Finally, the question arose, whether receptors containing 2 distinct α and/or 2 distinct β subunits actually occur in the brain. The existence of such receptors is suggested by the overlapping regional and cellular distribution of the α and β subunits, and by co-precipitation studies (Sieghart and Sperk, 2002). However, neither an overlapping regional and cellular distribution nor a co-precipitation of subunits necessarily means that the subunits are present in the same receptor subtypes. In the latter case they also could be present in different receptors associated with each other either directly or via their associated proteins. In addition, receptors containing 2 distinct α and/or 2 distinct β subunit so far have not been demonstrated to exhibit a specific pharmacology and function in the brain. But since Benke et al.(2004), using knock-in mice containing point-mutated α subunits, provided additional evidence for the existence of receptors containing different α subunits and reported upon the abundance of these receptors in an in vivo system, we decided to include at least receptors containing 2 different α subunits on the list of “tentative receptors”.
