The mechanisms by which N-methyl-D-aspartate (NMDA) receptor activation can modulate muscarinic receptor-stimulated phosphoinositide turnover have been studied in neonatal rat cerebral cortex slices. A maximally effective concentration of carbachol (1 mM) caused a large stimulation of both total [3H]inositol phosphate ([3H]InsPx) accumulation (30-40-fold over basal levels after 15 min in the presence of 5 mM LiCl) and inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] mass accumulation (consisting of a rapid peak increase of about 8-10-fold within 15 sec followed by a sustained plateau rise of 4-5-fold which persisted for > 10 min). Low concentrations of NMDA enhanced carbachol-stimulated [3H]InsPx and Ins(1,4,5)P3 accumulations with a maximal effect being observed at 10 microM NMDA. However, at higher concentrations of NMDA (30-300 microM) a dramatic inhibition of these indices of phosphoinositide turnover was observed. Time-course studies demonstrated that NMDA (100 microM) caused a significant enhancement of the initial increases in [3H]InsPx and Ins(1,4,5)P3 accumulations stimulated by carbachol, with the profound inhibitory effects becoming evident at longer incubation times. The modulatory effects of NMDA were antagonized by D-2-amino-5-phosphonopentanoate and MK-801. Reducing extracellular calcium concentration ([Ca2+]e) to the low micromolar range decreased basal Ins(1,4,5)P3 accumulation and attenuated the response to carbachol. Under these conditions NMDA (10-100 microM) caused only a potentiation of agonist-stimulated Ins(1,4,5)P3 accumulation. Under control conditions ([Ca2+]e = 1.3 mM), addition of MK-801 (1 microM) 10 min after carbachol + 100 microM NMDA challenge failed to reverse the inhibitory effect of NMDA on carbachol-stimulated [3H]InsPx accumulation. Furthermore, pre-incubation of cerebral cortex slices with 100 microM NMDA for 15 min (followed by extensive washing of slices to remove NMDA) dramatically decreased [3H]inositol incorporation into the cellular inositol phospholipid fraction and decreased basal and carbachol-stimulated Ins(1,4,5)P3 mass accumulations. We conclude that the enhancement of agonist-stimulated phosphoinositide turnover seen at concentrations of NMDA up to 10 microM may be due to Ca2+ entry and Ca2+ facilitation of phosphoinositide-specific phospholipase C activity. In contrast, the inhibitory effect of high concentrations of NMDA on agonist-stimulated phosphoinositide turnover may be due to progressive, irreversible and, at least in part, Ca2+-dependent damage to the cell populations in the slice preparation responding to muscarinic-receptor stimulation.