Evidence that the influence of ganglion cell axons on astrocyte morphology is mediated by action spike activity during development.

In many mammal retinas, the morphology of astrocytes is strongly influenced by nearby axons of ganglion cells. Astrocyte processes stretch along the axons, fine extensions of the processes contact node-like specialisation of the axon membrane and the morphology of the adult astrocyte is strongly determined by this relationship. The mechanism which attracts astrocyte processes to contact specific regions of the axon membrane is not known however. This study presents evidence that in the neonatal cat blocking the impulse activity of ganglion cells with the Na+-channel blocker tetrodotoxin (TTX) leads to a loss of the axon-related morphology of astrocytes. The morphological change induced in astrocytes by TTX was greater in younger animals and could not be detected in the adult. Conversely, if the TTX block was maintained for 4 postnatal weeks the changes induced in astrocytes persisted at least to 13 weeks. The TTX-induced loss of axon-related morphology in astrocytes suggests that the signal by which axons attract astrocyte processes to contact the axonal membrane in ways which modify astrocyte morphology is released by action spike activity during development.