Bioluminescent imaging of Ca2+ activity reveals spatiotemporal dynamics in glial networks of dark-adapted mouse retina

Glial Ca2+ excitability plays a key role in reciprocal neuron–glia communication. In the retina, neuron–glia signalling is expected to be maximal in the dark, but the glial Ca2+ signal characteristics under such conditions have not been evaluated. To address this question, we used bioluminescence imaging to monitor spontaneous Ca2+ changes under dark conditions selectively in Muller cells, the principal retinal glial cells. By combining this imaging approach with network analysis, we demonstrate that activity in Muller cells is organized in networks of coactive cells, involving 2–16 cells located distantly and/or in clusters. We also report that spontaneous activity of small networks (2–6 Muller cells) repeat over time, sometimes in the same sequential order, revealing specific temporal dynamics. In addition, we show that networks of coactive glial cells are inhibited by TTX, indicating that ganglion and/or amacrine neuronal cells probably regulate Muller cell network properties. These results represent the first demonstration that spontaneous activity in adult Muller cells is patterned into correlated networks that display repeated sequences of coactivations over time. Furthermore, our bioluminescence technique provides a novel tool to study the dynamic characteristics of glial Ca2+ events in the retina under dark conditions, which should greatly facilitate future investigations of retinal dark-adaptive processes.