Functional receptors for vasoactive intestinal polypeptide in cultured astroglia from neonatal rat brain

CREB-regulated transcription during glycogen synthesis in astrocytes

Glycogen storage, conversion and utilization in astrocytes play an important role in brain energy metabolism. The conversion of glycogen to lactate through glycolysis occurs through the coordinated activities of various enzymes and inhibition of this process can impair different brain processes including formation of long-lasting memories. To replenish depleted glycogen stores, astrocytes undergo glycogen synthesis, a cellular process that has been shown to require transcription and translation during specific stimulation paradigms. However, the detail nuclear signaling mechanisms and transcriptional regulation during glycogen synthesis in astrocytes remains to be explored. In this report, we study the molecular mechanisms of vasoactive intestinal peptide (VIP)-induced glycogen synthesis in astrocytes. VIP is a potent neuropeptide that triggers glycogenolysis followed by glycogen synthesis in astrocytes. We show evidence that VIP-induced glycogen synthesis requires CREB-mediated transcription that is calcium dependent and requires conventional Protein Kinase C but not Protein Kinase A. In parallel to CREB activation, we demonstrate that VIP also triggers nuclear accumulation of the CREB coactivator CRTC2 in astrocytic nuclei. Transcriptome profiles of VIP-induced astrocytes identified robust CREB transcription, including a subset of genes linked to glucose and glycogen metabolism. Finally, we demonstrate that VIP-induced glycogen synthesis shares similar as well as distinct molecular signatures with glucose-induced glycogen synthesis, including the requirement of CREB-mediated transcription. Overall, our data demonstrates the importance of CREB-mediated transcription in astrocytes during stimulus-driven glycogenesis.

A tribute to Leif Hertz: The historical context of his pioneering studies of the roles of astrocytes in brain energy metabolism, neurotransmission, cognitive functions, and pharmacology identifies important, unresolved topics for future studies

Leif Hertz, M.D., D.Sc. (honōris causā) (1930-2018), was one of the original and noteworthy participants in the International Conference on Brain Energy Metabolism (ICBEM) series since its inception in 1993. The biennial ICBEM conferences are organized by neuroscientists interested in energetics and metabolism underlying neural functions; they have had a high impact on conceptual and experimental advances in these fields and on promoting collaborative interactions among neuroscientists. Leif made major contributions to ICBEM discussions and understanding of metabolic and signaling characteristics of astrocytes and their roles in brain function. His studies ranged from uptake of K+ from extracellular fluid and its stimulation of astrocytic respiration, identification, and regulation of enzymes specifically or preferentially expressed in astrocytes in the glutamate-glutamine cycle of excitatory neurotransmission, a requirement for astrocytic glycogenolysis for fueling K+ uptake, involvement of glycogen in memory consolidation in the chick, and pharmacology of astrocytes. This tribute to Leif Hertz highlights his major discoveries, the high impact of his work on astrocyte-neuron interactions, and his unparalleled influence on understanding the cellular basis of brain energy metabolism. His work over six decades has helped integrate the roles of astrocytes into neurotransmission where oxidative and glycogenolytic metabolism during neurotransmitter glutamate turnover are key aspects of astrocytic energetics. Leif recognized that brain astrocytic metabolism is greatly underestimated unless the volume fraction of astrocytes is taken into account. Adjustment for pathway rates expressed per gram tissue for volume fraction indicates that astrocytes have much higher oxidative rates than neurons and astrocytic glycogen concentrations and glycogenolytic rates during sensory stimulation in vivo are similar to those in resting and exercising muscle, respectively. These novel insights are typical of Leif's astute contributions to the energy metabolism field, and his publications have identified unresolved topics that provide the neuroscience community with challenges and opportunities for future research.

Deciphering the Polyglucosan Accumulation Present in Lafora Disease Using an Astrocytic Cellular Model

Lafora disease (LD) is a neurological disorder characterized by progressive myoclonus epilepsy. The hallmark of the disease is the presence of insoluble forms of glycogen (polyglucosan bodies, or PGBs) in the brain. The accumulation of PGBs is causative of the pathophysiological features of LD. However, despite the efforts made by different groups, the question of why PGBs accumulate in the brain is still unanswered. We have recently demonstrated that, in vivo, astrocytes accumulate most of the PGBs present in the brain, and this could lead to astrocyte dysfunction. To develop a deeper understanding of the defects present in LD astrocytes that lead to LD pathophysiology, we obtained pure primary cultures of astrocytes from LD mice from the postnatal stage under conditions that accumulate PGBs, the hallmark of LD. These cells serve as novel in vitro models for studying PGBs accumulation and related LD dysfunctions. In this sense, the metabolomics of LD astrocytes indicate that they accumulate metabolic intermediates of the upper part of the glycolytic pathway, probably as a consequence of enhanced glucose uptake. In addition, we also demonstrate the feasibility of using the model in the identification of different compounds that may reduce the accumulation of polyglucosan inclusions.

P-Rex1 is a novel substrate of the E3 ubiquitin ligase Malin associated with Lafora disease

Laforin and Malin are two proteins that are encoded by the genes EPM2A and EPM2B, respectively. Laforin is a glucan phosphatase and Malin is an E3-ubiquitin ligase, and these two proteins function as a complex. Mutations occurring at the level of one of the two genes lead to the accumulation of an aberrant form of glycogen meant to cluster in polyglucosans that go under the name of Lafora bodies. Individuals affected by the appearance of these polyglucosans, especially at the cerebral level, experience progressive neurodegeneration and several episodes of epilepsy leading to the manifestation of a fatal form of a rare disease called Lafora disease (LD), for which, to date, no treatment is available. Despite the different dysfunctions described for this disease, many molecular aspects still demand elucidation. An effective way to unknot some of the nodes that prevent the achievement of better knowledge of LD is to focus on the substrates that are ubiquitinated by the E3-ubiquitin ligase Malin. Some substrates have already been provided by previous studies based on protein-protein interaction techniques and have been associated with some alterations that mark the disease. In this work, we have used an unbiased alternative approach based on the activity of Malin as an E3-ubiquitin ligase. We report the discovery of novel bonafide substrates of Malin and have characterized one of them more deeply, namely PIP3-dependent Rac exchanger 1 (P-Rex1). The analysis conducted upon this substrate sets the genesis of the delineation of a molecular pathway that leads to altered glucose uptake, which could be one of the origin of the accumulation of the polyglucosans present in the disease.

Brain Metabolic Alterations in Alzheimer's Disease

The brain is one of the most energy-consuming organs in the body. Satisfying such energy demand requires compartmentalized, cell-specific metabolic processes, known to be complementary and intimately coupled. Thus, the brain relies on thoroughly orchestrated energy-obtaining agents, processes and molecular features, such as the neurovascular unit, the astrocyte-neuron metabolic coupling, and the cellular distribution of energy substrate transporters. Importantly, early features of the aging process are determined by the progressive perturbation of certain processes responsible for adequate brain energy supply, resulting in brain hypometabolism. These age-related brain energy alterations are further worsened during the prodromal stages of neurodegenerative diseases, namely Alzheimer's disease (AD), preceding the onset of clinical symptoms, and are anatomically and functionally associated with the loss of cognitive abilities. Here, we focus on concrete neuroenergetic features such as the brain's fueling by glucose and lactate, the transporters and vascular system guaranteeing its supply, and the metabolic interactions between astrocytes and neurons, and on its neurodegenerative-related disruption. We sought to review the principles underlying the metabolic dimension of healthy and AD brains, and suggest that the integration of these concepts in the preventive, diagnostic and treatment strategies for AD is key to improving the precision of these interventions.

Advantages of Vasoactive Intestinal Peptide for the Future Treatment of Parkinson's Disease

Parkinson's disease is the second most common neurodegenerative disorder in adults over the age of 65. The characteristic symptoms of Parkinson's disease, such as resting tremor, muscular rigidity, bradykinesia, postural instability and gait imbalance, are thought to be a result of the progressive degeneration of the dopaminergic neurons of the substantia nigra compacta, resulting in insufficient dopamine integrated signalling on GABAergic medium spiny neurons in the striatum. Despite tremendous research, the molecular mechanisms underlying the pathogenesis of neurodegeneration in Parkinson's disease have remained largely unknown. Although a variety of possible pathogenic mechanisms have been proposed over the years, including excessive release of oxygen free radicals, impairment of mitochondrial function, loss of trophic support, abnormal kinase activity, disruption of calcium homeostasis, dysfunction of protein degradation and neuroinflammation, the pathogenesis is still largely uncertain, and there is currently no effective cure for Parkinson's disease. To develop potential therapies for Parkinson's disease, inflammatory processes, mitochondrial dynamics, oxidative stress, production of reactive aldehydes, excitotoxicity and synucleinopathies are to be targeted. In this respect, vasoactive intestinal peptide has beneficial effects that provide an advantage for the treatment of Parkinson's disease. Vasoactive intestinal peptide is a major neuropeptide-neurotransmitter having antioxidant, anti-inflammatory, neurotropic, neuromodulator, and anti-apoptotic properties. In addition to its direct neuroprotective actions regulating the activity of astrocytes, microglia and brain mast cells, it also plays important roles for neuronal adaptation, maintenance and survival.

Astrocytes: new players in progressive myoclonus epilepsy of Lafora type

Lafora disease (LD) is a fatal form of progressive myoclonus epilepsy characterized by the accumulation of insoluble poorly branched glycogen-like inclusions named Lafora bodies (LBs) in the brain and peripheral tissues. In the brain, since its first discovery in 1911, it was assumed that these glycogen inclusions were only present in affected neurons. Mouse models of LD have been obtained recently, and we and others have been able to report the accumulation of glycogen inclusions in the brain of LD animals, what recapitulates the hallmark of the disease. In this work we present evidence indicating that, although in mouse models of LD glycogen inclusions co-localize with neurons, as originally established, most of them co-localize with astrocytic markers such as glial fibrillary acidic protein (GFAP) and glutamine synthase. In addition, we have observed that primary cultures of astrocytes from LD mouse models accumulate higher levels of glycogen than controls. These results suggest that astrocytes may play a crucial role in the pathophysiology of Lafora disease, as the accumulation of glycogen inclusions in these cells may affect their regular functionality leading them to a possible neuronal dysfunction.

Effects of Monoamines and Antidepressants on Astrocyte Physiology: Implications for Monoamine Hypothesis of Depression

Major depressive disorder (MDD) is one of the most common neuropsychiatric disorders affecting over one-fifth of the population worldwide. Owing to our limited understanding of the pathophysiology of MDD, the quest for finding novel antidepressant drug targets is severely impeded. Monoamine hypothesis of MDD provides a robust theoretical framework, forming the core of a large jigsaw puzzle, around which we must look for the vital missing pieces. Growing evidence suggests that the glial loss observed in key regions of the limbic system in depressed patients, at least partly, accounts for the structural and cognitive manifestations of MDD. Studies in animal models have subsequently hinted at the possibility that the glial atrophy may play a causative role in the precipitation of depressive symptoms. Antidepressants as well as monoamine neurotransmitters exert profound effects on the gene expression and metabolism in astrocytes. This raises an intriguing possibility that the astrocytes may play a central role alongside neurons in the behavioral effects of antidepressant drugs. In this article, we discuss the gene expression and metabolic changes brought about by antidepressants in astrocytes, which could be of relevance to synaptic plasticity and behavioral effects of antidepressant treatments.

Astroglial 5-HT2B receptor in mood disorders

Introduction: Astroglia represent the main cellular homeostatic system of the central nervous system (CNS). Astrocytes are intimately involved in regulation and maintenance of neurotransmission by regulating neurotransmitters removal and turnover and by supplying neurons with neurotransmitters precursors. Astroglial cells are fundamental elements of monoaminergic transmission in the brain and in the spinal cord. Astrocytes receive monoaminergic inputs and control catabolism of monoamines through dedicated transporters and intracellular enzymatic pathways.Areas covered: Astroglial cells express serotonergic receptors; in this review, we provide an in-depth characterization of 5-HT_2B receptors. Activation of these receptors triggers numerous intracellular signaling cascades that regulate expression of multiple genes. Astroglial 5-HT_2B receptors are activated by serotonin-specific reuptake inhibitors, such as major anti-depressant fluoxetine. Expression of astroglial serotonin receptors undergoes remarkable changes in depression disorders, and these changes can be corrected by chronic treatment with anti-depressant drugs.Expert commentary: Depressive behaviors, which occur in rodents following chronic stress or in neurotoxic models of Parkinson disease, are associated with significant changes in the expression of astroglial, but not neuronal 5-HT_2B receptors; while therapy with anti-depressants normalizes both receptors expression and depressive behavioral phenotype. In summary, astroglial serotonin receptors are linked to mood disorders and may represent a novel target for cell- and molecule-specific therapies of depression and mood disorders.

Glia in brain energy metabolism: A perspective

Early views of glia as relatively inert, housekeeping cells have evolved, and glia are now recognized as dynamic cells that not only respond to neuronal activity but also sense metabolic changes and regulate neuronal metabolism. This evolution has been aided in part by technical advances permitting progressively better spatial and temporal resolution. Recent advances in cell-type specific genetic manipulation and sub-cellular metabolic probes promise to further this evolution by enabling study of metabolic interactions between intertwined fine neuronal and glial processes in vivo. Views of glia in disease processes have also evolved. Long considered purely reactive, glia and particularly microglia are now seen to play active roles in both promoting and limiting brain injury. At the same time, established concepts of glial energetics are now being linked to areas such as learning and neural network function, topics previously considered far removed from glial biology.

Peripheral administration of lactate produces antidepressant-like effects

In addition to its role as metabolic substrate that can sustain neuronal function and viability, emerging evidence supports a role for l-lactate as an intercellular signaling molecule involved in synaptic plasticity. Clinical and basic research studies have shown that major depression and chronic stress are associated with alterations in structural and functional plasticity. These findings led us to investigate the role of l-lactate as a potential novel antidepressant. Here we show that peripheral administration of l-lactate produces antidepressant-like effects in different animal models of depression that respond to acute and chronic antidepressant treatment. The antidepressant-like effects of l-lactate are associated with increases in hippocampal lactate levels and with changes in the expression of target genes involved in serotonin receptor trafficking, astrocyte functions, neurogenesis, nitric oxide synthesis and cAMP signaling. Further elucidation of the mechanisms underlying the antidepressant effects of l-lactate may help to identify novel therapeutic targets for the treatment of depression.

Brain energy metabolism: focus on astrocyte-neuron metabolic cooperation

The energy requirements of the brain are very high, and tight regulatory mechanisms operate to ensure adequate spatial and temporal delivery of energy substrates in register with neuronal activity. Astrocytes-a type of glial cell-have emerged as active players in brain energy delivery, production, utilization, and storage. Our understanding of neuroenergetics is rapidly evolving from a "neurocentric" view to a more integrated picture involving an intense cooperativity between astrocytes and neurons. This review focuses on the cellular aspects of brain energy metabolism, with a particular emphasis on the metabolic interactions between neurons and astrocytes.

Fluoxetine regulates the expression of neurotrophic/growth factors and glucose metabolism in astrocytes

RATIONALE

The pharmacological actions of most antidepressants are ascribed to the modulation of serotonergic and/or noradrenergic transmission in the brain. During therapeutic treatment for major depression, fluoxetine, one of the most commonly prescribed selective serotonin reuptake inhibitor (SSRI) antidepressants, accumulates in the brain, suggesting that fluoxetine may interact with additional targets. In this context, there is increasing evidence that astrocytes are involved in the pathophysiology of major depression.

OBJECTIVES

The aim of this study was to examine the effects of fluoxetine on the expression of neurotrophic/growth factors that have antidepressant properties and on glucose metabolism in cultured cortical astrocytes.

RESULTS

Treatment of astrocytes with fluoxetine and paroxetine, another SSRI antidepressant, upregulated brain-derived neurotrophic factor (BDNF), vascular endothelial growth factor (VEGF), and VGF mRNA expression. In contrast, the tricyclic antidepressants desipramine and imipramine did not affect the expression of these neurotrophic/growth factors. Analysis of the effects of fluoxetine on glucose metabolism revealed that fluoxetine reduces glycogen levels and increases glucose utilization and lactate release by astrocytes. Similar data were obtained with paroxetine, whereas imipramine and desipramine did not regulate glucose metabolism in this glial cell population. Our results also indicate that the effects of fluoxetine and paroxetine on glucose utilization, lactate release, and expression of BDNF, VEGF, and VGF are not mediated by serotonin-dependent mechanisms.

CONCLUSIONS

These data suggest that, by increasing the expression of specific astrocyte-derived neurotrophic factors and lactate release from astrocytes, fluoxetine may contribute to normalize the trophic and metabolic support to neurons in major depression.

VPAC and PAC receptors: From ligands to function

Vasoactive intestinal peptide (VIP) and the pituitary adenylate cyclase activating polypeptides (PACAPs) share 68% identity at the amino acid level and belong to the secretin peptide family. Following the initial discovery of VIP almost four decades ago a substantial amount of knowledge has been presented describing the mechanisms of action, distribution and pleiotropic functions of these related peptides. It is now known that the physiological actions of these widely distributed peptides are produced through activation of three common G-protein coupled receptors (VPAC(1), VPAC(2) and PAC(1)R) which preferentially stimulate adenylate cyclase and increase intracellular cAMP, although stimulation of other intracellular messengers, including calcium and phospholipase D, has been reported. Using a range of in vitro and in vivo approaches, including cell-based functional assays, transgenic animals and rodent models of disease, VPAC/PAC receptor activation has been associated with numerous physiological processes (e.g. control of circadian rhythms) and clinical conditions (e.g. pulmonary hypertension), which underlies on-going research efforts and makes these peptides and their cognate receptors attractive targets for the pharmaceutical industry. However, despite the considerable interest in VPAC/PAC receptors and the processes which they mediate, there is still a paucity of selective and available, non-peptide ligands, which has hindered further advances in this field both at the basic research and clinical level. This review summarises the current knowledge of VIP/PACAP and the VPAC/PAC receptors with regard to their distribution, pharmacology, signalling pathways, splice variants and finally, the utility of animal models in exploring their physiological roles.

Robust glycogen shunt activity in astrocytes: Effects of glutamatergic and adrenergic agents

The significance and functional roles of glycogen shunt activity in the brain are largely unknown. It represents the fraction of metabolized glucose that passes through glycogen molecules prior to entering the glycolytic pathway. The present study was aimed at elucidating this pathway in cultured astrocytes from mouse exposed to agents such as a high [K+], D-aspartate and norepinephrine (NE) known to affect energy metabolism in response to neurotransmission. Glycogen shunt activity was assessed employing [1,6-13C]glucose, and the glycogen phosphorylase inhibitor 1,4-dideoxy-1,4-imino-D-arabinitol (DAB) to block glycogen degradation. The label intensity in lactate, reflecting glycolytic activity, was determined by mass spectrometry. In the presence of NE a substantial glycogen shunt activity was observed, accounting for almost 40% of overall glucose metabolism. Moreover, when no metabolic stimulant was applied, a compensatory increase in glycolytic activity was seen when the shunt was inhibited by DAB. Actually the labeling in lactate exceeded that obtained when glycolysis and glycogen shunt both were operational, i.e. supercompensation. A similar phenomenon was seen when astrocytes were exposed to D-aspartate. In addition to glycolysis, tricarboxylic acid (TCA) cycle activity was monitored, analyzing labeling by mass spectrometry in glutamate which equilibrates with alpha-ketoglutarate. Both an elevated [K+] and D-aspartate induced an increased TCA cycle activity, which was altered when glycogen degradation was inhibited. Thus, the present study provides evidence that manipulation of glycogen metabolism affects both glycolysis and TCA cycle metabolism. Altogether, the results reveal a highly complex interaction between glycogenolysis and glycolysis, with the glycogen shunt playing a significant role in astrocytic energy metabolism.

Characterization of 1,4-dideoxy-1,4-imino-d-arabinitol (DAB) as an inhibitor of brain glycogen shunt activity

The pharmacological properties of 1,4-dideoxy-1,4-imino-d-arabinitol (DAB), a potent inhibitor of glycogen phosphorylase and synthase activity in liver preparations, were characterized in different brain tissue preparations as a prerequisite for using it as a tool to investigate brain glycogen metabolism. Its inhibitory effect on glycogen phosphorylase was studied in homogenates of brain tissue and astrocytes and IC50-values close to 400 nM were found. However, the concentration of DAB needed for inhibition of glycogen shunt activity, i.e. glucose metabolism via glycogen, in intact astrocytes was almost three orders of magnitude higher. Additionally, such complete inhibition required a pre-incubation period, a finding possibly reflecting a limited permeability of the astrocytic membrane. DAB did not affect the accumulation of 2-deoxyglucose-6-phosphate indicating that the transport of DAB is not mediated by the glucose transporter. DAB had no effect on enzymes involving glucose-6-phosphate, i.e. glucose-6-phosphate dehydrogenase, phosphoglucoisomerase and hexokinase. Furthermore, DAB was evaluated in a functional preparation of the isolated mouse optic nerve, in which its presence severely reduced the ability to sustain evoked compound action potentials in the absence of glucose, a condition in which glycogen serves as an important energy substrate. Based on the experimental findings, DAB can be used to evaluate glycogen shunt activity and its functional importance in intact brain tissue and cells at a concentration of 300-1000 muM and a pre-incubation period of 1 h.

Energy metabolism in astrocytes: high rate of oxidative metabolism and spatiotemporal dependence on glycolysis/glycogenolysis

Astrocytic energy demand is stimulated by K(+) and glutamate uptake, signaling processes, responses to neurotransmitters, Ca(2+) fluxes, and filopodial motility. Astrocytes derive energy from glycolytic and oxidative pathways, but respiration, with its high-energy yield, provides most adenosine 5' triphosphate (ATP). The proportion of cortical oxidative metabolism attributed to astrocytes ( approximately 30%) in in vivo nuclear magnetic resonance (NMR) spectroscopic and autoradiographic studies corresponds to their volume fraction, indicating similar oxidation rates in astrocytes and neurons. Astrocyte-selective expression of pyruvate carboxylase (PC) enables synthesis of glutamate from glucose, accounting for two-thirds of astrocytic glucose degradation via combined pyruvate carboxylation and dehydrogenation. Together, glutamate synthesis and oxidation, including neurotransmitter turnover, generate almost as much energy as direct glucose oxidation. Glycolysis and glycogenolysis are essential for astrocytic responses to increasing energy demand because astrocytic filopodial and lamellipodial extensions, which account for 80% of their surface area, are too narrow to accommodate mitochondria; these processes depend on glycolysis, glycogenolysis, and probably diffusion of ATP and phosphocreatine formed via mitochondrial metabolism to satisfy their energy demands. High glycogen turnover in astrocytic processes may stimulate glucose demand and lactate production because less ATP is generated when glucose is metabolized via glycogen, thereby contributing to the decreased oxygen to glucose utilization ratio during brain activation. Generated lactate can spread from activated astrocytes via low-affinity monocarboxylate transporters and gap junctions, but its subsequent fate is unknown. Astrocytic metabolic compartmentation arises from their complex ultrastructure; astrocytes have high oxidative rates plus dependence on glycolysis and glycogenolysis, and their energetics is underestimated if based solely on glutamate cycling.

Tracing the Neuroanatomical Profiles of Reward Pathways with Markers of Neuronal Activation

Functional neuroanatomical tools have played an important role in proposing which structures underlie brain stimulation reward circuitry. This review focuses on studies employing metabolic markers of neuronal and glial activation, including 2-deoxyglucose, cytochrome oxidase, and glycogen phosphorylase, and a marker of cellular activation, the immediate early gene c-fos. The principles underlying each method, their application to the study of brain stimulation reward, and their strengths and limitations are described. The usefulness of this strategy in identifying candidate structures, and the degree of overlap in the patterns of activation arising from different markers is addressed in detail. How these data have contributed to an understanding of the organization of reward circuitry and directed our thinking towards an alternative framework of neuronal arrangement is discussed in the final section.

Induction of mRNAs and proteins for Na/K ATPase alpha1 and beta1 subunits following hypoxia/reoxygenation in astrocytes

Characteristics of the cellular response to oxygen deprivation and subsequent reoxygenation (hypoxia/reoxygenation) include redirection of energy metabolism, increased glucose utilization and expression of oxygen-regulated proteins. Inhibition of protein synthesis during early reoxygenation period prevented effective astrocyte adaptation to hypoxia/reoxygenation, resulting in eventual cell death. To elucidate the role of astrocytes in the central nervous system in response to hypoxia/reoxygenation, we analyzed the cDNA library derived from the cultured rat astrocytes subjected to 24 h of hypoxia followed by reoxygenation by differential display, and isolated a cDNA corresponding to Na/K ATPase alpha1 subunit. The expression of Na/K ATPase alpha1 subunit mRNA as well as beta1subunit mRNA was transiently increased after reoxygenation, whereas hypoxia itself did not induce any gene expression change. Na/K ATPase alpha1 subunit protein was transiently increased, whereas the protein expression for Na/K ATPase beta1 subunit showed sustained induction after reoxygenation. Overexpression of beta1 subunit in HEK 293 cells subjected to hypoxia/reoxygenation promoted survival of the cells. These findings suggest that Na/K ATPases may contribute to maintain the cellular environment of astrocytes subjected to hypoxia/reoxygenation.

Effects of adrenergic agonists on glycogenolysis in primary cultures of glycogen body cells and telencephalon astrocytes of the chick

The glycogen body (GB) is at the dorsal area of the lumbosacral spinal cord in birds and is composed of uniform cells that are characterized by high-glycogen storage. Previous morphological and embryological examinations suggest that the GB is derived from the neuroepithelium and contains many blood vessels and a few nerve fibers. However, the function of the GB and role of the glycogen are unknown. Mammalian astrocytes are major sites for glycogen stores in the central nervous system. The metabolic features of astrocytes have been defined by using cultured cells. As a first step toward investigating the function of GB, we established primary culture of chicken GB cells and telencephalon astrocytes. The cultured GB cells maintained high glycogen content and glial fibrillary acidic protein (GFAP) in the cytoplasm. The glycogen content of GB cells significantly increased with the glucose concentration in the medium. The effects of adrenergic agonists on glycogenolysis were different between GB cells and telencephalon astrocytes. The telencephalon astrocytes shared similar characteristics of glycogenolysis with mouse astrocytes, which are mainly affected by beta adrenergic receptor. Although GB cells were affected by noradrenalin (both alpha and beta adrenergic agonist), they were not affected by beta adrenergic agonist. These results showed that cultured GB cells were considered as one lineage of astrocytes because of their reactivity to antibody against GFAP; however, the metabolic features of GB cells were different from those of telencephalon astrocytes.

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.

Differential regulation of adenine nucleotide translocators by hypertonicity in the brain

To determine the gene(s) induced by hypertonicity in the brain, we performed a differential display analysis using RNA isolated from isotonic and hypertonic rat astrocytes. One cDNA rapidly up-regulated by hypertonicity was isolated, and the DNA sequence revealed that it was identical to adenine nucleotide translocator (ANT)2. ANT2 protein exchanges intramitochondrial ATP for cytoplasmic ADP. Among three ANT isoforms, only ANT2 mRNA was up-regulated markedly from 1 to 4 h after exposure to hypertonicity. Induction of the mRNA did not require de novo protein synthesis. Furthermore, ADP translocase activity in mitochondria of astrocytes was increased significantly by hypertonicity. To see the localization and regulation of ANT2 mRNA in the brain, we performed in situ hybridization of rat brain after intraperitoneal injection of a high concentration of NaCl. Although there were only weak signals in the control, intense hybridization signals were seen in hypertonic rat whole brain. Microscopic examination showed that ANT2 signals were present in the neurons, as well as glial cells. These results suggest that ANT2 may play a role in brain cells to adapt to the hypertonic environment.

A comparison of the release of a vasoactive-intestinal-peptide-like peptide and acetylcholine in the giant axon-Schwann cell preparation of the tropical squid Sepioteuthis sepioidea

A vasoactive intestinal peptide (VIP)-like peptide is released by axonal stimulation in the giant axon-Schwann cell preparation from the tropical squid Sepioteuthis sepioidea. It is also released by direct application of l-glutamate, the giant axon-Schwann cell signalling molecule in this preparation. The release of the peptide parallels the release of acetylcholine from the Schwann cells themselves in this preparation in a number of different ways. The release of both acetylcholine and the VIP-like peptide have the same threshold (between 2×10(−10) and 5×10(−10)mol l-1) for l-glutamate application and the same recovery time after inhibition of release by exposure of the preparation to a prolonged pulse of l-glutamate. A prolonged l-glutamate pulse of 10(−8)mol l-1 releases both substances for as long as the pulse is applied to the preparation, whereas a prolonged pulse of 10(−9)mol l-1 l-glutamate releases acetylcholine in the same way but releases the VIP-like peptide only transiently. The VIP-like peptide is likely to be co-released with acetylcholine from the Schwann cells.

Ultrastructural evidence for combined action of noradrenaline and vasoactive intestinal polypeptide upon neurons, astrocytes, and blood vessels of the rat cerebral cortex

The intracortical organization of the noradrenaline (NA) and vasoactive intestinal polypeptide (VIP) systems provides ample opportunity for functional convergence, and accumulated evidence indicates that NA and VIP share certain cellular actions upon both neuronal and nonneuronal cortical elements. In the present study, a double immunolabeling method was combined with a silver-gold intensification procedure to examine the ultrastructural relationships of the NA coeruleocortical afferents and the intrinsic VIP neurons with three main constituents of the cortex: neurons, astrocytes, and blood vessels. Electron microscopy of singly or doubly labeled material indicated that NA and VIP boutons are engaged in a variety of anatomical relationships with both neuronal and nonneuronal elements. Dendritic shafts and perikarya of nonpyramidal neurons, some of which are VIP positive, receive combined NA and VIP synapses. A significant number of cortical microvessels are in intimate contact with NA or VIP profiles. NA axons often form perivascular loops, and VIP dendritic shafts of large diameter are frequently observed to bend around the vessel circumference. Serial section examination demonstrates that some NA boutons are directly apposed to the capillary wall at sites of glial end-feet discontinuities, whereas VIP boutons contact astrocytic sleeves of capillaries but never cross the perivascular astroglial barrier. Some VIP dendrites containing coated vesicles make intimate contact with the capillary basal lamina. Astrocytic perikarya, mainly in the supragranular layers, are also directly apposed to NA and/or VIP elements. These complex anatomical relationships provide a structural basis for the known interactions between NA and VIP in the control of cortical metabolism and function.

Peptide receptors on astrocytes

In recent years, it has become apparent that astrocytes (at least in vitro) harbor functional receptors to almost all possible neurotransmitters (with the potential noticeable exception of acetylcholine nicotinic receptors). Peptides are no exception, since receptors to all neuropeptides known to be produced in the CNS have been found on cultured astrocytes, and the presence of many of these has been confirmed on astrocytes in vivo. A variety of methodologies have been used to detect peptide receptors on astrocytes, as summarized in the current review. Special emphasis is also put on the possible roles that peptides may play in the regulation of astrocyte functions. These include proliferation, morphology, release of eicosanoids and arachidonic acid, induction of calcium transients and calcium waves, and control of internal pH, glucose uptake, glycogen metabolism, and gap junctional conductance. Recent data concerning the effects of natriuretic peptides on astrocytes are reviewed, and why these peptides may constitute priviledged tools to test the effects of peptides on astrocyte-neuron interactions is also discussed.

Cloning of a gene, YT521, for a novel RNA splicing-related protein induced by hypoxia/reoxygenation

To elucidate the role of astrocytes in the stress response of the central nervous system to ischemia, early gene expression was examined in rat cultured astrocytes after the exposure to hypoxia/reoxygenation, and we have previously cloned a novel RNA binding protein, RA301, from the reoxygenated astrocytes. Furthermore, we have now cloned a new gene for RA301 binding protein, termed YT521, by a yeast two-hybrid screening technique to explore RA301 functions. The YT521 cDNA is about 3200 bp long with an open reading frame encoding 712 amino acids. This amino acid sequence contains arginine-aspartic acid-glutamic acid rich region and glutamic acid rich one, and has a low degree of homology with RNA binding proteins such as U1-70k. Northern blot analysis revealed that YT521 mRNA expression was up-regulated in reoxygenated astrocytes. Induction of YT521 mRNA was mediated by endogenously generated reactive oxygen species, as it was suppressed by treatment of the cells with diphenyl iodonium which blocks oxygen-free radical formation by astrocytes. These expression patterns resembled those of RA301 mRNA. Far Western blot analysis showed that YT521 protein was not only interacting with RA301 protein, but also with SC35 and SF2, both of which are splicing factors. These results suggest that YT521 is a novel candidate for RNA splicing-related protein.

Neurobehavioral development of neonatal mice following blockade of VIP during the early embryonic period

Previous work has shown that blockade of VIP function in the early postimplantation embryo results in growth retardation and microcephaly. In the present work, the neurobehavioral development of neonatal mice was examined following treatment of dams with a VIP antagonist during this period. Inhibition of VIP functions during early embryogenesis impaired the performance of 5 of 10 developmental behaviors. These behaviors included developmental milestones (first appearance of ear twitch and eye opening) and complex motor behaviors (negative geotaxis, surface righting, and air righting). The retardation of neurobehavioral development produced by inhibition of VIP action indicates that this peptide is important to the progression of embryonic development.

Peptidergic transmission: from morphological correlates to functional implications

Like non-peptidergic transmitters, neuropeptides and their receptors display a wide distribution in specific cell types of the nervous system. The peptides are synthesized, typically as part of a larger precursor molecule, on the rough endoplasmic reticulum in the cell body. In the trans-Golgi network, they are sorted to the regulated secretory pathway, packaged into so-called large dense-core vesicles, and concentrated. Large dense-core vesicles are preferentially located at sites distant from active zones of synapses. Exocytosis may occur not only at synaptic specializations in axonal terminals but frequently also at nonsynaptic release sites throughout the neuron. Large dense-core vesicles are distinguished from small, clear synaptic vesicles, which contain "classical' transmitters, by their morphological appearance and, partially, their biochemical composition, the mode of stimulation required for release, the type of calcium channels involved in the exocytotic process, and the time course of recovery after stimulation. The frequently observed "diffuse' release of neuropeptides and their occurrence also in areas distant to release sites is paralleled by the existence of pronounced peptide-peptide receptor mismatches found at the light microscopic and ultrastructural level. Coexistence of neuropeptides with other peptidergic and non-peptidergic substances within the same neuron or even within the same vesicle has been established for numerous neuronal systems. In addition to exerting excitatory and inhibitory transmitter-like effects and modulating the release of other neuroactive substances in the nervous system, several neuropeptides are involved in the regulation of neuronal development.

Peptide T prevents NBM lesion-induced cortical atrophy in aged rats

Because VIP is known to be neurotrophic in vitro, the present study tested whether peptide T (PT), an octapeptide with a pentapeptide sequence homologous to VIP, could prevent nucleus basalis (NBM)-induced degenerative changes in the parietal neocortex of aged rats. Aged (20-21 months old) Sprague-Dawley rats were given bilateral neurotoxic lesions of the NBM, and injected daily with PT (1 mg, IP) or vehicle solution for 5 months. Compared to unoperated controls, vehicle-treated NBM lesioned animals had: 1) a significant 17% decrease in overall cortical thickness, 2) significant decreases of 13-29% in the thickness of cortical layers II-IV, V, and VI, and 3) significant neuronal and glial cell loss in layer V. PT treatment prevented or attenuated these lesion-induced decreases in cortical thickness and attenuated the accompanying loss of large neurons in layer V. These results provide evidence that PT1 perhaps acting via VIP receptor stimulation, is neurotrophic and important for the integrity of brain tissue following denervation.

Cloning of a novel RNA binding polypeptide (RA301) induced by hypoxia/reoxygenation

Astrocytes have a critical role in the neuronal response to ischemia, as their production of neurotrophic mediators can favorably impact on the extreme sensitivity of nervous tissue to oxygen deprivation. Using a differential display method, a novel putative RNA binding protein, RA301, was cloned from reoxygenated astrocytes. Analysis of the deduced amino acid sequence showed two ribonucleoprotein domains and serine/arginine-rich domains, suggestive of their function as RNA splicing factor. Northern analysis displayed striking induction only in cultured astrocytes within 15 min of reoxygenation and reached a maximum by 60 min after hypoxia/reoxygenation. Immunoblotting demonstrated expression of an immunoreactive polypeptide of the expected molecular mass, 36 kDa, in lysates of hypoxia/reoxygenated astrocytes. Induction of RA301 mRNA was mediated, in large part, by endogenously generated reactive oxygen species, as shown by diphenyl iodonium, an inhibitor of neutrophil-type nicotinamide adenine dinucleotide phosphate oxidase which blocks oxygen-free radical formation by astrocytes. Similarly, increased expression of RA301 in supporting a neurotrophic function of astrocytes was suggested by inhibition of interleukin-6 elaboration, a neuroprotective cytokine, in the presence of antisense oligonucleotide for RA301. These studies provide a first step in characterizing a novel putative RNA binding protein, whose expression is induced by oxygen-free radicals generated during hypoxia/reoxygenation, and which may have an important role in redirection of biosynthetic events observed in the ischemic tissues.

Pharmacological characteristics of potassium-induced, glycogenolysis in astrocytes

Elevated extracellular concentrations of the potassium ion ([K+]o) stimulate glycogenolysis in primary cultures of mouse astrocytes that have been grown in the presence of dibutyryl cyclic AMP but not in corresponding cultures which have not been treated in this manner. The response is potently inhibited by nifedipine, suggesting that it is evoked by entry of calcium ions through voltage dependent L-channels. The benzodiazepine midazolam, which is known to enhance calcium entry at concentrations of [K+]o causing submaximum calcium entry, increases the glycogenolytic effect by such levels of [K+]o.

Vasoactive intestinal peptide increases intracellular calcium in astroglia: synergism with alpha-adrenergic receptors

In type I astrocytes from rat cerebral cortex, vasoactive intestinal peptide (VIP) at concentrations below 1 nM evoked an increase in intracellular calcium ion concentration. This response, however, was observed in only 18% of the astrocytes examined. alpha-Adrenergic stimulation with phenylephrine or norepinephrine also resulted in an intracellular calcium response in these cells and the threshold sensitivity of astrocytes to phenylephrine was vastly different from cell to cell. Treatment of these astrocytes with VIP (0.1 nM) together with phenylephrine at subthreshold concentrations produced large increases in intracellular Ca2+ concentration ([Ca2+]i) and oscillations. The continued occupation of the alpha-adrenergic receptor was required for sustained synergism. Both alpha-receptor stimulation and stimulation with the mixture of agonists induced the cellular calcium response by triggering release of calcium from cellular stores, since the response persisted in the absence of extracellular calcium. Furthermore, thapsigargin pretreatment, which depletes intracellular stores, abolished the agonist-induced [Ca2+]i response. VIP (0.1 nM) and phenylephrine were found to increase cellular levels of inositol phosphates; however, there was no apparent additivity in this response when the agonists were added together. These observations suggest a calcium-mediated second messenger system for the high-affinity VIP receptor in astrocytes and that alpha-adrenergic receptors act synergistically with the VIP receptor to augment an intracellular calcium signal. The synergism between diverse receptor types may constitute an important mode of cellular signaling in astroglia.

Substance P receptors on human astrocytoma cells are linked to glycogen breakdown

In this study we report that substance P stimulated [3H]glycogen breakdown and elevation of intracellular Ca2+ concentration in the human astrocytoma cell line UC-11MG. Both effects were dose dependent, and completely blocked by CP-96,345 suggesting the involvement of an NK1 receptor. Our previous studies indicated that norepinephrine and histamine stimulate glycogenolysis via cAMP and Ca2+ respectively. Combined stimulation with substance P and norepinephrine or histamine resulted in additive effects suggesting that there is no interaction between these neurotransmitters in regulating glycogenolysis in these cells. These results confirm that UC-11MG cells are a useful model system to investigate the functional role of neurotransmitter receptors in astroglial cells.

Induction by vasoactive intestinal peptide of interferon alpha/beta synthesis in glial cells but not in neurons

Vasoactive intestinal peptide (VIP), a 28-amino acid peptide, plays a multifunctional neuromodulatory role in both peripheral and central nervous systems. We have recently reported that VIP induces interferon (IFN) alpha/beta synthesis in human colon adenocarcinoma cell line HT-29. It has been reported that VIP may counteract HIV-induced neuronal cell death; therefore, we postulated that the action of VIP may be mediated by a cascade regulation, involving the production of some cytokines such as IFN. Here we demonstrate that primary cultures of rat mesencephalic neurons and glial cells respond differently to VIP. Thus VIP enhanced 2'5' oligoadenylate (2'5' A) synthetase activity and inhibited vesicular stomatitis virus multiplication in glial cultures only. However, both cell cultures had functional adenylate cyclase coupled receptors for VIP. The increase in 2'5'A synthetase activity in glial cultures reached a maximum with 10(-6) M VIP and required cellular RNA and protein synthesis. Anti-IFN alpha/beta, but not anti-IFN gamma, antibodies abolished the induction of the antiviral and 2'5'A synthetase activities by VIP in rat glial-enriched cultures, suggesting that these inductions were mediated through IFN alpha/beta synthesis. Moreover, VIP or poly (i). poly (C12U) caused, in the glial cultures, the induction and secretion of an IFN of type alpha/beta with a titer value of 16 and 32 units/ml respectively. In contrast, neither of these two substances was able to induce IFN synthesis in neurons, which were, however, sensitive to IFN alpha/beta produced by VIP-treated glial cells. IFN produced by VIP in glial cells may therefore play an important role in defending the brain against viruses.

Thrombin receptor peptides induce shape change in neonatal murine astrocytes in culture

Astrocytes appear star-shaped in the brain, increasingly so after injury. When astroglia are cultured in serum-containing medium, they exhibit a flat, fibroblast-like morphology. In serum-free medium, astrocytes become stellate, with many long processes. The serine protease alpha-thrombin mimics the effects of serum at subnanomolar concentrations, whereas the thrombin-inhibiting serpin, protease nexin I (PNI), reverses the thrombin effect. In our current experiments, murine neonatal spinal cord astrocytes became stellate after 4 hr in serum-free medium, while cortical astrocytes required 12 hr in serum-free medium for stellation. Astrocytes from either region flattened after 60 min in medium containing 3.0 to 300 pM proteolytically active human alpha-thrombin. After 12 hr in thrombin-containing medium, 98% of the astrocytes had a flattened morphology. No flattening occurred if alpha-thrombin was replaced by gamma-thrombin, which has its fibrinogen-recognition exosite disrupted. PNI added at 1 nM to serum-containing medium caused stellation after 3 hr, and astroglia were 50% stellate by 12 hr. The effect of thrombin was mimicked by a 7-amino acid peptide (TRP-7) from the cleavage site of the human thrombin receptor. This peptide caused 40% of the astrocytes in serum-free medium to exhibit a flattened morphology after 6 hr. PNI had no effect on TRP-7 action on astrocytes. These results indicate that astrocytes possess a cell-surface receptor for thrombin, similar to that described for platelets, endothelial cells, and neurons.

Gonadal steroids as promoters of neuro-glial plasticity

Estradiol induces coordinated modifications in the extension of glial and neuronal processes in the arcuate nucleus of the hypothalamus of adult female rats. This hormonal effect results in natural fluctuations in the ensheathing of arcuate neurons by glial processes and these glial changes are linked to a remodelling of inhibitory GABAergic synapses during the estrous cycle. Hormonally induced glial and synaptic changes appear to be dependent on specific recognition or adhesion molecules on the neuronal and/or glial membranes.

Gonadal hormone regulation of glial fibrillary acidic protein immunoreactivity and glial ultrastructure in the rat neuroendocrine hypothalamus

The influence of gonadal steroids on the ultrastructure of glial cells and on the immunoreactivity for the specific astrocytic marker glial fibrillary acidic protein (GFAP) has been assessed in the neuroendocrine hypothalamus. The following parameters were analyzed in the arcuate nucleus of adult female rats: the number and the surface density of cells immunoreactive for GFAP, the number of glial profiles showing bundles of glial filaments, the size of the bundles of glial filaments, and the proportion of neuronal perikaryal membrane apposed by glial processes. These parameters were studied during the different phases of the estrous cycle, after ovariectomy, and after the administration of estradiol or progesterone to ovariectomized rats. No significant differences were detected in the number of GFAP-immunoreactive cells among the different experimental groups. The surface density of GFAP-immunoreactive material, the number of glial profiles in the neuropil, and the proportion of neuronal perikaryal membrane covered by glia were increased in the afternoon of proestrus and in the morning of estrus compared with other phases of the estrous cycle or to ovariectomized rats and showed a rapid (5 h) and reversible increase in ovariectomized rats injected with 17 beta estradiol, with a maximal effect by 24 h after the administration of the hormone. In contrast, the size of the bundles of glial filaments was decreased in the afternoon of proestrus, in the morning of estrus, and by the administration of estradiol to ovariectomized rats. The parameters studied were not affected by the administration of progesterone. However, progesterone (300 micrograms/rat) blocked the effects of 17 beta estradiol (1, 10, and 300 micrograms). The results suggest that glial cells may be actively involved in the modulation of neuroendocrine events by the hypothalamus.

Dipeptidyl peptidase II in astrocytes of the rat brain. Meningeal cells increase enzymic activity in cultivated astrocytes

Astrocytes grown in media conditioned by meningeal cells (MCM) develop cellular processes and markedly increased protein per cell. One protein component affected is the dipeptidyl peptidase II (DPP II). The increase of DPP II activity is dose- and time-dependent and can also be elicited by the second messenger cAMP. More mature astrocytes express higher levels of DPP II than immature proliferating astrocytes. The rate of proliferation of astrocytes is markedly enhanced by enriched MCM. These observations lead to the assumption that DPP II has a function within the catabolic processes of cellular differentiation. To assess whether the in vitro results may reflect in vivo conditions, we investigated the postnatal development of DPP II in the rat brain. Differentiating astrocytes in vivo are especially found early postnatally and, indeed, during this period high specific activities are found in brain. Depending on the region investigated DPP II activities decrease within the first ten days to one fourth of their P2 level and finally reach at about similar levels in all brain regions. Exceptions are the hypothalamus, where the activity is generally 1.5- to 3-fold higher than elsewhere in brain, and pons and mesencephalon, where the perinatal activity peak is lacking. The bulk activity of DPP II in immature rat brains is attributed to differentiating astrocytes loosing it in later postnatal stages due to a neuronal influence.

Endothelins modulate dibutyryl cAMP-induced stellation of cultured astrocytes

Effect of endothelin-3 (ET-3) on dibutyryl cAMP (DBcAMP)-induced stellation of rat cerebral cultured astrocytes was examined. Treatment with 1 mM DBcAMP, 10 microM forskolin, 100 microM isoproterenol and 500 nM phorbol 12-myristate 13-acetate changed protoplasmic cultured astrocytes into process-bearing ones. ET-3 (1 nM) completely prevented the astrocytic stellation induced by these agents. The effect of ET-3 showed a dose-dependence, where IC50 value and maximal effective dose were 49 pM and about 0.1 nM, respectively. ET-1 and sarafotoxin (SRTX) S6b prevented the DBcAMP-induced astrocytic stellation with potencies similar to that of ET-3. ET-3 (1 nM) did not affect the cAMP accumulation after DBcAMP treatment in cultured astrocytes. Stellate astrocytes were reversed to the protoplasmic type cells by addition of 1 nM ET-3 in the presence of DBcAMP. ET-1 and SRTX similarly reversed the astrocytic stellation. ET-3 reversed the astrocytic stellation in the absence of extracellular Ca2+. Pre-loading of BAPTA-AM, a permeable Ca2+ chelator, on stellate astrocytes had no effect on the reversal by ET-3. ET-3 did not increase intracellular free Ca2+ concentration ([Ca2+]i) of most astrocytes tested at 0.1 nM. A high concentration (100 nM) of ET-3 increased astrocytic [Ca2+]i which was negated by Ca(2+)-free and BAPTA-AM loading. These results suggest that ETs modulate morphological changes in astrocytes through cAMP- and Ca(2+)-independent mechanisms.

Sensory stimulation induces local cerebral glycogenolysis: demonstration by autoradiography

Brain glycogen stores are localized primarily to glia and undergo continuous utilization and resynthesis. To study the function of glycogen under normal conditions in brain, we developed an autoradiographic method of demonstrating local-glycogen utilization in the awake rat. The method employs labeling of brain glycogen with 14C(3,4)glucose, in situ microwave fixation of brain metabolism, and anhydrous tissue preparation. With this technique, tactile stimulation of the rat face and vibrissae was found to accelerate the utilization of labeled glycogen in brain regions known to receive sensory input from face and vibrissae: the contralateral somatosensory cortex and the ipsilateral trigeminal, sensory and motor nuclei. These findings demonstrate a link between neuronal activity and local glycogen utilization in mammalian brain and suggest that, like other tissues, brain may respond to sudden increases in energy demand in part by rapid glycolytic metabolism of glycogen. As cerebral glycogen is restricted primarily to glia, these observations also support a close coupling of glial energy metabolism with neuronal activity.

Histamine stimulates glycogenolysis in human astrocytoma cells by increasing intracellular free calcium

Astrocytes from a variety of sources, including the human UC-11MG astrocytoma line, express receptors for histamine on their plasma membranes, but the function of these receptors is largely unknown. Here we report studies on the effect of histamine on newly synthesized glycogen in the human astrocytoma-derived cell line, UC-11MG. We have found [3H]glycogen hydrolysis with a EC50 of 2 microM and a maximum effect of 30% at 300 microM histamine. The glycogenolytic effect of histamine was completely blocked by the H1 receptor antagonist, mepyramine, and was insensitive to the H2 receptor antagonist, cimetidine. Histamine-induced glycogenolysis was significantly reduced in the absence of extracellular Ca2+ and the residual response could be accounted for by Ca2+ released from intracellular stores. The Ca2+ ionophore, ionomycin, induced a similar concentration-dependent increase in both intracellular Ca2+ concentration and in glycogenolysis. These results suggest that one function of astrocytic histamine receptors in vivo may be the stimulation of glucose release from astrocytes, and that this process is mediated by increased intracellular free Ca2+. The glycogenolytic effect of histamine and other neurotransmitters in different systems, and the possible implication of astrocytic glycogenolysis in the pathophysiology of ischemia are discussed.

Somatostatin binding to a fresh rat astrocyte-enriched suspension

Since there are conflicting reports regarding the effects of somatostatin (SS) on cyclic AMP levels in astrocytes derived from rat cerebral cortex and, to date, the SS binding to mature astrocytes is unknown, the present study has determined SS binding and its effect on cyclic AMP accumulation in a fresh astrocyte-rich suspension from rat cerebral cortex. 125I-Tyr11-SS binding was inhibited by SS in a dose-dependent manner. The Scatchard analysis of binding data was linear and yielded a dissociation constant of 0.95 +/- 0.15 nM with a maximal binding capacity of 122 +/- 13 fmol/mg protein. Vasoactive intestinal peptide (VIP) stimulated cyclic AMP accumulation up to 2.3 times above the basal levels whereas SS had no effect. This effect at any of the VIP concentrations. Likewise, SS did not inhibit the stimulation of cyclic AMP accumulation provoked by other effectors such as isoproterenol and forskolin. In view of our results and those of other authors, SS receptor localized in astrocytes must be able to couple with signal transduction systems other than adenylate cyclase, in order to carry out its biological actions in the cell.

VIP receptor subtypes in mouse cerebral cortex: evidence for a differential localization in astrocytes, microvessels and synaptosomal membranes

The binding characteristics of a monoiodinated form of vasoactive intestinal peptide (M-[125I]VIP) to the membranes of astrocytes, intraparenchymal microvessels and synaptosomes were analyzed in mouse cerebral cortex. Binding to astrocytes, studied in primary cultures, indicates the presence of a single class of high affinity binding sites with a Kd of 3.3 nM and a Bmax of 565 fmol/mg protein. The structurally related peptide secretin does not compete for sites labeled by M-[125I]VIP. In cultured astrocytes, VIP has been previously shown to promote glycogenolysis. Secretin, despite its lack of interaction with sites labeled by M-[125I]VIP, stimulates glycogenolysis with an EC50 of 0.5 nM, thus demonstrating the presence in astrocytes of functional secretin receptors independent from those for VIP. Trypsinization of the primary astrocyte cultures followed by replating as secondary cultures, reveals a second class of low affinity binding sites, with a Kd of 41.3 nM and a Bmax of 881 fmol/mg protein. Secretin does not compete for this class of low affinity binding sites either. Binding of M-[125I]VIP to intraparenchymal microvessels reveals the presence of two classes of binding sites with Kd of 1.4 and 30.3 nM, and Bmax of 7.1 and 73.8 pmol/mg protein, respectively. Similar to what is observed in primary or secondary astrocyte cultures, secretin does not interact with these sites. In this cell type VIP stimulates cAMP formation with an EC50 of 18 nM, while secretin is ineffective. Finally, in agreement with previous reports in rat and guinea pig cerebral cortex, two classes of binding sites are observed in synaptosomal membranes: a high affinity class with a Kd of 4.9 nM and a Bmax of 316 fmol/mg protein, and a low affinity class with a Kd of 42.8 nM and a Bmax of 1578 fmol/mg protein. In contrast to what is observed in non-neuronal membranes, in synaptosomal membranes, secretin effectively competes for sites labeled by M-[125I]VIP with an EC50 of approximately 150 nM. These results indicate that secretin may represent a useful tool to discriminate between neuronal and non-neuronal VIP binding sites, since it competes with M-[125I]VIP exclusively for the neuronal class of binding sites.

Vasoactive intestinal polypeptide modulates neuronal excitability in hippocampal slices of the rat

Vasoactive intestinal polypeptide added at submicromolar concentrations to the perfusion fluid of rat hippocampal slices and slice cultures enhanced the excitability of CA1 and CA3 pyramidal cells in several ways. Specifically, cells were depolarized and the Ca(2+)- and cyclic AMP-dependent potassium conductance was blocked as demonstrated by reduction of the long-lasting afterhyperpolarization and the accommodation of firing. This was also found in tetrodotoxin-containing medium. In low Ca(2+)-high Mg2+ medium (in synaptic isolation) the firing rate was increased. Synaptic transmission was potentiated: extracellularly registered excitatory postsynaptic potentials and population spikes in response to stratum radiatum stimulation and intracellularly recorded excitatory postsynaptic potential-inhibitory postsynaptic potential sequences were enhanced. These results are in keeping with the known stimulation of adenylate cyclase by vasoactive intestinal polypeptide.