Ability of retinal Muller glial cells to protect neurons against excitotoxicity in vitro depends upon maturation and neuron-glial interactions

Glutamate is the most abundant excitatory amino acid in the central nervous system. It has also been described as a potent toxin when present in high concentrations because excessive stimulation of its receptors leads to neur onal death. Glial influence on neuronal survival has already been shown in the central nervous system, but the mechanisms underlying glial neuroprotec tion are only partly known. When cells isolated from newborn rat retina wer e maintained in culture as enriched neuronal populations, 80% of the cells were destroyed by application of excitotoxic concentrations of glutamate. M assive neuronal death was also observed in newborn retinal cultures contain ing large numbers of glia, or when neurons were seeded onto feeder layers o f purified cells prepared from immature (postnatal 8 day) rat retina. When newborn retinal neurons were seeded onto feeder layers of purified glial ce lls prepared from adult retinas, application of excitotoxic amino acids no longer led to neuronal death. Furthermore, neuronal death was not observed in mixed neuron/glial cultures prepared from adult retina. However, in all cases (newborn and adult) application of kainate led to amacrine cell-speci fic death. Activity of glutamine synthetase, a key glial enzyme involved in glutamate detoxification, was assayed in these cultures in the presence or absence of exogenous glutamate. Whereas pure glial cultures alone (from yo ung or adult retina) showed low activity that was not stimulated by glutama te addition, mixed or co-cultured neurons and adult glia exhibited up to th reefold higher levels of activity following glutamate treatment. These data indicate that two conditions must be satisfied to observe glial neuroprote ction: maturation of glutamine synthetase expression, and neuron-glial sign alling through glutamate-elicited responses. (C) 1999 Wiley-Liss, Inc.