STIMULATORY AND INHIBITORY EFFECTS OF N-METHYL-D-ASPARTATE ON H-3 INOSITOL POLYPHOSPHATE ACCUMULATION IN RAT CORTICAL SLICES

The actions of the excitatory amino acid N-methyl-D-aspartate (NMDA) on the accumulation of H-3-inositol polyphosphate isomers in rat cerebral cortex slices have been examined over short (less than 5 min ) incubation periods. NMDA caused the dose-dependent accumulation of only [H-3]inositol monophosphate and [H-3]inositol bisphosphate (maximal effect between 0.3 and 1 mM), with no increase in [H-3]inositol trisphosphate ([H-3]InsP3) and [H-3]inositol tetrakisphosphate ([H-3]InsP4). HPLC analysis confirmed this, showing no increases in the breakdown products of [H-3]Ins(1,3,4,5)P4. When present with the muscarinic agonist carbachol (1 mM), high concentrations of NMDA (1 mM) could almost totally inhibit carbachol-induced accumulation of H-3-inositol polyphosphates. In contrast, at lower concentrations of NMDA (10-mu-M), the inhibitory effect was replaced with a synergistic accumulation of inositol polyphosphates, especially [H-3]InsP4 and [H-3]InsP3. The inhibitory effects of NMDA were only apparent when extracellular Ca2+ was present, although incubation in media with no added Ca2+ resulted in somewhat reduced stimultory responses to NMDA alone, but suppressed totally the inhibitory effects of 1 mM NMDA and reduced the synergistic effects of 10-mu-M NMDA on carbachol responses. These studies, therefore, reveal Ca2+-dependent effecs of NMDA indicative of indirect mechanisms of action and show that care must be made in interpreting the effects of NMDA on phosphoinositide metabolism unless the inositol polyphosphate composition has been fully characterised.