GABAA receptor diversity revealed in freeze‐fracture replica (Commentary on Kasugai et al.)

GABAergic inputs from at least 18 types of inhibitory interneurons regulate and coordinate the activity of pyramidal cells in the hippocampal area CA1 (Klausberger et al., 2005), which in turn express at least 14 subunits of the GABAA receptor (GABAAR) with varying affinity to GABA and other ligands (Persohn et al., 1992; Wisden et al., 1992; Sperk et al., 1997; Ogurusu et al., 1999). Thus, the subunit composition determines the local response of the GABAAR to synaptically released GABA. Elucidating the subunit composition of synaptic and extrasynaptic GABAAR is also crucial in understanding the phasic vs. tonic postsynaptic responses evoked by GABA. In their elegant study, Kasugai et al. (2010) performed a series of double-labeling experiments to demonstrate for the first time that virtually all somatic inhibitory synapses in the rat hippocampal CA1 pyramidal cell contained a1, a2, and b3 subunits of GABAAR. The authors developed a new antibody against the a1 subunit to be used for a sensitive immunocytochemical method, freeze-fracture replica-immunogold labeling, that allows for quantitative analyses of transmembrane protein distribution (Fujimoto, 1995; Masugi-Tokita & Shigemoto, 2007). Their finding is consistent with previous post-embedding immunogold studies (Nusser et al., 1996; Somogyi et al., 1996). The presence of the three subunits in all somatic synapses does not imply that all synaptic GABAARs consist of these subunits. However, this finding raises an interesting question regarding the relative proportion of the subunits at these synapses, because previous studies showed that a1 and a2 subunits preferentially mediate inputs from fast-spiking and regular-spiking basket cells, respectively (Pawelzik et al., 1999; Thomson et al., 2000; Nyiri et al., 2001; Klausberger et al., 2002). Kasugai et al. (2010) also conducted single-labeling experiments to examine carefully the density of a1, a2 and b3 subunits in the synaptic and extrasynaptic plasma membrane of the pyramidal cells. Thirty to 50% of total labeling was found in synapses with 50–70% being extrasynaptic, suggesting that these subunits are well distributed between synaptic and extrasynaptic membrane. This is perhaps not too surprising as GABAARs diffuse laterally in the plasma membrane (Bannai et al., 2009). However, in light of previous studies suggesting that tonic inhibition is mediated by extrasynaptic GABAARs containing the a4, a5, a6 and ⁄ or d subunits (Belelli et al., 2009), one might wonder how a1or a2-containing extrasynaptic receptors are different from the synaptic ones in their subunit composition, targeting and functions. Elucidating the native subunit composition of GABAARs at identified synapses and extrasynaptic membrane will require a combination of research tools, including subunit-specific antibodies against different epitopes (intracellular vs. extracellular), transgenic animals (with specific subunits deleted or mutated in specific neuronal populations) and subunit-specific allosteric modulators. Studies on the interactions between specific subunit and scaffolding proteins would also provide insight into the mechanisms that target and regulate GABAARs. The data from this study provide a solid anatomical substrate to understand the functions of a1, a2 and b3 subunits in synaptic and extrasynaptic plasma membrane. They begin to elucidate how a diverse population of GABAARs and inhibitory interneurons may shape cortical network activity in health and disease.