Spatiotemporal calcium dynamics in single astrocytes and its modulation by neuronal activity

Abstract

Astrocytes produce a complex repertoire of Ca²⁺ events that coordinate their major functions. The principle of Ca²⁺ events integration in astrocytes, however, is unknown. Here we analyze whole Ca²⁺ events, which were defined as spatiotemporally interconnected transient Ca²⁺ increases. Using such analysis in single hippocampal astrocytes in culture and in slices we found that spreads and durations of Ca²⁺ events follow power law distributions, a fingerprint of scale-free systems. A mathematical model demonstrated that such Ca²⁺ dynamics can arise from intracellular inositol-3-phosphate diffusion. The power law exponent (α) was decreased by activation of metabotropic glutamate receptors (mGluRs) either by specific receptor agonist or by low frequency stimulation of glutamatergic fibers in hippocampal slices. Decrease in α indicated an increase in proportion of large Ca²⁺ events. Notably, mGluRs activation did not increase the frequency of whole Ca²⁺ events. This result suggests that neuronal activity does not trigger new Ca²⁺ events in astrocytes (detectable by our methods), but modulates the properties of existing ones. Thus, our results provide a new perspective on how astrocyte responds to neuronal activity by changing its Ca²⁺ dynamics, which might further affect local network by triggering release of gliotransmitters and by modulating local blood flow.

Introduction

Astrocytes are electrically non-excitable cells that form a support network for neurons in the brain. Astrocyte-neuron communication plays an important role in the operation of neuronal networks [1], [2]. In response to neuronal activity, astrocytes can release a number of neuroactive substances, or gliotransmitters (e.g., glutamate [3], GABA [4], ATP [5], d-serine [6], [7] etc) that alter synaptic transmission and neuronal excitability. Gliotransmitter release is often sensitive to, or directly triggered by cytosolic Ca²⁺ elevations in astrocytes [2]. Ca²⁺ regulated K⁺ uptake by astrocytes is another mechanism that can be involved in modulation of synaptic signaling [8]. Indeed, intrasynaptic K⁺ accumulation during activation of postsynaptic NMDA receptors can change presynaptic release probability [9]. Recently, we reported that the level of Ca²⁺ activity in astrocytes determines coverage of synapses by the astrocytic processes and perisynaptic glutamate uptake [10]. In addition, astrocytic Ca²⁺ events convey information about local brain activity levels to the vasculature [11]. Thus, Ca²⁺ dynamics comprise a major cellular signaling mechanism in astrocytes. The principle of Ca²⁺ events integration in astrocytes, however, is unknown.

Astrocytes maintain a complex repertoire of Ca²⁺ events that can be generated by several routes. In particular, the activation of metabotropic glutamate receptors (mGluRs), results in the elevation of intracellular levels of inositol-3-phosphate (IP₃), triggering the release of Ca²⁺ from endogenous stores [12], [13], [14]. A similar form of endogenous Ca²⁺ release can be triggered by G-protein coupled purinergic (P2Y) receptors [15]. Such Ca²⁺ events can spread by a sequential activation of neighboring IP₃ receptors. Additionally, Ca²⁺ entry can occur through astrocytic ligand-gated Ca²⁺ channels (ionotropic glutamate receptors and P2X receptors), transient receptor potential (TRP) channels and reverse operation of Na⁺/Ca²⁺ exchanger (NCX) [16], [17], [18], [19], [20]. Finally, astrocytes can generate spontaneous Ca²⁺ events independently of receptor activation in the plasma membrane [13], [21]. According to their spread, astrocytic Ca²⁺ events are classified as focal, expanded, or generalized (i.e. occupying most of the cell) [13], [21]. The relationship between these different types of Ca²⁺ events, however, has not been systematically studied. Here we provide an evidence that neuronal activity modulates the spread and duration of the events. This may occur without any increase in frequency of Ca²⁺ events (i.e., without appearance of new Ca²⁺ events), which believed to be the main astrocytic response to neuronal activity.