Inositol phospholipid hydrolysis in cultured astrocytes and oligodendrocytes

Systematic Review of Pharmacological Properties of the Oligodendrocyte Lineage

Oligodendrogenesis and oligodendrocyte precursor maturation are essential processes during the course of central nervous system development, and lead to the myelination of axons. Cells of the oligodendrocyte lineage are generated in the germinal zone from migratory bipolar oligodendrocyte precursor cells (OPCs), and acquire cell surface markers as they mature and respond specifically to factors which regulate proliferation, migration, differentiation, and survival. Loss of myelin underlies a wide range of neurological disorders, some of an autoimmune nature—multiple sclerosis probably being the most prominent. Current therapies are based on the use of immunomodulatory agents which are likely to promote myelin repair (remyelination) indirectly by subverting the inflammatory response, aspects of which impair the differentiation of OPCs. Cells of the oligodendrocyte lineage express and are capable of responding to a diverse array of ligand-receptor pairs, including neurotransmitters and nuclear receptors such as γ-aminobutyric acid, glutamate, adenosine triphosphate, serotonin, acetylcholine, nitric oxide, opioids, prostaglandins, prolactin, and cannabinoids. The intent of this review is to provide the reader with a synopsis of our present state of knowledge concerning the pharmacological properties of the oligodendrocyte lineage, with particular attention to these receptor-ligand (i.e., neurotransmitters and nuclear receptor) interactions that can influence oligodendrocyte migration, proliferation, differentiation, and myelination, and an appraisal of their therapeutic potential. For example, many promising mediators work through Ca²⁺ signaling, and the balance between Ca²⁺ influx and efflux can determine the temporal and spatial properties of oligodendrocytes (OLs). Moreover, Ca²⁺ signaling in OPCs can influence not only differentiation and myelination, but also process extension and migration, as well as cell death in mature mouse OLs. There is also evidence that oligodendroglia exhibit Ca²⁺ transients in response to electrical activity of axons for activity-dependent myelination. Cholinergic antagonists, as well as endocannabinoid-related lipid-signaling molecules target OLs. An understanding of such pharmacological pathways may thus lay the foundation to allow its leverage for therapeutic benefit in diseases of demyelination.

Muscarinic receptor subtypes as potential targets to modulate oligodendrocyte progenitor survival, proliferation, and differentiation

Acetylcholine (ACh) is a major neurotransmitter but also an important signaling molecule in neuron-glia interactions. Expression of ACh receptors has been reported in several glial cell populations, including oligodendrocytes (OLs). Nonetheless, the characterization of muscarinic receptors in these cells, as well as the description of the cholinergic effects at different stages of OL development, is still incomplete. In this study, we characterized the pattern of expression of muscarinic receptor subtypes in primary cultures of rat oligodendrocyte progenitor cells (OPC) and mature OLs, at both mRNA and protein levels. We found that muscarinic receptor expression is developmentally regulated. M1, M3, and M4 receptors were the main subtypes expressed in OPC, whereas all receptor subtypes were expressed at low levels in mature OLs. Exposure of OPC to muscarine enhanced cell proliferation, an effect mainly due to M1, M3, and M4 receptor subtypes as demonstrated by pharmacological competition with selective antagonists. Conversely, M2 receptor activation impaired OPC survival. In line with the mitogenic activity, muscarinic receptor activation increased the expression of platelet derived growth factor receptor α. Muscarine stimulation increased CX32 and myelin basic protein expression, left unaffected that of myelin proteolipid protein (PLP), and decreased member of the family of epidermal growth factor receptor (EGFR) ErbB3/ErbB4 receptor expression indicating a predominant role of muscarinic receptors in OPC. These findings suggest that ACh may contribute to the maintenance of an immature proliferating progenitor pool and impair the progression toward mature stage. This hypothesis is further supported by increased expression of Notch-1 in OL on muscarinic activation.

Bradykinin-induced microglial migration mediated by B1-bradykinin receptors depends on Ca2+ influx via reverse-mode activity of the Na+/Ca2+ exchanger

Bradykinin (BK) is produced and acts at the site of injury and inflammation. In the CNS, migration of microglia toward the lesion site plays an important role pathologically. In the present study, we investigated the effect of BK on microglial migration. Increased motility of cultured microglia was mimicked by B1 receptor agonists and markedly inhibited by a B1 antagonist but not by a B2 receptor antagonist. BK induced chemotaxis in microglia isolated from wild-type and B2-knock-out mice but not from B1-knock-out mice. BK-induced motility was not blocked by pertussis toxin but was blocked by chelating intracellular Ca2+ or by low extracellular Ca2+, implying that Ca2+ influx is prerequisite. Blocking the reverse mode of Na+/Ca2+ exchanger (NCX) completely inhibited BK-induced migration. The involvement of NCX was further confirmed by using NCX+/- mice; B1-agonist-induced motility and chemotaxis was decreased compared with that in NCX+/+ mice. Activation of NCX seemed to be dependent on protein kinase C and phosphoinositide 3-kinase, and resultant activation of intermediate-conductance (IK-type) Ca2+-dependent K+ currents (I(K(Ca))) was activated. Despite these effects, BK did not activate microglia, as judged from OX6 staining. Using in vivo lesion models and pharmacological injection to the brain, it was shown that microglial accumulation around the lesion was also dependent on B1 receptors and I(K(Ca)). These observations support the view that BK functions as a chemoattractant by using the distinct signal pathways in the brain and, thus, attracts microglia to the lesion site in vivo.

Endogenous alpha2-adrenergic receptor-mediated neuroprotection after severe hypoxia in preterm fetal sheep

Central alpha-adrenergic receptor activity is important for fetal adaptation to hypoxia before birth. It is unclear whether it is also important during recovery. We therefore tested the hypothesis that an infusion of the specific alpha(2)-adrenergic receptor antagonist idazoxan (1 mg/kg/h i.v.) from 15 min to 4 h after profound hypoxia induced by 25 min umbilical cord occlusion in fetal sheep at 70% of gestation (equivalent to the 28-32 weeks in humans) would increase neural injury. After 3 days' recovery, idazoxan infusion was associated with a significant increase in neuronal loss in the hippocampus (P<0.05), expression of cleaved caspase-3 (P<0.05), and numbers of activated microglia (P<0.05). There was no significant effect on other neuronal regions or on loss of O4-positive premyelinating oligodendrocytes in the subcortical white matter. Idazoxan was associated with an increase in evolving epileptiform electroencephalographic (EEG) transient activity after occlusion (difference at peak 2.5+/-1.0 vs. 11.7+/-4.7 counts/min, P<0.05) and significantly reduced average spectral edge frequency, but not EEG intensity, from 54 until 72 h after occlusion (P<0.05). Hippocampal neuronal loss was correlated with total numbers of epileptiform transients during idazoxan infusion (P<0.01; r(2)=0.7). In conclusion, endogenous inhibitory alpha(2)-adrenergic receptor activation after severe hypoxia appears to significantly limit evolving hippocampal damage in the immature brain.

Anti-inflammatory effects of kinins via microglia in the central nervous system

Kinins are important biologically active peptides that are up-regulated after lesions in both the peripheral and central (CNS) nervous systems. Microglia are immune cells in the CNS and play an important role in the defense of the neuronal parenchyma. In cultured murine microglia, bradykinin (BK) induces mobilization of intracellular Ca2+, microglial migration, and increases the release of nitric oxide and prostaglandin E2. On the other hand, BK attenuates lipopolysaccharide-activated TNF-α and IL-1β release. These results suggest that BK functions as a signal in brain trauma and may have an anti-inflammatory role in the CNS.

Kinin receptors in cultured rat microglia

Kinins are produced and act at the site of injury and inflammation in various tissues. They are likely to initiate a particular cascade of inflammatory events, which evokes physiological and pathophysiological responses including an increase in blood flow and plasma leakage. In the central nervous system (CNS), kinins are potent stimulators of the production and release of pro-inflammatory mediators represented by prostanoids and cytotoxins. They are known to induce neural tissue damage. Many of the cytotoxins such as cytokines and free radicals and prostanoids are released from glial cells. Among glial cells, astrocytes and oligodendrocytes are known to possess bradykinin (BK) B(2) receptors that phosphoinositide (PI) turnover and raise intracellular Ca(2+) concentration. The presence of bradykinin receptors in microglia has been of great significance. We recently showed that rat primary microglia express kinin receptors. In resting microglia, B(2) receptors but not B(1) receptors are expressed. When the microglia are activated by bradykinin, B(1) receptors are up-regulated, while B(2) receptors are down-regulated. As observed in other glial cells, electrophysiological measurements suggest that B(2) receptors in phosphoinositide turnover and intracellular Ca(2+) concentration in microglia. Release of cytotoxins is likely consequent upon the activation of BK receptors. Our study provides the first evidence that microglia express functional kinin receptors and suggests that microglia play an important role in CNS inflammatory responses.

Regulation of muscarinic receptor function in developing oligodendrocytes by agonist exposure

1 Oligodendrocytes, the myelin forming cells in the CNS, express muscarinic acetylcholine receptors (mAChR), primarily M3, coupled to various signal transduction pathways. 2 In the present study we have investigated whether mAChR undergo functional agonist-induced regulation in cultured oligodendrocyte progenitors and differentiated oligodendrocytes. 3 The muscarinic agonist, carbachol (CCh) caused a time-dependent desensitization of phosphoinositide (PI) hydrolysis, and the internalization and down-regulation of receptors. Short-time desensitization (5 min) of PI hydrolysis occurred without receptor internalization and reached 54% by 1 h. The same treatment decreased cell surface receptors labelled with the non-permeable ligand [(3)H]-NMS by 47%, while total receptor density ([(3)H]-scopolamine binding) decreased by 30%. Longer CCh treatment down-regulated receptors by 70% and desensitized the PI response by 80%. 4 Although protein kinase C (PKC) activation desensitized mAChR, CCh-mediated desensitization was independent of PKC. 5 Inhibition of receptor endocytosis by low temperature during the pre-stimulation period or in the presence of hyperosmotic sucrose (0.5 M) blocked desensitization, receptor internalization and down-regulation. 6 Recovery of surface mAChR and their functional activity following down-regulation was slow, returning to control levels by 24 h after agonist removal. In progenitor cells, dose-response curves for CCh-mediated PI hydrolysis and c-fos mRNA expression showed that newly synthesized mAChR were supersensitive after recovery. 7 Overall, the present results provide evidence of functional agonist-mediated mAChR regulation in brain oligodendroglial cells.

Pharmacological and functional characterization of muscarinic receptor subtypes in developing oligodendrocytes

This study focused on the molecular and pharmacological characterization of muscarinic acetylcholine receptors expressed by progenitors and differentiated oligodendrocytes. We also analyzed the role of muscarinic receptors in regulating downstream signal transduction pathways and the functional significance of receptor expression in oligodendrocytes. RT-PCR analysis revealed the expression of transcripts for M3, and to a lesser extent M4, followed by M1, M2 and M5 receptor subtypes in both progenitors and differentiated oligodendrocytes. Competition binding experiments using [(3)H]N-methylscopolamine and several antagonists, as well as inhibition of carbachol-mediated phosphoinositide hydrolysis, showed that M3 is the main subtype expressed in these cells. In progenitors the activation of p42/44-mitogen-activated protein kinase (MAPK) and cAMP-response element binding protein (CREB) as well as c-fos mRNA expression were blocked by the M3 relatively selective antagonist, 4-DAMP, and its irreversible analogue, 4-DAMP-mustard. Carbachol increased proliferation of progenitors, an effect prevented by atropine and 4-DAMP, as well as by the MAPK kinase inhibitor PD98059. These results indicate that carbachol modulates oligodendrocyte progenitor proliferation through M3 receptors, involving activation of a MAPK signaling pathway. Receptor density and phosphoinositide hydrolysis are down-regulated during oligodendrocyte differentiation. Functional consequences of these events are a reduction in carbachol-stimulated p42/44(MAPK) and CREB phosphorylation, as well as induction of c-fos.

Chronic treatment with lithium and pretreatment with excess inositol reduce inositol pool size in astrocytes by different mechanisms

Chronic treatment with a lithium salt is the classical treatment for manic-depressive disorder. It is hypothesized that the therapeutic action of lithium is caused by its inhibition of inositol phosphatases which leads to a relative deficiency of inositol and, therefore, an impairment of inositol recycling and production of precursor for the second messengers inositol triphosphate (IP3) and diacylglycerol (DAG). However, peculiarly enough, treatment with high doses of inositol also has an antidepressant effect. In the present work, we have studied the acute and chronic effects of lithium and of excess inositol, in separation or together, on accumulation of 50 microM [3H]inositol (a physiologically relevant concentration) into primary cultures of mouse astrocytes. Two parameters were investigated: (1) rate of unidirectional uptake across the cell membrane (measured during short-term exposure to the radioisotope), and (2) magnitude of the intracellular pool of inositol, equilibrating with extracellular inositol (measured during long-term exposure to the radioisotope). Inositol uptake was highly concentrative and occurred with a Km of approximately 500 microM and a Vmax of 1.5 nmol/min/mg protein. The uptake rate was not affected by either acute or chronic treatment with LiCl (or both), but it was substantially reduced ('down-regulated') after pretreatment with a high concentration of inositol. The inositol pool size was decreased to a similar extent as the uptake rate by previous exposure to excess inositol. In spite of the fact that inositol uptake rate was unaffected by lithium, the magnitude of the inositol pool was significantly decreased by chronic treatment with a pharmacologically relevant concentration of LiCl (1 mM), but not by treatment with lower concentrations. This decrease is likely to reflect a reduction in either inositol synthesis or replenishment of inositol from IP3, due to the inhibition of inositol phosphatases by the lithium ion. In agreement with the different mechanisms by which lithium and pretreatment with excess inositol appear to reduce the pool size of inositol, the effects of pretreatment with excess inositol and of LiCl were additive. It is noteworthy that both effects could be observed in astrocytes, suggesting that there might be a significant astrocytic target during clinical treatment.

ATP stimulates release of excitatory amino acids from cultured Schwann cells

The release of excitatory amino acids from Schwann cell cultures in the rat was monitored using high-performance liquid chromatography. The basal concentration of glutamate and aspartate was 33 +/- 4 nM (mean +/- S.E.M., n = 12) and 8 +/- 1 nM (mean +/- S.E.M., n = 12), respectively. ATP (100 microM) caused a receptor-mediated increase in release of glutamate and aspartate from Schwann cell cultures. Bath application of adenosine (100 microM) was without effect on release of excitatory amino acids suggesting involvement of P2 receptors. Suramin, a competitive antagonist at P2 receptors, prevented the response to ATP. The release of excitatory amino acids evoked by ATP was not abolished in calcium-depleted saline. Pretreatment of the Schwann cultures with 50 microM 1,2-bis(2-aminophenoxy)ethane-N,N,N'N'-tetracetic acid-acetoxymethyl ester (BAPTA-AM) abolished the effect of ATP. ATP-evoked release of glutamate from cultured Schwann cells was significantly reduced by thapsigargin (1 microM), an inhibitor of Ca(2+)-ATPase of the Ca2+ pump of internal stores. U73122, a selective inhibitor of receptor-coupled phospholipase C-dependent processes, abolished stimulatory effect of ATP suggesting that ATP's action is mediated through an inositol 1,4,5-triphosphate-sensitive calcium store. The action of ATP was not blocked by L-trans-pyrrolidine-2,4-dicarboxylate, an inhibitor of the electrogenic glutamate transporter, nor was it blocked in Na(+)-free medium, and glutamate release was not stimulated by a depolarizing stimulus, suggesting that ATP-evoked release of glutamate from Schwann cells is not due to the reversal of the glutamate uptake. An anion transport blocker, furosemide, reduced ATP-induced glutamate release. These results suggest that ATP-stimulated glutamate and aspartate release from Schwann cells may be through a calcium-dependent furosemide-sensitive mechanism.

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.

Acetylcholine agonists stimulate mitogen-activated protein kinase in oligodendrocyte progenitors by muscarinic receptors

Oligodendrocytes, the myelin-producing cells of the central nervous system, express muscarinic acetylcholine receptors (mAChR). Activation of this neurotransmitter receptor by the stable acetylcholine analog carbachol (CCh) triggers transducing events, modulating c-fos expression and cellular proliferation. To elucidate the signal transduction pathways involved in the transmission of these cellular events, we examined the ability of CCh to activate mitogen-activated protein kinase (MAPK) in primary cultures of oligodendrocyte progenitors prepared from newborn rat brain. CCh produced a concentration- and time-dependent increase in MAPK activity (predominantly the p42mapk or ERK2) as determined by in-gel MBP kinase assays. Using the non-selective muscarinic antagonist atropine we determined that MAPK-activation by CCH is mediated by muscarinic receptors. In the presence of PD098059, a specific inhibitor of MAPK kinase (MEK), MAPK activity was blocked. Similarly, the presence of extracellular calcium was required for CCh-mediated MAPK activation. To further elucidate the mechanisms involved in MAPK activation by CCh, the role of PKC was studied. In cells in which protein kinase had been downregulated by chronic treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA), the effect of carbachol on MAPK activation was maintained. In contrast, the response to CCh was blocked by the PKC inhibitors H7 and bisindolylmaleimide GF109203X. Our results suggest that MAPK is implicated in the transmission of the signal for mACh receptors and involves a TPA-insensitive PKC pathway. Further work is required to define the upstream and downstream events which result in CCh-mediated MAPK activation and proliferation of oligodendrocyte progenitors.

Effects of maturation on the phospholipid and phospholipid fatty acid compositions in primary rat cortical astrocyte cell cultures

Phospholipid and phospholipid fatty acid compositional changes were studied in rat cortical astrocytes during dibutyryl cyclic adenosine monophosphate (dBcAMP, 0.25 mM) treatment starting after 14 days in culture (DIC). After 15 DIC, ethanolamine- and choline glycerophospholipid levels were increased 1.2- and 1.3-fold, respectively in treated compared to control cells. However, after 21 and 28 DIC, these levels were not significantly different between groups. Both groups had an increase in phosphatidylserine levels with increasing time in culture. Similarly, ethanolamine plasmalogen levels were transiently elevated after 21 DIC, but returned to previous levels after 28 DIC. The phospholipid fatty acid compositions for the acid stable and labile ethanolamine- and choline glycerophospholipids indicated that in dBcAMP treated cells, 20:4 n-6 and 22:6 n-3 proportions were elevated with increasing time in culture relative to control cells. As 20:4 n-6 proportions increased, there was a concomitant decrease in 20:3 n-9 proportions, suggesting an up regulation of n-6 series elongation and desaturation. In contrast, in control cells, the 20:4 n-6 proportions decreased with a corresponding increase in the 20:3 n-9 proportions. Thus, in treated cells, the cellular phospholipid fatty acid composition was dramatically different than control cells, suggesting that dBcAMP treatment may act to increase fatty acid elongation and desaturation.

Carbachol stimulates c-fos expression and proliferation in oligodendrocyte progenitors

To determine if muscarinic receptor-activation plays a role in oligodendrocyte development, the effect of carbachol a stable acetylcholine analog, on gene expression and proliferation was investigated. Using Northern blot analysis we showed that carbachol caused a time and concentration-dependent increase in c-fos mRNA. This effect was blocked by atropine, a non-selective muscarinic antagonist. In addition, the muscarinic-stimulated c-fos increase was inhibited by 1-(5-isoquinoline-sulfonyl)-2-methylpiperazine (H-7), a potent inhibitor of protein kinase C (PKC), but not by N-2-(p-bromocinnamylamino)-ethyl-5-isoquinoline-sulfonamide (H-89), a potent inhibitor of protein kinase A, suggesting the involvement of PKC in mediating the response. Down-regulation of PKC by overnight pre-treatment with 12-O-tetradecanoylphorbol 13-acetate (TPA) blocked only the phorbol ester-stimulated c-fos accumulation while no effect was observed in the carbachol-induced response. These results suggested that carbachol stimulated an H-7 sensitive PKC pathway which may be different than that activated by TPA. Further evidence for two separate mechanisms of proto-oncogene induction was provided by the additive effect of carbachol and TPA. Induction of c-fos mRNA by carbachol was dependent on both influx of extracellular Ca2+ and release from intracellular stores, as both EDTA and BAPTA blocked the response. Since activation of muscarinic receptors can affect cell division in other cellular systems, the effect of carbachol on [3H]thymidine and bromodeoxyuridine incorporation into oligodendrocyte DNA was measured. Carbachol stimulated DNA synthesis in oligodendrocyte progenitors. This effect was mediated by muscarinic receptors as [3H]thymidine incorporation was prevented or significantly reduced by the addition of atropine. In conclusion, the present findings suggest that, the neurotransmitter, acetylcholine may act as a trophic factor in developing oligodendrocytes, regulating their growth and development in the central nervous system.

Mitochondria support inositol 1,4,5-trisphosphate-mediated Ca2+ waves in cultured oligodendrocytes

We have examined the spatial and temporal nature of Ca2+ signals activated via the phosphoinositide pathway in oligodendrocytes and the cellular specializations underlying oligodendrocyte Ca2+ response characteristics. Cultured cortical oligodendrocytes were incubated with fluo 3 or fura 2, and digital video fluorescence microscopy was used to study the effect of methacholine on [Ca2+]i. Single peaks, oscillations, and steady-state plateau [Ca2+]i elevations were evoked by increasing agonist concentration. The peaks and oscillations were found to be Ca2+ wave fronts, which propagate via distinct amplification regions in the cell where the kinetics of Ca2+ release (amplitude and rate of rise of response) are elevated. Staining with 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolecarbocyanine++ + iodide (JC-1) and 3,3'-dihexyloxacarbocyanine iodide revealed that mitochondria are found in groups of three or more in oligodendrocyte processes and that the groups are distributed with considerable distance separating them. Cross-correlation analysis showed a high degree of correlation between sites where mitochondria are present and peaks in the amplitude and rate of rise of the Ca2+ response. Intramitochondrial Ca2+ concentration, measured using rhod 2, increased upon treatment with methacholine. Methacholine also evoked a rapid change in mitochondrial membrane potential as measured by the J-aggregate fluorescence of JC-1. Pretreatment with the mitochondrial inhibitors carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (1 microM, 2 min) or antimycin (2 microg/ml, 2 min) altered the methacholine-evoked Ca2+ response in most cells studied, responses being either markedly potentiated or inhibited. The results of this study demonstrate that stimulation of phosphoinositide-coupled muscarinic acetylcholinoceptors activates propagating Ca2+ wave fronts in oligodendrocytes and that the characteristics of these waves are dependent on mitochondrial location and function.

Calcium signaling in cultured rat oligodendrocytes

The syntax of neuronal-glial or axonal-glial interaction is frequently communicated through transient changes in internal calcium (Cai). We examined mechanisms for Cai signaling and intercellular propagation of Cai responses in cultured oligodendrocytes (OLGs) derived from adult spinal cord (SC), postnatal day 21 (P21) SC, and P21 brain. We found that (1) cultured OLGs exhibited a heterogeneous response to norepinephrine, carbachol, ATP, histamine, and glutamate; (2) receptor-mediated Cai increases were derived from both Ca2+ influx and intracellular Ca2+ release; (3) the percentage of responders to neuroligands varied as a function of cell origin; (4) cultured OLGs exhibited a thapsigargin-sensitive, but not a caffeine-sensitive, intracellular Ca2+ pool; and (5) gap junctional contacts between OLGs permitted limited intercellular propagation of mechanically stimulated Cai responses. Receptor-mediated Cai signaling appears to occur not only in cultured OLGs but also in acutely dissociated OLGs. The heterogeneity in Cai responses as a function of cell origin may reflect the existence of OLG subsets or differences in the maturation stage of OLGs.

Receptors on astrocytes--what possible functions?

Receptors for transmitters, as varied as those expressed by neurons, have been described on primary astrocyte cultures prepared from new-born rats and mice. A variety of functional effects and considerable cell-to-cell and regional heterogeneity have been observed for such receptors in vitro. The various systems available for studying the presence and properties of receptors on astrocytes in situ, and the results from these studies, are discussed. Much fewer studies using these more difficult systems have been done. So far, some resemblances and differences between in situ and in vitro work have been observed. More of these in situ studies, to supplement the ongoing in vitro work, are needed to enable us to determine unequivocally which receptors are present on astrocytes, and their functions in vivo. If there is cell-to-cell and CNS regional heterogeneity in vivo comparable to that seen in vitro, these analyses will be very complex. To illustrate the importance and variety of receptor-linked functions, a number of suggestions are made in this commentary, based on current proposals for the roles of astrocytes. However, it is argued that we need to have a more complete understanding of astrocyte functions in vivo, before we can really understand the functional significance of astrocyte receptors.

Kinins and kinin receptors in the nervous system

Kinins, including bradykinin and kallidin, are peptides that are produced and act at the site of tissue injury or inflammation. They induce a variety of effects via the activation of specific B1 or B2 receptors that are coupled to a number of biochemical transduction mechanisms. In the periphery the actions of kinins include vasodilatation, increased vascular permeability and the stimulation of immune cells and peptide-containing sensory neurones to induce pain and a number of neuropeptide-induced reflexes. Mechanisms for kinin synthesis are also present in the CNS where kinins are likely to initiate a similar cascade of events, including an increase in blood flow and plasma leakage. Kinins are potent stimulators of neural and neuroglial tissues to induce the synthesis and release of other pro-inflammatory mediators such as prostanoids and cytotoxins (cytokines, free radicals, nitric oxide). These events lead to neural tissue damage as well as long lasting disturbances in blood-brain barrier function. Animal models for CNS trauma and ischaemia show that increases in kinin activity can be reversed either by kinin receptor antagonists or by the inhibition of kinin production. A number of other central actions have been attributed to kinins including an effect on pain signalling, both within the brain (which may be related to vascular headache) and within the spinal dorsal horn where primary afferent nociceptors can be stimulated. Kinins also appear to play a role in cardiovascular regulation especially during chronic spontaneous hypertension. Presently, however, direct evidence is lacking for the release of kinins in pathophysiological conditions of the CNS and it is not known whether spinal or central neurones, other than afferent nerve terminals, are sensitive to kinins. A more detailed examination of the effects of kinins and their central pharmacology is necessary. It is also important to determine whether the inhibition of kinin activity will alleviate CNS inflammation and whether kinin receptor antagonists are useful in pathological conditions of the CNS.

Developmental expression of protein kinase C isozymes in oligodendrocytes and their differential modulation by 4 beta-phorbol-12,13-dibutyrate

Myelin gene expression in normal oligodendrocytes (OLG) depends on developmentally regulated protein kinase C (PKC) enzyme activity (Asotra and Macklin: J Neurosci Res 34:571-588, 1993). We studied the developmental expression of the Ca(++)-dependent PKC-alpha, -beta 1, -beta II and -gamma isozymes, and the Ca(++)-independent PKC-delta, -epsilon, -zeta and -eta isozymes in enriched rat brain OLG cultures. In A2B5+ O-2A progenitors, only PKC-delta, PKC-epsilon and PKC-zeta were detected immunocytochemically. In 04+ proligondendrocytes, PKC-beta I, -delta and -zeta were expressed moderately and low levels of PKC-alpha and -epsilon were detected. GD3+ OLG, GC+ OLG and MBP+ OLG showed increased levels of PKC-alpha, -beta I, -delta and -zeta isozymes. PKC-beta II, -gamma and -eta were poorly expressed in OLG. On immunoblots, PKC-alpha was present early and increased continually up to 18 days but PKC-beta I increased until 12 days in cultured OLG. High levels of PKC-delta, PKC-epsilon and PKC-zeta, the most abundant PKC isozymes in OLG, were maintained up to 12 days and were then slightly reduced. Interestingly, relatively high levels of PKC-alpha, PKC-beta I, PKC-beta II, PKC-gamma and PKC-epsilon isozymes were detected in purified myelin membrane although greater levels of PKC-delta were found in OLG than in purified myelin. Thus, most of the PKC isozymes found in cultured OLG were also present in myelin, although at different levels. Treatment with 50 nM 4 beta-phorbol-12,13-dibutyrate (PDB) caused a delayed downregulation of PKC-delta levels after 8 hr without modulating the expression of other PKC isozymes in 1-day OLG; in the 3-day-old and 6-day-old OLG, PDB downmodulated PKC-beta I, -delta and epsilon isozymes with only a minor effect on PKC-alpha and no reduction in PKC-zeta. Induction or downmodulation of individual PKC isozymes by phorbol esters appears to depend on the differentiation state of OLG. These data suggest that PKC-beta I, -delta and -epsilon isozymes have an important function in different cellular events of OLG differentiation. We conclude that the PKC-dependent modulation of myelin gene expression in OLG results predominantly from the Ca(++)-dependent PKC-beta I isozyme activity and the CA(++)-independent PKC-delta and PKC-epsilon activitives in a cell differentiation state-dependent manner.(ABSTRACT TRUNCATED AT 400 WORDS)

Rat oligodendrocytes express muscarinic receptors coupled to phosphoinositide hydrolysis and adenylyl cyclase

Muscarinic receptors expressed by rat oligodendrocyte primary cultures were examined by measuring changes in second messengers following exposure to carbachol, an acetylcholine analog, and by polymerase chain reaction. Inositol phosphate levels were measured in [3H]myo-inositol-labelled young oligodendrocyte cultures following stimulation with carbachol. Atropine, a specific muscarinic antagonist, prevented the carbachol-induced accumulation of inositol phosphates. The formation of inositol trisphosphate was concentration- and time-dependent, with the peak at 100 microM carbachol and 10 min. Carbachol increased intracellular calcium levels, which were dependent both on the mobilization of intracellular stores and influx of extracellular calcium. In initial experiments with more selective antagonists, the mobilization of intracellular calcium was preferentially inhibited by pirenzepine, a selective M1 antagonist, but not methoctramine, a selective M2 antagonist, suggesting M1 muscarinic receptor involvement. A role for protein kinase C in the regulation of carbachol-stimulated inositol phosphate formation and intracellular calcium mobilization was demonstrated, as acute pretreatment with phorbol-12,13-myristate acetate abolished the formation of both second messengers. Pretreatment with 100 microM carbachol abolished the 40% increase in the cyclic AMP accumulation stimulated by isoproterenol, a specific beta-adrenergic agonist. In turn, the inhibition was alleviated by pretreatment with atropine, suggesting muscarinic receptor involvement. Polymerase chain reaction carried out with specific m1 and m2 muscarinic receptor oligonucleotide primers, confirmed that these cells express, at least, the two muscarinic receptor subtypes. Without excluding the expression of other subtypes, these results suggest that developing oligodendrocytes express m1 (M1) and m2 (M2) muscarinic receptors capable of mediating phosphoinositide hydrolysis, mobilization of intracellular calcium and the attenuation of beta-adrenergic stimulation of cyclic AMP formation.

Glutamate-induced calcium signaling in astrocytes

Astrocytes respond to the excitatory neurotransmitter glutamate with dynamic spatio-temporal changes in intracellular calcium [Ca2+]i. Although they share a common wave-like appearance, the different [Ca2+]i changes--an initial spike, sustained elevation, oscillatory intracellular waves, and regenerative intercellular waves--are actually separate and distinct phenomena. These separate components of the astrocytic Ca2+ response appear to be generated by two different signal transduction pathways. The metabotropic response evokes an initial spatial Ca2+ spike that can propagate rapidly from cell to cell and appears to involve IP3. The metabotropic response can also produce oscillatory intracellular waves of various amplitudes and frequencies that propagate within cells and are sustained only in the presence of external Ca2+. The ionotropic response, however, evokes a sustained elevation in [Ca2+]i associated with receptor-mediated Na+ and Ca2+ influx, depolarization, and voltage-dependent Ca2+ influx. In addition, the ionotropic response can lead to regenerative intercellular waves that propagate smoothly and nondecrementally from cell to cell, possibly involving Na+/Ca2+ exchange. All these astrocytic [Ca2+]i changes tend to appear wave-like, traveling from region to region as a transient rise in [Ca2+]i. Nevertheless, as our understanding of the cellular events that underlie these [Ca2+]i changes grows, it becomes increasingly clear that glutamate-induced Ca2+ signaling is a composite of separate and distinct phenomena, which may be distinguished not based on appearance alone, but rather on their underlying mechanisms.

Accumulation of inositol phosphates in low-passage human meningioma cells following treatment with epidermal growth factor

In order to elucidate some of the signal transduction processes in human meningioma cells, the authors studied the effect of epidermal growth factor (EGF) and bromocriptine on inositol phospholipid hydrolysis, using low-passage human meningioma cells in culture. Epidermal growth factor is a well-studied mitogenic factor for meningioma cells, whereas bromocriptine is known to have an inhibitory effect on meningioma cell proliferation. The addition of EGF to meningioma cells caused stimulation of inositol phosphate accumulation in a dose-dependent manner at 60 minutes posttreatment, with the maximum effect (120% to 167% of control) achieved at a concentration of 10 ng/ml. Extraction of separate inositol phosphates accumulation in a dose-dependent manner at 60 minutes posttreatment, with the maximum effect (120% to 167% of control) achieved at a concentration of 10 ng/ml. Extraction of separate inositol phosphates revealed that inositol monophosphate (IP1) and inositol bisphosphate (IP2), but not inositol trisphosphate (IP3), accounted for the increase at 60 minutes. Kinetic analysis of EGF-stimulated inositol phospholipid hydrolysis showed that a sharp and transient increase in IP3 from 5 to 12 minutes post-EGF and a transient but more gradual increase in IP2 from 2 to 12 minutes post-EGF were followed by a gradual and steady increase in IP1, which was significantly greater than control after 5 minutes. On the other hand, long-term studies showed a down-regulation of inositol phosphate accumulation (a 64% decrease vs. control) after 7 days of treatment with EGF (10 ng/ml). Bromocriptine (5 microM) exhibited no significant effect on inositol phosphate accumulation at 60 minutes in four of five meningiomas studied. However, of two meningiomas studied with bromocriptine in combination with EGF, both showed a significant additive increase in inositol phosphate accumulation compared to those treated with EGF alone. The results suggest a close involvement of inositol phospholipid turnover in human meningioma cells in response to mitogenic stimulation by EGF.

Role of phosphoinositide hydrolysis in astrocyte volume regulation

Astrocytes exposed to hypoosmotic stress swell and subsequently reduce their size to almost their original volume, a phenomenon called regulatory volume decrease (RVD). We found that during hypoosmotic swelling there was a twofold increase in phosphatidylinositol (PI) hydrolysis. This increase was inhibited by the phospholipase C inhibitor, U-73122 (10 microM). Inhibition of PI hydrolysis resulted in blockage of RVD. We also examined whether agents that stimulate PI hydrolysis would enhance RVD. These agents significantly accelerated RVD. The rank order of potency was endothelin (20 nM) > or = norepinephrine (100 microM) > endothelin-3 (7 nM) > thrombin (1 U/ml) > or = ATP (500 microM) > bradykinin (20 microM) > or = carbachol (500 microM), as indicated by RVD rate constants. The extent of PI hydrolysis induced by these agents at the beginning of RVD exhibited a logarithmic relationship with the magnitude of RVD enhancement. Also, there was a linear relationship between the rate of PI hydrolysis and RVD rate constants. Our results suggest that stimulated PI hydrolysis is involved in the regulation of cell volume in astrocytes.

Substance P receptors on O-2A progenitor cells and type-2 astrocytes in vitro

Bradykinin- and substance P (SP)-stimulated second messenger studies in isolated subsets of neuroglia showed bradykinin-stimulated synthesis of phosphoinositides (PI) in type-1 astrocytes and oligodendrocytes. SP-stimulated PI accumulation was restricted to oligodendrocyte/type-2 astrocyte progenitor cells and type-2 astrocytes. These data were confirmed by analysis of calcium transients in single cells. In a regional study, SP-stimulated PI accumulation in primary astrocyte cultures was restricted to white matter. We conclude that regional heterogeneity in the expression of peptide receptors in cultures of primary astrocytes arises from a restricted distribution on subsets of macroglia. SP receptors restricted on cells of the oligodendrocyte/type-2 astrocyte type-2 lineage in vitro, coupled with in vivo observations by others, suggests that SP receptor expression is conserved on subsets of macroglia in vitro and possibly reactive astrocytes in vivo.

Effects of hypoxia on oligodendrocyte signal transduction

We have previously established that 21-day-old postnatal rat oligodendrocytes, maintained in monolayer culture and subjected to 6 h of hypoxia, show reversible inhibition of synthesis of alpha-hydroxy fatty acid and myelin basic protein but a dramatic induction of a 22-kDa protein, suggesting that this is a good model to study the mechanism of CNS demyelination caused by hypoxic injury. We now report that hypoxia also dramatically inhibits the basal protein kinase C-mediated phosphorylation of myelin basic protein and myelin 2',3'-cyclic nucleotide phosphohydrolase by 80%, but that the inhibition of phosphorylation can be reversed by addition of a protein kinase C activator, phorbol 12-myristate 13-acetate. The mechanism of action appears to involve the uncoupling of signal transduction at a site before phospholipase C, because hypoxia did not affect protein kinase C activity or its translocation to the membrane fraction. The most potent activator of phospholipase C (as measured by inositol phosphate release) was carbachol (muscarinic M1 receptor agonist), followed by L-phenylephrine (alpha 1-adrenergic receptor agonist) in normal oligodendrocytes. Excitatory amino acids and histamine were ineffective. Hypoxia for 6 h completely inhibited both muscarinic and alpha 1-adrenergic receptor-mediated inositol monophosphate release but did not affect phospholipase D-coupled phosphatidylethanol production in response to carbachol. We therefore conclude from this and earlier work that early, reversible changes in oligodendrocyte metabolism result not simply from ATP depletion, but may specifically target GTP binding protein-mediated processes.

Stimulation of phosphoinositide hydrolysis by trans-(+/-)-ACPD is greatly enhanced when astrocytes are cultured in a serum-free defined medium

Recent studies have demonstrated that astrocytes have much greater abilities to produce and respond to signalling molecules in the CNS than had been previously estimated. We now report a dramatic enhancement in the ability of a glutamate metabotropic receptor agonist, 1-aminocyclopentane-trans-(+/-)-1,3-dicarboxylic acid (trans-(+/-)-ACPD, to stimulate phosphoinositide hydrolysis in astrocytes cultured in a serum-free defined medium compared with astrocytes cultured in conventional serum-containing medium (43.2 +/- 3.6 vs. 3.2 +/- 0.48-fold of basal, respectively). This enhancement was selective to trans-(+/-)-ACPD as little or no difference in the response to carbachol or norepinephrine was seen between the two culture conditions. These results indicate a great potential for the phosphoinositide pathway in astrocyte glutamatergic signal transduction.

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.

Bradykinin inhibition of cyclic AMP accumulation in D384 astrocytoma cells. Evidence against a role of cyclic GMP

The present studies were performed in order to examine the possible role of cyclic GMP-stimulated phosphodiesterase (cGMP-PDE) activity in the inhibitory action of the inflammatory peptide bradykinin on cyclic AMP (cAMP) accumulation in D384 cells. Bradykinin decreased the forskolin-stimulated cAMP accumulation in the presence of the phosphodiesterase inhibitor rolipram, and caused a transient 50% rise in cellular cGMP in the presence of the nonselective PDE inhibitor 3-isobutyl-1-methylxanthine (IBMX). Both basal and bradykinin-stimulated cGMP accumulation were about 8 times higher in the presence of IBMX than in the presence of rolipram. Sodium nitroprusside, which caused a 20-70-fold increase in cGMP levels reduced forskolin stimulated cAMP accumulation, whereas hydroxylamine, which maximally caused a 16-fold increase in cGMP, did not. 8-bromo-cGMP or dibutyryl cGMP had no effect on cAMP accumulation induced by forskolin. The inhibitory effect of nitroprusside was totally reversed by blocking the soluble guanylate cyclase activity by methylene blue treatment; however, the inhibitory action of bradykinin on cAMP accumulation was not changed by this treatment. Additionally, inhibition of nitric oxide synthesis, which is known to be regulated by Ca2+ and in turn stimulates cGMP production, by N omega-nitro-L-arginine (L-NAME) treatment did not alter the inhibitory effect of bradykinin on forskolin-induced cAMP accumulation. These results indicate that large increases in cGMP may regulate cAMP via cGMP-PDE whereas the small increase induced by bradykinin is insufficient and that cGMP is not involved in the inhibitory action of bradykinin on cAMP levels in D384 cells.

Norepinephrine-mediated protein phosphorylation in astrocytes

Studies were conducted to determine if norepinephrine activates both protein kinase C and the cyclic AMP-dependent protein kinase in cultured rat astrocytes using phosphoproteins as markers. Norepinephrine was found to decrease 32P incorporation into an acidic 80,000 M(R) protein. A similar response was observed with isoproterenol and cyclic AMP analogs. In contrast, phorbol myristate acetate (PMA) increased 32P incorporation into this protein. Further studies looked at phosphorylation sites on glial fibrillary acidic protein and vimentin using two-dimensional tryptic phosphopeptide maps. The pattern of phosphorylation of these two proteins by norepinephrine resembles that of 8-bromo cyclic AMP and isoproterenol, and not that of PMA. Additionally, the effect of norepinephrine on the phosphorylation of GFAP and vimentin was blocked by alprenolol. One difference noted between norepinephrine and isoproterenol was the phosphorylation of an 18,000 M(R) protein. Norepinephrine increased, and isoproterenol decreased, 32P incorporation into this protein; however, the mechanism which mediates the norepinephrine effect remains to be determined. Overall, these studies indicate that the most prominent phosphorylation events mediated by norepinephrine are the consequence of the activation of cyclic AMP-dependent protein kinase.

Regulation of phosphoinositide hydrolysis in cultured astrocytes by sphingosine and psychosine

The effects of sphingosine and psychosine on phosphoinositide hydrolysis in primary cultured astrocytes were determined. Exposure to sphingosine produced a dose-dependent stimulation of phosphoinositide hydrolysis requiring the presence of external Ca++ for optimal activity. The addition of 10 microM norepinephrine resulted in a stimulation additional to that with sphingosine. The alpha 1-antagonist prazosin completely inhibited norepinephrine-induced phosphoinositide hydrolysis but had no effect on that produced by sphingosine. Psychosine (108 microM), when co-incubated with sphingosine, produced complete inhibition of sphingosine-induced phosphoinositide hydrolysis at all doses of sphingosine tested (33-668 microM). Likewise, psychosine totally inhibited norepinephrine-induced phosphoinositide hydrolysis. The protein kinase C inhibitor staurosporine (1 microM) had no effect on sphingosine-induced phosphoinositide hydrolysis. These findings suggest that lysosphingolipids such as sphingosine and psychosine may play an important role in the regulation of phosphoinositide turnover in astrocytes by a mechanism dependent on extracellular Ca++ and independent of the alpha 1-adrenergic receptor and protein kinase C.

Characterization of a cell line derived from a human oligodendroglioma

A novel clonal cell line derived from a human glioma (HOG) was found to express some oligodendrocyte-specific proteins including a 15-kDa form of myelin basic protein (MBP) and high 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) activity. Expression of the myelin lipids galactosylceramide and sulfogalactosylceramide (sulfatide) was low. HOG cells did not express the characteristic astrocyte markers glial fibrillary acidic protein (GFAP) or significant glutamine synthetase (GS) activity. After initial plating, HOG cells were flat and epitheloid and thus showed a limited oligodendrocyte-like morphology. However, after cells became more confluent, some cells were phase-bright and elaborated short processes. Receptor types expressed by HOG cells included A2-adenosine, prostaglandin E1 (PGE1), and beta 2-adrenergic receptors (beta-ARs) linked to stimulation of adenylate cyclase, and muscarinic cholinergic and H1-histamine coupled to phosphatidyinositol turnover (Post and Dawson, 1991). HOG cells should therefore provide a useful model for studying the extracellular regulation and phosphorylation of oligodendrocyte-specific proteins.

Catecholamine effects on dissociated tiger salamander Muller (glial) cells

Dissociated Muller (glial) cells from the neotenous tiger salamander retina respond electrogenically and rheogenically to three putative catecholamine (CA) neurotransmitters, epinephrine, norepinephrine and dopamine. All 3 CAs stimulate a net inward current and an increase in input resistance (Rn). The Muller cell response to the CAs is concentration dependent. At high concentrations ascorbate, an antioxidant used to protect the CAs from oxidation, stimulated a net outward current and a decrease in Rn. When the effects of ascorbate were considered, the CA response at 1 mM was no larger than the response at 100 microM, indicating that 100 microM CA maximally stimulated the Muller cell response.

Regulation of carbachol- and histamine-induced inositol phospholipid hydrolysis in a human oligodendroglioma

A stable cell line derived from a human oligodendroglioma (HOG) was used to study the regulation of muscarinic- and histamine receptor-mediated phosphoinositide hydrolysis. Both carbachol and histamine increased inositol monophosphate (InsP) accumulation in a dose- and time-dependent manner in the presence of lithium and the effect of simultaneous addition of carbachol and histamine was additive, implying independent signal transduction pathways. Homologous desensitization of muscarinic, but not histamine receptors, could be demonstrated although neither receptor type appeared to be heterologously desensitized. [3H]InsP accumulation in HOG cells was also stimulated by fluoride, suggesting guanosine triphosphate (GTP)-binding protein involvement, but phosphoinositide (PtdIns) hydrolysis was not sensitive to pertussis toxin. Phorbol ester-activation of protein kinase C (PKC) inhibited both muscarinic and histamine receptor-stimulated InsP release but did not attenuate either the fluoride-induced release of InsP nor beta-adrenergic receptor-mediated stimulation of adenylate cyclase activity. Taken together, we conclude that muscarinic and histamine receptors are differentially regulated through both PKC-dependent and -independent mechanisms, and that feedback inhibition of PtdIns turnover occurs proximal to the GTP binding proteins.

Comparison of calcium ionophore and receptor-activated inositol phosphate formation in primary glial cell cultures

The possible role of Ca2+ influx in alpha 1-adrenoceptor-stimulated [3H]inositol phosphate [( 3H]InsP) formation was examined in primary cultures of glial cells from 1-day-old rat brain. The Ca2+ ionophore A23187 caused a concentration- and time-dependent increase in [3H]InsP formation similar in magnitude to that caused by norepinephrine (NE). Responses to A23187 and NE were both completely dependent on extracellular Ca2+, with a similar concentration dependence. However, cadmium was more potent in blocking the response to A23187 than to NE. Lanthanum (1 mM) blocked the response to NE, although cobalt (5 mM) did not. The [3H]InsP response to A23187 was not additive with the response to NE or to the muscarinic agonist carbachol, although responses to NE and carbachol were addictive Both A23187 and ionomycin inhibited the additive stimulation caused by a combination of NE and carbachol, and this inhibition was potentiated by cadmium. Ionomycin stimulated [3H]InsP formation at concentrations lower than those inhibiting receptor-mediated responses, and this stimulation was not additive with responses to NE or carbachol. High-performance liquid chromatography separation showed similar patterns of [3H]InsPs formed in response to both Ca2+ ionophore and receptor agonists. These results raise the possibility that receptor-activated Ca2+ influx may be involved in stimulation of [3H]InsP formation in these cells.

Identification of B2 bradykinin binding sites on cultured cortical astrocytes

Bradykinin was found to bind to specific high-affinity sites in cultured cortical astrocytes from rat brain, and this binding appeared to be specific for the B2 bradykinin receptor subtype. Nonlinear regression analysis of saturation experiments using a computer programme revealed a single KD of 16.6 +/- 2.6 nM and a Bmax of 352.2 +/- 30.7 fmol/mg of protein. These results indicate that astrocytes possess bradykinin receptors and that these are predominantly of the B2 subtype.

Distribution of protein kinase C isozymes in rat optic nerves

Light (LM) and electron (EM) microscopic immunocytochemical methods were used to study the distribution of protein kinase C (PKC) isozymes in adult rat optic nerves. In cryostat and vibratome sections examined by LM, type II (beta) isozyme was localized almost exclusively in the axons. In the EM, immunoreaction products were found to associate with microtubules and neurofilaments. The inner surface of axonal membranes were occasionally stained. Analysis of PKC isozyme composition of the optic nerves by using immunoblot techniques revealed that type II (beta) isozyme accounted for approximately 80% of the total immunoreactivity. By contrast, type III (alpha) isozyme, which accounted for the remaining 20% of PKC, was found mainly in the astrocytes. Astrocytic processes next to blood vessels and between myelinated axons were stained. In the EM, immunoreaction products were found in the cytoplasm and along astroglial filaments. Segments of plasma membranes also were stained; but nuclei were unstained. Adult glial cells were not stained by an antibody to type II (beta) isozyme except for the occurrence of a few punctate cytoplasmic densities in occasional astrocytes. Very faint or no immunostaining was observed in sections treated with a monoclonal antibody to type I (gamma) isozyme. Immunoblot analyses also did not reveal this subspecies. The absence of type I (gamma) isozyme in optic nerves is not due to a down-regulation of the enzyme during development. In developing (5 and 11 day) rats, immunoreactivity of protein kinase C was very faint or absent. After 15 days, reaction products of both type III (alpha) and type II (beta) isozymes were found throughout the nerve. These findings suggest that type II (beta) isozyme may be involved in axonal transport whereas type III (alpha) isozyme may play a role in some astrocyte functions in mature optic nerves.

Ca2+ waves in astrocytes

The glial cell is the most numerous cell type in the central nervous system and is believed to play an important role in guiding brain development and in supporting adult brain function. One type of glial cell, the astrocyte also may be an integral computational element in the brain since it undergoes neurotransmitter-triggered signalling. Here we review the role of the astrocyte in the central nervous system, emphasizing receptor-mediated Ca2+ physiology. One focus is the recent discovery that the neurotransmitter glutamate induces a variety of intracellular Ca2+ changes in astrocytes. Simple Ca2+ spikes or intracellular Ca2+ oscillations often appear spatially uniform. However, in many instances, the Ca2+ rise has a significant spatial dimension, beginning in one part of the cell it spreads through the rest of the cell in the form of a wave. With high enough agonist concentration an astrocyte syncitium supports intercellular waves which propagate from cell to cell over relatively long distances. We present results of experiments using more specific pharmacological glutamate receptor agonists. In addition to describing the intercellular Ca2+ wave we present evidence for another form of intercellular signalling. Some possible functions of a long-range glial signalling system are also discussed.

Glutamate-stimulated, guanine nucleotide-mediated phosphoinositide turnover in astrocytes is inhibited by cyclic AMP

The potential for cross-talk between the adenyl cyclase and phosphoinositide (PPI) lipid second messenger system was investigated in astrocytes cultured from neonatal rat brain. Glutamate-stimulated PPI turnover, measured by the formation of total inositol phosphates from myo-[3H]inositol-labeled lipids, was inhibited in a concentration-dependent manner by the elevation of intracellular cyclic AMP levels produced either by stimulation of the isoproterenol receptor linked to adenyl cyclase or by its direct activation by forskolin. N6,2'-O-Dibutyryl cyclic AMP, an analogue that can also activate cyclic AMP-dependent kinase, inhibited glutamate-stimulated PPI turnover in a concentration-dependent manner as well, a result suggesting that cyclic AMP-dependent kinase is involved in mediating the inhibition. Inclusion of an inhibitor of cyclic AMP-dependent kinase, 1-(5-isoquinolinesulfonyl)-2 methylpiperazine dihydrochloride or N-(2-guanidinoethyl)-5-isoquinolinesulfonamide hydrochloride, blocked the cyclic AMP-mediated inhibition in a concentration-dependent manner, a finding further supporting this hypothesis. The site of inhibition of the phosphoinositol lipid pathway by cyclic AMP was probed using a digitonin-permeabilized cell system. Guanosine 5'-O-(3-thiotriphosphate), a nonhydrolyzable analogue of GTP, stimulated PPI turnover and potentiated glutamate-stimulated PPI turnover, and guanosine 5'-O-(3-thiodiphosphate) inhibited glutamate-stimulated PPI turnover in these cells, results providing evidence that glutamate receptors are coupled to phospholipase C by a guanine nucleotide binding protein in astrocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Phosphoinositide breakdown in isolated myelin is stimulated by GTP analogues and calcium

Purified myelin from rat brainstem, prelabeled in vivo by intracerebral injection of [3H]myoinositol, showed enhanced breakdown of phosphoinositides on treatment with 5'-guanylylimidodiphosphate [Gpp-(NH)p] and Ca2+. Concentration variation of the former in the presence of Ca2+ showed a dose-dependent release of inositol 1,4-bisphosphate (IP2) and inositol 1,4,5-trisphosphate (IP3), while inositol 1-phosphate (IP) release was erratic. Concentration-dependent release of IP2 and IP3 was also observed with Ca2+ as the variable in the presence of Gpp(NH)p. Carbachol, when present, did not enhance the stimulatory effect of Gpp(NH)p alone. Addition of diphosphoglycerate during incubation enhanced IP3 at the expense of IP2, suggesting the presence of IP3 phosphatase in myelin.

Alpha 1-adrenergic receptors in the brain: characterization in astrocytic glial cultures and comparison with neuronal cultures

Binding of [125I]HEAT to membranes prepared from primary cultures of astrocytic glial cells was time-dependent and 70-85% specific. Various adrenergic agonists and antagonists competed for [125I]HEAT binding according to the potencies of prazosin greater than, yohimbine greater than or equal to, clonidine, norepinephrine (NE), and propranolol. Scatchard analysis showed the Bmax of 209 fmol/mg protein and a Kd of 184 pM for [125I]HEAT binding by astrocytic glial membranes. Pretreatment of astrocytes with NE resulted in a dose-dependent downregulation of [125I]HEAT binding sites with a maximal response observed after 8 h at 100 microM NE. Removal of NE from cultures after pretreatment resulted in a time- and protein synthesis-dependent recovery of binding sites to control levels within 120 h. Incubation of astrocytic glial cultures with NE stimulated phosphoinositide (PI) hydrolysis in a time- and dose-dependent manner with a maximal stimulation of 2-fold observed in 60 min by 100 microM NE. Clonidine expressed differential effects on alpha 1-adrenergic receptors of the neuronal and astrocytic glial cultures. Pretreatment with 10 microM clonidine caused a 40% decrease in the Bmax of [125I]HEAT binding without influencing the Kd value in neuronal cultures. This downregulatory effect of clonidine was associated with a reduction in the ability of NE to stimulate PI hydrolysis in clonidine pretreated cells. In contrast to neuronal cultures, clonidine neither downregulated [125I]HEAT binding sites nor stimulated PI hydrolysis in glial cultures.

Differential suppression of interferon-gamma-induced Ia antigen expression on cultured rat astroglia and microglia by second messengers

The roles of intracellular second messengers in interferon-gamma (IFN-gamma)-induced Ia antigen (Ag) expression by astroglia and microglia were examined. Ia Ag on both glia types was induced by IFN-gamma. Reagents known to increase intracellular cAMP or activate intracellular protein kinase C (PKC) reduced IFN-gamma-induced Ia Ag expression by astroglia. In contrast, increasing intracellular cAMP had no suppressive effect on Ia Ag expression by microglia. These results indicate (1) cAMP and PKC negatively regulate IFN-gamma-induced Ia expression on astroglia, and (2) Ia expression is regulated differentially in astroglia vs. microglia. These findings may explain the frequent observation of Ia+ microglia (or macrophages) but not astroglia in various neurodegenerative diseases.

Comparison of alpha 1-adrenergic receptor-stimulated inositol phosphate formation in primary neuronal and glial cultures

alpha 1-Adrenergic receptor binding sites and norepinephrine-stimulated 3H-inositol phosphate (3H-InsP) accumulation were measured in primary cultures of neurons and glia from 1-day-old rat brains. The density of alpha 1-adrenergic receptor binding sites was approximately three times higher in membranes from neurons compared to glia. Although norepinephrine was slightly more potent in stimulating 3H-InsP formation in neurons than in glia, the maximal response was greater in glial cells. Norepinephrine-stimulated 3H-InsP formation remained constant for [3H]inositol prelabelling periods of 1-14 days in neurons, whereas the response increased with time in glia and was maximal after 7-10 days of prelabelling. Both the incorporation of [3H]inositol into lipid and basal levels of 3H-InsPs were lower in glial cells than in neurons, which accounted for the greater percent stimulation in glia. Pretreatment with phenoxybenzamine decreased norepinephrine-stimulated 3H-InsP formation in a dose-dependent manner in both neurons and glia by decreasing the maximal response without altering potency. HPLC separation showed that similar types of 3H-InsPs were accumulated in neurons and glial cells. These results demonstrate that alpha 1-adrenergic receptors exist on both neurons and glial cells and activate 3H-InsP accumulation in both cell types. Although receptor density is higher in neurons than in glia, the 3H-InsP response is higher in glia. This difference does not appear to be due to different receptor reserves, but may be due to differential coupling mechanisms in the two cell types.

Interactions between cyclic AMP and inositol phosphate transduction systems in astrocytes in primary culture

Astroglial cells in primary culture possess receptors with cyclic AMP and inositol phosphates (IP) as second messengers. The beta-receptor agonist, isoproterenol induces an increase in the accumulation of cyclic AMP, the alpha 2-receptor agonist clonidine inhibits the isoproterenol-induced accumulation of cyclic AMP, while the alpha 1-receptor agonist phenylephrine acts only on the inositol phosphate system. 5-Hydroxytryptamine (5-HT) stimulates, the formation of inositol phosphate, while isoproterenol and clonidine per se do not affect the inositol phosphate system. In the present paper the possibility of interactions between the cyclic AMP and the inositol phosphate transduction systems were investigated. In the presence of 10(-5) M 5-HT, in itself ineffective on the formation of cyclic AMP, isoproterenol stimulated the accumulation of cyclic AMP far more than in the absence of 5-HT. The potentiation was blocked by the 5-HT2 receptor antagonist ketanserin. On the other hand, there were no indications for a beta-receptor influence on the 5-HT-induced inositol phosphate formation. Stimulation of the alpha 2-receptor did not induce accumulation of inositol phosphate but significantly potentiated 5-HT2-receptor transduction, as measured by hydrolysis of phosphoinositide and formation of inositol phosphate. Stimulation by 5-HT also increased the formation of inositol phosphate after adrenergic stimulation and this effect was found to be synergistic at certain concentrations of adrenergic agonists. In addition, there was a statistically significant accumulation of cyclic AMP in the presence of both 5-HT and phenylephrine, none of which stimulated cyclic AMP alone. The results suggest specific interactions between the cyclic AMP and inositol phosphate systems on cultured astroglial cells.

Glutamate receptors activate Ca2+ mobilization and Ca2+ influx into astrocytes

We measured changes in the molar concentration of cytosolic Ca2+ ([Ca2+]i) in individual astrocytes in culture produced by the glutamate analog quisqualate (QA) and related substances by using fura-2 digital fluorescence microscopy. In cells cultured from the cortex, hippocampus, and cerebellum, the QA analog alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA; 10 microM) produced a slow increase in [Ca2+]i that was modest in amplitude (approximately 200 nM). These effects were completely abolished by 10 microM 6-nitro-7-cyano-quinoxaline-2,3-dione (CNQX). In cerebellar astrocytes, similar effects were produced by QA. However, in cortical and hippocampal astrocytes, the response to QA was much more complex. In these cells, QA produced an initial [Ca2+]i spike that was followed by a sustained influx of Ca2+ ("plateau"). In the absence of extracellular Ca2+, this plateau was abolished but the spike remained. CNQX did not block the spike and only slightly reduced the size of the plateau in some cells. Ni2+ (10 microM) but not nimodipine (10 microM) reduced the amplitude of the plateau. Pretreatment with 100 nM phorbol 12-myristate 13-acetate for 15 min abolished the spike but not the plateau portion of the QA response. Treatment with pertussis toxin at 250 ng/ml for 12-16 hr failed to alter the response. In some instances, the latency of the QA response differed considerably for individual cells in a group. It appeared that the response began in one cell and then spread to neighboring cells. Thus, QA appears to trigger a complex response in some astrocytes consisting of Ca2+ mobilization from intracellular stores and also Ca2+ influx resulting from the activation of AMPA-sensitive and -insensitive pathways.

Channel-mediated and carrier-mediated uptake of K+ into cultured ovine oligodendrocytes

Uptake of radioactive K+ by mature ovine oligodendrocytes (OLGs) maintained in primary culture was measured under steady-state conditions, i.e., in cells maintained in a normal tissue culture medium (5.4 mM K+), and in cells after depletion of intracellular K+ to less than 15% of its normal value by pre-incubation in K(+)-free medium. The latter value is dominated by an active, carrier-mediated uptake (although it may include some diffusional uptake), whereas the former, in addition to active uptake, also reflects passive K+ diffusion through ion selective channels and possible self-exchange between extracellular and intracellular K+, which may be carrier-mediated. The total uptake rate was 144 +/- 10 nmol/min/mg protein, and the uptake after K+ depletion was 60 +/- 2 nmol/min/mg protein, much lower rates than previously observed in astrocytes. The uptake into K(+)-depleted cells was inhibited by about 80% in the presence of ouabain (1 mM) and about 30% in the presence of furosemide (2 mM). Activators of protein kinase C (phorbol esters) and cAMP-dependent protein kinase (forskolin) have been shown to alter the myelinogenic metabolism as well as outward K+ current in cultured OLGs. The present study demonstrates that K+ homeostasis in OLGs is modulated through similar second messenger pathways. Active uptake was inhibited by about 60% in the presence of active phorbol esters (100 nM) but was not affected by forskolin (100 nM). Forskolin likewise had no effect on total uptake, whereas phorbol esters caused a much larger inhibition than expected from their effect on carrier-mediated uptake alone, suggesting that channel-mediated uptake was also reduced.(ABSTRACT TRUNCATED AT 250 WORDS)