Radially expanding transglial calcium waves in the intact cerebellum

185

166

The synchronization of neuronal assemblies during cortical UP states has been implicated in computational and homeostatic processes, but the mechanisms by which this occurs remain unknown. To investigate potential roles of astrocytes in synchronizing cortical circuits, we electrically activated astrocytes while monitoring the activity of the surrounding network with electrophysiological recordings and calcium imaging. Stimulating a single astrocyte activates other astrocytes in the local circuit and can trigger UP state synchronizations of neighboring neurons. Moreover, interfering with astrocytic activity with intracellular injections of a calcium chelator into individual astrocytes inhibits spontaneous and stimulated UP states. Finally, both astrocytic activity and neuronal UP states are regulated by purinergic signaling in the circuit. These results demonstrate that astroglia can play a causal role in regulating the synchronized activation of neuronal ensembles.T he transient synchrony of distributed groups of neurons is an important operational characteristic of the cerebral cortex for sensory processing and internal computational functions (1-4). One important type of synchrony-the multineuronal, network-driven fluctuations in membrane potential known as UP and DOWN states-occurs in neocortex both in vitro and in vivo (5-11). During UP states, neurons are depolarized for up to hundreds of milliseconds, sometimes firing barrages of action potentials (12). The function of these UP states is unknown, although they underlie synchronization among distant cortical territories (13). UP states in vivo are observed at similar frequencies as the "resting" spontaneous activity that is evident in functional MRI and EEG recordings from human subjects when not involved in sensory motor tasks (14-16), so they may be a cellular basis of this activity.The mechanisms by which UP states occur remain unclear, although recurrent excitatory activity is important (6, 9). Because astrocytes regulate extracellular glutamate (17), we explored their effects on cortical UP states using electrophysiology and population calcium imaging. The effects of astroglia on synaptic transmission and plasticity have been intensely studied, but their role in coherent function of the circuit has received much less attention, even though glia constitute almost half of all of the cells in the adult human brain (17) and are well-suited for longrange, network-wide signaling. Astrocytes also tile the cortex with near-complete coverage (18,19), are connected into an extensive syncytium via gap junctions (20,21), and communicate by intercellular calcium signaling (22), and processes from a single astrocyte can contact up to tens of thousands of synapses (18), making them attractive candidates for mediating neuronal synchronization. Here we describe a role of astrocytes in the cortical circuit, demonstrating their importance in the modulation of neuronal UP states.Results UP States Are Network-Wide Events. We performed whole-cell patch clamping of neurons i...

Section: Stimulation Of Individual Astrocytes Increases Network Astromentioning

confidence: 83%

Astrocytic regulation of cortical UP states

Poskanzer

Yuste²

2011

185

166

“…However, formation of the typically narrow and elongate PAP structure in Bergmann glia depends on Ca 2+ influx through AMPA receptors (8,15) not identified in hippocampal or cortical astrocytes (38). Here, mGluR-mediated intracellular PAP motility mechanisms might similarly work by an increase in intracellular Ca 2+ concentration through release from intracellular stores triggered by direct and indirect mGluR3 or 5 signaling (30).…”

Section: Discussionmentioning

confidence: 99%

“…At the ultrastructural level, the PAPs, although abundant in the neuropil, specifically prefer contacting synapses and dendrites versus axons (4). The synaptic wrapping is highly dynamic (5-9) and also activity-dependent even in the context of physiological functions, such as motor learning, daily fluctuations of the circadian clock, lactation, parturition, or dehydration (10)(11)(12)(13)(14)(15). Here, we asked two questions about the structural basis of glia-synaptic interaction: What is the stimulus for PAP plasticity, and what are the intracellular mechanisms accomplishing the motility and the formation of the PAP?…”

mentioning

confidence: 99%

Structural plasticity of perisynaptic astrocyte processes involves ezrin and metabotropic glutamate receptors

Lavialle

Aumann

Anlauf

et al. 2011

228

264

The peripheral astrocyte process (PAP) preferentially associates with the synapse. The PAP, which is not found around every synapse, extends to or withdraws from it in an activity-dependent manner. Although the pre-and postsynaptic elements have been described in great molecular detail, relatively little is known about the PAP because of its difficult access for electrophysiology or light microscopy, as they are smaller than microscopic resolution. We investigated possible stimuli and mechanisms of PAP plasticity. Immunocytochemistry on rat brain sections demonstrates that the actin-binding protein ezrin and the metabotropic glutamate receptors (mGluRs) 3 and 5 are compartmentalized to the PAP but not to the GFAP-containing stem process. Further experiments applying ezrin siRNA or dominant-negative ezrin in primary astrocytes indicate that filopodia formation and motility require ezrin in the membrane/cytoskeleton bound (i.e., T567-phosphorylated) form. Glial processes around synapses in situ consistently display this ezrin form. Possible motility stimuli of perisynaptic glial processes were studied in culture, based on their similarity with filopodia. Glutamate and glutamate analogues reveal that rapid (5 min), glutamate-induced filopodia motility is mediated by mGluRs 3 and 5. Ultrastructurally, these mGluR subtypes were also localized in astrocytes in the rat hippocampus, preferentially in their fine PAPs. In vivo, changes in glutamatergic circadian activity in the hamster suprachiasmatic nucleus are accompanied by changes of ezrin immunoreactivity in the suprachiasmatic nucleus, in line with transmitter-induced perisynaptic glial motility. The data suggest that (i) ezrin is required for the structural plasticity of PAPs and (ii) mGluRs can stimulate PAP plasticity.A strocytes act as a third partner in synaptic signal processing by responding to neurotransmitters and by releasing "gliotransmitters" (1, 2). Structurally, the astrocyte functions mainly through its peripheral processes, which constitute approximately 80% of the cell's membrane (3). These peripheral astrocyte processes (PAPs) are frequently extremely fine (<50 nm) and display an extreme surface-to-volume ratio. They are rarely studied in live tissue, as they are not directly accessible to electrophysiology, cannot be isolated for biochemistry, and are smaller than microscopic resolution. However, they also wrap synapses, but most studies on glia-synaptic interaction can only indiscriminately refer to them as the astrocyte. At the ultrastructural level, the PAPs, although abundant in the neuropil, specifically prefer contacting synapses and dendrites versus axons (4). The synaptic wrapping is highly dynamic (5-9) and also activity-dependent even in the context of physiological functions, such as motor learning, daily fluctuations of the circadian clock, lactation, parturition, or dehydration (10-15). Here, we asked two questions about the structural basis of glia-synaptic interaction: What is the stimulus for PAP plasticity, and what are the intra...

“…Computational modeling has suggested that such synaptically induced activity would cause strikingly large depolarization (up to 40 mV) of the BG membrane processes local to the activated sites (7). Furthermore, PF burst activity elicits transient and local Ca 2+ increases in BG processes via AMPA and ATP receptor-mediated mechanisms (2,(8)(9)(10)(11). Despite the accumulating evidence of neuron-to-glia communication in the cerebellum, if the communication between these cells is a one-way relationship, the glia would only be a "listener" and it would not be able to actively participate in a layer of information processing.…”

mentioning

confidence: 99%

Application of an optogenetic byway for perturbing neuronal activity via glial photostimulation

Sasaki

Beppu

Tanaka

et al. 2012

145

172

Dynamic activity of glia has repeatedly been demonstrated, but if such activity is independent from neuronal activity, glia would not have any role in the information processing in the brain or in the generation of animal behavior. Evidence for neurons communicating with glia is solid, but the signaling pathway leading back from glialto-neuronal activity was often difficult to study. Here, we introduced a transgenic mouse line in which channelrhodopsin-2, a light-gated cation channel, was expressed in astrocytes. Selective photostimulation of these astrocytes in vivo triggered neuronal activation. Using slice preparations, we show that glial photostimulation leads to release of glutamate, which was sufficient to activate AMPA receptors on Purkinje cells and to induce long-term depression of parallel fiber-to-Purkinje cell synapses through activation of metabotropic glutamate receptors. In contrast to neuronal synaptic vesicular release, glial activation likely causes preferential activation of extrasynaptic receptors that appose glial membrane. Finally, we show that neuronal activation by glial stimulation can lead to perturbation of cerebellar modulated motor behavior. These findings demonstrate that glia can modulate the tone of neuronal activity and behavior. This animal model is expected to be a potentially powerful approach to study the role of glia in brain function.cerebellum | Bergmann glia | gliotransmitter | plasticity | c-fos