Astrocytes have for decades been considered as non-excitable support cells of the central nervous system (CNS). However, this view has changed radically during the last thirty years. Astrocytes have come to the forefront of neuroscience in our attempt to gain a better understanding of the cellular mechanisms underlying synaptic transmission. Astrocytes express a variety of surface receptors for common neurotransmitters, e.g., glutamate and acetylcholine. Additionally, astrocytes respond to stimulation by releasing gliotransmitters such as glutamate. These properties have led to the concept of tripartite synapse involving bidirectional communication between neurons and astrocytes. This review aims at summarizing current evidence for how astrocytes are involved in modulating synaptic transmission within cortical regions of the CNS. The activation of metabotropic- and ionotropic receptors expressed in astrocytes triggers an increase in their intracellular Ca2+ concentration, which in turn promotes the release of glutamate. The astrocyte released glutamate stimulate either group I or II and III metabotropic glutamate receptors (mGluRs) on presynaptic terminals. Depending on the type of activated presynaptic mGluR, astrocytic glutamate may either potentiate or depress synaptic transmission. In summary, astrocytes are actively participating in the modulation of synaptic transmission within cortical regions of the CNS. Astrocytes may consequently be a potential therapeutic target for brain pathological illnesses, e.g., neurodegeneration during ischemic stroke.

1. Astrocytes in the Synapse
- Nurse or Key-Player?
Rune Rasmussen
Supervisor: Assoc. Prof. Jean-Francois Perrier
Department of Neuroscience and Pharmacology

2. Agenda
• Introduction to Astrocytes and Tripartite synapse
• Aim of project
• Main findings (exemplified through studies)
• Therapeutic applications
• Considerations
• Summary

3. Introduction
Glial cells (Greek “Glue”): Astrocytes,
Oligodendrocytes and Microglia (CNS)
Astrocytes the most numerous glial cell type
Do not fire action potentials
(Obenheim et al., 2009)
Astrocytes maintain synaptic homeostasis:
e.g., K+ buffering (Kuffler, 1967)
Occopy different spatial territories
(Volterra & Meldolesi, 2005)
(Volterra & Meldolesi, 2005)

4. Introduction
Astrocyte
Presynaptic
neuron
Gt
Astrocytes release gliotransmitters
(Bezzi et al., 1998; Navarrete et al., 2012)
Nt
Gt
Postsynaptic
neuron
Astrocytes express receptors for
neurotransmitters
(Mothet at al., 2005; Araque, 2008)
= Concept of a Tripartite Synapse
(Araque Haydon, 1999)
(Home-made illustration)

5. Aim of Project
i) Summarize main properties of cortical astrocytes
ii) Describe how neurotransmitters cause the release
of the gliotransmitter glutamate
iii) Discuss how this glutamate is involved in modulation of synaptic transmission

6. Aim of Project
i) Summarize main properties of cortical astrocytes
ii) Describe how neurotransmitters cause the release
of the gliotransmitter glutamate
iii) Discuss how this glutamate is involved in modulation of synaptic transmission

7. Aim of Project
i) Summarize main properties of cortical astrocytes
ii) Describe how neurotransmitters cause the release
of the gliotransmitter glutamate
iii) Discuss how this glutamate is involved in modulation of synaptic transmission

8. Properties of Astrocytes

9. Properties of Astrocytes
Express metabotropic and ionotropic receptors (Rs)
(Porter McCarthy, 1996; Araque 2008)
Many metabotropic Rs coupled to Gαq and PLC-β
Trigger intracellular Ca2+ elevation
(Home-made illustration)

10. Properties of Astrocytes
Astrocytes base their excitability on intracellular variations in Ca2+ (Local or Global)
(Volterra Meldolesi, 2005)
Calcium signal in vivo
Calcium signal in culture
(Nedergaard et al. 2013; Molnár et al. 2011)

11. Astrocytes release Glutamate

12. Astrocytes release Glutamate
Uncaging NP-EGTA in astrocyte
trigger Ca2+ elevation
(Liu et al., 2004, PNAS)

13. Astrocytes release Glutamate
Uncaging NP-EGTA in astrocyte
trigger Ca2+ elevation
.. And this trigger glutamate
release (activate Kainate Rs)
(Liu et al., 2004, PNAS)

14. Astrocytes release Glutamate
AMPA and mGluR agonist trigger
glutamate release
(Bezzi et al., 1998, Nature)

15. Astrocytes release Glutamate
Mediated through type I mGluR and AMPAR activation,
and in a Ca2+-dependent mechanism
(Bezzi et al., 1998, Nature)

16. Modulation of Synaptic Transmission

17. Modulation of Synaptic Transmission
Perea et al. (2007) setup
(Home-made illustration)

18. Modulation of Synaptic Transmission
ATP application triggered Ca2+
elevation in astrocytes
(Perea et al., 2007, Science)

19. Modulation of Synaptic Transmission
ATP application triggered Ca2+
elevation in astrocytes
And increased neurotransmitter
release probability
(Perea et al., 2007, Science)

20. Modulation of Synaptic Transmission
ATP application triggered Ca2+
elevation in astrocytes
And increased neurotransmitter
release probability
Through neuronal type I mGluR
activation
(Perea et al., 2007, Science)

21. Therapeutic application

22. Therapeutic application
Excitotoxicity plays a central role in the neuronal damage through activation of
extrasynaptic NMDARs (Hardingham Bading, 2010)

23. Therapeutic application
Excitotoxicity plays a central role in the neuronal damage through activation of
extrasynaptic NMDARs (Hardingham Bading, 2010)
Enhanced Ca2+ activity in astrocytic
networks play a key role in the
activation of extrasynaptic NMDARs
in hippocampal slice neurons (Dong
et al., 2013)
(Home-made illustration)

24. Considerations
Astrocytic Ca2+ variations through the IP3 pathway does not affect synaptic
transmission (Agulhon et al., 2010)

25. Considerations
Astrocytic Ca2+ variations through the IP3 pathway does not affect synaptic
transmission (Agulhon et al., 2010)
Photolysis of caged Ca2+, but not receptor-mediated Ca2+ signaling triggers
astrocytic glutamate release (Smith et al., 2013, submitted)

26. Considerations
Astrocytic Ca2+ variations through the IP3 pathway does not affect synaptic
transmission (Agulhon et al., 2010)
Photolysis of caged Ca2+, but not receptor-mediated Ca2+ signaling triggers
astrocytic glutamate release (Smith et al., 2013, submitted)
Adult mice ( 3 weeks) do not
express mGluR subtype 5
(Wei Sun et al. 2013)

27. Summary
• Astrocytes express metabotropic Rs (ATP and
Glutamate) and base their excitability on variations in
Ca2+ level

28. Summary
• Astrocytes express metabotropic Rs (ATP and
Glutamate) and base their excitability on variations in
Ca2+ level
• Astrocytes release the gliotransmitter
glutamate (IP3-Ca2+ dependent?)

29. Summary
• Astrocytes express metabotropic Rs (ATP and
Glutamate) and base their excitability on variations in
Ca2+ level
• Astrocytes release the gliotransmitter
glutamate (IP3-Ca2+ dependent?)
• Release of astrocytic glutamate may potentiate
synaptic transmission (Presynaptic mGluRs)

30. Summary
• Astrocytes express metabotropic Rs (ATP and
Glutamate) and base their excitability on variations in
Ca2+ level
• Astrocytes release the gliotransmitter
glutamate (IP3-Ca2+ dependent?)
• Release of astrocytic glutamate may potentiate
synaptic transmission (Presynaptic mGluRs)
• Bi-directional communication between astrocytes and
neurons might open up new therapeutic targets

31. Rune Rasmussen
M.Sc. Stud. Human Biology
(Hippocampal astrocytes: Bushong et al., 2002, J. Neurosci)