Sustained Ca²⁺-induced Ca²⁺-release underlies the post-glutamate lethal Ca²⁺ plateau in older cultured hippocampal neurons

Several studies have shown that a prolonged Ca²⁺ elevation follows a glutamate-mediated excitotoxic insult in cultured neurons, and may be associated with impending cell death. Recently, we showed that the prolonged Ca²⁺ elevation that emerges as neurons age in culture is specifically linked to an age-related increase in excitotoxic vulnerability. However, the multiple sources of Ca²⁺ that contribute to Ca²⁺ elevation during and after glutamate exposure are not well understood. Here, we examined the Ca²⁺ sources of the age-related prolonged Ca²⁺ elevation in cultured hippocampal neurons. Studies with caffeine showed that the ryanodine receptor-dependent releasable pool of Ca²⁺ from intracellular stores was similar in older and younger neurons. Thapsigargin, which inhibits intracellular store refilling, did not mimic the age-related prolonged Ca²⁺ elevation and, in fact, partially reduced it. Ryanodine, which blocks Ca²⁺-induced Ca²⁺-release (CICR) from stores, completely blocked the age-related prolonged Ca²⁺ elevation following glutamate exposure but did not alter maximal Ca²⁺ elevation during the glutamate exposure. Thus, we conclude that sustained CICR plays a selective and key role in generating the lethal, age-related, prolonged Ca²⁺ elevation, and is the likely mechanism underlying age-related, enhanced vulnerability to excitotoxicity in neurons.