Modulatory effects of NMDA on phosphoinositide responses evoked by the metabotropic glutamate receptor agonist 1S,3R‐ACPD in neonatal rat cerebral cortex

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Abstract
1 The effect of NMDA-receptor stimulation on phosphoinositide signalling in response to the metabotropic glutamate receptor agonist 1-aminocyclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD) has been examined in neonatal rat cerebral cortex slices. 2 Total [3H]-inositol phosphate ([3H]-InsPx) accumulation, in the presence of 5 mm LiCl, in [3H]-inositol pre-labelled slices was concentration-dependently increased by 1S,3R-ACPD (EC50 16.6 μm) and, at a maximally effective concentration, 1S,3R-ACPD (300 μm) increased [3H]-InsPx accumulation by 12.8 fold over basal values. 3 [3H]-InsPx accumulation stimulated by 1S,1R-ACPD was enhanced by low concentrations of NMDA (3–30 μm), but not by higher concentrations (> 30 μm). [3H]-InsPx accumulations stimulated by 1S,3R-ACPD in the absence or presence of 10 μm NMDA were linear with time, at least over the 15 min period examined; however, in the presence of 100 μm NMDA the initial enhancement of 1S,3R-ACPD-stimulated phosphoinositide hydrolysis progressively decreased with time. 4 In the presence of a maximal enhancing concentration of NMDA (10 μm), the response to 1S,3R-ACPD (300 μm) was increased 1.9 fold and the EC50 for agonist-stimulated [3H]-InsPx accumulation decreased about 4 fold. The enhanced response to the metabotropic agonist was concentration-dependently inhibited by competitive and uncompetitive antagonists of NMDA-receptor activation. 5 1S,3R-ACPD also stimulated inositol 1,4,5-trisphosphate (Ins(1,4,5)P3) mass accumulation with an initial peak response (5–6 fold over basal) at 15 s decaying to a smaller (2 fold), but persistent elevated accumulation (1–10 min). 6 Co-addition of 10 or 100 μm NMDA enhanced the initial peak Ins(1,4,5)P3 response to 1S,3R-ACPD. However, the enhancing effect was only maintained over 10 min in the presence of 10 μm NMDA, whilst in contrast, 100 μm NMDA ceased to cause a significant enhancement of the metabotropic response by 5 min and completely suppressed 1S,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation at 10 min. 7 Both basal and 1S,3R-ACPD-stimulated Ins(1,4,5)P3 accumulations were reduced when slices were incubated in nominally Ca2+-free medium. Under these conditions only a concentration-dependent enhancement of the response was observed (EC50 for NMDA facilitation of 1S,3R-ACPD-stimulated Ins(1,4,5)P3 accumulation of 32 μm). 8 These experiments have revealed that at low concentrations, NMDA can dramatically potentiate 1S,3R-ACPD-stimulated phosphoinositide hydrolysis, probably by a Ca2+-dependent facilitation of agonist-stimulated phosphoinositide-specific phospholipase C activity. Higher concentrations of NMDA result in time-dependent inhibition of the metabotropic agonist-stimulated response. We believe the former effect could be fundamental in glutamate receptor ‘cross-talk’, whereas the latter may reflect a Ca2+-dependent neurotoxic effect of NMDA on the neonatal cerebral cortex slices.