Where the Thoughts Dwell: the Physiology of Neuronal–Glial “Diffuse Neural Net”

BRAIN RESEARCH REVIEWS 66 (2011) 133– 151

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Review

Where the thoughts dwell: The physiology of neuronal–glial “diffuse neural net”

Alexei Verkhratskya,b,d,⁎, Vladimir Parpurac, José J. Rodríguezb,d,e aFaculty of Life Sciences, The University of Manchester, Manchester, UK bInstitute of Experimental Medicine, ASCR, Prague, Czech Republic cDepartment of Neurobiology, Center for Glial Biology in Medicine, Civitan International Research Center, Atomic Force Microscopy and Nanotechnology Laboratories, and Evelyn F. McKnight Brain Institute, University of Alabama, Birmingham, USA dIKERBASQUE, Basque Foundation for Science, 48011, Bilbao, Spain eDepartment of Neurosciences, University of the Basque Country UPV/EHU, 48940, Leioa, Spain

ARTICLE INFO ABSTRACT

Article history: The mechanisms underlying the production of thoughts by exceedingly complex cellular Accepted 17 May 2010 networks that construct the human brain constitute the most challenging problem of Available online 9 June 2010 natural sciences. Our understanding of the brain function is very much shaped by the neuronal doctrine that assumes that neuronal networks represent the only substrate for Keywords: cognition. These neuronal networks however are embedded into much larger and probably Human brain more complex network formed by neuroglia. The latter, although being electrically silent, Glia employ many different mechanisms for intercellular signalling. It appears that astrocytes Neurone can control synaptic networks and in such a capacity they may represent an integral History component of the computational power of the brain rather than being just brain “connective Evolution of glia tissue”. The fundamental question of whether neuroglia is involved in cognition and Cognition information processing remains, however, open. Indeed, a remarkable increase in the Neuronal–glial network number of glial cells that distinguishes the human brain can be simply a result of Homeostasis exceedingly high specialisation of the neuronal networks, which delegated all matters of Neuropathology survival and maintenance to the neuroglia. At the same time potential power of analogue processing offered by internally connected glial networks may represent the alternative mechanism involved in cognition. © 2010 Elsevier B.V. All rights reserved.

Contents

1. The neural cells ...... 134 2. Neuroglia organises and maintains the nervous system ...... 138 2.1. Evolutionary aspect ...... 138 2.2. Developmental aspect ...... 140 2.3. Anatomical aspect ...... 140 2.4. Homeostatic aspect ...... 140

⁎ Corresponding author. The University of Manchester, Oxford Road, Manchester, M13 9PT, UK. E-mail address: [email protected] (A. Verkhratsky).

2.4.1. Ion and water homeostasis ...... 140 2.4.2. Neurotransmitter homeostasis ...... 141 2.4.3. Control of local blood flow and metabolic support ...... 141 2.5. Physiological aspect: neuroglia and information processing in the brain ...... 141 2.5.1. Glial receptors ...... 141 2.5.2. Synaptic transmission onto glia ...... 142 2.5.3. Signalling in glial syncytia ...... 142 2.5.4. Gliotransmission and exocytotic release ...... 143 2.5.5. Neuroglia controls connectivity in neural networks ...... 144 2.6. Pathophysiological aspect...... 144 3. Conclusion: the neuronal–glial networks form the substrate of brain function...... 145 Acknowledgments ...... 145 References...... 145

1. The neural cells tian Gottfried Ehrenberg who was investigating the nervous system of the leech (Ehrenberg, 1836) and by Johann Evange- “Omnis cellula e cellula” this aphorism initially coined by lista Purkinje (Figs. 1B and C) who was studying cerebellum and François-Vincent Raspail and subsequently popularised by described the cell named after him (Purkinje, 1837). Purkinje's Rudolf Virchow is an epitome of the biological revolution of pupil, Gustav Gabriel Valentin came with the first published 19th century, which begun from the identification of cellular drawing of the neurone (which he called Kugeln — Fig. 1D) nature of life and ended in theoretical understanding of where the nucleus and other intracellular structures were evolution and genetic code. visible (Valentin, 1836). Incidentally, Valentin also was the first The general idea about elementary units of life, from which to suggest that the nervous system contains both active and all tissues and organisms are formed, began to circulate in the passive elements. early 17th century, being promulgated, for example, by Pierre The passive elements of the brain were soon conceptua- Gassendi and Robert Boyle. The origins of cellular theory are lised by Rudolf Virchow who introduced connective tissue to rooted in the discoveries of the first microscopists (e.g. Robert the nervous system under the name of neuroglia ((Virchow, Hooke, Marcello Malpighi and Nehemiah Grew) made in the 1856; Virchow, 1858), for the more detailed account on the 17th century. It all began with the microscopic observations of history of glia see Kettenmann and Ransom (2005), Ketten- the plants, and it was Robert Hooke, who, when observing the mann and Verkhratsky (2008), Verkhratsky (2006b) and fine structures of cork, visualised the regular structures that Verkhratsky (2009)). Virchow did not recognise neuroglia as a reminded him of the monk's cells in the monasterial specific class of neural cells; the drawings of some round dormitories — and the term “cell” was born (Hooke, 1665). structures, which he reproduced in the Die Cellularpathologie, The first animal cells were discovered, to all likelihood, by show, if anything, activated microglia. This did not matter, Antonius van Leeuwenhoek, who in his many letters to the because for Virchow the neuroglia was a connective tissue of Royal Society described bacteria (and named them animalcules mesodermal origin; the misconception, which was embraced or little animals) and erythrocytes, observed single muscle by many histologists (e.g. by Andriezen, Robertson and fibres, followed the movements of live spermatozoids, and Weigart) and which was still believed by Cajal as late as in was the first to see the regular structure (representing single 1920 (Ramón y Cajal, 1920). axons) in the sagittal slice of the peripheral nerve (Fig. 1, In the meantime discoveries of the neural cells continued. (Bentivoglio, 1996; Leeuwenhoek, 1673–1696; Leeuwenhoek, In 1838 Robert Remak made the description of nerve fibres and 1798)). Leeuwenhoek reflected on the latter observation made visualised the covering sheath around them (Remak, 1838). In in 1717: “Often and not without pleasure, I have observed the 1851 Heinrich Müller (1851) produced the first images of retinal structure of the nerves to be composed of very slender vessels radial glia (the cells which were subsequently named Müller of an indescribable fineness, running lengthwise to form the cells by Rudolf Albert von Kölliker (1852b). Von Kölliker by nerve” (cited from Bentivoglio, 1996). The fine cylindrical nerve himself also described nerve elements in the acoustic nerve of fibres were also described by Felice Gaspar Ferdinand Fontana, the ox (Kölliker, 1852a). In 1858 Max Schulze made a detailed who mechanically dissected the nerve and observed his investigation of the Müller cells and produced probably the preparations under 700× magnification (Bentivoglio, 1996). best possible drawings of them in the pre-staining era of More cell types were discovered at the beginning of 19th histology. Simultaneously Karl Bergmann (1857) had identified century, and around 1830 Robert Brown found the nucleus radial glial cells in the cerebellum (the Bergmann glial cells). In (Ford, 1992). The cellular theory was then formalised by 1862 the neuro-muscular junction was described by Wilhelm Theodor Schwann and Matthias Jakob Schleiden (Schleiden, Friedrich Kühne, who named it the “endplate” (Kühne, 1862). 1838; Schwann, 1839; Schwann and Schleiden, 1847) and was Slightly later the very detailed images of both neurones and rapidly embraced by the scientific community. stellate glial cells (that to all probability were astrocytes — The first cells of the nervous system were visualised in Figs. 1E, F and G) were made by young (who untimely deceased 1830-ies. Probably the first observations were done by Chris- at 29) Otto Deiters (1865), and some years later Jacob Henle and BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 135

Fig. 1 – First images of neural cells. A. Antonius van Leeuwenhoek (1632–1723) and his drawing of a “small Nerve (BCDEF),” composed by many “vessels” in which “the lines or strokes denote the cavities or orifices of those vessels. This Nerve is surrounded, in part, by five other Nerves (GGGGG)” in which only “external coats” are represented. The image was kindly provided by Prof. Marina Bentivoglio. B, C. Johann Evangelista Purkinje (1787–1869) and the first drawings of the Purkinje neurone made by him for the Congress of Physicians and Scientists Conference in Prague, in 1837. Reproduced from Lopez-Munoz et al. (2006). D. The first published drawing of neurone made by Gustav Gabriel Valentin (1810–1883) (Valentin, 1836). E, F, G. Otto Friedrich Karl Deiters (1834–1863) and his drawings (Deiters, 1865) of motoneurones and “connective tissue cells” (astrocytes). Reproduced with permission from Bentivoglio (1996) and Kettenmann and Ransom (2005). H. Section of the spinal cord of the ox demonstrating the network of stellate cells. Cells were stained by carmine after chromic acid preparation (Henle and Merkel, 1869). Reproduced with permission from Kettenmann and Ransom (2005).

Friedrich Merkel visualised the glial network in the grey nera” or black staining — (Golgi, 1873; Golgi, 1903)), which, matter (Fig. 1H — (Henle and Merkel, 1869)). for the first time, allowed neurohistologists to obtain images It has to be stressed that all these early cellular images of neural cells in their entity (Fig. 2; for a comprehensive and were done mostly on unstained preparations following vividly written account of the dissemination of Golgi staining painstaking cells isolation by microsurgery. The histological technique through the world see Mazzarello (2010)). revolution occurred in 1873 when Camillo Golgi developed the As early as 1871 Golgi recognised glial cells and he was the silver-chromate staining technique (the famous “reazione first to demonstrate that the glia represent cellular population 136 BRAIN RESEARCH REVIEWS 66 (2011) 133– 151

Fig. 2 – Astroglial cells stained by the silver-chromate technique. A: Protoplasmic astroglial cells in the grey matter stained and drawn by Camillo Golgi; the astrocytes form numerous contacts (the endfeet) with brain capillaries (Golgi, 1883). The image was kindly provided by Prof. Paolo Mozzarello. B. Morphological heterogeneity of human astrocytes. The astrocytes in the brain slices of human foetuses were stained by silver-chromate technique (Retzius, 1894). BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 137 distinct from nervous cells, and that the glial cells (which were that was specific for both protoplasmic and fibrous astro- to all probability the protoplasmic astrocytes) send the cytes (Garcia-Marin et al., 2007). We now know that this processes to the blood vessels where they establish the stain targeted intermediate filaments consisting mainly of endfoot structures (Fig. 2A); he was also the first to propose glial fibrillary acidic protein (GFAP), a protein used today as that the glial cells are instrumental in transferring the an astrocytic marker (reviewed in Kimelberg, 2004). Using metabolic substances from the blood to the brain parenchyma this technique Cajal confirmed earlier ideas of the origin of (Golgi, 1875; Golgi, 1894), this concept subsequently gained the astrocytes from radial glia, and also demonstrated that universal acceptance. The application of Golgi staining led to a astrocytes can divide in the adult brain, thus laying the further in depth characterisation of glial cells of the grey basis for much later discoveries of the stem properties of matter, and in 1891 the term “astrocyte” was introduced by astroglia (Ramón y Cajal, 1913a; Ramón y Cajal, 1913b; Michael von Lenhossek (Lenhossek, 1891; Lenhossek, 1893); Ramón y Cajal, 1916). Subsequently Pío del Río-Hortega slightly later the astroglia was further subdivided into identified two other principal classes of glial cells, initially protoplasmic (Andriezen, 1893) and fibrous (Kölliker, 1893) considered by Cajal as the “third element” (a group of astrocytes. At the same years Wilhelm His made a funda- “adendritic” cells) and what represented, in fact, the mental discovery, when, in 1889, he found the neuronal origin oligodendrocytes and the microglia. of neuroglia and directly demonstrated that both nerve cells Oligodendrocytes were initially observed and described by and neuroglia derive from the neuroectoderm (His, 1889a; His, William Ford Robertson who developed for this purpose a 1889b). specific platinum stain technique; Robertson however did not The end of 1880-ies marked the arrival of the neurone and realise the myelinating capacity and role of these cells and of Santiago Ramón y Cajal, who was to champion and develop thought them external invaders to the brain, hence identifying the neuronal doctrine over the next decades. The first papers them as mesoglia (Robertson, 1899; Robertson, 1900). Almost of Cajal dedicated to the fine structure of the nervous system 20 years later Río-Hortega rediscovered these cells and gave begun to appear in 1888, about one year after he learned the them a name of oligodendrocytes (Del Río-Hortega, 1921; black staining technique. With characteristic determination Gibson, 1994); he also demonstrated that oligodendrocytes and originality Cajal made a special journal for his papers, the were myelinating cells in the CNS being thus analogous to the Revista Truimestral de Histologia Normal y Patologica, of which he Schwann cells in the periphery. It was also Río-Hortega who naturally became the Editor-in-Chief, and the first issues of described the microglial cells, the only glia members of non- this journal were almost entirely occupied by his papers neuronal origin (for staining of both oligodendrocytes and (Mazzarello, 2010). In the first paper published in the first issue microglia Río-Hortega deployed specifically developed ammo- of his journal Cajal made the seminal statement that “each niacal silver carbonate method (Del Río-Hortega, 1917)). nerve cell is a totally autonomous physiological canton” Initially Río-Hortega called these cells “garbage collectors”;in (Ramón y Cajal, 1888). One year later, in October of 1889, the literature they also were known as “cells of Hortega” Cajal attended the German Anatomical congress where he (Gibson, 1993; Gibson, 1994). Del Río-Hortega gave microglia a demonstrated his numerous microscopic preparations (Fig. 3) very detailed and comprehensive characterisation. He found to the delegates; this instantly made his international that microglial precursors invade the brain shortly after birth, reputation. In 1891 Heinrich Wilhelm von Waldeyer, a great disseminate over the brain parenchyma and develop a distinct admirer and follower of Cajal, coined the term “neurone” phenotype. He also found that these cells reacted to brain (Waldeyer von, 1891) and neuronal doctrine begun to conquer damage by morphological and functional modification known the minds of neuroscientists. The neurone was endowed with as microglial activation (Del Río-Hortega, 1919a; Del Río- dendrites (the term was introduced by Wilhelm His in 1889) Hortega, 1919b; Del Río-Hortega, 1920; Del Río-Hortega, 1927; and very soon it acquired the axon (which was named so by Del Río-Hortega, 1932). Alfred Kölliker in 1896). Thus by the 1920-ies the overall understanding of mor- The previous (and the first) grand theory about the brain phological organisation of the brain and the spinal cord has functional organisation was formalised by Josef von Gerlach been generally completed, and the following developments (1871), who proposed the diffuse network of neurofilaments were mostly dedicated to uncovering the functional mechan- and neural cell processes, that are internally connected and isms by which neural cells communicate and by which they act in concert. This “reticular” theory dominated neurosci- may perform higher brain functions. By then the neuronal enceforgood20yearsandrecruitedmanysupporters doctrine became absolutely dominating, and the synapse, the including Golgi, who was very much convinced in the name of which was introduced by Charles Scott Sherrington existence of “diffuse neural net”. The history of the neuron- (Foster and Sherrington, 1897; Sherrington, 1906) has received ism–reticularism conflict was widely popularised, and the a particular attention as the principal place of integration in curios reader may find all the dramatic details in several the nervous system. Rapidly developing electrophysiological comprehensive treatises (Jacobson, 2005; Lopez-Munoz et al., experimental techniques allowed monitoring of the functional 2006; Mazzarello, 2007; Mazzarello, 2010; Ramón y Cajal, activity of neurones; and the image of neuronal network 1933; Shepherd, 1991). where binary signals (action potentials) swiftly convey the Here we shall continue with a brief outline of further information though neuronal membranes and activate syn- discoveries of glial cells (Fig. 3), which, curiously enough, are aptic release of neurotransmitters that subsequently trigger very much connected with Cajal's school and with his changes in postsynaptic potential, which, in turn, provide for pupils, Nicolás Achúcarro and Pío del Río-Hortega. First, the electronic summation thus integrating the incoming Cajal developed gold chloride-sublimate staining method signals, was universally accepted. 138 BRAIN RESEARCH REVIEWS 66 (2011) 133– 151

Fig. 3 – Glial cells by the eyes of Santiago Ramón y Cajal and Pío del Río-Hortega. A: Cajal's drawing of Golgi impregnated glia showing human cortical neuroglial cells of the plexiform layer (A–D), cells of the second and third layers (E–H and K, R) and perivascular glia (I, J). B, C: Astrocytes in the stratum lucidum of the human CA1 area of the hippocampus with particular emphasis on the anatomy of perivascular astrocytes in the CA1 stratum radiatum. D, E: Drawings of Pío del Río-Hortega showing the different morphological types of microglial cells in the rabbit Ammon's horn and cortical perivascular neuroglia.

signalling events that occur between and within the cells as well 2. Neuroglia organises and maintains the as comprehension of the relevance of all these immensely nervous system complex processes in the generations of thoughts, emotions and creative ideas remain very far from understanding. The neuronal “It may seem strange that, since I have always been doctrine regards neuronal networks as a sole substance for opposed to the neurone theory – although acknowledging information processing; the knowledge accumulated in the last that its starting-point is to be found in my own work – I 2 decades indicates that the overall picture can be more complex. have chosen this question of the neurone as the subject of Here we shall briefly overview the cell biology and physiology of my lecture, and that it comes at a time when this doctrine neuroglia from the perspective of the integrated neuronal–glial is generally recognized to be going out of favour.”(Camillo networks as a substrate of brain function. Golgi starting the Nobel lecture (Golgi, 1906)) 2.1. Evolutionary aspect The nature of information processing in the human brain and the role of different cell types and their temporal and spatial The primordial glial cells appeared early in evolution (Fig. 4), relations,theroleoffastandslowprocessesandthenatureof probably already at the stage of diffuse nervous system when BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 139

Fig. 4 – Evolution of the neuroglia; the tree of life is taken from Haeckel (1879).

they assisted ancestral neurones in their functions (Bacaj et CNS results in neuronal demise. The high degree of special- al., 2008; Reichenbach and Pannicke, 2008). Development of isation acquired by neurones in the centralised nervous neuronal masses organised first in ganglia and then in the system renders them helpless in the absence of the glial functionally segregated centralised nervous system, stimulat- support; this support (which contributes at many levels of ed the evolution of supporting cells, the proto-astrocytes, brain homeostasis) is the ancestral function of astrocytes that which controlled the extracellular homeostasis, provided remains throughout the evolution. Another important func- metabolic support, regulated development and assisted neu- tion that appeared very early in the evolutionary ladder was rones in fulfilling their primary functions. In Caenorhabditis the isolation of the nervous tissue from the rest of the body by elegans worm the artificial elimination of glia (some of the glial the blood–brain barrier (BBB), which is made solely by cells are associated with the nematode sensory organ) does astrocytes in crustacean, insects and cephalopods. Interest- not result in wide-spread neuronal death, but dramatically ingly that astroglial blood–brain barrier is also present in the alters function and sensitivity of sensory neurones (Bacaj et brain of sharks; some of the capillaries are completely al., 2008). Incidentally a number of signalling and enzymatic surrounded by astroglial process being thus endocellular cascades present in glial cells are exceptionally highly vessels (Long et al., 1968). At the later stages of phylogenesis conserved in evolution, being similar in the worms, insects the BBB function is shifted to endothelial cells, the latter and mammals (Barres, 2008). At the higher evolutionary stages nonetheless remain under astrocytic control (Abbott, 2005; astroglial cells remain an important part of both sensory Abbott and Pichon, 1987). organs and the CNS, where they assume multiple roles, and An increase in the size of living organisms naturally much higher significance — elimination of astroglia from the presented an additional survivalist task — an increase of the 140 BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 speed of communication between the periphery and the adult neuro- and glio-genesis (see Alvarez-Buylla et al. (2001), central neural organs. Demand for an increased velocity of Doetsch et al. (1999), Gotz and Huttner (2005) and Mori et al. electrical propagation through the axons led to an appearance (2005) for a comprehensive review). In addition neuroglial cells of myelin isolators provided by oligodendrocytes and are instrumental in promoting neuronal survival at different Schwann cells. The myelin isolation appeared sometimes in developmental stages through the release of numerous Ordovican period (Fields, 2008a; Hartline, 2008; Roots, 2008)in neurotrophic factors (e.g. epidermal growth factor, EGF, Glial the ancestral invertebrates (e.g. in oligochaetes (earthworms)), cell-derived neurotrophic factor, GDNF, etc); and by the in malacostraca (shrimps and prawns), in copepods (small activation of astrocytic transcriptome-associated genes such crustaceans) and in gnathostomes (the jawed fishes; although as ApoE, ApoJ, MFGE8 and cystatin C (Banker, 1980; Barres, the class includes not only sharks and rays but also 2008; Wagner et al., 2006). tetrapodes). Finally, the isolated position of the nervous tissue required 2.3. Anatomical aspect self-defence system and the immune cells begun to invade the ganglia in bivalve, crustacean and insects thus forming the In mammalian brain the astroglial cells define the micro- ancestral microglia, which is present in brain parenchyma at architecture of the parenchyma by dividing the grey matter all subsequent evolutionary stages (Liu et al., 1996; Magazine (through the process known as “tiling” (Bushong et al., 2004)) et al., 1996; Stefano et al., 1996). into relatively independent structural units. The protoplasmic Neuroglia attained maximal development in mammals, astrocytes occupy their own territory and create the micro- and moreover an increased complexity of the brain, together anatomical domains within the limits of their processes with an increased intellectual power was accompanied with (Bushong et al., 2002; Halassa et al., 2007b; Nedergaard et al., remarkable increase in both numbers and complexity of glia 2003; Oberheim et al., 2009; Ogata and Kosaka, 2002; Wilhelms- (Oberheim et al., 2006; Reichenbach, 1989; Verkhratsky, 2009; son et al., 2006). Within the confines of these anatomical Verkhratsky and Butt, 2007). Astrocytes became the most domains the membrane of the astrocyte covers synapses and numerous cells in the human brain, outnumbering neurones. neuronal membranes as well as sends the process to plaster For example, in layers I and IV of the rat cerebral cortex, the wall of the neighbouring blood vessel with the endfoot. neurones/glial cells (astrocytes and oligodendrocytes) ratio is The individual astroglial domains are integrated into the ∼2.5 to 1 (Bass et al., 1971). This is inversed in the humans: the superstructure of astroglial syncytia through gap junctions glia to neurones ratio in human cortex is ∼1.65:1 (Nedergaard (Giaume and Venance, 1998) localised on the peripheral et al., 2003; Sherwood et al., 2006). Furthermore, the morphol- processes of astroglial cells. These astroglial syncytia are ogy of human astrocytes is unique when compared to all other also anatomically segregated being formed within defined mammals (Oberheim et al., 2009); the size of human proto- anatomical structures, for example in individual barrels of the plasmic astrocyte is about 2.5–3 times and fibrous astrocytes somatosensory cortex (Giaume et al., 2009). ∼2.2 times larger than in rodents. The human protoplasmic The second main class of neuroglia, the oligodendroglia, is astrocytes have ∼10 times more primary processes, and central in moulding and maintaining the white matter. In the correspondingly much more complex processes arborisation human brain the white matter is much larger as compared to than rodent astroglia. As a result, human protoplasmic other species; it makes more that 50% of the whole brain, astrocyte contacts and integrates ∼2 millions of synapses which most likely reflects a high demand for interneuronal residing in its territorial domain, whereas rodent astrocytes connections (see Fields (2008b) for comprehensive review). cover ∼20,000–120,000 synaptic contacts. The oligodendrocytes demonstrate an exceptionally high degree of plasticity and myelination can be rapidly modulated 2.2. Developmental aspect in response to various learning paradigms (Fields, 2008b). The oligodendrocytes are actively communicating with neurones, Neurones and neuroglia both originate from the neuroepithe- and this reciprocal signalling, details of which only begin to be lial cells (Alvarez-Buylla et al., 2001), which phylogenetically uncovered, profoundly contributes to overall brain plasticity correspond to the ancestral neuroepithelium. The latter and cognitive processes. comprised secretory epithelial cells, which were connected by gap junctions and probably expressed some sets of voltage- 2.4. Homeostatic aspect gated channels. All in all, the very primordial neural elements were, most likely, physiologically closer to neuroglia, than to Astroglia represents the main cellular element of extracellular the neurones judging on their excitability and syncytia-driven homeostasis in the brain. Indeed the control over the concen- signal propagation. trations of ions, neurotransmitters, neuromodulators, metabo- Be it all as it may, the neuroepithelial cells transform into lites and other active substances is of a paramount importance the radial glia, which act as a universal neural precursor; the for normal operation of neural networks, because even rela- asymmetrical division of radial glia produce neuronal pre- tively small changes in a number of molecules may result in cursors that migrate to their destinations using the processes substantial shifts of their respective concentrations in tiny and of their progenitor as a scaffolding guide-line. The radial glia often diffusionally isolated extracellular compartments. also act as progenitors (through several transitional forms) for both astrocytes and oligodendrocytes. Some of the astrocytes, 2.4.1. Ion and water homeostasis dwelling in the germinal centres of the adult brain, retain the Astroglia assume the leading role in regulation of extracellular stem cell properties throughout the life span and underlie the K+. The concentration of the latter may rise up to 10–12 mM BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 141 following frequent action potential firing that may substan- is transformed into glutamate. It is also of importance that tially affect the excitability of neuronal networks. Astrocytes astrocytes possess the enzyme pyruvate carboxylase, and remove the excess of extracellular K+ via local K+ uptake thus act as a main source for de novo glutamate synthesis (accomplished by inward rectifier K+ channels) and K+ spatial (Hertz et al., 1999). buffering (Coles and Orkand, 1983; Kofuji and Newman, 2004; Newman, 1995). The latter mechanism provides for the 2.4.3. Control of local blood flow and metabolic support + + redistribution of K from the areas with elevated [K ]o to the The astroglial cells are the central elements of the neurovas- + regions with low [K ]o and may occur either in the single glial cular units that integrate neural circuitry with local blood flow cells (K+ siphoning in retinal Müller cells (Newman et al., 1984)) and metabolic support. The astrocyte, which determines the or in the glial syncytia. The glial syncytia and aquaporine confines of the neurovascular unit, bridges the brain paren- channels expressed in astrocytes also play the most critical chyma and local vasculature by virtue of perivascular process role in regulation of brain water homeostasis (Simard and and the endfeet. Increased activity of neurones integrated into Nedergaard, 2004). the neurovascular unit triggers Ca2+ signals in the astrocyte, which, in turn, stimulate release of vasocoactive agents that 2.4.2. Neurotransmitter homeostasis regulate the local blood flow (Metea and Newman, 2006; The second important homeostatic task accomplished by Mulligan and MacVicar, 2004; Zonta et al., 2003). Similarly, astroglial cells is the control over extracellular concentration astrocytes are also responsible for local metabolic support of of neurotransmitters, and most importantly, glutamate. The active neurones though the postulated glucose–lactate shut- latter, when released in excess or for a long-time, acts as a tle; the latter being under the control of cytosolic Na+ powerful neurotoxin that triggers neuronal cell death in a concentration (Magistretti, 2006; Magistretti, 2009). multitude of acute and chronic brain lesions (Caudle and Zhang, 2009; Obrenovitch et al., 2000; Schousboe et al., 1997; 2.5. Physiological aspect: neuroglia and information Westbrook, 1993). Interestingly, the glial function to “chemi- processing in the brain cally split or take up” transmitters was speculated upon by an Italian psychiatrist Ernesto Lugaro in 1907 (Lugaro, 1907). The glia is electrically non-excitable, and therefore is unable to Astrocytes are responsible for the bulk of glutamate removal use the plasmalemmal ion conductances to rapidly convey the from the extracellular space: they accumulate 80% of the information. Nonetheless, glial cells are capable of expressing glutamate released, whereas the remaining 20% goes to the excitable molecules: the voltage-gated ion channels, the neurones (Swanson, 2005; Verkhratsky and Kirchhoff, 2007a). neurotransmitter receptors, and the intracellular signalling Astrocytic uptake of glutamate from extracellular space is also chains. Furthermore, the neuroglia is capable of developing an important modulator of the time-course of synaptic propagating signals either through the gap junctions or neurotransmission (Mennerick and Zorumski, 1994). through the release of gliotransmitters. Conceptually, glial There are five types of glutamate transporters operative in cells can perform all physiological processes as neurones do the human brain (Gadea and Lopez-Colome, 2001); these are (recognise the incoming information, generate propagating known as EAAT1 to EAAT5 (where EAAT stands for excitatory signals, and secrete the transmitters that form the informa- amino acid transporter). Astroglial cells express almost tional output), albeit the neuroglia executes these physiolog- exclusively the EAAT1 and EAAT2 (known in rodent brain as ical processes in a specific and distinct way. glutamate/aspartate transporter, GLAST, and glutamate transporter-1, GLT-1) that accomplish the bulk of glutamate uptake 2.5.1. Glial receptors (Danbolt, 2001); some expression of GLT-1a has been recently Neuroglial cells are capable to express virtually every receptor reported at neuronal pre-synaptic terminals (Furness et al., to neurotransmitters, neurohormones and neuromodulators. 2008; Melone et al., 2009). The glutamate transporters are co- This fundamentally important property was discovered in the transporters which utilise the energy saved in the form of multitude of early experiments on cultured glial cells that transmembrane Na+ gradient so that the transport of a single demonstrated the sensitivity of glia to a variety of stimulating glutamate molecule requires an influx of 3 Na+ ions and 1 H+ agents (Condorelli et al., 1999; Cornell Bell et al., 1990; Dave et ion coupled with the efflux of 1 K+ ion (Kirischuk et al., 2007). al., 1991; Fraser et al., 1995; Gallo and Ghiani, 2000; Verkh- The substantial sodium accumulation accompanying gluta- ratsky et al., 1998; Verkhratsky and Steinhauser, 2000). This mate accumulation is counterbalanced by Na+ efflux through promiscuity of cultured glia towards neuroactive ligands is Na+/Ca2+ exchanger working in the reverse mode (Kirischuk et generally regarded as an obstacle, and indeed the cultured al., 1997; Kirischuk et al., 2007); both the Na+/Ca2+ exchanger neuroglial cells are very poor models of the glia in vivo, both and glutamate transporters are co-localised in perisynaptic morphologically and functionally. Nonetheless, these early astroglial processes (Minelli et al., 2007). experiments were specifically important because they Astroglial glutamate transport is crucial for neuronal showed, beyond any doubt, that the neuroglia is endowed glutamatergic transmission by operating the glutamate– with molecular machinery to participate in chemical trans- glutamine shuttle (Verkhratsky and Butt, 2007). Glutamate, mission in neural networks. accumulated by astrocytes is enzymatically converted into The receptor patterns expressed by glial cells in situ are very glutamine by the astrocytic-specific glutamine synthetase different, as the complement of receptors is quite restrictive (Martinez-Hernandez et al., 1977). Glutamine can be safely depending on the brain region, and is, most likely, controlled transported to pre-synaptic terminals through the extracellu- by the local neurotransmitter environment. Usually the lar space; after entering the neuronal compartment glutamine modality of neurotransmitter receptors expressed by astroglia 142 BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 is similar to that expressed in their neuronal neighbours The two main receptor systems, glutamatergic and pur- (Verkhratsky and Kettenmann, 1996; Verkhratsky et al., 1998). inergic, expressed in glial cells are complemented by almost The most abundant neurotransmitter receptors expressed by limitless combinations of other receptors that allow glia to neuroglia are glutamate receptors and purinoceptors, al- accurately receive all types of chemical transmission that though the expression of particular subtypes varies between occurs in the brain. In addition, the morphological proximity brain regions. Most of astrocytes and oligodendrocytes of astroglial membranes and synaptic structures (which in fact express metabotropic glutamate receptors of mGluR3 and make the central synapses tripartite — i.e. comprising pre- mGluR5 variety; the latter receptors are instrumental for and postsynaptic compartments as well as astroglial perisy- generation of glial Ca2+ signals (Hamilton et al., 2008; Kirischuk naptic process (Araque, 2008; Araque et al., 1999; Halassa et al., et al., 1999; Tamaru et al., 2001; Verkhratsky and Kirchhoff, 2007a)) almost certainly indicates the continuous involvement 2007a). Astro- and oligodendroglia in hippocampus, corpus of glia in the ongoing synaptic transmission. callosum, optic nerve and cerebellum show relatively high levels of expression of ionotropic AMPA receptors, quite often 2.5.2. Synaptic transmission onto glia of high-Ca2+-permeable form (Garcia-Barcina and Matute, Astrocytes are able to receive inputs from synaptic transmis- 1998; Matute and Miledi, 1993; Muller et al., 1992; Seifert and sion during the spill-over of a neurotransmitter for which they Steinhauser, 2001; Steinhäuser and Gallo, 1996; Verkhratsky express appropriate receptors. Dani et al. (1992),using and Kirchhoff, 2007a). Cortical astrocytes and oligodendro- cultured organotypic hippocampal slices, have demonstrated cytes in the corpus callosum, cerebellum and the optic nerve that neuronal activity at mossy fibre—CA3 synapses can “ ” 2+ 2+ express glial NMDA receptors characterised by weak Mg trigger [Ca ]i increases and intercellular waves in an astro- block (Karadottir et al., 2005; Lalo et al., 2006; Micu et al., 2006; cytic network mediated by glutamate released from synaptic Salter and Fern, 2005; Verkhratsky and Kirchhoff, 2007b). terminals. Similarly, astrocytes can respond to neurotrans- All glial cells express a broad spectrum of purinoceptors. mitter release in acute slices. Porter and McCarthy (1996), The purinergic neurotransmission (Abbracchio et al., 2009; using acutely isolated hippocampal slices, have observed that North and Verkhratsky, 2006), operative in many regions of electrical stimulation of Schaffer collaterals caused increases – – 2+ the brain, is particularly important for neuronal glial, glial in [Ca ]i in astrocytes located in the stratum radiatum of the glial and glial–neuronal signalling. The purinergic signalling CA1 region. At the lower level of synaptic activity this effect system employing ATP, adenosine and adenine nucleotides was mediated by mGluRs, while the higher levels of neuronal as mediators represents the ancient system for intercellular activity recruited both mGluRs and AMPA/kainate ionotropic communications, and as such it is exceptionally broadly GluRs on astrocytes. Glutamate is not the only mediator of this distributed in different types of cells and tissues (Burnstock et neurone–astrocyte signalling via spill-over. For example 2+ al., 2010; Burnstock and Verkhratsky, 2009). The glial cells, synaptically released acetylcholine also evokes [Ca ]i eleva- arguably, inherited the sensitivity to purines from their tions in astrocytes in hippocampal slices through the activa- epithelial ancestors; be it as it may, all glial cells have tion of metabotropic acetylcholine receptors (Araque et al., sensitivity to purines mediated through several types of P2 2002). (ATP/nucleotide) and/or P1 (adenosine) receptors. Activation Besides glial cells sensing spill-over of synaptically re- of purinoceptors controls many vital functions of glial cells leased neurotransmitters, they can receive direct synapse-like being instrumental for glial signalling, release of gliotrans- (synaptoid) or even classical synaptic connections. Stimula- mitters, initiation of astrogliosis and activation of microglia tion of pituitary stalk can depolarize stellate astrocyte-like (Verkhratsky et al., 2009). The actual mapping of purinocep- glia, pituicytes, in situ, an effect mediated by γ-aminobutyric tors in various types of glia has not been accomplished; acid (GABA) and dopamine receptors on this glial cell type nonetheless majority of astrocytes and oligodendrocytes (Mudrick-Donnon et al., 1993) and occurring via direct inputs possess metabotropic A1,2A,2B,3 adenosine receptors and from neurones through synaptoid contacts, where axonal

P2Y1,2,4,6 nucleotide receptors coupled to the InsP3-signalling projections end on pituicytes (Buijs et al., 1987; van Leeuwen et chain and Ca2+ signalling (Abbracchio and Burnstock, 1998; al., 1983; Wittkowski and Brinkmann, 1974). Similarly, norepi- Boison et al., in press; Kirischuk et al., 1995a; Kirischuk et al., nephrine terminals make synaptoid contacts onto septohip- 1995b; Verkhratsky et al., 2009). Glial expression of ionotropic pocampal astrocytes (Milner et al., 1995). Direct neuronal

P2X receptors is much less clear; the P2X1/5 mediated currents glutamatergic and GABAergic synapses made onto oligoden- were found in cortical astroglia (Lalo et al., 2008), yet P2X drocyte precursor cells were observed in the hippocampus currents were detected neither in hippocampal astrocytes (Bergles et al., 2000; Lin and Bergles, 2004). (Jabs et al., 2007) nor in Bergmann glial cells (Kirischuk et al., 1995a). There is some evidence of functional expression of 2.5.3. Signalling in glial syncytia

P2X7 receptors in oligodendrocytes, although they may The extended complement of plasmalemmal receptors, operate mostly in pathological conditions (Domercq et al., expressed by glial cells is coupled to intracellular signalling

2010; Matute, 2008). Finally, multiple P2Y and P2X4,7 receptors cascades, which provide glia with specific excitability are abundantly expressed in microglia; these receptors mechanisms. The glial excitability, as we can perceive it control activation, motility, release of cytokines and exert today, is based on the excitability of the endomembrane numerous trophic effects upon microglia (Farber and Ketten- (which forms the endoplasmic reticulum, the ER) endowed 2+ mann, 2006b; Haas et al., 1996; Hanisch and Kettenmann, with Ca release channels, represented by InsP3 receptors and 2007; Inoue, 2008; Inoue and Tsuda, 2009; Inoue et al., 2005; ryanodine receptors (Berridge et al., 2003; Kostyuk and Moller et al., 2000). Verkhratsky, 1994; Verkhratsky, 2005). Stimulation of the BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 143 majority of metabotropic receptors expressed in neuroglia Since the first demonstration of the release of GABA from 2+ induces formation of InsP3, which in turn triggers Ca release glial cells in superior cervical ganglia (Bowery et al., 1976), a from the ER producing thus Ca2+ signals; these Ca2+ signals are quest for an understanding the mechanisms and conditions generally associated with glial activation and act as a that underlie transmitter release is underway. Several differ- substrate for glial excitability (Deitmer et al., 1998; Farber ent mechanisms have been implicated in release of a and Kettenmann, 2006a; Verkhratsky et al., 1998). The Ca2+ gliotransmitter: (i) through channels like anion channel release channels are regulated by both cytosolic and intra-ER opening, induced by cell swelling (Pasantes Morales and Ca2+ concentrations (Burdakov et al., 2005); importantly this Schousboe, 1988), release through functional unpaired “hemi- Ca2+ sensitivity (together with the internal continuity of the ER channels” (connexins/pannexins) located at the plasma mem- 2+ Ca store (Jones et al., 2009; Petersen and Verkhratsky, 2007; brane (Cotrina et al., 1998; Iglesias et al., 2009) and P2X7 Solovyova and Verkhratsky, 2003)) is instrumental in gener- ionotropic purinergic receptors (Duan et al., 2003); (ii) through ating intracellular propagating Ca2+ waves. These propagating transporters, e.g. by reversal of uptake by plasma membrane waves of endomembrane excitation cross cell-to-cell bound- excitatory amino acid transporters (Szatkowski et al., 1990), aries in the astroglial syncytia thus providing the means for exchange via the cystine–glutamate antiporter (Warr et al., long-range signalling (Cornell Bell et al., 1990; Haas et al., 2006; 1999) or organic anion transporters (Rosenberg et al., 1994); Scemes and Giaume, 2006; Verkhratsky and Toescu, 2006). The and (iii) through Ca2+-dependent exocytosis (Parpura et al., actual mechanisms of generation and maintenance of inter- 1994). Gliotransmitters can be divided into three general 2+ cellular Ca waves are complex and involve InsP3 diffusion groups: (i) amino acids and their derivatives, such as through gap junctions as well as gliotransmitter (most notably glutamate and D-serine; (ii) nucleotides, like ATP, and (iii) ATP) release (Anderson et al., 2004; Arcuino et al., 2002; peptides (recently reviewed in Parpura and Zorec, 2010). At Guthrie et al., 1999; Haas et al., 2006; Scemes and Giaume, present, Ca2+-dependent vesicular release of glutamate and 2006; Suadicani et al., 2004; Verkhratsky, 2006a). ATP from astrocytes can readily occur under physiological The gap junctional route can also form the pathways for conditions, while there are questions raised as to whether alternative signalling in the astroglial syncytia, which may other mechanisms might solely operate during pathophysio- involve various second messengers, metabolic substrates logical circumstances. (Rouach et al., 2008), and maybe some other molecules. We The first documented description of general secretion from know very little about these alternative pathways, and yet glial cells came from the work of a French neuroanatomist- these can be important for the plasticity and information physician Jean Nageotte who proposed that these cells may processing by neuroglia. Furthermore not only Ca2+ but also release substances into the blood, acting like an endocrine other ions may serve as glial signallers. This, for example, may gland (Nageotte, 1910). He observed various secretory granules be an important function of Na+ ions. The concentration of Na+ in glial cells of grey matter (astrocytes) using the Altmann is often increased in glia following activation of transporter method of fucsin labelling (Fig. 5). systems and ionotropic receptors (Deitmer and Rose, 2010; He reported: “The facts that I have just observed seems Kirischuk et al., 2007; Langer and Rose, 2009). Sodium ions are to me shed new light on the physiology of the neuroglial capable of activating enzymatic systems. An increase in cells, not only of cells that are associated with neuronal + +– + astroglial [Na ]i activates Na K ATPase, which, in turn, cells, which have been named satellite cells, but also, and stimulates phosphoglycerate kinase thus initiating produc- particularly, of cells that are in connection with the tion of lactate and lactate shuttling to neurones. vasculature walls. The glial networks established by the plethora of gap Indeed, I was able to present evidence of robust and active junctions formed between glial cells can also connect them to secretion phenomenon in the protoplasm of these cells in neuronal networks. Although gap junctions between glial cells rabbit and guinea pig. This observation was visible especially and neurones are not common, their existence has been within the protoplasmic expansions which cross the empty demonstrated in crayfish (Peracchia, 1981). Additionally, space created by the retraction of tissues around the vascular direct heterocellular coupling of mammalian neurones and walls, on which the neuroglial cells attach using an enlarged glia has been demonstrated in neuronal-glial co-cultures foot. (Nedergaard, 1994; Froes et al., 1999), in acute slices from the In a previous note, I have described the mitochondria that locus coeruleus (Alvarez-Maubecin et al., 2000) and in the exist in these protoplasmic expansions, and I have shown that cerebellum (Pakhotin and Verkhratsky, 2005). many, and maybe all of the granulations located in the grey substance outside the protoplasm of neuronal cells, in reality 2.5.4. Gliotransmission and exocytotic release belong to the neuroglia. Today, I am poised to follow the Astrocytes and other glial cells can release a variety of evolution that occurs within these granulations and to show transmitters into the extracellular space. The criteria for a their progressive transformation into secretion grains. These chemical released from glia to be classified as a gliotransmitter phenomena are exactly similar as those described by Altmann have been defined (Do et al., 1997; Martin et al., 2007; Volterra in the glandular cells; the granulations observed are of three and Meldolesi, 2005). Gliotransmitters are classified based on a types: 1° round grains excessively small that, by the Altmann working set of criteria: (i) synthesis by and/or storage in glia; (ii) method, colour intensively in red; 2° more voluminous grains, regulated release triggered by physiological and/or patholog- with clear centres; 3° grains that do not colour with fucsin. The ical stimuli; (iii) activation of rapid (milliseconds to seconds) last ones are slightly smaller than the more voluminous red responses in neighbouring cells; and (iv) a role in (patho) grains. All intermediates exist between the three types, which physiological processes (Parpura and Zorec, 2010). represent the successive phases of the transformation of 144 BRAIN RESEARCH REVIEWS 66 (2011) 133– 151

astrocyte–neurone signalling pathway have been observed in neurones of different brain regions, including visual cortex, hippocampus, thalamus and nucleus accumbens. Besides glutamate there are several other vesicularly released mediators of astrocyte–neurone signalling, most notably D-serine and ATP. Astrocytes can exocytotically release D-serine (Mothet et al., 2000) an endogenous NMDA receptor agonist (Schell et al., 1995). Indeed, release of D-serine from glia can contribute to the physiological activation of NMDA receptors expressed on retinal ganglion cells (Stevens et al., 2003), as well as to the hippocampal LTP (Yang et al., 2003). Similarly, ATP can be released from astrocytes and it serves as the major humoral factor in support of astroglial Ca2+ wave propagation (Arcuino et al., 2002; Cotrina et al., 2000; Guthrie et al., 1999; Stout et al., 2002). ATP can also contribute to astrocyte–microglia (Verderio and Matteoli, 2001), astrocyte–endothelial cell (Braet et al., 2001), and astrocyte–neurone signalling (Newman, 2003). For example, in the case of astrocyte–neurone signalling, stimulated astrocytes in the whole mount retina can release ATP into extracellular space, where it is rapidly converted by ectoen- zymes to adenosine. Subsequent activation of adenosine Fig. 5 – Neuroglial cells of grey matter in rabbit medulla, most receptors located on retinal ganglion cells induces hyperpolar- likely astrocytes, contain various secretory granules. Scale ization of these cells thus reducing spontaneous firing rate of bar, 5 μm. Reproduced from Nageotte (1910). neurones exhibiting spontaneous spike activity. Similarly, ATP released from astrocytes can cause tonic and activity-dependent suppression of excitatory synaptic transmission in acute mitochondria in to secretion grains.” (Translated from French; slices (Zhang et al., 2003). Taken together, glia–neurone Nageotte, 1910). signalling pathway might be a wide-spread phenomenon Only recently, however, has there been accumulating throughout the brain with glia/astrocytes having the means evidence of biochemical and morphological correlates of the (by releasing various gliotransmitters) to participate in many exocytotic process in astrocytes (reviewed in Montana et al. information processing functions of the CNS. 2006) as briefed below. Astrocytes express proteins of the core SNARE complex: 2.5.5. Neuroglia controls connectivity in neural networks synaptobrevin 2, syntaxin 1, synaptosome-associated protein The operational connectivity between the elements of the of 23 kDa, as well as proteins important for sequestering nervous system are of a paramount importance for its glutamate/ATP into vesicles: the vacuolar type of proton function; by and by the maintenance and plastic remodelling ATPase (V-ATPase), which drives protons into the vesicular of the connectivity status is the raison d'etre of the nervous lumen creating the proton concentration gradient necessary tissue. The connections in the brain are represented by many for glutamate/ATP transport into vesicles, and the three known structures that include chemical and electrical synapses, the isoforms of vesicular glutamate transporters (VGLUTs) 1, 2 and neuronal–glial and glial–glial contacts. The synapse appear- 3 and vesicular nucleotide transporter (VNUT) (reviewed in ance, maintenance survival and death — these all are Parpura and Zorec, 2010). Immunoelectron microscopy studies controlled by the neuroglia. Indeed, astrocytes secrete numer- demonstrated that VGLUTs 1 or 2 in astrocytes in situ associate ous factors indispensable for synaptogenesis (Liauw et al., with small clear vesicles with a mean diameter of ∼30 nm 2008; Nieweg et al., 2009), and without astrocytes formation of (Bezzi et al., 2004). Astrocytes also display large dense core synapses is greatly depressed (Pfrieger, 2009; Pfrieger and granules with diameters of ∼115 nm, containing the secretory Barres, 1996). The astroglia in the tripartite synapse (which is peptide secretogranin II (Calegari et al., 1999) and ATP (Coco et the most common in the CNS) maintains the efficacy of al., 2003). These two populations of vesicles appear to be synaptic transmission through homeostatic control over the biochemically and morphologically distinct. synaptic cleft contents and through metabolic support. The consequences of Ca2+-dependent exocytotic glutamate Removal of astroglial coverage results in synaptic shutdown release from astrocytes can be seen in neurones as: (i) an and synapse elimination (Nagele et al., 2004). The second level 2+ elevation of neuronal [Ca ]i (Parpura et al., 1994); (ii) a slow of CNS connectivity accomplished by the white matter is inward current (Araque et al., 1998a; Araque et al., 1998b); almost exclusively maintained by the oligodendrocytes, and (iii) an increase of excitability (Hassinger et al., 1995); oligodendroglial failure severely alters the brain function. (iv) modulation of spontaneous and action potential evoked synaptic transmission (Araque et al., 1998a; Araque et al., 2.6. Pathophysiological aspect 1998b); (v) synchronization of synaptic events (Fellin et al., 2004), induction of long-term potentiation, LTP (Perea and The pathological potential of neuroglia was recognised very Araque, 2007) or various combinations of the above (reviewed early, and several prominent neurologists of 19th–beginning in Ni et al., 2007). Such effects of the glutamate-mediated of 20th centuries have stressed the role of glial cells in the BRAIN RESEARCH REVIEWS 66 (2011) 133– 151 145 progression of various neurological diseases (Alzheimer, 1910; neuroglia. The latter, although being electrically silent, Del Río-Hortega, 1919a; Frommann, 1878; Nissl, 1899). Our employ many different mechanisms for inter- and intracel- knowledge of the pathological importance of neuroglia lular signalling. It appears that astrocytes can control remains fragmentary, because of a long-lasting prevalence of synaptic networks and in such a capacity they may neurocentric views in neurology and neuropathology. represent an integral component of the computational Despite substantial gaps in our knowledge we may safely power of the brain rather than being just a brain “connective conclude that neuroglia takes the leading role in initiation, tissue”. Consequently, it is not surprising that in an progression and outcome of neurological diseases; further- ascending phylogeny, the glia (astrocytes):neurones ratio more we can regard neurological diseases primarily as the increases and peaks within the human brain, which has both gliopathologies (for the state of the art reviews and compre- relatively and absolutely the greatest number of glia. hensive literature see Fields (2008b), Giaume et al. (2007), Interestingly, Albert Einstein's brain contained a significant- Halassa et al. (2007a), Heneka et al. (2010), Matute (2010), ly higher ratio of glia:neurons in his left Brodmann's area 39 Nedergaard and Dirnagl (2005), Nedergaard et al. (2010), (Diamond et al., 1985). This makes us wonder. Rodriguez et al. (2009), Rossi and Volterra (2009) and Seifert The fundamental question of whether neuroglia is in- et al. (2010)). Indeed, the ultimate specialisation of neurones, volved in cognition and information processing remains, which underlie the sophistication of neuronal network in the however, open. Indeed, the “glial explosion” that distinguishes human brain, robbed the former from the house-holding the human brain can be simply a result of exceedingly high abilities. Indeed in the higher mammals, the neuroglia is fully specialisation of the neuronal networks that delegated all responsible for brain homeostasis, for the brain metabolic matters of survival and maintenance to the neuroglia; to do so support and for the brain defence, and every type of lesion to the latter obviously is in need of sensing the neuronal activity the nervous system tests the ability of neuroglia to withstand and communicating with fellow glia. At the same time and repair the damage. potential power of analogue processing offered by internally The neuroglial cells are involved in all types of brain connected glial networks may represent the alternative pathology from acute lesions (trauma or stroke) to chronic mechanism of cognitive amplification. Where is the truth? neurodegenerative processes (such as Alzheimer's disease, This is the future, which holds the answer. Parkinson's disease, multiple sclerosis and many others) and psychiatric diseases. The pathologically relevant neuroglial processes are many, and they include various programmes of Acknowledgments activation, which are essential for limiting the areas of damage, producing neuro-immune responses and for the post-insult We thank Drs. Vedrana Montana and Mathieu Lesort for their remodelling and recovery of neural function (Hanisch and translation of Nageotte's publication. Authors' research was Kettenmann, 2007; Pekny and Nilsson, 2005; Rolls et al., 2009). supported by the Alzheimer's Research Trust (UK) Programme Recent studies also emphasised the role of astroglial degener- grant (ART/PG2004A/1) to AV and JJR; by the National Institute ation and atrophy in the early stages of various neurodegener- of Health (NIH) grant to VP (R01 MH 069791); by the National ative disorders, which may be important for cognitive Science Foundation (CBET 0943343) grant to VP, and by the impairments (see Heneka et al., 2010; Olabarria et al., 2010; Grant Agency of the Czech Republic (GACR 309/09/1696) to JJR Rodriguez et al., 2009; Rossi and Volterra, 2009). Similarly, and (GACR 305/08/1381, GACR 305/08/1384) to AV. microglia being the innate CNS defence system is intimately involved in all types of brain pathology including the neurodegenerative diseases (Heneka et al., 2010; Rodríguez et al., 2010). REFERENCES

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