Alpha-receptor and cholinergic receptor-linked changes in cytosolic Ca2+ and membrane potential in primary rat astrocytes

Role of Glycogenolysis in Memory and Learning: Regulation by Noradrenaline, Serotonin and ATP

This paper reviews the role played by glycogen breakdown (glycogenolysis) and glycogen re-synthesis in memory processing in two different chick brain regions, (1) the hippocampus and (2) the avian equivalent of the mammalian cortex, the intermediate medial mesopallium (IMM). Memory processing is regulated by the neuromodulators noradrenaline and serotonin soon after training glycogen breakdown and re-synthesis. In day-old domestic chicks, memory formation is dependent on the breakdown of glycogen (glycogenolysis) at three specific times during the first 60 min after learning (around 2.5, 30, and 55 min). The chicks learn to discriminate in a single trial between beads of two colors and tastes. Inhibition of glycogen breakdown by the inhibitor of glycogen phosphorylase 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) given at specific times prior to the formation of long-term memory prevents memory forming. Noradrenergic stimulation of cultured chicken astrocytes by a selective β₂-adrenergic (AR) agonist reduces glycogen levels and we believe that in vivo this triggers memory consolidation at the second stage of glycogenolysis. Serotonin acting at 5-HT_2B receptors acts on the first stage, but not on the second. We have shown that noradrenaline, acting via post-synaptic α₂-ARs, is also responsible for the synthesis of glycogen and our experiments suggest that there is a readily accessible labile pool of glycogen in astrocytes which is depleted within 10 min if glycogen synthesis is inhibited. Endogenous ATP promotion of memory consolidation at 2.5 and 30 min is also dependent on glycogen breakdown. ATP acts at P₂Y₁ receptors and the action of thrombin suggests that it causes the release of internal calcium ([Ca²⁺]_i) in astrocytes. Glutamate and GABA, the primary neurotransmitters in the brain, cannot be synthesized in neurons de novo and neurons rely on astrocytic glutamate synthesis, requiring glycogenolysis.

Adult astroglia is competent for Na+/Ca2+ exchanger-operated exocytotic glutamate release triggered by mild depolarization

Glutamate release induced by mild depolarization was studied in astroglial preparations from the adult rat cerebral cortex, that is acutely isolated glial sub-cellular particles (gliosomes), cultured adult or neonatal astrocytes, and neuron-conditioned astrocytes. K+ (15, 35 mmol/L), 4-aminopyridine (0.1, 1 mmol/L) or veratrine (1, 10 micromol/L) increased endogenous glutamate or [3H]D-aspartate release from gliosomes. Neurotransmitter release was partly dependent on external Ca2+, suggesting the involvement of exocytotic-like processes, and partly because of the reversal of glutamate transporters. K+ increased gliosomal membrane potential, cytosolic Ca2+ concentration [Ca2+]i, and vesicle fusion rate. Ca2+ entry into gliosomes and glutamate release were independent from voltage-sensitive Ca2+ channel opening; they were instead abolished by 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiurea (KB-R7943), suggesting a role for the Na+/Ca2+ exchanger working in reverse mode. K+ (15, 35 mmol/L) elicited increase of [Ca2+]i and Ca2+-dependent endogenous glutamate release in adult, not in neonatal, astrocytes in culture. Glutamate release was even more marked in in vitro neuron-conditioned adult astrocytes. As seen for gliosomes, K+-induced Ca2+ influx and glutamate release were abolished by KB-R7943 also in cultured adult astrocytes. To conclude, depolarization triggers in vitro glutamate exocytosis from in situ matured adult astrocytes; an aptitude grounding on Ca2+ influx driven by the Na+/Ca2+ exchanger working in the reverse mode.

The role of nucleotides in the neuron--glia communication responsible for the brain functions

Accumulating findings indicate that nucleotides play an important role in cell-to-cell communication through P2 purinoceptors, even though ATP is recognized primarily to be a source of free energy and nucleotides are key molecules in cells. P2 purinoceptors are divided into two families, ionotropic receptors (P2X) and metabotropic receptors (P2Y). P2X receptors (7 types; P2X(1)-P2X(7)) contain intrinsic pores that open by binding with ATP. P2Y (8 types; P2Y(1, 2, 4, 6, 11, 12, 13,) and (14)) are activated by nucleotides and couple to intracellular second-messenger systems through heteromeric G-proteins. Nucleotides are released or leaked from non-excitable cells as well as neurons in physiological and pathophysiological conditions. One of the most exciting cells in non-excitable cells is the glia cells, which are classified into astrocytes, oligodendrocytes, and microglia. Astrocytes express many types of P2 purinoceptors and release the 'gliotransmitter' ATP to communicate with neurons, microglia and the vascular walls of capillaries. Microglia also express many types of P2 purinoceptors and are known as resident macrophages in the CNS. ATP and other nucleotides work as 'warning molecules' especially through activating microglia in pathophysiological conditions. Microglia play a key role in neuropathic pain and show phagocytosis through nucleotide-evoked activation of P2X(4) and P2Y(6) receptors, respectively. Such strong molecular, cellular and system-level evidence for extracellular nucleotide signaling places nucleotides in the central stage of cell communications in glia/CNS.

Evidence that α7 nicotinic receptor modulates glutamate release from mouse neocortical gliosomes

The presence of nicotinic receptors on astrocytes in human and rat brain has been previously demonstrated however their possible functional role is still poorly understood. In this study we investigated on the presence of nicotinic receptors on gliosomes, purified from mouse cortex, and on their role in eliciting glutamate release. Epibatidine significantly increased basal release of [3H]D-aspartate and of endogenous glutamate from mouse gliosomes but not from synaptosomes. This effect was prevented by methyllycaconitine, alpha-bungarotoxin and mecamylamine but not by dihydro-beta-erythroidine. Epibatidine provoked also a significant increase of calcium concentration in gliosomes but not in synaptosomes; the increase in [Ca2+]i induced by epibatidine and KCl in gliosomes was very similar to each other. The present results indicate that alpha7 nicotinic receptors exist on mouse cortical glial particles and stimulate glutamate release.

Glia re-sealed particles freshly prepared from adult rat brain are competent for exocytotic release of glutamate

Glial subcellular re-sealed particles (referred to as gliosomes here) were purified from rat cerebral cortex and investigated for their ability to release glutamate. Confocal microscopy showed that the glia-specific proteins glial fibrillary acidic protein (GFAP) and S-100, but not the neuronal proteins 95-kDa postsynaptic density protein (PSD-95), microtubule-associated protein 2 (MAP-2) and beta-tubulin III, were enriched in purified gliosomes. Furthermore, gliosomes exhibited labelling neither for integrin-alphaM nor for myelin basic protein, which are specific for microglia and oligodendrocytes respectively. The Ca2+ ionophore ionomycin (0.1-5 microm) efficiently stimulated the release of tritium from gliosomes pre-labelled with [3H]d-aspartate and of endogenous glutamate in a Ca(2+)-dependent and bafilomycin A1-sensitive manner, suggesting the involvement of an exocytotic process. Accordingly, ionomycin was found to induce a Ca(2+)-dependent increase in the vesicular fusion rate, when exocytosis was monitored with acridine orange. ATP stimulated [3H]d-aspartate release in a concentration- (0.1-3 mm) and Ca(2+)-dependent manner. The gliosomal fraction contained proteins of the exocytotic machinery [syntaxin-1, vesicular-associated membrane protein type 2 (VAMP-2), 23-kDa synaptosome-associated protein (SNAP-23) and 25-kDa synaptosome-associated protein (SNAP-25)] co-existing with GFAP immunoreactivity. Moreover, GFAP or VAMP-2 co-expressed with the vesicular glutamate transporter type 1. Consistent with ultrastructural analysis, several approximately 30-nm non-clustered vesicles were present in the gliosome cytoplasm. It is concluded that gliosomes purified from adult brain contain glutamate-accumulating vesicles and can release the amino acid by a process resembling neuronal exocytosis.

Regulation of cell-to-cell communication mediated by astrocytic ATP in the CNS

It has become apparent that glial cells, especially astrocytes, not merely supportive but are integrative, being able to receive inputs, assimilate information and send instructive chemical signals to other neighboring cells including neurons. At first, the excitatory neurotransmitter glutamate was found to be a major extracellular messenger that mediates these communications because it can be released from astrocytes in a Ca(2+)-dependent manner, diffused, and can stimulate extra-synaptic glutamate receptors in adjacent neurons, leading to a dynamic modification of synaptic transmission. However, recently extracellular ATP has come into the limelight as an important extracellular messenger for these communications. Astrocytes express various neurotransmitter receptors including P2 receptors, release ATP in response to various stimuli and respond to extracellular ATP to cause various physiological responses. The intercellular communication "Ca(2+) wave" in astrocytes was found to be mainly mediated by the release of ATP and the activation of P2 receptors, suggesting that ATP is a dominant "gliotransmitter" between astrocytes. Because neurons also express various P2 receptors and synapses are surrounded by astrocytes, astrocytic ATP could affect neuronal activities and even dynamically regulate synaptic transmission in adjacent neurons as if forming a "tripartite synapse". In this review, we summarize the role of astrocytic ATP, as compared with glutamate, in gliotransmission and synaptic transmission in neighboring cells, mainly focusing on the hippocampus. Dynamic communication between astrocytes and neurons mediated by ATP would be a key event in the processing or integration of information in the CNS.

Identification and characterisation of functional bombesin receptors in human astrocytes

Reverse transcription polymerase chain reaction (RT-PCR) demonstrated the presence of bombesin BB2 receptor mRNA but not bombesin BB1 receptor or bombesin BB3 receptor mRNA in cultured human astrocytes. Neuromedin C hyperpolarised human astrocytes in whole-cell current and voltage clamp recordings and increased the intracellular free Ca(2+) ion concentration ([Ca(2+)](i)) in single astrocytes. Treatment with neuromedin C caused larger and more frequent increases in [Ca(2+)](i) than those triggered by neuromedin B, with 96% and 78% of cells responding, respectively. The stimulatory effects of neuromedin C were inhibited significantly by treatment with U73122 or the bombesin BB2 receptor antagonist [D-Phe(6), des-Met(14)]bombesin-(6-14) ethylester. A Fluorometric Imaging Plate Reader (FLIPR) was used to measure [Ca(2+)](i) in cell populations. Neuromedin C was approximately 50-fold more potent than neuromedin B in elevating [Ca(2+)](i) in astrocytes and Chinese hamster ovary (CHO) cells expressing human bombesin BB2 receptors (hBB2-CHO). However, in CHO cells expressing the bombesin BB1 receptor hBB1-CHO, neuromedin B was 32-fold more potent than neuromedin C. [D-Phe(6), des-Met(14)]bombesin-(6-14) ethylester was a partial agonist in hBB1-CHO cells (E(max)=55%) but was a noncompetitive antagonist in both hBB2-CHO cells and astrocytes. These studies report the first identification of functional bombesin receptors on cultured human astrocytes and have demonstrated that the bombesin BB2 receptor contributes significantly to astrocyte physiology.

Inflammatory cytokines enhance muscarinic-mediated arachidonic acid release through p38 mitogen-activated protein kinase in A2058 cells

The human melanoma cell line A2058 expresses the Gq-coupled M5 subtype of muscarinic receptor. Stimulation with the cholinergic agonist, carbachol, induces a dose-dependent increase in arachidonic acid release. The carbachol-induced arachidonate release is potentiated two- to threefold by pretreatment of A2058 cells with either of the inflammatory cytokines, tumor necrosis factor-alpha or interleukin-1beta . Cytokine-induced enhancement of muscarinic-mediated arachidonic acid release peaks near 1 h. Western analysis suggests that both cytokines are capable of activating the nuclear factor-kappaB (NF-kappaB) and p38 mitogen-activated protein kinase (MAPK) pathways. Anisomycin (1 microM) treatment mimics the cytokine-induced enhancement of arachidonic acid production and activates the p38 MAPK pathway, but does not activate the NF-kappaB pathway. Furthermore, pre-treatment of A2058 cells with the putative p38 MAPK inhibitor, SB202190, ablates the cytokine-dependent augmentation without interfering with the muscarinic-mediated arachidonic acid release in untreated cells. Moreover, cytokine treatment does not affect other M5-coupled pathways (e.g., phospholipase C activity or intracellular Ca2+ mobilization), suggesting that p38 MAPK activation principally modulates muscarinic-mediated phospholipase A2 activity. Finally, in primary cultures of cells taken from rat cerebellum, key aspects of this finding are repeated in cultures enriched for glia, but not in cultures enriched for granule neurons.

Colocalization of neurotransmitter receptors on astrocytes in explant cultures of rat CNS

In recent years evidence has accumulated that astrocytes express functional receptors for a variety of neurotransmitters/neuromodulators. By means of electrophysiological and combined autoradiographic and immunohistochemical methods we have demonstrated the colocalization of cholinergic, adrenergic and peptidergic receptors on astrocytes in explant cultures from various regions of rat central nervous system. A great number of biochemical and electrophysiological studies from other laboratories have shown that most of the neurotransmitters exert their effects on second messenger systems and on Ca2+-activated K+-channels. Furthermore, certain neurotransmitters are involved in the regulation of energy metabolism by stimulating enzymatic breakdown of glycogen in astrocytes. It was suggested that there is a cross-talk between the various neurotransmitter receptors on the glial membrane and that these receptors act in a synergistic or antagonistic way. The coexistence of cholinergic and peptidergic receptors on astrocytes is of great interest since both neurotransmitter systems are involved in cognitive functions and are impaired in patients with Alzheimer's dementia. The question is therefore raised whether not only neurones but also astrocytes might be involved in neurodegenerative disorders such as Alzheimer's disease.

Muscarinic acetylcholine receptors in the hippocampus, neocortex and amygdala: a review of immunocytochemical localization in relation to learning and memory

Immunocytochemical mapping studies employing the extensively used monoclonal anti-muscarinic acetylcholine receptor (mAChR) antibody M35 are reviewed. We focus on three neuronal muscarinic cholinoceptive substrates, which are target regions of the cholinergic basal forebrain system intimately involved in cognitive functions: the hippocampus; neocortex; and amygdala. The distribution and neurochemistry of mAChR-immunoreactive cells as well as behaviorally induced alterations in mAChR-immunoreactivity (ir) are described in detail. M35+ neurons are viewed as cells actively engaged in neuronal functions in which the cholinergic system is typically involved. Phosphorylation and subsequent internalization of muscarinic receptors determine the immunocytochemical outcome, and hence M35 as a tool to visualize muscarinic receptors is less suitable for detection of the entire pool of mAChRs in the central nervous system (CNS). Instead, M35 is sensitive to and capable of detecting alterations in the physiological condition of muscarinic receptors. Therefore, M35 is an excellent tool to localize alterations in cellular cholinoceptivity in the CNS. M35-ir is not only determined by acetylcholine (ACh), but by any substance that changes the phosphorylation/internalization state of the mAChR. An important consequence of this proposition is that other neurotransmitters than ACh (especially glutamate) can regulate M35-ir and the cholinoceptive state of a neuron, and hence the functional properties of a neuron. One of the primary objectives of this review is to provide a synthesis of our data and literature data on mAChR-ir. We propose a hypothesis for the role of muscarinic receptors in learning and memory in terms of modulation between learning and recall states of brain areas at the postsynaptic level as studied by way of immunocytochemistry employing the monoclonal antibody M35.

5-Hydroxytryptamine and glutamate modulate velocity and extent of intercellular calcium signalling in hippocampal astroglial cells in primary cultures

The effects of 5-hydroxytryptamine or glutamate treatment on mechanically induced intercellular calcium waves were studied in gap junction-coupled astroglial cells using rat astroglial-neuronal primary cultures from hippocampus. Imaging software was developed to study amplitude, velocity and extent of wave propagation. Velocity software was designed to find the cell contours automatically and to calculate travelled distance and time-delay of the calcium wave as it propagates from the stimulated cell to all other cells. Propagation analyses were performed to calculate the area of wave propagation. Mechanical stimulation of a single astroglial cell induced an intercellular calcium wave spreading from cell to cell in the astroglial syncytium. When registering the appearances of calcium signals in individual cells along the wave path upon re-stimulation of the same cell, 44.7% of the cells responded with similar calcium signal appearances the second time as the first time. A second wave from the opposite direction resulted in similar calcium signal appearances in 27.3% of the studied cells. Both amplitude and velocity of the calcium signal decreased most prominently in the first part and showed a later flattening out. Treatment with 5-hydroxytryptamine or glutamate for 20-30 s before mechanical stimulation increased the velocity of the calcium waves. 5-Hydroxytryptamine treatment for varying times decreased the propagation area of the calcium waves. In contrast, glutamate treatment increased the propagation area.

Neuronal nitric oxide synthase expression is induced in neocortical astrocytes after spreading depression

Spreading depression (SD) confers either increased susceptibility to ischemic injury or a delayed protection. Because nitric oxide modulates ischemic injury, we investigated if altered expression of nitric oxide synthase (NOS) by SD could account for the effect of SD on ischemia. Furthermore, the identity of cells expressing NOS after SD is important, since SD results in heterogeneous, cell type-specific changes in intracellular environment, which can control NOS activity. Immunohistochemical, computer-based image analyses and Western blotting show that the number of neuronal NOS (nNOS)-positive cells in the somatosensory cortex was significantly increased at 6 hours and 3 days after SD (P < 0.05 and 0.01, respectively), whereas inducible NOS expression remained unchanged. Double-labeling of nNOS and glial fibrillary acidic protein identified these nNOS-positive cells as astrocytes. The effect of altered NO production on induced nNOS expression was examined by treating rats with sodium nitroprusside or NA-nitro-L-arginine methyl ester (LNAM) during SD. Increased nNOS expression was prevented by sodium nitroprusside and phenylephrine or phenylephrine alone, but not LNAM. Because SD increased astrocytic nNOS expression at time points correlating with both ischemic hypersensitivity and ischemic tolerance, the ability of SD to modulate ischemic injury must be complex, perhaps involving NOS but other factors as well.

Differences in the stimulation of the phosphoinositide cycle by amine neurotransmitters in cultured rat forebrain neurones and astrocytes

In this study, we compared the stimulation by carbachol (CCh), noradrenaline (NA), and histamine (HA) of phosphoinositide hydrolysis in rat forebrain neuronal and glial cultures. When Ca2+ was omitted from the stimulation buffer (low microM extracellular Ca2+), amine-induced [3H]inositol phosphate accumulation was reduced to a higher extent in astrocytes (70-80% for CCh and NA and 100% for HA) than in neurones (around 50-60% for all the amines). Furthermore, guanosine 5'-[gamma-thio]trisphosphate (GTP[S]) stimulation of phosphoinositidase C (PIC) in membranes was 5-fold higher in neurones than in astrocytes. These results indicate differences in the mechanism of PIC stimulation in the two cell types. After 30 min stimulation in the presence of 10 mM Li+, a higher accumulation of [3H]inositol 4-monophosphate and [3H]inositol 1,4-bisphosphate than of [3H]inositol 1/3-monophosphate occurred for all agonists in neurones, whereas the opposite was observed in astrocytes. Moreover, in these cells stimulation for 5 min in the absence of Li+ produced a 2-3-fold accumulation of all metabolites of the 3-kinase pathway of inositol-1,4,5-trisphosphate metabolism but not of those of the 5-phosphatase pathway. Thus, regardless of the amine receptor stimulated, the 3-kinase route appeared to prevail in astrocytes and the 5-phosphatase pathway in neurones. The histamine response in neurones differed from that of the other agonists in that it rapidly declined. Taken together these results indicate that the heterogeneity in amine stimulation of the phosphoinositide cycle previously observed in brain slices could arise to a great extent from the cellular diversity of this preparation and be related to the differential contribution of the amine receptors located in neurones and astrocytes.

Muscarinic Ca2+ responses resistant to muscarinic antagonists at perisynaptic Schwann cells of the frog neuromuscular junction

1. Acetylcholine causes a rise of intracellular Ca2+ in perisynaptic Schwann cells (PSCs) of the frog neuromuscular junction. The signalling pathway was characterized using the fluorescent Ca2+ indicator fluo-3 and fluorescence microscopy. 2. Nicotinic antagonists had no effect on Ca2+ responses evoked by ACh and no Ca2+ responses were evoked with the nicotinic agonist nicotine. The muscarinic agonists muscarine and oxotremorine-M induced Ca2+ signals in PSCs. 3. Ca2+ responses remained unchanged when extracellular Ca2+ was removed, indicating that they are due to the release of Ca2+ from internal stores. Incubation with pertussis toxin did not alter the Ca2+ signals induced by muscarine, but did block depression of transmitter release induced by adenosine and prevented Ca2+ responses in PSCs induced by adenosine. 4. The general muscarinic antagonists atropine, quinuclidinyl benzilate and N-methyl-scopolamine failed to block Ca2+ responses to muscarinic agonists. Atropine (at 20,000-fold excess concentration) also failed to reduce the proportion of cells responding to a threshold muscarine concentration sufficient to cause responses in less than 50% of cells. Only the allosteric, non-specific blocker, gallamine (1-10 microM) was effective in blocking muscarine-induced Ca2+ responses. 5. In preparations denervated 7 days prior to experiments, low concentrations of atropine reversibly and completely blocked Ca2+ responses to muscarine. 6. The lack of blockade by general muscarinic antagonists in innervated, in situ preparations suggests that muscarinic Ca2+ responses at PSCs are not mediated by any of the five known muscarinic receptors or that post-translational modification prevented antagonist binding.

Cytosolic phospholipase A2 is coupled to muscarinic receptors in the human astrocytoma cell line 1321N1: characterization of the transducing mechanism

The cholinergic agonist carbachol induced the release of arachidonic acid in the 1321N1 astrocytoma cell line, and this was blocked by atropine, suggesting the involvement of muscarinic receptors. To assess the mechanisms of signalling involved in the response to carbachol, a set of compounds characterized by eliciting responses through different mechanisms was tested. A combination of 4beta-phorbol 12beta-myristate 13alpha-acetate and thapsigargin, an inhibitor of endomembrane Ca2+-ATPase that induces a prolonged elevation of cytosolic Ca2+ concentration, induced an optimal response, suggesting at first glance that both protein kinase C (PKC) and Ca2+ mobilization were involved in the response. This was consistent with the observation that carbachol elicited Ca2+ mobilization and PKC-dependent phosphorylation of cytosolic phospholipase A2 (cPLA2; phosphatide sn-2-acylhydrolase, EC 3.1.1.4) as measured by a decrease in electrophoretic mobility. Nevertheless, the release of arachidonate induced by carbachol was unaltered in media containing decreased concentrations of Ca2+ or in the presence of neomycin, a potent inhibitor of phospholipase C which blocks phosphoinositide turnover and Ca2+ mobilization. Guanosine 5'-[gamma-thio]triphosphate added to the cell-free homogenate induced both [3H]arachidonate release and cPLA2 translocation to the cell membrane fraction in the absence of Ca2+, thus suggesting the existence of an alternative mechanism of cPLA2 translocation dependent on G-proteins and independent of Ca2+ mobilization. From the combination of experiments utilizing biochemical and immunological tools the involvement of cPLA2 was ascertained. In summary, these data indicate the existence in the astrocytoma cell line 1321N1 of a pathway involving the cPLA2 which couples the release of arachidonate to the occupancy of receptors for a neurotransmitter, requires PKC activity and G-proteins and might operate in the absence of Ca2+ mobilization.

Learning-induced alterations in hippocampal PKC-immunoreactivity: a review and hypothesis of its functional significance

1. To localize protein kinase C (PKC) in the hippocampus, PKC activity measures, mRNA in situ hybridization, and [3H]phorbol ester binding techniques were used until in the 1980s antibodies became available for in situ immunocytochemistry. In the late 1980s, PKC-isoform-specific antibodies were first used to map hippocampal PKC at the cellular and subcellular level. The mammalian hippocampus contains all four Ca(2+)-dependent PKC isoforms, but the (sub)cellular localization is both isoform- and species-specific. 2. Hippocampally-dependent spatial and associative learning in rat, mice and rabbit induce an increase in PKC immunoreactivity (ir) in hippocampal principal cells studied 24 hours after the animals had learned the task. Among the four Ca(2+)-dependent PKC subtypes, this increase is selective for the gamma-isoform. The presence of the gamma-isoform in dendritic spines (the most likely site for synaptic plasticity and information storage), in contrast to PKC alpha, beta 1, and beta 2, may underlie the isoform-selectivity. 3. Compared to fully trained animals, subjects halfway training showed intermediate levels of increased PKC gamma-ir. Poor learners that were not able to learn the task showed considerably less enhanced PKC gamma-ir as compared to good learners. 4. Associative learning induced a decrease in astroglial PKC beta 2 and gamma-ir in those regions where a simultaneous increase in neuronal PKC gamma-ir was observed. This decrease most likely reflects PKC down-regulation, enabling the astrocytes to maintain their K+ buffering capacity necessary to support neuronal activity such as accompanying learning and memory. 5. Western blot analyses revealed that the increase in PKC gamma-ir was not due to an increase in total amount of PKC gamma, translocation, or the proteolytic generation of the fragment PKM. The increase in PKC gamma-ir must therefore reflect a learning-induced conformational change in the PKC gamma molecule that results in the exposure of the antigenic site(s). 6. Although a large number of hippocampal pyramidal cells display learning-induced enhancement of PKC gamma-ir at the 24 hours post-training time point, this does not indicate, however, that all synapses in these neurons are used, or that the maximal PKC signal transduction capacity per call has been reached. 7. The enhanced PKC gamma-ir may reflect a form of activated PKC, since PKC stimulation by phorbol esters (both in hippocampal slices and mildly aldehyde fixed sections) mimicked the increase in PKC gamma-ir similar as seen after learning. 8. The most likely transmitter systems which may have induced the altered PKC gamma-ir are acetylcholine and glutamate. Their contribution and interaction at the cellular level are depicted in a schematic circuit terminating on a CA1 pyramidal cell (Fig. 4). 9. Several functional roles for PKC gamma in learning and memory are discussed, and a hypothetical model is proposed based on an endogeneous PKC inhibitor protein that may explain altered antibody-binding to PKC gamma after learning (Fig. 6). 10. The immunocytochemical approach can contribute significantly to the ongoing attempts to decipher part of the cellular and biochemical mechanism of learning and memory. The development of ever more specific and better characterized antibodies reactive with different sites of proteins like PKC gamma will offer the necessary tools for further immunocytochemical research to help unravel complex brain functions.

Acetylcholine as a mitogen: muscarinic receptor-mediated proliferation of rat astrocytes and human astrocytoma cells

The mitogenic effect of muscarinic receptor agonists in glial cells has been characterized in rat cortical astrocytes and human 132 1N1 astrocytoma cells. The muscarinic receptor agonist carbachol caused a dose- and time-dependent increase in proliferation, as measured by [3H]thymidine incorporation. The mitogenic effect was mimicked by several muscarinic, but not nicotinic receptor agonists, and was blocked by muscarinic receptor antagonists. Reverse transcription-polymerase chain reaction (RT-PCR) experiments indicated the presence of m2, m3 and to a lesser degree, m5 muscarinic receptor mRNA in both astrocytes and astrocytoma cells. Proliferation experiments with subtype-specific muscarinic receptor antagonists suggest that carbachol-induced proliferation is due to activation of muscarinic M3 receptors. The phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) also stimulated glial cell proliferation. Down-regulation of protein kinase C, or the protein kinase C antagonist 1,5-(isoquinolynsulfanyl)-2-methylpiperazine dihydrochloride (H7) blocked proliferation induced by either TPA or carbachol. Of other neurotransmitters tested, histamine caused glial cell proliferation, norepinephrine and gamma-aminobutyric acid were ineffective, while serotonin and glutamate inhibited basal or serum-stimulated proliferation.

Intracellular acidification and Ca2+ transients in cultured rat cerebellar astrocytes evoked by glutamate agonists and noradrenaline

The effect of different neurotransmitters on the intracellular pH (pHi) and intracellular calcium (Ca2+i) was studied in cultured astrocytes from neonatal rat cerebellum, using the fluorescent dyes 2,7'-bis(carboxyethyl)-5,6-carboxy-fluorescein (BCECF) and Fura-2. Application of glutamate or kainate (100 microM) in a HEPES-buffered, CO2/HCO3(-) -free saline induced a decrease in pHi and an increase in Ca2+i. Amplitude and time course of the pHi and Ca2+i transients were different. Glutamate and kainate evoked a mean acidification of 0.22 +/- 0.05 (n = 29) and 0.20 +/- 0.09 (n = 12) pH units, respectively. The changes in pHi and Ca2+i induced by kainate, but not by glutamate, were inhibited by 6-cyano-7-dinitroquinozalin-2,3-dion (CNQX; 50 microM). In order to elucidate the mechanism of the agonist-induced acidification, whether the pHi changes were secondary to the Ca2+ rises was tested. In the absence of extracellular Ca2+, the kainate-induced Ca2+i transient was suppressed, while the intracellular acidification was only reduced by 13%. Removal of extracellular Ca2+ reduced the glutamate-induced pHi change by 8%, while the second component of the Ca2+i transient was abolished. Application of trans-( +/- )-1-amino-(1S,3R)-cyclopentadicarboxylic acid (t-ACPD, 100 microM), a metabotropic glutamate receptor agonist, and of noradrenaline (20 microM) evoked a Ca2+i increase, but no change of pHi. D-aspartate, which has a low affinity to glutamate receptors, but is known to be transported by the glutamate uptake system in some astrocytes, evoked an intracellular acidification, similar to that induced by glutamate, but no Ca2+i transient. The results suggest that the kainate-induced acidification is only partly due to the concomitant Ca2+i rise, while the glutamate/aspartate-induced acidification is mainly due to the activation of the glutamate uptake system.

Binding of cholecystokinin, bombesin and muscarine to neurons and astrocytes in explant cultures of rat central nervous system: autoradiographic and immunohistochemical studies

The cellular localization of binding sites for the gastrointestinal peptides [3H]cholecystokinin and [125I]bombesin as well as the cholecystokininB-antagonist [3H]L-365,260 was investigated in explant cultures of rat cortex, cerebellum, brainstem and spinal cord using autoradiographic techniques. Many neurons in cortical, brainstem and spinal cord cultures revealed intense labelling of the radioligands whereas cerebellar neurons showed only little binding. In addition to neurons, binding sites for these peptides were also observed on astrocytes. Labelling of glial cells in cerebellar cultures was usually weaker than in the other CNS areas studied, suggesting a certain specialization of astrocytes in various brain regions. By means of combined immunohistochemical and autoradiographic techniques it was demonstrated that many neurons and astrocytes which expressed binding sites for [3H]cholecystokinin, [3H]L-365,260 and [125I]bombesin were also immunostained by the monoclonal muscarinic receptor antibody M 35 providing evidence for a co-localization of peptidergic and cholinergic receptors on the membrane of these cells. Our autoradiographic findings suggesting the presence of receptors for cholecystokinin and bombesin on astrocytes are supported by electrophysiological studies demonstrating that both peptides induce a hyperpolarization of glial cells.

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.

Glial receptors and their intervention in astrocyto-astrocytic and astrocyto-neuronal interactions

As shown on cultured astrocytes from the mouse, in the presence of adenosine deaminase, 2-chloroadenosine by acting on A1-adenosine receptors potentiated the activation of phospholipase C induced by the alpha 1-adrenergic agonist, methoxamine. This potentiation required the presence of external calcium and was blocked by pertussis toxin. Moreover, this potentiation resulted from a cascade of events: activation (by calcium and protein kinase C) of a phospholipase A2 coupled to A1-adenosine receptors, release of arachidonic acid, which inhibited the reuptake of glutamate into astrocytes and finally additional activation of phospholipase C by externally accumulated glutamate through metabotropic receptors. The effects of 2-chloroadenosine and methoxamine were respectively mimicked by somatostatin and substance P while endothelins reproduced the combined effects of 2-chloroadenosine and methoxamine. Conditioned media from treated astrocytes enriched in glutamate stimulated phospholipase C in cultured striatal neurones. In addition, glutamate alone was also found to stimulate phospholipase A2 in astrocytes through receptors exhibiting a pharmacological profile distinct from metabotropic receptors coupled to phospholipase C and the glutamate response was potentiated by ATP. Moreover, the neuronal arachidonic acid production evoked by glutamate was potentiated by acetylcholine. Finally, the combined application of 2-chloroadenosine and methoxamine on striatal astrocytes reduced the permeability of gap junctions between astrocytes and this response was mimicked by arachidonic acid. Together, these results emphasized the contribution of astrocytes in the regulation of glutamatergic transmission.

Electrophysiological evidence for the presence of receptors for cholecystokinin and bombesin on cultured astrocytes of rat central nervous system

The action of cholecystokinin (CCK) and bombesin (Bom) was studied on the membrane potential of astrocytes in explant cultures of rat cortex, cerebellum, brain stem and spinal cord. Both peptides (10(-8) and 10(-7) M) caused a hyperpolarization of most astrocytes studied. The hyperpolarization by CCK was markedly reduced or blocked by the CCKB-antagonist L-365,260 whereas addition of the Bom-antagonist [D-Phe12,Leu14]-Bom antagonized the effects of Bom, suggesting a specific action of these peptides. When CCK and Bom were tested on the same cell, both peptides were effective, indicating a colocalization of receptors for CCK and Bom on the glial membrane. Our electrophysiological investigations provide strong evidence for the existence of functional CCK and Bom receptors on astrocytes.

Calcium-mobilizing and electrophysiological effects of bradykinin on cortical astrocyte subtypes in culture

The possible consequences of activating bradykinin (Bk) receptors on identified astrocyte subtypes derived from rat cortex have been investigated in terms of: 1) mobilization of intracellular Ca2+; and 2) electrophysiological response. Bk induced a rapid, transient rise in intracellular Ca2+ in 63% of cortical type-1-like astrocytes and 44% of type-2 astrocytes tested. Experiments involving Ca(2+)-free conditions suggested that the release occurred largely from internal stores in both astrocyte subtypes. Bk receptor activation resulted in an inward current in approximately 10% of each astrocyte subtype tested using the whole-cell patch clamp technique. The use of perforated patch recording confirmed a similar population of cells responsive to Bk. The Bk-induced current was associated with a decrease in membrane conductance. The reversal potential was close to the K+ equilibrium potential, consistent with the closure of K+ channels. The current demonstrated similar EC50 values and Hill coefficients and also a marked degree of desensitization for both astrocyte subtypes. Recordings from type-1-like astrocytes indicated that the receptor mediating the electrophysiological response was of the B2 subtype. The data were consistent with the functional translation of Bk receptor occupation to physiological responses in distinct sub-populations of cortical type-1-like and type-2 astrocytes.

Glial calcium

This review summarizes current knowledge relating intracellular calcium and glial function. During steady state, glia maintain a low cytosolic calcium level by pumping calcium into intracellular stores and by extruding calcium across the plasma membrane. Glial Ca2+ increases in response to a variety of physiological stimuli. Some stimuli open membrane calcium channels, others release calcium from intracellular stores, and some do both. The temporal and spatial complexity of glial cytosolic calcium changes suggest that these responses may form the basis of an intracellular or intercellular signaling system. Cytosolic calcium rises effect changes in glial structure and function through protein kinases, phospholipases, and direct interaction with lipid and protein constituents. Ultimately, calcium signaling influence glial gene expression, development, metabolism, and regulation of the extracellular milieu. Disturbances in glial calcium homeostasis may have a role in certain pathological conditions. The discovery of complex calcium-based glial signaling systems, capable of sensing and influencing neural activity, suggest a more integrated neuro-glial model of information processing in the central nervous system.

Receptor-mediated phospholipase D activity in primary astroglial cultures

Phospholipase D, an enzyme involved in signal transduction cascades, catalyses the formation of phosphatidic acid and, when ethanol is present, the formation of phosphatidylethanol. In the present study we demonstrate that stimulation of muscarinic acetylcholine receptors as well as P2-purinergic receptors induces activation of phospholipase D in primary cultures of astroglial cells. Both the hydrolysis and the transphosphatidylation reactions were stimulated by receptor agonists. Carbachol and ATP induced a rapid increase in the amount of [3H]phosphatidic acid in astroglial cells prelabelled with [3H]oleic acid. When ethanol (150 mM) was present, phosphatidylethanol was formed. Furthermore, the receptor-mediated increase in the concentration of phosphatidic acid was inhibited by ethanol, indicating that the phosphatidic acid production was indeed mediated by phospholipase D. The formation of phosphatidylethanol was concentration dependent, with a half-maximal effective concentration of 5 x 10(-5) M for carbachol and 10(-5) M for ATP. The carbachol-induced response was almost completely inhibited by atropine, indicating activation of phospholipase D via muscarinic receptors. The purinergic response is most probably mediated via P2-receptors since ADP was almost as efficient as ATP in inducing phosphatidylethanol formation, whereas AMP was significantly less potent. We conclude that astroglial cells in primary culture display muscarinic and purinergic receptors coupled to phospholipase D. The relationship to cell function needs to be further investigated.

Electrophysiological and biochemical evidence for bradykinin receptors on cultured rat cortical oligodendrocytes

The effects of the neuropeptide bradykinin (Bk) were examined on antigenically-identified rat cortical oligodendrocytes. Bk significantly increased the incorporation of [3H]myo-inositol into phospholipids, indicating the turnover of phosphatidyl inositol (PI). Ca2+ flux analysis experiments confirmed that this effect was accompanied by an increase in intracellular Ca2+. Using the whole-cell patch clamp technique, Bk was shown to induce an inward current associated with a decrease in membrane conductance, indicating a closure of ion channels. The reversal potential of the current was close to the potassium equilibrium potential, consistent with an effect on a K+ conductance in these cells. These results show that oligodendrocytes possess Bk receptors that may be of functional relevance.

Receptor subtype involved and mechanism of norepinephrine-induced stimulation of glutamate uptake into primary cultures of rat brain astrocytes

Glutamate uptake into rat brain astrocytes is potently stimulated by addition of norepinephrine (NE). This effect is mediated by alpha 1-adrenergic receptors expressed by these cells (Hansson and Rönnbäck: Life Sci 44:27, 1989; Brain Res 548:215, 1991). The present study was undertaken in order to identify the adrenergic receptor subtype involved, and to determine the sequence of events following receptor activation. NE increased glutamate uptake rates in a dose- and time-dependent manner (EC50 = 6 microM). Both, the selective alpha 1-receptor antagonist prazosin (IC50 = 2.5 microM) and the alpha 1b-adrenergic receptor subtype specific alkylating agent chloroethyl-clonidine (CEC, 100 microM) prevented NE (100 microM) evoked stimulation of glutamate uptake. Furthermore, omission of Ca2+ from the extracellular medium had no significant influence on NE-induced increase in glutamate uptake, indicating that the stimulatory effect is mediated by alpha 1b-adrenergic receptors. Treatment of cells with pertussis toxin (PTX) for 24 h or with 12-O-tetradecanoylphorbol-13-acetate (TPA) for 30-45 min prior to NE addition abolished the NE-mediated effect on glutamate uptake. Addition of TPA alone resulted in a rapid increase of glutamate uptake, which declined to control levels when TPA was applied 30 min prior to uptake initiation by glutamate. The increase in glutamate uptake elicited by TPA and NE added at the same time showed no additivity of the stimulatory effect resulting from treatment with each agent alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of noradrenaline-stimulated cyclic GMP formation in brain astrocytes in culture

Cyclic GMP accumulation induced by noradrenaline in astrocyte-enriched primary cultures from rat cerebrum involves synthesis of NO, as evidenced by the competitive inhibition exerted by the NO synthase inhibitor NG-monomethyl-L-arginine (IC50 = 3 microM). Furthermore, the noradrenaline effect was potently inhibited by haemoglobin (IC50 = 25 nM) and potentiated by superoxide dismutase, indicating that NO synthesis and cyclic GMP formation may occur in different subsets of astrocytes. Investigation of the receptors implicated by using selective adrenoceptor agonists and antagonists indicates that about 75% of the NO-dependent noradrenaline response is mediated by alpha 1-adrenoceptors and the rest by beta-adrenoceptors, with no evidence for potentiating effects between the two receptor types. This noradrenaline effect appears to require Ca2+ entry, since it is strongly dependent on extracellular Ca2+ but is not affected by conditions that will abolish intracellular Ca2+ mobilization (incubation with neomycin or pretreatment with carbachol). Inhibition by pretreatment with pertussis toxin is in agreement with involvement of the alpha 1A-adrenoceptor subtype in this Ca(2+)-dependent effect. However, implication of an unknown alpha 1-adrenoceptor subtype cannot be disregarded, because a similar inhibition is exerted by the presumably selective alpha 1B- and alpha 1C-adrenoceptor blocking agent chloroethylclonidine. Treatment of the cultures with the protein kinase C activator phorbol 12-myristate 13-acetate inhibits to a great extent the noradrenaline-induced cyclic GMP formation.

Histamine H1 receptors in UC-11MG astrocytes and their regulation of cytoplasmic Ca2+

Experiments were carried out on UC-11MG human astrocytoma cells, a continuous cell line that expresses a broad range of the biochemical and electrophysiological properties found in well-differentiated astrocytes. Because of a number of recent reports that astrocytes may express receptors for a variety of neuro-active substances, we measured the effects of 12 different neurotransmitters on intracellular free Ca2+ (Ca2+i) in UC-11MG cells. Of these neurotransmitters only histamine was found to have a significant effect. Further characterization of the nature of the histamine response showed that UC-11MG cells express mepyramine-sensitive H1 receptors the activation of which causes both mobilization of Ca2+ from intracellular stores and entry of Ca2+ from the extracellular solution. No evidence was found for the presence of H2 receptors. The Ca2+i response was maximal at 300 microM histamine and was attenuated by increasing cell density. We suggest that this neurotransmitter may play a role in astrocytic function in the human CNS.

Single K+ channels in adrenal zona glomerulosa cells. II. Inhibition by angiotensin II

The effects of angiotensin II (ANG II) on single K+ channels were studied in rat and bovine adrenal zona glomerulosa (ZG) cells, using the patch-clamp technique. ANG II (0.1-10 nM) induced substantial inhibition of inward rectifier and delayed rectifier K+ channel activities in rat and bovine ZG cells. Analysis of single-channel activities showed that the ANG II-induced channel-blocking effect involved reductions in the probability of the open state (Po) and the mean open time. The changes in these channel parameters occurred at all test voltages, indicating that the effect of ANG II was voltage independent. ANG II could not interact directly with the extracellular sides of the membranes in these experiments using cell-attached patches. Therefore, the effect of ANG II on K+ channels must occur through an indirect cytosolic transduction pathway. The ANG II-induced block of K+ channels will result in membrane depolarization, which may activate voltage-dependent Ca2+ channels, thereby increasing cytosolic free Ca2+ and stimulating aldosterone secretion. These channel-modulating actions of ANG II may be an important step in the initial sequence of events underlying its transduction mechanism.

Autonomic control of neuronal-astrocytic interactions, regulating metabolic activities, and ion fluxes in the CNS

It is generally assumed that the brain, in contrast to all other organs, is not equipped with an autonomic nervous system, regulating blood supply, and cellular activities. This may be because systemic administration of most drugs acting on monoaminergic or cholinergic receptors have little or no effect on cerebral blood flow and metabolism. However, intrathecal administration of noradrenaline does, indeed, influence both blood flow and energy metabolism in the brain. The present review focuses on effects of noradrenaline or serotonin on energy metabolism, turnover of amino acid transmitters and ion homeostasis, with special emphasis on the cellular localization. Noradrenergic agonists stimulate brain metabolism in vivo as well as many aspects of energy metabolism, Na+,K(+)-ATPase activity and uptake of transmitter amino acids in astrocytes in primary cultures, with little or no effect on corresponding preparations of neurons. Serotonin acts differently, decreasing potassium-induced release of glutamate from both neurons and astrocytes. Little is known about the effects of acetylcholine. The functional significance of these effects is discussed.

Activation of protein kinase C blocks astroglial gap junction communication and inhibits the spread of calcium waves

The following two processes related to astrocytes are thought to depend on intercellular coupling through gap junctions: the spatial buffering of K+o and the spread of calcium waves in the astrocytic syncytium. We have used the following two independent methods to measure the open state of gap junctions: injection of lucifer yellow, and optical calcium imaging of calcium waves in response to probing the cells with a micropipette. The spread of lucifer yellow and calcium waves was inhibited if the cells were treated with either phorbol 12-myristate 13-acetate (PMA) or a synthetic diacylglycerol that activates protein kinase C. Down-regulation of protein kinase C by a 24-h treatment with PMA inhibited the uncoupling effect of PMA, supporting a direct involvement of protein kinase C in the regulation of astroglial gap junctions. Purinergic P2Y receptors, which are coupled to the inositol phospholipid pathway, are expressed by most astroglia in culture. Activation of the P2Y purinergic receptor with the selective agonist 2-methylthio-ATP uncoupled astroglia in a manner similar to the effect of treatment with PMA. Modulation of gap junctional conductance could isolate specific pathways within the astrocytic syncytium to form an extraneuronal information transfer network in brain.

Transmitter release increases intracellular calcium in perisynaptic Schwann cells in situ

Glial cells isolated from the nervous system are sensitive to neurotransmitters and may therefore be involved in synaptic transmission. The sensitivity of individual perisynaptic Schwann cells to activity of a single synapse was investigated, in situ, at the frog neuromuscular junction by monitoring changes in intracellular Ca2+ in the Schwann cells. Motor nerve stimulation induced an increase in intracellular Ca2+ in these Schwann cells; this increase was greatly reduced when transmitter release was blocked. Furthermore, local application of the cotransmitters acetylcholine and ATP evoked Ca2+ responses even in the absence of extracellular Ca2+. Successive trains of nerve stimuli or applications of transmitters resulted in progressively smaller Ca2+ responses. We conclude that transmitter released during synaptic activity can evoke release of intracellular Ca2+ in perisynaptic Schwann cells. This Ca2+ signal may play a role in the maintenance or modulation of a synapse. These data show that synaptic transmission involves three cellular components with both postsynaptic and glial components responding to transmitter secretion.

Different receptors mediate stimulation of nitric oxide-dependent cyclic GMP formation in neurons and astrocytes in culture

The ability of various compounds to stimulate cyclic GMP accumulation was studied in neuronal and astrocyte-enriched primary cultures from rat cerebrum. Glutamate was the only agonist eliciting a response in neurons whereas several agonists had an effect in astrocytes but only those due to norepinephrine and glutamate were of considerable magnitude. The responses were markedly inhibited by the nitric oxide synthase inhibitor NG-monomethyl-L-arginine. The effect of glutamate appears to be mediated predominantly by NMDA receptors in neurons and by quisqualate AMPA-insensitive receptors in astrocytes.

Activation of endothelin receptors by sarafotoxin regulates Ca2+ homeostasis in cerebellar astrocytes

We carried out experiments designed to investigate the effects of sarafotoxin-6B (SFTx) on [Ca2+]i in cerebellar astrocytes using the Ca2+ indicator fura-2. Both endothelin-1 and sarafotoxin-6B increased [Ca2+]i in individual cerebellar astrocytes in cell culture. The shape of the response was variable but usually consisted of an initial peak of [Ca2+]i followed by an extended plateau increase in [Ca2+]i. In Ca(2+)-free medium only the initial peak was observed. If Ca2+ was subsequently readmitted to the external medium a plateau was now formed. When external Ca2+ was removed during a plateau, [Ca2+]i rapidly declined; replacing the external Ca2+ reversed this decline. The plateau was also reversibly reduced by addition of Ni2+ (5 mM) to the external medium. Addition of 50 mM K+ produced a small increase in [Ca2+]i in most cells. This response was blocked by nimodipine. However, nimodipine only slightly blocked the plateau increase in [Ca2+]i that was formed following activation of endothelin receptors. Furthermore, perfusion of cells with 50 mM K+ during the plateau portion of a response to SFTx reduced [Ca2+]i. In some cells addition of a phorbol ester produced a sustained increase in [Ca2+]i that was blocked by nimodipine. In conclusion, activation of endothelin receptors by SFTx in cerebellar astrocytes produces both Ca2+ mobilization and Ca2+ influx. The pathway for Ca2+ influx is predominantly a non-voltage-dependent one, although some entry through a dihydropyridine-sensitive pathway also appears to occur. Furthermore, activation of protein kinase C in cerebellar astrocytes activates voltage-sensitive Ca2+ channels.

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.

An ion channel locus for the protein kinase C potentiation of transmitter glutamate release from guinea pig cerebrocortical synaptosomes

The mechanism by which protein kinase C (PKC) activates transmitter release from guinea pig cerebrocortical synaptosomes was investigated by employing parallel fluorescent assays of glutamate release, cytoplasmic free Ca2+, and plasma membrane potential. 4 beta-Phorbol dibutyrate (4 beta-PDBu) enhances the Ca(2+)-dependent, 4-aminopyridine (4AP)-evoked release of glutamate from synaptosomes, the 4AP-evoked elevation of cytoplasmic free Ca2+, and the 4AP-evoked depolarization of the plasma membrane. 4 beta-PDBu itself causes a slow depolarization, which may underlie the small effect of 4 beta-PDBu on spontaneous, KCl-evoked, and Ca(2+)-independent/4AP-evoked glutamate release. Because 4AP (but not KCl) generates spontaneous, tetrodotoxin-sensitive action potentials in synaptosomes, a major locus of presynaptic PKC action is to enhance these action potentials, perhaps by inhibiting delayed rectifier K+ channels.

Synergistic effects in the alpha 1- and beta 1-adrenergic regulations of intracellular calcium levels in striatal astrocytes

1. Using indo-1 as a calcium fluorescent probe, we have observed the following in striatal astrocytes in primary culture. 2. The stimulation of alpha-adrenoceptors induces a rapid rise in cytosolic calcium resulting from an internal calcium mobilization followed by an external calcium influx (4-min duration). 3. The stimulation of beta 1-adrenoceptors evokes only a slight internal calcium mobilization (90-sec duration). 4. The simultaneous stimulation of beta 1- and alpha 1-adrenoceptors induces a more prolonged calcium influx (10 min). The latter phenomenon could explain the calcium-dependent synergistic effects of alpha 1 and beta stimulation on cAMP production already described in the brain.

Evidence for electrogenic sodium-dependent ascorbate transport in rat astroglia

The dependence of ascorbate uptake on external cations was studied in primary cultures of rat cerebral astrocytes. Initial rates of ascorbate uptake were diminished by lowering the external concentrations of either Ca2+ or Na+. The Na(+)-dependence of astroglial ascorbate uptake gave Hill coefficients of approximately 2, consistent with a Na(+)-ascorbate cotransport system having stoichiometry of 2 Na+:1 ascorbate anion. Raising external K+ concentration incrementally from 5.4 to 100 mM, so as to depolarize the plasma membrane, decreased the initial rate of ascorbate uptake, with the degree of inhibition depending on the level of K+. The depolarizing ionophores gramicidin and nystatin slowed ascorbate uptake by astrocytes incubated in 5.4 mM K+; whereas, the nondepolarizing ionophore valinomycin did not. Qualitatively similar results were obtained whether or not astrocytes were pretreated with dibutyryl cyclic AMP (0.25 mM for 2 weeks) to induce stellation. These data are consistent with the existence of an electrogenic Na(+)-ascorbate cotransport system through which the rate of ascorbate uptake is modulated by endogenous agents, such as K+, that alter astroglial membrane potential.