Hara Masato, Zhou Zhen-Yu, Hemmings Hugh C
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.).
Anesthesiology. 2016 Sep;125(3):535-46. doi: 10.1097/ALN.0000000000001213.
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.
有证据表明，α2肾上腺素能受体（AR）激动剂的麻醉节省效应涉及非肾上腺素能神经元上的α2A-AR异源受体。由于挥发性麻醉剂通过减少突触小泡（SV）胞吐作用来抑制神经递质释放，因此作者推测α2-AR激动剂抑制非肾上腺素能SV胞吐作用，从而增强异氟烷对突触前胞吐作用的抑制。
利用动作电位诱发的SV胞吐（突触素-pHluorin）和Ca内流（GCaMP6）荧光生物传感器的定量成像，来表征临床使用的α2-AR激动剂右美托咪定和可乐定在培养的大鼠海马神经元中的突触前作用，以及它们与异氟烷的相互作用。
右美托咪定（0.1μM，n = 10）或可乐定（0.5μM，n = 8）抑制动作电位诱发的胞吐作用（分别为对照的54±5%和59±8%；P < 0.001）。对胞吐作用的影响被非选择性α2-AR拮抗剂阿替美唑或α2A-AR选择性拮抗剂BRL 44408阻断，但未被α2C-AR选择性拮抗剂JP 1302阻断。右美托咪定抑制胞吐作用和突触前Ca内流，而不影响Ca与胞吐作用的偶联，这与Ca-胞吐作用偶联上游的效应一致。与N型和P/Q型Ca通道偶联的胞吐作用均被右美托咪定或可乐定抑制。右美托咪定增强了0.7 mM异氟烷对胞吐作用的抑制（降至42±5%，而异氟烷单独作用时为63±8%；P < 0.05）。
海马SV胞吐作用因α2A-AR激活而受到抑制，与Ca内流减少成比例。这些效应与异氟烷的效应相加，这与α2A-AR突触前异源受体抑制非肾上腺素能突触传递在α2A-AR激动剂的麻醉节省效应中的作用一致。
