Phosphoinositide Turnover Associated with Synaptic Transmission

Abstract: Although pharmacological stimulation of a wide variety of transmitter receptors triggers phosphoinositide (PI) turnover, little is known about the type of synaptic activity required to activate this system. To investigate this question, we have used primary cultures of embryonic cortical neurons, which develop functional glutamate and GABA synapses during maturation in vitro. Mature cultures display spontaneous synaptic activity that is totally suppressed by tetrodotoxin (TTX). PI turnover, assayed by the lithium‐sensitive accumulation of [³H]CDP‐diacylglycerol, was readily detected under basal conditions and was abolished by TTX. Increased excitatory synaptic activity induced by picrotoxin, an antagonist of GABA_A receptor‐mediated inhibition, further stimulated PI turnover. Similar results were obtained when PI turnover was assayed using [³H]inositol labeling. With either assay, the magnitude of synaptically induced PI turnover was comparable to maximal responses produced by muscarinic receptor stimulation. Although a component of the spontaneous synaptic currents is sensitive to N‐methyl‐D‐aspartate (NMDA)‐preferring glutamate receptor antagonists, blockade of NMDA receptors did not affect PI turnover associated with synaptic transmission. To assess the time course of synaptically mediated PI turnover, the amplitude and duration of spontaneous synaptic currents were reduced by lowering the extracellular Ca²⁺ concentration from 2.25 to 0.5 mM, a maneuver that suppresses basal PI turnover. Increases in PI turnover were detected as early as 5 min following restoration of the extracellular Ca²⁺ concentration to 2.25 mM. Taken together, these findings indicate that activation of the PI system is associated with physiological levels of glutamatergic synaptic transmission.