We have investigated the functional properties of four rat neuronal nicotinic acetylcholine receptor types expressed in Xenopus oocytes after injection of pairwise combinations of mRNA encoding alpha 2 or alpha 3 receptor subunits with mRNA encoding beta 2 or beta 4 receptor subunits. Current responses evoked by rapid application of cholinergic agonists (acetylcholine, nicotine, or 1,1-dimethyl-4-phenylpiperazinium) were recorded from voltage-clamped oocytes. Substituting BaCl2 for CaCl2 in the external solution increased the apparent Kd values of beta 4 subunit-containing receptors for acetylcholine but decreased the apparent Kd values of beta 2 subunit-containing receptors. Inhibition curves for the cholinergic antagonist (+)-tubocurarine were measured in BaCl2 medium at low agonist concentrations. (+)-Tubocurarine was a competitive antagonist of acetylcholine at neuronal nicotinic acetylcholine receptors that coexpressed the beta 2 subunit; the estimated Kb values were 3.6 microM (alpha 2 beta 2 receptors) and 390 nM (alpha 3 beta 2 receptors). In contrast, (+)-tubocurarine enhanced the peak responses evoked by low acetylcholine concentrations at alpha 2 beta 4 and alpha 3 beta 4 neuronal nicotinic acetylcholine receptors, without being a partial agonist. The maximal increase was observed at 5 microM and 10 microM (+)-tubocurarine for alpha 2 beta 4 and alpha 3 beta 4 receptors, respectively. Higher (+)-tubocurarine concentrations inhibited cholinergic responses, thus yielding a "bell-shaped" concentration-response curve.