Astroglial processes show spontaneous motility at active synaptic terminals in situ

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Abstract
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.