Another candidate qualifying for this list is the α5β3γ2 receptor subtype. The α5 and β3 subunits appear to be co-depleted in mice lacking either the β3, or α5 subunits (Olsen and Homanics, 2000), and the properties of recombinant α5β3γ2 receptors appear to reflect those of a native subtype found in CA1 pyramidal neurons (that are enriched in both α5 and β3 subunits) and are distinct from receptors in dentate gyrus granule cells (Burgard et al., 1996; Sur et al., 1998; McClellan and Twyman, 1999; Stell et al., 2003;Caraiscos et al., 2004). Such studies are not conclusive evidence for this combination, so we place this subtype in the second list. Another example in this category is the α1β3γ2 subtype. If we specify that β2 is the most common partner of α1γ2, and list it in category A, we need to ask whether α1 can partner with other β subunits. While strong evidence for β1 is lacking, the properties of α1β3γ2 receptors in recombinant studies resemble those in certain neurons that express these subunits, some of which lack the β2 subunit (Whiting et al., 2000; Sieghart and Sperk, 2002).
