Maitra R, Reynolds J N
Department of Pharmacology and Toxicology, Queen's University, Kingston, Ontario, Canada.
Brain Res. 1999 Feb 20;819(1-2):75-82. doi: 10.1016/s0006-8993(98)01316-x.
Neurosteroids are potent, endogenous modulators of GABAA receptor function in the central nervous system. The endogenous progesterone metabolite allopregnanolone (ALP) and the synthetic steroid compound alphaxalone (AFX) have been shown to both directly activate and potentiate GABAA receptor-activated membrane current (IGABA). The role of different alpha and gamma subunit subtypes in modulation of IGABA by ALP and AFX was investigated using recombinant GABAA receptor isoforms expressed in Xenopus oocytes. Changing or removal of the alpha subunit subtype altered the efficacy of both ALP and AFX (alpha2beta1gamma2L>alpha1beta1gamma2L>>beta1gamma2L) to potentiate IGABA, but did not alter the potency of the neuroactive steroids at these receptor isoforms. The efficacy of ALP to enhance IGABA was also dependent on the gamma subunit subtype (alpha1beta1gamma3>alpha1beta1gamma2L = alpha1beta1gamma1). AFX also had higher efficacy in the alpha1beta1gamma3 receptor isoform compared to alpha1beta1gamma1. In contrast to ALP, the potency of AFX was greater in the alpha1beta1gamma3 and alpha1beta1gamma1 receptor isoforms compared to alpha1beta1gamma2L. This study provides evidence that the alpha subunit subtype determines the efficacy, but not the potency, of these neuroactive steroids to potentiate IGABA. The gamma3 subunit subtype increases the maximal efficacy of neuroactive steroids compared to other gamma subunit subtypes. These results suggest that the heteromeric assembly of different GABAA receptor isoforms containing different subunit subtypes results in multiple steroid recognition sites on GABAA receptors that in turn produce distinctly different modulatory interactions between neuroactive steroids acting at the GABAA receptor.
神经甾体是中枢神经系统中γ-氨基丁酸A（GABAA）受体功能的强效内源性调节剂。内源性孕酮代谢物别孕烯醇酮（ALP）和合成甾体化合物alphaxalone（AFX）已被证明既能直接激活GABAA受体激活的膜电流（IGABA），又能增强该电流。利用非洲爪蟾卵母细胞中表达的重组GABAA受体亚型，研究了不同α和γ亚基亚型在ALP和AFX对IGABA调节中的作用。改变或去除α亚基亚型会改变ALP和AFX增强IGABA的效能（α2β1γ2L>α1β1γ2L>>β1γ2L），但不会改变这些神经活性甾体在这些受体亚型上的效力。ALP增强IGABA的效能也取决于γ亚基亚型（α1β1γ3>α1β1γ2L = α1β1γ1）。与α1β1γ1相比，AFX在α1β1γ3受体亚型中也具有更高的效能。与ALP相反，与α1β1γ2L相比，AFX在α1β1γ3和α1β1γ1受体亚型中的效力更大。这项研究提供了证据，表明α亚基亚型决定了这些神经活性甾体增强IGABA的效能，而非效力。与其他γ亚基亚型相比，γ3亚基亚型增加了神经活性甾体的最大效能。这些结果表明，含有不同亚基亚型的不同GABAA受体亚型的异源组装导致GABAA受体上存在多个甾体识别位点，进而在作用于GABAA受体的神经活性甾体之间产生明显不同的调节相互作用。
