Inhibition of Muscarinic‐Coupled Phosphoinositide Hydrolysis by N‐Methyl‐d‐Aspartate Is Dependent on Depolarization via Channel Activation

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Abstract
The intent of this work was to elucidate the mechanism by which N-methyl-D-aspartate (NMDA) receptor agonists inhibit a second messenger system, namely, the stimulation of phosphoinositide (PI) hydrolysis activated by muscarinic cholinergic receptor agonsits. NMDA inhibited cholinergic stimulation of PI hydrolysis in a dose- and time dependent manner. NMDA exerts this effect indirectly through channel activation, because both MK-801 and N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) prevented this action. Prevention of the NMD effect by removal of sodium, but not calcium, from the incubation buffer suggested that depolarization alone proved sufficient in inhibit cholinergic activation of PI hydrolysis, because both veratridine and an elevated extracellular potassium level inhibited cholinergic stimulation of PI-hydrolysis. The effect of NMDA appeared to require sodium flux through NMDA channels rather than through voltage-dependent sodium channels, because tetrodotoxin failed to inhibit the effect of NMDA. In correlative electrophysiologic experiments, NMDA profoundly inhibited evoked excitatory postsynaptic potentials and population action potentials of CA1 neurons, and effect almost certainly due to depolarization. The dose and time course of the electrophysiologic effects correlated well with the biochemical effects. Taken together, the data support the assertion that NMDA receptor activation inhibits PI hydrolysis by depolarization mediated by sodium flux through NMDA channels.