06 at P13-14. The developmental decrease of the activity of Ca2+-activated K+ channels is assumed to be mainly caused by alteration of the resting membrane potential which developed from low values (-49 mV) at P1-6 to high adult values (-84 mV) after P13. The activity of each distinct type of K+ channel investigated is differently modulated by developmental regulation. This may reflect different functional requirements of immature and mature Müller cells.     

Role of Kir4.1 channels in growth control of glia

The inwardly rectifying potassium channel Kir4.1 is widely expressed by astrocytes throughout the brain. Kir4.1 channels are absent in immature, proliferating glial cells. The progressive expression of Kir4.1 correlates with astrocyte differentiation and is characterized by the establishment of a negative membrane potential (> -70 mV) and an exit from the cell cycle. Despite some correlative evidence, a mechanistic interdependence between Kir4.1 expression, membrane hyperpolarization, and control of cell proliferation has not been demonstrated. To address this question, we used astrocyte-derived tumors (glioma) that lack functional Kir4.1 channels, and generated two glioma cell lines that stably express either AcGFP-tagged Kir4.1 channels or AcGFP vectors only. Kir4.1 expression confers the same K+ conductance to glioma membranes and a similar responsiveness to changes in [K+]o that characterizes differentiated astrocytes. Kir4.1 expression was sufficient to move the resting potential of gliomas from -50 to -80 mV. Importantly, Kir4.1 expression impaired cell growth by shifting a significant number of cells from the G2/M phase into the quiescent G0/G1 stage of the cell cycle. Furthermore, these effects could be nullified entirely if Kir4.1 channels were either pharmacologically inhibited by 100 microM BaCl2 or if cells were chronically depolarized by 20 mM KCl to the membrane voltage of growth competent glioma cells. These studies therefore demonstrate directly that Kir4.1 causes a membrane hyperpolarization that is sufficient to account for the growth attenuation, which in turn induces cell maturation characterized by a shift of the cells from G2/M into G0/G1.     

Astrocyte membrane responses and potassium accumulation during neuronal activity

Older studies suggest that astrocytes act as potassium electrodes and depolarize with the potassium efflux accompanying neuronal activity. Newer studies suggest that astrocytes depolarize in response to neuronal glutamate release and the activity of electrogenic glial glutamate transporters, thus casting doubt on the fidelity with which astrocytes might sense extracellular potassium rises. Any K(+)-induced astrocyte depolarization might reflect a spatial buffering effect of astrocytes during neuronal activity. For these reasons, we studied stimulus-evoked currents in hippocampal CA1 astrocytes. Hippocampal astrocytes exhibited stimulus-evoked transient glutamate transporter currents and slower Ba(2+)-sensitive inward rectifier potassium (K(ir)) currents. In whole-cell astrocyte recordings, Ba(2+) blocked a very weakly rectifying component of the astrocyte membrane conductance. The slow stimulus-elicited current, like measurements from K(+)-sensitive electrodes under the same conditions, predicted small bulk [K(+)](o) increases (<0.5 mM) following the termination of short-stimulus trains. These currents indicate the potential for astrocyte spatial K(+) buffering. However, Ba(2+) did not significantly affect resting [K(+)](o) or the [K(+)](o) rises detected by the K(+)-sensitive electrode. To test whether local K(+) rises may be significantly higher than those detected by glial recordings or by K(+) electrodes, we assayed EPSCs and fiber volleys, two measures very sensitive to K(+) increases. We found that Ba(2+) had little effect on neuronal axonal or synaptic function during short-stimulus trains, indicating that K(ir)s do not influence local [K(+)](o) rises enough, under these conditions to affect synaptic transmission. In conclusion, our results indicate that hippocampal astrocytes are faithful sensors of [K(+)](o) rises, but we find little evidence for physiologically relevant spatial K(+) buffering during brief bursts of presynaptic activity.     

K(+) inward rectifier currents in reactive astrocytes from adult rat brain.

We characterized inward rectifier (K(IR)) currents in reactive astrocytes activated by CNS injury. We used primary cultures of reactive astrocytes obtained from gelatin sponge implants in adult rat brains, a system that yielded highly purified, homogeneous cultures with >95% of cells positive for GFAP, vimentin, and S-100beta. Ionic channels were studied in 1-21-day-old primary cultures using a nystatin-perforated patch clamp technique. Fast Na(+) currents were identified in <2% of cells. Most cells exhibited outward currents positive to -50 mV, with one component being sensitive to charybdotoxin, iberiotoxin, and tetraethylammonium chloride, and another component being sensitive to 4-aminopyridine. Two populations of cells were distinguished, based on presence or absence of Ba(2+)-sensitive K(IR) current negative to the K(+) reversal potential (E(k)), with >80% of cells expressing K(IR) currents. In contrast to previous reports on mammalian astrocytes, the current-voltage curve showed no appreciable current between E(k) and -50 mV, reflecting strong rectification by K(IR) channels. The magnitude of K(IR) current at -130 mV (I(-)(130)) did not change significantly during 21 days in culture (123 cells), suggesting constitutive expression of K(IR) channels. The fraction of K(IR)-negative cells was not affected by serum-starvation for 16-24 h. In cells with I(-)(130) >/= -30 pA, the membrane potential was invariably near E(k) and depolarized appreciably on addition of Ba(2+), but in cells with I(-)(130) < -30 pA, resting potentials ranged from -40 mV to -90 mV. We conclude that most adult reactive astrocytes constitutively express K(IR) channel(s) that exhibit strong rectification not previously observed in mammalian astrocytes.     

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.     

Ionic basis of the resting membrane potential in cultured rat sympathetic neurons

The conductances which determine the resting membrane potential of rat superior cervical ganglia (SCG) neurons were investigated using perforated voltage- and current-clamp whole-cell techniques. The resting potential of SCG cells varied from -47 to -80 mV (-58.3 +/- 0.8 mV, n = 55). Blockade of M and h currents induced a depolarisation (7.4 +/- 0.7 mV, n = 22) and a hyperpolarisation (7.2 +/- 0.7 mV, n = 20) respectively; however, no correlation between the amplitude of these currents and the resting potential was found. The inhibition of the Na/K pump also induced membrane depolarisation (3.2 +/- 0.2 mV, n = 8). Inhibition of voltage-gated currents unmasked a voltage-independent resting conductance reversing at -50 mV. The reversal potential of the voltage-independent conductance, which included the electrogenic contribution of the Na/K pump, was strongly correlated with the resting potential (R = 0.87, p < 0.0001, n = 30). Ionic substitution experiments confirmed the existence of a voltage-independent conductance (leakage) with four components, a main potassium conductance, two minor sodium and chloride conductances and a small contribution of the Na/K pump. It is concluded that the resting potential of SCG cells strongly depends on the reversal potential of the voltage-independent conductance, with voltage-activated M and h currents playing a prominent stabilising role.     

Voltage-dependent Ca2+ currents in rat cardiac dorsal root ganglion neurons

This study presents the kinetic and pharmacological properties of voltage-gated Ca(2+) currents in anatomically defined cardiac dorsal root ganglion (DRG) neurons in rats. The neurons were labelled by prior injection of fluorescent tracer Fast Blue into the pericardial sack. There were three distinct groups of neurons with respect to cell size: small (27% of total; cell capacitance <30 pF), medium (65% of total; capacitance 30-80 pF) and large neurons (8% of total; capacitance >80 pF). The properties of Ca(2+) currents were tested in small and medium-sized neurons. In large neurons currents could not be adequately controlled and were not analysed. Ca(2+) currents did not completely inactivate during 100 ms depolarising voltage steps. The activation thresholds in small (-36.9+/-1.3 mV) and medium (-39.0+/-1.3 mV) size neurons were similar. Current densities were 105.8 pA/pF in small and 97.4 pA/pF in large neurons and also did not differ. The kinetic properties of activation and inactivation did not differ between small and medium-sized cardiac DRG neurons. At membrane potentials between -50 and -60 mV (the expected resting membrane potential in these neurons) 55 to 70% of Ca(2+) currents in small and medium-sized neurons were available for activation. Both, small and medium-sized neurons expressed similar proportions of L (7.5%), N (25%) and P/Q (36%) type Ca(2+) currents. We conclude that small and medium-sized cardiac DRG neurons are homogeneous with respect to the expression and properties of voltage-gated Ca(2+) currents. Voltage-gated Ca(2+) currents probably play an important role in action potential generation in cardiac DRG neurons due to their availability for activation at resting membrane potential, their high density and voltage threshold close to the threshold for voltage-gated Na(+) currents.     

The development and pharmacological characterization of calcium channel currents in cultured embryonic rat septal cells

We characterized the development and pharmacology of Ca(2+) channel currents in NGF-treated embryonic day 21 cultured rat septal cells. Using standard whole-cell voltage clamp techniques, cells were held at -80 mV and depolarized to construct current-voltage relations in conditions that eliminated Na(+) or K(+) currents. Barium (10 mM) was used as the charge carrier. Maximum current was produced when cells were depolarized to 0 or +10 mV. Recordings from 77 cells revealed that Ca(2+) channel current density increases over time in culture from nearly 0 pA/pF on day 2 in vitro (0.65+/-0.65 pA/pF) to (6.95+/-1.59 pA/pF) on days 6-8. This was followed by a period where currents became nearly 3 times more dense (21.05+/-7.16 pA/pF) at days 9-17. There was little or no evidence for low voltage activated currents. Bath application of 50-100 microM CdCl(2) abolished approximately 95% of the current. Application of 10 microM nimodipine produced a 50.5+/-3.22% reduction in current, 2 microM omega-CTx-GVIA produced a 32.4+/-7.3% reduction, and application of 4 microM omega-Aga-IVA produced a 29.5+/-5.73% reduction in current. When all three inhibitors (10 microM nimodipine, 2 microM omega-CTx-GVIA, and 4 microM omega-Aga-IVA) were applied simultaneously, a residual current remained that was 18.0+/-4.9% of the total current and was completely abolished by application of CdCl(2). This is the first report to characterize Ca(2+) channel currents in cultured embryonic septal cells. These data indicate that there is a steady increase in Ca(2+) channel expression over time in vitro, and show that like other cultured neuronal cells, septal cells express multiple Ca(2+) channel types including L, N, P/Q and R-type channels.     

Chlorotoxin-sensitive Ca2+-activated Cl- channel in type R2 reactive astrocytes from adult rat brain

Astrocytes express four types of Cl(-) or anion channels, but Ca(2+)-activated Cl(-) (Cl(Ca)) channels have not been described. We studied Cl(-) channels in a morphologically distinct subpopulation ( approximately 5% of cells) of small (10-12 micro m, 11.8 +/- 0.6 pF), phase-dark, GFAP-positive native reactive astrocytes (NRAs) freshly isolated from injured adult rat brains. Their resting potential, -57.1 +/- 4.0 mV, polarized to -72.7 +/- 4.5 mV with BAPTA-AM, an intracellular Ca(2+) chelator, and depolarized to -30.7 +/- 6.1 mV with thapsigargin, which mobilizes Ca(2+) from intracellular stores. With nystatin-perforated patch clamp, thapsigargin activated a current that reversed near the Cl(-) reversal potential, which was blocked by Cl(-) channel blockers, 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) and Zn(2+), by I(-) (10 mM), and by chlorotoxin (EC(50) = 47 nM). With conventional whole-cell clamp, NPPB- and Zn(2+)-sensitive currents became larger with increasing [Ca(2+)](i) (10, 150, 300 nM). Single-channel recordings of inside-out patches confirmed Ca(2+) sensitivity of the channel and showed open-state conductances of 40, 80, 130, and 180 pS, and outside-out patches confirmed sensitivity to chlorotoxin. In primary culture, small phase-dark NRAs developed into small GFAP-positive bipolar cells with chlorotoxin-sensitive Cl(Ca) channels. Imaging with biotinylated chlorotoxin confirmed the presence of label in GFAP-positive cells from regions of brain injury, but not from uninjured brain. Chlorotoxin-tagged cells isolated by flow cytometry and cultured up to two passages exhibit positive labeling for GFAP and vimentin, but not for prolyl 4-hydroxylase (fibroblast), A2B5 (O2A progenitor), or OX-42 (microglia). Expression of a novel chlorotoxin-sensitive Cl(Ca) channel in a morphologically distinct subpopulation of NRAs distinguishes these cells as a new subtype of reactive astrocyte.     

Ion channel expression by white matter glia: I. Type 2 astrocytes and oligodendrocytes

White matter is a compact structure consisting primarily of neuronal axons and glial cells. As in other parts of the nervous system, the function of glial cells in white matter is poorly understood. We have explored the electrophysiological properties of two types of glial cells found predominantly in white matter: type 2 astrocytes and oligodendrocytes. Whole-cells and single-channel patch-clamp techniques were used to study these cell types in postnatal rat optic nerve cultures prepared according to the procedures of Raff et al. (Nature, 303:390-396, 1983b). Type 2 astrocytes in culture exhibit a "neuronal" channel phenotype, expressing at least six distinct ion channel types. With whole-cell recording we observed three inward currents: a voltage-sensitive sodium current qualitatively similar to that found in neurons and both transient and sustained calcium currents. In addition, type 2 astrocytes had two components of outward current: a delayed potassium current which activated at 0 mV and an inactivating calcium-dependent potassium current which activated at -30 mV. Type 2 astrocytes in culture could be induced to fire single regenerative potentials in response to injections of depolarizing current. Single-channel recording demonstrated the presence of an outwardly rectifying chloride channel in both type 2 astrocytes and oligodendrocytes, but this channel could only be observed in excised patches. Oligodendrocytes expressed only one other current: an inwardly rectifying potassium current that is mediated by 30- and 120-pS channels. Because these channels preferentially conduct potassium from outside to inside the cell, and because they are open at the resting potential of the cell, they would be appropriate for removing potassium from the extracellular space; thus it is proposed that oligodendrocytes, besides myelinating axons, play an important role in potassium regulation in white matter. The conductances present in oligodendrocytes suggest a "modulated Boyle and Conway mechanism" of potassium accumulation.     

Developmental downregulation of ATP-sensitive potassium conductance in astrocytes in situ

In many neural and non-neural cells, ATP-sensitive potassium (K(ATP)) channels couple the membrane potential to energy metabolism. We investigated the activation of K(ATP) currents in astrocytes of different brain regions (hippocampus, cerebellum, dorsal vagal nucleus) by recording whole-cell currents with the patch-clamp technique in acute rat brain slices. Pharmacological tools, hypoglycemia and specific compounds in the pipette solution (cAMP, UDP), were used to modulate putative K(ATP) currents. The highest rate of K(ATP) specific currents was observed with a pipette solution containing cAMP and external stimulation with diazoxide (0.3 mM). The diazoxide-activated current had a reversal potential negative to -80 mV and was inhibited by tolbutamide (0.2 mM). We found that not all cells activated a K(ATP) current, and that the portion of cells with functional K(ATP) channel expression was developmentally downregulated. Whereas diazoxide activated K(ATP) currents in 57% of the astrocytes in rats aged 8-11 days (n = 21), the rate decreased to 38% at 12-15 days (n = 29) and to 8% at 16-19 days (n = 12). No significant difference was observed for the three brain regions. In recordings without cAMP in the internal solution, only 21% (12-15 days; n = 19) or none (16-19 days; n = 7), respectively, showed a potassium current upon diazoxide application. This metabolically regulated potassium conductance may be of importance, particularly in immature astrocytes with a complex current pattern, which have a relatively high input resistance: K(ATP) currents activated by energy depletion may hyperpolarize the cells, or stabilize a negative resting potential during depolarizing stimuli mediated, e.g., by glutamate receptors and/or uptake carriers.     

Characterization of depolarization-evoked ERG K+ currents in interstitial cells of Cajal

Interstitial cells of Cajal (ICC) harbour the ether-a-go-go related gene (ERG) channel as shown by its characteristic rapidly deactivating current upon hyperpolarization. This property, however, does not explain the marked increase in cell excitability by ERG channel blockers, namely an increase in slow wave plateau duration and action potential generation. The objective of the present study was to characterize the depolarization-activated, E4031-sensitive ERG currents in murine ICC within a range of physiologically relevant membrane potentials. Whole cell currents were recorded from ICC isolated from murine neonatal jejunum, superfused with a physiological salt solution and with high intracellular Cs(+) to block most other K(+) currents. Upon depolarizing the cell from the resting membrane potential (approximately -60 mV) towards the region of the slow wave plateau (approximately -30 mV), significant sustained (window) current was generated between the potentials of -40 to 0 mV (maximal at -30 mV) and inhibited by the ERG specific blocker E4031. Channel activation followed by rapid inactivation produced a steady state conductance at -30 mV which was 51.6 +/- 11% of the hyperpolarization-evoked peak conductance value at -100 mV. When the cell repolarized from -30 mV, again, significant currents were generated, indicating recovery from inactivation, a typical characteristic of ERG channels. These data provide evidence that the ERG channel is of significance in the regulation of ICC excitability and provide the mechanism by which ERG channel blockade increases the slow wave duration.     

Glial depolarization evokes a larger potassium accumulation around oligodendrocytes than around astrocytes in gray matter of rat spinal cord slices.

The cell membrane of astrocytes and oligodendrocytes is almost exclusively permeable for K+. Depolarizing and hyperpolarizing voltage steps produce in oligodendrocytes, but not in astrocytes, decaying passive currents followed by large tail currents (Itail) after the offset of a voltage jump. The aim of the present study was to characterize the properties of Itail in astrocytes, oligodendrocytes, and their respective precursors in the gray matter of spinal cord slices. Studies were carried out on 5- to 11-day-old rats, using the whole-cell patch clamp technique. The reversal potential (Vrev) of Itail evoked by membrane depolarization was significantly more positive in oligodendrocytes (-31.7+/-2.58 mV, n = 53) than in astrocytes (-57.9+/-2.43 mV, n = 21), oligodendrocyte precursors (-41.2+/-3.44 mV, n = 36), or astrocyte precursors (-52.1+/-1.32 mV, n = 43). Analysis of the Itail (using a variable amplitude and duration of the de- and hyperpolarizing prepulses as well as an analysis of the time constant of the membrane currents during voltage steps) showed that the Itail in oligodendrocytes arise from a larger shift of K+ across their membrane than in other cell types. As calculated from the Nernst equation, changes in Vrev revealed significantly larger accumulation of the extracellular K+ concentration ([K+]e) around oligodendrocytes than around astrocytes. The application of 50 mM K+ or hypotonic solution, used to study the effect of cell swelling on the changes in [K+]e evoked by a depolarizing prepulse, produced in astrocytes an increase in [K+]e of 201% and 239%, respectively. In oligodendrocytes, such increases (22% and 29%) were not found. We conclude that K+ tail currents, evoked by a larger accumulation of K+ in the vicinity of the oligodendrocyte membrane, could result from a smaller extracellular space (ECS) volume around oligodendrocytes than around astrocytes. Thus, in addition to the clearance of K+ from the ECS performed by astrocytes, the presence of the K+ tail currents in oligodendrocytes indicates that they might also contribute to efficient K+ homeostasis.     

Quantitative differences between kinetic properties of Na(+) currents in postganglionic sympathetic neurones projecting to muscular and cutaneous effectors

The activity of muscular and cutaneous sympathetic neurones has been shown to be differentially regulated. The differences may partially stem from the different ionic channel expression and current kinetics in these neurones, particularly that of Na(+) channels, which play a critical role in action potential generation and modulation of neuronal excitability. The whole cell patch-clamp technique was used to compare the kinetic properties of Na(+) currents in two groups of sympathetic neurones identified by the fluorescent tracer Fast Blue: putative muscular sympathetic neurones (PMSN) and putative cutaneous sympathetic neurones (PSSN). The tracer was injected into the muscular part of the diaphragm (to mark PMSN) and into the skin of the ear (to mark PSSN). Both kinds of neurones expressed fast activating, fast inactivating, voltage dependent and TTX sensitive Na(+) currents. However, the electrical characteristics of the cells were markedly different: (1) The capacitance of PMSN (21.7 pF) was larger than PSSN (12.7 pF). Maximum current in PMSN (3.1 nA) was also larger than in PSSN (2.0 nA). Calculated current density was smaller in PMSN (148.0 pA/pF) than in PSSN (181.1 pA/pF). Slope conductance was larger in PMSN compared to PSSN (102.7 nS and 73.6 nS respectively). (2) V(1/2) of activation for PMSN (-20.9 mV) was more negative than the potential recorded for PSSN (-16.7 mV); the slope factors were not different. (3) V(1/2) for inactivation was more negative for PMSN than for PSSN (-66.3 vs. -60.8 mV); again, the slope factors for inactivation were not different. (4) The rate of recovery from inactivation could be described by the sum of two exponential functions. In PMSN the fast and slow recovery exponential factors tau(f) and tau(s) were 12.6 (66%) and 83.9 (34%) ms, while in PSSN they were shorter and equalled 8.2 (62%) and 41.9 (38%) ms, respectively. We conclude that the Na(+) currents of PMSN and PSSN have different kinetic properties.     

Single Channel Recording from Glial Cells on the Untreated Surface of the Frog Optic Nerve

The patch clamp technique has been used to record single channel currents from the untreated surface of the intact frog optic nerve after the meninges and basal lamina have been mechanically removed. Cells filled via dialysis with Lucifer yellow (LY) from the patch pipette had a typical astrocyte morphology and were dye-coupled to adjacent astrocytes. This is consistent with the electron-microscopic observation that all the cells on the surface of this nerve are astrocytes. Two types of ion channels were studied in detached patches. One, identified as a K+ channel, had a conductance of 88 +/- 4 (S.E.) n=9 pS and an equilibrium potential of -59 +/- 8 mV in physiological K+ solutions. The steady-state open probability was not significantly altered by changing the membrane potential. A second channel had a large conductance of 300 - 1200 pS, a reversal potential of approximately 0 mV in symmetrical and non-symmetrical solutions, and was open only in the voltage range of +/-20 mV. These are the characteristics of a large anionic channel described in other preparations including cultured astrocytes.     

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)     

Analysis of K+ accumulation reveals privileged extracellular region in the vicinity of glial cells in situ

Astrocytes and oligodendrocytes in rat and mouse spinal cord slices, characterized by passive membrane currents during de- and hyperpolarizing stimulation pulses, express a high resting K+ conductance. In contrast to the case for astrocytes, a depolarizing prepulse in oligodendrocytes produces a significant shift of reversal potential (Vrev) to positive values, arising from the larger accumulation of K+ in the vicinity of the oligodendrocyte membrane. As a result, oligodendrocytes express large tail currents (Itail) after a depolarizing prepulse due to the shift of K+ into the cell. In the present study, we used a mathematical model to calculate the volume of the extracellular space (ECS) in the vicinity of astrocytes and oligodendrocytes (ESVv), defined as the volume available for K+ accumulation during membrane depolarization. A mathematical analysis of membrane currents revealed no differences between glial cells from mouse (n = 59) or rat (n = 60) spinal cord slices. We found that the Vrev of a cell after a depolarizing pulse increases with increasing Itail, expressed as the ratio of the integral inward current (Qin) after the depolarizing pulse to the total integral outward current (Qout) during the pulse. In astrocytes with small Itail and Vrev ranging from -50 to -70 mV, the Qin was only 3-19% of Qout, whereas, in oligodendrocytes with large Itail and Vrev between -20 and 0 mV, Qin/Qout was 30-75%. On the other hand, ESVv decreased with increasing values of Vrev. In astrocytes, ESVv ranged from 2 to 50 microm3, and, in oligodendrocytes, it ranged from 0.1 to 2.0 microm3. Cell swelling evoked by the application of hypotonic solution shifted Vrev to more positive values by 17.2 +/- 1.8 mV and was accompanied by a decrease in ESVv of 3.6 +/- 1.3 microm3. Our mathematical analysis reveals a 10-100 times smaller region of the extracellular space available for K+ accumulation during cell depolarization in the vicinity of oligodendrocytes than in the vicinity of astrocytes. The presence of such privileged regions around cells in the CNS may affect the accumulation and diffusion of other neuroactive substances and alter communication between cells in the CNS.     

Sodium channel currents in maturing acutely isolated rat hippocampal CA1 neurones.

Sodium channel currents were recorded in excised inside-out patches from immature (P(4-10)) and older (P(20-46)) rat CA1 neurones. Channel conductance was 16.6+/-0.013 pS (P(20-46)) and 19.0+/-0.031 pS (P(4-10)). Opening patterns varied with step voltage and with age. In some patches bursting was apparent at voltages positive to -30 mV. Non-bursting behaviour was more dominant in patches from younger animals. In older animals mean open time (m.o.t.) was best described by two exponentials especially in the older cells; in the immature, there were fewer cases with two exponentials. The time constant of inactivation (tau(h)) estimated in ensemble averages was best described by two exponentials (tau(hf) and tau(hs)) in most patches from older cells. tau(hf) decreased with depolarization; tau(hs) increased in the range -30 to 0 mV. The voltage dependence of tau(hf) in the older cells is identical to that of the single tau(h) found in the younger; the results indicate a dominance of tau(hf) in the younger. Patches from younger cells more often showed one apparent active channel; in such cases, m.o.t. was described by a single exponential. However, in two cases, channels showed bursting behaviour with one of these channels showing a shift between bursting and non-bursting modes. Our findings are consistent with a heterogeneous channel population and with changes in the population in the course of maturation.     

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.     

Characteristics of IA currents in adult rabbit cerebellar Purkinje cells

Classical conditioning the rabbit nictitating membrane involves changes in synaptic and intrinsic membrane properties of cerebellar Purkinje cell dendrites, and a 4-aminopyridine (4-AP)-sensitive potassium channel underlies these membrane properties. We characterized I(A) currents in adult, rabbit Purkinje cells to determine whether I(A) is the target channel involved in learning. Whole-cell recordings of Purkinje cell somas and dendrites revealed a fast activating and inactivating current with half maximal activation at -27.08 +/- 3.48 mV and -25.51 +/- 1.15 mV in somas and dendrites, respectively; half maximal inactivation at -58.91 +/- 2.34 mV and -49.90 +/- 2.58 mV; and a recovery time constant of 22.81 +/- 1.92 ms and 16.60 +/- 4.26 ms. Outside-out patch recordings from cerebellar Purkinje cell somas confirmed these 4-AP-sensitive currents with half maximal activation at -13.85 +/- 1.17 mV and half maximal inactivation at -55.07 +/- 5.54 mV. More importantly, there was an overlap of activation and incomplete inactivation at potentials from -60 to -40 mV, suggesting a "window" current that was responsible for subthreshold variations of membrane potential and might underlie conditioning-specific increases in Purkinje cell excitability. The potassium current was inhibited by 4-AP and by Heteropodatoxin, a specific blocker of Kv4.2 and Kv4.3 channels, but not by Stromatoxin, a blocker of Kv4.2 channels. Mouse monoclonal antibody labeling identified both Kv4.3 and Kv4.2 subunits in the granule cell layer but only Kv4.3 subunits in the molecular layer. This is the first demonstration of A-type currents in adult, rabbit Purkinje cells that may play a role in regulating membrane potential and firing frequency and comprise the target channel mediating conditioning-specific changes of excitability in rabbit Purkinje cell dendrites.