Voltage-Jump Relaxation Kinetics for Wild-type and Chimeric β Subunits of Neuronal Nicotinic Receptors

We have studied the voltage-jump relaxation currents for a series of neuronal nicotinic acetylcholine receptors resulting from the coexpression of wild-type and chimeric β4/β2 subunits with α3 subunits in Xenopus oocytes. With acetylcholine as the agonist, the wild-type α3β4 receptors displayed five- to eightfold slower voltage-jump relaxations than did the wild-type α3β2 receptors. In both cases, the relaxations could best be described by two exponential components of approximately equal amplitudes over a wide range of [ACh]'s. Relaxation rate constants increased with [ACh] and saturated at 20- to 30-fold lower concentrations for the α3β2 receptor than for the α3β4 receptor, as observed previously for the peak steady state conductance. Furthermore, the chimeric β4/β2 subunits showed a transition in the concentration dependence of the rate constants in the region between residues 94 and 109, analogous to our previous observation with steady state conductances. However, our experiments with a series of β-subunit chimeras did not localize residues that govern the absolute value of the kinetic parameters. Hill coefficients for the relaxations also differed from those previously measured for steady state responses. The data reinforce previous conclusions that the region between residues 94 and 109 on the β subunit plays a role in binding agonist but also show that other regions of the receptor control gating kinetics subsequent to the binding step.