title: α2-Adrenergic Receptor and Isoflurane Modulation of Presynaptic Ca2+ Influx and Exocytosis in Hippocampal Neurons.
abstract: 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.
  
  
  
  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.
  
  
  
  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).
  
  
  
  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.
authors.affiliation: 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.).
authors.forename: Masato
authors.initials: M
authors.lastname: Hara
authors.forename: Zhen-Yu
authors.initials: ZY
authors.lastname: Zhou
authors.forename: Hugh C
authors.initials: HC
authors.lastname: Hemmings
mesh_terms: D058647:Adrenergic alpha-2 Receptor Agonists / Q000494:pharmacology*; D018685:Anesthetics, Inhalation / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D003000:Clonidine / Q000494:pharmacology; D020927:Dexmedetomidine / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects*; D005260:Female; D006624:Hippocampus / Q000187:drug effects* / Q000378:metabolism; D007530:Isoflurane / Q000494:pharmacology*; D008297:Male; D023421:Models, Animal; D009474:Neurons / Q000187:drug effects* / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley
chemical_list: D058647:Adrenergic alpha-2 Receptor Agonists; D018685:Anesthetics, Inhalation; D020927:Dexmedetomidine; D007530:Isoflurane; D003000:Clonidine; D002118:Calcium