Suramin affects capsaicin responses and capsaicin-noxious heat interactions in rat dorsal root ganglia neurones

The effect of suramin, an inhibitor of G protein regulated signalling, was studied on the membrane currents induced by noxious heat and by capsaicin in cultured dorsal root ganglia neurones isolated from neonatal rats. Whole-cell responses induced by a heat ramp (24-52 degrees C) were little affected by suramin. The noxious heat-activated currents were synergistically facilitated in the presence of 0.3 microM capsaicin 13.2-fold and 6.3-fold at 40 degrees C and 50 degrees C, respectively. In 65% of neurones, the capsaicin-induced facilitation was inhibited by 10 microM suramin to 35 +/- 6% and 53 +/- 6% of control at 40 degrees C and 50 degrees C (S.E.M., n = 15). Suramin 30 microM caused a significant increase in the membrane current produced by a nearly maximal dose (1 microM) of capsaicin over the whole recorded temperature range (2.4-fold at 25 degrees C and 1.2-fold at 48 degrees C). The results demonstrate that suramin differentially affects the interaction between capsaicin and noxious heat in DRG neurones and thus suggest that distinct transduction pathways may participate in vanilloid receptor activation mechanisms.

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.

Functional expression of Kir 6.1/SUR1-K(ATP) channels in frog retinal Müller glial cells

The retinae and brains of larval and adult amphibians survive long-lasting anoxia; this finding suggests the presence of functional K(ATP) channels. We have previously shown with immunocytochemistry studies that retinal glial (Müller) cells in adult frogs express the K(ATP) channel and receptor proteins, Kir6.1 and SUR1, while retinal neurons display Kir6.2 and SUR2A/B (Skatchkov et al., 2001a: NeuroReport 12:1437-1441; Eaton et al., in press: NeuroReport). Using both immunocytochemistry and electrophysiology, we demonstrate the expression of Kir6.1/SUR1 (K(ATP)) channels in adult frog and tadpole Müller cells. Using conditions favoring the activation of K(ATP) channels (i.e., ATP- and spermine-free cytoplasm-dialyzing solution containing gluconate) in Müller cells isolated from both adult frogs and tadpoles, we demonstrate the following. First, using the patch-clamp technique in whole-cell recordings, tolbutamide, a blocker of K(ATP) channels, blocks nearly 100% of the transient and about 30% of the steady-state inward currents and depolarizes the cell membrane by 5-12 mV. Second, inside-out membrane patches display a single-channel inward current induced by gluconate (40 mM) and blocked by ATP (200 microM) at the cytoplasmic side. The channels apparently show two sublevels (each of approximately 27-32 pS) with a total of 85-pS maximal conductance at -80 mV; the open probability follows a two-exponential mechanism. Thus, functional K(ATP) channels, composed of Kir6.1/SUR1, are present in frog Müller cells and contribute a significant part to the whole-cell K+ inward currents in the absence of ATP. Other inwardly rectifying channels, such as Kir4.1 or Kir2.1, may mediate the remaining currents. K(ATP) channels may help maintain glial cell functions during ATP deficiency.

The effects of capsaicin and acidity on currents generated by noxious heat in cultured neonatal rat dorsal root ganglion neurones

1. The effects of capsaicin, acidic pH, ATP, kainate and GABA on currents generated by noxious heat were studied in cultured dorsal root ganglion (DRG) neurones (< 20 microm in diameter) isolated from neonatal rats. The patch clamp technique was used to record membrane currents or changes of membrane potential. 2. In agreement with previous results, inward membrane currents (I(heat)) induced by a 3 s ramp of increasing temperature from room temperature (approximately 23 degrees C) to over 42 degrees C varied greatly between cells (-100 pA to -2.4 nA at 48 degrees C) and had a temperature coefficient (Q(10)) > 10 over the range of 43-52 degrees C. 3. Capsaicin potentiated the heat-induced current even when capsaicin, at room temperature, had little or no effect on its own. In cells in which capsaicin induced no or very small membrane current at room temperature (< 50 pA), I(heat) exhibited detectable activation above 40 degrees C and increased 5.1 +/- 1.1 (n = 37) and 6.3 +/- 2.0 (n = 18) times at 0.3 and 1 microM capsaicin, respectively. 4. A rapid decrease in extracellular pH from 7.3 to 6.8, 6.3 or 6.1 produced an inward current which inactivated in ~5 s either completely (pH 6.8 or 6.3) or leaving a small current (approximately 50 pA) for more than 2 min (pH 6.1). After inactivation of the initial low pH-induced current, I(heat) at 48 degrees C increased 2.3 +/- 0.4 times at pH 6.8, 4.0 +/- 0.6 times at pH 6.3 and 4.8 +/- 0.8 times at pH 6.1 with a Q(10) > 10 (n = 16). 5. ATP (n = 22), kainate (n = 7) and GABA (n = 8) at 100 microM, produced an inactivating inward current in all heat-sensitive DRG neurones tested. During inactivation and in the presence of the drug, I(heat) was increased slightly with ATP and unaffected with kainate and GABA. These agents apparently do not directly affect the noxious heat receptor. 6. The results indicate a novel class of capsaicin-sensitive cells, in which capsaicin evokes no or very small inward current but nevertheless increases sensitivity to noxious heat.

Membrane currents and morphological properties of neurons and glial cells in the spinal cord and filum terminale of the frog

Using the patch-clamp technique in the whole-cell configuration combined with intracellular dialysis of the fluorescent dye Lucifer yellow (LY), the membrane properties of cells in slices of the lumbar portion of the frog spinal cord (n=64) and the filum terminale (FT, n=48) have been characterized and correlated with their morphology. Four types of cells were found in lumbar spinal cord and FT with membrane and morphological properties similar to those of cells that were previously identified in the rat spinal cord (Chvátal, A., Pastor, A., Mauch, M., Syková, E., Kettenmann, H., 1995. Distinct populations of identified glial cells in the developing rat spinal cord: Ion channel properties and cell morphology. Eur. J. Neurosci. 7, 129-142). Neurons, in response to a series of symmetrical voltage steps, displayed large repetitive voltage-dependent Na(+) inward currents and K(+) delayed rectifying outward currents. Three distinct types of non-neuronal cells were found. First, cells that exhibited passive symmetrical non-decaying currents were identified as astrocytes. These cells immunostained for GFAP and typically had at least one thick process and a number of fine processes. Second, cells with the characteristic properties of rat spinal cord oligodendrocytes, with passive symmetrical decaying currents and large tail currents after the end of the voltage step. These cells exhibited either long parallel or short hairy processes. Third, cells that expressed small brief inward currents in response to depolarizing steps, delayed rectifier outward currents and small sustained inward currents identical to rat glial precursor cells. Morphologically, they were characterized by round cell bodies with a number of finely branched processes. LY dye-coupling in the frog spinal cord gray matter and FT was observed in neurons and in all glial populations. All four cell types were found in both the spinal cord gray matter and FT. The glia/neuron ratio in the spinal cord was 0.78, while in FT it was 2.0. Moreover, the overall cell density was less in the FT than in the spinal cord. The present study shows that the membrane and morphological properties of glial cells in the frog and rat spinal cords are similar. Such striking phylogenetic similarity suggests a significant contribution from distinct glial cell populations to various spinal cord functions, particularly ionic and volume homeostasis in both mammals and amphibians.

Müller glial cells in anuran retina

Whereas in the brain, the activity of the neurons is supported by several types of glial cells such as astrocytes, oligodendrocytes, and ependymal cells, the retina (evolving from the brain during ontogenesis) contains only one type of macroglial cell, the Müller (radial glial) cells, in most vertebrates including the anurans. These cells span the entire thickness of the tissue, and thereby contact and ensheath virtually every type of neuronal cell body and process. This intimate topographical relationship is reflected by a multitude of functional interactions between retinal neurons and Müller glial cells. Müller cells are the principal stores of retinal glycogen, and are thought to fuel retinal neurons with substrate (lactate/pyruvate) for their oxidative metabolism. Furthermore, Müller cells are involved in the control and homeostasis of many constituents of the extracellular space, such as potassium and perhaps other ions, signaling molecules, and of the extracellular pH. They also seem to play important roles in recycling mechanisms of photopigment molecules and neurotransmitter molecules such as glutamate and GABA. By containing the main retinal stores of glutathione, Müller cells may protect retinal neurons against free radicals. Moreover, Müller cells express receptors for many neuroactive substances, and may also release such substances to their neighbouring neurons. Thus, Müller cells exert many functions crucial for signal processing in the normal retina. Moreover, Müller cells change their properties in cases of retinal disease and injury, and may either support the survival of neuronal cells or accelerate the progress of neuronal degeneration.

Spatial distribution of spermine/spermidine content and K(+)-current rectification in frog retinal glial (Müller) cells

Previous studies in retinal glial (Müller) cells have suggested that (1) the dominant membrane currents are mediated by K(+) inward-rectifier (Kir) channels (Newman and Reichenbach, Trends Neurosci 19:307-312, 1996), and (2) rectification of these Kir channels is due largely to a block of outward currents by endogenous polyamines such as spermine/spermidine (SPM/SPD) (Lopatin et al., Nature 372:366-369, 1994). In frog Müller cells, the degree of rectification of Kir-mediated currents is significantly higher in the endfoot than in the somatic membrane (Skatchkov et al., Glia 27:171-181, 1999). This article shows that in these cells there is a topographical correlation between the local cytoplasmic SPM/SPD immunoreactivity and the ratio of inward to outward K(+) currents through the surrounding membrane area. Throughout the retina, Müller cell endfeet display a high SPM/SPD immunolabel (assessed by densitometry) and a large inward rectification of K(+) currents, as measured by the ratio of inward to outward current produced by step changes in [K(+)](o). In the retinal periphery, Müller cell somata are characterized by roughly one-half of the SPM/SPD immunoreactivity and K(+)-current rectification as the corresponding endfeet. In the retinal center, Müller cell somata are virtually devoid of both SPM/SPD immunolabel and K(+)-current inward rectification. Comparing one region of the retina with another, we find an exponential correlation between the local K(+) rectification and the local SPM/SPD content. This finding suggests that the degree of inward rectification in a given membrane area is determined by the local cytoplasmic polyamine concentration.

Potassium buffering by Müller cells isolated from the center and periphery of the frog retina

Müller (radial glial) cells span the retina from the outer to the inner limiting membranes. They are the only glial cells found in the amphibian retina. The thickness of the frog (Rana pipiens) retina decreases by a factor of about four from the center to the periphery. Thus, Müller cells were isolated, by enzymatic dissociation, with stalk lengths from 20 to 140 microm. Their ability to transfer K(+) via the stalk between soma and endfoot was studied. Membrane currents were recorded using the whole-cell voltage-clamp technique with the pipette sealed to either the endfoot or the soma. Inward (I(KIN)) or outward (I(KO)) currents were elicited by rapid increases (3 to 10 mM) or decreases (3 to 1 mM) of the extracellular K(+) concentration ([K(+)](o)) either by local application (close or distant to the recording pipette) or around the entire cell (whole cell perfusion). For the long central cells, the ratio I(KIN)/I(KO) was 4.6 +/- 0.6 SE (n = 9) at the endfoot and 1.7 +/- 0.1 SE (n = 8) at the soma. In cells from the retinal periphery, the ratio I(KIN)/I(KO) was higher, 7.0 +/- 0.27 (n = 8) at the endfoot and 3.2 +/- 0.1 (n = 10) at the soma. The results suggest that there is less inward rectification in the somatic than in the endfoot membrane. As expected from previous studies, the sensitivity of the cells to K(+) was higher at the endfoot than at the soma. The amplitude of I(KIN) at the endfoot compared to the soma was about 8-fold for the long central cells but only about 1.5-fold for the short peripheral cells. Currents spread readily from endfoot to soma in the peripheral cells. In the long central Müller cells the soma and endfoot appeared electrotonically isolated. The "functional length constant", lambda, of cell stalk processes was about 70 microm. The relative decrement of large inward currents was stronger than that of smaller outward currents; this difference ("artificial rectification") is explained by a simple model, where larger currents (inward) are attenuated more than smaller (outward) currents. The data support the hypothesis that in the retinal periphery, Müller cells provide extensive spatial K(+) buffering from both plexiform layers into the vitreous body. In the central retina, however, such currents are limited within a short (interlaminar) range.

Temperature coefficient of membrane currents induced by noxious heat in sensory neurones in the rat

1. Membrane currents induced by noxious heat (Iheat) were studied in cultured dorsal root ganglion (DRG) neurones from newborn rats using ramps of increasing temperature of superfusing solutions. 2. Iheat was observed in about 70 % of small (< 25 microm) DRG neurones. At -60 mV, Iheat exhibited a threshold at about 43 C and reached its maximum, sometimes exceeding 1 nA, at 52 C (716 +/- 121 pA; n = 39). 3. Iheat exhibited a strong temperature sensitivity (temperature coefficient over a 10 C temperature range (Q10) = 17.8 +/- 2.1, mean +/- s.d., in the range 47-51 C; n = 41), distinguishing it from the currents induced by capsaicin (1 microM), bradykinin (5 microM) and weak acid (pH 6.1 or 6.3), which exhibited Q10 values of 1.6-2.8 over the whole temperature range (23-52 C). Repeated heat ramps resulted in a decrease of the maximum Iheat and the current was evoked at lower temperatures. 4. A single ramp exceeding 57 C resulted in an irreversible change in Iheat. In a subsequent trial, maximum Iheat was decreased to less than 50 %, its threshold was lowered to a temperature just above that in the bath and its maximum Q10 was markedly lower (5.6 +/- 0.8; n = 8). 5. DRG neurones that exhibited Iheat were sensitive to capsaicin. However, four capsaicin-sensitive neurones out of 41 were insensitive to noxious heat. There was no correlation between the amplitude of capsaicin-induced responses and Iheat. 6. In the absence of extracellular Ca2+, Q10 for Iheat was lowered from 25.3 +/- 7.5 to 4. 2 +/- 0.4 (n = 7) in the range 41-50 C. The tachyphylaxis, however, was still observed. 7. A high Q10 of Iheat suggests a profound, rapid and reversible change in a protein structure in the plasma membrane of heat-sensitive nociceptors. It is hypothesized that this protein complex possesses a high net free energy of stabilization (possibly due to ionic bonds) and undergoes disassembly when exposed to noxious heat. The liberated components activate distinct cationic channels to generate Iheat. Their affinity to form the complex at low temperatures irreversibly decreases after one exposure to excessive heat.

Procaine excites nociceptors in cultures from dorsal root ganglion of the rat

Procaine, a classical local anesthetic, produces, at low concentration (2-200 microM), excitation in a distinct population of small sensory neurons isolated from newborn rats (2D) and cultured for 1-5 days. The excitation or inward current (>50 pA) induced by procaine was observed in 59 out of 78 neurons. Nearly all these procaine-sensitive neurons (56 of 59) were also sensitive to capsaicin while 8 procaine-insensitive neurons responded to capsaicin (1 microM). In procaine-sensitive neurons tested for responsiveness to noxious heat, a 10 s temperature ramp from 24 to 48 degrees C induced an inward current of 413 +/- 47 pA (SEM, n = 27) and this current was enhanced, in the presence of procaine, about 3-fold (2.8 +/- 0.4, SEM, n = 27). The responses to procaine were concentration dependent and underwent pronounced tachyphylaxis after repeated applications. The voltage-current relationship exhibited outward rectification and the apparent reversal at 25 +/- 4.2 mV (SEM, n = 9) suggesting that the current is carried by cations including Ca2+. This procaine effect may offer an explanation for toxic consequence of the clinical use of local anesthetics.

K+ Channel density increases selectively in the endfoot of retinal glial cells during development of Rana catesbiana

The radial glial cells that span the retina, described by Müller in 1851, have a remarkable distribution of ion channels in adult amphibia that mediate extracellular K+ spatial buffering. 94% of the total membrane conductance of these cells resides in inward rectifier K+ channels in the endfoot processes apposed to the vitreous humour. We now report that this regional specialization is found in Müller cells isolated from adult (>120 day old) bullfrogs but to a far less extent in those from 10-20 day old tadpoles (stages 34-36). Using the cell attached configuration of the patch-clamp technique, we found, in agreement with previous studies in salamanders, that the endfoot of adult cells had 19.2+/-2.4 (mean +/- S.E., n = 81) channels/patch, whereas the soma had 1.81+/-0.28 (n = 21) channels/patch. In the tadpole, the respective values were 4.29+/-0.26 (n = 79) for the endfoot and 2.26+/-0.24 (n = 27) for the soma. The slope conductance of the inward rectifier K+ channel in 115 mM K+, 19.2+/-0.25 pS (n = 205), channel kinetics and the resting membrane potential (-69+/-2.7 mV, n = 224) were similar at both the endfoot and soma of both adults and embryos. We conclude that during development, the K+ conductance of the Müller cell endfoot, but not of the soma, increases due to a selective clustering of inwardly rectifying K+ channels in that specific region of the cell membrane. The properties of the channels change little during the transformation from tadpole to adult bullfrog.

Glial calcium: homeostasis and signaling function

Glial cells respond to various electrical, mechanical, and chemical stimuli, including neurotransmitters, neuromodulators, and hormones, with an increase in intracellular Ca2+ concentration ([Ca2+]i). The increases exhibit a variety of temporal and spatial patterns. These [Ca2+]i responses result from the coordinated activity of a number of molecular cascades responsible for Ca2+ movement into or out of the cytoplasm either by way of the extracellular space or intracellular stores. Transplasmalemmal Ca2+ movements may be controlled by several types of voltage- and ligand-gated Ca(2+)-permeable channels as well as Ca2+ pumps and a Na+/Ca2+ exchanger. In addition, glial cells express various metabotropic receptors coupled to intracellular Ca2+ stores through the intracellular messenger inositol 1,4,5-triphosphate. The interplay of different molecular cascades enables the development of agonist-specific patterns of Ca2+ responses. Such agonist specificity may provide a means for intracellular and intercellular information coding. Calcium signals can traverse gap junctions between glial cells without decrement. These waves can serve as a substrate for integration of glial activity. By controlling gap junction conductance, Ca2+ waves may define the limits of functional glial networks. Neuronal activity can trigger [Ca2+]i signals in apposed glial cells, and moreover, there is some evidence that glial [Ca2+]i waves can affect neurons. Glial Ca2+ signaling can be regarded as a form of glial excitability.

Changes in glial K+ currents with decreased extracellular volume in developing rat white matter

Whole cell patch-clamp recordings of K+ currents from oligodendrocyte precursors in 10-day-old rats (P10) and, following myelination, in mature oligodendrocytes from 20-day-old rats (P20) were correlated with extracellular space (ECS) diffusion parameters measured by the local diffusion of iontophoretically injected tetramethylammonium ions (TMA+). The aim of this study was to find an explanation for the changes in glial currents that occur with myelination. Oligodendrocyte precursors (P10) in slices from corpus callosum were characterized by the presence of A-type K+ currents, delayed and inward rectifier currents, and lack of tail currents after the offset of a voltage jump. Mature oligodendrocytes in corpus callosum slices from P20 rats were characterized by passive, decaying currents and large tail currents after the offset of a voltage jump. Measurements of the reversal potential for the tail currents indicate that they result from increases in [K+]e by an average of 32 mM during a 20 msec 100 mV voltage step. Concomitant with the change in oligodendrocyte electrophysiological behavior after myelination there is a decrease in the ECS of the corpus callosum. ECS volume decreases from 36% (P9-10) to 25% (P20-21) of total tissue volume. ECS tortuosity lambda = (D/ADC)0.5, where D is the free diffusion coefficient and ADC is the apparent diffusion coefficient of TMA+ in the brain, increases as measured perpendicular to the axons from 1.53 +/- 0.02 (n = 6, mean +/- SEM) to 1.70 +/- 0.02 (n = 6). TMA+ non-specific uptake (k') was significantly larger at P20 (5.2 +/- 0.6 x 10(-3) s(-1), n = 6) than at P10 (3.5 +/- 0.4 x 10(-3) s(-1), n = 6). It can be concluded that membrane potential changes in mature oligodendrocytes are accompanied by rapid changes in the K+ gradient resulting from K+ fluxes across the glial membrane. As a result of the reduced extracellular volume and increased tortuosity, the membrane fluxes produce larger changes in [K+]e in the more mature myelinated corpus callosum than before myelination. These conclusions also account for differences between membrane currents in cells in slices compared to those in tissue culture where the ECS is essentially infinite. The size and geometry of the ECS influence the membrane current patterns of glial cells and may have consequences for the role of glial cells in spatial buffering.

Glial-neuronal interactions in non-synaptic areas of the brain: studies in the optic nerve

Optic nerves, like other CNS tracts, consist of axons closely apposed across narrow extracellular clefts to the cell bodies and processes of glial cells. Despite the anatomical simplicity of these pathways and the absence of synapses, a surprising range of interactions occurs between axons and glial cells mediated by changes in the chemical composition of the extracellular fluid produced by glial or neuronal stimulation. Some of the interactions are relatively brief, resulting from alterations in extracellular ions such as K+ or H+, or alterations of small molecules like glutamate or ATP. Other interactions involve much longer time periods and presumably larger signaling molecules, like peptides or proteins. These play a role not only in the development of axonal pathways but also in the processes of degeneration and regeneration that follow brain injury or disease.

Potassium currents in cultured glia of the frog optic nerve

The processes that participate in clearing increases in [K+]o produced by active neurons include KCl uptake, Na pump stimulation, and spatial buffering. The latter process requires glial cells to carry: 1) inward K+ currents in regions where K+ is elevated at a glial membrane potential more negative than EK; and 2) outward K+ currents at normal K+ and glial membrane potential more positive than EK (Orkand et al: J Neurophysiol 29:788, 1966). Techniques for isolation and culturing glial cells brought new possibilities for studying ionic channels involved in spatial buffering. However, they raised the question of the extent to which the properties of ionic channels are changed due to the process of culturing when glial cells are exposed to an artificial environment and deprived of direct interaction with neurons. We studied potassium currents in glial cells from the frog optic nerve that were cultured for 1-8 days. At 24-48 h, cells exhibited an inwardly rectifying Cs+ blocked current (IK(IN)) that increased in amplitude and shifted its threshold of activation to EK when [K+]o was increased from 3 to 6 or 10 mM. IK(IN), diminished after 3 days in culture and virtually disappeared after 5 days. At 24-48 h, a potassium delayed rectifier current (IKD) was relatively small but became large at 3 days, and was practically the only current present after 5 days. IKD was activated at -8.5 +/- 0.58 mV(SE, n = 48) and 58 +/- 2.2% (SE, n = 48) blocked by 20 mM tetraethylammonium. The results of this study support the idea that the inward rectifying potassium channels (Kir) are responsible for carrying K+ into glial cells whenever [K+]o increases. However, the delayed rectifier potassium channels (KD) cannot provide the pathway for outward K+ current during spatial buffering, and another mechanism must be involved in this process. Our study provides further evidence that culture conditions can greatly influence functional expression of ionic channels in glial cells.

Effect of cutting the optic nerve on K+ currents in endfeet of Muller cells isolated from frog retina

Membrane currents were recorded from Muller cells isolated from normal retinas and from retinas whose ganglion cell axons had been cut in the optic nerve 30-60 days previously. The surgical procedure did not block the retinal blood supply and did not allow the axons to regenerate. The principal finding was that after severing the optic nerve there was less inward rectification in response to voltage commands. That is, the maintained inward current (I K(IN)) produced in response to a hyperpolarizing voltage command decreased leading to a decrease in the ratio I K(IN)/I K(OUT) In 98 mM [K+]O, this ratio was 2.86 +/- 0.21 (mean +/- SE; n = 24) in controls and 1.13 +/- 0.13 (n = 21) in Muller cells from denervated retinae. Barium, a blocker of the potassium inward rectifier (I (KIR)), eliminated this difference. Moreover, severing the optic nerve also decreased the resting potentials of Muller cells in 2.5 mM [K+]O from -83 +/- 7 mV to -63 +/- 9 mV. The results suggest that the voltage-dependent behavior and selectivity of K+ inward rectifying channels (K (ir)) in the endfeet depends on the integrity of the closely apposed ganglion cells.

Nerve impulses increase glial intercellular permeability

Coordinating the activity of neurons and their satellite glial cells requires mechanisms by which glial cells detect neuronal activity and change their properties as a result. This study monitors the intercellular diffusion of the fluorescent dye Lucifer Yellow (LY), following its injection into glial cells of the frog optic nerve, and demonstrates that nerve impulses increase the permeability of interglial gap junctions. Consequently, the spatial buffer capacity of the neuroglial cell syncytium for potassium, other ions, and small molecules will be enhanced; this may facilitate glial function in maintaining homeostasis of the neuronal microenvironment.

Effects of low doses of caffeine on [Ca2+]i in voltage-clamped snail (Helix aspersa) neurones

1. We have measured cytosolic free Ca2+ concentrations ([Ca2+]i) in voltage-clamped snail neurones using fura-2. Transient increases in [Ca2+]i were induced by depolarizing voltage steps of 20-60 mV for 0.1-10 s from a holding potential of -50 or -60 mV. 2. Low doses of caffeine, 0.2-1 mM, increased the size of the [Ca2+]i transients by both increasing the peak and producing an undershoot. 3. Ryanodine, an inhibitor of Ca2+ release from the intracellular Ca2+ stores, and cyclopiazonic acid (CPA), an inhibitor of the Ca(2+)-ATPase of the intracellular Ca2+ stores, both reduced the size of the [Ca2+]i transients and blocked the effects of caffeine on the transients. 4. The effects of caffeine and CPA were greater on transients produced by long, small, rather than short, large depolarizations. This suggests that calcium-induced calcium release (CICR) played a greater role in the [Ca2+]i increase resulting from longer, smaller depolarizations. 5. Increasing the extracellular pH from 7.5 to over 9, which inhibits the plasmalemmal Ca(2+)-H(+)-ATPase, increased the resting [Ca2+]i level. Depolarization-induced [Ca2+]i transients became much larger while the two effects of caffeine remained. CPA was ineffective at high pH. 6. In some experiments the increase in basal [Ca2+]i caused by alkaline pH was reduced by 0.2 or 0.5 mM caffeine. The increase in basal [Ca2+]i caused by maintained depolarization was reduced, after a transient increase, by 0.5 mM caffeine. Both reduction and increase were blocked by CPA. 7. We conclude that low doses of caffeine can increase uptake by intracellular Ca2+ stores. Caffeine could also release Ca2+ from ryanodine-insensitive Ca(2+)-ATPase-dependent stores as well as facilitating normal ryanodine-sensitive CICR.

Potassium currents in endfeet of isolated Müller cells from the frog retina

Voltage dependent potassium currents were recorded using the whole-cell mode of the patch-clamp technique for the first time from endfeet of Müller cells dissociated from the frog retina. Recordings from intact cells and isolated endfeet indicate that the inward rectifier potassium channel is the dominant ion channel in these cells and that the density of these channels is highest in the endfoot as has been previously reported for several other species. The present study uses rapid changes in [K+]o to understand the behavior of these channels in buffering [K+]o in the retina. With rapid changes in [K+]o, it was found that, at a membrane potential of -90mV, which is close to EK, increasing [K+]o from 3 to 10 mM produced an inward K+ current 5.48 +/- 0.89 SD (n = 9) times larger than outward current induced by decreasing [K+]o from 3 to 1 mM. The outward current was maximal at a holding potential of about -80mV and exhibited inactivation at more positive potentials. At -40 mV both the inward and outward currents are markedly reduced. The current voltage curve for the inward current was linear at holding potentials from -50 mV to -140 mV. Using 20 mV voltage steps, it was found that the voltage dependent K+ currents were unaffected by the addition of 2 mM Cd2+, a blocker of Ca(2+)-activated potassium currents, decreasing [Cl-]o from 120 mM to 5 mM or the substitution of 30 mM Na+ by TEA. The addition of 5 mM [Cs+]o blocked only the inward current. Both the outward and the inward currents disappeared in the absence of intracellular and extracellular K+; 0.3 mM [Ba2+]o blocked the inward current completely and strongly inhibited the outward current in a time and voltage dependent manner. We conclude that at physiological [K+]o and membrane potential, the K+ channels in the Müller cell endfoot are well suited to carry K+ both inward and outward across the membrane as required for spatial buffering.

Serotonin- and proton-induced and modified ionic currents in frog sensory neurons

Using the whole-cell patch-clamp technique, the effects of serotonin (5-HT) and increased acidity to produce membrane currents and to modify high threshold voltage-dependent calcium currents were studied in isolated dorsal root ganglion (DRG) cells of the frog maintained in short-term culture. DRG cells were classified by morphology into two types: (1) cells with a large number of dark rusty brown granules, and (2) cells devoid of these granules or with few scattered pale granules. Fast application of 5-HT (10-30 microM) induced a rapidly desensitizing inward current with a reversal potential at about 0 mV in 38 of 50 granule-containing neurons (76%) which was never observed (0/35) in "clear" neurons. This current was blocked by 10 nM (+)-tubocurarine. In addition, a small noninactivating outward current was also observed in most DRG neurons during 5-HT superfusion. A sudden decrease of pH from 7.4 to 6 or 5.8 induced a fast inactivating inward current of 100-300 pA in 74% of the "clear" neurons and only 24% of the granule-containing neurons. Small noninactivating membrane currents induced by lowering pH were observed in all neurons. Both 5-HT and increased extracellular H+ reduced the magnitude of high threshold calcium currents in all DRG neurons. It is suggested that the 5-HT receptors are expressed on a morphologically distinct population of neurons while the cells with channels responsible for the fast inactivating proton-induced current cannot be related to any distinct morphological cell type.

Glial Function in Homeostasis of the Neuronal Microenvironment

Neuroglia buffer changes in the concentrations of ions and small molecules in the tortuous network of narrow extracellular clefts that constitutes the functional environment of neurons in the central nervous system. The large area of glial membrane bordering this space exhibits specific membrane transport systems for homeostasis.

NMDA receptors on adult frog spinal motoneurons in culture

Whole-cell membrane currents induced by superfusion of NMDA were examined in cultured motoneurons from the spinal cord of the adult frog in Mg(2+)-free Ringer solution containing 10 microM glycine. The amplitude of the response to 100 microM NMDA was 280 +/- 37 pA (mean +/- S.D.; n = 24) with a reversal potential +6.1 +/- 3.0 mV (mean +/- S.D.; n = 6). At a membrane potential of -60 mV, the response to 100 microM NMDA was blocked by 0.1 mM Mg2+ or 100 microM AP5. From the dose-response curve, the estimated EC50 was 77 microM and the calculated Hill coefficient was 1.6. NMDA receptors on frog motoneurons appear to have properties similar to those of mammals but may be expressed at lower density.

Facilitation of sodium currents in frog neuroglia by nerve impulses: dependence on external calcium

Facilitation of voltage-gated sodium currents in glial membranes by nerve impulses has been studied by using both the whole cell and loose-patch clamp techniques in the isolated intact optic nerve of the frog. During facilitation there is a shift in the voltage dependence of glial Na+ channels such that a given depolarization produces a larger inward Na+ current. Decreasing external calcium from 4 times normal to 0.2 times normal produced a similar shift in the current-voltage relation. Increasing the external calcium concentration to 4-5 times normal blocks facilitation. In reduced calcium, 0.1-0.2 times normal, the peak of facilitation was unaffected, but its decay was slowed. The addition of 1 mM nickel and 2 mM cobalt or 2 mM cadmium, to prevent depletion of extracellular calcium that might result from voltage-dependent entry of calcium into the axons, did not block the facilitation. The results suggest that, even though facilitation is blocked by high extracellular calcium, a decrease in extracellular calcium produced by axon impulses is not the cause of the facilitation.

Changes in ultrastructure and voltage-dependent currents at the glia limitans of the frog optic nerve following retinal ablation

The surface of the frog optic nerve consists of astrocytic processes separated by narrow extracellular clefts underlying a pial sheath of loose connective tissue. Macroscopic voltage dependent currents can be recorded from this surface using the loose patch-clamp technique. In this study the changes in ultrastructure and voltage dependent Na currents have been studied for up to 1 year following removal of the retina. During the first 1-4 weeks, many of the myelinated and unmyelinated axons of the retinal ganglion cells degenerate, and the debris is phagocytosed by macrophages and glial cells. However, some morphologically intact axons remain even 12 weeks after surgery. Finally, after 16 weeks all the axons have disappeared, leaving a nerve consisting only of glial cells, some of which contain phagosomes. At 40-52 weeks after enucleation, the nerve persists, at 20-40% of the normal diameter, consisting mostly of normal looking astrocytes. The amplitude of the voltage dependent Na currents recorded from nerves during the first 1-4 weeks after enucleation, with the pial sheath intact, decreases by about 50%. After 8 weeks, the Na current recorded from the surface is about 30% of control. At 16-52 weeks after removal of the retina, when there are no intact axons, the Na current is reduced by 90%. If, however, the pial sheath is stripped away, the Na currents recorded from the glial surface are 40-50% of control during this same 16- to 52-week period, suggesting that in the all-glia nerve, the currents are shunted by the relatively thicker pial sheath.(ABSTRACT TRUNCATED AT 250 WORDS)

Characteristics of activity-dependent potassium accumulation in mammalian peripheral nerve in vitro

Ion-sensitive microelectrodes were used to study the behavior of extracellular ions in rat sciatic nerve during and following activity. Nerve stimulation produced increases in [K+]o that were dependent upon the frequency and duration of stimulation; no change in extracellular pH occurred with stimulation. Increases in [K+]o depended on axonal discharge since they were blocked by inhibiting sodium channels with tetrodotoxin. At 22 degrees C, stimulation could induce increases in [K+]o of several mM; at 36 degrees C, stimulation rarely produced increases in [K+]o greater than 1 mM. Stimulated increases in [K+]o dissipated very slowly (i.e. t 1/2 = 50-100 s) and the rate of dissipation was not significantly affected by anoxia, changes in temperature, changes in extracellular pH, or the application of a blocker of Na+, K(+)-ATPase (ouabain) or a K+ channel blocker (Ba2+). In comparison to the central nervous system, neural activity in rat sciatic nerve produced smaller increases in [K+]o and these increases dissipated much more slowly. The primary mechanism of K+ dissipation appeared to be diffusion, probably facilitated by the larger extracellular space in peripheral nerve compared to the central nervous system, but impeded by diffusion barriers imposed by the blood-nerve barrier.

Further studies of electrogenic Na+/HCO3- cotransport in glial cells of Necturus optic nerve: regulation of pHi

In the presence of Ba++, an increase in the bath HCO3- at constant CO2 (i.e., variable bath pH) produced a hyperpolarization. The hyperpolarizing effect of adding HCO3-/CO2 at constant bath pH was not significantly affected by the presence of 50 mumol/l strophanthidin. In the absence of Ba++, addition of HCO3-/CO2 at constant bath pH produced a Na(+)-dependent hyperpolarization. Therefore, CO2 movements, electrogenic Na+/K+ pump activity and changes in Ba++ binding do not contribute significantly to the hyperpolarization induced by HCO3-. These results along with the results of previous studies (Astion et al: J Gen Physiol 93:731, 1989) strongly suggest that the hyperpolarization induced by the addition of HCO3- is due to an electrogenic Na+/HCO3- cotransporter, which transports Na+, HCO3- (or its equivalent), and net negative charge across the glial membrane. To study the role of electrogenic Na+/HCO3- cotransport in the regulation of pHi in glial cells, we used intracellular double-barreled, pH-sensitive microelectrodes. At a bath pH of 7.5, the mean initial intracellular pH (pHi) was 7.32 (SD 0.03, n = 6) in HEPES-buffered Ringer's solution and 7.39 (SD 0.1, n = 6) in HCO3-/CO2 buffered solution. These values for pHi are more than 1.2 pH units alkaline to the pHi predicted from a passive distribution of protons; thus, these cells actively regulate pHi. Superfusion and withdrawal of 15 mmol/l NH4+ induced an acidification of 0.2 to 0.3 pH units, which recovered toward the original steady-state pHi.(ABSTRACT TRUNCATED AT 250 WORDS)

Na+/H+ exchange in glial cells of Necturus optic nerve

Single and double-barreled pH-sensitive electrodes were used to study intracellular pH (pHi) regulation in glial cells of Necturus optic nerve in the nominal absence of HCO3-/CO2. After the cells were acidified by the addition and withdrawal of NH4+, the pHi recovered toward the original steady-state pHi. The recovery from acidification was Na+-dependent and inhibited by 1 mM amiloride. These results suggest the existence in intact vertebrate glial cells of a Na+/H+ exchanger which functions in acid extrusion.

Facilitation of voltage-gated ion channels in frog neuroglia by nerve impulses

The functions of glial cells in the nervous system are not well defined, with the exception of myelin production by oligodendrocytes, uptake of amino-acid synaptic transmitters, and a contribution to extracellular potassium homeostasis. Neuroglia have receptors for neurotransmitters which may be involved in neuron-glia interactions. Recent studies have demonstrated voltage-gated ion channels in glial membranes. In a study of the optic nerve of the frog, small areas of the surface were examined with the loose patch-clamp method, and voltage-gated Na+ and K+ channels, presumably located in the membranes of the astrocytes forming the glia limitans, were identified. We now report that nerve impulses in the axons of the frog optic nerve transiently alter the properties of the voltage-dependent membrane channels of the surface glial cells (astrocytes), a demonstration of a new form of neuron-glia interaction.

Effects of barium and bicarbonate on glial cells of Necturus optic nerve. Studies with microelectrodes and voltage-sensitive dyes

We have studied the effects of Ba++, a known K+ channel blocker, on the electrophysiological properties of the glial cells of Necturus optic nerve. The addition of Ba++ reversibly depolarized glial cells by 25-50 mV; the half maximal deplorization was obtained with a Ba++ concentration of approximately 0.3 mM. In the presence of Ba++, the sensitivity of the membrane to changes in K+ was reduced and there was evidence of competition between K+ and Ba++ for the K+ channel. These effects, which were accompanied by a large increase in the input resistance of the glial cells, indicate that Ba++ blocks the K+ conductance in glial cells of Necturus optic nerve. With the K+ conductance reduced, we were able to investigate the presence of other membrane conductances. We found that in the presence of Ba++, the addition of HCO3- caused a Na+-dependent hyperpolarization that was sensitive to the disulfonic stilbene SITS (4-acetamido-4'-isothiocyanostilbene-2, 2'-disulfonic acid). Removal of Na+ resulted in a HCO3- -dependent, SITS-sensitive depolarization. These results are consistent with the presence in the glial membrane of an electrogenic Na+/HCO3- cotransporter in which Na+, HCO3-, and net negative charge are transported in the same direction. In Cl- -free solutions, the Ba++-induced depolarization increased, suggesting a small permeability to Cl-. Using voltage-sensitive dyes and a photodiode array for multiple site optical recording, the distribution of potential changes in response to square pulses of intracellularly injected current were recorded before and after the addition of increased and the decay of amplitude as a function of distance decreased. Such results indicate that Ba++ increases the membrane resistance more than the resistance of the intercellular junctions.

Chloride enters glial cells and photoreceptors in response to light stimulation in the retina of the honey bee drone

Double-barrelled ion-selective microelectrodes were used to measure free [Cl-] in photoreceptors, extracellular space, and glial cells in superfused slices of drone retina. Tests indicated that with normal superfusate the intracellular electrode signal was due essentially to Cl- and not to some other interfering anion. The results indicate that Cl- is more concentrated in both photoreceptors and glial cells than would be predicted for a passive electrochemical distribution. When the photoreceptors were stimulated by a standard train of 20 ms flashes, 1/s for 90 s, their intracellular free [Cl-] (Cli) rose by 8 +/- 1 mM. At the end of stimulation Cli usually continued to rise for up to a further 2 min and then returned toward the baseline over about 10 min. During light stimulation Cli in the glia rose. The magnitude of the increase was 5.1 +/- 0.4 mM, about half the increase in Ki. In some extracellular recording sites, light stimulation caused [Cl-] to increase and in others to decrease. The mean change was -0.7 mM, SD 6.5 mM. The Cl- that entered the photoreceptors and the glia was presumably made available by the shrinking of the extracellular space. When the cells were depolarized by increasing [K+] in the superfusate from 7.5 mM to 18 mM, Cli increased. The half-time of the change in Cli was longer than the half-time of the depolarization by 10-30 s in the glia and 50-250s in the photoreceptors. During superfusion with 0 Cl- Ringer's solution, the light-induced rise in extracellular [K+] was greater by a factor of 1.4-2.7, and the clearance after the end of the stimulation was slower. The rate of increase in glial Ki during light stimulation fell; the rate of increase of glial Ki caused by superfusion with raised [K+] (in the absence of Cl-) fell more. We conclude that when extracellular [K+] is increased, entry of Cl- into the glia is necessary for part, but not all, of the net uptake of K+. During light stimulation, the observed movement of CL- into glia contributes to homeostasis of extracellular [K+], and the cell swelling associated with movement of Cl- into both glia and photoreceptors contributes to homeostasis of extracellular [Na+].

Voltage-gated currents recorded from the surface of the frog optic nerve

Macroscopic voltage-dependent currents were recorded from the surface of the intact optic nerve of Rana pipiens using the loose patch clamp technique. Depolarizing steps of more than 40 mV produced sodium-dependent TTX sensitive inward currents and a 4-AP and sodium sensitive fast outward current in addition to a slower outward current. Since the surface of the nerve is a glia limitans, it appears that the membranes of these astrocytes contain both voltage-sensitive sodium and potassium channels.

Effects of carbon dioxide on extracellular potassium accumulation and volume in isolated frog spinal cord

A 6-10-fold increase in pCO2 in the superfusing Ringer solution increased the volume of the extracellular space (ECS) and changed the spatial distribution and amplitude of the extracellular K+ accumulation which resulted from dorsal root stimulation. Using the increase in tetraethylammonium concentration [( TEA+]) resulting from iontophoretic injection of that ion in the extracellular fluid as an indication of the volume of the ECS, it was found that in high pCO2 the ECS volume in spinal dorsal horn increased by more than 60%. In addition, in the presence of raised pCO2 we also observed the following: (1) The rate of diffusion of TEA+ into the dorsal horn increased. (2) The accumulation of K+ evoked by single or tetanic stimulation of the dorsal root was less. (3) The clearance of K+ was slowed down. (4) The regions where K+ accumulated were more restricted. (5) The K+ evoked depolarization of the primary afferent fibres decreased. (6) In contrast to TEA+, the rate of diffusion of K+ into the dorsal horn decreased. The effects of an increase in pCO2 on K+ accumulation and clearance appear to result from an increase in ECS volume and a possible decrease in glial electrical coupling which interferes with glial spatial buffering of K+.

K+ accumulation in the space between giant axon and Schwann cell in the squid Alloteuthis. Effects of changes in osmolarity

In a train of impulses in squid giant axon, accumulation of extracellular potassium causes successive afterhyperpolarizations to be progressively less negative. In Loligo, Frankenhaeuser and Hodgkin had satisfactorily accounted for the characteristics of this effect with a model in which the axon is surrounded by a space, width theta, and a barrier of permeability P. In axons isolated from Alloteuthis, we found that the model fitted the observations quite well. Superfusing the axon with hypotonic artificial seawater (ASW) caused theta and P to decrease, and, conversely, hypertonic ASW caused them to increase: this would be the case if both the space and the pathway through the barrier were extracellular. In some cases, in normal ASW, the afterhyperpolarizations in a train decreased very little, less than 0.7 mV. In these extreme cases, theta was estimated to be 190 nm and P to be 7 x 10(-4) cm s-1, both several times the values of 30 nm and 6 x 10(-5) cm s-1 estimated by Frankenhaeuser and Hodgkin. We suggest that in vivo the periaxonal space may be considerably wider than that seen in conventionally fixed squid tissue.

Optical recording of electrical activity from axons and glia of frog optic nerve: potentiometric dye responses and morphometrics

Voltage-sensitive dyes were used to study the changes in membrane potential in axons and glial cells of the frog optic nerve following electrical stimulation. The lack of a signal in the unstained nerve and the multiphasic action spectra after staining indicated that the optical responses were from the extrinsic dyes. Changes in dye absorption and fluorescence had rapid and slow phases. The rapid phases resulted from action potentials in myelinated and unmyelinated axons. The kinetics of the slow phase of the optical response were similar to the depolarization recorded from the glial cells with intracellular electrodes. The ratio of the amplitudes of the fast and slow phases was characteristic for each type of dye. Pharmacological analysis of the action potential of the unmyelinated axons revealed tetrodotoxin-sensitive sodium channels and 4-aminopyridine-sensitive potassium channels. Repeated exposure of the stained preparation to light led to photodynamic damage as shown by a block of recovery of the glial depolarization. An electron microscopic morphometric study of the nerve was carried out in an effort to understand the contribution of the various anatomical elements to the compound optical response. The ratio of unmyelinated axon membrane to glial membrane was much greater than was the ratio of the fast and slow components of the signal, suggesting that the dyes either had a higher affinity for glial membrane or did not penetrate the nerve uniformly.

Electrogenic Na+/HCO3- cotransport in neuroglia

Membrane potential recording from glial cells in Necturus optic nerve in the presence of 2 mM Ba++, which was added to block the K+ conductance, gave the following results. 1) In HCO3- -free, low-Na+ solutions (11% of control; Na+ replaced with N-methyl-D-glucamine), the hyperpolarizing effect of adding 10 mM HCO3- was reduced by approximately 80%. 2) 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid (SITS, 0.1 or 1 mM) reduced the effect of HCO3- by at least 50%. 3) In the presence of HCO3-, reduction of Na+ caused a depolarization which was much larger than that observed in nominally HCO3- -free solutions. These observations indicate the presence in the glial membrane of an electrogenic Na+/HCO3- cotransporter in which the stoichiometry of HCO3- to Na+ is greater than 1.

Glial potassium uptake following depletion by intracellular ionophoresis

The K+ uptake processes of immunologically identified oligodendrocytes from embryonic mouse spinal cord were studied in primary culture by injecting ions and recording membrane potential changes and, in some experiments, K+ ion activity with intracellular electrodes. When Na+ was injected [K+]i decreased. Immediately before and after current injection the membrane potential was close to the K+ equilibrium potential (EK) and this finding was used to study K+ uptake following its depletion by intracellular ionophoresis. The uptake of K+ following Na+ injection was blocked by ouabain and unaffected by removal of extracellular Cl- or Cl- transport blockers. This suggests that recovery comes about mostly through the activity of the Na+/K+ -ATPase stimulated by either the increase in [Na+]i or the decrease in [K+]i. Pump current could be determined by clamping at different membrane potentials and was found to increase in proportion to the depolarization of the cell resulting from [K+]i depletion. The time course of recovery of membrane potential following either Li+ or tetramethylammonium (TMA+) injection was similar to that after Na+ injection, indicating that injection of these ions to produce a comparable decrease in [K+]i leads to a similar stimulation of the Na+/K+ -ATPase. In addition, the recovery of membrane potential following injection of TMA+, but not of Na+ or Li+, was blocked when the external Na+ was removed. Internal Na+ or Li+ appears necessary for Na+/K+ -ATPase-activity, but under conditions of normal or low [Na+]i the rate of Na+/K+ -ATPase activity seems to be sensitive to [K+]i and/or membrane potential.

Effects of bicarbonate on glial cell membrane potential in Necturus optic nerve

Intracellular electrodes were used to continuously monitor the membrane potential of glial cells in the isolated Necturus optic nerve. Addition of up to 10 mM extracellular bicarbonate (with CO2), at constant pH, produced a hyperpolarization of up to 10 mV (with a time course almost as fast as that of a K+ depolarization) that returned toward baseline during the following 2-15 min. Upon bicarbonate withdrawal, the potential transiently became more positive. The bicarbonate effects were magnified when the K+ conductance was decreased and the cell depolarized by the addition of barium. Similar bicarbonate effects were observed in Cl- free solutions. These results suggest to us that: glial cells have a bicarbonate permeability of the same order as that to K+ and glial cells buffer transient changes in acid base balance in the neuronal microenvironment at the expense of their internal pH.

Voltage-sensitive dyes measure potential changes in axons and glia of the frog optic nerve

Changes in dye absorption and fluorescence produced by electrical stimulation were measured in frog optic nerves stained with voltage-sensitive dyes. Following a single maximal stimulus applied through a suction electrode, the change in transmitted light intensity consisted of two components: one representing an axonal compound action potential and the second a slow depolarizing afterpotential which appeared to arise from the glial cells. The following results support this interpretation: during a train of stimuli the depolarizing potentials sum and can exceed 80% of the initial spike amplitude while the spike amplitude itself remains essentially constant. Thus, the axons cannot have undergone significant depolarization during the train. Optical recordings with simultaneous microelectrode recordings from the glial cells indicate that the change in glial membrane potential during the train has a time-course similar to that of the slow optical response. We conclude that voltage-sensitive dyes can monitor potential changes in both neurons and glia.

Glutamate depolarization of glial cells in Necturus optic nerve

L-Glutamate, 10(-5) to 10(-2) M, depolarized glial cells in the normal and axon-free optic nerve of Necturus by up to 80 mV. The depolarization was maintained if lithium, but not tetramethylammonium, replaced sodium in the bathing solution. It was not blocked by either strophanthidin or tetrodotoxin. The response was accompanied by a membrane conductance increase and was followed by a strophanthidin-sensitive hyperpolarization. These results suggest that the depolarization resulted from sodium entry. D-Glutamate and L-aspartate were less potent but produced a comparable depolarization. gamma-Aminobutyric acid, glycine, taurine or glutamine (up to 10(-3) M) were ineffective.

Changes in sodium activity during light stimulation in photoreceptors, glia and extracellular space in drone retina

Ion-selective micro-electrodes were used to measure Na+ activity, aNa, in the two types of cell, photoreceptors and glial cells, and in the extracellular space, in superfused slices of the retina of the honey-bee drone, Apis mellifera male. Movements of Na+ were induced by light stimulation, or by increasing [K+] in the superfusate. In the dark, aNa in the photoreceptors was 10 mM (S.E. of the mean = 1 mM); in the glial cells it was higher: 37 +/- 2 mM. We estimate that in this preparation about 2/3 of the free Na+ in the tissue is in the glial cells. Stimulation with a train of light flashes, 1 s-1 for 90 s caused aNa in the photoreceptors to increase by 16 +/- 2 mM. K+ activity, aK, decreased by 21 +/- 3 mM. During the standard train of light flashes, aNa in glial cells decreased by only 1.5 +/- 0.3 mM, much less than the increase in aK (7 +/- 2 mM). One possible interpretation of this result is that most of the increase in aK is due to K+ uptake by a mechanism other than Na+-K+ exchange. In extracellular fluid, stimulation caused aNa to fall to a relatively steady value in about 10 s. Unlike aK, there was no tendency for aNa to return to the base line during the remainder of the 90 s stimulation. The fall in aNa was 14 +/- 1 mM: a greater fall is prevented by extracellular electric currents and a decrease in extracellular volume. When [K+] in the superfusate was increased from 7.5 to 18 mM, aNa decreased in the glial cells but not in the photoreceptors. In this tissue, stimulation causes changes in aNa in the neurones that might be large enough to modify the biochemistry of the cells. But in the glia, the fractional changes are small.

Active calcium responses recorded optically from nerve terminals of the frog neurohypophysis

Voltage-sensitive dyes were used to record by optical means membrane potential changes from nerve terminals in the isolated frog neurohypophysis. Following the block of voltage-sensitive Na+ channels by tetrodotoxin (TTX) and K+ channels by tetraethylammonium (TEA), direct electric field stimulation of the nerve terminals still evoked large active responses. These responses were reversibly blocked by the addition of 0.5 mM CdCl2. At both normal and low [Na+]o, the regenerative response appeared to increase with increasing [Ca++]o (0.1-10 mM). There was a marked decrease in the size of the response, as well as in its rate of rise, at low [Ca++]o (0.2 mM) when [Na+]o was reduced from 120 to 8 mM (replaced by sucrose), but little if any effect of this reduction of [Na+]o at normal [Ca++]o. In normal [Ca++]o, these local responses most probably arise from an inward Ca++ current associated with hormone release from these nerve terminals. At low [Ca++]o, Na+ appears to contribute to the TTX-insensitive inward current.

Coupling and uncoupling of amphibian neuroglia

Glial cells in the optic nerve of Necturus are coupled to each other by low resistance pathways which also permit the diffusion of the fluorescent dye Lucifer Yellow CH among the cells. The spread of dye is readily observed as nuclei of cells distant from the site of intracellular injection are stained. By contrast, horseradish peroxidase does not traverse the intercellular pathways. This protein remains in the injected cell. The addition of weak acids (carbonic or propionic) to the bathing medium reversibly uncouples the glia; it blocks the spread of ionic current and Lucifer Yellow among the cells. A block of ionic coupling will block the spatial buffering of potassium by the glial syncytium.

Electrical properties of oligodendrocytes in culture

The electrical properties of immunocytologically identified oligondendrocytes from embryonic mouse spinal cord maintained in culture for 3 to 6 weeks studied by passing current and recording potential changes with two separate intracellular electrodes. The average input resistance was 3.3 M omega and ranged from 0.7 to 16 M omega (n = 35). The input resistance increased by 19% with depolarization and decreased by 9% with hyperpolarization of 25 mV. The membrane time constant determined from the slope of the late exponential tail was 3.45 +/- 2.5 ms SD (n = 15). The specific membrane resistance of three cells was determined by a simplified square pulse analysis combined with measurement of membrane area. Membrane area was estimated from photomicrographs of cells injected with Lucifer Yellow CH and stained with the cell surface-reactive antibody 04 and from electron micrographs. An average specific membrane resistance of 1.3 X 10(3) omega cm2 and specific capacitance of 1.7 mu F/cm2 were calculated. Increasing [K+]o depolarized the cells and decreased the input resistance and the time constant.

Light-induced changes in extracellular volume in the retina of the drone, Apis mellifera

Slices of drone retina were superfused with a Ringer solution containing 1 mM tetraethylammonium (TEA), and the concentration of this ion in the extracellular space [( TEA]0) was measured with ion-sensitive microelectrodes. A train of light flashes for 90 s caused [TEA] to increase by 48 +/- 4% (S.E.), n = 12. Since water crosses cell membranes more readily than TEA does this indicates a volume decrease of at least 32%. Measurements of Ca2+ activity under similar conditions showed an increase of 32 +/- 4% (S.E.), n = 14. Since this is less than the increase in [TEA]0 it suggests that the total amount of Ca2+ in the extracellular space actually decreased.

Some properties of single potassium channels in cultured oligodendrocytes

K+ channels were studied in oligodendrocytes in cultures of mouse spinal cord. Single channel currents were measured using the gigaseal technique. The conductance of the channels varied greatly i.e. from 6 to 125 pS (38 +/- 28 SD, N = 21). In some patches there were up to three current levels of the same size. At -70 mV the open state probability was 0.51 +/- 0.17 and the average duration of an opening 70 +/- 20 ms for 4 channels with conductance from 16-57 pS. These analyses exclude brief flickering (less than 2 ms) or long closed periods (seconds to minutes). These times were not markedly affected by pulling the patch off the cell or by superfusing the isolated patch with media containing 10 mmol X 1(-1) TEA or EGTA without Ca2+. At membrane potentials between -90 and -30 mV there was a small but consistent effect of depolarization to increase the open state probability. Large positive or negative voltage steps decreased the open state probability. Current voltage measurements on intact cells showed a striking decrease in membrane conductance at these large membrane potentials. The leakage conductance of the patch also exhibited some K+ selectivity. The oligodendrocyte membrane appears to contain about one K+ channel per 5 micron 2. The known electrical properties of cultured oligodendrocytes can essentially be explained by the distribution and properties of these K+ channels.