**13. Distribution**

**Figure 6.** GABAA receptor transmembrane topology. Each subunit consists of four transmembrane domains. The N‐ and C‐terminus lie on the extracellular side, with the N‐terminus being the site of action for several drugs. A large intracellular domain exists between TM3 and TM4, providing a hub for a majority of the protein‐protein interactions as

**Figure 7.** GABAB receptor structure. GABAB receptors are heterodimers composed of either a 1a or 1b subunit and a mandatory 2 subunit. GABAB receptors are G‐protein coupled and are activated by the ligand GABA, which binds to the 1 subunit. Following GABA ligand binding, G‐protein activation induces opening of postsynaptic potassium channels

and closing of presynaptic calcium channels, hyperpolarizing the target cell (Modified from Emson [64]).

well as posttranslational modifications (palmitoylation, ubiquitination, phosphorylation) [65, 69].

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GABAA subunit expression has been well characterized in the rat brain [66]. Several subunits exhibit broad expression throughout the nervous system, fluctuating across development and regions. However, a few subunits demonstrate regional or cell‐type specificity. For example, α6 subunit is exclusively expressed in cerebellar granule cells and the ρ subunit is largely restricted to the retina. Peripheral expression of GABAA receptors has been demonstrated in the liver, smooth airway muscles, the lung, immune cells, and the intestines [68]. GABAB receptors are localized in the striatum, brainstem, thalamus, hippocampus, cerebellum, and cortex. The 1b subunit is the most prevalently expressed across all brains regions, except for the striatum in which the 1a subunit is the most abundant [64].
