Weir C J, Ling A T Y, Belelli D, Wildsmith J A W, Peters J A, Lambert J J
Department of Anaesthesia, Division of Pathology and Neuroscience, Ninewells Hospital and Medical School, The University of Dundee, Dundee DD1 9SY, UK.
Br J Anaesth. 2004 May;92(5):704-11. doi: 10.1093/bja/aeh125. Epub 2004 Mar 19.
Anaesthetic steroids are established positive allosteric modulators of GABAA receptors, but little is known concerning steroid modulation of strychnine-sensitive glycine receptors, the principal mediators of fast, inhibitory neurotransmission in the brain stem and spinal cord. This study compared the modulatory actions of five anaesthetic pregnane steroids and two non-anaesthetic isomers at human recombinant alpha1 glycine and alpha1beta2gamma2L GABAA receptors.
Recombinant alpha1 glycine or alpha1beta2gamma2L GABAA receptors were expressed in Xenopus laevis oocytes and agonist-evoked currents recorded under voltage-clamp. Steroid modulation of currents evoked by GABA, or glycine, was quantified by determining the potency (EC50) and maximal effect of the compounds.
The anaesthetics minaxolone (EC50=1.3 microM), Org20599 (EC50=1.1 microM) and alphaxalone (EC50=2.2 microM) enhanced currents mediated by GABAA receptors. The anaesthetics also enhanced currents mediated by glycine receptors, although with higher EC50 values (minaxolone 13.1 microM; Org20599=22.9 microM and alphaxalone=27.8 microM). The maximal enhancement (to 780-950% of control) produced by the three steroids acting at the GABAA receptor was similar, but currents evoked by glycine were potentiated with increasing effectiveness by alphaxalone (199%) <Org20599 (525%) <minaxolone (1197%). The anaesthetic isomers, 5alpha-pregnan-3alpha-ol-20-one and 5beta-pregnan-3alpha-ol-20-one (eltanolone) enhanced GABAA receptor-mediated currents with similar potency and efficacy, but only the former enhanced glycine, the latter causing inhibition. The non-anaesthetic steroids 5alpha-pregnan-3beta-ol-20-one and 5beta-pregnan-3beta-ol-20-one modulated neither GABAA, nor glycine, receptors.
The data demonstrate that structure-activity relationships for steroid modulation at glycine and GABAA receptors differ. Comparing the EC50 values reported here with free plasma concentrations during steroid-induced anaesthesia indicates that a selective modulation of GABAA receptor activity is likely to occur in vivo.
麻醉性甾体是已确定的GABAA受体的正变构调节剂，但关于甾体对士的宁敏感的甘氨酸受体的调节作用知之甚少，甘氨酸受体是脑干和脊髓中快速抑制性神经传递的主要介质。本研究比较了五种麻醉性孕烷甾体和两种非麻醉性异构体对人重组α1甘氨酸受体和α1β2γ2L GABAA受体的调节作用。
重组α1甘氨酸受体或α1β2γ2L GABAA受体在非洲爪蟾卵母细胞中表达，并在电压钳下记录激动剂诱发的电流。通过测定化合物的效价（EC50）和最大效应来量化甾体对GABA或甘氨酸诱发电流的调节作用。
麻醉药米那索龙（EC50 = 1.3 μM）、Org20599（EC50 = 1.1 μM）和阿法沙龙（EC50 = 2.2 μM）增强了GABAA受体介导的电流。这些麻醉药也增强了甘氨酸受体介导的电流，尽管其EC50值更高（米那索龙为13.1 μM；Org20599为22.9 μM；阿法沙龙为27.8 μM）。三种甾体作用于GABAA受体产生的最大增强作用（达到对照的780 - 950%）相似，但甘氨酸诱发的电流被阿法沙龙（199%）< Org20599（525%）<米那索龙（1197%）以递增的效力增强。麻醉性异构体5α-孕烷-3α-醇-20-酮和5β-孕烷-3α-醇-20-酮（依托那olone）以相似的效价和效力增强GABAA受体介导的电流，但只有前者增强甘氨酸电流，后者引起抑制。非麻醉性甾体5α-孕烷-3β-醇-20-酮和5β-孕烷-3β-醇-20-酮对GABAA受体和甘氨酸受体均无调节作用。
数据表明甾体对甘氨酸受体和GABAA受体的调节作用的构效关系不同。将此处报道EC的值与甾体诱导麻醉期间的游离血浆浓度进行比较表明，体内可能发生对GABAA受体活性的选择性调节。
