The β subunit dominates the relaxation kinetics of heteromeric neuronal nicotinic receptors

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Abstract
The ACh-induced voltage-jump relaxation currents of the nicotinic receptors formed by pair-wise expression of the rat alpha2, alpha3, or alpha4 subunits with the beta2 or beta4 subunit in Xenopus oocytes were fitted best by the sum of two exponentials and a constant between -60 and -150 mV. As the ACh concentration approached zero, the relaxation time constants approached limiting values that should equal the single-channel burst duration at low ACh concentrations and the synaptic current decay time constants. beta4 co-expression prolonged the zero ACh concentration limits for the relaxation time constants. The fast beta4 zero ACh concentration limits ranged from 40 to 121 ms between -60 and -150 mV, and the slow beta4 zero ACh concentration limits ranged from 274 to 1039 ms. In contrast, the fast beta2 limits were 4-6 ms over the same voltage range and the slow beta2 limits were 30-53 ms. Expression with the beta4 subunit increased the voltage sensitivity of the alpha2, alpha3 and slow alpha4 relaxation time constants but not that of the fast alpha4 relaxation time constant. Reducing the temperature from 22 C to 8-9 C increased the alpha4beta2 and alpha3beta4 relaxation time constants 2.3- to 6.6-fold and reduced the fractional amplitude of the fast relaxation component. It also increased the voltage dependence of the fast alpha3beta4 relaxation time constant and decreased that of the slow time constant. The Q10 for alpha4beta2 and alpha3beta4 relaxation time constants ranged from 1.9 to 3.9 between 10 and 20 C. The beta subunit appears to have a dominant influence on the voltage-jump relaxation kinetics of heteromeric neuronal nicotinic receptors.