Ryanodine Receptor Activation Induces Long-Term Plasticity of Spine Calcium Dynamics

A key feature of signalling in dendritic spines is the synapse-specific transduction of short electrical signals into biochemical responses. Ca2+ is a major upstream effector in this transduction cascade, serving both as a depolarising electrical charge carrier at the membrane and an intracellular second messenger. Upon action potential firing, the majority of spines are subject to global back-propagating action potential (bAP) Ca2+ transients. These transients translate neuronal suprathreshold activation into intracellular biochemical events. Using a combination of electrophysiology, two-photon Ca2+ imaging, and modelling, we demonstrate that bAPs are electrochemically coupled to Ca2+ release from intracellular stores via ryanodine receptors (RyRs). We describe a new function mediated by spine RyRs: the activity-dependent long-term enhancement of the bAP-Ca2+ transient. Spines regulate bAP Ca2+ influx independent of each other, as bAP-Ca2+ transient enhancement is compartmentalized and independent of the dendritic Ca2+ transient. Furthermore, this functional state change depends exclusively on bAPs travelling antidromically into dendrites and spines. Induction, but not expression, of bAP-Ca2+ transient enhancement is a spine-specific function of the RyR. We demonstrate that RyRs can form specific Ca2+ signalling nanodomains within single spines. Functionally, RyR mediated Ca2+ release in these nanodomains induces a new form of Ca2+ transient plasticity that constitutes a spine specific storage mechanism of neuronal suprathreshold activity patterns

Synapse Geometry and Receptor Dynamics Modulate Synaptic Strength

Synaptic transmission relies on several processes, such as the location of a released vesicle, the number and type of receptors, trafficking between the postsynaptic density (PSD) and extrasynaptic compartment, as well as the synapse organization. To study the impact of these parameters on excitatory synaptic transmission, we present a computational model for the fast AMPA-receptor mediated synaptic current. We show that in addition to the vesicular release probability, due to variations in their release locations and the AMPAR distribution, the postsynaptic current amplitude has a large variance, making a synapse an intrinsic unreliable device. We use our model to examine our experimental data recorded from CA1 mice hippocampal slices to study the differences between mEPSC and evoked EPSC variance. The synaptic current but not the coefficient of variation is maximal when the active zone where vesicles are released is apposed to the PSD. Moreover, we find that for certain type of synapses, receptor trafficking can affect the magnitude of synaptic depression. Finally, we demonstrate that perisynaptic microdomains located outside the PSD impacts synaptic transmission by regulating the number of desensitized receptors and their trafficking to the PSD. We conclude that geometrical modifications, reorganization of the PSD or perisynaptic microdomains modulate synaptic strength, as the mechanisms underlying long-term plasticity

Function and expression of Kv channels and neurotransmitter receptors on microglial cells

0\. Titelblatt und Inhaltsverzeichnis 1\. Einleitung 2\. Zielsetzung der Dissertation 3\. Material und Methoden 4\. Ergebnisse 5\. Diskussion 6\. Zusammenfassung 7\. Summary 8\. Tabellen 9\. Literaturverzeichnis 10\. Abkürzungsverzeichnis 1\. Publikationen und Posterbeiträge 12\. Curriculum vitaeMikroglia sind spezialisierte Immunzellen im Gehirn, deren Funktion hauptsächlich darin besteht, die hochempfindlichen Neurone vor Erkrankungen zu schützen und deren normale Funktionen durch Sekretion neurotrophischer Stoffe zu unterstützen. Mikroglia tasten mit ihren beweglichen Zellausläufern ständig das umgebende Gewebe ab und können bei Veränderungen sofort reagieren. Demnach müssen diese Zellen besonders schnelle Aktivierungsmechanismen aufweisen. Neurone erreichen schnelle Informationsweiterleitung durch Aktionspotentiale, d.h. eine Depolarisation des Membranpotentials durch das Öffnen von Na+-Kanälen. Möglicherweise nutzen auch Mikroglia Ionenkanäle um sehr schnell auf Veränderungen ihrer Umgebung reagieren zu können. Im Rahmen dieser Arbeit konnte gezeigt werden, dass die, durch LPS-Stimulation aktivierten spannungsgesteuerten, auswärtsrektifizierenden K+-Kanäle (voltage-gated K+ channels Kv) Kv1.5 und Kv1.3 wichtige Zellfunktionen wie Proliferation und NO- Freisetzung unterschiedlich modulieren. Um jedoch adäquat auf ein neuronales Signal reagieren zu können, müssen Mikroglia kontinuierlich über den Zustand der umliegenden Neurone informiert sein. Dazu sollten Mikroglia neuronale Signale wahrnehmen können. Neurone kommunizieren untereinander und auch mit Astrozyten über die Ausschüttung von Neurotransmittern. Deshalb war es nahe liegend zu untersuchen ob auch Mikroglia neuronale Signale in Form von Neurotransmittern wahrnehmen können. Im Rahmen dieser Arbeit konnten funktionelle Rezeptoren für Dopamin, Noradrenalin, Histamin und Serotonin auf der Mikroglia in vitro und in situ, sowohl auf amöboiden als auch auf ramifizierten Zellen nachgewiesen werden. Die Stimulation dieser Rezeptoren bewirkt ebenfalls eine Änderung des K+-Stromprofils, hierbei handelt es sich jedoch vermutlich um einen G-Protein gekoppelten K+ -Kanal. Demnach scheint die Aktivierung von Mikroglia maßgeblich durch die Expression unterschiedlicher K+ -Kanäle beeinflusst werden.The main functions of microglia, the immunocompetent cells in the brain, are to protect the very fragile neurons against pathological events and to support them with neurotrophic factors. Microglia continually survey their microenvironment with extremely motile processes and can immediately respond to changes. Therefore they require a rapid activation system. Neurons can respond very fast through the activation of Na+ channels and a massive depolarization of the membrane potential. Potentially microglia use similar mechanisms for a fast response. As shown in this work microglia respond to LPS activation with the induction of a voltage gated outwardly rectifying K+ current mediated by two channel proteins Kv1.5 and Kv1.3. These channels can modulate distinct immune functions such as nitric oxide release or proliferation. These channels are possibly co-expressed on microglia cells and lead to the precise immune response. For an adequate immune response the microglia need information about the state of health of the surrounding neurons. Hence they need a communication system. Neurons communicate among each other and with astrocytes via neurotransmitter release. Consequently the question arises wether microglial cells are able to sense neurotransmitters as well. I could show that microglia in vitro and in situ express functional receptors for some of the main neurotransmitters, dopamine, noradrenaline, serotonin and histamine. Ramified as well as ameboid microglia respond in the same way to dopamine, noradrenalin and histamine application, indicating that there is a common mechanism for neurotransmitter action on microglia. Receptor stimulation results again, as shown for LPS activation, in a change in the K+ current profile mediated by a G-protein coupled mechanism. According to these observations one could conclude that the activation of microglial cells is decisively influenced by the expression of K+ currents and that these curents modulate distinct functions

Astroglial potassium clearance contributes to short-term plasticity of synaptically evoked currents at the tripartite synapse

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Astroglial connexin 43 sustains glutamatergic synaptic efficacy

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Astroglial gap junctions shape neuronal network activity

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Astroglial networks promote neuronal coordination

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Activation of serotonin receptors promotes microglial injury-induced motility but attenuates phagocytic activity

Microglia, the brain immune cell, express several neurotransmitter receptors which modulate microglial functions. In this project we studied the impact of serotonin receptor activation on distinct microglial properties as serotonin deficiency not only has been linked to a number of psychiatric disease like depression and anxiety but may also permeate from the periphery through blood-brain barrier openings seen in neurodegenerative disease. First, we tested the impact of serotonin on the microglial response to an insult caused by a laser lesion in the cortex of acute slices from Cx3Cr1-GFP mice. In the presence of serotonin the microglial processes moved more rapidly towards the laser lesion which is considered to be a chemotactic response to ATP. Similarly, the chemotactic response of cultured microglia to ATP was also enhanced by serotonin. Quantification of phagocytic activity by determining the uptake of microspheres showed that the amoeboid microglia in slices from early postnatal animals or microglia in culture respond to serotonin application with a decreased phagocytic activity whereas we could not detect any significant change in ramified microglia in situ. The presence of microglial serotonin receptors was confirmed by patch-clamp experiments in culture and amoeboid microglia and by qPCR analysis of RNA isolated from primary cultured and acutely isolated adult microglia. These data suggest that microglia express functional serotonin receptors linked to distinct microglial properties