Fritschy J M, Mohler H
Institute of Pharmacology, University of Zürich, Switzerland.
J Comp Neurol. 1995 Aug 14;359(1):154-94. doi: 10.1002/cne.903590111.
GABAA-receptors display an extensive structural heterogeneity based on the differential assembly of a family of at least 15 subunits (alpha 1-6, beta 1-3, gamma 1-3, delta, rho 1-2) into distinct heteromeric receptor complexes. The subunit composition of receptor subtypes is expected to determine their physiological properties and pharmacological profiles, thereby contributing to flexibility in signal transduction and allosteric modulation. In heterologous expression systems, functional receptors require a combination of alpha-, beta-, and gamma-subunit variants, the gamma 2-subunit being essential to convey a classical benzodiazepine site to the receptor. The subunit composition and stoichiometry of native GABAA-receptor subtypes remain unknown. The aim of this study was to identify immunohistochemically the main subunit combinations expressed in the adult rat brain and to allocate them to identified neurons. The regional and cellular distribution of seven major subunits (alpha 1, alpha 2, alpha 3, alpha 5, beta 2,3, gamma 2, delta) was visualized by immunoperoxidase staining with subunit-specific antibodies (the beta 2- and beta 3-subunits were covisualized with the monoclonal antibody bd-17). Putative receptor subtypes were identified on the basis of colocalization of subunits within individual neurons, as analyzed by confocal laser microscopy in double- and triple-immunofluorescence staining experiments. The results reveal an extraordinary heterogeneity in the distribution of GABAA-receptor subunits, as evidenced by abrupt changes in immunoreactivity along well-defined cytoarchitectonic boundaries and by pronounced differences in the cellular distribution of subunits among various types of neurons. Thus, functionally and morphologically diverse neurons were characterized by a distinct GABAA-receptor subunit repertoire. The multiple staining experiments identified 12 subunit combinations in defined neurons. The most prevalent combination was the triplet alpha 1/beta 2,3/gamma 2, detected in numerous cell types throughout the brain. An additional subunit (alpha 2, alpha 3, or delta) sometimes was associated with this triplet, pointing to the existence of receptors containing four subunits. The triplets alpha 2/beta 2,3/gamma 2, alpha 3/beta 2,3/gamma 2, and alpha 5/beta 2,3/gamma 2 were also identified in discrete cell populations. The prevalence of these seven combinations suggest that they represent major GABAA-receptor subtypes. Five combinations also apparently lacked the beta 2,3-subunits, including one devoid of gamma 2-subunit (alpha 1/alpha 2/gamma 2, alpha 2/gamma 2, alpha 3/gamma 2, alpha 2/alpha 3/gamma 2, alpha 2/alpha 5/delta).(ABSTRACT TRUNCATED AT 400 WORDS)
GABAA受体基于至少15种亚基（α1 - 6、β1 - 3、γ1 - 3、δ、ρ1 - 2）家族的差异组装形成不同的异聚体受体复合物，呈现出广泛的结构异质性。受体亚型的亚基组成有望决定其生理特性和药理学特征，从而有助于信号转导和变构调节的灵活性。在异源表达系统中，功能性受体需要α -、β - 和γ - 亚基变体的组合，γ2亚基对于将经典苯二氮䓬位点传递给受体至关重要。天然GABAA受体亚型的亚基组成和化学计量仍不清楚。本研究的目的是通过免疫组织化学方法鉴定成年大鼠脑中表达的主要亚基组合，并将它们分配到已识别的神经元中。用亚基特异性抗体通过免疫过氧化物酶染色观察七种主要亚基（α1、α2、α3、α5、β2,3、γ2、δ）的区域和细胞分布（β2和β3亚基用单克隆抗体bd - 17共定位观察）。在双免疫荧光和三免疫荧光染色实验中，通过共聚焦激光显微镜分析，根据单个神经元内亚基的共定位来鉴定假定的受体亚型。结果显示GABAA受体亚基分布存在异常异质性，这通过沿明确的细胞构筑边界免疫反应性的突然变化以及不同类型神经元中亚基细胞分布的明显差异得以证明。因此，功能和形态多样的神经元具有独特的GABAA受体亚基组成。多重染色实验在特定神经元中鉴定出12种亚基组合。最常见的组合是三联体α1/β2,3/γ2，在全脑的多种细胞类型中都能检测到。有时额外的一个亚基（α2、α3或δ）会与这个三联体相关联，这表明存在包含四个亚基的受体。三联体α2/β2,3/γ2、α3/β2,3/γ2和α5/β2,3/γ2也在离散的细胞群体中被鉴定出来。这七种组合的普遍性表明它们代表主要的GABAA受体亚型。还有五种组合显然缺乏β2,3亚基，包括一种缺乏γ2亚基的组合（α1/α2/γ2、α2/γ2、α3/γ2、α2/α3/γ2、α2/α5/δ）。
