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Hypothesis

Cell Biology International 10.1002/cbin.10326

Mechanisms through which hippocampal astrocytes might mediate spatial

memory and theta rhythm by gliotransmitters and growth factors† HosseinHassanpoor, Ali Fallah, PhD

Department of Bioelectrics, Faculty of Biomedical Engineering, Amirkabir University of

Technology, Tehran, IR Iran.

Mohsin Raza, MD PhD

Section of Neurosciences, Department of Neurology, Faculty of Medicine, Baqiyatallah

University of Medical Sciences, Tehran, IR Iran

Corresponding author:

Mohsin Raza, MD PhD

Section of Neurosciences, Department of Neurology

Faculty of Medicine

Baqiyatallah University of Medical Sciences, MollaSadra Avenue

Tehran, IR Iran

†

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the Version of Record. Please cite this article as doi: [10.1002/cbin.10326]

This article is protected by copyright. All rights reserved Received 04 March 2014; Revised 09 May 2014; Accepted 20 May 2014

E-mail: [email protected]

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Phone / Fax: +9821-81264150

Financial Support: PhD student grant for H. Hassanpoor

Abstract

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Orknowledge about encoding and maintenance of spatial memory emphasizes the integrated

functional role of the grid cells and the place cells of the hippocampus in the generation of theta

rhythm in spatial memory formation. However, the role of astrocytes in these processes is often

underestimated in their contribution to the required structural and functional characteristics of

hippocampal neural network operative in spatial memory. We show that hippocampal astrocytes,

by the secretion of gliotransmitters, such as glutamate, D-serine, and ATP and growth factors

such as BDNF and by the expression of receptors and channels such as those of TNFα and

aquaporin,haveseveral diverse fuctions in spatial memory. We specifically focus on the role of

astrocytes on 5phases of spatial memory: 1) Theta rhythm generation 2) Theta phase precession

3) Formation of spatial memory by mapping data of entorhinal grid cells into the place cells 4)

Storage of spatial information 5) Maintenance of spatial memory. Finally, by reviewing the

literature, we propose specific mechanisms mentioned in the form of a hypothesis suggesting that

astrocytes are important in spatial memory formation.

Introduction/Background

The hippocampusiscritical in several types of memory, including spatial memory which is a type

of declarative memory relatedto learning, encoding, storage and recall of spatial

locations(Jarrard, 1993). However, the exact neural mechanisms underlying these processes

remain unclear.

Hippocampal place cells, located in CA1 region, are the neurons with spatially localized

activities(O'Keefe and Dostrovsky, 1971). These neurons have been identified in several other

species (Moser et al., 2008, Sargolini and Moser, 2007, Witter and Moser, 2006).Place cells only

fire whenever an animal is within a certain specific location in the environment, called theplace

field(O'Keefe and Dostrovsky, 1971). Place fields signal the location of the animal within its

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environment by high-frequency discharge of place cells whenever animaltraverses the specific

region(Mizumori, 2008). This representation can be specific to the environment with different

cells being active in different environments or the same cell being active at different locations in

different environments(Wills et al., 2005).

Another type of neuron known as the grid cell located in the dorsocaudal region of medial

entorhinalcortex shows an activity pattern that correlates with animal's position(Moser, Kropff,

2008). Firing of grid cells has specific properties including spacing, orientation, and phase shift

of the nodes of its grid.These cells increase their firing frequency at multiple regions in the

environment arranged in regular triangular grids. Within a local anatomical region,spacing and

orientation properties are same; however, spatial phases are different (Hasselmo, 2008). These

cells are the main inputs from medial entorhinal cortex to the place cells of the CA1 area of

hippocampus.Hippocampal place fields are formed by the integration of grid cell inputs.(Solstad

et al., 2006).

Construction of single localized firing patterns of place cells from multiple firing fields of grid

cells has been the focus of attention of investigators. Simulation studies have used mathematical

models based on mechanisms of attractor dynamics, competitive learning, radiallybasedfunction

network method and Bayesian position reconstruction to map the activity of grid cell network in

the medial entorhinal cortex, whichis essential for the place specific-firing of the CA1 place cells

(Guanella and Verschure, 2006, Guanella and Verschure, 2007, Rolls et al., 2006, Saeidi and

Towhidkhah, 2008, Solstad, Moser, 2006)., there are direct and indirectneural pathways that

connectgrid cells of entorhinal cortex andthe place cells located in the CA1 regions of

hippocampus(Brun et al., 2008; figure 1).

Figure 1 here

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The hippocampaltheta rhythm with a characteristic frequency of 7-9 Hz is observed in local field

potential (LFP) when animals orient, rear, or engage in exploratory sniffing in a particular

environment as well as REM sleep(Skaggs et al., 1996).

Place and grid cells firing shows temporal organization pattern known as ‘theta phase

precession’whereby the relationship between the phase of the spike bursts of grid cells and the

rhythm of the LFPchanges systematically as the animal moves through a particular place

field(Burgess and O'Keefe, 2011, O'Keefe et al., 1993). This phase shifting is due to the

difference of frequency rate between the place cell firing and the LFP (Burgess and O'Keefe,

2011).There isa linear relationship betweenthe distance travelled within the place field and the

spike phase and it is independent of theanimal’s running speed. This phenomenon is important

for the coordinated activity ofhippocampal neurons, space coding and drives hippocampal

remapping during which the locations of rodent hippocampal place fields may alternate from one

place to another (Geisler et al., 2007, Monaco et al., 2011). Computational modeling suggests

that the spatial activity of the grid cells is produced by interference between neuronal oscillators

(Barry et al., 2012). While the exact mechanisms of these processes remain unclear, it is now

known that the altered number of astrocytes and their laminar distribution in the CA1

hippocampal field affects spatial learning and memory (Frota de Almeida et al., 2012).

Astrocytes exert significant effect on the synaptic activity and hippocampal neuronal output by

ensheathing the synapsesof these neurons and by producing gliotransmitters. These

gliotransmittershave complex and diverse effects on adjacent neurons including place and grid

cells (Brun and Leutgeb, 2008, Fellin et al., 2006, Jacobson et al., 2008). For example, D-serine

and glutamate released by astrocytes can modify NMDA receptor–mediated current, resulting in

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an excitatory feedback to neurons (Bains and Oliet, 2007, Volterra and Meldolesi, 2005). At

CA1 synapses, NMDA receptor-dependent synaptic plasticity is essential for both the formation

of place fields in CA1 and the spatial memoryacquisition(Wilson and Tonegawa,

1997).Astrocytes also secrete ATP, which is rapidly converted to adenosine that acts on

adenosine A1 receptors to inhibit synaptic transmission(Pascual et al., 2005). Astrocytes have

receptors

for neurotransmitters such as serotonin and acetylcholine (ACh). These

neurotransmittersare simultaneously secreted during the spatial memory task, which suggests

role of astrocytes in these neurotransmitter systems that regulate behavioral and cognitive

functions via theirvarious receptors(Albuquerque et al., 2009, Hasselmo, 2006, Stancampiano et

al., 1999). More specifically, AChfunctions in memory formation related to environmental

novelty, associated with strong cholinergic drive and induces expansion in the firing pattern of

grid cells(Barry, Heys, 2012, Deiana et al., 2011). This is also associated with a reduction in the

frequency of theta rhythm (Burgess and O'Keefe, 2005).

Hippocampal astrocytes respond to ACh released atthe synaptic terminals(Araque et al.,

2002).The synaptically released ACh acts on muscarinic ACh receptors (mAChRs) present on

the astrocytes, releasing Ca2+ from the intracellular stores(Araque, Martı, 2002). Astrocyte Ca2+

elevations lead to the release of gliotransmitters such as glutamate and ATP from these glial cells

at the synapse and alter synaptic plasticity (Haydon et al., 2009).Furthermore,during spatial

acquisition learning, ACh efflux occurs immediately into the extracellular space in the

hippocampus and cortex, which leads to the consolidation of memory formation. (Deiana, Platt,

2011).

At the cellular level, cholinergic inputs play a key role in the generation of the hippocampal theta

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rhythm (Stan Leung, 1998, Vertes and Kocsis, 1997). Hippocampal cholinergic transmission is

known to be involved in the long-term potentiation (LTP) which underlies learning and

memory(Araque,

Martı,

2002).

Cholinergic-induced

LTP

requires

astrocyte

Ca2+

elevations(Perea and Araque, 2005)and results from the temporal coincidence of the postsynaptic

activity while the astrocyte Ca2+ signal is also simultaneously evoked by cholinergic activity(de

Sevilla et al., 2010). Thus, it is plausible that astrocyte Ca2+ signal is necessary for cholinergic-

induced synaptic plasticity and indicates that astrocytes are rather directly involved in neuronal

storage of information and theta rhythm generation(Navarrete et al., 2012).

Finally, because of the ability of astrocytes to communicate with neural network, many

theoretical studies proposethat the astrocytesare pivotal in information processing in various

states including consciousness (Pereira and Furlan, 2010), formation of memories (Banaclocha,

2007), intentionality (Mitterauer, 2007), and development of motor responses (Hassanpoor et al.,

2012).

The Hypothesis

Astrocytesare important in theta rhythm generation, theta phase precession and formation,

leading to the formation and consolidation of spatial memory. We suggest that these five

processes (outlined in Fig 2a and illustrated in detail in Fig 2b) are important in spatial memory.

Figure 2a and b here

The basis of our hypothesis is elaborated in the following 5sections.

1- Theta rhythm generation:

Theta rhythm of hippocampus and ACh neurotransmission are essential for spatial memory.

Hippocampal CA1area intrinsically generates theta population oscillations in response to the

activation of metabotropic glutamate receptor under conditions of reduced AMPA receptor

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activation (Gillies et al., 2002, Goutagny et al., 2009). Theta oscillations in CA1 are travelling

waves that propagate coarsely along the septotemporal axis of the hippocampus (Lubenov and

Siapas, 2009). This relationship between astrocyte and theta rhythm is more obvious in sleep

(Fellin et al., 2009, Florian et al., 2011, Halassa et al., 2009).

Astrocytes help generate theta rhythm bysynchronization and modulation of local field potential

(LFP) by inhibitory and excitatory effect of gliotransmitters on synaptic space.Thisinfluences

theta generator activation leading to the propagation of waves to other hippocampal areas.

Underlying mechanisms are as follows:

1.1 ATP released from astrocytes is degraded to adenosine and activates presynaptic adenosine

P2Y1 or A1 receptors that leads to an increase or decrease in its release probability (Panatier

et al., 2011). Activation of A1 receptors is followed by the inhibition of adenylyl cyclase that

leads to decrease incAMP and consequent reduction in AMPA receptor activation (Fields and

Burnstock, 2006). Activation of these receptorsalso leads to activation of phospholipase C

(PLC) and production of Ip3 and causes Ca2+ oscillation in astrocyte releasing glutamate

(Banke et al., 2000).

1.2 Activation of G-protein-regulated inwardly rectifying K+ channels (GIRKs) and inhibition of

Ca2+ channels, that finally leads to inhibition of synaptic transmission (Dunwiddie and

Masino, 2001). Furthermore, astrocytes, by regulating the extracellular K+ in hippocampus,

can modulate excitability of neural network (Min et al., 2012). They decrease frequency and

increase fidelity of excitatory synaptic transmission by decreasing extracellular K+

concentration,leads to transient hyperpolarization of the adjacent neurons (Wang et al.,

2012). Hyperpolarization of neuron decreases the release of glutamate (Dunwiddie and

Masino, 2001, Fellin et al., 2012), which then causes less excitable postsynaptic neuron and

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reduction of power of high frequency LFP spectrum and its shift towards theta rhythm

frequency range. All these processes predominantly generate neuronal synchrony in the

hippocampus (Angulo et al., 2004, Carmignoto and Fellin, 2006) whereby low frequency

theta oscillations dominate the frequency range of LFP spectrum of the hippocampus.

1.3 Synchronization of neuronal firing by theta wave is essential in the separation of encoding

and retrieval processes into separate theta cycles. This is potentiated by the hippocampal

astrocytes, which respond to the ACh released by the synaptic terminals during spatial

memory task (Winkler et al., 1995). The synaptically released ACh then acts on muscarinic

receptors, mobilizing Ca2+ from the intracellular stores (Araque, Martı, 2002). An increase in

intracellular Ca2+ concentration ([Ca2+]i) is sufficient as well as necessary to cause glutamate

and adenosine release from astrocytes(Malarkey and Parpura, 2008). On the other hand

Global Ca2+ signaling (spread~12µm range) (Di Castro et al., 2011) in astrocyte network

with glutamate secretion (Carmignoto and Fellin, 2006) may beimportant in potentiating and

propagating theta wave and therefore prepare conditions for store spatial information. Thus,

we can conclude that astrocytes seem to be directly associated with genesis and propagation

of theta wave by local and as well as global effect. The role of AChis probablyto increase

theta rhythm oscillations, leading to state of enhanced encoding (Hasselmo, 2006). However,

we suggest that ACh instead also activates astrocytes and then they secrete glutamate leading

to the synchronization of the neuronal activity in the hippocampus, which in turn increases

the amplitude of theta wave. These mechanisms are illustrated in Figure 3 (pink and blue

pathway).

Figure 3 here

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2- Theta phase precession:

Theta oscillations gate synaptic plasticity and offer macroscopic access to the internal clock

of the hippocampal circuit, responsible for temporally patterning its operation (Lubenov and

Siapas, 2009). Clocking is essential for the temporal coding of spatial information by place

cells as evidenced in theta phase precession (Huxter et al., 2003). In the normal brain, local

TNFα levels are low and astrocytic G protein-coupled receptor (GPCR) agonist such

as the chemokine stromal-derived factor-1α (SDF1α/CXCL12) induces a local Ca2+

signaling (spread~4 µm range) (Di Castro, Chuquet, 2011) in the astrocyte. This process

occurs upon the activationof C-X-C chemokine type 4 (CXCR4) receptors. The result of this

activation is the moderation ofglutamate release leading to increased synaptic activity of a

single neuron and amplification of the amplitude of high frequency oscillations(Santello et

al., 2011). Astrocytes are also capable of reducing activity of presynaptic neurons and

attenuation of the amplitude of high frequency single neuron oscillations by uptaking again

the glutamate from synaptic space and secretion of adenosine. Therefore, astrocytes via

controlling the frequencies of single neuron oscillation, mediate in theta phase precession.

They modulate the frequencies of neural firing action potential locally and cause shift of the

local frequency towards global frequency (theta rhythm) leading to phase precession. This

process (Fig 3, yellow pathway) takes place as an animal traverses a place field which is

originally encoded in the hippocampal place cells (Malhotra et al., 2012).

3- Encoding and consolidation of spatial memory by mapping data of entorhinal grid cells

into the hippocampal place cells:

The exact relationship between single localized firing patterns of place cells from multiple firing

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fields of grid cells, that lead to spatial memory formation, are still unclear. One possible

mechanism is the important role of astrocyte specific aquaporin 4 (AQP4) channels that may be

involved in the formation of spatial memoryin hippocampus (Scharfman and Binder, 2013).

Additionally, we suggest that astrocytes by changing the physical characteristics of neural

network, communication and synaptic strength (changing functional characteristics of neural

networks) lead to learning of diverse patterns and mapping of firing fields pattern of grid cells as

input that then leads to firing patterns of place cells as output. Astrocytes can also help in pattern

separation and pattern completion, which are complementary processes in associative memory.

These mechanisms are as follows:

3.1 Astrocytes change physical characteristicsofneuron-astrocytenetwork between CA1and CA3

regions of hippocampus and dentate gyrusduring learning process of spatial memory formation

by 2mechanisms.

3.1.1

By mediating Ephrin (Eph) signaling, which is essential in spatial memory

formation through EphARs, astrocytes receive the signals from neural activity and

induce

outgrowth

of

filopodial

processes

within

minutes

in

rat

hippocampus(Nestor et al., 2007; Fig 4, red pathway).

3.1.2

By secreting growth factors such as BDNF that regulate synaptogenesis and lead

to the formation of more connections in neuron-astrocyte network (Vicario-

Abejon et al., 2002; Fig 4, orange pathway).

These mechanisms lead to anatomical alterations in neuron-astrocyte network

andincrease the number and distribution of astrocytes in dentate gyrus, CA1 and CA3

regions during spatial learning(Diniz et al., 2010, Jahanshahi et al., 2008). Based on the

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artificial neural network data, learningcapacity of network may be altered and formation

of new network capable of generating more diverse and complex output is possible by

astrocytic BDNF secretion. Theseuch activities may also affect the overall functions of

brain such as storage and retrieval of information.(Nedergaard et al., 2003)

3.2 Astrocytes are also capable of changing the function of neural network bymanipulation of

synaptic plasticity. In nearly all models of learning and memory, such as spatial memory and

synaptic plasticity, have a central role and lead to changes in synaptic function during

learning process (Neves et al., 2008, Paixão et al., 2010, Silva, 2003). Astrocytes, by

secretion of different gliotransmitters such as glutamate, D-serine, ATP and adenosine,

mediate synaptic plasticity (Paixão, Klein, 2010, Santello and Volterra, 2009, Yang et al.,

2003).

By the following mechanisms, astrocytes change the characteristics of neural network and

increase the learning capacity of neurons to produce more diverse patterns in response to

stimulations.

3.2.1

By releasing and controlling the level of TNFα, promote the insertion of AMPA

receptors in the membrane of the postsynaptic neurons (Perea and Araque, 2009,

Stellwagen and Malenka, 2006; Fig 4 yellow pathway)

3.2.2

By the secretion of Ca2+ dependent D-serine,induce LTP and LTD, enhancing the

spatial memory retrieval via NMDA receptors (Duffy et al., 2008, Henneberger et al.,

2010, Zhang et al., 2008; Fig 4 violet pathway).

3.2.3

By the secretion of glutamate (Stevens, 2008) and regulation of glutamate uptake, that

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integrates and processes synaptic information (Perea et al., 2009), and by enveloping

3.2.4

the synapses and using glutamate transporters, limit the glutamate spillover and

modulate synaptic transmission and plasticity (illustrated in Fig 4, blue pathway).

By influencing the polarity of long-term synaptic plasticity of the Schaffer collateral pathway

in hippocampus via AQP4 channels (Scharfman and Binder, 2013; Fig 4 pink pathway).

In conclusion, astrocytes areessential in mapping the firing patterns of entorhinal grid cells into the

activities of place cells, and ultimately lead to formation and encoding of spatial memory (Levenson

et al., 2002, Pita-almenar et al., 2006, Yang et al., 2005; summarised inFigure 4).

Figure 4 here

4- Storage of spatial information:

Cholinergic-induced LTP requires astrocyte Ca2+ elevationinvolved in spatial learning.

Stimulation of astrocytes by the activation of cholinergic fibers leads to an increase of release

ofD-serine from the astrocytes, which causes an increasedavailability of neuronal NMDARs for

activation(Takata et al., 2011). Thus, less neuronal activity is required for the induction of LTP

and LTD as astrocytes assist neurons in these processes.

Ca2+ elevation in astrocytes modulates probability of transmitter release and evokes short and

long-term synaptic plasticity at single CA3-CA1 hippocampal synapses, involved in the

formation of spatial memory (Perea and Araque, 2007).

5- Maintenance of spatial memory:

Astrocytes release lactate via glycogenolysis, which provides reserve fuel for neurons in

hippocampus, essential for long-term memory formation and its maintenance. Astrocytic lactate

release is also increased during spatial memory tasks indicating that astrocytes play pivotal role

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not only in the formation, but in the maintenance of spatial memory in hippocampus (Newman et

al., 2011, Suzuki et al., 2011).

How the hypothesis is different from current thinking

There is circumstantial evidence of possible roles of astrocytes in spatial memory and lacks

single unified concept on the behavior of these cells in the acquisition of spatial memory. Our

hypothesis has classified the reported evidences into 5underlying mechanisms that might be

involved in induction and formation of spatial memory in hippocampusby astrocytes. Based on

this hypothesis, we suggest that astrocytesare key in spatial memory induction and

formation.More specifically, we have alsohighlighted action of astrocytes on relevant aspects of

theta rhythm in the spatial memory.

Importance

If hypothesis is true, the function of astrocytes in spatial memory can shed morelight on

understanding the navigation system of human brain in health and disease. It may be useful in

the treatment of disorders, such as hemi spatial neglect(inability to detect stimuli and sensations

on the contralateral side of space, deficit in spatial awareness and poor prognosis for long-term

recovery),where the exact nature of the pathogenesis is unclear (Byrne et al., 2007, Verdon et al.,

2010). It may also help in our understanding of the possible mechanisms of spatial working

memory deficits in childrenborn prematurely without major neurological deficit(Vicari et al.,

2004) or impairment of memoryinadult attention-deficit/hyperactivity disorder (ADHD)

(Dowson et al., 2004).

Evaluation of the hypothesis/idea

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After gathering and analysingthe experimental evidence mentioned in Table 1, we have put

forward our hypothesis. We have generated a computational model (a 3-layered neural network

with one hidden layer consisting of pyramidal neurons and astrocytes) and have shown that the

astrocytes by using inhibitory and stimulatory neurotransmitters, such as glutamate and ATP can

generate certain patterns, modulate and balance activity of synaptic space across the neural

network(Hassanpoor et al. ).By rapid communication with other astrocytesvia Ca2+ signaling,

these cells synchronize neural network that leads to change in dominant frequency of brain wave

similar to theta wave frequency. Also, by modeling BDNF secretion and considering its effect on

the characteristics of neural network, we have shown that it leads to alteration as well as

enhancement in learning (Hassanpoor et al. ). For experimental validation in living system, a

suitable approach is to block astrocyte receptors (such as mAChR, A1 or EphAR) during training

or learning sessions and evaluate its effect on spatial memory..

Table 1 here

Consequences of the hypothesis and discussion

In conclusion, we have attempted to describe the different roles of astrocyte in spatial memory.

However, the exact mechanisms relevant to astrocyte in this process especially the mechanism of

formation of place field from grid cell patterns remain unclear. The role of astrocytes in

modulation of brain waves such as theta waves needs further research. Finally, indeed, if our

hypothesis is correct, astrocytes are very important in spatial memory.

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Accepted Article

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Accepted Article

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Accepted Article

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Captions to illustrations

Figure 1. Schematic View of the Direct Input (Purple) from Layer III in Entorhinal Cortex

to CA1, and the Indirect Input (green) from Layer II via the Dentate Gyrus (DG) and CA3

Both perforant-path (pp) and temporoammonic-tract (TA) axons are depicted.

Figure 2.A) Basic mechanisms underlying the Hypothesis

B) Schematic view of mechanisms through which astrocytes mediate spatial memory

Figure 3. Mechanisms of theta wave generation, propagation and phase precessionby

astrocytes. Abbreviation: phospholipase C (PLC), Inwardly Rectifying K+ Channels

(GIRK), G Protein-Coupled Receptor (GPCR), C-X-C chemokine receptor type 4

(CXCR4), Muscarinic ACh Receptors (mAChRs)

Figure 4. Mechanisms which astrocytesmediate encoding and consolidation of spatial

memory by mapping data of entorhinal grid cells into the hippocampal place cells.

Abbreviation: Slow Inward Current (SIC), Excitatory Postsynaptic Current (EPSC),

Aquaporin 4 (AQP4)

Table 1. Evidence from published literature for the validation of hypothesis on the role of

Accepted Article

astrocyte in spatial memory

Table 2Evidence from published literature for the validation of hypothesis on the role of

astrocyte in spatial memory

#

1

2

3

4

5

Possible physiological role of astrocyte in spatial memory

Implication on spatial memory

Ref

(Frota de Almeida, de Siqueira Mendes, 2012, Jahanshahi, Sadeghi, 2008)

Number, formation and distribution of astrocytes in CA1 region of hippocampus play important role in spatial memory. Furthermore, the number of astrocytes increase due to spatial learning in hippocampus.

Altered astrocyte laminar distribution and number in the CA1 hippocampus can lead to impaired spatial learning and memory.

Ca2+ elevation in astrocytes modulates probability of transmitter release and evokes short and long-term synaptic plasticity at single CA3-CA1 hippocampal synapses.

Transfer and storage of synaptic information in CA3-CA1 area of hippocampus, which are involved in spatial memory, actively mediated directly by astrocytes.

(Perea and Araque, 2007)

Astrocytic EphARs mediate neuron-glia plasticity in hippocampus and lead to change in structural and functional plasticity of neural astrocyte network.

Ephrin (Eph) signaling via Eph receptors affects neuronal structure and function, which is essential in spatial memory.

(Nestor, Mok, 2007)

Astrocytes have glutamate transporter subtype 1 (GLT1) which is one of the main glutamate transporters in the hippocampus. GLTI with two isoforms (GLT1a and GLT1b), play important role in glutamate uptake from synapse.

Regulation of glutamate uptake plays important role in synaptic plasticity as well as memory formation. Furthermore, changes in glutamate uptake lead to changes in synaptic efficacy.

(Levenson, Weeber, 2002, Pita-almenar, Collado, 2006, Yang, Huang, 2005)

Astrocytes actively participate in synaptic transmission and plasticity by secreting neuroactive substances and by actively

In nearly all models of learning and memory such as spatial memory, synaptic plasticity has a

(Neves, Cooke, 2008, Paixão, Klein, 2010,

Accepted Article 6

7

8

9

Klein, 2010, Silva, 2003)

neuroactive substances and by actively removing synaptically released neurotransmitters from hippocampal synapses.

memory, synaptic plasticity has a central role and leads to changes in synaptic function during learning process.

Number and laminar distribution of astrocytes and neurons in different areas of hippocampal formation alter during aging process, which correlates with decline in cognitive abilities and memory in old age.

Anatomical alterations in neuronastrocyte network within hippocampal formation during aging process leads to decline in their functional characteristics necessary for learning and memory including spatial memory.

(Diniz, Foro, 2010, Jacobson, Zhang, 2008)

Astrocytes promote the development and plasticity of synaptic circuits and regulate the wiring of the brain during development in brain regions such as entorhinal cortex.

Crucial role of the entorhinal cortex in spatial representation and navigation indicates functional role of astrocytes in spatial memory.

(Stevens, 2008, Witter and Moser, 2006)

Astrocytes release ATP, which is a rapidly metabolized to adenosine. Accumulation of adenosine and its resultant inhibitory effect via A1 receptors contribute to alteration of hippocampal synaptic plasticity and cognitive deficits seen in Sleep Deprivation (SD).

Decline in cognitive function and hippocampal plasticity during SD via astrocytic adenosine can also lead to deficits in spatial memory and navigation.

Hippocampal astrocytes respond to Ach released by synaptic terminals via muscarinic receptors, mobilizing Ca2+ from the intracellular stores, which leads to release of gliotransmitters from astrocytes and alters synaptic plasticity.

Astrocytes play important role in hippocampal spatial memory via their muscarinic Ach receptors.

Astrocytes spontaneously release glutamate, which acts extrasynaptically on NMDA receptors on pyramidal neurons in hippocampus and synchronizes the neuronal 10 activity in hippocampus leading to decrease in brain wave frequencies. This occurs via generation of slow transient currents (STCs) and synchronous, slow inward currents (SICs) in pyramidal neurons by the released

Astrocyte can synchronize neuronal networks in hippocampus leading to the theta wave generation, which is important in the acquisition of spatial memory.

(Fellin, Ellenbogen, 2012, Florian, Vecsey, 2011)

(Araque, Martı, 2002, Winkler, Suhr, 1995)

(Angulo, Kozlov, 2004, Carmignoto and Fellin, 2006)

glutamate.

Sleep modulation by astrocyte via various mechanisms leads to consolidation of memories including spatial memory and reduction of theta rhythm can impair it.

Astrocytes release lactate via glycogenolysis, which provides reserve fuel for neurons in hippocampus, which is essential for long-term 12 memory formation and maintenance. Additionally, astrocytic lactate release also increase during spatial memory tasks.

Astrocytes play pivotal role not only in formation but also in maintenance of spatial memory in hippocampus.

Astrocytes in hippocampus exocytotically release Ca2+ dependent D-serine, which plays important role in synaptic plasticity by the 13 induction of LTP and LTD and enhancing spatial memory retrieval via NMDA receptors.

Astrocytes play pivotal role in spatial learning and memory and retrieval as well as navigation via secretion of D-serine.

Accepted Article

Astrocytes modulate the accumulation of sleep pressure and its cognitive consequences through a pathway involving A1 receptors indicating role of gliotransmitters sleep 11 homeostasis. Inhibition of gliotransmitters reduces slow wave activity, particularly that in the low-frequency range which is similar as theta wave.

Astrocytes regulate network dynamics, cortical low- frequency rhythmogenesis and sleep. They modulate the cortical slow oscillation, which is fundamental to sleep and therefore plays role in spatial memory consolidation.

14

These rhythmic brain activities are generated by the coordinated action of the neuronal and astrocytes networks.

Sleep is a state in which spatial by offline reactivation of the neurons involved in memory encoding during recent wakefulness consolidate memories. Hippocampal theta rhythm is crucial for spatial memory and is generated by extrinsic and intrinsic inputs, which may consider astrocyte as intrinsic effect, atropine-resistant theta generators in CA1.

(Halassa, Florian, 2009, Suzuki, Stern, 2011)

(Newman, Korol, 2011, Suzuki, Stern, 2011)

(Duffy, Labrie, 2008, Henneberger, Papouin, 2010, Zhang, Gong, 2008)

(Fellin, Ellenbogen, 2012, Fellin, Halassa, 2009, Goutagny, Jackson, 2009)

Accepted Article Fig 1.

Accepted Article Fig 2_a.

Accepted Article Fig 2_b .

Accepted Article Fig 3.

Accepted Article Fig 4.