Isoflurane inhibits synaptic vesicle exocytosis through reduced Ca2+ influx, not Ca2+-exocytosis coupling.
Identifying presynaptic mechanisms of general anesthetics is critical to understanding their effects on synaptic transmission. We show that the volatile anesthetic isoflurane inhibits synaptic vesicle (SV) exocytosis at nerve terminals in dissociated rat hippocampal neurons through inhibition of presynaptic Ca(2+) influx without significantly altering the Ca(2+) sensitivity of SV exocytosis. A clinically relevant concentration of isoflurane (0.7 mM) inhibited changes in [Ca(2+)]i driven by single action potentials (APs) by 25 ± 3%, which in turn led to 62 ± 3% inhibition of single AP-triggered exocytosis at 4 mM extracellular Ca(2+) ([Ca(2+)]e). Lowering external Ca(2+) to match the isoflurane-induced reduction in Ca(2+) entry led to an equivalent reduction in exocytosis. These data thus indicate that anesthetic inhibition of neurotransmitter release from small SVs occurs primarily through reduced axon terminal Ca(2+) entry without significant direct effects on Ca(2+)-exocytosis coupling or on the SV fusion machinery. Isoflurane inhibition of exocytosis and Ca(2+) influx was greater in glutamatergic compared with GABAergic nerve terminals, consistent with selective inhibition of excitatory synaptic transmission. Such alteration in the balance of excitatory to inhibitory transmission could mediate reduced neuronal interactions and network-selective effects observed in the anesthetized central nervous system.
Department of Anesthesiology, Weill Cornell Medical College, New York, NY 10065;
Joel P
JP
Baumgart
Department of Anesthesiology, Weill Cornell Medical College, New York, NY 10065;
Zhen-Yu
http://orcid.org/0000-0002-5024-5263
ZY
Zhou
Department of Anesthesiology, Weill Cornell Medical College, New York, NY 10065; Department of Anesthesiology, Kurume University School of Medicine, Kurume, Fukuoka 830-0011, Japan;
Masato
M
Hara
Department of Anesthesiology, Weill Cornell Medical College, New York, NY 10065;
Daniel C
DC
Cook
Department of Biochemistry, Weill Cornell Medical College, New York, NY 10065; Department of Biology, Dartmouth College, Hanover, NH 03755;
Michael B
MB
Hoppa
Department of Anesthesiology, Weill Cornell Medical College, New York, NY 10065; Department of Biochemistry, Weill Cornell Medical College, New York, NY 10065;
Timothy A
TA
Ryan
Department of Anesthesiology, Weill Cornell Medical College, New York, NY 10065; Department of Pharmacology, Weill Cornell Medical College, New York, NY 10065 hchemmi@med.cornell.edu.
Hugh C
HC
Hemmings
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D005089:Exocytosis / Q000187:drug effects*; D059330:GABAergic Neurons / Q000187:drug effects / Q000378:metabolism; D005971:Glutamates / Q000378:metabolism; D007530:Isoflurane / Q000494:pharmacology*; D007700:Kinetics; D017729:Presynaptic Terminals / Q000187:drug effects / Q000378:metabolism; D017207:Rats, Sprague-Dawley; D013572:Synaptic Vesicles / Q000187:drug effects / Q000378:metabolism*
D005971:Glutamates; D007530:Isoflurane; D002118:Calcium
GCaMP3; live cell imaging; mechanisms of anesthesia; pHlourin; presynaptic