High- and low-sensitivity subforms of alpha4beta2 and alpha3beta2 nAChRs.

Previous studies in other laboratories have shown that alpha4beta2 nicotinic acetylcholine receptor (nAChR) exhibits a biphasic concentration-response relationship for ACh with low and high EC50 components, and that the low EC50 component can be augmented by decreasing the alpha4:beta2 message ratio or incubating overnight in nicotine or at low temperature (Zwart and Vijverberg, 1998; Covernton and Connolly, 2000; Buisson and Bertrand, 2001; Nelson et al., 2003; Zhou et al., 2003). In the process of cloning ferret nAChR subunits, we found alpha4 and beta2 messages with long untranslated regions (UTRs), as well as those with no UTRs. Combinations of these messages revealed that the presence of UTRs influenced the ability to exclusively express high-sensitivity subforms of alpha4beta2 and alpha3beta2 nAChRs. Injection of oocytes with alpha4 and beta2 RNAs lacking UTRs (1:1 ratio) led to expression of a biphasic concentration-response relationship for ACh with EC50 values of 0.5 (high sensitivity) and 114 microM(low sensitivity). Decreasing the alpha4:beta2 message ratio to as much as 1:120 increased the high-sensitivity component slightly, but the ACh concentration response remained biphasic. In contrast, injection of messages with UTRs (1:1 ratio) led to expression of a monophasic concentration response to ACh and a high-sensitivity EC50 value of 2.3 microM, as shown in Fig. 1.