Isoflurane Alters Presynaptic Endoplasmic Reticulum Calcium Dynamics in Wild-Type and Malignant Hyperthermia-Susceptible Rodent Hippocampal Neurons

Volatile anesthetics reduce excitatory synaptic transmission by both presynaptic and postsynaptic mechanisms which include inhibition of depolarization-evoked increases in presynaptic Ca(2+) concentration and blockade of postsynaptic excitatory glutamate receptors. The presynaptic sites of action leading to reduced electrically evoked increases in presynaptic Ca(2+) concentration and Ca(2+)-dependent exocytosis are unknown. Endoplasmic reticulum (ER) of Ca(2+) release via ryanodine receptor 1 (RyR1) and uptake by SERCA are essential for regulation intracellular Ca(2+) and are potential targets for anesthetic action. Mutations in sarcoplasmic reticulum (SR) release channels mediate volatile anesthetic-induced malignant hyperthermia (MH), a potentially fatal pharmacogenetic condition characterized by unregulated Ca(2+) release and muscle hypermetabolism. However, the impact of MH mutations on neuronal function are unknown. We used primary cultures of postnatal hippocampal neurons to analyze volatile anesthetic-induced changes in ER Ca(2+) dynamics using a genetically encoded ER-targeted fluorescent Ca(2+) sensor in both rat and mouse wild-type (WT) neurons and in mouse mutant neurons harboring the RYR1 T4826I MH-susceptibility mutation. The volatile anesthetic isoflurane reduced both baseline and electrical stimulation-evoked increases in ER Ca(2+) concentration in neurons independent of its depression of presynaptic cytoplasmic Ca(2+) concentrations. Isoflurane and sevoflurane, but not propofol, depressed depolarization-evoked increases in ER Ca(2+) concentration significantly more in mouse RYR1 T4826I mutant neurons than in wild-type neurons. The RYR1 T4826I mutant neurons also showed markedly greater isoflurane-induced reductions in presynaptic cytosolic Ca(2+) concentration and synaptic vesicle (SV) exocytosis. These findings implicate RyR1 as a molecular target for the effects of isoflurane on presynaptic Ca(2+) handling.