Finally, evidence accumulated for the existence of receptors composed of α and β subunits, only. First, Bencsits et al., (1999) demonstrated that a large part of the α4 receptors (about 50%) are not associated with γ, or δ, subunits and are possibly composed of α and β subunits only. This was in contrast to α1 receptors. Second, in several knockout mice it was demonstrated that receptors composed of only α and β subunits do exist, for example in γ2 knockout mice (Günther et al., 1995), δ knockout mice (Tretter et al., 2001), or α1 knockout mice (Ogris et al., 2006). Third, in several brain regions α and β occur in the absence of γ and δ subunits. And fourth, Mortensen and Smart (2006) demonstrated by electrophysiological studies that there are extrasynaptic αβ receptors on rat hippocampal pyramidal neurons. In the absence of any evidence for a specific αβ subunit combination, we placed one undefined αβ subtype into category B.
