Astroglial processes show spontaneous motility at active synaptic terminals in situ

Within the tripartite structure of vertebrate synapses, enwrapping astroglial processes regulate synaptic transmission by transmitter uptake and by direct transmitter release. We applied confocal and two-photon laser scanning microscopy to acutely isolated slices prepared from the brainstem of transgenic TgN(GFAP-EGFP) mice. In transversal sections fluorescently labelled astrocytes are evenly distributed throughout the tissue. Astroglial processes contacted neuronal somata and enwrapped active synaptic terminals as visualized using FM1-43 staining in situ. Here, at these synaptic regions astroglial process endings displayed a high degree of dynamic morphological changes. Two defined modes of spontaneous motility could be distinguished: (i) gliding of thin lamellipodia-like membrane protrusions along neuronal surfaces and (ii) transient extensions of filopodia-like processes into the neuronal environment. Our observations highlight the active role of astrocytes in direct modulation of synaptic transmission.