Distinct physiologic properties of microglia and blood-borne cells in rat brain slices after permanent middle cerebral artery occlusion.

The authors investigated the time course of leukocyte infiltration compared with microglial activation in adult rat brain slices after permanent middle cerebral artery occlusion (MCAO). To distinguish peripheral leukocytes from microglia, the blood cells were prelabeled in vivo with Rhodamine 6G (Rhod6G) i.v. before induction of ischemia. At specific times after infarct, invading leukocytes, microglia, and endothelial cells were labeled in situ with isolectin (IL)B4-FITC (ILB4). Six hours after MCAO only a few of the ILB4+ cells were colabeled by Rhod6G. These cells expressed the voltage-gated inwardly and outwardly rectifying K+ currents characteristic of macrophages. The majority of the ILB4+ cells were Rhod6G- and expressed a lack of voltage-gated channels, recently described for ramified microglial cells in brain slices, or exhibited only an inward rectifier current, a unique marker for cultured (but unstimulated) microglia. Forty-eight hours after MCAO, all blood-borne and the majority of Rhod6G- cells expressed outward and inward currents indicating that the intrinsic microglial population exhibited physiologic features of stimulated, cultured microglia. The ILB4+/Rhod6G- intrinsic microglial population was more abundant in the border zone of the infarct and their morphology changed from radial to ameboid. Within this zone, the authors observed rapidly migrating cells and recorded this movement by time-lapse microscopy. The current findings indicate that microglial cells acquire physiologic features of leukocytes at a later time point after MCAO.

Incorporation of N-propanoylneuraminic acid leads to calcium oscillations in oligodendrocytes upon the application of GABA.

Sialylation of glycoproteins and glycolipids plays an important role during development, regeneration and pathogenesis. It has been shown that unnatural sialylation within glial cell cultures can have distinct effects on their proliferation and antigenic profiles. These cultures metabolize N-propanoylmannosamine (N-propanoylneuraminic acid precursor=P-NAP), a synthetic non-physiological precursor of neuraminic acid, resulting in the expression of N-propanoylneuraminic acid in glycoconjugates of their cell membranes [Schmidt, C., Stehling, P., Schnitzer, J., Reutter, W. and Horstkorte, R. (1998) J. Biol. Chem. 273, 19146-19152]. To determine whether these biochemically engineered sialic acids influence calcium concentrations in cells of the oligodendrocyte lineage, mixed glial cultures of oligodendrocytes growing on top of an astrocyte monolayer were exposed to glutamate, histamine, adrenaline, gamma-aminobutyric acid (GABA), high potassium (high K(+)) and ATP. Calcium responses in P-NAP-treated oligodendrocytes were determined by confocal microscopy with the calcium indicator fluo-3 AM, and compared with control cultures. We showed that P-NAP differentially modulated the calcium responses of individual oligodendrocytes when GABA was applied. GABA induced calcium oscillations with up to four spikes per min in 60% of oligodendrocytes when treated with P-NAP.

Expression of myelin-associated glycoprotein transcripts in murine oligodendrocytes.

The recognition molecule myelin-associated glycoprotein is expressed by oligodendrocytes, the myelinating cells of the central nervous system. The myelin-associated glycoprotein gene gives rise to two alternatively spliced transcript variants ("early" and "late" message) which are developmentally regulated. In this study, using mice, we investigated whether both transcripts can be expressed in an individual oligodendrocyte or whether different oligodendrocyte populations exist expressing either one or the other myelin-associated glycoprotein messenger RNA. For this purpose the cytoplasmic RNA content of single oligodendrocytes derived either from cultures of embryonic mouse brain or from the corpus callosum murine slice preparation was harvested during patch-clamping in the whole-cell recording mode by applying negative pressure to the patch pipette. After reverse transcription, cDNA fragments were amplified by the polymerase chain reaction and analysed by agarose gel electrophoresis and restriction enzyme maps. Expression of myelin-associated glycoprotein transcripts could first be detected in those oligodendrocytes which already had acquired a more mature developmental stage. This stage could electrophysiologically be characterized by the dominance of passive K+ currents. In addition to oligodendrocytes expressing only the late or the early transcript, many cells were found expressing simultaneously both transcripts with varying levels. The myelin-associated glycoprotein transcript expression is therefore found to be developmentally regulated at a stage when oligodendrocytes have already acquired the channel properties of the adult.

Modulation of potassium currents in cultured murine microglial cells by receptor activation and intracellular pathways.

The electrophysiological properties of ameboid microglia from rodent brain are dominated by inwardly rectifying potassium channels and by the lack of outward currents. This channel pattern results in a distinct physiological behavior: depolarizing events, e.g. following adenosine triphosphate receptor activation, can lead to a long lasting membrane depolarization. Here we address the question whether this resting K+ channel activity can be modulated. Intracellular application of guanosine 5'-O-(3-thiotriphosphate) induced an outward current and led to a complete disappearance of the inward current inward rectifier potassium current as measured with the patch clamp technique. Moreover, an elevation in cytosolic calcium concentration (to 1.6 microM) via intracellular perfusion reversibly blocked the inward current. The inhibition of inward currents by guanosine 5'-O-(3-thiotriphosphate) could be enhanced by additional adenosine triphosphate receptor activation. Adenosine triphosphate or tumor necrosis factor receptor activation alone could lead to a transient partial block of the inward rectifier and to the transient appearance of a delayed outward current. We conclude that the activity of the microglia K+ channels and thus the physiological behavior of microglia can be modulated on a time scale of seconds by receptor activation and distinct intracellular pathways.

AMPA/kainate receptor activation in murine oligodendrocyte precursor cells leads to activation of a cation conductance, calcium influx and blockade of delayed rectifying K+ channels.

Studies during the last few years have shown that glial cells can express a large repertoire of neurotransmitter receptors. In this study, we have characterized the properties of a glutamate receptor in oligodendrocytes and their precursor cells from cultures of mouse brain, using the patch-clamp technique to measure ligand-activated currents and a fura-2 imaging system to determine changes in free cytosolic Ca2+ concentration ([Ca2+]i). The precursor cells were identified by their characteristic morphology and their voltage-gated currents as described previously [Sontheimer H. et al. (1989) Neuron 2, 1135-1145]. The ligands kainate, domoate and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), as well as L-glutamate but not trans-1-amino-1,3-cyclopentanedicarboxylate elicited inward currents at a holding potential of -70 mV and the antagonist 6-cyano-7-nitroquinoxaline-2,3-dione blocked the glutamate- and kainate-induced response reversibly, indicating the expression of an AMPA/kainate-type glutamate receptor. The response is due to the activation of a cationic conductance as revealed by analysing the reversal potential of the kainate-activated current. Receptor activation is accompanied by two additional responses: (i) an increase in [Ca2+]i mediated by depolarization and a subsequent activation of voltage-gated Ca2+ channels and (ii) a transient blockade of a delayed rectifying K+ current, but not of the A-type K+ current. The blockade of the K+ current was not due to the increase in [Ca2+]i since it was also observed in Ca(2+)-free bathing solution when no increase in [Ca2+]i was detectable after exposure to kainate. In contrast to precursor cells, oligodendrocytes responded weakly or not at all to glutamate or related ligands. We conclude that glutamate activates a complex pattern of physiological events in the glial precursor cells, which may play a role during the differentiation process of these cells.

NMDA-activated currents in Bergmann glial cells.

NMDA receptors play a crucial role in synaptic plasticity of the central nervous system and were thought to be exclusive to neurones. In this study we provide evidence that Bergmann glial cells from mouse cerebellar slices show intrinsic responses to NMDA. As in neurones, NMDA increased membrane conductance and the responses were blocked by the NMDA antagonist ketamine, but not by the non-NMDA glutamate receptor antagonist CNQX. In contrast to responses in neurones, the current voltage relation of the glial NMDA-induced current was linear, reversed at -40 mV, currents were not blocked by Mg2+ or enhanced by glycine and NMDA did not induce an increase in cytosolic Ca2+ as recorded with a fura-2 imaging system. These data imply the presence of distinct NMDA receptors on Bergmann glial cells; these glial receptors could be the substitute for complex neurone-glia interactions in the cerebellum.

Bradykinin receptors in cultured astrocytes from neonatal rat brain are linked to physiological responses.

Specific binding sites for bradykinin (BK) have recently been demonstrated on astrocytes of primary cultures from neonatal rat brain. In this study we demonstrate that BK induces membrane currents in concert with an elevation of [Ca2+]i. In 67% of astrocytes, BK induced an inward current as determined with the perforated patch-clamp technique in the whole-cell recording configuration. In a small population of astrocytes (20%), a BK-activated outward current was observed, while in the remainder of the cells (13%) no apparent current responses were detected. As recorded by fura-2 microfluorimetry, the peptide induced a transient rise of [Ca2+/bdi even when the extracellular calcium was removed. In the majority of astrocytes, the selective B1-agonist des-Arg9-BK elicited physiological responses with a much lower potency, indicating that the BK receptors are predominantly of the B2 subtype. A minor population of astrocytes was present which only responded to des-Arg9-BK.

Diagnosis of Trichomonas vaginalis in adolescent females: InPouch TV culture versus wet-mount microscopy.

PURPOSE: This study compared the InPouch TV culture to wet-mount, Diamond's culture medium, and Papanicolaou (Pap) smear for the diagnosis of trichomonas infection in sexually active adolescents. METHODS: A total of 467 subjects were recruited among 12-18-year-old girls who received pelvic examinations at two urban adolescent clinics. All girls were tested by wet-mount and InPouch TV. In addition 339 of 467 had cultures in Diamond's medium and 366 of 467 had Pap smears. Specimens were collected for InPouch TV and Diamond's cultures and read at 24-48 h and 5 days, and in the case of Diamond's cultures, also at 7 days. In a subset of subjects (268 of 467) who had all four tests done, sensitivities and specificities were calculated using Diamond's culture as the "gold standard." RESULTS: In the 467 subjects, 73 (15.6%) tested positive for trichomonas by at least one method. In the subset with all four tests done, sensitivities of the wet-mount and InPouch TV were 36% and 81%, respectively; while that of the Pap smear was 56%. The culture media were equally efficient in identifying Trichomonas vaginalis. There were no differences found between subjects with or without trichomonas infections in gynecological symptoms, previous history of sexually transmitted diseases, or use of a condom at last intercourse. CONCLUSIONS: InPouch TV culture is a good diagnostic method for T. vaginalis because of its long shelf-life, relatively low expense, and high sensitivity (over twice as sensitive as wet-mount).

Intracellular pH regulation in cultured microglial cells from mouse brain.

Intracellular pH (pHi) and the mechanisms of pHi regulation have been investigated in cultured microglial cells from mouse brain using the pH-sensitive fluorescent dye 2',7'-bis-(2-carboxyethyl)-5-(6)-carboxyfluorescein (BCECF). Cells were acidified by a pulse of NH4+ (4-5 min; 20 mM) and the subsequent pHi recovery from an acidification was studied. In HCO3(-)-free saline, pH regulation was dependent on extracellular [Na+] and sensitive to amiloride, indicating the involvement of the Na+/H+ exchanger. In HCO3(-)-containing solution 2 mM amiloride slowed but did not block pHi recovery; the recovery however was dependent on extracellular [Na+] and sensitive to 0.3 mM DIDS, suggesting the presence of Na+/HCO3 cotransporter and/or Na(+)-dependent Cl-/HCO3-exchanger. The involvement of a Na-dependent Cl-/HCO3-exchanger was inferred from the observation that removal of Cl- or application of 1 mM furosemide decreased but did not block the recovery rate. Increasing [K+]0 resulted in an alkalinization by a process that was neither HCO3- nor Na(+)-dependent, nor DIDS- and amiloride-inhibitable. In conclusion, microglial cells express a distinct set of pH regulatory carriers which control for a defined level of pHi. An increase in [K+]0 can offset this level.

A fast perfusion system for single cell physiology optimized for microscopes with water immersion objectives.

A perfusion system was constructed which allows the fast application of different solutes underneath a water immersion objective. The perfusion system is mounted into the immersion objective by milling a slot into the frontal metal plate of the lens holder. It consists of a five-channel pipette fixed to the objective and solution reservoirs gated by computer controlled magnetic valves. Up to five different solutions can be applied to the specimen under study. The solution between objective and specimen is completely exchanged after 1-2 s as determined from fluorescence measurements. This arrangement is optimized for [Ca2+] measurements with a fluorescence measurement system in tissue slices, where upright microscopes are required. It offers the advantage of saving a micromanipulator for the perfusion pipette and facilitates a fast, reproducible and precise positioning of the perfusion system.

Extracellular ATP-induced currents in astrocytes: involvement of a cation channel.

Whole-cell currents were measured with the perforated patch clamp technique in cultured rat astrocytes to analyze the underlying ionic mechanism for a P2-purinoceptor-mediated depolarization. ATP (100 microM) induced an inward current with a mean amplitude of 130 pA and an EC50 of 17 microM. The response desensitized during a 1 min application. Replacement of extracellular Na+ with NMDG or K+ abolished the ATP-evoked inward current. Replacement of Na+ with choline, however, resulted in an ATP-evoked response of one-third the amplitude in normal solution. This is indicative of a cation rather than Na+ channel. However, due to difficulties in voltage-clamping these gap junction-coupled cells at voltages different from the membrane resting potential, the current reversal potential could not be determined. Measurements with K(+)-sensitive microelectrodes showed that 100 microM ATP lowered the intracellular K+ concentration. Replacement of extracellular Ca2+ or Cl- did not alter the ATP-induced inward currents. Fura-2 imaging experiments revealed a transient rise of the intracellular Ca2+ concentration during ATP application. Removal of extracellular Ca2+ did not influence the peak response; it did, however, shorten the time course. These results and previous observations that the permeability changes are caused by a P2x receptor are indicative of an ATP-sensitive cation conductance. In addition, cytoplasmic Ca2+ is increased by mobilization from intracellular stores, and by additional influx across the cell membrane. Extracellular ATP released by neurons could evoke K+ release from astrocytes as well as be a mediator for cation changes that signal cell activation processes when released by damaged cells.

Adult rat optic nerve oligodendrocyte progenitor cells express a distinct repertoire of voltage- and ligand-gated ion channels.

Cultured oligodendrocyte progenitor cells derived from the developing central nervous system (CNS) express a pattern of ion channels that is distinct from mature oligodendrocytes and other cell types of the CNS. In the present study, we used the whole-cell patch-clamp technique and the fura-2-based Ca++ imaging system to study the ion channel expression of oligodendrocyte progenitor cells derived from the optic nerves of adult rats. We found that the adult oligodendrocyte progenitor cell membrane is dominated by K+ currents, both delayed outward and inward rectifying. The inwardly rectifying K+ currents were often as large as the outward delayed rectifying K+ currents. The delayed rectifying outward currents were partially blocked by 50 mM tetraethylammonium or 1 mM 4-aminopyridine, but not by 2 or 5 mM BaCl2. This suggests that the delayed rectifier channels expressed by adult progenitor cells are different from those expressed by perinatal cells. Most adult oligodendrocyte progenitor cells showed no or only small A-type K+ currents. Both Ca++ and Na+ channels were also detected in these cells. Furthermore, adult progenitor cells responded to the neurotransmitters GABA and kainate and the pharmacology of these responses indicated that these cells express GABAA receptors and kainate receptors that are Ca(++)-permeable. Our study suggests that adult oligodendrocyte progenitor cells are electrophysiologically distinct and that these cells share electrophysiological characteristics with both perinatal progenitor cells and immature oligodendrocytes.

Regionally distinct regulation of astroglial neurotransmitter receptors by fibroblast growth factor-2.

Fibroblast growth factor (FGF)-2 is an abundant astroglial cytokine. We have previously shown that FGF-2 downregulates gap junctions in primary astroglial cultures (B. Reuss et al., 1998, Glia 22, 19-30). We demonstrate now that FGF-2 induces astroglial dopamine (DA) sensitivity and D1 dopamine-receptor (D1DR) antigen and message in cortical and striatal astroglial cultures. On the functional level 10 micromol/L DA triggered transient increases in astroglial [Ca(2+)](i). In gap-junction-coupled cells, no FGF-2-dependent changes in proportions of DA-responsive cells were observable. However, uncoupling with octanol or 18alpha-glycirrhetinic acid isolated the smaller population of astrocytes intrinsically sensitive to DA which was significantly increased by FGF-2 in cortical and striatal cultures. Administration of DR-specific substances revealed that FGF-2 upregulated D1DR. These results indicate that downregulation of astroglial gap junctions by FGF-2 is accompanied by an upregulation of D1DR and DA sensitivity, adding a new aspect to the role of FGF-2 in the regulation of brain functions.

Analysis of motile oligodendrocyte precursor cells in vitro and in brain slices.

Oligodendrocyte precursor cells are purported to migrate over long distances into the various brain regions where they differentiate into oligodendrocytes and fulfill their appropriate tasks, i.e., myelination of axons. Here we characterize motile oligodendrocyte precursor cells in detail. Video-time lapse analysis was performed on isolated precursor cells in single cell cultures, in co-culture with cerebellar microexplants, and in living brain slices. Motility analysis of individual cells was combined with electrophysiological, immunological, and morphological characterizations. Translocation of the cell bodies was not continuous but occurred in waves. All motile cells exhibited a simple morphology and most, but not all, of them expressed the A2B5 epitope in vitro. Patch clamp analysis of the motile cells confirmed that they belong to the O-2A lineage. The percentage of motile cells, as well as their velocities, were enhanced on substrate-coated laminin in comparison to poly-L-lysine. Motility was not influenced by the presence of cerebellar microexplants. O-2A progenitor cells did not migrate strictly along neurite fascicles which were projected from the microexplants. Glial progenitor cells in situ also did not strictly migrate along the main direction of the axonal fibers of the corpus callosum but rather traversed the fibers with an overall direction toward the cortex. After Lucifer Yellow filling of the motile progenitor cells in situ, we could demonstrate that they were dye-coupled to yet unidentified cells of the corpus callosum.

Extracellular ATP activates a cation conductance and a K+ conductance in cultured microglial cells from mouse brain.

Microglial cells have important functions during regenerative processes after brain injury. It is well established that they rapidly respond to damage to the brain tissue. Stages of activation are associated with changes of cellular properties such as proliferation rate or expression of surface antigens. Yet, nothing is known about signal substances leading to the rapid changes of membrane properties, which may be required to initiate the transition from one cell stage into another. From our present study, using the patch-clamp technique, we report that cultured microglial cells obtained from mouse or rat brain respond to extracellularly applied ATP with the activation of a cation conductance. Additionally, in the majority of cells an outwardly directed K+ conductance was activated with some delay. Since ADP, AMP, and adenosine (in descending order) were less potent or ineffective in inducing the cation conductance, the involvement of a P2 purinergic receptor is proposed. The receptor activation is accompanied by an increase of cytosolic Ca2+ as determined by a fura-2-based Ca(2+)-imaging system. This ATP receptor could enable microglial cells to respond to transmitter release from nerve endings with ATP as a transmitter or cotransmitter or to the death of cells with resulting leakage of ATP.

Purification and analysis of in vivo-differentiated oligodendrocytes expressing the green fluorescent protein.

A complete understanding of the molecular mechanisms involved in the formation and repair of the central nervous system myelin sheath requires an unambiguous identification and isolation of in vivo-differentiated myelin-forming cells. In order to develop a novel tool for the analysis of in vivo-differentiated oligodendrocytes, we generated transgenic mice expressing a red-shifted variant of the green fluorescent protein under the control of the proteolipid protein promoter. We demonstrate here that green fluorescent protein-derived fluorescence in the central nervous system of 9-day- to 7-week-old mice is restricted to mature oligodendrocytes, as determined by its spatiotemporal appearance and by both immunocytochemical and electrophysiological criteria. Green fluorescent protein-positive oligodendrocytes could easily be visualized in live and fixed tissue. Furthermore, we show that this convenient and reliable identification now allows detailed physiological analyses of differentiated oligodendrocytes in situ. In addition, we developed a novel tissue culture system for in vivo-differentiated oligodendrocytes. Initial data using this system indicate that, for oligodendrocytes isolated after differentiation in vivo, as yet unidentified factors secreted by astrocytes are necessary for survival and/or reappearance of a mature phenotype in culture.

GFAP promoter-controlled EGFP-expressing transgenic mice: a tool to visualize astrocytes and astrogliosis in living brain tissue.

We have generated transgenic mice in which astrocytes are labeled by the enhanced green fluorescent protein (EGFP) under the control of the human glial fibrillary acidic protein (GFAP) promoter. In all regions of the CNS, such as cortex, cerebellum, striatum, corpus callosum, hippocampus, retina, and spinal cord, EGFP-positive cells with morphological properties of astrocytes could be readily visualized by direct fluorescence microscopy in living brain slices or whole mounts. Also in the PNS, nonmyelinating Schwann cells from the sciatic nerve could be identified by their bright green fluorescence. Highest EGFP expression was found in the cerebellum. Already in acutely prepared whole brain, the cerebellum appeared green-yellowish under normal daylight. Colabeling with GFAP antibodies revealed an overlap with EGFP in the majority of cells. Some brain areas, however, such as retina or hypothalamus, showed only low levels of EGFP expression, although the astrocytes were rich in GFAP. In contrast, some areas that were poor in immunoreactive GFAP were conspicuous for their EGFP expression. Applying the patch clamp technique in brain slices, EGFP-positive cells exhibited two types of membrane properties, a passive membrane conductance as described for astrocytes and voltage-gated channels as described for glial precursor cells. Electron microscopical investigation of ultrastructural properties revealed EGFP-positive cells enwrapping synapses by their fine membrane processes. EGFP-positive cells were negative for oligodendrocyte (MAG) and neuronal markers (NeuN). As response to injury, i.e., by cortical stab wounds, enhanced levels of EGFP expression delineated the lesion site and could thus be used as a live marker for pathology.