Therefore, using a stringent classification, we only have 9 different GABAA receptor subtypes (α1-6βγ2, α4βδ, α6βδ, ρ) unequivocally identified in the brain (Olsen and Sieghart, 2008). To keep the possibility alive that there are also receptors composed of ρ1, ρ2, and ρ3 subunits, we added these three receptor types to a list of “tentative receptors”. In this list we did not include receptors containing mixed ρ subunits, because only recombinant receptor studies so far provided evidence for their possible existence, and in such studies receptors also can be formed that might not be formed under native conditions (Olsen and Sieghart, 2008). Out of the same reason, we also did not include possible receptors formed from a combination of ρ and γ2 or glycine receptor subunits (Pan et al., 2000; Milligan et al., 2004).
