Excitatory amino acid receptor-stimulated phosphoinositide turnover in primary cerebrocortical cultures

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Abstract
1 Characterization of excitatory amino acid-induced accumulation of [3H]-phosphoinositides was carried out in primary cerebrocortical cultures isolated from foetal rats. 2 All of the excitatory amino acid receptor agonists examined caused concentration-dependent enhancement of phosphoinositide (PI) formation. The most potent excitatory amino acid receptor agonists were quisqualate, (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid ((1S,3R)-ACPD), ibotenate and glutamate with mean EC50 values of 0.9 ± 0.4 μm, 15 ± 5 μm, 15 ± 3 μm and 41 ± 8 μm respectively. 3 The selective ionotropic receptor antagonists kynurenic acid (1 mm), 2,3-dihydroxy-6-nitro-7-sulphamoyl-benzo(F)quinoxaline (NBQX, 10 μm) and (±)-4-(3-phosphonopropyl)-2 piperazinecarboxylic acid (CPP, 100 μm), failed to block responses to quisqualate, (1S,3R)-ACPD or glutamate. d,l-2-Amino-3-phosphonopropionate (d,l-AP3) did not block 1S,3R-ACPD or quisqualate-induced PI turnover, but had an additive effect with quisqualate or (1S,3R)-ACPD. 4 Exposure of cultures to agonists in the absence of added extracellular calcium reduced the maximal quisqualate response by approximately 45%, revealing a two-component concentration-response curve. Concentration-response curves to ibotenate and glutamate became flattened by omission of extracellular calcium, whereas (1S,3R)-ACPD-stimulated PI turnover was unaffected. 5 Pretreatment of cultures with pertussis toxin markedly inhibited PI responses evoked by (1S,3R)-ACPD. 6 These results suggest that excitatory amino acid-stimulated PI turnover in cerebrocortical cultures is independent of ionotropic receptor activation and is mediated via specific G-protein-linked metabotropic receptors. The partial dependence of the responses to quisqualate, ibotenate and glutamate on the presence of extracellular calcium suggests that the effects of these agonists may be mediated by more than one receptor subtype.

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