Figure 7.

Openings by nonliganded α_twiβδγ receptors expressed in Xenopus oocytes. A, Outside-out patch recordings of α_twiβδγ receptors before (top trace) and after (bottom trace) addition of ACh (n = 6 patches). B, Recordings of macroscopic current in oocytes expressing α_twiβδγ and held at −50 mV. Shown are the baseline current in the absence of ACh, the decrease in holding current upon addition of 100 μm quinidine (arrow) and, following washout of the quinidine, the large inward current that occurred following addition of 300 nm ACh (arrow) to the oocyte. C, Overall comparisons of the effect of quinidine to reduce the holding current in the absence of ACh activation of the different receptor isoforms. The fractional peak current was determined on the basis of reduction in holding current in the presence of 100 μm quinidine relative to total ACh-activated current following washout of quinidine. The mean ± SD are shown and the number of oocytes tested is indicated. D, Traces from an oocyte expressing α_twiβδγ receptors showed decreases in holding current in response to individual application of either 1 μm α-btx (black trace) or 100 μm quinidine. The response to the first application of quinidine was allowed to recover before the oocyte was treated with α-btx. Following treatment with α-btx, the oocyte showed no response to further application of quinidine (gray arrow).