The beta subunit dominates the relaxation kinetics of heteromeric neuronalnicotinic receptors

1. The ACh-induced voltage-jump relaxation currents of the nicotinic recept ors formed by pairwise expression of the rat alpha 2, alpha 3, or alpha 4 s ubunits with the beta 2 or beta 4 subunit in Xenopus oocytes were fitted be st by the sum of two exponentials and a constant between -60 and -150 mV. 2. As the ACh concentration approached zero, the relaxation time constants approached limiting values that should equal the single-channel burst durat ion at low ACh concentrations and the synaptic current decay time constants . beta 4 co-expression prolonged the zero ACh concentration limits for the relaxation time constants. The fast beta 4 zero ACh concentration limits ra nged from 40 to 121 ms between -60 and -150 mV, and the slow beta 4 zero AC h concentration limits ranged from 274 to 1039 ms. In contrast, the fast be ta 2 limits were 4-6 ms over the same voltage range and the slow beta 2 lim its were 30-53 ms. 3. Expression with the beta 4 subunit increased the voltage sensitivity of the alpha 2, alpha 3 and slow alpha 4 relaxation time constants but not tha t of the fast alpha 4 relaxation time constant. 4. Reducing the temperature from 22 degrees C to 8-9 degrees C increased th e alpha 4 beta 2 and alpha 3 beta 4 relaxation time constants 2.3- to 6.6-f old and reduced the fractional amplitude of the fast relaxation component. It also increased the voltage dependence of the fast alpha 3 beta 4 relaxat ion time constant and decreased that of the slow time constant. The Q(10) f or alpha 4 beta 2 and alpha 3 beta 4 relaxation time constants ranged from 1.9 to 3.9 between 10 and 20 degrees C. 5. The beta subunit appears to have a dominant influence on the voltage-jum p relaxation kinetics of heteromeric neuronal nicotinic receptors.