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

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 Müller glia, which are specialized radial astroglia that are
the predominant glial type of the vertebrate retina. Müller 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, Müller 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 Müller 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 Müller glial process
morphology. Hence, outgrowth of Müller glial lateral processes into
synaptic layers is determined by factors that are independent of neuronal
activity.