Belelli D, Lambert J J, Peters J A, Wafford K, Whiting P J
Neurosciences Institute, Department of Pharmacology and Neuroscience, Ninewells Hospital and Medical School, Dundee University, Dundee, DD1 9SY, United Kingdom.
Proc Natl Acad Sci U S A. 1997 Sep 30;94(20):11031-6. doi: 10.1073/pnas.94.20.11031.
The gamma-aminobutyric acid type A (GABAA) receptor is a transmitter-gated ion channel mediating the majority of fast inhibitory synaptic transmission within the brain. The receptor is a pentameric assembly of subunits drawn from multiple classes (alpha1-6, beta1-3, gamma1-3, delta1, and epsilon1). Positive allosteric modulation of GABAA receptor activity by general anesthetics represents one logical mechanism for central nervous system depression. The ability of the intravenous general anesthetic etomidate to modulate and activate GABAA receptors is uniquely dependent upon the beta subunit subtype present within the receptor. Receptors containing beta2- or beta3-, but not beta1 subunits, are highly sensitive to the agent. Here, chimeric beta1/beta2 subunits coexpressed in Xenopus laevis oocytes with human alpha6 and gamma2 subunits identified a region distal to the extracellular N-terminal domain as a determinant of the selectivity of etomidate. The mutation of an amino acid (Asn-289) present within the channel domain of the beta3 subunit to Ser (the homologous residue in beta1), strongly suppressed the GABA-modulatory and GABA-mimetic effects of etomidate. The replacement of the beta1 subunit Ser-290 by Asn produced the converse effect. When applied intracellularly to mouse L(tk-) cells stably expressing the alpha6beta3gamma2 subunit combination, etomidate was inert. Hence, the effects of a clinically utilized general anesthetic upon a physiologically relevant target protein are dramatically influenced by a single amino acid. Together with the lack of effect of intracellular etomidate, the data argue against a unitary, lipid-based theory of anesthesia.
γ-氨基丁酸A型（GABAA）受体是一种递质门控离子通道，介导大脑中大部分快速抑制性突触传递。该受体是由多个类别（α1-6、β1-3、γ1-3、δ1和ε1）的亚基组成的五聚体。全身麻醉药对GABAA受体活性的正向变构调节是中枢神经系统抑制的一种合理机制。静脉全身麻醉药依托咪酯调节和激活GABAA受体的能力独特地取决于受体中存在的β亚基亚型。含有β2或β3亚基而非β1亚基的受体对该药物高度敏感。在此，在非洲爪蟾卵母细胞中与人α6和γ2亚基共表达的嵌合β1/β2亚基确定了细胞外N端结构域远端的一个区域是依托咪酯选择性的决定因素。β3亚基通道结构域内的一个氨基酸（Asn-289）突变为Ser（β1中的同源残基），强烈抑制了依托咪酯的GABA调节和GABA模拟作用。将β1亚基的Ser-290替换为Asn产生了相反的效果。当细胞内应用于稳定表达α6β3γ2亚基组合的小鼠L(tk-)细胞时，依托咪酯无活性。因此，一种临床使用的全身麻醉药对生理相关靶蛋白的作用受到单个氨基酸的显著影响。连同细胞内依托咪酯缺乏作用的数据，这些数据反对单一的基于脂质的麻醉理论。
