Excitatory neurotransmission activates compartmentalized calcium transients in Müller glia without affecting lateral process motility

Abstract

Neural activity has been implicated in the motility and outgrowth of glial cell processes throughout the central nervous system. Here we explore this phenomenon in Muller glia, which are specialized radial astroglia that are the predominant glial type of the vertebrate retina. Muller glia extend fine filopodia-like processes into retinal synaptic layers, in similar fashion to brain astrocytes and radial glia which exhibit perisynaptic processes. Using two-photon volumetric imaging, we found that during the second postnatal week, Muller glial processes were highly dynamic, with rapid extensions and retractions that were mediated by cytoskeletal rearrangements. During this same stage of development, retinal waves led to increases in cytosolic calcium within Muller glial lateral processes and stalks. These comprised distinct calcium compartments, distinguished by variable participation in waves, timing, and sensitivity to an M1 muscarinic acetylcholine receptor antagonist. However, we found that motility of lateral processes was unaffected by the presence of pharmacological agents that enhanced or blocked wave-associated calcium transients. Finally, we found that mice lacking normal cholinergic waves in the first postnatal week also exhibited normal Muller glial process morphology. Hence, outgrowth of Muller glial lateral processes into synaptic layers is determined by factors that are independent of neuronal activity.