Figure 3: Schematic and putative binding sites of γ-aminobutyric acid receptor. (a) A single subunit of the γ-aminobutyric acid receptor, highlighting topology. M1– M4 represent transmembrane domains. The M2 transmembrane domain (gray) forms an important part of the chloride channel pore. (b) Pentameric structure of a typical γ-aminobutyric acid receptor. Several putative sites of γ-aminobutyric acid and modulatory drugs, including neurosteroids, are shown. The indication that steroids act on the γ-aminobutyric acid receptor from within the transmembrane domains is supported by pharmacological studies and by recent site-directed mutagenesis studies.