Interactions between cerebellar Purkinje cells and their associated astrocytes.

Some neurons, including cerebellar Purkinje cells, are completely ensheathed by astrocytes. When granule cell neurons and functional glia were eliminated from newborn mouse cerebellar cultures by initial exposure to a DNA synthesis inhibitor, Purkinje cells lacked glial sheaths and there was a tremendous sprouting of Purkinje cell recurrent axon collaterals, terminals of which hyperinnervated Purkinje cell somata, including persistent somatic spines, and formed heterotypical synapses with Purkinje cell dendritic spines, sites usually occupied by parallel fiber (granule cell axon) terminals. Purkinje cells in such preparations failed to develop complex spikes when recorded from intracellularly, and their membrane input resistances were low, making them less sensitive to inhibitory input. If granule cells and oligodendrocytes were eliminated, but astrocytes were not compromised, sprouting of recurrent axon collaterals occurred and their terminals projected to Purkinje cell dendritic spines, but the Purkinje cells had astrocytic sheaths, their somata were not hyperinnervated, the somatic spines had disappeared, complex spike discharges predominated, and membrane input resistance was like that of Purkinje cells in untreated control cultures. When cerebellar cultures without granule cells and glia were transplanted with granule cells and/or glia from another source, a series of changes occurred that included stripping of excess Purkinje cell axosomatic synapses by astrocytic processes, reduction of heterotypical axospinous synapses in the presence of astrocytes, disappearance of Purkinje cell somatic spines with astrocytic ensheathment, and proliferation of Purkinje cell dendritic spines after the introduction of astrocytes. Dendritic spine proliferation was followed by formation of homotypical axospinous synapses when granule cells were present or persistence as unattached spines in the absence of granule cells. The results of these studies indicate that astrocytes regulate the numbers of Purkinje cell axosomatic and axospinous synapses, induce Purkinje cell dendritic spine proliferation, and promote the structural and functional maturation of Purkinje cells.