MODULATORY EFFECTS OF NMDA ON PHOSPHOINOSITIDE RESPONSES EVOKED BY THE METABOTROPIC GLUTAMATE-RECEPTOR AGONIST 1S,3R-ACPD IN NEONATAL RAT CEREBRAL-CORTEX

1 The effect of NMDA-receptor stimulation on phosphoinositide signalli ng in response to the metabotropic glutamate receptor agonist 1-aminoc yclopentane-1S,3R-dicarboxylic acid (1S,3R-ACPD) has been examined in neonatal rat cerebral cortex slices. 2 Total H-3-inositol phosphate (H-3-InsP(x)) accumulation, in the presence of 5mM LiCl, in H-3-in ositol pre-labelled slices was concentration-dependently increased by 1S,3R-ACPD (EC(50) 16.6 mu M) and, at a maximally effective concentrat ion, 1S,3R-ACPD (300 mu M) increased H-3-InsP(x), accumulation by 12 .8 fold over basal values. 3 H-3-InsP(x) accumulation stimulated by 1S,1R-ACPD was enhanced by low concentrations of NMDA (3-30 mu M), but not by higher concentrations (> 30 mu M). H-3-InsP(x) accumulations stimulated by 1S,3R-ACPD in the absence or presence of 10 mu M NMDA w ere linear with time, at least over the 15 min period examined; howeve r, in the presence of 100 mu M NMDA the initial enhancement of 1S,3R-A CPD-stimulated phosphoinositide hydrolysis progressively decreased wit h time. 4 In the presence of a maximal enhancing concentration of NMDA (10 mu M), the response to 1S,3R-ACPD (300 mu M) was increased 1.9 fo ld and the EC(50) for agonist-stimulated 3H-InsP, accumulation decre ased about 4 fold. The enhanced response to the metabotropic agonist w as concentration-dependently inhibited by competitive and uncompetitiv e antagonists of NMDA-receptor activation. 5 1S,3R-ACPD also stimulate d inositol 1,4,5-trisphosphate (Ins(1,4,5)P,) mass accumulation with a n initial peak response (5-6 fold over basal) at 15 s decaying to a sm aller (2 fold), but persistent elevated accumulation (1-10 min). 6 Co- addition of 10 or 100 mu M NMDA enhanced the initial peak Ins(1,4,5)P- 3 response to 1S,3R-ACPD. However, the enhancing effect was only maint ained over 10 min in the presence of 10 mu M NMDA, whilst in contrast, 100 mu M NMDA ceased to cause a significant enhancement of the metabo tropic response by 5 min and completely suppressed 1S,3R-ACPD-stimulat ed Ins(1,4,5)P-3 accumulation at 10 min. 7 Both basal and 1S,3R-ACPD-s timulated Zns(1,4,5)P-3 accumulations were reduced when slices were in cubated in nominally Ca2+-free medium. Under these conditions only a c oncentration-dependent enhancement of the response was observed (EC(50 ) for NMDA facilitation of 1S,3R-ACPD-stimulated Ins(1,4,5)P-3 accumul ation of 32 mu M). 8 These experiments have revealed that at low conce ntrations, NMDA can dramatically potentiate 1S,3R-ACPD-stimulated phos phoinositide 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 ef fect could be fundamental in glutamate receptor 'cross-talk', whereas the latter may reflect a Ca2+-dependent neurotoxic effect of NMDA on t he neonatal cerebral cortex slices.