Spontaneous Ca2+ events are linked to the development of neuronal firing during maturation in mice primary hippocampal culture cells.

Calcium is one of the most vital intracellular secondary messengers that tightly regulates a variety of cell physiology processes, especially in the brain. Using a fluorescent Ca2+-sensitive Oregon Green probe, we revealed three different amplitude distributions of spontaneous Ca2+ events (SCEs) in neurons between 15 and 26 days in vitro (DIV) culture maturation. We detected a series of amplitude events: micro amplitude SCE (microSCE) 25% increase from the baseline, intermediate amplitude SCE (interSCE) as 25-75%, and macro amplitude SCE (macroSCE) - over 75%. The SCEs were fully dependent on extracellular Ca2+ and neuronal network activity and vanished in the Ca2+-free solution, 10 mM Mg2+-block, or in the presence of voltage-gated Na+-channel blocker, tetrodotoxin. Combined patch-clamp and Ca2+-imaging techniques revealed that microSCE match single action potential (AP), interSCE - burst of 3-12 APs, and macroSCE - 'superburst' of 10+ APs. MicroSCEs were blocked by a common α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainic acid (KA) receptor antagonist, CNQX. The γ-aminobutyric acid (GABA) A-type receptor (GABAAR) picrotoxin blockade and L-type voltage-dependent Ca2+-channel inhibitor diltiazem significantly reduced microSCE frequency. InterSCEs were inhibited by CNQX, but picrotoxin treatment significantly increased its amplitude. The N-methyl-d-aspartate (NMDA) receptor antagonist, D-APV, voltage-gated K+-channel blocker, tetraethylammonium, noticeably suppressed interSCE amplitude. We also demonstrate that macroSCEs were AMPA/KA receptor-independent.