The effects of beta3 subunit incorporation on the pharmacology and single channel properties of oocyte-expressed human alpha3beta4 neuronal nicotinic receptors.

We compared the main properties of human recombinant alpha3beta4beta3 neuronal nicotinic receptors with those of alpha3beta4 receptors, expressed in Xenopus oocytes. beta3 incorporation decreased the channel mean open time (from 5.61 to 1.14 ms, after approximate correction for missed gaps) and burst length. There was also an increase in single channel slope conductance from 28.8 picosiemens (alpha3beta4) to 46.7 picosiemens (alpha3beta4beta3; in low divalent external solution). On the other hand, the calcium permeability (determined by a reversal potential method in chloride-depleted oocytes) and the pharmacological properties of beta3-containing receptors differed little from those of alpha3beta4. The main pharmacological difference in alpha3beta4beta3 "triplet" receptors was a 3-fold decrease in the potency of lobeline relative to acetylcholine. Nevertheless, there was no change in the rank order of potency for agonists (epibatidine >> lobeline > cytisine, 1,1-dimethyl-4-phenylpiperazinium iodide, nicotine > acetylcholine > carbachol for both receptors; measured at low agonist concentrations). Sensitivity to the competitive antagonists trimetaphan (0.2-1 microM) and dihydro-beta-erythroidine (30 microM) was similar for the two combinations, with a Schild KB for trimetaphan of 76 and 66 nM on alpha3beta4 and alpha3beta4beta3, respectively. The change in single channel conductance confirms that beta3 replaces a beta4 subunit in the pentamer. The absence of pronounced differences in the pharmacological profile of the triplet receptor argues against a role for the beta3 subunit in the formation of agonist binding sites, whereas the changes in channel kinetics suggest an important effect on receptor gating. The shortening of the burst length of beta3-containing receptors implies that any synaptic currents mediated by such channels would have faster decay kinetics.