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Ollgodendrocytes myelinate axons In the Central Nervous System This cell type mainly develops shortly atter the majority of neurons are generated and therefore. it is iikely that neurons play an important role in controlling oligodendrocyte development. Thus. it is known that the proliferation and survival of oligodendrocytes depends on electricai activity in axons and In the reciprocal contacts they establish. respectively (for a recent review, see Barres and Raff, 1996). Axon to oligodendrocyte signalling underlies the generation of the precise number of oligodendrocytes necessary to myelinate the entire extent of a given population ofaxons. Unfortunately, little Information is available about the molecules participating in this signalling process. Here, we tested whether glutamate, the most abundant excitatory neurotransmitter in the brain. is involved in the shaping of the oligodendrocyte population, by studying the effects of glutamate agonists In oligodendrocyte cultures of the developing rat optic nerve. Glutamate activates different receptor subtypes that can be grouped into two categories, lonotropic and metabotropic receptors. lonotroplc receptors contain Integral cationic ion channels that are associated with the ligand binding site, and are classified into alfa-amlno-3-hydroxy-5-methylisoxazole-4propionic acid (AMPA), kainate and N-methyl-D-aspartate (NMDA) subtypes. according to their preferred agonist (Hollmann and Heinemann, 1994). In addition to their presence In neurons, non-NMDA glutamate receptors have also been found in glial cells (for a recent review, see Steinhauser and Gallo, 1996). In the current study, we first investigated by immunocytochemistry the types of lonotropic glutamate receptors expressed by ollgodendrocytes in vitro. Cultures were derived from P7 rat optic nerves and grown in chemically~ defined medium (Barres et ai., 1992). After 5 days, these cultures were highly enriched In oligodendrocytes (>95%) as assessed by employing antibodies to galactocerebroside C, and used for immunolabelling following a previously described procedure (Puchalski et ai., 1994). Briefly, cells were fixed in 4% paraformaldehyde and permeabilized in PBS containing 1% Triton X-100. After blocking with 5% donor serum, primary antibodies to ionotropic glutamate receptor subunits were applied and the immunodetection procedure was carried out using biotinylated secondary antibodies followed by incubations with ABC complex and with the peroxidase substrate. The use of specific subunit antibodies Indicated that the glutamate receptors expressed in the oligodendrocyte cultures were made of AMP A subunits (GluR2-4), kainate subunits (GluR5-7, KA2) and the NMDA subunrt NMDAR1 (Fig. 1). Because functional NMDA receptors are heteromeric in nature (Hollmann and Heinemann, 1994), we infer that the main ionotropic glutamate receptors expressed in ollgodendrocytes are those belonging to the AMPA and kainate subfamilies. GC GFAP

Activation of AMPA and kainate glutamate receptors impairs the viability of oligodendrocytes in vitro.