Isoflurane Inhibits Dopaminergic Synaptic Vesicle Exocytosis Coupled to CaV2.1 and CaV2.2 in Rat Midbrain Neurons

Volatile anesthetics affect neuronal signaling by poorly understood mechanisms. Activation of central dopaminergic pathways has been implicated in emergence from general anesthesia. The volatile anesthetic isoflurane differentially inhibits glutamatergic and GABAergic synaptic vesicle (SV) exocytosis by reducing presynaptic Ca2+ influx without affecting the Ca2+-exocytosis relationship, but its effects on dopaminergic exocytosis are unclear. We tested the hypothesis that isoflurane inhibits exocytosis in dopaminergic neurons. We used electrical stimulation or depolarization by elevated extracellular KCl to evoke exocytosis measured by quantitative live-cell fluorescence imaging in cultured rat ventral tegmental area neurons. Using trains of electrically evoked action potentials (APs), isoflurane inhibited exocytosis in dopaminergic neurons to a greater extent (30 +/- 4% inhibition; p < 0.0001) than in non-dopaminergic neurons (15 +/- 5% inhibition; p = 0.014). Isoflurane also inhibited exocytosis evoked by elevated KCl in dopaminergic neurons (35 +/- 6% inhibition; p = 0.0007), but not in non-dopaminergic neurons (2 +/- 4% inhibition). Pharmacological isolation of presynaptic Ca2+ channel subtypes showed that isoflurane inhibited KCl-evoked exocytosis mediated exclusively by either Ca(V)2.1 (P/Q-type Ca2+ channels; 30 +/- 5% inhibition; p = 0.0002) or by Ca(V)2.2 (N-type Ca2+ channels; 35 +/- 11% inhibition; p = 0.015). Additionally, isoflurane inhibited single AP-evoked Ca2+ influx by 41 +/- 3% and single AP-evoked exocytosis by 34 +/- 6%. Comparable reductions in exocytosis and Ca2+ influx were produced by lowering extracellular [Ca2+]. Thus, isoflurane inhibits exocytosis from dopaminergic neurons by a mechanism distinct from that in non-dopaminergic neurons involving reduced Ca2+ entry through Ca(V)2.1 and/or Ca(V)2.2.