On the role of voltage-dependent calcium channels in calcium signaling of astrocytes in situ

Electrically-evoked oscillating calcium transients in mono- and co-cultures of iPSC glia and sensory neurons

Activated glia are known to exhibit either neuroprotective or neurodegenerative effects, depending on their phenotype, while participating in chronic pain regulation. Until recently, it has been believed that satellite glial cells and astrocytes are electrically slight and process stimuli only through intracellular calcium flux that triggers downstream signaling mechanisms. Though glia do not exhibit action potentials, they do express both voltage- and ligand-gated ion channels that facilitate measurable calcium transients, a measure of their own phenotypic excitability, and support and modulate sensory neuron excitability through ion buffering and secretion of excitatory or inhibitory neuropeptides (i.e., paracrine signaling). We recently developed a model of acute and chronic nociception using co-cultures of iPSC sensory neurons (SN) and spinal astrocytes on microelectrode arrays (MEAs). Until recently, only neuronal extracellular activity has been recorded using MEAs with a high signal-to-noise ratio and in a non-invasive manner. Unfortunately, this method has limited compatibility with simultaneous calcium transient imaging techniques, which is the most common method for monitoring the phenotypic activity of astrocytes. Moreover, both dye-based and genetically encoded calcium indicator imaging rely on calcium chelation, affecting the culture’s long-term physiology. Therefore, it would be ideal to allow continuous and simultaneous direct phenotypic monitoring of both SNs and astrocytes in a high-to-moderate throughput non-invasive manner and would significantly advance the field of electrophysiology. Here, we characterize astrocytic oscillating calcium transients (OCa2+Ts) in mono- and co-cultures of iPSC astrocytes as well as iPSC SN-astrocyte co-cultures on 48 well plate MEAs. We demonstrate that astrocytes exhibit OCa2+Ts in an electrical stimulus amplitude- and duration-dependent manner. We show that OCa2+Ts can be pharmacologically inhibited with the gap junction antagonist, carbenoxolone (100 μM). Most importantly, we demonstrate that both neurons and glia can be phenotypically characterized in real time, repeatedly, over the duration of the culture. In total, our findings suggest that calcium transients in glial populations may serve as a stand-alone or supplemental screening technique for identifying potential analgesics or compounds targeting other glia-mediated pathologies.

Endocannabinoid signaling in astrocytes

The study of the astrocytic contribution to brain functions has been growing in popularity in the neuroscience field. In the last years, and especially since the demonstration of the involvement of astrocytes in synaptic functions, the astrocyte field has revealed multiple functions of these cells that seemed inconceivable not long ago. In parallel, cannabinoid investigation has also identified different ways by which cannabinoids are able to interact with these cells, modify their functions, alter their communication with neurons and impact behavior. In this review, we will describe the expression of different endocannabinoid system members in astrocytes. Moreover, we will relate the latest findings regarding cannabinoid modulation of some of the most relevant astroglial functions, namely calcium (Ca²⁺ ) dynamics, gliotransmission, metabolism, and inflammation.

Complementary encoding of spatial information in hippocampal astrocytes

Calcium dynamics into astrocytes influence the activity of nearby neuronal structures. However, because previous reports show that astrocytic calcium signals largely mirror neighboring neuronal activity, current information coding models neglect astrocytes. Using simultaneous two-photon calcium imaging of astrocytes and neurons in the hippocampus of mice navigating a virtual environment, we demonstrate that astrocytic calcium signals encode (i.e., statistically reflect) spatial information that could not be explained by visual cue information. Calcium events carrying spatial information occurred in topographically organized astrocytic subregions. Importantly, astrocytes encoded spatial information that was complementary and synergistic to that carried by neurons, improving spatial position decoding when astrocytic signals were considered alongside neuronal ones. These results suggest that the complementary place dependence of localized astrocytic calcium signals may regulate clusters of nearby synapses, enabling dynamic, context-dependent variations in population coding within brain circuits.

Astrocytic contribution to glutamate-related central respiratory chemoreception in vertebrates

Central respiratory chemoreceptors play a key role in the respiratory homeostasis by sensing CO₂ and H⁺ in brain and activating the respiratory neural network. This ability of specific brain regions to respond to acidosis and hypercapnia is based on neuronal and glial mechanisms. Several decades ago, glutamatergic transmission was proposed to be involved as a main mechanism in central chemoreception. However, a complete identification of mechanism has been elusive. At the rostral medulla, chemosensitive neurons of the retrotrapezoid nucleus (RTN) are glutamatergic and they are stimulated by ATP released by RTN astrocytes in response to hypercapnia. In addition, recent findings show that caudal medullary astrocytes in brainstem can also contribute as CO₂ and H⁺ sensors that release D-serine and glutamate, both gliotransmitters able to activate the respiratory neural network. In this review, we describe the mammalian astrocytic glutamatergic contribution to the central respiratory chemoreception trying to trace in vertebrates the emergence of several components involved in this process.

Negative Feedback Role of Astrocytes in Shaping Excitation in Brain Cell Co-cultures

Glial cells play an important role in maintaining neuronal homeostasis and may thus influence excitability in epileptogenesis. These cells in the brain have glutamate (Glu) transporters, which remove this neurotransmitter from the extracellular space. Lack of negative (-) feedback makes local neuronal circuits more excitable and potentially contributing to epileptogenic phenomena. In this study, the role of glial cells in providing (-) feedback is shown through different models of brain cells in culture imaged for intracellular calcium concentration [(Ca²⁺)_i]. Moreover, here we study the individual cells by putting them in categories. Neuronal networks with high and low (-) feedback were established by using anti-mitotics to deplete glial cells. Separate stimuli with very low subthreshold concentrations of Glu (250-750 nM) were added to cultures to test if the order of stimulations matter in regard to calcium dynamics outcomes. Additionally, KCl and ATP were used to stimulate glial cells. We found that for cultures high in (-) feedback, order of the stimulus was not important in predicting cellular responses and because of the complexity of networks in low (-) feedback cultures the order of stimulus matters. As an additional method for analysis, comparison of high (-) feedback cultures, and pure astrocytes was also considered. Glial cells in pure astrocyte cultures tend to be larger in size than glial cells in high (-) feedback cultures. The potential effect of (-) feedback at the blood brain barrier (BBB) was also considered for the inflammatory responses of nitric oxide (NO) production and [Ca²⁺]_i regulation using brain microvascular endothelial cells (BMVECs). The inflammatory and calcium signaling pathways both indicate the negative feedback role of astrocytes, poised between the BBB and structures deeper within the brain, where neuronal synapses are homeostatically maintained by glial uptake of neurotransmitters.

Two-photon calcium imaging of neuronal and astrocytic responses: the influence of electrical stimulus parameters and calcium signaling mechanisms

Objective. Electrical brain stimulation has been used to ameliorate symptoms associated with neurologic and psychiatric disorders. The astrocytic activation and its interaction with neurons may contribute to the therapeutic effects of electrical stimulation. However, how the astrocytic activity is affected by electrical stimulation and its calcium signaling mechanisms remain largely unknown. This study is to explore the influence of electrical stimulus parameters on cellular calcium responses and corresponding calcium signaling mechanisms, with a focus on the heretofore largely overlooked astrocytes.Approach. Usingin vivotwo-photon microscopy in mouse somatosensory cortex, the calcium activity in neurons and astrocytes were recorded.Main results. The cathodal stimulation evoked larger responses in both neurons and astrocytes than anodal stimulation. Both neuronal and astrocytic response profiles exhibited the unimodal frequency dependency, the astrocytes prefer higher frequency stimulation than neurons. Astrocytes need longer pulse width and higher current intensity than neurons to activate. Compared to neurons, the astrocytes were not capable of keeping sustained calcium elevation during prolonged electrical stimulation. The neuronal Ca²⁺influx involves postsynaptic effects and direct depolarization. The Ca²⁺surge of astrocytes has a neuronal origin, the noradrenergic and glutamatergic signaling act synergistically to induce astrocytic activity.Significance. The astrocytic activity can be regulated by manipulating stimulus parameters and its calcium activation should be fully considered when interpreting the mechanisms of action of electrical neuromodulation. This study brings considerable benefits in the application of electrical stimulation and provides useful insights into cortical signal transduction, which contributes to the understanding of mechanisms underlying the therapeutic efficacy of electrical stimulation for neurorehabilitation applications.

Altered astrocytic function in experimental neuroinflammation and multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) that affects about 2.5 million people worldwide. In MS, the patients' immune system starts to attack the myelin sheath, leading to demyelination, neurodegeneration, and, ultimately, loss of vital neurological functions such as walking. There is currently no cure for MS and the available treatments only slow the initial phases of the disease. The later-disease mechanisms are poorly understood and do not directly correlate with the activity of immune system cells, the main target of the available treatments. Instead, evidence suggests that disease progression and disability are better correlated with the maintenance of a persistent low-grade inflammation inside the CNS, driven by local glial cells, like astrocytes and microglia. Depending on the context, astrocytes can (a) exacerbate inflammation or (b) promote immunosuppression and tissue repair. In this review, we will address the present knowledge that exists regarding the role of astrocytes in MS and experimental animal models of the disease.

Astrocytic Calcium Dynamics Along the Pain Pathway

Astrocytes, once thought to be passive cells merely filling the space between neurons in the nervous system, are receiving attention as active modulators of the brain and spinal cord physiology by providing nutrients, maintaining homeostasis, and modulating synaptic transmission. Accumulating evidence indicates that astrocytes are critically involved in chronic pain regulation. Injury induces astrocytes to become reactive, and recent studies suggest that reactive astrocytes can have either neuroprotective or neurodegenerative effects. While the exact mechanisms underlying the transition from resting astrocytes to reactive astrocytes remain unknown, astrocytic calcium increase, coordinated by inflammatory molecules, has been suggested to trigger this transition. In this mini review article, we will discuss the roles of astrocytic calcium, channels contributing to calcium dynamics in astrocytes, astrocyte activations along the pain pathway, and possible relationships between astrocytic calcium dynamics and chronic pain.

Molecular characterization of frog vocal neurons using constellation pharmacology

Identification and characterization of neuronal cell classes in motor circuits are essential for understanding the neural basis of behavior. It is a challenging task, especially in a non-genetic-model organism, to identify cell-specific expression of functional macromolecules. Here, we performed constellation pharmacology, calcium imaging of dissociated neurons to pharmacologically identify functional receptors expressed by vocal neurons in adult male and female African clawed frogs, Xenopus laevis. Previously we identified a population of vocal neurons called fast trill neurons (FTNs) in the amphibian parabrachial nucleus (PB) that express N-methyl-d-aspartate (NMDA) receptors and GABA and/or glycine receptors. Using constellation pharmacology, we identified four cell classes of putative fast trill neurons (pFTNs, responsive to both NMDA and GABA/glycine applications). We discovered that some pFTNs responded to the application of substance P (SP), acetylcholine (ACh), or both. Electrophysiological recordings obtained from FTNs using an ex vivo preparation verified that SP and/or ACh depolarize FTNs. Bilateral injection of ACh, SP, or their antagonists into PBs showed that ACh receptors are not sufficient but necessary for vocal production, and SP receptors play a role in shaping the morphology of vocalizations. Additionally, we discovered that the PB of adult female X. laevis also contains all the subclasses of neurons at a similar frequency as in males, despite their sexually distinct vocalizations. These results reveal novel neuromodulators that regulate X. laevis vocal production and demonstrate the power of constellation pharmacology in identifying the neuronal subtypes marked by functional expression of cell-specific receptors in non-genetic-model organisms.NEW & NOTEWORTHY Molecular profiles of neurons are critical for understanding the neuronal functions, but their identification is challenging especially in non-genetic-model organisms. Here, we characterized the functional expression of membrane macromolecules in vocal neurons of African clawed frogs, Xenopus laevis, using a technique called constellation pharmacology. We discovered that receptors for acetylcholine and/or substance P are expressed by some classes of vocal neurons, and their activation plays a role in the production of normal vocalizations.

Response to pregabalin and progesterone differs in male and female rat models of neuropathic and cancer pain

Background: Cancer pain involves nervous system damage and pathological neurogenesis. Neuropathic pain arises from damage to the nervous system and is driven by ectopic signaling. Both progesterone and pregabalin are neuroprotective in animal models, and there is evidence that both drugs bind to and inhibit voltage-gated calcium channels. Aims: This study was designed to characterize the effects of progesterone and pregabalin in preclinical models of cancer and neuropathic pain in both sexes. Methods: We measured peripheral sensory signaling by intracellular in vivo electrophysiology and behavioral indicators of pain in rat models of cancer-induced bone pain and neuropathic pain. Results: Female but not male models of cancer pain showed a behavioral response to treatment and pregabalin reduced excitability in C and A high-threshold but not low-threshold sensory neurons of both sexes. Male models of neuropathic pain treated with pregabalin demonstrated higher signaling thresholds only in A high-threshold neurons, and behavioral data indicated a clear recovery to baseline mechanical withdrawal thresholds in all treatment groups. Female rat treatment groups did not show excitability changes in sensory neurons, but all demonstrated higher mechanical withdrawal thresholds than vehicle-treated females, although not to baseline levels. Athymic female rat models of neuropathic pain showed no behavioral or electrophysiological responses to treatment. Conclusions: Both pregabalin and progesterone showed evidence of efficacy in male models of neuropathic pain. These results add to the evidence demonstrating differential effects of treatments for pain in male and female animals and widely differing responses in models of cancer and neuropathic pain.

Calcium Signaling in Glioma Cells: The Role of Nucleotide Receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y₁ and P2Y₁₂ nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Curcumane C and (±)-curcumane D, an unusual seco-cadinane sesquiterpenoid and a pair of unusual nor-bisabolane enantiomers with significant vasorelaxant activity from Curcuma longa

A new seco-cadinane sesquiterpenoid (curcumane C, 1) and a pair of new nor-bisabolene enantiomers [(+)- and (-)-curcumane D, 2a and 2b] were isolated from C. longa. Compound 1 possesses an unusual 4,5-seco-cadinane skeleton with a tetrahydrophthalide moiety, while 2a and 2b contain an unusual 15-nor-bisabolene skeleton with a chromone core. All compounds exhibited significant vasorelaxant effects against KCl-induced contraction of rat aortic rings. Compound 1 also exhibited a vasorelaxant effect against phenylephrine-induced contraction of rat aortic rings. Meanwhile, compound 1 showed a stronger vasorelaxant effect in endothelium-intact rat aortic rings compared with endothelium-denuded rat aortic rings, indicating that vasodilation by 1 involved both endothelium-dependent and endothelium-independent pathways. Furthermore, compound 1 increased the NO content in human umbilical vein endothelial cells and its vasorelaxant effect could be attenuated by treatment with L-NAME, an endothelium NO synthase inhibitor. Thus, the underlying vasodilatory mechanisms of 1 may be mediated via abrogation of extracellular Ca²⁺ influx and regulation of NO release in vascular endothelial cells.

The Role of Astrocytes in the Central Nervous System Focused on BK Channel and Heme Oxygenase Metabolites: A Review

Astrocytes outnumber neurons in the human brain, and they play a key role in numerous functions within the central nervous system (CNS), including glutamate, ion (i.e., Ca²⁺, K⁺) and water homeostasis, defense against oxidative/nitrosative stress, energy storage, mitochondria biogenesis, scar formation, tissue repair via angiogenesis and neurogenesis, and synapse modulation. After CNS injury, astrocytes communicate with surrounding neuronal and vascular systems, leading to the clearance of disease-specific protein aggregates, such as β-amyloid, and α-synuclein. The astrocytic big conductance K⁺ (BK) channel plays a role in these processes. Recently, potential therapeutic agents that target astrocytes have been tested for their potential to repair the brain. In this review, we discuss the role of the BK channel and antioxidant agents such as heme oxygenase metabolites following CNS injury. A better understanding of the cellular and molecular mechanisms of astrocytes’ functions in the healthy and diseased brains will greatly contribute to the development of therapeutic approaches following CNS injury, such as Alzheimer’s disease, Parkinson’s disease, and stroke.

Huntingtin-Associated Protein 1A Regulates Store-Operated Calcium Entry in Medium Spiny Neurons From Transgenic YAC128 Mice, a Model of Huntington's Disease

Huntington's disease (HD) is a hereditary neurodegenerative disease that is caused by polyglutamine expansion within the huntingtin (HTT) gene. One of the cellular activities that is dysregulated in HD is store-operated calcium entry (SOCE), a process by which Ca²⁺ release from the endoplasmic reticulum (ER) induces Ca²⁺ influx from the extracellular space. HTT-associated protein-1 (HAP1) is a binding partner of HTT. The aim of the present study was to examine the role of HAP1A protein in regulating SOCE in YAC128 mice, a transgenic model of HD. After Ca²⁺ depletion from the ER by the activation of inositol-(1,4,5)triphosphate receptor type 1 (IP₃R1), we detected an increase in the activity of SOC channels when HAP1 protein isoform HAP1A was overexpressed in medium spiny neurons (MSNs) from YAC128 mice. A decrease in the activity of SOC channels in YAC128 MSNs was observed when HAP1 protein was silenced. In YAC128 MSNs that overexpressed HAP1A, an increase in activity of IP₃R1 was detected while the ionomycin-sensitive ER Ca²⁺ pool decreased. 6-Bromo-N-(2-phenylethyl)-2,3,4,9-tetrahydro-1H-carbazol-1-amine hydrochloride (C₂₀H₂₂BrClN₂), identified in our previous studies as a SOCE inhibitor, restored the elevation of SOCE in YAC128 MSN cultures that overexpressed HAP1A. The IP₃ sponge also restored the elevation of SOCE and increased the release of Ca²⁺ from the ER in YAC128 MSN cultures that overexpressed HAP1A. The overexpression of HAP1A in the human neuroblastoma cell line SK-N-SH (i.e., a cellular model of HD (SK-N-SH HTT138Q)) led to the appearance of a pool of constitutively active SOC channels and an increase in the expression of STIM2 protein. Our results showed that HAP1A causes the activation of SOC channels in HD models by affecting IP₃R1 activity.

Patchouli alcohol isolated from Pogostemon cablin mediates endothelium-independent vasorelaxation by blockade of Ca2+ channels in rat isolated thoracic aorta

ETHNOPHARMACOLOGICAL RELEVANCE

The aerial parts of Pogostemon cablin (Blanco) Benth. for the treatment of cardiodynia have been documented in Mingyi Bielu of late Han Dynasty, in addition to that the Ca²⁺ antagonized activities of P. cablin and its critically pharmacological ingredient patchouli alcohol (PA) were reported previously.

AIM OF THE STUDY

To investigate the relaxant effects of PA on rat isolated thoracic aortas and further explore its potential mechanisms of actions.

MATERIALS AND METHODS

The aortas with endothelium and without endothelium were prepared and suspended in the organ bath for isometric tension recordings. The responses to accumulative concentrations of PA in endothelium-intact (E + ) aortas with basal tension and in different treated aortas pre-contracted with potassium chloride (KCl) or phenylephrine (PHE) were observed; the effects of L-NAME and indomethacin in aortas with intact endothelium, and of L-NAME, propranolol, tetraethtylamine (TEA), 4-aminopyridine (4-AP), barium chloride (BaCl₂), glyburide in aortas with endothelial denudation on PA-produced vasorelaxation were assessed; the influences of PA on extracellular Ca²⁺ influx and intracellular Ca²⁺ release were measured in Ca²⁺-free medium. Finally, the abilities of PA to inhibit KCl- and PHE-induced contractions were compared to that of verapamil in E- aortas.

RESULTS

PA produced vasorelaxant effects in KCl- and PHE-precontracted E + aortas in a concentration-dependent manner, which had no statistically different from that in KCl- and PHE-precontracted E- aortas. PA (10 μM) significantly reduced KCl-induced contractions while PHE-induced contractions were significantly reduced by 100 μM of PA instead of 10 μM and 30 μM in aortas with endothelium. Pre-incubation of E + aortas with L-NAME as well as indomethacin and of E- aortas with L-NAME, propranolol, TEA, 4-AP, BaCl₂ and glyburide had no obvious effects on vasorelaxation of PA. In endothelium-removed aortas around Ca²⁺-free solution, PA remarkably lowered the contractions stimulated with Ca²⁺ and PHE, and application of ruthenium red and heparin further enhanced the abilities of PA to reduce PHE-caused contractions. In aortas without endothelium, 100 μM of PA markedly attenuated KCl-induced contractions but not affect PHE-induced contractions. Verapamil at the equal dose markedly attenuated KCl- and PHE-induced contractions, and the inhibitory effects on KCl-induced contractions were more forceful than that on PHE-induced contractions. In combined usage, the inhibitory effects on the contractions elicited by KCl were evidently weaker than that of verapamil alone and indifferently stronger than that of PA alone, and the inhibitory effects on the contractions elicited by PHE were evidently weaker than that of single verapamil but stronger than that of single PA.

CONCLUSION

PA may act as a Ca²⁺ antagonist to exert an intensively vasorelaxant effects through endothelium-independent pathway, and its mechanisms underlying the vasoactivities seem to be associated with the blockade of extracellular Ca²⁺ influx through VDCCs and ROCCs in vascular smooth muscle cells (VSMCs) membrane and intracellular Ca²⁺ releases through IP₃R- and RYR-mediated Ca²⁺ channels in sarcolemma.

GABA and Gap Junctions in the Development of Synchronized Activity in Human Pluripotent Stem Cell-Derived Neural Networks

The electrical activity of the brain arises from single neurons communicating with each other. However, how single neurons interact during early development to give rise to neural network activity remains poorly understood. We studied the emergence of synchronous neural activity in human pluripotent stem cell (hPSC)-derived neural networks simultaneously on a single-neuron level and network level. The contribution of gamma-aminobutyric acid (GABA) and gap junctions to the development of synchronous activity in hPSC-derived neural networks was studied with GABA agonist and antagonist and by blocking gap junctional communication, respectively. We characterized the dynamics of the network-wide synchrony in hPSC-derived neural networks with high spatial resolution (calcium imaging) and temporal resolution microelectrode array (MEA). We found that the emergence of synchrony correlates with a decrease in very strong GABA excitation. However, the synchronous network was found to consist of a heterogeneous mixture of synchronously active cells with variable responses to GABA, GABA agonists and gap junction blockers. Furthermore, we show how single-cell distributions give rise to the network effect of GABA, GABA agonists and gap junction blockers. Finally, based on our observations, we suggest that the earliest form of synchronous neuronal activity depends on gap junctions and a decrease in GABA induced depolarization but not on GABA_A mediated signaling.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Physiology of Astroglia

Astrocytes are neural cells of ectodermal, neuroepithelial origin that provide for homeostasis and defense of the central nervous system (CNS). Astrocytes are highly heterogeneous in morphological appearance; they express a multitude of receptors, channels, and membrane transporters. This complement underlies their remarkable adaptive plasticity that defines the functional maintenance of the CNS in development and aging. Astrocytes are tightly integrated into neural networks and act within the context of neural tissue; astrocytes control homeostasis of the CNS at all levels of organization from molecular to the whole organ.

Glycogenolysis, an Astrocyte-Specific Reaction, is Essential for Both Astrocytic and Neuronal Activities Involved in Learning

In brain glycogen, formed from glucose, is degraded (glycogenolysis) in astrocytes but not in neurons. Although most of the degradation follows the same pathway as glucose, its breakdown product, l-lactate, is released from astrocytes in larger amounts than glucose when glycogenolysis is activated by noradrenaline. However, this is not the case when glycogenolysis is activated by high potassium ion (K⁺) concentrations - possibly because noradrenaline in contrast to high K⁺ stimulates glycogenolysis by an increase not only in free cytosolic Ca²⁺ concentration ([Ca²⁺]_i) but also in cyclic AMP (c-AMP), which may increase the expression of the monocarboxylate transporter through which it is released. Several transmitters activate glycogenolysis in astrocytes and do so at different time points after training. This stimulation is essential for memory consolidation because glycogenolysis is necessary for uptake of K⁺ and stimulates formation of glutamate from glucose, and therefore is needed both for removal of increased extracellular K⁺ following neuronal excitation (which initially occurs into astrocytes) and for formation of transmitter glutamate and GABA. In addition the released l-lactate has effects on neurons which are essential for learning and for learning-related long-term potentiation (LTP), including induction of the neuronal gene Arc/Arg3.1 and activation of gene cascades mediated by CREB and cofilin. Inhibition of glycogenolysis blocks learning, LTP and all related molecular events, but all changes can be reversed by injection of l-lactate. The effect of extracellular l-lactate is due to both astrocyte-mediated signaling which activates noradrenergic activity on all brain cells and to a minor uptake, possibly into dendritic spines.

Severe Convulsions and Dysmyelination in Both Jimpy and Cx32/47 -/- Mice may Associate Astrocytic L-Channel Function with Myelination and Oligodendrocytic Connexins with Internodal Kv Channels

The Jimpy mouse illustrates the importance of interactions between astrocytes and oligodendrocytes. It has a mutation in Plp coding for proteolipid protein and DM20. Its behavior is normal at birth but from the age of ~2 weeks it shows severe convulsions associated with oligodendrocyte/myelination deficits and early death. A normally occurring increase in oxygen consumption by highly elevated K+ concentrations is absent in Jimpy brain slices and cultured astrocytes, reflecting that Plp at early embryonic stages affects common precursors as also shown by the ability of conditioned medium from normal astrocytes to counteract histological abnormalities. This metabolic response is now known to reflect opening of L-channels for Ca2+. The resulting deficiency in Ca2+ entry has many consequences, including lack of K+-stimulated glycogenolysis and release of gliotransmitter ATP. Lack of purinergic stimulation compromises oligodendrocyte survival and myelination and affects connexins and K+ channels. Mice lacking the oligodendrocytic connexins Cx32 and 47 show similar neurological dysfunction as Jimpy. This possibly reflects that K+ released by intermodal axonal Kv channels is transported underneath a loosened myelin sheath instead of reaching the extracellular space via connexin-mediated transport to oligodendrocytes, followed by release and astrocytic Na+,K+-ATPase-driven uptake with subsequent Kir4.1-facilitated release and neuronal uptake.

Tetrahydrocarbazoles decrease elevated SOCE in medium spiny neurons from transgenic YAC128 mice, a model of Huntington's disease

Huntington's disease (HD) is a hereditary neurodegenerative disease caused by a polyglutamine expansion within the huntingtin (HTT) gene. One of the cellular functions that is dysregulated in HD is store-operated calcium entry (SOCE), a process in which the depletion of Ca²⁺ from the endoplasmic reticulum (ER) induces Ca²⁺ influx from the extracellular space. We detected an enhanced activity of SOC channels in medium spiny neurons (MSNs) from YAC128 mice, a transgenic model of HD, and investigated whether this could be reverted by tetrahydrocarbazoles. The compound 6-bromo-N-(2-phenylethyl)-2,3,4,9-tetrahydro-1H-carbazol-1-amine hydrochloride was indeed able to restore the disturbed Ca²⁺ homeostasis and stabilize SOCE in YAC128 MSN cultures. We also detected a beneficial effect of this compound on the mitochondrial membrane potential. Since dysregulated Ca²⁺ homeostasis is believed to be one of the pathological hallmarks of HD, this compound might be a lead structure for HD treatment.

L-type voltage-operated calcium channels contribute to astrocyte activation In vitro

We have found a significant upregulation of L-type voltage-operated Ca(++) channels (VOCCs) in reactive astrocytes. To test if VOCCs are centrally involved in triggering astrocyte reactivity, we used in vitro models of astrocyte activation in combination with pharmacological inhibitors, siRNAs and the Cre/lox system to reduce the activity of L-type VOCCs in primary cortical astrocytes. The endotoxin lipopolysaccharide (LPS) as well as high extracellular K(+) , glutamate, and ATP promote astrogliosis in vitro. L-type VOCC inhibitors drastically reduce the number of reactive cells, astrocyte hypertrophy, and cell proliferation after these treatments. Astrocytes transfected with siRNAs for the Cav1.2 subunit that conducts L-type Ca(++) currents as well as Cav1.2 knockout astrocytes showed reduce Ca(++) influx by ∼80% after plasma membrane depolarization. Importantly, Cav1.2 knock-down/out prevents astrocyte activation and proliferation induced by LPS. Similar results were found using the scratch wound assay. After injuring the astrocyte monolayer, cells extend processes toward the cell-free scratch region and subsequently migrate and populate the scratch. We found a significant increase in the activity of L-type VOCCs in reactive astrocytes located in the growing line in comparison to quiescent astrocytes situated away from the scratch. Moreover, the migration of astrocytes from the scratching line as well as the number of proliferating astrocytes was reduced in Cav1.2 knock-down/out cultures. In summary, our results suggest that Cav1.2 L-type VOCCs play a fundamental role in the induction and/or proliferation of reactive astrocytes, and indicate that the inhibition of these Ca(++) channels may be an effective way to prevent astrocyte activation. GLIA 2016. GLIA 2016;64:1396-1415.

Association of rat thoracic aorta dilatation by astragaloside IV with the generation of endothelium-derived hyperpolarizing factors and nitric oxide, and the blockade of Ca2+ channels

The aim of the present study was to elucidate the roles of endothelium-derived hyperpolarizing factors (EDHFs) and nitric oxide (NO) in mediating the vasodilatation response to astragaloside IV and the effects of astragaloside IV on voltage-dependent Ca²⁺ channels and receptor-operated Ca²⁺ channels in rat thoracic aortic rings precontracted with potassium chloride (KCl; 60 mM) or phenylephrine (PHE; 1 µM). The results showed that astragaloside IV (1×10⁻⁴-3×10⁻¹ g/l) concentration-dependently relaxed the contraction induced by KCl (10–90 mM) or PHE (1×10⁻⁹-3×10⁻⁵ µM) and inhibited concentration-contraction curves for the two vasoconstrictors in the aortic rings. Preincubation with N^ω-nitro-L-arginine methyl ester (L-NAME, 100 µM) significantly attenuated astragaloside IV-induced relaxation in the endothelium-intact and -denuded arterial rings precontracted with PHE. Astragaloside IV, following preincubation with L-NAME (100 µM) plus indomethacin (10 µM), exerted vasodilatation, which was depressed by tetraethtylamine (1 mM) and propargylglycine (100 µM), but not by carbenoxolone (10 µM), catalase (500 U/ml) or proadifen hydrochloride (10 µM). The action mode of astragaloside IV was evident in comparison to nifedipine. Inhibition of PHE-induced contraction by astragaloside IV (100 mg/l) was more potent compared to inhibition of KCl-induced contraction, while inhibition of KCl-induced contraction by nifedipine (100 mg/l) was more potent compared to inhibition of PHE-induced contraction by nifedipine (100 mg/l). In addition, the combination of astragaloside IV and nifedipine exhibited synergistic and additive inhibitory effects on contraction evoked by KCl, which was similar to PHE. In conclusion, astragaloside IV, as a Ca²⁺ antagonist, relaxes the vessels through the blockade of superior receptor-operated Ca²⁺ and inferior voltage-dependent Ca²⁺ channels, which modulate NO from vascular endothelial cells and vascular smooth muscle cells, and EDHFs including K⁺ and hydrogen sulfide.

Calcium Imaging of AM Dyes Following Prolonged Incubation in Acute Neuronal Tissue

Calcium-imaging is a sensitive method for monitoring calcium dynamics during neuronal activity. As intracellular calcium concentration is correlated to physiological and pathophysiological activity of neurons, calcium imaging with fluorescent indicators is one of the most commonly used techniques in neuroscience today. Current methodologies for loading calcium dyes into the tissue require prolonged incubation time (45-150 min), in addition to dissection and recovery time after the slicing procedure. This prolonged incubation curtails experimental time, as tissue is typically maintained for 6-8 hours after slicing. Using a recently introduced recovery chamber that extends the viability of acute brain slices to more than 24 hours, we tested the effectiveness of calcium AM staining following long incubation periods post cell loading and its impact on the functional properties of calcium signals in acute brain slices and wholemount retinae. We show that calcium dyes remain within cells and are fully functional >24 hours after loading. Moreover, the calcium dynamics recorded >24 hrs were similar to the calcium signals recorded in fresh tissue that was incubated for <4 hrs. These results indicate that long exposure of calcium AM dyes to the intracellular cytoplasm did not alter the intracellular calcium concentration, the functional range of the dye or viability of the neurons. This data extends our previous work showing that a custom recovery chamber can extend the viability of neuronal tissue, and reliable data for both electrophysiology and imaging can be obtained >24hrs after dissection. These methods will not only extend experimental time for those using acute neuronal tissue, but also may reduce the number of animals required to complete experimental goals.

Astrocytes regulate heterogeneity of presynaptic strengths in hippocampal networks

Dendrites are neuronal structures specialized for receiving and processing information through their many synaptic inputs. How input strengths are modified across dendrites in ways that are crucial for synaptic integration and plasticity remains unclear. We examined in single hippocampal neurons the mechanism of heterosynaptic interactions and the heterogeneity of synaptic strengths of pyramidal cell inputs. Heterosynaptic presynaptic plasticity that counterbalances input strengths requires N-methyl-d-aspartate receptors (NMDARs) and astrocytes. Importantly, this mechanism is shared with the mechanism for maintaining highly heterogeneous basal presynaptic strengths, which requires astrocyte Ca(2+) signaling involving NMDAR activation, astrocyte membrane depolarization, and L-type Ca(2+) channels. Intracellular infusion of NMDARs or Ca(2+)-channel blockers into astrocytes, conditionally ablating the GluN1 NMDAR subunit, or optogenetically hyperpolarizing astrocytes with archaerhodopsin promotes homogenization of convergent presynaptic inputs. Our findings support the presence of an astrocyte-dependent cellular mechanism that enhances the heterogeneity of presynaptic strengths of convergent connections, which may help boost the computational power of dendrites.

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

The inhibitory neurotransmitter GABA evokes long-lasting Ca(2+) oscillations in cortical astrocytes

Studies over the last decade provided evidence that in a dynamic interaction with neurons glial cell astrocytes contribut to fundamental phenomena in the brain. Most of the knowledge on this derives, however, from studies monitoring the astrocyte Ca²⁺ response to glutamate. Whether astrocytes can similarly respond to other neurotransmitters, including the inhibitory neurotransmitter GABA, is relatively unexplored. By using confocal and two photon laser‐scanning microscopy the astrocyte response to GABA in the mouse somatosensory and temporal cortex was studied. In slices from developing (P15‐20) and adult (P30‐60) mice, it was found that in a subpopulation of astrocytes GABA evoked somatic Ca²⁺ oscillations. This response was mediated by GABA_B receptors and involved both G_i/o protein and inositol 1,4,5‐trisphosphate (IP₃) signalling pathways. In vivo experiments from young adult mice, revealed that also cortical astrocytes in the living brain exibit GABA_B receptor‐mediated Ca²⁺ elevations. At all astrocytic processes tested, local GABA or Baclofen brief applications induced long‐lasting Ca²⁺ oscillations, suggesting that all astrocytes have the potential to respond to GABA. Finally, in patch‐clamp recordings it was found that Ca²⁺ oscillations induced by Baclofen evoked astrocytic glutamate release and slow inward currents (SICs) in pyramidal cells from wild type but not IP₃R2^−/− mice, in which astrocytic GABA_B receptor‐mediated Ca²⁺ elevations are impaired. These data suggest that cortical astrocytes in the mouse brain can sense the activity of GABAergic interneurons and through their specific recruitment contribut to the distinct role played on the cortical network by the different subsets of GABAergic interneurons. GLIA 2016;64:363–373

Loose excitation-secretion coupling in astrocytes

Astrocytes play an important housekeeping role in the central nervous system. Additionally, as secretory cells, they actively participate in cell-to-cell communication, which can be mediated by membrane-bound vesicles. The gliosignaling molecules stored in these vesicles are discharged into the extracellular space after the vesicle membrane fuses with the plasma membrane. This process is termed exocytosis, regulated by SNARE proteins, and triggered by elevations in cytosolic calcium levels, which are necessary and sufficient for exocytosis in astrocytes. For astrocytic exocytosis, calcium is sourced from the intracellular endoplasmic reticulum store, although its entry from the extracellular space contributes to cytosolic calcium dynamics in astrocytes. Here, we discuss calcium management in astrocytic exocytosis and the properties of the membrane-bound vesicles that store gliosignaling molecules, including the vesicle fusion machinery and kinetics of vesicle content discharge. In astrocytes, the delay between the increase in cytosolic calcium activity and the discharge of secretions from the vesicular lumen is orders of magnitude longer than that in neurons. This relatively loose excitation-secretion coupling is likely tailored to the participation of astrocytes in modulating neural network processing.

A long Stokes shift red fluorescent Ca2+ indicator protein for two-photon and ratiometric imaging

The introduction of calcium ion (Ca(2+)) indicators based on red fluorescent proteins (RFPs) has created new opportunities for multicolour visualization of intracellular Ca(2+) dynamics. However, one drawback of these indicators is that they have optimal two-photon excitation outside the near-infrared window (650-1,000 nm) where tissue is most transparent to light. To address this shortcoming, we developed a long Stokes shift RFP-based Ca(2+) indicator, REX-GECO1, with optimal two-photon excitation at <1,000 nm. REX-GECO1 fluoresces at 585 nm when excited at 480 nm or 910 nm by a one- or two-photon process, respectively. We demonstrate that REX-GECO1 can be used as either a ratiometric or intensiometric Ca(2+) indicator in organotypic hippocampal slice cultures (one- and two-photon) and the visual system of albino tadpoles (two-photon). Furthermore, we demonstrate single excitation wavelength two-colour Ca(2+) and glutamate imaging in organotypic cultures.

P2Y13 receptor-mediated rapid increase in intracellular calcium induced by ADP in cultured dorsal spinal cord microglia

P2Y receptors have been implicated in the calcium mobilization by the response to neuroexcitatory substances in neurons and astrocytes, but little is known about P2Y receptors in microglia cells. In the present study, the effects of ADP on the intracellular calcium concentration ([Ca(2+)]i) in cultured dorsal spinal cord microglia were detected with confocal laser scanning microscopy using fluo-4/AM as a calcium fluorescence indicator that could monitor real-time alterations of [Ca(2+)]i. Here we show that ADP (0.01-100 μM) causes a rapid increase in [Ca(2+)]i with a dose-dependent manner in cultured microglia. The action of ADP on [Ca(2+)]i was significantly blocked by MRS2211 (a selective P2Y13 receptor antagonist), but was unaffected by MRS2179 (a selective P2Y1 receptor antagonist) or MRS2395 (a selective P2Y12 receptor antagonist), which suggest that P2Y13 receptor may be responsible for ADP-evoked Ca(2+) mobilization in cultured microglia. P2Y13-evoked Ca(2+) response can be obviously inhibited by BAPTA-AM and U-73122, respectively. Moreover, removal of extracellular Ca(2+) (by EGTA) also can obvious suppress the Ca(2+) mobilization. These results means both intracellular calcium and extracellular calcium are potentially important mechanisms in P2Y13 receptor-evoked Ca(2+) mobilization. However, P2Y13 receptor-evoked Ca(2+) response was not impaired after CdCl2 and verapamil administration, which suggest that voltage-operated Ca(2+) channels may be not related with P2Y13-evoked Ca(2+) response. In addition, Ca(2+) mobilization induced by ADP was abolished by different store-operated Ca(2+) channels (SOCs) blocker, 2-APB (50 μM) and SKF-96365 (1 mM), respectively. These observations suggest that the activation of P2Y13 receptor might be involved in the effect of ADP on [Ca(2+)]i in cultured dorsal spinal cord microglia. Furthermore, our results raise a possibility that P2Y13 receptor activation causes Ca(2+) release from Ca(2+) store, which leads to the opening of SOCs.

Neurovascular signaling in the brain and the pathological consequences of hypertension

The execution and maintenance of all brain functions are dependent on a continuous flow of blood to meet the metabolic needs of the tissue. To ensure the delivery of resources required for neural processing and the maintenance of neural homeostasis, the cerebral vasculature is elaborately and extensively regulated by signaling from neurons, glia, interneurons, and perivascular nerves. Hypertension is associated with impaired neurovascular regulation of the cerebral circulation and culminates in neurodegeneration and cognitive dysfunction. Here, we review the physiological processes of neurovascular signaling in the brain and discuss mechanisms of hypertensive neurovascular dysfunction.

Mechanisms for L-channel-mediated increase in [Ca(2+)]i and its reduction by anti-bipolar drugs in cultured astrocytes combined with its mRNA expression in freshly isolated cells support the importance of astrocytic L-channels

The importance of Ca(2+) signaling in astrocytes is undisputed but a potential role of Ca(2+) influx via L-channels in the brain in vivo is disputed, although expression of these channels in cultured astrocytes is recognized. This study shows that an increase in free cytosolic Ca(2+) concentration ([Ca(2+)]i) in astrocytes in primary cultures in response to an increased extracellular K(+) concentration (45mM) is inhibited not only by nifedipine (confirming previous observations) but also to a very large extent by ryanodine, inhibiting ryanodine receptor-mediated release of Ca(2+), known to occur in response to an elevation in [Ca(2+)]i. This means that the actual influx of Ca(2+) is modest, which may contribute to the difficulty in demonstrating L-channel-mediated Ca(2+) currents in astrocytes in intact brain tissue. Chronic treatment with any of the 3 conventional anti-bipolar drugs lithium, carbamazepine or valproic acid similarly causes a pronounced inhibition of K(+)-mediated increase in [Ca(2+)]i. This is shown to be due to an inhibition of capacitative Ca(2+) influx, reflected by decreased mRNA and protein expression of the 'transient receptor potential channel' (TRPC1), a constituent of store-operated channels (SOCEs). Literature data are cited (i) showing that depolarization-mediated Ca(2+) influx in response to an elevated extracellular K(+) concentration is important for generation of Ca(2+) oscillations and for the stimulatory effect of elevated K(+) concentrations in intact, non-cultured brain tissue, and (ii) that Ca(2+) channel activity is dependent upon availability of metabolic substrates, including glycogen. Finally, expression of mRNA for Cav1.3 is demonstrated in freshly separated astrocytes from normal brain.

Iron uptake in quiescent and inflammation-activated astrocytes: a potentially neuroprotective control of iron burden

Astrocytes play a crucial role in proper iron handling within the central nervous system. This competence can be fundamental, particularly during neuroinflammation, and neurodegenerative processes, where an increase in iron content can favor oxidative stress, thereby worsening disease progression. Under these pathological conditions, astrocytes undergo a process of activation that confers them either a beneficial or a detrimental role on neuronal survival. Our work investigates the mechanisms of iron entry in cultures of quiescent and activated hippocampal astrocytes. Our data confirm that the main source of iron is the non-transferrin-bound iron (NTBI) and show the involvement of two different routes for its entry: the resident transient receptor potential (TRP) channels in quiescent astrocytes and the de novo expressed divalent metal transporter 1 (DMT1) in activated astrocytes, which accounts for a potentiation of iron entry. Overall, our data suggest that at rest, but even more after activation, astrocytes have the potential to buffer the excess of iron, thereby protecting neurons from iron overload. These findings further extend our understanding of the protective role of astrocytes under the conditions of iron-mediated oxidative stress observed in several neurodegenerative conditions.

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Non-transferrin-bound iron (NTBI) is the main source of iron for astrocytes.

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TRPC channels represent an entry pathway for Fe^2 + in resting astrocytes.

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Activation process increases the competence of astrocytes to uptake iron.

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DMT1 expression accounts for potentiation of iron ingress in activated astrocytes.

Calcium signaling in glioma cells--the role of nucleotide receptors

Calcium signaling is probably one of the evolutionary oldest and the most common way by which the signal can be transmitted from the cell environment to the cytoplasmic calcium binding effectors. Calcium signal is fast and due to diversity of calcium binding proteins it may have a very broad effect on cell behavior. Being a crucial player in neuronal transmission it is also very important for glia physiology. It is responsible for the cross-talk between neurons and astrocytes, for microglia activation and motility. Changes in calcium signaling are also crucial for the behavior of transformed glioma cells. The present Chapter summarizes molecular mechanisms of calcium signal formation present in glial cells with a strong emphasis on extracellular nucleotide-evoked signaling pathways. Some aspects of glioma C6 signaling such as the cross-talk between P2Y(1) and P2Y(12) nucleotide receptors in calcium signal generation will be discussed in-depth, to show complexity of machinery engaged in formation of this signal. Moreover, possible mechanisms of modulation of the calcium signal in diverse environments there will be presented herein. Finally, the possible role of calcium signal in glioma motility is also discussed. This is a very important issue, since glioma cells, contrary to the vast majority of neoplastic cells, cannot spread in the body with the bloodstream and, at least in early stages of tumor development, may expand only by means of sheer motility.

Bidirectional scaling of astrocytic metabotropic glutamate receptor signaling following long-term changes in neuronal firing rates

Very little is known about the ability of astrocytic receptors to exhibit plasticity as a result of changes in neuronal activity. Here we provide evidence for bidirectional scaling of astrocytic group I metabotropic glutamate receptor signaling in acute mouse hippocampal slices following long-term changes in neuronal firing rates. Plasticity of astrocytic mGluRs was measured by recording spontaneous and evoked Ca²⁺ elevations in both astrocytic somata and processes. An exogenous astrocytic Gq G protein-coupled receptor was resistant to scaling, suggesting that the alterations in astrocyte Ca²⁺ signaling result from changes in activity of the surface mGluRs rather than a change in intracellular G protein signaling molecules. These findings suggest that astrocytes actively detect shifts in neuronal firing rates and adjust their receptor signaling accordingly. This type of long-term plasticity in astrocytes resembles neuronal homeostatic plasticity and might be important to ensure an optimal or expected level of input from neurons.

Modeling the contributions of Ca2+ flows to spontaneous Ca2+ oscillations and cortical spreading depression-triggered Ca2+ waves in astrocyte networks

Astrocytes participate in brain functions through Ca(2+) signals, including Ca(2+) waves and Ca(2+) oscillations. Currently the mechanisms of Ca(2+) signals in astrocytes are not fully clear. Here, we present a computational model to specify the relative contributions of different Ca(2+) flows between the extracellular space, the cytoplasm and the endoplasmic reticulum of astrocytes to the generation of spontaneous Ca(2+) oscillations (CASs) and cortical spreading depression (CSD)-triggered Ca(2+) waves (CSDCWs) in a one-dimensional astrocyte network. This model shows that CASs depend primarily on Ca(2+) released from internal stores of astrocytes, and CSDCWs depend mainly on voltage-gated Ca(2+) influx. It predicts that voltage-gated Ca(2+) influx is able to generate Ca(2+) waves during the process of CSD even after depleting internal Ca(2+) stores. Furthermore, the model investigates the interactions between CASs and CSDCWs and shows that the pass of CSDCWs suppresses CASs, whereas CASs do not prevent the generation of CSDCWs. This work quantitatively analyzes the generation of astrocytic Ca(2+) signals and indicates different mechanisms underlying CSDCWs and non-CSDCWs. Research on the different types of Ca(2+) signals might help to understand the ways by which astrocytes participate in information processing in brain functions.

Calcium signalling in astroglia

Astroglia possess excitability based on movements of Ca(2+) ions between intracellular compartments and plasmalemmal Ca(2+) fluxes. This "Ca(2+) excitability" is controlled by several families of proteins located in the plasma membrane, within the cytosol and in the intracellular organelles, most notably in the endoplasmic reticulum (ER) and mitochondria. Accumulation of cytosolic Ca(2+) can be caused by the entry of Ca(2+) from the extracellular space through ionotropic receptors and store-operated channels expressed in astrocytes. Plasmalemmal Ca(2+) ATP-ase and sodium-calcium exchanger extrude cytosolic Ca(2+) to the extracellular space; the exchanger can also operate in reverse, depending of the intercellular Na(+) concentration, to deliver Ca(2+) to the cytosol. The ER internal store possesses inositol 1,4,5-trisphosphate receptors which can be activated upon stimulation of astrocytes through a multiple plasma membrane metabotropic G-protein coupled receptors. This leads to release of Ca(2+) from the ER and its elevation in the cytosol, the level of which can be modulated by mitochondria. The mitochondrial uniporter takes up Ca(2+) into the matrix, while free Ca(2+) exits the matrix through the mitochondrial Na(+)/Ca(2+) exchanger as well as via transient openings of the mitochondrial permeability transition pore. One of the prominent consequences of astroglial Ca(2+) excitability is gliotransmission, a release of transmitters from astroglia which can lead to signalling to adjacent neurones.

Deep brain stimulation and the role of astrocytes

Deep brain stimulation (DBS) has emerged as a powerful surgical therapy for the management of treatment-resistant movement disorders, epilepsy and neuropsychiatric disorders. Although DBS may be clinically effective in many cases, its mode of action is still elusive. It is unclear which neural cell types are involved in the mechanism of DBS, and how high-frequency stimulation of these cells may lead to alleviation of the clinical symptoms. Neurons have commonly been a main focus in the many theories explaining the working mechanism of DBS. Recent data, however, demonstrates that astrocytes may be active players in the DBS mechanism of action. In this review article, we will discuss the potential role of reactive and neurogenic astrocytes (neural progenitors) in DBS.

Models of neurovascular coupling via potassium and EET signalling

Functional hyperemia is an important metabolic autoregulation mechanism by which increased neuronal activity is matched by a rapid and regional increase in blood supply. This mechanism is facilitated by a process known as "neurovascular coupling"--the orchestrated communication system involving neurons, astrocytes and arterioles. Important steps in this process are the production of EETs in the astrocyte and the release of potassium, via two potassium channels (BK and KIR), into the perivascular space. We provide a model which successfully accounts for several observations seen in experiment. The model is capable of simulating the approximate 15% arteriolar dilation caused by a 60-s neuronal activation (modelled as a release of potassium and glutamate into the synaptic cleft). This model also successfully emulates the paradoxical experimental finding that vasoconstriction follows vasodilation when the astrocytic calcium concentration (or perivascular potassium concentration) is increased further. We suggest that the interaction of the changing smooth muscle cell membrane potential and the changing potassium-dependent resting potential of the KIR channel are responsible for this effect. Finally, we demonstrate that a well-controlled mechanism of potassium buffering is potentially important for successful neurovascular coupling.

Morphology and properties of astrocytes

Astrocytes were identified about 150 years ago, and, for the longest time, were considered to be supporting cells in the brain providing trophic, metabolic, and structural support for neural networks. Research in the last 2 decades has uncovered many novel molecules in astrocytes and the finding that astrocytes communicate with neurons via Ca2+ signaling, which leads to release of chemical transmitters, termed gliotransmitters, has led to renewed interest in their biology. This chapter will briefly review the unique morphology and molecular properties of astrocytes. The reader will be introduced to the role of astrocytes in blood-brain barrier (BBB) maintenance, in Ca2+ signaling, in synaptic transmission, in CNS synaptogenesis, and as neural progenitor cells. Mention is also made of the diseases in which astrocyte dysfunction has a role.

The endocannabinoid system in rat gliosomes and its role in the modulation of glutamate release

The endocannabinoid system and endocannabinoid receptor-driven modulation of glutamate release were studied in rat brain cortex astroglial gliosomes. These preparations contained the endocannabinoids N-arachidonoylethanolamine (anandamide) and 2-arachidonoylglycerol, as well their major biosynthetic (N-acyl-phosphatidylethanolamines-hydrolyzing-phospholipase D and diacylglycerol-lipase) and catabolic (fatty acid amide-hydrolase and monoacylglycerol-lipase) enzymes. Gliosomes expressed type-1 (CB1R), type-2 (CB2R) cannabinoid, and type-1 vanilloid (TRPV1) receptors, as ascertained by Western blotting and confocal microscopy. Methanandamide, a stable analogue of anandamide acting as CB1R, CB2R, and TRPV1 agonist, stimulated or inhibited the depolarization-evoked gliosomal [(3)H]D: -aspartate release, at lower and higher concentrations, respectively. Experiments with ACEA (arachidonyl-2'-chloroethylamide), JWH133 ((6aR,10aR)-3-(1,1-dimethylbutyl)-6a,7,10,10a-tetrahydro-6,6,9-trimethyl-6H-dibenzo[b,d]-pyran) and capsaicin, selective agonists at CB1R, CB2R and TRPV1, respectively, demonstrated that potentiation of [(3)H]D: -aspartate release was due to CB1R while inhibition to CB2R and TRPV1 engagement. These findings were confirmed by using selective receptor antagonists. Furthermore, CB1R activation caused increase of intracellular IP3 and Ca(2+) concentration, suggesting an involvement of phospholipase C.