GLIA MODULATE NMDA-MEDIATED SIGNALING IN PRIMARY CULTURES OF CEREBELLAR GRANULE CELLS

Excessive activation of N-methyl-D-aspartate (NMDA) receptor channels (NRs) is a major cause of neuronal death associated with stroke and is chemia. Cerebellar granule neurons in vivo, but not in culture, are re latively resistant to toxicity, possibly owing to protective effects o f glia, To evaluate whether NR-mediated signaling is modulated when de veloping neurons are cocultured with glia, the neurotoxic responses of rat cerebellar granule cells to applied NMDA or glutamate were compar ed in astrocyte-rich and astrocyte-poor cultures. In astrocyte-poor cu ltures, significant neurotoxicity was observed in response to NMDA or glutamate and was inhibited by an NR antagonist. Astrocyte-rich neuron al cultures demonstrated three significant differences, compared with astrocyte-poor cultures: (a) Neuronal viability was increased; (b) glu tamate-mediated neurotoxicity was decreased, consistent with the prese nce of a sodium-coupled glutamate transport system in astrocytes; and (c) NMDA- but not kainate-mediated neurotoxicity was decreased, in a m anner that depended on the relative abundance of glia in the culture. Because glia do not express NRs or an NMDA transport system, the mecha nism of protection is distinct from that observed in response to gluta mate. No differences in NR subunit composition (evaluated using RT-PCR assays for NR1 and NR2 subunit mRNAs), NR sensitivity (evaluated by m easuring NR-mediated changes in intracellular Ca2+ levels), or glycine availability as a coagonist (evaluated in the presence and absence of exogenous glycine) were observed between astrocyte-rich and astrocyte -poor cultures, suggesting that glia do not directly modulate NR compo sition or function. Nordihydroguaiaretic acid, a lipoxygenase inhibito r, blocked NMDA-mediated toxicity in astrocyte-poor cultures, raising the possibility that glia effectively reduce the accumulation of highl y diffusible and toxic arachidonic acid metabolites in neurons. Altern atively, glia may alter neuronal development/phenotype in a manner tha t selectively reduces susceptibility to NR-mediated toxicity.