Perioperative Medicine  |   September 2016
α2-Adrenergic Receptor and Isoflurane Modulation of Presynaptic Ca2+ Influx and Exocytosis in Hippocampal Neurons
Author Notes
  • From the Departments of Anesthesiology (M.H., Z.-Y.Z., H.C.H.) and Pharmacology (H.C.H.), Weill Cornell Medical College, New York, New York, USA; and 3Department of Anesthesiology, Kurume University School of Medicine, Kurume, Fukuoka, Japan (M.H.).
  • Submitted for publication January 8, 2016. Accepted for publication May 23, 2016.
    Submitted for publication January 8, 2016. Accepted for publication May 23, 2016.×
  • Address correspondence to Dr. Hemmings: Weill Cornell Medical College, Department of Anesthesiology, Box 124, 1300 York Avenue, New York, New York. hchemmi@med.cornell.edu. Information on purchasing reprints may be found at www.anesthesiology.org or on the masthead page at the beginning of this issue. Anesthesiology’s articles are made freely accessible to all readers, for personal use only, 6 months from the cover date of the issue.
Article Information
Perioperative Medicine / Basic Science / Central and Peripheral Nervous Systems / Pharmacology
Perioperative Medicine   |   September 2016
α2-Adrenergic Receptor and Isoflurane Modulation of Presynaptic Ca2+ Influx and Exocytosis in Hippocampal Neurons
Anesthesiology 9 2016, Vol.125, 535-546. doi:10.1097/ALN.0000000000001213
Anesthesiology 9 2016, Vol.125, 535-546. doi:10.1097/ALN.0000000000001213
Abstract

Background: Evidence indicates that the anesthetic-sparing effects of α2-adrenergic receptor (AR) agonists involve α2A-AR heteroreceptors on nonadrenergic neurons. Since volatile anesthetics inhibit neurotransmitter release by reducing synaptic vesicle (SV) exocytosis, the authors hypothesized that α2-AR agonists inhibit nonadrenergic SV exocytosis and thereby potentiate presynaptic inhibition of exocytosis by isoflurane.

Methods: Quantitative imaging of fluorescent biosensors of action potential–evoked SV exocytosis (synaptophysin-pHluorin) and Ca2+ influx (GCaMP6) were used to characterize presynaptic actions of the clinically used α2-AR agonists dexmedetomidine and clonidine, and their interaction with isoflurane, in cultured rat hippocampal neurons.

Results: Dexmedetomidine (0.1 μM, n = 10) or clonidine (0.5 μM, n = 8) inhibited action potential–evoked exocytosis (54 ± 5% and 59 ± 8% of control, respectively; P < 0.001). Effects on exocytosis were blocked by the subtype-nonselective α2-AR antagonist atipamezole or the α2A-AR–selective antagonist BRL 44408 but not by the α2C-AR–selective antagonist JP 1302. Dexmedetomidine inhibited exocytosis and presynaptic Ca2+ influx without affecting Ca2+ coupling to exocytosis, consistent with an effect upstream of Ca2+–exocytosis coupling. Exocytosis coupled to both N-type and P/Q-type Ca2+ channels was inhibited by dexmedetomidine or clonidine. Dexmedetomidine potentiated inhibition of exocytosis by 0.7 mM isoflurane (to 42 ± 5%, compared to 63 ± 8% for isoflurane alone; P < 0.05).

Conclusions: Hippocampal SV exocytosis is inhibited by α2A-AR activation in proportion to reduced Ca2+ entry. These effects are additive with those of isoflurane, consistent with a role for α2A-AR presynaptic heteroreceptor inhibition of nonadrenergic synaptic transmission in the anesthetic-sparing effects of α2A-AR agonists.