α2-Adrenergic Receptor and Isoflurane Modulation of Presynaptic Ca2+ Influx and Exocytosis in Hippocampal Neurons

Anesthesiology. 2016 Sep;125(3):535-46. 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 Ca 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 Ca influx without affecting Ca coupling to exocytosis, consistent with an effect upstream of Ca-exocytosis coupling. Exocytosis coupled to both N-type and P/Q-type Ca 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 Ca 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.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adrenergic alpha-2 Receptor Agonists / pharmacology*
  • Anesthetics, Inhalation / pharmacology
  • Animals
  • Calcium / metabolism*
  • Clonidine / pharmacology
  • Dexmedetomidine / pharmacology
  • Exocytosis / drug effects*
  • Female
  • Hippocampus / drug effects*
  • Hippocampus / metabolism
  • Isoflurane / pharmacology*
  • Male
  • Models, Animal
  • Neurons / drug effects*
  • Neurons / metabolism
  • Rats
  • Rats, Sprague-Dawley

Substances

  • Adrenergic alpha-2 Receptor Agonists
  • Anesthetics, Inhalation
  • Dexmedetomidine
  • Isoflurane
  • Clonidine
  • Calcium