Excitotoxic damage of retinal glial cells depends upon normal neuron-glial interactions

Glutamate, the principal retinal neurotransmitter, can also act as a toxin when present in excessive concentrations as may occur in pathologies such as retinal ischemia or more generally in cerebral neuronal degenerative disease. As glial cells play pivotal roles in transfer of blood-borne molecules and in glutamate clearance, we investigated the effects of the excitatory amino acids glutamic and kainic acid on different in vitro preparations of retinal Müller glial cells. Glial viability or morphology were not influenced by excitatory amino acid exposure in either pure glial cultures or in monolayer cultures of mixed neonatal neurons and glia, whereas kainic acid specifically lysed amacrine cells in mixed or pure neuronal cultures. When retinal fragments were pre-incubated in excitatory amino acids prior to dissociation and seeding into culture, under these conditions Müller glial cells exhibited a dramatic loss of their normal epithelioid form to a retracted morphology. However, glial cell viability was not compromised, and rapid restoration of epithelioid in vitro glial morphology could be achieved by addition of exogenous epidermal and basic fibroblast growth factor to the culture medium. This study demonstrates that glial cells are structurally perturbed by excitotoxic conditions and that such effects are dependent on normal glial-neuronal interactions.