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Notes: Abstract: Excitatory amino acids stimulated inositol phospholipid hydrolysis in primary cultures of astrocytes, as reflected by an increased formation of [3H]inositol monophosphate ([3H]InsP) in the presence of 10 mM Li+. Quisqualate was the most potent activator of inositol phospholipid hydrolysis, followed by glutamate and ibotenate. Kainate exhibited low activity, whereas N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) were inactive. The increase in [3H]InsP formation induced by glutamate was potentiated after 12-h exposure to the proliferative agent epidermal growth factor (EGF), suggesting that activation of the mitotic cycle leads to an enhanced coupling of glutamate recognition sites with phospholipase C. To study how glutamate receptors are involved in regulating cell proliferation, we have measured [methyl-3H]thymidine incorporation in cultured astrocytes. Excitatory amino acids reduced thymidine incorporation with a pharmacological profile similar to that observed for the stimulation of inositol phospholipid hydrolysis. Quisqualate acted as a potent antiproliferative agent, both under basal conditions and in cells stimulated to proliferate by addition of EGF or phorbol 12-tetradecanoate 13-acetate. Glutamate and ibotenate reduced [methyl-3H]-thymidine incorporation at high concentrations, whereas kainate, AMPA, and NMDA were virtually inactive. The action of quisqualate on both inositol phospholipid hydrolysis and thymidine incorporation was attenuated by 2-amino-4-phosphonobutyrate, which acted as a weak agonist/competitive antagonist. Other excitatory amino acid receptor antagonists were not effective. Inhibition of [methyl-3H]thymidine incorporation by quisqualate required a lag time of about 4 h and, in cells synchronized to proliferate, occurred when the drug was added during the transition between G0 and G1, but not during the S phase of the mitotic cycle. This suggests that an inducible factor may be involved in the antiproliferative effect of excitatory amino acids. Accordingly, activation of quisqualate receptors led to a rapid and transient increase in mRNA levels of the early inducible gene, c-fos. These results suggest that activation of a specific class of “quisqualate-preferring”excitatory amino acid receptors reduces proliferation of astrocytes in primary cultures.