Glutamate‐Induced Increase in Intracellular Ca2+ in Cerebral Cortex Neurons Is Transient in Immature Cells but Permanent in Mature Cells

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Abstract
The free cytosolic Ca2+ concentration ([Ca2+],) of cultured cerebral cortex neurons was determined using a fluorescent Ca2+chelator (Fluo‐3) after exposure of the neurons to glutamate. Mature neurons (8 days in culture) responded within 45 s to 100 μM glutamate by an increase in [Ca2+]j from 75 to 340 nM, an increase that during the following 6 min of exposure reached 400 nM. This increase in [Ca2+]j could not be reversed by removal of glutamate. In the absence of extracellular CaCl2, only part of the initial, rapid, glutamate‐induced increase in [Ca2+]j was observed in these neurons. In contrast to these findings, neurons cultured for only 2 days (immature neurons) exhibited only a small (from 75 to 173 nM) increase in [Ca2+]j after exposure to 100 μM glutamate, and this rapid increase in [Ca2+]j tended to decline on prolonged exposure to glutamate. Moreover, after removal of glutamate, the increase in [Ca2+]j was fully reversible. Pharmacological characterization of the response to glutamate in mature neurons showed that the yN‐methyl‐D‐aspartate (NMDA) receptor antagonists phencyclidine and D‐2‐amino‐5‐phosphonovalerate blocked 75 and 90%, respectively, of the response, whereas the non‐NMDA antagonist 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione had little effect.