Modulation of Neuronal Signal Transduction Systems by Extracellular ATP

: The secretion of ATP by stimulated nerves is well documented. Following repetitive stimulation, extracellular ATP at the synapse can accumulate to levels estimated to be well over 100 μM. The present study examined the effects of extracellular ATP in the concentration range of 0.1–1.0 mM on second‐messenger‐generating systems in cultured neural cells of the clones NG108‐15 and NIE‐115. Cells in a medium mimicking the physiological extracellular environment were used to measure 45Ca2+ uptake, changes in free intracellular Ca2+ levels by the probes aequorin and Quin‐2, de novo generation of cyclic GMP and cyclic AMP from intracellular GTP and ATP pools prelabeled with [3H]guanosine and [3H]adenine, respectively, and phosphoinositide metabolism in cells preloaded with [3H]inositol and assayed in the presence of LiCI. Extracelluar ATP induced a concentration‐dependent increase of 45Ca2+ uptake by intact cells, which was additive with the uptake induced by K+ depolarization. The increased uptake involved elevation of intracellular free Ca2+ ions, evidenced by measuring aequorin and Quin‐2 signals. At the same concentration range (0.1–1.0 mM), extracellular ATP induced an increase in [3H]cyclic GMP formation, and a decrease in prostaglandin E1‐stimulated [3H]cyclic AMP generation. In addition, extracellular ATP (1 mM) caused a large (15‐fold) increase in [3H]inositol phosphates accumulation, and this effect was blocked by including La3+ ions in the assay medium. In parallel experiments, we found in NG 108–15 cells surface protein phosphorylation activity that had an apparent Km for extracellular ATP at the same concentration required to produce half‐maximal effects on Ca2+ uptake. Extracellular ATP at concentrations that can be produced in the synaptic cleft by repetitive stimulation but not during routine transmission can thus initiate a unique chain of events, which may play a role in the induction of long‐term adaptive changes in neuronal function.